Parathyroid hormone-dependent signaling pathways regulating genes in bone cells

Mesenchymal stem cells for cartilage regeneration: Insights into molecular mechanism and therapeutic strategies

The use of mesenchymal stem cells (MSCs) in cartilage regeneration has gained significant attention in regenerative medicine. This paper reviews the molecular mechanisms underlying MSC-based cartilage regeneration and explores various therapeutic strategies to enhance the efficacy of MSCs in this context. MSCs exhibit multipotent capabilities and can differentiate into various cell lineages under specific microenvironmental cues. Chondrogenic differentiation, a complex process involving signaling pathways, transcription factors, and growth factors, plays a pivotal role in the successful regeneration of cartilage tissue. The chondrogenic differentiation of MSCs is tightly regulated by growth factors and signaling pathways such as TGF-β, BMP, Wnt/β-catenin, RhoA/ROCK, NOTCH, and IHH (Indian hedgehog). Understanding the intricate balance between these pathways is crucial for directing lineage-specific differentiation and preventing undesirable chondrocyte hypertrophy. Additionally, paracrine effects of MSCs, mediated by the secretion of bioactive factors, contribute significantly to immunomodulation, recruitment of endogenous stem cells, and maintenance of chondrocyte phenotype. Pre-treatment strategies utilized to potentiate MSCs, such as hypoxic conditions, low-intensity ultrasound, kartogenin treatment, and gene editing, are also discussed for their potential to enhance MSC survival, differentiation, and paracrine effects. In conclusion, this paper provides a comprehensive overview of the molecular mechanisms involved in MSC-based cartilage regeneration and outlines promising therapeutic strategies. The insights presented contribute to the ongoing efforts in optimizing MSC-based therapies for effective cartilage repair.

Targeted silencing of GNAS in a human model of osteoprogenitor cells results in the deregulation of the osteogenic differentiation program

Introduction

The dysregulation of cell fate toward osteoprecursor cells associated with most GNAS-based disorders may lead to episodic de novo extraskeletal or ectopic bone formation in subcutaneous tissues. The bony lesion distribution suggests the involvement of abnormal differentiation of mesenchymal stem cells (MSCs) and/or more committed precursor cells. Data from transgenic mice support the concept that GNAS is a crucial factor in regulating lineage switching between osteoblasts (OBs) and adipocyte fates. The mosaic nature of heterotopic bone lesions suggests that GNAS genetic defects provide a sensitized background for ectopic osteodifferentiation, but the underlying molecular mechanism remains largely unknown.

Methods

The effect of GNAS silencing in the presence and/or absence of osteoblastic stimuli was evaluated in the human L88/5 MSC line during osteodifferentiation. A comparison of the data obtained with data coming from a bony lesion from a GNAS-mutated patient was also provided.

Results

Our study adds some dowels to the current fragmented notions about the role of GNAS during osteoblastic differentiation, such as the premature transition of immature OBs into osteocytes and the characterization of the differences in the deposed bone matrix.

Conclusion

We demonstrated that our cell model partially replicates the in vivo behavior results, resulting in an applicable human model to elucidate the pathophysiology of ectopic bone formation in GNAS-based disorders.

Engineering immunomodulatory hydrogels and cell-laden systems towards bone regeneration

The well-known synergetic interplay between the skeletal and immune systems has changed the design of advanced bone tissue engineering strategies. The immune system is essential during the bone lifetime, with macrophages playing multiple roles in bone healing and biomaterial integration. If in the past, the most valuable aspect of implants was to avoid immune responses of the host, nowadays, it is well-established how important are the crosstalks between immune cells and bone-engineered niches for an efficient regenerative process to occur. For that, it is essential to recapitulate the multiphenotypic cellular environment of bone tissue when designing new approaches. Indeed, the lack of osteoimmunomodulatory knowledge may be the explanation for the poor translation of biomaterials into clinical practice. Thus, smarter hydrogels incorporating immunomodulatory bioactive factors, stem cells, and immune cells are being proposed to develop a new generation of bone tissue engineering strategies. This review highlights the power of immune cells to upgrade the development of innovative engineered strategies, mainly focusing on orthopaedic and dental applications.

Immunology of osteoporosis: relevance of inflammatory targets for the development of novel interventions

Osteoporosis is recognized as low bone mass and deteriorated bone microarchitecture. It is the leading cause of fractures and consequent morbidity globally. The established pathophysiological evidence favors the endocrine factors for osteoporosis and the role of the immune system on the skeletal system has been recently identified. Due to the common developmental niche bone and immune system interactions have led to the emergence of osteoimmunology. Immune dysregulation can initiate inflammatory conditions that adversely affect bone integrity. The role of immune cells, such as T-lymphocytes subsets (Th17), cannot be neglected in the pathogenesis of osteoporosis. Local inflammation within the bone from any cause attracts immune cells that participate in the activation of osteoclasts. This work summarizes the present knowledge of osteoimmunology in reference to osteoporosis and identifies novel targets for immunotherapy of osteoporosis.

Loss of Parathyroid Hormone Receptor Signaling in Osteoprogenitors Is Associated With Accumulation of Multiple Hematopoietic Lineages in the Bone Marrow

Osteoblasts and their progenitors play an important role in the support of hematopoiesis within the bone marrow (BM) microenvironment. We have previously reported that parathyroid hormone receptor (PTH1R) signaling in osteoprogenitors is required for normal B cell precursor differentiation, and for trafficking of maturing B cells out of the BM. Cells of the osteoblast lineage have been implicated in the regulation of several other hematopoietic cell populations, but the effects of PTH1R signaling in osteoprogenitors on other maturing hematopoietic populations have not been investigated. Here we report that numbers of maturing myeloid, T cell, and erythroid populations were increased in the BM of mice lacking PTH1R in Osx-expressing osteoprogenitors (PTH1R-OsxKO mice; knockout [KO]). This increase in maturing hematopoietic populations was not associated with an increase in progenitor populations or proliferation. The spleens of PTH1R-OsxKO mice were small with decreased numbers of all hematopoietic populations, suggesting that trafficking of mature hematopoietic populations between BM and spleen is impaired in the absence of PTH1R in osteoprogenitors. RNA sequencing (RNAseq) of osteoprogenitors and their descendants in bone and BM revealed increased expression of vascular cell adhesion protein 1 (VCAM-1) and C-X-C motif chemokine ligand 12 (CXCL12), factors that are involved in trafficking of several hematopoietic populations. © 2022 American Society for Bone and Mineral Research (ASBMR).

Inactivation of Osteoblast PKC Signaling Reduces Cortical Bone Mass and Density and Aggravates Renal Osteodystrophy in Mice with Chronic Kidney Disease on High Phosphate Diet

Chronic kidney disease (CKD) frequently leads to hyperphosphatemia and hyperparathyroidism, mineral bone disorder (CKD-MBD), ectopic calcifications and cardiovascular mortality. PTH activates the osteoanabolic Gα_s/PKA and the Gα_q/11/PKC pathways in osteoblasts, the specific impact of the latter in CKD-MBD is unknown. We generated osteoblast specific Gα_q/11 knockout (KO) mice and established CKD-MBD by subtotal nephrectomy and dietary phosphate load. Bone morphology was assessed by micro-CT, osteoblast function by bone planar scintigraphy at week 10 and 22 and by histomorphometry. Osteoblasts isolated from Gα_q/11 KO mice increased cAMP but not IP3 in response to PTH 1-34, demonstrating the specific KO of the PKC signaling pathway. Osteoblast specific Gα_q/11 KO mice exhibited increased serum calcium and reduced bone cortical thickness and mineral density at 24 weeks. CKD Gα_q/11 KO mice had similar bone morphology compared to WT, while CKD Gα_q/11-KO on high phosphate diet developed decreased metaphyseal and diaphyseal cortical thickness and area, as well as a reduction in trabecular number. Gα_q/11-KO increased bone scintigraphic tracer uptake at week 10 and mitigated tracer uptake in CKD mice at week 22. Histological bone parameters indicated similar trends. Gα_q/11-KO in osteoblast modulates calcium homeostasis, bone formation rate, bone morphometry, and bone mineral density. In CKD and high dietary phosphate intake, osteoblast Gα_q/11/PKC KO further aggravates mineral bone disease.

