Memory Consolidation

Conscious memory for a new experience is initially dependent on information stored in both the hippocampus and neocortex. Systems consolidation is the process by which the hippocampus guides the reorganization of the information stored in the neocortex such that it eventually becomes independent of the hippocampus. Early evidence for systems consolidation was provided by studies of retrograde amnesia, which found that damage to the hippocampus-impaired memories formed in the recent past, but typically spared memories formed in the more remote past. Systems consolidation has been found to occur for both episodic and semantic memories and for both spatial and nonspatial memories, although empirical inconsistencies and theoretical disagreements remain about these issues. Recent work has begun to characterize the neural mechanisms that underlie the dialogue between the hippocampus and neocortex (e.g., “neural replay,” which occurs during sharp wave ripple activity). New work has also identified variables, such as the amount of preexisting knowledge, that affect the rate of consolidation. The increasing use of molecular genetic tools (e.g., optogenetics) can be expected to further improve understanding of the neural mechanisms underlying consolidation.Memory consolidation refers to the process by which a temporary, labile memory is transformed into a more stable, long-lasting form. Memory consolidation was first proposed in 1900 (Müller and Pilzecker 1900; Lechner et al. 1999) to account for the phenomenon of retroactive interference in humans, that is, the finding that learned material remains vulnerable to interference for a period of time after learning. Support for consolidation was already available in the facts of retrograde amnesia, especially as outlined in the earlier writings of Ribot (1881). The key observation was that recent memories are more vulnerable to injury or disease than remote memories, and the significance of this finding for consolidation was immediately appreciated.

In normal memory a process of organization is continually going on—a physical process of organization and a psychological process of repetition and association. In order that ideas may become a part of permanent memory, time must elapse for these processes of organization to be completed. (Burnham 1903, p. 132)

It is useful to note that the term consolidation has different contemporary usages that derive from the same historical sources. For example, the term is commonly used to describe events at the synaptic/cellular level (e.g., protein synthesis), which stabilize synaptic plasticity within hours after learning. In contrast, systems consolidation, which is the primary focus of this review, refers to gradual reorganization of the brain systems that support memory, a process that occurs within long-term memory itself (Squire and Alvarez 1995; Dudai and Morris 2000; Dudai 2012).Systems consolidation is typically, and accurately, described as the process by which memories, initially dependent on the hippocampus, are reorganized as time passes. By this process, the hippocampus gradually becomes less important for storage and retrieval, and a more permanent memory develops in distributed regions of the neocortex. The idea is not that memory is literally transferred from the hippocampus to the neocortex, for information is encoded in the neocortex as well as in hippocampus at the time of learning. The idea is that gradual changes in the neocortex, beginning at the time of learning, establish stable long-term memory by increasing the complexity, distribution, and connectivity among multiple cortical regions. Recent findings have enriched this perspective by emphasizing the dynamic nature of long-term memory (Dudai and Morris 2013). Memory is reconstructive and vulnerable to error, as in false remembering (Schacter and Dodson 2001). Also, under some conditions, long-term memory can transiently return to a labile state (and then gradually stabilize), a phenomenon termed reconsolidation (Nader et al. 2000; Sara 2000; Alberini 2005). In addition, the rate of consolidation can be influenced by the amount of prior knowledge that is available about the material to be learned (Tse et al. 2007; van Kesteren et al. 2012).Neurocomputational models of consolidation (McClelland et al. 1995; McClelland 2013) describe how the acquisition of new knowledge might proceed and suggest a purpose for consolidation. As originally described, elements of information are first stored in a fast-learning hippocampal system. This information directs the training of a “slow learning” neocortex, whereby the hippocampus gradually guides the development of connections between the multiple cortical regions that are active at the time of learning and that represent the memory. Training of the neocortex by the hippocampus (termed “interleaved” training) allows new information to be assimilated into neocortical networks with a minimum of interference. In simulations (McClelland et al. 1995), rapid learning of new information, which was inconsistent with prior knowledge, was shown to cause interference and disrupt previously established representations (“catastrophic interference”). The gradual incorporation of information into the neocortex during consolidation avoids this problem. In a recent revision of this framework (McClelland 2013), neocortical learning is characterized, not so much as fast or slow, but as dependent on prior knowledge. If the information to be learned is consistent with prior knowledge, neocortical learning can be more rapid.This review considers several types of evidence that illuminate the nature of the consolidation process: studies of retrograde amnesia in memory-impaired patients, studies of healthy volunteers with neuroimaging, studies of sleep and memory, studies of experimental animals, both with lesions or other interventions, and studies that track neural activity as time passes after learning.     

Task-induced neural covariability as a signature of approximate Bayesian learning and inference

Perception is often characterized computationally as an inference process in which uncertain or ambiguous sensory inputs are combined with prior expectations. Although behavioral studies have shown that observers can change their prior expectations in the context of a task, robust neural signatures of task-specific priors have been elusive. Here, we analytically derive such signatures under the general assumption that the responses of sensory neurons encode posterior beliefs that combine sensory inputs with task-specific expectations. Specifically, we derive predictions for the task-dependence of correlated neural variability and decision-related signals in sensory neurons. The qualitative aspects of our results are parameter-free and specific to the statistics of each task. The predictions for correlated variability also differ from predictions of classic feedforward models of sensory processing and are therefore a strong test of theories of hierarchical Bayesian inference in the brain. Importantly, we find that Bayesian learning predicts an increase in so-called “differential correlations” as the observer’s internal model learns the stimulus distribution, and the observer’s behavioral performance improves. This stands in contrast to classic feedforward encoding/decoding models of sensory processing, since such correlations are fundamentally information-limiting. We find support for our predictions in data from existing neurophysiological studies across a variety of tasks and brain areas. Finally, we show in simulation how measurements of sensory neural responses can reveal information about a subject’s internal beliefs about the task. Taken together, our results reinterpret task-dependent sources of neural covariability as signatures of Bayesian inference and provide new insights into their cause and their function.     

