Analysis of the orientation of primary cilia in growth plate cartilage: a mathematical method based on multiphoton microscopical images

Modelling acute myeloid leukemia (AML): What's new? A transition from the classical to the modern

Acute myeloid leukemia (AML) is a heterogeneous malignancy affecting myeloid cells in the bone marrow (BM) but can spread giving rise to impaired hematopoiesis. AML incidence increases with age and is associated with poor prognostic outcomes. There has been a disconnect between the success of novel drug compounds observed in preclinical studies of hematological malignancy and less than exceptional therapeutic responses in clinical trials. This review aims to provide a state-of-the-art overview on the different preclinical models of AML available to expand insights into disease pathology and as preclinical screening tools. Deciphering the complex physiological and pathological processes and developing predictive preclinical models are key to understanding disease progression and fundamental in the development and testing of new effective drug treatments. Standard scaffold-free suspension models fail to recapitulate the complex environment where AML occurs. To this end, we review advances in scaffold/matrix-based 3D models and outline the most recent advances in on-chip technology. We also provide an overview of clinically relevant animal models and review the expanding use of patient-derived samples, which offer the prospect to create more "patient specific" screening tools either in the guise of 3D matrix models, microphysiological "organ-on-chip" tools or xenograft models and discuss representative examples.

Chondrocyte-specific response to stiffness-mediated primary cilia formation and centriole positioning

Mechanical stress and the stiffness of the extracellular matrix are key drivers of tissue development and homeostasis. Aberrant mechanosensation is associated with a wide range of pathologies, including osteoarthritis. Matrix (or substrate) stiffness plays a major role in cell spreading, adhesion, proliferation and differentiation. However, how specific cells sense substrate stiffness still remains unclude. The primary cilium is an essential cellular organelle that senses and integrates mechanical and chemical signals from the extracellular environment. We hypothesised that the primary cilium dynamically alters its length and position to fine-tune cell mechanosignalling based on substrate stiffness alone. We used a hydrogel system of varying substrate stiffness to examine the role of stiffness on cilia frequency, length and centriole position as well as cell and nuclei area over time. Contrary to other cell types, we show that chondrocyte primary cilia shorten on softer substrates demonstrating tissue-specific mechanosensing which is aligned with the tissue stiffness the cells originate from. We further show that stiffness determines centriole positioning to either the basal or apical membrane during attachment and spreading, with centriole positioned towards the basal membrane on stiffer substrates. These phenomena are mediated by force generation actin-myosin stress fibres in a time-dependent manner. Finally we show on stiff substrates, that primary cilia are involved in tension-mediated cell spreading. We propose that substrate stiffness plays a role in cilia positioning, regulating cellular responses to external forces, and may be a key driver of mechanosignalling-associated diseases.

Physiological and Pathophysiological Aspects of Primary Cilia-A Literature Review with View on Functional and Structural Relationships in Cartilage

Cilia are cellular organelles that project from the cell. They occur in nearly all non-hematopoietic tissues and have different functions in different tissues. In mesenchymal tissues primary cilia play a crucial role in the adequate morphogenesis during embryological development. In mature articular cartilage, primary cilia fulfil chemo- and mechanosensitive functions to adapt the cellular mechanisms on extracellular changes and thus, maintain tissue homeostasis and morphometry. Ciliary abnormalities in osteoarthritic cartilage could represent pathophysiological relationships between ciliary dysfunction and tissue deformation. Nevertheless, the molecular and pathophysiological relationships of ‘Primary Cilia’ (PC) in the context of osteoarthritis is not yet fully understood. The present review focuses on the current knowledge about PC and provide a short but not exhaustive overview of their role in cartilage.

