Jasplakinolide induces primary cilium formation through cell rounding and YAP inactivation

Functions of actin-binding proteins in cilia structure remodeling and signaling

Cilia are microtubule-based organelles found on the surfaces of many types of cells, including cardiac fibroblasts, vascular endothelial cells, human retinal pigmented epithelial-1 (RPE-1) cells, and alveolar epithelial cells. These organelles can be classified as immotile cilia, referred to as primary cilia in mammalian cells, and motile cilia. Primary cilia are cellular sensors that detect extracellular signals; this is a critical function associated with ciliopathies, which are characterized by the typical clinical features of developmental disorders. Cilia are extensively studied organelles of the microtubule cytoskeleton. However, the ciliary actin cytoskeleton has rarely been studied. Clear evidence has shown that highly regulated actin cytoskeleton dynamics contribute to normal ciliary function. Actin-binding proteins (ABPs) play vital roles in filamentous actin (F-actin) morphology. Here, we discuss recent progress in understanding the roles of ABPs in ciliary structural remodeling and further downstream ciliary signaling with a focus on the molecular mechanisms underlying actin cytoskeleton-related ciliopathies.

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.

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.

Reduced expression of TAZ inhibits primary cilium formation in renal glomeruli

Renal primary cilia are antenna-like organelles that maintain cellular homeostasis via multiple receptors clustered along their membranes. Recent studies have revealed that YAP/TAZ, key paralogous effectors of the Hippo pathway, are involved in ciliogenesis; however, their independent roles need to be further investigated. Here, we analyzed the renal phenotypes of kidney-specific TAZ knockout mice and observed ciliary defects only in glomeruli where mild cysts were formed. This finding prompted us to verify the role of TAZ specifically in renal tubule ciliary regulation. Therefore, we investigated the effects of TAZ silencing and compared them to those of YAP knockdown using three different types of renal tubular cells. We found that the absence of TAZ prevented proper cilia formation in glomerular cells, whereas it had a negligible effect in collecting duct and proximal tubule cells. IFT and NPHP protein levels were altered because of TAZ deficiency, accompanied by ciliary defects in glomerular cells, and ciliary recovery was identified by regulating some NPHP proteins. Although our study focused on TAZ, ciliogenesis, and other ciliary genes, the results suggest the very distinct roles of YAP and TAZ in kidneys, specifically in terms of ciliary regulation.

The roles of two regulatory proteins in the kidneys have been further clarified and provide insights into cilia defects and cyst formation. Cilia are organelles that act as ‘antennae’ for cell signaling in many tissues. Recent studies have highlighted two proteins involved in kidney cilia formation, YAP and TAZ, but little is known about their roles. Jong Hoon Park and co-workers at Sookmyung Women’s University in Seoul, South Korea, examined the role of TAZ in the regulation of kidney tubule cilia in mice. They explored the effects of silencing TAZ or YAP expression in different types of kidney tubule cells. TAZ deficiency but not YAP deficiency prevented correct cilia formation in the glomeruli, blood vessels that filter waste in the kidneys, and the resulting defects led to mild cyst generation.

LUZP1 and the tumor suppressor EPLIN modulate actin stability to restrict primary cilia formation

Cilia and flagella are microtubule-based cellular projections with important sensory and motility functions. Their absence or malfunction is associated with a growing number of human diseases collectively referred to as ciliopathies. However, the fundamental mechanisms underpinning cilia biogenesis and functions remain only partly understood. Here, we show that depleting LUZP1 or its interacting protein, EPLIN, increases the levels of MyosinVa at the centrosome and primary cilia formation. We further show that LUZP1 localizes to both actin filaments and the centrosome/basal body. Like EPLIN, LUZP1 is an actin-stabilizing protein that regulates actin dynamics, at least in part, by mobilizing ARP2 to the centrosomes. Both LUZP1 and EPLIN interact with known ciliogenesis and cilia-length regulators and as such represent novel players in actin-dependent centrosome to basal body conversion. Ciliogenesis deregulation caused by LUZP1 or EPLIN loss may thus contribute to the pathology of their associated disease states.

