Principal component analysis of ensemble recordings reveals cell assemblies at high temporal resolution

Simultaneous recordings of many single neurons reveals unique insights into network processing spanning the timescale from single spikes to global oscillations. Neurons dynamically self-organize in subgroups of coactivated elements referred to as cell assemblies. Furthermore, these cell assemblies are reactivated, or replayed, preferentially during subsequent rest or sleep episodes, a proposed mechanism for memory trace consolidation. Here we employ Principal Component Analysis to isolate such patterns of neural activity. In addition, a measure is developed to quantify the similarity of instantaneous activity with a template pattern, and we derive theoretical distributions for the null hypothesis of no correlation between spike trains, allowing one to evaluate the statistical significance of instantaneous coactivations. Hence, when applied in an epoch different from the one where the patterns were identified, (e.g. subsequent sleep) this measure allows to identify times and intensities of reactivation. The distribution of this measure provides information on the dynamics of reactivation events: in sleep these occur as transients rather than as a continuous process.     

radish encodes a phospholipase-A2 and defines a neural circuit involved in anesthesia-resistant memory

BACKGROUND: In both vertebrate and invertebrate animals, anesthetic agents cause retrograde amnesia for recently experienced events. In contrast, older memories are resistant to the same treatments. In Drosophila, anesthesia-resistant memory (ARM) and long-term memory (LTM) are genetically distinct forms of long-lasting memory that exist in parallel for at least a day after training. ARM is disrupted in radish mutants but is normal in transgenic flies overexpressing a CREB repressor transgene. In contrast, LTM is normal in radish mutants but is disrupted in CREB repressor transgenic flies. To date, nothing is known about the molecular, genetic, or cell biological pathways underlying ARM. RESULTS: Here, we report the molecular identification of radish as a phospholipase-A2, providing the first clue about signaling pathways underlying ARM in any animal. An enhancer-trap allele of radish (C133) reveals expression in a novel anatomical pathway. Transgenic expression of PLA2 under control of C133 restores normal levels of ARM to radish mutants, whereas transient disruption of neural activity in C133 neurons inhibits memory retention. Notably, expression of C133 is not in mushroom bodies, the primary anatomical focus of olfactory memory research in Drosophila. CONCLUSIONS: Identification of radish as a phospholipase-A2 and the neural expression pattern of an enhancer-trap allele significantly broaden our understanding of the biochemistry and anatomy underlying olfactory memory in Drosophila.     

Simulation studies of a temporal sequence memory model

Models of circuit action in the mammalian hippocampus have led us to a study of habituation circuits. In order to help model the process of habituation we consider here a memory network designed to learn sequences of inputs separated by various time intervals and to repeat these sequences when cued by their initial portions. The structure of the memory is based on the anatomy of the dentate gyrus region of the mammalian hippocampus. The model consists of a number of arrays of cells called lamellae. Each array consists of four lines of model cells coupled uniformly to neighbors within the array and with some randomness to cells in other lamellae. All model cells operate according to first-order differential equations. Two of the lines of cells in each lamella are coupled such that sufficient excitation by a system input generates a wave of activity that travels down the lamella. Such waves effect dynamic storage of the representation of each input, allowing association connections to form that code both the set of cells stimulated by each input and the time interval between successive inputs. Results of simulation of two networks are presented illustrating the model's operating characteristics and memory capacity.     

The medial temporal lobe supports conceptual implicit memory

The medial temporal lobe (MTL) is generally thought to be critical for explicit, but not implicit, memory. Here, we demonstrate that the perirhinal cortex (PRc), within the MTL, plays a role in conceptually-driven implicit memory. Amnesic patients with MTL lesions that converged on the left PRc exhibited deficits on two conceptual implicit tasks (i.e., exemplar generation and semantic decision). A separate functional magnetic resonance imaging (fMRI) study in healthy subjects indicated that PRc activation during encoding of words was predictive of subsequent exemplar generation. Moreover, across subjects, the magnitude of the fMRI and behavioral conceptual priming effects were directly related. Additionally, the PRc region implicated in the fMRI study was the same region of maximal lesion overlap in the patients with impaired conceptual priming. These patient and imaging results converge to suggest that the PRc plays a critical role in conceptual implicit memory, and possibly conceptual processing in general.     

Associative memory model with temporal spike coding and active dendrite

We propose a neuron model whose internal state is integrated on a two dimensional phase space composed of time and dendritic space. Here, the postsynaptic potential is given as a curved surface on the phase space. Using the proposed neuron model, we introduce a continuous-time associative memory model with spike propagation delay and multiple synaptic sites on the dendrite. We show by numerical simulation that the memory capacity is doubled due to the effects of temporal spike coding and active dendrite compared to the sparse coding associative memory model.     

