Fibroblast growth factor receptor influences primary cilium length through an interaction with intestinal cell kinase

Ccrk-Mak/Ick signaling is a ciliary transport regulator essential for retinal photoreceptor survival

Primary cilia are microtubule-based sensory organelles whose dysfunction causes ciliopathies in humans. The formation, function, and maintenance of primary cilia depend crucially on intraflagellar transport (IFT); however, the regulatory mechanisms of IFT at ciliary tips are poorly understood. Here, we identified that the ciliopathy kinase Mak is a ciliary tip-localized IFT regulator that cooperatively acts with the ciliopathy kinase Ick, an IFT regulator. Simultaneous disruption of Mak and Ick resulted in loss of photoreceptor ciliary axonemes and severe retinal degeneration. Gene delivery of Ick and pharmacological inhibition of FGF receptors, Ick negative regulators, ameliorated retinal degeneration in Mak -/- mice. We also identified that Ccrk kinase is an upstream activator of Mak and Ick in retinal photoreceptor cells. Furthermore, the overexpression of Mak, Ick, and Ccrk and pharmacological inhibition of FGF receptors suppressed ciliopathy-related phenotypes caused by cytoplasmic dynein inhibition in cultured cells. Collectively, our results show that the Ccrk-Mak/Ick axis is an IFT regulator essential for retinal photoreceptor maintenance and present activation of Ick as a potential therapeutic approach for retinitis pigmentosa caused by MAK mutations.

detectCilia: An R Package for Automated Detection and Length Measurement of Primary Cilia

Background and objective

The primary cilium is a small protrusion found on most mammalian cells. It acts as a cellular antenna, being involved in various cell signaling pathways. The length of the primary cilium affects its function. To study the impact of physical or chemical stimuli on cilia, their lengths must be determined easily and reproducibly.

Methods

We have developed and evaluated an open-source R package called detectCilia to detect and measure primary cilia automatically. As a case study to demonstrate the capability of our tool, we compared the influence of 4 different cell culture media compositions on the lengths of primary cilia in human chondrocytes. These media compositions include (1) insulin-transferrin-selenium (ITS); (2) ITS and dexamethasone (Dexa); (3) ITS, Dexa, insulin-like growth factor 1 (IGF-1), and transforming growth factor beta 1 (TGF-β1); and (4) fetal bovine serum (FBS).

Results

The assessment of detectCilia included a comparison with 2 similar tools: ACDC (Automated Cilia Detection in Cells) and CiliaQ. Several differences and advantages of our package make it a valuable addition to these tools. In the case study, we have observed variations in the ciliary lengths associated with using different media compositions.

Conclusions

We conclude that detectCilia can automatically and reproducibly detect and measure primary cilia in confocal microscopy images with low false-positive rates without requiring extensive user interaction.

Ciliogenesis-associated Kinase 1 Promotes Breast Cancer Cell Proliferation and Chemoresistance via Phosphorylating ERK1

Ciliogenesis-associated kinase 1 (CILK1) plays a key role in the ciliogenesis and ciliopathies. It remains totally unclear whether CILK1 is involved in tumor progression and therapy resistance. Here, we report that the aberrant high-expression of CILK1 in breast cancer is required for tumor cell proliferation and chemoresistance. Two compounds, CILK1-C30 and CILK1-C28, were identified with selective inhibitory effects towards the Tyr-159/Thr-157 dual-phosphorylation of CILK1, pharmacological inhibition of CILK1 significantly suppressed tumor cell proliferation and overcame chemoresistance in multiple experimental models. Large-scale screen of CILK1 substrates confirmed that the kinase directly phosphorylates ERK1, which is responsible for CILK1-mediated oncogenic function. CILK1 is also indicated to be responsible for the chemoresistance of small-cell lung cancer cells. Our data highlight the importance of CILK1 in cancer, implicating that targeting CILK1/ERK1 might offer therapeutic benefit to cancer patients.

