Intricate molecular interactions between neurons and glial cells form the underlying basis of axonal insulation across species. Mutations in human genes that affect insulation of axons are associated with profound disturbances in normal impulse conduction and significant neurological disabilities. We are investigating the genetic and molecular basis of complex and reciprocal interactions between various types of glial cells, which play a key role in axonal insulation, blood-brain barrier formation and axon guidance during Drosophila development. Our lab identified Neurexin IV, Contactin and Neuroglian as key molecular components of the glial- and axo-glial septate junctions and showed that these proteins are crucial for the organization and function of the septate junctions. Recent studies in our lab have uncovered additional molecular components which link the midline glial scaffold with midline neurons to bring about commissural axon insulation and proper midline axon guidance.
We have extended these Drosophila studies to vertebrates, where axonal insulation is achieved by myelination carried out by glial cells (Schwann cells and oligodendrocytes). The myelinated nerve fibers are organized into distinct domains that are necessary for rapid saltatory conduction. These domains include the nodes of Ranvier and the flanking paranodal regions where myelin loops closely appose and form axo-glial septate junctions. We identified the vertebrate homologs of the Drosophila septate junction proteins and demonstrated a conserved role for these proteins in the organization and function of the axo-glial septate junctions in myelinated axons. We generated Caspr, and Neurofascin (homologs of Drosophila nrx IV and nrg, respectively) mutant mice and demonstrated that in these mutants, paranodal axo-glial septate junctions fail to form and the axonal domain organization is disrupted. These defects result in severe motor deficits, decrease in nerve conduction velocity and axonal degeneration, thus demonstrating a critical role for these proteins in axon-glial interactions in myelinated axons. Our recent studies in mice, using neuron- and myelinating glia-specific inducible-Cre lines, show that axo-glial junction disruption in adults results in slow but progressive neurological disabilities leading to paralysis. These adult mouse mutants serve as models for human myelin-related pathologies.
We are using genetic and biochemical methods to identify and characterize additional molecular complexes that are involved at the interface of axons and glial cells in Drosophila and mice, and how loss of these molecules affects conduction of nerve impulses and synaptogenesis.