RPGR regulates motile cilia by interfering with actin dynamics

Cilia are highly conserved cellular organelles extruding from the surface of cell types carrying either sensory (signaling) or motile functions. These include photoreceptor cells and airway epithelia, where they function in light sensation and mucociliary clearance respectively. Retinitis pigmentosa GTPase regulator (RPGR) variants affect both photoreceptor sensory cilia and airway motile cilia, leading to retinitis pigmentosa (RP) and in some cases, primary ciliary dyskinesia (PCD), both debilitating conditions. Not all patients develop PCD and it is unclear which RPGR variants predispose patients to PCD and why this happens. In this study, using nasal biopsy samples of patients with RPGR-related RP, we leverage 2D organoid cell culturing, super-resolution microscopy, and live cell imaging to characterize the multiciliated cells from patients with different RPGR variants, healthy human nasal and bronchial multiciliated cells with CRISPR-modified RPGR function. We demonstrate for the first time that multiciliated cells with RPGR variants may have reduced ciliation, shorter cilia, significantly impaired cilia beat, or cilia beat incoordination, which could lead to defective mucociliary clearance and lung disease. In addition, we show the regulation of motile cilia by RPGR involves F-actin, as evidenced by temporarily reduced Gelsolin and undissolved condensed actin meshwork at the apical surface of RPGR-deficient multiciliated cells. In support, we show that the motile cilia defect can be ameliorated by treating with the actin polymerization inhibitor Latrunculin A. Though PCD was observed only in patients with variants that affect both main isoforms (RPGR1-19 and RPGRORF15), patients with variants affecting only RPGRORF15 also showed cilia and airway anomalies. Though all RPGR variants affected motile cilia in one way or another, RPGR loss of function variants affecting both isoforms are associated with more severe cilia and systemic phenotypes, the mechanisms of which involve the accumulation of apical F-actin. One Sentence Summary: Loss of RPGR, through the mechanism of apical F-actin accumulation in airway multiciliated cells, leads to reduced ciliation with short cilia that have an impaired beat, leading to defective mucociliary clearance.