Evidence for reduced neurogenesis in the aging human hippocampus despite stable stem cell markers

Reduced neurogenesis in the aging mammalian hippocampus has been linked to cognitive deficits and increased risk of dementia. We utilized postmortem human hippocampal tissue from 26 subjects aged 18–88 years to investigate changes in expression of six genes representing different stages of neurogenesis across the healthy adult lifespan. Progressive and significant decreases in mRNA levels of the proliferation marker Ki67 (MKI67) and the immature neuronal marker doublecortin (DCX) were found in the healthy human hippocampus over the lifespan. In contrast, expression of genes for the stem cell marker glial fibrillary acidic protein delta and the neuronal progenitor marker eomesodermin was unchanged with age. These data are consistent with a persistence of the hippocampal stem cell population with age. Age‐associated expression of the proliferation and immature neuron markers MKI67 and DCX, respectively, was unrelated, suggesting that neurogenesis‐associated processes are independently altered at these points in the development from stem cell to neuron. These data are the first to demonstrate normal age‐related decreases at specific stages of adult human hippocampal neurogenesis.     

Adult hippocampal neurogenesis in aging and Alzheimer's disease

Adult neurogenesis occurs in the subgranular zone of the hippocampal dentate gyrus and the subventricular zone of the lateral ventricles. This process is highly regulated by intrinsic and extrinsic factors, which may control the proliferation and/or maturation of neural progenitor cells. Adult-born neurons are integrated in preexisting networks and may have functional implications for adult brain. Here we attempt to summarize relevant findings concerning the physiological role of adult neurogenesis mainly focused on the subgranular zone, and to discuss the reduced neurogenesis observed during aging and the factors that have been involved in this phenomenon. Finally, we focus on hippocampal neurogenesis in Alzheimer's disease, reviewing animal models of the disease used for the study of this process and the conclusions that have been drawn in this context.     

Adult neurogenesis in the short-lived teleost Nothobranchius furzeri: localization of neurogenic niches, molecular characterization and effects of aging

We studied adult neurogenesis in the short‐lived annual fish Nothobranchius furzeri and quantified the effects of aging on the mitotic activity of the neuronal progenitors and the expression of glial fibrillary acid protein (GFAP) in the radial glia. The distribution of neurogenic niches is substantially similar to that of zebrafish and adult stem cells generate neurons, which persist in the adult brain. As opposed to zebrafish, however, the N. furzeri genome contains a doublecortin (DCX) gene. Doublecortin is transiently expressed by newly generated neurons in the telencephalon and optic tectum (OT). We also analyzed the expression of the microRNA miR‐9 and miR‐124 and found that they have complementary expression domains: miR‐9 is expressed in the neurogenic niches of the telencephalon and the radial glia of the OT, while miR‐124 is expressed in differentiated neurons. The main finding of this paper is the demonstration of an age‐dependent decay in adult neurogenesis. Using unbiased stereological estimates of cell numbers, we detected an almost fivefold decrease in the number of mitotically active cells in the OT between young and old age. This reduced mitotic activity is paralleled by a reduction in DCX labeling. Finally, we detected a dramatic up‐regulation of GFAP in the radial glia of the aged brain. This up‐regulation is not paralleled by a similar up‐regulation of S100B and Musashi‐1, two other markers of the radial glia. In summary, the brain of N. furzeri replicates two typical hallmarks of mammalian aging: gliosis and reduced adult neurogenesis.     

Adult neurogenesis in the mammalian brain

New neurons are generated throughout life in distinct areas of the mammalian brain. This process, called adult neurogenesis, has challenged previously held concepts about adult brain plasticity and opened novel therapeutic avenues to treat certain neuro-psychiatric diseases. Here, we review the current knowledge regarding the fate and potency of neural stem cells (NSCs), as well as the mechanisms underlying neuronal differentiation and subsequent integration. Furthermore, we discuss the functional significance of adult neurogenesis in health and disease, and offer brief insight into the future directions of the adult neurogenesis field.     

