Delivery and deployment of a ciliary actin dynamics module by PCARE drives the neogenesis of phototransductive membrane disks

Abstract

Purpose : Various loss-of-function mutations in C2orf71 are known to cause inherited retinal dystrophies such as retinitis pigmentosa (RP). Yet, very little is still known about the protein encoded by this gene. In this study, we aimed to unravel the molecular function of C2orf71, and understand the pathophysiological mechanism underlying C2orf71-associated RP.

Methods : We have performed an affinity proteomics approach to identify binding partners of the protein product of the C2orf71 gene. Interactions were confirmed via co-immunoprecipitation and immunofluorescence studies, in ciliated cells as well as in a C2orf71^-/- mutant mouse model. The intriguing occurrence of ciliary evaginations was further studied via co-transfections and high-resolution microscopy.

Results : We identified that an extensive actin assembly module was recruited to the tip of the connecting cilium stalk at the base of the outer segment by C2orf71, which we therefore coined PCARE (photoreceptor cilium actin regulator). Nucleated by the Arp2/3 activator WASF3, a dynamic branched F-actin network, resembling that of lamellipodia, drove the membrane evagination at the very onset of OS disk neogenesis, a process that was abrogated in C2orf71/Pcare^-/- mice. Ectopic co-expression of PCARE and WASF3 in ciliated cells mimicked this evagination process at the ciliary tip. Furthermore, this process was disrupted by a retinal dystrophy-associated missense mutation in PCARE, emphasizing that disturbance of this actin dynamics-driven process can underlie RP.

Conclusions : The observation that several proteins involved in non-syndromic and syndromic retinal ciliopathies take part in the ciliary membrane evagination suggests that it is an important common denominator in the retinal pathomechanisms of these hereditary disorders.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.