Primary Cilia, Ciliogenesis and the Actin Cytoskeleton: A Little Less Resorption, A Little More Actin Please

Primary cilia are microtubule-based organelles that extend from the apical surface of most mammalian cells, forming when the basal body (derived from the mother centriole) docks at the apical cell membrane. They act as universal cellular "antennae" in vertebrates that receive and integrate mechanical and chemical signals from the extracellular environment, serving diverse roles in chemo-, mechano- and photo-sensation that control developmental signaling, cell polarity and cell proliferation. Mutations in ciliary genes cause a major group of inherited developmental disorders called ciliopathies. There are very few preventative treatments or new therapeutic interventions that modify disease progression or the long-term outlook of patients with these conditions. Recent work has identified at least four distinct but interrelated cellular processes that regulate cilia formation and maintenance, comprising the cell cycle, cellular proteostasis, signaling pathways and structural influences of the actin cytoskeleton. The actin cytoskeleton is composed of microfilaments that are formed from filamentous (F) polymers of globular G-actin subunits. Actin filaments are organized into bundles and networks, and are attached to the cell membrane, by diverse cross-linking proteins. During cell migration, actin filament bundles form either radially at the leading edge or as axial stress fibers. Early studies demonstrated that loss-of-function mutations in ciliopathy genes increased stress fiber formation and impaired ciliogenesis whereas pharmacological inhibition of actin polymerization promoted ciliogenesis. These studies suggest that polymerization of the actin cytoskeleton, F-actin branching and the formation of stress fibers all inhibit primary cilium formation, whereas depolymerization or depletion of actin enhance ciliogenesis. Here, we review the mechanistic basis for these effects on ciliogenesis, which comprise several cellular processes acting in concert at different timescales. Actin polymerization is both a physical barrier to both cilia-targeted vesicle transport and to the membrane remodeling required for ciliogenesis. In contrast, actin may cause cilia loss by localizing disassembly factors at the ciliary base, and F-actin branching may itself activate the YAP/TAZ pathway to promote cilia disassembly. The fundamental role of actin polymerization in the control of ciliogenesis may present potential new targets for disease-modifying therapeutic approaches in treating ciliopathies.

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.

Functional aspects of primary cilium in signaling, assembly and microenvironment in cancer

The primary cilium is an antennae‐like structure extent outside the cell surface. It has an important role in regulating cell‐signaling transduction to affect proliferation, differentiation and migration. Evidence is accumulating that ciliary defects lead to ciliopathies and ciliary deregulation also play crucial roles in cancer formation and progression. Interestingly, restoring the cilia can suppress proliferation in some cancer cell. However, t he role of primary cilia in cancer still be debated. In this article, we review the role of the primary cilium in cancer through architecture, signaling pathways, cilia assembly and disassembly regulators, and summarized the new findings of the primary cilium in tumor microenvironments and different cancers, highlighting novel possibilities for therapeutic target in cancer.

Furry protein suppresses nuclear localization of yes-associated protein (YAP) by activating NDR kinase and binding to YAP

The Hippo signaling pathway suppresses cell proliferation and tumorigenesis. In the canonical Hippo pathway, large tumor suppressor kinases 1/2 (LATS1/2) phosphorylate the transcriptional coactivator yes-associated protein (YAP) and thereby suppress its nuclear localization and co-transcriptional activity. Nuclear Dbf2-related kinases 1/2 (NDR1/2), which are closely related to LATS1/2, also phosphorylate and inactivate YAP by suppressing its nuclear localization. Furry (FRY) is a cytoplasmic protein that associates with NDR1/2 and activates them, but its role in the nuclear/cytoplasmic localization of YAP remains unknown. Here, we constructed FRY-knockout cell lines to examine the role of FRY in YAP's cytoplasmic localization. FRY depletion markedly increased YAP nuclear localization and decreased NDR1/2 kinase activity and YAP phosphorylation levels, but did not affect LATS1/2 kinase activity. This indicated that FRY suppresses YAP's nuclear localization by promoting its phosphorylation via NDR1/2 activation. NDR1/2 depletion also promoted YAP nuclear localization, but depletion of both FRY and NDR1/2 increased the number of cells with YAP nuclear localization more strongly than did depletion of NDR1/2 alone, suggesting that FRY suppresses YAP nuclear localization by a mechanism in addition to NDR1/2 activation. Co-precipitation assays revealed that Fry uses its N-terminal 1-2400-amino-acid-long region to bind to YAP. Expression of full-length FRY or its 1-2400 N-terminal fragment restored YAP cytoplasmic localization in FRY-knockout cells. Taken together, these results suggest that FRY plays a crucial role in YAP cytoplasmic retention by promoting YAP phosphorylation via NDR1/2 kinase activation and by binding to YAP, leading to its cytoplasmic sequestration.

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.

