Cell Mechanics of Craniosynostosis

Atypical sagittal suture craniosynostosis: pathological considerations for early closure of the anterior part of the sagittal suture

Sagittal suture synostosis is one of the most common craniosynostoses and is often diagnosed by characteristic narrow and long skull shape, scaphocephaly. However, some patients with sagittal suture synostosis do not present with typical scaphocephaly, making early diagnosis difficult. In this study, five cases of characteristic skull deformity showing a narrowing of the cranium posterior to the coronal suture on computed tomography (CT) are presented. The three older children presented with papilledema and intellectual disability and a closed sagittal suture on CT. The two infant cases were diagnosed with the characteristic cranial deformities with aggravation of the deformity over time, but sagittal suture closure was not evident on CT. All patients underwent cranial remodeling surgery. In the two infant cases, the histopathological findings showed that the anterior part of the sagittal suture was firmly fused with fibrous tissue without bony fusion. These findings suggested that narrowing of the cranium posterior to the coronal suture might be due to functional fusion of the anterior portion of the sagittal suture prior to bony fusion. In an infant presenting with such a deformity that shows aggravation of the deformity over time, surgical treatment should be considered.

Bone mineral density loci specific to the skull portray potential pleiotropic effects on craniosynostosis

Skull bone mineral density (SK-BMD) provides a suitable trait for the discovery of key genes in bone biology, particularly to intramembranous ossification, not captured at other skeletal sites. We perform a genome-wide association meta-analysis (n ~ 43,800) of SK-BMD, identifying 59 loci, collectively explaining 12.5% of the trait variance. Association signals cluster within gene-sets involved in skeletal development and osteoporosis. Among the four novel loci (ZIC1, PRKAR1A, AZIN1/ATP6V1C1, GLRX3), there are factors implicated in intramembranous ossification and as we show, inherent to craniosynostosis processes. Functional follow-up in zebrafish confirms the importance of ZIC1 on cranial suture patterning. Likewise, we observe abnormal cranial bone initiation that culminates in ectopic sutures and reduced BMD in mosaic atp6v1c1 knockouts. Mosaic prkar1a knockouts present asymmetric bone growth and, conversely, elevated BMD. In light of this evidence linking SK-BMD loci to craniofacial abnormalities, our study provides new insight into the pathophysiology, diagnosis and treatment of skeletal diseases.

Physiology and Clinical Manifestations of Pathologic Cranial Suture Widening

Cranial sutures are complex structures integrating mechanical forces with osteogenesis which are often affected in craniofacial syndromes. While premature fusion is frequently described, rare pathological widening of cranial sutures is a comparatively understudied phenomenon. This narrative review aims to bring to light the biologically variable underlying causes of widened sutures and persistent fontanelles leading to a common outcome. The authors herein present four syndromes, selected from a literature review, and their identified biological mechanisms in the context of altered suture physiology, exploring the roles of progenitor cell differentiation, extracellular matrix production, mineralization, and bone resorption. This article illustrates the gaps in understanding of complex craniofacial disorders, and the potential for further unification of genetics, cellular biology, and clinical pillars of health science research to improve treatment outcomes for patients.

Localized glucose import, glycolytic processing, and mitochondria generate a focused ATP burst to power basement-membrane invasion

Invasive cells use transient, energy-consuming protrusions to breach basement membrane (BM) barriers. Using the ATP sensor PercevalHR during anchor cell (AC) invasion in Caenorhabditis elegans, we show that BM invasion is accompanied by an ATP burst from mitochondria at the invasive front. RNAi screening and visualization of a glucose biosensor identified two glucose transporters, FGT-1 and FGT-2, which bathe invasive front mitochondria with glucose and facilitate the ATP burst to form protrusions. FGT-1 localizes at high levels along the invasive membrane, while FGT-2 is adaptive, enriching most strongly during BM breaching and when FGT-1 is absent. Cytosolic glycolytic enzymes that process glucose for mitochondrial ATP production cluster with invasive front mitochondria and promote higher mitochondrial membrane potential and ATP levels. Finally, we show that UNC-6 (netrin), which polarizes invasive protrusions, also orients FGT-1. These studies reveal a robust and integrated energy acquisition, processing, and delivery network that powers BM breaching.

The developing mouse coronal suture at single-cell resolution

Sutures separate the flat bones of the skull and enable coordinated growth of the brain and overlying cranium. The coronal suture is most commonly fused in monogenic craniosynostosis, yet the unique aspects of its development remain incompletely understood. To uncover the cellular diversity within the murine embryonic coronal suture, we generated single-cell transcriptomes and performed extensive expression validation. We find distinct pre-osteoblast signatures between the bone fronts and periosteum, a ligament-like population above the suture that persists into adulthood, and a chondrogenic-like population in the dura mater underlying the suture. Lineage tracing reveals an embryonic Six2+ osteoprogenitor population that contributes to the postnatal suture mesenchyme, with these progenitors being preferentially affected in a Twist1+/-; Tcf12+/- mouse model of Saethre-Chotzen Syndrome. This single-cell atlas provides a resource for understanding the development of the coronal suture and the mechanisms for its loss in craniosynostosis.