Primary cilia are necessary for Prx1-expressing cells to contribute to postnatal skeletogenesis

Although Prx1 (also known as PRRX1)-expressing cells and their primary cilia are critical for embryonic development, they have yet to be studied in the context of postnatal skeletogenesis owing to the lethality of mouse models. A tamoxifen-inducible Prx1 model has been developed, and we determined that expression directed by this promoter is highly restricted to the cambium layers in the periosteum and perichondrium after birth. To determine the postnatal role of these cambium layer osteochondroprogenitors (CLOPs) and their primary cilia, we developed models to track the fate of CLOPs (Prx1CreER-GFP;Rosa26^tdTomato) and selectively disrupt their cilia (Prx1CreER-GFP;Ift88^fl/fl). Our tracking studies revealed that CLOPs populate cortical and trabecular bone, the growth plate and secondary ossification centers during the normal program of postnatal skeletogenesis. Furthermore, animals lacking CLOP cilia exhibit stunted limb growth due to disruptions in endochondral and intramembranous ossification. Histological examination indicates that growth is stunted due to limited differentiation, proliferation and/or abnormal hypertrophic differentiation in the growth plate. Collectively, our results suggest that CLOPs are programmed to rapidly populate distant tissues and produce bone via a primary cilium-mediated mechanism in the postnatal skeleton.

Key developments that impacted the field of mechanobiology and mechanotransduction

Advances in mechanobiology have evolved through insights from multiple disciplines including structural engineering, biomechanics, vascular biology, and orthopaedics. In this paper, we reviewed the impact of key reports related to the study of applied loads on tissues and cells and the resulting signal transduction pathways. We addressed how technology has helped advance the burgeoning field of mechanobiology (over 33,600 publications from 1970 to 2016). We analyzed the impact of critical ideas and then determined how these concepts influenced the mechanobiology field by looking at the citation frequency of these reports as well as tracking how the overall number of citations within the field changed over time. These data allowed us to understand how a key publication, idea, or technology guided or enabled the field. Initial observations of how forces acted on bone and soft tissues stimulated the development of computational solutions defining how forces affect tissue modeling and remodeling. Enabling technologies, such as cell and tissue stretching, compression, and shear stress devices, allowed more researchers to explore how deformation and fluid flow affect cells. Observation of the cell as a tensegrity structure and advanced methods to study genetic regulation in cells further advanced knowledge of specific mechanisms of mechanotransduction. The future of the field will involve developing gene and drug therapies to simulate or augment beneficial load regimens in patients and in mechanically conditioning organs for implantation. Here, we addressed a history of the field, but we limited our discussions to advances in musculoskeletal mechanobiology, primarily in bone, tendon, and ligament tissues. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:605-619, 2018.

Biological and Chemical Removal of Primary Cilia Affects Mechanical Activation of Chondrogenesis Markers in Chondroprogenitors and Hypertrophic Chondrocytes

Chondroprogenitors and hypertrophic chondrocytes, which are the first and last stages of the chondrocyte differentiation process, respectively, are sensitive to mechanical signals. We hypothesize that the mechanical sensitivity of these cells depends on the cell surface primary cilia. To test this hypothesis, we removed the primary cilia by biological means with transfection with intraflagellar transport protein 88 (IFT88) siRNA or by chemical means with chloral hydrate treatment. Transfection of IFT88 siRNA significantly reduced the percentage of ciliated cells in both chondroprogenitor ATDC5 cells as well as primary hypertrophic chondrocytes. Cyclic loading (1 Hz, 10% matrix deformation) of ATDC5 cells in three-dimensional (3D) culture stimulates the mRNA levels of chondrogenesis marker Type II collagen (Col II), hypertrophic chondrocyte marker Type X collagen (Col X), and a molecular regulator of chondrogenesis and chondrocyte hypertrophy bone morphogenetic protein 2 (BMP-2). The reduction of ciliated chondroprogenitors abolishes mechanical stimulation of Col II, Col X, and BMP-2. In contrast, cyclic loading stimulates Col X mRNA levels in hypertrophic chondrocytes, but not those of Col II and BMP-2. Both biological and chemical reduction of ciliated hypertrophic chondrocytes reduced but failed to abolish mechanical stimulation of Col X mRNA levels. Thus, primary cilia play a major role in mechanical stimulation of chondrogenesis and chondrocyte hypertrophy in chondroprogenitor cells and at least a partial role in hypertrophic chondrocytes.