The CPLANE protein Fuzzy regulates ciliogenesis by suppressing actin polymerization at the base of the primary cilium via p190A RhoGAP

The primary cilium decorates most eukaryotic cells and regulates tissue morphogenesis and maintenance. Structural or functional defects of primary cilium result in ciliopathies, congenital human disorders affecting multiple organs. Pathogenic variants in the ciliogenesis and planar cell polarity effectors (CPLANE) genes FUZZY, INTU and WDPCP disturb ciliogenesis, causing severe ciliopathies in humans and mice. Here, we show that the loss of Fuzzy in mice results in defects of primary cilia, accompanied by increased RhoA activity and excessive actin polymerization at the basal body. We discovered that, mechanistically, Fuzzy interacts with and recruits the negative actin regulator ARHGAP35 (also known as p190A RhoGAP) to the basal body. We identified genetic interactions between the two genes and found that a mutant ArhGAP35 allele increases the severity of phenotypic defects observed in Fuzzy-/- mice. Based on our findings, we propose that Fuzzy regulates ciliogenesis by recruiting ARHGAP35 to the basal body, where the latter likely restricts actin polymerization and modifies the actin network. Our study identifies a mechanism whereby CPLANE proteins control both actin polymerization and primary cilium formation.

Illumination of understudied ciliary kinases

Cilia are cellular signaling hubs. Given that human kinases are central regulators of signaling, it is not surprising that kinases are key players in cilia biology. In fact, many kinases modulate ciliogenesis, which is the generation of cilia, and distinct ciliary pathways. Several of these kinases are understudied with few publications dedicated to the interrogation of their function. Recent efforts to develop chemical probes for members of the cyclin-dependent kinase like (CDKL), never in mitosis gene A (NIMA) related kinase (NEK), and tau tubulin kinase (TTBK) families either have delivered or are working toward delivery of high-quality chemical tools to characterize the roles that specific kinases play in ciliary processes. A better understanding of ciliary kinases may shed light on whether modulation of these targets will slow or halt disease onset or progression. For example, both understudied human kinases and some that are more well-studied play important ciliary roles in neurons and have been implicated in neurodevelopmental, neurodegenerative, and other neurological diseases. Similarly, subsets of human ciliary kinases are associated with cancer and oncological pathways. Finally, a group of genetic disorders characterized by defects in cilia called ciliopathies have associated gene mutations that impact kinase activity and function. This review highlights both progress related to the understanding of ciliary kinases as well as in chemical inhibitor development for a subset of these kinases. We emphasize known roles of ciliary kinases in diseases of the brain and malignancies and focus on a subset of poorly characterized kinases that regulate ciliary biology.

Leukemia inhibitory factor shortens primary cilia by upregulating C-C motif chemokine 2 in human neural stem/progenitor cells

Primary cilia on neural stem/progenitor cells (NSPCs) play an important role in determining cell fate, although the regulatory mechanisms involved in the ciliogenesis remain largely unknown. In this study, we analyzed the effect of the leukemia inhibitory factor (LIF) for the primary cilia in immortalized human NSPCs. LIF withdrawal elongated the primary cilia length, whereas the addition of LIF shortened it. Microarray gene expression analysis revealed that differentially expressed genes (DEGs) associated with LIF treatment were related with the multiple cytokine signaling pathways. Among the DEGs, C-C motif chemokine 2 (CCL2) had the highest ranking and its increase in the protein concentration in the NSPCs-conditioned medium after the LIF treatment was confirmed by ELISA. Interestingly, we found that CCL2 was a negative regulator of cilium length, and LIF-induced shortening of primary cilia was antagonized by CCL2-specific antibody, suggesting that LIF could influence cilia length via upregulating CCL2. The shortening effect of LIF and CCL2 on primary cilia was also observed in SH-SY5Y cells. The results of the study suggested that the LIF-CCL2 axis may well be a regulator of NSPCs and its primary cilia length, which could affect multiple cellular processes, including NSPC proliferation and differentiation.

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.

