MIR, a novel ERM-like protein in the nervous system

Sammanfattning: Proteins of the band 4.1 superfamily are characterized by their sequence similarity to the ERM proteins ezrin, radixin and moesin, which are involved in cell motility, adhesion of cells, and signal transduction events. Little is however known of the function of ERM proteins in the nervous system, though an essential role for radixin and moesin in neuronal growth cone motility has been suggested. This thesis is focused on the cloning, functional characterization and description of the tissue distribution in rat brain of MIR, a novel member of the band 4.1 superfamily. The cDNA of MIR encods a protein of 445 amino acids which is composed of an ERM-homology domain and a RING finger, separated by an interregion. To reveal the cellular function of MIR, PC12 cell lines overexpressing MIR was generated and observed to inhibit NGF stimulated neurite outgrowth. To elucidate the signal transduction of MIR by which it exerts its physiological activity, the yeast two-hybrid system was employed to screen for proteins that interact with MIR. A number of interactors known to regulate the cytoskeleton was obtained - among them myosin regulatory light chain-B which controls the actomyosin complex - and a novel type 2 membrane protein denoted NSAP for its similarity to saposin A-D. Overexpressed NSAP induced neurite outgrowth in PC12 cells and enhanced cell adhesion in fibroblasts. The tissue distribution of MIR in rat brain, as determined by immunohistochemistry studies, showed that MIR is localized especially to neurons in hippocampus and cerebellum. The chromosomal localization of the MIR gene was assessed to 6p22.3-23, a region lost in the 6p23 deletion syndrome. These results suggests that MIR is expressed in neurons in discrete regions of rat brain where it may regulate neurite outgrowth by modulating the cytoskeleton.