Cell cycle-dependent ciliogenesis and cancer

Effects of Sesquiterpene Lactones on Primary Cilia Formation (Ciliogenesis)

Sesquiterpene lactones (SLs), plant-derived metabolites with broad spectra of biological effects, including anti-tumor and anti-inflammatory, hold promise for drug development. Primary cilia, organelles extending from cell surfaces, are crucial for sensing and transducing extracellular signals essential for cell differentiation and proliferation. Their life cycle is linked to the cell cycle, as cilia assemble in non-dividing cells of G0/G1 phases and disassemble before entering mitosis. Abnormalities in both primary cilia (non-motile cilia) and motile cilia structure or function are associated with developmental disorders (ciliopathies), heart disease, and cancer. However, the impact of SLs on primary cilia remains unknown. This study evaluated the effects of selected SLs (grosheimin, costunolide, and three cyclocostunolides) on primary cilia biogenesis and stability in human retinal pigment epithelial (RPE) cells. Confocal fluorescence microscopy was employed to analyze the effects on primary cilia formation (ciliogenesis), primary cilia length, and stability. The effects on cell proliferation were evaluated by flow cytometry. All SLs disrupted primary cilia formation in the early stages of ciliogenesis, irrespective of starvation conditions or cytochalasin-D treatment, with no effect on cilia length or cell cycle progression. Interestingly, grosheimin stabilized and promoted primary cilia formation under cilia homeostasis and elongation treatment conditions. Thus, SLs have potential as novel drugs for ciliopathies and tumor treatment.

Primary cilia: a novel research approach to overcome anticancer drug resistance

Primary cilia are cellular organelles that consist of a microtubule skeleton surrounded by a membrane filled with cell signaling receptors. Many studies have shown that primary cilia are cellular antennas, which serve as signaling hubs and their assembly and disassembly are dynamically regulated throughout the cell cycle, playing an important role in regulating cellular homeostasis. Aberrant control of primary cilia dynamics causes a number of genetic disorders known as ciliopathies and is closely associated with tumorigenesis. Anticancer drug resistance is a primary cause of chemotherapy failure, although there is no apparent remedy. The recent identification of a relationship between anticancer drug resistance and primary ciliary dynamics has made primary cilia an important target subcellular organelle for overcoming anticancer drug resistance. Therefore, the research on primary ciliary dynamics may provide new strategies to overcome anticancer drug resistance, which is urgently needed. This review aims to summarize research on the relevance of primary cilia and anticancer drug resistance, as well as future possibilities for research on overcoming anticancer drug resistance utilizing primary cilia dynamics.

Regulation of CD8 T-cell signaling, metabolism, and cytotoxic activity by extracellular lysophosphatidic acid

Lysophosphatidic acid (LPA) is an endogenous bioactive lipid that is produced extracellularly and signals to cells via cognate LPA receptors, which are G-protein coupled receptors (GPCRs). Mature lymphocytes in mice and humans express three LPA receptors, LPA2 , LPA5, and LPA6 , and work from our group has determined that LPA5 signaling by T lymphocytes inhibits specific antigen-receptor signaling pathways that ultimately impair lymphocyte activation, proliferation, and function. In this review, we discuss previous and ongoing work characterizing the ability of an LPA-LPA5 axis to serve as a peripheral immunological tolerance mechanism that restrains adaptive immunity but is subverted during settings of chronic inflammation. Specifically, LPA-LPA5 signaling is found to regulate effector cytotoxic CD8 T cells by (at least) two mechanisms: (i) regulating the actin-microtubule cytoskeleton in a manner that impairs immunological synapse formation between an effector CD8 T cell and antigen-specific target cell, thus directly impairing cytotoxic activity, and (ii) shifting T-cell metabolism to depend on fatty-acid oxidation for mitochondrial respiration and reducing metabolic efficiency. The in vivo outcome of LPA5 inhibitory activity impairs CD8 T-cell killing and tumor immunity in mouse models providing impetus to consider LPA5 antagonism for the treatment of malignancies and chronic infections.

Regulation of Primary Cilium Length by O-GlcNAc during Neuronal Development in a Human Neuron Model

The primary cilium plays critical roles in the homeostasis and development of neurons. Recent studies demonstrate that cilium length is regulated by the metabolic state of cells, as dictated by processes such as glucose flux and O-GlcNAcylation (OGN). The study of cilium length regulation during neuron development, however, has been an area left largely unexplored. This project aims to elucidate the roles of O-GlcNAc in neuronal development through its regulation of the primary cilium. Here, we present findings suggesting that OGN levels negatively regulate cilium length on differentiated cortical neurons derived from human-induced pluripotent stem cells. In neurons, cilium length increased significantly during maturation (after day 35), while OGN levels began to drop. Long-term perturbation of OGN via drugs, which inhibit or promote its cycling, during neuron development also have varying effects. Diminishing OGN levels increases cilium length until day 25, when neural stem cells expand and undergo early neurogenesis, before causing cell cycle exit defects and multinucleation. Elevating OGN levels induces greater primary cilia assembly but ultimately results in the development of premature neurons, which have higher insulin sensitivity. These results indicate that OGN levels and primary cilium length are jointly critical in proper neuron development and function. Understanding the interplays between these two nutrient sensors, O-GlcNAc and the primary cilium, during neuron development is important in paving connections between dysfunctional nutrient-sensing and early neurological disorders.

Evidence for intraflagellar transport in butterfly spermatocyte cilia

In the model organism insect Drosophila melanogaster short cilia assemble on spermatocytes that elaborate into 1.8 mm long flagella during spermatid differentiation. A unique feature of these cilia/flagella is their lack of dependence on intraflagellar transport (IFT) for their assembly. Here, we show that in the common butterfly Pieris brassicae, the spermatocyte cilia are exceptionally long: about 40 μm compared to less than 1 μm in Drosophila. By transmission electron microscopy, we show that P. brassicae spermatocytes display several features not found in melanogaster, including compelling evidence of IFT structures and features of motile cilia.

lncRNA XIST/miR‑129‑2‑3p axis targets CCP110 to regulate the proliferation, invasion and migration of endometrial cancer cells

