Evidence that the cells responsible for marrow fibrosis in a rat model for hyperparathyroidism are preosteoblasts

Increased Osteoblast GαS Promotes Ossification by Suppressing Cartilage and Enhancing Callus Mineralization During Fracture Repair in Mice

GαS, the stimulatory G protein α-subunit that raises intracellular cAMP levels by activating adenylyl cyclase, plays a vital role in bone development, maintenance, and remodeling. Previously, using transgenic mice overexpressing GαS in osteoblasts (GS-Tg), we demonstrated the influence of osteoblast GαS level on osteogenesis, bone turnover, and skeletal responses to hyperparathyroidism. To further investigate whether alterations in GαS levels affect endochondral bone repair, a postnatal bone regenerative process that recapitulates embryonic bone development, we performed stabilized tibial osteotomy in male GS-Tg mice at 8 weeks of age and examined the progression of fracture healing by micro-CT, histomorphometry, and gene expression analysis over a 4-week period. Bone fractures from GS-Tg mice exhibited diminished cartilage formation at the time of peak soft callus formation at 1 week post-fracture followed by significantly enhanced callus mineralization and new bone formation at 2 weeks post-fracture. The opposing effects on chondrogenesis and osteogenesis were validated by downregulation of chondrogenic markers and upregulation of osteogenic markers. Histomorphometric analysis at times of increased bone formation (2 and 3 weeks post-fracture) revealed excess fibroblast-like cells on newly formed woven bone surfaces and elevated osteocyte density in GS-Tg fractures. Coincident with enhanced callus mineralization and bone formation, GS-Tg mice showed elevated active β-catenin and Wntless proteins in osteoblasts at 2 weeks post-fracture, further substantiated by increased mRNA encoding various canonical Wnts and Wnt target genes, suggesting elevated osteoblastic Wnt secretion and Wnt/β-catenin signaling. The GS-Tg bony callus at 4 weeks post-fracture exhibited greater mineral density and decreased polar moment of inertia, resulting in improved material stiffness. These findings highlight that elevated GαS levels increase Wnt signaling, conferring an increased osteogenic differentiation potential at the expense of chondrogenic differentiation, resulting in improved mechanical integrity. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Pathophysiology of bone disease in chronic kidney disease: from basics to renal osteodystrophy and osteoporosis

Chronic kidney disease (CKD) is a highly prevalent disease that has become a public health problem. Progression of CKD is associated with serious complications, including the systemic CKD-mineral and bone disorder (CKD-MBD). Laboratory, bone and vascular abnormalities define this condition, and all have been independently related to cardiovascular disease and high mortality rates. The "old" cross-talk between kidney and bone (classically known as "renal osteodystrophies") has been recently expanded to the cardiovascular system, emphasizing the importance of the bone component of CKD-MBD. Moreover, a recently recognized higher susceptibility of patients with CKD to falls and bone fractures led to important paradigm changes in the new CKD-MBD guidelines. Evaluation of bone mineral density and the diagnosis of "osteoporosis" emerges in nephrology as a new possibility "if results will impact clinical decisions". Obviously, it is still reasonable to perform a bone biopsy if knowledge of the type of renal osteodystrophy will be clinically useful (low versus high turnover-bone disease). However, it is now considered that the inability to perform a bone biopsy may not justify withholding antiresorptive therapies to patients with high risk of fracture. This view adds to the effects of parathyroid hormone in CKD patients and the classical treatment of secondary hyperparathyroidism. The availability of new antiosteoporotic treatments bring the opportunity to come back to the basics, and the knowledge of new pathophysiological pathways [OPG/RANKL (LGR4); Wnt-ß-catenin pathway], also affected in CKD, offers great opportunities to further unravel the complex physiopathology of CKD-MBD and to improve outcomes.

Abaloparatide Has the Same Catabolic Effects on Bones of Mice When Infused as PTH (1-34)

Abaloparatide is a peptide analog of parathyroid hormone-related protein (PTHrP 1-34) and was approved in 2017 as the second osteoanabolic peptide for treating osteoporosis. We previously showed that intermittent abaloparatide is equally as effective as PTH (1-34). This study was designed to compare the catabolic effects of PTH (1-34) and abaloparatide on bone in young female wild-type mice. Two-month-old C57Bl/6J female mice were continuously infused with human PTH (1-34) or abaloparatide at 80 μg/kg BW/day or vehicle for 2 weeks. At euthanasia, DEXA-PIXImus was performed to assess bone mineral density (BMD) in the whole body, femurs, tibiae, and vertebrae. Bone turnover marker levels were measured in sera, femurs were harvested for micro-computer tomography (μCT) analyses and histomorphometry, and tibiae were separated into cortical and trabecular fractions for gene expression analyses. Our results demonstrated that the infusion of abaloparatide resulted in a similar decrease in BMD as infused PTH (1-34) at all sites. μCT and histomorphometry analyses showed similar decreases in cortical bone thickness and BMD associated with an increase in bone turnover from the increased bone formation rate found by in vivo double labeling and serum P1NP and increased bone resorption as shown by osteoclast numbers and serum cross-linked C-telopeptide. Trabecular bone did not show major changes with either treatment. Osteoblastic gene expression analyses of trabecular and cortical bone revealed that infusion of PTH (1-34) or abaloparatide led to similar and different actions in genes of osteoblast differentiation and activity. As with intermittent and in vitro treatment, both infused PTH (1-34) and abaloparatide similarly regulated downstream genes of the PTHR1/SIK/HDAC4 pathway such as Sost and Mmp13 but differed for those of the PTHR1/SIK/CRTC pathway. Taken together, at the same dose, infused abaloparatide causes the same high bone turnover as infused PTH (1-34) with a net resorption in female wild-type mice. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Generalized Uncoupled Bone Remodeling Associated With Delayed Healing of Fatigue Fractures

Fatigue fractures in bones are common injuries with load‐bearing activities, during which the remodeling aimed at removing microdamage has been suggested to play a role in increasing related fracture risk. Much attention has been given to the uncoupling between osteoclastic bone resorption and osteoblastic osteogenesis in fatigue fracture cases; however, the underlying pathophysiologic mechanisms of impaired fracture healing are yet unknown. Here we report multiple fatigue fractures in a physically active woman receiving contraceptive pills for years. Her fracture healing was remarkably slow, although she has been otherwise healthy. The patient underwent bone biopsy of the iliac crest that showed remarkable peritrabecular fibrosis with increased osteoclastic bone resorption combined with relatively low bone formation. Analysis of bone biochemical composition revealed a more complex picture: First, notable declines in bone mineral content–based parameters indicating abnormal mineralization were evident in both cancellous and cortical bone. Second, there was elevation in mineral crystal size, perfection, and collagen maturity in her bone tissues from different anatomical sites. To our knowledge, this is the first report showing generalized uncoupling in bone remodeling, increased peritrabecular fibrosis, and bone compositional changes associated with delayed healing of fatigue fractures. These results may explain delayed healing of fatigue and stress fractures. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Skeletal and mineral metabolic effects of risedronate in a rat model of high-turnover renal osteodystrophy

INTRODUCTION

High-turnover bone disease is a major consequence of SHPT and may explain the high risk for fracture in patients with advanced chronic kidney disease (CKD). Bisphosphonates suppress bone turnover and improve bone strength, but their effects have not been fully characterized in advanced CKD with severe SHPT. Bisphosphonates also increase 1,25-dihydroxyvitamin D levels in normal and uremic rats, but the underlying mechanism remains to be determined.

