NESPRIN1 IS A NOVEL COMPONENT OF CILIARY ROOTLETS

Abstract

Purpose : SYNE1 encodes multiple isoforms of Nesprin1 that homogenously populate the outer nuclear membrane of the nuclear envelope (NE). Through their direct interaction with Sun proteins, which populate the inner nuclear membrane of the NE, multiple isoforms of Nesprin1 position the nucleus during organogenesis. To date, this well-established biological function of Nesprin1 has failed to provide any hypothesis related to the wide spectrum of human pathologies linked to biallelic truncations of SYNE1. Based on our recent findings in mouse photoreceptors, we hypothesized novel functions for Nesprin1 beyond the NE.

Methods : Using conditional inactivation SYNE1 in rod photoreceptors, multiple isoforms of Nesprin1 were characterized at the transcript level by RT-PCR and at the protein level by immunoblotting and immunolocalization in light and electron microscopy. The functional relevance of SYNE1 in rods was evaluated by electroretinography. Transfection of cultured NIH3t3 fibroblasts was used to induce the noncanonical localization of endogenous Nesprin1 in a cell culture model.

Results : Nesprin1α (120 kDa) forms long filaments that line the whole inner segments of photoreceptors where they colocalize with rootletin. Nesprin1α also localizes asymmetrically at the NE of rods where it specifically recruits Sun2 and anchors finite rootletin filaments at the nuclear surface. In cones, Nesprin1α formed similar filamentous structures that capped the apical side of nuclei and further extended all the way up to basal bodies. We further show that targeted depletion of Nesprin1α in rods significantly decreases scotopic a- and b-waves in 2.5 month-old mice. The formation of Nesprin1 filaments is induced by rootletin itself as a large endogenous isoform of Nesprin1 that localizes homogenously at the NE of untransfected NIH3t3 fibroblasts is redistributed into filamentous structures both at and beyond the NE upon rootletin expression. In line with these results, we uncovered that Nesprin1 localizes to ciliary rootlets of multiciliated cells.

Conclusions : Multiple isoforms of Nesprin1 associate with ciliary rootlets. This novel functional paradigm suggests that the wide spectrum of human pathologies linked to nonsense mutations of SYNE1 may in part originate from ciliary defects.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.