How Microtubules And Motor Proteins Enable The Maintenance And Functioning Of Cilia

Introduction

Cilia are highly conserved, microtubule-based organelles present on the vast majority of human cells.¹ The two major types are primary cilia, also known as sensory or nonmotile cilia, and motile cilia (Figure 1a). Most cells have a single primary cilium² that serves sensory and signaling functions, whereas motile cilia are a particular feature of specialized multiciliated cells and beat in a coordinated fashion to drive extracellular fluid flow.^1,3 Ciliary beating enables mucus clearance by the respiratory epithelium, cerebrospinal fluid circulation via the ependyma, and egg transportation in fallopian tubes.⁴

Regardless of type, all cilia are anchored intracellularly by a modified centriolar structure known as the basal body.⁴ The extracellular protrusion is referred to as the axoneme⁵ and is supported by a cylindrical bundle of nine microtubule doublets (Figure 1b). In motile cilia, an additional central doublet is present and the combined axonemal microtubules act as a scaffold for other structural components,^3,5 most notably the axonemal dynein assemblies that power ciliary beating.⁶

In this newsletter, we will briefly review the unique roles of key ciliary motor proteins including kinesin-2, dynein-2, and axonemal dynein arms, and also consider the pathological consequences of mutations that affect their normal functioning.⁷