Conditional inactivation of FGF receptor 2 reveals an essential role for FGF signaling in the regulation of osteoblast function and bone growth

Human craniosynostosis syndromes, resulting from activating or neomorphic mutations in fibroblast growth factor receptor 2 (FGFR2), underscore an essential role for FGFR2 signaling in skeletal development. Embryos harboring homozygous null mutations in FGFR2 die prior to skeletogenesis. To address the role of FGFR2 in normal bone development, a conditional gene deletion approach was adopted. Homologous introduction of cre recombinase into the Dermo1 (Twist2) gene locus resulted in robust expression of CRE in mesenchymal condensations giving rise to both osteoblast and chondrocyte lineages. Inactivation of a floxed Fgfr2 allele with Dermo1-cre resulted in mice with skeletal dwarfism and decreased bone density. Although differentiation of the osteoblast lineage was not disturbed, the proliferation of osteoprogenitors and the anabolic function of mature osteoblasts were severely affected.

The role of Axin2 in calvarial morphogenesis and craniosynostosis

Axin1 and its homolog Axin2/conductin/Axil are negative regulators of the canonical Wnt pathway that suppress signal transduction by promoting degradation of beta-catenin. Mice with deletion of Axin1 exhibit defects in axis determination and brain patterning during early embryonic development. We show that Axin2 is expressed in the osteogenic fronts and periosteum of developing sutures during skull morphogenesis. Targeted disruption of Axin2 in mice induces malformations of skull structures, a phenotype resembling craniosynostosis in humans. In the mutants, premature fusion of cranial sutures occurs at early postnatal stages. To elucidate the mechanism of craniosynostosis, we studied intramembranous ossification in Axin2-null mice. The calvarial osteoblast development is significantly affected by the Axin2 mutation. The Axin2 mutant displays enhanced expansion of osteoprogenitors, accelerated ossification, stimulated expression of osteogenic markers and increases in mineralization. Inactivation of Axin2 promotes osteoblast proliferation and differentiation in vivo and in vitro. Furthermore, as the mammalian skull is formed from cranial skeletogenic mesenchyme, which is derived from mesoderm and neural crest, our data argue for a region-specific effect of Axin2 on neural crest dependent skeletogenesis. The craniofacial anomalies caused by the Axin2 mutation are mediated through activation of beta-catenin signaling, suggesting a novel role for the Wnt pathway in skull morphogenesis.

The BMP antagonist noggin regulates cranial suture fusion

During skull development, the cranial connective tissue framework undergoes intramembranous ossification to form skull bones (calvaria). As the calvarial bones advance to envelop the brain, fibrous sutures form between the calvarial plates. Expansion of the brain is coupled with calvarial growth through a series of tissue interactions within the cranial suture complex. Craniosynostosis, or premature cranial suture fusion, results in an abnormal skull shape, blindness and mental retardation. Recent studies have demonstrated that gain-of-function mutations in fibroblast growth factor receptors (fgfr) are associated with syndromic forms of craniosynostosis. Noggin, an antagonist of bone morphogenetic proteins (BMPs), is required for embryonic neural tube, somites and skeleton patterning. Here we show that noggin is expressed postnatally in the suture mesenchyme of patent, but not fusing, cranial sutures, and that noggin expression is suppressed by FGF2 and syndromic fgfr signalling. Since noggin misexpression prevents cranial suture fusion in vitro and in vivo, we suggest that syndromic fgfr-mediated craniosynostoses may be the result of inappropriate downregulation of noggin expression.

