Activation of Wnt Planar Cell Polarity (PCP) signaling promotes growth plate column formation in vitro

ROR2-Related Skeletal Dysplasia Reveals Disrupted Chondrocyte Polarity through Modulation of BMP/TGF-β Signaling

Genetic studies have shown that Robinow syndrome (RS), a rare skeletal dysplasia, is caused by ROR2 mutation. However, the cell origin and molecular mechanisms underlying this disease remain elusive. We established a conditional knockout system by crossing Prx1cre and Osxcre with Ror2 flox/flox mice. and conducted histological and immunofluorescence analyses to investigate the phenotypes during skeletal development. In the Prx1cre line, we observed RS-like skeletal abnormities, including short stature and an arched skull. Additionally, we found inhibition of chondrocyte differentiation and proliferation. In the Osxcre line, loss of ROR2 in osteoblast lineage cells led to reduced osteoblast differentiation during both embryonic and postnatal stages. Furthermore, ROR2 mutant mice exhibited increased adipogenesis in the bone marrow compared to their littermate controls. To further explore the underlying mechanisms, bulk RNA-seq analysis of Prx1cre; Ror2 flox/flox embryos was performed, results revealed decreased BMP/TGF-β signaling. Immunofluorescence analysis further confirmed the decreased expression of p-smad1/5/8, accompanied by disrupted cell polarity in the developing growth plate. Pharmacological treatment using FK506 partially rescued the skeletal dysplasia and resulted in increased mineralization and osteoblast differentiation. By modeling the phenotype of RS in mice, our findings provide evidence for the involvement of mesenchymal progenitors as the cell origin and highlight the molecular mechanism of BMP/TGF-β signaling in skeletal dysplasia.

Loss of dlx5a/dlx6a Locus Alters Non-Canonical Wnt Signaling and Meckel's Cartilage Morphology

The dlx genes encode transcription factors that establish a proximal-distal polarity within neural crest cells to bestow a regional identity during craniofacial development. The expression regions of dlx paralogs are overlapping yet distinct within the zebrafish pharyngeal arches and may also be involved in progressive morphologic changes and organization of chondrocytes of the face. However, how each dlx paralog of dlx1a, dlx2a, dlx5a and dlx6a affects craniofacial development is still largely unknown. We report here that the average lengths of the Meckel's, palatoquadrate and ceratohyal cartilages in different dlx mutants were altered. Mutants for dlx5a-/- and dlx5i6-/-, where the entire dlx5a/dlx6a locus was deleted, have the shortest lengths for all three structures at 5 days post fertilization (dpf). This phenotype was also observed in 14 dpf larvae. Loss of dlx5i6 also resulted in increased proliferation of neural crest cells and expression of chondrogenic markers. Additionally, altered expression and function of non-canonical Wnt signaling were observed in these mutants suggesting a novel interaction between dlx5i6 locus and non-canonical Wnt pathway regulating ventral cartilage morphogenesis.

Suppressing Chondrocyte Hypertrophy to Build Better Cartilage

Current clinical strategies for restoring cartilage defects do not adequately consider taking the necessary steps to prevent the formation of hypertrophic tissue at injury sites. Chondrocyte hypertrophy inevitably causes both macroscopic and microscopic level changes in cartilage, resulting in adverse long-term outcomes following attempted restoration. Repairing/restoring articular cartilage while minimizing the risk of hypertrophic neo tissue formation represents an unmet clinical challenge. Previous investigations have extensively identified and characterized the biological mechanisms that regulate cartilage hypertrophy with preclinical studies now beginning to leverage this knowledge to help build better cartilage. In this comprehensive article, we will provide a summary of these biological mechanisms and systematically review the most cutting-edge strategies for circumventing this pathological hallmark of osteoarthritis.

TNF overexpression and dexamethasone treatment impair chondrogenesis and bone growth in an additive manner

Children with chronic inflammation are often treated with glucocorticoids (GCs) and many of them experience growth retardation. It is poorly understood how GCs interact with inflammatory cytokines causing growth failure as earlier experimental studies have been performed in healthy animals. To address this gap of knowledge, we used a transgenic mouse model where human TNF is overexpressed (huTNFTg) leading to chronic polyarthritis starting from the first week of age. Our results showed that femur bone length and growth plate height were significantly decreased in huTNFTg mice compared to wild type animals. In the growth plates of huTNFTg mice, increased apoptosis, suppressed Indian hedgehog, decreased hypertrophy, and disorganized chondrocyte columns were observed. Interestingly, the GC dexamethasone further impaired bone growth, accelerated chondrocyte apoptosis and reduced the number of chondrocyte columns in huTNFTg mice. We conclude that TNF and dexamethasone separately suppress chondrogenesis and bone growth when studied in an animal model of chronic inflammation. Our data give a possible mechanistic explanation to the commonly observed growth retardation in children with chronic inflammatory diseases treated with GCs.

Growing Pains: The Need for Engineered Platforms to Study Growth Plate Biology

Growth plates, or physis, are highly specialized cartilage tissues responsible for longitudinal bone growth in children and adolescents. Chondrocytes that reside in growth plates are organized into three distinct zones essential for proper function. Modeling key features of growth plates may provide an avenue to develop advanced tissue engineering strategies and perspectives for cartilage and bone regenerative medicine applications and a platform to study processes linked to disease progression. In this review, a brief introduction of the growth plates and their role in skeletal development is first provided. Injuries and diseases of the growth plates as well as physiological and pathological mechanisms associated with remodeling and disease progression are discussed. Growth plate biology, namely, its architecture and extracellular matrix organization, resident cell types, and growth factor signaling are then focused. Next, opportunities and challenges for developing 3D biomaterial models to study aspects of growth plate biology and disease in vitro are discussed. Finally, opportunities for increasingly sophisticated in vitro biomaterial models of the growth plate to study spatiotemporal aspects of growth plate remodeling, to investigate multicellular signaling underlying growth plate biology, and to develop platforms that address key roadblocks to in vivo musculoskeletal tissue engineering applications are described.

