Soluble levels of cytosolic tubulin regulate ciliary length control

NME7 maintains primary cilium assembly, ciliary microtubule stability, and Hedgehog signaling

NME7 (nucleoside diphosphate kinase 7), a lesser studied member of the non-metastatic expressed (NME) family, has been reported as a potential subunit of the γ-tubulin ring complex (γTuRC). However, its role in the cilium assembly and function remains unclear. Our research demonstrated that NME7 is located at the centrosome, including at the spindle poles during metaphase and at the basal bodies during cilium assembly. Notably, a small fraction of NME7 localizes within the cilium. Detailed analysis of cilium assembly after NME7 knockdown and knockout revealed that NME7 is required for this process. NME7 knockout cells exhibited sensitivity to nocodazole, indicating its role in ciliary microtubule stability. In addition, NME7 deficiency impacted the Hedgehog signaling pathway, evident from reduced smoothened (Smo) fluorescence within primary cilia. This role of NME7 in Hedgehog signaling may depend on its nucleoside diphosphate kinase activity and γTuRC association. In conclusion, these findings enhance our understanding of the γTuRC roles in primary cilia in mammalian cells, highlighting the importance of NME7 in ciliary functions and signaling pathways.

Structural Disruption of Cilia and Increased Cytoplasmic Tubulin in Biliary Atresia-An Exploratory Study Focusing on Early Postoperative Prognosis Following Portoenterostomy

Introduction: Biliary atresia (BA) is a progressive hepatobiliary disease in infants, leading to liver failure and the need for transplantation. While its etiopathogenesis remains unclear, recent studies suggest primary cilia (PC) disruption plays a role. This study investigates correlations between PC and cytoplasmic tubulin (TUBA4A) alterations with hypoxia in patients with the isolated form of BA, focusing on native liver survival. Methods: Using qualitative and quantitative digital image analysis of immunofluorescence-stained liver samples, we assessed PC and TUBA4A features correlating these findings with HIF-1α nuclear positivity, clinical-laboratory data, and early native liver survival. Liver samples from fourteen BA patients and six controls with another liver disease were analyzed by digital image analysis, with data evaluated using Spearman's correlation and independent t-tests. Results: HIF-1α positivity in cholangiocytes was observed in 42.8% of BA patients. While the PC ratio per biliary structure (cilia ratio status, CRs) was similar between BA patients and controls, PC length was decreased in BA patients. Cytoplasmic TUBA4A levels were elevated in BA patients. CRs positively correlated with lower cytoplasmic TUBA4A expression and was higher in patients without HIF-1α nuclear positivity. Reduced cilia length correlated with higher bilirubin levels at portoenterostomy. Predictors of early poor prognosis (death or need for transplantation until 1 year of life) included HIF-1α positivity, elevated direct bilirubin levels, decreased cilia length, PC bending, and increased TUBA4A expression. Conclusions: Reduced PC length, PC bending, and increased intensity of cytoplasmic TUBA4A expression occur in the isolated BA clinical type and negatively impact the early prognosis after post-portoenterostomy. These findings suggest the existence of a disruption in the tubulin transport between cytoplasm and PC. The detrimental effect of HIF-1alpha pathway activation over early native liver survival was confirmed, although independently from PC or cytoplasmic tubulin features.

The cilium like region of the Drosophila bifurca spermatocyte: Elongation of a giant axoneme without intraflagellar transport

The growth of the ciliary axonemes mainly depends on the evolutionary conserved intraflagellar transport (IFT) machinery. However, insect spermatocytes are characterized by cilium-like regions (CLRs) that elongate in the absence of IFT. It is generally believed that the dynamics of these structures relies on the free diffusion of soluble tubulin from the cytoplasm. However, this passive process could allow the elongation of short ciliary axonemes, but it is unclear whether simple diffusion of tubulin molecules can ensure the correct assembly of elongated ciliary structures. To decipher this point we analyzed the assembly of the CLRs held by the primary spermatocytes of Drosophila bifurca. These ciliary structures consist of a very elongated axoneme that grows without IFT and, therefore, could represent a good model in which to evaluate the role played by the free diffusion of soluble tubulin. The observation of wavy microtubules in the axonemal lumen of fully elongated CLRs of D. bifurca may be consistent with the diffusion of tubulin within the axonemal lumen. Progressive consumption of soluble tubulin used for axoneme growth at the apical tip of the CLRs could result in a gradient sufficient to move tubulin from the cytoplasm to the apical end of the forming ciliary structure. When the axoneme reaches its full length, tubulin molecules are not drawn to the tip of the CLRs and accumulate at the base of the axoneme, where its concentration may exceed the threshold need for microtubule polymerization. The presence of γ-TuRCs at the proximal ends of the supernumerary microtubules could enhance their nucleation.

Vinblastine Resistance Is Associated with Nephronophthisis 3-Mediated Primary Cilia via Intraflagellar Transport Protein 88 and Apoptosis-Antagonizing Transcription Factor

Primary cilia (PC) are microtubule-based organelles that function as cellular antennae to sense and transduce extracellular signals. Nephronophthisis 3 (NPHP3) is localized in the inversin compartment of PC. Mutations in NPHP3 are associated with renal-hepatic-pancreatic dysplasia. In this study, we investigated whether vinblastine (VBL), a microtubule destabilizer, induces anticancer drug resistance through NPHP3-associated PC formation in HeLa human cervical cancer cells. A considerable increase in PC frequency was observed in HeLa cells under serum-deprived (SD) conditions, which led to the inhibition of VBL-induced cell death. VBL-resistant cells were established by repetitive treatments with VBL and showed an increase in PC frequency. NPHP3 expression was also increased by VBL treatment under serum starvation as well as in VBL-resistant cells. NPHP3 expression and PC-associated resistance were positively correlated with apoptosis-antagonizing transcription factor (AATF) and negatively correlated with inhibition of NPHP3. In addition, AATF-mediated NPHP3 expression is associated with PC formation via the regulation of intraflagellar transport protein 88 (IFT88). VBL resistance ability was reduced by treating with ciliobrevin A, a well-known ciliogenesis inhibitor. Collectively, cancer cell survival following VBL treatment is regulated by PC formation via AATF-mediated expression of IFT88 and NPHP3. Our data suggest that the activation of AATF and IFT88 could be a novel regulator to induce anticancer drug resistance through NPHP3-associated PC formation.

MI-181 Modulates Cilia Length and Restores Cilia Length in Cells with Defective Shortened Cilia

Primary cilia are membrane-covered microtubule-based structures that protrude from the cell surface and are critical for cell signaling and homeostasis during human development and adulthood. Dysregulation of cilia formation, length, and function can lead to a spectrum of human diseases and syndromes known as ciliopathies. Although some genetic and chemical screens have been performed to define important factors that modulate cilia biogenesis and length control, there are currently no clinical treatments that restore cilia length in patients. We report that the microtubule-targeting agent MI-181(mitotic inhibitor-181) is a potent modulator of cilia length and biogenesis. Treatment of retinal pigment epithelial-1 cells with MI-181 induced an increase in the average size of cilia and in the percent ciliated cells under nonstarved conditions. Importantly, MI-181 was effective at rescuing cilia length and ciliation defects in cells that had been treated with the intraflagellar transport inhibitor Ciliobrevin D or the O-GlcNAc transferase inhibitor OSMI-1. Most importantly, MI-181 induced an increase in cilia length and restored ciliation in cells with compromised shortened cilia at low nanomolar concentrations and did not show an inhibitory response at high concentrations. Therefore, MI-181 represents a lead molecule for developing drugs targeting ciliopathies characterized by shortened cilia.