Fgfr2 and osteopontin domains in the developing skull vault are mutually exclusive and can be altered by locally applied FGF2

Mutations in the human fibroblast growth factor receptor type 2 (FGFR2) gene cause craniosynostosis, particularly affecting the coronal suture. We show here that, in the fetal mouse skull vault, Fgfr2 transcripts are most abundant at the periphery of the membrane bones; they are mutually exclusive with those of osteopontin (an early marker of osteogenic differentiation) but coincide with sites of rapid cell proliferation. Fibroblast growth factor type 2 (FGF2) protein, which has a high affinity for the FGFR2 splice variant associated with craniosynostosis, is locally abundant; immunohistochemical detection showed it to be present at low levels in Fgfr2 expression domains and at high levels in differentiated areas. Implantation of FGF2-soaked beads onto the fetal coronal suture by ex utero surgery resulted in ectopic osteopontin expression, encircled by Fgfr2 expression, after 48 hours. We suggest that increased FGF/FGFR signalling in the developing skull, whether due to FGFR2 mutation or to ectopic FGF2, shifts the cell proliferation/differentiation balance towards differentiation by enhancing the normal paracrine down-regulation of Fgfr2.

Twist1 dimer selection regulates cranial suture patterning and fusion

Saethre-Chotzen syndrome is associated with haploinsufficiency of the basic-helix-loop-helix (bHLH) transcription factor TWIST1 and is characterized by premature closure of the cranial sutures, termed craniosynostosis; however, the mechanisms underlying this defect are unclear. Twist1 has been shown to play both positive and negative roles in mesenchymal specification and differentiation, and here we show that the activity of Twist1 is dependent on its dimer partner. Twist1 forms both homodimers (T/T) and heterodimers with E2A E proteins (T/E) and the relative level of Twist1 to the HLH inhibitor Id proteins determines which dimer forms. On the basis of the expression patterns of Twist1 and Id1 within the cranial sutures, we hypothesized that Twist1 forms homodimers in the osteogenic fronts and T/E heterodimers in the mid-sutures. In support of this hypothesis, we have found that genes regulated by T/T homodimers, such as FGFR2 and periostin, are expressed in the osteogenic fronts, whereas genes regulated by T/E heterodimers, such as thrombospondin-1, are expressed in the mid-sutures. The ratio between these dimers is altered in the sutures of Twist1+/- mice, favoring an increase in homodimers and an expansion of the osteogenic fronts. Of interest, the T/T to T/E ratio is greater in the coronal versus the sagittal suture, and this finding may contribute to making the coronal suture more susceptible to fusion due to TWIST haploinsufficiency. Importantly, we were able to inhibit suture fusion in Twist1+/- mice by modulating the balance between these dimers toward T/E formation, by either increasing the expression of E2A E12 or by decreasing Id expression. Therefore, we have identified dimer partner selection as an important mediator of Twist1 function and provide a mechanistic understanding of craniosynostosis due to TWIST haploinsufficiency.

Cell mixing at a neural crest-mesoderm boundary and deficient ephrin-Eph signaling in the pathogenesis of craniosynostosis

Boundaries between cellular compartments often serve as signaling interfaces during embryogenesis. The coronal suture is a major growth center of the skull vault and develops at a boundary between cells derived from neural crest and mesodermal origin, forming the frontal and parietal bones, respectively. Premature fusion of these bones, termed coronal synostosis, is a common human developmental anomaly. Known causes of coronal synostosis include haploinsufficiency of TWIST1 and a gain of function mutation in MSX2. In Twist1(+/-) mice with coronal synostosis, we found that the frontal-parietal boundary is defective. Specifically, neural crest cells invade the undifferentiated mesoderm of the Twist1(+/-) mutant coronal suture. This boundary defect is accompanied by an expansion in Msx2 expression and reduction in ephrin-A4 distribution. Reduced dosage of Msx2 in the Twist1 mutant background restores the expression of ephrin-A4, rescues the suture boundary and inhibits craniosynostosis. Underlining the importance of ephrin-A4, we identified heterozygous mutations in the human orthologue, EFNA4, in three of 81 patients with non-syndromic coronal synostosis. This provides genetic evidence that Twist1, Msx2 and Efna4 function together in boundary formation and the pathogenesis of coronal synostosis.

