Interplay between CaSR and PTH1R signaling in skeletal development and osteoanabolism

Single-cell transcriptomic analysis of glioblastoma reveals pericytes contributing to the blood-brain-tumor barrier and tumor progression

The blood-brain barrier is often altered in glioblastoma (GBM) creating a blood-brain-tumor barrier (BBTB) composed of pericytes. The BBTB affects chemotherapy efficacy. However, the expression signatures of BBTB-associated pericytes remain unclear. We aimed to identify BBTB-associated pericytes in single-cell RNA sequencing data of GBM using pericyte markers, a normal brain pericyte expression signature, and functional enrichment. We identified parathyroid hormone receptor-1 (PTH1R) as a potential marker of pericytes associated with BBTB function. These pericytes interact with other cells in GBM mainly through extracellular matrix-integrin signaling pathways. Compared with normal pericytes, pericytes in GBM exhibited upregulation of several ECM genes (including collagen IV and FN1), and high expression levels of these genes were associated with a poor prognosis. Cell line experiments showed that PTH1R knockdown in pericytes increased collagen IV and FN1 expression levels. In mice models, the expression levels of PTH1R, collagen IV, and FN1 were consistent with these trends. Evans Blue leakage and IgG detection in the brain tissue suggested a negative correlation between PTH1R expression levels and blood-brain barrier function. Further, a risk model based on differentially expressed genes in PTH1R+ pericytes had predictive value for GBM, as validated using independent and in-house cohorts.

3,3'-Diindolylmethane promotes bone formation - A assessment in MC3T3-E1 cells and zebrafish

Osteoporosis is a common degenerative bone disease in middle-aged and elderly people. The current drugs used to treat osteoporosis have many side effects and low patient compliance. Phytochemotherapy may be safer and more effective. 3,3'-diindolemethane (DIM) is the digestive product of indole-3-methanol in cruciferous vegetables in the stomach, which is a kind of anti-tumor and anti-oxidation phytochemical. However, the effects of DIM on osteoblasts and the mechanism by which DIM regulates bone formation are not fully understood. The aim of this study was to investigate the effects of DIM on the bone formation of mouse preosteoblasts MC3T3-E1 and zebrafish. DIM promotes proliferation and osteogenic differentiation of MC3T3-E1 cells in vitro, and also plays a bone promoting role by increasing the interaction between BRCA1-Associated Protein 1(BAP1) and Inositol 1,4,5-Trisphosphate Receptor(IP3R), up-regulating the expression of BAP1 and IP3R and downstream storage operation calcium entry (SOCE) related protein Recombinant Stromal Interaction Molecule 1(STIM1). The effect of DIM on osteoporosis was confirmed in zebrafish osteoporosis model, and its molecular mechanism may be related to BAP1/IP3R/SOCE signaling pathway. These findings highlight the potential therapeutic value of DIM in the prevention and treatment of osteoporosis.

Prg4-Expressing Chondroprogenitor Cells in the Superficial Zone of Articular Cartilage

Joint-resident chondrogenic precursor cells have become a significant therapeutic option due to the lack of regenerative capacity in articular cartilage. Progenitor cells are located in the superficial zone of the articular cartilage, producing lubricin/Prg4 to decrease friction of cartilage surfaces during joint movement. Prg4-positive progenitors are crucial in maintaining the joint's structure and functionality. The disappearance of progenitor cells leads to changes in articular hyaline cartilage over time, subchondral bone abnormalities, and the formation of ectopic ossification. Genetic labeling cell technology has been the main tool used to characterize Prg4-expressing progenitor cells of articular cartilage in vivo through drug injection at different time points. This technology allows for the determination of the origin of progenitor cells and the tracking of their progeny during joint development and cartilage damage. We endeavored to highlight the currently known information about the Prg4-producing cell population in the joint to underline the significance of the role of these cells in the development of articular cartilage and its homeostasis. This review focuses on superficial progenitors in the joint, how they contribute to postnatal articular cartilage formation, their capacity for regeneration, and the consequences of Prg4 deficiency in these cells. We have accumulated information about the Prg4+ cell population of articular cartilage obtained through various elegantly designed experiments using transgenic technologies to identify potential opportunities for further research.

Exploring Adaptive Phenotypes for the Human Calcium-Sensing Receptor Polymorphism R990G

Rainforest hunter-gatherers from Southeast Asia are characterized by specific morphological features including a particularly dark skin color (D), short stature (S), woolly hair (W), and the presence of steatopygia (S)-fat accumulation localized in the hips (DSWS phenotype). Based on previous evidence in the Andamanese population, we first characterized signatures of adaptive natural selection around the calcium-sensing receptor gene in Southeast Asian rainforest groups presenting the DSWS phenotype and identified the R990G substitution (rs1042636) as a putative adaptive variant for experimental follow-up. Although the calcium-sensing receptor has a critical role in calcium homeostasis by directly regulating the parathyroid hormone secretion, it is expressed in different tissues and has been described to be involved in many biological functions. Previous works have also characterized the R990G substitution as an activating polymorphism of the calcium-sensing receptor associated with hypocalcemia. Therefore, we generated a knock-in mouse for this substitution and investigated organismal phenotypes that could have become adaptive in rainforest hunter-gatherers from Southeast Asia. Interestingly, we found that mouse homozygous for the derived allele show not only lower serum calcium concentration but also greater body weight and fat accumulation, probably because of enhanced preadipocyte differentiation and lipolysis impairment resulting from the calcium-sensing receptor activation mediated by R990G. We speculate that such differential features in humans could have facilitated the survival of hunter-gatherer groups during periods of nutritional stress in the challenging conditions of the Southeast Asian tropical rainforests.

Identification of a novel heterozygous PTH1R variant in a Chinese family with incomplete penetrance

BACKGROUND

Mutations in PTH1R are associated with Jansen-type metaphyseal chondrodysplasia (JMC), Blomstrand osteochondrodysplasia (BOCD), Eiken syndrome, enchondroma, and primary failure of tooth eruption (PFE). Inheritance of the PTH1R gene can be either autosomal dominant or autosomal recessive, indicating the complexity of the gene. Our objective was to identify the phenotypic differences in members of a family with a novel PTH1R mutation.

METHODS

The proband was a 13-year, 6-month-old girl presenting with short stature, abnormal tooth eruption, skeletal dysplasia, and midface hypoplasia. The brother and father of the proband presented with short stature and abnormal tooth eruption. High-throughput sequencing was performed on the proband, and the variant was confirmed in the proband and other family members by Sanger sequencing. Amino acid sequence alignment was performed using ClustalX software. Three-dimensional structures were analyzed and displayed using the I-TASSER website and PyMOL software.

RESULTS

High-throughput genome sequencing and Sanger sequencing validation showed that the proband, her father, and her brother all carried the PTH1R (NM_000316) c.1393G>A (p.E465K) mutation. The c.1393G>A (p.E465K) mutation was novel, as it has not been reported in the literature database. According to the American College of Medical Genetics and Genomics (ACMG) guidelines, the p.E465K variant was considered to have uncertain significance. Biological information analysis demonstrated that this identified variant was highly conserved and highly likely pathogenic.