Excess microtubule and F-actin formation mediates shortening and loss of primary cilia in response to a hyperosmotic milieu

The primary cilium is a small organelle protruding from the cell surface that receives signals from the extracellular milieu. Although dozens of studies have reported that several genetic factors can impair the structure of primary cilia, evidence for environmental stimuli affecting primary cilia structures is limited. Here, we investigated an extracellular stress that affected primary cilia morphology and its underlying mechanisms. Hyperosmotic shock induced reversible shortening and disassembly of the primary cilia of murine intramedullary collecting duct cells. The shortening of primary cilia caused by hyperosmotic shock followed delocalization of the pericentriolar material (PCM). Excessive microtubule and F-actin formation in the cytoplasm coincided with the hyperosmotic shock-induced changes to primary cilia and the PCM. Treatment with a microtubule-disrupting agent, nocodazole, partially prevented the hyperosmotic shock-induced disassembly of primary cilia and almost completely prevented delocalization of the PCM. An actin polymerization inhibitor, latrunculin A, also partially prevented the hyperosmotic shock-induced shortening and disassembly of primary cilia and almost completely prevented delocalization of the PCM. We demonstrate that hyperosmotic shock induces reversible morphological changes in primary cilia and the PCM in a manner dependent on excessive formation of microtubule and F-actin.

Microtubule-associated protein MAP7 promotes tubulin posttranslational modifications and cargo transport to enable osmotic adaptation

Cells remodel their cytoskeletal networks to adapt to their environment. Here, we analyze the mechanisms utilized by the cell to tailor its microtubule landscape in response to changes in osmolarity that alter macromolecular crowding. By integrating live-cell imaging, ex vivo enzymatic assays, and in vitro reconstitution, we probe the impact of cytoplasmic density on microtubule-associated proteins (MAPs) and tubulin posttranslational modifications (PTMs). We find that human epithelial cells respond to fluctuations in cytoplasmic density by modulating microtubule acetylation, detyrosination, or MAP7 association without differentially affecting polyglutamylation, tyrosination, or MAP4 association. These MAP-PTM combinations alter intracellular cargo transport, enabling the cell to respond to osmotic challenges. We further dissect the molecular mechanisms governing tubulin PTM specification and find that MAP7 promotes acetylation and inhibits detyrosination. Our data identify MAP7 in modulating the tubulin code, resulting in microtubule cytoskeleton remodeling and alteration of intracellular transport as an integrated mechanism of cellular adaptation.

STMND1 is a phylogenetically ancient stathmin which localizes to motile cilia and exhibits nuclear translocation that is inhibited when soluble tubulin concentration increases

Stathmins are small, unstructured proteins that bind tubulin dimers and are implicated in several human diseases, but whose function remains unknown. We characterized a new stathmin, STMND1 (Stathmin Domain Containing 1) as the human representative of an ancient subfamily. STMND1 features a N-terminal myristoylated and palmitoylated motif which directs it to membranes and a tubulin-binding stathmin-like domain (SLD) that contains an internal nuclear localization signal. Biochemistry and proximity labeling showed that STMND1 binds tubulin, and live imaging showed that tubulin binding inhibits translocation from cellular membranes to the nucleus. STMND1 is highly expressed in multiciliated epithelial cells, where it localizes to motile cilia. Overexpression in a model system increased the length of primary cilia. Our study suggests that the most ancient stathmins have cilium-related functions that involve sensing soluble tubulin.

Determinants of cytoplasmic microtubule depolymerization during ciliogenesis in Chlamydomonas

At the core of cilia are microtubules which establish length and assist ciliary assembly and disassembly; however, microtubules outside of the cilium can regulate ciliogenesis. The microtubule cytoskeleton polymerizes and depolymerizes rapidly. These processes have been studied across various organisms with chemical and genetic perturbations. However, these have generated conflicting data in terms of the role of cytoplasmic microtubules (CytoMTs) and free tubulin dynamics during ciliogenesis. Here, we look at the relationship between ciliogenesis and CytoMT dynamics in Chlamydomonas reinhardtii using chemical and mechanical perturbations. We find that not only can stabilized CytoMTs allow for normal ciliary assembly, but high calcium concentrations and low pH-induced deciliation cause CytoMTs to depolymerize separately from ciliary shedding. In addition, ciliary shedding through mechanical shearing allows cilia to regenerate earlier despite intact CytoMTs. Our data suggest that CytoMTs are not a sink for a limiting pool of cytoplasmic tubulin in Chlamydomonas, depolymerization after deciliation is a consequence rather than a requirement for ciliogenesis, and intact tubulin in the cytoplasm and proximal cilium support more efficient ciliary assembly.

Regulation of the length of neuronal primary cilia and its potential effects on signalling

Primary cilia protrude from most vertebrate cell bodies and act as specialized 'signalling antennae' that can substantially lengthen or retract in minutes to hours in response to specific stimuli. Here, we review the conditions and mechanisms responsible for regulating primary cilia length (PCL) in mammalian nonsensory neurons, and propose four models of how they could affect ciliary signalling and alter cell state and suggest experiments to distinguish between them. These models include (i) the passive indicator model, where changes in PCL have no consequence; (ii) the rheostat model, in which a longer cilium enhances signalling; (iii) the local concentration model, where ciliary shortening increases the local protein concentration to facilitate signalling; and (iv) the altered composition model where changes in PCL skew signalling.

Primary cilia in skeletal development and disease

Primary cilia are non-motile, microtubule-based sensory organelle present in most vertebrate cells with a fundamental role in the modulation of organismal development, morphogenesis, and repair. Here we focus on the role of primary cilia in embryonic and postnatal skeletal development. We examine evidence supporting its involvement in physiochemical and developmental signaling that regulates proliferation, patterning, differentiation and homeostasis of osteoblasts, chondrocytes, and their progenitor cells in the skeleton. We discuss how signaling effectors in mechanotransduction and bone development, such as Hedgehog, Wnt, Fibroblast growth factor and second messenger pathways operate at least in part at the primary cilium. The relevance of primary cilia in bone formation and maintenance is underscored by a growing list of rare genetic skeletal ciliopathies. We collate these findings and summarize the current understanding of molecular factors and mechanisms governing primary ciliogenesis and ciliary function in skeletal development and disease.

Determinants of cytoplasmic microtubule depolymerization during ciliogenesis in Chlamydomonas reinhardtii

At the core of cilia are microtubules which are important for establishing length and assisting ciliary assembly and disassembly; however, another role for microtubule regulation on ciliogenesis lies outside of the cilium. The microtubule cytoskeleton is a highly dynamic structure which polymerizes and depolymerizes rapidly to assist in cellular processes. These processes have been studied across various organisms with chemical as well as genetic perturbations. However, these have generated conflicting data in terms of the role of cytoplasmic microtubules (CytoMTs) and free tubulin dynamics during ciliogenesis. Here we look at the relationship between ciliogenesis and cytoplasmic microtubule dynamics in Chlamydomonas reinhardtii using chemical and mechanical perturbations. We find that not only can stabilized CytoMTs allow for normal ciliary assembly, but high calcium concentrations and low pH-induced deciliation cause CytoMTs to depolymerize separately from ciliary shedding. In addition, we find that ciliary shedding through mechanical shearing, cilia regenerate earlier despite intact CytoMTs. Our data suggests that cytoplasmic microtubules are not a sink for a limiting pool of cytoplasmic tubulin in Chlamydomonas, depolymerization that occurs following deciliation is a consequence rather than a requirement for ciliogenesis, and intact CytoMTs in the cytoplasm and the proximal cilium support more efficient ciliary assembly.