Place Cells,Grid Cells,and Memory

The hippocampal system is critical for storage and retrieval of declarative memories, including memories for locations and events that take place at those locations. Spatial memories place high demands on capacity. Memories must be distinct to be recalled without interference and encoding must be fast. Recent studies have indicated that hippocampal networks allow for fast storage of large quantities of uncorrelated spatial information. The aim of the this article is to review and discuss some of this work, taking as a starting point the discovery of multiple functionally specialized cell types of the hippocampal–entorhinal circuit, such as place, grid, and border cells. We will show that grid cells provide the hippocampus with a metric, as well as a putative mechanism for decorrelation of representations, that the formation of environment-specific place maps depends on mechanisms for long-term plasticity in the hippocampus, and that long-term spatiotemporal memory storage may depend on offline consolidation processes related to sharp-wave ripple activity in the hippocampus. The multitude of representations generated through interactions between a variety of functionally specialized cell types in the entorhinal–hippocampal circuit may be at the heart of the mechanism for declarative memory formation.The scientific study of human memory started with Herman Ebbinghaus, who initiated the quantitative investigation of associative memory processes as they take place (Ebbinghaus 1885). Ebbinghaus described the conditions that influence memory formation and he determined several basic principles of encoding and recall, such as the law of frequency and the effect of time on forgetting. With Ebbinghaus, higher mental functions were brought to the laboratory. In parallel with the human learning tradition that Ebbinghaus started, a new generation of experimental psychologists described the laws of associative learning in animals. With behaviorists like Pavlov, Watson, Hull, Skinner, and Tolman, a rigorous program for identifying the laws of animal learning was initiated. By the middle of the 20th century, a language for associative learning processes had been developed, and many of the fundamental relationships between environment and behavior had been described. What was completely missing, though, was an understanding of the neural activity underlying the formation of the memory. The behaviorists had deliberately shied away from physiological explanations because of the intangible nature of neural activity at that time.Then the climate began to change. Karl Lashley had shown that lesions in the cerebral cortex had predictable effects on behavior in animals (Lashley 1929, 1950), and Donald Hebb introduced concepts and ideas to account for complex brain functions at the neural circuit level, many of which have retained a place in modern neuroscience (Hebb 1949). Both Lashley and Hebb searched for the engram, but they found no specific locus for it. A significant turning point was reached when Scoville and Milner (1957) reported severe loss of memory in an epileptic patient, patient H.M., after bilateral surgical removal of the hippocampal formation and the surrounding medial temporal lobe areas. “After operation this young man could no longer recognize the hospital staff nor find his way to the bathroom, and he seemed to recall nothing of the day-to-day events of his hospital life.” This tragic misfortune inspired decades of research on the function of the hippocampus in memory. H.M.’s memory impairment could be reproduced in memory tasks in animals and studies of H.M., as well as laboratory animals, pointed to a critical role for the hippocampus in declarative memory—memory, which, in humans, can be consciously recalled and declared, such as memories of experiences and facts (Milner et al. 1968; Mishkin 1978; Cohen and Squire 1980; Squire 1992; Corkin 2002). What was missing from these early studies, however, was a way to address the neuronal mechanisms that led information to be stored as memory.The aim of this article is to show how studies of hippocampal neuronal activity during the past few decades have brought us to a point at which a mechanistic basis of memory formation is beginning to surface. An early landmark in this series of investigations was the discovery of place cells, cells that fire selectively at one or few locations in the environment. At first, these cells seemed to be part of the animal’s instantaneous representation of location, independent of memory, but gradually, over the course of several decades, it has become clear that place cells express current as well as past and future locations. In many ways, place cells can be used as readouts of the memories that are stored in the hippocampus. More recent work has also shown that place cells are part of a wider network of spatially modulated neurons, including grid, border, and head direction cells, each with distinct roles in the representation of space and spatial memory. In this article, we shall discuss potential mechanisms by which these cell types, particularly place and grid cells, in conjunction with synaptic plasticity, may form the basis of a mammalian system for fast high-capacity declarative memory.     

Reconsolidation and the Dynamic Nature of Memory

Memory reconsolidation is the process in which reactivated long-term memory (LTM) becomes transiently sensitive to amnesic agents that are effective at consolidation. The phenomenon was first described more than 50 years ago but did not fit the dominant paradigm that posited that consolidation takes place only once per LTM item. Research on reconsolidation was revitalized only more than a decade ago with the demonstration of reconsolidation in a well-defined behavioral protocol (auditory fear conditioning in the rat) subserved by an identified brain circuit (basolateral amygdala). Since then, reconsolidation has been shown in many studies over a range of species, tasks, and amnesic agents, and cellular and molecular correlates of reconsolidation have also been identified. In this review, I will first define the evidence on which reconsolidation is based, and proceed to discuss some of the conceptual issues facing the field in determining when reconsolidation does and does not occur. Last, I will refer to the potential clinical implications of reconsolidation.Learning and memory are commonly depicted as going through a set of phases. There is the learning or encoding phase, in which information is acquired, by stabilization phase, in which specific mechanisms are engaged to stabilize initially unstable new information (referred to as synaptic consolidation) (Glickman 1961; McGaugh 1966), the “storage” or maintenance phase, during which other mechanisms are involved to maintain the memory, and the retrieval phase, in which specific mechanisms permit a memory to be retrieved (Miller and Springer 1973; Spear 1973). For a long time, from a neurobiological perspective, only acquisition and memory stabilization (Martin et al. 2000; Kandel 2001; Dudai 2004) were considered to be active phases, in the sense that neurons had to perform certain computations or synthesize new RNA and proteins for these phases of memory processing to be performed successfully. After acquisition and stabilization, all other phases were implicitly thought by many to be passive readout of changes in the circuits mediating the long-term memory (LTM). However, the picture has now changed and the maintenance of memory is portrayed as an active process. One of the reasons for this change is the demonstration that a consolidated LTM can become susceptible to disruption and restoration, a process termed “reconsolidation” (Spear 1973; Nader et al. 2000; Sara 2000). There are now detailed molecular and cellular models of this time-dependent active memory phase.This review will first describe the logic of the findings that brought the existence of the consolidation process to light. I will then describe how we concluded that a consolidated memory undergoes reconsolidation in a well-defined behavioral protocol (auditory fear conditioning in the rat). I will then refer to the range of species, tasks, and treatments in which reconsolidation have been reported. One aspect of reconsolidation that has attracted experimental attention involves the finding that there seem to be conditions that facilitate, inhibit, or even prevent reconsolidation from occurring. I present an approach that could help to identify such conditions. Last, I will discuss potential clinical implications of reconsolidation.     

Dynamic patterns of correlated activity in the prefrontal cortex encode information about social behavior

New technologies make it possible to measure activity from many neurons simultaneously. One approach is to analyze simultaneously recorded neurons individually, then group together neurons which increase their activity during similar behaviors into an “ensemble.” However, this notion of an ensemble ignores the ability of neurons to act collectively and encode and transmit information in ways that are not reflected by their individual activity levels. We used microendoscopic GCaMP imaging to measure prefrontal activity while mice were either alone or engaged in social interaction. We developed an approach that combines a neural network classifier and surrogate (shuffled) datasets to characterize how neurons synergistically transmit information about social behavior. Notably, unlike optimal linear classifiers, a neural network classifier with a single linear hidden layer can discriminate network states which differ solely in patterns of coactivity, and not in the activity levels of individual neurons. Using this approach, we found that surrogate datasets which preserve behaviorally specific patterns of coactivity (correlations) outperform those which preserve behaviorally driven changes in activity levels but not correlated activity. Thus, social behavior elicits increases in correlated activity that are not explained simply by the activity levels of the underlying neurons, and prefrontal neurons act collectively to transmit information about socialization via these correlations. Notably, this ability of correlated activity to enhance the information transmitted by neuronal ensembles is diminished in mice lacking the autism-associated gene Shank3. These results show that synergy is an important concept for the coding of social behavior which can be disrupted in disease states, reveal a specific mechanism underlying this synergy (social behavior increases correlated activity within specific ensembles), and outline methods for studying how neurons within an ensemble can work together to encode information.

Behaviorally-specific patterns of correlated activity between prefrontal neurons normally enhance the information that neuronal ensembles transmit about social behavior. This study shows that in a mouse model of autism, individual neurons continue to encode social information, but this additional information carried by patterns of correlated activity is lost.     

