Identification of IGF-I in the calvarial suture of young rats: histochemical analysis of the cranial sagittal sutures in a hyperthyroid rat model

Thyroid Hormone and Skeletal Development

Thyroid hormone (TH) is essential for skeletal development from the late fetal life to the onset of puberty. During this large window of actions, TH has key roles in endochondral and intramembranous ossifications and in the longitudinal bone growth. There is evidence that TH acts directly in skeletal cells but also indirectly, specially via the growth hormone/insulin-like growth factor-1 axis, to control the linear skeletal growth and maturation. The presence of receptors, plasma membrane transporters, and activating and inactivating enzymes of TH in skeletal cells suggests that direct actions of TH in these cells are crucial for skeletal development, which has been confirmed by several in vitro and in vivo studies, including mouse genetic studies, and clinical studies in patients with resistance to thyroid hormone due to dominant-negative mutations in TH receptors. This review examines progress made on understanding the mechanisms by which TH regulates the skeletal development.

Thyroxine Exposure Effects on the Cranial Base

Thyroid hormone is important for skull bone growth, which primarily occurs at the cranial sutures and synchondroses. Thyroid hormones regulate metabolism and act in all stages of cartilage and bone development and maintenance by interacting with growth hormone and regulating insulin-like growth factor. Aberrant thyroid hormone levels and exposure during development are exogenous factors that may exacerbate susceptibility to craniofacial abnormalities potentially through changes in growth at the synchondroses of the cranial base. To elucidate the direct effect of in utero therapeutic thyroxine exposure on the synchondroses in developing mice, we provided scaled doses of the thyroid replacement drug, levothyroxine, in drinking water to pregnant C57BL6 wild-type dams. The skulls of resulting pups were subjected to micro-computed tomography analysis revealing less bone volume relative to tissue volume in the synchondroses of mouse pups exposed in utero to levothyroxine. Histological assessment of the cranial base area indicated more active synchondroses as measured by metabolic factors including Igf1. The cranial base of the pups exposed to high levels of levothyroxine also contained more collagen fiber matrix and an increase in markers of bone formation. Such changes due to exposure to exogenous thyroid hormone may drive overall morphological changes. Thus, excess thyroid hormone exposure to the fetus during pregnancy may lead to altered craniofacial growth and increased risk of anomalies in offspring.

Effects of In Utero Thyroxine Exposure on Murine Cranial Suture Growth

Large scale surveillance studies, case studies, as well as cohort studies have identified the influence of thyroid hormones on calvarial growth and development. Surveillance data suggests maternal thyroid disorders (hyperthyroidism, hypothyroidism with pharmacological replacement, and Maternal Graves Disease) are linked to as much as a 2.5 fold increased risk for craniosynostosis. Craniosynostosis is the premature fusion of one or more calvarial growth sites (sutures) prior to the completion of brain expansion. Thyroid hormones maintain proper bone mineral densities by interacting with growth hormone and aiding in the regulation of insulin like growth factors (IGFs). Disruption of this hormonal control of bone physiology may lead to altered bone dynamics thereby increasing the risk for craniosynostosis. In order to elucidate the effect of exogenous thyroxine exposure on cranial suture growth and morphology, wild type C57BL6 mouse litters were exposed to thyroxine in utero (control = no treatment; low ~167 ng per day; high ~667 ng per day). Thyroxine exposed mice demonstrated craniofacial dysmorphology (brachycranic). High dose exposed mice showed diminished area of the coronal and widening of the sagittal sutures indicative of premature fusion and compensatory growth. Presence of thyroid receptors was confirmed for the murine cranial suture and markers of proliferation and osteogenesis were increased in sutures from exposed mice. Increased Htra1 and Igf1 gene expression were found in sutures from high dose exposed individuals. Pathways related to the HTRA1/IGF axis, specifically Akt and Wnt, demonstrated evidence of increased activity. Overall our data suggest that maternal exogenous thyroxine exposure can drive calvarial growth alterations and altered suture morphology.

Effects of thyroxine exposure on the Twist 1 +/- phenotype: A test of gene-environment interaction modeling for craniosynostosis

BACKGROUND

Craniosynostosis, the premature fusion of one or more of the cranial sutures, is estimated to occur in 1:1800 to 2500 births. Genetic murine models of craniosynostosis exist, but often imperfectly model human patients. Case, cohort, and surveillance studies have identified excess thyroid hormone as an agent that can either cause or exacerbate human cases of craniosynostosis.

