lan4 is required for mitochondrial integrity and T cell survival

Apoptosis is a regulated cell death program controlled by extrinsic and intrinsic signaling pathways. The intrinsic pathway involves stress signals that activate pro-apoptotic members of the Bcl-2 family, inducing permeabilization of mitochondria and release of apoptogenic factors. These proteins localize to the outer mitochondrial membrane. lan4, a mitochondrial outer membrane protein with GTP-bincling activity, is normally present in thymocytes, T cells, and B cells. We and others have recently discovered that a mutation in the rat lan4 gene results in severe T cell lymphopenia that is associated with the expression of autoimmune diabetes. The mechanism by which lan4 controls T cell homeostasis is unknown. Here we show that the absence of lan4 in T cells causes mitochondrial dysfunction, increased mitochondrial levels of stress-inducible chaperonins and a leucine-rich protein, and T cell-specific spontaneous apoptosis. T cell activation and caspase 8 inhibition both prevented apoptosis, whereas transfection of T cells with lan4-specific small interfering RNA recapitulated the apoptotic phenotype. The findings establish lan4 as a tissue-specific regulator of mitochondrial integrity.