1-Indanone retards cyst development in ADPKD mouse model by stabilizing tubulin and down-regulating anterograde transport of cilia

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited kidney disease. Cyst development in ADPKD involves abnormal epithelial cell proliferation, which is affected by the primary cilia-mediated signal transduction in the epithelial cells. Thus, primary cilium has been considered as a therapeutic target for ADPKD. Since ADPKD exhibits many pathological features similar to solid tumors, we investigated whether targeting primary cilia using anti-tumor agents could alleviate the development of ADPKD. Twenty-four natural compounds with anti-tumor activity were screened in MDCK cyst model, and 1-Indanone displayed notable inhibition on renal cyst growth without cytotoxicity. This compound also inhibited cyst development in embryonic kidney cyst model. In neonatal kidney-specific Pkd1 knockout mice, 1-Indanone remarkably slowed down kidney enlargement and cyst expansion. Furthermore, we demonstrated that 1-Indanone inhibited the abnormal elongation of cystic epithelial cilia by promoting tubulin polymerization and significantly down-regulating expression of anterograde transport motor protein KIF3A and IFT88. Moreover, we found that 1-Indanone significantly down-regulated ciliary coordinated Wnt/β-catenin, Hedgehog signaling pathways. These results demonstrate that 1-Indanone inhibits cystic cell proliferation by reducing abnormally prolonged cilia length in cystic epithelial cells, suggesting that 1-Indanone may hold therapeutic potential to retard cyst development in ADPKD.

Time-lapse imaging of primary cilium behavior with physiological expression of fluorescent ciliary proteins

Almost all cell types of mammals have a small protrusion named a primary cilium on their surface. Primary cilia are enriched by cilia-specific ion channels and G-protein-coupled receptors. They are known to regulate various cellular functions that contribute to the development and homeostasis of living organisms by receiving extracellular signals and transfusing them to the cell body. All functions are performed when the structure of the primary cilia is maintained properly. Abnormalities in primary cilia or their signaling can lead to a collection of diseases in various organs called ciliopathies. The primary cilium is dynamic, static, or fixed. The length of primary cilia varies as the cell cycle progresses and is also altered by extracellular stimuli. Ligand binding to cilia-specific receptors is also known to alter the length. Thus, there is a need for a method to study the morphological changes of the primary cilium in a time-dependent manner, especially under stimuli or mechanical shocks. Time-lapse imaging of primary cilia is one of the most powerful methods to capture the time-dependent behavior of primary cilia. Overexpression of ciliary proteins fused to fluorescent proteins is commonly used for the time-lapse imaging of primary cilia. However, overexpression has drawbacks in terms of artifacts. In addition, the time-lapse imaging of the tiny primary cilia requires some technical tricks. Here, we present a detailed description of the methods for time-lapse imaging of primary cilium, from the generation of cell lines that stably express fluorescent protein-labeled cilia-localized proteins at the physiological level to image analysis, including quantification through image acquisition.

Islet cilia and glucose homeostasis

Diabetes is a growing pandemic affecting over ten percent of the U.S. population. Individuals with all types of diabetes exhibit glucose dysregulation due to altered function and coordination of pancreatic islets. Within the critical intercellular space in pancreatic islets, the primary cilium emerges as an important physical structure mediating cell-cell crosstalk and signal transduction. Many events leading to hormone secretion, including GPCR and second-messenger signaling, are spatiotemporally regulated at the level of the cilium. In this review, we summarize current knowledge of cilia action in islet hormone regulation and glucose homeostasis, focusing on newly implicated ciliary pathways that regulate insulin exocytosis and intercellular communication. We present evidence of key signaling proteins on islet cilia and discuss ways in which cilia might functionally connect islet endocrine cells with the non-endocrine compartments. These discussions aim to stimulate conversations regarding the extent of cilia-controlled glucose homeostasis in health and in metabolic diseases.

Nuclear factor (erythroid-derived 2)-like 2 counter-regulates thymosin beta-4 expression and primary cilium formation for HeLa cervical cancer cell survival

We investigated the function of thymosin beta-4 (TB4) expression and primary cilium (PC) formation via the underlying Nrf2-dependent mechanism for cervical cancer cell (CC) survival under conditions of serum deprivation (SD). TB4 silencing was achieved using RNA interference. The percentage of PC formation was analyzed by immunofluorescence staining. Nrf2 expression was modified by the preparation of stable Nrf2-knockdown cells with shNrf2 and the overexpression of Nrf2 with pcDNA-Nrf2 plasmids. Gene expression was measured using reverse-transcription PCR, Gaussia luciferase assay, and western blotting. Cell viability was assessed using the MTT assay or CellTiter Glo assay. Reactive oxygen species (ROS) were detected with flow cytometry. CCs incubated in SD without fetal bovine serum remained viable, and SD increased PC formation and TB4 transcription. CC viability was further decreased by treatment with ciliobrevin A to inhibit PC formation or TB4-siRNA. SD increased ROS, including H₂O₂. N-acetylcysteine inhibited ROS production following H₂O₂ treatment or SD, which also decreased PC formation and TB4 transcription. Meanwhile, H₂O₂ increased PC formation, which was attenuated in response to TB4 siRNA. Treatment with H₂O₂ increased Nrf2 expression, antioxidant responsive element (ARE) activity, and PC formation, which were inhibited by the Nrf2 inhibitor clobestasol propionate. Nrf2 knockdown via expression of Tet-On shNrf2 enhanced ROS production, leading to increased PC formation and decreased TB4 expression; these effects were counteracted by Nrf2 overexpression. Our data demonstrate that Nrf2 counter-regulates TB4 expression and PC formation for CC survival under conditions of SD, suggesting cervical CC survival could be upregulated by PC formation via Nrf2 activation and TB4 expression.

Primary Cilia Influence Progenitor Function during Cortical Development

Corticogenesis is an intricate process controlled temporally and spatially by many intrinsic and extrinsic factors. Alterations during this important process can lead to severe cortical malformations. Apical neuronal progenitors are essential cells able to self-amplify and also generate basal progenitors and/or neurons. Apical radial glia (aRG) are neuronal progenitors with a unique morphology. They have a long basal process acting as a support for neuronal migration to the cortical plate and a short apical process directed towards the ventricle from which protrudes a primary cilium. This antenna-like structure allows aRG to sense cues from the embryonic cerebrospinal fluid (eCSF) helping to maintain cell shape and to influence several key functions of aRG such as proliferation and differentiation. Centrosomes, major microtubule organising centres, are crucial for cilia formation. In this review, we focus on how primary cilia influence aRG function during cortical development and pathologies which may arise due to defects in this structure. Reporting and cataloguing a number of ciliary mutant models, we discuss the importance of primary cilia for aRG function and cortical development.