MATERIALS AND METHODS

We investigated the skeletal and mineral metabolic effects of RIS, a pyridinyl bisphosphonate, in rats with severe SHPT induced by 5/6 nephrectomy plus a high phosphate diet.

RESULTS

Nephrectomized rats developed severe SHPT, along with hyperphosphatemia, low 1,25-dihydroxyvitamin D, and markedly increased FGF23. Moreover, these rats exhibited characteristic features of high-turnover renal osteodystrophy, including increased indices of trabecular bone turnover, decreased cortical bone thickness, inferior cortical biomechanical properties, and a prominent increase in peritrabecular fibrosis. RIS treatment increased bone volume and partially attenuated trabecular bone remodeling, cortical bone loss, and mechanical properties, whereas it produced a marked improvement in peritrabecular fibrosis along with a corresponding decrease in osteogenic gene markers. RIS treatment also suppressed the elevation of FGF23, which was associated with increased 1,25-dihydroxyvitamin D.

CONCLUSIONS

In a rat model of severe SHPT, treatment with RIS partially attenuated histological manifestations of high-turnover bone disease. RIS treatment also suppressed the elevation of FGF23, which may explain the increased 1,25-dihydroxyvitamin D production during the treatment.

Magnesium: An old player revisited in the context of CKD-MBD

Chronic kidney disease (CKD) is associated with a wide number of abnormalities in mineral metabolism. Often, these alterations are the leading players in the development of comorbidities associated with CKD, which are risk factors of mortality. In this context, mineral and bone disorder associated with CKD (CKD-MBD) are highlighted, connecting bone, renal, and cardiovascular disorders. Many studies have been led to propose strategies to avoid, reduce, or slow down CKD-MBD progression using different compositions of metallic elements-based P binders such as aluminum, magnesium, or calcium. Magnesium, the aim of this review, has been used by nephrologists to treat CKD-MBD with a variable acceptation due mainly to different results on bone homeostasis. Nowadays, we have new evidence about the efficacy of magnesium supplementation on vascular calcification, renal function, and bone disorders, suggesting potential beneficial effects of Magnesium in the management of CKD-MBD.

Parathyroid Hormone Shifts Cell Fate of a Leptin Receptor-Marked Stromal Population from Adipogenic to Osteoblastic Lineage

Intermittent parathyroid hormone (iPTH) treatment induces bone anabolic effects that result in the recovery of osteoporotic bone loss. Human PTH is usually given to osteoporotic patients because it induces osteoblastogenesis. However, the mechanism by which PTH stimulates the expansion of stromal cell populations and their maturation toward the osteoblastic cell lineage has not be elucidated. Mouse genetic lineage tracing revealed that iPTH treatment induced osteoblastic differentiation of bone marrow (BM) mesenchymal stem and progenitor cells (MSPCs), which carried the leptin receptor (LepR)-Cre. Although these findings suggested that part of the PTH-induced bone anabolic action is exerted because of osteoblastic commitment of MSPCs, little is known about the in vivo mechanistic details of these processes. Here, we showed that LepR⁺ MSPCs differentiated into type I collagen (Col1)⁺ mature osteoblasts in response to iPTH treatment. Along with osteoblastogenesis, the number of Col1⁺ mature osteoblasts increased around the bone surface, although most of them were characterized as quiescent cells. However, the number of LepR-Cre-marked lineage cells in a proliferative state also increased in the vicinity of bone tissue after iPTH treatment. The expression levels of SP7/osterix (Osx) and Col1, which are markers for osteoblasts, were also increased in the LepR⁺ MSPCs population in response to iPTH treatment. In contrast, the expression levels of Cebpb, Pparg, and Zfp467, which are adipocyte markers, decreased in this population. Consistent with these results, iPTH treatment inhibited 5-fluorouracil- or ovariectomy (OVX)-induced LepR⁺ MSPC-derived adipogenesis in BM and increased LepR⁺ MSPC-derived osteoblasts, even under the adipocyte-induced conditions. Treatment of OVX rats with iPTH significantly affected the osteoporotic bone tissue and expansion of the BM adipose tissue. These results indicated that iPTH treatment induced transient proliferation of the LepR⁺ MSPCs and skewed their lineage differentiation from adipocytes toward osteoblasts, resulting in an expanded, quiescent, and mature osteoblast population. © 2019 American Society for Bone and Mineral Research.

Silicosis decreases bone mineral density in rats

Silicosis is the most common occupational lung disease in China, and is associated with a variety of complications, many of which are poorly understood. For example, recent data indicate that silicosis associates with the development of osteopenia, and in some cases this bone loss is severe, meeting criteria for osteoporosis. Although many factors are likely to contribute to this relationship, including a sedentary lifestyle in patients with advanced silicotic lung disease, we hypothesized that silica might directly reduce bone mineral density. In the present study, six Wistar rats were exposed to silica for 24 weeks in order to induce pulmonary silicosis and examine the relationship to bone mineral density. As expected, all rats exposed to silica developed severe pulmonary fibrosis, as manifested by the formation of innumerable silicotic nodules and the deposition of large amounts of interstitial collagen. Moreover, micro-CT results showed that bone mineral density (BMD) was also significantly reduced in rats exposed to silica when compared control animals and this associated with a modest reduction in serum calcium and 25-hydroxyvitamin D levels. In addition, we found that decreased BMD was also linked to increased osteoclast activity as well as fibrosis-like changes, and to the deposition of silica within bone marrow. In summary, our findings support the hypothesis that silicosis reduces bone mineral density and provide support for ongoing investigations into the mechanisms causing osteopenia in silicosis patients.

Osteogenic Factor Runx2 Marks a Subset of Leptin Receptor-Positive Cells that Sit Atop the Bone Marrow Stromal Cell Hierarchy

Bone marrow mesenchymal stem and progenitor cells (BM-MSPCs) maintain homeostasis of bone tissue by providing osteoblasts. Although several markers have been identified for labeling of MSPCs, these labeled cells still contain non-BM-MSPC populations. Studies have suggested that MSPCs are observed as leptin receptor (LepR)-positive cells, whereas osteoblasts can be classified as positive for Runx2, a master regulator for osteoblastogenesis. Here, we demonstrate, using Runx2-GFP reporter mice, that the LepR-labeled population contains Runx2-GFP^low sub-population, which possesses higher fibroblastic colony-forming units (CFUs) and mesensphere capacity, criteria for assessing stem cell activity, than the Runx2-GFP⁻ population. In response to parathyroid hormone (PTH), a bone anabolic hormone, LepR⁺Runx2-GFP^low cells increase Runx2 expression and form multilayered structures near the bone surface. Subsequently, the multilayered cells express Osterix and Type I collagen α, resulting in generation of mature osteoblasts. Therefore, our results indicate that Runx2 is weakly expressed in the LepR⁺ population without osteoblastic commitment, and the LepR⁺Runx2-GFP^low stromal cells sit atop the BM stromal hierarchy.

Renal Osteodystrophy-Time for Common Nomenclature

PURPOSE OF REVIEW

The term renal osteodystrophy has been used to describe a wide variety of bone problems facing patients with chronic kidney disease (CKD). Here, we review the history of the use of this term.