Rab11a Regulates the Development of Cilia and Establishment of Planar Cell Polarity in Mammalian Vestibular Hair Cells

Vestibular organs have unique planar cell polarity (Figure 1A), and their normal development and function are dependent on the regular polarity of cilia (Figure 1B) requires. Rab11a is a small G protein that participates in the transportation of intracellular and extracellular materials required for polarity formation; however, our understanding of the mechanisms of the actions of Rab11a in vestibular organs is limited. Here, we showed that the general shape of the utricle was abnormal in Rab11a ^CKO/CKO mice. These mice also showed abnormal morphology of the stereocilia bundles, which were reduced in both length and number, as well as disturbed tissue-level polarity. Rab11a affected the distribution of polarity proteins in the vestibular organs, indicating that the normal development of cilia requires Rab11a and intraflagellar transportation. Furthermore, small G protein migration works together with intraflagellar transportation in the normal development of cilia. FIGURE 1Morphological changes of stereocilia in the extrastriolar hair cells from Rab11a single or Rab11a/IFT88 double-mutant utricles. (A) Medial view of a mouse left inner ear with its five vestibular sensory organs (gray). Enlarged are the utricle showing their subdivisions, LPR (yellow line), and striola (blue). LES, lateral extrastriola; MES, medial extrastriola; LPR, line of polarity reversal. (B) Schematic view of vestibular hair cell. Kinocilium is marked with ace-tubulin. Basal body is marked with γ-tubulin. (C,C1,D,D1) Normal appearance of the stereocilia of extrastriolar hair cells of wild-type controls. (E,E1,F,F1) Altered morphology in Rab11a ^CKO/CKO animals. (G,G1,H,H1) The changes in the stereocilia morphology were more severe in Rab11a ^CKO/CKO /IFT 88 ^CKO/+ mice. (I-L) Higher magnification of confocal images of hair cells. (M-P) Scanning electron microscopy images of hair cells from wild-type controls and Rab11a mutants. (I,M) Morphology of normal. hair cells of wild-type controls. (J,N) The number of stereocilia on a single hair cell was deceased in the Rab11a mutant. (K,O) Stereocilia were shorter in mutants compared to the wild-type controls. (L,P) The staircase-like hair bundle architecture of hair cells was lost in Rab11a mutant mice. (Q) The percentage of hair cells with abnormal development of static cilia bundles in the extrastriola region was counted as a percentage of the total (n = 5). The percentage of abnormal hair cells was higher in Rab11a ^CKO/CKO , IFT88 ^CKO/+ mice compared to Rab11a ^CKO/CKO . The abnormal ratios of single and double knockout hair cells were 42.1 ± 5.7 and 71.5 ± 10.4, respectively. In (A-J), for all primary panels, hair cell stereociliary bundles were marked with phalloidin (green), the actin-rich cuticular plate of hair cells was labeled with β-spectrin (red), while the basal body of the hair cell was labeled with γ-tubulin (blue). Scale bars: 10 μm (C-H1), 5 μm (J-N). *P < 0.05.

WNT5B in cellular signaling pathways

The Wnt signaling ligand WNT5B is implicated in various developmental pathways, both in normal and pathological physiology. Most of the research on WNT5B has been associated with expression analysis and disease states, leaving the signaling pathways underexplored. Here, we review the current understandings of WNT5B's regulation of signal transduction, from receptors to downstream mediators and transcription factors. We also describe its roles in β-catenin-dependent and β-catenin-independent (Planar Cell Polarity and Wnt/Ca²⁺) Wnt signaling.

Chondrocytes in the resting zone of the growth plate are maintained in a Wnt-inhibitory environment

Chondrocytes in the resting zone of the postnatal growth plate are characterized by slow cell cycle progression, and encompass a population of parathyroid hormone-related protein (PTHrP)-expressing skeletal stem cells that contribute to the formation of columnar chondrocytes. However, how these chondrocytes are maintained in the resting zone remains undefined. We undertook a genetic pulse-chase approach to isolate slow cycling, label-retaining chondrocytes (LRCs) using a chondrocyte-specific doxycycline-controllable Tet-Off system regulating expression of histone 2B-linked GFP. Comparative RNA-seq analysis identified significant enrichment of inhibitors and activators for Wnt signaling in LRCs and non-LRCs, respectively. Activation of Wnt/β-catenin signaling in PTHrP+ resting chondrocytes using Pthlh-creER and Apc-floxed allele impaired their ability to form columnar chondrocytes. Therefore, slow-cycling chondrocytes are maintained in a Wnt-inhibitory environment within the resting zone, unraveling a novel mechanism regulating maintenance and differentiation of PTHrP+ skeletal stem cells of the postnatal growth plate.

Wdpcp regulates cellular proliferation and differentiation in the developing limb via hedgehog signaling

Background

Mice with a loss of function mutation in Wdpcp were described previously to display severe birth defects in the developing heart, neural tube, and limb buds. Further characterization of the skeletal phenotype of Wdpcp null mice was limited by perinatal lethality.

