Growth of the normal skull vault and its alteration in craniosynostosis: insights from human genetics and experimental studies

Transcriptional analysis of human cranial compartments with different embryonic origins

OBJECTIVE

Previous investigations suggest that the embryonic origins of the calvarial tissues (neural crest or mesoderm) may account for the molecular mechanisms underlying sutural development. The aim of this study was to evaluate the differences in the gene expression of human cranial tissues and assess the presence of an expression signature reflecting their embryonic origins.

METHODS

Using microarray technology, we investigated global gene expression of cells from the frontal and parietal bones and the metopic and sagittal intrasutural mesenchyme (ISM) of four human foetal calvaria. qRT-PCR of a selected group of genes was done to validate the microarray analysis. Paired comparison and correlation analyses were performed on microarray results.

RESULTS

Of six paired comparisons, frontal and parietal compartments (distinct tissue types of calvaria, either bone or intrasutural mesenchyme) had the most different gene expression profiles despite being composed of the same tissue type (bone). Correlation analysis revealed two distinct gene expression profiles that separate frontal and metopic compartments from parietal and sagittal compartments. TFAP2A, TFAP2B, ICAM1, SULF1, TNC and FOXF2 were among differentially expressed genes.

CONCLUSION

Transcriptional profiles of two groups of tissues, frontal and metopic compartments vs. parietal and sagittal compartments, suggest differences in proliferation, differentiation and extracellular matrix production. Our data suggest that in the second trimester of human foetal development, a gene expression signature of neural crest origin still exists in frontal and metopic compartments while gene expression of parietal and sagittal compartments is more similar to mesoderm.

Enzyme replacement for craniofacial skeletal defects and craniosynostosis in murine hypophosphatasia

Hypophosphatasia (HPP) is an inborn-error-of-metabolism disorder characterized by deficient bone and tooth mineralization due to loss-of function mutations in the gene (Alpl) encoding tissue-nonspecific alkaline phosphatase (TNAP). Alpl(-/-) mice exhibit many characteristics seen in infantile HPP including long bone and tooth defects, vitamin B6 responsive seizures and craniosynostosis. Previous reports demonstrated that a mineral-targeted form of TNAP rescues long bone, vertebral and tooth mineralization defects in Alpl(-/-) mice. Here we report that enzyme replacement with mineral-targeted TNAP (asfotase-alfa) also prevents craniosynostosis (the premature fusion of cranial bones) and additional craniofacial skeletal abnormalities in Alpl(-/-) mice. Craniosynostosis, cranial bone volume and density, and craniofacial shape abnormalities were assessed by microscopy, histology, digital caliper measurements and micro CT. We found that craniofacial shape defects, cranial bone mineralization and craniosynostosis were corrected in Alpl(-/-) mice injected daily subcutaneously starting at birth with recombinant enzyme. Analysis of Alpl(-/-) calvarial cells indicates that TNAP deficiency leads to aberrant osteoblastic gene expression and diminished proliferation. Some but not all of these cellular abnormalities were rescued by treatment with inorganic phosphate. These results confirm an essential role for TNAP in craniofacial skeletal development and demonstrate the efficacy of early postnatal mineral-targeted enzyme replacement for preventing craniofacial abnormalities including craniosynostosis in murine infantile HPP.

Does an elevated bony ridge along the course of the metopic suture equal metopic synostosis? Implications for management

Metopic synostosis represents an increasingly prevalent form of nonsyndromic craniosynostosis. Premature fusion of the metopic suture classically results in trigonocephaly, hypotelorism, temporal narrowing, and a pronounced midline forehead ridge. However, as varying degrees of skull deformity exist, there is confusion regarding the appropriate management for an infant with a metopic ridge. We report on a 2-month-old infant with clinical manifestations of metopic synostosis but with a patent metopic suture documented on computed tomography scan. We examine the implications for management related to fusion of the suture, age of the patient, and severity of the head deformity.

The brain and the braincase: a spatial analysis on the midsagittal profile in adult humans

The spatial relationships between brain and braincase represent a major topic in surgery and evolutionary neuroanatomy. In paleoneurology, neurocranial landmarks are often used as references for brain areas. In this study, we analyze the variation and covariation of midsagittal brain and skull coordinates in a sample of adult modern humans in order to demonstrate spatial associations between hard and soft tissues. The correlation between parietal lobe size and parietal bone size is very low, and there is a marked individual variation. The distances between lobes and bones are partially influenced by the dimensions of the parietal lobes. The main pattern of morphological variability among individuals, associated with the size of the precuneus, apparently does not influence the position of the neurocranial sutures. Therefore, variations in precuneal size modify the distance between the paracentral lobule and bregma, and between the parietal lobe and lambda. Hence, the relative position of the cranial and cerebral landmarks can change as a function of the parietal dimensions. The slight correlation and covariation among these elements suggests a limited degree of spatial integration between soft and hard tissues. Therefore, although the brain influences the cranial size and shape during morphogenesis, the specific position of the cerebral components is sensitive to multiple effects and local factors, without a strict correspondence with the bone landmarks. This absence of correspondent change between brain and skull boundaries suggests caution when making inferences about the brain areas from the position of the cranial sutures. The fact that spatial relationships between cranial and brain areas may vary according to brain proportions must be considered in paleoneurology, when brain anatomy is inferred from cranial evidence.

Transforming Growth Factor-β3 Therapy Delays Postoperative Reossification and Improves Craniofacial Growth in Craniosynostotic Rabbits

Postoperative reossification is a common clinical correlate following surgery. It has been suggested that an underexpression of transforming growth factor-β3 (TGF-β3) may be related to craniosynostosis and postoperative reossification. Adding TGF-β3 may delay reossification and improve postoperative growth. The present study was designed to test this hypothesis. Thirty 10-day-old New Zealand white rabbits with hereditary coronal suture synostosis were divided into three groups: (1) suturectomy controls (n = 14), (2) suturectomy treated with bovine serum albumin (n = 8), and (3) suturectomy treated with TGF-β3 protein (n = 8). At 10 days of age, a 3-mm × 15-mm coronal suturectomy was performed, and serial three-dimensional (3D) computed tomography (CT) scans and cephalographs were taken at 10, 25, 42, and 84 days of age. Calvaria were harvested at 84 days of age for histomorphometric analysis. Mean differences were analyzed using a group by age analysis of variance. Analysis of the 3D CT scan data revealed that sites treated with TGF-β3 had significantly (P < .05) greater defect areas and significantly (P < .05) greater intracranial volumes through 84 days of age compared with controls. Histomorphometry showed that sites treated with TGF-β3 had patent suturectomy sites and significantly (P < .001) less new bone in the suturectomy site compared with controls. Serial radiograph data revealed significant (P < .05) differences in craniofacial growth from 25 to 84 days in TGF-β3-treated rabbits compared with controls. Data show that TGF-β3 administration delayed reossification and improved craniofacial growth in this rabbit model. These findings also suggest that this molecular-based therapy may have potential clinical use.

Mechanical properties of calvarial bones in a mouse model for craniosynostosis

The mammalian cranial vault largely consists of five flat bones that are joined together along their edges by soft fibrous tissues called sutures. Premature closure of the cranial sutures, craniosynostosis, can lead to serious clinical pathology unless there is surgical intervention. Research into the genetic basis of the disease has led to the development of various animal models that display this condition, e.g. mutant type Fgfr2^C342Y/+ mice which display early fusion of the coronal suture (joining the parietal and frontal bones). However, whether the biomechanical properties of the mutant and wild type bones are affected has not been investigated before. Therefore, nanoindentation was used to compare the elastic modulus of cranial bone and sutures in wild type (WT) and Fgfr2^C342Y/+mutant type (MT) mice during their postnatal development. Further, the variations in properties with indentation position and plane were assessed. No difference was observed in the elastic modulus of parietal bone between the WT and MT mice at postnatal (P) day 10 and 20. However, the modulus of frontal bone in the MT group was lower than the WT group at both P10 (1.39±0.30 vs. 5.32±0.68 GPa; p<0.05) and P20 (5.57±0.33 vs. 7.14±0.79 GPa; p<0.05). A wide range of values was measured along the coronal sutures for both the WT and MT samples, with no significant difference between the two groups. Findings of this study suggest that the inherent mechanical properties of the frontal bone in the mutant mice were different to the wild type mice from the same genetic background. These differences may reflect variations in the degree of biomechanical adaptation during skull growth, which could have implications for the surgical management of craniosynostosis patients.

