Analysis of Mouse Growth Plate Development

Osteoblastic erythropoietin is not required for bone mass accrual

Erythropoietin (EPO), primarily produced by interstitial fibroblasts in the kidney during adulthood, and its receptor are well-known for their crucial role in regulating erythropoiesis. Recent research has unveiled an additional function of circulating EPO in the control of bone mass accrual and homeostasis through its receptor, which is expressed in both osteoblasts and osteoclasts. Notably, cells of the osteoblast lineage can produce and secrete functional EPO upon activation of the hypoxia signaling pathway. However, the physiological relevance of osteoblastic EPO remains to be fully elucidated. This study aimed to investigate the potential role of osteoblastic EPO in regulating bone mass accrual and erythropoiesis in young adult mice. To accomplish this, we employed a mutant mouse model lacking EPO specifically in mesenchymal progenitors and their descendants. Our findings indicate that in vivo loss of EPO in the osteoblast lineage does not significantly affect either bone mass accrual or erythropoiesis in young adult mice. Further investigations are necessary to comprehensively understand the potential contribution of EPO produced and secreted by osteoblast cells during aging, repair, and under pathological conditions.

The NAD salvage pathway in mesenchymal cells is indispensable for skeletal development in mice

NAD is an essential co-factor for cellular energy metabolism and multiple other processes. Systemic NAD+ deficiency has been implicated in skeletal deformities during development in both humans and mice. NAD levels are maintained by multiple synthetic pathways but which ones are important in bone forming cells is unknown. Here, we generate mice with deletion of Nicotinamide Phosphoribosyltransferase (Nampt), a critical enzyme in the NAD salvage pathway, in all mesenchymal lineage cells of the limbs. At birth, NamptΔPrx1 exhibit dramatic limb shortening due to death of growth plate chondrocytes. Administration of the NAD precursor nicotinamide riboside during pregnancy prevents the majority of in utero defects. Depletion of NAD post-birth also promotes chondrocyte death, preventing further endochondral ossification and joint development. In contrast, osteoblast formation still occurs in knockout mice, in line with distinctly different microenvironments and reliance on redox reactions between chondrocytes and osteoblasts. These findings define a critical role for cell-autonomous NAD homeostasis during endochondral bone formation.

Matrix in Medicine: Health Consequences of Mutant Cartilage Oligomeric Matrix Protein and its relationship to abnormal growth and to joint degeneration

Cartilage oligomeric matrix protein (COMP), an extracellular matrix protein, has been shown to enhance proliferation and mechanical integrity in the matrix, supporting functions of the growth plate and articular cartilage. Mutations in COMP cause pseudoachondroplasia (PSACH), a severe dwarfing condition associated with premature joint degeneration and significant lifelong joint pain. The MT (mutant)-COMP mouse mimics PSACH with decreased limb growth, early joint degeneration and pain. Ablation of endoplasmic reticulum stress CHOP signaling eliminated pain and prevented joint degeneration. The health effects of mutant COMP are discussed in relation to cellular/chondrocyte stress in the growth plate, articular cartilage and nearby tissues, and the implications for therapeutic approaches. There are many similarities between osteoarthritis and mutant-COMP protein-induced joint degeneration, suggesting that the relevance of findings in the joints may extend beyond PSACH to idiopathic primary OA.

Joint Degeneration in a Mouse Model of Pseudoachondroplasia: ER Stress, Inflammation, and Block of Autophagy

