Temporomandibular joint formation requires two distinct hedgehog-dependent steps

Replacing Shox2 with human SHOX leads to congenital disc degeneration of the temporomandibular joint in mice

The temporomandibular joint (TMJ) consists in the glenoid fossa arising from the otic capsule through intramembranous ossification, the fibrocartilaginous disc and the condyle, which is derived from the secondary cartilage by endochondral ossification. We have reported previously that cranial neural-crest-specific inactivation of the homeobox gene Shox2, which is expressed in the mesenchymal cells of the maxilla-mandibular junction and later in the progenitor cells and perichondrium of the developing chondyle, leads to dysplasia and ankylosis of the TMJ and that replacement of the mouse Shox2 with the human SHOX gene rescues the dysplastic and ankylosis phenotypes but results in a prematurely worn out articular disc. In this study, we investigate the molecular and cellular bases for the prematurely worn out articular disc in the TMJ of mice carrying the human SHOX replacement allele in the Shox2 locus (termed Shox2 (SHOX-KI/KI)). We find that the developmental process and expression of several key genes in the TMJ of Shox2 (SHOX-KI/KI) mice are similar to that of controls. However, the disc of the Shox2 (SHOX-KI/KI) TMJ exhibits a reduced level of Collagen I and Aggrecan, accompanied by increased activities of matrix metalloproteinases and a down-regulation of Ihh expression. Dramatically increased cell apoptosis in the disc was also observed. These combinatory cellular and molecular defects appear to contribute to the observed disc phenotype, suggesting that, although human SHOX can exert similar functions to mouse Shox2 in regulating early TMJ development, it apparently has a distinct function in the regulation of those molecules that are involved in tissue homeostasis.

SAD kinases control the maturation of nerve terminals in the mammalian peripheral and central nervous systems

Axons develop in a series of steps, beginning with specification, outgrowth, and arborization, and terminating with formation and maturation of presynaptic specializations. We found previously that the SAD-A and SAD-B kinases are required for axon specification and arborization in subsets of mouse neurons. Here, we show that following these steps, SAD kinases become localized to synaptic sites and are required within presynaptic cells for structural and functional maturation of synapses in both peripheral and central nervous systems. Deleting SADs from sensory neurons can perturb either axonal arborization or nerve terminal maturation, depending on the stage of deletion. Thus, a single pair of kinases plays multiple, sequential roles in axonal differentiation.

Interaction between Foxc1 and Fgf8 during mammalian jaw patterning and in the pathogenesis of syngnathia

Syngnathia (bony fusion of the upper and lower jaw) is a rare human congenital condition, with fewer than sixty cases reported in the literature. Syngnathia typically presents as part of a complex syndrome comprising widespread oral and maxillofacial anomalies, but it can also occur in isolation. Most cartilage, bone, and connective tissue of the head and face is derived from neural crest cells. Hence, congenital craniofacial anomalies are often attributed to defects in neural crest cell formation, survival, migration, or differentiation. The etiology and pathogenesis of syngnathia however remains unknown. Here, we report that Foxc1 null embryos display bony syngnathia together with defects in maxillary and mandibular structures, and agenesis of the temporomandibular joint (TMJ). In the absence of Foxc1, neural crest cell derived osteogenic patterning is affected, as osteoblasts develop ectopically in the maxillary prominence and fuse with the dentary bone. Furthermore, we observed that the craniofacial musculature is also perturbed in Foxc1 null mice, which highlights the complex tissue interactions required for proper jaw development. We present evidence that Foxc1 and Fgf8 genetically interact and that Fgf8 dosage is associated with variation in the syngnathic phenotype. Together our data demonstrates that Foxc1 – Fgf8 signaling regulates mammalian jaw patterning and provides a mechanistic basis for the pathogenesis of syngnathia. Furthermore, our work provides a framework for understanding jaw patterning and the etiology of other congenital craniofacial anomalies, including temporomandibular joint agenesis.

Approximately one-third of all babies born with congenital defects, exhibit malformations of the head and face. Anomalies can include cleft lip, cleft palate, and abnormal development of bones and muscles. Such defects result in significant infant mortality, as well as life-long physical and social consequences for patients. Improved repair and the development of prevention strategies requires a thorough understanding of the underlying genetic, molecular, and environmental factors that contribute to normal craniofacial development and the pathogenesis of disease. In this study, we report the first genetic model of syngnathia, a rare human craniofacial defect characterized by bony fusion of the upper and lower jaw. We discovered that Foxc1 is required for normal development of the bones and muscles of the jaw as well as the jaw joint. Our studies provide a mechanistic basis for understanding the cause of human syngnathia as well as the failure of jaw joint formation. Furthermore, our work enhances our knowledge of jaw development and may inform treatment strategies for patients with syngnathia and related craniofacial malformation conditions.