Self-avoidance and Tiling: Mechanisms of Dendrite and Axon Spacing

The spatial pattern of branches within axonal or dendritic arbors and the relative arrangement of neighboring arbors with respect to one another impact a neuron''s potential connectivity. Although arbors can adopt diverse branching patterns to suit their functions, evenly spread branches that avoid clumping or overlap are a common feature of many axonal and dendritic arbors. The degree of overlap between neighboring arbors innervating a surface is also characteristic within particular neuron types. The arbors of some populations of neurons innervate a target with a comprehensive and nonoverlapping “tiled” arrangement, whereas those of others show substantial territory overlap. This review focuses on cellular and molecular studies that have provided insight into the regulation of spatial arrangements of neurite branches within and between arbors. These studies have revealed principles that govern arbor arrangements in dendrites and axons in both vertebrates and invertebrates. Diverse molecular mechanisms controlling the spatial patterning of sister branches and neighboring arbors have begun to be elucidated.Axonal and dendritic arbors adopt complex and morphologically diverse shapes that influence neural connectivity and information processing. In this article we review anatomical and molecular studies that elucidate how the arrangements of branches within neuronal arbors are established during development (isoneuronal spacing) and how the relative spacing of arbors is determined when multiple neurons together innervate a defined territory (heteroneuronal spacing). Together these mechanisms ensure that arbors achieve functionally appropriate coverage of input or output territories.Isoneuronal and heteroneuronal processes display a variety of spacing arrangements, suggesting a diversity of underlying molecular mechanisms. Self-avoidance can occur between branches that arise from a single soma (Yau 1976; Kramer and Kuwada 1983; Kramer and Stent 1985), implying that neurons are able to discriminate “self,” which they avoid, from “nonself” arbors, with which they coexist (Kramer and Kuwada 1983). Similarly, arbors from different cells that share the same function and together innervate a defined territory can create a pattern of minimally overlapping neighboring dendritic or axonal fields, known as tiling. Such spacing mechanisms ensure that arbors maximize their spread across a territory while minimizing the redundancy with which the territory is innervated. In contrast, adhesive interactions between arbors can operate to maintain coherence of dendrites at specific targets (Zhu and Luo 2004), or to bundle functionally similar processes and possibly coordinate their activity (Campbell et al. 2009). Understanding how processes are patterned relative to one another can help to uncover the functional logic of neural circuit organization.Here we focus primarily on mechanisms of isoneuronal and heteroneuronal avoidance that result in complete and nonredundant innervation of sensory or synaptic space. Such mechanisms have been studied extensively in systems where neuronal arbors innervate a two-dimensional plane, such as the retina or body wall (Wassle et al. 1981; Perry and Linden 1982; Hitchcock 1989; Lin and Masland 2004; Fuerst et al. 2009; Kramer and Stent 1985; Grueber et al. 2003; Sugimura et al. 2003; Sagasti et al. 2005). However, the principles regulating process spacing in these regions likely also apply in three dimensions, most prominently where processes are segregated into nonoverlapping domains or columns (Huckfeldt et al. 2009). It is also notable that nonneuronal cell types might similarly engage in self-avoidance and form tiling arrangements, including leech comb cells (Jellies and Kristan 1991) and mammalian astrocytes (Bushong et al. 2002; Ogata and Kosaka 2002; Livet et al. 2007). Elucidating the mechanisms of process spacing during development is therefore relevant for understanding principles of tissue organization inside and outside of the nervous system.     

Presynaptic Membrane Retrieval and Endosome Biology: Defining Molecularly Heterogeneous Synaptic Vesicles

The release and uptake of neurotransmitters by synaptic vesicles is a tightly controlled process that occurs in response to diverse stimuli at morphologically disparate synapses. To meet these architectural and functional synaptic demands, it follows that there should be diversity in the mechanisms that control their secretion and retrieval and possibly in the composition of synaptic vesicles within the same terminal. Here we pay particular attention to areas where such diversity is generated, such as the variance in exocytosis/endocytosis coupling, SNAREs defining functionally diverse synaptic vesicle populations and the adaptor-dependent sorting machineries capable of generating vesicle diversity. We argue that there are various synaptic vesicle recycling pathways at any given synapse and discuss several lines of evidence that support the role of the endosome in synaptic vesicle recycling.Chemical synapses contain discrete numbers of synaptic vesicles, which are capable of sustaining neurotransmitter release. Sustained neurotransmission occurs despite the secretory demands imposed by persistent and diverse patterns of neuronal electrical activity. Maintaining synaptic vesicle numbers requires local mechanisms to regenerate these vesicles to prevent their exhaustion, preserve plasma membrane surface area, and to maintain the molecularly distinct identity of a vesicle versus plasma membrane. Rizzoli and Betz (2005) eloquently draw a parallel between chemical neurotransmission with synapse chatter saying that some synapses “whisper,” whereas others “shout.” The “louder” the synapse, the more synaptic vesicles are required, extending from a few hundred vesicles (whisperers) to nearly thousands (shouters). This beautiful analogy implies that every synapse has just one “voice” or species of vesicle. Here we will present the case that synapses are more like choirs in which multiple vesicle species or “voices” contribute to the “pianissimo” or “fortissimo” parts of chemical neurotransmission.Synaptic terminals show a range of structural and functional differences in distinct regions of the brain, suggesting that the mechanisms for exocytosis/endocytosis coupling, as well as local vesicle recycling, may also be diverse. On one side, the Calyx of Held nerve terminal participates in fast and sustained synaptic transmission at high frequency (800 Hz), which is crucial for sound localization in the auditory brainstem (Taschenberger and von Gersdorff 2000; Borst and Soria van Hoeve 2012). The Calyx of Held houses ∼70,000 synaptic vesicles with nearly 3000 vesicles docked per Calyx terminal. These docked vesicles are distributed across the ∼500 active zones that exist per Calyx where vesicle fusion occurs (Satzler et al. 2002). On the other hand, hippocampal synapses fire action potentials at ∼0.5 Hz in bursts (Dobrunz and Stevens 1999). This synapse contains ∼200 synaptic vesicles and one active zone with ∼10 vesicles docked (Schikorski and Stevens 1997). With such a wide functional and structural gamut of synapses, it is reasonable to hypothesize that synaptic vesicles may differ in their retrieval mechanisms, not just at the rate at which the process occurs but also in the molecular pathways used.Two synaptic vesicle retrieval mechanisms, namely clathrin/AP-2/dynamin-dependent biogenesis and kiss-and-run, have been summarized in outstanding recent reviews (see, for example, Augustine et al. 2006; Rizzoli and Jahn 2007; Smith et al. 2008; Royle and Lagnado 2010; Ferguson and De Camilli 2012; Saheki and De Camilli 2012). Therefore, here we focus on the coupling of secretion and membrane retrieval, as well as endosome sorting. We will discuss new developments supporting the existence of diverse functional and molecular pools of synaptic vesicles and how endocytosis and endosome retrieval mechanisms may generate these vesicle pools.     