CONCLUSIONS

We identified a novel heterozygous mutation in the PTH1R gene leading to clinical manifestations with incomplete penetrance that expands the spectrum of known PTH1R mutations.

CaSR modulates proliferation of the superficial zone cells in temporomandibular joint cartilage via the PTHrP nuclear localization sequence

The superficial zone cells in mandibular condylar cartilage are proliferative. The present purpose was to delineate the relation of calcium-sensing receptor (CaSR) and parathyroid hormone-related peptide nuclear localization sequence (PTHrP87-139 ), and their role in the proliferation behaviors of the superficial zone cells. A gain- and loss-of-function strategy were used in an in vitro fluid flow shear stress (FFSS) model and an in vivo bilateral elevation bite model which showed mandibular condylar cartilage thickening. CaSR and PTHrP87-139 were modulated through treating the isolated superficial zone cells with activator/SiRNA and via deleting CaSR or parathyroid hormone-related peptide (PTHrP) gene in mice with the promoter gene of proteoglycan 4 (Prg4-CreERT2 ) in the tamoxifen-inducible pattern with or without additional injection of Cinacalcet, the CaSR agonist, or PTHrP87-139 peptide. FFSS stimulated CaSR and PTHrP expression, and accelerated proliferation of the Prg4-expressing superficial zone cells, in which process CaSR acted as an up-streamer of PTHrP. Proteoglycan 4 specific knockout of CaSR or PTHrP reduced the cartilage thickness, suppressed the proliferation and early differentiation of the superficial zone cells, and inhibited cartilage thickening and matrix production promoted by bilateral elevation bite. Injections of CaSR agonist Cinacalcet could not improve the phenotype caused by PTHrP mutation. Injections of PTHrP87-139 peptide rescued the cartilage from knockout of CaSR gene. CaSR modulates proliferation of the superficial zone cells in mandibular condylar cartilage through activation of PTHrP nuclear localization sequence. Our data support the therapeutic target of CaSR in promoting PTHrP production in superficial zone cartilage.

Advanced applications of strontium-containing biomaterials in bone tissue engineering

Strontium (Sr) and strontium ranelate (SR) are commonly used therapeutic drugs for patients suffering from osteoporosis. Researches have showed that Sr can significantly improve the biological activity and physicochemical properties of materials in vitro and in vivo. Therefore, a large number of strontium containing biomaterials have been developed for repairing bone defects and promoting osseointegration. In this review, we provide a comprehensive overview of Sr-containing biomaterials along with the current state of their clinical use. For this purpose, the different types of biomaterials including calcium phosphate, bioactive glass, and polymers are discussed and provided future outlook on the fabrication of the next-generation multifunctional and smart biomaterials.

Alkaline earth metals for osteogenic scaffolds: From mechanisms to applications

Regeneration of bone defects is a significant challenge today. As alternative approaches to the autologous bone, scaffold materials have remarkable features in treating bone defects; however, the various properties of current scaffold materials still fall short of expectations. Due to the osteogenic capability of alkaline earth metals, their application in scaffold materials has become an effective approach to improving their properties. Furthermore, numerous studies have shown that combining alkaline earth metals leads to better osteogenic properties than applying them alone. In this review, the physicochemical and physiological characteristics of alkaline earth metals are introduced, mainly focusing on their mechanisms and applications in osteogenesis, especially magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Furthermore, this review highlights the possible cross-talk between pathways when alkaline earth metals are combined. Finally, some of the current drawbacks of scaffold materials are enumerated, such as the high corrosion rate of Mg scaffolds and defects in the mechanical properties of Ca scaffolds. Moreover, a brief perspective is also provided regarding future directions in this field. It is worth exploring that whether the levels of alkaline earth metals in newly regenerated bone differs from those in normal bone. The ideal ratio of each element in the bone tissue engineering scaffolds or the optimal concentration of each elemental ion in the created osteogenic environment still needs further exploration. The review not only summarizes the research developments in osteogenesis but also offers a direction for developing new scaffold materials.

Etiology and Pathophysiology of Hypoparathyroidism: A Narrative Review

The approach utilized a systematic review of the medical literature executed with specifically designed criteria that focused on the etiologies and pathogenesis of hypoparathyroidism. Enhanced attention by endocrine surgeons to new knowledge about parathyroid gland viability are reviewed along with the role of intraoperative parathyroid hormone (ioPTH) monitoring during and after neck surgery. Nonsurgical etiologies account for a significant proportion of cases of hypoparathyroidism (~25%), and among them, genetic etiologies are key. Given the pervasive nature of PTH deficiency across multiple organ systems, a detailed review of the skeletal, renal, neuromuscular, and ocular complications is provided. The burden of illness on affected patients and their caregivers contributes to reduced quality of life and social costs for this chronic endocrinopathy. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Therapeutic Treatments for Osteoporosis-Which Combination of Pills Is the Best among the Bad?

Osteoporosis is a chronical, systemic skeletal disorder characterized by an increase in bone resorption, which leads to reduced bone density. The reduction in bone mineral density and therefore low bone mass results in an increased risk of fractures. Osteoporosis is caused by an imbalance in the normally strictly regulated bone homeostasis. This imbalance is caused by overactive bone-resorbing osteoclasts, while bone-synthesizing osteoblasts do not compensate for this. In this review, the mechanism is presented, underlined by in vitro and animal models to investigate this imbalance as well as the current status of clinical trials. Furthermore, new therapeutic strategies for osteoporosis are presented, such as anabolic treatments and catabolic treatments and treatments using biomaterials and biomolecules. Another focus is on new combination therapies with multiple drugs which are currently considered more beneficial for the treatment of osteoporosis than monotherapies. Taken together, this review starts with an overview and ends with the newest approaches for osteoporosis therapies and a future perspective not presented so far.

Biased and allosteric modulation of bone cell-expressing G protein-coupled receptors as a novel approach to osteoporosis therapy

On the cellular level, osteoporosis (OP) is a result of imbalanced bone remodeling, in which osteoclastic bone resorption outcompetes osteoblastic bone formation. Currently available OP medications include both antiresorptive and bone-forming drugs. However, their long-term use in OP patients, mainly in postmenopausal women, is accompanied by severe side effects. Notably, the fundamental coupling between bone resorption and formation processes underlies the existence of an undesirable secondary outcome that bone anabolic or anti-resorptive drugs also reduce bone formation. This drawback requires the development of anti-OP drugs capable of selectively stimulating osteoblastogenesis and concomitantly reducing osteoclastogenesis. We propose that the application of small synthetic biased and allosteric modulators of bone cell receptors, which belong to the G-protein coupled receptors (GPCR) family, could be the key to resolving the undesired anti-OP drug selectivity. This approach is based on the capacity of these GPCR modulators, unlike the natural ligands, to trigger signaling pathways that promote beneficial effects on bone remodeling while blocking potentially deleterious effects. Under the settings of OP, an optimal anti-OP drug should provide fine-tuned regulation of downstream effects, for example, intermittent cyclic AMP (cAMP) elevation, preservation of Ca²⁺ balance, stimulation of osteoprotegerin (OPG) and estrogen production, suppression of sclerostin secretion, and/or preserved/enhanced canonical β-catenin/Wnt signaling pathway. As such, selective modulation of GPCRs involved in bone remodeling presents a promising approach in OP treatment. This review focuses on the evidence for the validity of our hypothesis.

