No presynaptic membrane proteins which anchor nerve terminals to the extracellular matrix (ECM) are known. The goal of my proposed research is to identify and characterize some of these proteins involved with anchoring nerve terminals. Such proteins could mediate the well-known trophic interactions between the nerve terminal and basal lamina during nerve regeneration. Of special interest, therefore, will be those proteins that are specific for a subgroup of synapses. We have identified a proteoglycan from elasmobranch electric organ which has some of the characteristics of such an anchoring protein. It is found on the nerve terminal surface restricted to the synaptic region. It is tightly bound to and enriched in an extracellular matrix (ECM) fraction. Intriguingly, this proteoglycan contains an antigenic determinant which is only associated with the electric organ neurons. Finally, synaptic vesicles also contain a transmembrane proteoglycan which shares this antigenic determinant. Thus, synaptic vesicles also might contain this same proteoglycan. From this data we hypothesize that this proteoglycan is presynaptic membrane protein which links the nerve terminal to the ECM. In addition, the synaptic vesicles may act as vehicles for shuttling this molecule to and from the nerve terminal surface. This research proposal has two basic goals: 1) to determine the validity of the above hypothesis; 2) to identify and characterize other presynaptic membrane proteins which anchor the synaptic ECM. To accomplish the first goal several steps are required. We will determine whether the proteoglycan is an integral membrane protein by the presence or absence of a hydrophobic tail. We will identify the ECM components which bind the proteoglycan. If the proteoglycan is an integral membrane protein and binds ECM components, it must link the nerve terminal to the matrix. If this linkage is through the pathway specific antigenic determinant, this linkage could be specific to this synapse. A biochemical comparison of the synaptic vesicle and ECM proteoglycan protein cores will determine whether these two molecules could share a precursor-product relationship. To achieve the second goal we will use the ECM fraction, in which the nerve terminal proteoglycan was originally found. We will isolate a subfraction containing the integral membrane proteins contained in this ECM material. If presynaptic membrane proteins (other than the proteoglycan) bind to the ECM, they should be present in this subfraction. By making monoclonal antibodies against this subfraction, we should be able to identify them.
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