Tunicamycin-induced dysgenesis of retinal rod outer segment membranes. I. A scanning electron microscopy study

Incubation of Xenopus retinas with tunicamycin has been shown to block the glycosylation of opsin, the rod visual pigment apoglycoprotein, with concomitant accumulation of vesicular membrane material in the compartment between the rod inner and outer segments (i.e., the intersegmental space) (Fliesler et al, J Cell Biol 100:574-587, 1985). To further assess the morphology, topology, and cellular origin of this membranous material, Xenopus retinas were incubated in the presence or absence of tunicamycin and the photoreceptor cells were examined by scanning electron microscopy. The material which accumulated in the intersegmental space appeared to be a complex of membranous structures consisting of cisternae with numerous tubular projections, as well as closely associated individual vesicles of various sizes. This tubulo-vesicular material was exclusively associated with the basal surface of the rod outer segment. The connecting cilium, periciliary ridge complex, and the apical surface of the rod inner segment were devoid of such membrane material. Nascent (open) discs (i.e., evaginations of the plasma membrane at the base of the outer segment) often observed in control retinas were not present in tunicamycin-treated tissue. These results support the hypothesis that the membranous material which accumulates in the intersegmental space of rods in tunicamycin-treated retinas represents incompletely and aberrantly formed nascent disc membranes. The formation of this material is apparently a consequence of a deficiency in newly synthesized, asparagine-linked membrane glycoconjugates (e.g., the oligosaccharide chains of opsin) at the site of disc assembly.