(The) role and mechanism of ESCRT proteins as regulators of primary cilia

Abstract

Primary cilia are one of the subcellular sensory organelle present in many types of cells in the human body. Primary cilia transmit signals from the outside to the inside of the cell, and they are involved in the growth and the homeostasis of the cell. When cilia formation or function is abnormal, a variety of defects appear in the body, and these defects are clinical features of cilia related genetic diseases called the ciliopathy. Therefore, efforts are continuing to search for genes involved in the development and maintenance of cilia. Recently, a number of genes related to ciliogenesis have been found through whole genome screening using siRNA library. Based on the screening data, I focused on components of the endosomal sorting complexes required for transport (ESCRT) machinery, and identified CHMP4B as a protein essential for the formation of primary cilia. Immunofluorescence staining showed ciliary localization of CHMP4B. A loss-of-function study using siRNA revealed ciliogenesis defect and abnormal distribution of actin fibers in CHMP4B depleted cells. The defect of ciliary formation upon CHMP4B depletion was partially restored by actin disassembly drug Cytochalasin D. These findings suggest that CHMP4B participates in the dynamics of the actin cytoskeleton and, as a result, influences the formation of cilia. In addition, I found that ciliary localization of CHMP4B is affected by dissociation of actin fibers, which suggest that actin and CHMP4B interact with each other at cilia. Interestingly, microscopic live-cell imaging showed that CHMP4B not only affects the formation of cilia but also is involved in ciliary stability and maintenance. In the embryonic Zebrafish, morpholino-mediated CHMP4B depletion caused hydrocephalus and deformed otolith, which are clinical features of the ciliopathy. These results demonstrate that CHMP4B is a new regulator essential for the formation and maintenance of cilia.