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S. Karan, S. Li, W. Baehr, J. M. Frederick; Consequences of Deletion of Guanylate Cyclase 1 (GC1/GC-E) and Guanylate Cyclase 2 (GC2/GC-F) on Photoreceptor Outer Segment Structure. Invest. Ophthalmol. Vis. Sci. 2007;48(13):595.
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To determine the consequence of deletion of the two mouse guanylate cyclases (GC1 and GC2) on the photoreceptor outer and inner segment structure.
Generation of GC1/GC2 double knockout mice; immunocytochemistry, western blotting, electron microscopy.
GCs are key enzymes in producing the second messenger of phototransduction, cyclic GMP. We previously showed that deletion of both GC1 and GC2 (GC double knockout, or GCdko) caused recessive retinal degneration (LCA), and rendered rod and cone photoreceptors nonfunctional. Further we showed that a number of peripheral membrane proteins (GCAPs, rod and cone PDE, cone transducin subunits) were depleted. Here we set out to analyze the structure of GCdko ROS and GC1-/- COS by electron microscopy. The results show that in GC1-/- cones, apical inner segments appeared retracted from connecting cilia and surrounded by extracellular ‘blebs’. At the distal connecting cilia, membrane components appeared to partition into layered stacks or vesicles/tubules and in severely degenerated cases, these membranes were detached. Adjacent to the retinal pigmented epithelium, particles consisting of an inner collection of vesicles enclosed by layers of dense membrane were often observed. GCdko rods were surrounded by ‘blebs’ in the interphotoreceptor matrix. Distal to the connecting cilia, the rod outer segments were shorter and narrower than their WT counterparts, and appeared "banded" with alternating regions of dense membrane layers and lumen-containing tubules suggesting unconventional ROS membrane assembly.
Apart from enzyme activity, the severe consequences of GC double knockout affect disk assembly and outer segment structure. This observation suggests that GCs may have additional roles in supporting disk assembly and structure.
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