Nmp4/CIZ suppresses the parathyroid hormone anabolic window by restricting mesenchymal stem cell and osteoprogenitor frequency

PTH and the Regulation of Mesenchymal Cells within the Bone Marrow Niche

Parathyroid hormone (PTH) plays a pivotal role in maintaining calcium homeostasis, largely by modulating bone remodeling processes. Its effects on bone are notably dependent on the duration and frequency of exposure. Specifically, PTH can initiate both bone formation and resorption, with the outcome being influenced by the manner of PTH administration: continuous or intermittent. In continuous administration, PTH tends to promote bone resorption, possibly by regulating certain genes within bone cells. Conversely, intermittent exposure generally favors bone formation, possibly through transient gene activation. PTH's role extends to various aspects of bone cell activity. It directly influences skeletal stem cells, osteoblastic lineage cells, osteocytes, and T cells, playing a critical role in bone generation. Simultaneously, it indirectly affects osteoclast precursor cells and osteoclasts, and has a direct impact on T cells, contributing to its role in bone resorption. Despite these insights, the intricate mechanisms through which PTH acts within the bone marrow niche are not entirely understood. This article reviews the dual roles of PTH-catabolic and anabolic-on bone cells, highlighting the cellular and molecular pathways involved in these processes. The complex interplay of these factors in bone remodeling underscores the need for further investigation to fully comprehend PTH's multifaceted influence on bone health.

Loss of Nmp4 enhances bone gain from sclerostin antibody administration

Severe osteoporosis is often treated with one of three Food and Drug Administration (FDA)-approved osteoanabolics. These drugs act by (1) parathyroid hormone (PTH) receptor stimulation using analogues to PTH (teriparatide) or PTH-related peptide (abaloparatide) or by (2) monoclonal antibody neutralization of sclerostin, an innate Wnt inhibitor (Scl-mAb, romosozumab-aqqg). The efficacies of both strategies wane over time. The transcription factor Nmp4 (Nuclear Matrix Protein 4) is expressed in all tissues yet mice lacking this gene are healthy and exhibit enhanced PTH-induced bone formation. Conditional deletion of Nmp4 in mesenchymal stem progenitor cells (MSPCs) phenocopies the elevated response to PTH in global Nmp4-/- mice. However, targeted deletion in later osteoblast stages does not replicate this response. In this study we queried whether loss of Nmp4 improves Scl-mAb potency. Experimental cohorts included global Nmp4-/- and Nmp4+/+ littermates and three conditional knockout models. Nmp4-floxed (Nmp4fl/fl) mice were crossed with mice harboring one of three Cre-drivers (i) Prx1Cre+ targeting MSPCs, (ii) BglapCre+ (mature osteocalcin-expressing osteoblasts), and (iii) Dmp1Cre+ (osteocytes). Female mice were treated with Scl-mAb or 0.9 % saline vehicle for 4 or 7 weeks from 10 weeks of age. Skeletal response was assessed using micro-computed tomography, dual-energy X-ray absorptiometry, bone histomorphometry, and serum analysis. Global Nmp4-/- mice exhibited enhanced Scl-mAb-induced increases in trabecular bone in the femur and spine and a heightened increase in whole body areal bone mineral density compared to global Nmp4+/+ controls. This improved Scl-mAb potency was primarily driven by enhanced increases in bone formation. Nmp4fl/fl;PrxCre+ mice showed an exaggerated Scl-mAb-induced increase in femoral bone but not in the spine since Prrx1 is not expressed in vertebra. The Nmp4fl/fl;BglapCre+ and Nmp4fl/fl;Dmp1Cre+ mice did not exhibit an improved Scl-mAb response. We conclude that Nmp4 expression in MSPCs interferes with the bone anabolic response to anti-sclerostin therapy.

NMP4, an Arbiter of Bone Cell Secretory Capacity and Regulator of Skeletal Response to PTH Therapy

The skeleton is a secretory organ, and the goal of some osteoporosis therapies is to maximize bone matrix output. Nmp4 encodes a novel transcription factor that regulates bone cell secretion as part of its functional repertoire. Loss of Nmp4 enhances bone response to osteoanabolic therapy, in part, by increasing the production and delivery of bone matrix. Nmp4 shares traits with scaling factors, which are transcription factors that influence the expression of hundreds of genes to govern proteome allocation for establishing secretory cell infrastructure and capacity. Nmp4 is expressed in all tissues and while global loss of this gene leads to no overt baseline phenotype, deletion of Nmp4 has broad tissue effects in mice challenged with certain stressors. In addition to an enhanced response to osteoporosis therapies, Nmp4-deficient mice are less sensitive to high fat diet-induced weight gain and insulin resistance, exhibit a reduced disease severity in response to influenza A virus (IAV) infection, and resist the development of some forms of rheumatoid arthritis. In this review, we present the current understanding of the mechanisms underlying Nmp4 regulation of the skeletal response to osteoanabolics, and we discuss how this unique gene contributes to the diverse phenotypes among different tissues and stresses. An emerging theme is that Nmp4 is important for the infrastructure and capacity of secretory cells that are critical for health and disease.