METHODS

Here we investigate the influence of in utero and in vitro exogenous thyroid hormone exposure on a murine model of craniosynostosis, Twist 1 +/-.

RESULTS

By 15 days post-natal, there was evidence of coronal suture fusion in the Twist 1 +/- model, regardless of exposure. With the exception of craniofacial width, there were no significant effects of exposure; however, the Twist 1 +/- phenotype was significantly different from the wild-type control. Twist 1 +/- cranial suture cells did not respond to thyroxine treatment as measured by proliferation, osteogenic differentiation, and gene expression of osteogenic markers. However, treatment of these cells did result in modulation of thyroid associated gene expression.

CONCLUSION

Our findings suggest the phenotypic effects of the genetic mutation largely outweighed the effects of thyroxine exposure in the Twist 1 +/- model. These results highlight difficultly in experimentally modeling gene-environment interactions for craniosynostotic phenotypes. Birth Defects Research (Part A) 106:803-813, 2016. © 2016 Wiley Periodicals, Inc.

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.

IGF1R variants associated with isolated single suture craniosynostosis

The genetic contribution to the pathogenesis of isolated single suture craniosynostosis is poorly understood. The role of mutations in genes known to be associated with syndromic synostosis appears to be limited. We present our findings of a candidate gene resequencing approach to identify rare variants associated with the most common forms of isolated craniosynostosis. Resequencing of the coding regions, splice junction sites, and 5' and 3' untranslated regions of 27 candidate genes in 186 cases of isolated non-syndromic single suture synostosis revealed three novel and two rare sequence variants (R406H, R595H, N857S, P190S, M446V) in insulin-like growth factor I receptor (IGF1R) that are enriched relative to control samples. Mapping the resultant amino acid changes to the modeled homodimer protein structure suggests a structural basis for segregation between these and other disease-associated mutations found in IGF1R. These data suggest that IGF1R mutations may contribute to the risk and in some cases cause single suture craniosynostosis.

Ectopic bone formation facilitated by human mesenchymal stem cells and osteogenic cytokines via nutrient vessel injection in a nude rat model

In vivo studies using bone marrow-derived mesenchymal stem cells are still uncommon. Applications for bone defect replacement in undesirable clinical circumstances such as large defects, bacterial or other pathogen-contaminated fields, and irradiated surgical wound bed necessitate vascularized bone regeneration. Use of a fascial flap including regenerated bone would be a very powerful tool for treatment. It would be especially beneficial in cases where normal bone regeneration is not expected due to a lack of sufficient blood supply, extensive surgical scarring, or bacterial contamination. In this study, we used nude rats in which the superficial epigastric flap of the experimental group was used to wrap around a mixture of human mesenchymal stem cells, bone morphogenetic protein-2, and basic fibroblast growth factor cytokines in a gelatin carrier. These rats showed significantly higher bone mineral density at 4 weeks compared to the other experimental groups containing phosphate buffered saline, human mesenchymal stem cells alone, or the two cytokines alone (p < 0.01). There were no remarkable histologic differences up to 7 days. At 2 weeks, more progressive vascularity and perivascular tissue deposits were seen in the experimental group. Basophilic mineral structure surrounded the fibroblast-like mesenchymal stem cells at 4 weeks, presumably osteoblastic or osteoclastic cell lining. Bone marker immunohistochemistry against alkaline phosphatase and osteocalcin revealed diffuse and distinct immunoreactivity in osteoblastic cells in the experimental group at 4 weeks. Further transcriptional expression of polyomavirus enhancer binding protein 2alphaA suggested that the human transplanted cells proceeded to osteogenic lineage in 4 weeks. These results may be useful as a new approach for bone regeneration.

Cytokine therapy for craniosynostosis

The birth prevalence of craniosynostosis (premature suture fusion) is 300-500 per 1,000,000 live births. Surgical management involves the release of the synostosed suture. In many cases, however, the suturectomy site rapidly reossifies, further restricts the growing brain and alters craniofacial growth. This resynostosis requires additional surgery, which increases patient morbidity and mortality. New findings in bone biology and molecular pathways involved with suture fusion, combined with novel tissue engineering techniques, may allow the design of targeted and complementary therapies to decrease complications inherent in high-risk surgical procedures. This paper selectively reviews recent advances in i) identifying genetic mutations and the aetiopathogenesis of a number of craniosynostotic conditions; ii) cranial suture biology and molecular biochemical pathways involved in suture fusion; and iii) the design, development and application of various vehicles and tissue engineered constructs to deliver cytokines and genes to cranial sutures. Such biologically based therapies may be used as surgical adjuncts to rescue fusing sutures or help manage postoperative resynostosis.

