Muscle force regulates bone shaping for optimal load-bearing capacity during embryogenesis

Early initiation of endochondral ossification of mouse femur cultured in hydrogel with different mechanical stiffness

Mineralization is one of the most important processes in normal bone tissue development and in disease condition. Developing a novel and standardized in vitro model system that can readily monitor both cellular dynamics and mineralization is crucial for better understanding the bone tissue development and growth. Recent studies indicated that the mechanical environment is a critical condition in mineralization. We hypothesized that hydrogel with different mechanical stiffness can provide a biomimetic mechanical environment that can modulate bone tissue growth and mineralization. A femur of mouse embryo (embryonic day 16) was embedded in agarose hydrogel (2-60 kPa) and cultured in an osteogenic medium for a week. Microcomputed tomography (μCT) results revealed enhanced mineralization was detected in the femur head cultured in the gel condition, whereas no mineralization in the femur head cultured in the control (floating culture) condition. The mineralized region was corresponding to the region of secondary ossification center. Both histological and quantitative analyses indicated that the mineralized region of femur head cultured in 10 kPa gel condition was the highest and the mineralized area was significantly larger than that cultured in 2, 40, and 60 kPa gel condition. Immunofluorescence results indicated the enhanced mineralization caused by the higher chondrogenic differentiation at that region. This enhancement mainly relating to the mechanical forces and not to the oxygen tension was also confirmed. Since this system enhances and shortens the mineralization procedure compared with the conventional two-dimensional or three-dimensional cell culture system, this hydrogel system would be one of the unique models for better understanding the mineralized tissue development.

A mechanical Jack-like Mechanism drives spontaneous fracture healing in neonatal mice

Treatment of fractured bones involves correction of displacement or angulation, known as reduction. However, angulated long-bone fractures in infants often heal and regain proper morphology spontaneously, without reduction. To study the mechanism underlying spontaneous regeneration of fractured bones, we left humeral fractures induced in newborn mice unstabilized, and rapid realignment of initially angulated bones was seen. This realignment was surprisingly not mediated by bone remodeling, but instead involved substantial movement of the two fragments prior to callus ossification. Analysis of gene expression profiles, cell proliferation, and bone growth revealed the formation of a functional, bidirectional growth plate at the concave side of the fracture. This growth plate acts like a mechanical jack, generating opposing forces that straighten the two fragments. Finally, we show that muscle force is important in this process, as blocking muscle contraction disrupts growth plate formation, leading to premature callus ossification and failed reduction.

Hox genes and limb musculoskeletal development

In the musculoskeletal system, muscle, tendon, and bone tissues develop in a spatially and temporally coordinated manner, and integrate into a cohesive functional unit by forming specific connections unique to each region of the musculoskeletal system. The mechanisms of these patterning and integration events are an area of great interest in musculoskeletal biology. Hox genes are a family of important developmental regulators and play critical roles in skeletal patterning throughout the axial and appendicular skeleton. Unexpectedly, Hox genes are not expressed in the differentiated cartilage or other skeletal cells, but rather are highly expressed in the tightly associated stromal connective tissues as well as regionally expressed in tendons and muscle connective tissue. Recent work has revealed a previously unappreciated role for Hox in patterning all the musculoskeletal tissues of the limb. These observations suggest that integration of the musculoskeletal system is regulated, at least in part, by Hox function in the stromal connective tissue. This review will outline our current understanding of Hox function in patterning and integrating the musculoskeletal tissues.

Relationship between bent long bones, bent scapulae, and wavy ribs: malformations or variations?

BACKGROUND

Shortened and bent long bones and bent scapulae are sometimes reported in fetuses with wavy ribs (Carney and Kimmel, ). Wavy ribs are typically seen in the presence of maternal and developmental toxicity, are transient and reversible postnatally, and are considered to be variations rather than malformations.

METHODS

We further assessed the literature cited in Kimmel and Carney () as well as papers published since then to determine under what conditions bent long bones in the absence of gross limb defects and bent scapulae were reported and whether information was available on the transient or permanent nature of these effects.

RESULTS

Long bone and/or scapular changes almost always occurred at a lower incidence than wavy ribs. In every case, maternal and fetal toxicity occurred at the same dose levels. In a few studies, pups were followed sequentially after birth and bent long bones and scapulae were transient in nature and appeared normal by the time of weaning. Rabbits were much less likely to show wavy ribs or long bone and scapular changes at birth, even in the presence of severe maternal and fetal toxicity. This species difference may be due in part to the great increase in bone mass and remodeling that occurs during the first few postnatal weeks in rodents, but which takes place during the longer fetal period in rabbits.