Results

We utilized Prx1-Cre mice to generate limb bud mesenchyme specific deletion of Wdpcp. These mice recapitulated the appendicular skeletal phenotype of the Wdpcp null mice including polydactyl and limb bud signaling defects. Examination of late stages of limb development demonstrated decreased size of cartilage anlagen, delayed calcification, and abnormal growth plates. Utilizing in vitro assays, we demonstrated that loss of Wdpcp in skeletal progenitors lead to loss of hedgehog signaling responsiveness and associated proliferative response. In vitro chondrogenesis assays showed this loss of hedgehog and proliferative response was associated with decreased expression of early chondrogenic marker N-Cadherin. E14.5 forelimbs demonstrated delayed ossification and expression of osteoblast markers Runx2 and Sp7. P0 growth plates demonstrated loss of hedgehog signaling markers and expansion of the hypertrophic zones of the growth plate. In vitro osteogenesis assays demonstrated decreased osteogenic differentiation of Wdpcp null mesenchymal progenitors in response to hedgehog stimulation.

Conclusions

These findings demonstrate how Wdpcp and associated regulation of the hedgehog signaling pathway plays an important role at multiple stages of skeletal development. Wdpcp is necessary for positive regulation of hedgehog signaling and associated proliferation is key to the initiation of chondrogenesis. At later stages, Wdpcp facilitates the robust hedgehog response necessary for chondrocyte hypertrophy and osteogenic differentiation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12861-021-00241-9.

Deer antler extract potentially facilitates xiphoid cartilage growth and regeneration and prevents inflammatory susceptibility by regulating multiple functional genes

BACKGROUND

Deer antler is a zoological exception due to its fantastic characteristics, including amazing growth rate and repeatable regeneration. Deer antler has been used as a key ingredient in traditional Chinese medicine relating to kidney and bone health for centuries. The aim of this study was to dissect the molecular regulation of deer antler extract (DAE) on xiphoid cartilage (XC).

METHODS

The DAE used in this experiment was same as the one that was prepared as previously described. The specific pathogen-free (SPF) grade Sprague-Dawley (SD) rats were randomly divided into blank group (n =10) and DAE group (n =10) after 1-week adaptive feeding. The DAE used in this experiment was same as the one that was prepared as previously described. The rats in DAE group were fed with DAE for 3 weeks at a dose of 0.2 g/kg per day according to the body surface area normalization method, and the rats in blank group were fed with drinking water. Total RNA was extracted from XC located in the most distal edge of the sternum. Illumina RNA sequencing (RNA-seq) in combination with quantitative real-time polymerase chain reaction (qRT-PCR) validation assay was carried out to dissect the molecular regulation of DAE on XC.

RESULTS

We demonstrated that DAE significantly increased the expression levels of DEGs involved in cartilage growth and regeneration, but decreased the expression levels of DEGs involved in inflammation, and mildly increased the expression levels of DEGs involved in chondrogenesis and chondrocyte proliferation.

CONCLUSIONS

Our findings suggest that DAE might serve as a complementary therapeutic regent for cartilage growth and regeneration to treat cartilage degenerative disease, such as osteoarthritis.

Silencing of Vangl2 attenuates the inflammation promoted by Wnt5a via MAPK and NF-κB pathway in chondrocytes

BACKGROUND

The Wnt planar cell polarity (PCP) pathway is implicated in osteoarthritis (OA) both in animals and in humans. Van Gogh-like 2 (Vangl2) is a key PCP protein that is required for the orientation and alignment of chondrocytes in the growth plate. However, its functional roles in OA still remain undefined. Here, we explored the effects of Vangl2 on OA chondrocyte in vitro and further elucidated the molecular mechanism of silencing Vangl2 in Wnt5a-overexpressing OA chondrocytes.

METHODS

Chondrocytes were treated with IL-1β (10 ng/mL) to simulate the inflammatory microenvironment of OA. The expression levels of Vangl2, Wnt5a, MMPs, and related proinflammatory cytokines were measured by RT-qPCR. Small interfering RNA (siRNA) of Vangl2 and the plasmid targeting Wnt5a were constructed and transfected into ATDC5 cells. Then, the functional roles of silencing Vangl2 in the OA chondrocytes were investigated by Western blotting, RT-qPCR, and immunocytochemistry (ICC). Transfected OA chondrocytes were subjected to Western blotting to analyze the relationship between Vangl2 and related signaling pathways.

RESULTS

IL-1β induced the production of Vangl2, Wnt5a, and MMPs in a time-dependent manner and the significantly increased expression of Vangl2. Vangl2 silencing effectively suppressed the expression of MMP3, MMP9, MMP13, and IL-6 at both gene and protein levels and upregulated the expression of type II collagen and aggrecan. Moreover, knockdown of Vangl2 inhibited the phosphorylation of MAPK signaling molecules (P38, ERK, and JNK) and P65 in Wnt5a-overexpressing OA chondrocytes.

CONCLUSIONS

For the first time, we demonstrate that Vangl2 is involved in the OA process. Vangl2 silencing can notably alleviate OA progression in vitro by inhibiting the expression of MMPs and increasing the formation of the cartilage matrix and can inhibit the proinflammatory effects of Wnt5a via MAPK and NF-κB pathway. This study provides new insight into the mechanism of cartilage inflammation.

