Chemical synapses are the primary means for transmitting information from one neuron to the next. Synapses are initially formed during development of the nervous system, and formation of appropriate synapses is crucial for establishment of neuronal circuits that underlie behavior and cognition. Minor irregularities during synapse formation can lead to developmental disorders such as autism, mental retardation and may contribute to psychological disorders. Most synapses in the vertebrate central nervous system (CNS) depend on the neurotransmitter glutamate, and thus glutamatergic synapses have been an important focus of study in trying to unravel these and other neurological disorders. A novel family of cell adhesion molecules (CAMs), the Synaptic Cell Adhesion Molecules (SynCAMs), has recently been proposed to mediate the formation of synapses. However, this work is based on experiments in neuronal culture, and knock-out mouse data so far does not corroborate this. Furthermore, it remains unknown how the SynCAMs, and CAMs in general, bring about the recruitment of synaptic elements to new adhesive contacts. We propose to test a model describing specific mechanisms through which SynCAM family members 1 and 2 can recruit both synaptic vesicles (SVs) to the presynaptic terminal and glutamate receptors to the postsynaptic specialization. We hypothesize that an interaction between SynCAM1 or 2 and CASK in axons can directly tether SV precursors to the site of SynCAM/SynCAM interaction. We also propose that binding of DAL-1 to SynCAM1 or 2 in dendrites results in formation of an actin/spectrin subsynaptic scaffold. These cytoskeletal elements then serve two functions: 1) strengthening of the adhesive nature of the synapse and morphological remodeling to generate a spine and 2) recruitment of NMDA type glutamate receptor transport packets via an actin-dependent transport mechanism. We propose to use a variety of techniques including biochemistry, immunolabeling, live-imaging, electrophysiology and behavioral tests, because a multidisciplinary approach will comprehensively test our model. We also propose to use various neuronal preparations for our experiments including cultured hippocampal neurons, cultured cerebellar granule cells (CGCs) and spinal cord neurons in zebrafish embryos in vivo. Testing our hypothesis in zebrafish will shed light on whether these proteins and their interactions are required for forming a specific circuit in a developing embryo that is required for a sensorimotor reflex. Our approach gives us the unprecedented opportunity to determine the mechanisms of glutamatergic synapse formation using behavioral, electrophysiological, genetic and biochemical approaches in both neuronal cultures and in a living vertebrate.
Synapses are sites at which nerve cells communicate with each other. Communication of nerve cells is absolutely necessary for nervous system function, ranging from simple reflexes to expressing philosophical thoughts. Errors during the formation of synapses are thought to be at the basis of nervous system disorders such as autism, mental retardation and schizophrenia. We propose to study two members of a family of genes, the Synaptic Cell Adhesion Molecules (SynCAMs) 1 and 2, which are thought to mediate the formation of synapses during development. Almost nothing is yet known about how SynCAMs make a contact site between two nerve cells become a place for active communication. We will investigate how these molecules carry out synapse formation by testing their function in rat nerve cells grown in culture and by testing their function in developing zebrafish embryos. These two systems allow us to determine the nature of the molecular interactions and to determine the importance of these interactions for the formation of synapses in a living organism, respectively. Our research will help understand the mechanisms by which synapses form and bring us closer to identifying the molecular deficits in individuals with autism and mental retardation.
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