RECENT FINDINGS

Bone disease resulting from CKD was first noticed in 1890. The term "renal osteodystrophy" was used to define the bone disease in 1942. Since then, important discoveries have increased our knowledge of the complexities of bone physiology in these patients. At the same time, secular changes in the disease have occurred. The terms used to describe the bone histological findings have changed as well, reflecting new understanding of the physiological processes. However, since different investigators used the terms in different ways, the need to standardize the nomenclature has become increasingly important. Ongoing international collaboration about nosography will allow more optimal communication among scientists and clinicians as we continue to make new discoveries.

Acute development of cortical porosity and endosteal naïve bone formation from the daily but not weekly short-term administration of PTH in rabbit

Teriparatide [human parathyroid hormone (1–34)], which exerts an anabolic effect on bone, is used for the treatment of osteoporosis in patients who are at a high risk for fracture. That the once-daily administration of teriparatide causes an increase in cortical porosity in animal models and clinical studies has been a matter of concern. However, it is not well documented that the frequency of administration and/or the total dose of teriparatide affect the cortical porosity. The present study developed 4 teriparatide regimens [20 μg/kg/day (D20), 40 μg/kg/day (D40), 140 μg/kg/week (W140) and 280 μg/kg/week (W280)] in the rabbit as a model animal with a well-developed Haversian system and osteons. The total weekly doses were equivalent in the low-dose groups (D20 and W140) and in the high-dose groups (D40 and W280). After the short-term (1 month) administration of TPDT, micro-CT, histomorphometry and three-dimensional second harmonic generation (3D-SHG) imaging to visualize the bone collagen demonstrated that daily regimens but not weekly regimens were associated with the significant development of cortical porosity and endosteal naïve bone formation by marrow fibrosis. We concomitantly monitored the pharmacokinetics of the plasma teriparatide levels as well as the temporal changes in markers of bone formation and resorption. The analyses in the present study suggested that the daily repeated administration of teriparatide causes more deleterious changes in the cortical microarchitecture than the less frequent administration of higher doses. The findings of the present study may have some implications for use of teriparatide in clinical treatment.

Nonproliferative and Proliferative Lesions of the Rat and Mouse Skeletal Tissues (Bones, Joints, and Teeth)

The INHAND (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) Project (www.toxpath.org/inhand.asp) is an initiative of the Societies of Toxicological Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP) and North America (STP) to develop an internationally accepted nomenclature for proliferative and nonproliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature for classifying microscopic lesions observed in the skeletal tissues and teeth of laboratory rats and mice, with color photomicrographs illustrating examples of many common lesions. The standardized nomenclature presented in this document is also available on the internet (http://www.goreni.org/). Sources of material were databases from government, academic and industrial laboratories throughout the world.

Skeletal inflammation and attenuation of Wnt signaling, Wnt ligand expression, and bone formation in atherosclerotic ApoE-null mice

ApoE-null (ApoE-KO) mice fed a high-fat diet (HFD) develop atherosclerosis, due in part to activation of vascular inflammation by oxidized low-density lipoprotein. Since bone loss also occurs in these mice, we used them to investigate the impact of oxidized lipids on bone homeostasis and to search for underlying pathogenic pathways. Four-month-old female ApoE-KO mice fed a HFD for three months exhibited increased levels of oxidized lipids in bone, as well as decreased femoral and vertebral trabecular and cortical bone mass, compared with ApoE-KO mice on normal diet. Despite HFD-induced increase in expression of Alox15, a lipoxygenase that oxidizes LDL and promotes atherogenesis, global deletion of this gene failed to ameliorate the skeletal impact of HFD. Osteoblast number and function were dramatically reduced in trabecular and cortical bone of HFD-fed mice, whereas osteoclast number was modestly reduced only in trabecular bone, indicating that an imbalance in favor of osteoclasts was responsible for HFD-induced bone loss. These changes were associated with decreased osteoblast progenitors and increased monocyte/macrophages in the bone marrow as well as increased expression of IL-1β, IL-6, and TNF. HFD also attenuated Wnt signaling as evidenced by reduced expression of Wnt target genes, and it decreased expression of pro-osteoblastogenic Wnt ligands. These results suggest that oxidized lipids decrease bone mass by increasing anti-osteoblastogenic inflammatory cytokines and decreasing pro-osteoblastogenic Wnt ligands.

Differential temporal effects of sclerostin antibody and parathyroid hormone on cancellous and cortical bone and quantitative differences in effects on the osteoblast lineage in young intact rats

Sclerostin antibody (Scl-Ab) and parathyroid hormone (PTH) are bone-forming agents that have different modes of action on bone, although a study directly comparing their effects has not been conducted. The present study investigated the comparative quantitative effects of these two bone-forming agents over time on bone at the organ, tissue, and cellular level; specifically, at the level of the osteoblast (Ob) lineage in adolescent male and female rats. Briefly, eight-week old male and female Sprague-Dawley rats were administered either vehicle, Scl-Ab (3 or 50mg/kg/week subcutaneously), or human PTH (1-34) (75 μg/kg/day subcutaneously) for 4 or 26 weeks. The 50mg/kg Scl-Ab and the PTH dose were those used in the respective rat lifetime pharmacology studies. Using robust stereological methods, we compared the effects of these agents specifically at the level of the Ob lineage in vertebrae from female rats. Using RUNX2 or nestin immunostaining, location, and morphology, the total number of osteoprogenitor subpopulations, Ob, and lining cells were estimated using the fractionator or proportionator estimators. Density estimates were also calculated referent to total bone surface, total Ob surface, or total marrow volume. Scl-Ab generally effected greater increases in cancellous and cortical bone mass than PTH, correlating with higher bone formation rates (BFR) at 4 weeks in the spine and mid-femur without corresponding increases in bone resorption indices. The increases in vertebral BFR/BS at 4 weeks attenuated with continued treatment to a greater extent with Scl-Ab than with PTH. At 4 weeks, both Scl-Ab and PTH effected equivalent increases in total Ob number (Ob.N). Ob density on the formative surfaces (Ob.N/Ob.S) remained similar across groups while mineral apposition rate (MAR) was significantly higher with Scl-Ab at week 4, reflecting an increase in individual Ob vigor relative to vehicle and PTH. After 26 weeks, Scl-Ab maintained BFR/BS with fewer Ob and lower Ob.N/Ob.S by increasing the Ob footprint (bone surface area occupied by an Ob) and increasing MAR, compared with PTH. The lower Ob.N and Ob.N/Ob.S with Scl-Ab at 26 weeks were associated with decreased osteoprogenitor numbers compared with both vehicle and PTH, an effect not evident at week 4. Osteoprogenitor numbers were generally positively correlated with Ob.N across groups and timepoints, suggesting dynamic coordination between the progenitor and Ob populations. The time-dependent reductions in subpopulations of the Ob lineage with Scl-Ab may be integral to the greater attenuation or self-regulation of bone formation observed at the vertebra, as PTH required more Ob at the formative site with correlative increased numbers of progenitors compared with Scl-Ab indicating potentially greater stimulus for progenitor pool proliferation or differentiation.

