p75(NTR) expression and nuclear localization of p75(NTR) intracellular domain in spiral ganglion Schwann cells following deafness correlate with cell proliferation.

Spiral ganglion Schwann cells (SGSCs) myelinate spiral ganglion neurons (SGNs) and represent a potential source of neurotrophic support for SGNs. Deafening due to loss of hair cells results in gradual degeneration and death of SGNs. Successful efforts to maintain or regenerate a functional auditory nerve may depend on a healthy population of SGSCs, yet the responses of SGSCs to neural injury remain largely unknown. Here we investigate the role of p75(NTR) in SGSC responses to gradual denervation. Following deafening, SGSCs in the osseous spiral lamina (OSL) and, subsequently, in Rosenthal's canal (RC) expressed elevated p75(NTR) compared to hearing controls. p75(NTR)-positive cells co-labeled with S100 and RIP antibodies (Schwann cell markers), but not with anti-neurofilament. The pattern of p75(NTR) expression mirrored the pattern of neural degeneration, beginning in the OSL of the cochlea base and later extending into the apex. SGSCs expressed sortilin, a p75(NTR) co-receptor for pro-neurotrophins. Both pro-nerve growth factor (pro-NGF) and pro-brain derived neurotrophic factor (proBDNF) induced apoptosis in cultured SGSCs. Deafened animals exhibited significantly higher levels of SGSC proliferation (as measured by BrdU uptake) compared to hearing animals while total Schwann cell density remained stable, suggesting a tight regulation of SGSC proliferation and cell death. SGSCs undergoing cell division lose p75(NTR) expression from the cell surface and demonstrate nuclear localization of the intracellular domain (ICD), raising the possibility that p75(NTR) cleavage and ICD nuclear localization regulate SGSC proliferation. These results suggest that p75(NTR) contributes to SGSC responses to deafening and neural degeneration.

Genome-wide analysis of epigenetic silencing identifies BEX1 and BEX2 as candidate tumor suppressor genes in malignant glioma.

Promoter hypermethylation and histone deacetylation are common epigenetic mechanisms implicated in the transcriptional silencing of tumor suppressor genes in human cancer. We treated two immortalized glioma cell lines, T98 and U87, and 10 patient-derived primary glioma cell lines with trichostatin A (TSA), a histone deacetylase inhibitor, or 5-aza-2'-deoxycytidine (5-AzaC), a DNA methyltransferase inhibitor, to comprehensively identify the cohort of genes reactivated through the pharmacologic reversal of these distinct but related epigenetic processes. Whole-genome microarray analysis identified genes induced by TSA (653) or 5-AzaC treatment (170). We selected a subset of reactivated genes that were markedly induced (greater than two-fold) after treatment with either TSA or 5-AzaC in a majority of glioma cell lines but not in cultured normal astrocytes. We then characterized the degree of promoter methylation and transcriptional silencing of selected genes in histologically confirmed human tumor and nontumor brain specimens. We identified two novel brain expressed genes, BEX1 and BEX2, which were silenced in all tumor specimens and exhibited extensive promoter hypermethylation. Viral-mediated reexpression of either BEX1 or BEX2 led to increased sensitivity to chemotherapy-induced apoptosis and potent tumor suppressor effects in vitro and in a xenograft mouse model. Using an integrated approach, we have established a novel platform for the genome-wide screening of epigenetically silenced genes in malignant glioma. This experimental paradigm provides a powerful new method for the identification of epigenetically silenced genes with potential function as tumor suppressors, biomarkers for disease diagnosis and detection, and therapeutically reversible modulators of critical regulatory pathways important in glioma pathogenesis.