Diffusion Tensor Imaging and Fiber Tractography in Children with Craniosynostosis Syndromes

BACKGROUND AND PURPOSE

Patients with craniosynostosis syndromes caused by mutations in FGFR-2, FGFR-3, and TWIST1 genes are characterized by having prematurely fused skull sutures and skull base synchondroses, which result in a skull deformity and are accompanied by brain anomalies, including altered white matter microarchitecture. In this study, the reliability and reproducibility of DTI fiber tractography was investigated in these patients. The outcomes were compared with those of controls.

MATERIALS AND METHODS

DTI datasets were acquired with a 1.5T MR imaging system with 25 diffusion gradient orientations (voxel size = 1.8 × 1.8 × 3.0 mm(3), b-value = 1000 s/mm(2)). White matter tracts studied included the following: corpus callosum, cingulate gyrus, fornix, corticospinal tracts, and medial cerebellar peduncle. Tract pathways were reconstructed with ExploreDTI in 58 surgically treated patients with craniosynostosis syndromes and 7 controls (age range, 6-18 years).

RESULTS

Because of the brain deformity and abnormal ventricular shape and size, DTI fiber tractography was challenging to perform in patients with craniosynostosis syndromes. To provide reliable tracts, we adapted standard tracking protocols. Fractional anisotropy was equal to that in controls (0.44 versus 0.45 ± 0.02, P = .536), whereas mean, axial, and radial diffusivity parameters of the mean white matter were increased in patients with craniosynostosis syndromes (P < .001). No craniosynostosis syndrome-specific difference in DTI properties was seen for any of the fiber tracts studied in this work.

CONCLUSIONS

Performing DTI fiber tractography in patients with craniosynostosis syndromes was difficult due to partial volume effects caused by an anisotropic voxel size and deformed brain structures. Although these patients have a normal fiber organization, increased diffusivity parameters suggest abnormal microstructural tissue properties of the investigated white matter tracts.

The aesthetic outcome of surgical correction for sagittal synostosis can be reliably scored by a novel method of preoperative and postoperative visual assessment

BACKGROUND

Aims of surgical correction for isolated sagittal synostosis are functional and aesthetic. Multiple surgical techniques exist; however, reliable assessment of aesthetic outcome is poorly documented, limiting direct comparisons. The pinched appearance of the temporal regions is particularly challenging to correct. A visual analogue scale was designed to grade skull shape in patients who had total or subtotal calvarial remodeling for isolated sagittal synostosis.

METHODS

Twenty-two assessors graded preoperative and postoperative photographs from 42 consecutive cases of sagittal synostosis under a single surgeon. Five aspects were graded (i.e., narrow elongated skull, frontal bossing, temporal pinching, occipital bullet, and overall shape) from 0 (normal) to 100 (severe). Interobserver and intraobserver agreement were analyzed by calculating within-subject standard deviation, coefficient of variation, and intraclass correlation coefficient. Linear regression analysis determined predictors of outcome.

RESULTS

Surgery improved outcome dramatically across all five aspects of skull shape, with a 72.6 to 76.4 percent decrease in severity score. Improvements in severity score were greater after total calvarial remodeling, and type of calvarial remodeling (total versus subtotal) was an independent predictor of outcome in all aspects of skull shape (p≤0.001). Temporal pinching was improved in a subset of patients who also had onlay bone grafts in this region.

CONCLUSIONS

Calvarial remodeling is a powerful technique for improving skull shape. A panel can detect gross and subtle aesthetic changes after surgical correction of sagittal synostosis using a visual analogue scale, with moderate interobserver and intraobserver agreement. This provides a tool for future outcome assessment.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Therapeutic, IV.

Functional diversity of fibroblast growth factors in bone formation

The functional significance of fibroblast growth factor (FGF) signaling in bone formation has been demonstrated through genetic loss-of-function and gain-of-function approaches. FGFs, comprising 22 family members, are classified into three subfamilies: canonical, hormone-like, and intracellular. The former two subfamilies activate their signaling pathways through FGF receptors (FGFRs). Currently, intracellular FGFs appear to be primarily involved in the nervous system. Canonical FGFs such as FGF2 play significant roles in bone formation, and precise spatiotemporal control of FGFs and FGFRs at the transcriptional and posttranscriptional levels may allow for the functional diversity of FGFs during bone formation. Recently, several research groups, including ours, have shown that FGF23, a member of the hormone-like FGF subfamily, is primarily expressed in osteocytes/osteoblasts. This polypeptide decreases serum phosphate levels by inhibiting renal phosphate reabsorption and vitamin D3 activation, resulting in mineralization defects in the bone. Thus, FGFs are involved in the positive and negative regulation of bone formation. In this review, we focus on the reciprocal roles of FGFs in bone formation in relation to their local versus systemic effects.

BCL11B expression in intramembranous osteogenesis during murine craniofacial suture development

Sutures, where neighboring craniofacial bones are separated by undifferentiated mesenchyme, are major growth sites during craniofacial development. Pathologic fusion of bones within sutures occurs in a wide variety of craniosynostosis conditions and can result in dysmorphic craniofacial growth and secondary neurologic deficits. Our knowledge of the genes involved in suture formation is poor. Here we describe the novel expression pattern of the BCL11B transcription factor protein during murine embryonic craniofacial bone formation. We examined BCL11B protein expression at E14.5, E16.5, and E18.5 in 14 major craniofacial sutures of C57BL/6J mice. We found BCL11B expression to be associated with all intramembranous craniofacial bones examined. The most striking aspects of BCL11B expression were its high levels in suture mesenchyme and increasingly complementary expression with RUNX2 in differentiating osteoblasts during development. BCL11B was also expressed in mesenchyme at the non-sutural edges of intramembranous bones. No expression was seen in osteoblasts involved in endochondral ossification of the cartilaginous cranial base. BCL11B is expressed to potentially regulate the transition of mesenchymal differentiation and suture formation within craniofacial intramembranous bone.

Intracranial volume in 15 children with bilateral coronal craniosynostosis

BACKGROUND

Intracranial volume (ICV) growth in patients with bilateral coronal craniosynostosis (BCS) is not well described. It is therefore important to evaluate the consequences of cranial surgery in children with this condition. The aim of the present study was to evaluate ICVs in patients operated on for BCS.

METHODS

A consecutive series of patients with BCS were operated on using spring-assisted cranioplasty, with computed tomography scans in 0.6-mm slices, were included. A MATLAB-based computer program capable of measuring ICV was used. Patients were compared with an age- and gender-matched control group of healthy children. Student's t test was used for statistical analysis.

RESULTS

Fifteen patients (7 girls and 8 boys) with 43 computed tomography scans were identified. The diagnoses were 13 syndromic BCS (3 Apert, 1 Crouzon, 6 Muenke, and 3 Saethre-Chotzen) and 2 nonsyndromic BCS. The mean preoperative volume at the age of 5 months (n = 15) was 887 mL (range, 687-1082). Mean volume at follow-up at the age of 3 years (n = 13) was 1369 mL (range, 1196-1616). In comparison, the mean ICVs for controls at the ages of 5 months (n = 30) and 3 years (n = 26) were 854 mL and 1358 mL, respectively. The differences were not statistically significant (P > 0.05).

CONCLUSIONS

Patients with BCS were operated on with spring-assisted cranioplasty seem to maintain their age-related ICV at 3 years of age when compared to normal children.