An RNA-seq protocol to identify mRNA expression changes in mouse diaphyseal bone: applications in mice with bone property altering Lrp5 mutations

Loss-of-function and certain missense mutations in the Wnt coreceptor low-density lipoprotein receptor-related protein 5 (LRP5) significantly decrease or increase bone mass, respectively. These human skeletal phenotypes have been recapitulated in mice harboring Lrp5 knockout and knock-in mutations. We hypothesized that measuring mRNA expression in diaphyseal bone from mice with Lrp5 wild-type (Lrp5(+/+) ), knockout (Lrp5(-/-) ), and high bone mass (HBM)-causing (Lrp5(p.A214V/+) ) knock-in alleles could identify genes and pathways that regulate or are regulated by LRP5 activity. We performed RNA-seq on pairs of tibial diaphyseal bones from four 16-week-old mice with each of the aforementioned genotypes. We then evaluated different methods for controlling for contaminating nonskeletal tissue (ie, blood, bone marrow, and skeletal muscle) in our data. These methods included predigestion of diaphyseal bone with collagenase and separate transcriptional profiling of blood, skeletal muscle, and bone marrow. We found that collagenase digestion reduced contamination, but also altered gene expression in the remaining cells. In contrast, in silico filtering of the diaphyseal bone RNA-seq data for highly expressed blood, skeletal muscle, and bone marrow transcripts significantly increased the correlation between RNA-seq data from an animal's right and left tibias and from animals with the same Lrp5 genotype. We conclude that reliable and reproducible RNA-seq data can be obtained from mouse diaphyseal bone and that lack of LRP5 has a more pronounced effect on gene expression than the HBM-causing LRP5 missense mutation. We identified 84 differentially expressed protein-coding transcripts between LRP5 "sufficient" (ie, Lrp5(+/+) and Lrp5(p.A214V/+) ) and "insufficient" (Lrp5(-/-) ) diaphyseal bone, and far fewer differentially expressed genes between Lrp5(p.A214V/+) and Lrp5(+/+) diaphyseal bone.

Three-dimensional condylar positions and forms associated with different anteroposterior skeletal patterns and facial asymmetry in Chinese adolescents

OBJECTIVE

To evaluate the association of condylar asymmetry and chin position with different anteroposterior skeletal patterns using three-dimensional models reconstructed from cone-beam computed tomography (CBCT) images.

MATERIALS AND METHODS

CBCT scans of 123 Chinese adolescents (aged 11-15 years, 68 girls and 55 boys) with 64 skeletal Class I, 46 Class II and 13 Class III were selected from scans of patients attending the orthodontic clinic. The condyles of the subjects were reconstructed bilaterally and 25 linear, angular and volumetric measurements were performed to evaluate the asymmetry of the condyles and position of the chin. The proportions of condylar asymmetry in the different skeletal groups were calculated by the absolute difference value between the left and right sides to the smaller side value. One-way analysis of variance and Pearson's correlations were used to analyse the data.

RESULTS

The values for RV, RCL, LCH, RCH, LCGM, RCGM, LCo-Me and RCo-Me were significantly different among the three skeletal groups (p < 0.05). There were significant positive correlations between Pog-Ss and Co-Sh, Co-Me in the Class I and II groups (p < 0.05). Asymmetries for Co-Ss, Co-Sh, CP and SP between the left and right condyles exceeded a ratio of 20% for more than 30% of the subjects.

CONCLUSION

Condylar asymmetry varied significantly among the three skeletal groups, with the vertical position of the condyle (Co-Sh) and height of the mandibular ramus (Co-Me) being significantly and positively related to the chin position.

