Proapoptotic Bid inhibits the Execution of Programmed Necrosis Affecting Hematopoietic and Intestinal Homeostasis

Abstract

Programmed cell death (PCD) is an important process necessary for the maintenance of tissues in adult organisms and the crafting of distinct tissues in development. The two main types of PCD, apoptosis and necroptosis (i.e. programmed necrosis), are characterized through differing morphologic presentations and outcomes. Death receptor signaling is a context in which both apoptotic or necroptotic outcomes can occur. Several recent studies implicate proteins involved in apoptotic signaling in the inhibition of necroptosis including Caspase-8, FADD, and cFlipL. Bid, a member of the BCL-2 family of proteins, is cleaved by Caspase-8 which promotes its activation and translocation to the mitochondrion, promoting apoptosis. To evaluate what role Bid might play in the necroptotic arm of death receptor signaling we developed a mouse with Bid and its apoptotic arm of function (Bax and Bak) removed in hematopoietic cells. Loss of these three proteins leads to loss of restraint of necroptosis leading to increased necroptotic death, inflammatory signaling, and perturbation of tissue homeostasis in the hematopoietic and gastrointestinal organ systems. These findings in mice have implications for Myelodysplastic Syndrome, a bone marrow failure disorder characterized by increased PCD, and Inflammatory Bowel Diseases, inflammatory diseases characterized by overwhelming inflammation in the gastrointestinal system.