Immunocytochemical localization of opsin in rod photoreceptors during periods of rapid disc assembly.

Transport of opsin from photoreceptor inner to outer segments has been assumed to occur via the connecting cilium, the only permanent structural connection between these two regions. However, in prior work, little or no immunoreactive opsin has been detected in the cilium, despite the high rate of transport of this protein. This suggests that immune epitopes are masked during passage through the cilium or that opsin is transported via an extra-ciliary route. In this study, we stained the photoreceptors of Xenopus laevis with well-characterized monoclonal antibodies directed at the N-terminal, C-terminal, and 5-6 loop regions of bovine opsin. This was done on isolated retinas incubated in vitro under conditions that support rapid disc assembly, to insure that opsin transport to forming discs was occurring at the time of fixation. Five MAbs that gave robust staining of Xenopus rod inner segment/rod outer segment preparations with the light microscope were utilized for electron microscopic studies on LR White embedded or cryo-ultrathin sections. Four of these stained outer segment discs and inner segment vesicles and plasma membrane. However, no significant staining of the connecting cilium was found. Furthermore, freeze-fractured mouse photoreceptors prepared by the 'fracture-label' technique showed extensive labelling of membrane compartments but lacked staining of the connecting cilium. Isolated retinas incubated under conditions that support robust rod disc synthesis contained many finger-like and vesicular projections of the apical inner segment plasma membrane and inner segment vesicles extending into them. Rod outer segment nascent discs usually made close contact with the inner segment. Both the vesicular profiles associated with the inner segment plasma membrane and the basal discs extending to the inner segment were heavily stained with all four anti-opsin antibodies. This suggests an alternate route for bulk transport of opsin to newly forming discs that involves direct transfer from apical inner segment plasma membrane to nascent discs.