Distinct roles for prominin-1 and photoreceptor cadherin in outer segment disc morphogenesis in CRISPR-altered X. laevis

Abstract

Mutations in prominin-1 (prom1) and photoreceptor cadherin (cdhr1) are associated with inherited retinal degenerative disorders but their functions remain unknown. Here, we used CRISPR-Cas9 to generate prom1-null, cdhr1-null, and prom1 plus cdhr1 double-null Xenopuslaevis and then documented the effects of these mutations on photoreceptor structure and function. Prom1-null mutations resulted in severely dysmorphic photoreceptors comprising overgrown and disorganized disc membranes. Cone outer segments were more severely affected than rods and had an impaired electroretinogram response. Cdhr1-null photoreceptors did not appear grossly dysmorphic, but ultrastructural analysis revealed that some disc membranes were overgrown or oriented vertically within the plasma membrane. Double-null mutants did not differ significantly from prom1-null mutants. Our results indicate that neither prom1 nor cdhr1 are necessary for outer segment disc membrane evagination or the fusion event that controls disc sealing. Rather, they are necessary for the higher-order organization of the outer segment. Prom1 may align and reinforce interactions between nascent disc leading edges, a function more critical in cones for structural support. Cdhr1 may secure discs in a horizontal orientation prior to fusion and regulate cone lamellae size.This article has an associated First Person interview with the first author of the paper.

Photoreceptor Disc Enclosure Is Tightly Controlled by Peripherin-2 Oligomerization

“…The incremental array of structural defects observed in young mice suggests a sequence of events leading to pathology in these and potentially other PRPH2 mutant mice. As discs remain open for an abnormally long period of time, they occasionally incorporate more than a normal amount of membranous material delivered to the outer segment, causing disc overgrowth comparable to that observed in other models (e.g., Yang et al, 2008;Schietroma et al, 2017;Sharif et al, 2018;Spencer et al, 2019;Carr et al, 2021). This disc overgrowth can become uncontrolled, leading to the formation of "membrane whorl" structures described for other mutations in PRPH2 (Hawkins et al, 1985;Tam et al, 2004;Chakraborty et al, 2016Chakraborty et al, , 2020Lewis et al, 2020) and other proteins (LaVail et al, 1972;Narfström and Nilsson, 1989;Leon and Curtis, 1990;White et al, 1993;Stiemke et al, 1994;Jablonski et al, 1999;Gao et al, 2002;Yefimova et al, 2002;Duncan et al, 2003;Rice et al, 2004;Lu et al, 2019;Spencer et al, 2019;Ying et al, 2019).…”

Early manifestations and differential gene expression associated with photoreceptor degeneration in Prom1-deficient retina

“…The expression of Edn2 has also been shown to be upregulated in other mouse models of RP ( Bramall et al, 2013 ), including retina-specific Cdhr1 -KO mice ( Rattner and Nathans, 2005 ), with Prom1 and Cdhr1 having been found to interact with each other ( Yang et al, 2008 ). Although a recent study demonstrated different phenotypes for the Prom1- and Cdhr1 -mutant retinas ( Carr et al, 2021 ), the functions of these two proteins may be mediated by similar signalling pathways.…”

“…Although a recent study demonstrated different phenotypes for the Prom1and Cdhr1-mutant retinas (Carr et al, 2021), the functions of these two proteins may be mediated by similar signalling pathways.…”

“…The retinal vasculature also becomes attenuated with disease progression ( Zacchigna et al, 2009 ). Markedly dysmorphic photoreceptors are also apparent in Prom1 -mutant frogs ( Carr et al, 2021 ), suggestive of a conserved role for Prom1 in photoreceptor function.…”

Visual pigment–deficient cones survive and mediate visual signaling despite the lack of outer segments

“…In addition to cone phototransduction proteins, we also examined the localization of two structural proteins, peripherin-2 (Prph2) and prominin-1 (Prom1). Both Prph2 and Prom1 are important for disk morphogenesis in rods and cones ( 11 – 17 ). As expected, Prom1 was normally restricted at the base of both ROS and COS in WT mice ( Fig.…”