Evaluating Osteogenic Differentiation of Osteoblastic Precursors Upon Intermittent Administration of PTH/IGFBP7

Parathyroid hormone (PTH) 1–34 is the first anabolic agent approved for the treatment of osteoporosis. Preclinical evidence shows a potential association between PTH and osteosarcoma. The mechanisms mediating the bone- and neoplasm-forming effects of PTH remain incompleted understood, few studies on the role of Insulin-like growth factor-binding protein 7 (IGFBP7) in mediating the anabolic effects of PTH has been reported. Intermittent PTH administration was found to increase the expression of IGFBP7 in mesenchymal stem cells (MSCs) and pre-osteoblasts. The results indicated that the anabolic effects of PTH were interrupted when knockdown of IGFBP7, while supplementation with IGFBP7 protein could enhance the bone-forming efficacy of PTH and regulate the signaling pathways. Moreover, bone healing was accelerated by the administration of IGFBP7 along with PTH in a mouse model of fracture. The obtained results proved that IGFBP7 was necessary for the anabolic effects of PTH, and combined administration of PTH and IGFBP7 showed stronger bone-forming effects relative to administration of PTH alone.

Amelioration of Fructus Ligustri Lucidi and its phenol glycosides on hypercalciuria via stimulating PTH1R/PKA/TRPV5 signaling

BACKGROUND

Our early studies performed on aged rats, ovariectomized (OVX) rats and diabetic mice, indicated the calciotropic role of Fructus Ligustri Lucidi (FLL), the fruit of Ligustrum lucidum Ait., in mediating calcium homeostasis which was partially attributed to its stimulation on renal calcium reabsorption.

PURPOSE

This study aimed to explicate the underlying molecular mechanism and explore the potential bioactive ingredients in FLL.

STUDY DESIGN AND METHODS

The OVX C57BL/6 J mice were orally administered with low (FL, 75 mg/kg), middle (FM, 225 mg/kg) or high (FH, 675 mg/kg) dose of extract of Fructus Ligustri Lucidi for 10 weeks. The biological properties of trabecular bone were measured by micro-CT and H&E staining. The molecular expression was assessed by immunoblotting and immunostaining. The potential active components were identified by cell membrane chromatography (CMC) and explored in renal tubular cells with Fluo-3/AM fluorescent staining to indicate intracellular calcium level. The male mice fed with high calcium diet (1.2% Ca) and orally treated with active components for 3 weeks.

RESULTS

Treatment of OVX mice with FLL extract suppressed the elevation in urinary calcium level (FH, 0.081 ± 0.012, vs. OVX, 0.189 ± 0.038 mg/mg), and increased bone mineral density (FH, 62.41 ± 2.57, vs. OVX, 43.72 ± 8.43 mg/ccm) and percentage of trabecular bone area. It also decreased circulating PTH level (FH, 66.69 ± 10.94, vs. OVX, 303.50 ± 26.56 pg/ml) and up-regulated TRPV5 expression in renal cortex of OVX mice as well as enhanced the expression of PTH receptor (PTH1R) and the ratio of p-PKA/PKA. The PKA inhibitor H89 abolished the induction of serum, prepared from rats treated with FLL extract, on PKA/TRPV5 signaling in renal tubular cells. The CMC identified phenol glycosides, including salidroside and oleuropein, which increased intracellular calcium content, promoted expression of PTH1R and TRPV5 and ratio of p-PKA/PKA as well as decreased calcium excretion in urine of mice fed with high calcium diet.

CONCLUSION

Salidroside and oleuropein are major ingredients contributing to the anti-hypercalciuria effects of FLL via acting on PTH1R/PKA/TRPV5 signaling in kidney. Further translational research would be required.

The phytochemical plumbagin reciprocally modulates osteoblasts and osteoclasts

Bone metabolism is essential for maintaining bone mineral density and bone strength through a balance between bone formation and bone resorption. Bone formation is associated with osteoblast activity whereas bone resorption is linked to osteoclast differentiation. Osteoblast progenitors give rise to the formation of mature osteoblasts whereas monocytes are the precursors for multi-nucleated osteoclasts. Chronic inflammation, auto-inflammation, hormonal changes or adiposity have the potential to disturb the balance between bone formation and bone loss. Several plant-derived components are described to modulate bone metabolism and alleviate osteoporosis by enhancing bone formation and inhibiting bone resorption. The plant-derived naphthoquinone plumbagin is a bioactive compound that can be isolated from the roots of the Plumbago genus. It has been used as traditional medicine for treating infectious diseases, rheumatoid arthritis and dermatological diseases. Reportedly, plumbagin exerts its biological activities primarily through induction of reactive oxygen species and triggers osteoblast-mediated bone formation. It is plausible that plumbagin's reciprocal actions - inhibiting or inducing death in osteoclasts but promoting survival or growth of osteoblasts - are a function of the synergy with bone-metabolizing hormones calcitonin, Parathormone and vitamin D. Herein, we develop a framework for plausible molecular modus operandi of plumbagin in bone metabolism.

New Approaches to Assess Mechanisms of Action of Selective Vitamin D Analogues

Recent studies of transcription have revealed an advanced set of overarching principles that govern vitamin D action on a genome-wide scale. These tenets of vitamin D transcription have emerged as a result of the application of now well-established techniques of chromatin immunoprecipitation coupled to next-generation DNA sequencing that have now been linked directly to CRISPR-Cas9 genomic editing in culture cells and in mouse tissues in vivo. Accordingly, these techniques have established that the vitamin D hormone modulates sets of cell-type specific genes via an initial action that involves rapid binding of the VDR-ligand complex to multiple enhancer elements at open chromatin sites that drive the expression of individual genes. Importantly, a sequential set of downstream events follows this initial binding that results in rapid histone acetylation at these sites, the recruitment of additional histone modifiers across the gene locus, and in many cases, the appearance of H3K36me3 and RNA polymerase II across gene bodies. The measured recruitment of these factors and/or activities and their presence at specific regions in the gene locus correlate with the emerging presence of cognate transcripts, thereby highlighting sequential molecular events that occur during activation of most genes both in vitro and in vivo. These features provide a novel approach to the study of vitamin D analogs and their actions in vivo and suggest that they can be used for synthetic compound evaluation and to select for novel tissue- and gene-specific features. This may be particularly useful for ligand activation of nuclear receptors given the targeting of these factors directly to genetic sites in the nucleus.