Higher rates of non-skeletal complications and greater healthcare needs in achondroplasia compared to the general UK population: a matched cohort study using the CPRD database

BACKGROUND

The natural history of skeletal complications in achondroplasia (ACH) is well-described. However, it remains unclear how the rates of non-skeletal complications, surgical procedures, healthcare needs and mortality differ between individuals with ACH and the general population. This study aimed to contextualise the extent of these outcomes by comparing event rates across the lifespan, between those with ACH and matched controls in a United Kingdom (UK) population.

METHODS

This retrospective, matched cohort study used data from national UK databases: the Clinical Practice Research Database (CPRD) GOLD from primary care, the secondary care Hospital Episode Statistics (HES) databases and the Office of National Statistics mortality records. ACH cases were identified using disorder-specific Read Codes or International Classification of Diseases 10th Revision codes. For each ACH case, up to four age- and sex-matched controls (defined as those without evidence of skeletal/growth disorders) were included. Event rates per 100 person-years were calculated for a pre-defined set of complications (informed by reviews of existing ACH literature and discussion with clinical authors), healthcare visits and mortality. Rate ratios (RRs) with 95% confidence intervals (CIs) were used to compare case and control cohorts.

RESULTS

541 ACH cases and 2052 controls were identified for the CPRD cohort; of these, 275 cases and 1064 matched controls had linkage to HES data. Approximately twice as many non-skeletal complications were reported among individuals with ACH versus controls (RR [95% CI] 1.80 [1.59-2.03]). Among ACH cases, a U-shaped distribution of complications was observed across age groups, whereby the highest complication rates occurred at < 11 and > 60 years of age. Individuals with ACH had greater needs for medication, GP referrals to specialist care, medical imaging, surgical procedures and healthcare visits versus controls, as well as a mortality rate of almost twice as high.

CONCLUSIONS

Patients with ACH experience high rates of a range of both skeletal and non-skeletal complications across their lifespan. To manage these complications, individuals with ACH have significantly increased healthcare needs compared to the general population. These results underscore the need for more coordinated and multidisciplinary management of people with ACH to improve health outcomes across the lifespan.

Chondrocyte primary cilia lengthening and shortening in response to mediators of osteoarthritis; a role for integrin α1β1 and focal adhesions

Objective

Integrin α1β1 protects against osteoarthritis when it is upregulated in the early stages of disease, however, the mechanism behind this is currently unknown. Hypo-osmotic stress, interleukin-1 (IL-1) and transforming growth factor β (TGFβ) influence chondrocyte signaling and are important mediators of osteoarthritis. Evidence for primary cilia as a signaling hub for these factors and the involvement of the F-actin cytoskeleton in this response is growing. The purpose of this study was to investigate the role of integrin α1β1 in the response of primary cilia and the F-actin cytoskeleton to these osteoarthritic mediators.

Design

Primary cilia length and the number of F-actin peaks were measured in ex vivo wild type and itga1-null chondrocytes in response to hypo-osmotic stress, IL-1, and TGFβ alone or in combination, and with or without focal adhesion kinase inhibitor.

Results

We show that integrin α1β1 and focal adhesions are necessary for cilial lengthening and increases in F-actin peaks with hypo-osmotic stress and IL-1, but are not required for cilial shortening with TGFβ. Furthermore, we established that the chondrocyte primary cilium has a resting length of 2.4 ​μm, a minimum length of 2.1 ​μm corresponding to the thickness of the pericellular matrix, and a maximum length of 3.0 ​μm.

Conclusions

While integrin α1β1 is not necessary for the formation of chondrocyte primary cilia and cilial shortening in response to TGFβ, it is necessary for the mediation of cilial lengthening and the formation of F-actin peaks in response to hypo-osmotic stress and IL-1.