Comprehensive evaluation of differential long non-coding RNA and gene expression in patients with cartilaginous endplate degeneration of cervical vertebra

Long non-coding RNAs (lncRNAs) are emerging as key regulators in gene expression; however, little is currently known regarding their role in cartilaginous endplate (CE) degeneration (CED) of cervical vertebra. The present study aimed to investigate the expression levels of lncRNAs and analyze their potential functions in CED of cervical vertebra in patients with cervical fracture and cervical spondylosis. Human competitive endogenous RNA (ceRNA) array was used to analyze lncRNA and mRNA expression levels in CE samples from patients with cervical fracture and cervical spondylosis, who received anterior cervical discectomy and fusion. Differentially expressed lncRNAs (DELs) or differentially expressed genes (DEGs) were identified and functionally analyzed, using Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses. An lncRNA-microRNA(miRNA)-mRNA ceRNA regulatory network was constructed based on the DELs and DEGs, and the ceRNA network was visualized using Cytoscape 3.7.2 software. In total, one downregulated mRNA, one upregulated miRNA and five downstream regulated lncRNAs were identified using reverse transcription-quantitative PCR in CED and healthy CE samples. A total of 369 lncRNAs and 246 mRNAs were identified as differentially expressed in CE. The GO and KEGG analyses demonstrated that the majority of GO and KEGG enrichments were associated with CED. Furthermore, a ceRNA network was established, including 168 putative miRNA response elements, 189 upregulated and 37 downregulated lncRNAs and 47 upregulated and 10dow regulated DEGs. The present study analyzed the function of DEGs in the ceRNA network and filtered out the same items as in DEG-function enrichment analysis. These results provide a new perspective for an improved understanding of ceRNA-mediated gene regulation in cervical spondylosis, and provide a novel theoretical basis for further studies on the function of lncRNA in cervical spondylosis. However, further experiments are required to validate the results of the present study.

Piezoelectric stimulation by ultrasound facilitates chondrogenesis of mesenchymal stem cells

A cellular stimulation device utilizing an AT-cut quartz coverslip mounted on an ultrasonic live imaging chamber is developed to investigate the effect of piezoelectric stimulation. Two types of chambers deliver ultrasound at intensities ranging from 1 to 20 mW/cm² to mesenchymal stem cells (MSCs) seeded on the quartz coverslip. The quartz coverslip imposes additionally localized electric charges as it vibrates with the stimulation. The device was applied to explore whether piezoelectric stimulation can facilitate chondrogenesis of MSCs. The results suggest piezoelectric stimulation drove clustering of MSCs and consequently facilitated chondrogenesis of MSCs without the use of differentiation media.

Disorganization of chondrocyte columns in the growth plate does not aggravate experimental osteoarthritis in mice

Osteoarthritis (OA) is a multifactorial joint disease mainly affecting articular cartilage (AC) with a relevant biomechanical component. During endochondral ossification growth plate (GP) chondrocytes arrange in columns. GPs do not ossify in skeletally mature rodents. In neonatal mice, an altered joint loading induces GP chondrocyte disorganization. We aimed to study whether experimental OA involves GP disorganization in adult mice and to assess if it may have additional detrimental effects on AC damage. Knee OA was induced by destabilization of the medial meniscus (DMM) in wild-type (WT) adult mice, and in Tamoxifen-inducible Ellis-van-Creveld syndrome protein (Evc) knockouts (Evc^cKO), used as a model of GP disorganization due to Hedgehog signalling disruption. Chondrocyte column arrangement was assessed in the tibial GP and expressed as Column Index (CI). Both DMM-operated WT mice and non-operated-Evc^cKO showed a decreased CI, indicating GP chondrocyte column disarrangement, although in the latter, it was not associated to AC damage. The most severe GP chondrocyte disorganization occurred in DMM-Evc^cKO mice, in comparison to the other groups. However, this altered GP structure in DMM-Evc^cKO mice did not exacerbate AC damage. Further studies are needed to confirm the lack of interference of GP alterations on the analysis of AC employing OA mice.

Analysis of -5p and -3p Strands of miR-145 and miR-140 During Mesenchymal Stem Cell Chondrogenic Differentiation

The chondrogenic differentiation of mesenchymal stem cells (MSCs) is mediated by transcription factors and small noncoding RNAs such as microRNAs (miRNAs). Each miRNA is initially transcribed as a long transcript, which matures to produce -5p and -3p strands. It is widely believed that the mature and functional miRNA from any given pre-miRNA, usually the -5p strand, is functional, while the opposing -3p strand is degraded. However, recent cartilage literature started to show functional -3p strands for a few miRNAs. This study aimed at examining both -5p and -3p strands of two key miRNAs miR-140 and miR-145, known to be involved in the chondrogenic differentiation of MSCs. The level (copy number) of both -5p and -3p strands of miR-145 and miR-140 along the time line of MSC chondrogenic differentiation was determined by polymerase chain reaction. The gene expression profiles of several genes related to MSC chondrogenesis were compared with these miRNA profiles along the same timeline. While miR-145-3p is declining in step with miR-145-5p in pellet cultures during the process, the -3p strand is only 1-2% of the total miR-145 products. In contrast, the mature -3p and -5p products of miR-140 are found to increase with near-equal molar expression throughout chondrogenic differentiation. Numerous genes are expressed by cartilage progenitor cells during development. One such target gene, Sox9, is a regulatory target of the dominant miR-145-5p, consistent with the data. Further experimental validations are warranted to confirm that ACAN, FOXO1, and RUNX3 as direct targets of miR-145-5p in the context of MSC chondrogenesis. Similarly, TRSP1 and ACAN are worth further validation as direct targets of miR-145-3p. For miR-140, SOX4 shall be further validated as a direct target of miR-140-5p, while KLF4, PTHLH, and WNT5A can be validated as direct targets of miR-140-3p.

