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The Molecular Basis of Brain Plasticity
Summary: Morgan Sheng seeks to understand the molecular and cellular mechanisms underlying the ability of the brain to change in response to experience, such as during development and learning.
The brain is a massive network of electrically active cells (neurons) that communicate with each other via specialized cell junctions (synapses). Especially during development but also in adult life, the brain responds to experience by adjusting the strength of communication at individual synapses and by changing the physical pattern of synaptic connections between neurons. In this way, information can be stored by the nervous system in the form of altered structure and chemistry of synapses and/or by the formation of new synapses and the elimination of old ones. This "plasticity" of synapses is believed to be the basis of learning and memory and other adaptive properties of the brain. Because of their central importance in information processing and storage, it is important to understand the molecular architecture of synapses and the cellular processes that govern synapse formation, growth, and elimination. By using biochemical, genetic, electrophysiological, and imaging approaches to study the dynamic organization of synapses, we seek to understand the molecular mechanisms by which the brain modifies itself and adapts to experience.
In a "bottoms-up" approach, we are systematically characterizing the individual protein components of synapses and elucidating how these proteins interact with each other to make up the synaptic junction. Synapses consist of a presynaptic neuron that releases a chemical messenger (neurotransmitter) across a narrow gap to stimulate the postsynaptic neuron. Our focus is on the postsynaptic side of excitatory synapses, which use glutamate as the neurotransmitter. Attached to the postsynaptic membrane is a specialized microscopic structure, the postsynaptic density (PSD). The PSD contains the receptors for the neurotransmitter glutamate (glutamate receptors), of which there are three major classes: NMDA receptors, AMPA receptors, and metabotropic glutamate receptors. Associated with these glutamate receptors are numerous scaffold proteins and enzymes that mediate postsynaptic signaling. Using mass spectrometry and electron microscopy, we are measuring the copy number of specific molecules in the PSD and imaging the three-dimensional structure of protein complexes in the PSD. A long-term goal is to achieve a quantitative three-dimensional description of the molecular architecture of the postsynaptic side of the synapse.
We are characterizing the network of proteins and signaling pathways that connect glutamate receptors to the interior of the neuron. By differential activation of postsynaptic signaling pathways, specific patterns of synaptic stimulation can either strengthen (long-term potentiation; LTP) or weaken synaptic transmission (long-term depression; LTD). LTP and LTD in a brain region called the hippocampus are two highly studied models of synaptic plasticity. A central puzzle has been how activation of one class of glutamate receptor (the NMDA receptor) and influx of the same second messenger (Ca2+ ions) can lead to opposite effects on synaptic strength (LTP or LTD). In one project, we are trying to understand how different NMDA receptor subtypes activate different signaling pathways in synapses. Our focus is the hypothesis that the cytoplasmic tails of the NR2A and NR2B subunits of NMDA receptors bind to, and are thereby coupled to, distinct signaling proteins. (A grant from the National Institutes of Health provides support for this project.)
We and others have found that the PSD contains several hundred different proteins. What are the specific functions of the individual components of the PSD with respect to synapse development and plasticity? We are taking several approaches to address these questions for selected key PSD proteins. First, we overexpress each protein in neurons and examine its effects on synapse morphology and function. Second, we introduce into neurons "dominant-negative" mutants of the protein that should poison the activity of the endogenous normal protein. Third, we use RNA interference to specifically suppress the expression of the protein in neurons. Finally, we disrupt the gene by genetic recombination and create a mutant mouse lacking the specific protein. In this last case, we evaluate the importance of the gene in terms of behavior and cognitive function (for instance, how the mutant mouse performs in learning and memory tasks).
Along these lines, we are studying a variety of PSD proteins that are involved in postsynaptic signal transduction, including scaffold proteins such as PSD-95; guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) that regulate small GTPases Ras, Rap, Rho, and Arf; and protein kinases, such as the MAP kinases. We are particularly interested in the involvement of these signaling molecules in LTP and LTD. We are also interested in the highly regulated mechanisms that control the mobility and/or degradation of PSD proteins. For instance, the distribution of the major PSD scaffold protein PSD-95 is regulated by phosphorylation on a specific serine residue, and the postsynaptic RapGAP called SPAR (spine-associated RapGAP) is degraded upon phosphorylation by the activity-inducible protein kinase Plk2. We are using molecular, mouse transgenic, microscopic imaging, proteomic, and electrophysiological techniques to pursue the mechanisms underlying these processes.
Dendritic spines—tiny protrusions found on the dendrites of many neurons—are specialized postsynaptic compartments on which most excitatory synapses are formed. These fascinating structures change in number, size, and shape, depending on a wide variety of factors such as brain activity, neurological disease, hormonal cycles, and aging. The size of spines correlates with the strength of synapses. It is believed that changes in dendritic spine number and morphology reflect synaptic plasticity, particularly changes in synaptic connections between neurons.
We have identified several key proteins within the PSD that regulate dendritic spine morphogenesis, including Shank, a family of scaffold proteins of the PSD that links NMDA receptor and metabotropic glutamate receptor complexes with the actin cytoskeleton. Mutations in Shank have been linked to human autism. We have generated genetically engineered mice that are deficient for the Shank1 protein. These mutant animals show smaller dendritic spines and weaker synaptic transmission but enhanced performance in spatial learning tasks, implying that Shank may be a suppressor of learning. (Funding from the RIKEN-MIT Neuroscience Research Center provides support for this project.)
Postsynaptic signaling pathways orchestrate the functional and morphological changes that underlie synaptic plasticity. Our goal is to understand the biochemical actions of these pathways in the PSD and in dendritic spines and how they work together to mediate the remarkable plasticity of synapses and the brain. Ultimately, we wish to understand the physiological significance of specific synaptic proteins and specific postsynaptic signaling events in animal behavior and cognition. Improved understanding of basic synaptic biology will illuminate the mechanisms that underlie human neurological disease and mental illness, many of which are manifestations of aberrant synaptic development or function.
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