OFD1 and flotillins are integral components of a ciliary signaling protein complex organized by polycystins in renal epithelia and odontoblasts

Recent advances in the understanding of cilia mechanisms and their applications as therapeutic targets

The primary cilium is a single immotile microtubule-based organelle that protrudes into the extracellular space. Malformations and dysfunctions of the cilia have been associated with various forms of syndromic and non-syndromic diseases, termed ciliopathies. The primary cilium is therefore gaining attention due to its potential as a therapeutic target. In this review, we examine ciliary receptors, ciliogenesis, and ciliary trafficking as possible therapeutic targets. We first discuss the mechanisms of selective distribution, signal transduction, and physiological roles of ciliary receptors. Next, pathways that regulate ciliogenesis, specifically the Aurora A kinase, mammalian target of rapamycin, and ubiquitin-proteasome pathways are examined as therapeutic targets to regulate ciliogenesis. Then, in the photoreceptors, the mechanism of ciliary trafficking which takes place at the transition zone involving the ciliary membrane proteins is reviewed. Finally, some of the current therapeutic advancements highlighting the role of large animal models of photoreceptor ciliopathy are discussed.

Hedgehog-Related Mutation Causes Bone Malformations with or without Hereditary Gene Mutations

The hedgehog (Hh) family consists of numerous signaling mediators that play important roles at various stages of development. Thus, the Hh pathway is essential for bone tissue development and tumorigenesis. Gorlin syndrome is a skeletal and tumorigenic disorder caused by gain-of-function mutations in Hh signaling. In this review, we first present the phenotype of Gorlin syndrome and the relationship between genotype and phenotype in bone and craniofacial tissues, including the causative gene as well as other Hh-related genes. Next, the importance of new diagnostic methods using next-generation sequencing and multiple gene panels will be discussed. We summarize Hh-related genetic disorders, including cilia disease, and the genetics of Hh-related bone diseases.

Do habitat and elevation promote hybridization during secondary contact between three genetically distinct groups of warbling vireo (Vireo gilvus)?

Following postglacial expansion, secondary contact can occur between genetically distinct lineages. These genetic lineages may be associated with specific habitat or environmental variables and therefore, their distributions in secondary contact could reflect such conditions within these areas. Here we used mtDNA, microsatellite, and morphological data to study three genetically distinct groups of warbling vireo (Vireo gilvus) and investigate the role that elevation and habitat play in their distributions. We studied two main contact zones and within each contact zone, we examined two separate transects. Across the Great Plains contact zone, we found that hybridization between eastern and western groups occurs along a habitat and elevational gradient, whereas hybridization across the Rocky Mountain contact zone was not as closely associated with habitat or elevation. Hybrids in the Great Plains contact zone were more common in transitional areas between deciduous and mixed-wood forests, and at lower elevations (<1000 m). Hybridization patterns were similar along both Great Plains transects indicating that habitat and elevation play a role in hybridization between distinct eastern and western genetic groups. The observed patterns suggest adaptation to different habitats, perhaps originating during isolation in multiple Pleistocene refugia, is facilitating hybridization in areas where habitat types overlap.

Polycystin-2 Is Required for Chondrocyte Mechanotransduction and Traffics to the Primary Cilium in Response to Mechanical Stimulation