Centromere coiled-coil protein 110 (CCP110) plays a role in the development of several types of cancer; however, its regulatory mechanism and role in endometrial cancer is unclear. The present study revealed that CCP110 is regulated by a signaling pathway involving microRNA (miR/miRNA)-129-2-3p and the long non-coding RNA (lncRNA) X-inactive-specific transcript (XIST), and plays a role in controlling the proliferation, migration and invasion of endometrial cancer cells. CCP110 was upregulated in human endometrial cancer tissues, as revealed by immunohistochemistry, and high expression of the protein was related to reduced overall survival of the patients. Genetic knockdown of CCP110 by small interfering RNA promoted apoptosis and suppressed the proliferation, migration, invasion and colony formation of endometrial cancer cells significantly in the endometrial cancer Ishikawa and HEC-1B cell lines, as assessed by flow cytometry, and Cell Counting Kit-8, Transwell and colony formation assays. A bioinformatics analysis and luciferase reporter assay revealed that CCP110 is a target of miR-129-2-3p. Overexpression of miR-129-2-3p mimic fragments inhibited the proliferation, migration and invasion of endometrial cancer cells significantly, while co-overexpression of CCP110 counteracted these inhibitory effects. The expression level of the lncRNA XIST was upregulated significantly in endometrial cancer tissues, as assessed by reverse transcription-quantitative PCR assay, while that of miR-129-2-3p was downregulated significantly. A bioinformatics analysis and luciferase reporter assay showed that XIST could inhibit miR-129-2-3p via a miRNA sponge effect. Furthermore, co-overexpression of XIST antagonized the inhibitory effect of the miR-129-2-3p mimic on the luciferase reporter gene signal and protein expression of CCP110. Co-overexpression of XIST also abolished the inhibitory effect of the miR-129-2-3p mimic on the proliferation, migration and invasion of endometrial cancer cells. Overall, these data identified a novel regulatory mechanism of CCP110 involving XIST and miR-129-2-3p, which affected the development of endometrial carcinoma. CCP110, XIST and miR-129-2-3p could represent novel targets for the clinical treatment of endometrial cancer.

An actin filament branching surveillance system regulates cell cycle progression, cytokinesis and primary ciliogenesis

Dysfunction of cell cycle control and defects of primary ciliogenesis are two features of many cancers. Whether these events are interconnected and the driving mechanism coordinating them remains elusive. Here, we identify an actin filament branching surveillance system that alerts cells of actin branching insufficiency and regulates cell cycle progression, cytokinesis and primary ciliogenesis. We find that Oral-Facial-Digital syndrome 1 functions as a class II Nucleation promoting factor to promote Arp2/3 complex-mediated actin branching. Perturbation of actin branching promotes OFD1 degradation and inactivation via liquid-to-gel transition. Elimination of OFD1 or disruption of OFD1-Arp2/3 interaction drives proliferating, non-transformed cells into quiescence with ciliogenesis by an RB-dependent mechanism, while it leads oncogene-transformed/cancer cells to incomplete cytokinesis and irreversible mitotic catastrophe via actomyosin ring malformation. Inhibition of OFD1 leads to suppression of multiple cancer cell growth in mouse xenograft models. Thus, targeting OFD1-mediated actin filament branching surveillance system provides a direction for cancer therapy.

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.

Exploring the recent trends in perturbing the cellular signaling pathways in cancer by natural products

Cancer is commonly thought to be the product of irregular cell division. According to the World Health Organization (WHO), cancer is the major cause of death globally. Nature offers an abundant supply of bioactive compounds with high therapeutic efficacy. Anticancer effects have been studied in a variety of phytochemicals found in nature. When Food and Drug Administration (FDA)-approved anticancer drugs are combined with natural compounds, the effectiveness improves. Several agents have already progressed to clinical trials based on these promising results of natural compounds against various cancer forms. Natural compounds prevent cancer cell proliferation, development, and metastasis by inducing cell cycle arrest, activating intrinsic and extrinsic apoptosis pathways, generating reactive oxygen species (ROS), and down-regulating activated signaling pathways. These natural chemicals are known to affect numerous important cellular signaling pathways, such as NF-B, MAPK, Wnt, Notch, Akt, p53, AR, ER, and many others, to cause cell death signals and induce apoptosis in pre-cancerous or cancer cells without harming normal cells. As a result, non-toxic “natural drugs” taken from nature’s bounty could be effective for the prevention of tumor progression and/or therapy of human malignancies, either alone or in combination with conventional treatments. Natural compounds have also been shown in preclinical studies to improve the sensitivity of resistant cancers to currently available chemotherapy agents. To summarize, preclinical and clinical findings against cancer indicate that natural-sourced compounds have promising anticancer efficacy. The vital purpose of these studies is to target cellular signaling pathways in cancer by natural compounds.

Hedgehog signaling regulates the development and treatment of glioblastoma (Review)

Glioblastoma (GBM) is the most common and fatal malignant tumor type of the central nervous system. GBM affects public health and it is important to identify biomarkers to improve diagnosis, reduce drug resistance and improve prognosis (e.g., personalized targeted therapies). Hedgehog (HH) signaling has an important role in embryonic development, tissue regeneration and stem cell renewal. A large amount of evidence indicates that both normative and non-normative HH signals have an important role in GBM. The present study reviewed the role of the HH signaling pathway in the occurrence and progression of GBM. Furthermore, the effectiveness of drugs that target different components of the HH pathway was also examined. The HH pathway has an important role in reversing drug resistance after GBM conventional treatment. The present review highlighted the relevance of HH signaling in GBM and outlined that this pathway has a key role in the occurrence, development and treatment of GBM.

Predictive modeling of resistance to SMO-inhibition in a patient-derived orthotopic xenograft model of SHH medulloblastoma

Background

Inhibition of the sonic hedgehog (SHH) pathway with Smoothened (SMO) inhibitors is a promising treatment strategy in SHH-activated medulloblastoma, especially in adult patients. However, the problem is that tumors frequently acquire resistance to the treatment. To understand the underlying resistance mechanisms and to find ways to overcome the resistance, preclinical models that became resistant to SMO inhibition are needed.

Methods

To induce SMO inhibitor resistant tumors, we have treated a patient-derived xenograft (PDX) model of SHH medulloblastoma, sensitive to SMO inhibition, with 20 mg/kg Sonidegib using an intermitted treatment schedule. Vehicle-treated and resistant models were subjected to whole-genome and RNA sequencing for molecular characterization and target engagement. In vitro drug screens (76 drugs) were performed using Sonidegib-sensitive and -resistant lines to find other drugs to target the resistant lines. One of the top hits was then validated in vivo.

Results

Nine independent Sonidegib-resistant PDX lines were generated. Molecular characterization of the resistant models showed that eight models developed missense mutations in SMO and one gained an inactivating point mutation in MEGF8, which acts downstream of SMO as a repressor in the SHH pathway. The in vitro drug screen with Sonidegib-sensitive and -resistant lines identified good efficacy for Selinexor in the resistant line. Indeed, in vivo treatment with Selinexor revealed that it is more effective in resistant than in sensitive models.

Conclusions

We report the first human SMO inhibitor resistant medulloblastoma PDX models, which can be used for further preclinical experiments to develop the best strategies to overcome the resistance to SMO inhibitors in patients.