Cranial bone defect healing is accelerated by mesenchymal stem cells induced by coadministration of bone morphogenetic protein-2 and basic fibroblast growth factor

Hypoalbuminemia is often claimed to impair wound healing, and therefore albumin has traditionally been administered to derive beneficial effects on general physiologic conditions including the nutritional state. However, the influence of albumin administration on systemic protein metabolism and wound healing is still unclear. Therefore, the objective of this study was to investigate the influence of albumin administration on protein metabolism and wound healing in burned rats. After receiving basic total parenteral nutrition (TPN) for 4 days, Sprague-Dawley rats underwent a 6-cm skin incision in the back and a burn involving 20 percent of the whole body surface. The rats were divided into three groups. Group I continued to receive basic TPN. Group II was given basic TPN, but 20 percent of the total nitrogen was replaced by albumin. Group III was administered basic TPN plus albumin equivalent to 20 percent of the total nitrogen of basic TPN. Group IV had the skin incision but no burn, receiving only basic TPN. All the groups were euthanized 4 days after the burn or skin incision. The wound healing potential in terms of tensile strength was enhanced by replacement and addition of albumin (groups II and III, respectively) after a 20 percent burn. Hydroxyproline levels in the wound tended to increase in group II, and significantly increased in group III. Whereas albumin replacement (group II) did not remarkably change the protein metabolism, albumin addition (group III) significantly increased both protein synthesis (S) and breakdown (B) with the S/B ratio and nitrogen balance remaining the same as with albumin-free nutrition (group I). The urinary 3-methyl-histidine/creatinine ratio significantly increased after burn in group III. We conclude that intravenous albumin administration enhanced incisional wound healing in burned rats. Increased protein synthesis with concurrent myolysis and protein breakdown by albumin addition (group III) was observed during wound healing.

The role of transforming growth factor-beta in the modulation of mouse cranial suture fusion

Transforming growth factor-beta (TGF-beta) is actively expressed during mouse calvarial suture fusion. However, the role TGF-beta plays in this process remains unclear. The present study was performed to investigate whether modulation of suture fusion can be achieved by blocking the bioavailability of TGF-beta. Both in vitro and in vivo models were studied. For the in vitro model, the posterior frontal sutures from 24-day-old mice were harvested and cultured for 2, 3, or 4 weeks in the presence of 20 microg/ml of pan-specific TGF-beta polyclonal antibody or rabbit IgG as a control. Culture media were changed every 48 hours and fresh antibody or rabbit IgG was added during each media change. Suture fusion was evaluated by histometric analysis. For the in vivo model, TGF-beta antisense plasmid DNA complexed with lipofectamine was injected into the subgaleal layer along the frontal suture of 22-day-old mice under anesthesia. For control groups, empty vector plasmid DNA+ complexed with lipofectamine was used. The posterior frontal sutures were harvested at various time points and examined by histometric analysis and reverse transcription and polymerase chain reaction for the detection of messenger RNA. The in vitro studies demonstrated that the presence of TGF-beta antibody in culture media delayed posterior frontal suture fusion. By 3 weeks in culture, new bone area was only 20 percent of that in control groups as determined by histometric analysis. By the end of the fourth week, suture fusion was only 25 percent completed compared with controls. The in vivo studies demonstrated that, compared with the control, posterior frontal suture fusion was significantly delayed in the animals injected with antisense plasmid DNA. Suture fusion was complete in control animals by postnatal day 45. There was a 70 percent inhibition of suture fusion (new bone area) in antisense groups measured with histometric analysis. Four days after antisense plasmid DNA injection (age of 26 days), messenger RNA expression for TGF-beta 1 was 77 percent lower than in the control group. We have demonstrated that modulation of TGF-beta affects calvarial suture formation in vitro and in vivo with TGF-beta antibody and TGF-beta1 antisense plasmid DNA treatment, respectively. These data indicate that TGF-beta may play a causative role in mouse posterior frontal suture fusion. Modulation TGF-beta 1 gene expression in vivo can alter the natural history of cranial suture fusion.

The faith of a coronal suture grafted onto midline synostosis inducing dura and deprived from tensile stress

OBJECTIVE

To discuss possible reasons for the synostosis of a coronal suture that was transplanted onto synostosis inducing dura in a scaphocephalic human cranium.