Ciliary Signalling and Mechanotransduction in the Pathophysiology of Craniosynostosis

Craniosynostosis (CS) is the second most prevalent inborn craniofacial malformation; it results from the premature fusion of cranial sutures and leads to dimorphisms of variable severity. CS is clinically heterogeneous, as it can be either a sporadic isolated defect, more frequently, or part of a syndromic phenotype with mendelian inheritance. The genetic basis of CS is also extremely heterogeneous, with nearly a hundred genes associated so far, mostly mutated in syndromic forms. Several genes can be categorised within partially overlapping pathways, including those causing defects of the primary cilium. The primary cilium is a cellular antenna serving as a signalling hub implicated in mechanotransduction, housing key molecular signals expressed on the ciliary membrane and in the cilioplasm. This mechanical property mediated by the primary cilium may also represent a cue to understand the pathophysiology of non-syndromic CS. In this review, we aimed to highlight the implication of the primary cilium components and active signalling in CS pathophysiology, dissecting their biological functions in craniofacial development and in suture biomechanics. Through an in-depth revision of the literature and computational annotation of disease-associated genes we categorised 18 ciliary genes involved in CS aetiology. Interestingly, a prevalent implication of midline sutures is observed in CS ciliopathies, possibly explained by the specific neural crest origin of the frontal bone.

Mathematical modeling of palatal suture pattern formation: morphological differences between sagittal and palatal sutures

The median palatal suture serves as a growth center for the maxilla; inadequate growth at this site causes malocclusion and dental crowding. However, the pattern formation mechanism of palatal sutures is poorly understood compared with that of calvarial sutures such as the sagittal suture. In the present study, therefore, we compared the morphological characteristics of sagittal and palatal sutures in human bone specimens. We found that palatal suture width was narrower than sagittal suture width, and the interdigitation amplitude of the palatal suture was lower than that of the sagittal suture. These tendencies were also observed in the neonatal stage. However, such differences were not observed in other animals such as chimpanzees and mice. We also used a mathematical model to reproduce the differences between palatal and sagittal sutures. After an extensive parameter search, we found two conditions that could generate the difference in interdigitation amplitude and suture width: bone differentiation threshold [Formula: see text] and growth speed c. We discuss possible biological interpretations of the observed pattern difference and its cause.

Single-suture craniosynostosis and the epigenome: current evidence and a review of epigenetic principles

Craniosynostosis (CS) is a congenital disease that arises due to premature ossification of single or multiple sutures, which results in skull deformities. The surgical management of single-suture CS continues to evolve and is driven by a robust body of clinical research; however, the molecular underpinnings of CS remain poorly understood. Despite long-standing hypotheses regarding the interaction of genetic predisposition and environmental factors, formal investigation of the epigenetic underpinnings of CS has been limited. In an effort to catalyze further investigation into the epigenetic basis of CS, the authors review the fundamentals of epigenetics, discuss recent studies that shed light on this emerging field, and offer hypotheses regarding the role of epigenetic mechanisms in the development of single-suture CS.

Serum nickel is associated with craniosynostosis risk: Evidence from humans and mice

BACKGROUND

To date, few studies have explored the effects of exposure to metal mixtures on adverse developmental outcomes, and no reported studies have linked metal exposure to craniosynostosis (CS). The purpose of this study is to investigate the association between metal exposure and the risk of CS by conducting epidemiological and experimental studies.

METHODS

Inductively coupled plasma mass spectrometry (ICP-MS) was used to measure the concentrations of 6 metals (chromium [Cr], nickel [Ni], tin [Sn], arsenic [As], thallium [Tl], and lead [Pb]) in serum samples from 174 CS patients and 85 control individuals. Non-syndromic patients with isolated sagittal suture closure were selected as the case group, and healthy children matched by sex and age were selected as controls. Bayesian kernel machine regression (BKMR) models were used to account for joint metal effects. Multiple logistic regression analysis was used to explore the association between metal concentration and CS occurrence, with adjustment for potential confounders. During pregnancy, mice were exposed to Ni (0, 0.05, or 0.1 g/kg/day) until weaning, and the widths of the sutures and shapes of the skull were analysed by micro-CT 3D imaging and histological analysis. MC3T3-E1 cells were treated with Ni (0, 0.005, or 0.05 μg/mL) for 72 h. Alkaline phosphate (ALP) staining and Alizarin red staining were performed to observe the development of osteoblasts. The expression levels of osteoblast-related genes were also detected.

RESULTS

A positive association between the metal mixture and CS risk was observed based on population data; the Ni group had the highest conditional posterior inclusion probability (PIP), at 0.8416, and in the fully adjusted model, the highest Ni exposure level had a more significant association with CS (coefficient = 2.65, 95% CI: 0.29, 5.02) than the lowest Ni exposure level. The mean widths of the sagittal sutures in mice were 8.8 ± 0.6 mm in the control group, 8.0 ± 0.8 mm in the 0.05 g/kg/day group and 6.8 ± 0.4 mm in the 0.1 g/kg/day group. After Ni exposure, ALP gene expression in skull tissue was increased, and ALP activity was increased in MC3T3-E1 cells. Moreover, increased collagen content in mouse skull sections and elevated osteocalcin (OCN) expression in MC3T3-E1 cells were observed in the Ni-treated groups compared to the control group.