CONCLUSION

Our conclusion from this review is that bent long bones and scapulae, like wavy ribs, appear to be secondary to maternal and developmental toxicity, are transient, and like wavy ribs should be considered variations rather than malformations.

Therapies for musculoskeletal disease: can we treat two birds with one stone?

Musculoskeletal diseases are highly prevalent with staggering annual health care costs across the globe. The combined wasting of muscle (sarcopenia) and bone (osteoporosis)-both in normal aging and pathologic states-can lead to vastly compounded risk for fracture in patients. Until now, our therapeutic approach to the prevention of such fractures has focused solely on bone, but our increasing understanding of the interconnected biology of muscle and bone has begun to shift our treatment paradigm for musculoskeletal disease. Targeting pathways that centrally regulate both bone and muscle (eg, GH/IGF-1, sex steroids, etc.) and newly emerging pathways that might facilitate communication between these 2 tissues (eg, activin/myostatin) might allow a greater therapeutic benefit and/or previously unanticipated means by which to treat these frail patients and prevent fracture. In this review, we will discuss a number of therapies currently under development that aim to treat musculoskeletal disease in precisely such a holistic fashion.

The role of muscle loading on bone (Re)modeling at the developing enthesis

Muscle forces are necessary for the development and maintenance of a mineralized skeleton. Removal of loads leads to malformed bones and impaired musculoskeletal function due to changes in bone (re)modeling. In the current study, the development of a mineralized junction at the interface between muscle and bone was examined under normal and impaired loading conditions. Unilateral mouse rotator cuff muscles were paralyzed using botulinum toxin A at birth. Control groups consisted of contralateral shoulders injected with saline and a separate group of normal mice. It was hypothesized that muscle unloading would suppress bone formation and enhance bone resorption at the enthesis, and that the unloading-induced bony defects could be rescued by suppressing osteoclast activity. In order to modulate osteoclast activity, mice were injected with the bisphosphonate alendronate. Bone formation was measured at the tendon enthesis using alizarin and calcein fluorescent labeling of bone surfaces followed by quantitative histomorphometry of histologic sections. Bone volume and architecture was measured using micro computed tomography. Osteoclast surface was determined via quantitative histomorphometry of tartrate resistant acid phosphatase stained histologic sections. Muscle unloading resulted in delayed initiation of endochondral ossification at the enthesis, but did not impair bone formation rate. Unloading led to severe defects in bone volume and trabecular bone architecture. These defects were partially rescued by suppression of osteoclast activity through alendronate treatment, and the effect of alendronate was dose dependent. Similarly, bone formation rate was increased with increasing alendronate dose across loading groups. The bony defects caused by unloading were therefore likely due to maintained high osteoclast activity, which normally decreases from neonatal through mature timepoints. These results have important implications for the treatment of muscle unloading conditions such as neonatal brachial plexus palsy, which results in shoulder paralysis at birth and subsequent defects in the rotator cuff enthesis and humeral head.

Mechanoadaptation of developing limbs: shaking a leg

The proportion of total limb length taken up by the individual skeletal elements (limb proportionality), varies widely between species. These diverse skeletal forms have evolved to allow for a range of limb uses and they first emerge as the embryo develops, to achieve the characteristic skeletal architecture of each species. During this time, the developing skeleton experiences mechanical loading as a result of embryonic muscle contraction. The possibility that adaptation to such mechanical input may allow embryos to coordinate the appearance of skeletal design with their expanding range of movements has so far received little attention. This is surprising, given the critical role exerted by embryo movement in normal skeletal development; stage-specific in ovo immobilisation of embryonic chicks results in joint contractures and a reduction in longitudinal bone growth in the limbs. Epigenetic mechanisms allow for selective activation of genes in response to environmental signals, resulting in the production of phenotypic complexity in morphogenesis; mechanical loading of bone during movement appears to be one such signal. It may be that 'mechanosensitive' genes under regulation of mechanical input adjust proportionality along the bone's proximo-distal axis, introducing a level of phenotypic plasticity. If this hypothesis is upheld, species with more elongated distal limb elements will have a greater dependence on mechanical input for the differences in their growth, and mechanosensitive bone growth in the embryo may have evolved as an additional source of phenotypic diversity during skeletal development.