Pseudoachondroplasia (PSACH), a short limb skeletal dysplasia associated with premature joint degeneration, is caused by misfolding mutations in cartilage oligomeric matrix protein (COMP). Here, we define mutant-COMP-induced stress mechanisms that occur in articular chondrocytes of MT-COMP mice, a murine model of PSACH. The accumulation of mutant-COMP in the ER occurred early in MT-COMP articular chondrocytes and stimulated inflammation (TNFα) at 4 weeks, and articular chondrocyte death increased at 8 weeks while ER stress through CHOP was elevated by 12 weeks. Importantly, blockage of autophagy (pS6), the major mechanism that clears the ER, sustained cellular stress in MT-COMP articular chondrocytes. Degeneration of MT-COMP articular cartilage was similar to that observed in PSACH and was associated with increased MMPs, a family of degradative enzymes. Moreover, chronic cellular stresses stimulated senescence. Senescence-associated secretory phenotype (SASP) may play a role in generating and propagating a pro-degradative environment in the MT-COMP murine joint. The loss of CHOP or resveratrol treatment from birth preserved joint health in MT-COMP mice. Taken together, these results indicate that ER stress/CHOP signaling and autophagy blockage are central to mutant-COMP joint degeneration, and MT-COMP mice joint health can be preserved by decreasing articular chondrocyte stress. Future joint sparing therapeutics for PSACH may include resveratrol.

ERα Signaling in GHRH/Kiss1 Dual-Phenotype Neurons Plays Sex-Specific Roles in Growth and Puberty

Gonadal steroids modulate growth hormone (GH) secretion and the pubertal growth spurt via undefined central pathways. GH-releasing hormone (GHRH) neurons express estrogen receptor α (ERα) and androgen receptor (AR), suggesting changing levels of gonadal steroids during puberty directly modulate the somatotropic axis. We generated mice with deletion of ERα in GHRH cells (GHRHΔERα), which displayed reduced body length in both sexes. Timing of puberty onset was similar in both groups, but puberty completion was delayed in GHRHΔERα females. Lack of AR in GHRH cells (GHRHΔAR mice) induced no changes in body length, but puberty completion was also delayed in females. Using a mouse model with two reporter genes, we observed that, while GHRHtdTom neurons minimally colocalize with Kiss1hrGFP in prepubertal mice, ∼30% of GHRH neurons coexpressed both reporter genes in adult females, but not in males. Developmental analysis of Ghrh and Kiss1 expression suggested that a subpopulation of ERα neurons in the arcuate nucleus of female mice undergoes a shift in phenotype, from GHRH to Kiss1, during pubertal transition. Our findings demonstrate that direct actions of gonadal steroids in GHRH neurons modulate growth and puberty and indicate that GHRH/Kiss1 dual-phenotype neurons play a sex-specific role in the crosstalk between the somatotropic and gonadotropic axes during pubertal transition.SIGNIFICANCE STATEMENT Late maturing adolescents usually show delayed growth and bone age. At puberty, gonadal steroids have stimulatory effects on the activation of growth and reproductive axes, but the existence of gonadal steroid-sensitive neuronal crosstalk remains undefined. Moreover, the neural basis for the sex differences observed in the clinical arena is unknown. Lack of ERα in GHRH neurons disrupts growth in both sexes and causes pubertal delay in females. Deletion of androgen receptor in GHRH neurons only delayed female puberty. In adult females, not males, a subset of GHRH neurons shift phenotype to start producing Kiss1. Thus, direct estrogen action in GHRH/Kiss1 dual-phenotype neurons modulates growth and puberty and may orchestrate the sex differences in endocrine function observed during pubertal transition.

Amplified morphogenetic and bone forming activity of amorphous versus crystalline calcium phosphate/polyphosphate