Skeletal diseases caused by mutations in PTH1R show aberrant differentiation of skeletal progenitors due to dysregulation of DEPTOR

Alterations in the balance between skeletogenesis and adipogenesis is a pathogenic feature in multiple skeletal disorders. Clinically, enhanced bone marrow adiposity in bones impairs mobility and increases fracture risk, reducing the quality of life of patients. The molecular mechanism that underlies the balance between skeletogenesis and adipogenesis is not completely understood but alterations in skeletal progenitor cells' differentiation pathway plays a key role. We recently demonstrated that parathyroid hormone (PTH)/PTH-related peptide (PTHrP) control the levels of DEPTOR, an inhibitor of the mechanistic target of rapamycin (mTOR), and that DEPTOR levels are altered in different skeletal diseases. Here, we show that mutations in the PTH receptor-1 (PTH1R) alter the differentiation of skeletal progenitors in two different skeletal genetic disorders and lead to accumulation of fat or cartilage in bones. Mechanistically, DEPTOR controls the subcellular localization of TAZ (transcriptional co-activator with a PDZ-binding domain), a transcriptional regulator that governs skeletal stem cells differentiation into either bone and fat. We show that DEPTOR regulation of TAZ localization is achieved through the control of Dishevelled2 (DVL2) phosphorylation. Depending on nutrient availability, DEPTOR directly interacts with PTH1R to regulate PTH/PTHrP signaling or it forms a complex with TAZ, to prevent its translocation to the nucleus and therefore inhibit its transcriptional activity. Our data point DEPTOR as a key molecule in skeletal progenitor differentiation; its dysregulation under pathologic conditions results in aberrant bone/fat balance.

Pharmacological intervention of the FGF-PTH axis as a potential therapeutic for craniofacial ciliopathies

Ciliopathies represent a disease class characterized by a broad range of phenotypes including polycystic kidneys and skeletal anomalies. Ciliopathic skeletal phenotypes are among the most common and most difficult to treat due to a poor understanding of the pathological mechanisms leading to disease. Using an avian model (talpid2) for a human ciliopathy with both kidney and skeletal anomalies (Orofaciodigital syndrome 14), we identified disruptions in the FGF23-PTH axis that resulted in reduced calcium uptake in the developing mandible and subsequent micrognathia. While pharmacological intervention with the FDA-approved pan-FGFR inhibitor AZD4547 alone rescued expression of the FGF target Sprouty2, it did not significantly rescue micrognathia. In contrast, treatment with a cocktail of AZD4547 and Teriparatide acetate, a PTH agonist and FDA-approved treatment for osteoporosis, resulted in a molecular, cellular, and phenotypic rescue of ciliopathic micrognathia in talpid2 mutants. Together, these data provide novel insight into pathological molecular mechanisms associated with ciliopathic skeletal phenotypes and a potential therapeutic strategy for a pleiotropic disease class with limited to no treatment options.

Expanding horizons of achondroplasia treatment: current options and future developments

Activating mutations in the FGFR3 receptor tyrosine kinase lead to most prevalent form of genetic dwarfism in humans, the achondroplasia. Many features of the complex function of FGFR3 in growing skeleton were characterized, which facilitated identification of therapy targets, and drove progress toward treatment. In August 2021, the vosoritide was approved for treatment of achondroplasia, which is based on a stable variant of the C-natriuretic peptide. Other drugs may soon follow, as several conceptually different inhibitors of FGFR3 signaling progress through clinical trials. Here, we review the current achondroplasia therapeutics, describe their mechanisms, and illuminate motivations leading to their development. We also discuss perspectives of curing achondroplasia, and options for repurposing achondroplasia drugs for dwarfing conditions unrelated to FGFR3.

Theobroma cacao improves bone growth by modulating defective ciliogenesis in a mouse model of achondroplasia

