The primary cilium as a signaling nexus for growth plate function and subsequent skeletal development

Skeletal ciliopathy: pathogenesis and related signaling pathways

Cilia are tiny organelles with conserved structures and components in eukaryotic cells. Ciliopathy is a set of diseases resulting from cilium dysfunction classified into first-order and second-order ciliopathy. With the advancement of clinical diagnosis and radiography, numerous skeletal phenotypes, including polydactyly, short limbs, short ribs, scoliosis, a narrow thorax, and numerous anomalies in bone and cartilage, have been discovered in ciliopathies. Mutation in genes encoding cilia core components or other cilia-related molecules have been found in skeletal ciliopathies. Meanwhile, various signaling pathways associated with cilia and skeleton development have been deemed to be significant for the occurrence and progression of diseases. Herein, we review the structure and key components of the cilium and summarize several skeletal ciliopathies with their presumable pathology. We also emphasize the signaling pathways involved in skeletal ciliopathies, which may assist in developing potential therapies for these diseases.

Primary cilia in osteoblasts and osteocytes are required for skeletal development and mechanotransduction

Primary cilia have been involved in the development and mechanosensation of various tissue types, including bone. In this study, we explored the mechanosensory role of primary cilia in bone growth and adaptation by examining two cilia specific genes, IFT88 and MKS5, required for proper cilia assembly and function. To analyze the role of primary cilia in osteoblasts, Osx1-GFP:Cre mice were bred with IFT88 LoxP/LoxP to generate mice with a conditional knockout of primary cilia in osteoblasts. A significant decrease in body weight was observed in both male (p=0.0048) and female (p=0.0374) conditional knockout (cKO) mice compared to the wild type (WT) controls. The femurs of cKO mice were significantly shorter than that of the WT mice of both male (p=0.0003) and female (p=0.0019) groups. Histological analysis revealed a significant difference in MAR (p=0.0005) and BFR/BS (p<0.0001) between female cKO and WT mice. The BFR/BS of male cKO mice was 58.03% lower compared to WT mice. To further investigate the role of primary cilia in osteocytes, Dmp1-8kb-Cre mice were crossed with MKS5 LoxP/LoxP to generate mice with defective cilia in osteocytes. In vivo axial ulnar loading was performed on 16-week-old mice for 3 consecutive days. The right ulnae were loaded for 120 cycles/day at a frequency of 2Hz with a peak force of 2.9N for female mice and 3.2N for male mice. Load-induced bone formation was measured using histomorphometry. The relative values of MS/BS, MAR and BFR/BS (loaded ulnae minus nonloaded ulnae) in male MKS5 cKO mice were decreased by 24.88%, 46.27% and 48.24%, respectively, compared to the controls. In the female groups, the rMS/BS was 52.5% lower, the rMAR was 27.58% lower, and the rBFR/BS was 41.54% lower in MKS5 cKO mice than the WT group. Histological analysis indicated that MKS5 cKO mice showed significantly decreased response to mechanical loading compared to the controls. Taken together, these data highlight a critical role of primary cilia in bone development and mechanotransduction, suggesting that the presence of primary cilia in osteoblasts play an important role in skeletal development, and primary cilia in osteocytes mediate mechanically induced bone formation.

Double-edged role of mechanical stimuli and underlying mechanisms in cartilage tissue engineering

Mechanical stimuli regulate the chondrogenic differentiation of mesenchymal stem cells and the homeostasis of chondrocytes, thus affecting implant success in cartilage tissue engineering. The mechanical microenvironment plays fundamental roles in the maturation and maintenance of natural articular cartilage, and the progression of osteoarthritis Hence, cartilage tissue engineering attempts to mimic this environment in vivo to obtain implants that enable a superior regeneration process. However, the specific type of mechanical loading, its optimal regime, and the underlying molecular mechanisms are still under investigation. First, this review delineates the composition and structure of articular cartilage, indicating that the morphology of chondrocytes and components of the extracellular matrix differ from each other to resist forces in three top-to-bottom overlapping zones. Moreover, results from research experiments and clinical trials focusing on the effect of compression, fluid shear stress, hydrostatic pressure, and osmotic pressure are presented and critically evaluated. As a key direction, the latest advances in mechanisms involved in the transduction of external mechanical signals into biological signals are discussed. These mechanical signals are sensed by receptors in the cell membrane, such as primary cilia, integrins, and ion channels, which next activate downstream pathways. Finally, biomaterials with various modifications to mimic the mechanical properties of natural cartilage and the self-designed bioreactors for experiment in vitro are outlined. An improved understanding of biomechanically driven cartilage tissue engineering and the underlying mechanisms is expected to lead to efficient articular cartilage repair for cartilage degeneration and disease.

