Nearly all cells in vertebrates and many cells in invertebrates contain primary cilia

Effects of Extracellular Osteoanabolic Agents on the Endogenous Response of Osteoblastic Cells

The complex multidimensional skeletal organization can adapt its structure in accordance with external contexts, demonstrating excellent self-renewal capacity. Thus, optimal extracellular environmental properties are critical for bone regeneration and inextricably linked to the mechanical and biological states of bone. It is interesting to note that the microstructure of bone depends not only on genetic determinants (which control the bone remodeling loop through autocrine and paracrine signals) but also, more importantly, on the continuous response of cells to external mechanical cues. In particular, bone cells sense mechanical signals such as shear, tensile, loading and vibration, and once activated, they react by regulating bone anabolism. Although several specific surrounding conditions needed for osteoblast cells to specifically augment bone formation have been empirically discovered, most of the underlying biomechanical cellular processes underneath remain largely unknown. Nevertheless, exogenous stimuli of endogenous osteogenesis can be applied to promote the mineral apposition rate, bone formation, bone mass and bone strength, as well as expediting fracture repair and bone regeneration. The following review summarizes the latest studies related to the proliferation and differentiation of osteoblastic cells, enhanced by mechanical forces or supplemental signaling factors (such as trace metals, nutraceuticals, vitamins and exosomes), providing a thorough overview of the exogenous osteogenic agents which can be exploited to modulate and influence the mechanically induced anabolism of bone. Furthermore, this review aims to discuss the emerging role of extracellular stimuli in skeletal metabolism as well as their potential roles and provide new perspectives for the treatment of bone disorders.

Nek2 kinase displaces distal appendages from the mother centriole prior to mitosis

Distal appendages (DAs) of the mother centriole are essential for the initial steps of ciliogenesis in G1/G0 phase of the cell cycle. DAs are released from centrosomes in mitosis by an undefined mechanism. Here, we show that specific DAs lose their centrosomal localization at the G2/M transition in a manner that relies upon Nek2 kinase activity to ensure low DA levels at mitotic centrosomes. Overexpression of active Nek2A, but not kinase-dead Nek2A, prematurely displaced DAs from the interphase centrosomes of immortalized retina pigment epithelial (RPE1) cells. This dramatic impact was also observed in mammary epithelial cells with constitutively high levels of Nek2. Conversely, Nek2 knockout led to incomplete dissociation of DAs and cilia in mitosis. As a consequence, we observed the presence of a cilia remnant that promoted the asymmetric inheritance of ciliary signaling components and supported cilium reassembly after cell division. Together, our data establish Nek2 as an important kinase that regulates DAs before mitosis.

LRGUK1 is part of a multiprotein complex required for manchette function and male fertility

Infertility occurs in 1 in 20 young men and is idiopathic in origin in most. We have reported that the leucine-rich repeat (LRR) and guanylate kinase-like domain containing, isoform (LRGUK)-1 is essential for sperm head shaping, via the manchette, and the initiation of sperm tail growth from the centriole/basal body, and thus, male fertility. Within this study we have used a yeast 2-hybrid screen of an adult testis library to identify LRGUK1-binding partners, which were then validated with a range of techniques. The data indicate that LRGUK1 likely achieves its function in partnership with members of the HOOK family of proteins (HOOK-1-3), Rab3-interacting molecule binding protein (RIMBP)-3 and kinesin light chain (KLC)-3, all of which are associated with intracellular protein transport as cargo adaptor proteins and are localized to the manchette. LRGUK1 consists of 3 domains; an LRR, a guanylate kinase (GUK)-like and an unnamed domain. In the present study, we showed that the GUK-like domain is essential for binding to HOOK2 and RIMBP3, and the LRR domain is essential for binding to KLC3. These findings establish LRGUK1 as a key component of a multiprotein complex with an essential role in microtubule dynamics within haploid male germ cells.-Okuda, H., DeBoer, K., O'Connor, A. E., Merriner, D. J., Jamsai, D., O'Bryan, M. K. LRGUK1 is part of a multiprotein complex required for manchette function and male fertility.