Temporomandibular joint pain: a critical role for Trpv4 in the trigeminal ganglion

Temporomandibular joint disorder (TMJD) is known for its mastication-associated pain. TMJD is medically relevant because of its prevalence, severity, chronicity, the therapy-refractoriness of its pain, and its largely elusive pathogenesis. Against this background, we sought to investigate the pathogenetic contributions of the calcium-permeable TRPV4 ion channel, robustly expressed in the trigeminal ganglion sensory neurons, to TMJ inflammation and pain behavior. We demonstrate here that TRPV4 is critical for TMJ-inflammation-evoked pain behavior in mice and that trigeminal ganglion pronociceptive changes are TRPV4-dependent. As a quantitative metric, bite force was recorded as evidence of masticatory sensitization, in keeping with human translational studies. In Trpv4(-/-) mice with TMJ inflammation, attenuation of bite force was significantly less than in wildtype (WT) mice. Similar effects were seen with systemic application of a specific TRPV4 inhibitor. TMJ inflammation and mandibular bony changes were apparent after injections of complete Freund adjuvant but were remarkably independent of the Trpv4 genotype. It was intriguing that, as a result of TMJ inflammation, WT mice exhibited significant upregulation of TRPV4 and phosphorylated extracellular-signal-regulated kinase (ERK) in TMJ-innervating trigeminal sensory neurons, which were absent in Trpv4(-/-) mice. Mice with genetically-impaired MEK/ERK phosphorylation in neurons showed resistance to reduction of bite force similar to that of Trpv4(-/-) mice. Thus, TRPV4 is necessary for masticatory sensitization in TMJ inflammation and probably functions upstream of MEK/ERK phosphorylation in trigeminal ganglion sensory neurons in vivo. TRPV4 therefore represents a novel pronociceptive target in TMJ inflammation and should be considered a target of interest in human TMJD.

Autonomous and nonautonomous roles of Hedgehog signaling in regulating limb muscle formation

Muscle progenitor cells migrate from the lateral somites into the developing vertebrate limb, where they undergo patterning and differentiation in response to local signals. Sonic hedgehog (Shh) is a secreted molecule made in the posterior limb bud that affects patterning and development of multiple tissues, including skeletal muscles. However, the cell-autonomous and non-cell-autonomous functions of Shh during limb muscle formation have remained unclear. We found that Shh affects the pattern of limb musculature non-cell-autonomously, acting through adjacent nonmuscle mesenchyme. However, Shh plays a cell-autonomous role in maintaining cell survival in the dermomyotome and initiating early activation of the myogenic program in the ventral limb. At later stages, Shh promotes slow muscle differentiation cell-autonomously. In addition, Shh signaling is required cell-autonomously to regulate directional muscle cell migration in the distal limb. We identify neuroepithelial cell transforming gene 1 (Net1) as a downstream target and effector of Shh signaling in that context.

Evolution of the mammalian middle ear and jaw: adaptations and novel structures

Having three ossicles in the middle ear is one of the defining features of mammals. All reptiles and birds have only one middle ear ossicle, the stapes or columella. How these two additional ossicles came to reside and function in the middle ear of mammals has been studied for the last 200 years and represents one of the classic example of how structures can change during evolution to function in new and novel ways. From fossil data, comparative anatomy and developmental biology it is now clear that the two new bones in the mammalian middle ear, the malleus and incus, are homologous to the quadrate and articular, which form the articulation for the upper and lower jaws in non-mammalian jawed vertebrates. The incorporation of the primary jaw joint into the mammalian middle ear was only possible due to the evolution of a new way to articulate the upper and lower jaws, with the formation of the dentary-squamosal joint, or TMJ in humans. The evolution of the three-ossicle ear in mammals is thus intricately connected with the evolution of a novel jaw joint, the two structures evolving together to create the distinctive mammalian skull.

Muenke syndrome mutation, FgfR3P²⁴⁴R, causes TMJ defects

Muenke syndrome is characterized by various craniofacial deformities and is caused by an autosomal-dominant activating mutation in fibroblast growth factor receptor 3 (FGFR3(P250R) ). Here, using mice carrying a corresponding mutation (FgfR3(P244R) ), we determined whether the mutation affects temporomandibular joint (TMJ) development and growth. In situ hybridization showed that FgfR3 was expressed in condylar chondroprogenitors and maturing chondrocytes that also expressed the Indian hedgehog (Ihh) receptor and transcriptional target Patched 1(Ptch1). In FgfR3(P244R) mutants, the condyles displayed reduced levels of Ihh expression, H4C-positive proliferating chondroprogenitors, and collagen type II- and type X-expressing chondrocytes. Primary bone spongiosa formation was also disturbed and was accompanied by increased osteoclastic activity and reduced trabecular bone formation. Treatment of wild-type condylar explants with recombinant FGF2/FGF9 decreased Ptch1 and PTHrP expression in superficial/polymorphic layers and proliferation in chondroprogenitors. We also observed early degenerative changes of condylar articular cartilage, abnormal development of the articular eminence/glenoid fossa in the TMJ, and fusion of the articular disc. Analysis of our data indicates that the activating FgfR3(P244R) mutation disturbs TMJ developmental processes, likely by reducing hedgehog signaling and endochondral ossification. We suggest that a balance between FGF and hedgehog signaling pathways is critical for the integrity of TMJ development and for the maintenance of cellular organization.