Ciliary IFT88 Protects Coordinated Adolescent Growth Plate Ossification From Disruptive Physiological Mechanical Forces

Compared with our understanding of endochondral ossification, much less is known about the coordinated arrest of growth defined by the narrowing and fusion of the cartilaginous growth plate. Throughout the musculoskeletal system, appropriate cell and tissue responses to mechanical force delineate morphogenesis and ensure lifelong health. It remains unclear how mechanical cues are integrated into many biological programs, including those coordinating the ossification of the adolescent growth plate at the cessation of growth. Primary cilia are microtubule-based organelles tuning a range of cell activities, including signaling cascades activated or modulated by extracellular biophysical cues. Cilia have been proposed to directly facilitate cell mechanotransduction. To explore the influence of primary cilia in the mouse adolescent limb, we conditionally targeted the ciliary gene Intraflagellar transport protein 88 (Ift88^fl/fl ) in the juvenile and adolescent skeleton using a cartilage-specific, inducible Cre (AggrecanCreER^T2 Ift88^fl/fl ). Deletion of IFT88 in cartilage, which reduced ciliation in the growth plate, disrupted chondrocyte differentiation, cartilage resorption, and mineralization. These effects were largely restricted to peripheral tibial regions beneath the load-bearing compartments of the knee. These regions were typified by an enlarged population of hypertrophic chondrocytes. Although normal patterns of hedgehog signaling were maintained, targeting IFT88 inhibited hypertrophic chondrocyte VEGF expression and downstream vascular recruitment, osteoclastic activity, and the replacement of cartilage with bone. In control mice, increases to physiological loading also impair ossification in the peripheral growth plate, mimicking the effects of IFT88 deletion. Limb immobilization inhibited changes to VEGF expression and epiphyseal morphology in Ift88cKO mice, indicating the effects of depletion of IFT88 in the adolescent growth plate are mechano-dependent. We propose that during this pivotal phase in adolescent skeletal maturation, ciliary IFT88 protects uniform, coordinated ossification of the growth plate from an otherwise disruptive heterogeneity of physiological mechanical forces. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

The functionality and translatability of neocartilage constructs are improved with the combination of fluid-induced shear stress and bioactive factors

Neocartilage tissue engineering aims to address the shortcomings of current clinical treatments for articular cartilage indications. However, advancement is required toward neocartilage functionality (mechanical and biochemical properties) and translatability (construct size, gross morphology, passage number, cell source, and cell type). Using fluid-induced shear (FIS) stress, a potent mechanical stimulus, over four phases, this work investigates FIS stress' efficacy toward creating large neocartilage derived from highly passaged minipig costal chondrocytes, a species relevant to the preclinical regulatory process. In Phase I, FIS stress application timing was investigated in bovine articular chondrocytes and found to improve the aggregate modulus of neocartilage by 151% over unstimulated controls when stimulated during the maturation stage. In Phase II, FIS stress stimulation was translated from bovine articular chondrocytes to expanded minipig costal chondrocytes, yielding a 46% improvement in aggregate modulus over nonstimulated controls. In Phase III, bioactive factors were combined with FIS stress to improve the shear modulus by 115% over bioactive factor-only controls. The translatability of neocartilage was improved in Phase IV by utilizing highly passaged cells to form constructs more than 9-times larger in the area (11 × 17 mm), yielding an improved aggregate modulus by 134% and a flat morphology compared to free-floating, bioactive factor-only controls. Overall, this study represents a significant step toward generating mechanically robust, large constructs necessary for animal studies, and eventually, human clinical studies.

Discovery and Characterization of Intercondylar Transphyseal Complexes and their Oncological Significance in Transphyseal Extension of Pediatric Osteosarcoma

Objective

To explore whether there exist undiscovered transphyseal vasculature‐canal compound structures in immature femurs and tibias, and reveal their potential oncological impact.