Calcium-Sensing Receptors Control CYP27B1-Luciferase Expression: Transcriptional and Posttranscriptional Mechanisms

25-hydroxyvitamin D 1α-hydroxylase (encoded by CYP27B1), which catalyzes the synthesis of 1,25-dihydroxyvitamin D₃, is subject to negative or positive modulation by extracellular Ca²⁺ (Ca²⁺_o) depending on the tissue. However, the Ca²⁺ sensors and underlying mechanisms are unidentified. We tested whether calcium-sensing receptors (CaSRs) mediate Ca²⁺_o-dependent control of 1α-hydroxylase using HEK-293 cells stably expressing the CaSR (HEK-CaSR cells). In HEK-CaSR cells, but not control HEK-293 cells, cotransfected with reporter genes for CYP27B1-Photinus pyralis (firefly) luciferase and control Renilla luciferase, an increase in Ca²⁺_o from 0.5mM to 3.0mM induced a 2- to 3-fold increase in firefly luciferase activity as well as mRNA and protein levels. Surprisingly, firefly luciferase was specifically suppressed at Ca²⁺_o ≥ 5.0mM, demonstrating biphasic Ca²⁺_o control. Both phases were mediated by CaSRs as revealed by positive and negative modulators. However, Ca²⁺_o induced simple monotonic increases in firefly luciferase and endogenous CYP27B1 mRNA levels, indicating that the inhibitory effect of high Ca²⁺_o was posttranscriptional. Studies with inhibitors and the CaSR C-terminal mutant T888A identified roles for protein kinase C (PKC), phosphorylation of T888, and extracellular regulated protein kinase (ERK)_1/2 in high Ca²⁺_o-dependent suppression of firefly luciferase. Blockade of both PKC and ERK_1/2 abolished Ca²⁺_o-stimulated firefly luciferase, demonstrating that either PKC or ERK_1/2 is sufficient to stimulate the CYP27B1 promoter. A key CCAAT box (−74 bp to −68 bp), which is regulated downstream of PKC and ERK_1/2, was required for both basal transcription and Ca²⁺_o-mediated transcriptional upregulation. The CaSR mediates Ca²⁺_o-dependent transcriptional upregulation of 1α-hydroxylase and an additional CaSR-mediated mechanism is identified by which Ca²⁺_o can promote luciferase and possibly 1α-hydroxylase breakdown.

Initiation and progression of dental-stimulated temporomandibular joints osteoarthritis

Temporomandibular joint (TMJ), a site that is often impacted by osteoarthritis (OA), is biomechanically linked with dental occlusion. Tissue responses in TMJ condyle to biomechanical stimulation could be investigated by intervention of the dental occlusion in animals. Unilateral anterior crossbite, an experimental malocclusion, has been demonstrated to induce TMJ-OA lesions, showing primarily as enhanced cartilage calcification and subchondral cortical bone formation at the osteochondral interface, causing the osteochondral interface thickening and stiffening. The changed interface would worsen the local biomechanical environment. At the cartilage side, the matrix degenerates. In the case of insufficient restoration of the matrix, the cells in the deep zone flow into the ones undergoing autophagy, apoptosis, and terminal differentiation while the cells in the superficial zone are promoted to differentiate to supply the loss of the deep zone cells. At the meantime, the bone marrow stromal cells are stimulated to bone formation in the subchondral cortical region which is uncoupled with the sites of the osteoclast-mediated resorption process that is predominantly observed at the subchondral trabecular bone region. Overall, the thickening and stiffening osteochondral interface, due greatly to the enhanced endochondral ossification in deep zone cartilage, should be a central pathological process that links with cartilage decay and subchondral bone remodelling in OA joints. The residual chondrocytes locating in the cartilage superficial zone have the progenitor-like qualities that can proliferate, and also differentiate into the deep zone chondrocytes, thus should be critical in progression and rehabilitation of TMJ-OA.

Effects of Parathyroid Hormone on Osteoporotic Fracture Healing in Mice via Non-Phospholipases C-Dependent Protein Kinase C Signaling Pathway

Objectives: This study was aimed to explore the effects of parathyroid hormone (PTH) on osteoporotic fracture healing in mice and the underlying mechanisms. Methods: Microarray analysis was conducted to analyze the gene expression level in MC3T3-E1 cells. Carboxyfluorescein succinimidyl ester (CFSE) staining and flow cytometry was adopted to analyze the proliferation and apopto-sis of MC3T3-E1 cells. qRT-PCR was used to analyze the mRNA expression level. Fluorescence resonance energy transfer (FRET) assay was conducted to detect PKC activity. The bone mineral density (BMD) and bone volume (BV)/total volume (TV) were determined via enzyme-linked immunosorbent assay (ELISA) and microscopic computed tomography (micro-CT). Results: ERK1/2 was abnormally expressed in MC3T3-E1 cells after GlylArg19hPTH (1-34) + KT5720 treatment. GlylArg19hPTH (1-34)+ KT5720 treatment promoted cell proliferation, inhibited cell apoptosis, and upregulatedthe expression of osteogenesis-related genes (ALP, OPN, Runx2 and OPG) in MC3T3-E1 cells, which were due to the activation of the non-PLC-dependent PKC signaling pathway and can be blocked by PKC inhibitor Go6983 or ERK1/2 inhibitor BVD-523. Moreover, the activity of PKC in MC3T3-E1 cells treated with GlylArg19hPTH (1-34) + KT5720 + Go6983 was alleviated by ERK1/2 inhibitor BVD-523. In vivo, specific activation of the non-PLC-dependent PKC signaling pathway increased the serum levels of APL and OPG in mice with osteoporotic fracture, which were reversed by PKC inhibitor Go6983 and ERK1/2 inhibitor BVD-523. Moreover, PKC inhibitor Go6983 and ERK1/2 inhibitor BVD-523 suppressed the elevation of BV/TV and BMD induced by specific activation of the non-PKC-dependent signaling pathway. Conclusions: Taken together, PTH stimulates osteoporotic fracture healing in mice through the non-PLC-dependent PKC signaling pathway in which ERK1/2 exerts a vital role.