Muscle-bone interactions: basic and clinical aspects

Muscle and bone are anatomically and functionally closely connected. The traditional concept that skeletal muscles serve to load bone and transform skeletal segments into a system of levers has been further refined into the mechanostat theory, according to which striated muscle is essential for bone development and maintenance, modelling and remodelling. Besides biomechanical function, skeletal muscle and bone are endocrine organs able to secrete factors capable of modulating biological function within their microenvironment, in nearby tissues or in distant organs. The endocrine properties of muscle and bone may serve to sense and transduce biomechanical signals such as loading, unloading or exercise, or systemic hormonal stimuli into biochemical signals. Nonetheless, given the close anatomical relationship between skeletal muscle and bone, paracrine interactions particularly at the periosteal interface can be hypothesized. These mechanisms can assume particular importance during bone and muscle healing after musculoskeletal injury. Basic studies in vitro and in rodents have helped to dissect the multiple influences of skeletal muscle on bone and/or expression of inside-organ metabolism and have served to explain clinical observations linking muscle-to-bone quality. Recent evidences pinpoint that also bone tissue is able to modulate directly or indirectly skeletal muscle metabolism, thus empowering the crosstalk hypothesis to be further tested in humans in vivo.

Interaction between Muscle and Bone

The clinical significance of sarcopenia and osteoporosis has increased with the increase in the population of older people. Sarcopenia is defined by decreased muscle mass and impaired muscle function, which is related to osteoporosis independently and dependently. Numerous lines of clinical evidence suggest that lean body mass is positively related to bone mass, which leads to reduced fracture risk. Genetic, endocrine and mechanical factors affect both muscle and bone simultaneously. Vitamin D, the growth hormone/insulin-like growth factor I axis and testosterone are physiologically and pathologically important as endocrine factors. These findings suggest the presence of interactions between muscle and bone, which might be very important for understanding the physiology and pathophysiology of sarcopenia and osteoporosis. Muscle/bone relationships include two factors: local control of muscle to bone and systemic humoral interactions between muscle and bone. As a putative local inducer of muscle ossification, we found Tmem119, a parathyroid hormone-responsive osteoblast differentiation factor. Moreover, osteoglycin might be one of the muscle-derived humoral bone anabolic factors. This issue may be important for the development of novel drugs and biomarkers for osteoporosis and sarcopenia. Further research will be necessary to clarify the details of the linkage of muscle and bone.

Identification of mechanosensitive genes during skeletal development: alteration of genes associated with cytoskeletal rearrangement and cell signalling pathways

Background

Mechanical stimulation is necessary for regulating correct formation of the skeleton. Here we test the hypothesis that mechanical stimulation of the embryonic skeletal system impacts expression levels of genes implicated in developmentally important signalling pathways in a genome wide approach. We use a mutant mouse model with altered mechanical stimulation due to the absence of limb skeletal muscle (Splotch-delayed) where muscle-less embryos show specific defects in skeletal elements including delayed ossification, changes in the size and shape of cartilage rudiments and joint fusion. We used Microarray and RNA sequencing analysis tools to identify differentially expressed genes between muscle-less and control embryonic (TS23) humerus tissue.

Results

We found that 680 independent genes were down-regulated and 452 genes up-regulated in humeri from muscle-less Spd embryos compared to littermate controls (at least 2-fold; corrected p-value ≤0.05). We analysed the resulting differentially expressed gene sets using Gene Ontology annotations to identify significant enrichment of genes associated with particular biological processes, showing that removal of mechanical stimuli from muscle contractions affected genes associated with development and differentiation, cytoskeletal architecture and cell signalling. Among cell signalling pathways, the most strongly disturbed was Wnt signalling, with 34 genes including 19 pathway target genes affected. Spatial gene expression analysis showed that both a Wnt ligand encoding gene (Wnt4) and a pathway antagonist (Sfrp2) are up-regulated specifically in the developing joint line, while the expression of a Wnt target gene, Cd44, is no longer detectable in muscle-less embryos. The identification of 84 genes associated with the cytoskeleton that are down-regulated in the absence of muscle indicates a number of candidate genes that are both mechanoresponsive and potentially involved in mechanotransduction, converting a mechanical stimulus into a transcriptional response.

