Actin-dependent regulation of cilia length by the inverted formin FHDC1

Evolutionary trajectory for nuclear functions of ciliary transport complex proteins

SUMMARYCilia and the nucleus were two defining features of the last eukaryotic common ancestor. In early eukaryotic evolution, these structures evolved through the diversification of a common membrane-coating ancestor, the protocoatomer. While in cilia, the descendants of this protein complex evolved into parts of the intraflagellar transport complexes and BBSome, the nucleus gained its selectivity by recruiting protocoatomer-like proteins to the nuclear envelope to form the selective nuclear pore complexes. Recent studies show a growing number of proteins shared between the proteomes of the respective organelles, and it is currently unknown how ciliary transport proteins could acquire nuclear functions and vice versa. The nuclear functions of ciliary proteins are still observable today and remain relevant for the understanding of the disease mechanisms behind ciliopathies. In this work, we review the evolutionary history of cilia and nucleus and their respective defining proteins and integrate current knowledge into theories for early eukaryotic evolution. We postulate a scenario where both compartments co-evolved and that fits current models of eukaryotic evolution, explaining how ciliary proteins and nucleoporins acquired their dual functions.

Identification of an FMNL2 Interactome by Quantitative Mass Spectrometry

Formin Homology Proteins (Formins) are a highly conserved family of cytoskeletal regulatory proteins that participate in a diverse range of cellular processes. FMNL2 is a member of the Diaphanous-Related Formin sub-group, and previous reports suggest FMNL2's role in filopodia assembly, force generation at lamellipodia, subcellular trafficking, cell-cell junction assembly, and focal adhesion formation. How FMNL2 is recruited to these sites of action is not well understood. To shed light on how FMNL2 activity is partitioned between subcellular locations, we used biotin proximity labeling and proteomic analysis to identify an FMNL2 interactome. The interactome identified known and new FMNL2 interacting proteins with functions related to previously described FMNL2 activities. In addition, our interactome predicts a novel connection between FMNL2 and extracellular vesicle assembly. We show directly that FMNL2 protein is present in exosomes.

Collagen VI Deficiency Impairs Tendon Fibroblasts Mechanoresponse in Ullrich Congenital Muscular Dystrophy

The pericellular matrix (PCM) is a specialized extracellular matrix that surrounds cells. Interactions with the PCM enable the cells to sense and respond to mechanical signals, triggering a proper adaptive response. Collagen VI is a component of muscle and tendon PCM. Mutations in collagen VI genes cause a distinctive group of inherited skeletal muscle diseases, and Ullrich congenital muscular dystrophy (UCMD) is the most severe form. In addition to muscle weakness, UCMD patients show structural and functional changes of the tendon PCM. In this study, we investigated whether PCM alterations due to collagen VI mutations affect the response of tendon fibroblasts to mechanical stimulation. By taking advantage of human tendon cultures obtained from unaffected donors and from UCMD patients, we analyzed the morphological and functional properties of cellular mechanosensors. We found that the length of the primary cilia of UCMD cells was longer than that of controls. Unlike controls, in UCMD cells, both cilia prevalence and length were not recovered after mechanical stimulation. Accordingly, under the same experimental conditions, the activation of the Hedgehog signaling pathway, which is related to cilia activity, was impaired in UCMD cells. Finally, UCMD tendon cells exposed to mechanical stimuli showed altered focal adhesions, as well as impaired activation of Akt, ERK1/2, p38MAPK, and mechanoresponsive genes downstream of YAP. By exploring the response to mechanical stimulation, for the first time, our findings uncover novel unreported mechanistic aspects of the physiopathology of UCMD-derived tendon fibroblasts and point at a role for collagen VI in the modulation of mechanotransduction in tendons.