Human NELL-1 expressed in unilateral coronal synostosis

Surgical correction of unilateral coronal synostosis offers a unique opportunity to examine the molecular differences between an abnormal and a normal cranial suture. We isolated and identified a cDNA fragment whose expression was up-regulated in the premature fusing and fused coronal sutures, as compared with normal coronal sutures. The nucleotide sequence of the full-length cDNA of this gene, human NELL-1, has approximately 61% homology with the chicken Nel gene. Both chicken Nel and human NELL-1 are comprised of six epidermal growth factor-like repeats. The human NELL-1 messages were localized primarily in the mesenchymal cells and osteoblasts at the osteogenic front, along the parasutural bone margins, and within the condensing mesenchymal cells of newly formed bone in sites of premature sutural fusion. Human multiorgan tissue mRNA blot showed that NELL-1 was specifically expressed in fetal brain but not in fetal kidney, liver, or lung. We also showed that Nell-1 was expressed in rat calvarial osteoprogenitor cells and was largely absent in rat tibiae and fibroblast cell cultures. In conclusion, our data suggest that the NELL-1 gene is preferentially expressed in cranial intramembranous bone and neural tissue (both of neural crest cell origin) and is up-regulated during unilateral premature closure of the coronal suture. The precise role of this gene is unknown.

Craniosynostosis in transgenic mice overexpressing Nell-1

Previously, we reported NELL-1 as a novel molecule overexpressed during premature cranial suture closure in patients with craniosynostosis (CS), one of the most common congenital craniofacial deformities. Here we describe the creation and analysis of transgenic mice overexpressing Nell-1. Nell-1 transgenic animals exhibited CS-like phenotypes that ranged from simple to compound synostoses. Histologically, the osteogenic fronts of abnormally closing/closed sutures in these animals revealed calvarial overgrowth and overlap along with increased osteoblast differentiation and reduced cell proliferation. Furthermore, anomalies were restricted to calvarial bone, despite generalized, non-tissue-specific overexpression of Nell-1. In vitro, Nell-1 overexpression accelerated calvarial osteoblast differentiation and mineralization under normal culture conditions. Moreover, Nell-1 overexpression in osteoblasts was sufficient to promote alkaline phosphatase expression and micronodule formation. Conversely, downregulation of Nell-1 inhibited osteoblast differentiation in vitro. In summary, Nell-1 overexpression induced calvarial overgrowth resulting in premature suture closure in a rodent model. Nell-1, therefore, has a novel role in CS development, perhaps as part of a complex chain of events resulting in premature suture closure. On a cellular level, Nell-1 expression may modulate and be both sufficient and required for osteoblast differentiation.

Immunolocalization of transforming growth factor beta 1, beta 2, and beta 3 and insulin-like growth factor I in premature cranial suture fusion

The etiology of craniosynostosis remains unknown. The beta group of transforming growth factors (TGF-beta) and insulin-like growth factors (IGF-I and IGF-II) are known to induce new bone formation and, when added exogenously, cause accelerated closure of calvarial defects. The possible roles of these bone growth factors in premature cranial suture fusion in humans have not been explored. We analyzed a total of 20 cranial suture biopsy samples (10 synostotic and 10 normal) from 10 infants with single-suture craniosynostosis undergoing cranial vault remodeling. Using isoform-specific antibodies for TGF-beta 1, -beta 2, and -beta 3 and IGF-I, we demonstrated immunoreactivity of these growth factors were present in human cranial sutures; the TGF-beta 2 isoform was the most intensely immunoreactive. Most importantly, the TGF-beta isoforms and IGF-I showed more intense immunoreactivity in the actively fusing craniosynostotic sutures compared with the control patent sutures. Specifically, the TGF-beta isoforms and IGF-I were intensely localized in the osteoblasts synthesizing new bone at the suture margin. It is noteworthy that although the patent sutures were less immunoreactive for TGF-beta isoforms than fused sutures, there was a distinct pattern of the TGF-beta 3 isoform that was immunolocalized to the margin of the normal patent sutures. This suggests a possible role for TGF-beta 3 in maintaining cranial suture patency. The increased immunoreactivity of both TGF-beta 2 and IGF-I in the actively fusing sutures compared with the patent control sutures indicates that these growth factors may play a role in the biology underlying premature suture closure. To our knowledge, this is the first study showing the presence of TGF-beta 1, -beta 2, and -beta 3 and IGF-I in prematurely fusing human cranial sutures. In the future, manipulating the local expression of these growth factors at the suture site may enable plastic surgeons to modulate premature suture fusion.