Calcium-Sensing Receptor (CaSR), Its Impact on Inflammation and the Consequences on Cardiovascular Health

The calcium Sensing Receptor (CaSR) is a cell surface receptor belonging to the family of G-protein coupled receptors. CaSR is mainly expressed by parathyroid glands, kidneys, bone, skin, adipose tissue, the gut, the nervous system, and the cardiovascular system. The receptor, as its name implies is involved in sensing calcium fluctuations in the extracellular matrix of cells, thereby having a major impact on the mineral homeostasis in humans. Besides calcium ions, the receptor is also activated by other di- and tri-valent cations, polypeptides, polyamines, antibiotics, calcilytics and calcimimetics, which upon binding induce intracellular signaling pathways. Recent studies have demonstrated that CaSR influences a wide variety of cells and processes that are involved in inflammation, the cardiovascular system, such as vascular calcification, atherosclerosis, myocardial infarction, hypertension, and obesity. Therefore, in this review, the current understanding of the role that CaSR plays in inflammation and its consequences on the cardiovascular system will be highlighted.

Role of the calcium-sensing receptor (CaSR) in cancer metastasis to bone: Identifying a potential therapeutic target

Cancer is a major cause of morbidity and mortality worldwide due to its ability to evade immune surveillance and metastasize from its origin to a secondary point of contact. Though several treatment techniques have been developed to suppress or manage cancer spread, a strategy for total control over the disease continues to evade researchers. In considering ways to control or prevent cancer from metastasizing to the bone, we analyze the impact of the calcium-sensing receptor (CaSR), whose primary role is to maintain calcium (Ca²⁺) homeostasis in cellular and systemic physiological processes. CaSR is a pleiotropic receptor capable of enhancing the proliferation of some cancers such as breast, lung, prostate and kidney cancers at its primary site(s) and stimulating bone metastasis, while exerting a suppressive effect in others such as colon cancer. The activity of CaSR not only increases cancer cell proliferation, migration and suppression of apoptosis in the organs indicated, but also increases the secretion of parathyroid hormone-related protein (PTHrP) and epiregulin, which induce osteolytic activity and osteoblastic suppression. In addition, released cytokines and Ca²⁺ from bone resorption are critical factors that further promote cancer proliferation. In this review, we seek to highlight previous viewpoints on CaSR, discuss its role in a new context, and consider its potential clinical application in cancer treatment.

Gene Expression Signatures of Synovial Fluid Multipotent Stromal Cells in Advanced Knee Osteoarthritis and Following Knee Joint Distraction

Osteoarthritis (OA) is the most common musculoskeletal disorder. Although joint replacement remains the standard of care for knee OA patients, knee joint distraction (KJD), which works by temporarily off-loading the joint for 6–8 weeks, is becoming a novel joint-sparing alternative for younger OA sufferers. The biological mechanisms behind KJD structural improvements remain poorly understood but likely involve joint-resident regenerative cells including multipotent stromal cells (MSCs). In this study, we hypothesized that KJD leads to beneficial cartilage-anabolic and anti-catabolic changes in joint-resident MSCs and investigated gene expression profiles of synovial fluid (SF) MSCs following KJD as compared with baseline. To obtain further insights into the effects of local biomechanics on MSCs present in late OA joints, SF MSC gene expression was studied in a separate OA arthroplasty cohort and compared with subchondral bone (SB) MSCs from medial (more loaded) and lateral (less loaded) femoral condyles from the same joints. In OA arthroplasty cohort (n = 12 patients), SF MSCs expressed lower levels of ossification- and hypotrophy-related genes [bone sialoprotein (IBSP), parathyroid hormone 1 receptor (PTH1R), and runt-related transcription factor 2 (RUNX2)] than did SB MSCs. Interestingly, SF MSCs expressed 5- to 50-fold higher levels of transcripts for classical extracellular matrix turnover molecules matrix metalloproteinase 1 (MMP1), a disintegrin and metalloproteinase with thrombospondin motifs 5 (ADAMTS5), and tissue inhibitor of metalloproteinase-3 (TIMP3), all (p < 0.05) potentially indicating greater cartilage remodeling ability of OA SF MSCs, compared with SB MSCs. In KJD cohort (n = 9 patients), joint off-loading resulted in sustained, significant increase in SF MSC colonies’ sizes and densities and a notable transcript upregulation of key cartilage core protein aggrecan (ACAN) (weeks 3 and 6), as well as reduction in pro-inflammatory C–C motif chemokine ligand 2 (CCL2) expression (weeks 3 and 6). Additionally, early KJD changes (week 3) were marked by significant increases in MSC chondrogenic commitment markers gremlin 1 (GREM1) and growth differentiation factor 5 (GDF5). In combination, our results reveal distinct transcriptomes on joint-resident MSCs from different biomechanical environments and show that 6-week joint off-loading leads to transcriptional changes in SF MSCs that may be beneficial for cartilage regeneration. Biomechanical factors should be certainly considered in the development of novel MSC-based therapies for OA.

International Union of Basic and Clinical Pharmacology. CVIII. Calcium-Sensing Receptor Nomenclature, Pharmacology, and Function

The calcium-sensing receptor (CaSR) is a class C G protein-coupled receptor that responds to multiple endogenous agonists and allosteric modulators, including divalent and trivalent cations, L-amino acids, γ-glutamyl peptides, polyamines, polycationic peptides, and protons. The CaSR plays a critical role in extracellular calcium (Ca2+ o) homeostasis, as demonstrated by the many naturally occurring mutations in the CaSR or its signaling partners that cause Ca2+ o homeostasis disorders. However, CaSR tissue expression in mammals is broad and includes tissues unrelated to Ca2+ o homeostasis, in which it, for example, regulates the secretion of digestive hormones, airway constriction, cardiovascular effects, cellular differentiation, and proliferation. Thus, although the CaSR is targeted clinically by the positive allosteric modulators (PAMs) cinacalcet, evocalcet, and etelcalcetide in hyperparathyroidism, it is also a putative therapeutic target in diabetes, asthma, cardiovascular disease, and cancer. The CaSR is somewhat unique in possessing multiple ligand binding sites, including at least five putative sites for the "orthosteric" agonist Ca2+ o, an allosteric site for endogenous L-amino acids, two further allosteric sites for small molecules and the peptide PAM, etelcalcetide, and additional sites for other cations and anions. The CaSR is promiscuous in its G protein-coupling preferences, and signals via Gq/11, Gi/o, potentially G12/13, and even Gs in some cell types. Not surprisingly, the CaSR is subject to biased agonism, in which distinct ligands preferentially stimulate a subset of the CaSR's possible signaling responses, to the exclusion of others. The CaSR thus serves as a model receptor to study natural bias and allostery. SIGNIFICANCE STATEMENT: The calcium-sensing receptor (CaSR) is a complex G protein-coupled receptor that possesses multiple orthosteric and allosteric binding sites, is subject to biased signaling via several different G proteins, and has numerous (patho)physiological roles. Understanding the complexities of CaSR structure, function, and biology will aid future drug discovery efforts seeking to target this receptor for a diversity of diseases. This review summarizes what is known to date regarding key structural, pharmacological, and physiological features of the CaSR.