The growth plate's response to load is partially mediated by mechano-sensing via the chondrocytic primary cilium

Mechanical load plays a significant role in bone and growth-plate development. Chondrocytes sense and respond to mechanical stimulation; however, the mechanisms by which those signals exert their effects are not fully understood. The primary cilium has been identified as a mechano-sensor in several cell types, including renal epithelial cells and endothelium, and accumulating evidence connects it to mechano-transduction in chondrocytes. In the growth plate, the primary cilium is involved in several regulatory pathways, such as the non-canonical Wnt and Indian Hedgehog. Moreover, it mediates cell shape, orientation, growth, and differentiation in the growth plate. In this work, we show that mechanical load enhances ciliogenesis in the growth plate. This leads to alterations in the expression and localization of key members of the Ihh-PTHrP loop resulting in decreased proliferation and an abnormal switch from proliferation to differentiation, together with abnormal chondrocyte morphology and organization. Moreover, we use the chondrogenic cell line ATDC5, a model for growth-plate chondrocytes, to understand the mechanisms mediating the participation of the primary cilium, and in particular KIF3A, in the cell's response to mechanical stimulation. We show that this key component of the cilium mediates gene expression in response to mechanical stimulation.

Centrosome fine ultrastructure of the osteocyte mechanosensitive primary cilium

The centrosome is the principal microtubule organization center in cells, giving rise to microtubule-based organelles (e.g., cilia, flagella). The aim was to study the osteocyte centrosome morphology at an ultrastructural level in relation to its mechanosensitive function. Osteocyte centrosomes and cilia in tibial cortical bone were explored by acetylated alpha-tubulin (AαTub) immunostaining under confocal microscopy. For the first time, fine ultrastructure and spatial orientation of the osteocyte centrosome were explored by transmission electron microscopy on serial ultrathin sections. AαTub-positive staining was observed in 94% of the osteocytes examined (222/236). The mother centriole formed a short primary cilium and was longer than the daughter centriole due to an intermediate zone between centriole and cilium. The proximal end of the mother centriole was connected with the surface of daughter centriole by striated rootlets. The mother centriole exhibited distal appendages that interacted with the cell membrane and formed a particular structure called "cilium membrane prolongation." The primary cilium was mainly oriented perpendicular to the long axis of bone. Mother and daughter centrioles change their original mutual orientation during the osteocyte differentiation process. The short primary cilium is hypothesized as a novel type of fluid-sensing organelle in osteocytes.

A truncating mutation of Alms1 reduces the number of hypothalamic neuronal cilia in obese mice

Primary cilia are ubiquitous cellular antennae whose dysfunction collectively causes various disorders, including vision and hearing impairment, as well as renal, skeletal, and central nervous system anomalies. One ciliopathy, Alström syndrome, is closely related to Bardet-Biedl syndrome (BBS), sharing amongst other phenotypic features morbid obesity. As the cellular and molecular links between weight regulation and cilia are poorly understood, we used the obese mouse strain foz/foz, bearing a truncating mutation in the Alström syndrome protein (Alms1), to help elucidate why it develops hyperphagia, leading to early onset obesity and metabolic anomalies. Our in vivo studies reveal that Alms1 localizes at the base of cilia in hypothalamic neurons, which are implicated in the control of satiety. Alms1 is lost from this location in foz/foz mice, coinciding with a strong postnatal reduction (∼70%) in neurons displaying cilia marked with adenylyl cyclase 3 (AC3), a signaling protein implicated in obesity. Notably, the reduction in AC3-bearing cilia parallels the decrease in cilia containing two appetite-regulating proteins, Mchr1 and Sstr3, as well as another established Arl13b ciliary marker, consistent with progressive loss of cilia during development. Together, our results suggest that Alms1 maintains the function of neuronal cilia implicated in weight regulation by influencing the maintenance and/or stability of the organelle. Given that Mchr1 and Sstr3 localization to remaining cilia is maintained in foz/foz animals but known to be lost from BBS knockout mice, our findings suggest different molecular etiologies for the satiety defects associated with the Alström syndrome and BBS ciliopathies.

Mechanical regulation of signaling pathways in bone

A wide range of cell types depend on mechanically induced signals to enable appropriate physiological responses. The skeleton is particularly dependent on mechanical information to guide the resident cell population towards adaptation, maintenance and repair. Research at the organ, tissue, cell and molecular levels has improved our understanding of how the skeleton can recognize the functional environment, and how these challenges are translated into cellular information that can site-specifically alter phenotype. This review first considers those cells within the skeleton that are responsive to mechanical signals, including osteoblasts, osteoclasts, osteocytes and osteoprogenitors. This is discussed in light of a range of experimental approaches that can vary parameters such as strain, fluid shear stress, and pressure. The identity of mechanoreceptor candidates is approached, with consideration of integrins, pericellular tethers, focal adhesions, ion channels, cadherins, connexins, and the plasma membrane including caveolar and non-caveolar lipid rafts and their influence on integral signaling protein interactions. Several mechanically regulated intracellular signaling cascades are detailed including activation of kinases (Akt, MAPK, FAK), β-catenin, GTPases, and calcium signaling events. While the interaction of bone cells with their mechanical environment is complex, an understanding of mechanical regulation of bone signaling is crucial to understanding bone physiology, the etiology of diseases such as osteoporosis, and to the development of interventions to improve bone strength.

