Actin cytoskeletal regulation of ciliogenesis in development and disease

Primary cilia are antenna-like sensory organelles that are evolutionarily conserved in nearly all modern eukaryotes, from the single-celled green alga, Chlamydomonas reinhardtii, to vertebrates and mammals. Cilia are microtubule-based cellular projections that have adapted to perform a broad range of species-specific functions, from cell motility to detection of light and the transduction of extracellular mechanical and chemical signals. These functions render cilia essential for organismal development and survival. The high conservation of cilia has allowed for discoveries in C. reinhardtii to inform our understanding of the basic biology of mammalian primary cilia, and to provide insight into the genetic etiology of ciliopathies. Over the last two decades, a growing number of studies has revealed that multiple aspects of ciliary homeostasis are regulated by the actin cytoskeleton, including centrosome migration and positioning, vesicle transport to the basal body, ectocytosis, and ciliary-mediated signaling. Here, we review actin regulation of ciliary homeostasis, and highlight conserved and divergent mechanisms in C. reinhardtii and mammalian cells. Further, we compare the disease manifestations of patients with ciliopathies to those with mutations in actin and actin-associated genes, and propose that primary cilia defects caused by genetic alteration of the actin cytoskeleton may underlie certain birth defects.