Viral and Transgenic Reporters and Genetic Analysis of Adult Neurogenesis

Stem and progenitor cells of the developing and adult brain can be effectively identified and manipulated using reporter genes, introduced into transgenic reporter mouse lines or recombinant viruses. Such reporters rely on an ever-increasing variety of fluorescent proteins and a continuously expanding list of regulatory elements and of mouse lines engineered for cell- or time-specific recombination. An important extension of stem-cell-based genetic strategies is an opportunity to explore the properties of newly generated neurons and their contribution to synaptic plasticity. Here, we review available strategies for marking and quantifying various classes of stem and progenitor cells in the adult brain, genetically tracing their progeny, and studying the properties of stem cells and new neurons. We compare various experimental approaches to labeling and investigating stem cells and their progeny and discuss caveats and limitations inherent to each approach.In adult humans and animals, neural stem cells maintained at specific locations in the adult brain, can produce new neurons that integrate into the existing neural circuits and contribute to neural plasticity. Neural stem cells are the only proven source of new neurons in the adult brain; therefore, our understanding of the features and the role of newly generated neurons depends on the ability to identify adult stem cells, trace their lineage, and reveal basic mechanisms governing their maintenance, division, differentiation, and death.There are various strategies to visualize, identify, and enumerate stem cells and their progeny in the adult brain in vivo. Traditionally, studies of neurogenesis relied on immunocytochemical staining of brain sections using stem-cell-specific antibodies and their combinations and on marking (“birth dating”) dividing stem cells and their progeny using thymidine analogs. These techniques are now complemented by powerful genetic approaches for ontogenetic labeling: generation of transgenic reporter animals constitutively expressing marker proteins; indelible labeling of stem cells and their progeny using inducible (usually Cre-based) recombination; and viral delivery of marker genes to stem cells and their progeny. The general strategy for all genetic approaches to neurogenesis is to drive the expression of live markers, such as fluorescent proteins (FPs) of various color, maturation time, stability, or localization, in defined subclasses of stem cells and their progeny. This review will focus on these genetic approaches, describing available genetic tools and their applications for studying adult neurogenesis (with a bias toward hippocampal neurogenesis) and discussing their advantages and limitations. Interested readers can also consult other reviews in this series, including a review on detection and phenotypic characterization of adult neurogenesis (Kuhn et al. 2015).     

Neurogenesis in the Adult Avian Song-Control System

New neurons are added throughout the forebrain of adult birds. The song-control system is a model to investigate the addition of new long-projection neurons to a cortical circuit that regulates song, a learned sensorimotor behavior. Neuroblasts destined for the song nucleus HVC arise in the walls of the lateral ventricle, and wander through the pallium to reach HVC. The survival of new HVC neurons is supported by gonadally secreted testosterone and its downstream effectors including neurotrophins, vascularization, and electrical activity of postsynaptic neurons in nucleus RA (robust nucleus of the arcopallium). In seasonal species, the HVC→RA circuit degenerates in nonbreeding birds, and is reconstructed by the incorporation of new projection neurons in breeding birds. There is a functional linkage between the death of mature HVC neurons and the birth of new neurons. Various hypotheses for the function of adult neurogenesis in the song system can be proposed, but this remains an open question.Song behavior in oscine birds is regulated by a network of pallial and striatal nuclei. The song-control system shows extensive plasticity in adults, including ongoing neurogenesis in several nuclei (Brenowitz 2008). The addition of new neurons to the adult brain of higher vertebrates was first suggested by the pioneering studies of Altman and Das (1965) and Kaplan and Hinds (1977). They reported that labeled cells were present in the dentate gyrus (DG) of rats following the injection of ³H-thymidine. Their claims, however, met with skepticism and the neuronal identity of the new cells that they observed was called into question (Gross 2000). In an influential study, Rakic (1985) injected adult rhesus monkeys with ³H-thymidine and reported that, “all neurons of the rhesus monkey brain are generated during prenatal and early postnatal life.” The study of neuronal addition to the adult brain, was subsequently dropped for ∼20 years in the face of the dogma that neurogenesis was largely completed by birth (Gross 2000). This prevailing view only started to be overturned when Nottebohm and colleagues published a series of studies showing that new cells are added to the cortical-like song nucleus HVC (Fig. 1) of adult canaries (Serinus canarius) (Goldman and Nottebohm 1983). These new cells have neuronal morphology, some of these cells fire action potentials in response to sound (Paton and Nottebohm 1984), receive synaptic input (Burd and Nottebohm 1985), may synapse on neurons in the efferent robust nucleus of the arcopallium (RA) (Alvarez-Buylla et al. 1990), and express neuron-specific proteins (Barami et al. 1995). Together, these studies in songbirds showed that new neurons are born and incorporated into functional circuits in the brains of adults of higher vertebrates (Nottebohm 2004). This research on adult neurogenesis in songbirds stimulated investigators to re-examine this topic in mammals. It soon became clear that new neurons are added throughout life to the DG and olfactory bulb of mammals including humans (Cameron and Gould 1994; Gould et al. 1997, 1999a; Lim et al. 1997; Eriksson et al. 1998). Because of these initial confirmatory reports, there has been explosive growth in study of the mechanisms and functions of adult neurogenesis in the mammalian DG and olfactory bulb.Open in a separate windowFigure 1.A schematic of the neurogenic regions in the avian brain overlaid on the avian song circuits. Neurogenic regions are shown in red. Note the proximity of HVC (and hippocampus [HC]) to the ventricular zone (VZ). A schematic version of the motor pathway for song production is shown in blue. A schematic of the ascending auditory pathway is shown in green. The dotted line indicates an indirect route through many nuclei of the ascending auditory pathway leading to field L in the telencephalon. The anterior forebrain circuit for song learning and plasticity is shown in yellow. NCM, Caudomedial nidopallium; RA, arcopallium; LMAN, lateral magnocellular nucleus of the anterior neostriatum; OB, olfactory bulb; DLM, dorsolateral medial; PAm, parambigualis; RAm, retroambigualisBirds continue to be a productive model for the study of neurogenesis in the adult brain, as discussed below. In this article, we will focus on neurogenesis in the song-control system as this is the most intensively studied model in birds. (For a review of neurogenesis in the avian hippocampus [HC], see Barnea and Pravosudov 2011.) We will discuss the mechanisms of neurogenesis in the song system, intrinsic and extrinsic factors that influence neuronal addition, a linkage between cell death and neurogenesis, seasonal plasticity, and consider potential functions of adult neurogenesis.     

