Caspases are a family of cysteine proteases implicated in the biochemical and morphological changes that occur during apoptosis (programmed cell death). The loop domain of Bcl-2 is cleaved at Asp34 by caspase-3 (CPP32) in vitro, in cells overexpressing caspase-3, and after induction of apoptosis by Fas ligation and interleukin-3 withdrawal. The carboxyl-terminal Bcl-2 cleavage product triggered cell death and accelerated Sindbis virus-induced apoptosis, which was dependent on the BH3 homology and transmembrane domains of Bcl-2. Inhibitor studies indicated that cleavage of Bcl-2 may further activate downstream caspases and contribute to amplification of the caspase cascade. Cleavage-resistant mutants of Bcl-2 had increased protection from interleukin-3 withdrawal and Sindbis virus-induced apoptosis. Thus, cleavage of Bcl-2 by caspases may ensure the inevitability of cell death.
The Bax protein is widely known as a pro-apoptotic Bcl-2 family member that when overexpressed can trigger apoptosis in multiple cell types and is important for the developmental cell death of neurons. However, Bax was found here to be a potent inhibitor of neuronal cell death in mice infected with Sindbis virus. Newborn mice, which are highly susceptible to a fatal infection with neurotropic Sindbis virus, were significantly protected from neuronal apoptosis and fatal disease when infected with a recombinant Sindbis virus encoding Bax. Deletion of the N terminus of Bax, which mimics cleaved Bax, converted Bax into a pro-apoptotic factor in vivo. As mice mature during the first week after birth, they acquire resistance to a fatal Sindbis virus infection. However, Bax-deficient mice remained very sensitive to fatal disease compared with their control littermates, indicating that endogenous Bax functions as a survival factor and contributes to age-dependent resistance to Sindbis virus-induced mortality. The protective effects of Bax were reproduced in cultured hippocampal neurons but not in cultured dorsal root ganglia neurons. These findings indicate that cell-specific factors determine the anti-apoptotic versus pro-apoptotic function of Bax.
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