Dual data and motif clustering improves the modeling and interpretation of phosphoproteomic data

Cell signaling is orchestrated in part through a network of protein kinases and phosphatases. Dysregulation of kinase signaling is widespread in diseases such as cancer and is readily targetable through inhibitors. Mass spectrometry-based analysis can provide a global view of kinase regulation, but mining these data is complicated by its stochastic coverage of the proteome, measurement of substrates rather than kinases, and the scale of the data. Here, we implement a dual data and motif clustering (DDMC) strategy that simultaneously clusters peptides into similarly regulated groups based on their variation and their sequence profile. We show that this can help to identify putative upstream kinases and supply more robust clustering. We apply this clustering to clinical proteomic profiling of lung cancer and identify conserved proteomic signatures of tumorigenicity, genetic mutations, and immune infiltration. We propose that DDMC provides a general and flexible clustering strategy for the analysis of phosphoproteomic data.

The Role of Centrosome Distal Appendage Proteins (DAPs) in Nephronophthisis and Ciliogenesis

The primary cilium is found in most mammalian cells and plays a functional role in tissue homeostasis and organ development by modulating key signaling pathways. Ciliopathies are a group of genetically heterogeneous disorders resulting from defects in cilia development and function. Patients with ciliopathic disorders exhibit a range of phenotypes that include nephronophthisis (NPHP), a progressive tubulointerstitial kidney disease that commonly results in end-stage renal disease (ESRD). In recent years, distal appendages (DAPs), which radially project from the distal end of the mother centriole, have been shown to play a vital role in primary ciliary vesicle docking and the initiation of ciliogenesis. Mutations in the genes encoding these proteins can result in either a complete loss of the primary cilium, abnormal ciliary formation, or defective ciliary signaling. DAPs deficiency in humans or mice commonly results in NPHP. In this review, we outline recent advances in our understanding of the molecular functions of DAPs and how they participate in nephronophthisis development.

Cilia, Centrosomes and Skeletal Muscle

Primary cilia are non-motile, cell cycle-associated organelles that can be found on most vertebrate cell types. Comprised of microtubule bundles organised into an axoneme and anchored by a mature centriole or basal body, primary cilia are dynamic signalling platforms that are intimately involved in cellular responses to their extracellular milieu. Defects in ciliogenesis or dysfunction in cilia signalling underlie a host of developmental disorders collectively referred to as ciliopathies, reinforcing important roles for cilia in human health. Whilst primary cilia have long been recognised to be present in striated muscle, their role in muscle is not well understood. However, recent studies indicate important contributions, particularly in skeletal muscle, that have to date remained underappreciated. Here, we explore recent revelations that the sensory and signalling functions of cilia on muscle progenitors regulate cell cycle progression, trigger differentiation and maintain a commitment to myogenesis. Cilia disassembly is initiated during myoblast fusion. However, the remnants of primary cilia persist in multi-nucleated myotubes, and we discuss their potential role in late-stage differentiation and myofiber formation. Reciprocal interactions between cilia and the extracellular matrix (ECM) microenvironment described for other tissues may also inform on parallel interactions in skeletal muscle. We also discuss emerging evidence that cilia on fibroblasts/fibro-adipogenic progenitors and myofibroblasts may influence cell fate in both a cell autonomous and non-autonomous manner with critical consequences for skeletal muscle ageing and repair in response to injury and disease. This review addresses the enigmatic but emerging role of primary cilia in satellite cells in myoblasts and myofibers during myogenesis, as well as the wider tissue microenvironment required for skeletal muscle formation and homeostasis.

Hedgehog/GLI Signaling Pathway: Transduction, Regulation, and Implications for Disease

Simple Summary

The Hedgehog/GLI (Hh/GLI) pathway plays a major role during development and it is commonly dysregulated in many diseases, including cancer. This highly concerted series of ligands, receptors, cytoplasmic signaling molecules, transcription factors, and co-regulators is involved in regulating the biological functions controlled by this pathway. Activation of Hh/GLI in cancer is most often through a non-canonical method of activation, independent of ligand binding. This review is intended to summarize our current understanding of the Hh/GLI signaling, non-canonical mechanisms of pathway activation, its implication in disease, and the current therapeutic strategies targeting this cascade.

Abstract

The Hh/GLI signaling pathway was originally discovered in Drosophila as a major regulator of segment patterning in development. This pathway consists of a series of ligands (Shh, Ihh, and Dhh), transmembrane receptors (Ptch1 and Ptch2), transcription factors (GLI1–3), and signaling regulators (SMO, HHIP, SUFU, PKA, CK1, GSK3β, etc.) that work in concert to repress (Ptch1, Ptch2, SUFU, PKA, CK1, GSK3β) or activate (Shh, Ihh, Dhh, SMO, GLI1–3) the signaling cascade. Not long after the initial discovery, dysregulation of the Hh/GLI signaling pathway was implicated in human disease. Activation of this signaling pathway is observed in many types of cancer, including basal cell carcinoma, medulloblastoma, colorectal, prostate, pancreatic, and many more. Most often, the activation of the Hh/GLI pathway in cancer occurs through a ligand-independent mechanism. However, in benign disease, this activation is mostly ligand-dependent. The upstream signaling component of the receptor complex, SMO, is bypassed, and the GLI family of transcription factors can be activated regardless of ligand binding. Additional mechanisms of pathway activation exist whereby the entirety of the downstream signaling pathway is bypassed, and PTCH1 promotes cell cycle progression and prevents caspase-mediated apoptosis. Throughout this review, we summarize each component of the signaling cascade, non-canonical modes of pathway activation, and the implications in human disease, including cancer.

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

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

Primary Cilium Is Involved in Stem Cell Differentiation and Renewal through the Regulation of Multiple Signaling Pathways

Signaling networks guide stem cells during their lineage specification and terminal differentiation. Primary cilium, an antenna-like protrusion, directly or indirectly plays a significant role in this guidance. All stem cells characterized so far have primary cilia. They serve as entry- or check-points for various signaling events by controlling the signal transduction and stability. Thus, defects in the primary cilia formation or dynamics cause developmental and health problems, including but not limited to obesity, cardiovascular and renal anomalies, hearing and vision loss, and even cancers. In this review, we focus on the recent findings of how primary cilium controls various signaling pathways during stem cell differentiation and identify potential gaps in the field for future research.

The emerging role of the KCTD proteins in cancer

The human family of Potassium (K+) Channel Tetramerization Domain (KCTD) proteins counts 25 members, and a significant number of them are still only partially characterized. While some of the KCTDs have been linked to neurological disorders or obesity, a growing tally of KCTDs are being associated with cancer hallmarks or involved in the modulation of specific oncogenic pathways. Indeed, the potential relevance of the variegate KCTD family in cancer warrants an updated picture of the current knowledge and highlights the need for further research on KCTD members as either putative therapeutic targets, or diagnostic/prognostic markers. Homology between family members, capability to participate in ubiquitination and degradation of different protein targets, ability to heterodimerize between members, role played in the main signalling pathways involved in development and cancer, are all factors that need to be considered in the search for new key players in tumorigenesis. In this review we summarize the recent published evidence on KCTD members' involvement in cancer. Furthermore, by integrating this information with data extrapolated from public databases that suggest new potential associations with cancers, we hypothesize that the number of KCTD family members involved in tumorigenesis (either as positive or negative modulator) may be bigger than so far demonstrated. Video abstract.