Conclusions

This work identifies key developmental regulatory genes impacted by altered mechanical stimulation, sheds light on the molecular mechanisms that interpret mechanical stimulation during skeletal development and provides valuable resources for further investigation of the mechanistic basis of mechanoregulation. In particular it highlights the Wnt signalling pathway as a potential point of integration of mechanical and molecular signalling and cytoskeletal components as mediators of the response.

Overview of skeletal development

Development of cartilage and bone, the core components of the mouse skeletal system, depends on the well-coordinated proliferation and differentiation of skeletogenic cells, including chondrocytes and osteoblasts. These cells differentiate from common progenitor cells originating in the mesoderm and neural crest. Multiple signaling pathways and transcription factors tightly regulate differentiation and proliferation of skeletal cells. In this chapter, we overview the process of mouse skeletal development and discuss major regulators of skeletal cells at each developmental stage.

Interdependence of muscle atrophy and bone loss induced by mechanical unloading

Mechanical unloading induces muscle atrophy and bone loss; however, the time course and interdependence of these effects is not well defined. We subjected 4-month-old C57BL/6J mice to hindlimb suspension (HLS) for 3 weeks, euthanizing 12 to 16 mice on day (D) 0, 7, 14, and 21. Lean mass was 7% to 9% lower for HLS versus control from D7-21. Absolute mass of the gastrocnemius (gastroc) decreased 8% by D7, and was maximally decreased 16% by D14 of HLS. mRNA levels of Atrogin-1 in the gastroc and quadriceps (quad) were increased 99% and 122%, respectively, at D7 of HLS. Similar increases in MuRF1 mRNA levels occurred at D7. Both atrogenes returned to baseline by D14. Protein synthesis in gastroc and quad was reduced 30% from D7-14 of HLS, returning to baseline by D21. HLS decreased phosphorylation of SK61, a substrate of mammalian target of rapamycin (mTOR), on D7-21, whereas 4E-BP1 was not lower until D21. Cortical thickness of the femur and tibia did not decrease until D14 of HLS. Cortical bone of controls did not change over time. HLS mice had lower distal femur bone volume fraction (-22%) by D14; however, the effects of HLS were eliminated by D21 because of the decline of trabecular bone mass of controls. Femur strength was decreased approximately 13% by D14 of HLS, with no change in tibia mechanical properties at any time point. This investigation reveals that muscle atrophy precedes bone loss during unloading and may contribute to subsequent skeletal deficits. Countermeasures that preserve muscle may reduce bone loss induced by mechanical unloading or prolonged disuse. Trabecular bone loss with age, similar to that which occurs in mature astronauts, is superimposed on unloading. Preservation of muscle mass, cortical structure, and bone strength during the experiment suggests muscle may have a greater effect on cortical than trabecular bone.

The linea aspera: a virtual case study testing emergence of form and function

The linea aspera (LA) forms a characteristic ridge along the posterior aspect of the human femur. Absent in youth, the LA emerges during early puberty and becomes more prominent with advancing age. Pauwels, a pioneer of mechanobiology, hypothesized that the LA forms in the precise location where axial intracortical stresses are greatest, effectively "stiffening" the femur in bending. This study reassesses the mechanical role of the LA in virtual models of human femora, accounting for increasing prominence of the LA at juvenile, young adult and aged stages. Using finite element analysis, peak stresses and the relationship between the LA, neutral axis and centroidal axes (CAs) are evaluated for cross-sections along the mid-diaphysis of the virtual femora. Additionally, the relationship between LA and CAs is studied in anatomical cross-sections from Pauwels' manuscript as well as aged cadaveric donors, indicating that his conclusion may have been stymied by lack of modern computational methods. The results of the current study do not support a mechanical role of the LA as its emergence results in less than a 3% decrease in peak stress, while increasing prominence of the LA also serves to rotate CAs away from calculated stress field, implicating a less "bone centric" view of form and function.

Mechanobiology and developmental control

Morphogenesis is the remarkable process by which cells self-assemble into complex tissues and organs that exhibit specialized form and function during embryological development. Many of the genes and chemical cues that mediate tissue and organ formation have been identified; however, these signals alone are not sufficient to explain how tissues and organs are constructed that exhibit their unique material properties and three-dimensional forms. Here, we review work that has revealed the central role that physical forces and extracellular matrix mechanics play in the control of cell fate switching, pattern formation, and tissue development in the embryo and how these same mechanical signals contribute to tissue homeostasis and developmental control throughout adult life.