Developmental covariation of human vault and base throughout postnatal ontogeny

In the present study, we analyzed postnatal ontogenetic integration among morphological traits of the human neurocranium. Particularly, the covariation between the vault and the base during postnatal life was assessed. Since the association between these regions may depend on the generalized change produced by allometry, we tested its effect on their covariation. On a sample of adults and subadults ranging from 0 to 31 years, 3D coordinates of neurocranial landmarks and semilandmarks were digitized and geometric morphometric technics were applied. Main aspects of shape variation were examined using Principal Components analysis. Covariation between the vault and the base was examined by Partial Least Squares analysis. According to our results, the vault and the base covary strongly during postnatal ontogeny and their relation depends largely on allometry. Two size variables were studied: centroid size, which was obtained from the recorded morphometric points, and endocranial volume, taken as an estimation of brain size. Although growing brain was found to be a developmental process that contributes to covariation among neurocranial traits, there would be other factors that exert their influence during ontogeny. These results lead to reconsider cranial morphological evolution taking into account the developmental constraints given by ontogenetic patterns of integration and reinforcing the idea that in human evolution a suite of relevant characters may be fuelled by few developmental processes.

Tissue-nonspecific alkaline phosphatase deficiency causes abnormal craniofacial bone development in the Alpl(-/-) mouse model of infantile hypophosphatasia

Tissue-nonspecific alkaline phosphatase (TNAP) is an enzyme present on the surface of mineralizing cells and their derived matrix vesicles that promotes hydroxyapatite crystal growth. Hypophosphatasia (HPP) is an inborn-error-of-metabolism that, dependent upon age of onset, features rickets or osteomalacia due to loss-of function mutations in the gene (Alpl) encoding TNAP. Craniosynostosis is prevalent in infants with HPP and other forms of rachitic disease but how craniosynostosis develops in these disorders is unknown.

OBJECTIVES

Because craniosynostosis carries high morbidity, we are investigating craniofacial skeletal abnormalities in Alpl(-/-) mice to establish these mice as a model of HPP-associated craniosynostosis and determine mechanisms by which TNAP influences craniofacial skeletal development.

METHODS

Cranial bone, cranial suture and cranial base abnormalities were analyzed by micro-CT and histology. Craniofacial shape abnormalities were quantified using digital calipers. TNAP expression was suppressed in MC3T3E1(C4) calvarial cells by TNAP-specific shRNA. Cells were analyzed for changes in mineralization, gene expression, proliferation, apoptosis, matrix deposition and cell adhesion.

RESULTS

Alpl(-/-) mice feature craniofacial shape abnormalities suggestive of limited anterior-posterior growth. Craniosynostosis in the form of bony coronal suture fusion is present by three weeks after birth. Alpl(-/-) mice also exhibit marked histologic abnormalities of calvarial bones and the cranial base involving growth plates, cortical and trabecular bone within two weeks of birth. Analysis of calvarial cells in which TNAP expression was suppressed by shRNA indicates that TNAP deficiency promotes aberrant osteoblastic gene expression, diminished matrix deposition, diminished proliferation, increased apoptosis and increased cell adhesion.

CONCLUSIONS

These findings demonstrate that Alpl(-/-) mice exhibit a craniofacial skeletal phenotype similar to that seen in infants with HPP, including true bony craniosynostosis in the context of severely diminished bone mineralization. Future studies will be required to determine if TNAP deficiency and other forms of rickets promote craniosynostosis directly through abnormal calvarial cell behavior, or indirectly due to deficient growth of the cranial base.

Expanding the mutation spectrum in 182 Spanish probands with craniosynostosis: identification and characterization of novel TCF12 variants

Craniosynostosis, caused by the premature fusion of one or more of the cranial sutures, can be classified into non-syndromic or syndromic and by which sutures are affected. Clinical assignment is a difficult challenge due to the high phenotypic variability observed between syndromes. During routine diagnostics, we screened 182 Spanish craniosynostosis probands, implementing a four-tiered cascade screening of FGFR2, FGFR3, FGFR1, TWIST1 and EFNB1. A total of 43 variants, eight novel, were identified in 113 (62%) patients: 104 (92%) detected in level 1; eight (7%) in level 2 and one (1%) in level 3. We subsequently screened additional genes in the probands with no detected mutation: one duplication of the IHH regulatory region was identified in a patient with craniosynostosis Philadelphia type and five variants, four novel, were identified in the recently described TCF12, in probands with coronal or multisuture affectation. In the 19 Saethre-Chotzen syndrome (SCS) individuals in whom a variant was detected, 15 (79%) carried a TWIST1 variant, whereas four (21%) had a TCF12 variant. Thus, we propose that TCF12 screening should be included for TWIST1 negative SCS patients and in patients where the coronal suture is affected. In summary, a molecular diagnosis was obtained in a total of 119/182 patients (65%), allowing the correct craniosynostosis syndrome classification, aiding genetic counselling and in some cases provided a better planning on how and when surgical intervention should take place and, subsequently the appropriate clinical follow up.

Difference in apical and basal growth of the frontal bone primordium in Foxc1ch/ch mice

The frontal and parietal bones form the major part of the calvarium and their primordia appear at the basolateral region of the head and grow apically. A spontaneous loss of Foxc1 function mutant mouse, congenital hydrocephalus (Foxc1(ch/ch)), results in congenital hydrocephalus accompanied by defects in the apical part of the skull vault. We found that during the initiation stage of apical growth of the frontal bone primordium in the Foxc1(ch/ch) mouse, the Runx2 expression domain extended only to the basal side and bone sialoprotein (Bsp) and N-cadherin expression domains appeared only in the basal region. Fluorescent dye (DiI) labeling of the frontal primordium by ex-utero surgery confirmed that apical extension of the frontal bone primordium of the mouse was severely retarded, while extension to the basal side underneath the brain was largely unaffected. Consistent with this observation, decreased cell proliferation activity was seen at the apical tip but not the basal tip of the frontal bone primordium as determined by double detection of Runx2 transcripts and BrdU incorporation. Furthermore, expression of the osteogenic-related genes Bmp4 and-7 was observed only in the basal part of the meninges during the initiation period of primordium growth. These results suggest that a loss of Foxc1 function affects skull bone formation of the apical region and that Bmp expression in the meninges might influence the growth of the calvarial bone primordium.

Insights into morphology and disease from the dog genome project

Although most modern dog breeds are less than 200 years old, the symbiosis between man and dog is ancient. Since prehistoric times, repeated selection events have transformed the wolf into man's guardians, laborers, athletes, and companions. The rapid transformation from pack predator to loyal companion is a feat that is arguably unique among domesticated animals. How this transformation came to pass remained a biological mystery until recently: Within the past decade, the deployment of genomic approaches to study population structure, detect signatures of selection, and identify genetic variants that underlie canine phenotypes is ushering into focus novel biological mechanisms that make dogs remarkable. Ironically, the very practices responsible for breed formation also spurned morbidity; today, many diseases are correlated with breed identity. In this review, we discuss man's best friend in the context of a genetic model to understand paradigms of heritable phenotypes, both desirable and disadvantageous.

Craniosynostosis: caring for infants and their families

Craniosynostosis is a developmental anomaly with premature closure of the cranial sutures causing an abnormally shaped skull in an infant. Recommended surgical treatment involves cranial vault reconstruction to open the closed suture, increase intracranial volume, and allow the brain to grow normally. Parents work with a multidisciplinary team during the evaluation process and face various preoperative and postoperative stressors. Critical care nurses can improve the care of the infants and their families by being knowledgeable about the anatomy, assessment, and surgical and nursing management of infants with this anomaly and its impact on the patients' families. This article discusses the definitions, diagnosis, and treatment of craniosynostosis and support for parents of infants with this malformation.