Amorphous Ca-phosphate (ACP) particles stabilized by inorganic polyphosphate (polyP) were prepared by co-precipitation of calcium and phosphate in the presence of polyP (15% [w/w]). These hybrid nanoparticles showed no signs of crystallinity according to X-ray diffraction analysis, in contrast to the particles obtained at a lower (5% [w/w]) polyP concentration or to hydroxyapatite. The ACP/15% polyP particles proved to be a suitable matrix for cell growth and attachment and showed pronounced osteoblastic and vasculogenic activity in vitro. They strongly stimulated mineralization of the human osteosarcoma cell line SaOS-2, as well as cell migration/microvascularization, as demonstrated in the scratch assay and the in vitro angiogenesis tube forming assay. The possible involvement of an ATP gradient, generated by polyP during tube formation of human umbilical vein endothelial cells, was confirmed by ATP-depletion experiments. In order to assess the morphogenetic activity of the hybrid particles in vivo, experiments in rabbits using the calvarial bone defect model were performed. The particles were encapsulated in poly(d,l-lactide-co-glycolide) microspheres. In contrast, to crystalline Ca-phosphate (containing only 5% [w/w] polyP) or to crystalline β-tricalcium phosphate, amorphous ACP/15% polyP particles caused pronounced osteoinductive activity already after a six-week healing period. The synthesis of new bone tissue was accompanied by an intense vascularization and an increased expression of mineralization/vascularization marker genes. The data show that amorphous polyP-stabilized ACP, which combines osteoinductive activity with the ability to act as a precursor of hydroxyapatite formation both in vitro and in vivo, is a promising material for bone regeneration.

Hox genes maintain critical roles in the adult skeleton

Hox genes are indispensable for the proper patterning of the skeletal morphology of the axial and appendicular skeleton during embryonic development. Recently, it has been demonstrated that Hox expression continues from embryonic stages through postnatal and adult stages exclusively in a skeletal stem cell population. However, whether Hox genes continue to function after development has not been rigorously investigated. We generated a Hoxd11 conditional allele and induced genetic deletion at adult stages to show that Hox11 genes play critical roles in skeletal homeostasis of the forelimb zeugopod (radius and ulna). Conditional loss of Hox11 function at adult stages leads to replacement of normal lamellar bone with an abnormal woven bone-like matrix of highly disorganized collagen fibers. Examining the lineage from the Hox-expressing mutant cells demonstrates no loss of stem cell population. Differentiation in the osteoblast lineage initiates with Runx2 expression, which is observed similarly in mutants and controls. With loss of Hox11 function, however, osteoblasts fail to mature, with no progression to osteopontin or osteocalcin expression. Osteocyte-like cells become embedded within the abnormal bony matrix, but they completely lack dendrites, as well as the characteristic lacuno-canalicular network, and do not express SOST. Together, our studies show that Hox11 genes continuously function in the adult skeleton in a region-specific manner by regulating differentiation of Hox-expressing skeletal stem cells into the osteolineage.

The use of commercially available adhesive tapes to preserve cartilage and bone tissue integrity during cryosectioning

The use of fluorescent tags to monitor protein expression and to lineage-trace cells has become a standard complement to standard histological techniques in the fields of embryology, pathology and regenerative medicine. Unfortunately, traditional paraffin embedding protocols can substantially diminish or abolish the native emission signal of the fluorophore of interest. To preserve the fluorescent signal, an alternative is to use cryosectioning; however, this can often result in undesirable artefacts such as tearing or shattering - particularly for mineralized tissues such as bone and cartilage. Here we present a method of using a commercially available tape to stabilize murine femur tissue, thus allowing for cryosectioning of cartilage and bone tissues carrying fluorescent tags without the need for demineralization.

Suppressing Mitochondrial Respiration Is Critical for Hypoxia Tolerance in the Fetal Growth Plate

Oxygen (O₂) is both an indispensable metabolic substrate and a regulatory signal that controls the activity of Hypoxia-Inducible Factor 1α (Hif1a), a mediator of the cellular adaptation to low O₂ tension (hypoxia). Hypoxic cells require Hif1a to survive. Additionally, Hif1a is an inhibitor of mitochondrial respiration. Hence, we hypothesized that enhancing mitochondrial respiration is detrimental to the survival of hypoxic cells in vivo. We tested this hypothesis in the fetal growth plate, which is hypoxic. Our findings show that mitochondrial respiration is dispensable for survival of growth plate chondrocytes. Furthermore, its impairment prevents the extreme hypoxia and the massive chondrocyte death observed in growth plates lacking Hif1a. Consequently, augmenting mitochondrial respiration affects the survival of hypoxic chondrocytes by, at least in part, increasing intracellular hypoxia. We thus propose that partial suppression of mitochondrial respiration is crucial during development to protect the tissues that are physiologically hypoxic from lethal intracellular anoxia.