The actin-bundling protein Fascin-1 modulates ciliary signalling

Primary cilia are microtubule-based cell organelles important for cellular communication. Since they are involved in the regulation of numerous signalling pathways, defects in cilia development or function are associated with genetic disorders, collectively called ciliopathies. Besides their ciliary functions, recent research has shown that several ciliary proteins are involved in the coordination of the actin cytoskeleton. Although ciliary and actin phenotypes are related, the exact nature of their interconnection remains incompletely understood. Here, we show that the protein BBS6, associated with the ciliopathy Bardet-Biedl syndrome, cooperates with the actin-bundling protein Fascin-1 in regulating filopodia and ciliary signalling. We found that loss of Bbs6 affects filopodia length potentially via attenuated interaction with Fascin-1. Conversely, loss of Fascin-1 leads to a ciliary phenotype, subsequently affecting ciliary Wnt signalling, possibly in collaboration with BBS6. Our data shed light on how ciliary proteins are involved in actin regulations and provide new insight into the involvement of the actin regulator Fascin-1 in ciliogenesis and cilia-associated signalling. Advancing our knowledge of the complex regulations between primary cilia and actin dynamics is important to understand the pathogenic consequences of ciliopathies.

Genome-wide association study reveals candidate markers related to field fertility and semen quality traits in Holstein-Friesian bulls

In vitro assessment of bull semen quality is routinely used in bull semen processing centres in order to ensure that semen destined to be used in the field has passed minimum standards. Despite these stringent quality control checks, individual bulls that pass the quality control checks can still vary in field fertility by up to 25%. A genome-wide association study was undertaken to determine genetic markers associated with prefreeze and post-thaw bull sperm quality traits as well as field fertility. Genome-wide association analysis was performed using a single nucleotide polymorphism (SNP) regression mixed linear model in WOMBAT. Genes within a 250 Kb span of a suggestive (P ≤ 1 × 10^-5) SNP were considered as candidate genes. One SNP was associated with adjusted pregnancy rate, and 21 SNPs were associated across the seven semen quality traits (P ≤ 1 × 10^-5). Functional candidate genes include SIPA1L2 which was associated with adjusted pregnancy rate. This encodes a Rap GTPase-activating protein involved in Rap1 signalling pathway and was previously found to play a role in the process of sperm differentiation. Gene ontology (GO) analysis also identified significantly enriched biological processes involved protein tyrosine kinase activity including genes such as DYRK1A, TEC and TXK that were associated with sperm motility prior to freezing. Another candidate gene associated with post-thaw sperm motility was FHDC1 which coordinates actin filament and microtubule dynamics. The induced 11 GO terms in the ejaculates rejected after freezing trait were related to ATPase, phosphatase and hydrolase activity. These results reveal novel specific genomic regions and candidate genes associated with economically important phenotypes such as field fertility and semen quality traits.

Cooperative assembly of filopodia by the formin FMNL2 and I-BAR domain protein IRTKS

Filopodia are long finger-like actin-based structures that project out from the plasma membrane as cells navigate and explore their extracellular environment. The initiation of filopodia formation requires release of tension at the plasma membrane followed by the coordinated assembly of long unbranched actin filaments. Filopodia growth is maintained by a tip complex that promotes actin polymerization and protects the growing barbed ends of the actin fibers from capping proteins. Filopodia growth also depends on additional F-actin bundling proteins to stiffen the actin filaments as well as extension of the membrane sheath projecting from the cell periphery. These activities can be provided by a number of actin-binding and membrane-binding proteins including formins such as formin-like 2 (FMNL2) and FMNL3, and Inverse-Bin-Amphiphysin-Rvs (I-BAR) proteins such as IRTKS and IRSp53, but the specific requirement for these proteins in filopodia assembly is not clear. We report here that IRTKS and IRSp53 are FMNL2-binding proteins. Coexpression of FMNL2 with either I-BAR protein promotes cooperative filopodia assembly. We find IRTKS, but not IRSp53, is required for FMNL2-induced filopodia assembly, and FMNL2 and IRTKS are mutually dependent cofactors in this process. Our results suggest that the primary function for FMNL2 during filopodia assembly is binding to the plasma membrane and that regulation of actin dynamics by its formin homology 2 domain is secondary. From these results, we conclude that FMNL2 initiates filopodia assembly via an unexpected novel mechanism, by bending the plasma membrane to recruit IRTKS and thereby nucleate filopodia assembly.

