Erin M. Schuman

Long-lasting neurotrophin-induced enhancement of synaptic transmission in the adult hippocampus

The neurotrophins are signaling factors important for the differentiation and survival of distinct neuronal populations during development. To test whether the neurotrophins also function in the mature nervous system, the effects of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophic factor 3 (NT-3) on the strength of synaptic transmission in hippocampal slices were determined. Application of BDNF or NT-3 produced a dramatic and sustained (2 to 3 hours) enhancement of synaptic strength at the Schaffer collateral-CA1 synapses; NGF was without significant effect. The enhancement was blocked by K252a, an inhibitor of receptor tyrosine kinases. BDNF and NT-3 decreased paired-pulse facilitation, which is consistent with a possible presynaptic modification. Long-term potentiation could still be elicited in slices previously potentiated by exposure to the neurotrophic factors, which implies that these two forms of plasticity may use at least partially independent cellular mechanisms.

A Requirement for Local Protein Synthesis in Neurotrophin-Induced Hippocampal Synaptic Plasticity

Two neurotrophic factors, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), are able to produce a long-lasting enhancement of synaptic transmission in the hippocampus. Unlike other forms of plasticity, neurotrophin-induced plasticity exhibited an immediate requirement for protein synthesis. Plasticity in rat hippocampal slices in which the synaptic neuropil was isolated from the principal cell bodies also required early protein synthesis. Thus, the neurotrophins may stimulate the synthesis of proteins in either axonal or dendritic compartments, allowing synapses to exert local control over the complement of proteins expressed at individual synaptic sites.

Dendritic Protein Synthesis, Synaptic Plasticity, and Memory

Considerable evidence suggests that the formation of long-term memories requires a critical period of new protein synthesis. Recently, the notion that some of these newly synthesized proteins originate through local translation in neuronal dendrites has gained some traction. Here, we review the experimental support for this idea and highlight some of the key questions outstanding in this area.

A rapamycin-sensitive signaling pathway contributes to long-term synaptic plasticity in the hippocampus

Many forms of long-lasting behavioral and synaptic plasticity require the synthesis of new proteins. For example, long-term potentiation (LTP) that endures for more than an hour requires both transcription and translation. The signal-transduction mechanisms that couple synaptic events to protein translational machinery during long-lasting synaptic plasticity, however, are not well understood. One signaling pathway that is stimulated by growth factors and results in the translation of specific mRNAs includes the rapamycin-sensitive kinase mammalian target of rapamycin (mTOR, also known as FRAP and RAFT-1). Several components of this translational signaling pathway, including mTOR, eukaryotic initiation factor-4E-binding proteins 1 and 2, and eukaryotic initiation factor-4E, are present in the rat hippocampus as shown by Western blot analysis, and these proteins are detected in the cell bodies and dendrites in the hippocampal slices by immunostaining studies. In cultured hippocampal neurons, these proteins are present in dendrites and are often found near the presynaptic protein, synapsin I. At synaptic sites, their distribution completely overlaps with a postsynaptic protein, PSD-95. These observations suggest the postsynaptic localization of these proteins. Disruption of mTOR signaling by rapamycin results in a reduction of late-phase LTP expression induced by high-frequency stimulation; the early phase of LTP is unaffected. Rapamycin also blocks the synaptic potentiation induced by brain-derived neurotrophic factor in hippocampal slices. These results demonstrate an essential role for rapamycin-sensitive signaling in the expression of two forms of synaptic plasticity that require new protein synthesis. The localization of this translational signaling pathway at postsynaptic sites may provide a mechanism that controls local protein synthesis at potentiated synapses.

Compartmentalized Synthesis and Degradation of Proteins in Neurons

An important aspect of gene expression in neurons involves the delivery of mRNAs to particular subcellular domains, where translation of the mRNAs is locally controlled. Local synthesis of protein within dendrites plays a key role in activity-dependent synaptic modifications. In growing axons, local synthesis in the growth cone is important for extension and guidance. Recent evidence also documents the existence of mechanisms permitting local protein degradation, providing bidirectional control of protein composition in local domains. Here, we summarize what is known about local synthesis and degradation of protein in dendrites and axons, highlighting key unresolved questions.