Nell-1-induced bone regeneration in calvarial defects

Many craniofacial birth defects contain skeletal components requiring bone grafting. We previously identified the novel secreted osteogenic molecule NELL-1, first noted to be overexpressed during premature bone formation in calvarial sutures of craniosynostosis patients. Nell-1 overexpression significantly increases differentiation and mineralization selectively in osteoblasts, while newborn Nell-1 transgenic mice significantly increase premature bone formation in calvarial sutures. In the current study, cultured calvarial explants isolated from Nell-1 transgenic newborn mice (with mild sagittal synostosis) demonstrated continuous bone growth and overlapping sagittal sutures. Further investigation into gene expression cascades revealed that fibroblast growth factor-2 and transforming growth factor-beta1 stimulated Nell-1 expression, whereas bone morphogenetic protein (BMP)-2 had no direct effect. Additionally, Nell-1-induced osteogenesis in MC3T3-E1 osteoblasts through reduction in the expression of early up-regulated osteogenic regulators (OSX and ALP) but induction of later markers (OPN and OCN). Grafting Nell-1 protein-coated PLGA scaffolds into rat calvarial defects revealed the osteogenic potential of Nell-1 to induce bone regeneration equivalent to BMP-2, whereas immunohistochemistry indicated that Nell-1 reduced osterix-producing cells and increased bone sialoprotein, osteocalcin, and BMP-7 expression. Insights into Nell-1-regulated osteogenesis coupled with its ability to stimulate bone regeneration revealed a potential therapeutic role and an alternative to the currently accepted techniques for bone regeneration.

Genotype-phenotype correlation for nucleotide substitutions in the IgII-IgIII linker of FGFR2

Dominantly acting, allelic mutations of the fibroblast growth factor receptor 2 (FGFR2) gene have been described in five craniosynostosis syndromes. In Apert syndrome, characterised by syndactyly of the hands and feet, recurrent mutations of a serine-proline dipeptide (either Ser252Trp or Pro253Arg) in the linker between the IgII and IgIII extracellular immunoglobulin-like domains, have been documented in more than 160 unrelated individuals. We have identified three novel mutations of this dipeptide, associated with distinct phenotypes. A C-->T mutation that predicts a Ser252Leu substitution, ascertained in a boy with mild Crouzon syndrome (craniosynostosis with normal limbs) is also present in three clinically normal members of his family. A CG-->TT mutation that predicts a Ser252Phe substitution results in a phenotype consistent with Apert syndrome. Finally, a CGC-->TCT mutation that predicts a double amino acid substitution (Ser252Phe and Pro253Ser) causes a Pfeiffer syndrome variant with mild craniosynostosis, broad thumbs and big toes, fixed extension of several digits, and only minimal cutaneous syndactyly. The observation that the Ser252Phe mutation causes Apert syndrome, whereas the other single or double substitutions are associated with milder or normal phenotypes, highlights the exquisitely specific molecular pathogenesis of the limb and craniofacial abnormalities associated with Apert syndrome. Ser252Phe is the first noncanonical mutation to be identified in this disorder, its rarity being explained by the requirement for two residues of the serine codon to be mutated. The description of independent, complex nucleotide substitutions involving identical nucleotides is unprecedented, and we speculate that this may result from functional selection of FGFR mutations in sperm.

