Abstract

The Brn-3a POU transcription factor is associated with survival and the differentiation of sensory neuronal cells during development. Brn-3a mediates its effects either by the direct regulation of target genes or indirectly upon interaction with proteins such as p53. Brn-3a differentially regulates p53-mediated gene expression and modifies its effect on cell fate. Here we show that, like Bax, Brn-3a antagonizes p53-mediated transcription of another proapoptotic target, Noxa, significantly reducing transactivation of the Noxa promoter by p53. This effect requires the p53 binding site, and electrophoretic mobility shift assay studies suggest that Brn-3a is associated with p53 when it is bound to its site in the Noxa promoter. The wild type but not the mutant promoter can be immunoprecipitated with Brn-3a in chromatin immunoprecipitation assays. Thus, Brn-3a may act by preventing the recruitment of cofactors required for p53 to transactivate this promoter. The co-expression of Brn-3a and p53 results in decreased endogenous Noxa protein in the neuronal cell line, ND7, suggesting a direct functional effect of this interaction. Moreover, there is a significant elevation of both proapoptotic Bax and Noxa proteins in sensory neuronal tissue taken from Brn-3a-/- embryos during development, compared with wild type controls. Striking changes occurred at embryonic day 14.5, a time that precedes a significant loss of specific neurons in the mutant embryos, but not at embryonic day 16.5 when Brn-3a-expressing cells are already lost by apoptosis. Therefore, the lack of antagonism by Brn-3a on activation of proapoptotic p53 target genes may contribute to the increased apoptosis seen in the Brn-3a-/- embryos. These results support a crucial role for Brn-3a in determining the pathway taken by p53 when co-expressed during development and thus in controlling the fate of these cells.

Highlights

  • During neuronal development, transcription factors, which are expressed in a temporal/spatial manner or in response to specific signals, play a critical role in determining the fate of specific progenitor cells in terms of proliferation, survival and differentiation, or apoptosis of excess cells

  • Striking changes occurred at embryonic day 14.5, a time that precedes a significant loss of specific neurons in the mutant embryos, but not at embryonic day 16.5 when Brn-3aexpressing cells are already lost by apoptosis

  • Regulation of p53-mediated Activation of Noxa Promoter by Brn-3a—Because Brn-3a could block p53-mediated activation of the Bax promoter but enhance expression of the cell cycle arrest-associated gene p21cip1/waf[1], we examined whether Brn-3a might modulate the effect of p53 on the promoter of another target gene, Noxa

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Summary

Introduction

Transcription factors, which are expressed in a temporal/spatial manner or in response to specific signals, play a critical role in determining the fate of specific progenitor cells in terms of proliferation, survival and differentiation, or apoptosis of excess cells. This role is necessary to achieve the correct balance of specific neurons required for the normal innervation and function. Using a model of neonatal sciatic nerve crush, we have shown that the overexpression of Brn-3a can enhance neuronal survival in vivo (12)

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