Forum on bone and skeletal muscle interactions: summary of the proceedings of an ASBMR workshop

Annual costs are enormous for musculoskeletal diseases such as osteoporosis and sarcopenia and for bone and muscle injuries, costing billions annually in health care. Although it is clear that muscle and bone development, growth, and function are connected, and that muscle loads bone, little is known regarding cellular and molecular interactions between these two tissues. A conference supported by the National Institutes of Health (NIH) and the American Society for Bone and Mineral Research (ASBMR) was held in July 2012 to address the enormous burden of musculoskeletal disease. National and international experts in either bone or muscle presented their findings and their novel hypotheses regarding muscle-bone interactions to stimulate the exchange of ideas between these two fields. The immediate goal of the conference was to identify critical research themes that would lead to collaborative research interactions and grant applications focusing on interactions between muscle and bone. The ultimate goal of the meeting was to generate a better understanding of how these two tissues integrate and crosstalk in both health and disease to stimulate new therapeutic strategies to enhance and maintain musculoskeletal health.

Normal function of Myf5 during gastrulation is required for pharyngeal arch cartilage development in zebrafish embryos

Myf5, a myogenic regulatory factor, plays a key role in regulating muscle differentiation. However, it is not known if Myf5 has a regulatory role during early embryogenesis. Here, we used myf5-morpholino oligonucleotides [MO] to knock down myf5 expression and demonstrated a series of results pointing to the functional roles of Myf5 during early embryogenesis: (1) reduced head size resulting from abnormal morphology in the cranial skeleton; (2) decreased expressions of the cranial neural crest (CNC) markers foxd3, sox9a, dlx2, and col2a1; (3) defect in the chondrogenic neural crest similar to that of fgf3 morphants; (4) reduced fgf3/fgf8 transcripts in the cephalic mesoderm rescued by co-injection of myf5 wobble-mismatched mRNA together with myf5-MO1 during 12 h postfertilization; (5) abnormal patterns of axial and non-axial mesoderm causing expansion of the dorsal organizer, and (6) increased bmp4 gradient, but reduced fgf3/fgf8 marginal gradient, during gastrulation. Interestingly, overexpression of fgf3 could rescue the cranial cartilage defects caused by myf5-MO1, suggesting that Myf5 modulates craniofacial cartilage development through the fgf3 signaling pathway. Together, the loss of Myf5 function results in a cascade effect that begins with abnormal formation of the dorsal organizer during gastrulation, causing, in turn, defects in the CNC and cranial cartilage of myf5-knockdown embryos.

Bone and skeletal muscle: neighbors with close ties

The musculoskeletal system evolved in mammals to perform diverse functions that include locomotion, facilitating breathing, protecting internal organs, and coordinating global energy expenditure. Bone and skeletal muscles involved with locomotion are both derived from somitic mesoderm and accumulate peak tissue mass synchronously, according to genetic information and environmental stimuli. Aging results in the progressive and parallel loss of bone (osteopenia) and skeletal muscle (sarcopenia) with profound consequences for quality of life. Age-associated sarcopenia results in reduced endurance, poor balance, and reduced mobility that predispose elderly individuals to falls, which more frequently result in fracture because of concomitant osteoporosis. Thus, a better understanding of the mechanisms underlying the parallel development and involution of these tissues is critical to developing new and more effective means to combat osteoporosis and sarcopenia in our increasingly aged population. This perspective highlights recent advances in our understanding of mechanisms coupling bone and skeletal muscle mass, and identify critical areas where further work is needed.

Notch signalling is required for the formation of structurally stable muscle fibres in zebrafish

BACKGROUND

Accurate regulation of Notch signalling is central for developmental processes in a variety of tissues, but its function in pectoral fin development in zebrafish is still unknown.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that core elements necessary for a functional Notch pathway are expressed in developing pectoral fins in or near prospective muscle territories. Blocking Notch signalling at different levels of the pathway consistently leads to the formation of thin, wavy, fragmented and mechanically weak muscles fibres and loss of stress fibres in endoskeletal disc cells in pectoral fins. Although the structural muscle genes encoding Desmin and Vinculin are normally transcribed in Notch-disrupted pectoral fins, their proteins levels are severely reduced, suggesting that weak mechanical forces produced by the muscle fibres are unable to stabilize/localize these proteins. Moreover, in Notch signalling disrupted pectoral fins there is a decrease in the number of Pax7-positive cells indicative of a defect in myogenesis.