Closing the Gap: Genetic and Genomic Continuum from Syndromic to Nonsyndromic Craniosynostoses

Craniosynostosis, a condition that includes the premature fusion of one or multiple cranial sutures, is a relatively common birth defect in humans and the second most common craniofacial anomaly after orofacial clefts. There is a significant clinical variation among different sutural synostoses as well as significant variation within any given single-suture synostosis. Craniosynostosis can be isolated (i.e., nonsyndromic) or occurs as part of a genetic syndrome (e.g., Crouzon, Pfeiffer, Apert, Muenke, and Saethre-Chotzen syndromes). Approximately 85 % of all cases of craniosynostosis are nonsyndromic. Several recent genomic discoveries are elucidating the genetic basis for nonsyndromic cases and implicate the newly identified genes in signaling pathways previously found in syndromic craniosynostosis. Published epidemiologic and phenotypic studies clearly demonstrate that nonsyndromic craniosynostosis is a complex and heterogeneous condition supporting a strong genetic component accompanied by environmental factors that contribute to the pathogenetic network of this birth defect. Large population, rather than single-clinic or hospital-based studies is required with phenotypically homogeneous subsets of patients to further understand the complex genetic, maternal, environmental, and stochastic factors contributing to nonsyndromic craniosynostosis. Learning about these variables is a key in formulating the basis of multidisciplinary and lifelong care for patients with these conditions.

Generating anatomical variation through mutations in networks - implications for evolution

Genetic mutation leads to anatomical variation only indirectly because many proteins involved in generating anatomical structures in embryos operate cooperatively within molecular networks. These include gene-regulatory or control networks (CNs) for timing, signaling and patterning together with the process networks (PNs) for proliferation, apoptosis, differentiation and morphogenesis that they control. This paper argues that anatomical variation is achieved through a two-stage process: mutation alters the outputs of CNs and perhaps the proliferation network, and such changed outputs alter the ways that PNs construct tissues. This systems-biology approach has several implications: first, because networks contain many cooperating proteins, they amplify the effects of genetic variation so enabling mutation to generate a wider range of phenotypes than a single changed protein acting alone could. Second, this amplification helps explain how novel phenotypes can be produced relatively rapidly. Third, because even organisms with novel anatomical phenotypes derive from variants in standard networks, there is no genetic barrier to their producing viable offspring. This approach also clarifies a terminological difficulty: classical evolutionary genetics views genes in terms of phenotype heritability rather than as DNA sequences. This paper suggests that the molecular phenotype of the classical concept of a gene is often a protein network, with a mutation leading to an alteration in that network's dynamics.

Functional craniology and brain evolution: from paleontology to biomedicine

Anatomical systems are organized through a network of structural and functional relationships among their elements. This network of relationships is the result of evolution, it represents the actual target of selection, and it generates the set of rules orienting and constraining the morphogenetic processes. Understanding the relationship among cranial and cerebral components is necessary to investigate the factors that have influenced and characterized our neuroanatomy, and possible drawbacks associated with the evolution of large brains. The study of the spatial relationships between skull and brain in the human genus has direct relevance in cranial surgery. Geometrical modeling can provide functional perspectives in evolution and brain physiology, like in simulations to investigate metabolic heat production and dissipation in the endocranial form. Analysis of the evolutionary constraints between facial and neural blocks can provide new information on visual impairment. The study of brain form variation in fossil humans can supply a different perspective for interpreting the processes behind neurodegeneration and Alzheimer’s disease. Following these examples, it is apparent that paleontology and biomedicine can exchange relevant information and contribute at the same time to the development of robust evolutionary hypotheses on brain evolution, while offering more comprehensive biological perspectives with regard to the interpretation of pathological processes.

Os odontoideum in identical twins: Comparative gene expression analysis

Background:

Os odontoideum is a well identified anomaly of the craniovertebral junction. Since its initial description, there has been a continuous debate regarding the nature of its etiology: Whether congenital or traumatic. We sought to compare the gene expression profiles in patients with congenital os odontoideum, those with traumatic os odontoideum and controls.

Methods:

We have evaluated a pair of identical twins both with os odontoideum. We identified two additional patients with and four subjects without os odontoideum. We analyzed the gene expression profiles in these patients using a custom TaqMan microarray and quantitative reverse transcriptase polymerase chain reaction (qRT-PCR). The relative gene expression profiles in the two identical twins, the two nontwin patients with os odontoideum and the controls were assessed.

Results:

A total of 213 genes with significantly different expression between the twin os odontoideum patients and the subjects without os odontoideum were detected. CACNG6, PHEX, CACNAD3, IL2, FAS, TUFT1, KIT, TGFBR2, and IGF2 were expressed at levels greater than 100-fold more in the twins. There were six genes with significantly different expression profiles in the twins as compared with the nontwin os odontoideum patients: CMK4, ATF1, PLCG1, TAB1, E2F3, and ATF4. There were no statistically significant differences in gene expression in the four patients with os odontoideum and the subjects without. Trends, however, were noted in MMP8, KIT, HIF1A, CREB3, PWHAZ, TGFBR1, NFKB2, FGFR1, IPO8, STAT1, COL1A1, and BMP3.

Conclusions:

Os odontoideum has multiple etiologies, both traumatic and congenital and perhaps some represent a combination of the two. This work has identified a number of genes that show increased expression in a pair of twins with congenital os odontoideum and also demonstrates trends in gene expression profiles between a larger group of os odontoideum patients and non-os patients. A number of these genes are related to bone morphogenesis and maintenance.

Identification of skull base sutures and craniofacial anomalies in children with craniosynostosis: utility of multidetector CT

PURPOSE

Craniosynostosis is a condition characterised by the premature fusion of one or more of the cranial sutures. The aim of the study was to identify, by multidetector computed tomography (CT), the involvement of vault sutures as well as of the skull base sutures (named "minor" sutures). The latter ones are involved in development of craniofacial and skull base deformities.

MATERIALS AND METHODS

We retrospectively reviewed 27 children with complex synostosis (n = 21) and anterior synostotic plagiocephaly (n = 6). High-resolution CT images with bone definition algorithm and tridimensional volume rendering reconstructions were assessed.

RESULTS

In 27 children we found different sutures involved in the synostotic process, including both major and minor skull suture synostosis, and synostosis of synchondroses. Superior orbital rim deformity, nasal root deviation, anterior endocranial axis deviation (ethmoidal axis) are found in children with coronal arch synostosis, while reduced size of the posterior fossa and Chiari 1 malformation are noted in children with lambdoid arch synostosis.

CONCLUSIONS

High-resolution CT allows an accurate identification of both "major" and "minor" skull base suture synostosis and it represents the gold standard for the diagnosis of craniostenosis and for planning the proper surgical approach.

Embryonic craniofacial bone volume and bone mineral density in Fgfr2(+/P253R) and nonmutant mice

BACKGROUND

Quantifying multiple phenotypic aspects of individual craniofacial bones across early osteogenesis illustrates differences in typical bone growth and maturation and provides a basis for understanding the localized and overall influence of mutations associated with disease. We quantify the typical pattern of bone growth and maturation during early craniofacial osteogenesis and determine how this pattern is modified in Fgfr2(+/P253R) Apert syndrome mice.

RESULTS

Early differences in typical relative bone density increase are noted between intramembranous and endochondral bones, with endochondral bones normally maturing more quickly during the prenatal period. Several craniofacial bones, including the facial bones of Fgfr2(+/P253R) mice, display lower volumes during the earliest days of osteogenesis and lower relative densities until the perinatal period relative to unaffected littermates.

CONCLUSIONS

Estimates of bone volume and linear measures describing morphology do not necessarily covary, highlighting the value of quantifying multiple facets of gross osteological phenotypes when exploring the influence of a disease causing mutation. Differences in mechanisms of osteogenesis likely underlie differences in intramembranous and endochondral relative density increase. The influence of the FGFR2 P253R mutation on bone volume changes across the prenatal period and again after birth, while its influence on relative bone density is more stable.

Neural crest cell signaling pathways critical to cranial bone development and pathology

Neural crest cells appear early during embryogenesis and give rise to many structures in the mature adult. In particular, a specific population of neural crest cells migrates to and populates developing cranial tissues. The ensuing differentiation of these cells via individual complex and often intersecting signaling pathways is indispensible to growth and development of the craniofacial complex. Much research has been devoted to this area of development with particular emphasis on cell signaling events required for physiologic development. Understanding such mechanisms will allow researchers to investigate ways in which they can be exploited in order to treat a multitude of diseases affecting the craniofacial complex. Knowing how these multipotent cells are driven towards distinct fates could, in due course, allow patients to receive regenerative therapies for tissues lost to a variety of pathologies. In order to realize this goal, nucleotide sequencing advances allowing snapshots of entire genomes and exomes are being utilized to identify molecular entities associated with disease states. Once identified, these entities can be validated for biological significance with other methods. A crucial next step is the integration of knowledge gleaned from observations in disease states with normal physiology to generate an explanatory model for craniofacial development. This review seeks to provide a current view of the landscape on cell signaling and fate determination of the neural crest and to provide possible avenues of approach for future research.