Trps1 is necessary for normal temporomandibular joint development

Mutation of the human TRPS1 gene leads to trichorhinophalangeal syndrome (TRPS), which is characterized by an abnormal development of various organs including the craniofacial skeleton. Trps1 has recently been shown to be expressed in the jaw joints of zebrafish; however, whether Trps1 is expressed in the mammalian temporomandibular joint (TMJ), or whether it is necessary for TMJ development is unknown. We have analyzed (1) the expression pattern of Trps1 during TMJ development in mice and (2) TMJ development in Trps1 knockout animals. Trps1 is expressed in the maxillo-mandibular junction at embryonic day (E) 11.5. At E15.5, expression is restricted to the developing condylar cartilage and to the surrounding joint disc progenitor cells. In Trps1 knockout mice, the glenoid fossa of the temporal bone forms relatively normally but the condylar process is extremely small and the joint disc and cavities do not develop. The initiation of condyle formation is slightly delayed in the mutants at E14.5; however, at E18.5, the flattened chondrocyte layer is narrowed and most of the condylar chondrocytes exhibit precocious chondrocyte maturation. Expression of Runx2 and its target genes is expanded toward the condylar apex in the mutants. These observations underscore the indispensable role played by Trps1 in normal TMJ development in supporting the differentiation of disc and synoviocyte progenitor cells and in coordinating condylar chondrocyte differentiation.

Spry1 and spry2 are essential for development of the temporomandibular joint

The temporomandibular joint (TMJ) is a specialized synovial joint essential for the function of the mammalian jaw. The main components of the TMJ are the mandibular condyle, the glenoid fossa of the temporal bone, and a fibrocartilagenous disc interposed between them. The genetic program for the development of the TMJ remains poorly understood. Here we show the crucial role of sprouty (Spry) genes in TMJ development. Sprouty genes encode intracellular inhibitors of receptor tyrosine kinase (RTK) signaling pathways, including those triggered by fibroblast growth factors (Fgfs). Using in situ hybridization, we show that Spry1 and Spry2 are highly expressed in muscles attached to the TMJ, including the lateral pterygoid and temporalis muscles. The combined inactivation of Spry1 and Spry2 results in overgrowth of these muscles, which disrupts normal development of the glenoid fossa. Remarkably, condyle and disc formation are not affected in these mutants, demonstrating that the glenoid fossa is not required for development of these structures. Our findings demonstrate the importance of regulated RTK signaling during TMJ development and suggest multiple skeletal origins for the fossa. Notably, our work provides the evidence that the TMJ condyle and disc develop independently of the mandibular fossa.

A Wnt-bmp feedback circuit controls intertissue signaling dynamics in tooth organogenesis

Many vertebrate organs form through the sequential and reciprocal exchange of signaling molecules between juxtaposed epithelial and mesenchymal tissues. We undertook a systems biology approach that combined the generation and analysis of large-scale spatiotemporal gene expression data with mouse genetic experiments to gain insight into the mechanisms that control epithelial-mesenchymal signaling interactions in the developing mouse molar tooth. We showed that the shift in instructive signaling potential from dental epithelium to dental mesenchyme was accompanied by temporally coordinated genome-wide changes in gene expression in both compartments. To identify the mechanism responsible, we developed a probabilistic technique that integrates regulatory evidence from gene expression data and from the literature to reconstruct a gene regulatory network for the epithelial and mesenchymal compartments in early tooth development. By integrating these epithelial and mesenchymal gene regulatory networks through the action of diffusible extracellular signaling molecules, we identified a key epithelial-mesenchymal intertissue Wnt-Bmp (bone morphogenetic protein) feedback circuit. We then validated this circuit in vivo with compound genetic mutations in mice that disrupted this circuit. Moreover, mathematical modeling demonstrated that the structure of the circuit accounted for the observed reciprocal signaling dynamics. Thus, we have identified a critical signaling circuit that controls the coordinated genome-wide expression changes and reciprocal signaling molecule dynamics that occur in interacting epithelial and mesenchymal compartments during organogenesis.