A 51 bp indel polymorphism within the PTH1R gene is significantly associated with chicken growth and carcass traits

Parathyroid hormone (PTH) is a crucial regulator of calcium homeostasis and bone remodeling, and the parathyroid hormone 1 receptor (PTH1R) belongs to a class II G-protein-coupled receptor. PTH activates PTH1R, which mediates catabolic and anabolic processes in the skeleton. However, the functional mechanism of PTH1R has not been thoroughly elucidated in organisms. This study identified a 51 bp indel mutation in the first intron of the PTH1R gene and elucidated the effect of this gene mutation on the growth and carcass traits in chickens. The results indicated that the 51 bp indel was significantly associated with subcutaneous fat thickness, abdominal fat weight, body weight and daily gain over 4-8 weeks. Furthermore, we found that PTH1R gene expression was highest in the kidney and liver tissues, and it showed a trend of decreasing in leg and breast muscle tissues at different embryonic stages. In addition, we examined the expression of the three genotypes of the PTH1R gene in the liver, breast muscle and abdominal fat and found that the II genotype was significantly higher than the DD and ID genotypes. In summary, these findings suggest that the PTH1R gene can serve as a potential molecular marker for chicken breeding.

Pathological calcification in osteoarthritis: an outcome or a disease initiator?

In the progression of osteoarthritis, pathological calcification in the affected joint is an important feature. The role of these crystallites in the pathogenesis and progression of osteoarthritis is controversial; it remains unclear whether they act as a disease initiator or are present as a result of joint damage. Recent studies reported that the molecular mechanisms regulating physiological calcification of skeletal tissues are similar to those regulating pathological or ectopic calcification of soft tissues. Pathological calcification takes place when the equilibrium is disrupted. Calcium phosphate crystallites are identified in most affected joints and the presence of these crystallites is closely correlated with the extent of joint destruction. These observations suggest that pathological calcification is most likely to be a disease initiator instead of an outcome of osteoarthritis progression. Inhibiting pathological crystallite deposition within joint tissues therefore represents a potential therapeutic target in the management of osteoarthritis.

The CaSR in Pathogenesis of Breast Cancer: A New Target for Early Stage Bone Metastases

The Ca2+-sensing receptor (CaSR) is a class-C G protein-coupled receptor which plays a pivotal role in calciotropic processes, primarily in regulating parathyroid hormone secretion to maintain systemic calcium homeostasis. Among its non-calciotropic roles, where the CaSR sits at the intersection of myriad processes, it has steadily garnered attention as an oncogene or tumor suppressor in different organs. In maternal breast tissues the CaSR promotes lactation but in breast cancer it acts as an oncoprotein and has been shown to drive the pathogenesis of skeletal metastases from breast cancer. Even though research has made great strides in treating primary breast cancer, there is an unmet need when it comes to treatment of metastatic breast cancer. This review focuses on how the CaSR leads to the pathogenesis of breast cancer by contrasting its role in healthy tissues and tumorigenesis, and by drawing brief parallels with the tissues where it has been implicated as an oncogene. A class of compounds called calcilytics, which are CaSR antagonists, have also been surveyed in the instances where they have been used to target the receptor in cancerous tissues and constitute a proof of principle for repurposing them. Current clinical therapies for treating bone metastases from breast cancer are limited to targeting osteoclasts and a deeper understanding of the CaSR signaling nexus in this context can bolster them or lead to novel therapeutic interventions.

Physiologic Leg Bowing is not a Physiologic Condition but Instead is Associated with Vitamin D Disorders in Toddlers

When children around 2-year-old show leg bowing without lower-limb radiographic abnormalities for rickets, the leg bowing is classified as "physiologic" genu varum without conducting a blood test. However, it has recently been suggested that toddlers who are diagnosed with physiologic genu varum may in fact have some form of bone metabolic disorder. In this 1:2 case-control study, blood samples were obtained from 33 toddlers with genu varum without radiographic abnormalities for rickets and 66 age- and gender-matched healthy children. Serum alkaline phosphatase (sALP), intact parathyroid hormone (siPTH), 25-hydroxy vitamin D [s25(OH)D], calcium (sCa), and inorganic phosphate (sP) were measured. s25(OH)D of the subjects with genu varum (24.8 ng/ml) were significantly lower than those of the control (33.6 ng/ml) (p < 0.001). The frequency of vitamin D insufficiency/deficiency (< 20 ng/ml) of the subjects with genu varum (39%) was significantly higher than that in the control (14%) (p = 0.004) (odds ratio by vitamin D insufficiency/deficiency: 4.1 [1.5-11.1, p = 0.004]). sCa in subjects with genu varum (10.2 ng/ml) were significantly higher than in control (9.8 ng/ml) (p < 0.001), as were sALP (1057 IU/l) and siPTH (28.4 pg/ml) (740 IU/l and 8.8 pg/ml in control, respectively; p < 0.001). siPTH levels were associated with s25(OH)D levels in subjects with genu varum (r = - 0.57, p < 0.001), while no association was observed in the control (r = 0.11, p = 0.36). Genu varum without radiographic abnormalities of rickets was associated with both vitamin D and bone-metabolic disorders in toddlers, indicating that physiologic genu varum is not a physiologic condition in toddlers.

Calcium-Sensing Receptors in Chondrocytes and Osteoblasts Are Required for Callus Maturation and Fracture Healing in Mice

Calcium and its putative receptor (CaSR) control skeletal development by pacing chondrocyte differentiation and mediating osteoblast (OB) function during endochondral bone formation-an essential process recapitulated during fracture repair. Here, we delineated the role of the CaSR in mediating transition of callus chondrocytes into the OB lineage and subsequent bone formation at fracture sites and explored targeting CaSRs pharmacologically to enhance fracture repair. In chondrocytes cultured from soft calluses at a closed, unfixed fracture site, extracellular [Ca²⁺ ] and the allosteric CaSR agonist (NPS-R568) promoted terminal differentiation of resident cells and the attainment of an osteoblastic phenotype. Knockout (KO) of the Casr gene in chondrocytes lengthened the chondrogenic phase of fracture repair by increasing cell proliferation in soft calluses but retarded subsequent osteogenic activity in hard calluses. Tracing growth plate (GP) and callus chondrocytes that express Rosa26-tdTomato showed reduced chondrocyte transition into OBs (by >80%) in the spongiosa of the metaphysis and in hard calluses. In addition, KO of the Casr gene specifically in mature OBs suppressed osteogenic activity and mineralizing function in bony calluses. Importantly, in experiments using PTH (1-34) to enhance fracture healing, co-injection of NPS-R568 not only normalized the hypercalcemic side effects of intermittent PTH (1-34) treatment in mice but also produced synergistic osteoanabolic effects in calluses. These data indicate a functional role of CaSR in mediating chondrogenesis and osteogenesis in the fracture callus and the potential of CaSR agonism to facilitate fracture repair. © 2019 American Society for Bone and Mineral Research.