Primary cilia and associated intraflagellar transport are essential for skeletal development, joint homeostasis, and the response to mechanical stimuli, although the mechanisms remain unclear. Polycystin-2 (PC2) is a member of the transient receptor potential polycystic (TRPP) family of cation channels, and together with Polycystin-1 (PC1), it has been implicated in cilia-mediated mechanotransduction in epithelial cells. The current study investigates the effect of mechanical stimulation on the localization of ciliary polycystins in chondrocytes and tests the hypothesis that they are required in chondrocyte mechanosignaling. Isolated chondrocytes were subjected to mechanical stimulation in the form of uniaxial cyclic tensile strain (CTS) in order to examine the effects on PC2 ciliary localization and matrix gene expression. In the absence of strain, PC2 localizes to the chondrocyte ciliary membrane and neither PC1 nor PC2 are required for ciliogenesis. Cartilage matrix gene expression (Acan, Col2a) is increased in response to 10% CTS. This response is inhibited by siRNA-mediated loss of PC1 or PC2 expression. PC2 ciliary localization requires PC1 and is increased in response to CTS. Increased PC2 cilia trafficking is dependent on the activation of transient receptor potential cation channel subfamily V member 4 (TRPV4) activation. Together, these findings demonstrate for the first time that polycystins are required for chondrocyte mechanotransduction and highlight the mechanosensitive cilia trafficking of PC2 as an important component of cilia-mediated mechanotransduction.

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

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

Ciliary exclusion of Polycystin-2 promotes kidney cystogenesis in an autosomal dominant polycystic kidney disease model

The human PKD2 locus encodes Polycystin-2 (PC2), a TRPP channel that localises to several distinct cellular compartments, including the cilium. PKD2 mutations cause Autosomal Dominant Polycystic Kidney Disease (ADPKD) and affect many cellular pathways. Data underlining the importance of ciliary PC2 localisation in preventing PKD are limited because PC2 function is ablated throughout the cell in existing model systems. Here, we dissect the ciliary role of PC2 by analysing mice carrying a non-ciliary localising, yet channel-functional, PC2 mutation. Mutants develop embryonic renal cysts that appear indistinguishable from mice completely lacking PC2. Despite not entering the cilium in mutant cells, mutant PC2 accumulates at the ciliary base, forming a ring pattern consistent with distal appendage localisation. This suggests a two-step model of ciliary entry; PC2 first traffics to the cilium base before TOP domain dependent entry. Our results suggest that PC2 localisation to the cilium is necessary to prevent PKD.

The molecular role of ciliary Polycystin-2 (PC2) in cyst formation and polycystic kidney disease (ADKPD) is unclear. Here, the authors identify a PC2 mutant lacking ciliary localisation but with active Ca2+ channel function in mice, that is sufficient to generate an ADPKD phenotype.

[Current situation of researches on a sensor organelle, primary cilium, to understand the pathogenesis of ciliopathy]

Primary cilium is a membrane-protruding immotile sensory organelle. It had been supposed that the cilium was a static organelle for long periods. However, recent studies have uncovered that the cilium is dynamically organized organelle in a cell cycle-dependent manner; it is formed during G₀/G₁ phase and resorbed when the cells enter cell division cycle. Despite the primary cilium is very short and its surface area is extremely small, the cilium possesses a few kinds of G protein-coupled receptors, growth factor receptors and ion channels. Therefore, it can function as a signaling receptor for selective bioactive ligands and mechanical stresses. Dysregulation of the ciliary dynamics is linked with hereditary disorders, so called "ciliopathy", with clinical manifestations of microcephaly, polycystic kidney, situs inversus, polydactyly, and so on. No effective medical treatment for the ciliopathies has been available. Increasing evidences about the molecular mechanisms of ciliary dynamics and ciliary functions have revealed that enormous number of molecules regulate a cycle of ciliogenesis, cilium-derived signaling, ciliary resorption and elimination. However, it is a fact that research progress is far inferior to the full disclosure of the molecular mechanisms. Further studies are required to clarify the pathogenesis of the cilipathies. Moreover, efficient medical treatments are expected to be developed by pharmacological approaches.

Primary Cilia as Signaling Hubs in Health and Disease

Primary cilia detect extracellular cues and transduce these signals into cells to regulate proliferation, migration, and differentiation. Here, the function of primary cilia as signaling hubs of growth factors and morphogens is in focus. First, the molecular mechanisms regulating the assembly and disassembly of primary cilia are described. Then, the role of primary cilia in mediating growth factor and morphogen signaling to maintain human health and the potential mechanisms by which defects in these pathways contribute to human diseases, such as ciliopathy, obesity, and cancer are described. Furthermore, a novel signaling pathway by which certain growth factors stimulate cell proliferation through suppression of ciliogenesis is also described, suggesting novel therapeutic targets in cancer.