CONCLUSIONS

This study is the first to provide evidence that increased serum Ni was associated with an increased risk of CS. Early life exposure to Ni promoted osteogenesis during skull growth, which may contribute to the development of CS.

Vibrational spectral fingerprinting for chemical recognition of biominerals

Pathologies associated with calcified tissue, such as osteoporosis, demand in vivo and/or in situ spectroscopic analysis to assess the role of chemical substitutions in the inorganic component. High energy X-ray or NMR spectroscopies are impractical or damaging in biomedical conditions. Low energy spectroscopies, such as IR and Raman techniques, are often the best alternative. In apatite biominerals, the vibrational signatures of the phosphate group are generally used as fingerprint of the materials although they provide only limited information. Here, we have used first principles calculations to unravel the complexity of the complete vibrational spectra of apatites. We determined the spectroscopic features of all the phonon modes of fluoroapatite, hydroxy-apatite, and carbonated fluoroapatite beyond the analysis of the phosphate groups, focusing on the effect of local corrections induced by the crystalline environment and the specific mineral composition. This provides a clear and unique reference to discriminate structural and chemical variations in biominerals, opening the way to a widespread application of non-invasive spectroscopies for in vivo diagnostics, and biomedical analysis.

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.

A coupled reaction-diffusion-strain model predicts cranial vault formation in development and disease

How cells utilize instructions provided by genes and integrate mechanical forces generated by tissue growth to produce morphology is a fundamental question of biology. Dermal bones of the vertebrate cranial vault are formed through the direct differentiation of mesenchymal cells on the neural surface into osteoblasts through intramembranous ossification. Here we join a self-organizing Turing mechanism, computational biomechanics, and experimental data to produce a 3D representative model of the growing cerebral surface, cranial vault bones, and sutures. We show how changes in single parameters regulating signaling during osteoblast differentiation and bone formation may explain cranial vault shape variation in craniofacial disorders. A key result is that toggling a parameter in our model results in closure of a cranial vault suture, an event that occurred during evolution of the cranial vault and that occurs in craniofacial disorders. Our approach provides an initial and important step toward integrating biomechanics into the genotype phenotype map to explain the production of variation in head morphology by developmental mechanisms.

Characterizing and Modeling Bone Formation during Mouse Calvarial Development

The newborn mammalian cranial vault consists of five flat bones that are joined together along their edges by soft fibrous tissues called sutures. Early fusion of these sutures leads to a medical condition known as craniosynostosis. The mechanobiology of normal and craniosynostotic skull growth is not well understood. In a series of previous studies, we characterized and modeled radial expansion of normal and craniosynostotic (Crouzon) mice. Here, we describe a new modeling algorithm to simulate bone formation at the sutures in normal and craniosynostotic mice. Our results demonstrate that our modeling approach is capable of predicting the observed ex vivo pattern of bone formation at the sutures in the aforementioned mice. The same approach can be used to model different calvarial reconstruction in children with craniosynostosis to assist in the management of this complex condition.

Predicting calvarial growth in normal and craniosynostotic mice using a computational approach

During postnatal calvarial growth the brain grows gradually and the overlying bones and sutures accommodate that growth until the later juvenile stages. The whole process is coordinated through a complex series of biological, chemical and perhaps mechanical signals between various elements of the craniofacial system. The aim of this study was to investigate to what extent a computational model can accurately predict the calvarial growth in wild-type (WT) and mutant type (MT) Fgfr2C342Y/+ mice displaying bicoronal suture fusion. A series of morphological studies were carried out to quantify the calvarial growth at P3, P10 and P20 in both mouse types. MicroCT images of a P3 specimen were used to develop a finite element model of skull growth to predict the calvarial shape of WT and MT mice at P10. Sensitivity tests were performed and the results compared with ex vivo P10 data. Although the models were sensitive to the choice of input parameters, they predicted the overall skull growth in the WT and MT mice. The models also captured the difference between the ex vivoWT and MT mice. This modelling approach has the potential to be translated to human skull growth and to enhance our understanding of the different reconstruction methods used to manage clinically the different forms of craniosynostosis, and in the long term possibly reduce the number of re-operations in children displaying this condition and thereby enhance their quality of life.

Integrated Multiscale Biomaterials Experiment and Modeling

The integration of modeling and experimentation is an integral component of all engineering design. Developing the technologies to achieve this represents a critical challenge in biomaterials because of the hierarchical structures that comprise them and the spectra of timescales upon which they operate. Progress in integrating modeling and experiment in biomaterials represents progress towards harnessing them for engineering application. We present here a summary of the state of the art, and outlooks for the field as a whole.