Cross species analysis of Prominin reveals a conserved cellular role in invertebrate and vertebrate photoreceptor cells

Abstract

The two fundamental types of photoreceptor cells have evolved unique structures to expand the apical membrane to accommodate the phototransduction machinery, exemplified by the cilia-based outer segment of the vertebrate photoreceptor cell and the microvilli-based rhabdomere of the invertebrate photoreceptor. The morphogenesis of these compartments is integral for photoreceptor cell integrity and function. However, little is known about the elementary cellular and molecular mechanisms required to generate these compartments. Here we investigate whether a conserved cellular mechanism exists to create the phototransduction compartments by examining the functional role of a photoreceptor protein common to both rhabdomeric and ciliated photoreceptor cells, Prominin. First and foremost we demonstrate that the physiological role of Prominin is conserved between rhabdomeric and ciliated photoreceptor cells. Human Prominin1 is not only capable of rescuing the corresponding rhabdomeric Drosophila prominin mutation but also demonstrates a conserved genetic interaction with a second photoreceptor protein Eyes Shut. Furthermore, we demonstrate the Prominin homologs in vertebrate and invertebrate photoreceptors require the same structural features and post-translational modifications for function. Moreover, expression of mutant human Prominin1, associated with autosomal dominant retinal degeneration, in rhabdomeric photoreceptor cells disrupts morphogenesis in ways paralleling retinal degeneration seen in ciliated photoreceptors. Taken together, our results suggest the existence of an ancestral Prominin-directed cellular mechanism to create and model the apical membranes of the two fundamental types of photoreceptor cells into their respective phototransduction compartments.

The Actomyosin Machinery Is Required for Drosophila Retinal Lumen Formation

“…Nonetheless, the exact mechanism of its anti-adhesive properties remains ambiguous. We cannot separate the role of N-glycosylation from proper trafficking of Prom to the membrane [18]. Furthermore we have not been able to confirm whether Prom's interaction with EYS is essential for membrane separation or whether the anti–adhesive properties of Prom is sufficient to prevent the interaction of Chaoptin between rhabdomeres as long as a force is supplied to keep the membranes apart.…”

“…In Drosophila photoreceptors, Prom has the potential to act as an anti-adhesive molecule as described for Mucin1 and members of the CD34 family of proteins (reviewed in [2]). Prom is a five-transmembrane protein with two large extracellular loops containing a minimum of four sites that are N-glycosylated [18]. Prom is present on the apical surface before secretion of the extracellular matrix and in combination with EYS provides a barrier to prevent interactions between the adhesive molecule Chp [17], [20], [22].…”

“…Furthermore, the actomyosin contraction observed in Drosophila photoreceptors may also extend to vertebrate ciliary photoreceptors. Previous work has demonstrated the functional conservation of EYS and Prom among rhabdomeric and ciliary photoreceptors [18]. Interestingly, both Prominin1 and MyoII localize to the basal region of the outer segment where the nascent discs are formed in mammalian photoreceptors [59]–[61].…”

“…Nonetheless, secretion of EYS is not sufficient for the generation of a continuous lumen. In addition to being secreted, EYS must be localized around the developing apical rhabdomeres through an interaction with the five-pass transmembrane protein Prominin (Prom) [17], [18]. In the absence of Prom, the lumen space is present but not continuous and the residual fusion between rhabdomeres is the result of the adhesion between the rhabdomeric apical membranes mediated by the GPI-anchored membrane protein Chp [17], [19], [20].…”

chaoptin, prominin, eyes shut and crumbs form a genetic network controlling the apical compartment of Drosophila photoreceptor cells

“…Several mechanisms are discussed, which prevent retinal degeneration in flies (Colley, 2012; Nie et al, 2012; Raghu et al, 2012). Accumulation of intracellular Rh1 has been suggested to trigger light dependent PRC degeneration in crb mutant PRCs (Pocha et al, 2011; Hollingsworth and Gross, 2012).…”