Recent Progresses in the Treatment of Osteoporosis

Osteoporosis (OP) is a chronic bone disease characterized by aberrant microstructure and macrostructure of bone, leading to reduced bone mass and increased risk of fragile fractures. Anti-resorptive drugs, especially, bisphosphonates, are currently the treatment of choice in most developing countries. However, they do have limitations and adverse effects, which, to some extent, helped the development of anabolic drugs such as teriparatide and romosozumab. In patients with high or very high risk for fracture, sequential or combined therapies may be considered with the initial drugs being anabolic agents. Great endeavors have been made to find next generation drugs with maximal efficacy and minimal toxicity, and improved understanding of the role of different signaling pathways and their crosstalk in the pathogenesis of OP may help achieve this goal. Our review focused on recent progress with regards to the drug development by modification of Wnt pathway, while other pathways/molecules were also discussed briefly. In addition, new observations made in recent years in bone biology were summarized and discussed for the treatment of OP.

Signaling Pathways in Bone Development and Their Related Skeletal Dysplasia

Bone development is a tightly regulated process. Several integrated signaling pathways including HH, PTHrP, WNT, NOTCH, TGF-β, BMP, FGF and the transcription factors SOX9, RUNX2 and OSX are essential for proper skeletal development. Misregulation of these signaling pathways can cause a large spectrum of congenital conditions categorized as skeletal dysplasia. Since the signaling pathways involved in skeletal dysplasia interact at multiple levels and have a different role depending on the time of action (early or late in chondrogenesis and osteoblastogenesis), it is still difficult to precisely explain the physiopathological mechanisms of skeletal disorders. However, in recent years, significant progress has been made in elucidating the mechanisms of these signaling pathways and genotype–phenotype correlations have helped to elucidate their role in skeletogenesis. Here, we review the principal signaling pathways involved in bone development and their associated skeletal dysplasia.

Parathyroid hormone levels and aging: Effect on balance

Falls result in multiple adverse health outcomes including mortality in older persons. Impaired balance is known to increase the risk of falls. Numerous factors play a role in the etiology of balance disturbance. Among the calciotropic hormones, the role of vitamin D in falls risk has been extensively studied. There is evolving interest in the role of parathyroid hormone (PTH) in this area. Elevated PTH hormone levels have been associated with age-related syndromes such as frailty, osteoporosis, and sarcopenia. Among the existing studies, most have focused on muscle strength with few studies evaluating its role in balance disturbances. In this chapter, we will summarize the aspects of PTH and aging, its biological effects on muscle, and the known associations between PTH and balance.

Inactivation of parathyroid hormone: perspectives of drug discovery to combating hyperparathyroidism

Hormonal coordination is tightly regulated within the human body and thus regulates human physiology. The parathyroid hormone (PTH), a member of the endocrine system, regulates the calcium and phosphate level within the human body. Under non-physiological conditions, PTH levels get upregulated (hyperparathyroidism) or downregulated (hypoparathyroidism) due to external or internal factors. In the case of hyperparathyroidism, elevated PTH stimulates cellular receptors present in the bones, kidneys, and intestines to increase the blood calcium level, leading to calcium deposition. This eventually causes various symptoms including kidney stones. Currently, there is no known medication that directly targets PTH in order to suppress its function. Therefore, it is of great interest to find novel small molecules or any other means that can modulate PTH function. The molecular signaling of PTH starts by binding of its N-terminus to the G-protein coupled PTH1/2 receptor. Therefore, any intervention that affects the N-terminus of PTH could be a lead candidate for treating hyperparathyroidism. As a proof-of-concept, there are various possibilities to inhibit molecular PTH function by (i) a small molecule, (ii) N-terminal PTH phosphorylation, (iii) fibril formation and (iv) residue-specific mutations. These modifications put PTH into an inactive state, which will be discussed in detail in this review article. We anticipate that exploring small molecules or other means that affect the N-terminus of PTH could be lead candidates in combating hyperparathyroidism.

Emerging insights into the comparative effectiveness of anabolic therapies for osteoporosis

Over the past three decades, the mainstay of treatment for osteoporosis has been antiresorptive agents (such as bisphosphonates), which have been effective with continued administration in lowering fracture risk. However, the clinical landscape has changed as adherence to these medications has declined due to perceived adverse effects. As a result, decreases in hip fracture rates that followed the introduction of bisphosphonates have now levelled off, which is coincident with a decline in the use of the antiresorptive agents. In the past two decades, two types of anabolic agents (including three new drugs), which represent a novel approach to improving bone quality by increasing bone formation, have been approved. These therapies are expected to lead to a new clinical paradigm in which anabolic agents will be used either alone or in combination with antiresorptive agents to build new bone and reduce fracture risk. This Review examines the mechanisms of action for these anabolic agents by detailing their receptor-activating properties for key cell types in the bone and marrow niches. Using these advances in bone biology as context, the comparative effectiveness of these anabolic agents is discussed in relation to other therapeutic options for osteoporosis to better guide their clinical application in the future.

hPTH(3-34)(29-34) selectively activated PKC and mimicked osteoanabolic effects of hPTH(1-34)

Teriparatide (hPTH(1-34)) exhibits both osteoanabolic and osteocatabolic effects. We generated a novel PTH analog by duplicating the PTH(29-34) domain to hPTH(3-34) (named MY-1), which was identified to activate PKC but not PLC and cAMP/PKA signaling. It increased osteo-differentiation but did not affect osteoclastogenesis and RANKL expression in primary osteoblasts or bone marrow cells. MY-1 and hPTH(1-34) increased the synthesis and decreased the degradation οf β-catenin protein in osteoblasts, while PKC inhibitor blunted such effects. In vivo results indicated that intermittent MY-1 and hPTH(1-34) prevented bone loss in ovariectomized mice, and that MY-1 infusion increased bone volume in normal mice. Histological analysis observed more osteoclasts surrounding the cancellous bone surface in hPTH(1-34), but not MY-1 treated mice. We conclude that MY-1 mimicked the osteoanabolic but not the osteocatabolic effects of hPTH(1-34), which is related to PKC and β-catenin signaling. Such anabolic-only analog provides a new strategy to study PTH's versatile functions and design new medicines to treat osteoporosis and bone defects.

Effect of parathyroid hormone-related protein on odontogenic differentiation in human dental pulp cells

BACKGROUND

Parathyroid hormone-related protein (PTHrP) plays an important role in many physiological processes, including bone regeneration. The function of PTHrP is similar to PTH. It promotes osteogenic differentiation in MC3T3-E1 cells. The aim of this study was to investigate whether PTHrP might have odontogenic differentiation ability in human dental pulp cells (hDPCs).

METHODS

The viability of hDPCs after stimulation with PTHrP was measured. Real-time polymerase chain reaction and Western blot analysis were performed to evaluate the expression levels of odontogenic markers and activation of protein kinase B (PKB/AKT), extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase (MAPK). To evaluate mineralized nodule formation, alkaline phosphatase (ALP) staining and alizarin red S staining were performed.

RESULTS

PTHrP promoted odontogenic differentiation as evidenced by the formation of mineralized nodules, the induction of ALP activity, and the upregulation of odontogenic markers (dentin sialophosphoprotein and dentin matrix protein-1). The phosphorylation of AKT, ERK, JNK, and p38 was increased by PTHrP. However, an AKT inhibitor (LY294002), an ERK inhibitor (U0126), a JNK inhibitor (SP600125), and a p38 inhibitor (SB203580) inhibited the increase of mineralization induced by PTHrP.

