Tension–Induced Reduction in Connexin 43 Expression in Cranial Sutures Is Linked to Transcriptional Regulation by TBX2

FACEts of mechanical regulation in the morphogenesis of craniofacial structures

During embryonic development, organs undergo distinct and programmed morphological changes as they develop into their functional forms. While genetics and biochemical signals are well recognized regulators of morphogenesis, mechanical forces and the physical properties of tissues are now emerging as integral parts of this process as well. These physical factors drive coordinated cell movements and reorganizations, shape and size changes, proliferation and differentiation, as well as gene expression changes, and ultimately sculpt any developing structure by guiding correct cellular architectures and compositions. In this review we focus on several craniofacial structures, including the tooth, the mandible, the palate, and the cranium. We discuss the spatiotemporal regulation of different mechanical cues at both the cellular and tissue scales during craniofacial development and examine how tissue mechanics control various aspects of cell biology and signaling to shape a developing craniofacial organ.

Recent Advances in Craniosynostosis

Craniosynostosis is a pathologic craniofacial disorder and is defined as the premature fusion of one or more cranial (calvarial) sutures. Cranial sutures are fibrous joints consisting of nonossified mesenchymal cells that play an important role in the development of healthy craniofacial skeletons. Early fusion of these sutures results in incomplete brain development that may lead to complications of several severe medical conditions including seizures, brain damage, mental delay, complex deformities, strabismus, and visual and breathing problems. As a congenital disease, craniosynostosis has a heterogeneous origin that can be affected by genetic and epigenetic alterations, teratogens, and environmental factors and make the syndrome highly complex. To date, approximately 200 syndromes have been linked to craniosynostosis. In addition to being part of a syndrome, craniosynostosis can be nonsyndromic, formed without any additional anomalies. More than 50 nuclear genes that relate to craniosynostosis have been identified. Besides genetic factors, epigenetic factors like microRNAs and mechanical forces also play important roles in suture fusion. As craniosynostosis is a multifactorial disorder, evaluating the craniosynostosis syndrome requires and depends on all the information obtained from clinical findings, genetic analysis, epigenetic or environmental factors, or gene modulators. In this review, we will focus on embryologic and genetic studies, as well as epigenetic and environmental studies. We will discuss published studies and correlate the findings with unknown aspects of craniofacial disorders.

Tissue Dynamics: Lessons Learned From Sutural Morphogenesis and Cancer Growth

Why are cranial sutures the way they are? How do cancers grow? Merging physics and mathematics with biology, we develop equations describing these complex adaptive systems, to which all biological entities belong, calling them laws of tissue dynamics:Where t is time, E is energy, M is body mass, X is the biological characteristic of interest, C is a constant, a is an exponent.(1) is based on conservation of matter: for any given tissue, materials in must equal to materials out +/- assimilated or degraded. (2) is based on energy conservation. All living systems require energy, without which life becomes impossible. Equation (2) is a power spectrum.

OBJECTIVES

This study aimed to introduce the laws of tissue dynamics and to illustrate them using observations from craniofacial and cancer growth.

METHODS

We use cranial sutures as a model system to test Equation (1), we also measure the in vitro growth rate of normal murine liver and spleen cells, comparing them to B16F10 melanoma cells. We show the increase in compound growth rate and energetic requirement of malignant versus normal cells as partial proof of Equation (2).

RESULTS

The constant width and wavy form of cranial sutures are the inevitable results of repeated iteration from coupling of growth and stress. The compound growth rate of B10F16 melanoma cells exceeds that of normal cells by 1.0 to 1.5%, whereas their glucose uptake is equal to 3.6 billion glucose molecules/cell per minute.

SUMMARY

Living things are complex adaptive systems, thus a different way of thinking and investigating, going beyond the current reductive approach, is required.