Calvarial Versus Long Bone: Implications for Tailoring Skeletal Tissue Engineering

Tissue-engineered graft substitutes have shown great potential to treat large bone defects. While we usually assume that therapeutic approaches developed for appendicular bone healing could be similarly translated for application in craniofacial reconstruction and vice versa, this is not necessarily accurate. In addition to those more well-known healing-associated factors, such as age, lifestyle (e.g., nutrition and smoking), preexisting disease (e.g., diabetes), medication, and poor blood supply, the developmental origins and surrounding tissue of the wound sites can largely affect the fracture healing outcome as well as designed treatments. Therefore, the strategies developed for long bone fracture repair might not be suitable or directly applicable to skull bone repair. In this review, we discuss aspects of development, healing process, structure, and tissue engineering considerations between calvarial and long bones to assist in designing the tailored bone repair strategies. Impact Statement We summarized, in this review, the existing body of knowledge between long bone and calvarial bone with regard to their development and healing, tissue structure, and consideration of current tissue engineering strategies. By highlighting their similarities and differences, we propose that tailored tissue engineering strategies, such as scaffold features, growth factor usage, and the source of cells for tissue or region-specific bone repair, are necessary to ensure an optimized healing outcome.

The role of GPCRs in bone diseases and dysfunctions

The superfamily of G protein-coupled receptors (GPCRs) contains immense structural and functional diversity and mediates a myriad of biological processes upon activation by various extracellular signals. Critical roles of GPCRs have been established in bone development, remodeling, and disease. Multiple human GPCR mutations impair bone development or metabolism, resulting in osteopathologies. Here we summarize the disease phenotypes and dysfunctions caused by GPCR gene mutations in humans as well as by deletion in animals. To date, 92 receptors (5 glutamate family, 67 rhodopsin family, 5 adhesion, 4 frizzled/taste2 family, 5 secretin family, and 6 other 7TM receptors) have been associated with bone diseases and dysfunctions (36 in humans and 72 in animals). By analyzing data from these 92 GPCRs, we found that mutation or deletion of different individual GPCRs could induce similar bone diseases or dysfunctions, and the same individual GPCR mutation or deletion could induce different bone diseases or dysfunctions in different populations or animal models. Data from human diseases or dysfunctions identified 19 genes whose mutation was associated with human BMD: 9 genes each for human height and osteoporosis; 4 genes each for human osteoarthritis (OA) and fracture risk; and 2 genes each for adolescent idiopathic scoliosis (AIS), periodontitis, osteosarcoma growth, and tooth development. Reports from gene knockout animals found 40 GPCRs whose deficiency reduced bone mass, while deficiency of 22 GPCRs increased bone mass and BMD; deficiency of 8 GPCRs reduced body length, while 5 mice had reduced femur size upon GPCR deletion. Furthermore, deficiency in 6 GPCRs induced osteoporosis; 4 induced osteoarthritis; 3 delayed fracture healing; 3 reduced arthritis severity; and reduced bone strength, increased bone strength, and increased cortical thickness were each observed in 2 GPCR-deficiency models. The ever-expanding number of GPCR mutation-associated diseases warrants accelerated molecular analysis, population studies, and investigation of phenotype correlation with SNPs to elucidate GPCR function in human diseases.

Prevention of Injury-Induced Osteoarthritis in Rodent Temporomandibular Joint by Targeting Chondrocyte CaSR

Traumatic joint injuries produce osteoarthritic cartilage manifesting accelerated chondrocyte terminal differentiation and matrix degradation via unknown cellular and molecular mechanisms. Here we report the ability of biomechanical stress to increase expression of the calcium-sensing receptor (CaSR), a pivotal driver of chondrocyte terminal differentiation, in cultured chondrogenic cells subjected to fluid flow shear stress (FFSS) and in chondrocytes of rodent temporomandibular joint (TMJ) cartilage subjected to unilateral anterior cross-bite (UAC). In cultured ATDC5 cells or TMJ chondrocytes, FFSS induced Ca²⁺ loading and CaSR localization in endoplasmic reticulum (ER), casually accelerating cell differentiation that could be abrogated by emptying ER Ca²⁺ stores or CaSR knockdown. Likewise, acute chondrocyte-specific Casr knockout (KO) prevented the UAC-induced acceleration of chondrocyte terminal differentiation and matrix degradation in TMJ cartilage in mice. More importantly, local injections of CaSR antagonist, NPS2143, replicated the effects of Casr KO in preventing the development of osteoarthritic phenotypes in TMJ cartilage of the UAC-treated rats. Our study revealed a novel pathological action of CaSR in development of osteoarthritic cartilage due to aberrant mechanical stimuli and supports a therapeutic potential of calcilytics in preventing osteoarthritis in temporomandibular joints by targeting the CaSR. © 2018 American Society for Bone and Mineral Research.

Microporous polysaccharide multilayer coated BCP composite scaffolds with immobilised calcitriol promote osteoporotic bone regeneration both in vitro and in vivo

Incorporating a biomimetic coating and integrating osteoinductive biomolecules into basic bone substitutes are two common strategies to improve osteogenic capabilities in bone tissue engineering. Currently, the underlying mechanism of osteoporosis (OP)-related deficiency of osteogenesis remains unclear, and few treatments target at OP-related bone regeneration. Herein, we describe a self-assembling polyelectrolyte multilayered (PEM) film coating with local immobilisation of calcitriol (Cal) in biphasic calcium phosphate (BCP) scaffolds to promote osteoporotic bone regeneration by targeting the calcium sensing receptor (CaSR). Methods: The ovariectomy-induced functional changes in bone marrow mesenchymal stem cells (BMSCs), protective effects of Cal, and the potential mechanism were all verified. A PEM film composed of hyaluronic acid (HA) and chitosan (Chi) was prepared through layer-by-layer self-assembly. The morphology, growth behaviour, and drug retention capability of the composite scaffolds were characterised, and their biocompatibility and therapeutic efficacy for bone regeneration were systematically explored in vitro and in vivo.Results: The osteogenic differentiation, adhesion, and proliferation abilities of ovariectomised rat BMSCs (OVX-rBMSCs) decreased, in accordance with the deficiency of CaSR. Cal effectively activated osteogenesis in these OVX-rBMSCs by binding specifically to the active pocket of the CaSR structure, while the biomimetic PEM coating augmented OVX-rBMSCs proliferation and adhesion due to its porous surface structure. The PEM-coated scaffolds showed advantages in Cal loading and retention, especially at lower drug concentrations. HA/Chi PEM synergised with Cal to improve the proliferation, adhesion, and osteogenesis of OVX-rBMSCs and promote bone regeneration and BCP degradation in the critical-size calvarial bone defect model of OVX rats. Conclusion: A composite scaffold based on BCP, created by simply combining a biomimetic PEM coating and Cal immobilisation, could be clinically useful and has marked advantages as a targeted, off-the-shelf, cell-free treatment option for osteoporotic bone regeneration.