CONCLUSION

The present study revealed that PTHrP could promote odontogenic differentiation and mineralization through activating the AKT, ERK, JNK, and p38 signaling pathways. These results provide novel insights into the odontogenic action of PTHrP.

microRNA expression profiles and the potential competing endogenous RNA networks in NELL-1-induced human adipose-derived stem cell osteogenic differentiation

Studies have indicated that Nel-like molecule-1 (NELL-1) was an osteoblast-specific cytokine and some specific microRNAs (miRNAs) could serve as competing endogenous RNA (ceRNA) to partake in osteogenic differentiation of human adipose-derived stem cells (hASCs). The aim of this study was to explore the potential functional mechanisms of recombinant human NELL-1 protein (rhNELL-1) during hASCs osteogenic differentiation. rhNELL-1 was added to osteogenic medium to activate osteogenic differentiation of hASCs. High-throughput RNA sequencing (RNA-Seq) was performed and validated by real-time quantitative polymerase chain reaction. Gene ontology functional annotation and Kyoto Encyclopedia of Genes and Genomes pathway analysis were performed to detect the functions of differentially expressed miRNAs and genes. Coding-noncoding gene co-expression network and ceRNA networks were constructed to predict the potential regulatory role of miRNAs. A total of 1010 differentially expressed miRNAs and 1762 differentially expressed messenger RNAs (mRNAs) were detected. miRNA-370-3p, bone morphogenetic protein 2 (BMP2), and parathyroid hormone like hormone (PTHLH) were differentially expressed during NELL-1-induced osteogenesis. Bioinformatic analyses demonstrated that these differentially expressed miRNAs and mRNAs enriched in Rap1 signaling pathway, PI3K-Akt signaling pathway, p53 signaling pathway, Glucagon signaling pathway, and hypoxia-inducible factor-1 signaling pathway, which were important pathways related to osteogenic differentiation. In addition, miRNA-370-3p and has-miR-485-5p were predicted to interact with circ0001543, circ0002405, and ENST00000570267 in ceRNA networks. Based on the gain or loss of functional experiments by transfection, the results showed that miR-370-3p was a key regulator in osteogenic differentiation by targeting BMP2 and disturbing the expression of PTHLH, and participated in NELL-1-stimulated osteogenesis. The present study provided the primary data and evidence for further exploration on the roles of miRNAs and ceRNAs during NELL-1-induced ossification of hASCs.

Undifferentiated Pleomorphic Sarcoma and Hyperparathyroidism in an Adolescent Male: A Case Report and Review of Hyperparathyroidism-associated Sarcomas

The association between hyperparathyroidism and sarcoma is extremely rare with other reported cases describing the development of osteosarcoma and chondrosarcomas in middle-aged adults. This case describes an adolescent male with hyperparathyroidism and a pathologic fracture of a biopsy-proven brown tumor in the distal right femur. The fracture healed but later developed an undifferentiated pleomorphic sarcoma of the bone at the site of the known brown tumor. Although in vitro and in vivo studies have demonstrated the risks of elevated parathyroid hormone with development of sarcomas, there is limited evidence of a human association. The effects of elevated parathyroid hormone on the skeletally immature bone in the setting of sarcoma formation are currently not well understood without current description of adolescent hyperparathyroidism-associated sarcomas. This case highlights a sarcoma originating at a pathologically proven brown tumor within an adolescent male, discusses the association of sarcoma with hyperparathyroidism, and reviews the other nine reported cases in the literature.

Combined effects of systemic parathyroid hormone (1-34) and locally delivered neutral self-assembling peptide hydrogel in the treatment of periodontal defects: An experimental in vivo investigation

AIM

To evaluate in vivo combination therapy of systemic parathyroid hormone (PTH) and locally delivered neutral self-assembling peptide (SAP) hydrogel for periodontal treatment.

MATERIALS AND METHODS

Viability/proliferation of rat periodontal ligament cells in a neutral SAP nanofibre hydrogel (SPG-178) was evaluated using WST-1 assay. Periodontal defects were created mesially to the maxillary first molars in 40 Wistar rats. Defects were filled with 1.5% SPG-178 or left unfilled. Animals received PTH (1-34) or saline injections every 2 days. Microcomputed tomography, histological, and immunohistochemical examinations were used to evaluate healing at 2 or 4 weeks postoperative.

RESULTS

At 72 hr, cells in 1.5% SPG-178 showed increased viability/proliferation compared to cells in 0.8% SPG-178 or untreated controls. In vivo, systemic PTH resulted in significantly greater bone volume in the Unfilled group at 2 weeks (p = .01) and 4 weeks (p < .0001) than in the saline control. At 4 weeks, a significantly greater bone volume was observed in the PTH/SPG-178 (p = .0003) and PTH/Unfilled (p = .004) groups than in Saline/SPG-178 group. Histologically, greater bone formation was observed in PTH/SPG-178 at 4 weeks than in other groups. In the PTH/SPG-178 group, increased proportions of PCNA-, VEGF-, and Osterix-positive cells were observed in the treated sites.

CONCLUSIONS

These findings suggest that intermittent systemic PTH and locally delivered neutral SAP hydrogel enhance periodontal healing.

Brief exposure to full length parathyroid hormone-related protein (PTHrP) causes persistent generation of cyclic AMP through an endocytosis-dependent mechanism

Parathyroid hormone (PTH)-related protein (PTHrP) (gene name Pthlh) was discovered as the factor responsible for the humoral hypercalcemia of malignancy. It shares such sequence similarity with PTH in the amino-terminal region that the two are equally able to act through a single G protein-coupled receptor, PTH1R. A number of biological activities are ascribed to domains of PTHrP beyond the amino-terminal domain. PTH functions as a circulating hormone, but PTHrP is generated locally in many tissues including bone, where it acts as a paracrine factor on osteoblasts and osteocytes. The present study compares how PTH and PTHrP influence cyclic AMP (cAMP) formation through adenylyl cyclase, the first event in cell activation through PTH1R. Brief exposure to full length PTHrP(1-141) in several osteoblastic cell culture systems was followed by sustained adenylyl cyclase activity for more than an hour after ligand washout. This effect was dose-dependent and was not found with shorter PTHrP or PTH peptides even though they were fully able to activate adenylyl cyclase with acute treatment. The persistent activation response to PTHrP(1-141) was seen also with later events in the cAMP/PKA pathway, including persistent activation of CRE-luciferase and sustained regulation of several CREB-responsive mRNAs, up to 24 h after the initial exposure. Pharmacologic blockade of endocytosis prevented the persistent activation of cAMP and gene responses. We conclude that full length PTHrP, the likely local physiological effector in bone, differs in intracellular action to PTH by undergoing endosomal translocation to induce a prolonged adenylyl cyclase activation in its target cells.

Molecular imaging of endothelial activation and mineralization in a mouse model of accelerated atherosclerosis

Purpose

Preclinical imaging of endothelial activation and mineralization using both positron emission tomography (PET) and magnetic resonance (MR) remains scarce.

Procedures

A group of uremic ApoE^−/− (Ur), non-uremic ApoE^−/− (NUr), and control C57Bl/6 J mice (Ctl) were investigated. Mineralization process was assessed using sodium fluoride ([18F]NaF) PET, and MR imaging combined with intravenous injection of MPIO-αVCAM-1 was used to evaluate endothelial activation. Micro- and macrocalcifications were evaluated by flame atomic absorption spectroscopy and von Kossa staining, respectively.