Subcellular Organization of the cAMP Signaling Pathway

The field of cAMP signaling is witnessing exciting developments with the recognition that cAMP is compartmentalized and that spatial regulation of cAMP is critical for faithful signal coding. This realization has changed our understanding of cAMP signaling from a model in which cAMP connects a receptor at the plasma membrane to an intracellular effector in a linear pathway to a model in which cAMP signals propagate within a complex network of alternative branches and the specific functional outcome strictly depends on local regulation of cAMP levels and on selective activation of a limited number of branches within the network. In this review, we cover some of the early studies and summarize more recent evidence supporting the model of compartmentalized cAMP signaling, and we discuss how this knowledge is starting to provide original mechanistic insight into cell physiology and a novel framework for the identification of disease mechanisms that potentially opens new avenues for therapeutic interventions.

Single-Cell Reconstruction of Human Basal Cell Diversity in Normal and Idiopathic Pulmonary Fibrosis Lungs

Rationale: Declining lung function in patients with interstitial lung disease is accompanied by epithelial remodeling and progressive scarring of the gas-exchange region. There is a need to better understand the contribution of basal cell hyperplasia and associated mucosecretory dysfunction to the development of idiopathic pulmonary fibrosis (IPF).Objectives: We sought to decipher the transcriptome of freshly isolated epithelial cells from normal and IPF lungs to discern disease-dependent changes within basal stem cells.Methods: Single-cell RNA sequencing was used to map epithelial cell types of the normal and IPF human airways. Organoid and air-liquid interface cultures were used to investigate functional properties of basal cell subtypes.Measurements and Main Results: We found that basal cells included multipotent and secretory primed subsets in control adult lung tissue. Secretory primed basal cells include an overlapping molecular signature with basal cells obtained from the distal lung tissue of IPF lungs. We confirmed that NOTCH2 maintains undifferentiated basal cells and restricts basal-to-ciliated differentiation, and we present evidence that NOTCH3 functions to restrain secretory differentiation.Conclusions: Basal cells are dynamically regulated in disease and are specifically biased toward the expansion of the secretory primed basal cell subset in IPF. Modulation of basal cell plasticity may represent a relevant target for therapeutic intervention in IPF.

Loss of tubal ciliated cells as a risk for "ovarian" or pelvic serous carcinoma

Recent advances suggest the fallopian tube as the main anatomic site for high-grade ovarian or pelvic serous carcinoma (O/PSC). Many studies on the biologic role of tubal secretory cells in O/PSC development has been performed in the last decade. However, the role of tubal ciliated cells in this regard has rarely been explored. The purpose of this study was to determine if the change of the tubal ciliated cells is associated with serous neoplasia within the female pelvis. This study included 3 groups (low-risk or benign control, high-risk, and O/PSC) of patients and they were age-matched. Age of patients ranged from 20 to 85 and the age-associated data was stratified by 10-year intervals. The number of tubal ciliated cells was determined by microscopy and by tubulin immunohistochemical staining. The data was then professionally analyzed. The results showed that the absolute number of tubal ciliated cells decreased significantly with age within each age group. A reduction in ciliated cell counts within the tubal segments remained a significant risk factor for the development of serous cancers within the female pelvis after age adjustment. A dramatic decrease of tubal ciliated cells was identified in patients with high-risk and with O/PSC compared to those in the benign control or low-risk group (P < 0.001). Further, within the tubal fimbria, the number of ciliated cells reduction was more prominent in the high-risk group when compared to those of O/PSC patients. Our findings suggest that a decreased number of ciliated cells within women's fallopian tubes represents another histologic hallmark for early serous carcinogenesis. There is a relationship between loss of tubal ciliated cells and aging, the presence of high-risk factors for tubal-ovarian cancer, and co-existing O/PSCs. This represents an initial study identifying the role of tubal ciliated cells in the development of high-grade serous carcinoma in women's pelvis.

Therapeutic Targeting of Signaling Pathways Related to Cancer Stemness

The theory of cancer stem cells (CSCs) proposes that the different cells within a tumor, as well as metastasis deriving from it, are originated from a single subpopulation of cells with self-renewal and differentiation capacities. These cancer stem cells are supposed to be critical for tumor expansion and metastasis, tumor relapse and resistance to conventional therapies, such as chemo- and radiotherapy. The acquisition of these abilities has been attributed to the activation of alternative pathways, for instance, WNT, NOTCH, SHH, PI3K, Hippo, or NF-κB pathways, that regulate detoxification mechanisms; increase the metabolic rate; induce resistance to apoptotic, autophagic, and senescence pathways; promote the overexpression of drug transporter proteins; and activate specific stem cell transcription factors. The elimination of CSCs is an important goal in cancer therapeutic approaches because it could decrease relapses and metastatic dissemination, which are main causes of mortality in oncology patients. In this work, we discuss the role of these signaling pathways in CSCs along with their therapeutic potential.

Ciliogenesis and Hedgehog signalling are suppressed downstream of KRAS during acinar-ductal metaplasia in mouse

Pancreatic ductal adenocarcinoma (PDAC) is the third leading cause of cancer-related deaths worldwide, but has a 5-year survival rate of only 7% primarily due to late diagnosis and ineffective therapies. To treat or even prevent PDAC, it is vital that we understand the initiating events that lead to tumour onset. PDAC develops from preneoplastic lesions, most commonly pancreatic intraepithelial neoplasias (PanINs), driven by constitutive activation of KRAS. In patients, PanINs are associated with regions of acinar-to-ductal metaplasia (ADM) where, in response to inflammation, acini dedifferentiate to a pancreatic progenitor-like fate. In healthy tissue this process is reversible leading to regeneration of the pancreas; however, in the presence of oncogenic KRAS, regeneration is blocked and ADM can give rise to PanIN lesions. Here, we used a 3D mouse acinar culture that recapitulates ADM in vitro to explore how KRAS prevents regeneration. Regeneration is regulated by Hedgehog (Hh) signalling, which is transduced via the primary cilium. In wild-type acini, cilia assemble upon ADM and Hh target gene expression is upregulated; however, ciliogenesis and Hh signalling are suppressed during ADM in cells expressing oncogenic KRAS. We show that ciliogenesis fails due to ectopic activation of the cilium disassembly pathway, which is mediated by AurkA, a direct transcriptional target of KRAS. Inhibition of AurkA is able to rescue primary cilia and restore Hh signalling. We suggest that this could be used as a mechanism to prevent the formation of early lesions and thereby prevent progression to PDAC.

