Browsing by Subject "Synaptic plasticity"

The late phase of long-term synaptic potentiation (LTP) at glutamatergic synapses, which is thought to underlie the long lasting memory (at least hours), requires gene transcription in the nucleus. However, it remains elusive how signaling initiated at synapses during induction of LTP is transmitted into the nucleus to commence transcription. Using a combination of two-photon glutamate uncaging and a genetically encoded FRET sensor, I found that induction of synapse-specific LTP at only a few (3-7) dendritic spines leads to pronounced activation of extracellular signal-regulated kinase (ERK) in the nucleus and downstream phosphorylation of transcription factors, cAMP-response element-binding protein (CREB) and E26-like protein-1 (Elk-1). The underlying molecular mechanism of this nuclear ERK activation was investigated: it seems to involve activation of NMDA receptors, metabotrophic glutamate receptors, and the classical Ras pathway. I also found that the spatial pattern of synaptic stimulation matters: spatially dispersed stimulation over multiple dendritic branches activated nuclear ERK much more efficiently than clustered stimulation within a single dendritic branch. In sum, these results suggest that biochemical signals could be transmitted from individual spines to the nucleus following LTP induction and that such synapse-to-nucleus signaling requires integration across multiple dendritic branches.

Long-term plasticity, the long-lasting, activity-dependent change in synaptic efficacy, is a fundamental property of the nervous system. Presynaptic forms of long-term plasticity are widely expressed throughout the brain, having been described in regions such as the cortex, cerebellum, hippocampus, thalamus, amygdala and striatum. Presynaptic long-term potentiation (LTP) is associated with an increase in presynaptic release probability, but further evidence of the cellular basis for the change in release probability is not known. At the molecular level, presynaptic LTP is known to require protein kinase A, the synaptic vesicle protein, Rab3A, and the active zone protein, RIM1alpha. RIM1alpha, a presynaptic scaffold protein, binds to many molecules with known functions at different stages of the neurotransmitter release process and the synaptic vesicle cycle. Understanding which interactions of RIM1alpha mediate presynaptic LTP would shed light on the molecular and cellular mechanisms for presynaptic long-term plasticity.

Here I developed a novel platform to achieve robust acute genetic

manipulation of presynaptic proteins at hippocampal mossy fiber synapses, where presynaptic LTP is expressed. With this platform, I perform structure-function analysis of RIM1alpha in presynaptic LTP. I find that RIM1alpha phosphorylation by PKA at serine 413 is not required for mossy fiber LTP, nor does RIM1alpha-Rab3A interation. These findings suggest that RIM1alpha, Rab3A and PKA signaling, instead of functioning synergistically, may represent separate requirements for presynaptic long-term plasticity. I then tested whether Munc13-1, a priming protein, is an effector for RIM1alpha in presynaptic LTP and provide the first evidence for the involvement of Munc13-1 in presynaptic long-term synaptic plasticity. I further demonstrate that the interaction between RIM1alpha and Munc13-1 is required for this plasticity. These results further our understanding of the molecular mechanisms of presynaptic plasticity and suggest that modulation of vesicle priming may provide the cellular substrate for expression of LTP at mossy fiber synapses.

Long-term memory is believed to be stored in the human brain by changing synapses in a neuronal activity-dependent way. This idea has been implemented in the attractor neural network models, where the connectivity strength between neurons is determined by Hebbian synaptic plasticity rules. Classical studies of memory modeling synapses as continuous variables in networks of binary neurons have shown that such networks can have large storage capacities. However, a rising number of evidence suggests that synapses in brain structures involved in memory, such as the hippocampus and neocortex, are more digital than analog. Understanding how a large amount of information can be robustly stored with discrete-like synapses in the brain remains an open question in the field of computational neuroscience.

In this study, we explored a series of synaptic plasticity rules for discrete-like synapses and investigated how their application in attractor neural networks will affect the memory function of the system. We built mean-field equations to calculate the storage capacity of the network. We studied a network with a binarized Hebbian learning rule, showing that such networks can provide a near-optimal storage capacity, in the space of all possible binary connectivity matrices. We investigated a model with double-well synapses, where each synapse is described by a continuous variable that evolves in a potential with multiple minima. We showed that this model could interpolate between models with discrete synapses and models with continuous synapses by varying the shape of the potential. Our results indicated that discrete-like synapses could benefit neural networks by increasing their robustness with respect to noise. Furthermore, we incorporated the double-well synapses model with the memory consolidation mechanism. Our result showed that memory consolidation could significantly enhance the storage capacity of the network, leading to a power law decay of the memory forgetting curve as observed in psychological experiments.

Synaptic plasticity is the leading candidate for the cellular/molecular basis of learning and memory. One of the key molecules involved in synaptic plasticity is Calcium/calmodulin-dependent Kinase II (CaMKII). Synaptic plasticity can be expressed at a single dendritic spine independent of its neighboring dendritic spines. Here, we investigated how long the activity of CaMKII lasts during synaptic plasticity of single dendritic spines. We found that CaMKII activity lasted ~2 minutes during synaptic plasticity and was restricted to the dendritic spines undergoing synaptic plasticity while nearby dendritic spines did not show any change in the level of CaMKII activity. Our experimental data argue against the persistent activation of CaMKII in dendritic spines undergoing synaptic plasticity and suggest that the activity of CaMKII is a spine-specific biochemical signal necessary for synapse-specificity of synaptic plasticity. We provide a biophysical explanation of how spine-specific CaMKII activation can be achieved during synaptic plasticity. We also found that CaMKII is activated by highly localized calcium influx in the proximity of Voltage-dependent Calcium Channels (VDCCs) and a different set of VDCCs and their respective Ca2+ nanodomains are responsible for the differential activation of CaMKII between dendritic spines and dendritic shafts.

Learning and memory is one of the critical components of the human experience. In one model of memory, hippocampal LTP, it is believed that the trafficking of AMPA receptors to the synapse is a fundamental process, yet the spatiotemporal kinetics of the process remain under dispute. In this work, we imaged the trafficking of AMPA receptors by combining two-photon glutamate uncaging on single spines with a fluorescent reporter for surface AMPA receptors. We found that AMPA receptors are trafficked to the spine at the same time as the spine size is increasing. Using a bleaching protocol, we found that the receptors that reach the spine come from a combination of the surface and endosomal pools. Imaging exocytosis in real time, we found that the exocytosis rate increases briefly (~1 min.), both in the spine and neighbouring dendrite. Finally, we performed pharmacological and genetic manipulations of signaling pathways, and found that the Ras-ERK signaling pathway is necessary for AMPAR exocytosis.

In a set of related experiments, we also investigated the capacity of single spines to undergo potentiation multiple times. By stimulating spines twice using glutamate uncaging, we found that there is a refractory period for synaptic plasticity in spines during which they cannot further be potentiated. We furthermore found that inducing plasticity in a given spine inhibits plasticity at nearby spines.