Results

Ur mice showed an active and sustained mineralization process compared to Ctl mice (p = 0.002) using [18F]NaF PET imaging. Calcium plasma level was increased in Ur (2.54 ± 0.09 mM, n = 17) compared to NUr and Ctl mice (2.24 ± 0.01, n = 22, and 2.14 ± 0.02, n = 27, respectively; p < 0.0001). Likewise, vascular calcium content was increased in Ur (0.51 ± 0.06 μg Ca²⁺ per milligram of dry weight aorta, n = 11) compared to NUr (0.27 ± 0.05, n = 9, p = 0.013) and Ctl (0.28 ± 0.05, n = 11, p = 0.014). Ur mice also had a higher inflammatory state using MPIO-αVCAM-1 MR (p global = 0.01, post hoc analysis Ur vs. Ctl p = 0.003) associated with increased VCAM-1 expression (p global = 0.02). Aortic remodeling at the level of the brachiocephalic trunk, brachiocephalic trunk itself, and aortic arch in Ur mice was also demonstrated using MR.

Conclusions

Preclinical molecular imaging allowed in vivo characterization of the early phase of atherosclerosis. [18F]NaF PET showed early and sustained vascular mineralization in uremic ApoE^−/− mice. MPIO-αVCAM-1 MR imaging demonstrated aortic endothelial activation, predominantly in segments with vascular remodeling.

Strategies for elevating hematopoietic stem cells expansion and engraftment capacity

Hematopoietic stem cells (HSCs) are a rare cell population in adult bone marrow, mobilized peripheral blood, and umbilical cord blood possessing self-renewal and differentiation capability into a full spectrum of blood cells. Bone marrow HSC transplantation has been considered as an ideal option for certain disorders treatment including hematologic diseases, leukemia, immunodeficiency, bone marrow failure syndrome, genetic defects such as thalassemia, sickle cell anemia, autoimmune disease, and certain solid cancers. Ex vivo proliferation of these cells prior to transplantation has been proposed as a potential solution against limited number of stem cells. In such culture process, MSCs have also been shown to exhibit high capacity for secretion of soluble mediators contributing to the principle biological and therapeutic activities of HSCs. In addition, endothelial cells have been introduced to bridge the blood and sub tissues in the bone marrow, as well as, HSCs regeneration induction and survival. Cell culture in the laboratory environment requires cell growth strict control to protect against contamination, symmetrical cell division and optimal conditions for maximum yield. In this regard, microfluidic systems provide culture and analysis capabilities in micro volume scales. Moreover, two-dimensional cultures cannot fully demonstrate extracellular matrix found in different tissues and organs as an abstract representation of three dimensional cell structure. Microfluidic systems can also strongly describe the effects of physical factors such as temperature and pressure on cell behavior.

Intermittent parathyroid hormone promotes cementogenesis in a PKA- and ERK1/2-dependent manner

BACKGROUND

Intermittent parathyroid hormone (PTH) promotes cementogenesis and provides a promising biotherapeutic to rehabilitate resorbed roots. However, the underlying mechanisms remain inconclusive. Cyclic aenosine monophosphate (AMP)-dependent protein kinases A (PKA) and extracellular signal-regulated MAP kinases 1/2 (ERK1/2) are key regulators of bone remodeling. The present study aims to investigate whether PKA and ERK1/2 are involved in the process of intermittent PTH-promoted cementogenesis.

METHODS

Sprague-Dawley rats in experimental group (n = 30) received a daily subcutaneous injection of PTH and the control (n = 30) received placebo vehicle for 1, 2, 3, 4, and 5 weeks. Results were evaluated by hematoxylin and eosin and immunohistochemistry staining. In vitro, OCCM-30 cells were incubated with intermittent PTH. H89 and U0126 were used to determine the role of PKA and ERK1/2, respectively. The cementogenic results were analyzed by reverse transcription-polymerase chain reaction (RT-PCR), western blotting, alkaline phosphatase activity assay and Alizarin Red S staining. The interaction of PKA and p-ERK1/2 was determined by co-immunoprecipitation (Co-IP).

RESULTS

Intermittent PTH exerted anabolic effect on cellular cementum in developing teeth with elevated expression of osteocalcin, osteopontin, and PKA (catalytic subunit) in PTH injection group. The promoting effects of intermittent PTH on cementogenesis and osteogenic differentiation were abrogated by H89 and U0126 in vitro, respectively. Blocking of PKA pathway downregulated intermittent PTH-induced ERK1/2 phosphorylation.

CONCLUSIONS

Intermittent PTH promotes cementogenesis in a PKA- and ERK1/2-dependent manner. In this process, PKA and p-ERK1/2 interact with each other. These results support the future biotherapeutic applications of PTH in cementum resorption.

Role of microRNAs in osteogenesis of stem cells

Osteogenic differentiation is a controlled developmental process in which external and internal factors including cytokines, growth factors, transcription factors (TFs), signaling pathways and microRNAs (miRNAs) play important roles. Various stimulatory and inhibitory TFs contribute to osteogenic differentiation and are responsible for bone development. In addition, cross-talk between several complex signaling pathways regulates the osteogenic differentiation of some stem cells. Although much is known about regulatory genes and signaling pathways in osteogenesis, the role of miRNAs in osteogenic differentiation still needs to be explored. miRNAs are small, approximately 22 nucleotides, single-stranded nonprotein coding RNAs which are abundant in many mammalian cell types. They paly significant regulated roles in various biological processes and serve as promising biomarkers for disease states. Recently, emerging evidence have shown that miRNAs are the key regulators of osteogenesis of stem cells. They may endogenously regulate osteogenic differentiation of stem cells through direct targeting of positive or negative directors of osteogenesis and depending on the target result in the promotion or inhibition of osteogenic differentiation. This review aims to provide a general overview of miRNAs participating in osteogenic differentiation of stem cells and explain their regulatory effect based on the genes targeted with these miRNAs.

Analysis of Signaling Pathways by Western Blotting and Immunoprecipitation

This chapter describes the analysis of signaling pathways in bone cells by the use of western blotting and immunoprecipitation, including a step-by-step guide to cell culture techniques, cellular and subcellular fractionation, protein isolation, purification, measurement, electrophoretic transfer, and detection.

Anabolic Therapies in Osteoporosis and Bone Regeneration

Osteoporosis represents the most common bone disease worldwide and results in a significantly increased fracture risk. Extrinsic and intrinsic factors implicated in the development of osteoporosis are also associated with delayed fracture healing and impaired bone regeneration. Based on a steadily increasing life expectancy in modern societies, the global implications of osteoporosis and impaired bone healing are substantial. Research in the last decades has revealed several molecular pathways that stimulate bone formation and could be targeted to treat both osteoporosis and impaired fracture healing. The identification and development of therapeutic approaches modulating bone formation, rather than bone resorption, fulfils an essential clinical need, as treatment options for reversing bone loss and promoting bone regeneration are limited. This review focuses on currently available and future approaches that may have the potential to achieve these aims.

