Purpose: We have previously demonstrated that in RPE-deprived retinas, lactose, galactose, and structurally related glycans support the proper assembly of nascent photoreceptor outer segment membranes. Other glycans such as mannose and glucose have no effect on this process. While the ability to support outer segment assembly is highly specific, all of the permissive glycans we have tested are able to enter metabolic pathways within the retinal cells, thus the mechanism by which the glycans are functioning is still ambiguous. The present study was undertaken to determine if permissive glycan-mediated support of photoreceptor outer segment assembly occurred via a non-metabolic mechanism and if the phenomenon was reversible.
Methods: The RPE was removed from isolated Xenopus laevis embryonic eyes that were allowed to complete differentiation in (1) Niu-Twitty medium, (2) Niu-Twitty supplemented with 5x10-3 M lactose or mannose, (3) Niu-Twitty supplemented with lactose and 3H-galactose, (4) Niu-Twitty medium supplemented with isopropyl beta-D-thiogalactopyranoside (IPTG) at concentrations spanning five orders of magnitude, or (5) Niu-Twitty medium containing lactose or IPTG to which 3H-leucine was added for 2 days followed by an additional 2 days in Niu-Twitty alone. Control RPE-deprived and RPE-supported retinas were included for comparison. Under all experimental conditions, retinal photoreceptors were evaluated to determine the level of organized folding of outer segment membranes.
Results: Outer segment membranes of RPE-deprived retinas exposed to lactose were significantly more organized than both control RPE-deprived retinas and those exposed to mannose. In eyes exposed to the radiolabeled metabolizable glycan, the majority of the sugar was incorporated into Müller cells. IPTG, a non-metabolizable form of galactose, promoted the formation of organized outer segment assembly similar to lactose although at a 100 fold reduced concentration compared to metabolizable permissive glycans. Both IPTG and lactose supported outer segment assembly in a step-wise fashion with maximal support at 5x10(-5) M and 5x10(-3) M, respectively. Removal of the permissive glycans resulted in loss of support of outer segment assembly.
Conclusions: The ability of both lactose and IPTG to support outer segment assembly in the absence of the RPE is dose-dependent and the effect of these sugars upon membrane folding is reversible. Moreover, this effect is supported by a non-metabolic mechanism and is therefore not accounted for by simple provision of an energy source.