A gain-of-function mutation in the fibroblast growth factor receptor 3 gene (FGFR3) results in achondroplasia (ACH), the most frequent form of dwarfism. Constitutive activation of FGFR3 impairs bone formation and elongation and many signal transduction pathways. Identification of new and relevant compounds targeting the FGFR3 signaling pathway is of broad importance for the treatment of ACH, and natural plant compounds are prime drug candidate sources. Here, we found that the phenolic compound (-)-epicatechin, isolated from Theobroma cacao, effectively inhibited FGFR3's downstream signaling pathways. Transcriptomic analysis in an Fgfr3 mouse model showed that ciliary mRNA expression was modified and influenced significantly by the Indian hedgehog and PKA pathways. (-)-Epicatechin is able to rescue mRNA expression impairments that control both the structural organization of the primary cilium and ciliogenesis-related genes. In femurs isolated from a mouse model (Fgfr3^Y367C/+) of ACH, we showed that (-)-epicatechin eliminated bone growth impairment during 6 days of ex vivo culture. In vivo, we confirmed that daily subcutaneous injections of (-)-epicatechin to Fgfr3^Y367C/+ mice increased bone elongation and rescued the primary cilium defects observed in chondrocytes. This modification to the primary cilia promoted the typical columnar arrangement of flat proliferative chondrocytes and thus enhanced bone elongation. The results of the present proof-of-principle study support (-)-epicatechin as a potential drug for the treatment of ACH.

An updated SYSCILIA gold standard (SCGSv2) of known ciliary genes, revealing the vast progress that has been made in the cilia research field

Cilia are microtubule-based organelles with important functions in motility and sensation. They contribute to a broad spectrum of developmental disorders called ciliopathies and have recently been linked to common conditions such as cancers and congenital heart disease. There has been increasing interest in the biology of cilia and their contribution to disease over the past two decades. In 2013 we published a "Gold Standard" list of genes confirmed to be associated with cilia. This was published as part of the SYSCILIA consortium for systems biology study dissecting the contribution of cilia to human health and disease, and was named the Syscilia Gold Standard (SCGS). Since this publication, interest in cilia and understanding of their functions have continued to grow, and we now present an updated SCGS version 2. This includes an additional 383 genes, more than doubling the size of SCGSv1. We use this dataset to conduct a review of advances in understanding of cilia biology 2013- 2021 and offer perspectives on the future of cilia research. We hope that this continues to be a useful resource for the cilia community.

Loss of Sprouty Produces a Ciliopathic Skeletal Phenotype in Mice Through Upregulation of Hedgehog Signaling

The Sprouty family is a highly conserved group of intracellular modulators of receptor tyrosine kinase (RTK)-signaling pathways, which have been recently linked to primary cilia. Disruptions in the structure and function of primary cilia cause inherited disorders called ciliopathies. We aimed to evaluate Sprouty2 and Sprouty4 gene-dependent alterations of ciliary structure and to focus on the determination of its association with Hedgehog signaling defects in chondrocytes. Analysis of the transgenic mice phenotype with Sprouty2 and Sprouty4 deficiency revealed several defects, including improper endochondral bone formation and digit patterning, or craniofacial and dental abnormalities. Moreover, reduced bone thickness and trabecular bone mass, skull deformities, or chondroma-like lesions were revealed. All these pathologies might be attributed to ciliopathies. Elongation of the ciliary axonemes in embryonic and postnatal growth plate chondrocytes was observed in Sprouty2^-/- and Sprouty2^+/- /Sprouty4^-/- mutants compared with corresponding littermate controls. Also, cilia-dependent Hedgehog signaling was upregulated in Sprouty2/4 mutant animals. Ptch1 and Ihh expression were upregulated in the autopodium and the proximal tibia of Sprouty2^-/- /Sprouty4^-/- mutants. Increased levels of the GLI3 repressor (GLI3R) form were detected in Sprouty2/4 mutant primary fibroblast embryonic cell cultures and tissues. These findings demonstrate that mouse lines deficient in Sprouty proteins manifest phenotypic features resembling ciliopathic phenotypes in multiple aspects and may serve as valuable models to study the association between overactivation of RTK and dysfunction of primary cilia during skeletogenesis. © 2021 American Society for Bone and Mineral Research (ASBMR).