Different duration of parathyroid hormone exposure distinctively regulates primary response genes Nurr1 and RANKL in osteoblasts

Parathyroid hormone (PTH) exerts dual effects, anabolic or catabolic, on bone when administrated intermittently or continuously, via mechanisms that remain largely unknown. PTH binding to cells induces PTH-responsive genes including primary response genes (PRGs). PRGs are rapidly induced without the need for de novo protein synthesis, thereby playing pivotal roles in directing subsequent molecular responses. In this study, to understand the role of PRGs in mediating osteoblastic cellular responses to PTH, we investigated whether various durations of PTH differentially induce PRGs in primary osteoblasts and MC3T3-E1. Nurr1 and RANKL, PRGs known for their anabolic and catabolic roles in bone metabolism respectively, presented distinctive transient vs. sustained induction kinetics. Corroborating their roles, maximum induction of Nurr1 was sufficiently achieved by brief PTH in as little as 30 minutes and continued beyond that, while maximum induction of RANKL was achieved only by prolonged PTH over 4 hours. Our data suggested distinctive regulatory mechanisms for Nurr1 and RANKL: PKA-mediated chromatin rearrangement for transcriptional regulation of both PRGs and ERK-mediated transcriptional regulation for RANKL but not Nurr1. Lastly, we classified PRGs into two groups based on the induction kinetics: The group that required brief PTH for maximum induction included Nur77, cox-2, and Nurr1, all of which are reported to play roles in bone formation. The other group that required prolonged PTH for maximum induction included IL-6 and RANKL, which play roles in bone resorption. Together, our data suggested the crucial role of PRG groups in mediating differential osteoblastic cellular responses to intermittent vs. continuous PTH. Continued research into the regulatory mechanisms of PKA and ERK for PRGs will help us better understand the molecular mechanisms underlying the dual effects of PTH, thereby optimizing the current therapeutic use of PTH for osteoporosis.

Examining the influence of PTH(1-34) on tissue strength and composition

The lacunar-canaliculi system is a network of channels that is created and maintained by osteocytes as they are embedded throughout cortical bone. As osteocytes modify their lacuna space, the local tissue composition and tissue strength are subject to change. Although continual exposure to parathyroid hormone (PTH) can induce adaptation at the lacunar wall, the impact of intermittent PTH treatment on perilacunar adaptation remains unclear. Therefore, the primary objective of this study was to establish how intermittent PTH(1-34) treatment influences perilacunar adaptation with respect to changes in tissue composition. We hypothesized that local changes in tissue composition following PTH(1-34) are associated with corresponding gains in tissue strength and resistance to microdamage at the whole bone level. Adult male C57BL/6J mice were treated daily with PTH(1-34) or vehicle for 3 weeks. In response to PTH(1-34), Raman spectroscopy revealed a significant decrease in the carbonate-to-phosphate ratio and crystallinity across the entire tissue, while the mineral-to-matrix ratio demonstrated a significant decrease in just the perilacunar region. The shift in perilacunar composition largely explained the corresponding increase in tissue strength, while the degree of new tissue added at the endosteum and periosteum did not produce any significant changes in cortical area or moment of inertia that would explain the increase in tissue strength. Furthermore, fatigue testing revealed a greater resistance to crack formation within the existing tissue following PTH(1-34) treatment. As a result, the shift in perilacunar composition presents a unique mechanism by which PTH(1-34) produces localized differences in tissue quality that allow more energy to be dissipated under loading, thereby increasing tissue strength and resistance to microdamage. In addition, our findings demonstrate the potential for PTH(1-34) to amplify osteocytes' mechanotransduction by producing a more compliant tissue. Overall, the present study demonstrates that changes in tissue composition localized at the lacuna wall contribute to the strength and fatigue resistance of cortical bone gained in response to intermittent PTH(1-34) treatment.

Phospholipase C signaling activated by parathyroid hormone mediates the rapid osteoclastogenesis in the fracture healing of orchiectomized mice

Background

The age-related osteoporosis is an increasing risk severely threatening the live quality of aged people. Human parathyroid hormone (hPTH) is applied to the therapy of osteoporosis successfully, however, the mechanism, especially the signaling pathway activated in the healing fracture by PTH is still unknown.

Methods

The once daily injections of hPTH(1–34) and GR (1–34) (the PLC deficient analog) into the orchiectomized male mice with bone fracture, were started at the second day after fracture and lasted for 4 weeks. To explore the role of phospholipase C signaling in the androgen-deficient fracture healing, the fracture healing were evaluated via radiography, micro-CT, biomechanics testing, serum biochemistry, bone marrow cell culture and gene expression quantification.

Results

After two weeks of fracture, both peptides significantly increased bone mineral density (BMD), bone mass content (BMC) and bone volume (BV/TV) in the healing area. However, compared to hPTH(1–34), GR(1–34) induced more woven bones, the higher BMC and BMD, as well as the less serum TRAP and osteoclasts. After four weeks of treatment, the effects of hPTH(1–34) on fracture healing showed no difference to those of GR(1–34). Consistently, GR(1–34) induced the similar osteogenesis but less osteoclastogenesis under the ex vivo condition immediately after administration compared to hPTH(1–34), which was verified by the weaker activation of RANKL, NFATC1, TRAP and Cathepsin K in GR(1–34) treatment.

Conclusion

These results indicated that the PLC signaling activated by the intermittent injection of hPTH(1–34) leads to the bone resorption by rapidly activating the osteoclastogenesis in the fracture healing zone.

Electronic supplementary material

The online version of this article (10.1186/s12891-018-2231-3) contains supplementary material, which is available to authorized users.

Regulation of Bone Remodeling by Parathyroid Hormone

Parathyroid hormone (PTH) exerts profound effects on skeletal homeostasis through multiple cellular and molecular mechanisms. Continuous hyperparathyroidism causes net loss of bone mass, despite accelerating bone formation by osteoblasts. Intermittent treatment with PTH analogs represents the only Food and Drug Administration (FDA)-approved bone anabolic osteoporosis treatment strategy. Functional PTH receptors are present on cells of the osteoblast lineage, ranging from early skeletal stem cells to matrix-embedded osteocytes. In addition, bone remodeling by osteoclasts liberates latent growth factors present within bone matrix. Here, we will provide an overview of the multiple cellular and molecular mechanisms through which PTH influences bone homeostasis. Notably, net skeletal effects of continuous versus intermittent can differ significantly. Where possible, we will highlight mechanisms through which continuous hyperparathyroidism leads to bone loss, and through which intermittent hyperparathyroidism boosts bone mass. Given the therapeutic usage of intermittent PTH (iPTH) treatment for osteoporosis, particular attention will be paid toward mechanisms underlying the bone anabolic effects of once daily PTH administration.

Ciliary parathyroid hormone signaling activates transforming growth factor-β to maintain intervertebral disc homeostasis during aging

Degenerative disc disease (DDD) is associated with intervertebral disc degeneration of spinal instability. Here, we report that the cilia of nucleus pulposus (NP) cells mediate mechanotransduction to maintain anabolic activity in the discs. We found that mechanical stress promotes transport of parathyroid hormone 1 receptor (PTH1R) to the cilia and enhances parathyroid hormone (PTH) signaling in NP cells. PTH induces transcription of integrin α_vβ₆ to activate the transforming growth factor (TGF)-β-connective tissue growth factor (CCN2)-matrix proteins signaling cascade. Intermittent injection of PTH (iPTH) effectively attenuates disc degeneration of aged mice by direct signaling through NP cells, specifically improving intervertebral disc height and volume by increasing levels of TGF-β activity, CCN2, and aggrecan. PTH1R is expressed in both mouse and human NP cells. Importantly, knockout PTH1R or cilia in the NP cells results in significant disc degeneration and blunts the effect of PTH on attenuation of aged discs. Thus, mechanical stress-induced transport of PTH1R to the cilia enhances PTH signaling, which helps maintain intervertebral disc homeostasis, particularly during aging, indicating therapeutic potential of iPTH for DDD.