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.

Functional Investigation of TUBB4A Variants Associated with Different Clinical Phenotypes

Dominant TUBB4A variants result in different phenotypes, including hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC), dystonia type 4 (DYT4), and isolated hypomyelination. Here, we report four new patients with a novel TUBB4A variant (p.K324T) and three new patients with previously reported variants (p.Q292K, p.V255I, p.E410K). The individual carrying the novel p.K324T variant exhibits epilepsy of infancy with migrating focal seizures (EIMFS), while the other three have isolated hypomyelination phenotype. We also present a study of the cellular effects of TUBB4A variants responsible for H-ABC (p.D249N), DYT4 (p.R2G), a severe combined phenotype with combination of hypomyelination and EIMFS (p.K324T), and isolated hypomyelination (p.Q292K and p.E410K) on microtubule stability and dynamics, neurite outgrowth, dendritic spine development, and kinesin binding. Cellular-based assays reveal that all variants except p.R2G increase microtubule stability, decrease microtubule polymerization rates, reduce axonal outgrowth, and alter the density and shape of dendritic spines. We also find that the p.K324T and p.E410K variants perturb the binding of TUBB4A to KIF1A, a neuron-specific kinesin required for transport of synaptic vesicle precursors. Taken together, our data suggest that impaired microtubule stability and dynamics, defected axonal growth, and dendritic spine development form the common molecular basis of TUBB4A-related leukodystrophy. Impairment of TUBB4A binding to KIF1A is more likely to be involved in the isolated hypomyelination phenotype, which suggests that alterations in kinesin binding may cause different phenotypes. In conclusion, our study extends the spectrum of TUBB4A mutations and related phenotypes and provides insight into why different TUBB4A variants cause distinct clinical phenotypes.

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.

Balancing the length of the distal tip by septins is key for stability and signalling function of primary cilia

Primary cilia are antenna-like organelles required for signalling transduction. How cilia structure is mechanistically maintained at steady-state to promote signalling is largely unknown. Here, we define that mammalian primary cilia axonemes are formed by proximal segment (PS) and distal segment (DS) delineated by tubulin polyglutamylation-rich and -poor regions, respectively. The analysis of proximal/distal segmentation indicated that perturbations leading to cilia over-elongation influenced PS or DS length with a different impact on cilia behaviour. We identified septins as novel repressors of DS growth. We show that septins control the localisation of MKS3 and CEP290 required for a functional transition zone (TZ), and the cilia tip accumulation of the microtubule-capping kinesin KIF7, a cilia-growth inhibitor. Live-cell imaging and analysis of sonic-hedgehog (SHH) signalling activation established that DS over-extension increased cilia ectocytosis events and decreased SHH activation. Our data underlines the importance of understanding cilia segmentation for length control and cilia-dependent signalling.

The dynamic organelle primary cilia: emerging roles in organ fibrosis

PURPOSE OF REVIEW

Primary cilia, the antenna-like organelles on most mammalian cells, host key components of multiple morphogen signal transduction pathways. Mutations in genes responsible for primary cilia assembly and function generally result in pathological conditions known as ciliopathies, which underlie several diseases, including various forms of fibrosis. Primary cilia modulate cellular responses to extracellular cues, including TGF-β and morphogens, such as Hedgehog. Aberrant morphogen signaling is recognized as essential for the transition of mesenchymal progenitor cells to myofibroblasts, the key step in fibrosis. This article aims to provide a critical overview of recent developments and insights in primary cilia biology relevant to fibrosis.

RECENT FINDINGS

Several studies have highlighted the association of altered primary cilia with various forms of fibrosis. In a rather complex manner, the presence of primary cilia seems to be required for initiation of myofibroblast transition, whereas its loss promotes myofibroblast transition at a later stage. Recent evidence also suggested that noncanonical functions of ciliary transport proteins may influence, such cellular transitions independently of primary cilia. The possibility of opposing signaling regulations being topologically separated between primary cilia and plasma membrane could also be critical for fibrosis.

SUMMARY

Recent progress in the field suggests that primary cilia are critical mediators of the pathogenesis of fibrosis. Understanding the potential role of primary cilia in fibrosis and the underlying mechanisms may pave the way for entirely new approaches for fibrosis prevention and treatment of SSc.

Post-translational modifications of tubulin: their role in cancers and the regulation of signaling molecules

Microtubules play an important role in regulating several vital cellular activities, including cell division and tissue organization, through their dynamic protofilament network. In addition to forming the cytoskeleton, microtubules regulate the intracellular trafficking of cytoplasmic components and various signaling molecules, depending on the presence of post-transitional modifications (PTMs) and binding proteins. Accumulating evidence indicates the significant role of microtubule PTMs on cancer behavior. The PTMs that frequently occur on microtubules include acetylation, detyrosination, tyrosination, polyglutamylation, and polyglycylation. Alterations in these PTMs cause global effects on intracellular signal transduction, strongly linked to cancer pathogenesis. This review provides an update on the role of microtubule PTMs in cancer aggressiveness, particularly regarding cell death, sensitivity to chemotherapy, cell migration, and invasion. Additionally, it provides a mechanistic explanation of the molecular signaling pathways involved. This information might prove useful for predictive or therapeutic purposes.

Identification of disease-relevant modulators of the SHH pathway in the developing brain

Pathogenic gene variants in humans that affect the sonic hedgehog (SHH) pathway lead to severe brain malformations with variable penetrance due to unknown modifier genes. To identify such modifiers, we established novel congenic mouse models. LRP2-deficient C57BL/6N mice suffer from heart outflow tract defects and holoprosencephaly caused by impaired SHH activity. These defects are fully rescued on a FVB/N background, indicating a strong influence of modifier genes. Applying comparative transcriptomics, we identified Pttg1 and Ulk4 as candidate modifiers upregulated in the rescue strain. Functional analyses showed that ULK4 and PTTG1, both microtubule-associated proteins, are positive regulators of SHH signaling, rendering the pathway more resilient to disturbances. In addition, we characterized ULK4 and PTTG1 as previously unidentified components of primary cilia in the neuroepithelium. The identification of genes that powerfully modulate the penetrance of genetic disturbances affecting the brain and heart is likely relevant to understanding the variability in human congenital disorders.

Renal Ciliopathies: Sorting Out Therapeutic Approaches for Nephronophthisis

Nephronophthisis (NPH) is an autosomal recessive ciliopathy and a major cause of end-stage renal disease in children. The main forms, juvenile and adult NPH, are characterized by tubulointerstitial fibrosis whereas the infantile form is more severe and characterized by cysts. NPH is caused by mutations in over 20 different genes, most of which encode components of the primary cilium, an organelle in which important cellular signaling pathways converge. Ciliary signal transduction plays a critical role in kidney development and tissue homeostasis, and disruption of ciliary signaling has been associated with cyst formation, epithelial cell dedifferentiation and kidney function decline. Drugs have been identified that target specific signaling pathways (for example cAMP/PKA, Hedgehog, and mTOR pathways) and rescue NPH phenotypes in in vitro and/or in vivo models. Despite identification of numerous candidate drugs in rodent models, there has been a lack of clinical trials and there is currently no therapy that halts disease progression in NPH patients. This review covers the most important findings of therapeutic approaches in NPH model systems to date, including hypothesis-driven therapies and untargeted drug screens, approached from the pathophysiology of NPH. Importantly, most animal models used in these studies represent the cystic infantile form of NPH, which is less prevalent than the juvenile form. It appears therefore important to develop new models relevant for juvenile/adult NPH. Alternative non-orthologous animal models and developments in patient-based in vitro model systems are discussed, as well as future directions in personalized therapy for NPH.