Cilia kinases in skeletal development and homeostasis

Primary cilia are dynamic compartments that regulate multiple aspects of cellular signaling. The production, maintenance, and function of cilia involve more than 1000 genes in mammals, and their mutations disrupt the ciliary signaling which manifests in a plethora of pathological conditions-the ciliopathies. Skeletal ciliopathies are genetic disorders affecting the development and homeostasis of the skeleton, and encompass a broad spectrum of pathologies ranging from isolated polydactyly to lethal syndromic dysplasias. The recent advances in forward genetics allowed for the identification of novel regulators of skeletogenesis, and revealed a growing list of ciliary proteins that are critical for signaling pathways implicated in bone physiology. Among these, a group of protein kinases involved in cilia assembly, maintenance, signaling, and disassembly has emerged. In this review, we summarize the functions of cilia kinases in skeletal development and disease, and discuss the available and upcoming treatment options.

Primary ciliary signaling: links with the cell cycle

Primary cilia are solitary, microtubule-based structures emanating from the surface of most vertebrate cells. Although it is understood that ciliary assembly and disassembly both depend upon and impact cell cycle progression, critical mechanistic details of these links remain unresolved. Accumulating evidence shows that the signaling pathways downstream of receptor tyrosine kinases and lysophosphatidic acid receptors control the dynamics of primary cilia. It has also become clear that primary cilia not only serve as signaling hubs but also regulate the composition of the surrounding membrane, which is likely to affect the response to growth factors. Here, we overview recent advances in understanding the interplay between primary cilia and the cell cycle, with a focus on growth factor signaling pathways.

Oncogenic FGFR Fusions Produce Centrosome and Cilia Defects by Ectopic Signaling

A single primary cilium projects from most vertebrate cells to guide cell fate decisions. A growing list of signaling molecules is found to function through cilia and control ciliogenesis, including the fibroblast growth factor receptors (FGFR). Aberrant FGFR activity produces abnormal cilia with deregulated signaling, which contributes to pathogenesis of the FGFR-mediated genetic disorders. FGFR lesions are also found in cancer, raising a possibility of cilia involvement in the neoplastic transformation and tumor progression. Here, we focus on FGFR gene fusions, and discuss the possible mechanisms by which they function as oncogenic drivers. We show that a substantial portion of the FGFR fusion partners are proteins associated with the centrosome cycle, including organization of the mitotic spindle and ciliogenesis. The functions of centrosome proteins are often lost with the gene fusion, leading to haploinsufficiency that induces cilia loss and deregulated cell division. We speculate that this complements the ectopic FGFR activity and drives the FGFR fusion cancers.

Cell cycle-related kinase is a crucial regulator for ciliogenesis and Hedgehog signaling in embryonic mouse lung development

Cell cycle-related kinase (CCRK) has a conserved role in ciliogenesis, and Ccrk defects in mice lead to developmental defects, including exencephaly, preaxial polydactyly, skeletal abnormalities, retinal degeneration, and polycystic kidney. Here, we found that Ccrk is highly expressed in mouse trachea and bronchioles. Ccrk mutants exhibited pulmonary hypoplasia and abnormal branching morphogenesis in respiratory organ development. Furthermore, we demonstrated that Ccrk mutant lungs exhibit not only impaired branching morphogenesis but also a significant sacculation deficiency in alveoli associated with reduced epithelial progenitor cell proliferation. In pseudoglandular stages, Ccrk mutant lungs showed a downregulation of Hedgehog (Hh) signaling and defects in cilia morphology and frequency during progenitor-cell proliferation. Interestingly, we observed that activation of the Hh signaling pathway by small-molecule smoothened agonist (SAG) partially rescued bud morphology during branch bifurcation in explants from Ccrk mutant lungs. Therefore, CCRK properly regulates respiratory airway architecture in part through Hh-signal transduction and ciliogenesis.

Post-translational modification enzymes as key regulators of ciliary protein trafficking

Primary cilia are evolutionarily conserved microtubule-based organelles that protrude from the surface of almost all cell types and decode a variety of extracellular stimuli. Ciliary dysfunction causes human diseases named ciliopathies, which span a wide range of symptoms, such as developmental and sensory abnormalities. The assembly, disassembly, maintenance and function of cilia rely on protein transport systems including intraflagellar transport (IFT) and lipidated protein intraflagellar targeting (LIFT). IFT is coordinated by three multisubunit protein complexes with molecular motors along the ciliary axoneme, while LIFT is mediated by specific chaperones that directly recognize lipid chains. Recently, it has become clear that several post-translational modification enzymes play crucial roles in the regulation of IFT and LIFT. Here, we review our current understanding of the roles of these post-translational modification enzymes in the regulation of ciliary protein trafficking as well as their regulatory mechanisms, physiological significance and involvement in human diseases.