Cloning of TgfβR1 and TgfβR2 and Likely Exclusion as Loci of Origin in a Rabbit Craniosynostotic Model

Objective

To determine whether TgfβR1 or TgfβR2 cause the craniosynostotic phenotype in a rabbit model of nonsyndromic craniosynostosis.

Design

Full-length TgfβR1 and TgfβR2 cDNAs were sequenced and real-time reverse-transcription polymerase chain reaction (RT-PCR) was performed to measure TgfβR1 and TgfβR2 transcripts in suturai tissue from wild type (WT) and craniosynostotic (CS) rabbits. Single nucleotide polymorphisms (SNP) were identified within TgfβR1 and TgfβR2 and were assayed for segregation with disease phenotype in 22 craniosynostotic animals.

Results

No structural mutations in TgfβR1 and TgfβR2 were identified in the craniosynostotic rabbits. Real-time RT-PCR quantification of TgfβR1 and TgfβR2 mRNA showed no significant difference in TgfβR1 expression between CS and WT animals, while TgfβR2 showed 50% elevation in the CS animals compared to WT ( P < .05). SNP analysis within the TgfβR1 and TgfβR2 genes suggested that neither locus is linked to the craniosynostotic phenotype because no allelic combination showed any specific correlation with disease phenotype for either TgfβR1 or TgfβR2.

Conclusions

Our data indicate that the craniosynostotic phenotype in this rabbit model does not arise from any structural mutation in TgfβR1 or TgfβR2, and SNP analysis also likely excludes these genes more broadly as the site of causative mutation.

Prx1 and 3.2kb Col1a1 promoters target distinct bone cell populations in transgenic mice

Bones consist of a number of cell types including osteoblasts and their precursor cells at various stages of differentiation. To analyze cellular organization within the bone, we generated Col1a1CreER-DsRed transgenic mice that express, in osteoblasts, CreER and DsRed under the control of a mouse 3.2kb Col1a1 promoter. We further crossed Col1a1CreER-DsRed mice with Prx1CreER-GFP mice that express CreER and GFP in osteochondro progenitor cells under the control of a 2.4kb Prx1 promoter. Since the 3.2kb Col1a1 promoter becomes active in osteoblasts at early stages of differentiation, and Prx1CreER-GFP-expressing periosteal cells show endogenous Col1a1 expression, we expected to find a cell population in which both the 2.4kb Prx1 promoter and the 3.2kb Col1a1 promoter are active. However, our histological and flow cytometric analyses demonstrated that these transgenes are expressed in distinct cell populations. In the periosteum of long bones, Col1a1CreER-DsRed is expressed in the innermost layer directly lining the bone surface, while Prx1CreER-GFP-expressing cells are localized immediately outside of the Col1a1CreER-DsRed-expressing osteoblasts. In the calvaria, Prx1CreER-GFP-expressing cells are also localized in the cranial suture mesenchyme. Our experiments further showed that Col1a1CreER-DsRed-expressing cells lack chondrogenic potential, while the Prx1CreER-GFP-expressing cells show both chondrogenic and osteogenic potential. Our results indicate that Col1a1CreER-DsRed-expressing cells are committed osteoblasts, while Prx1CreER-GFP-expressing cells are osteochondro progenitor cells. The Prx1CreER-GFP and Col1a1CreER-DsRed transgenes will offer novel approaches for analyzing lineage commitment and early stages of osteoblast differentiation under physiologic and pathologic conditions.

Models of cranial suture biology

Craniosynostosis is a common congenital defect caused by premature fusion of cranial sutures. The severe morphologic abnormalities and cognitive deficits resulting from craniosynostosis and the potential morbidity of surgical correction espouse the need for a deeper understanding of the complex etiology for this condition. Work in animal models for the past 20 years has been pivotal in advancing our understanding of normal suture biology and elucidating pathologic disease mechanisms. This article provides an overview of milestone studies in suture development, embryonic origins, and signaling mechanisms from an array of animal models including transgenic mice, rats, rabbits, fetal sheep, zebrafish, and frogs. This work contributes to an ongoing effort toward continued development of novel treatment strategies.

Deficiency of zebrafish fgf20a results in aberrant skull remodeling that mimics both human cranial disease and evolutionarily important fish skull morphologies

The processes that direct skull remodeling are of interest to both human-oriented studies of cranial dysplasia and evolutionary studies of skull divergence. There is increasing awareness that these two fields can be mutually informative when natural variation mimics pathology. Here we describe a zebrafish mutant line, devoid of blastema (dob), which does not have a functional fgf20a protein, and which also presents cranial defects similar to both adaptive and clinical variation. We used geometric morphometric methods to provide quantitative descriptions of the effects of the dob mutation on skull morphogenesis. In combination with "whole-mount in situ hybridization" labeling of normal fgf20a expression and assays for osteoblast and osteoclast activity, the results of these analyses indicate that cranial dysmorphologies in dob zebrafish are generated by aberrations in post-embryonic skull remodeling via decreased osteoblasotgenesis and increased osteoclastogenesis. Mutational effects include altered skull vault geometries and midfacial hypoplasia that are consistent with key diagnostic signs for multiple human craniofacial syndromes. These phenotypic shifts also mimic changes in the functional morphology of fish skulls that have arisen repeatedly in several highly successful radiations (e.g., damselfishes and East-African rift-lake cichlids). Our results offer the dob/fgf20a mutant as an experimentally tractable model with which to examine post-embryonic skull development as it relates to human disease and vertebrate evolution.

Boston type craniosynostosis: report of a second mutation in MSX2

We describe a family that segregated an autosomal dominant form of craniosynostosis characterized by variable expression and limited extra-cranial features. Linkage analysis and genome sequencing were performed to identify the underlying genetic mutation. A c.443C>T missense mutation in MSX2, which predicts p.Pro148Leu was identified and segregated with the disease in all affected family members. One other family with autosomal dominant craniosynostosis (Boston type) has been reported to have a missense mutation in MSX2. These data confirm that missense mutations altering the proline at codon 148 of MSX2 cause dominantly inherited craniosynostosis.

Regulation of bone morphogenetic protein signalling and cranial osteogenesis by Gpc1 and Gpc3

From birth, the vault of the skull grows at a prodigious rate, driven by the activity of osteoblastic cells at the fibrous joints (sutures) that separate the bony calvarial plates. One in 2500 children is born with a medical condition known as craniosynostosis because of premature bony fusion of the calvarial plates and a cessation of bone growth at the sutures. Bone morphogenetic proteins (BMPs) are potent growth factors that promote bone formation. Previously, we found that Glypican-1 (GPC1) and Glypican-3 (GPC3) are expressed in cranial sutures and are decreased during premature suture fusion in children. Although glypicans are known to regulate BMP signalling, a mechanistic link between GPC1, GPC3 and BMPs and osteogenesis has not yet been investigated. We now report that human primary suture mesenchymal cells coexpress GPC1 and GPC3 on the cell surface and release them into the media. We show that they inhibit BMP2, BMP4 and BMP7 activities, which both physically interact with BMP2 and that immunoblockade of endogenous GPC1 and GPC3 potentiates BMP2 activity. In contrast, increased levels of GPC1 and GPC3 as a result of overexpression or the addition of recombinant protein, inhibit BMP2 signalling and BMP2-mediated osteogenesis. We demonstrate that BMP signalling in suture mesenchymal cells is mediated by both SMAD-dependent and SMAD-independent pathways and that GPC1 and GPC3 inhibit both pathways. GPC3 inhibition of BMP2 activity is independent of attachment of the glypican on the cell surface and post-translational glycanation, and thus appears to be mediated by the core glypican protein. The discovery that GPC1 and GPC3 regulate BMP2-mediated osteogenesis, and that inhibition of endogenous GPC1 and GPC3 potentiates BMP2 responsiveness of human suture mesenchymal cells, indicates how downregulation of glypican expression could lead to the bony suture fusion that characterizes craniosynostosis.