Sensory structures found in the jelly-like space between spinal discs release a hormone that helps preserve back health in aging mice. Xu Cao from Johns Hopkins University in Baltimore, Maryland, USA, and colleagues observed that levels of a critical growth factor declined in the space between adjacent vertebrae as mice aged, and that injecting a naturally occurring hormone that activates this growth factor could attenuate disc degeneration in older animals. The researchers showed, in response to mechanical stresses, receptor proteins that respond to this hormone relocate themselves to particular sensory organelles known as cilia that found within cells of the intervertebral core. That results in elevated hormone signaling—and drugs designed to have the same effect could help treat degenerative disc disease, one of the most common causes of chronic back pain.

Lactate enhanced the effect of parathyroid hormone on osteoblast differentiation via GPR81-PKC-Akt signaling

Osteoblast uses aerobic glycolysis to meet the metabolic needs in differentiation process. Lactate, the end product of glycolysis, presents in the environment with elevated PTH and osteoblast differentiation. Although previous findings showed that lactate promoted osteoblast differentiation, whether lactate affects PTH-mediated osteoblast differentiation is unclear. To investigate this, pre-osteoblast cell line MC3T3-E1 was treated PTH with or without physiological dose of lactate. Lactate increases ALP positive cell formation, increases ALP activity and expression of differentiation related markers, enriches the CREB transcriptional factor target genes in PTH treated cells. Using inhibitors for MCT-1 reveales that lactate effects are MCT-1 independent. Lactate selectively increases Akt and p38 activation but not Erk1/2 and β-Catenin activation. The inhibitors for Akt and p38 inhibit lactate effects on PTH mediated osteoblast differentiation. Using inhibitors for Gαi signaling of GPR81 further increases Alp mRNA levels in lactate and PTH co-treatment cells. However, with the inhibitors for Gβγ-PLC-PKC signaling, the effect of lactate on PTH mediated osteoblast differentiation is inhibited. Our data demonstrate that lactate activates GPR81-Gβγ-PLC-PKC-Akt signaling to regulate osteoblast differentiation that mediated by PTH treatment.

Parathyroid hormone-induced down-regulation of miR-532-5p for matrix metalloproteinase-13 expression in rat osteoblasts

Parathyroid hormone (PTH) acts on osteoblasts and functions as an essential regulator of calcium homeostasis and as a mediator of bone remodeling. We previously reported that PTH stimulates the expression of matrix metalloproteinase-13 (MMP-13) in rat osteoblasts and that MMP-13 plays a key role in bone remodeling, endochondral bone formation, and bone repair. Recent evidence indicated that microRNAs (miRNAs) have regulatory functions in bone metabolism. In this study, we hypothesized that the down-regulation of miRNAs that target MMP-13 by PTH leads to the stimulation of MMP-13 expression in osteoblasts. We used various bioinformatic tools to identify miRNAs that putatively target rat MMP-13. Among these miRNAs, the expression of miR-532-5p in rat osteoblasts decreased at 4 h of PTH-treatment, whereas MMP-13 mRNA expression was maximal at the same time point. When an miR-532-5p mimic was transiently transfected into UMR-106-01 cells, MMP-13 mRNA and protein expression decreased. Using a luciferase reporter assay system, we also identified that miR-532-5p directly targeted the 3' UTRs of MMP-13 gene. Based on these results, we suggest that PTH-induced down-regulation of miR-532-5p resulted in the stimulation of MMP-13 expression in rat osteoblasts. This study identified a significant role of miRNA in controlling bone remodeling via PTH-stimulated MMP-13 expression. This finding enhances our understanding of bone metabolism and bone-related diseases and it could provide information regarding the usage of miRNAs as therapeutic agents or biomarkers.

Immunoporosis: Immunology of Osteoporosis-Role of T Cells

The role of immune system in various bone pathologies, such as osteoporosis, osteoarthritis, and rheumatoid arthritis is now well established. This had led to the emergence of a modern field of systems biology called as osteoimmunology, an integrated research between fields of immunology and bone biology under one umbrella. Osteoporosis is one of the most common inflammatory bone loss condition with more than 200 million individuals affected worldwide. T helper (Th) cells along with various other immune cells are major players involved in bone homeostasis. In the present review, we specifically discuss the role of various defined T lymphocyte subsets (Th cells comprising Th1, Th2, Th9, Th17, Th22, regulatory T cells, follicular helper T cells, natural killer T cells, γδ T cells, and CD8⁺ T cells) in the pathophysiology of osteoporosis. The study of the specific role of immune system in osteoporosis has now been proposed by our group as “immunoporosis: the immunology of osteoporosis” with special emphasis on the role of various subsets of T lymphocytes. The establishment of this new field had been need of the hour due to the emergence of novel roles of various T cell lymphocytes in accelerated bone loss observed during osteoporosis. Activated T cells either directly or indirectly through the secretion of various cytokines and factors modulate bone health and thereby regulate bone remodeling. Several studies have summarized the role of inflammation in pathogenesis of osteoporosis but very few reports had delineated the precise role of various T cell subsets in the pathobiology of osteoporosis. The present review thus for the first time clearly highlights and summarizes the role of various T lymphocytes in the development and pathophysiology of osteoporosis, giving birth to a new field of biology termed as “immunoporosis”. This novel field will thus provide an overview of the nexus between the cellular components of both bone and immune systems, responsible for the observed bone loss in osteoporosis. A molecular insight into the upcoming and novel field of immunoporosis would thus leads to development of innovative approaches for the prevention and treatment of osteoporosis.

IGF-I induced phosphorylation of PTH receptor enhances osteoblast to osteocyte transition

Parathyroid hormone (PTH) regulates bone remodeling by activating PTH type 1 receptor (PTH1R) in osteoblasts/osteocytes. Insulin-like growth factor type 1 (IGF-1) stimulates mesenchymal stem cell differentiation to osteoblasts. However, little is known about the signaling mechanisms that regulates the osteoblast-to-osteocyte transition. Here we report that PTH and IGF-I synergistically enhance osteoblast-to-osteocyte differentiation. We identified that a specific tyrosine residue, Y494, on the cytoplasmic domain of PTH1R can be phosphorylated by insulin-like growth factor type I receptor (IGF1R) in vitro. Phosphorylated PTH1R localized to the barbed ends of actin filaments and increased actin polymerization during morphological change of osteoblasts into osteocytes. Disruption of the phosphorylation site reduced actin polymerization and dendrite length. Mouse models with conditional ablation of PTH1R in osteoblasts demonstrated a reduction in the number of osteoctyes and dendrites per osteocyte, with complete overlap of PTH1R with phosphorylated-PTH1R positioning in osteocyte dendrites in wild-type mice. Thus, our findings reveal a novel signaling mechanism that enhances osteoblast-to-osteocyte transition by direct phosphorylation of PTH1R by IGF1R.

A key hormone and growth factor work together to help turn bone-forming cells into mature bone. Janet Crane and colleagues from Johns Hopkins University School of Medicine in Baltimore, Maryland, USA, tested the effects of parathyroid hormone (PTH) and insulin like-growth factor type 1 (IGF-1) signaling on the differentiation of bone-forming osteoblasts by modulating the activity of their receptors in genetically engineered mice. They found a specific part of the PTH type 1 receptor has a phosphate group added to it by the IGF-1 receptor. This chemical tagging leads to changes in the cytoskeleton of osteoblasts that enhance the formation of mature bone cells known as osteocytes. Mice without this PTH receptor had reduced numbers of osteocytes in their bone. The findings reveal a novel signaling mechanism behind this cellular transition during bone building.