Novel therapeutic approaches for the treatment of achondroplasia

Achondroplasia is the most common form of human dwarfism. The molecular basis of achondroplasia was elucidated in 1994 with the identification of the fibroblast growth factor receptor 3 (FGFR3) as the causative gene. Missense mutations causing achondroplasia result in activation of FGFR3 and its downstream signaling pathways, disturbing chondrogenesis, osteogenesis, and long bone elongation. A more accurate understanding of the clinical and molecular aspects of achondroplasia has allowed new therapeutic approaches to be developed. These are based on: clear understanding of the natural history of the disease; proof-of-concept preclinical studies in mouse models; and the current state of knowledge regarding FGFR3 and related growth plate homeostatic pathways. This review provides a brief overview of the preclinical mouse models of achondroplasia that have led to new, non-surgical therapeutic strategies being assessed and applied to children with achondroplasia through pioneering clinical trials.

Mutations in GRK2 cause Jeune syndrome by impairing Hedgehog and canonical Wnt signaling

Mutations in genes affecting primary cilia cause ciliopathies, a diverse group of disorders often affecting skeletal development. This includes Jeune syndrome or asphyxiating thoracic dystrophy (ATD), an autosomal recessive skeletal disorder. Unraveling the responsible molecular pathology helps illuminate mechanisms responsible for functional primary cilia. We identified two families with ATD caused by loss-of-function mutations in the gene encoding adrenergic receptor kinase 1 (ADRBK1 or GRK2). GRK2 cells from an affected individual homozygous for the p.R158* mutation resulted in loss of GRK2, and disrupted chondrocyte growth and differentiation in the cartilage growth plate. GRK2 null cells displayed normal cilia morphology, yet loss of GRK2 compromised cilia-based signaling of Hedgehog (Hh) pathway. Canonical Wnt signaling was also impaired, manifested as a failure to respond to Wnt ligand due to impaired phosphorylation of the Wnt co-receptor LRP6. We have identified GRK2 as an essential regulator of skeletogenesis and demonstrate how both Hh and Wnt signaling mechanistically contribute to skeletal ciliopathies.

Elucidation of protein interactions necessary for the maintenance of the BCR-ABL signaling complex

Many patients with chronic myeloid leukemia in deep remission experience return of clinical disease after withdrawal of tyrosine kinase inhibitors (TKIs). This suggests signaling of inactive BCR-ABL, which allows the survival of cancer cells, and relapse. We show that TKI treatment inhibits catalytic activity of BCR-ABL, but does not dissolve BCR-ABL core signaling complex, consisting of CRKL, SHC1, GRB2, SOS1, cCBL, p85a-PI3K, STS1 and SHIP2. Peptide microarray and co-immunoprecipitation results demonstrate that CRKL binds to proline-rich regions located in C-terminal, intrinsically disordered region of BCR-ABL, that SHC1 requires pleckstrin homology, src homology and tyrosine kinase domains of BCR-ABL for binding, and that BCR-ABL sequence motif located in disordered region around phosphorylated tyrosine 177 mediates binding of three core complex members, i.e., GRB2, SOS1, and cCBL. Further, SHIP2 binds to the src homology and tyrosine kinase domains of BCR-ABL and its inositol phosphatase activity contributes to BCR-ABL-mediated phosphorylation of SHC1. Together, this study characterizes protein-protein interactions within the BCR-ABL core complex and determines the contribution of particular BCR-ABL domains to downstream signaling. Understanding the structure and dynamics of BCR-ABL interactome is critical for the development of drugs targeting integrity of the BCR-ABL core complex.