Stretch force guides finger-like pattern of bone formation in suture

Mechanical tension is widely applied on the suture to modulate the growth of craniofacial bones. Deeply understanding the features of bone formation in expanding sutures could help us to improve the outcomes of clinical treatment and avoid some side effects. Although there are reports that have uncovered some biological characteristics, the regular pattern of sutural bone formation in response to expansion forces is still unknown. Our study was to investigate the shape, arrangement and orientation of new bone formation in expanding sutures and explore related clinical implications. The premaxillary sutures of rat, which histologically resembles the sutures of human beings, became wider progressively under stretch force. Micro-CT detected new bones at day 3. Morphologically, these bones were forming in a finger-like pattern, projecting from the maxillae into the expanded sutures. There were about 4 finger-like bones appearing on the selected micro-CT sections at day 3 and this number increased to about 18 at day 7. The average length of these projections increased from 0.14 mm at day 3 to 0.81 mm at day 7. The volume of these bony protuberances increased to the highest level of 0.12 mm³ at day 7. HE staining demonstrated that these finger-like bones had thick bases connecting with the maxillae and thin fronts stretching into the expanded suture. Nasal sections had a higher frequency of finger-like bones occuring than the oral sections at day 3 and day 5. Masson-stained sections showed stretched fibers embedding into maxillary margins. Osteocalcin-positive osteoblasts changed their shapes from cuboidal to spindle and covered the surfaces of finger-like bones continuously. Alizarin red S and calcein deposited in the inner and outer layers of finger-like bones respectively, which showed that longer and larger bones formed on the nasal side of expanded sutures compared with the oral side. Interestingly, these finger-like bones were almost paralleling with the direction of stretch force. Inclined force led to inclined finger-like bones formation and deflection of bilateral maxillae. Additionally, heavily compressive force caused fracture of finger-like bones in the sutures. These data together proposed the special finger-like pattern of bone formation in sutures guided by stretch force, providing important implications for maxillary expansion.

Tbx2a is required for specification of endodermal pouches during development of the pharyngeal arches

Tbx2 is a member of the T-box family of transcription factors essential for embryo- and organogenesis. A deficiency in the zebrafish paralogue tbx2a causes abnormalities of the pharyngeal arches in a p53-independent manner. The pharyngeal arches are formed by derivatives of all three embryonic germ layers: endodermal pouches, mesenchymal condensations and neural crest cells. While tbx2a expression is restricted to the endodermal pouches, its function is required for the normal morphogenesis of the entire pharyngeal arches. Given the similar function of Tbx1 in craniofacial development, we explored the possibility of an interaction between Tbx1 and Tbx2a. The use of bimolecular fluorescence complementation revealed the interaction between Tbx2a and Tbx1, thus providing support for the idea that functional interaction between different, co-expressed Tbx proteins could be a common theme across developmental processes in cell lineages and tissues. Together, this work provides mechanistic insight into the role of TBX2 in human disorders affecting the face and neck.

Timing of ectocranial suture activity in Gorilla gorilla as related to cranial volume and dental eruption

Research has shown that Pan and Homo have similar ectocranial suture synostosis patterns and a similar suture ontogeny (relative timing of suture fusion during the species ontogeny). This ontogeny includes patency during and after neurocranial expansion with a delayed bony response associated with adaptation to biomechanical forces generated by mastication. Here we investigate these relationships for Gorilla by examining the association among ectocranial suture morphology, cranial volume (as a proxy for neurocranial expansion) and dental development (as a proxy for the length of time that it has been masticating hard foods and exerting such strains on the cranial vault) in a large sample of Gorilla gorilla skulls. Two-hundred and fifty-five Gorilla gorilla skulls were examined for ectocranial suture closure status, cranial volume and dental eruption. Regression models were calculated for cranial volumes by suture activity, and Kendall's tau (a non-parametric measure of association) was calculated for dental eruption status by suture activity. Results suggest that, as reported for Pan and Homo, neurocranial expansion precedes suture synostosis activity. Here, Gorilla was shown to have a strong relationship between dental development and suture activity (synostosis). These data are suggestive of suture fusion extending further into ontogeny than brain expansion, similar to Homo and Pan. This finding allows for the possibility that masticatory forces influence ectocranial suture morphology.

Force-induced craniosynostosis in the murine sagittal suture

BACKGROUND

The cause of nonsyndromic craniosynostosis remains elusive. Although compressive forces have been implicated in premature suture fusion, conclusive evidence of force-induced craniosynostosis is lacking. The purpose of this study was to determine whether cyclical loading of the murine calvaria could induce suture fusion.