Function and regulation of primary cilia and intraflagellar transport proteins in the skeleton

Primary cilia are microtubule-based organelles that project from the cell surface to enable transduction of various developmental signaling pathways. The process of intraflagellar transport (IFT) is crucial for the building and maintenance of primary cilia. Ciliary dysfunction has been found in a range of disorders called ciliopathies, some of which display severe skeletal dysplasias. In recent years, interest has grown in uncovering the function of primary cilia/IFT proteins in bone development, mechanotransduction, and cellular regulation. We summarize recent advances in understanding the function of cilia and IFT proteins in the regulation of cell differentiation in osteoblasts, osteocytes, chondrocytes, and mesenchymal stem cells (MSCs). We also discuss the mechanosensory function of cilia and IFT proteins in bone cells, cilia orientation, and other functions of cilia in chondrocytes.

Automated Cell Detection and Morphometry on Growth Plate Images of Mouse Bone

Microscopy imaging of mouse growth plates is extensively used in biology to understand the effect of specific molecules on various stages of normal bone development and on bone disease. Until now, such image analysis has been conducted by manual detection. In fact, when existing automated detection techniques were applied, morphological variations across the growth plate and heterogeneity of image background color, including the faint presence of cells (chondrocytes) located deeper in tissue away from the image's plane of focus, and lack of cell-specific features, interfered with identification of cell. We propose the first method of automated detection and morphometry applicable to images of cells in the growth plate of long bone. Through ad hoc sequential application of the Retinex method, anisotropic diffusion and thresholding, our new cell detection algorithm (CDA) addresses these challenges on bright-field microscopy images of mouse growth plates. Five parameters, chosen by the user in respect of image characteristics, regulate our CDA. Our results demonstrate effectiveness of the proposed numerical method relative to manual methods. Our CDA confirms previously established results regarding chondrocytes' number, area, orientation, height and shape of normal growth plates. Our CDA also confirms differences previously found between the genetic mutated mouse Smad1/5^CKO and its control mouse on fluorescence images. The CDA aims to aid biomedical research by increasing efficiency and consistency of data collection regarding arrangement and characteristics of chondrocytes. Our results suggest that automated extraction of data from microscopy imaging of growth plates can assist in unlocking information on normal and pathological development, key to the underlying biological mechanisms of bone growth.

The extracellular matrix and ciliary signaling

The primary cilium protrudes like an antenna from the cell surface, sensing mechanical and chemical cues provided in the cellular environment. In some tissue types, ciliary orientation to lumens allows response to fluid flow; in others, such as bone, ciliary protrusion into the extracellular matrix allows response to compression forces. The ciliary membrane contains receptors for Hedgehog, Wnt, Notch, and other potent growth factors, and in some instances also harbors integrin and cadherin family members, allowing receipt of a robust range of signals. A growing list of ciliopathies, arising from deficient formation or function of cilia, includes both developmental defects and chronic, progressive disorders such as polycystic kidney disease (PKD); changes in ciliary function have been proposed to support cancer progression. Recent findings have revealed extensive signaling dialog between cilia and extracellular matrix (ECM), with defects in cilia associated with fibrosis in multiple contexts. Further, a growing number of proteins have been determined to possess multiple roles in control of cilia and focal adhesion interactions with the ECM, further coordinating functionality. We summarize and discuss these recent findings.

Development of a method for the measurement of primary cilia length in 3D

Background

Primary cilia length is an important measure of cell and tissue function. While accurate length measurements can be calculated from cells in 2D culture, measurements in tissue or 3D culture are inherently difficult due to optical distortions. This study uses a novel combination of image processing techniques to rectify optical distortions and accurately measure cilia length from 3D images.

