| Apoptosis accompanying negative selection is a central but poorly understood event in T cell development. The Nur77 nuclear steroid receptor and Bim, a pro-apoptotic BH3-only member of the Bcl-2 family, are two molecules implicated in this process. However, how they relate to each other and how Nur77 induces apoptosis remains unclear. In thymocytes, Nur77 has been shown to induce cell death through a transcriptional-dependent pathway but in cancer cell lines, Nur77 was reported to induce apoptosis through conversion of Bcl-2 into a killer protein at the mitochondria. Whether this Nur77 transcriptional-independent pathway actually occurs in vivo remains controversial. Using an optimized fractionation protocol for thymocytes, here we report that stimulation of CD4+CD8+ thymocytes results in translocation of Nur77 and its family member Nor-1 to the mitochondria, leading to their association with Bcl-2 and exposure of the Bcl-2 pro-apoptotic BH3 domain. In two T-cell receptor transgenic models of negative selection, F5 and H-Y, a conformational change of the Bcl-2 molecule in the negatively selected T cell population was similarly observed. Thus, the Nur77 family and Bim pathways converge at mitochondria to mediate negative selection. Though Nur77 has been reported to be phosphorylated by various kinases, the functional consequence of this modulation has remained controversial. Here we show, Nur77 and Nor-1 mitochondrial targeting is mediated by PKC proteins in thymocytes undergoing apoptosis. |