Biology of FGFRL1, the fifth fibroblast growth factor receptor

FGFRL1 (fibroblast growth factor receptor like 1) is the most recently discovered member of the FGFR family. It contains three extracellular Ig-like domains similar to the classical FGFRs, but it lacks the protein tyrosine kinase domain and instead contains a short intracellular tail with a peculiar histidine-rich motif. The gene for FGFRL1 is found in all metazoans from sea anemone to mammals. FGFRL1 binds to FGF ligands and heparin with high affinity. It exerts a negative effect on cell proliferation, but a positive effect on cell differentiation. Mice with a targeted deletion of the Fgfrl1 gene die perinatally due to alterations in their diaphragm. These mice also show bilateral kidney agenesis, suggesting an essential role for Fgfrl1 in kidney development. A human patient with a frameshift mutation exhibits craniosynostosis, arguing for an additional role of FGFRL1 during bone formation. FGFRL1 contributes to the complexity of the FGF signaling system.

Recent progress in genetics of Marfan syndrome and Marfan-associated disorders

Marfan syndrome (MFS, OMIM #154700) is a hereditary connective tissue disorder, clinically presenting with cardinal features of skeletal, ocular, and cardiovascular systems. In classical MFS, changes in connective tissue integrity can be explained by defects in fibrillin-1, a major component of extracellular microfibrils. However, some of the clinical manifestations of MFS cannot be explained by mechanical properties alone. Recent studies manipulating mouse Fbn1 have provided new insights into the molecular pathogenesis of MFS. Dysregulation of transforming growth factor beta (TGFbeta) signaling in lung, mitral valve and aortic tissues has been implicated in mouse models of MFS. TGFBR2 and TGFBR1 mutations were identified in a subset of patients with MFS (MFS2, OMIM #154705) and other MFS-related disorders, including Loeys-Dietz syndrome (LDS, #OMIM 609192) and familial thoracic aortic aneurysms and dissections (TAAD2, #OMIM 608987). These data indicate that genetic heterogeneity exists in MFS and its related conditions and that regulation of TGFbeta signaling plays a significant role in these disorders.

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.

Mutation associated with Crouzon syndrome causes ligand-independent dimerization and activation of FGF receptor-2

FGF signaling is clearly important for proper bone development, and several autosomally dominant forms of genetic bone disorders have been mapped to FGF receptors 1, 2, and 3. We have studied the biological effects of the most commonly mutated cysteine residue in FGFR-2 which is detected in individuals with Crouzon syndrome, an autosomally dominant trait which causes premature fusion of the skull bones (craniosynostosis). This Crouzon mutation replaces the cysteine at position 342 with tyrosine, thus disrupting the formation of the third immunoglobulin (Ig)-like loop in the extracellular portion of the receptor. By transfecting mutated and wild-type receptors into a variety of cell lines, we have shown that the C342Y mutation in FGFR-2 produces a receptor which is constitutively activated and capable of transforming NIH3T3 cells and preventing the differentiation of C2 myoblasts in the absence of ligand. Constitutive activation appears to result from the ability of this receptor to form stable interreceptor dimers which involve disulfide bonds between the remaining free cysteine in the mutant receptor. The altered conformation of the third Ig-like domain in the mutated receptor also results in a drastically reduced ability to bind FGF-1 or FGF-2 and in a reduced level of receptor glycosylation. Thus it appears that Crouzon syndrome results from constitutive activation of FGFR-2 and that uncontrolled FGF signaling produces alterations of intramembranous bone development and premature closing of cranial sutures.