Involvement of miR-9/MCPIP1 axis in PDGF-BB-mediated neurogenesis in neuronal progenitor cells

Highly conserved microRNA-9 (miR-9) has a critical role in various cellular processes including neurogenesis. However, its regulation by neurotropins that are known to mediate neurogenesis remains poorly defined. In this study, we identify platelet-derived growth factor-BB (PDGF-BB)-mediated upregulation of miR-9, which in turn downregulates its target gene monocyte chemotactic protein-induced protein 1 (MCPIP1), as a key player in modulating proliferation, neuronal differentiation as well as migration of neuronal progenitor cells (NPCs). Results indicate that miR-9-mediated NPC proliferation and neuronal differentiation involves signaling via the nuclear factor-kappa B (NF-κB) and cAMP response element-binding protein (CREB) pathways, and that NPC migration involves CREB but not the NF-κB signaling. These findings thus suggest that miR-9-mediated downregulation of MCPIP1 acts as a molecular switch regulation of neurogenesis.     

Age‐related changes in stem cell dynamics,neurogenesis, apoptosis,and gliosis in the adult brain: A novel teleost fish model of negligible senescence

Adult neurogenesis, the generation of new neurons in the adult central nervous system, is a reported feature of all examined vertebrate species. However, a dramatic decline in the rates of cell proliferation and neuronal differentiation occurs in mammals, typically starting near the onset of sexual maturation. In the present study, we examined possible age‐related changes associated with adult neurogenesis in the brain of brown ghost knifefish (Apteronotus leptorhynchus), a teleost fish distinguished by its enormous neurogenic potential. Contrary to the well‐established alterations in the mammalian brain during aging, in the brain of this teleostean species we could not find evidence for any significant age‐related decline in the absolute levels of stem/progenitor cell proliferation, neuronal and glial differentiation, or long‐term survival of newly generated cells. Moreover, there was no indication that the amount of glial fibrillary acidic protein or the number of apoptotic cells in the brain was altered significantly over the course of adult life. We hypothesize that this first demonstration of negligible cellular senescence in the vertebrate brain is related to the continued growth of this species and to the lack of reproductive senescence during adulthood. The establishment of the adult brain of this species as a novel model of negligible senescence provides new opportunities for the advancement of our understanding of the biology of aging and the fundamental mechanisms that underlie senescence in the brain. © 2013 Wiley Periodicals, Inc. Develop Neurobiol 74: 514–530, 2014     

Aging brain microenvironment decreases hippocampal neurogenesis through Wnt-mediated survivin signaling

Accumulating evidence suggests that adult hippocampal neurogenesis relies on the controlled and continued proliferation of neural progenitor cells (NPCs). With age, neurogenesis decreases through mechanisms that remain unclear but are believed to involve changes in the NPC microenvironment. Here, we provide evidence that NPC proliferation in the adult brain is in part regulated by astrocytes via Wnt signaling and that this cellular cross-talk is modified in the aging brain, leading to decreased proliferation of NPCs. Furthermore, we show that astrocytes regulate the NPC cell cycle by acting on the expression levels of survivin, a known mitotic regulator. Among cell cycle genes found down-regulated in aged NPCs, survivin was the only one that restored NPC proliferation in the aged brain. Our results provide a mechanism for the gradual loss of neurogenesis in the brain associated with aging and suggest that targeted modulation of survivin expression directly or through Wnt signaling could be used to stimulate adult neurogenesis.     

The awakening of dormant neuronal precursors in the adult and aged brain

Beyond the canonical neurogenic niches, there are dormant neuronal precursors in several regions of the adult mammalian brain. Dormant precursors maintain persisting post-mitotic immaturity from birth to adulthood, followed by staggered awakening, in a process that is still largely unresolved. Strikingly, due to the slow rate of awakening, some precursors remain immature until old age, which led us to question whether their awakening and maturation are affected by aging. To this end, we studied the maturation of dormant precursors in transgenic mice (DCX-CreER^T2/flox-EGFP) in which immature precursors were labelled permanently in vivo at different ages. We found that dormant precursors are capable of awakening at young age, becoming adult-matured neurons (AM), as well as of awakening at old age, becoming late AM. Thus, protracted immaturity does not prevent late awakening and maturation. However, late AM diverged morphologically and functionally from AM. Moreover, AM were functionally most similar to neonatal-matured neurons (NM). Conversely, late AM were endowed with high intrinsic excitability and high input resistance, and received a smaller amount of spontaneous synaptic input, implying their relative immaturity. Thus, late AM awakening still occurs at advanced age, but the maturation process is slow.     