A Role for the Cadherin Family of Cell Adhesion Molecules in Hippocampal Long-Term Potentiation

The cadherins are a family of cell-cell adhesion molecules that mediate Ca2+-dependent homophilic interactions between cells and transduce signals by interacting with cytoplasmic proteins. In the hippocampus, immunostaining combined with confocal microscopy revealed that both neural- (N-) and epithelial- (E-) cadherin are present at synaptic sites, implying a role in synaptic function. Pretreatment of hippocampal slices with antibodies (Abs) raised against the extracellular domain of either N-cad or E-cad had no effect on basal synaptic properties but significantly reduced long-term potentiation (LTP). Infusion of antagonistic peptides containing the His-Ala-Val (HAV) consensus sequence for cadherin dimerization also attenuated LTP induction without affecting previously established LTP. Because the intense synaptic stimulation associated with LTP induction might transiently deplete extracellular Ca2+ and hence potentially destabilize cadherin-cadherin interactions, we examined whether slices could be protected from inhibition by N-cad Abs or HAV peptides by raising the extracellular Ca2+ concentration. Indeed, we found that high extracellular Ca2+ prevented the block of LTP by these agents. Taken together, these results indicate that cadherins are involved in synaptic plasticity, and the stability of cadherin-cadherin bonds may be regulated by synaptic stimulation.

Human memory strength is predicted by theta-frequency phase-locking of single neurons

Learning from novel experiences is a major task of the central nervous system. In mammals, the medial temporal lobe is crucial for this rapid form of learning. The modification of synapses and neuronal circuits through plasticity is thought to underlie memory formation. The induction of synaptic plasticity is favoured by coordinated action-potential timing across populations of neurons. Such coordinated activity of neural populations can give rise to oscillations of different frequencies, recorded in local field potentials. Brain oscillations in the theta frequency range (3–8 Hz) are often associated with the favourable induction of synaptic plasticity as well as behavioural memory. Here we report the activity of single neurons recorded together with the local field potential in humans engaged in a learning task. We show that successful memory formation in humans is predicted by a tight coordination of spike timing with the local theta oscillation. More stereotyped spiking predicts better memory, as indicated by higher retrieval confidence reported by subjects. These findings provide a link between the known modulation of theta oscillations by many memory-modulating behaviours and circuit mechanisms of plasticity.

Depolarization drives β-catenin into neuronal spines promoting changes in synaptic structure and function

Activity-induced changes in adhesion molecules may coordinate presynaptic and postsynaptic plasticity. Here, we demonstrate that β-catenin, which mediates interactions between cadherins and the actin cytoskeleton, moves from dendritic shafts into spines upon depolarization, increasing its association with cadherins. β-catenins redistribution was mimicked or prevented by a tyrosine kinase or phosphatase inhibitor, respectively. Point mutations of β-catenins tyrosine 654 altered the shaft/spine distribution: Y654F-β-catenin-GFP (phosphorylation-prevented) was concentrated in spines, whereas Y654E-β-catenin-GFP (phosphorylation-mimic) accumulated in dendritic shafts. In Y654F-expressing neurons, the PSD-95 or associated synapsin-I clusters were larger than those observed in either wild-type-β-catenin or also Y654E-expressing neurons. Y654F-expressing neurons exhibited a higher minifrequency. Thus, neural activity induces β-catenins redistribution into spines, where it interacts with cadherin to influence synaptic size and strength.

The Local Transcriptome in the Synaptic Neuropil Revealed by Deep Sequencing and High-Resolution Imaging

In neurons, dendritic protein synthesis is required for many forms of long-term synaptic plasticity. The population of mRNAs that are localized to dendrites, however, remains sparsely identified. Here, we use deep sequencing to identify the mRNAs resident in the synaptic neuropil in the hippocampus. Analysis of a neuropil data set yielded a list of 8,379 transcripts of which 2,550 are localized in dendrites and/or axons. Using a fluorescent barcode strategy to label individual mRNAs, we show that their relative abundance in the neuropil varies over 3 orders of magnitude. High-resolution in situ hybridization validated the presence of mRNAs in both cultured neurons and hippocampal slices. Among the many mRNAs identified, we observed a large fraction of known synaptic proteins including signaling molecules, scaffolds and receptors. These results reveal a previously unappreciated enormous potential for the local protein synthesis machinery to supply, maintain and modify the dendritic and synaptic proteome.

Ubiquitin, the proteasome and protein degradation in neuronal function and dysfunction

Eukaryotic protein degradation by the proteasome and the lysosome is a dynamic and complex process in which ubiquitin has a key regulatory role. The distinctive morphology of the postmitotic neuron creates unique challenges for protein degradation systems with respect to cell-surface protein turnover and substrate delivery to proteolytic machineries that are required for both synaptic plasticity and self-renewal. Moreover, the discovery of ubiquitin-positive protein aggregates in a wide spectrum of neurodegenerative diseases underlines the importance and vulnerability of the degradative system in neurons. In this article, we discuss the molecular mechanism of protein degradation in the neuron with respect to both its function and its dysfunction.