Mechanical loading regulates organization of the actin cytoskeleton and column formation in postnatal growth plate

Longitudinal growth of bones occurs at the growth plates where chondrocytes align into columns that allow directional growth. Little is known about the mechanisms controlling the ability of chondrocytes to form columns. We hypothesize that mechanical load and the resulting force on chondrocytes are necessary during active growth for proper growth plate development and limb length. To test this hypothesis, we created a mouse model in which a portion of the sciatic nerve from one hind limb was transected at postnatal day 8 to cause paralysis to that limb. At 6 and 12 wk postsurgery, the hind limb had significantly less bone mineral density than contralateral controls, confirming reduced load. At 8 and 14 wk postsurgery, tibiae were significantly shorter than controls. The paralyzed growth plate showed disruptions to column organization, with fewer and shorter columns. Polarized light microscopy indicated alterations in collagen fiber organization in the growth plate. Furthermore, organization of the actin cytoskeleton in growth plate chondrocytes was disrupted. We conclude that mechanical load and force on chondrocytes within the growth plate regulate postnatal development of the long bones.

A Tunable, Three-Dimensional In Vitro Culture Model of Growth Plate Cartilage Using Alginate Hydrogel Scaffolds

Defining the final size and geometry of engineered tissues through precise control of the scalar and vector components of tissue growth is a necessary benchmark for regenerative medicine, but it has proved to be a significant challenge for tissue engineers. The growth plate cartilage that promotes elongation of the long bones is a good model system for studying morphogenetic mechanisms because cartilage is composed of a single cell type, the chondrocyte; chondrocytes are readily maintained in culture; and growth trajectory is predominately in a single vector. In this cartilage, growth is generated via a differentiation program that is spatially and temporally regulated by an interconnected network composed of long- and short-range signaling mechanisms that together result in the formation of functionally distinct cellular zones. To facilitate investigation of the mechanisms underlying anisotropic growth, we developed an in vitro model of the growth plate cartilage by using neonatal mouse growth plate chondrocytes encapsulated in alginate hydrogel beads. In bead cultures, encapsulated chondrocytes showed high viability, cartilage matrix deposition, low levels of chondrocyte hypertrophy, and a progressive increase in cell proliferation over 7 days in culture. Exogenous factors were used to test functionality of the parathyroid-related protein-Indian hedgehog (PTHrP-IHH) signaling interaction, which is a crucial feedback loop for regulation of growth. Consistent with in vivo observations, exogenous PTHrP stimulated cell proliferation and inhibited hypertrophy, whereas IHH signaling stimulated chondrocyte hypertrophy. Importantly, the treatment of alginate bead cultures with IHH or thyroxine resulted in formation of a discrete domain of hypertrophic cells that mimics tissue architecture of native growth plate cartilage. Together, these studies are the first demonstration of a tunable in vitro system to model the signaling network interactions that are required to induce zonal architecture in growth plate chondrocytes, which could also potentially be used to grow cartilage cultures of specific geometries to meet personalized patient needs.

Smad4 regulates growth plate matrix production and chondrocyte polarity

Smad4 is an intracellular effector of the TGFβ family that has been implicated in Myhre syndrome, a skeletal dysplasia characterized by short stature, brachydactyly and stiff joints. The TGFβ pathway also plays a critical role in the development, organization and proliferation of the growth plate, although the exact mechanisms remain unclear. Skeletal phenotypes in Myhre syndrome overlap with processes regulated by the TGFβ pathway, including organization and proliferation of the growth plate and polarity of the chondrocyte. We used in vitro and in vivo models of Smad4 deficiency in chondrocytes to test the hypothesis that deregulated TGFβ signaling leads to aberrant extracellular matrix production and loss of chondrocyte polarity. Specifically, we evaluated growth plate chondrocyte polarity in tibiae of Col2-Cre^+/−;Smad4^fl/fl mice and in chondrocyte pellet cultures. In vitro and in vivo, Smad4 deficiency decreased aggrecan expression and increased MMP13 expression. Smad4 deficiency disrupted the balance of cartilage matrix synthesis and degradation, even though the sequential expression of growth plate chondrocyte markers was intact. Chondrocytes in Smad4-deficient growth plates also showed evidence of polarity defects, with impaired proliferation and ability to undergo the characteristic changes in shape, size and orientation as they differentiated from resting to hypertrophic chondrocytes. Therefore, we show that Smad4 controls chondrocyte proliferation, orientation, and hypertrophy and is important in regulating the extracellular matrix composition of the growth plate.

Summary: Smad4 is a key regulator of extracellular matrix production and chondrocyte proliferation, shape and orientation in the growth plate. Smad4 dysregulation results in skeletal dysplasias, such as Myhre syndrome.