Regulatory interactions during early neurogenesis in Drosophila

During neurogenesis in Drosophila, ectodermal cells are endowed with the capacity to become neuronal precursors. Following their selection, these cells initiate neuronal lineage development and differentiation. The processes of neuronal precursor specification and neuronal lineage development require the activities of several groups of genes functioning in a complex, hierarchical regulatory network. Whereas the proneural genes promote neurogenic potential, neurogenic genes restrict the acquisition of this identity to a subset of ectodermal cells. Following their selection, these cells express the pan neural neuronal precursor genes and a set of neuronal lineage identity genes. While lineage identity genes allow the various lineages to acquire specific identities, neuronal precursor genes presumably regulate functional and developmental characteristics common to all neuronal precursor cells. © 1996 Wiley-Liss, Inc.     

Hippocampal neurogenesis is associated with migratory behaviour in adult but not juvenile sparrows (Zonotrichia leucophrys ssp.)

It has been hypothesized that individuals who have higher demands for spatially based behaviours should show increases in hippocampal attributes. Some avian species have been shown to use a spatially based representation of their environment during migration. Further, differences in hippocampal attributes have been shown between migratory and non-migratory subspecies as well as between individuals with and without migratory experience (juveniles versus adults). We tested whether migratory behaviour might also be associated with increased hippocampal neurogenesis, and whether potential differences track previously reported differences in hippocampal attributes between a migratory (Zonotrichia leucophrys gambelii) and non-migratory subspecies (Z. l. nuttalli) of white-crowned sparrows. We found that non-migratory adults had relatively fewer numbers of immature hippocampal neurons than adult migratory birds, while adult non-migrants had a lower density of new hippocampal neurons than adult and juvenile migratory birds and juvenile non-migratory birds. Our results suggest that neurogenesis decreases with age, as juveniles, regardless of migratory status, exhibit similar and higher levels of neurogenesis than non-migratory adults. However, our results also suggest that adult migrants may either seasonally increase or maintain neurogenesis levels comparable to those found in juveniles. Our results thus suggest that migratory behaviour in adults is associated with maintained or increased neurogenesis and the differential production of new neurons may be the mechanism underpinning changes in the hippocampal architecture between adult migratory and non-migratory birds.     

Abca7 deletion does not affect adult neurogenesis in the mouse

ATP-binding cassette transporter A7 (ABCA7) is highly expressed in the brain. Recent genome-wide association studies (GWAS) have identified ABCA7 single nucleotide polymorphisms (SNPs) that increase Alzheimer''s disease (AD) risk, however, the mechanisms by which ABCA7 may control AD risk remain to be fully elucidated. Based on previous research suggesting that certain ABC transporters may play a role in the regulation of neurogenesis, we conducted a study of cell proliferation and neurogenic potential using cellular bromodeoxyuridine (BrdU) incorporation and doublecortin (DCX) immunostaining in adult Abca7 deficient mice and wild-type-like (WT) littermates. In the present study counting of BrdU-positive and DCX-positive cells in an established adult neurogenesis site in the dentate gyrus (DG) indicated there were no significant differences when WT and Abca7 deficient mice were compared. We also measured the area occupied by immunohistochemical staining for BrdU and DCX in the DG and the subventricular zone (SVZ) of the same mice and this confirmed that ABCA7 does not play a significant role in the regulation of cell proliferation or neurogenesis in the adult mouse.     

Cellular organization of adult neurogenesis in the Common Marmoset

Adult neurogenesis within the subgranular zone (SGZ) of the hippocampal dentate gyrus and the subventricular zone (SVZ) of the lateral ventricle (LV) has been most intensely studied within the brains of rodents such as mice and rats. However, little is known about the cell types and processes involved in adult neurogenesis within primates such as the common marmoset (Callithrix jacchus). Moreover, substantial differences seem to exist between the neurogenic niche of the LV between rodents and humans. Here, we set out to use immunohistochemical and autogradiographic analysis to characterize the anatomy of the neurogenic niches and the expression of cell type-specific markers in those niches in the adult common marmoset brain. Moreover, we demonstrate significant differences in the activity of neurogenesis in the adult marmoset brain compared to the adult mouse brain. Finally, we provide evidence for ongoing proliferation of neuroblasts within both the SGZ and SVZ of the adult brain and further show that the age-dependent decline of neurogenesis in the hippocampus is associated with a decrease in neuroblast cells.     