Cytoskeletal networks in primary cilia: Current knowledge and perspectives

Primary cilia, microtubule-based protrusions present on the surface of most mammalian cells, function as sensory organelles that monitor extracellular signals and transduce them into intracellular biochemical responses. There is renewed research interest in primary cilia due to their essential roles in development, tissue homeostasis, and human diseases. Primary cilia dysfunction causes a large spectrum of human diseases, collectively known as ciliopathies. Despite significant advances in our understanding of primary cilia, there are still no effective agents for treating ciliopathies. Primary ciliogenesis is a highly ordered process involving membrane trafficking, basal body maturation, vesicle docking and fusion, transition zone assembly, and axoneme extension, in which actin and microtubule networks play critical and multiple roles. Actin and microtubule network architecture, isotropy, and dynamics are tightly controlled by cytoskeleton-associated proteins, a growing number of which are now recognized as responsible for cilium formation and maintenance. Here we summarize the roles of actin and microtubules and their associated proteins in primary ciliogenesis and maintenance. In doing so, we highlight that targeting cytoskeleton-associated proteins may be a promising therapeutic strategy for the treatment of ciliopathies.

LUZP1: A new player in the actin-microtubule cross-talk

LUZP1 (leucine zipper protein 1) was first described as being important for embryonic development. Luzp1 null mice present defective neural tube closure and cardiovascular problems, which cause perinatal death. Since then, LUZP1 has also been implicated in the etiology of diseases like the 1p36 and the Townes-Brocks syndromes, and the molecular mechanisms involving this protein started being uncovered. Proteomics studies placed LUZP1 in the interactomes of the centrosome-cilium interface, centriolar satellites, and midbody. Concordantly, LUZP1 is an actin and microtubule-associated protein, which localizes to the centrosome, the basal body of primary cilia, the midbody, actin filaments and cellular junctions. LUZP1, like its interactor EPLIN, is an actin-stabilizing protein and a negative regulator of primary cilia formation. Moreover, through the regulation of actin, LUZP1 has been implicated in the regulation of cell cycle progression, cell migration and epithelial cell apical constriction. This review discusses the latest findings concerning LUZP1 molecular functions and implications in disease development.

Roles of the actin cytoskeleton in ciliogenesis

Primary cilia play a key role in the ability of cells to respond to extracellular stimuli, such as signaling molecules and environmental cues. These sensory organelles are crucial to the development of many organ systems, and defects in primary ciliogenesis lead to multisystemic genetic disorders, known as ciliopathies. Here, we review recent advances in the understanding of several key aspects of the regulation of ciliogenesis. Primary ciliogenesis is thought to take different pathways depending on cell type, and some recent studies shed new light on the cell-type-specific mechanisms regulating ciliogenesis at the apical surface in polarized epithelial cells, which are particularly relevant for many ciliopathies. Furthermore, recent findings have demonstrated the importance of actin cytoskeleton dynamics in positively and negatively regulating multiple stages of ciliogenesis, including the vesicular trafficking of ciliary components and the positioning and docking of the basal body. Finally, studies on the formation of motile cilia in multiciliated epithelial cells have revealed requirements for actin remodeling in this process too, as well as showing evidence of an additional alternative ciliogenesis pathway.