FGF and FGFR signaling in chondrodysplasias and craniosynostosis

The first experimental mouse model for FGF2 in bone dysplasia was made serendipitously by overexpression of FGF from a constitutive promoter. The results were not widely accepted, rightfully drew skepticism, and were difficult to publish; because of over 2,000 studies published on FGF-2 at the time (1993), only a few reported a role of FGF-2 in bone growth and differentiation. However, mapping of human dwarfisms to mutations of the FGFRs shortly, thereafter, made the case that bone growth and remodeling was a major physiological function for FGF. Subsequent production of numerous transgenic and targeted null mice for several genes in the bone growth and remodeling pathways have marvelously elucidated the role of FGFs and their interactions with other genes. Indeed, studies of the FGF pathway present one of the best success stories for use of experimental genetics in functionally parsing morphogenetic regulatory pathways. What remains largely unresolved is the pleiotropic nature of FGF-2. How does it accelerate growth in one cell then stimulate apoptosis or retard growth for another cell in the same type of tissue? Some of the answers may come through distinguishing the FGF-2 protein isoforms, made from alternative translation start sites, these appear to have substantially different functions. Although we have made substantial progress, there is still much to be learned regarding FGF-2 as a most complex, enigmatic protein. Studies of genetic models in mice and human FGFR mutations have provided strong evidence that FGFRs are important modulators of osteoblast function during membranous bone formation. However, there is some controversy regarding the effects of FGFR signaling in human and murine genetic models. Although significant progress has been made in our understanding of FGFR signaling, several questions remain concerning the signaling pathways involved in osteoblast regulation by activated FGFR. Additionally, little is known about the specific role of FGFR target genes involved in cranial bone formation. These issues need to be addressed in future in in vitro and in vivo approaches to better understand the molecular mechanisms of action of FGFR signaling in osteoblasts that result in anabolic effects in bone formation.

Increased bone formation and osteoblastic cell phenotype in premature cranial suture ossification (craniosynostosis)

Craniosynostosis is a heterogeneous disorder characterized by premature fusion of the skull bone sutures. To evaluate the pathogenesis of premature cranial suture ossification in craniosynostosis, we have evaluated the histologic indices of bone formation and the characteristics of osteoblastic cells derived from normal and affected cranial sutures in 47 infants and children, aged 3-18 months, with nonsyndromic craniosynostosis. The histomorphometric analysis of normal and fused sutures showed an age-related decline in the extent of endosteal bone surface covered with osteoid and osteoblasts during postnatal suture ossification. Bone formation was 20-50% higher at 3-6 months of age in fused sutures compared with normal sutures in the same patients. Cells derived from normal and fused sutures displayed characteristics of the osteoblast phenotype in culture. Analysis of [3H]thymidine incorporation into DNA from 1-14 days of culture showed an age-related decrease in osteoblastic cell growth in both normal and affected sutures. The proliferation of osteoblastic cells isolated from fused sutures was similar at all ages to that of cells isolated from normal sutures in the same patients. In contrast, alkaline phosphatase activity and osteocalcin production by osteoblastic cells cultured in basal conditions and after stimulation with 1,25-dihydroxyvitamin D (1,25[OH]2D3), were 53-74% higher in fused sutures compared with cells isolated from normal sutures in the same patients. The results indicate that bone formation activity at the suture site is locally increased in craniosynostosis, and this disorder is associated with increased in vitro parameters of osteoblastic cell differentiation, suggesting that an increased maturation of osteoblastic cells at the site of the suture leads to the premature ossification in nonsyndromic craniosynostosis.

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.