Role of Primary Cilia in Odontogenesis

Primary cilium is a solitary organelle that emanates from the surface of most postmitotic mammalian cells and serves as a sensory organelle, transmitting the mechanical and chemical cues to the cell. Primary cilia are key coordinators of various signaling pathways during development and maintenance of tissue homeostasis. The emerging evidence implicates primary cilia function in tooth development. Primary cilia are located in the dental epithelium and mesenchyme at early stages of tooth development and later during cell differentiation and production of hard tissues. The cilia are present when interactions between both the epithelium and mesenchyme are required for normal morphogenesis. As the primary cilium coordinates several signaling pathways essential for odontogenesis, ciliary defects can interrupt the latter process. Genetic or experimental alterations of cilia function lead to various developmental defects, including supernumerary or missing teeth, enamel and dentin hypoplasia, or teeth crowding. Moreover, dental phenotypes are observed in ciliopathies, including Bardet-Biedl syndrome, Ellis-van Creveld syndrome, Weyers acrofacial dysostosis, cranioectodermal dysplasia, and oral-facial-digital syndrome, altogether demonstrating that primary cilia play a critical role in regulation of both the early odontogenesis and later differentiation of hard tissue-producing cells. Here, we summarize the current evidence for the localization of primary cilia in dental tissues and the impact of disrupted cilia signaling on tooth development in ciliopathies.

Mechanosensor polycystin-1 potentiates differentiation of human osteoblastic cells by upregulating Runx2 expression via induction of JAK2/STAT3 signaling axis

Polycystin-1 (PC1) has been proposed as a chief mechanosensing molecule implicated in skeletogenesis and bone remodeling. Mechanotransduction via PC1 involves proteolytic cleavage of its cytoplasmic tail (CT) and interaction with intracellular pathways and transcription factors to regulate cell function. Here we demonstrate the interaction of PC1-CT with JAK2/STAT3 signaling axis in mechanically stimulated human osteoblastic cells, leading to transcriptional induction of Runx2 gene, a master regulator of osteoblastic differentiation. Primary osteoblast-like PC1-expressing cells subjected to mechanical-stretching exhibited a PC1-dependent increase of the phosphorylated(p)/active form of JAK2. Specific interaction of PC1-CT with pJAK2 was observed after stretching while pre-treatment of cells with PC1 (anti-IgPKD1) and JAK2 inhibitors abolished JAK2 activation. Consistently, mechanostimulation triggered PC1-mediated phosphorylation and nuclear translocation of STAT3. The nuclear phosphorylated(p)/DNA-binding competent pSTAT3 levels were augmented after stretching followed by elevated DNA-binding activity. Pre-treatment with a STAT3 inhibitor either alone or in combination with anti-IgPKD1 abrogated this effect. Moreover, PC1-mediated mechanostimulation induced elevation of Runx2 mRNA levels. ChIP assays revealed direct regulation of Runx2 promoter activity by STAT3/Runx2 after mechanical-stretching that was PC1-dependent. Our findings show that mechanical load upregulates expression of Runx2 gene via potentiation of PC1-JAK2/STAT3 signaling axis, culminating to possibly control osteoblastic differentiation and ultimately bone formation.