PIP2 determines length and stability of primary cilia by balancing membrane turnovers

Primary cilia are sensory organelles on many postmitotic cells. The ciliary membrane is continuous with the plasma membrane but differs in its phospholipid composition with phosphatidylinositol 4,5-bisposphate (PIP2) being much reduced toward the ciliary tip. In order to determine the functional significance of this difference, we used chemically induced protein dimerization to rapidly synthesize or degrade PIP2 selectively in the ciliary membrane. We observed ciliary fission when PIP2 was synthesized and a growing ciliary length when PIP2 was degraded. Ciliary fission required local actin polymerisation in the cilium, the Rho kinase Rac, aurora kinase A (AurkA) and histone deacetylase 6 (HDAC6). This pathway was previously described for ciliary disassembly before cell cycle re-entry. Activating ciliary receptors in the presence of dominant negative dynamin also increased ciliary PIP2, and the associated vesicle budding required ciliary PIP2. Finally, ciliary shortening resulting from constitutively increased ciliary PIP2 was mediated by the same actin – AurkA – HDAC6 pathway. Taken together, changes in ciliary PIP2 are a unifying point for ciliary membrane stability and turnover. Different stimuli increase ciliary PIP2 to secrete vesicles and reduce ciliary length by a common pathway. The paucity of PIP2 in the distal cilium therefore ensures ciliary stability.

Cep55 regulation of PI3K/Akt signaling is required for neocortical development and ciliogenesis

Homozygous nonsense mutations in CEP55 are associated with several congenital malformations that lead to perinatal lethality suggesting that it plays a critical role in regulation of embryonic development. CEP55 has previously been studied as a crucial regulator of cytokinesis, predominantly in transformed cells, and its dysregulation is linked to carcinogenesis. However, its molecular functions during embryonic development in mammals require further investigation. We have generated a Cep55 knockout (Cep55-/-) mouse model which demonstrated preweaning lethality associated with a wide range of neural defects. Focusing our analysis on the neocortex, we show that Cep55-/- embryos exhibited depleted neural stem/progenitor cells in the ventricular zone as a result of significantly increased cellular apoptosis. Mechanistically, we demonstrated that Cep55-loss downregulates the pGsk3β/β-Catenin/Myc axis in an Akt-dependent manner. The elevated apoptosis of neural stem/progenitors was recapitulated using Cep55-deficient human cerebral organoids and we could rescue the phenotype by inhibiting active Gsk3β. Additionally, we show that Cep55-loss leads to a significant reduction of ciliated cells, highlighting a novel role in regulating ciliogenesis. Collectively, our findings demonstrate a critical role of Cep55 during brain development and provide mechanistic insights that may have important implications for genetic syndromes associated with Cep55-loss.

Ciliary GPCR-based transcriptome as a key regulator of cilia length control

The primary cilium is a plasma membrane-protruding sensory organelle that efficiently conveys signaling cascades in a highly ordered microenvironment. Its signaling is mediated, in part, by a limited set of GPCRs preferentially enriched in the cilium membrane. This includes melanin-concentrating hormone (MCH) receptor 1 (MCHR1), which plays a role in feeding and mood. In addition to its receptor composition, the length of the cilium is a characteristic parameter that is implicated in its function. We previously found that MCH can dynamically shorten cilia length via the Gi/o and Akt pathways in both MCHR1-expressing hTERT-RPE1 cells (hRPE1 cells) and rat hippocampal neurons. However, the detailed mechanisms by which MCH regulates cilia length through ciliary MCHR1 remains unclear. In this study, we aimed to determine the transcriptome changes in MCHR1-expressing hRPE1 cells in response to MCH to identify the target molecules involved in cilia length regulation via MCHR1 activation. RNA sequencing analysis of ciliated cells subjected to MCH treatment showed upregulation of 424 genes and downregulation of 112 genes compared with static control cells. Validation by quantitative real-time PCR, knocking down, and CRISPR/Cas9-mediated knockout technology identified a molecule, PDZ and LIM domain-containing protein 5 (PDLIM5). Thus, it was considered as the most significant key factor for MCHR1-mediated shortening of cilia length. Additional analyses revealed that the actin-binding protein alpha-actinin 1/4 is a crucial downstream target of the PDLIM5 signaling pathway that exerts an effect on MCHR1-induced cilia shortening. In the endogenous MCHR1-expressing hippocampus, transcriptional upregulation of PDLIM5 and actinin 1/4, following the application of MCH, was detected when the MCHR1-positive cilia were shortened. Together, our transcriptome study based on ciliary MCHR1 function uncovered a novel and important regulatory step underlying cilia length control. These results will potentially serve as a basis for understanding the mechanism underlying the development of obesity and mood disorders.