DESIGN

Case report.

SETTING

Supraregional teaching hospital, center for craniofacial anomalies.

PATIENT

A bathmocephalic boy, followed from age 7(1/2) to 26 months.

INTERVENTION

Radical synostosectomy, radial osteotomies in the parietal bone with outward fracturing of the barrel staves, and left-sided coronal suture transplantation onto the midline was undertaken at the age of 11 months.

METHODS

Computer tomography and clinical follow-up.

RESULTS

The sutural graft, initially deprived from tensile stress and quickly exposed to the anomalous dura, turned synostotic in one year.

CONCLUSIONS

Both cell signaling and biomechanical theories on calvarial morphogenesis, sutural development, and synostosis can apply. An animal experiment is recommended to test which hypothesis prevails.

Osteogenesis in cranial defects: reassessment of the concept of critical size and the expression of TGF-beta isoforms

Transforming growth factor-betas (TGF-beta) have been demontstrated to be upregulated during osteoblast function in vitro and during cranial suture fusion in vivo. The authors hypothesized that spontaneous reossification of calvarial defects was also associated with upregulation of TGF-beta. The present study was designed to (1) evaluate the concept of a critical-size defect within the calvaria in an adult guinea pig model and (2) investigate the association between the ossification of calvarial defects and TGF-beta upregulation. Paired circular parietal defects with diameters of 3 and 5 mm and single parietal defects with diameters of 8 or 12 mm were made in 45 six-month-old skeletally mature guinea pigs. Three animals per defect size were killed after survival periods of 3 days, 1 week, 4 weeks, 8 weeks, or 12 weeks. New bone ingrowth was evaluated by assessing for linear closure by a traditional linear method and by a modified cross-sectional area method using an image analysis system in which the thickness of new bone was taken into account. Immunohistochemistry was performed using rabbit polyclonal antibodies to localize TGF-beta1, -beta2, and -beta3. All specimens were photographed, and the intensity of immunostaining was graded based on subjective photographic assessment by three independent reviewers. No defect demonstrated any measurable bone replacement after a survival period of 3 days. All 3- and 5-mm defects were completely reossified after 12 weeks based on the linear analysis of new bone, indicating these defects to be less than critical size. However, new bone formation in the 5-mm defects never exceeded a mean of 40 percent by cross-sectional area of new bone. Percent of new bone formation by cross-sectional area was significantly higher within 3-mm defects than in all larger defects 4 weeks after the craniotomy, reaching a mean of 89 percent new bone by 12 weeks. Persistent gaps were noted on linear analysis of the 8- and 12-mm wounds by 12 weeks, and mean percent new bone by cross-sectional area remained below 30 percent. Immunolocalization demonstrated osteogenic fronts at the advancing bone edge and the endocranial side, in which the osteoblasts stained strongly for all isoforms of TGF-beta. The intensity of osteoblast expression waned considerably after the majority of the defect had reossified. These data indicate that histometric analysis based on cross-sectional area more accurately reflects the osteogenic potential of a cranial defect than does linear inspection of defect closure. Although the interpretation of immunolocalization studies is highly subjective, independent assessment by three reviewers indicates that isoforms of TGF-beta were upregulated during a limited "window" of time corresponding to the period of active calvarial reossification, and expression of TGF-beta corresponded to osteoblast activity within osteogenic fronts.

Pregnant rat uterus expresses high levels of the type 3 iodothyronine deiodinase

Although thyroid hormones are critically important for the coordination of morphogenic processes in the fetus and neonate, premature exposure of the embryo to levels of the hormones present in the adult is detrimental and can result in growth retardation, malformations, and even death. We report here that the pregnant rat uterus expresses extremely high levels of the type 3 iodothyronine deiodinase (D3), which inactivates thyroxine and 3,3', 5-triiodothyronine by 5-deiodination. Both D3 mRNA and activity were present at the implantation site as early as gestational day 9 (E9), when expression was localized using in situ hybridization to uterine mesometrial and antimesometrial decidual tissue. At later stages of gestation, uterine D3 activity remained very high, and the levels exceeded those observed in the placenta and in fetal tissues. After days E12 and E13, as decidual tissues regressed, D3 expression became localized to the epithelial cells lining the recanalized uterine lumen that surrounds the fetal cavity. These findings strongly suggest that the pregnant uterus, in addition to the placenta, plays a critical role in determining the level of exposure of the fetus to maternal thyroid hormones.