During development of the retina, programmed cell death helps to establish the final size and distribution of various cell classes in distinct layers of the tissue. Here we show that dying cells in the developing ganglion and inner nuclear layers are clustered spatially and that gap junction inhibitors decrease the clustering of dying cells. To confirm the role of gap junctions in cell death, we induced targeted cell death via intracellular cytochrome c (Cc) and examined the induced cells and their neighbors for apoptotic morphology or caspase-3 cleavage. These studies indicate that bystander killing extends to coupled cells. Quantitative studies of bystander killing were performed by scrape-loading retinas with Cc in the presence of rhodamine dextran (RD; to identify Cc-loaded cells) and by counting pyknotic cells in cryosections. Although only 1.5% of control scrape-loaded cells (RD alone) showed apoptotic morphology, 97% of Cc scrape-loaded cells were pyknotic. Moreover, bystander killing extended to neighboring cells, not labeled with RD, and was reduced significantly by the gap junction inhibitors octanol and carbenoxolone. We hypothesize that dying cells in the retina generate a gap junction-permeant apoptotic signal that mediates bystander killing. This novel finding of naturally occurring bystander cell death may have important implications in the histogenesis and pathology of the nervous system.

Progression of retinal degeneration in a mouse model was studied in vivo with high-resolution spectral-domain optical coherence tomography (SD-OCT). Imaging in 3D with high depth resolution (<3 mum), SD-OCT resolved all the major layers of the retina of control C57BL/6J mice. Images of transgenic mice having a null mutation of the rhodopsin gene revealed the anatomical consequences of retinal degeneration: thinning of the outer retina, including the outer plexiform layer (OPL), outer nuclear layer (ONL), and inner and outer segments (IS/OS). We monitored the progression of retinal degeneration in rd1 mice (C3H/HeJ) by periodically imaging the same mice from the time the pups opened their eyes on P13 to P34. SD-OCT images showed that the outer retina (OPL, ONL, IS/OS) had already thinned by 73% (100 to 27 mum) at eye opening. The retina continued to degenerate, and by P20 the outer retina was not resolvable. The thickness of entire retina decreased from 228 mum (control) to 152 mum on P13 and to 98 mum by P34, a 57% reduction with the complete loss in the outer retina. In summary, we show that SD-OCT can monitor the progression of retinal degeneration in transgenic mice.

Tonic synapses are specialized for sustained calcium entry and transmitter release, allowing them to operate in a graded fashion over a wide dynamic range. We identified a novel plasma membrane calcium entry mechanism that extends the range of rod photoreceptor signalling into light-adapted conditions. The mechanism, which shares molecular and physiological characteristics with store-operated calcium entry (SOCE), is required to maintain baseline