Myocardin-related transcription factor and serum response factor regulate cilium turnover by both transcriptional and local mechanisms

Turnover of the primary cilium (PC) is critical for proliferation and tissue homeostasis. Each key component of the PC resorption machinery, the HEF1/Aurora kinase A (AurA)/HDAC6 pathway harbors cis-elements potentially targeted by the transcriptional co-activator myocardin-related transcription factor (MRTF) and/or its partner serum response factor (SRF). Thus we investigated if MRTF and/or SRF regulate PC turnover. Here we show that (1) both MRTF and SRF are indispensable for serum-induced PC resorption, and (2) they act via both transcriptional and local mechanisms. Intriguingly, MRTF and SRF are present in the basal body and/or the PC, and serum facilitates ciliary MRTF recruitment. MRTF promotes the stability and ciliary accumulation of AurA and facilitates SRF phosphorylation. Ciliary SRF interacts with AurA and HDAC6. MRTF also inhibits ciliogenesis. It interacts with and is required for the correct localization of the ciliogenesis modulator CEP290. Thus, MRTF and SRF are critical regulators of PC assembly and/or disassembly, acting both as transcription factors and as PC constituents.

Epigenetically Mediated Ciliogenesis and Cell Cycle Regulation, and Their Translational Potential

Primary cilia biogenesis has been closely associated with cell cycle progression. Cilia assemble when cells exit the cell cycle and enter a quiescent stage at the post-mitosis phase, and disassemble before cells re-enter a new cell cycle. Studies have focused on how the cell cycle coordinates with the cilia assembly/disassembly process, and whether and how cilia biogenesis affects the cell cycle. Appropriate regulation of the functions and/or expressions of ciliary and cell-cycle-associated proteins is pivotal to maintaining bodily homeostasis. Epigenetic mechanisms, including DNA methylation and histone/chromatin modifications, are involved in the regulation of cell cycle progression and cilia biogenesis. In this review, first, we discuss how epigenetic mechanisms regulate cell cycle progression and cilia biogenesis through the regulation of DNA methylation and chromatin structures, to either promote or repress the transcription of genes associated with those processes and the modification of cytoskeleton network, including microtubule and actin. Next, we discuss the crosstalk between the cell cycle and ciliogenesis, and the involvement of epigenetic regulators in this process. In addition, we discuss cilia-dependent signaling pathways in cell cycle regulation. Understanding the mechanisms of how epigenetic regulators contribute to abnormal cell cycle regulation and ciliogenesis defects would lead to developing therapeutic strategies for the treatment of a wide variety of diseases, such as cancers, polycystic kidney disease (PKD), and other ciliopathy-associated disorders.

Nonredundant roles of DIAPHs in primary ciliogenesis

Primary cilia are hubs for several signaling pathways, and disruption in cilia function and formation leads to a range of diseases collectively known as ciliopathies. Both ciliogenesis and cilia maintenance depend on vesicle trafficking along a network of microtubules and actin filaments toward the basal body. The DIAPH (Diaphanous-related) family of formins promote both actin polymerization and microtubule (MT) stability. Recently, we showed that the formin DIAPH1 is involved in ciliogenesis. However, the role of other DIAPH family members in ciliogenesis had not been investigated. Here we show that depletion of either DIAPH2 or DIAPH3 also disrupted ciliogenesis and cilia length. DIAPH3 depletion also reduced trafficking within cilia. To specifically examine the role of DIAPH3 at the base, we used fused full-length DIAPH3 to centrin, which targeted DIAPH3 to the basal body, causing increased trafficking to the ciliary base, an increase in cilia length, and formation of bulbs at the tips of cilia. Additionally, we confirmed that the microtubule-stabilizing properties of DIAPH3 are important for its cilia length functions and trafficking. These results indicate the importance of DIAPH proteins in regulating cilia maintenance. Moreover, defects in ciliogenesis caused by DIAPH depletion could only be rescued by expression of the specific family member depleted, indicating nonredundant roles for these proteins.