The Signaling Pathways Involved in Chondrocyte Differentiation and Hypertrophic Differentiation

Chondrocytes communicate with each other mainly via diffusible signals rather than direct cell-to-cell contact. The chondrogenic differentiation of mesenchymal stem cells (MSCs) is well regulated by the interactions of varieties of growth factors, cytokines, and signaling molecules. A number of critical signaling molecules have been identified to regulate the differentiation of chondrocyte from mesenchymal progenitor cells to their terminal maturation of hypertrophic chondrocytes, including bone morphogenetic proteins (BMPs), SRY-related high-mobility group-box gene 9 (Sox9), parathyroid hormone-related peptide (PTHrP), Indian hedgehog (Ihh), fibroblast growth factor receptor 3 (FGFR3), and β-catenin. Except for these molecules, other factors such as adenosine, O₂ tension, and reactive oxygen species (ROS) also have a vital role in cartilage formation and chondrocyte maturation. Here, we outlined the complex transcriptional network and the function of key factors in this network that determine and regulate the genetic program of chondrogenesis and chondrocyte differentiation.

BMPR1A antagonist differentially affects cartilage and bone formation during fracture healing

A soluble form of BMP receptor type 1A (mBMPR1A-mFC) acts as an antagonist to endogenous BMPR1A and has been shown to increase bone mass in mice. The goal of this study was to examine the effects of mBMPR1A-mFC on secondary fracture healing. Treatment consisted of 10 mg/kg intraperitoneal injections of mBMPR1A-mFC twice weekly in male C57BL/6 mice. Treatment beginning at 1, 14, and 21 days post-fracture assessed receptor function during endochondral bone formation, at the onset of secondary bone formation, and during coupled remodeling, respectively. Control animals received saline injections. mBMPR1A-mFC treatment initiated on day 1 delayed cartilage maturation in the callus and resulted in large regions of fibrous tissue. Treatment initiated on day 1 also increased the amount of mineralized tissue and up-regulated many bone-associated genes (p = 0.002) but retarded periosteal bony bridging and impaired strength and toughness at day 35 (p < 0.035). Delaying the onset of treatment to day 14 or 21 partially mitigated these effects and produced evidence of accelerated coupled remodeling. These results indicate that inhibition of the BMPR1A-mediated signaling has negative effects on secondary fracture healing that are differentially manifested at different stages of healing and within different cell populations. These effects are most pronounced during the endochondral period and appear to be mediated by selective inhibition of BMPRIA signaling within the periosteum. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2096-2105, 2016.

Articular cartilage and joint development from embryogenesis to adulthood

Within each synovial joint, the articular cartilage is uniquely adapted to bear dynamic compressive loads and shear forces throughout the joint's range of motion. Injury and age-related degeneration of the articular cartilage often lead to significant pain and disability, as the intrinsic repair capability of the tissue is extremely limited. Current surgical and biological treatment options have been unable to restore cartilage de novo. Before successful clinical cartilage restoration strategies can be developed, a better understanding of how the cartilage forms during normal development is essential. This review focuses on recent progress made towards addressing key questions about articular cartilage morphogenesis, including the origin of synovial joint progenitor cells, postnatal development and growth of the tissue. These advances have provided novel insight into fundamental questions about the developmental biology of articular cartilage, as well as potential cell sources that may participate in joint response to injury.

Wnt-Frizzled/planar cell polarity signaling: cellular orientation by facing the wind (Wnt)

The establishment of planar cell polarity (PCP) in epithelial and mesenchymal cells is a critical, evolutionarily conserved process during development and organogenesis. Analyses in Drosophila and several vertebrate model organisms have contributed a wealth of information on the regulation of PCP. A key conserved pathway regulating PCP, the so-called core Wnt-Frizzled PCP (Fz/PCP) signaling pathway, was initially identified through genetic studies of Drosophila. PCP studies in vertebrates, most notably mouse and zebrafish, have identified novel factors in PCP signaling and have also defined cellular features requiring PCP signaling input. These studies have shifted focus to the role of Van Gogh (Vang)/Vangl genes in this molecular system. This review focuses on new insights into the core Fz/Vangl/PCP pathway and recent advances in Drosophila and vertebrate PCP studies. We attempt to integrate these within the existing core Fz/Vangl/PCP signaling framework.

WNT10B enhances proliferation through β-catenin and RAC1 GTPase in human corneal endothelial cells

The cornea is the anterior, transparent tissue of the human eye that serves as its main refractive element. Corneal endothelial cells are arranged as a monolayer on the posterior surface of the cornea and function as a pump to counteract the leakiness of its basement membrane. Maintaining the cornea in a slightly dehydrated state is critical for the maintenance of corneal transparency. Adult human corneal endothelial cells are G1-arrested, even in response to injury, leading to an age-dependent decline in endothelial cell density. Corneal edema and subsequent vision loss ensues when endothelial cell density decreases below a critical threshold. Vision loss secondary to corneal endothelial dysfunction is a common indication for transplantation in developed nations. An impending increase in demand for and a current global shortage of donor corneas will necessitate the development of treatments for vision loss because of endothelial dysfunction that do not rely on donor corneas. Wnt ligands regulate many critical cellular functions, such as proliferation, making them attractive candidates for modulation in corneal endothelial dysfunction. We show that WNT10B causes nuclear transport and binding of RAC1 and β-catenin in human corneal endothelial cells, leading to the activation of Cyclin D1 expression and proliferation. Our findings indicate that WNT10B promotes proliferation in human corneal endothelial cells by simultaneously utilizing both β-catenin-dependent and -independent pathways and suggest that its modulation could be used to treat vision loss secondary to corneal endothelial dysfunction.