Effects of thyroxine exposure on osteogenesis in mouse calvarial pre-osteoblasts

The incidence of craniosynostosis is one in every 1,800–2500 births. The gene-environment model proposes that if a genetic predisposition is coupled with environmental exposures, the effects can be multiplicative resulting in severely abnormal phenotypes. At present, very little is known about the role of gene-environment interactions in modulating craniosynostosis phenotypes, but prior evidence suggests a role for endocrine factors. Here we provide a report of the effects of thyroid hormone exposure on murine calvaria cells. Murine derived calvaria cells were exposed to critical doses of pharmaceutical thyroxine and analyzed after 3 and 7 days of treatment. Endpoint assays were designed to determine the effects of the hormone exposure on markers of osteogenesis and included, proliferation assay, quantitative ALP activity assay, targeted qPCR for mRNA expression of Runx2, Alp, Ocn, and Twist1, genechip array for 28,853 targets, and targeted osteogenic microarray with qPCR confirmations. Exposure to thyroxine stimulated the cells to express ALP in a dose dependent manner. There were no patterns of difference observed for proliferation. Targeted RNA expression data confirmed expression increases for Alp and Ocn at 7 days in culture. The genechip array suggests substantive expression differences for 46 gene targets and the targeted osteogenesis microarray indicated 23 targets with substantive differences. 11 gene targets were chosen for qPCR confirmation because of their known association with bone or craniosynostosis (Col2a1, Dmp1, Fgf1, 2, Igf1, Mmp9, Phex, Tnf, Htra1, Por, and Dcn). We confirmed substantive increases in mRNA for Phex, FGF1, 2, Tnf, Dmp1, Htra1, Por, Igf1 and Mmp9, and substantive decreases for Dcn. It appears thyroid hormone may exert its effects through increasing osteogenesis. Targets isolated suggest a possible interaction for those gene products associated with calvarial suture growth and homeostasis as well as craniosynostosis.

The morphology of the mouse masticatory musculature

The mouse has been the dominant model organism in studies on the development, genetics and evolution of the mammalian skull and associated soft-tissue for decades. There is the potential to take advantage of this well studied model and the range of mutant, knockin and knockout organisms with diverse craniofacial phenotypes to investigate the functional significance of variation and the role of mechanical forces on the development of the integrated craniofacial skeleton and musculature by using computational mechanical modelling methods (e.g. finite element and multibody dynamic modelling). Currently, there are no detailed published data of the mouse masticatory musculature available. Here, using a combination of micro-dissection and non-invasive segmentation of iodine-enhanced micro-computed tomography, we document the anatomy, architecture and proportions of the mouse masticatory muscles. We report on the superficial masseter (muscle, tendon and pars reflecta), deep masseter, zygomaticomandibularis (anterior, posterior, infraorbital and tendinous parts), temporalis (lateral and medial parts), external and internal pterygoid muscles. Additionally, we report a lateral expansion of the attachment of the temporalis onto the zygomatic arch, which may play a role in stabilising this bone during downwards loading. The data presented in this paper now provide a detailed reference for phenotypic comparison in mouse models and allow the mouse to be used as a model organism in biomechanical and functional modelling and simulation studies of the craniofacial skeleton and particularly the masticatory system.

Reconsidering the evolution of brain, cognition, and behavior in birds and mammals

Despite decades of research, some of the most basic issues concerning the extraordinarily complex brains and behavior of birds and mammals, such as the factors responsible for the diversity of brain size and composition, are still unclear. This is partly due to a number of conceptual and methodological issues. Determining species and group differences in brain composition requires accounting for the presence of taxon-cerebrotypes and the use of precise statistical methods. The role of allometry in determining brain variables should be revised. In particular, bird and mammalian brains appear to have evolved in response to a variety of selective pressures influencing both brain size and composition. “Brain” and “cognition” are indeed meta-variables, made up of the variables that are ecologically relevant and evolutionarily selected. External indicators of species differences in cognition and behavior are limited by the complexity of these differences. Indeed, behavioral differences between species and individuals are caused by cognitive and affective components. Although intra-species variability forms the basis of species evolution, some of the mechanisms underlying individual differences in brain and behavior appear to differ from those between species. While many issues have persisted over the years because of a lack of appropriate data or methods to test them; several fallacies, particularly those related to the human brain, reflect scientists' preconceptions. The theoretical framework on the evolution of brain, cognition, and behavior in birds and mammals should be reconsidered with these biases in mind.

Cranial sutures work collectively to distribute strain throughout the reptile skull

The skull is composed of many bones that come together at sutures. These sutures are important sites of growth, and as growth ceases some become fused while others remain patent. Their mechanical behaviour and how they interact with changing form and loadings to ensure balanced craniofacial development is still poorly understood. Early suture fusion often leads to disfiguring syndromes, thus is it imperative that we understand the function of sutures more clearly. By applying advanced engineering modelling techniques, we reveal for the first time that patent sutures generate a more widely distributed, high level of strain throughout the reptile skull. Without patent sutures, large regions of the skull are only subjected to infrequent low-level strains that could weaken the bone and result in abnormal development. Sutures are therefore not only sites of bone growth, but could also be essential for the modulation of strains necessary for normal growth and development in reptiles.

Systems biology - the broader perspective

Systems biology has two general aims: a narrow one, which is to discover how complex networks of proteins work, and a broader one, which is to integrate the molecular and network data with the generation and function of organism phenotypes. Doing all this involves complex methodologies, but underpinning the subject are more general conceptual problems about upwards and downwards causality, complexity and information storage, and their solutions provide the constraints within which these methodologies can be used. This essay considers these general aspects and the particular role of protein networks; their functional outputs are often the processes driving phenotypic change and physiological function—networks are, in a sense, the units of systems biology much as proteins are for molecular biology. It goes on to argue that the natural language for systems-biological descriptions of biological phenomena is the mathematical graph (a set of connected facts of the general form <state 1> [process] <state 2> (e.g., <membrane-bound delta> [activates] <notch pathway>). Such graphs not only integrate events at different levels but emphasize the distributed nature of control as well as displaying a great deal of data. The implications and successes of these ideas for physiology, pharmacology, development and evolution are briefly considered. The paper concludes with some challenges for the future.

Molecular Analysis of Twist1 and FGF Receptors in a Rabbit Model of Craniosynostosis: Likely Exclusion as the Loci of Origin

Craniosynostosis is the premature fusion of the cranial vault sutures. We have previously described a colony of rabbits with a heritable pattern of nonsyndromic, coronal suture synostosis; however, the underlying genetic defect remains unknown. We now report a molecular analysis to determine if four genes implicated in human craniosynostosis, TWIST1 and fibroblast growth factor receptors 1–3 (FGFR1–3), could be the loci of the causative mutation in this unique rabbit model. Single nucleotide polymorphisms (SNPs) were identified within the Twist1, FGFR1, and FGFR2 genes, and the allelic patterns of these silent mutations were examined in 22 craniosynostotic rabbits. SNP analysis of the Twist1, FGFR1, and FGFR2 genes indicated that none were the locus of origin of the craniosynostotic phenotype. In addition, no structural mutations were identified by direct sequence analysis of Twist1 and FGFR3 cDNAs. These data indicate that the causative locus for heritable craniosynostosis in this rabbit model is not within the Twist1, FGFR1, and FGFR2 genes. Although a locus in intronic or flanking sequences of FGFR3 remains possible, no direct structural mutation was identified for FGFR3.

Cranial sutures: a multidisciplinary review

INTRODUCTION

Progress in cranial suture research is shaping our current understanding of the topic; however, emphasis has been placed on individual contributing components rather than the cranial sutural system as a whole. Improving our holistic view helps further guide clinicians who treat cranial sutural abnormalities as well as researchers who study them.

MATERIALS AND METHODS

Information from anatomy, anthropology, surgery, and computed modeling was integrated to provide a perspective to interpret suture formation and variability within the cranial functional and structural system.