Ciliary Genes in Renal Cystic Diseases

Cilia are microtubule-based organelles, protruding from the apical cell surface and anchoring to the cytoskeleton. Primary (nonmotile) cilia of the kidney act as mechanosensors of nephron cells, responding to fluid movements by triggering signal transduction. The impaired functioning of primary cilia leads to formation of cysts which in turn contribute to development of diverse renal diseases, including kidney ciliopathies and renal cancer. Here, we review current knowledge on the role of ciliary genes in kidney ciliopathies and renal cell carcinoma (RCC). Special focus is given on the impact of mutations and altered expression of ciliary genes (e.g., encoding polycystins, nephrocystins, Bardet-Biedl syndrome (BBS) proteins, ALS1, Oral-facial-digital syndrome 1 (OFD1) and others) in polycystic kidney disease and nephronophthisis, as well as rare genetic disorders, including syndromes of Joubert, Meckel-Gruber, Bardet-Biedl, Senior-Loken, Alström, Orofaciodigital syndrome type I and cranioectodermal dysplasia. We also show that RCC and classic kidney ciliopathies share commonly disturbed genes affecting cilia function, including VHL (von Hippel-Lindau tumor suppressor), PKD1 (polycystin 1, transient receptor potential channel interacting) and PKD2 (polycystin 2, transient receptor potential cation channel). Finally, we discuss the significance of ciliary genes as diagnostic and prognostic markers, as well as therapeutic targets in ciliopathies and cancer.

Identification of a new aggressive axis driven by ciliogenesis and absence of VDAC1-ΔC in clear cell Renal Cell Carcinoma patients

Rationale: Renal cell carcinoma (RCC) accounts for about 2% of all adult cancers, and clear cell RCC (ccRCC) is the most common RCC histologic subtype. A hallmark of ccRCC is the loss of the primary cilium, a cellular antenna that senses a wide variety of signals. Loss of this key organelle in ccRCC is associated with the loss of the von Hippel-Lindau protein (VHL). However, not all mechanisms of ciliopathy have been clearly elucidated.

Methods: By using RCC4 renal cancer cells and patient samples, we examined the regulation of ciliogenesis via the presence or absence of the hypoxic form of the voltage-dependent anion channel (VDAC1-ΔC) and its impact on tumor aggressiveness. Three independent cohorts were analyzed. Cohort A was from PREDIR and included 12 patients with hereditary pVHL mutations and 22 sporadic patients presenting tumors with wild-type pVHL or mutated pVHL; Cohort B included tissue samples from 43 patients with non-metastatic ccRCC who had undergone surgery; and Cohort C was composed of 375 non-metastatic ccRCC tumor samples from The Cancer Genome Atlas (TCGA) and was used for validation. The presence of VDAC1-ΔC and legumain was determined by immunoblot. Transcriptional regulation of IFT20/GLI1 expression was evaluated by qPCR. Ciliogenesis was detected using both mouse anti-acetylated α-tubulin and rabbit polyclonal ARL13B antibodies for immunofluorescence.

Results: Our study defines, for the first time, a group of ccRCC patients in which the hypoxia-cleaved form of VDAC1 (VDAC1-ΔC) induces resorption of the primary cilium in a Hypoxia-Inducible Factor-1 (HIF-1)-dependent manner. An additional novel group, in which the primary cilium is re-expressed or maintained, lacked VDAC1-ΔC yet maintained glycolysis, a signature of epithelial-mesenchymal transition (EMT) and more aggressive tumor progression, but was independent to VHL. Moreover, these patients were less sensitive to sunitinib, the first-line treatment for ccRCC, but were potentially suitable for immunotherapy, as indicated by the immunophenoscore and the presence of PDL1 expression.

Conclusion: This study provides a new way to classify ccRCC patients and proposes potential therapeutic targets linked to metabolism and immunotherapy.

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

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

Functional role of hedgehog pathway in osteoarthritis

The hedgehog signalling pathway is one of the key regulators of metazoan development, and it plays an important role in the regulation of a variety of developmental and physiological processes. But it is aberrantly activated in many human diseases, including osteoarthritis (OA). In this study, we have reviewed the association of hedgehog signalling pathway in the development and progression of OA and evaluated the efforts to target this pathway for the prevention of OA. Usually in OA, activation of hedgehog induces up-regulation of the expression of hypertrophic markers, including type X collagen, increases production of nitric oxide and prostaglandin E2, several matrix-degrading enzymes including matrix metalloproteinase and a disintegrin and metalloproteinase with thrombospondin motifs in human knee joint cartilage leading to cartilage degeneration, and thus contributes in OA. Targeting hedgehog signalling might be a viable strategy to prevent or treat OA. Chemical inhibitors of hedgehog signalling is promising, but they cause severe side effects. Knockdown of HH gene is not an option for OA treatment in humans because it is not possible to delete HH in larger animals. Efficient knockdown of HH achieved by local delivery of small interfering RNA in future studies utilizing large animal OA models might be a more efficient approach for the prevention of OA. However, it remains a major problem to develop one single scaffold due to the different physiological functions of cartilage and subchondral bones possess. More studies are necessary to identify selective inhibitors for efficiently targeting the hedgehog pathway in clinical conditions.

Ciliogenesis-coupled accumulation of IFT-B proteins in a novel cytoplasmic compartment

Cluap1/IFT38 is a ciliary protein that belongs to the IFT-B complex and is required for ciliogenesis. In this study, we have examined the behaviors of Cluap1 protein in nonciliated and ciliated cells. In proliferating cells, Cluap1 is located at the distal appendage of the mother centriole. When cells are induced to form cilia, Cluap1 is found in a novel noncentriolar compartment, the cytoplasmic IFT spot, which mainly exists once in a cell. Other IFT-B proteins such as IFT46 and IFT88 are colocalized in this spot. The cytoplasmic IFT spot is present in mouse embryonic fibroblasts (MEFs) but is absent in ciliogenesis-defective MEFs lacking Cluap1, Kif3a or Odf2. The cytoplasmic IFT spot is also found in mouse embryos but is absent in the Cluap1 mutant embryo. When MEFs are induced to form cilia, the cytoplasmic IFT spot appears at an early step of ciliogenesis but starts to disappear when ciliogenesis is mostly completed. These results suggest that IFT-B proteins such as Cluap1 accumulate in a previously undescribed cytoplasmic compartment during ciliogenesis.

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.