Gene expression profiling at early organogenesis reveals both common and diverse mechanisms in foregut patterning

The thyroid and lungs originate as neighboring bud shaped outgrowths from the midline of the embryonic foregut. When and how organ specific programs regulate development into structures of distinct shapes, positions and functions is incompletely understood. To characterize, at least in part, the genetic basis of these events, we have employed laser capture microdissection and microarray analysis to define gene expression in the mouse thyroid and lung primordia at E10.5. By comparing the transcriptome of each bud to that of the whole embryo as well as to each other, we broadly describe the genes that are preferentially expressed in each developing organ as well as those with an enriched expression common to both. The results thus obtained provide a valuable resource for further analysis of genes previously unrecognized to participate in thyroid and lung morphogenesis and to discover organ specific as well as common developmental mechanisms. As an initial step in this direction we describe a regulatory pathway involving the anti-apoptotic gene Bcl2 that controls cell survival in early thyroid development.

Tissue interaction is required for glenoid fossa development during temporomandibular joint formation

The mammalian temporomandibular joint (TMJ) develops from two distinct mesenchymal condensations that grow toward each other and ossify through different mechanisms, with the glenoid fossa undergoing intramembranous ossification while the condyle being endochondral in origin. In this study, we used various genetically modified mouse models to investigate tissue interaction between the condyle and glenoid fossa during TMJ formation in mice. We report that either absence or dislocation of the condyle results in an arrested glenoid fossa development. In both cases, glenoid fossa development was initiated, but failed to sustain, and became regressed subsequently. However, condyle development appears to be independent upon the presence of the forming glenoid fossa. In addition, we show that substitution of condyle by Meckel's cartilage is able to sustain glenoid fossa development. These observations suggest that proper signals from the developing condyle or Meckel's cartilage are required to sustain the glenoid fossa development.

Clinical findings in patients with GLI2 mutations--phenotypic variability

Mutations in the human GLI2 gene were first reported in association with defective anterior pituitary formation, panhypopituitarism, and forebrain anomalies represented by typical holoprosencephaly (HPE) and holoprosencephaly-like (HPE-L) phenotypes and postaxial polydactyly. Subsequently, anophthalmia plus orbital anomalies, heminasal aplasia, branchial arch anomalies and polydactyly have also been incorporated into the general phenotype. Here we described six Brazilian patients with phenotypic manifestations that range from isolated cleft lip/palate with polydactyly, branchial arch anomalies to semi-lobar holoprosencephaly. Novel sequence variants were found in the GLI2 gene in patients with marked involvement of the temporomandibular joint (TMJ), a new clinical finding observed with mutations of this gene. Clinical, molecular and genetic aspects are discussed.

Synovial joint morphogenesis requires the chondrogenic action of Sox5 and Sox6 in growth plate and articular cartilage

The mechanisms underlying synovial joint development remain poorly understood. Here we use complete and cell-specific gene inactivation to identify the roles of the redundant chondrogenic transcription factors Sox5 and Sox6 in this process. We show that joint development aborts early in complete mutants (Sox5(-/-)6(-/-)). Gdf5 and Wnt9a expression is punctual in articular progenitor cells, but Sox9 downregulation and cell condensation in joint interzones are late. Joint cell differentiation is unsuccessful, regardless of lineage, and cavitation fails. Sox5 and Sox6 restricted expression to chondrocytes in wild-type embryos and continued Erg expression and weak Ihh expression in Sox5(-/-)6(-/-) growth plates suggest that growth plate failure contribute to this Sox5(-/-)6(-/-) joint morphogenesis block. Sox5/6 inactivation in specified joint cells and chondrocytes (Sox5(fl/fl)6(fl/fl)Col2Cre) also results in a joint morphogenesis block, whereas Sox5/6 inactivation in specified joint cells only (Sox5(fl/fl)6(fl/fl)Gdf5Cre) results in milder joint defects and normal growth plates. Sox5(fl/fl)6(fl/fl)Gdf5Cre articular chondrocytes remain undifferentiated, as shown by continued Gdf5 expression and pancartilaginous gene downregulation. Along with Prg4 downregulation, these defects likely account for joint tissue overgrowth and incomplete cavitation in adult mice. Together, these data suggest that synovial joint morphogenesis relies on essential roles for Sox5/6 in promoting both growth plate and articular chondrocyte differentiation.