Methods

Point spread functions and experimental resolutions were calculated from subresolution microspheres embedded in 3D agarose gels for both wide-field fluorescence and confocal laser scanning microscopes. The degree of axial smearing and spherical aberration was calculated from xy:xz diameter ratios of 3D image data sets of 4 μm microspheres that had undergone deconvolution and/or Gaussian blurring. Custom-made 18 and 50 μm fluorescent microfibers were also used as calibration objects to test the suitability of processed image sets for 3D skeletonization. Microfiber length in 2D was first measured to establish an original population mean. Fibers were then embedded in 3D agarose gels to act as ciliary models. 3D image sets of microfibers underwent deconvolution and Gaussian blurring. Length measurements within 1 standard deviation of the original 2D population mean were deemed accurate. Finally, the combined method of deconvolution, Gaussian blurring and skeletonization was compared to previously published methods using images of immunofluorescently labeled renal and chondrocyte primary cilia.

Results

Deconvolution significantly improved contrast and resolution but did not restore the xy:xz diameter ratio (0.80). Only the additional step of Gaussian blurring equalized xy and xz resolutions and yielded a diameter ratio of 1.02. Following image processing, skeletonization successfully estimated microfiber boundaries and allowed reliable and repeatable measurement of fiber lengths in 3D. We also found that the previously published method of calculating length from 2D maximum projection images significantly underestimated ciliary length.

Conclusions

This study used commercial and public domain image processing software to rectify a long-standing problem of 3D microscopy. We have shown that a combination of deconvolution and Gaussian blurring rectifies optical distortions inherent in 3D images and allows accurate skeletonization and length measurement of microfibers and primary cilia that are bent or curved in 3D space.

Modulation of Wnt and Hedgehog signaling pathways is linked to retinoic acid-induced amelioration of chronic allograft dysfunction

Chronic renal allograft damage (CAD) is manifested by a smoldering inflammatory process that leads to transplant glomerulopathy, diffuse interstitial fibrosis and tubular atrophy with loss of tubular structures. Using a Fischer 344 (RT1lvl) to Lewis (RT1l) rat renal allograft model, transcriptomic profiling and pathway mapping, we have previously shown that dynamic dysregulation of the Wnt signaling pathways may underlie progressive CAD. Retinoic acid, an important regulator of differentiation during vertebrate embryogenesis, can moderate the damage observed in this experimental model of CAD. We show here that subsets of the Hedgehog (Hh) and canonical Wnt signaling pathways are linked to the pathophysiology of progressive fibrosis, loss of cilia in epithelia and chronic dysfunction. Oral treatment with 13cis retinoic acid (13cRA) was found to selectively ameliorate the dysregulation of the Hh and canonical Wnt pathways associated with CAD, and lead to a general preservation of cilial structures. Interplay between these pathways helps explain the therapeutic effects of retinoic acid treatment in CAD, and suggests future targets for moderating chronic fibrosing organ damage.

Axonemal positioning and orientation in three-dimensional space for primary cilia: what is known, what is assumed, and what needs clarification

Two positional characteristics of the ciliary axoneme--its location on the plasma membrane as it emerges from the cell, and its orientation in three-dimensional (3D) space--are known to be critical for optimal function of actively motile cilia (including nodal cilia), as well as for modified cilia associated with special senses. However, these positional characteristics have not been analyzed to any significant extent for primary cilia. This review briefly summarizes the history of knowledge of these two positional characteristics across a wide spectrum of cilia, emphasizing their importance for proper function. Then the review focuses what is known about these same positional characteristics for primary cilia in all major tissue types where they have been reported. The review emphasizes major areas that would be productive for future research for understanding how positioning and 3D orientation of primary cilia may be related to their hypothesized signaling roles within different cellular populations.