RESULTS

Evidence from experimental settings, simulations, and evolution suggest a multifactorial morphogenetic process associated with functions and morphology of the sutures. Despite molecular influences, the biomechanical cranial environment has a main role in both the ontogenetic and phylogenetic suture dynamics.

CONCLUSIONS

Furthering our holistic understanding of the intricate cranial sutural system promises to expand our knowledge and enhance our ability to treat associated anomalies.

FGF18 accelerates osteoblast differentiation by upregulating Bmp2 expression

Fibroblast growth factor (FGF) signaling is involved in skeletal development. Among total 22 FGFs, it is suggested that FGF18 functions in promotion of osteoblast differentiation. In order to elucidate the mechanism of FGF18-dependent acceleration of osteogenesis, we implanted rhFGF18 soaked beads over mouse fetal coronal sutures using ex-utero surgery. The coronal suture area comprises the peripheries of the developing frontal and parietal bones, separated by the sutural mesenchyme. rhFGF18 accelerated osteogenesis by promoting connection of the frontal and parietal bone domains, resulting in elimination of the sutural mesenchyme. Expression of Fgf receptors, Fgfr1, -2 and -3 involved in skeletal development, was maintained or upregulated in the developing bone domains, consistent with enhanced osteogenesis. Bone morphogenetic protein (Bmp) 2 was specifically upregulated in the skeletogenic layer and the application of Bmp antagonist, rmNoggin, inhibited rhFGF18-dependent upregulation of osteoblast markers. These results suggest that FGF18 accelerates osteogenesis by upregulation of Bmp2 as well as maintenance or upregulation of Fgfr1, -2 and -3 expression in osteoblasts.

Augmentation of Smad-dependent BMP signaling in neural crest cells causes craniosynostosis in mice

Craniosynostosis describes conditions in which one or more sutures of the infant skull are prematurely fused, resulting in facial deformity and delayed brain development. Approximately 20% of human craniosynostoses are thought to result from gene mutations altering growth factor signaling; however, the molecular mechanisms by which these mutations cause craniosynostosis are incompletely characterized, and the causative genes for diverse types of syndromic craniosynostosis have yet to be identified. Here, we show that enhanced bone morphogenetic protein (BMP) signaling through the BMP type IA receptor (BMPR1A) in cranial neural crest cells, but not in osteoblasts, causes premature suture fusion in mice. In support of a requirement for precisely regulated BMP signaling, this defect was rescued on a Bmpr1a haploinsufficient background, with corresponding normalization of Smad phosphorylation. Moreover, in vivo treatment with LDN-193189, a selective chemical inhibitor of BMP type I receptor kinases, resulted in partial rescue of craniosynostosis. Enhanced signaling of the fibroblast growth factor (FGF) pathway, which has been implicated in craniosynostosis, was observed in both mutant and rescued mice, suggesting that augmentation of FGF signaling is not the sole cause of premature fusion found in this model. The finding that relatively modest augmentation of Smad-dependent BMP signaling leads to premature cranial suture fusion suggests an important contribution of dysregulated BMP signaling to syndromic craniosynostoses and potential strategies for early intervention.

Craniosynostosis-associated Fgfr2(C342Y) mutant bone marrow stromal cells exhibit cell autonomous abnormalities in osteoblast differentiation and bone formation

We recently reported that cranial bones of Fgfr2^C342Y/+ craniosynostotic mice are diminished in density when compared to those of wild type mice, and that cranial bone cells isolated from the mutant mice exhibit inhibited late stage osteoblast differentiation. To provide further support for the idea that craniosynostosis-associated Fgfr mutations lead to cell autonomous defects in osteoblast differentiation and mineralized tissue formation, here we tested bone marrow stromal cells isolated from Fgfr2^C342Y/+ mice for their ability to differentiate into osteoblasts. Additionally, to determine if the low bone mass phenotype of Crouzon syndrome includes the appendicular skeleton, long bones were assessed by micro CT. Fgfr2^C342Y/+ cells showed increased osteoblastic gene expression during early osteoblastic differentiation but decreased expression of alkaline phosphatase mRNA and enzyme activity, and decreased mineralization during later stages of differentiation, when cultured under 2D in vitro conditions. Cells isolated from Fgfr2^C342Y/+ mice also formed less bone when allowed to differentiate in a 3D matrix in vivo. Cortical bone parameters were diminished in long bones of Fgfr2^C342Y/+ mice. These results demonstrate that marrow stromal cells of Fgfr2^C342Y/+ mice have an autonomous defect in osteoblast differentiation and bone mineralization, and that the Fgfr2^C342Y mutation influences both the axial and appendicular skeletons.

Further analysis of the Crouzon mouse: effects of the FGFR2(C342Y) mutation are cranial bone-dependent

Crouzon syndrome is a debilitating congenital disorder involving abnormal craniofacial skeletal development caused by mutations in fibroblast growth factor receptor-2 (FGFR2). Phenotypic expression in humans exhibits an autosomal dominant pattern that commonly involves premature fusion of the coronal suture (craniosynostosis) and severe midface hypoplasia. To further investigate the biologic mechanisms by which the Crouzon syndrome-associated FGFR2(C342Y) mutation leads to abnormal craniofacial skeletal development, we created congenic BALB/c FGFR2(C342Y/+) mice. Here, we show that BALB/c FGFR2(C342Y/+) mice have a consistent craniofacial phenotype including partial fusion of the coronal and lambdoid sutures, intersphenoidal synchondrosis, and multiple facial bones, with minimal fusion of other craniofacial sutures. This phenotype is similar to the classic and less severe form of Crouzon syndrome that involves significant midface hypoplasia with limited craniosynostosis. Linear and morphometric analyses demonstrate that FGFR2(C342Y/+) mice on the BALB/c genetic background differ significantly in form and shape from their wild-type littermates and that in this genetic background the FGFR2(C342Y) mutation preferentially affects some craniofacial bones and sutures over others. Analysis of cranial bone cells indicates that the FGFR2(C342Y) mutation promotes aberrant osteoblast differentiation and increased apoptosis that is more severe in frontal than parietal bone cells. Additionally, FGFR2(C342Y/+) frontal, but not parietal, bones exhibit significantly diminished bone volume and density compared to wild-type mice. These results confirm that FGFR2-associated craniosynostosis occurs in association with diminished cranial bone tissue and may provide a potential biologic explanation for the clinical finding of phenotype consistency that exists between many Crouzon syndrome patients.

Hand in glove: brain and skull in development and dysmorphogenesis

The brain originates relatively early in development from differentiated ectoderm that forms a hollow tube and takes on an exceedingly complex shape with development. The skull is made up of individual bony elements that form from neural crest- and mesoderm-derived mesenchyme that unite to provide support and protection for soft tissues and spaces of the head. The meninges provide a protective and permeable membrane between brain and skull. Across evolutionary and developmental time, dynamic changes in brain and skull shape track one another so that their integration is evidenced in two structures that fit soundly regardless of changes in biomechanical and physiologic functions. Evidence for this tight correspondence is also seen in diseases of the craniofacial complex that are often classified as diseases of the skull (e.g., craniosynostosis) or diseases of the brain (e.g., holoprosencephaly) even when both tissues are affected. Our review suggests a model that links brain and skull morphogenesis through coordinated integration of signaling pathways (e.g., FGF, TGFβ, Wnt) via processes that are not currently understood, perhaps involving the meninges. Differences in the earliest signaling of biological structure establish divergent designs that will be enhanced during morphogenesis. Signaling systems that pattern the developing brain are also active in patterning required for growth and assembly of the skull and some members of these signaling families have been indicated as causal for craniofacial diseases. Because cells of early brain and skull are sensitive to similar signaling families, variation in the strength or timing of signals or shifts in patterning boundaries that affect one system (neural or skull) could also affect the other system and appropriate co-adjustments in development would be made. Interactions of these signaling systems and of the tissues that they pattern are fundamental to the consistent but labile functional and structural association of brain and skull conserved over evolutionary time obvious in the study of development and disease.