Methods

This investigation was divided into a morphological study and a clinical study. In the morphological part, a new‐identified anatomic structure was investigated by using radiographical, anatomical, and histological methodologies. Twenty‐eight 1‐mm‐slice thickness magnetic resonance images of pediatric knees were generated and 10 pediatric knees were dissected to verify the existence and universality, observe the radiographic and anatomic characteristics, and determined the located region of this structure. Hematoxylin–eosin staining, immunofluorescence, and angiography procedures were performed to illustrate its histological feature, molecular identification, and vascular origination, respectively. In the clinical part, 38 pediatric osteosarcoma patients were enrolled from January 2014 to December 2020. A descriptive clinical study including 13 typical participants was conducted to investigate the oncological significance of this new‐identified structure. Meanwhile, the discrepancy in transphyseal osteosarcoma extension between different physeal regions was evaluated in a cross‐sectional study.

Results

In the morphological study, we discovered a new‐found vasculature‐canal compound structure, intercondylar transphyseal complex (ITC), which originated from the middle genicular vessels, traversed the whole epiphysis, and breached the intact open physis in the immature proximal tibia or distal femur. The components of ITC included the juxta‐articular, epiphyseal, and transphyseal segments of vessels, the canals that traverse the entire epiphysis and physis and enclosed the vessels, vascular foramina on articular facet and foramina‐covered synovium. Depending on the location, ITCs can be divided into three types: femoral ITC, anterior tibial ITC, and posterior tibial ITC. Clinically, the ITC may facilitate intercondylar transphyseal sarcomatous dissemination without damaging the adjacent physeal cartilage. Compared to bilateral condylar physes, more osteosarcomas transgressed the open growth plates through intercondylar regions in which ITC was located (P = 0.022).

Conclusion

As the “gap” on intact open physis, ITC, which is a new‐identified compound structure in intercondylar regions of immature femur or tibia, may promote intercondylar transphyseal tumor extension. Moreover, the identification and characterization of ITC subvert some traditional comprehensions about physis and may provide novel perspectives for pediatric osteosarcomas.

Primary cilia in hard tissue development and diseases

Bone and teeth are hard tissues. Hard tissue diseases have a serious effect on human survival and quality of life. Primary cilia are protrusions on the surfaces of cells. As antennas, they are distributed on the membrane surfaces of almost all mammalian cell types and participate in the development of organs and the maintenance of homeostasis. Mutations in cilium-related genes result in a variety of developmental and even lethal diseases. Patients with multiple ciliary gene mutations present overt changes in the skeletal system, suggesting that primary cilia are involved in hard tissue development and reconstruction. Furthermore, primary cilia act as sensors of external stimuli and regulate bone homeostasis. Specifically, substances are trafficked through primary cilia by intraflagellar transport, which affects key signaling pathways during hard tissue development. In this review, we summarize the roles of primary cilia in long bone development and remodeling from two perspectives: primary cilia signaling and sensory mechanisms. In addition, the cilium-related diseases of hard tissue and the manifestations of mutant cilia in the skeleton and teeth are described. We believe that all the findings will help with the intervention and treatment of related hard tissue genetic diseases.

The role of IFT140 in early bone healing of tooth extraction sockets

OBJECTIVES

Primary cilium is a key organelle of regulating bone development and maintenance. The aim of this study is to investigate whether ciliary intraflagellar transporter protein 140 (IFT140) plays a positive role in extraction socket healing by promoting bone formation.

MATERIALS AND METHODS

A left maxillary first molar extraction model was established using 6-week-old Ift140^flox/flox (Ctrl group) and Ift140^flox/flox , Osx-cre (cKO group) mice. The maxillary bone samples from 1, 2, and 3 weeks were postoperatively evaluated by micro-CT, molecular biology, and histomorphometry analysis. Alveolar bone marrow stromal cells (aBMSCs) from 4-week-old mice were cultured in vitro and tested for proliferation and osteogenic ability.

RESULTS

Ciliated cells were predominantly observed in the early socket healing stage with highly expressed ciliary protein IFT140. Compared with the Ctrl group, the healing of extraction sockets in the cKO group was significantly delayed. The proliferation and osteogenic differentiation ability of aBMSCs were reduced in the cKO group.

CONCLUSION

IFT140 has a facilitating role in the early osteogenesis of extraction socket healing and is involved in regulating the proliferation and osteogenic differentiation of aBMSCs.