Cilia in vertebrate development and disease

Through the combined study of model organisms, cell biology, cell signaling and medical genetics we have significantly increased our understanding of the structure and functions of the vertebrate cilium. This ancient organelle has now emerged as a crucial component of certain signaling and sensory perception pathways in both developmental and homeostatic contexts. Here, we provide a snapshot of the structure, function and distribution of the vertebrate cilium and of the pathologies that are associated with its dysfunction.

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.

Effect of in vitro stress-deprivation and cyclic loading on the length of tendon cell cilia in situ

To determine the effect of loading conditions on the length of primary cilia in tendon cells in situ, freshly harvested rat tail tendons were stress-deprived (SD) for up to 72 h, cyclically loaded at 3% strain at 0.17 Hz for 24 h, or SD for 24 h followed by cyclic loading (CL) for 24 h. Tendon sections were stained for tubulin, and cilia measured microscopically. In fresh control tendons, cilia length ranged from 0.6 to 2.0 µm with a mean length of 1.1 µm. Following SD, cilia demonstrated an increase (p < 0.001) in overall length at 24 h when compared to controls. Cilia length did not increase with time of SD (p = 0.329). Cilia in cyclically loaded tendons were shorter (p < 0.001) compared to all SD time periods, but were not different from 0 time controls (p = 0.472). CL for 24 h decreased cilia length in 24 h SD tendons (p < 0.001) to levels similar to those of fresh controls (p = 0.274). The results of this study demonstrate that SD resulted in an immediate and significant increase in the length of primary cilia of tendon cells, which can be reversed by cyclic tensile loading. This suggests that, as in other tissues, cilia length in tendon cells is affected by mechanical signaling from the extracellular matrix.

Common botanical compounds inhibit the hedgehog signaling pathway in prostate cancer

Many botanical compounds have been proposed to prevent cancer. We investigated the cancer treatment and prevention abilities of apigenin, baicalein, curcumin, epigallocatechin 3-gallate (EGCG), genistein, quercetin, and resveratrol both in vivo in transgenic adenocarcinoma of the mouse prostate (TRAMP) mice as well as in vitro in prostate cancer cell lines. In our experiments, these seven compounds act similarly to the Hedgehog antagonist cyclopamine, a teratogenic plant alkaloid, which had been previously shown to "cure" prostate cancer in a mouse xenograft model. With IC(50) values ranging from <1 to 25 mumol/L, these compounds can inhibit Gli1 mRNA concentration by up to 95% and downregulate Gli reporter activity by 80%. We show that four compounds, genistein, curcumin, EGCG, and resveratrol, inhibit Hedgehog signaling as monitored by real-time reverse transcription-PCR analysis of Gli1 mRNA concentration or by Gli reporter activity. Three compounds, apigenin, baicalein, and quercetin, decreased Gli1 mRNA concentration but not Gli reporter activity. Our results show that these compounds are also able to reduce or delay prostate cancer in vivo in TRAMP mice. All seven compounds, when fed in combination as pure compounds or as crude plant extracts, inhibit well-differentiated carcinoma of the prostate by 58% and 81%, respectively. In vitro, we show that all seven compounds also inhibit growth in human and mouse prostate cancer cell lines. Mechanistically, we propose the Hedgehog signaling pathway to be a direct or indirect target of these compounds. These botanicals at pharmacologic concentrations are potentially safer and less expensive alternatives to cyclopamine and its pharmaceutical analogues for cancer therapy.