Erk pathway and activator protein 1 play crucial roles in FGF2-stimulated premature cranial suture closure

Cranial sutures are an important growth center of the cranial bones, and the suture space must be maintained to permit the cranial adjustments needed to accommodate brain growth. Craniosynostosis, characterized by premature suture closure, mainly results from mutations that generate constitutively active fibroblast growth factor (FGF) receptors. FGF signaling, thus, is responsible for the pathogenesis of craniosynostosis. Even though FGF activates many different signaling pathways, the one involved in premature suture closure has not been defined. We observed that placing FGF2-soaked bead on the osteogenic fronts of cultured mouse calvaria accelerates cranial suture closure and strongly induces the expression of osteopontin, an early marker of differentiated osteoblasts. FGF2 treatment also induced fos and jun mRNAs and later increased the nuclear levels of activator protein 1 (AP1). FGF2 stimulates the expression of osteopontin by inducing expression of AP1, which then binds to its response element in the osteopontin promoter. Blocking of the Erk pathway by PD98059 suppressed the AP1 and osteopontin expression stimulated by FGF2. Coincidently, blocking of the Erk pathway also significantly retarded FGF2-accelerated cranial suture closure. Thus, the Erk pathway mediates FGF/FGF receptor-stimulated cranial suture closure, probably by stimulating synthesis of AP1 that then stimulates the differentiation of osteoblasts.

COLEC10 is mutated in 3MC patients and regulates early craniofacial development

3MC syndrome is an autosomal recessive heterogeneous disorder with features linked to developmental abnormalities. The main features include facial dysmorphism, craniosynostosis and cleft lip/palate; skeletal structures derived from cranial neural crest cells (cNCC). We previously reported that lectin complement pathway genes COLEC11 and MASP1/3 are mutated in 3MC syndrome patients. Here we define a new gene, COLEC10, also mutated in 3MC families and present novel mutations in COLEC11 and MASP1/3 genes in a further five families. The protein products of COLEC11 and COLEC10, CL-K1 and CL-L1 respectively, form heteromeric complexes. We show COLEC10 is expressed in the base membrane of the palate during murine embryo development. We demonstrate how mutations in COLEC10 (c.25C>T; p.Arg9Ter, c.226delA; p.Gly77Glufs*66 and c.528C>G p.Cys176Trp) impair the expression and/or secretion of CL-L1 highlighting their pathogenicity. Together, these findings provide further evidence linking the lectin complement pathway and complement factors COLEC11 and COLEC10 to morphogenesis of craniofacial structures and 3MC etiology.

The 3MC syndrome is a unifying term amalgamating four rare recessive genetic disorders with overlapping features namely; Mingarelli, Malpuech, Michels and Carnevale syndromes. It is characterised by facial malformations including, high-arched eyebrows, cleft lip/palate, hypertelorism, developmental delay and hearing loss. We previously reported that lectin complement pathway genes COLEC11 and MASP1/3 were mutated in 3MC syndrome patients. Here we describe a new gene from the same pathway, COLEC10, mutated in 3MC patients. Our results show that COLEC10 is expressed in craniofacial tissues during development. We demonstrate how CL-L1, the protein expressed by COLEC10, can act as a cellular chemoattractant in vitro, controlling cell movement and migration. We overexpressed constructs carrying COLEC10 non-sense mutations found in our patients, CL-L1 failed to be expressed and secreted. Moreover, when we expressed a missense COLEC10 construct, CL-L1 was expressed but failed to be secreted. In sum, we discovered a new gene, COLEC10, mutated in 3MC syndrome and we propose a pathogenic mechanism for 3MC relating to the failure of CL-L1 function and its craniofacial developmental consequences.

Craniosynostosis and altered patterns of fetal TGF-beta expression induced by intrauterine constraint