Ckip-1 regulates C3H10T1/2 mesenchymal cell proliferation and osteogenic differentiation via Lrp5

Casein kinase-2 interaction protein-1 (Ckip-1) is a negative regulator of bone formation. The identification of novel Ckip-1-related targets and their associated signaling pathways that regulate mesenchymal stem cell (MSC) osteogenic differentiation is required. The present study aimed to evaluate the effects of Ckip-1 knockdown on C3H10T1/2 MSC proliferation and osteogenic differentiation, and to explore the role of the canonical Wnt-signaling receptor Lrp5. Ckip-1-knockdown (shCkip-1), Ckip-1-overexpression (Ckip-1) and their corresponding control [shCtrl and empty vector (EV), respectively] cell groups were used in the present study. Immunofluorescence localization of Ckip-1 was observed. The expression of the key molecules of the canonical Wnt signaling pathway was examined in C3H10T1/2 cells following osteogenic induction. Moreover, the effects of Lrp5 knockdown in the presence or absence of Ckip-1 knockdown were examined on C3H10T1/2 cell proliferation and osteogenic differentiation. The results indicated an increase in cell proliferation and osteogenic differentiation in the shCkip-1 group compared with the shCtrl group. The expression levels of LDL receptor related protein 5 (Lrp5), lymphoid enhancer binding factor 1 (Lef1) and transcription factor 1 in C3H10T1/2 cells were significantly increased in shCkip-1 cells following 7-day osteoinduction compared with shCtrl cells. Moreover, the involvement of Lrp5 in shCkip-1-induced osteogenic differentiation of C3H10T1/2 cells was further verified. The results indicated that Ckip-1 reduced C3H10T1/2 MSC proliferation and osteogenic differentiation via the canonical Wnt-signaling receptor Lrp5, which is essential for the improvement of bone tissue engineering.

Prx1-expressing cells contributing to fracture repair require primary cilia for complete healing in mice

Bone is a dynamic organ that is continuously modified during development, load-induced adaptation, and fracture repair. Understanding the cellular and molecular mechanisms for natural fracture healing can lead to therapeutics that enhance the quality of newly formed tissue, advance the rate of healing, or replace the need for invasive surgical procedures. Prx1-expressing cells in the periosteum are thought to supply the majority of osteoblasts and chondrocytes in the fracture callus, but the exact mechanisms for this behavior are unknown. The primary cilium is a sensory organelle that is known to mediate several signaling pathways involved in fracture healing and required for Prx1-expressing cells to contribute to juvenile bone development and adult load-induced bone formation. We therefore investigated the role of Prx1-expressing cell primary cilia in fracture repair by developing a mouse model that enabled us to simultaneously track Prx1 lineage cell fate and disrupt Prx1-expressing cell primary cilia in vivo. The cilium KO mice exhibited abnormally large calluses with significantly decreased bone formation and persistent cartilage nodules. Analysis of mRNA expression in the early soft callus revealed downregulation of osteogenesis, Hh signaling, and Wnt signaling, and upregulation of chondrogenesis and angiogenesis. The mutant mice also exhibited decreased Osx and Periostin but increased αSMA and PECAM-1 protein expression in the hard callus. We further used a Gli1LacZ reporter and found that Hh signaling was significantly upregulated in the mutant callus at later stages of healing. Interestingly, altered protein expression and Hh signaling did not correlate with labeled Prx1-lineage cells, suggesting loss of cilia altered Hh signaling non-autonomously. Overall, cilium KO mice demonstrated severely delayed and incomplete fracture healing, and our findings suggest Prx1-expressing cell primary cilia are necessary to tune Hh signaling for proper fracture repair.

A role for primary cilia in coral calcification?

Cilia are evolutionarily conserved organelles that extend from the surface of cells and are found in diverse organisms from protozoans to multicellular organisms. Motile cilia play various biological functions by their beating motion, including mixing fluids and transporting food particles. Non-motile cilia act as sensors that signal cells about their microenvironment. In corals, cilia have been described in some of the cell layers but never in the calcifying epithelium, which is responsible for skeleton formation. In the present study, we used scanning electron microscopy and immunolabelling to investigate the cellular ciliature of the different tissue layers of the coral Stylophora pistillata, with a focus on the calcifying calicoblastic ectoderm. We show that the cilium of the calcifying cells is different from the cilium of the other cell layers. It is much shorter, and more importantly, its base is structurally distinct from the base observed in cilia of the other tissue layers. Based on these structural observations, we conclude that the cilium of the calcifying cells is a primary cilium. From what is known in other organisms, primary cilia are sensors that signal cells about their microenvironment. We discuss the implications of the presence of a primary cilium in the calcifying epithelium for our understanding of the cellular physiology driving coral calcification and its environmental sensitivity.