SOXC Transcription Factors Induce Cartilage Growth Plate Formation in Mouse Embryos by Promoting Noncanonical WNT Signaling

Growth plates are specialized cartilage structures that ensure the elongation of most skeletal primordia during vertebrate development. They are made by chondrocytes that proliferate in longitudinal columns and then progress in a staggered manner towards prehypertrophic, hypertrophic and terminal maturation. Complex molecular networks control the formation and activity of growth plates, but remain incompletely understood. We investigated here the importance of the SoxC genes, which encode the SOX4, SOX11 and SOX12 transcription factors, in growth plates. We show that the three genes are expressed robustly in perichondrocytes and weakly in growth plate chondrocytes. SoxC(Prx1Cre) mice, which deleted SoxC genes in limb bud skeletogenic mesenchyme, were born with tiny appendicular cartilage primordia because of failure to form growth plates. In contrast, SoxC(Col2Cre) and SoxC(ATC) mice, which deleted SoxC genes primarily in chondrocytes, were born with mild dwarfism and fair growth plates. Chondrocytes in the latter mutants matured normally, but formed irregular columns, proliferated slowly and died ectopically. Asymmetric distribution of VANGL2 was defective in both SoxC(Prx1Cre) and SoxC(ATC) chondrocytes, indicating impairment of planar cell polarity, a noncanonical WNT signaling pathway that controls growth plate chondrocyte alignment, proliferation and survival. Accordingly, SoxC genes were necessary in perichondrocytes for expression of Wnt5a, which encodes a noncanonical WNT ligand required for growth plate formation, and in chondrocytes and perichondrocytes for expression of Fzd3 and Csnk1e, which encode a WNT receptor and casein kinase-1 subunit mediating planar cell polarity, respectively. Reflecting the differential strengths of the SOXC protein transactivation domains, SOX11 was more powerful than SOX4, and SOX12 interfered with the activity of SOX4 and SOX11. Altogether, these findings provide novel insights into the molecular regulation of skeletal growth by proposing that SOXC proteins act cell- and non-cell-autonomously in perichondrocytes and chondrocytes to establish noncanonical WNT signaling crosstalk essential for growth plate induction and control.

A hypomorphic allele reveals an important role of inturned in mouse skeletal development

BACKGROUND

Cilia are important for Hedgehog signaling in vertebrates and many genes that encode proteins involved in ciliogenesis have been studied for their roles in embryonic development. Null mutations in many of these genes cause early embryonic lethality, hence an understanding of their roles in postnatal development is limited.

RESULTS

The Inturned (Intu) gene is required for ciliogenesis and here we report a recessive hypomorphic mutation, resulting in substitution of a conserved hydrophobic residue (I813N) near the C-terminus, that sheds light on later functions of Intu. Mice homozygous for this Double-thumb (Intu(Dtm)) allele exhibit polydactyly, retarded growth, and reduced survival. There is a moderate loss of cilia in Intu(Dtm/Dtm) mutants, and Intu(I813N) exhibits compromised ability to increase ciliogenesis in cultured Intu null mutant cells. Intu(Dtm) mutants show rib defects and delay of endochondral ossification in long bones, digits, vertebrae, and the sternum. These skeletal defects correlate with a decrease in Hh signaling. However, patterning of the neural tube and planar cell polarity appear to be normal.

CONCLUSIONS

This hypomorphic Intu allele highlights an important role of Intu in mouse skeletal development.

Planar cell polarity aligns osteoblast division in response to substrate strain

Exposure of bone to dynamic strain increases the rate of division of osteoblasts and also influences the directional organization of the cellular and molecular structure of the bone tissue that they produce. Here, we report that brief exposure to dynamic substrate strain (sufficient to rapidly stimulate cell division) influences the orientation of osteoblastic cell division. The initial proliferative response to strain involves canonical Wnt signaling and can be blocked by sclerostin. However, the strain-related orientation of cell division is independently influenced through the noncanonical Wnt/planar cell polarity (PCP) pathway. Blockade of Rho-associated coiled kinase (ROCK), a component of the PCP pathway, prevents strain-related orientation of division in osteoblast-like Saos-2 cells. Heterozygous loop-tail mutation of the core PCP component van Gogh-like 2 (Vangl2) in mouse osteoblasts impairs the orientation of division in response to strain. Examination of bones from Vangl2 loop-tail heterozygous mice by µCT and scanning electron microscopy reveals altered bone architecture and disorganized bone-forming surfaces. Hence, in addition to the well-accepted role of PCP involvement in response to developmental cues during skeletal morphogenesis, our data reveal that this pathway also acts postnatally, in parallel with canonical Wnt signaling, to transduce biomechanical cues into skeletal adaptive responses. The simultaneous and independent actions of these two pathways appear to influence both the rate and orientation of osteoblast division, thus fine-tuning bone architecture to meet the structural demands of functional loading.

Interleukin-1β-induced Wnt5a enhances human corneal endothelial cell migration through regulation of Cdc42 and RhoA

Wnt5a can activate β-catenin-independent pathways for regulation of various cellular functions, such as migration, that play critical roles in wound repair. Investigation of Wnt5a signaling may help identify therapeutic targets for enhancing corneal endothelial wound healing that could provide an alternative to corneal transplantation in patients with blindness from endothelial dysfunction. However, Wnt5a signaling in corneal endothelial cells (CECs) has not been well characterized. In this study, we show transient induction of Wnt5a by interleukin-1β (IL-1β) stimulation proceeds through NF-κB in human CECs. This leads to binding of Fzd5 to Ror2, resulting in activation of disheveled protein (Dvl) and subsequently disheveled-associated activator of morphogenesis 1 (DAAM1). This leads to activation of Cdc42 and subsequent inhibition of RhoA. Inhibition of RhoA leads to parallel dephosphorylation and inactivation of LIM domain kinase 2 along with dephosphorylation and activation of slingshot 1, resulting in dephosphorylation and activation of cofilin and leading to enhanced cell migration. These findings suggest that Wnt5a enhances cell migration through activation of Cdc42 and inactivation of RhoA in human CECs.

