Apoptosis in the developing visual system

Programed cellular death is a widespread phenomenon during development of the nervous system. Two classes of molecules are particularly important in the context of apoptosis control in the nervous system: intracellular effecters homologous to the Caenorhabditis elegans Ced-3, -4, and -9 proteins, which in mammals correspond to the proteases of the caspase family, Apaf-1, and the members of the Bcl-2 protein family, and neurotrophic factors. Retinal ganglion cells lend a convenient model system with which to investigate apoptosis in central neurons during development as well as after injury. In this review, we discuss the role of these molecules in the control of programed cellular death in the retinotectal system. Transgenic animal models and expression studies have shown that caspases, Bcl-2, Bax, and possibly Bcl-X are necessary players for the control of programed cellular death in retinal ganglion cells. Bax and caspase 3 expression in retinal ganglion cells is upregulated after injury, and inhibition of Bax or caspase 3 increases the survival of injured retinal ganglion cells. Neurotrophins can support the survival of injured retinal ganglion cells, but this effect is transient. The physiological role of neurotrophins in the development of the retinocollicular system seems more related to the topographic refinement of retinocollicular projections, a process that is mediated, at least partially, by selective elimination of retinal ganglion cells making inappropriate topographic projections.