Brad E. Pfeiffer

Hippocampal place-cell sequences depict future paths to remembered goals

Effective navigation requires planning extended routes to remembered goal locations. Hippocampal place cells have been proposed to have a role in navigational planning, but direct evidence has been lacking. Here we show that before goal-directed navigation in an open arena, the rat hippocampus generates brief sequences encoding spatial trajectories strongly biased to progress from the subject’s current location to a known goal location. These sequences predict immediate future behaviour, even in cases in which the specific combination of start and goal locations is novel. These results indicate that hippocampal sequence events characterized previously in linearly constrained environments as ‘replay’ are also capable of supporting a goal-directed, trajectory-finding mechanism, which identifies important places and relevant behavioural paths, at specific times when memory retrieval is required, and in a manner that could be used to control subsequent navigational behaviour.

Rapid Translation of Arc/Arg3.1 Selectively Mediates mGluR-Dependent LTD through Persistent Increases in AMPAR Endocytosis Rate

Salient stimuli that modify behavior induce transcription of activity-regulated cytoskeleton-associated protein (Arc/Arg3.1) and transport Arc mRNA into dendrites, suggesting that local Arc translation mediates synaptic plasticity that encodes such stimuli. Here, we demonstrate that long-term synaptic depression (LTD) in hippocampal neurons induced by group 1 metabotropic glutamate receptors (mGluRs) relies on rapid translation of Arc. mGluR-LTD induction causes long-term increases in AMPA receptor endocytosis rate and dendritic synthesis of Arc, a component of the AMPAR endocytosis machinery. Knockdown of Arc prevents mGluRs from triggering AMPAR endocytosis or LTD, and acute blockade of new Arc synthesis with antisense oligonucleotides blocks mGluR-LTD and AMPAR trafficking. In contrast, LTD induced by NMDA receptors does not persistently alter AMPAR endocytosis rate, induce Arc synthesis, or require Arc protein. These data demonstrate a role for local Arc synthesis specifically in mGluR-LTD and suggest that mGluR-LTD may be one consequence of Arc mRNA induction during experience.

Antiviral Effect and Virus-Host Interactions in Response to Alpha Interferon, Gamma Interferon, Poly(I)-Poly(C), Tumor Necrosis Factor Alpha, and Ribavirin in Hepatitis C Virus Subgenomic Replicons

ABSTRACT The recently developed hepatitis C virus (HCV) subgenomic replicon system was utilized to evaluate the efficacy of several known antiviral agents. Cell lines that persistently maintained a genotype 1b replicon were selected. The replicon resident in each cell line had acquired adaptive mutations in the NS5A region that increased colony-forming efficiency, and some replicons had acquired NS3 mutations that alone did not enhance colony-forming efficiency but were synergistic with NS5A mutations. A replicon constructed from the infectious clone of the HCV-1 strain (genotype 1a) was not capable of inducing colony formation even after the introduction of adaptive mutations identified in the genotype 1b replicon. Alpha interferon (IFN-α), IFN-γ, and ribavirin exhibited antiviral activity, while double-stranded RNA (dsRNA) and tumor necrosis factor alpha did not. Analysis of transcript levels for a series of genes stimulated by IFN (ISGs) or dsRNA following treatment with IFN-α, IFN-γ, and dsRNA revealed that both IFNs increased ISG transcript levels, but that some aspect of the dsRNA response pathway was defective in Huh7 cells and replicon cell lines in comparison to primary chimpanzee and tamarin hepatocytes. The colony-forming efficiency of the replicon was reduced or eliminated following replication in the presence of ribavirin, implicating the induction of error-prone replication. The potential role of error-prone replication in the synergy observed between IFN-α and ribavirin in attaining sustained viral clearance is discussed. These studies reveal characteristics of Huh7 cells that may contribute to their unique capacity to support HCV RNA synthesis and demonstrate the utility of the replicon system for mechanistic studies on antiviral agents.

The State of Synapses in Fragile X Syndrome

Fragile X syndrome (FXS) is the most common inherited form of mental retardation and a leading genetic cause of autism. There is increasing evidence in both FXS and other forms of autism that alterations in synapse number, structure, and function are associated and contribute to these prevalent diseases. FXS is caused by loss of function of the Fmr1 gene, which encodes the RNA binding protein, fragile X mental retardation protein (FMRP). Therefore, FXS is a tractable model to understand synaptic dysfunction in cognitive disorders. FMRP is present at synapses where it associates with mRNA and polyribosomes. Accumulating evidence finds roles for FMRP in synapse development, elimination, and plasticity. Here, the authors review the synaptic changes observed in FXS and try to relate these changes to what is known about the molecular function of FMRP. Recent advances in the understanding of the molecular and synaptic function of FMRP, as well as the consequences of its loss, have led to the development of novel therapeutic strategies for FXS.

Multiple Gq-Coupled Receptors Converge on a Common Protein Synthesis-Dependent Long-Term Depression That Is Affected in Fragile X Syndrome Mental Retardation

Gq-coupled, M1 muscarinic acetylcholine receptors (mAChRs) facilitate hippocampal learning, memory, and synaptic plasticity. M1 mAChRs induce long-term synaptic depression (LTD), but little is known about the underlying mechanisms of mAChR-dependent LTD and its link to cognitive function. Here, we demonstrate that chemical activation of M1 mAChRs induces LTD in hippocampal area CA1, which relies on rapid protein synthesis, as well as the extracellular signal-regulated kinase and mammalian target of rapamycin translational activation pathways. Synaptic stimulation of M1 mAChRs, alone, or together with the Gq-coupled glutamate receptors (mGluRs), also results in protein synthesis-dependent LTD. New proteins maintain mAChR-dependent LTD through a persistent decrease in surface AMPA receptors. mAChRs stimulate translation of the RNA-binding protein, Fragile X mental retardation protein (FMRP) and FMRP target mRNAs. In mice without FMRP (Fmr1 knock-out), a model for human Fragile X syndrome mental retardation (FXS), both mGluR- and mAChR-dependent protein synthesis and LTD are affected. Our results reveal that multiple Gq-coupled receptors converge on a common protein synthesis-dependent LTD mechanism, which is aberrant in FXS. These findings suggest novel therapeutic strategies for FXS in the form of mAChR antagonists.