Effect of localization, length and orientation of chondrocytic primary cilium on murine growth plate organization

The research investigates the role of the immotile chondrocytic primary cilium in the growth plate. This study was motivated by (i) the recent evidence of the mechano-sensorial function of the primary cilium in kidney tubule epithelial cells and (ii) the distinct three-dimensional orientation patterns that the chondrocytic primary cilium forms in articular cartilage in the presence or the absence of loading. For our investigation, we used the Smad1/5(CKO) mutant mouse, whose disorganized growth plate is due to the conditional deletion of Smad 1 and 5 proteins that also affect the so-called Indian Hedgehog pathway, whose physical and functional topography has been shown to be partially controlled by the primary cilium. Fluorescence and confocal microscopy on stained sections visualized ciliated chondrocytes. Morphometric data regarding position, orientation and eccentricity of chondrocytes, and ciliary localization on cell membrane, length and orientation, were collected and reconstructed from images. We established that both localization and orientation of the cilium are definite, and differently so, in the Smad1/5(CKO) and control mice. The orientation of the primary cilium, relative to the major axis of the chondrocyte, clusters at 80° with respect to the anterior-posterior direction for the Smad1/5(CKO) mice, showing loss of the additional clustering present in the control mice at 10°. We therefore hypothesized that the clustering at 10° contains information of columnar organization. To test our hypothesis, we prepared a mathematical model of relative positioning of the proliferative chondrocytic population based on ciliary orientation. Our model belongs to the category of "interactive particle system models for self-organization with birth". The model qualitatively reproduced the experimentally observed chondrocytic arrangements in growth plate of each of the Smad1/5(CKO) and control mice. Our mathematically predicted cell division process will need to be observed experimentally to advance the identification of ciliary function in the growth plate.

Orientation of primary cilia of articular chondrocytes in three-dimensional space

Primary cilia have functions as sensory organelles integral to signal transduction and establishment of cell polarity. In articular cartilage the primary cilium has been hypothesized to function as an antenna to sense the biomechanical environment, regulate the secretion of extracellular matrix components, and maintain cellular positional information, leading to high tissue anisotropy. We used analysis of electron microscopy serial sections to demonstrate positional attributes of the primary cilium of adult equine articular chondrocytes in situ. Data for ~500 axonemes, comparing superficial to radiate chondrocytes from both load-bearing and non-load-bearing regions, were graphed using spherical co-ordinates θ, φ. The data demonstrate the axoneme has a definable orientation in 3D space differing in superficial and radiate zone chondrocytes, cells that differ by 90° in the orientation of their major axes to the articular surface. Axonemal orientation is more definable in load-bearing than in non-load-bearing areas. The position of emergence of the axoneme from the cell also is variable. In load-bearing regions of the superficial zone, extension of the axoneme is from the cellular side facing the subchondral bone. In radiate zone cells, axonemes extend from either face of the chondrocyte, that is, both toward the articular surface or toward the subchondral bone. In non-load-bearing regions this consistency is lost. These observations relate to current hypotheses concerning establishment of tissue anisotropy in articular cartilage during development, involving both migration of cells from the joint periphery and a restricted zone of division within the tissue resulting in the columnar arrangement of radiate zone cells.

Ofd1 is required in limb bud patterning and endochondral bone development

Oral-facial-digital type I (OFDI) syndrome is an X-linked male lethal developmental disorder. It is ascribed to ciliary dysfunction and characterized by malformation of the face, oral cavity, and digits. Conditional inactivation using different Cre lines allowed us to study the role of the Ofd1 transcript in limb development. Immunofluorescence and ultrastructural studies showed that Ofd1 is necessary for correct ciliogenesis in the limb bud but not for cilia outgrowth, in contrast to what was previously shown for the embryonic node. Mutants with mesenchymal Ofd1 inactivation display severe polydactyly with loss of antero-posterior (A/P) digit patterning and shortened long bones. Loss of digit identity was found to be associated with a progressive loss of Shh signaling and an impaired processing of Gli3, whereas defects in limb outgrowth were due to defective Ihh signaling and to mineralization defects during endochondral bone formation. Our data demonstrate that Ofd1 plays a role in regulating digit number and identity during limb and skeletal patterning increasing insight on the functional role of primary cilia during development.

