Mechanism of primary ciliogenesis by polarized epithelial cells

Abstract

The primary cilium is a highly conserved membrane extension protruding from the cell surface of most mammalian cells. It consists of a ciliary membrane that surrounds a microtubule-based structure termed the axoneme, which is nucleated from the older of the two centrioles. Although its function has been an enigma for a long time, nowadays it is known to act as a biosensor regulating multiple signaling pathways during development and tissue homeostasis. The physiological and clinical relevance of cilia is evident, since defects in primary cilium function cause a wide spectrum of genetic diseases collectively grouped under the term of ciliopathies. Among the disorders produced by primary cilium dysfunction are cystic kidney disease, blindness, deafness, obesity, and developmental and skeletal abnormalities. The kidney is the organ most frequently affected in ciliopathies. However, despite its importance in the kidney, primary cilium biogenesis has mainly been studied in non-polarized cells. Almost 50 years ago, it was proposed that the process of primary ciliogenesis in polarized epithelia, such as that in kidney tubules, takes place entirely at the plasma membrane, in contrast to fibroblasts that assemble the cilium intracellularly. Using the renal epithelial MDCK cell line, I have investigated the unexplored process of primary cilium biogenesis in polarized epithelial cells. I observed that the midbody, which is a microtubule-based structure that occupies the central part of the intercellular bridge connecting the two sister cells during the final stages of cell division, is inherited by one of the cells as a remnant that localizes at the periphery of the apical membrane, and that accumulates important machinery for ciliary biogenesis. The remnant then moves along the apical plasma membrane to a central position to encounter the centrosome. Once the two organelles have met, the remnant enables the centrosome for primary cilium formation. These findings reveal a biological mechanism that functionally links the midbody with the centrosome and the primary cilium, which are the other two main microtubule-based organelles. I have also investigated the role of MAL, a component of the machinery of apical transport, in primary cilium assembly. The results indicate that MAL is required for correct membrane condensation at the ciliary base, which, in turn, is essential for efficient primary cilium elongation. In summary, the work presented establishes a novel pathway of primary ciliogenesis in renal polarized epithelial cells and establishes the importance of the condensation of the membranes at the ciliary base.