NRP B, A NOVEL NUCLEAR MATRIX PROTEIN, ASSOCIATES WITH P110(RB) AND IS INVOLVED IN NEURONAL DIFFERENTIATION/

The nuclear matrix is defined as the insoluble framework of the nucleu s and has been implicated in the regulation of gene expression, the ce ll cycle, and nuclear structural integrity via linkage to intermediate filaments of the cytoskeleton. We have discovered a novel nuclear mat rix protein, NRP/B (nuclear restricted protein/brain), which contains two major structural elements: a BTB domain-like structure in the pred icted NH2 terminus, and a ''kelch motif'' in the predicted COOH-termin al domain. NRP/B mRNA (5.5 kb) is predominantly expressed in human fet al and adult brain with minor expression in kidney and pancreas. Durin g mouse embryogenesis, NRP/B mRNA expression is upregulated in the ner vous system. The NRP/B protein is expressed in rat primary hippocampal neurons, but not in primary astrocytes. NRP/B expression was upregula ted during the differentiation of murine Neuro 2A and human SH-SY5Y ne uroblastoma cells. Overexpression of NRP/B in these cells augmented ne uronal process formation. Treatment with antisense NRP/B oligodeoxynuc leotides inhibited the neurite development of rat primary hippocampal neurons as well as the neuronal process formation during neuronal diff erentiation of PC-12 cells. Since the hypophosphorylated form of retin oblastoma protein (p110(RB)) is found to be associated with the nuclea r matrix and overexpression of p110(RB) induces neuronal differentiati on, we investigated whether NRP/B is associated with p110(RB). Both in vivo and in vitro experiments demonstrate that NRP/B can be phosphory lated and can bind to the functionally active hypophosphorylated form of the p110(RB) during neuronal differentiation of SH-SY5Y neuroblasto ma cells induced by retinoic acid. Our studies indicate that NRP/B is a novel nuclear matrix protein, specifically expressed in primary neur ons, that interacts with p110(RB) and participates in the regulation o f neuronal process formation.