Alterations in intrinsic neuronal excitability during normal aging

Normal aging subjects, including humans, have difficulty learning hippocampus-dependent tasks. For example, at least 50% of normal aging rabbits and rats fail to meet a learning criterion in trace eyeblink conditioning. Many factors may contribute to this age-related learning impairment. An important cause is the reduced intrinsic excitability observed in hippocampal pyramidal neurons from normal aging subjects, as reflected by an enlarged postburst afterhyperpolarization (AHP) and an increased spike-frequency adaptation (accommodation). In this review, we will focus on the alterations in the AHP and accommodation during learning and normal aging. We propose that age-related increases in the postburst AHP and accommodation in hippocampal pyramidal neurons play an integral role in the learning impairment observed in normal aging subjects.     

Mapping neurogenesis onset in the optic tectum of Xenopus laevis

Neural progenitor cells have a central role in the development and evolution of the vertebrate brain. During early brain development, neural progenitors first expand their numbers through repeated proliferative divisions and then begin to exhibit neurogenic divisions. The transparent and experimentally accessible optic tectum of Xenopus laevis is an excellent model system for the study of the cell biology of neurogenesis, but the precise spatial and temporal relationship between proliferative and neurogenic progenitors has not been explored in this system. Here we construct a spatial map of proliferative and neurogenic divisions through lineage tracing of individual progenitors and their progeny. We find a clear spatial separation of proliferative and neurogenic progenitors along the anterior‐posterior axis of the optic tectum, with proliferative progenitors located more posteriorly and neurogenic progenitors located more anteriorly. Since individual progenitors are repositioned toward more anterior locations as they mature, this spatial separation likely reflects an increasing restriction in the proliferative potential of individual progenitors. We then examined whether the transition from proliferative to neurogenic behavior correlates with cellular properties that have previously been implicated in regulating neurogenesis onset. Our data reveal that the transition from proliferation to neurogenesis is associated with a small change in cleavage plane orientation and a more pronounced change in cell cycle kinetics in a manner reminiscent of observations from mammalian systems. Our findings highlight the potential to use the optic tectum of Xenopus laevis as an accessible system for the study of the cell biology of neurogenesis. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1328–1341, 2016     

Plasticity of hippocampal stem/progenitor cells to enhance neurogenesis in response to kainate-induced injury is lost by middle age

A remarkable up-regulation of neurogenesis through increased proliferation of neural stem/progenitor cells (NSCs) is a well-known plasticity displayed by the young dentate gyrus (DG) following brain injury. To ascertain whether this plasticity is preserved during aging, we quantified DG neurogenesis in the young adult, middle-aged and aged F344 rats after kainic acid induced hippocampal injury. Measurement of new cells that are added to the dentate granule cell layer (GCL) between post-injury days 4 and 15 using 5'-bromodeoxyuridine labeling revealed an increased addition of new cells in the young DG but not in the middle-aged and aged DG. Quantification of newly born neurons using doublecortin immunostaining also demonstrated a similar trend. Furthermore, the extent of ectopic migration of new neurons into the dentate hilus was dramatically increased in the young DG but was unaltered in the middle-aged and aged DG. However, there was no change in neuronal fate-choice decision of newly born cells following injury in all age groups. Similarly, comparable fractions of new cells that are added to the GCL after injury exhibited 5-month survival and expressed the mature neuronal marker NeuN, regardless of age or injury at the time of their birth. Thus, hippocampal injury does not adequately stimulate NSCs in the middle-aged and aged DG, resulting in no changes in neurogenesis after injury. Interestingly, rates of both neuronal fate-choice decision and long-term survival of newly born cells remain stable with injury in all age groups. These results underscore that the ability of the DG to increase neurogenesis after injury is lost as early as middle age.     