The Primary Cilium as a Therapeutic Target in Ocular Diseases

Primary cilia are microtubule-based cellular structures located on the surfaces of most mammalian cells and play important roles in detecting external stimuli, signal transduction, and cell cycle regulation. Primary cilia are also present in several structures of the eye, and their abnormal development or dysfunction can cause various ocular diseases. The rapid development of proteomics and metabolomics technologies have helped in the identification of many ocular disease-related proteins, some of which are dysregulated in primary cilia. This review focuses on ciliary dysregulation in a number of ocular diseases and discusses the potential of targeting primary cilia in gene and stem cell therapy for these diseases.

Functional Alterations in Ciliogenesis-Associated Kinase 1 (CILK1) that Result from Mutations Linked to Juvenile Myoclonic Epilepsy

Ciliopathies are a group of human genetic disorders associated with mutations that give rise to the dysfunction of primary cilia. Ciliogenesis-associated kinase 1 (CILK1), formerly known as intestinal cell kinase (ICK), is a conserved serine and threonine kinase that restricts primary (non-motile) cilia formation and length. Mutations in CILK1 are associated with ciliopathies and are also linked to juvenile myoclonic epilepsy (JME). However, the effects of the JME-related mutations in CILK1 on kinase activity and CILK1 function are unknown. Here, we report that JME pathogenic mutations in the CILK1 N-terminal kinase domain abolish kinase activity, evidenced by the loss of phosphorylation of kinesin family member 3A (KIF3A) at Thr672, while JME mutations in the C-terminal non-catalytic domain (CTD) have little effect on KIF3A phosphorylation. Although CILK1 variants in the CTD retain catalytic activity, they nonetheless lose the ability to restrict cilia length and also gain function in promoting ciliogenesis. We show that wild type CILK1 predominantly localizes to the base of the primary cilium; in contrast, JME variants of CILK1 are distributed along the entire axoneme of the primary cilium. These results demonstrate that JME pathogenic mutations perturb CILK1 function and intracellular localization. These CILK1 variants affect the primary cilium, independent of CILK1 phosphorylation of KIF3A. Our findings suggest that CILK1 mutations linked to JME result in alterations of primary cilia formation and homeostasis.

Ciliogenesis associated kinase 1: targets and functions in various organ systems

Ciliogenesis associated kinase 1 (CILK1) was previously known as intestinal cell kinase because it was cloned from that origin. However, CILK1 is now recognized as a widely expressed and highly conserved serine/threonine protein kinase. Mutations in the human CILK1 gene have been associated with ciliopathies, a group of human genetic disorders with defects in the primary cilium. In mice, both Cilk1 knock-out and Cilk1 knock-in mutations have recapitulated human ciliopathies. Thus, CILK1 has a fundamental role in the function of the cilium. Several candidate substrates have been proposed for CILK1 and the challenge is to relate these to the mutant phenotypes. In this review, we summarize what is known about CILK1 functions and targets, and discuss gaps in current knowledge that motivate further experimentation to fully understand the role of CILK1 in organ development in humans.

Ciliopathy-Associated Protein Kinase ICK Requires Its Non-Catalytic Carboxyl-Terminal Domain for Regulation of Ciliogenesis

Loss-of-function mutations in the human ICK (intestinal cell kinase) gene cause dysfunctional primary cilia and perinatal lethality which are associated with human ciliopathies. The enzyme that we herein call CAPK (ciliopathy-associated protein kinase) is a serine/threonine protein kinase that has a highly conserved MAPK-like N-terminal catalytic domain and an unstructured C-terminal domain (CTD) whose functions are completely unknown. In this study, we demonstrate that truncation of the CTD impairs the ability of CAPK to interact with and phosphorylate its substrate, kinesin family member 3A (KIF3A). We also find that deletion of the CTD of CAPK compromises both localization to the primary cilium and negative regulation of ciliogenesis. Thus, CAPK substrate recognition, ciliary targeting, and ciliary function depend on the non-catalytic CTD of the protein which is predicted to be intrinsically disordered.