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.

SPECC1L: a cytoskeletal protein that regulates embryonic tissue dynamics.

Many structural birth defects occur due to failure of tissue movement and fusion events during embryogenesis. Examples of such birth defects include failure of closure of the neural tube, palate, and ventral body wall. Actomyosin forces play a pivotal role in these closure processes, making proteins that regulate actomyosin dynamics a priority when studying the etiology of structural birth defects. SPECC1L (sperm antigen with calponin homology and coiled-coil domains 1 like) cytoskeletal protein associates with microtubules, filamentous actin, non-muscle myosin II (NMII), as well as membrane-associated components of adherens junctions. Patients with SPECC1L mutations show a range of structural birth defects affecting craniofacial development (hypertelorism, cleft palate), ventral body wall (omphalocele), and internal organs (diaphragmatic hernia, bicornuate uterus). Characterization of mouse models indicates that these syndromic mutations utilize a gain-of-function mechanism to affect intra- and supra-cellular actin organization. Interestingly, SPECC1L deficiency appears to affect the efficiency of tissue dynamics, making it an important cytoskeletal regulator to study tissue movement and fusion events during embryonic development. Here we summarize the SPECC1L-related syndrome mutations, phenotypes of Specc1l mouse models, and cellular functions of SPECC1L that highlight how it may regulate embryonic tissue dynamics.

Genetic Interaction of Thm2 and Thm1 Shapes Postnatal Craniofacial Bone.

Ciliopathies are genetic syndromes that link skeletal dysplasias to the dysfunction of primary cilia. Primary cilia are sensory organelles synthesized by intraflagellar transport (IFT)-A and B complexes, which traffic protein cargo along a microtubular core. We have reported that the deletion of the IFT-A gene, Thm2, together with a null allele of its paralog, Thm1, causes a small skeleton with a small mandible or micrognathia in juvenile mice. Using micro-computed tomography, here we quantify the craniofacial defects of Thm2-/-; Thm1aln/+ triple allele mutant mice. At postnatal day 14, triple allele mutant mice exhibited micrognathia, midface hypoplasia, and a decreased facial angle due to shortened upper jaw length, premaxilla, and nasal bones, reflecting altered development of facial anterior-posterior elements. Mutant mice also showed increased palatal width, while other aspects of the facial transverse, as well as vertical dimensions, remained intact. As such, other ciliopathy-related craniofacial defects, such as cleft lip and/or palate, hypo-/hypertelorism, broad nasal bridge, craniosynostosis, and facial asymmetry, were not observed. Calvarial-derived osteoblasts of triple allele mutant mice showed reduced bone formation in vitro that was ameliorated by Hedgehog agonist, SAG. Together, these data indicate that Thm2 and Thm1 genetically interact to regulate bone formation and sculpting of the postnatal face. The triple allele mutant mice present a novel model to study craniofacial bone development.