The strawberry gene Cyf1 encodes a phytocystatin with antifungal properties

An EST, encoding a strawberry phytocystatin (PhyCys) obtained from a developing fruit of Fragariaxananassa cv. Elsanta has been characterized. The corresponding gene (Cyf1) had three introns interrupting its ORF that codes for a protein (FaCPI-1) of 235 amino acid residues with a putative signal peptide of 29 residues and an estimated molecular mass for the mature protein of 23.1 kDa. This protein contains, besides a C-terminal extension, several motifs conserved in all members of the PhyCys superfamily: (i) a GG and LARFAV-like motifs towards the N-terminal part of the protein; (ii) the reactive site QVVAG, and (iii) a conserved PW, downstream of the reactive site. Northern blot and in situ hybridization analyses indicated that the Cyf1 gene was expressed in fully expanded leaves, in roots and in achenes, but not in the receptacle (pseudocarp) during fruit development. The recombinant FaCPI-1 protein expressed in E. coli efficiently inhibited papain (K(i) 1.9 x 10(-9) M) and less so cathepsin H (K(i) 4.7 x 10(-7) M) and cathepsin B (K(i) 3.3 x 10(-6) M), and was a good inhibitor of the in vitro growth of phytopathogenic fungi Botrytis cinerea (EC(50): 1.90 microM) and Fusarium oxysporum (EC(50): 2.28 microM).     

Expression profiling reveals differential gene induction underlying specific and non-specific memory for pheromones in mice

Memory for the mating male’s pheromones in female mice is thought to require synaptic changes in the accessory olfactory bulb (AOB). Induction of this memory depends on release of glutamate in response to pheromonal exposure coincident with release of norepinephrine (NE) in the AOB following mating. A similar memory for pheromones can also be induced artificially by local infusion of the GABA_A receptor antagonist bicuculline into the AOB. The natural memory formed by exposure to pheromones during mating is specific to the pheromones sensed by the female during mating. In contrast, the artificial memory induced by bicuculline is non-specific and results in the female mice recognizing all pheromones as if they were from the mating male. Although protein synthesis has been shown to be essential for development of pheromone memory, the gene expression cascades critical for memory formation are not known. We investigated changes in gene expression in the AOB using oligonucleotide microarrays during mating-induced pheromone memory (MIPM) as well as bicuculline-induced pheromone memory (BIPM). We found the set of genes induced during MIPM and BIPM are largely non-overlapping and Ingenuity Pathway Analysis revealed that the signaling pathways in MIPM and BIPM also differ. The products of genes induced during MIPM are associated with synaptic function, indicating the possibility of modification at specific synapses, while those induced during BIPM appear to possess neuron-wide functions, which would be consistent with global cellular changes. Thus, these results begin to provide a mechanistic explanation for specific and non-specific memories induced by pheromones and bicuculline infusion respectively.     

The crystal structure of a fusagenic sperm protein reveals extreme surface properties

Sp18 is an 18 kDa protein that is released from abalone sperm during the acrosome reaction. It coats the acrosomal process where it is thought to mediate fusion between sperm and egg cell membranes. Sp18 is evolutionarily related to lysin, a 16 kDa abalone sperm protein that dissolves the vitelline envelope surrounding the egg. The two proteins were generated by gene duplication followed by rapid divergence by positive selection. Here, we present the crystal structure of green abalone sp18 resolved to 1.86 A. Sp18 is composed of a bundle of five alpha-helices with surface clusters of basic and hydrophobic residues, giving it a large dipole moment and making it extremely amphipathic. The large clusters of hydrophobic surface residues and domains of high positive electrostatic surface charge explain sp18's ability as a potent fusagen of liposomes. The overall fold of sp18 is similar to that of green abalone lysin; however, the surface features of the proteins are quite different, accounting for their different roles in fertilization. This is the first crystal structure of a protein implicated in sperm-egg fusion during animal fertilization.     

Visualization of single mRNAs reveals temporal association of proteins with microRNA-regulated mRNA

Although many proteins are known to function in microRNA (miRNA)-based translational repression, we lack a comprehensive understanding of temporal relationships between the mRNA, miRNA and their constituent proteins. To understand the dynamics of miRNA and protein interactions, we created a synthetic inducible miRNA system in mammalian cells. By visualizing single mRNAs and observing their co-localization with proteins over time, we produced a temporal association map of miRNA-associated factors. Argonaute2, Dcp1a, hedls and Rck co-localize with miRNA-regulated mRNA after 24 h of miRNA induction, and RNAi knockdown of any one of these proteins affected the co-localization of any of the other proteins with miRNA-regulated mRNA, demonstrating that these proteins could interact with each other in a complex. We identified Argonaute2 and hedls as proteins that co-localize and interact with miRNA-regulated mRNA, indicating that processing body components are involved in long-term storage of miRNA-regulated mRNA.     