OFD1, as a Ciliary Protein, Exhibits Neuroprotective Function in Photoreceptor Degeneration Models

Ofd1 is a newly identified causative gene for Retinitis pigmentosa (RP), a photoreceptor degenerative disease. This study aimed to examine Ofd1 localization in retina and further to investigate its function in photoreceptor degeneration models. Ofd1 localization in rat retina was examined using immunofluorescence. N-methyl-N-nitrosourea (MNU)-induced rats and Royal College of Surgeons (RCS) rats were used as photoreceptor degeneration models. The expression pattern of Ofd1, other ciliary associated genes and Wnt signaling pathway genes were examined in rat models. Furthermore, pEGFP-Ofd1-CDS and pSUPER-Ofd1-shRNA were constructed to overexpress and knockdown the expression level in 661W and R28 cells. MNU was also used to induce cell death. Cilia formation was observed using immunocytochemistry (ICC). Reactive oxygen species (ROS) were detected using the 2', 7'-Dichlorofluorescin diacetate (DCFH-DA) assay. Apoptosis genes expression was examined using qRT-PCR, Western blotting and fluorescence-activated cell sorting (FACS). Ofd1 localized to outer segments of rat retina photoreceptors. Ofd1 and other ciliary proteins expression levels increased from the 1^st and 4^th postnatal weeks and decreased until the 6^th week in the RCS rats, while their expression consistently decreased from the 1^st and 7^th day in the MNU rats. Moreover, Wnt signaling pathway proteins expression was significantly up-regulated in both rat models. Knockdown of Ofd1 expression resulted in a smaller population, shorter length of cell cilia, and lower cell viability. Ofd1 overexpression partially attenuated MNU toxic effects by reducing ROS levels and mitigating apoptosis. To the best of our knowledge, this is the first study demonstrating Ofd1 localization and its function in rat retina and in retinal degeneration rat models. Ofd1 plays a role in controlling photoreceptor cilium length and number. Importantly, it demonstrates a neuroprotective function by protecting the photoreceptor from oxidative stress and apoptosis. These data have expanded our understanding of Ofd1 function beyond cilia, and we concluded that ofd1 neuroprotection could be a potential treatment strategy in retina degeneration models.

Charming neighborhoods on the cell surface: plasma membrane microdomains regulate receptor tyrosine kinase signaling

Receptor tyrosine kinases (RTK) are an important family of growth factor and hormone receptors that regulate many aspects of cellular physiology. Ligand binding by RTKs at the plasma membrane elicits activation of many signaling intermediates. The spatial and temporal regulation of RTK signaling within cells is an important determinant of receptor signaling outcome. In particular, the compartmentalization of the plasma membrane into a number of microdomains allows context-specific control of RTK signaling. Indeed various RTKs are recruited to and enriched within specific plasma membrane microdomains under various conditions, including lipid-ordered domains such as caveolae and lipid rafts, clathrin-coated structures, tetraspanin-enriched microdomains, and actin-dependent protrusive membrane microdomains such as dorsal ruffles and invadosomes. We examine the evidence for control of RTK signaling by each of these plasma membrane microdomains, as well as molecular mechanisms for how this spatial organization controls receptor signaling.

The role of cilia in the pathogenesis of cystic kidney disease

PURPOSE OF REVIEW

Primary (immotile) cilia are specialized organelles present on most cell types. Almost all of proteins associated with a broad spectrum of human cystic kidney diseases have been localized to the region in or around the cilia. Abnormal cilia structure and function have both been reported in animal models and human cystic kidneys. The goal of this review is to discuss current understanding of the mechanisms by which abnormal genes/proteins and cilia interact to potentially influence renal cystogenesis.

RECENT FINDINGS

Novel direct recording of cilia calcium levels/channel activity suggests that cilia form a calcium-mediated signaling microenvironment separate from the cytoplasm, which could provide a mechanism for cilia-specific downstream signaling. Genetic-based studies confirm that cilia are not required for cystogenesis, but modulate cystic kidney disease severity through a novel, undefined mechanism. Mechanisms by which both cilia-associated and noncilia-associated proteins can alter cilia structure/function have also been identified.

SUMMARY

Considerable progress has been made in defining the mechanisms by which abnormal genes and proteins affect cilia structure and function. However, the exact mechanisms by which these interactions cause renal cyst formation and progression of cystic kidney disease are still unknown.