Exome sequencing analysis identifies frequent oligogenic involvement and FLNB variants in adolescent idiopathic scoliosis

Background

Adolescent idiopathic scoliosis (AIS) is a genetically heterogeneous disease characterised by three-dimensional deformity of the spine in the absence of a congenital spinal anomaly or neurological musculoskeletal disorder. The clinical variability and incomplete penetrance of some genes linked with AIS indicate that this disease constitutes an oligogenic trait.

Objective

We aimed to explore the oligogenic nature of this disease and identify novel AIS genes.

Methods

We analysed rare damaging variants within AIS-associated genes by using exome sequencing in 40 AIS trios and 183 sporadic patients.

Results

Multiple variants within AIS-associated genes were identified in eight AIS trios, and five individuals harboured rare damaging variants in the FLNB gene. The patients showed more frequent oligogenicity than the controls. In the gene-based burden test, the top signal resided in FLNB. In functional studies, we found that the AIS-associated FLNB variants altered the protein’s conformation and subcellular localisation and its interaction with other proteins (TTC26 and OFD1) involved in AIS. The most compelling evidence of an oligogenic basis was that the number of rare damaging variants was recognised as an independent prognostic factor for curve progression in Cox regression analysis.

Conclusion

Our data indicate that AIS is an oligogenic disease and identify FLNB as a susceptibility gene for AIS.

Interactions between Muscle and Bone-Where Physics Meets Biology

Muscle and bone interact via physical forces and secreted osteokines and myokines. Physical forces are generated through gravity, locomotion, exercise, and external devices. Cells sense mechanical strain via adhesion molecules and translate it into biochemical responses, modulating the basic mechanisms of cellular biology such as lineage commitment, tissue formation, and maturation. This may result in the initiation of bone formation, muscle hypertrophy, and the enhanced production of extracellular matrix constituents, adhesion molecules, and cytoskeletal elements. Bone and muscle mass, resistance to strain, and the stiffness of matrix, cells, and tissues are enhanced, influencing fracture resistance and muscle power. This propagates a dynamic and continuous reciprocity of physicochemical interaction. Secreted growth and differentiation factors are important effectors of mutual interaction. The acute effects of exercise induce the secretion of exosomes with cargo molecules that are capable of mediating the endocrine effects between muscle, bone, and the organism. Long-term changes induce adaptations of the respective tissue secretome that maintain adequate homeostatic conditions. Lessons from unloading, microgravity, and disuse teach us that gratuitous tissue is removed or reorganized while immobility and inflammation trigger muscle and bone marrow fatty infiltration and propagate degenerative diseases such as sarcopenia and osteoporosis. Ongoing research will certainly find new therapeutic targets for prevention and treatment.

Primary cilia: Versatile regulator in cartilage development

Cartilage is a connective tissue in the skeletal system and has limited regeneration ability and unique biomechanical reactivity. The growth and development of cartilage can be affected by different physical, chemical and biological factors, such as mechanical stress, inflammation, osmotic pressure, hypoxia and signalling transduction. Primary cilia are multifunctional sensory organelles that regulate diverse signalling transduction and cell activities. They are crucial for the regulation of cartilage development and act in a variety of ways, such as react to mechanical stress, mediate signalling transduction, regulate cartilage‐related diseases progression and affect cartilage tumorigenesis. Therefore, research on primary cilia‐mediated cartilage growth and development is currently extremely popular. This review outlines the role of primary cilia in cartilage development in recent years and elaborates on the potential regulatory mechanisms from different aspects.

Ellis-van Creveld syndrome in a patient from Tanzania

We report an African infant with Ellis-van Creveld (EVC) syndrome. EVC syndrome is a chondral and ectodermal dysplasia with autosomal recessive transmission. The baby presented with polydactyly, short limbs and atrioventricular septal defect, but was withdrawn from clinical follow up for the first year of life. Initial hematological abnormalities could not be explained and normalized later. EVC syndrome was confirmed by genetic analysis that showed two pathogenic mutations in the EVC2 gene, c.653_654del, p.Val218Glyfs*12 in exon 5, and c.2710C>T, p.Gln904* in exon 16. The variant c.653_654del; p.Val218Glyfs*12 in exon 5 has not been described before. Our review of medical literature suggested this is the first molecularly confirmed case of EVC syndrome in sub-Saharan Africa.