Postnatal ex vivo rat model for longitudinal bone growth investigations

BACKGROUND

Chondrocytes in the growth plate (GP) undergo increases in volume during different cascades of cell differentiation during longitudinal bone growth. The volume increase is reported to be the most significant variable in understanding the mechanism of long bone growth.

METHODS

Forty-five postnatal Sprague-Dawley rat pups, 7-15 days old were divided into nine age groups (P7-P15). Five pups were allocated to each group. The rats were sacrificed and tibia and metatarsal bones were harvested. Bone lengths were measured after 0, 24, 48, and 72 hours of ex vivo incubation. Histology of bones was carried out, and GP lengths and chondrocyte densities were determined.

RESULTS

There were significant differences in bone length among the age groups after 0 and 72 hours of incubation. Histological sectioning was possible in metatarsal bone from all age groups, and in tibia from 7- to 13-day-old rats. No significant differences in tibia and metatarsal GP lengths were seen among different age groups at 0 and 72 hours of incubation. Significant differences in chondrocyte densities along the epiphyseal GP of the bones between 0 and 72 hours of incubation were observed in most of the age groups.

CONCLUSION

Ex vivo growth of tibia and metatarsal bones of rats aged 7-15 days old is possible, with percentage growth rates of 23.87 ± 0.80% and 40.38 ± 0.95% measured in tibia and metatarsal bone, respectively. Histological sectioning of bones was carried out without the need for decalcification in P7-P13 tibia and P7-P15 metatarsal bone. Increases in chondrocyte density along the GP influence overall bone elongation.

Hypoxia-inducible factor 2α is a negative regulator of osteoblastogenesis and bone mass accrual

Osteoblasts, which are the bone-forming cells, operate in a hypoxic environment. The transcription factors hypoxia-inducible factor-1α (HIF1) and HIF2 are key mediators of the cellular response to hypoxia. Both are expressed in osteoblasts. HIF1 is known to be a positive regulator of bone formation. Conversely, the role of HIF2 in the control osteoblast biology is still poorly understood. In this study, we used mouse genetics to demonstrate that HIF2 is an inhibitor of osteoblastogenesis and bone mass accrual. Moreover, we provided evidence that HIF2 impairs osteoblast differentiation at least in part, by upregulating the transcription factor Sox9. Our findings constitute a paradigm shift, as activation of the hypoxia-signaling pathway has traditionally been associated with increased bone formation through HIF1. Inhibiting HIF2 could thus represent a therapeutic approach for the treatment of the low bone mass observed in chronic diseases, osteoporosis, or aging.

The Ex Vivo Time of Fresh Autologous Cartilage Before Transplantation and Cartilage Absorption Degree

OBJECTIVE

This study aims to determine the relationship between the time of autogenous cartilage in vitro and the degree of absorption in animal experiments.

METHODS

New Zealand white rabbits were randomly divided into 3 groups according to the time of cartilage in vitro: 1-hour group, 2-hour group, and 3-hour group. A volume of ear cartilage was taken and transplanted into the back, according to the group. After 1 month, the volume was taken out and remeasured. Then, these were compared by scanning electron microscopy and hematoxylin and eosin staining.

RESULTS

The cartilage bulk absorption level of different groups is different (P < 0.05). There was statistical significance when the 3-hour group was compared with the other 2 groups (P < 0.05). This shows that cartilage volume absorption level becomes higher after 3 hours. Scanning electron microscopy revealed that before and after transplantation, the arrangement of collagen fibers and the gap between these fibers changed. Hematoxylin and eosin staining revealed that there were some morphological changes in chondrocytes, and the degree of chondrocyte apoptosis increased with time, which was accompanied by granulation tissue formation. In addition, the cartilage tissue survived after transplantation.