Autophagy and Neuronal Cell Death in Neurological Disorders

Autophagy is implicated in the pathogenesis of major neurodegenerative disorders although concepts about how it influences these diseases are still evolving. Once proposed to be mainly an alternative cell death pathway, autophagy is now widely viewed as both a vital homeostatic mechanism in healthy cells and as an important cytoprotective response mobilized in the face of aging- and disease-related metabolic challenges. In Alzheimer’s, Parkinson’s, Huntington’s, amyotrophic lateral sclerosis, and other diseases, impairment at different stages of autophagy leads to the buildup of pathogenic proteins and damaged organelles, while defeating autophagy’s crucial prosurvival and antiapoptotic effects on neurons. The differences in the location of defects within the autophagy pathway and their molecular basis influence the pattern and pace of neuronal cell death in the various neurological disorders. Future therapeutic strategies for these disorders will be guided in part by understanding the manifold impact of autophagy disruption on neurodegenerative diseases.Soon after the discovery of lysosomes by de Duve in the 1950s, electron microscopists recognized the presence of cytoplasmic organelles within membrane-limited vacuoles (Clark 1957) and observed what appeared to be the progressive breakdown of these contents (Ashford and Porter 1962). Proposing that “prelysosomes” containing sequestered cytoplasm matured to autolysosomes by fusion with primary lysosomes, de Duve and colleagues (de Duve 1963; de Duve and Wattiaux 1966) named this process “autophagy” (self-eating). Neurons, as cells particularly rich in acid phosphatase-positive lysosomes, were a preferred model in the initial investigations of autophagy. Early studies of pathologic states such as neuronal chromatolysis (Holtzman and Novikoff 1965; Holtzman et al. 1967) linked neurodegenerative phenomena to robust proliferation of autophagic vacuoles (AVs) and lysosomes. Although de Duve appreciated the importance of lysosomes for maintaining cell homeostasis, he was especially intrigued with their potential as “suicide bags” capable of triggering cell death by releasing proteases into the cytoplasm. Despite some support for this notion (Brunk and Brun 1972), the concept was not significantly embraced until many decades later. Instead, for many years, lysosomes and autophagy were mainly considered to perform cellular housekeeping and to scavenge and clean up debris during neurodegeneration in preparation for regenerative processes. The connection between autophagy and neuronal cell death reemerged in the 1970s from observations of Clarke and colleagues, who presented evidence that the developing brain deployed autophagy as a form of programmed neuronal cell death during which autophagy was massively up-regulated to eliminate cytoplasmic components, at once killing the neuron and reducing its cell mass for easy removal. Self-degradation was suggested as a more efficient elimination mechanism than apoptosis, which requires a large population of phagocytic cells and access of these cells to the dying region (Baehrecke 2005). Indeed, the best evidence for this process is in the context of massive cell death, as in metamorphosis and involutional states (Das et al. 2012).Clarke proposed that autophagic cell death (ACD)—type 2 programmed cell death (PCD)—could be a relatively common alternative route to death distinct from apoptosis—type 1 PCD (Clarke 1990)—or caspase-independent cell death—type 3 PCD (Fig. 1). The distinguishing features of ACD are marked proliferation of AVs and progressive disappearance of organelles but relative preservation of cytoskeletal and nuclear integrity until late in the process (Schweichel and Merker 1973; Hornung et al. 1989). In this original concept of ACD or type 2 PCD, death is achieved by autophagic digestion of organelles and essential regulatory molecules and elimination of death inhibitory factors (Baehrecke 2005). With the advent of the molecular era of autophagy research in the 1990s, it became possible to verify the most important implication of ACD, namely, that the death could be prevented by inhibiting autophagy genetically or pharmacologically. Meanwhile, reports of prominent lysosomal/autophagic pathology in Alzheimer’s disease (AD) (Cataldo et al. 1997; Nixon et al. 2000, 2005) and other neuropathic states (Anglade et al. 1997; Rubinsztein et al. 2005) raised important questions about whether autophagy pathology signifies a prodeath program or an attempt to maintain survival—a critical question for any potential therapy based on autophagy modulation. In this article, we will examine evidence for the various neuroprotective roles of autophagy and review our current understanding of how specific stages of autophagy may become disrupted and influence the neurodegenerative pattern seen in major adult-onset neurological diseases. We will particularly focus on how neurons regulate the balance between prosurvival autophagy and well-established cell death mechanisms in making life or death decisions.Open in a separate windowFigure 1.Neuronal cell death: three general morphological types of dying cells in the developing nervous system, as initially classified by Schweichel and Merker (1973) and later Clarke (1990). (A,B) Type 1 (“apoptotic”) cell death: (A) A neuron, from the brain of a postnatal day 6 mouse pup, in the middle of apoptotic degeneration showing cell shrinkage, cytoplasmic condensation, ruffled plasma membrane, and a highly electron-dense nucleus. Endoplasmic reticulum (ER) is still recognizable and some are dilated. A small number of autophagic vacuoles (AVs) can be seen (arrows). (B) A late-stage apoptotic neuron displaying electron-dense chromatin balls (CB), each surrounded by a small amount of highly condensed cytoplasm. (Panel from Yang et al. 2008; reprinted, with permission, from the American Association of Pathologists and Bacteriologists.) (C) Type 2 (“autophagic”) cell death: a deafferented isthmo-optic neuron in developing chick brain after uptake of horseradish peroxidase to highlight (electron dense) endocytic and autophagic compartments. The cell death pattern features pyknosis, abundant AVs, and sometimes dilated ER and mitochondria. (Panel from Hornung et al. 1989; reproduced, with permission, from John Wiley & Sons) (D) Type 3 (“cytoplasmic, nonlysosomal”) cell death: a motoneuron displaying markedly dilated rough ER, Golgi, and nuclear envelope, late vacuolization, and increased chromatin granularity. (Panel from Chu-Wang and Oppenheim 1978; reproduced, with permission, from John Wiley & Sons) Scale bars, 1 µm (A,B); 2 µm (C,D).     