Molecular characterization of centriole assembly in ciliated epithelial cells

Ciliated epithelial cells have the unique ability to generate hundreds of centrioles during differentiation. We used centrosomal proteins as molecular markers in cultured mouse tracheal epithelial cells to understand this process. Most centrosomal proteins were up-regulated early in ciliogenesis, initially appearing in cytoplasmic foci and then incorporated into centrioles. Three candidate proteins were further characterized. The centrosomal component SAS-6 localized to basal bodies and the proximal region of the ciliary axoneme, and depletion of SAS-6 prevented centriole assembly. The intraflagellar transport component polaris localized to nascent centrioles before incorporation into cilia, and depletion of polaris blocked axoneme formation. The centriolar satellite component PCM-1 colocalized with centrosomal components in cytoplasmic granules surrounding nascent centrioles. Interfering with PCM-1 reduced the amount of centrosomal proteins at basal bodies but did not prevent centriole assembly. This system will help determine the mechanism of centriole formation in mammalian cells and how the limitation on centriole duplication is overcome in ciliated epithelial cells.

Hedgehog signaling in mammary gland development and breast cancer

The Hedgehog pathway is critical for many developmental processes, including the formation of several epidermal appendages. In the mammary gland strict regulation of the Hedgehog pathway is required for normal development. Alterations in Hedgehog signaling result in defects in both the embryonic and postnatal mammary gland. Activation of Hedgehog signaling either by mutation or misexpression of pathway members can lead to the development and/or progression of cancers in multiple organs. This review addresses the current understanding and controversies of Hedgehog signaling in mammary gland development and its potential role in promoting breast carcinogenesis and cancer progression.

Knockdown of the intraflagellar transport protein IFT46 stimulates selective gene expression in mouse chondrocytes and affects early development in zebrafish

Bone morphogenetic proteins (BMPs) act as multifunctional regulators in morphogenesis during development. In particular they play a determinant role in the formation of cartilage molds and their replacement by bone during endochondral ossification. In cell culture, BMP-2 favors chondrogenic expression and promotes hypertrophic maturation of chondrocytes. In mouse chondrocytes we have identified a BMP-2-sensitive gene encoding a protein of 301 amino acids. This protein, named mIFT46, is the mouse ortholog of recently identified Caenorhabditis elegans and Chlamydomonas reinhardtii intraflagellar transport (IFT) proteins. After generation of a polyclonal antibody against mIFT46, we showed for the first time that the endogenous protein is located in the primary cilium of chondrocytes. We also found that mIFT46 is preferentially expressed in early hypertrophic chondrocytes located in the growth plate. Additionally, mIFT46 knockdown by small interfering RNA oligonucleotides in cultured chondrocytes specifically stimulated the expression of several genes related to skeletogenesis. Furthermore, Northern blotting analysis indicated that mIFT46 is also expressed before chondrogenesis in embryonic mouse development, suggesting that the role of mIFT46 might not be restricted to cartilage. To explore the role of IFT46 during early development, we injected antisense morpholino oligonucleotides in Danio rerio embryos to reduce zebrafish IFT46 protein (zIFT46) synthesis. Dramatic defects in embryonic development such as a dorsalization and a tail duplication were observed. Thus our results taken together indicate that the ciliary protein IFT46 has a specific function in chondrocytes and is also essential for normal development of vertebrates.

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

The chondrocytic primary cilium has been hypothesized to act as a mechano-sensor, analogously to primary cilium of cells in epithelial tissues. We hypothesize that mechanical inputs during growth, sensed through the primary cilium, result in directed secretion of the extracellular matrix, thereby establishing tissue anisotropy in growth plate cartilage. The cilium, through its orientation in three-dimensional space, is hypothesized to transmit to the chondrocyte the preferential direction for matrix secretion. This paper reports on the application of classical mathematical methods to develop an algorithm that addresses the particular challenges relative to the assessment of the orientation of the primary cilium in growth plate cartilage, based on image analysis of optical sections visualized by multiphoton microscopy. Specimens are prepared by rapid cold precipitation-based fixation to minimize possible artifactual post-mortem alterations of ciliary orientation. The ciliary axoneme is localized by immunocytochemistry with antibody acetylated-alpha-tubulin. The method is applicable to investigation of ciliary orientation in different zones of the growth plate, under either normal or altered biomechanical environments. The methodology is highly flexible and adaptable to other connective tissues where tissue anisotropy and directed secretion of extracellular matrix components are hypothesized to depend on the tissue's biomechanical environment during development and growth.

Planar cell polarity, ciliogenesis and neural tube defects

Cilia are microtubule-based protrusions that are found on the surface of most vertebrate cells. Long studied by cell biologists, these organelles have recently caught the attention of developmental biologists and human geneticists. In this review, I will discuss recent findings suggesting a link between cilia and the planar cell polarity signaling cascade. In particular, I will focus on how this interaction may influence the process of neural tube closure and how these results may be relevant to our understanding of common human birth defects in which neural tube closure is compromised.