Vascular Calcification: Current Genetics Underlying This Complex Phenomenon

OBJECTIVE

Vascular calcification is the consequence of the complex interaction between genetic, environmental, and vascular factors, which ultimately lead to the deposition of calcium in the tunica intima (atherosclerotic calcification) or tunica media (Mönckenberg's sclerosis). Vascular calcification is also closely related to other pathologies, such as diabetes mellitus, dyslipidemia, and chronic kidney disease. It has been concluded that the degree of vascular calcification may vary from person to person, even if the associated pathologies and environmental factors are the same. Therefore, this suggests an important genetic contribution to the development of vascular calcification. This review aimed to find the most recent evidence about vascular calcification pathophysiology regarding the genetic aspects and molecular pathways.

DATA SOURCES

We conducted an exhaustive search in Scopus, EBSCO, and PubMed with the keywords "genetics and vascular calcification", "molecular pathways, genetic and vascular calcification" and included the main articles from January 1995 up to August 2016. We focused on the most recent evidence about vascular calcification pathophysiology regarding the genetic aspects and molecular pathways.

STUDY SELECTION

The most valuable published original and review articles related to our objective were selected.

RESULTS

Vascular calcification is a multifactorial disease; thus, its pathophysiology cannot be explained by a single specific factor, rather than by the result of the association of several genetic variants, molecular pathway interactions, and environmental factors that promote its development.

CONCLUSION

Although several molecular aspects of this mechanism have been elucidated, there is still a need for a better understanding of the factors that predispose to this disease.

Lrp6 is a target of the PTH-activated αNAC transcriptional coregulator

In the nucleus of differentiated osteoblasts, the alpha chain of nascent polypeptide associated complex (αNAC) interacts with cJUN transcription factors to regulate the expression of target genes, including Osteocalcin (Bglap2). PTH induces the phosphorylation of αNAC on serine 99 through a Gαs-PKA dependent pathway. This leads to activation of αNAC and expression of Bglap2. To identify additional target genes regulated by PTH-activated αNAC, we performed ChIP-Seq against αNAC in PTH-treated MC3T3-E1 cells. This identified Low density lipoprotein receptor-Related Protein 6 (Lrp6) as a potential αNAC target. LRP6 acts as a co-receptor for the PTH receptor to allow optimal activation of PTH signaling. PTH increased Lrp6 mRNA levels in primary osteoblasts. Conventional quantitative ChIP confirmed the ChIP-Seq results. To assess whether αNAC plays a critical role in PTH-stimulated Lrp6 expression, we knocked-down Naca expression in MC3T3-E1 cells. Reduction of αNAC levels decreased basal expression of Lrp6 by 30% and blocked the stimulation of Lrp6 expression by PTH. We cloned the proximal mouse Lrp6 promoter (-2523/+120 bp) upstream of the luciferase reporter. Deletion and point mutations analysis in electrophoretic mobility shift assays and transient transfections identified a functional αNAC binding site centered around -343 bp. ChIP and ChIP-reChIP against JUND and αNAC showed that they cohabit on the proximal Lrp6 promoter. Luciferase assays confirmed that PTH-activated αNAC potentiated JUND-mediated Lrp6 transcription and Jund knockdown abolished this response. This study identified a novel αNAC target gene induced downstream of PTH signaling and represents the first characterization of the regulation of Lrp6 transcription in osteoblasts.

Cellular and molecular mechanisms of alcohol-induced osteopenia

Alcoholic beverages are widely consumed, resulting in a staggering economic cost in different social and cultural settings. Types of alcohol consumption vary from light occasional to heavy, binge drinking, and chronic alcohol abuse at all ages. In general, heavy alcohol consumption is widely recognized as a major epidemiological risk factor for chronic diseases and is detrimental to many organs and tissues, including bones. Indeed, recent findings demonstrate that alcohol has a dose-dependent toxic effect in promoting imbalanced bone remodeling. This imbalance eventually results in osteopenia, an established risk factor for osteoporosis. Decreased bone mass and strength are major hallmarks of osteopenia, which is predominantly attributed not only to inhibition of bone synthesis but also to increased bone resorption through direct and indirect pathways. In this review, we present knowledge to elucidate the epidemiology, potential pathogenesis, and major molecular mechanisms and cellular effects that underlie alcoholism-induced bone loss in osteopenia. Novel therapeutic targets for correcting alcohol-induced osteopenia are also reviewed, such as modulation of proinflammatory cytokines and Wnt and mTOR signaling and the application of new drugs.

Intermittent Administration of Parathyroid Hormone 1-34 Enhances Osteogenesis of Human Mesenchymal Stem Cells by Regulating Protein Kinase Cδ

Human mesenchymal stem cells (hMSCs) can differentiate into osteoblasts and are regulated by chemical cues. The recombinant N-terminal (1–34 amino acids) fragment of the parathyroid hormone (PTH (1–34)) is identified to promote osteogenesis. The osteoanabolic effects of intermittent PTH (1–34) treatment are linked to a complex consisting of signaling pathways; additionally, protein kinase C (PKC) act as mediators of multifunctional signaling transduction pathways, but the role of PKC δ (PKCδ), a downstream target in regulating osteoblast differentiation during intermittent administration of PTH (1–34) is less studied and still remains elusive. The purpose of this study is to examine the role of PKCδ during intermittent and continuous PTH (1–34) administration using osteoblast-lineage-committed hMSCs. Relative gene expression of osteoblast-specific genes demonstrated significant upregulation of RUNX2, type I Collagen, ALP, and Osterix and increased alkaline phosphatase activity in the presence of PTH (1–34). Intermittent PTH (1–34) administration increased PKC activity at day 7 of osteogenic differentiation, whereas inhibition of PKC activity attenuated these effects. In addition, the specific isoform PKCδ was activated upon treatment. These findings demonstrate that intermittent PTH (1–34) treatment enhances the osteogenesis of hMSCs by upregulating osteoblast-specific genes via PKCδ activation.

Adrenomedullin promotes the odontogenic differentiation of dental pulp stem cells through CREB/BMP2 signaling pathway

Adrenomedullin (AM) could promote the proliferation, the odontogenic differentiation and inhibit the apoptosis of dental pulp stem cells (DPSCs). AM in combination with DPSCs may be an effective strategy for pulp repair. However, there was no report on the mechanisms of AM in the odontogenic differentiation of DPSCs. The aim of this study is to investigate the molecular mechanisms through which AM promotes the odontogenic differentiation of DPSCs. Freshly extracted wisdom teeth were obtained from 27 patients. Cells at passage 3 to passage 5 were used in this study. DPSCs were treated with or without 10-7 M AM in Dulbecco's modified Eagle's medium culture, and then the accumulated calcium deposition was analyzed after 21 days by using alizarin red S staining. Odontogenic differentiation markers were determined by western blot analysis and quantitative real-time PCR. Western blot analysis results showed that AM had the capability of promoting the odontogenic differentiation of DPSCs and AM could enhance the phosphorylation of CREB and up-regulate the expression of BMP2. H89 is a CREB inhibitor which can inhibit the odontogenic differentiation of DPSCs through inhibiting the phosphorylation of CREB. Noggin could inhibit the odontogenic differentiation of DPSCs through inhibiting the activity of BMP2. These results indicated that AM could promote the odontogenic differentiation of DPSCs by upregulating the expression of BMP2 through the CREB signaling pathway.