Primary cilia are not calcium-responsive mechanosensors

Abstract

Primary cilia are solitary, generally non-motile, hair-like protrusions that extend from the surface of cells between cell divisions. Their antenna-like structure leads naturally to the assumption that they sense the surrounding environment, the most common hypothesis being sensation of mechanical force through calcium-permeable ion channels within the cilium1. This Ca2+- Responsive MechanoSensor (CaRMS) hypothesis for primary cilia has been invoked to explain a large range of biological responses, from control of left-right axis determination in embryonic development to adult progression of polycystic kidney disease and some cancers2,3. Here, we report the complete lack of mechanically induced calcium increases in primary cilia, in tissues upon which this hypothesis has been based. First, we developed a transgenic mouse, Arl13b-mCherry-GECO1.2, expressing a ratiometric genetically encoded calcium indicator (GECI) in all primary cilia. We then measured responses to flow in primary cilia of cultured kidney epithelial cells, kidney thick ascending tubules, crown cells of the embryonic node, kinocilia of inner ear hair cells, and several cell lines. Cilia-specific Ca2+ influxes were not observed in physiological or even highly supraphysiological levels of fluid flow. We conclude that mechanosensation, if it originates in primary cilia, is not via calcium signaling.

Ciliary Mechanisms of Cyst Formation in Polycystic Kidney Disease

“…It has long been proposed to be a flow sensor (Nauli et al 2003) although recent studies have called that into question (Delling et al 2016). More recently, it has been proposed to be a Wnt ligand coreceptor (Kim et al 2016).…”

Present Status and Advances in Autosomal Dominant Polycystic Kidney Disease

“…Thus, besides the absence of polycystins that leads to polycystic kidney disease phenotype, IFT system deletions such as deactivating the IFT system components Kif3a, Ift20 and Ift88 in mouse models also cause polycystic kidney disease phenotype [54]. Although cilia loss is associated with cystic kidney disease, recent studies have shown that there is no evidence of primary cilia-associated Ca 2+ influx in both the renal tubules and cell lines stimulated by fluid flow at physiological levels [55]. Therefore, the role and function of fibril in ADPKD remains to be further studied and elucidated.…”

Introduction to provocative questions in left–right asymmetry

“…mechanosensitive cilia, morphogen gradients or vesicular particles. More recent studies have shown that primary cilia are not in fact calcium-responsive mechanosensors [8], and that asymmetries in cleavage-stage frog embryos have not only consistently asymmetric metabolic profiles [9], but also protein components with functional roles in asymmetry development [10]. Consistent with the existence of L/R asymmetry at such an early stage of development, Dasgupta and Amack [6] used mosaic cell labelling and time-lapse imaging in the zebrafish embryo to demonstrate that ciliated cells within Kupffer's vesicle-a structure that is functionally analogous to the embryonic node and alternatively termed the 'L/R organizer'-are non-stochastically arranged, suggesting that they may be propagating L/R positional information that exists prior to L/R Organizer morphogenesis and hence, generation of directional fluid flow.…”