Control of programmed cell death in the inner nuclear layer of the developing retina

Neurons in the inner nuclear layer (INL) of the developing retina undergo massive programmed cell death during differentiation. The determinants of this cell death remain unknown. This study has examined the role of intercellular communication in the survival of INL cells.; The role of retinal ganglion cells, a synaptic partner of many INL cells, was determined. The removal of retinal ganglion cells by optic nerve transection increased cell death in the INL at two distinct time points, during the loss of ganglion cells, and later during the normal peak of INL cell death. This suggests that ganglion cells play multiple roles in the survival of INL cells.; The role of the neurotrophic factor, BDNF (brain derived neurotrophic factor), in the survival of amacrine neurons was determined. Culturing retinas with BDNF decreased cell death in the amacrine cell layer at postnatal day 7 (P7) but not at P5. Treatment with a soluble form of TrkB (the BDNF receptor), to decrease endogenous BDNF, increased cell death in the amacrine cell layer. These data suggest that amacrine cells are dependent on BDNF for survival.; Spatial distributions of dying cells show a clustered arrangement in sections of developing retina, suggesting that neighboring cells die simultaneously. Nearest Neighbor analysis of dying cells in whole-mounted retinas showed clustering. A bystander effect was induced by scrapeloading whole-mounted retinas with cytochrome c (a protein that induces cell death) and rhodamine dextran (a marker to follow which cells were loaded). These molecules remain in the cell because they are too large to pass through gap junctions, intercellular channels that pass small molecules. Scrapeloading with cytochrome c induced a bystander effect. Inhibitors of gap junctions (octanol, 8-Br-cAMP, carbenoxolone, and SNAP) decreased this effect. Single-cell injection of cytochrome c induced a bystander effect in coupled cells, that was inhibited by carbenoxolone. These data indicate that a gap junction mediated bystander effect of cell death occurs in the developing retina.; Three main findings are reported: retinal ganglion cells support the survival of INL cells; amacrine cells depended on BDNF for survival; and a bystander effect of cell death occurs during retinal development.