CONCLUSION

The change in cartilage volume was more obvious after 3 hours of autogenous fresh cartilage transplantation, when compared with that of the first 2 hours. The longer the time of light microscopy was, the longer the apoptosis of cartilage cells, the more serious the destruction of collagen fibers and the cartilage matrix, and the greater the absorption of cartilage and the new chondrocytes.

Knock-In of the Recurrent R368X Mutation of PRKAR1A that Represses cAMP-Dependent Protein Kinase A Activation: A Model of Type 1 Acrodysostosis

In humans, activating mutations in the PRKAR1A gene cause acrodysostosis 1 (ACRDYS1). These mutations result in a reduction in PKA activation caused by an impaired ability of cAMP to dissociate mutant PRKAR1A from catalytic PKA subunits. Two striking features of this rare developmental disease are renal resistance to PTH and chondrodysplasia resulting from the constitutive inhibition of PTHR1/Gsa/AC/cAMP/PKA signaling. We developed a knock-in of the recurrent ACRDYS1 R368X PRKAR1A mutation in the mouse. No litters were obtained from [R368X]/[+] females (thus no homozygous [R368X]/[R368X] mice). In [R368X]/[+] mice, Western blot analysis confirmed mutant allele heterozygous expression. Growth retardation, peripheral acrodysostosis (including brachydactyly affecting all digits), and facial dysostosis were shown in [R368X]/[+] mice by weight curves and skeletal measurements (μCT scan) as a function of time. [R368X]/[+] male and female mice were similarly affected. Unexpected, however, whole-mount skeletal preparations revealed a striking delay in mineralization in newborn mutant mice, accompanied by a decrease in the height of terminal hypertrophic chondrocyte layer, an increase in the height of columnar proliferative prehypertrophic chondrocyte layer, and changes in the number and spatial arrangement of proliferating cell nuclear antigen (PCNA)-positive chondrocytes. Plasma PTH and basal urinary cAMP were significantly higher in [R368X]/[+] compared to WT mice. PTH injection increased urinary cAMP similarly in [R368X]/[+] and WT mice. PRKACA expression was regulated in a tissue (kidney not bone and liver) manner. This model, the first describing the germline expression of a PRKAR1A mutation causing dominant repression of cAMP-dependent PKA, reproduced the main features of ACRDYS1 in humans. It should help decipher the specificity of the cAMP/PKA signaling pathway, crucial for numerous stimuli. In addition, our results indicate that PRKAR1A, by tempering intracellular cAMP levels, is a molecular switch at the crossroads of signaling pathways regulating chondrocyte proliferation and differentiation. © 2016 American Society for Bone and Mineral Research.

Development of a Representative Mouse Model with Nonalcoholic Steatohepatitis

Non-alcoholic fatty liver disease (NAFLD) is the most prevalent liver disease in the Western world. It represents a disease spectrum ranging from isolated steatosis to non-alcoholic steatohepatitis (NASH). In particular, NASH can evolve to fibrosis, cirrhosis, hepatocellular carcinoma, and liver failure. The development of novel treatment strategies is hampered by the lack of representative NASH mouse models. Here, we describe a NASH mouse model, which is based on feeding non-genetically manipulated C57BL6/J mice a 'Western style' high-fat/high-sucrose diet (HF-HSD). HF-HSD leads to early obesity, insulin resistance, and hypercholesterolemia. After 12 weeks of HF-HSD, all mice exhibit the complete spectrum of features of NASH, including steatosis, hepatocyte ballooning, and lobular inflammation, together with fibrosis in the majority of mice. Hence, this model closely mimics the human disease. Implementation of this mouse model will lead to a standardized setup for the evaluation of (i) underlying mechanisms that contribute to the progression of NAFLD to NASH, and (ii) therapeutic interventions for NASH. © 2016 by John Wiley & Sons, Inc.