A dynamic cell adhesion surface regulates tissue architecture in growth plate cartilage

The architecture and morphogenetic properties of tissues are founded in the tissue-specific regulation of cell behaviors. In endochondral bones, the growth plate cartilage promotes bone elongation via regulated chondrocyte maturation within an ordered, three-dimensional cell array. A key event in the process that generates this cell array is the transformation of disordered resting chondrocytes into clonal columns of discoid proliferative cells aligned with the primary growth vector. Previous analysis showed that column-forming chondrocytes display planar cell divisions, and the resulting daughter cells rearrange by ∼90° to align with the lengthening column. However, these previous studies provided limited information about the mechanisms underlying this dynamic process. Here we present new mechanistic insights generated by application of a novel time-lapse confocal microscopy method along with immunofluorescence and electron microscopy. We show that, during cell division, daughter chondrocytes establish a cell-cell adhesion surface enriched in cadherins and β-catenin. Rearrangement into columns occurs concomitant with expansion of this adhesion surface in a process more similar to cell spreading than to migration. Column formation requires cell-cell adhesion, as reducing cadherin binding via chelation of extracellular calcium inhibits chondrocyte rearrangement. Importantly, physical indicators of cell polarity, such as cell body alignment, are not prerequisites for oriented cell behavior. Our results support a model in which regulation of adhesive surface dynamics and cortical tension by extrinsic signaling modifies the thermodynamic landscape to promote organization of daughter cells in the context of the three-dimensional growth plate tissue.

Regulation of cell polarity in the cartilage growth plate and perichondrium of metacarpal elements by HOXD13 and WNT5A

The morphology of bones is genetically determined, but the molecular mechanisms that control shape, size and the overall gestalt of bones remain unclear. We previously showed that metacarpals in the synpolydactyly homolog (spdh) mouse, which carries a mutation in Hoxd13 similar to the human condition synpolydactyly (SPD), were transformed to carpal-like bones with cuboid shape that lack cortical bone and a perichondrium and are surrounded by a joint surface. Here we provide evidence that spdh metacarpal growth plates have a defect in cell polarization with a random instead of linear orientation. In parallel prospective perichondral cells failed to adopt the characteristic flattened cell shape. We observed a similar cell polarity defect in metacarpals of Wnt5a(-/-) mice. Wnt5a and the closely related Wnt5b were downregulated in spdh handplates, and HOXD13 induced expression of both genes in vitro. Concomitant we observed mislocalization of core planar cell polarity (PCP) components DVL2 and PRICKLE1 in spdh metacarpals indicating a defect in the WNT/PCP pathway. Conversely the WNT/β-CATENIN pathway, a hallmark of joint cells lining carpal bones, was upregulated in the perichondral region. Finally, providing spdh limb explant cultures with cells expressing either HOXD13 or WNT5A led to a non-cell autonomous partial rescue of cell polarity the perichondral region and restored the expression of perichondral markers. This study provides a so far unrecognized link between HOX proteins and cell polarity in the perichondrium and the growth plate, a failure of which leads to transformation of metacarpals to carpal-like structures.

Prickle1 stunts limb growth through alteration of cell polarity and gene expression

BACKGROUND

Wnt/PCP signaling plays a critical role in multiple developmental processes, including limb development. Wnt5a, a ligand of the PCP pathway, signals through the Ror2/Vangl2 or the Vangl2/Ryk complex to regulate limb development along the proximal-distal axis in mice. Based on the interaction between Van Gogh and Prickle in Drosophila, we hypothesized the vertebrate Prickle1 has a similar function as Vangl2 in limb development.

RESULTS

We show Prickle1 is expressed in the skeletal condensates that will differentiate into chondrocytes and later form bones. Disrupted Prickle1 function in Prickle1(C251X/C251X) mouse mutants alters expression of genes such as Bmp4, Fgf8, Vangl2, and Wnt5a. These expression changes correlate with shorter and wider bones in the limbs and loss of one phalangeal segment in digits 2-5 of Prickle1C251X mutants. These growth defects along the proximal-distal axis are also associated with increased cell death in the growing digit tip, reduced cell death in the interdigital membrane, and disrupted chondrocyte polarity.

CONCLUSIONS

We suggest Prickle1 is part of the Wnt5a/PCP signaling, regulating cell polarity and affecting expression of multiple factors to stunt limb growth through altered patterns of gene expression, including the PCP genes Wnt5a and Vangl2.

Planar cell polarity in vertebrate limb morphogenesis

Studies of the vertebrate limb development have contributed significantly to understanding the fundamental mechanisms underlying growth, patterning, and morphogenesis of a complex multicellular organism. In the limb, well-defined signaling centers interact to coordinate limb growth and patterning along the three axes. Recent analyses of live imaging and mathematical modeling have provided evidence that polarized cell behaviors governed by morphogen gradients play an important role in shaping the limb bud. Furthermore, the Wnt/planar cell polarity (PCP) pathway that controls uniformly polarized cell behaviors in a field of cells has emerged to be critical for directional morphogenesis in the developing limb. Directional information coded in the morphogen gradient may be interpreted by responding cells through regulating the activities of PCP components in a Wnt morphogen dose-dependent manner.