The molecular structure of mammalian primary cilia revealed by cryo-electron tomography

Primary cilia are microtubule-based organelles that are important for signaling and sensing in eukaryotic cells. Unlike the thoroughly studied motile cilia, the three-dimensional architecture and molecular composition of primary cilia are largely unexplored. Yet, studying these aspects is necessary to understand how primary cilia function in health and disease. We developed an enabling method for investigating the structure of primary cilia isolated from MDCK-II cells at molecular resolution by cryo-electron tomography. We show that the textbook '9 + 0' arrangement of microtubule doublets is only present at the primary cilium base. A few microns out, the architecture changes into an unstructured bundle of EB1-decorated microtubules and actin filaments, putting an end to a long debate on the presence or absence of actin filaments in primary cilia. Our work provides a plethora of insights into the molecular structure of primary cilia and offers a methodological framework to study these important organelles.

The entangled relationship between cilia and actin

Primary cilia are microtubule-based sensory cell organelles that are vital for tissue and organ development. They act as an antenna, receiving and transducing signals, enabling communication between cells. Defects in ciliogenesis result in severe genetic disorders collectively termed ciliopathies. In recent years, the importance of the direct and indirect involvement of actin regulators in ciliogenesis came into focus as it was shown that F-actin polymerisation impacts ciliation. The ciliary basal body was further identified as both a microtubule and actin organising centre. In the current review, we summarize recent studies on F-actin in and around primary cilia, focusing on different actin regulators and their effect on ciliogenesis, from the initial steps of basal body positioning and regulation of ciliary assembly and disassembly. Since primary cilia are also involved in several intracellular signalling pathways such as planar cell polarity (PCP), subsequently affecting actin rearrangements, the multiple effectors of this pathway are highlighted in more detail with a focus on the feedback loops connecting actin networks and cilia proteins. Finally, we elucidate the role of actin regulators in the development of ciliopathy symptoms and cancer.

DIAPH1 regulates ciliogenesis and trafficking in primary cilia

Primary cilia are critical hubs for several signaling pathways, and defects in ciliogenesis or cilia maintenance produce a range of diseases collectively known as ciliopathies. Ciliogenesis requires vesicle trafficking along a network of microtubules and actin filaments to the basal body. The DIAPH1 (Diaphanous-related formin) family of formins promotes both actin polymerization and EB1-dependent microtubule (MT) stability. EB1 and EB3 have previously been implicated in cilia biogenesis to carry out centrosome-related functions. However, the role of DIAPH1 proteins had not been examined. Here we show that the depletion of DIAPH1 decreased ciliogenesis, cilia length, and reduced trafficking within cilia. Additionally, both actin nucleating and microtubule-stabilizing properties of DIAPH1 are important for their cilia functions. To assess their roles in ciliogenesis in isolation, we targeted DIAPH1 specifically to the basal body, which caused an increase in cilia length and increased trafficking within cilia. Intriguingly, expression of DIAPH1 mutants associated with human deafness and microcephaly impaired ciliation and caused cilia elongation and bulb formation. These results suggest that the actin and microtubule functions of DIAPH1 proteins regulate cilia maintenance in part by regulating vesicular trafficking to the base of the primary cilia.