Suppression of IGF‐I signals in neural stem cells enhances neurogenesis and olfactory function during aging

Downregulation of insulin-like growth factor (IGF) pathways prolongs lifespan in various species, including mammals. Still, the cellular mechanisms by which IGF signaling controls the aging trajectory of individual organs are largely unknown. Here, we asked whether suppression of IGF-I receptor (IGF-1R) in adult stem cells preserves long-term cell replacement, and whether this may prevent age-related functional decline in a regenerating tissue. Using neurogenesis as a paradigm, we showed that conditional knockout of IGF-1R specifically in adult neural stem cells (NSC) maintained youthful characteristics of olfactory bulb neurogenesis within an aging brain. We found that blocking IGF-I signaling in neural precursors increased cumulative neuroblast production and enhanced neuronal integration into the olfactory bulb. This in turn resulted in neuro-anatomical changes that improved olfactory function. Interestingly, mutants also displayed long-term alterations in energy metabolism, possibly related to IGF-1R deletion in NSCs throughout lifespan. We explored Akt and ERK signaling cascades and revealed differential regulation downstream of IGF-1R, with Akt phosphorylation preferentially decreased in IGF-1R^−/− NSCs within the niche, and ERK pathway downregulated in differentiated neurons of the OB. These challenging experimental results were sustained by data from mathematical modeling, predicting that diminished stimulation of growth is indeed optimal for tissue aging. Thus, inhibiting growth and longevity gene IGF-1R in adult NSCs induced a gain-of-function phenotype during aging, marked by optimized management of cell renewal, and enhanced olfactory sensory function.     

BNIP3 as a new tool to promote healthy brain aging

The article “Neuronal induction of BNIP3-mediated mitophagy slows systemic aging in Drosophila” reveals BCL2-interacting protein 3 as a therapeutic target to counteract brain aging and prolong overall organismal health with age. In this spotlight, we consider the roles of BNIP3, a mitochondrial outer membrane protein, in the adult nervous system, including its induction of mitophagy and prevention of dysfunctional mitochondria in the aged brain. Implications for other tissue types to reduce the burden of aging are further considered.     

Where's my dinner? Adult neurogenesis in free‐living food‐storing rodents

Postnatal hippocampal neurogenesis in wild mammals may play an essential role in spatial memory. We compared two species that differ in their reliance on memory to locate stored food. Yellow-pine chipmunks use a single cache to store winter food; eastern gray squirrels use multiple storage sites. Gray squirrels had three times the density of proliferating cells in the dentate gyrus (determined by Ki-67 immunostaining) than that found in chipmunks, but similar density of young neurons (determined by doublecortin immunostaining). Three explanations may account for these results. First, the larger population of young cells in squirrels may increase the flexibility of the spatial memory system by providing a larger pool of cells from which new neurons can be recruited. Second, squirrels may have a more rapid cell turnover rate. Third, many young cells in the squirrels may mature into glia rather than neurons. The densities of young neurons were higher in juveniles than in adults of both species. The relationship between adult age and cell density was more complex than that has been found in captive populations. In adult squirrels, the density of proliferating cells decreased exponentially with age, whereas in adult chipmunks the density of young neurons decreased exponentially with age.     

Enriched environment increases neurogenesis in the adult rat dentate gyrus and improves spatial memory

The fetal and even the young brain possesses a considerable degree of plasticity. The plasticity and rate of neurogenesis in the adult brain is much less pronounced. The present study was conducted to investigate whether housing conditions affect neurogenesis, learning, and memory in adult rats. Three‐month‐old rats housed either in isolation or in an enriched environment were injected intraperitoneally with bromodeoxyuridine (BrdU) to detect proliferation among progenitor cells and to follow their fate in the dentate gyrus. The rats were sacrificed either 1 day or 4 weeks after BrdU injections. This experimental paradigm allows for discrimination between proliferative effects and survival effects on the newborn progenitors elicited by different housing conditions. The number of newborn cells in the dentate gyrus was not altered 1 day after BrdU injections. In contrast, the number of surviving progenitors 1 month after BrdU injections was markedly increased in animals housed in an enriched environment. The relative ratio of neurogenesis and gliogenesis was not affected by environmental conditions, as estimated by double‐labeling immunofluorescence staining with antibodies against BrdU and either the neuronal marker calbindin D28k or the glial marker GFAp, resulting in a net increase in neurogenesis in animals housed in an enriched environment. Furthermore, we show that adult rats housed in an enriched environment show improved performance in a spatial learning test. The results suggest that environmental cues can enhance neurogenesis in the adult hippocampal region, which is associated with improved spatial memory. © 1999 John Wiley & Sons, Inc. J Neurobiol 39: 569–578, 1999