METHODS

Calvarial coupons from postnatal day-21, B6CBA, wild-type mice (n = 18) were harvested and cultured. A custom appliance capable of delivering controlled, cyclical, compressive loads was applied perpendicular to the sagittal suture within the coupon in vitro. Nine coupons were subjected to 0.3 g of force for 30 minutes each day for a total of 14 days. A control group of nine coupons was clamped in the appliance without loading. Analysis of suture phenotype was performed using alkaline phosphatase and hematoxylin and eosin staining techniques and in situ hybridization analysis using bone sialoprotein.

RESULTS

Control group sagittal sutures-which normally remain patent in mice-showed their customary histologic appearance. In contradistinction, sagittal sutures subjected to cyclic loading showed histologic evidence of premature fusion (craniosynostosis). In addition, alkaline phosphatase activity and bone sialoprotein expression were observed to be increased in the experimental group when compared with matched controls.

CONCLUSIONS

An in vitro model of force-induced craniosynostosis has been devised. Premature fusion of the murine sagittal suture was induced with the application of controlled, cyclical, compressive loads. These results implicate abnormal forces in the development of nonsyndromic craniosynostosis, which supports our global hypothesis that epigenetic phenomena play a crucial role in the pathogenesis of craniosynostosis.

Mechanical influences on suture development and patency

In addition to their role in skull growth, sutures are sites of flexibility between the more rigid bones. Depending on the suture, predominant loading during life may be either tensile or compressive. Loads are transmitted across sutures via collagenous fibers and a fluid-rich extracellular matrix and can be quasi-static (growth of neighboring tissues) or intermittent (mastication). The mechanical properties of sutures, while always viscoelastic, are therefore quite different for tensile versus compressive loading. The morphology of individual sutures reflects the nature of local loading, evidently by a process of developmental adaptation. In vivo or ex vivo, sutural cells respond to tensile or cyclic loading by expressing markers of proliferation and differentiation, whereas compressive loading appears to favor osteogenesis. Braincase and facial sutures exhibit similar mechanical behavior and reactions despite their different natural environments.

Msx1 and Msx2 are functional interacting partners of T-box factors in the regulation of Connexin43

AIMS

T-box factors Tbx2 and Tbx3 play key roles in the development of the cardiac conduction system, atrioventricular canal, and outflow tract of the heart. They regulate the gap-junction-encoding gene Connexin43 (Cx43) and other genes critical for heart development and function. Discovering protein partners of Tbx2 and Tbx3 will shed light on the mechanisms by which these factors regulate these gene programs.

METHODS AND RESULTS

Employing an yeast 2-hybrid screen and subsequent in vitro pull-down experiments we demonstrate that muscle segment homeobox genes Msx1 and Msx2 are able to bind the cardiac T-box proteins Tbx2, Tbx3, and Tbx5. This interaction, as that of the related Nkx2.5 protein, is supported by the T-box and homeodomain alone. Overlapping spatiotemporal expression patterns of Msx1 and Msx2 together with the T-box genes during cardiac development in mouse and chicken underscore the biological significance of this interaction. We demonstrate that Msx proteins together with Tbx2 and Tbx3 suppress Cx43 promoter activity and down regulate Cx43 gene activity in a rat heart-derived cell line. Using chromatin immunoprecipitation analysis we demonstrate that Msx1 can bind the Cx43 promoter at a conserved binding site located in close proximity to a previously defined T-box binding site, and that the activity of Msx proteins on this promoter appears dependent in the presence of Tbx3.

CONCLUSION

Msx1 and Msx2 can function in concert with the T-box proteins to suppress Cx43 and other working myocardial genes.