Primary cilia biogenesis and associated retinal ciliopathies

The primary cilium is a ubiquitous microtubule-based organelle that senses external environment and modulates diverse signaling pathways in different cell types and tissues. The cilium originates from the mother centriole through a complex set of cellular events requiring hundreds of distinct components. Aberrant ciliogenesis or ciliary transport leads to a broad spectrum of clinical entities with overlapping yet highly variable phenotypes, collectively called ciliopathies, which include sensory defects and syndromic disorders with multi-organ pathologies. For efficient light detection, photoreceptors in the retina elaborate a modified cilium known as the outer segment, which is packed with membranous discs enriched for components of the phototransduction machinery. Retinopathy phenotype involves dysfunction and/or degeneration of the light sensing photoreceptors and is highly penetrant in ciliopathies. This review will discuss primary cilia biogenesis and ciliopathies, with a focus on the retina, and the role of CP110-CEP290-CC2D2A network. We will also explore how recent technologies can advance our understanding of cilia biology and discuss new paradigms for developing potential therapies of retinal ciliopathies.

Prominins control ciliary length throughout the animal kingdom: New lessons from human prominin-1 and zebrafish prominin-3

Prominins (proms) are transmembrane glycoproteins conserved throughout the animal kingdom. They are associated with plasma membrane protrusions, such as primary cilia, as well as extracellular vesicles derived thereof. Primary cilia host numerous signaling pathways affected in diseases known as ciliopathies. Human PROM1 (CD133) is detected in both somatic and cancer stem cells and is also expressed in terminally differentiated epithelial and photoreceptor cells. Genetic mutations in the PROM1 gene result in retinal degeneration by impairing the proper formation of the outer segment of photoreceptors, a modified cilium. Here, we investigated the impact of proms on two distinct examples of ciliogenesis. First, we demonstrate that the overexpression of a dominant-negative mutant variant of human PROM1 (i.e. mutation Y819F/Y828F) significantly decreases ciliary length in Madin-Darby canine kidney cells. These results contrast strongly to the previously observed enhancing effect of WT PROM1 on ciliary length. Mechanistically, the mutation impeded the interaction of PROM1 with ADP-ribosylation factor-like protein 13B, a key regulator of ciliary length. Second, we observed that in vivo knockdown of prom3 in zebrafish alters the number and length of monocilia in the Kupffer's vesicle, resulting in molecular and anatomical defects in the left-right asymmetry. These distinct loss-of-function approaches in two biological systems reveal that prom proteins are critical for the integrity and function of cilia. Our data provide new insights into ciliogenesis and might be of particular interest for investigations of the etiologies of ciliopathies.

C3G localizes to mother centriole dependent on cenexin, and regulates centrosome duplication and primary cilia length

C3G (RapGEF1) plays a role in cell differentiation and is essential for early embryonic development in mice. In this study, we identify C3G as a centrosomal protein colocalizing with cenexin at the mother centriole in interphase cells. C3G interacts through its catalytic domain with cenexin, and they show interdependence for localization to the centrosome. C3G depletion caused a decrease in cellular cenexin levels. Centrosomal localization is lost as myocytes differentiate to form myotubes. Stable clone of cells depleted of C3G by CRISPR/Cas9 showed the presence of supernumerary centrioles. Overexpression of C3G, or a catalytically active deletion construct inhibited centrosome duplication. Cilia length is longer in C3G knockout cells, and the phenotype could be reverted upon reintroduction of C3G or its catalytic domain. Association of C3G with the basal body is dynamic, decreasing upon serum starvation, and increasing upon reentry into the cell cycle. C3G inhibits cilia formation and length dependent on its catalytic activity. We conclude that C3G inhibits centrosome duplication and maintains ciliary homeostasis, properties that may be important for its role in embryonic development.