Extra-intestinal calcium handling contributes to normal serum calcium levels when intestinal calcium absorption is suboptimal

The active form of vitamin D, 1,25(OH)2D, is a crucial regulator of calcium homeostasis, especially through stimulation of intestinal calcium transport. Lack of intestinal vitamin D receptor (VDR) signaling does however not result in hypocalcemia, because the increased 1,25(OH)2D levels stimulate calcium handling in extra-intestinal tissues. Systemic VDR deficiency, on the other hand, results in hypocalcemia because calcium handling is impaired not only in the intestine, but also in kidney and bone. It remains however unclear whether low intestinal VDR activity, as observed during aging, is sufficient for intestinal calcium transport and for mineral and bone homeostasis. To this end, we generated mice that expressed the Vdr exclusively in the gut, but at reduced levels. We found that ~15% of intestinal VDR expression greatly prevented the Vdr null phenotype in young-adult mice, including the severe hypocalcemia. Serum calcium levels were, however, in the low-normal range, which may be due to the suboptimal intestinal calcium absorption, renal calcium loss, insufficient increase in bone resorption and normal calcium incorporation in the bone matrix. In conclusion, our results indicate that low intestinal VDR levels improve intestinal calcium absorption compared to Vdr null mice, but also show that 1,25(OH)2D-mediated fine-tuning of renal calcium reabsorption and bone mineralization and resorption is required to maintain fully normal serum calcium levels.

Stem cells and bone diseases: new tools, new perspective

Postnatal skeletal stem cells are a unique class of progenitors with biological properties that extend well beyond the limits of stemness as commonly defined. Skeletal stem cells sustain skeletal tissue homeostasis, organize and maintain the complex architectural structure of the bone marrow microenvironment and provide a niche for hematopoietic progenitor cells. The identification of stem cells in the human post-natal skeleton has profoundly changed our approach to the physiology and pathology of this system. Skeletal diseases have been long interpreted essentially in terms of defective function of differentiated cells and/or abnormal turnover of the matrix that they produce. The notion of a skeletal stem cell has brought forth multiple, novel concepts in skeletal biology that provide potential alternative concepts. At the same time, the recognition of the complex functions played by skeletal progenitors, such as the structural and functional organization of the bone marrow, has provided an innovative, unifying perspective for understanding bone and bone marrow changes simultaneously occurring in many disorders. Finally, the possibility to isolate and highly enrich for skeletal progenitors, enables us to reproduce perfectly normal or pathological organ miniatures. These, in turn, provide suitable models to investigate and manipulate the pathogenetic mechanisms of many genetic and non-genetic skeletal diseases. This article is part of a Special Issue entitled Stem cells and Bone.

Stk11 (Lkb1) deletion in the osteoblast lineage leads to high bone turnover, increased trabecular bone density and cortical porosity

The mTOR pathway couples energy homeostasis to growth, division and survival of the cell. Stk11/Lkb1 is a critical serine-threonine protein kinase in the inhibition of mTOR pathway action. In the mammalian skeleton, Stk11 regulates the transition between immature and hypertrophic chondrocytes. Here, we have focused on the action of Stk11in the osteoblast lineage through osteoblast specific-removal of Stk11 activity. In the mouse model system, specification and primary organization of the neonatal boney skeleton is independent of Stk11. However, histological, molecular and micro-CT analysis revealed a marked perturbation of normal bone development evident in the immediate post-natal period. Cortical bone was unusually porous displaying a high rate of turnover with new trabeculae forming in the endosteal space. Trabecular bone also showed enhanced turnover and marked increase in the density of trabeculae and number of osteoclasts. Though mutants showed an expansion of bone volume and trabecular number, their bone matrix comprised large amounts of osteoid and irregularly deposited woven bone highlighted by diffuse fluorochrome labeling. Additionally, we observed an increase in fibroblast-like cells associated with trabecular bone in Stk11 mutants. Stk11 down-regulates mTORC1 activity through control of upstream modulators of the AMP kinase family: an increase in the levels of the phosphorylated ribosomal protein S6, a target of mTORC1-mediated kinase activity, on osteoblast removal of Stk11 suggests deregulated mTORC1 activity contributes to the osteoblast phenotype. These data demonstrate Stk11 activity within osteoblasts is critical for the development of normally structured bone regulating directly the number and coordinated actions of osteoblasts, and indirectly osteoclast number.

Acute exposure to high dose γ-radiation results in transient activation of bone lining cells

The present studies investigated the cellular mechanisms for the detrimental effects of high dose whole body γ-irradiation on bone. In addition, radioadaptation and bone marrow transplantation were assessed as interventions to mitigate the skeletal complications of irradiation. Increased trabecular thickness and separation and reduced cancellous bone volume fraction, connectivity density, and trabecular number were detected in proximal tibia and lumbar vertebra 14days following γ-irradiation with 6Gy. To establish the cellular mechanism for the architectural changes, vertebrae were analyzed by histomorphometry 1, 3, and 14days following irradiation. Marrow cell density decreased within 1day (67% reduction, p<0.0001), reached a minimum value after 3days (86% reduction, p<0.0001), and partially rebounded by 14days (30% reduction, p=0.0025) following irradiation. In contrast, osteoblast-lined bone perimeter was increased by 290% (1day, p=0.04), 1230% (3days, p<0.0001), and 530% (14days, p=0.003), respectively. There was a strong association between radiation-induced marrow cell death and activation of bone lining cells to express the osteoblast phenotype (Pearson correlation -0.85, p<0.0001). An increase (p=0.004) in osteoclast-lined bone perimeter was also detected with irradiation. A priming dose of γ-radiation (0.5mGy), previously shown to reduce mortality, had minimal effect on the cellular responses to radiation and did not prevent detrimental changes in bone architecture. Bone marrow transplantation normalized marrow cell density, bone turnover, and most indices of bone architecture following irradiation. In summary, radiation-induced death of marrow cells is associated with 1) a transient increase in bone formation due, at least in part, to activation of bone lining cells, and 2) an increase in bone resorption due to increased osteoclast perimeter. Bone marrow transplantation is effective in mitigating the detrimental effects of acute exposure to high dose whole body γ-radiation on bone turnover.

Evaluation of markers of bone turnover during lactation in African-Americans: a comparison with Caucasian lactation

CONTEXT

The African-American skeleton is resistant to PTH; whether it is also resistant to PTHrP and the hormonal milieu of lactation is unknown.

OBJECTIVES

The objective of the study was to assess bone turnover markers in African-Americans during lactation vs Caucasians.

DESIGN AND PARTICIPANTS

A prospective cohort study with repeated measures of markers of bone turnover in 60 African-American women (3 groups of 20: lactating, bottle feeding, and healthy controls), compared with historic Caucasian women.

SETTING

The study was conducted at a university medical center.

OUTCOME MEASURES

Biochemical markers of bone turnover and calcium metabolism were measured.

RESULTS

25-Hydroxyvitamin D (25-OHD) and PTH were similar among all 3 African-American groups, but 25-OHD was 30%-50% lower and PTH 2-fold higher compared with Caucasians (P < .001, P < .002), with similar 1,25 dihydroxyvitamin D [1,25(OH)(2)D] values. Formation markers [amino-terminal telopeptide of procollagen-1 (P1NP) and bone-specific alkaline phosphatase (BSAP)] increased significantly (2- to 3-fold) in lactating and bottle-feeding African-American women (P1NP, P < .001; BSAP, P < .001), as did resorption [carboxy-terminal telopeptide of collagen-1 (CTX) and serum amino-terminal telopeptide of collagen 1 (NTX), both P < .001]. P1NP and BSAP were comparable in African-American and Caucasian controls, but CTX and NTX were lower in African-American vs Caucasian controls. African-American lactating mothers displayed quantitatively similar increases in markers of bone formation but slightly lower increases in markers of resorption vs Caucasians (P = .036).