Bone metabolism in pediatric burned patients: A review

Severe burns in children can lead to growth delays, bone loss, and wasting of lean body mass and muscle with subsequent long-term effects such as osteoporosis. The following review examines 11 randomized, placebo-controlled, prospective clinical trials in pediatric burns between 1995 and 2017. These studies included approximately 250 burned children, and they were conducted to evaluate the impact of severe burn on markers of bone formation and bone metabolism. Some trials also analyzed current therapy regimens such as pamidronate and vitamin D. The clinical utility of these outlined biomarkers is uncertain with regard to acute burn care, as the current literature remains unclear. This review thus serves to address the impact of severe burn on markers of bone formation and bone metabolism in pediatric patients but will not focus on the clinical utility of the markers. The aim of this review is to summarize the findings of the trials to guide the future care of burned patients to maximize bone recovery.

Familial hypocalciuric hypercalcemia and related disorders

Familial hypocalciuric hypercalcemia (FHH) causes hypercalcemia by three genetic mechanisms: inactivating mutations in the calcium-sensing receptor, the G-protein subunit α₁₁, or adaptor-related protein complex 2, sigma 1 subunit. While hypercalcemia in other conditions causes significant morbidity and mortality, FHH generally follows a benign course. Failure to diagnose FHH can result in unwarranted treatment or surgery for the mistaken diagnosis of primary hyperparathyroidism (PHPT), given the significant overlap of biochemical features. Determinations of urinary calcium excretion greatly aid in distinguishing PHPT from FHH, but overlap still exists in certain cases. It is important that 24-h urine calcium and creatinine be included in the initial workup of hypercalcemia. FHH should be considered if low or even low normal urinary calcium levels are found in what is typically an asymptomatic hypercalcemic patient. The calcimimetic cinacalcet has been used to treat hypercalcemia in certain symptomatic causes of FHH.

The art of building bone: emerging role of chondrocyte-to-osteoblast transdifferentiation in endochondral ossification

There is a worldwide epidemic of skeletal diseases causing not only a public health issue but also accounting for a sizable portion of healthcare expenditures. The vertebrate skeleton is known to be formed by mesenchymal cells condensing into tissue elements (patterning phase) followed by their differentiation into cartilage (chondrocytes) or bone (osteoblasts) cells within the condensations. During the growth and remodeling phase, bone is formed directly via intramembranous ossification or through a cartilage to bone conversion via endochondral ossification routes. The canonical pathway of the endochondral bone formation process involves apoptosis of hypertrophic chondrocytes followed by vascular invasion that brings in osteoclast precursors to remove cartilage and osteoblast precursors to form bone. However, there is now an emerging role for chondrocyte-to-osteoblast transdifferentiation in the endochondral ossification process. Although the concept of "transdifferentiation" per se is not recent, new data using a variety of techniques to follow the fate of chondrocytes in different bones during embryonic and post-natal growth as well as during fracture repair in adults have identified three different models for chondrocyte-to-osteoblast transdifferentiation (direct transdifferentiation, dedifferentiation to redifferentiation, and chondrocyte to osteogenic precursor). This review focuses on the emerging models of chondrocyte-to-osteoblast transdifferentiation and their implications for the treatment of skeletal diseases as well as the possible signaling pathways that contribute to chondrocyte-to-osteoblast transdifferentiation processes.

Hyperstimulation of CaSR in human MSCs by biomimetic apatite inhibits endochondral ossification via temporal down-regulation of PTH1R

In adult bone injuries, periosteum-derived mesenchymal stem/stromal cells (MSCs) form bone via endochondral ossification (EO), whereas those from bone marrow (BM)/endosteum form bone primarily through intramembranous ossification (IMO). We hypothesized that this phenomenon is influenced by the proximity of MSCs residing in the BM to the trabecular bone microenvironment. Herein, we investigated the impact of the bone mineral phase on human BM-derived MSCs' choice of ossification pathway, using a biomimetic bone-like hydroxyapatite (BBHAp) interface. BBHAp induced hyperstimulation of extracellular calcium-sensing receptor (CaSR) and temporal down-regulation of parathyroid hormone 1 receptor (PTH1R), leading to inhibition of chondrogenic differentiation of MSCs even in the presence of chondroinductive factors, such as transforming growth factor-β1 (TGF-β1). Interestingly rescuing PTH1R expression using human PTH fragment (1-34) partially restored chondrogenesis in the BBHAp environment. In vivo studies in an ectopic site revealed that the BBHAp interface inhibits EO and strictly promotes IMO. Furthermore, CaSR knockdown (CaSR KD) disrupted the bone-forming potential of MSCs irrespective of the absence or presence of the BBHAp interface. Our findings confirm the expression of CaSR in human BM-derived MSCs and unravel a prominent role for the interplay between CaSR and PTH1R in regulating MSC fate and the choice of pathway for bone formation.

The art of building bone: emerging role of chondrocyte-to-osteoblast transdifferentiation in endochondral ossification

There is a worldwide epidemic of skeletal diseases causing not only a public health issue but also accounting for a sizable portion of healthcare expenditures. The vertebrate skeleton is known to be formed by mesenchymal cells condensing into tissue elements (patterning phase) followed by their differentiation into cartilage (chondrocytes) or bone (osteoblasts) cells within the condensations. During the growth and remodeling phase, bone is formed directly via intramembranous ossification or through a cartilage to bone conversion via endochondral ossification routes. The canonical pathway of the endochondral bone formation process involves apoptosis of hypertrophic chondrocytes followed by vascular invasion that brings in osteoclast precursors to remove cartilage and osteoblast precursors to form bone. However, there is now an emerging role for chondrocyte-to-osteoblast transdifferentiation in the endochondral ossification process. Although the concept of "transdifferentiation" per se is not recent, new data using a variety of techniques to follow the fate of chondrocytes in different bones during embryonic and post-natal growth as well as during fracture repair in adults have identified three different models for chondrocyte-to-osteoblast transdifferentiation (direct transdifferentiation, dedifferentiation to redifferentiation, and chondrocyte to osteogenic precursor). This review focuses on the emerging models of chondrocyte-to-osteoblast transdifferentiation and their implications for the treatment of skeletal diseases as well as the possible signaling pathways that contribute to chondrocyte-to-osteoblast transdifferentiation processes.

