BCL-X(L) IS THE MAJOR BCL-X MESSENGER-RNA FORM EXPRESSED DURING MURINE DEVELOPMENT AND ITS PRODUCT LOCALIZES TO MITOCHONDRIA

Most examples of cell death in animals are controlled by a genetic pro gram that is activated within the dying cell. The apoptotic process is further regulated by a set of genes that act as repressors of cell de ath. Of these, bcl-2 is expressed in a variety of embryonic and postna tal tissues which suggests a critical role for bcl-2 in organogenesis and tissue homeostasis. Surprisingly, mutant mice with targeted disrup tion of bcl-2 appear normal at birth and complete maturation of lympho id tissues before succumbing to fulminant lymphopenia and polycystic r enal disease by 2-5 weeks of age. This suggests that there may be gene s other than bcl-2 that can regulate apoptosis during development. To begin to investigate this possibility, we have cloned and characterize d the murine bcl-x gene, whose human counterpart displays striking hom ology to bcl-2. The predicted murine bcl-x(L) gene product exhibits a high level of amino acid identity (97%) to its human counterpart. Just like Bcl-2, the murine bcl-x(L) gene product can act as a dominant in hibitor of cell death upon growth factor withdrawal. In addition, the bulk of the bcl-x(L) product localizes to the periphery of mitochondri a as assessed by a bcl-x(L)-tag expression system, suggesting that bot h Bcl-2 and Bcl-x(L) proteins prevent cell death by a similar mechanis m. bcl-x(L) is the most abundant bcl-x mRNA species expressed in embry onic and adult tissues. The levels of bcl-x(L) mRNA appear higher than those of bcl-2 during embryonal development and in several adult orga ns including bone marrow, brain, kidney and thymus. In addition to bcl -x(L), we have identified another form of bcl-x mRNA, bcl-x beta, that results from an unspliced bcl-x transcript. bcl-x beta mRNA is expres sed in various embryonic and postnatal tissues. Surprisingly, the expr ession of bcl-x(S) (a negative regulator of programmed cell death) was undetectable by a sensitive S1-nuclease assay and polymerase chain re action analysis of mouse tissues. Based on its tissue and developmenta l patterns of expression, it appears that bcl-x may play an important role in the regulation of cell death during development and tissue hom eostasis.