Novel functions of the primary cilium in bone disease and cancer

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

Periosteal progenitors contribute to load-induced bone formation in adult mice and require primary cilia to sense mechanical stimulation

BACKGROUND

The fully developed adult skeleton adapts to mechanical forces by generating more bone, usually at the periosteal surface. Progenitor cells in the periosteum are believed to differentiate into bone-forming osteoblasts that contribute to load-induced adult bone formation, but in vivo evidence does not yet exist. Furthermore, the mechanism by which periosteal progenitors might sense physical loading and trigger differentiation is unknown. We propose that periosteal osteochondroprogenitors (OCPs) directly sense mechanical load and differentiate into bone-forming osteoblasts via their primary cilia, mechanosensory organelles known to be involved in osteogenic differentiation.

METHODS

We generated a diphtheria toxin ablation mouse model and performed ulnar loading and dynamic histomorphometry to quantify the contribution of periosteal OCPs in adult bone formation in vivo. We also generated a primary cilium knockout model and isolated periosteal cells to study the role of the cilium in periosteal OCP mechanosensing in vitro. Experimental groups were compared using one-way analysis of variance or student's t test, and sample size was determined to achieve a minimum power of 80%.

RESULTS

Mice without periosteal OCPs had severely attenuated mechanically induced bone formation and lacked the mineralization necessary for daily skeletal maintenance. Our in vitro results demonstrate that OCPs in the periosteum uniquely sense fluid shear and exhibit changes in osteogenic markers consistent with osteoblast differentiation; however, this response is essentially lost when the primary cilium is absent.

CONCLUSIONS

Combined, our data show that periosteal progenitors are a mechanosensitive cell source that significantly contribute to adult skeletal maintenance. More importantly, an OCP population persists in the adult skeleton and these cells, as well as their cilia, are promising targets for bone regeneration strategies.

The primary cilium - once a "rudimentary" organelle that is now a ubiquitous sensory cellular structure involved in many pathological disorders

This article looks mostly at the steps that have led to the primary cilium finding its place in our understanding of cell biology, developmental biology, and medical syndromes due to its aberrations. It is a personal account that stresses, if nothing else, the value of the adage "stick to your guns". My obsession with this organelle, following on from fascination with the centriole, has led to a whole career devoted to determining the nature and role of primary cilia in basic cell biology, which has proved much more important than had been appreciated for almost a century. They are heavily involved in very many aspects of cell physiology that have much wider implications with regard to human biology and probably throughout the animal kingdom. That aberrations, to the surprise of many researchers in their structure or functioning has led to their being implicated or perhaps deeply involved in an extraordinary range of medical conditions. This invitation allows me to raise crucial questions that need answers regarding the regulation of their genesis, their cache of both intracellular and extracellular signal, and their association with a multitude of development processes from embryo to adult status.

A Murine Model for Human ECO Syndrome Reveals a Critical Role of Intestinal Cell Kinase in Skeletal Development

An autosomal-recessive inactivating mutation R272Q in the human intestinal cell kinase (ICK) gene caused profound multiplex developmental defects in human endocrine-cerebro-osteodysplasia (ECO) syndrome. ECO patients exhibited a wide variety of skeletal abnormalities, yet the underlying mechanisms by which ICK regulates skeletal development remained largely unknown. The goal of this study was to understand the structural and mechanistic basis underlying skeletal anomalies caused by ICK dysfunction. Ick R272Q knock-in transgenic mouse model not only recapitulated major ECO skeletal defects such as short limbs and polydactyly but also revealed a deformed spine with defective intervertebral disk. Loss of ICK function markedly reduced mineralization in the spinal column, ribs, and long bones. Ick mutants showed a significant decrease in the proliferation zone of long bones and the number of type X collagen-expressing hypertrophic chondrocytes in the spinal column and the growth plate of long bones. These results implicate that ICK plays an important role in bone and cartilage development by promoting chondrocyte proliferation and maturation. Our findings provided new mechanistic insights into the skeletal phenotype of human ECO and ECO-like syndromes.