Paradox of Mother's Curse and the Maternally Provisioned Offspring Microbiome

Strict maternal transmission creates an “asymmetric sieve” favoring the spread of mutations in organelle genomes that increase female fitness, but diminish male fitness. This phenomenon, called “Mother''s Curse,” can be viewed as an asymmetrical case of intralocus sexual conflict. The evolutionary logic of Mother''s Curse applies to each member of the offspring microbiome, the community of maternally provisioned microbes, believed to number in the hundreds, if not thousands, of species for host vertebrates, including humans. Taken together, these observations pose a compelling evolutionary paradox: How has maternal transmission of an offspring microbiome become a near universal characteristic of the animal kingdom when the genome of each member of that community poses a potential evolutionary threat to the fitness of host males? I review features that limit or reverse Mother''s Curse and contribute to resolving this paradox. I suggest that the evolution of vertical symbiont transmission requires conditions that mitigate the evolutionary threat to host males.The genomes of mitochondria, chloroplasts, and many symbiotic microbes are transmitted maternally by host females to their offspring. Maternal transmission can be transovariole (intracellular, within the egg) or contagious, during gestation, birth, or feeding (Sonneborn 1950; Smith and Dunn 1991; Gillham 1994; O’Neill et al. 1997). Vertically transmitted (VT) symbiont lineages tend to be genetically homogeneous within hosts (Birky et al. 1983, 1989; Funk et al. 2000). Maternal uniparental transmission creates an “asymmetric sieve” wherein mutations advantageous for females, but harmful for males, can spread through a population (Cosmides and Tooby 1981; Frank and Hurst 1996; Zeh and Zeh 2005; Burt and Trivers 2006). Such mutations spread because deleterious male-specific fitness effects do not affect the response to natural selection of the maternally transmitted entities. This adaptive process favoring the transmitting sex is called Mother''s Curse (MC) (Gemmell et al. 2004) and it has been referred to as an irreconcilable instance of intralocus conflict: “… exclusively maternal transmission of cytoplasmic genes (e.g., in mitochondria) can result in suboptimal mitochondrial function in males … a form of [intralocus sexual conflict] that apparently cannot be resolved, because selection on mitochondria in males cannot produce a response” (Bonduriansky and Chenoweth 2009, p. 285).Mitochondria are ubiquitous in animals and despite the indisputable evolutionary logic of MC (Frank and Hurst 1996) there are no reported cases of sperm-killing or son-killing mitochondria (Burt and Trivers 2006). Moreover, many species of animals possess an offspring microbiome, a community of microbes transmitted uniparentally from mother to offspring at some point in development, whether prefertilization, postfertilization, or postnatal (Funkhouser and Bordenstein 2013). In some vertebrates, including humans, this community is believed to number in the hundreds of species (Funkhouser and Bordenstein 2013). Prolonged periods of maternal care, as in mammals and birds, as well as kin-structured sociality, afford many opportunities for maternal provisioning of microbes to developing offspring. The social insects, in particular, show obligate mutualisms with a microbiome that confers important nutritional benefits for its host (Baumann 2005; Engel and Moran 2013), the termites being a classic example (Ikeda-Ohtsubo and Brune 2009).Together, the evolutionary logic of MC and the widespread existence of maternally transmitted hereditary symbioses pose a paradox for evolutionary biology. The maternally provisioned microbiome (MC) consists of tens to hundreds of genomes affording ample opportunity, along with mitochondrial and organelle genomes, for the occurrence of mutations that benefit females while harming host males. Assembling a VT community as a host nutritional or defensive adaptation requires evading MC not once, but from a continuous siege over evolutionary time. This is the Mother''s Curse–microbiome (MC–MB) paradox. It conceptually affiliated with the “paradox of mutualism,” the persistence of interspecific mutualisms despite the advantages of cheating by one or the other member of the mutualism (Heath and Stinchcombe 2014). Symbiont “cheating” on only half the members of a host species, the males, might offer marginal benefits relative to wholesale cheating on both host sexes. Nevertheless, the MC–MB paradox deserves research attention.In this review, I discuss inbreeding, kin selection, compensatory evolution, and defensive advantages against more virulent pathogens (or predators and herbivores) as means for resolving the MC–MB paradox. First, I review the simple population genetics of MC. I discuss how host inbreeding and kin selection (Unckless and Herren 2009; Wade and Brandvain 2009), alone or in concert, allow for a response to selection on male fertility and viability fitness effects of maternally transmitted genomes. As a result, inbreeding and kin selection can limit or prevent the spread of mutations in a hereditary symbiosis (Cowles 1915) that are harmful to males. I will show that, for both inbreeding and kin selection, there exist conditions that “favor the spread of maternally transmitted mutations harmful to females”; a situation that is the reverse of MC. However, many outbreeding, asocial species harbor maternally provisioned microbiomes and these solutions cannot be applied to them.I also consider the evolution of compensatory nuclear mutations that mitigate or eliminate the harm to males of organelles or symbionts, spreading via MC dynamics. However, I find that the relative rate of compensatory evolution is only 1/4 the rate of evolution of male-harming symbionts. Thus, an evolutionary rescue of host males via compensatory host nuclear mutations requires that there be fourfold or more opportunities for compensation offered by a larger host nuclear genome. The larger the number of species in a host microbiome, the more difficult it is to entertain host nuclear compensatory mutations as a resolution of the MC–MB paradox.Next, I consider the situation in which a deleterious, VT symbiont harms its host but prevents host infection by a more severely deleterious contagiously transmitted pathogen (Lively et al. 2005; see also Clay 1988). This is a case in which absolute harm to a host by a maternally provisioned symbiont becomes a “relative” fitness advantage. This is a scenario that may be common in hosts with speciose microbial communities, especially if each microbial species increases host resistance or outright immunity to infectious, virulent pathogens.Finally, I discuss models of symbiont domestication and capture via the evolution of vertical transmission from an ancestral state of horizontal transmission (Drown et al. 2013). I show that the evolution of vertical transmission requires conditions that tend to restrict the capacity for male harming by symbionts. Each of these scenarios significantly expands the range of evolutionary possibilities permitted for the coevolution of host–symbiont assemblages, especially those microbial communities that are maternally, uniparentally transmitted across host generations. Unfortunately, current data do not permit discriminating among these various evolutionary responses to MC, so none can be definitively considered a resolution of the MC–MB paradox.     

Social Diversity in Humans: Implications and Hidden Consequences for Biological Research

Humans are both similar and diverse in such a vast number of dimensions that for human geneticists and social scientists to decide which of these dimensions is a worthy focus of empirical investigation is a formidable challenge. For geneticists, one vital question, of course, revolves around hypothesizing which kind of social diversity might illuminate genetic variation—and vice versa (i.e., what genetic variation illuminates human social diversity). For example, are there health outcomes that can be best explained by genetic variation—or for social scientists, are health outcomes mainly a function of the social diversity of lifestyles and social circumstances of a given population? Indeed, what is a “population,” how is it bounded, and are those boundaries most appropriate or relevant for human genetic research, be they national borders, religious affiliation, ethnic or racial identification, or language group, to name but a few? For social scientists, the matter of what constitutes the relevant borders of a population is equally complex, and the answer is demarcated by the goal of the research project. Although race and caste are categories deployed in both human genetics and social science, the social meaning of race and caste as pathways to employment, health, or education demonstrably overwhelms the analytic and explanatory power of genetic markers of difference between human aggregates.Two contradictory magnetic poles pull medical research on humans in opposite directions, producing a tension that will never be resolved. On the one hand, there is a universalizing impulse—based on a legitimate assumption that human bodies are sufficiently similar that vaccines, catheters, pasteurizing processes, and tranquilizers that work in one population will work in others. On the other hand, and unless and until research protocols establish and confirm specific similarities across populations, there is sufficient human variation that targeting medicines for specific populations can be a legitimate—even vital—empirically driven task. The theoretical question, of course, is why a particular population or subpopulation is to be so targeted? Because of folk theories about different groups’ biological difference, or because of their social and political standing? Age, gender, and race leap to the forefront. The history of research on ailments as disparate as breast and prostate cancer (Rothenberg 1997; Wailoo 2011), heart disease (Cooper et al. 2005), and syphilis (Jones 1981; Reverby 2009) provides strong evidence that the answer is not either/or but both. So, on what grounds do we choose one strategy over the other?And it is precisely on this point that Steven Epstein (2007) raises the most fundamental question:

Out of all the ways by which people differ from one another, why should it be assumed that sex and gender, race and ethnicity, and age are the attributes of identity that are most medically meaningful? Why these markers of identity and not others? (Epstein 2007, p. 10)

The answer is profoundly social and political, economic, and cultural. The United States is the only country in the world that, as public health policy, does not operate on the assumption of the single standard human.Moreover, by highlighting certain categories, there is the unassailable truth that other categories are thereby ignored. But more to the theoretical point, because each of the categories noted above has a potential or real biological base in either scientific or common sense understandings (Schutz 1962), when scientists report findings indicating differences, the danger is that these findings can seductively divert policymakers from seeking alternative interventions that could better address health disparities (Krieger 2011).The goal of Epstein’s monograph was to (a) better understand how ways of thinking about differences in human populations paved the way to try to “improve medical research by making it inclusive,” and (b) explain how and why the strategies of exclusiveness got institutionalized:

Academic researchers receiving federal funds, and pharmaceutical manufacturers hoping to win regulatory approval for their company''s products, are now enjoined to include women, racial and ethnic minorities, children, and the elderly as research subjects in many forms of clinical research … and question the presumption that findings derived from the study of any single group, such as middle-aged white men, might be generalized to other populations. (Epstein 2007, p. 5)

This shift has occurred only in the last two and a half decades, beginning with regulations that were developed first in 1986. Once again, it is important to restate the relatively unique feature of this development as it applies mainly to the United States (Epstein 2007, p. 7). The rest of the world has continued to act on the presupposition of the standard human, at least until now. As we shall see, that is about to change.     