Multiple postnatal craniofacial anomalies are characterized by conditional loss of polycystic kidney disease 2 (Pkd2)

Polycystin 2 (Pkd2), which belongs to the transient receptor potential family, plays a critical role in development. Pkd2 is mainly localized in the primary cilia, which also function as mechanoreceptors in many cells that influence multiple biological processes including Ca(2+) influx, chemical activity and signalling pathways. Mutations in many cilia proteins result in craniofacial abnormalities. Orofacial tissues constantly receive mechanical forces and are known to develop and grow through intricate signalling pathways. Here we investigate the role of Pkd2, whose role remains unclear in craniofacial development and growth. In order to determine the role of Pkd2 in craniofacial development, we located expression in craniofacial tissues and analysed mice with conditional deletion of Pkd2 in neural crest-derived cells, using Wnt1Cre mice. Pkd2 mutants showed many signs of mechanical trauma such as fractured molar roots, distorted incisors, alveolar bone loss and compressed temporomandibular joints, in addition to abnormal skull shapes. Significantly, mutants showed no indication of any of these phenotypes at embryonic stages when heads perceive no significant mechanical stress in utero. The results suggest that Pkd2 is likely to play a critical role in craniofacial growth as a mechanoreceptor. Pkd2 is also identified as one of the genes responsible for autosomal dominant polycystic kidney disease (ADPKD). Since facial anomalies have never been identified in ADPKD patients, we carried out three-dimensional photography of patient faces and analysed these using dense surface modelling. This analysis revealed specific characteristics of ADPKD patient faces, some of which correlated with those of the mutant mice.

Reduced dosage of ERF causes complex craniosynostosis in humans and mice and links ERK1/2 signaling to regulation of osteogenesis

The extracellular signal-related kinases 1 and 2 (ERK1/2) are key proteins mediating mitogen-activated protein kinase signaling downstream of RAS: phosphorylation of ERK1/2 leads to nuclear uptake and modulation of multiple targets. Here, we show that reduced dosage of ERF, which encodes an inhibitory ETS transcription factor directly bound by ERK1/2 (refs. 2,3,4,5,6,7), causes complex craniosynostosis (premature fusion of the cranial sutures) in humans and mice. Features of this newly recognized clinical disorder include multiple-suture synostosis, craniofacial dysmorphism, Chiari malformation and language delay. Mice with functional Erf levels reduced to ∼30% of normal exhibit postnatal multiple-suture synostosis; by contrast, embryonic calvarial development appears mildly delayed. Using chromatin immunoprecipitation in mouse embryonic fibroblasts and high-throughput sequencing, we find that ERF binds preferentially to elements away from promoters that contain RUNX or AP-1 motifs. This work identifies ERF as a novel regulator of osteogenic stimulation by RAS-ERK signaling, potentially by competing with activating ETS factors in multifactor transcriptional complexes.

Foxc1 controls the growth of the murine frontal bone rudiment by direct regulation of a Bmp response threshold of Msx2

The mammalian skull vault consists of several intricately patterned bones that grow in close coordination. The growth of these bones depends on the precise regulation of the migration and differentiation of osteogenic cells from undifferentiated precursor cells located above the eye. Here, we demonstrate a role for Foxc1 in modulating the influence of Bmp signaling on the expression of Msx2 and the specification of these cells. Inactivation of Foxc1 results in a dramatic reduction in skull vault growth and causes an expansion of Msx2 expression and Bmp signaling into the area occupied by undifferentiated precursor cells. Foxc1 interacts directly with a Bmp responsive element in an enhancer upstream of Msx2, and acts to reduce the occupancy of P-Smad1/5/8. We propose that Foxc1 sets a threshold for the Bmp-dependent activation of Msx2, thus controlling the differentiation of osteogenic precursor cells and the rate and pattern of calvarial bone development.

Lim mineralization protein is involved in the premature calvarial ossification in sporadic craniosynostoses

Sporadic mono-sutural craniosynostosis represents a highly prevalent regional bone disorder, where a single cranial suture undergoes premature ossification due to a generally unclear etiopathogenesis. The LIM mineralization protein (LMP) was recently described as an efficient osteogenic molecule involved in osteoblast differentiation, expressed in calvarial tissues upon corticosteroid-osteogenic induction and used as a potent inducer of bone formation in several animal models. In this study, calvarial cells isolated from both prematurely fused and physiologically patent sutures of children with sporadic craniosynostosis, were used as an in vitro paradigmatic model for the study of the molecular events involved in calvarial osteogenesis, focusing on the possible role of the LMP-related osteogenic signaling. Calvarial cells isolated from both patent and fused sutures expressed a mesenchymal-like immunophenotype. Cells isolated from fused sutures displayed an increased osteogenic potential, being able to undergo spontaneous mineralization and premature response to osteogenic induction, leading to in vitro bone nodule formation. The expression of LMP and its target genes (bone morphogenetic protein-2, osteocalcin and Runt-related transcription factor 2) was significantly up-regulated in cells derived from the fused sutures. Upon silencing the expression of LMP in fused suture-derived cells, the osteogenic potential along with the expression of osteo-specific transcription factors decreased, restoring the "physiologic" cell behavior. These results suggested that: 1. mesenchymal cells residing in fused sutures display a constitutionally active osteogenic disposition leading to the premature suture ossification; 2. the molecular basis of the overactive osteogenic process may at least in part involve a deregulation of the LMP-related pathway in calvarial cells.

Novel reporter alleles of GSK-3α and GSK-3β

Glycogen Synthase Kinase 3 (GSK-3) is a key player in development, physiology and disease. Because of this, GSK-3 inhibitors are increasingly being explored for a variety of applications. In addition most analyses focus on GSK-3β and overlook the closely related protein GSK-3α. Here, we describe novel GSK-3α and GSK-3β mouse alleles that allow us to visualise expression of their respective mRNAs by tracking β-galactosidase activity. We used these new lacZ alleles to compare expression in the palate and cranial sutures and found that there was indeed differential expression. Furthermore, both are loss of function alleles and can be used to generate homozygous mutant mice; in addition, excision of the lacZ cassette from GSK-3α creates a Cre-dependent tissue-specific knockout. As expected, GSK3α mutants were viable, while GSK3β mutants died after birth with a complete cleft palate. We also assessed the GSK-3α mutants for cranial and sternal phenotypes and found that they were essentially normal. Finally, we observed gestational lethality in compound GSK-3β^−/−; GSK3α^+/− mutants, suggesting that GSK-3 dosage is critical during embryonic development.

Skeletogenic fate of zebrafish cranial and trunk neural crest

The neural crest (NC) is a major contributor to the vertebrate craniofacial skeleton, detailed in model organisms through embryological and genetic approaches, most notably in chick and mouse. Despite many similarities between these rather distant species, there are also distinct differences in the contribution of the NC, particularly to the calvariae of the skull. Lack of information about other vertebrate groups precludes an understanding of the evolutionary significance of these differences. Study of zebrafish craniofacial development has contributed substantially to understanding of cartilage and bone formation in teleosts, but there is currently little information on NC contribution to the zebrafish skeleton. Here, we employ a two-transgene system based on Cre recombinase to genetically label NC in the zebrafish. We demonstrate NC contribution to cells in the cranial ganglia and peripheral nervous system known to be NC-derived, as well as to a subset of myocardial cells. The indelible labeling also enables us to determine NC contribution to late-forming bones, including the calvariae. We confirm suspected NC origin of cartilage and bones of the viscerocranium, including cartilages such as the hyosymplectic and its replacement bones (hymandibula and symplectic) and membranous bones such as the opercle. The cleithrum develops at the border of NC and mesoderm, and as an ancestral component of the pectoral girdle was predicted to be a hybrid bone composed of both NC and mesoderm tissues. However, we find no evidence of a NC contribution to the cleithrum. Similarly, in the vault of the skull, the parietal bones and the caudal portion of the frontal bones show no evidence of NC contribution. We also determine a NC origin for caudal fin lepidotrichia; the presumption is that these are derived from trunk NC, demonstrating that these cells have the ability to form bone during normal vertebrate development.