CONCLUSIONS

Despite reported resistance to PTH, lactating African-American women have a significant increase in markers of bone resorption and formation in response the hormonal milieu of lactation. This response is similar to that reported in Caucasian women despite racial differences in 25-OHD and PTH. Whether this is associated with similar bone loss in African-Americans as in Caucasians during lactation is unknown and requires further study.

High phosphate feeding promotes mineral and bone abnormalities in mice with chronic kidney disease

BACKGROUND

Chronic kidney disease-mineral bone disorder (CKD-MBD) is a systemic syndrome characterized by imbalances in mineral homeostasis, renal osteodystrophy (ROD) and ectopic calcification. The mechanisms underlying this syndrome in individuals with chronic kidney disease (CKD) are not yet clear.

METHODS

We examined the effect of normal phosphate (NP) or high phosphate (HP) feeding in the setting of CKD on bone pathology, serum biochemistry and vascular calcification in calcification-prone dilute brown non-agouti (DBA/2) mice.

RESULTS

In both NP and HP-fed CKD mice, elevated serum parathyroid hormone and alkaline phosphatase (ALP) levels were observed, but serum phosphorus levels were equivalent compared with sham controls. CKD mice on NP diet showed trabecular alterations in the long bone consistent with high-turnover ROD, including increased trabecular number with abundant osteoblasts and osteoclasts. Despite trabecular bone and serum biochemical changes, CKD/NP mice did not develop vascular calcification. In contrast, CKD/HP mice developed arterial medial calcification (AMC), more severe trabecular bone alterations and cortical bone abnormalities that included decreased cortical thickness and density, and increased cortical porosity. Cortical bone porosity and trabecular number strongly correlated with the degree of aortic calcification.

CONCLUSIONS

HP feeding was required to induce the full spectrum of CKD-MBD symptoms in CKD mice.

Osteoblastic expansion induced by parathyroid hormone receptor signaling in murine osteocytes is not sufficient to increase hematopoietic stem cells

Microenvironmental expansion of hematopoietic stem cells (HSCs) is induced by treatment with parathyroid hormone (PTH) or activation of the PTH receptor (PTH1R) in osteoblastic cells; however, the osteoblastic subset mediating this action of PTH is unknown. Osteocytes are terminally differentiated osteoblasts embedded in mineralized bone matrix but are connected with the BM. Activation of PTH1R in osteocytes increases osteoblastic number and bone mass. To establish whether osteocyte-mediated PTH1R signaling expands HSCs, we studied mice expressing a constitutively active PTH1R in osteocytes (TG mice). Osteoblasts, osteoclasts, and trabecular bone were increased in TG mice without changes in BM phenotypic HSCs or HSC function. TG mice had progressively increased trabecular bone but decreased HSC function. In severely affected TG mice, phenotypic HSCs were decreased in the BM but increased in the spleen. TG osteocytes had no increase in signals associated with microenvironmental HSC support, and the spindle-shaped osteoblastic cells that increased with PTH treatment were not present in TG bones. These findings demonstrate that activation of PTH1R signaling in osteocytes does not expand BM HSCs, which are instead decreased in TG mice. Therefore, osteocytes do not mediate the HSC expansion induced by PTH1R signaling. Further, osteoblastic expansion is not sufficient to increase HSCs.

A novel population of cells expressing both hematopoietic and mesenchymal markers is present in the normal adult bone marrow and is augmented in a murine model of marrow fibrosis

Bone marrow (BM) fibrosis is a feature of severe hyperparathyroidism. Consistent with this observation, mice expressing constitutively active parathyroid hormone (PTH)/PTH-related peptide receptors (PPR) in osteoblasts (PPR*Tg) display BM fibrosis. To obtain insight into the nature of BM fibrosis in such a model, a double-mutant mouse expressing constitutively active PPR and green fluorescent protein (GFP) under the control of the type I collagen promoter (PPR*Tg/GFP) was generated. Confocal microscopy and flow cytometry revealed the presence of a cell population expressing GFP (GFP(+)) that was also positive for the hematopoietic marker CD45 in the BM of both PPR*Tg/GFP and control animals. This cell population was expanded in PPR*Tg/GFP. The existence of cells expressing both type I collagen and CD45 in the adult BM was confirmed by IHC and fluorescence-activated cell sorting. An analysis of total RNA extracted from sorted GFP(+)CD45(+) cells showed that these cells produced type I collagen and PTH/PTH-related peptide receptor and receptor activator for NF-κB mRNAs, further supporting their features of being both mesenchymal and hematopoietic lineages. Similar cells, known as fibrocytes, are also present in pathological fibroses. Our findings, thus, indicate that the BM is a permissive microenvironment for the differentiation of fibrocyte-like cells and raise the possibility that these cells could contribute to the pathogenesis of BM fibrosis.

A 7-day continuous infusion of PTH or PTHrP suppresses bone formation and uncouples bone turnover

Human in vivo models of primary hyperparathyroidism (HPT), humoral hypercalcemia of malignancy (HHM), or lactational bone mobilization for more than 48 hours have not been described previously. We therefore developed 7-day continuous-infusion models using human parathyroid hormone(1-34) [hPTH(1-34)] and human parathyroid hormone-related protein(1-36) [hPTHrP(1-36)] in healthy human adult volunteers. Study subjects developed sustained mild increases in serum calcium (10.0 mg/dL), with marked suppression of endogenous PTH(1-84). The maximal tolerated infused doses over a 7-day period (2 and 4 pmol/kg/h for PTH and PTHrP, respectively) were far lower than in prior, briefer human studies (8 to 28 pmol/kg/h). In contrast to prior reports using higher PTH and PTHrP doses, both 1,25-dihydroxyvitamin D(3) [1,25(OH)(2) D(3) ] and tubular maximum for phosphorus (TmP/GFR) remained unaltered with these low doses despite achievement of hypercalcemia and hypercalciuria. As expected, bone resorption increased rapidly and reversed promptly with cessation of the infusion. However, in contrast to events in primary HPT, bone formation was suppressed by 30% to 40% for the 7 days of the infusions. With cessation of PTH and PTHrP infusion, bone-formation markers abruptly rebounded upward, confirming that bone formation is suppressed by continuous PTH or PTHrP infusion. These studies demonstrate that continuous exposure of the human skeleton to PTH or PTHrP in vivo recruits and activates the bone-resorption program but causes sustained arrest in the osteoblast maturation program. These events would most closely mimic and model events in HHM. Although not a perfect model for lactation, the increase in resorption and the rebound increase in formation with cessation of the infusions are reminiscent of the maternal skeletal calcium mobilization and reversal that occur following lactation. The findings also highlight similarities and differences between the model and HPT.

Differential effects of oral doxercalciferol (Hectorol) or paricalcitol (Zemplar) in the Cyp27b1-null mouse model of uremia

BACKGROUND/AIMS

Kidney disease patients experience declining calcitriol levels and develop secondary hyperparathyroidism (SHPT). Animal models of uremia based on 5/6 nephrectomy (NTX) do not consistently reproduce this calcitriol deficiency. We developed an animal model, the NTX Cyp27b1-null mouse, which completely lacks endogenous calcitriol, and examined the suitability of this model for evaluation of treatment with vitamin D analogs in uremia.

