Identification of membrane proteins that comprise the plasmalemmal junction between migrating neurons and radial glial cells

We have initiated studies to identify membrane polypeptides of radial glial cells that contribute to the selective cell-cell recognition and migration events in developing brain. Of several polyclonal antisera evaluated, one (D4), developed against formaldehyde fixed type 1 cerebellar glial cells, immunolabels the free surface of cortical and cerebellar astroglial and radial glial cells in a patchy pattern. In dissociated glial- neuronal cell cocultures, microdomains of immunoreactivity are detected at the site where the somal region of cells with the morphology of migrating neurons is in contact with an elongated glial cell fiber. Microdomains are absent from oligodendrocytes, process-bearing astrocytes, and neurons. The surface microdomains do not colocalize with components that compose focal adhesion plaques - integrin subunits, vinculin, or actin - and their integrity appears to require an intact microtubule rather than actin cytoskeleton. Furthermore, microdomain structure is maintained in the absence of extracellular Ca and Mg ions. Immunoblot analyses using antibodies affinity purified to individual proteins indicate that the microdomains are composed of two antigens with apparent molecular mass of ~48 kDa and ~72 kDa. The 48 kDa antigen is not observed in non-neural epithelial tissues and is detected in cortical and cerebellar tissues only at a developmental period that coincides with the stage of active neuronal cell migration. In contrast, the 72 kDa antigen is expressed in many neural and non-neural tissues at late developmental and adult stages. Our data suggest that the identified membrane proteins may contribute to the formation of the junction between migrating neurons and radial glial cell processes and that this junctional complex is linked to the microtubule cytoskeleton.