Primary Cilia and Atherosclerosis

In artery tree, endothelial function correlates with the distribution of shear stress, a dragging force generated by flowing blood. In laminar shear stress areas, endothelial cells (ECs) are available to prevent atherosclerosis, however, ECs in disturbed shear stress sites are featured with proinflammation and atherogenesis. Basic studies in the shear stress field that focused on the mechanosensors of ECs have attracted the interest of researchers. Among all the known mechanosensors, the primary cilium is distinctive because it is enriched in disturbed shear stress regions and sparse in laminar shear stress areas. The primary cilium, a rod liked micro-organelle, can transmit extracellular mechanical and chemical stimuli into intracellular space. In the cardiovascular system, primary cilia are enriched in disturbed shear stress regions, where blood flow is slow and oscillatory, such as the atrium, downstream of the aortic valve, branches, bifurcations, and inner curves of the artery. However, in the atrioventricular canal and straight vessels, blood flow is laminar, and primary cilia can barely be detected. Primary cilia in the heart cavity prevent ECs from mesenchymal transition and calcification by suppressing transforming growth factor (TGF) signaling. Besides, primary cilia in the vascular endothelium protected ECs against disturbed shear stress-induced cellular damage by triggering Ca²⁺ influx as well as nitric oxide (NO) release. Moreover, primary cilia inhibit the process of atherosclerosis. In the current review, we discussed ciliogenesis, ciliary structure, as well as ciliary distribution, function and the coordinate signal transduction with shear stress in the cardiovascular system.

Impact of the 'tubulin economy' on the formation and function of the microtubule cytoskeleton

The microtubule cytoskeleton is assembled from a finite pool of α,β-tubulin, the size of which is controlled by an autoregulation mechanism. Cells also tightly regulate the architecture and dynamic behavior of microtubule arrays. Here, we discuss progress in our understanding of how tubulin autoregulation is achieved and highlight work showing that tubulin, in its unassembled state, is relevant for regulating the formation and organization of microtubules. Emerging evidence suggests that tubulin regulates microtubule-associated proteins and kinesin motors that are critical for microtubule nucleation, dynamics, and function. These relationships create feedback loops that connect the tubulin assembly cycle to the organization and dynamics of microtubule networks. We term this concept the 'tubulin economy', which emphasizes the idea that tubulin is a resource that can be deployed for the immediate purpose of creating polymers, or alternatively as a signaling molecule that has more far-reaching consequences for the organization of microtubule arrays.

Primary cilium: a paradigm for integrating mathematical modeling with experiments and numerical simulations in mechanobiology

Primary cilia are non-motile, solitary (one per cell) microtubule-based organelles that emerge from the mother centriole after cells have exited the mitotic cycle. Identified as a mechanosensing organelle that responds to both mechanical and chemical stimuli, the primary cilium provides a fertile ground for integrative investigations of mathematical modeling, numerical simulations, and experiments. Recent experimental findings revealed considerable complexity to the underlying mechanosensory mechanisms that transmit extracellular stimuli to intracellular signaling many of which include primary cilia. In this invited review, we provide a brief survey of experimental findings on primary cilia and how these results lead to various mathematical models of the mechanics of the primary cilium bent under an external forcing such as a fluid flow or a trap. Mathematical modeling of the primary cilium as a fluid-structure interaction problem highlights the importance of basal anchorage and the anisotropic moduli of the microtubules. As theoretical modeling and numerical simulations progress, along with improved state-of-the-art experiments on primary cilia, we hope that details of ciliary regulated mechano-chemical signaling dynamics in cellular physiology will be understood in the near future.

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.

Centrosome and ciliary abnormalities in fetal akinesia deformation sequence human fibroblasts

Ciliopathies are clinical disorders of the primary cilium with widely recognised phenotypic and genetic heterogeneity. Here, we found impaired ciliogenesis in fibroblasts derived from individuals with fetal akinesia deformation sequence (FADS), a broad spectrum of neuromuscular disorders arising from compromised foetal movement. We show that cells derived from FADS individuals have shorter and less primary cilia (PC), in association with alterations in post-translational modifications in α-tubulin. Similarly, siRNA-mediated depletion of two known FADS proteins, the scaffold protein rapsyn and the nucleoporin NUP88, resulted in defective PC formation. Consistent with a role in ciliogenesis, rapsyn and NUP88 localised to centrosomes and PC. Furthermore, proximity-ligation assays confirm the respective vicinity of rapsyn and NUP88 to γ-tubulin. Proximity-ligation assays moreover show that rapsyn and NUP88 are adjacent to each other and that the rapsyn-NUP88 interface is perturbed in the examined FADS cells. We suggest that the perturbed rapsyn-NUP88 interface leads to defects in PC formation and that defective ciliogenesis contributes to the pleiotropic defects seen in FADS.

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.

Intraflagellar transport protein 74 is essential for spermatogenesis and male fertility in mice†

Intraflagellar transport protein 74 (IFT74) is a component of the core intraflagellar transport complex, a bidirectional movement of large particles along the axoneme microtubules for cilia formation. In this study, we investigated its role in sperm flagella formation and discovered that mice deficiency in Ift74 gene in male germ cells were infertile with low sperm count and immotile sperm. The few developed spermatozoa displayed misshaped heads and short tails. Transmission electron microscopy revealed abnormal flagellar axonemes in the seminiferous tubules where sperm are made. Clusters of unassembled microtubules were present in the spermatids. Testicular expression levels of IFT27, IFT57, IFT81, IFT88, and IFT140 proteins were significantly reduced in the conditional Ift74 mutant mice, with the exception of IFT20 and IFT25. The levels of outer dense fiber 2 and sperm-associated antigen 16L proteins were also not changed. However, the processed A-Kinase anchor protein, a major component of the fibrous sheath, a unique structure of sperm tail, was significantly reduced. Our study demonstrates that IFT74 is essential for mouse sperm formation, probably through assembly of the core axoneme and fibrous sheath, and suggests that IFT74 may be a potential genetic factor affecting male reproduction in man.

Targeting Primary Ciliogenesis with Small-Molecule Inhibitors

The primary cilium is generally a non-motile solitary organelle that protrudes from a basal body at the cell surface in various cell types in multicellular organisms. This microtubule-based structure acts as a cell signaling platform to control key cellular processes, including cell proliferation and differentiation in development and in adult tissues. Elongated and/or dysfunctional primary cilia cause developmental disorders termed ciliopathies and cancers. The genetic inhibition of ciliogenesis inducers can block the progression of these diseases in model organisms. Thus, pharmacological inhibition of primary ciliogenesis has emerged as a potential strategy to treat these pathological conditions. Pharmacological inhibitors that affect cilium assembly, and have an impact on other cellular processes, have been identified. Here, we review some of these tools and discuss their value and limitations in the study of primary cilium biology, as well as for the treatment of some ciliopathies and cancers.