Molecular and Cellular Mechanisms of Lamina-specific Axon Targeting

The specificity of synaptic connections is directly related to the functional integrity of neural circuits. Long-range axon guidance and topographic mapping mechanisms bring axons into spatial proximity of target cells and thus limit the number of potential synaptic partners. Synaptic specificity is then achieved by extracellular short-range guidance cues and cell-surface recognition cues. Neural activity may enhance the precision and strength of specific circuit connections. Here, we focus on one of the final steps of synaptic matchmaking: the targeting of synaptic layers and the mutual recognition of axons and dendrites within these layers.Perception and behavior are critically dependent on synaptic communication between specific neurons. Understanding how neurons achieve such “synaptic specificity” is therefore one of the most fundamental issues in developmental neuroscience. Langley’s notion of “chemical relations” between synaptically connected neurons (Langley 1892) and Sperry’s “chemoaffinity” hypothesis (Sperry 1963) provided a conceptual framework for the development of precise synaptic connections in the central nervous system. Sperry postulated that molecular interactions between neurons and their extracellular environment (including between and amongst axons and dendrites) ensure that connections form only between “appropriate” synaptic partners (Sperry 1963). This hypothesis has been confirmed by experimental work over the last four decades, most importantly by the identification of molecular cues that provide synaptic specificity (see Sanes and Yamagata 2009 for a recent comprehensive review). However, within this broad framework, a number of alternate mechanisms have been shown or proposed to play roles in specific aspects of such targeting processes. Here, we focus on mechanisms that underlie the formation of synaptic layers, a prominent anatomical feature of the visual system as well as many other areas of the CNS.As reviewed previously (O''Leary 2010), the chemoaffinity principle underlies the developmental process of topographic mapping. Indeed, the precision with which neurons preserve the spatial relationships between the visual world and its representation in the brain is remarkable: Across animals ranging from flies to vertebrates, axons that bear signals from adjacent points in visual space invariably choose adjacent targets in the brain (Braitenberg 1967; Lemke and Reber 2005; Sperry 1963). Thus, position-dependent guidance of axons ensures that a visuotopic map develops. However, position in space is just one attribute of a visual stimulus; others include color, brightness, edge detection, and movement. If position in visual space is encoded by localized activation within a two-dimensional field of neurons, then these other features are encoded by local circuits that act both in series and in parallel and are reiterated many times across the field (Fig. 1). These local circuit modules are often envisioned as “columns” that lie orthogonal to the topographic map, with each column corresponding to a pixel in visual space and each level of the column representing a different, specific visual feature within that pixel, such as brightness, color, etc. (Fig. 1). How these columns acquire their laminated structure represents a developmental challenge of extraordinary scale. Although long-range axon guidance and topographic mapping no doubt contribute to restricting the astronomical number of potential synaptic partners, these mechanisms are clearly not sufficient; additional mechanisms must (and do) exist that act on a local scale to provide an additional level of positional information and cell-type-specific “chemoaffinity.”Open in a separate windowFigure 1.Laminae are a fundamental organizing unit of neural circuits. Each column corresponds to a single topographic position (e.g., location on the retina). Within each column, different cell types (shown type A: blue, and type B: red) respond to different features in the visual world, such as motion or luminance. These pixels are repeated many times over and thus cover all of visual space. A simple rule of “Cell type A connects to Layer A, etc.” ensures that functional segregation is maintained in the connections from the retina to the target (parallel processing). Each pixel P1, P2, and P3 connects to a single column (C1, C2, and C3), establishing serial processing. Within each column, there are local circuits that, too, are layer-specific. Thus, laminae ensure functional specificity of both afferent-target connections and local circuit connections.A prominent principle, which guides the formation of connections between specific cell types and is a characteristic feature of CNS architecture, is the concentration of synapses in small areas. These synapse clusters can take the form of planar layers or spherical glomeruli. Although glomeruli are a specialization that appears most prominent in the olfactory system, layers, or laminae, are an almost ubiquitous feature of central nervous system architecture. Indeed, even crude histological stains reveal that axons and dendrites often accumulate in neuropil (cell-body-free areas). Cell-type-specific or single-cell labeling has shown that, within individual neuropil layers, neurites and synapses are not distributed randomly. Rather, synaptic connections arising between neurons with the same or similar functional properties are localized to particular sublaminae that distinguish synapses with different properties (Fig. 1). The structural underpinnings of this functional principle are provided by mechanisms that ensure the lamina-specific branching of the corresponding neurites. How this enormous precision is achieved is the subject of intense investigations in the Drosophila, zebrafish, chick, and mouse visual systems. We will begin by describing three anatomical regions in these model organisms. Then, we will discuss three broad principles of layer-specific targeting in the visual system, namely cell–cell recognition, guidance by matrix cues, and activity-dependent sorting of axon terminals.     

Epilepsy and Adult Neurogenesis

Seizure activity in the hippocampal region strongly affects stem cell-associated plasticity in the adult dentate gyrus. Here, we describe how seizures in rodent models of mesial temporal lobe epilepsy (mTLE) affect multiple steps in the developmental course from the dividing neural stem cell to the migrating and integrating newborn neuron. Furthermore, we discuss recent evidence indicating either that seizure-induced aberrant neurogenesis may contribute to the epileptic disease process or that altered neurogenesis after seizures may represent an attempt of the injured brain to repair itself. Last, we describe how dysfunction of adult neurogenesis caused by chronic seizures may play an important role in the cognitive comorbidities associated with mTLE.The epilepsies are a diverse group of neurological disorders that share the central feature of spontaneous recurrent seizures. Some epilepsies result from inherited mutations in single or multiple genes, termed idiopathic or primary epilepsies, whereas symptomatic or secondary epilepsies develop as a consequence of acquired brain abnormalities, such as from tumor, trauma, stroke, infection, or developmental malformation. Of acquired epilepsies, mesial temporal lobe epilepsy (mTLE) is a particularly common and often intractable form. In addition to pharmacoresistant seizures, the syndrome of mTLE almost always involves impairments in cognitive function (Helmstaedter 2002; Elger et al. 2004; von Lehe et al. 2006) that may progress even with adequate seizure control (Blume 2006).Seizure activity in mTLE subjects typically arises from the hippocampus or other mesial temporal lobe structures. Simple and complex partial seizures, the most common seizure types in this epilepsy syndrome, often become medically refractory and may respond only to surgical resection of the epileptogenic tissue. Patients usually also have secondarily generalized tonic–clonic seizures, although these are often controlled by anticonvulsants. Hippocampi in pharmacoresistant mTLE usually show substantial structural abnormalities that include pyramidal cell loss, astrogliosis, dentate granule cell axonal reorganization (mossy fiber sprouting), and dispersion of the granule cell layer (GCL) (Blumcke et al. 1999, 2012).Humans with mTLE often have a history of an early “precipitating” insult, such as a prolonged or complicated febrile seizure, followed by a latent period and then the development of epilepsy in later childhood or adolescence. These historical findings have led to the development of what are currently the most common animal models, the status epilepticus (SE) models, used to study epileptogenic mechanisms in mTLE. In these models, a prolonged seizure induced by chemoconvulsant (typically kainic acid or pilocarpine) treatment or electrical stimulation leads to an initial brain injury, followed, after a latent period of days to weeks, by spontaneous recurrent seizures. These models recapitulate much of the pathology of human mTLE (reviewed in Buckmaster 2004). Experimental paradigms are necessary to investigate mechanisms underlying mTLE, as surgical specimens from mTLE cases are collected at late stages of the disease and, thus, are unlikely to reveal early features critical for the disease process. Studies of experimental mTLE indicate that excess neural activity in the course of seizures not only damages existing, mature structures of the hippocampal formation but also dramatically affects endogenous neural stem cells (NSCs) within the adult rodent dentate gyrus (Bengzon et al. 1997; Parent et al. 1997; Scott et al. 1998). In the following, we will discuss the consequences of seizure activity on proliferation of NSCs, maturation and integration of newborn neurons, and the functional relevance of seizure-induced neurogenesis.     

