| In neuronal populations, Bax is the central mediator of apoptosis. An essential role for Bax expression in the process of retinal ganglion cell (RGC) death in development, after acute nerve trauma, and in models of neurodegenerative disease, has been documented. RGC death and the degeneration of their axons in the optic nerve, are the principal characteristics of blinding optic neuropathies.;Although Bax was required for cell death, the effects of Bax gene dosage were not as well-defined. Investigations in 129B6 and DBA/2J mice heterozygous for the mutant Bax allele indicated that 129B6 mice exhibited RGC death after optic nerve damage, while DBA/2J mice exhibited RGC survival. Neurons of 129B6 expressed approximately twice the level of Bax mRNA and protein as DBA/2J neurons, thus suggesting that a threshold level of Bax expression was required for RGC death. To further investigate the implications of the expression of the Bax gene in cell death, I have addressed the following concerns: (1) the threshold response for cell death, and (2) the underlying mechanism for the differential expression of Bax in DBA/2J and 129B6 mice.;The level of BAX was titrated in BAX-deficient HCT116 colorectal cancer cells, which require BAX for cell death in response to an apoptotic stimulus. A threshold level of BAX expression required for cell death was observed in these cells. Maximal cell death was obtained when the threshold level of BAX expression was reached. The mechanism underlying the threshold was due to the inability of BAX to translocate to the mitochondria at sub-threshold levels. This indicated that small alterations in the level of BAX gene expression could be critical for cell death. Thus, the mechanism for differential Bax expression in 129B6 and DBA/2J neurons was investigated. A single nucleotide polymorphism was identified in the mouse Bax promoter at position -515, changing a T (in 129B6) to a C (in DBA/2J). In vitro, the 129B6 promoter directed a 2-fold greater level of expression of a Luciferase reporter gene, than the DBA/2J promoter. In addition, in vitro binding assays indicated that nuclear proteins interacted more efficiently with the polymorphism in the 129B6 promoter. |