Primary cilia act as microgravity sensors by depolymerizing microtubules to inhibit osteoblastic differentiation and mineralization

Microgravity-induced bone deterioration is a major challenge in long-term spaceflights since the underlying mechanisms remain elusive. Previously, we reported that primary cilia of osteoblasts gradually disappeared in microgravity conditions, and cilia abrogation was necessary for the inhibition of osteogenesis induced by microgravity. However, the precise roles of primary cilia have not been fully elucidated. Here, we report that microgravity depolymerizes the microtubule network of rat calvarial osteoblasts (ROBs) reversibly but has no effect on the architecture of actin filaments. Preventing primary ciliogenesis by chloral hydrate or a small interfering RNA sequence (siRNA) targeting intraflagellar transport protein 88 (IFT88) effectively relieves microgravity-induced microtubule depolymerization, whereas the stabilization of microtubules using pharmacological approaches cannot prevent the disappearance of primary cilia in microgravity conditions. Furthermore, quantification of the number of microtubules emerging from the ciliary base body shows that microgravity significantly decreases the number of basal microtubules, which is dependent on the existence of primary cilia. Finally, microgravity-induced repression of the differentiation, maturation, and mineralization of ROBs is abrogated by the stabilization of cytoplasmic microtubules. Taken together, these data suggest that primary cilia-dependent depolymerization of microtubules is responsible for the inhibition of osteogenesis induced by microgravity. Our study provides a new perspective regarding the mechanism of microgravity-induced bone loss, supporting the previously established role of primary cilia as a sensor in bone metabolism.

Autophagy lipidation machinery regulates axonal microtubule dynamics but is dispensable for survival of mammalian neurons

Neurons maintain axonal homeostasis via employing a unique organization of the microtubule (MT) cytoskeleton, which supports axonal morphology and provides tracks for intracellular transport. Abnormal MT-based trafficking hallmarks the pathology of neurodegenerative diseases, but the exact mechanism regulating MT dynamics in axons remains enigmatic. Here we report on a regulation of MT dynamics by AuTophaGy(ATG)-related proteins, which previously have been linked to the autophagy pathway. We find that ATG proteins required for LC3 lipid conjugation are dispensable for survival of excitatory neurons and instead regulate MT stability via controlling the abundance of the MT-binding protein CLASP2. This function of ATGs is independent of their role in autophagy and requires the active zone protein ELKS1. Our results highlight a non-canonical role of ATG proteins in neurons and suggest that pharmacological activation of autophagy may not only promote the degradation of cytoplasmic material, but also impair axonal integrity via altering MT stability.

In neurons, the microtubule cytoskeleton provides support and directionality of axons. Here, the authors report that microtubule dynamics in axons may be regulated by the autophagy proteins (ATGs) independently of their known role in autophagy.

Differential HDAC6 Activity Modulates Ciliogenesis and Subsequent Mechanosensing of Endothelial Cells Derived from Pluripotent Stem Cells

The role of primary cilia in mechanosensation is essential in endothelial cell (EC) shear responsiveness. Here, we find that venous, capillary, and progenitor ECs respond to shear stress in vitro in a cilia-dependent manner. We then demonstrate that primary cilia assembly in human induced pluripotent stem cell (hiPSC)-derived ECs varies between different cell lines with marginal influence of differentiation protocol. hiPSC-derived ECs lacking cilia do not align to shear stress, lack stress fiber assembly, have uncoordinated migration during wound closure in vitro, and have aberrant calcium influx upon shear exposure. Transcriptional analysis reveals variation in regulatory genes involved in ciliogenesis among different hiPSC-derived ECs. Moreover, inhibition of histone deacetylase 6 (HDAC6) activity in hiPSC-ECs lacking cilia rescues cilia formation and restores mechanical sensing. Taken together, these results show the importance of primary cilia in hiPSC-EC mechano-responsiveness and its modulation through HDAC6 activity varies among hiPSC-ECs.

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.

Primary cilium loss in mammalian cells occurs predominantly by whole-cilium shedding

The primary cilium is a central signaling hub in cell proliferation and differentiation and is built and disassembled every cell cycle in many animal cells. Disassembly is critically important, as misregulation or delay of cilia loss leads to cell cycle defects. The physical means by which cilia are lost are poorly understood but are thought to involve resorption of ciliary components into the cell body. To investigate cilium loss in mammalian cells, we used live-cell imaging to comprehensively characterize individual events. The predominant mode of cilium loss was rapid deciliation, in which the membrane and axoneme of the cilium was shed from the cell. Gradual resorption was also observed, as well as events in which a period of gradual resorption was followed by rapid deciliation. Deciliation resulted in intact shed cilia that could be recovered from culture medium and contained both membrane and axoneme proteins. We modulated levels of katanin and intracellular calcium, two putative regulators of deciliation, and found that excess katanin promotes cilia loss by deciliation, independently of calcium. Together, these results suggest that mammalian ciliary loss involves a tunable decision between deciliation and resorption.

Mechanical loading inhibits cartilage inflammatory signalling via an HDAC6 and IFT-dependent mechanism regulating primary cilia elongation

OBJECTIVE

Physiological mechanical loading reduces inflammatory signalling in numerous cell types including articular chondrocytes however the mechanism responsible remains unclear. This study investigates the role of chondrocyte primary cilia and associated intraflagellar transport (IFT) in the mechanical regulation of interleukin-1β (IL-1β) signalling.

DESIGN

Isolated chondrocytes and cartilage explants were subjected to cyclic mechanical loading in the presence and absence of the cytokine IL-1β. Nitric oxide (NO) and prostaglandin E₂ (PGE₂) release were used to monitor IL-1β signalling whilst Sulphated glycosaminoglycan (sGAG) release provided measurement of cartilage degradation. Measurements were made of HDAC6 activity and tubulin polymerisation and acetylation. Effects on primary cilia were monitored by confocal and super resolution microscopy. Involvement of IFT was analysed using ORPK cells with hypomorphic mutation of IFT88.

RESULTS

Mechanical loading suppressed NO and PGE₂ release and prevented cartilage degradation. Loading activated HDAC6 and disrupted tubulin acetylation and cilia elongation induced by IL-1β. HDAC6 inhibition with tubacin blocked the anti-inflammatory effects of loading and restored tubulin acetylation and cilia elongation. Hypomorphic mutation of IFT88 reduced IL-1β signalling and abolished the anti-inflammatory effects of loading indicating the mechanism is IFT-dependent. Loading reduced the pool of non-polymerised tubulin which was replicated by taxol which also mimicked the anti-inflammatory effects of mechanical loading and prevented cilia elongation.

CONCLUSIONS

This study reveals that mechanical loading suppresses inflammatory signalling, partially dependent on IFT, by activation of HDAC6 and post transcriptional modulation of tubulin.

