The ciliopathy-associated protein homologs RPGRIP1 and RPGRIP1L are linked to cilium integrity through interaction with Nek4 serine/threonine kinase

Recent studies have established ciliary dysfunction as the underlying cause of a broad range of multi-organ phenotypes, known as 'ciliopathies'. Ciliopathy-associated proteins have a common site of action in the cilium, however, their overall importance for ciliary function differs, as implied by the extreme variability in ciliopathy phenotypes. The aim of this study was to gain more insight in the function of two ciliopathy-associated protein homologs, RPGR interacting protein 1 (RPGRIP1) and RPGRIP1-like protein (RPGRIP1L). Mutations in RPGRIP1 lead to the eye-restricted disease Leber congenital amaurosis, while mutations in RPGRIP1L are causative for Joubert and Meckel syndrome, which affect multiple organs and are at the severe end of the ciliopathy spectrum. Using tandem affinity purification in combination with mass spectrometry, we identified Nek4 serine/threonine kinase as a prominent component of both the RPGRIP1- as well as the RPGRIP1L-associated protein complex. In ciliated cells, this kinase localized to basal bodies, while in ciliated organs, the kinase was predominantly detected at the ciliary rootlet. Down-regulation of NEK4 in ciliated cells led to a significant decrease in cilium assembly, pointing to a role for Nek4 in cilium dynamics. We now hypothesize that RPGRIP1 and RPGRIP1L function as cilium-specific scaffolds that recruit a Nek4 signaling network which regulates cilium stability. Our data are in line with previously established roles in the cilium of other members of the Nek protein family and define NEK4 as a ciliopathy candidate gene.