Actin-based regulation of ciliogenesis - The long and the short of it

The primary cilia is found on the mammalian cell surface where it serves as an antenna for the reception and transmission of a variety of cellular signaling pathways. At its core the cilium is a microtubule-based organelle, but it is clear that its assembly and function are dependent upon the coordinated regulation of both actin and microtubule dynamics. In particular, the discovery that the centrosome is able to act as both a microtubule and actin organizing centre implies that both cytoskeletal networks are acting directly on the process of cilia assembly. In this review, we set our recent results with the formin FHDC1 in the context of current reports that show each stage of ciliogenesis is impacted by changes in actin dynamics. These include direct effects of actin filament assembly on basal body positioning, vesicle trafficking to and entry into the cilium, cilia length, cilia membrane organization and cilia-dependent signaling.

Divergent roles of the Wnt/PCP Formin Daam1 in renal ciliogenesis

Kidneys are composed of numerous ciliated epithelial tubules called nephrons. Each nephron functions to reabsorb nutrients and concentrate waste products into urine. Defects in primary cilia are associated with abnormal formation of nephrons and cyst formation in a wide range of kidney disorders. Previous work in Xenopus laevis and zebrafish embryos established that loss of components that make up the Wnt/PCP pathway, Daam1 and ArhGEF19 (wGEF) perturb kidney tubulogenesis. Dishevelled, which activates both the canonical and non-canonical Wnt/PCP pathway, affect cilia formation in multiciliated cells. In this study, we investigated the role of the noncanoncial Wnt/PCP components Daam1 and ArhGEF19 (wGEF) in renal ciliogenesis utilizing polarized mammalian kidney epithelia cells (MDCKII and IMCD3) and Xenopus laevis embryonic kidney. We demonstrate that knockdown of Daam1 and ArhGEF19 in MDCKII and IMCD3 cells leads to loss of cilia, and Daam1's effect on ciliogenesis is mediated by the formin-activity of Daam1. Moreover, Daam1 co-localizes with the ciliary transport protein Ift88 and is present in cilia. Interestingly, knocking down Daam1 in Xenopus kidney does not lead to loss of cilia. These data suggests a new role for Daam1 in the formation of primary cilia.

Caveolin-1α regulates primary cilium length by controlling RhoA GTPase activity

The primary cilium is a single non-motile protrusion of the plasma membrane of most types of mammalian cell. The structure, length and function of the primary cilium must be tightly controlled because their dysfunction is associated with disease. Caveolin 1 (Cav1), which is best known as a component of membrane invaginations called caveolae, is also present in non-caveolar membrane domains whose function is beginning to be understood. We show that silencing of α and β Cav1 isoforms in different cell lines increases ciliary length regardless of the route of primary ciliogenesis. The sole expression of Cav1α, which is distributed at the apical membrane, restores normal cilium size in Cav1 KO MDCK cells. Cells KO for only Cav1α, which also show long cilia, have a disrupted actin cytoskeleton and reduced RhoA GTPase activity at the apical membrane, and a greater accumulation of Rab11 vesicles at the centrosome. Subsequent experiments showed that DIA1 and ROCK help regulate ciliary length. Since MDCK cells lack apical caveolae, our results imply that non-caveolar apical Cav1α is an important regulator of ciliary length, exerting its effect via RhoA and its effectors, ROCK and DIA1.

Formins, Golgi, and the Centriole

Formin homology proteins (formins) are a highly conserved family of cytoskeletal remodeling proteins that are involved in a diverse array of cellular functions. Formins are best known for their ability to regulate actin dynamics, but the same functional domains also govern stability and organization of microtubules. It is thought that this dual activity allows them to coordinate the activity of these two major cytoskeletal networks and thereby influence cellular architecture. Golgi ribbon assembly is dependent upon cooperative interactions between actin filaments and cytoplasmic microtubules originating both at the Golgi itself and from the centrosome. Similarly, centrosome assembly, centriole duplication, and centrosome positioning are also reliant on a dialogue between both cytoskeletal networks. As presented in this chapter, a growing body of evidence suggests that multiple formin proteins play essential roles in these central cellular processes.