Proliferative signaling by ERBB proteins and RAF/MEK/ERK effectors in polycystic kidney disease

A primary pathological feature of polycystic kidney disease (PKD) is the hyperproliferation of epithelial cells in renal tubules, resulting in formation of fluid-filled cysts. The proliferative aspects of the two major forms of PKD-autosomal dominant PKD (ADPKD), which arises from mutations in the polycystins PKD1 and PKD2, and autosomal recessive PKD (ARPKD), which arises from mutations in PKHD1-has encouraged investigation into protein components of the core cell proliferative machinery as potential drivers of PKD pathogenesis. In this review, we examine the role of signaling by ERBB proteins and their effectors, with a primary focus on ADPKD. The ERBB family of receptor tyrosine kinases (EGFR/ERBB1, HER2/ERBB2, ERBB3, and ERBB4) are activated by extracellular ligands, inducing multiple pro-growth signaling cascades; among these, activation of signaling through the RAS GTPase, and the RAF, MEK1/2, and ERK1/2 kinases enhance cell proliferation and restrict apoptosis during renal tubuloepithelial cyst formation. Characteristics of PKD include overexpression and mislocalization of the ERBB receptors and ligands, leading to enhanced activation and increased activity of downstream signaling proteins. The altered regulation of ERBBs and their effectors in PKD is influenced by enhanced activity of SRC kinase, which is promoted by the loss of cytoplasmic Ca²⁺ and an increase in cAMP-dependent PKA kinase activity that stimulates CFTR, driving the secretory phenotype of ADPKD. We discuss the interplay between ERBB/SRC signaling, and polycystins and their depending signaling, with emphasis on thes changes that affect cell proliferation in cyst expansion, as well as the inflammation-associated fibrogenesis, which characterizes progressive disease. We summarize the current progress of preclinical and clinical trials directed at inhibiting this signaling axis, and discuss potential future strategies that may be productive for controlling PKD.

Coordination of WNT signaling and ciliogenesis during odontogenesis by piezo type mechanosensitive ion channel component 1

Signal transmission from the mechanical forces to the various intracellular activities is a fundamental process during tissue development. Despite their critical role, the mechanism of mechanical forces in the biological process is poorly understood. In this study, we demonstrated that in the response to hydrostatic pressure (HP), the piezo type mechanosensitive ion channel component 1 (PIEZO1) is a primary mechanosensing receptor for odontoblast differentiation through coordination of the WNT expression and ciliogenesis. In stem cells from human exfoliated deciduous teeth (SHED), HP significantly promoted calcium deposition as well as the expression of odontogenic marker genes, PANX3 and DSPP, and WNT related-genes including WNT5b and WNT16, whereas HP inhibited cell proliferation and enhanced primary cilia expression. WNT signaling inhibitor XAV939 and primary cilia inhibitor chloral hydrate blocked the HP-induced calcium deposition. The PIEZO1 activator Yoda1 inhibited cell proliferation but induced ciliogenesis and WNT16 expression. Interestingly, HP and Yoda1 promoted nuclear translocation of RUNX2, whereas siRNA-mediated silencing of PIEZO1 decreased HP-induced nuclear translocation of RUNX2. Taken together, these results suggest that PIEZO1 functions as a mechanotransducer that connects HP signal to the intracellular signalings during odontoblast differentiation.

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.

Cullin-3-KCTD10-mediated CEP97 degradation promotes primary cilium formation

Primary cilia are antenna-like sensory organelles that transmit various extracellular signals. Ciliogenesis requires the removal of CP110 and its interactor CEP97 from the mother centriole for initiating ciliary axoneme extension, but the underlying mechanism remains unknown. Here we show that, upon serum starvation, CEP97 is partially degraded by the ubiquitin-proteasome system. CEP97 was polyubiquitylated in serum-starved cells, and overexpression of a non-ubiquitylatable CEP97 mutant effectively blocked CP110 removal and ciliogenesis induced by serum-starvation. Through several screening steps, we identified the cullin-3-RBX1-KCTD10 complex as the E3 ligase that mediates CEP97 degradation and removal from the mother centriole. Depletion of each component of this E3 complex caused aberrant accumulation of CEP97 on the centrosome, suppressed the removal of CEP97 and CP110 from the mother centriole, and impaired ciliogenesis. Moreover, KCTD10 was specifically localized to the mother centriole. These results suggest that CEP97 degradation by the cullin-3-RBX1-KCTD10 complex plays a crucial role in serum-starvation-induced CP110 removal and ciliogenesis.

Cilium structure, assembly, and disassembly regulated by the cytoskeleton

The cilium, once considered a vestigial structure, is a conserved, microtubule-based organelle critical for transducing extracellular chemical and mechanical signals that control cell polarity, differentiation, and proliferation. The cilium undergoes cycles of assembly and disassembly that are controlled by complex inter-relationships with the cytoskeleton. Microtubules form the core of the cilium, the axoneme, and are regulated by post-translational modifications, associated proteins, and microtubule dynamics. Although actin and septin cytoskeletons are not major components of the axoneme, they also regulate cilium organization and assembly state. Here, we discuss recent advances on how these different cytoskeletal systems­ affect cilium function, structure, and organization.