Unexpected Activities in Regulating Ciliation Contribute to Off-target Effects of Targeted Drugs

PURPOSE

For many tumors, signaling exchanges between cancer cells and other cells in their microenvironment influence overall tumor signaling. Some of these exchanges depend on expression of the primary cilium on nontransformed cell populations, as extracellular ligands including Sonic Hedgehog (SHH), PDGFRα, and others function through receptors spatially localized to cilia. Cell ciliation is regulated by proteins that are themselves therapeutic targets. We investigated whether kinase inhibitors of clinical interest influence ciliation and signaling by proteins with ciliary receptors in cancer and other cilia-relevant disorders, such as polycystic kidney disease (PKD).

EXPERIMENTAL DESIGN

We screened a library of clinical and preclinical kinase inhibitors, identifying drugs that either prevented or induced ciliary disassembly. Specific bioactive protein targets of the drugs were identified by mRNA depletion. Mechanism of action was defined, and activity of select compounds investigated.

RESULTS

We identified multiple kinase inhibitors not previously linked to control of ciliation, including sunitinib, erlotinib, and an inhibitor of the innate immune pathway kinase, IRAK4. For all compounds, activity was mediated through regulation of Aurora-A (AURKA) activity. Drugs targeting cilia influenced proximal cellular responses to SHH and PDGFRα. In vivo, sunitinib durably limited ciliation and cilia-related biological activities in renal cells, renal carcinoma cells, and PKD cysts. Extended analysis of IRAK4 defined a subset of innate immune signaling effectors potently affecting ciliation.

CONCLUSIONS

These results suggest a paradigm by which targeted drugs may have unexpected off-target effects in heterogeneous cell populations in vivo via control of a physical platform for receipt of extracellular ligands.

Primary Cilia-An Underexplored Topic in Major Mental Illness

Though much progress has been made in recent years towards understanding the function and physiology of primary cilia, they remain a somewhat elusive organelle. Some studies have explored the role of primary cilia in the developing nervous system, and their dysfunction has been linked with several neurosensory deficits. Yet, very little has been written on their potential role in psychiatric disorders. This article provides an overview of some of the functions of primary cilia in signalling pathways, and demonstrates that they are a worthy candidate in psychiatric research. The links between primary cilia and major mental illness have been demonstrated to exist at several levels, spanning genetics, signalling pathways, and pharmacology as well as cell division and migration. The primary focus of this review is on the sensory role of the primary cilium and the neurodevelopmental hypothesis of psychiatric disease. As such, the primary cilium is demonstrated to be a key link between the cellular environment and cell behaviour, and hence of key importance in the considerations of the nature and nurture debate in psychiatric research.

Emerging Roles of Primary Cilia in Glioma

Primary cilia are microtubule-based organelles that are typically present on cells during the G0 or G1-S/G2 phases of the cell cycle. Recent studies of glioblastoma (GBM) biopsies, a brain tumor that is notorious for its aggressive growth and resistance to treatment, show that many cells in the tumor lack cilia. At this point, it remains unclear whether primary cilia promote or suppress glioma tumorigenesis. In this review, we will discuss the different roles that have been proposed for primary cilia in glioma and how cilia may contribute to the resistance of these tumors to current therapies.

Bardet-Biedl Syndrome proteins regulate cilia disassembly during tissue maturation

Primary cilia are conserved organelles that mediate cellular communication crucial for organogenesis and homeostasis in numerous tissues. The retinal pigment epithelium (RPE) is a ciliated monolayer in the eye that borders the retina and is vital for visual function. Maturation of the RPE is absolutely critical for visual function and the role of the primary cilium in this process has been largely ignored to date. We show that primary cilia are transiently present during RPE development and that as the RPE matures, primary cilia retract, and gene expression of ciliary disassembly components decline. We observe that ciliary-associated BBS proteins protect against HDAC6-mediated ciliary disassembly via their recruitment of Inversin to the base of the primary cilium. Inhibition of ciliary disassembly components was able to rescue ciliary length defects in BBS deficient cells. This consequently affects ciliary regulation of Wnt signaling. Our results shed light onto the mechanisms by which cilia-mediated signaling facilitates tissue maturation.

The Plk1 kinase negatively regulates the Hedgehog signaling pathway by phosphorylating Gli1

Hedgehog (Hh) signaling is a highly conserved cell signaling pathway important for cell life, development and tumorigenesis. Increasing evidence suggests that the Hh signaling pathway functions in certain phases of the cell cycle. However, the coordination between Hh signaling and cell cycle control remains poorly understood. Here, we show that polo-like kinase-1 (Plk1), a critical protein kinase regulating many processes during the cell cycle, also regulates Hh signaling by phosphorylating and inhibiting Gli1, a downstream transcription factor of the Hh signaling pathway. Gli1 expression increases along with Hh signaling activation, leading to upregulation of Hh target genes, including cyclin E, during the G1 and S phases. Gli1 is phosphorylated at S481 by Plk1, and this phosphorylation facilitates the nuclear export and binding of Gli1 with its negative regulator Sufu, leading to a reduction in Hh signaling activity. Inhibition of Plk1 kinase activity led to Gli1 maintaining is role in promoting downstream gene expression. Collectively, our data reveal a novel mechanism regarding the crosstalk between Hh signaling and cell cycle control.

The Roles of Primary Cilia in Cardiovascular Diseases

Primary cilia are microtubule-based organelles found in most mammalian cell types. Cilia act as sensory organelles that transmit extracellular clues into intracellular signals for molecular and cellular responses. Biochemical and molecular defects in primary cilia are associated with a wide range of diseases, termed ciliopathies, with phenotypes ranging from polycystic kidney disease, liver disorders, mental retardation, and obesity to cardiovascular diseases. Primary cilia in vascular endothelia protrude into the lumen of blood vessels and function as molecular switches for calcium (Ca²⁺) and nitric oxide (NO) signaling. As mechanosensory organelles, endothelial cilia are involved in blood flow sensing. Dysfunction in endothelial cilia contributes to aberrant fluid-sensing and thus results in vascular disorders, including hypertension, aneurysm, and atherosclerosis. This review focuses on the most recent findings on the roles of endothelial primary cilia within vascular biology and alludes to the possibility of primary cilium as a therapeutic target for cardiovascular disorders.

Effects of Repeated Aurora-A siRNA Transfection on Cilia Generation and Proliferation of SK-MES-1 or A549 Cells

BACKGROUND

Suppression of Aurora kinase A (Aurora-A, AURKA) by siRNA of Aurora-A (siAurora-A, siA) has been used in lung tumor treatment. However, the dose and frequency of gene transfection still need to be confirmed further. We imitated multiple administration of solid tumor and attempted to make out the effects of thrice transfection of siAurora-A on cilia generation and apoptosis of SK-MES-1 cells (SK) or A549 cells.

METHODS

The Aurora-A mRNA levels of cells cultured with serum for 6 d or without serum for 2, 4, or 6 d were examined with real-time quantitative PCR; Cells were transfected single or repeatedly with siAurora-A or siControl (siC), their Aurora-A mRNA levels were determined with PCR; Their cilia were examined with immunohistochemistry. Cell viability was measured with the MTT assay. Protein expression was analyzed with western blot.