METHODS

NTX was performed at 2 months of age. One week post-NTX, animals were treated for 4 weeks with vehicle; doxercalciferol at 30, 100 or 300 pg/g body weight (b.w.); or paricalcitol at 100, 300 or 1,000 pg/g b.w. by gavage 3 times per week.

RESULTS

Serum blood urea nitrogen and creatinine were elevated. Vehicle-treated NTX null mice had hypocalcemia and SHPT. Doxercalciferol at 100 or 300 pg/g b.w. normalized serum calcium and parathyroid hormone (PTH) levels. Paricalcitol at 300 or 1,000 pg/g normalized serum calcium, but PTH levels remained elevated. Osteomalacia was corrected by 100 pg/g b.w. of doxercalciferol or 1,000 pg/g b.w. of paricalcitol. The highest dose of doxercalciferol, but not of paricalcitol, significantly reduced osteitis fibrosa.

CONCLUSION

Our results reveal the differential efficacy of doxercalciferol and paricalcitol in this novel animal model incorporating both calcitriol deficiency and renal insufficiency.

PTH and stem cells

At least 2 different types of cells, hematopoietic and mesenchymal, are present in the adult bone marrow, in addition to endothelial cells. Hematopoietic and mesenchymal cells are believed to originate from hematopoietic stem cells (HSC) and mesenchymal stem cells (MSC), respectively. The bone marrow stroma, a cellular microenvironment that supports HSC, is composed of non-hematopoietic cells and contains MSC. A unique expansion of the bone marrow stroma, also known as marrow fibrosis, is the hallmark of a variety of disorders including hyperparathyroidism and fibrous dysplasia. PTH is the first bone anabolic agent approved by US Food and Drug Administration for the treatment of osteoporosis. Recent studies have suggested that PTH treatment may affect the number of hematopoietic stem cells in the bone marrow and their mobilization into the bloodstream. In addition, cells with classical features of mesenchymal stem cells/progenitors have been shown to express receptors for PTH, and to increase in number and undergo redistribution in the adult bone marrow upon PTH treatment. In this review, we will summarize the up-to-date knowledge on PTH and its relation to stem cells. We will also discuss the contribution of different cell types to the development of marrow fibrosis and the involvement of PTH signaling in this pathology.

Continuous elevation of PTH increases the number of osteoblasts via both osteoclast-dependent and -independent mechanisms

Sustained parathyroid hormone (PTH) elevation stimulates bone remodeling (ie, both resorption and formation). The former results from increased RANKL synthesis, but the cause of the latter has not been established. Current hypotheses include release of osteoblastogenic factors from osteoclasts or from the bone matrix during resorption, modulation of the production and activity of osteoblastogenic factors from cells of the osteoblast lineage, and increased angiogenesis. To dissect the contribution of these mechanisms, 6-month-old Swiss-Webster mice were infused for 5 days with 470 ng/h PTH(1-84) or 525 ng/h soluble RANKL (sRANKL). Both agents increased osteoclasts and osteoblasts in vertebral cancellous bone, but the ratio of osteoblasts to osteoclasts and the increase in bone formation was greater in PTH-treated mice. Cancellous bone mass was maintained in mice receiving PTH but lost in mice receiving sRANKL, indicating that maintenance of balanced remodeling requires osteoblastogenic effects beyond those mediated by osteoclasts. Consistent with this contention, PTH, but not sRANKL, decreased the level of the Wnt antagonist sclerostin and increased the expression of the Wnt target genes Nkd2, Wisp1, and Twist1. Furthermore, PTH, but not sRANKL, increased the number of blood vessels in the bone marrow. Weekly injections of the RANKL antagonist osteoprotegerin at 10 µg/g for 2 weeks prior to PTH infusion eliminated osteoclasts and osteoblasts and prevented the PTH-induced increase in osteoclasts, osteoblasts, and blood vessels. These results indicate that PTH stimulates osteoclast-dependent as well as osteoclast-independent (Wnt signaling) pro-osteoblastogenic pathways, both of which are required for balanced focal bone remodeling in cancellous bone.

Phospholipase C signaling via the parathyroid hormone (PTH)/PTH-related peptide receptor is essential for normal bone responses to PTH

We have previously shown that differentiation of hypertrophic chondrocytes is delayed in mice expressing a mutated PTH/PTHrP receptor (PTHR) (called DSEL here) that stimulates adenylyl cyclase normally but fails to activate phospholipase C (PLC). To better understand the role of PLC signaling via the PTHR in skeletal and mineral homeostasis, we examined these mice fed a normal or calcium-deficient diet. On a standard diet, DSEL mice displayed a modest decrease in bone mass. Remarkably, when fed a low-calcium diet or infused with PTH, DSEL mice exhibited strikingly curtailed peritrabecular stromal cell responses and attenuated new bone formation when compared with Wt mice. Attenuated in vitro colony formation was also observed in bone marrow cells derived from DSEL mice fed a low-calcium diet. Furthermore, PTH stimulated proliferation and increased mRNAs encoding cyclin D1 in primary osteoblasts derived from Wt but not from DSEL mice. Our data indicate that PLC signaling through the PTHR is required for skeletal homeostasis.

The Effects of a Vitamin D–deficient Diet on Chronic Cadmium Exposure in Rats

Itai-itai disease (IID) of humans is one of the most severe forms of chronic cadmium (Cd) intoxication. Itai-itai disease occurs mainly in post-menopausal women and is characterized by osteoporosis with osteomalacia, renal tubular disorder, and renal anemia. Some researchers insist the major cause of IID is not Cd, but rather malnutrition, especially hypovitaminosis D. We administrated a low concentration of Cd chloride intravenously to ovariectomized female rats that were fed a vitamin D–deficient diet or a normal diet for fifty weeks. The vitamin D–deficient diet decreased serum concentration of vitamin D, but it did not affect the metabolism of the kidney or bone. Cadmium treatment alone induced a decrease in serum concentration of vitamin D, as well as renal dysfunction, renal anemia, and abnormal bone metabolism. Osteoporosis with osteomalacia, tubular nephropathy, fibrous osteodystrophy, and bone marrow hyperplasia occurred following Cd treatment. In rats treated with Cd and administered a vitamin D–deficient diet, the toxic effects of Cd on kidney, bone, and hematopoiesis were enhanced in comparison to rats treated with Cd and a normal diet. The present experiment demonstrated that hypovitaminosis D did not evoke morphologic features of IID in humans but did enhance Cd-induced toxicity in the rat model of this disease.

The role of mast cells in parathyroid bone disease

Chronic hyperparathyroidism (HPT) is a common cause of metabolic bone disease. These studies investigated the underlying cellular and molecular mechanisms responsible for the detrimental actions of elevated parathyroid hormone (PTH) on the skeleton. Bone biopsies from hyperparathyroid patients revealed an association between parathyroid bone disease and increased numbers of bone marrow mast cells. We therefore evaluated the role of mast cells in the etiology of parathyroid bone disease in a rat model for chronic HPT. In rats, mature mast cells were preferentially located at sites undergoing bone turnover, and the number of mast cells at the bone-bone marrow interface was greatly increased following treatment with PTH. Time-course studies and studies employing parathyroid hormone-related peptide (PTHrP), as well as inhibitors of platelet-derived growth factor-A (PDGF-A, trapidil), kit (gleevec), and PI3K (wortmannin) signaling revealed that mature mast cell redistribution from bone marrow to bone surfaces precedes and is associated with osteitis fibrosa, a hallmark of parathyroid bone disease. Importantly, mature mast cells were not observed in the bone marrow of mice. Mice, in turn, were resistant to the development of PTH-induced bone marrow fibrosis. These findings suggest that the mast cell may be a novel target for treatment of metabolic bone disease.