Spatial and temporal distribution of a Drosophila melanogaster embryonic variable nuclear antigen

A polyclonal antibody has been prepared that specifically recognises a nuclear protein antigen in Drosophila embryos. During development, the antigen appears initially to be uniformly distributed but by nuclear division cycle 10 is seen to accumulate in nuclei in a manner suggesting that it is destroyed or becomes modified upon transition from S- to M phase of the nuclear division cycle. This conclusion is supported by the observed disappearance of the antigen from the postblastoderm nuclei in a manner that reflects the pattern of the first asynchronous postblastoderm cell division and persistence in the polyploid nuclei of the amnioserosa that do not undergo further cell or nuclear division. In Western blot experiments, the antibody detects specifically a 105 kDa nuclear protein that probably corresponds to the antigen detected in embryos by immunocytochemical means.     

A gene product of the mouse t complex with chemical properties of a cell surface-associated component of the extracellular matrix

p63/6.9 is a major testicular cell protein coded for by a gene, called Tcp-1, within the mouse t complex. All wild-type chromosomes carry the Tcp-1^b allele which codes for a basic form of this protein, while all complete mutant t haplotypes carry the Tcp-1^a allele which codes for an acidic form of this protein. Genetic studies have demonstrated a correlation between the Tcp-1 gene and certain t haplotype effects on sperm differentiation and maturation. In this report, an initial biochemical analysis of the p63/6.9 protein is presented. The data provide evidence that p63/6.9 is closely associated with the external surface of testicular cells but not as an integral membrane component. Some properties of the testicular form of this t complex gene product are similar to those reported for the matrix proteins fibronectin and laminin. The possibility is suggested that primary effects of t haplotypes on sperm differentiation could be exerted through the extracellular matrix. p63/6.9 is also present at a lower level within the cytoplasm and membranes of F9 teratocarcinoma cells. It appears that the level of p63/6.9 synthesis and the exact nature of p63/6.9 intra- and/or intermolecular interactions are under tissue-specific control.     

The Arabidopsis elch mutant reveals functions of an ESCRT component in cytokinesis

Recently, an alternative route to the proteasomal protein-degradation pathway was discovered that specifically targets transmembrane proteins marked with a single ubiquitin to the endosomal multivesicular body (MVB) and, subsequently, to the vacuole (yeast) or lysosome (animals), where they are degraded by proteases. Vps23p/TSG101 is a key component of the ESCRT I-III machinery in yeast and animals that recognizes mono-ubiquitylated proteins and sorts them into the MVB. Here, we report that the Arabidopsis ELCH (ELC) gene encodes a Vps23p/TSG101 homolog, and that homologs of all known ESCRT I-III components are present in the Arabidopsis genome. As with its animal and yeast counterparts, ELC binds ubiquitin and localizes to endosomes. Gel-filtration experiments indicate that ELC is a component of a high-molecular-weight complex. Yeast two-hybrid and immunoprecipitation assays showed that ELC interacts with Arabidopsis homologs of the ESCRT I complex. The elc mutant shows multiple nuclei in various cell types, indicating a role in cytokinesis. Double-mutant analysis with kaktus shows that increased ploidy levels do not influence the cytokinesis effect of elc mutants, suggesting that ELC is only important during the first endoreduplication cycle. Double mutants with tubulin folding cofactor a mutants show a synergistic phenotype, suggesting that ELC regulates cytokinesis through the microtubule cytoskeleton.     

Recognition memory and the medial temporal lobe: a new perspective

Recognition memory is widely viewed as consisting of two components, recollection and familiarity, which have been proposed to be dependent on the hippocampus and the adjacent perirhinal cortex, respectively. Here, we propose an alternative perspective: we suggest that the methods traditionally used to separate recollection from familiarity instead separate strong memories from weak memories. A review of work with humans, monkeys and rodents finds evidence for familiarity signals (as well as recollection signals) in the hippocampus and recollection signals (as well as familiarity signals) in the perirhinal cortex. We also indicate ways in which the functions of the medial temporal lobe structures are different, and suggest that these structures work together in a cooperative and complementary way.