Recent work has demonstrated that fusion of the calvarial sutures is mediated by locally elaborated soluble growth factors, including the transforming growth factor-betas (TGF-betas), leading some to speculate that external biomechanical forces play little role in suture development. Clinical evidence has long suggested, however, that fetal head constraint may play a critical role in the pathogenesis of many cases of nonsyndromic craniosynostosis. The purpose of these experiments was to test the hypothesis that intrauterine constraint leads to an alteration in normal patterns of TGF-beta expression and that these alterations are associated with craniosynostosis. Fetal constraint was induced by allowing C57Bl/6 murine fetuses to grow for 2.5 days beyond the normal 20-day gestation by performing uterine cerclage on the eighteenth day. Cranial suture morphology was examined in hematoxylin and eosin-stained sections and in cleared whole-mount specimens, double stained with alizarin red S and Alcian blue. Expression patterns of TGF-beta1 and TGF-beta3 were examined by immunohistochemical techniques. Gross and microscopic examination of the cranial sutures of 17 constrained fetuses revealed changes that ranged from narrowing to complete osseous obliteration of the coronal and squamosal sutures. All sutures of 14 nonconstrained control pups remained patent. Fetal head constraint was associated with increased TGF-beta1 immunoreactivity within the new bone and the underlying dura when compared with nonconstrained age-matched controls. TGF-beta3 immunoreactivity was associated with the dura underlying patent, nonconstrained sutures, whereas constraint-induced synostosis was characterized by down-regulation of dural TGF-beta3 expression. These experiments confirm the ability of intrauterine constraint to induce premature fusion of the cranial sutures and provide evidence that intrauterine head constraint induces the expression of osteogenic growth factors in fetal calvarial bone and the underlying dura.

Intracellular retention, degradation, and signaling of glycosylation-deficient FGFR2 and craniosynostosis syndrome-associated FGFR2C278F

Fibroblast growth factors (FGFs) and their receptors (FGFRs) are known to play a critical role in a variety of fundamental processes, including wound healing, angiogenesis, and development of multiple organ systems. Mutations in the FGFR gene family have been linked to a series of syndromes (the craniosynostosis syndromes) whose primary phenotype involves aberrant development of the craniofacial skeleton. Craniosynostosis syndrome-linked FGFR mutations have been shown to be gain of function in terms of receptor activation and have been presumed to result in increased levels of FGF/FGFR signaling. Unfortunately, studies attempting to link expression of mutant FGFRs with changes in cellular phenotype have yielded conflicting results. In an effort to better understand the biochemical consequences of these mutations on receptor function, here we have investigated the effect of the FGFR2C278F mutation of Crouzon craniosynostosis syndrome on receptor trafficking, ubiquitination, degradation, and signaling. We find that FGFR2C278F exhibits diminished glycosylation, increased degradation, and limited cellular sublocalization in the osteoblastic cell line, MC3T3E1(C4). Additionally, we show that trafficking and autoactivation of wild type FGFR2 is glycosylation-dependent. Both FGFR2C278F and unglycosylated wild type FGFR2 signal through phospholipase Cgamma in a ligand-independent manner as well as exhibit dramatically increased binding to the adaptor protein, Frs2. These findings suggest that autoactive FGFR2 can signal from intracellular compartments. Based upon our results, we propose that the functional signaling of craniosynostosis mutant, autoactive receptors is limited in some cell types by protective cellular responses, such as increased trafficking to lysosomes and proteasomes for degradation.

Beyond the closed suture in apert syndrome mouse models: evidence of primary effects of FGFR2 signaling on facial shape at birth

Apert syndrome is a congenital disorder caused mainly by two neighboring mutations on fibroblast growth factor receptor 2 (FGFR2). Premature closure of the coronal suture is commonly considered the identifying and primary defect triggering or preceding the additional cranial malformations of Apert phenotype. Here we use two transgenic mouse models of Apert syndrome, Fgfr2(+/S252W) and Fgfr2(+/P253R), to explore variation in cranial phenotypes in newborn (P0) mice. Results show that the facial skeleton is the most affected region of the cranium. Coronal suture patency shows marked variation that is not strongly correlated with skull dysmorphology. The craniofacial effects of the FGFR2 mutations are similar, but Fgfr2(+/S252W) mutant mice display significantly more severe dysmorphology localized to the posterior palate. Our results demonstrate that coronal suture closure is neither the primary nor the sole locus of skull dysmorphology in these mouse models for Apert syndrome, but that the face is also primarily affected.