The cilium is an essential structural element of almost all eukaryotic cells. It resembles hair-like projections on the cell surface. Cilia sense environmental cues, play crucial roles in organ development, and are responsible for moving cells and fluids. Problems that affect how cilia work cause a diverse set of ‘ciliopathies’, including polycystic kidney disease, infertility, airway diseases and hearing defects. Calcium ion (Ca2+) signaling is central for controlling how cilia move and respond to signals received from outside the cell (Phua et al., 2015(link is external)), and the key principles of Ca2+ signaling differ significantly between different types of cilia (Doerner et al., 2015(link is external)).

Cilia are categorized as either primary cilia or undulipodia based on obvious structural differences (Figure 1(link is external)). Primary cilia are not able to move themselves, but help cells detect external cues and serve as a signaling hub during development (Goetz and Anderson, 2010(link is external)). In contrast, undulipodia are able to move themselves and are subdivided into motile cilia that move fluid over the outer surface of the cell, and flagellum that propel the cell through fluids (Fliegauf et al., 2007(link is external)). Ciliary movement is directly associated with the axoneme – a structure made of microtubules that serves as a ‘skeleton’ and extends along the length of the cilium. All primary cilia contain nine pairs of microtubules that are arranged in a circle to give the so called 9+0 structure. Undulipodia have two additional microtubules in the center of the axoneme (the 9+2 arrangement; Figure 1(link is external)). The exception from the rule is a nodal cilia, which is motile cilium with 9+0 structure. Sperm flagellum is an excellent model to understand how different ion channels and transporters regulate ciliary/flagellar movement and transport environmental signals to the whole cell that ultimately changes its physiology and navigation.

We are particularly interested in understanding the molecular mechanism of flagellar-regulated sperm chemotaxis. We are also dissecting the molecular mechanisms of sperm temperature activation, and are working on understanding the molecular architecture of the cilia and flagella.

 The ciliary diversity.

Figure 1. The ciliary diversity.

There are two types of cilia, primary cilia and undulipodia, which are distinguished by differences in the structure of their axoneme. The level of calcium ions (Ca2+) inside the cilia is regulated by specialized ion channels. (A) The main Ca2+ channel for primary cilia has been identified as the PKD1L1/PKD2L1 complex. (B) There are two types of undulipodia: motile cilia and flagella. The main Ca2+ channel of mammalian flagella was identified as CatSper in 2001. L-type voltage-gated Ca2+ channel (Cav1 family) is the main Ca2+ channel for motile cilia. (From eLife 2015).