Michael V.L. Bennett

Molecular and cellular physiology of glutamatergic transmission and gap junction mediated intercellular communication.

Our laboratory has collaborated extensively with the laboratory of Dr. R. Suzanne Zukin in cloning and characterization of glutamate receptors. Glutamatergic transmission is the primary mode of excitation in the nervous system and plays important roles in disease processes as well. Change in glutamate receptor expression appears to mediate at least some instances of delayed neuronal death, for example the degeneration in the hippocampal CA1 following global ischemia and in CA3/4 following kainate induced status epilepticus. In situ hybridization and immunocytochemistry indicate reduction in expression of GluR2, the AMPA receptor subunit that limits calcium permeability of these receptors, and measurements with Ca2+ indicators demonstrate increased Ca2+ influx through them. Increased Ca2+ influx in response to endogenous glutamate may then trigger apoptosis. NMDA receptors mediate some forms of long term potentiation which underlies at least one form of memory. Determination of the properties of receptor subtypes comprised of different combinations of NR1 splice variants and NR2 subunits has been a recent theme of our work. Site directed mutagenesis is being used in structure-function studies and has identified amino acid residues that are critical for agonist affinity and potentiation by spermine, PCP and zinc. Sites of phosphorylation and potentiation by protein kinase C are also under investigation.

Studies of gap junction mediated communication include phylogenetic analyses that have identified regions of conservation and variation in connexins. Conserved regions must represent functionally important regions in common. Divergent regions account for functional differences or are neutral. These data have guided structure function studies (in collaboration with V.K. Verselis and T.A. Bargiello). Identification of connexin types expressed in different tissues and analysis of functional differences is proceeding. A human genetic disease, X-linked Charcot-Marie-Tooth disease, has recently been found to be due to mutations in connexin 32. The phenotype is a demyelinating peripheral neuropathy, generally with onset in late childhood. We are analyzing how this mutation affects myelination (connexin 32 is expressed by Schwann cells) and whether there are any subclinical effects in the numerous other tissues that express this connexin. In other studies the role of gap junctions in CNS functions including communication between astrocytes is being investigated.

 

Selected Publications