The bone histology spectrum in experimental renal failure: adverse effects of phosphate and parathyroid hormone disturbances

Bone disease is a common disorder of bone remodeling and mineral metabolism, which affects patients with chronic kidney disease. Minor changes in the serum level of a given mineral can trigger compensatory mechanisms, making it difficult to evaluate the role of mineral disturbances in isolation. The objective of this study was to determine the isolated effects that phosphate and parathyroid hormone (PTH) have on bone tissue in rats. Male Wistar rats were subjected to parathyroidectomy and 5/6 nephrectomy or were sham-operated. Rats were fed diets in which the phosphate content was low, normal, or high. Some rats received infusion of PTH at a physiological rate, some received infusion of PTH at a supraphysiological rate, and some received infusion of vehicle only. All nephrectomized rats developed moderate renal failure. High phosphate intake decreased bone volume, and this effect was more pronounced in animals with dietary phosphate overload that received PTH infusion at a physiological rate. Phosphate overload induced hyperphosphatemia, hypocalcemia, and changes in bone microarchitecture. PTH at a supraphysiological rate minimized the phosphate-induced osteopenia. These data indicate that the management of uremia requires proper control of dietary phosphate, together with PTH adjustment, in order to ensure adequate bone remodeling.

Suppression of Wnt signaling by Dkk1 attenuates PTH-mediated stromal cell response and new bone formation

Parathyroid hormone (PTH) suppresses Dickkopf 1 (Dkk1) expression in osteoblasts. To determine whether this suppression is essential for PTH-mediated Wnt signaling and bone formation, we examined mice that overexpress Dkk1 in osteoblasts (Dkk1 mice). Dkk1 mice were osteopenic due to abnormal osteoblast and osteoclast activity. When fed a low-calcium diet, and in two other models of hyperparathyroidism, these mice failed to develop the peritrabecular stromal cell response ("osteitis fibrosis") and new bone formation seen in wild-type mice. Despite these effects of Dkk1 overexpression, PTH still activated Wnt signaling in Dkk1 mice and in osteoblastic cells cultured from these mice. In cultured MC3T3E1 preosteoblastic cells, PTH dramatically suppressed Dkk1 expression, induced PKA-mediated phosphorylation of beta-catenin, and significantly enhanced Lef1 expression. Our findings indicate that the full actions of PTH require intact Wnt signaling but that PTH can activate the Wnt pathway despite overexpression of Dkk1.

Review article: Bone biopsy in chronic kidney disease: patient level end-point or just another test?

The reduction of renal function in chronic kidney disease leads to disturbed calcium and phosphorus metabolism, impaired action of calcitriol, increased parathyroid hormone, FGF-23 levels and ultimately bone disorders. These disturbances have been traditionally termed renal osteodystrophy, which evaluation and diagnosis require a bone biopsy. In the last four decades, researchers from different countries have developed new techniques and have introduced concepts that allowed the development of bone histomorphometry, considered the key tool to study bone metabolism, remodelling and structure. In this review we focus on the relevance of bone biopsy and its respective histomorphometric analysis to help nephrologists to evaluate patients with chronic kidney disease.

Osteoprotegerin abrogated cortical porosity and bone marrow fibrosis in a mouse model of constitutive activation of the PTH/PTHrP receptor

Intracortical porosities and marrow fibrosis are hallmarks of hyperparathyroidism and are present in bones of transgenic mice expressing constitutively active parathyroid hormone/parathyroid hormone-related protein receptors (PPR*Tg). Cortical porosity is the result of osteoclast activity; however, the etiology of marrow fibrosis is poorly understood. While osteoclast numbers and activity are regulated by osteoprotegerin (OPG), bisphosphonates suppress osteoclast activity but not osteoclast numbers. We therefore used OPG and bisphosphonates to evaluate the extent to which osteoclasts, as opposed to bone resorption, regulate marrow fibrosis in PPR*Tg mice after treatment of animals with vehicle, OPG, alendronate, or zoledronate. All three agents similarly increased trabecular bone volume in both PPR*Tg and control mice, suggesting that trabecular bone resorption was comparably suppressed by these agents. However, the number of trabecular osteoclasts was greatly decreased by OPG but not by either alendronate or zoledronate. Furthermore, intracortical porosity and marrow fibrosis were virtually abolished by OPG treatment, whereas alendronate and zoledronate only partially reduced these two parameters. The greater reductions in cortical porosity and increments in cortical bone mineral density with OPG in PPR*Tg mice were associated with greater improvements in bone strength. The differential effect of OPG versus bisphosphonates on marrow fibrosis, despite similar effects on trabecular bone volume, suggests that marrow fibrosis was related not only to bone resorption but also to the presence of osteoclasts.

Osteitis fibrosa is mediated by Platelet-Derived Growth Factor-A via a phosphoinositide 3-kinase-dependent signaling pathway in a rat model for chronic hyperparathyroidism

Abnormal secretion of PTH by the parathyroid glands contributes to a variety of common skeletal disorders. Prior studies implicate platelet-derived growth factor-A (PDGF-A) as an important mediator of selective PTH actions on bone. The present studies used targeted gene profiling and small-molecule antagonists directed against candidate gene products to elucidate the roles of specific PTH-regulated genes and signaling pathways. A group of 29 genes in rats continuously infused with PTH and cotreated with the PDGF receptor antagonist trapidil were differentially expressed compared with PTH treatment alone. Several of the identified genes were functionally clustered as regulators of fibroblast differentiation and extracellular matrix modeling, including the matrix cross-linking enzyme lysyl oxidase (LOX). Treatment with beta-aminopropionitrile, an irreversible inhibitor of LOX activity, dramatically reduced diffuse mineralization but had no effect on PTH-induced fibrosis. In contrast, the receptor tyrosine kinase inhibitor Gleevec and the phosphoinositide 3-kinase inhibitor wortmannin each reduced bone marrow fibrosis. In summary, the present studies support the hypotheses that PTH-induced bone marrow fibrosis is mediated by PDGF-A via a phosphoinositide 3-kinase-dependent signaling pathway and that increased LOX gene expression plays a key role in abnormal mineralization, a hallmark of chronic hyperparathyroidism.

Pancytopenia and secondary myelofibrosis could be induced by primary hyperparathyroidism

Hyperparathyroidism may be a precipitating factor important to the development of myelofibrosis: however, there has been only a few reports regarding myelofibrosis secondary to primary hyperparathyroidism. Recently, a rare case of pancytopenia caused by myelofibrosis in a 41-year-old woman who complained of general weakness and arthralgia presented to our clinical service. The patient was diagnosed with primary hyperparathyroidism with pancytopenia. Bone marrow biopsy revealed myelofibrosis. Right parathyroidectomy was performed and a parathyroid adenoma was totally excised. After surgery, the CBC counts and other clinical abnormalities gradually improved without further intervention. We concluded that the pancytopenia was because of bone marrow fibrosis resulting from primary hyperparathyroidism. Therefore, physicians should consider myelofibrosis secondary to primary hyperparathyroidism as a cause of pancytopenia in hypercalcemic patients, even though it is rare.