CONCLUSIONS/SIGNIFICANCE

We propose that by controlling the differentiation of myogenic progenitor cells, Notch signalling might secure the formation of structurally stable muscle fibres in the zebrafish pectoral fin.

Mechanical regulation of skeletal development

Development of the various components of a normal skeleton requires highly regulated signalling systems that co-ordinate spatial and temporal patterns of cell division, cell differentiation, and morphogenesis. Much work in recent decades has revealed cascades of molecular signalling, acting through key transcription factors to regulate, for example, organized chondrogenic and osteogenic differentiation. It is now clear that mechanical stimuli are also required for aspects of skeletogenesis but very little is known about how the mechanical signals are integrated with classic biochemical signalling. Spatially organized differentiation is vital to the production of functionally appropriate tissues contributing to precise, region specific morphologies, for example transient chondrogenesis of long bone skeletal rudiments, which prefigures osteogenic replacement of the cartilage template, compared with the production of permanent cartilage at the sites of articulation. Currently a lack of understanding of how these tissues are differentially regulated hampers efforts to specifically regenerate stable bone and cartilage. Here, we review current research revealing the influence of mechanical stimuli on specific aspects of skeletal development and refer to other developing systems to set the scene for current and future work to uncover the molecular mechanisms involved. We integrate this with a brief overview of the effects of mechanical stimulation on stem cells in culture bringing together developmental and tissue engineering aspects of mechanoregulation of cell behavior. A better understanding of the molecular mechanisms that link mechanical stimuli to transcriptional control guiding cell differentiation will lead to new ideas about how to effectively prime stem cells for tissue engineering and regenerative therapies.

Embryology of Early Jurassic dinosaur from China with evidence of preserved organic remains

Fossil dinosaur embryos are surprisingly rare, being almost entirely restricted to Upper Cretaceous strata that record the late stages of non-avian dinosaur evolution. Notable exceptions are the oldest known embryos from the Early Jurassic South African sauropodomorph Massospondylus and Late Jurassic embryos of a theropod from Portugal. The fact that dinosaur embryos are rare and typically enclosed in eggshells limits their availability for tissue and cellular level investigations of development. Consequently, little is known about growth patterns in dinosaur embryos, even though post-hatching ontogeny has been studied in several taxa. Here we report the discovery of an embryonic dinosaur bone bed from the Lower Jurassic of China, the oldest such occurrence in the fossil record. The embryos are similar in geological age to those of Massospondylus and are also assignable to a sauropodomorph dinosaur, probably Lufengosaurus. The preservation of numerous disarticulated skeletal elements and eggshells in this monotaxic bone bed, representing different stages of incubation and therefore derived from different nests, provides opportunities for new investigations of dinosaur embryology in a clade noted for gigantism. For example, comparisons among embryonic femora of different sizes and developmental stages reveal a consistently rapid rate of growth throughout development, possibly indicating that short incubation times were characteristic of sauropodomorphs. In addition, asymmetric radial growth of the femoral shaft and rapid expansion of the fourth trochanter suggest that embryonic muscle activation played an important role in the pre-hatching ontogeny of these dinosaurs. This discovery also provides the oldest evidence of in situ preservation of complex organic remains in a terrestrial vertebrate.

A novel syndrome caused by the E410K amino acid substitution in the neuronal β-tubulin isotype 3

Missense mutations in TUBB3, the gene that encodes the neuronal-specific protein β-tubulin isotype 3, can cause isolated or syndromic congenital fibrosis of the extraocular muscles, a form of complex congenital strabismus characterized by cranial nerve misguidance. One of the eight TUBB3 mutations reported to cause congenital fibrosis of the extraocular muscles, c.1228G>A results in a TUBB3 E410K amino acid substitution that directly alters a kinesin motor protein binding site. We report the detailed phenotypes of eight unrelated individuals who harbour this de novo mutation, and thus define the 'TUBB3 E410K syndrome'. Individuals harbouring this mutation were previously reported to have congenital fibrosis of the extraocular muscles, facial weakness, developmental delay and possible peripheral neuropathy. We now confirm by electrophysiology that a progressive sensorimotor polyneuropathy does indeed segregate with the mutation, and expand the TUBB3 E410K phenotype to include Kallmann syndrome (hypogonadotropic hypogonadism and anosmia), stereotyped midface hypoplasia, intellectual disabilities and, in some cases, vocal cord paralysis, tracheomalacia and cyclic vomiting. Neuroimaging reveals a thin corpus callosum and anterior commissure, and hypoplastic to absent olfactory sulci, olfactory bulbs and oculomotor and facial nerves, which support underlying abnormalities in axon guidance and maintenance. Thus, the E410K substitution defines a new genetic aetiology for Moebius syndrome, Kallmann syndrome and cyclic vomiting. Moreover, the c.1228G>A mutation was absent in DNA from ∼600 individuals who had either Kallmann syndrome or isolated or syndromic ocular and/or facial dysmotility disorders, but who did not have the combined features of the TUBB3 E410K syndrome, highlighting the specificity of this phenotype-genotype correlation. The definition of the TUBB3 E410K syndrome will allow clinicians to identify affected individuals and predict the mutation based on clinical features alone.