Fragile X mental retardation protein induces synapse loss through acute postsynaptic translational regulation.

Fragile X syndrome, as well as other forms of mental retardation and autism, is associated with altered dendritic spine number and structure. Fragile X syndrome is caused by loss-of-function mutations in Fragile X mental retardation protein (FMRP), an RNA-binding protein that regulates protein synthesis in vivo. It is unknown whether FMRP plays a direct, cell-autonomous role in regulation of synapse number, function, or maturation. Here, we report that acute postsynaptic expression of FMRP in Fmr1 knock-out (KO) neurons results in a decrease in the number of functional and structural synapses without an effect on their synaptic strength or maturational state. Similarly, neurons endogenously expressing FMRP (wild-type) have fewer synapses than neighboring Fmr1 KO neurons. An intact K homology domain 2 (KH2) RNA-binding domain and dephosphorylation of FMRP at S500 were required for the effects of FMRP on synapse number, indicating that FMRP interaction with RNA and translating polyribosomes leads to synapse loss.

Current Advances in Local Protein Synthesis and Synaptic Plasticity

Local or dendritic protein synthesis is required for long-term functional synaptic change, such as long-term potentiation (LTP) and long-term depression (LTD). LTP and LTD both rely on similar signal transduction cascades, which regulate translation initiation. Current research indicates that the specificity by which new proteins participate in either LTP or LTD may be determined in part by specific RNA-binding proteins as well as activity-dependent capture.

Fragile X Mental Retardation Protein Is Required for Synapse Elimination by the Activity-Dependent Transcription Factor MEF2

Fragile X syndrome (FXS), the most common genetic form of mental retardation and autism, is caused by loss-of-function mutations in an RNA-binding protein, Fragile X Mental Retardation Protein (FMRP). Neurons from patients and the mouse Fmr1 knockout (KO) model are characterized by an excess of dendritic spines, suggesting a deficit in excitatory synapse elimination. In response to neuronal activity, myocyte enhancer factor 2 (MEF2) transcription factors induce robust synapse elimination. Here, we demonstrate that MEF2 activation fails to eliminate functional or structural excitatory synapses in hippocampal neurons from Fmr1 KO mice. Similarly, inhibition of endogenous MEF2 increases synapse number in wild-type but not Fmr1 KO neurons. MEF2-dependent synapse elimination is rescued in Fmr1 KO neurons by acute postsynaptic expression of wild-type but not RNA-binding mutants of FMRP. Our results reveal that active MEF2 and FMRP function together in an acute, cell-autonomous mechanism to eliminate excitatory synapses.

Autoassociative dynamics in the generation of sequences of hippocampal place cells

Memory storage in neural networks Neuronal networks can store and retrieve discrete memories, but often fail to retrieve stored sequences. This is because error decreases over time for a static attractor, but builds up drastically over time if patterns are not trained to retrieve themselves but to retrieve the next item in a sequence. Pfeiffer and Foster studied brain activity in awake but immobile rats. Recording simultaneously from a large number of place cells in the hippocampal formation, they found that internally generated sequences alternated between periods of hovering in place while being strengthened, and periods of abrupt transition to a new place. Science, this issue p. 180 Place cell representations of spatial trajectories in awake but immobile animals jump from one location to another. Neuronal circuits produce self-sustaining sequences of activity patterns, but the precise mechanisms remain unknown. Here we provide evidence for autoassociative dynamics in sequence generation. During sharp-wave ripple (SWR) events, hippocampal neurons express sequenced reactivations, which we show are composed of discrete attractors. Each attractor corresponds to a single location, the representation of which sharpens over the course of several milliseconds, as the reactivation focuses at that location. Subsequently, the reactivation transitions rapidly to a spatially discontiguous location. This alternation between sharpening and transition occurs repeatedly within individual SWRs and is locked to the slow-gamma (25 to 50 hertz) rhythm. These findings support theoretical notions of neural network function and reveal a fundamental discretization in the retrieval of memory in the hippocampus, together with a function for gamma oscillations in the control of attractor dynamics.

Reverse Replay of Hippocampal Place Cells Is Uniquely Modulated by Changing Reward

Hippocampal replays are episodes of sequential place cell activity during sharp-wave ripple oscillations (SWRs). Conflicting hypotheses implicate awake replay in learning from reward and in memory retrieval for decision making. Further, awake replays can be forward, in the same order as experienced, or reverse, in the opposite order. However, while the presence or absence of reward has been reported to modulate SWR rate, the effect of reward changes on replay, and on replay direction in particular, has not been examined. Here we report divergence in the response of forward and reverse replays to changing reward. While both classes of replays were observed at reward locations, only reverse replays increased their rate at increased reward or decreased their rate at decreased reward, while forward replays were unchanged. These data demonstrate a unique relationship between reverse replay and reward processing and point to a functional distinction between different directions of replay. VIDEO ABSTRACT.