The toxic effect of cytostatics on primary cilia frequency and multiciliation

The primary cilium is considered as a key component of morphological cellular stability. However, cancer cells are notorious for lacking primary cilia in most cases, depending upon the tumour type. Previous reports have shown the effect of starvation and cytostatics on ciliogenesis in normal and cancer cells although with limited success, especially when concerning the latter. In this study, we evaluated the presence and frequency of primary cilia in breast fibroblasts and in triple-negative breast cancer cells after treatment with cytostatics finding that, in the case of breast fibroblasts, primary cilia were detected at their highest incidence 72 hours after treatment with 120 nM doxorubicin. Further, multiciliated cells were also detected after treatment with 80 nM doxorubicin. On the other hand, treatment with taxol increased the number of ciliated cells only at low concentrations (1.25 and 3.25 nM) and did not induce multiciliation. Interestingly, triple-negative breast cancer cells did not present primary cilia after treatment with either doxorubicin or taxol. This is the first study reporting the presence of multiple primary cilia in breast fibroblasts induced by doxorubicin. However, the null effect of these cytostatics on primary cilia incidence in the evaluated triple negative breast carcinomas cell lines requires further research.

Fbxo41 Promotes Disassembly of Neuronal Primary Cilia

Neuronal primary cilia are signaling organelles with crucial roles in brain development and disease. Cilia structure is decisive for their signaling capacities but the mechanisms regulating it are poorly understood. We identify Fbxo41 as a novel Skp1/Cullin1/F-box (SCF) E3-ligase complex subunit that targets to neuronal centrioles where its accumulation promotes disassembly of primary cilia, and affects sonic hedgehog signaling, a canonical ciliary pathway. Fbxo41 targeting to centrioles requires its Coiled-coil and F-box domains. Levels of Fbxo41 at the centrioles inversely correlate with neuronal cilia length, and mutations that disrupt Fbxo41 targeting or assembly into SCF-complexes also disturb its function in cilia disassembly and signaling. Fbxo41 dependent cilia disassembly in mitotic and post-mitotic cells requires rearrangements of the actin-cytoskeleton, but requires Aurora A kinase activation only in mitotic cells, highlighting important mechanistical differences controlling cilia size between mitotic and post-mitotic cells. Phorbol esters induce recruitment of overexpressed Fbxo41 to centrioles and cilia disassembly in neurons, but disassembly can also occur in absence of Fbxo41. We propose that Fbxo41 targeting to centrosomes regulates neuronal cilia structure and signaling capacity in addition to Fbxo41-independent pathways controlling cilia size.

Novel functions of the primary cilium in bone disease and cancer

The primary cilium, a sensory organelle that emanates from the cell surface of most mammalian cell types during growth arrest, has attracted the attention of many researchers over the past decade. Recently, a large number of new findings have assigned novel functions and roles to the primary cilium in signal transduction and related diseases, which has greatly augmented the importance of the cilium in human health and development. Here, we review emerging evidence supporting the primary cilium as a sensory organelle in signal transduction in microgravity, electromagnetic field sensing, chemosensation and tumorigenesis. We also present an overview of signal transduction crosstalk associated with the primary cilium in bone disease and cancer, including primary cilium-related Ca²⁺ signaling, parathyroid hormone signaling, cAMP signaling, BMP/Smad1/5/8 signaling and Wnt signaling. We anticipate that emerging discoveries about the function of the primary cilium will provide novel insight into the molecular mechanisms of stimulus sensation, signal transduction and pathogenesis.

Partially Redundant Actin Genes in Chlamydomonas Control Transition Zone Organization and Flagellum-Directed Traffic

The unicellular green alga Chlamydomonas reinhardtii is a biflagellated cell with two actin genes: one encoding a conventional actin (IDA5) and the other encoding a divergent novel actin-like protein (NAP1). Here, we probe how actin redundancy contributes to flagellar assembly. Disrupting a single actin allows complete flagellar assembly. However, when disrupting both actins using latrunculin B (LatB) treatment on the nap1 mutant background, we find that actins are necessary for flagellar growth from newly synthesized limiting flagellar proteins. Under total actin disruption, transmission electron microscopy identified an accumulation of Golgi-adjacent vesicles. We also find that there is a mislocalization of a key transition zone gating and ciliopathy protein, NPHP-4. Our experiments demonstrate that each stage of flagellar biogenesis requires redundant actin function to varying degrees, with an absolute requirement for these actins in transport of Golgi-adjacent vesicles and flagellar incorporation of newly synthesized proteins.

Thymosin β-4 is a novel regulator for primary cilium formation by nephronophthisis 3 in HeLa human cervical cancer cells

Thymosinβ-4(Tβ4) is an actin-sequestering protein involved in tumor malignancy. Primary cilia, microtubule-based organelles, are present in most eukaryotic cells, which might be related to tumor cell transformation. Here, we investigated whether ciliogenesis is affected by Tβ4 in HeLa human cervical cancer cells. The inhibition of Tβ4 attenuated primary cilia formation. The frequency of cilia was increased by Tβ4 overexpression. When yeast two-hybrid assay was performed by using Tβ4 as a bait, we rescued nephronophthisis 3(NPHP3), one of the components of primary cilia. Interaction of Tβ4 with NPHP3 in mammalian cells was confirmed by GST-pulldown assay. Their intracellular co-localization was observed by immunofluorescence staining at peripheral surface of cells. In addition, the number of ciliated cells was reduced by the inhibition of NPHP3. Moreover, NPHP3 expression was decreased by the inhibition of Tβ4 but it was increased by Tβ4 overexpression. Taken together, the results demonstrate that primary cilia formation could be regulated by Tβ4 through its interaction with NPHP3 and/or the control of NPHP3 expression. It suggests that Tβ4 is a novel regulator for primary cilia formation by NPHP3. It also suggests that tumorigenesis could be associated with inappropriate regulation of Tβ4 and/or NPHP3 expression to maintain primary cilia formation normally.

Centrosomal protein FOR20 is essential for cilia-dependent development in zebrafish embryos

Centrosomal proteins play critical roles in ciliogenesis. Mutations in many centrosomal proteins have been documented to contribute to developmental defects and cilium-related diseases. Centrosomal protein fibroblast growth factor receptor 1 oncogene partner-related protein of 20 kDa (FOR20) is crucial for ciliogenesis in mammalian cells and the unicellular eukaryote Paramecium; however, the biologic significance of FOR20 in vertebrate development remains unclear. We cloned the zebrafish homolog of the for20 gene and found that for20 mRNA is enriched in ciliated tissues during early zebrafish development. Knockdown of for20 by morpholino oligonucleotides in zebrafish results in multiple ciliary phenotypes, including curved body, hydrocephaly, pericardial edema, kidney cysts, and left-right asymmetry defects. for20 morphants show reduced number and length of cilia in Kupffer's vesicle and pronephric ducts. High-speed video microscopy reveals that cilia in most for20 morphants are consistently paralyzed or beat arrhythmically. To confirm the ciliary phenotypes of for20 morphants, we used the CRISPR/Cas9 system to disrupt for20 gene in zebrafish. for20 mutants exhibit multiple ciliary phenotypes resembling the defects in for20 morphants. All of these phenotypes in for20 morphants and mutants are significantly reversed by exogenous expression of for20 mRNA. Taken together, these data suggest that FOR20 is required for cilium-mediated processes during zebrafish embryogenesis.-Xie, S., Jin, J., Xu, Z., Huang, Y., Zhang, W., Zhao, L., Lo, L. J., Peng, J., Liu, W., Wang, F., Shu, Q., Zhou, T. Centrosomal protein FOR20 is essential for cilia-dependent development in zebrafish embryos.