RESULTS

Cell viability showed a downward trend along with the prolongation of starvation time to the second, fourth, and even to the sixth day in both types of cells. But, the expression level of Aurora-A mRNA flipped to rise at the sixth day instead of decreasing at the fourth day. Protein expression trend of total Aurora-A in the two groups was consistent with Aurora-A mRNA expression trend. Compared with siC-3 group (transfected three times with siControl), siAurora-A significantly reduced the Aurora-A mRNA expression in siA-3 group (transfected three times with siAurora-A). Similarly, the cell viability of siA-3 group was lower than that of siC-3 group. The cell viability of siC-3 group was higher than that of serum-free-6d group, but, levels of Aurora-A mRNA expression of siC-3 group had no difference with serum-free-6d group. Finally, among groups transfected once or three times or starved for 6 d, there was no significant difference of ciliated cell proportions in both types of cells respectively.

CONCLUSIONS

Repeated siAurora-A transfection decreased Aurora-A expression that resulted in effective suppression proliferation of SK-MES-1 or A549 cells, but did not affect cilia generation.

Activation of the Gi protein-RHOA axis by non-canonical Hedgehog signaling is independent of primary cilia

Primary cilia are solitary organelles that emanate from the plasma membrane during growth arrest in almost all mammalian cells. The canonical Hedgehog (HH) pathway requires trafficking of the G protein-coupled receptor SMOOTHENED (SMO) and the GLI transcription factors to the primary cilium upon binding of a HH ligand to PATCHED1. However, it is unknown if activation of the small GTPase RHOA by SMO coupling to heterotrimeric Gi proteins, a form of non-canonical HH signaling, requires localization of SMO in the primary cilium. In this study, we compared RHOA and Gi protein stimulation by activation of SMO or sphingosine 1-phosphate receptor (S1P) receptors in WT and KIF3A-deficient mouse embryonic fibroblasts that lack primary cilia. We found that activation of SMO in response to Sonic HH (SHH) or purmorphamine (PUR), a small molecule agonist of SMO, stimulates Gi proteins and RHOA independently of the presence of primary cilia, similar to the effects of S1P. However, while S1P induced a fast activation of AKT that is sensitive to the Gi inhibitor pertussis toxin, HH pathway activators did not significantly activate AKT, suggesting that RHOA activation is not downstream of AKT. Our findings demonstrate that early events in some forms of non-canonical HH signaling occur in extraciliary membranes, which might be particularly relevant for actively-cycling cells, for some cancers characterized by loss of primary cilia, and in ciliopathies.

Signaling interplay between primary cilia and nitric oxide: A mini review

New discoveries into the functional role of primary cilia are on the rise. In little more than 20 years, research has shown the once vestigial organelle is a signaling powerhouse involved in a vast number of essential cellular processes. In the same decade that interest in primary cilia was burgeoning, nitric oxide won molecule of the year and a Nobel prize for its role as a near ubiquitous signaling molecule. Although primary cilia and nitric oxide are both involved in signaling, a direct relationship has not been investigated; however, after a quick review of the literature, parallels between their functions can be drawn. This review aims to suggest a possible interplay between primary cilia and nitric oxide signaling especially in the areas of vascular tissue homeostasis and cellular proliferation.

Ciliary signalling in cancer

Although tumours initiate from oncogenic changes in a cancer cell, subsequent tumour progression and therapeutic response depend on interactions between the cancer cells and the tumour microenvironment (TME). The primary monocilium, or cilium, provides a spatially localized platform for signalling by Hedgehog, Notch, WNT and some receptor tyrosine kinase pathways and mechanosensation. Changes in ciliation of cancer cells and/or cells of the TME during tumour development enforce asymmetric intercellular signalling in the TME. Growing evidence indicates that some oncogenic signalling pathways as well as some targeted anticancer therapies induce ciliation, while others repress it. The links between the genomic profile of cancer cells, drug treatment and ciliary signalling in the TME likely affect tumour growth and therapeutic response.

Counterregulation of cAMP-directed kinase activities controls ciliogenesis

The primary cilium emanates from the cell surface of growth-arrested cells and plays a central role in vertebrate development and tissue homeostasis. The mechanisms that control ciliogenesis have been extensively explored. However, the intersection between GPCR signaling and the ubiquitin pathway in the control of cilium stability are unknown. Here we observe that cAMP elevation promotes cilia resorption. At centriolar satellites, we identify a multimeric complex nucleated by PCM1 that includes two kinases, NEK10 and PKA, and the E3 ubiquitin ligase CHIP. We show that NEK10 is essential for ciliogenesis in mammals and for the development of medaka fish. PKA phosphorylation primes NEK10 for CHIP-mediated ubiquitination and proteolysis resulting in cilia resorption. Disarrangement of this control mechanism occurs in proliferative and genetic disorders. These findings unveil a pericentriolar kinase signalosome that efficiently links the cAMP cascade with the ubiquitin-proteasome system, thereby controlling essential aspects of ciliogenesis.

EGF receptor kinase suppresses ciliogenesis through activation of USP8 deubiquitinase

Ciliogenesis is generally inhibited in dividing cells, however, it has been unclear which signaling cascades regulate the phenomenon. Here, we report that epidermal growth factor receptor (EGFR) kinase suppresses ciliogenesis by directly phosphorylating the deubiquitinase USP8 on Tyr-717 and Tyr-810 in RPE1 cells. These phosphorylations elevate the deubiquitinase activity, which then stabilizes the trichoplein-Aurora A pathway, an inhibitory mechanism of ciliogenesis. EGFR knockdown and serum starvation result in ciliogenesis through downregulation of the USP8-trichoplein-Aurora A signal. Moreover, primary cilia abrogation, which is induced upon IFT20 or Cep164 depletion, ameliorates the cell cycle arrest of EGFR knockdown cells. The present data reveal that the EGFR-USP8-trichoplein-Aurora A axis is a critical signaling cascade that restricts ciliogenesis in dividing cells, and functions to facilitate cell proliferation. We further show that usp8 knockout zebrafish develops ciliopathy-related phenotypes including cystic kidney, suggesting that USP8 is a regulator of ciliogenesis in vertebrates.