PTH1R-CaSR Cross Talk: New Treatment Options for Breast Cancer Osteolytic Bone Metastases

Metastatic breast cancer (BrCa) is currently incurable despite great improvements in treatment of primary BrCa. The incidence of skeletal metastases in advanced BrCa occurs up to 70%. Recent findings have established that the distribution of BrCa metastases to the skeleton is not a random process but due to the favorable microenvironment for tumor invasion and growth. The complex interplay among BrCa cells, stromal/osteoblastic cells, and osteoclasts in the osseous microenvironment creates a bone-tumor vicious cycle (a feed-forward loop) that results in excessive bone destruction and progressive tumor growth. Both the type 1 PTH receptor (PTH1R) and extracellular calcium-sensing receptor (CaSR) participate in the vicious cycle and influence the skeletal metastatic niche. Thus, this review focuses on how the PTH1R and CaSR signaling pathways interact and contribute to the pathogenesis of BrCa bone metastases. The effects of intermittent PTH and allosteric modulators of CaSR for the use of bone-anabolic agents and prevention of BrCa bone metastases constitute a proof of principle for therapeutic consideration. Understanding the interplay between PTH1R and CaSR signaling in the development of BrCa bone metastases could lead to a novel therapeutic approach to control both osteolysis and tumor burden in the bone.

Comparison of calcimimetic R568 and calcitriol in mineral homeostasis in the Hyp mouse, a murine homolog of X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) caused by mutations in the Phex gene is the most common human inherited phosphate wasting disorder characterized by enhanced synthesis of fibroblast growth factor 23 (FGF23) in bone, renal phosphate wasting, 1,25(OH)₂D₃ (1,25D) deficiency, rickets and osteomalacia. Here we studied the effects of calcimimetic R568 and calcitriol treatment in the Hyp mouse, a murine homolog of XLH. We hypothesized that mineral homeostasis is differentially affected by R568 and 1,25D with respect to the PTH-vitamin D-FGF23-Klotho axis and bone health. Four-week-old male Hyp mice received R568 in different doses, 1,25D or vehicle for 28days. Vehicle-treated wild-type mice served as controls. Both R568 and 1,25D reduced PTH levels, yet only 1,25D raised serum phosphate levels in Hyp mice. 1,25D increased calciuria and further enhanced FGF23 synthesis in bone and circulating FGF23 levels. By contrast, R568 reduced bone FGF23 expression and serum total but not intact FGF23 concentrations. Renal 1,25D metabolism was further impaired by 1,25D and improved although not normalized by R568. Hyp mice showed reduced renal Klotho levels, which were increased by 1,25D and high dose R568. 1,25D, but not R568, significantly improved femur growth, and weight gain, and partially restored growth plate morphology and bone mineralization. Although a significant improvement of trabecular bone was noted by μCT, compared to 1,25D the effects of R568 on bone histomophometric parameters were marginal. Our data indicate that monotherapy with R568 reduced PTH and FGF23 synthesis in bone, but failed to restore vitamin D and phosphate metabolism and skeletal abnormalities in Hyp mice. By contrast, 1,25D improved body growth, and defective mineralization despite further enhancement of skeletal FGF23 synthesis thereby highlighting the importance of vitamin D in bone mineralization in Hyp mice.

The Calcium-Sensing Receptor and the Parathyroid: Past, Present, Future

Parathyroid hormone (PTH) defends the extracellular fluid from hypocalcemia and has powerful and well-documented actions on the skeleton and renal tubular system. To achieve a satisfactory stable plasma calcium level, the secretion of PTH, and the resulting serum PTH level, is titrated carefully to the prevailing plasma ionized Ca²⁺ concentration via a Ca²⁺ sensing mechanism that mediates feedback inhibition of PTH secretion. Herein, I consider the properties of the parathyroid Ca²⁺ sensing mechanism, the identity of the Ca²⁺ sensor, the intracellular biochemical mechanisms that it controls, the manner of its integration with other components of the PTH secretion control mechanism, and its modulation by other nutrients. Together the well-established, recently elucidated, and yet-to-be discovered elements of the story constitute the past, present, and future of the parathyroid and its calcium-sensing receptor (CaSR).

A Convenient In Vivo Model Using Small Interfering RNA Silencing to Rapidly Assess Skeletal Gene Function

It is difficult to study bone in vitro because it contains various cell types that engage in cross-talk. Bone biologically links various organs, and it has thus become increasingly evident that skeletal physiology must be studied in an integrative manner in an intact animal. We developed a model using local intraosseous small interfering RNA (siRNA) injection to rapidly assess the effects of a target gene on the local skeletal environment. In this model, 160-g male Sprague-Dawley rats were treated for 1-2 weeks. The left tibia received intraosseous injection of a parathyroid hormone 1 receptor (Pth1r) or insulin-like growth factor 1 receptor (Igf-1r) siRNA transfection complex loaded in poloxamer 407 hydrogel, and the right tibia received the same volume of control siRNA. All the tibias received an intraosseous injection of recombinant human parathyroid hormone (1-34) (rhPTH (1-34)) or insulin-like growth factor-1 (IGF-1). Calcein green and alizarin red were injected 6 and 2 days before euthanasia, respectively. IGF-1R and PTH1R expression levels were detected via RT-PCR assays and immunohistochemistry. Bone mineral density (BMD), microstructure, mineral apposition rates (MARs), and strength were determined by dual-energy X-ray absorptiometry, micro-CT, histology and biomechanical tests. The RT-PCR and immunohistochemistry results revealed that IGF-1R and PTH1R expression levels were dramatically diminished in the siRNA-treated left tibias compared to the right tibias (both p<0.05). Using poloxamer 407 hydrogel as a controlled-release system prolonged the silencing effect of a single dose of siRNA; the mRNA expression levels of IGF-1R were lower at two weeks than at one week (p<0.01). The BMD, bone microstructure parameters, MAR and bone strength were significantly decreased in the left tibias compared to the right tibias (all p<0.05). This simple and convenient local intraosseous siRNA injection model achieved gene silencing with very small quantities of siRNA over a short treatment period (≤7 days).

The Emergence of Physiology and Form: Natural Selection Revisited

Natural Selection describes how species have evolved differentially, but it is descriptive, non-mechanistic. What mechanisms does Nature use to accomplish this feat? One known way in which ancient natural forces affect development, phylogeny and physiology is through gravitational effects that have evolved as mechanotransduction, seen in the lung, kidney and bone, linking as molecular homologies to skin and brain. Tracing the ontogenetic and phylogenetic changes that have facilitated mechanotransduction identifies specific homologous cell-types and functional molecular markers for lung homeostasis that reveal how and why complex physiologic traits have evolved from the unicellular to the multicellular state. Such data are reinforced by their reverse-evolutionary patterns in chronic degenerative diseases. The physiologic responses of model organisms like Dictyostelium and yeast to gravity provide deep comparative molecular phenotypic homologies, revealing mammalian Target of Rapamycin (mTOR) as the final common pathway for vertical integration of vertebrate physiologic evolution; mTOR integrates calcium/lipid epistatic balance as both the proximate and ultimate positive selection pressure for vertebrate physiologic evolution. The commonality of all vertebrate structure-function relationships can be reduced to calcium/lipid homeostatic regulation as the fractal unit of vertebrate physiology, demonstrating the primacy of the unicellular state as the fundament of physiologic evolution.