A temporary decrease in mineral density in perinatal mouse long bones

Fetal and postnatal bone development in humans is traditionally viewed as a process characterized by progressively increasing mineral density. Yet, a temporary decrease in mineral density has been described in the long bones of infants in the immediate postnatal period. The mechanism that underlies this phenomenon, as well as its causes and consequences, remain unclear. Using daily μCT scans of murine femora and tibiae during perinatal development, we show that a temporary decrease in tissue mineral density (TMD) is evident in mice. By monitoring spatial and temporal structural changes during normal growth and in a mouse strain in which osteoclasts are non-functional (Src-null), we show that endosteal bone resorption is the main cause for the perinatal decrease in TMD. Mechanical testing revealed that this temporary decrease is correlated with reduced stiffness of the bones. We also show, by administration of a progestational agent to pregnant mice, that the decrease in TMD is not the result of parturition itself. This study provides a comprehensive view of perinatal long bone development in mice, and describes the process as well as the consequences of density fluctuation during this period.

The skeletal site-specific role of connective tissue growth factor in prenatal osteogenesis

BACKGROUND

Connective tissue growth factor (CTGF/CCN2) is a matricellular protein that is highly expressed during bone development. Mice with global CTGF ablation (knockout, KO) have multiple skeletal dysmorphisms and perinatal lethality. A quantitative analysis of the bone phenotype has not been conducted.

RESULTS

We demonstrated skeletal site-specific changes in growth plate organization, bone microarchitecture, and shape and gene expression levels in CTGF KO compared with wild-type mice. Growth plate malformations included reduced proliferation zone and increased hypertrophic zone lengths. Appendicular skeletal sites demonstrated decreased metaphyseal trabecular bone, while having increased mid-diaphyseal bone and osteogenic expression markers. Axial skeletal analysis showed decreased bone in caudal vertebral bodies, mandibles, and parietal bones in CTGF KO mice, with decreased expression of osteogenic markers. Analysis of skull phenotypes demonstrated global and regional differences in CTGF KO skull shape resulting from allometric (size-based) and nonallometric shape changes. Localized differences in skull morphology included increased skull width and decreased skull length. Dysregulation of the transforming growth factor-β-CTGF axis coupled with unique morphologic traits provides a potential mechanistic explanation for the skull phenotype.

CONCLUSIONS

We present novel data on a skeletal phenotype in CTGF KO mice, in which ablation of CTGF causes site-specific aberrations in bone formation.

Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension

Convergent extension driven by mediolateral intercalation of chondrocytes is a key process that contributes to skeletal growth and morphogenesis. While progress has been made in deciphering the molecular mechanism that underlies this process, the involvement of mechanical load exerted by muscle contraction in its regulation has not been studied. Using the zebrafish as a model system, we found abnormal pharyngeal cartilage morphology in both chemically and genetically paralyzed embryos, demonstrating the importance of muscle contraction for zebrafish skeletal development. The shortening of skeletal elements was accompanied by prominent changes in cell morphology and organization. While in control the cells were elongated, chondrocytes in paralyzed zebrafish were smaller and exhibited a more rounded shape, confirmed by a reduction in their length-to-width ratio. The typical columnar organization of cells was affected too, as chondrocytes in various skeletal elements exhibited abnormal stacking patterns, indicating aberrant intercalation. Finally, we demonstrate impaired chondrocyte intercalation in growth plates of muscle-less Sp(d) mouse embryos, implying the evolutionary conservation of muscle force regulation of this essential morphogenetic process.Our findings provide a new perspective on the regulatory interaction between muscle contraction and skeletal morphogenesis by uncovering the role of muscle-induced mechanical loads in regulating chondrocyte intercalation in two different vertebrate models.