Cilia Distal Domain: Diversity in Evolutionarily Conserved Structures

Eukaryotic cilia are microtubule-based organelles that protrude from the cell surface to fulfill sensory and motility functions. Their basic structure consists of an axoneme templated by a centriole/basal body. Striking differences in ciliary ultra-structures can be found at the ciliary base, the axoneme and the tip, not only throughout the eukaryotic tree of life, but within a single organism. Defects in cilia biogenesis and function are at the origin of human ciliopathies. This structural/functional diversity and its relationship with the etiology of these diseases is poorly understood. Some of the important events in cilia function occur at their distal domain, including cilia assembly/disassembly, IFT (intraflagellar transport) complexes' remodeling, and signal detection/transduction. How axonemal microtubules end at this domain varies with distinct cilia types, originating different tip architectures. Additionally, they show a high degree of dynamic behavior and are able to respond to different stimuli. The existence of microtubule-capping structures (caps) in certain types of cilia contributes to this diversity. It has been proposed that caps play a role in axoneme length control and stabilization, but their roles are still poorly understood. Here, we review the current knowledge on cilia structure diversity with a focus on the cilia distal domain and caps and discuss how they affect cilia structure and function.

Spinocerebellar ataxia type 11-associated alleles of Ttbk2 dominantly interfere with ciliogenesis and cilium stability

Spinocerebellar ataxia type 11 (SCA11) is a rare, dominantly inherited human ataxia characterized by atrophy of Purkinje neurons in the cerebellum. SCA11 is caused by mutations in the gene encoding the Serine/Threonine kinase Tau tubulin kinase 2 (TTBK2) that result in premature truncations of the protein. We previously showed that TTBK2 is a key regulator of the assembly of primary cilia in vivo. However, the mechanisms by which the SCA11-associated mutations disrupt TTBK2 function, and whether they interfere with ciliogenesis were unknown. In this work, we present evidence that SCA11-associated mutations are dominant negative alleles and that the resulting truncated protein (TTBK2^SCA11) interferes with the function of full length TTBK2 in mediating ciliogenesis. A Ttbk2 allelic series revealed that upon partial reduction of full length TTBK2 function, TTBK2^SCA11 can interfere with the activity of the residual wild-type protein to decrease cilia number and interrupt cilia-dependent Sonic hedgehog (SHH) signaling. Our studies have also revealed new functions for TTBK2 after cilia initiation in the control of cilia length, trafficking of a subset of SHH pathway components, including Smoothened (SMO), and cilia stability. These studies provide a molecular foundation to understand the cellular and molecular pathogenesis of human SCA11, and help account for the link between ciliary dysfunction and neurodegenerative diseases.

Defects in primary cilia structure and function are linked to a number of recessive genetic disorders, now collectively referred to as ciliopathies. Most of the characteristics of these disorders arise from disruptions to embryonic development, with the requirements for primary cilia in adult tissues being less well-defined. We previously showed that a kinase associated with an adult-onset neurodegenerative condition is required for cilium assembly and ciliary signaling during development. Here, we show that the human disease-associated mutations act as mild dominant negatives, interfering with the function of the full-length protein in cilia formation and ciliary signaling.

Ciliotherapy Treatments to Enhance Biochemically- and Biophysically-Induced Mesenchymal Stem Cell Osteogenesis: A Comparison Study

Introduction

New approaches to treat osteoporosis have focused on promoting bone formation through the targeting of osteoblasts and their progenitors, mesenchymal stem cells (MSCs). The primary cilium is a singular cellular extension known to play an important role in biochemical and biophysical osteogenic induction of MSCs. Defects in ciliary structure have been associated with a plethora of diseases. Therefore targeting the cilium therapeutically (ciliotherapies) has emerged as a potential new treatment modality. Therefore, this study performed a comparison analysis on known ciliotherapies and their potential effects in mediating MSC osteogenic differentiation.

Methods

MSCs were treated with forskolin, lithium chloride (LiCl) or fenoldopam to investigate the effect on ciliogenesis and cilia-associated signalling. Moreover, both early and long term biochemical and biophysical (fluid shear) induced osteogenic differentiation was examined in terms of osteogenic gene expression and bone matrix deposition following each treatment.

Results

LiCl and fenoldopam were found to enhance MSC ciliogenesis to a similar degree. LiCl significantly altered hedgehog (HH) and Wnt signalling which was associated with inhibited osteogenic gene expression, while fenoldopam demonstrated enhanced early osteogenesis. Long term treatment with both ciliotherapies did not enhance osteogenesis, however LiCl had detrimental effects on cell viability. Intriguingly both ciliotherapies enhanced MSC mechanosensitivity as demonstrated by augmented osteogenic gene expression in response to fluid shear, which over longer durations resulted in enhanced matrix deposition per cell.

Conclusions

Therefore, ciliotherapies can be utilised to enhance MSC ciliogenesis resulting in enhanced mechanosensitivity, however, only fenoldopam is a viable ciliotherapeutic option to enhance MSC osteogenesis.

Fragmentation of kidney epithelial cell primary cilia occurs by cisplatin and these cilia fragments are excreted into the urine

The primary cilium, which protrudes from the cell surface, is associated with the pathogenesis of various diseases, including acute kidney injury (AKI). Primary cilium length dynamically changes during the progression of diseases. However, its relevance in disease and the underlying mechanism are largely unknown. In this study, we investigated the role of primary cilia in AKI induced by cisplatin, an effective anticancer drug, and the underlying mechanisms. In addition, we evaluated the usefulness of length alteration and deciliation of primary cilia into the urine for the diagnosis of AKI. Cisplatin induced shortening, elongation, and normalization of the primary cilia in kidney epithelial cells over time. During shortening, primary cilia fragments and ciliary proteins were excreted into the urine. During deciliation, cell proliferation and the expression of cyclin-dependent kinase inhibitor and proliferating cell nuclear antigen were not significantly changed. Shortening and deciliation of primary cilia were observed before significant increases in plasma creatinine and blood urea nitrogen concentration occurred. Pretreatment with Mito-Tempo, a mitochondria-targeted antioxidant, prevented cisplatin-induced primary cilium shortening and inhibited the increases in superoxide formation, lipid peroxidation, blood urea nitrogen, and tissue damage. In contrast, isocitrate dehydrogenase 2 (Idh2) gene deletion, which results in defect of the NADPH-associated mitochondrial antioxidant system, exacerbated cisplatin-induced changes in mice. Taken together, our findings demonstrate that cisplatin induces deciliation into the urine and antioxidant treatment prevents this deciliation, renal dysfunction, and tissue damage after cisplatin injection. These results suggest that cisplatin-induced AKI is associated with primary cilia and urine primary cilia proteins might be a non-invasive biomarker of kidney injury.

The regulation of cilium assembly and disassembly in development and disease

The primary cilium is an antenna-like organelle assembled on most types of quiescent and differentiated mammalian cells. This immotile structure is essential for interpreting extracellular signals that regulate growth, development and homeostasis. As such, ciliary defects produce a spectrum of human diseases, termed ciliopathies, and deregulation of this important organelle also plays key roles during tumor formation and progression. Recent studies have begun to clarify the key mechanisms that regulate ciliary assembly and disassembly in both normal and tumor cells, highlighting new possibilities for therapeutic intervention. Here, we review these exciting new findings, discussing the molecular factors involved in cilium formation and removal, the intrinsic and extrinsic control of cilium assembly and disassembly, and the relevance of these processes to mammalian cell growth and disease.