The Relationship between Sexual Selection and Sexual Conflict

Evolutionary conflicts of interest arise whenever genetically different individuals interact and their routes to fitness maximization differ. Sexual selection favors traits that increase an individual’s competitiveness to acquire mates and fertilizations. Sexual conflict occurs if an individual of sex A’s relative fitness would increase if it had a “tool” that could alter what an individual of sex B does (including the parental genes transferred), at a cost to B’s fitness. This definition clarifies several issues: Conflict is very common and, although it extends outside traits under sexual selection, sexual selection is a ready source of sexual conflict. Sexual conflict and sexual selection should not be presented as alternative explanations for trait evolution. Conflict is closely linked to the concept of a lag load, which is context-dependent and sex-specific. This makes it possible to ask if one sex can “win.” We expect higher population fitness if females win.Many published studies ask if sexual selection or sexual conflict drives the evolution of key reproductive traits (e.g., mate choice). Here we argue that this is an inappropriate question. By analogy, G. Evelyn Hutchinson (1965) coined the phrase “the ecological theatre and the evolutionary play” to capture how factors that influence the birth, death, and reproduction of individuals (studied by ecologists) determine which individuals reproduce, and “sets the stage” for the selective forces that drive evolutionary trajectories (studied by evolutionary biologists). The more modern concept of “eco-evolutionary feedback” (Schoener 2011) emphasizes that selection changes the character of the actors over time, altering their ecological interactions. No one would sensibly ask whether one or the other shapes the natural world, when obviously both interact to determine the outcome.So why have sexual conflict and sexual selection sometimes been elevated to alternate explanations? This approach is often associated with an assumption that sexual conflict affects traits under direct selection, favoring traits that alter the likelihood of a potential mate agreeing or refusing to mate because it affects the bearer’s immediate reproductive output, whereas “traditional” sexual selection is assumed to favor traits that are under indirect selection because they increase offspring fitness. These “traditional” models are sometimes described as “mutualistic” (e.g., Pizzari and Snook 2003; Rice et al. 2006), although this term appears to be used only when contrasting them with sexual conflict models. The investigators of the original models never describe them as “mutualistic,” which is hardly surprising given that some males are rejected by females.In this review, we first define sexual conflict and sexual selection. We then describe how the notion of a “lag load” can reveal which sex currently has greater “power” in a sexual conflict over a specific resource. Next, we discuss why sexual conflict and sexual selection are sometimes implicitly (or explicitly) presented as alternative explanations for sexual traits (usually female mate choice/resistance). To illustrate the problems with the assumptions made to take this stance, we present a “toy model” of snake mating behavior based on a study by Shine et al. (2005). We show that empirical predictions about the mating behavior that will be observed if females seek to minimize direct cost of mating or to obtain indirect genetic benefits were overly simplistic. This allows us to make the wider point that whom a female is willing to mate with and how often she mates are often related questions. Finally, we discuss the effect of sexual conflict on population fitness.     

Endoreplication

Developmentally programmed polyploidy occurs by at least four different mechanisms, two of which (endoreduplication and endomitosis) involve switching from mitotic cell cycles to endocycles by the selective loss of mitotic cyclin-dependent kinase (CDK) activity and bypassing many of the processes of mitosis. Here we review the mechanisms of endoreplication, focusing on recent results from Drosophila and mice.Eukaryotic cells proliferate by undergoing a sequence of events termed the “mitotic cell cycle” in which the genome is duplicated once and only once between cell divisions. The result is a population of cells with two copies of each chromosome (diploid, or 2C). Agents that interfere with the mechanisms that govern genome duplication frequently induce reinitiation of nuclear DNA replication during S phase. This phenomenon, termed “DNA rereplication,” is an aberrant event that produces a population of cells with a heterogeneous DNA content that reflects incomplete chromosome duplication, stalled replication forks, and DNA damage. In most cells, these events can lead to inducing the cell’s DNA damage response and can lead to apoptosis (Lee et al. 2010).Remarkably, some cells are developmentally programmed to exit their mitotic cell cycle in response either to environmental signals or to injury or stress, and then differentiate into nonproliferating, viable, polyploid cells. This phenomenon, termed “developmentally programmed polyploidy,” is a normal part of animal and plant development that occurs frequently in ferns, flowering plants, mollusks, arthropods, amphibians, and fish, although rarely in mammals. In contrast to DNA rereplication, developmentally programmed polyploidy produces cells with a DNA content of >4C, but in integral multiples of 4C (e.g., 8C, 16C, 32C, etc.), consistent with multiple S phases in the absence of cytokinesis. These cells typically stop proliferating but remain viable in a terminally differentiated state that may serve to regulate tissue size or organization, to trigger cell differentiation or morphogenesis, to increase the number of genes dedicated to tissue-specific functions without increasing the number of cells, or to adapt to environmental conditions. Mitotic divisions of polyploid cells are common for plant species, but they are rarely found in animals. Although known for decades, polyploid mitosis in insects remained mostly unstudied until it was recently shown that the cells of the rectal papilla in Drosophila undergo mitosis after executing two or more endocycles (Fox et al. 2010). Thus, polyploidy is not an irreversible process, although the benefit of this cell cycle variant remains to be elucidated.Developmentally programmed polyploidy occurs by at least four different mechanisms (Ullah et al. 2009). Proliferating cells in the syncytial blastoderm of Drosophila embryos and some hepatocytes in the postnatal liver of mammals become multinucleated and therefore polyploid by failing to undergo cytokinesis after mitosis (“acytokinetic mitosis”). Differentiation of skeletal muscle myoblasts into myotubes, monocytes into osteoclasts, and formation of placental syncytiotrophoblasts involves “cell fusion” to produce multinucleated, terminally differentiated cells that are similarly polyploid. Alternatively, cells may exit their mitotic cell cycle by arresting mitosis during anaphase and failing to undergo cytokinesis. This phenomenon, termed “endomitosis,” produces cells with a single giant nucleus that may subsequently fragment into a multinuclear appearance. Endomitosis occurs in mammals when megakaryoblasts differentiate into megakaryocytes (Bluteau et al. 2009), and in some plant cells (Weingartner et al. 2004). However, the primary mechanism for developmentally programmed polyploidy in arthropods (Smith and Orr-Weaver 1991; Edgar and Orr-Weaver 2001), plants (de la Paz Sanchez et al. 2012), and possibly mammals (Ullah et al. 2009) is “endoreplication” (also referred to as “endoreduplication”). Endoreplication occurs when a cell exits the mitotic cell cycle in G₂ phase and undergoes multiple S phases without entering mitosis and undergoing cytokinesis. The result is a giant cell with a single, enlarged, polyploid nucleus.