Connexin 32 and 43 gap junctions differentially modulate tenocyte response to cyclic mechanical load

Gap junctions allow rapid exchange of ions and small metabolites between cells. They can occur between connective tissue cells, and in tendons there are two prominent types, composed of connexin 32 or 43. These form distinct networks - tenocyte rows are linked by both longitudinally, but only by connexin 43 laterally. We hypothesised that the junctions had different roles in cell response to mechanical loading, and measured the effects of inhibitors of gap junction function on secretion of collagen by tenocyte cultures exposed to mechanical strain. Chicken tendon fibroblasts were exposed to cyclic tensile loading in the presence or absence of general gap junction inhibitors (halothane or the biomimetic peptide gap27), or antisense oligonucleotides to chicken connexin 32 or 43. Untreated cultures increased collagen secretion by around 25% under load. Halothane eliminated this response but caused cell damage. Gap27 peptide reduced secretion but maintained loading effects - strained cultures secreting more collagen than unstrained. Antisense downregulation showed major differences between connexins: antisense 32 reduced, and antisense 43 increased, collagen secretion. In both cases loading effects were maintained. This shows that (i) gap junctional integration of signals is important in load response of tenocyte populations - mechanotransduction occurs in individual cells but integration of signals markedly enhances it and (ii) communication via connexin 32 and 43 have differential effects on the load response, with connexin 32 being stimulatory and connexin 43 being inhibitory. Cells coordinate and control their response to mechanical signals at least in part by differential actions of these two types of gap junction.

Cellular response to force application at craniofacial sutures

OBJECTIVES

To provide a comprehensive review of the literature describing research done on the responses of suture cells to force application in vitro and in vivo.

DESIGN AND RESULTS

This review outlines the types of forces that can be applied, methods of applying the forces, the sutures used in experiments, and the changes in morphology, molecular biology (gene and protein expression), and cell biology (proliferation, differentiation, apoptosis) in response to these forces.

CONCLUSION

The molecular response of sutures to force needs to be further investigated as these molecules can be used to enhance the way in which craniofacial sutures respond to mechanical force during orthopedic-orthodontic treatment.

Connexin 40, a target of transcription factor Tbx5, patterns wrist, digits, and sternum

Haploinsufficiency of T-box transcription factor 5 (TBX5) causes human Holt-Oram syndrome (HOS), a developmental disorder characterized by skeletal and heart malformations. Mice carrying a Tbx5 null allele (Tbx5(+/Delta)) have malformations in digits, wrists, and sternum joints, regions where Tbx5 is expressed. We demonstrate that mice deficient in connexin 40 (Cx40), a Tbx5-regulated gap junction component, shared axial and appendicular skeletal malformations with Tbx5(+/Delta) mice. Although no role in skeleton patterning has been described for gap junctions, we demonstrate here that Cx40 is involved in formation of specific joints, as well as bone shape. Even a 50% reduction in either Tbx5 or Cx40 produces bone abnormalities, demonstrating their crucial control over skeletal development. Further, we demonstrate that Tbx5 exerts in part its key regulatory role in bone growth and maturation by controlling via Cx40 the expression of Sox9 (a transcription factor essential for chondrogenesis and skeleton growth). Our study strongly suggests that Cx40 deficiency accounts for many skeletal malformations in HOS and that Tbx5 regulation of Cx40 plays a critical role in the exquisite developmental patterning of the forelimbs and sternum.

“Culture shock” from the bone cell's perspective: emulating physiological conditions for mechanobiological investigations

Bone physiology can be examined on multiple length scales. Results of cell-level studies, typically carried out in vitro, are often extrapolated to attempt to understand tissue and organ physiology. Results of organ- or organism-level studies are often analyzed to deduce the state(s) of the cells within the larger system(s). Although phenomena on all of these scales—cell, tissue, organ, system, organism—are interlinked and contribute to the overall health and function of bone tissue, it is difficult to relate research among these scales. For example, groups of cells in an exogenous, in vitro environment that is well defined by the researcher would not be expected to function similarly to those in a dynamic, endogenous environment, dictated by systemic as well as organismal physiology. This review of the literature on bone cell culture describes potential causes and components of cell “culture shock,” i.e., behavioral variations associated with the transition from in vivo to in vitro environment, focusing on investigations of mechanotransduction and experimental approaches to mimic aspects of bone tissue on a macroscopic scale. The state of the art is reviewed, and new paradigms are suggested to begin bridging the gap between two-dimensional cell cultures in petri dishes and the three-dimensional environment of living bone tissue.