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J. Cell Biol.,
Volume 141, Number 3, May 4, 1998 553-566

* Divisions of Experimental Medicine and Hematology/Oncology, Beth Israel Deaconess Medical Center, Harvard Institutes of
Medicine, Boston, Massachusetts 02115; and The nuclear matrix is defined as the insoluble framework of the nucleus and has been implicated
in the regulation of gene expression, the cell cycle, and
nuclear structural integrity via linkage to intermediate
filaments of the cytoskeleton. We have discovered a
novel nuclear matrix protein, NRP/B (nuclear restricted protein/brain), which contains two major structural elements: a BTB domain-like structure in the predicted NH2 terminus, and a "kelch motif" in the
predicted COOH-terminal domain. NRP/B mRNA (5.5 kb) is predominantly expressed in human fetal and
adult brain with minor expression in kidney and pancreas. During mouse embryogenesis, NRP/B mRNA
expression is upregulated in the nervous system. The NRP/B protein is expressed in rat primary hippocampal
neurons, but not in primary astrocytes. NRP/B expression was upregulated during the differentiation of murine Neuro 2A and human SH-SY5Y neuroblastoma
cells. Overexpression of NRP/B in these cells augmented neuronal process formation. Treatment with
antisense NRP/B oligodeoxynucleotides inhibited the
neurite development of rat primary hippocampal neurons as well as the neuronal process formation during
neuronal differentiation of PC-12 cells. Since the hypophosphorylated form of retinoblastoma protein
(p110RB) is found to be associated with the nuclear matrix and overexpression of p110RB induces neuronal differentiation, we investigated whether NRP/B is associated with p110RB. Both in vivo and in vitro experiments
demonstrate that NRP/B can be phosphorylated and
can bind to the functionally active hypophosphorylated
form of the p110RB during neuronal differentiation of
SH-SY5Y neuroblastoma cells induced by retinoic acid.
Our studies indicate that NRP/B is a novel nuclear matrix protein, specifically expressed in primary neurons,
that interacts with p110RB and participates in the regulation of neuronal process formation.
Biomedical Imaging Laboratory, Harvard School of Public Health, Boston,
Massachusetts 02215
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