Programmed cell death.

Programmed cell death is an integral component of C. elegans development. Genetic studies in C. elegans have led to the identification of more than two dozen genes that are important for the specification of which cells should live or die, the activation of the suicide program, and the dismantling and removal of dying cells. Molecular and biochemical studies have revealed the underlying conserved mechanisms that control these three phases of programmed cell death. In particular, an interplay of transcriptional regulatory cascades and networks involving CES-1, CES-2, HLH-1/HLH-2, TRA-1, and other transcriptional regulators is crucial in activating the expression of the key death-inducing gene egl-1 in cells destined to die. A protein interaction cascade involving EGL-1, CED-9, CED-4 and CED-3 results in the activation of the key cell death protease CED-3. The activation of CED-3 initiates the cell disassembly process and nuclear DNA fragmentation, which is mediated by the release of apoptogenic mitochondrial factors (CPS-6 and WAH-1) and which involves multiple endo- and exo-nucleases such as NUC-1 and seven CRN nucleases. The recognition and removal of the dying cell is mediated by two partially redundant signaling pathways involving CED-1, CED-6 and CED-7 in one pathway and CED-2, CED-5, CED-10, CED-12 and PSR-1 in the other pathway. Further studies of programmed cell death in C. elegans will continue to advance our understanding of how programmed cell death is regulated, activated, and executed in multicellular organisms.