Primary cilia organization reflects polarity in the growth plate and implies loss of polarity and mosaicism in osteochondroma

Primary cilia are specialized cell surface projections found on most cell types. Involved in several signaling pathways, primary cilia have been reported to modulate cell and tissue organization. Although they have been implicated in regulating cartilage and bone growth, little is known about the organization of primary cilia in the growth plate cartilage and osteochondroma. Osteochondromas are bone tumors formed along the growth plate, and they are caused by mutations in EXT1 or EXT2 genes. In this study, we show the organization of primary cilia within and between the zones of the growth plate and osteochondroma. Using confocal and electron microscopy, we found that in both tissues, primary cilia have a similar formation but a distinct organization. The shortest ciliary length is associated with the proliferative state of the cells, as confirmed by Ki-67 immunostaining. Primary cilia organization in the growth plate showed that non-polarized chondrocytes (resting zone) are becoming polarized (proliferating and hypertrophic zones), orienting the primary cilia parallel to the longitudinal axis of the bone. The alignment of primary cilia forms one virtual axis that crosses the center of the columns of chondrocytes reflecting the polarity axis of the growth plate. We also show that primary cilia in osteochondromas are found randomly located on the cell surface. Strikingly, the growth plate-like polarity was retained in sub-populations of osteochondroma cells that were organized into small columns. Based on this, we propose the existence of a mixture ('mosaic') of normal lining (EXT(+/-) or EXT(wt/wt)) and EXT(-/-) cells in the cartilaginous cap of osteochondromas.

Primary cilia are highly oriented with respect to collagen direction and long axis of extensor tendon

Skeletal tissues adapt to their mechanical environments by modulating gene expression, cell metabolism, and extracellular matrix (ECM) architecture; however, the mechanosensory mechanisms for these processes are incompletely understood. Primary cilia have emerged as critical components of the cellular mechanosensory apparatus and have been hypothesized to participate in establishment of cellular and ECM orientation, but their function in skeletal tissues is just beginning to be examined. Here we focused on tendon, a tissue with an oriented matrix that is ideal for analysis of spatial relationships between primary cilia and the ECM. The objective of this study was to characterize the incidence and orientation of tenocyte primary cilia in their native ECM. Primary cilia, nuclei, and collagen were analyzed three-dimensionally in immunofluorescently labeled rat extensor tendon using multiphoton microscopy and semiautomated morphometry. Primary cilia were observed in 64% of tenocytes. The cilia were highly oriented with respect to the ECM: cilia were aligned parallel to the collagen fibers and the long axis of the tendon. This study represents the first quantification of the in situ incidence and orientation of primary cilia in tendon.

Analyzing primary cilia by multiphoton microscopy

In this chapter, a technique is outlined for the use of immunohistochemistry (IHC) followed by multiphoton microscopy (MPM) for the analysis of incidence, length, and 3D orientation of the axoneme of the primary cilium. Although the application presented specifically emphasizes localizations in tenocytes and chondrocytes, the technique is applicable to cells in a wide range of connective tissues. The primary advantages of utilizing MPM as opposed to TEM for these kinds of ciliary analyses are the rapidity of the technique for preparation of the samples and the ability to collect data from multiple cells simultaneously. Using MPM, the axoneme, basal body, and associated centriole can be visualized by specific IHC with localizing antibodies. However, the resolution achieved through TEM analyses allows the complex morphology of the primary cilium to be visualized, and this remains the primary advantage of TEM versus MPM. SHG, which occurs only with MPM, allows visualization of collagen fibrils and is particularly advantageous for localizing primary cilia associated with cells in connective tissues. This, and the deep penetration with less photobleaching, are the primary advantages of MPM compared to confocal microscopy. As with any microscopical technique, the protocol needs to be optimized for any given tissue. In particular, additional antigen retrieval techniques to enhance the unmasking of specific epitopes for antibody binding may be required for adaptation of this approach to other dense connective tissues with complex spatial organizations such as intervertebral disc or meniscus.

