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© The Rockefeller University Press, 0021-9525/1998//553 $5.00
The Journal of Cell Biology, Volume 141, Number 3, , 1998 553-566

Articles

NRP/B, a Novel Nuclear Matrix Protein, Associates With p110RB and Is Involved in Neuronal Differentiation



Tae-Aug Kim*, Jinkyu Lim*, Setsuo Ota*, Sandhya Raja*, Rick Rogers{ddagger}, Benjamin Rivnay*, Hava Avraham*, and Shalom Avraham*

* Divisions of Experimental Medicine and Hematology/Oncology, Beth Israel Deaconess Medical Center, Harvard Institutes of Medicine, Boston, Massachusetts 02115; and {ddagger} Biomedical Imaging Laboratory, Harvard School of Public Health, Boston, Massachusetts 02215

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.

Abbreviations used in this paper: CDK, cyclin-dependent kinase; NRP/B, nuclear restricted protein/brain; NuMA, nuclear-mitotic apparatus; p110RB, retinoblastoma protein; pc, post coitum; RA, retinoic acid; RCR-1, Ring canal–related protein.

This paper is supported in part by National Institutes of Health grants HL55445, HL51456, and HL43510-06A.

This paper is dedicated to Raphael Recanati and his family for their friendship and support for our research program.

Address all correspondence to Dr. Shalom Avraham, Divisions of Experimental Medicine and Hematology/Oncology, Beth Israel Deaconess Medical Center, Harvard Institutes of Medicine, 4 Blackfan Circle, 3rd Floor, Boston, MA 02115. Tel.: (617) 667-0063. Fax: (617) 975-5240.

Sequence data are available from GenBank/EMBL/DDBJ under accession number AF059611.


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