The role of the Hedgehog signaling pathway in cancer: A comprehensive review

The Hedgehog (Hh) signaling pathway was first identified in the common fruit fly. It is a highly conserved evolutionary pathway of signal transmission from the cell membrane to the nucleus. The Hh signaling pathway plays an important role in the embryonic development. It exerts its biological effects through a signaling cascade that culminates in a change of balance between activator and repressor forms of glioma-associated oncogene (Gli) transcription factors. The components of the Hh signaling pathway involved in the signaling transfer to the Gli transcription factors include Hedgehog ligands (Sonic Hh [SHh], Indian Hh [IHh], and Desert Hh [DHh]), Patched receptor (Ptch1, Ptch2), Smoothened receptor (Smo), Suppressor of fused homolog (Sufu), kinesin protein Kif7, protein kinase A (PKA), and cyclic adenosine monophosphate (cAMP). The activator form of Gli travels to the nucleus and stimulates the transcription of the target genes by binding to their promoters. The main target genes of the Hh signaling pathway are PTCH1, PTCH2, and GLI1. Deregulation of the Hh signaling pathway is associated with developmental anomalies and cancer, including Gorlin syndrome, and sporadic cancers, such as basal cell carcinoma, medulloblastoma, pancreatic, breast, colon, ovarian, and small-cell lung carcinomas. The aberrant activation of the Hh signaling pathway is caused by mutations in the related genes (ligand-independent signaling) or by the excessive expression of the Hh signaling molecules (ligand-dependent signaling - autocrine or paracrine). Several Hh signaling pathway inhibitors, such as vismodegib and sonidegib, have been developed for cancer treatment. These drugs are regarded as promising cancer therapies, especially for patients with refractory/advanced cancers.

Calcium-axonemal microtubuli interactions underlie mechanism(s) of primary cilia morphological changes

We have used cell culture of astrocytes aligned within microchannels to investigate calcium effects on primary cilia morphology. In the absence of calcium and in the presence of flow of media (10 μL.s^-1) the majority (90%) of primary cilia showed reversible bending with an average curvature of 2.1 ± 0.9 × 10^-4 nm^-1. When 1.0 mM calcium was present, 90% of cilia underwent bending. Forty percent of these cilia demonstrated strong irreversible bending, resulting in a final average curvature of 3.9 ± 1 × 10^-4 nm^-1, while 50% of cilia underwent bending similar to that observed during calcium-free flow. The average length of cilia was shifted toward shorter values (3.67 ± 0.34 μm) when exposed to excess calcium (1.0 mM), compared to media devoid of calcium (3.96 ± 0.26 μm). The number of primary cilia that became curved after calcium application was reduced when the cell culture was pre-incubated with 15 μM of the microtubule stabilizer, taxol, for 60 min prior to calcium application. Calcium caused single microtubules to curve at a concentration ≈1.0 mM in vitro, but at higher concentration (≈1.5 mM) multiple microtubule curving occurred. Additionally, calcium causes microtubule-associated protein-2 conformational changes and its dislocation from the microtubule wall at the location of microtubule curvature. A very small amount of calcium, that is 1.45 × 10¹¹ times lower than the maximal capacity of TRPPs calcium channels, may cause gross morphological changes (curving) of primary cilia, while global cytosol calcium levels are expected to remain unchanged. These findings reflect the non-linear manner in which primary cilia may respond to calcium signaling, which in turn may influence the course of development of ciliopathies and cancer.

The Biology of Ciliary Dynamics

The cilium is an evolutionally conserved apical membrane protrusion that senses and transduces diverse signals to regulate a wide range of cellular activities. The cilium is dynamic in length, structure, and protein composition. Dysregulation of ciliary dynamics has been linked with ciliopathies and other human diseases. The cilium undergoes cell-cycle-dependent assembly and disassembly, with ciliary resorption linked with G₁-S transition and cell-fate choice. In the resting cell, the cilium remains sensitive to environmental cues for remodeling during tissue homeostasis and repair. Recent findings further reveal an interplay between the cilium and extracellular vesicles and identify bioactive cilium-derived vesicles, posing a previously unrecognized role of cilia for sending signals. The photoreceptor outer segment is a notable dynamic cilium. A recently discovered protein transport mechanism in photoreceptors maintains light-regulated homeostasis of ciliary length.

Lengthening primary cilia enhances cellular mechanosensitivity

The primary cilium is a mechanosensor in a variety of mammalian cell types, initiating and directing intracellular signalling cascades in response to external stimuli. When primary cilia formation is disrupted, cells have diminished mechanosensitivity and an abrogated response to mechanical stimulation. Due to this important role, we hypothesised that increasing primary cilia length would enhance the downstream response and therefore, mechanosensitivity. To test this hypothesis, we increased osteocyte primary cilia length with fenoldopam and lithium and found that cells with longer primary cilia were more mechanosensitive. Furthermore, fenoldopam treatment potentiated adenylyl cyclase activity and was able to recover primary cilia form and sensitivity in cells with impaired cilia. This work demonstrates that modulating the structure of the primary cilium directly impacts cellular mechanosensitivity. Our results implicate cilium length as a potential therapeutic target for combating numerous conditions characterised by impaired cilia function.

Mechanisms for nonmitotic activation of Aurora-A at cilia

Overexpression of the Aurora kinase A (AURKA) is oncogenic in many tumors. Many studies of AURKA have focused on activities of this kinase in mitosis, and elucidated the mechanisms by which AURKA activity is induced at the G2/M boundary through interactions with proteins such as TPX2 and NEDD9. These studies have informed the development of small molecule inhibitors of AURKA, of which a number are currently under preclinical and clinical assessment. While the first activities defined for AURKA were its control of centrosomal maturation and organization of the mitotic spindle, an increasing number of studies over the past decade have recognized a separate biological function of AURKA, in controlling disassembly of the primary cilium, a small organelle protruding from the cell surface that serves as a signaling platform. Importantly, these activities require activation of AURKA in early G1, and the mechanisms of activation are much less well defined than those in mitosis. A better understanding of the control of AURKA activity and the role of AURKA at cilia are both important in optimizing the efficacy and interpreting potential downstream consequences of AURKA inhibitors in the clinic. We here provide a current overview of proteins and mechanisms that have been defined as activating AURKA in G1, based on the study of ciliary disassembly.

Dynamic Remodeling of Membrane Composition Drives Cell Cycle through Primary Cilia Excision

The life cycle of a primary cilium begins in quiescence and ends prior to mitosis. In quiescent cells, the primary cilium insulates itself from contiguous dynamic membrane processes on the cell surface to function as a stable signaling apparatus. Here, we demonstrate that basal restriction of ciliary structure dynamics is established by the cilia-enriched phosphoinositide 5-phosphatase, Inpp5e. Growth induction displaces ciliary Inpp5e and accumulates phosphatidylinositol 4,5-bisphosphate in distal cilia. This change triggers otherwise-forbidden actin polymerization in primary cilia, which excises cilia tips in a process we call cilia decapitation. While cilia disassembly is traditionally thought to occur solely through resorption, we show that an acute loss of IFT-B through cilia decapitation precedes resorption. Finally, we propose that cilia decapitation induces mitogenic signaling and constitutes a molecular link between the cilia life cycle and cell-division cycle. This newly defined ciliary mechanism may find significance in cell proliferation control during normal development and cancer.