Primary Cilia of Odontoblasts: Possible Role in Molar Morphogenesis

A primary cilium, a sensory organelle present in almost every vertebrate cell, is regularly described in odontoblasts, projecting from the surfaces of the cells. Based on the hypothesis that the primary cilium is crucial both for dentin formation and possibly in tooth pain transmission, we have investigated the expression and localization of the main cilium components and involvement of the OFD1 gene in tooth morphogenesis. Odontoblasts in vitro express tubulin, inversin, rootletin, OFD1, BBS4, BBS6, ALMS1, KIF3A, PC1, and PC2. In vivo, cilia are aligned parallel to the dentin walls, with the top part oriented toward the pulp core. Close relationships between cilium and nerve fibers are evidenced. Calcium channels are concentrated in the vicinity of the basal body. Analysis of these data suggests a putative role of cilia in sensing the microenvironment, probably related to dentin secretion. This hypothesis is enhanced by the huge defects observed on molars from Ofd1 knockout mice, showing undifferentiated dentin-forming cells.

Primary cilia in osteoarthritic chondrocytes: from chondrons to clusters

Osteoarthritis (OA) is a common joint disease characterized by articular cartilage degeneration. The etiology of OA is unknown. Because several previous studies have shown that primary cilia play critical roles in joint development, this study examined the incidence and morphology of primary cilia in chondrocytes during joint degeneration in a naturally occurring bovine model of OA. Primary cilia were detected using antibodies to acetylated alpha-tubulin in normal cartilage as well as in mild and severe OA tissue. In normal cartilage, cilia number and length were lowest in the superficial zone and increased with distance from the articular surface. In OA tissue, the incidence and length of cilia increased at the eroding articulating surface, resulting in an overall increased proportion of cilia. This is the first study to show that primary cilia are present on chondrocytes throughout OA progression and that the overall percentage of ciliated cells within the degenerating cartilage increases with OA severity.

Role of intraflagellar transport and primary cilia in skeletal development

Primary cilia are nonmotile microtubule-based appendages extending from the surface of almost all vertebrate cells. The process of intraflagellar transport (IFT) is responsible for building and maintaining the structure and function of primary cilia. Disruption of Kif3a, a component of the Kinesin-II motor complex, disables anterograde IFT and leads to failure in the formation and maintenance of cilia. Likewise, the absence of IFT88/Tg737/Polaris, a core component of the IFT particle, results in the loss of cilia. Although primary cilia were described on chondrocytes almost 40 years ago, only recently has the functional significance of IFT and cilia in skeletal development been uncovered through the use of mouse models containing mutations or deletions in genes required to make and maintain cilia. Together, the results indicate that primary cilia/IFT are involved in coordinating multiple signaling pathways within the skeleton.

Cilia involvement in patterning and maintenance of the skeleton

Although the expression of cilia on chondrocytes was described over 40 years ago, the importance of this organelle in skeletal development and maintenance has only recently been recognized. Primary cilia are found on most mammalian cells and have been shown to play a role in chemosensation and mechanosensation. A growing number of human pleiotropic syndromes have been shown to be associated with ciliary or basal body dysfunction. Skeletal phenotypes, including alterations in limb patterning, endochondral bone formation, craniofacial development, and dentition, have been described in several of these syndromes. Additional insights into the potential roles and mechanisms of cilia action in the mammalian skeleton have been provided by research in model organisms including mouse and zebrafish. In this article we describe what is currently known about the localization of cilia in the skeleton as well as the roles and underlying molecular mechanisms of cilia in skeletal development.