Research in the Brachmann Lab:
Bcl-2 Proteins in Drosophila

      The induction of apoptosis results in a cascade of caspase activation that ultimately kills the cell (Fig. 1). Death stimuli originate from within the cell, as is the case for DNA damage, or from neighboring cells as has been shown in the immune system for the removal of unnecessary T cells through FAS.

     
Regulating the apoptotic process are pro- and anti-apoptotic members of the Bcl-2 family of proteins. One of the central unanswered questions is how the Bcl-2 family members exert their influence on apoptosis and currently, there are two theories. In C. elegans, the Bcl-2 protein Ced-9 binds to and inhibits the Apaf-1 ortholog, Ced-4, from activating the caspase Ced-3, the workhorse of apoptosis that cleaves many essential cellular proteins. Ced-9 is released from Ced-4 through interactions with the pro-apoptotic Egl-1 protein. However, this mechanism of Bcl-2 protein function is likely not evolutionarily conserved. The second theory is that pro-apoptotic Bcl-2 family members Bax and tBid, similar in structure to pore-forming bacterial toxins, induce efflux of cytochrome c from the mitochondrion, directly or indirectly through insertion into the mitochondrial membrane. Release of cytochrome c induces the apoptosome to activate apical caspases to either initiate or propagate the caspase cascade. In support of this, some Bcl-2 proteins reside in the mitochondrial membrane and some move to the mitochondria following a death signal. However, the relevance of the pore-forming ability of the Bcl-2 proteins is controversial and is unlikely to represent their only function. Significantly, in some cells, microinjection of cytochrome c does not kill all cell types and Bcl-2 can protect cells after much of the mitochondrial cytochrome c has been released.

Only recently has it become apparent that the fly executes apoptosis using a complex system of effector proteins that is very similar to mammals. We identified dBorg-1, the first Bcl-2 family member in Drosophila and showed that it functions in embryonic develoment. dBorg-1 was pro-apoptotic in cultured cells, but intriguingly, required activation by UV-irradiation to direct cell death in the fly eye (Figure, both eyes are UV-irradiated). In ongoing research, we are biochemically investigating this activation, looking for post-transcriptional modifications of dBorg-1 and protein partners with which dBorg-1 may cooperate.

What does the study of dBorg-1 add to existing investigations of Bcl-2 family members? Clearly, an understanding of dBorg-1 function in the developing fly will be crucial for the clarification and interpretation of conflicting data on mammalian Bcl-2 protein function. But most importantly, dBorg-1 provides an unparalleled opportunity to determine genetically, in a complex apoptotic system, protein partners and pathway members involved in the function of a Bcl-2 family member. We are using the power of fly genetics to discover the molecules that keep the intrinsic apoptotic pathway in check, that activate apoptosis through dBorg-1 and that act downstream of dBorg-1 to kill the cell.