PHOSPHORUS METABOLISM AND MANAGEMENT IN CHRONIC KIDNEY DISEASE: Renal Osteodystrophy, Phosphate Homeostasis, and Vascular Calcification

New advances in the pathogenesis of renal osteodystrophy (ROD) change the perspective from which many of its features and treatment are viewed. Calcium, phosphate, parathyroid hormone (PTH), and vitamin D have been shown to be important determinants of survival associated with kidney diseases. Now ROD dependent and independent of these factors is linked to survival more than just skeletal frailty. This review focuses on recent discoveries that renal injury impairs skeletal anabolism decreasing the osteoblast compartment of the skeleton and consequent bone formation. This discovery and the discovery that PTH regulates the hematopoietic stem cell niche alters our view of secondary hyperparathyroidism in chronic kidney disease (CKD) from that of a disease to that of a necessary adaptation to renal injury that goes awry. Furthermore, ROD is shown to be an underappreciated factor in the level of the serum phosphorus in CKD. The discovery and the elucidation of the mechanism of hyperphosphatemia as a cardiovascular risk in CKD change the view of ROD. It is now recognized as more than a skeletal disorder, it is an important component of the mortality of CKD that can be treated.

Benign parathyroid adenoma presenting with unusual parathyroid crisis, anemia and myelofibrosis

Although the clinical symptoms of patients with benign parathyroid adenoma are usually nonspecific and benign, a malignant presentation of the benign disease may sometimes occur. Here, we report a case of a 58-year-old woman who presented with aggravated sacrum pain, general malaise, and polydipsia. Initial laboratory findings revealed hypercalcemia, normocytic anemia, and impaired renal function. Acute hypercalcemic crisis manifested and primary hyperparathyroidism was diagnosed together with myelofibrosis on account of the result of bone marrow biopsy. Excision of a parathyroid adenoma was performed, and the anemia and bone marker regressed later. These findings suggested that benign parathyroid adenoma may mimic the clinical presentation of parathyroid carcinoma, releasing excess parathyroid hormone and resulting in hyperparathyroid crisis. In addition, primary hyperparathyroidism can be associated with anemia and myelofibrosis.

Abnormal muscle and hematopoietic gene expression may be important for clinical morbidity in primary hyperparathyroidism

In primary hyperparathyroidism (PHPT), excess PTH secretion by adenomatous or hyperplastic parathyroid glands leads to elevated serum [Ca(2+)]. Patients present complex symptoms of muscular fatigue, various neuropsychiatric, neuromuscular, and cardiovascular manifestations, and, in advanced disease, kidney stones and metabolic bone disease. Our objective was to characterize changes in muscle and hematopoietic gene expression in patients with reversible mild PHPT after parathyroidectomy and possibly link molecular pathology to symptoms. Global mRNA profiling using Affymetrix gene chips was carried out in biopsies obtained before and 1 yr after parathyroidectomy in seven patients discovered by routine blood [Ca(2+)] screening. The tissue distribution of PTH receptor (PTHR1 and PTHR2) mRNAs were quantitated using real-time RT-PCR in unrelated persons to define PTH target tissues. Of about 10,000 expressed genes, 175 muscle, 169 hematological, and 99 bone-associated mRNAs were affected. Notably, the major part of muscle-related mRNAs was increased whereas hematological mRNAs were predominantly decreased during disease. Functional and molecular network analysis demonstrated major alterations of several tissue characteristic groups of mRNAs as well as those belonging to common cell signaling and major metabolic pathways. PTHR1 and PTHR2 mRNAs were more abundantly expressed in muscle and brain than in hematopoietic cells. We suggest that sustained stimulation of PTH receptors present in brain, muscle, and hematopoietic cells have to be considered as one independent, important cause of molecular disease in PHPT leading to profound alterations in gene expression that may help explain symptoms like muscle fatigue, cardiovascular pathology, and precipitation of psychiatric illness.

Cannabinoids stimulate fibroblastic colony formation by bone marrow cells indirectly via CB2 receptors

Recently, the cannabinoid receptors CB(1) and CB(2) were shown to modulate bone formation and resorption in vivo, although little is known of the mechanisms underlying this. The effects of cannabinoids on mesenchymal stem cell (MSC) recruitment in whole bone marrow were investigated using either the fibroblastic colony-forming unit (CFU-f) assay or high-density cultures of whole bone marrow. Levels of the CB(1) and CB(2) receptors were assessed by flow cytometry. Treatment of CFU-f cultures with the endocannabinoid 2-arachidonylglycerol (2-AG) dose-dependently increased fibroblastic and differentiated colony formation along with colony size. The nonspecific agonists CP 55,940 and WIN 55,212 both increased colony numbers, as did the CB(2) agonists BML190 and JWH015. The CB(1)-specific agonist ACEA had no effect, whereas the CB(2) antagonist AM630 blocked the effect of the natural cannabinoid tetrahydrocannabivarin, confirming mediation via the CB(2) receptor. Treatment of primary bone marrow cultures with 2-AG stimulated proliferation and collagen accumulation, whereas treatment of subcultures of MSC had no effect, suggesting that the target cell is not the MSC but an accessory cell present in bone marrow. Subcultures of MSCs were negative for CB(1) and CB(2) receptors as shown by flow cytometry, whereas whole bone marrow contained a small population of cells positive for both receptors. These data suggest that cannabinoids may stimulate the recruitment of MSCs from the bone marrow indirectly via an accessory cell and mediated via the CB(2) receptor. This recruitment may be one mechanism responsible for the increased bone formation seen after cannabinoid treatment in vivo.

Expression of measles virus nucleocapsid protein in osteoclasts induces Paget's disease-like bone lesions in mice

We targeted the MVNP gene to the OCL lineage in transgenic mice. These mice developed abnormal OCLs and bone lesions similar to those found in Paget's patients. These results show that persistent expression of MVNP in OCLs can induce pagetic-like bone lesions in vivo.

INTRODUCTION

Paget's disease (PD) of bone is the second most common bone disease. Both genetic and viral factors have been implicated in its pathogenesis, but their exact roles in vivo are unclear. We previously reported that transfection of normal human osteoclast (OCL) precursors with the measles virus nucleocapsid (MVNP) or measles virus (MV) infection of bone marrow cells from transgenic mice expressing a MV receptor results in formation of pagetic-like OCLs.

MATERIALS AND METHODS

Based on these in vitro studies, we determined if the MVNP gene from either an Edmonston-related strain of MV or a MVNP gene sequence derived from a patient with PD (P-MVNP), when targeted to cells in the OCL lineage of transgenic mice with the TRACP promoter (TRACP/MVNP mice), induced changes in bone similar to those found in PD.

RESULTS

Bone marrow culture studies and histomorphometric analysis of bones from these mice showed that their OCLs displayed many of the features of pagetic OCLs and that they developed bone lesions that were similar to those in patients with PD. Furthermore, IL-6 seemed to be required for the development of the pagetic phenotype in OCLs from TRACP/MVNP mice.

CONCLUSIONS

These results show that persistent expression of the MVNP gene in cells of the OCL lineage can induce pagetic-like bone lesions in vivo.