Conditional Loss of Nmp4 in Mesenchymal Stem Progenitor Cells Enhances PTH-Induced Bone Formation

Activation of bone anabolic pathways is a fruitful approach for treating severe osteoporosis, yet FDA-approved osteoanabolics, eg, parathyroid hormone (PTH), have limited efficacy. Improving their potency is a promising strategy for maximizing bone anabolic output. Nmp4 (Nuclear Matrix Protein 4) global knockout mice exhibit enhanced PTH-induced increases in trabecular bone but display no overt baseline skeletal phenotype. Nmp4 is expressed in all tissues; therefore, to determine which cell type is responsible for driving the beneficial effects of Nmp4 inhibition, we conditionally removed this gene from cells at distinct stages of osteogenic differentiation. Nmp4-floxed (Nmp4fl/fl ) mice were crossed with mice bearing one of three Cre drivers including (i) Prx1Cre+  to remove Nmp4 from mesenchymal stem/progenitor cells (MSPCs) in long bones; (ii) BglapCre+  targeting mature osteoblasts, and (iii) Dmp1Cre+  to disable Nmp4 in osteocytes. Virgin female Cre+  and Cre- mice (10 weeks of age) were sorted into cohorts by weight and genotype. Mice were administered daily injections of either human PTH 1-34 at 30 μg/kg or vehicle for 4 weeks or 7 weeks. Skeletal response was assessed using dual-energy X-ray absorptiometry, micro-computed tomography, bone histomorphometry, and serum analysis for remodeling markers. Nmp4fl/fl ;Prx1Cre+  mice virtually phenocopied the global Nmp4-/- skeleton in the femur, ie, a mild baseline phenotype but significantly enhanced PTH-induced increase in femur trabecular bone volume/total volume (BV/TV) compared with their Nmp4fl/fl ;Prx1Cre- controls. This was not observed in the spine, where Prrx1 is not expressed. Heightened response to PTH was coincident with enhanced bone formation. Conditional loss of Nmp4 from the mature osteoblasts (Nmp4fl/fl ;BglapCre+ ) failed to increase BV/TV or enhance PTH response. However, conditional disabling of Nmp4 in osteocytes (Nmp4fl/fl ;Dmp1Cre+ ) increased BV/TV without boosting response to hormone under our experimental regimen. We conclude that Nmp4-/- Prx1-expressing MSPCs drive the improved response to PTH therapy and that this gene has stage-specific effects on osteoanabolism. © 2022 American Society for Bone and Mineral Research (ASBMR).

Nmp4, a Regulator of Induced Osteoanabolism, Also Influences Insulin Secretion and Sensitivity

A bidirectional and complex relationship exists between bone and glycemia. Persons with type 2 diabetes (T2D) are at risk for bone loss and fracture, however, heightened osteoanabolism may ameliorate T2D-induced deficits in glycemia as bone-forming osteoblasts contribute to energy metabolism via increased glucose uptake and cellular glycolysis. Mice globally lacking nuclear matrix protein 4 (Nmp4), a transcription factor expressed in all tissues and conserved between humans and rodents, are healthy and exhibit enhanced bone formation in response to anabolic osteoporosis therapies. To test whether loss of Nmp4 similarly impacted bone deficits caused by diet-induced obesity, male wild-type and Nmp4-/- mice (8 weeks) were fed either low-fat diet or high-fat diet (HFD) for 12 weeks. Endpoint parameters included bone architecture, structural and estimated tissue-level mechanical properties, body weight/composition, glucose-stimulated insulin secretion, glucose tolerance, insulin tolerance, and metabolic cage analysis. HFD diminished bone architecture and ultimate force and stiffness equally in both genotypes. Unexpectedly, the Nmp4-/- mice exhibited deficits in pancreatic β-cell function and were modestly glucose intolerant under normal diet conditions. Despite the β-cell deficits, the Nmp4-/- mice were less sensitive to HFD-induced weight gain, increases in % fat mass, and decreases in glucose tolerance and insulin sensitivity. We conclude that Nmp4 supports pancreatic β-cell function but suppresses peripheral glucose utilization, perhaps contributing to its suppression of induced skeletal anabolism. Selective disruption of Nmp4 in peripheral tissues may provide a strategy for improving both induced osteoanabolism and energy metabolism in comorbid patients.

Anabolic actions of PTH in murine models: two decades of insights

Parathyroid hormone (PTH) is produced by the parathyroid glands in response to low serum calcium concentrations where it targets bones, kidneys, and indirectly, intestines. The N-terminus of PTH has been investigated for decades for its ability to stimulate bone formation when administered intermittently (iPTH) and is used clinically as an effective anabolic agent for the treatment of osteoporosis. Despite great interest in iPTH and its clinical use, the mechanisms of PTH action remain complicated and not fully defined. More than 70 gene targets in more than 90 murine models have been utilized to better understand PTH anabolic actions. Because murine studies utilized wild-type mice as positive controls, a variety of variables were analyzed to better understand the optimal conditions under which iPTH functions. The greatest responses to iPTH were in male mice, with treatment starting later than 12 weeks of age, a treatment duration lasting 5-6 weeks, and a PTH dose of 30-60 μg/kg/day. This comprehensive study also evaluated these genetic models relative to the bone formative actions with a primary focus on the trabecular compartment revealing trends in critical genes and gene families relevant for PTH anabolic actions. The summation of these data revealed the gene deletions with the greatest increase in trabecular bone volume in response to iPTH. These included PTH and 1-α-hydroxylase (Pth;1α(OH)ase, 62-fold), amphiregulin (Areg, 15.8-fold), and PTH related protein (Pthrp, 10.2-fold). The deletions with the greatest inhibition of the anabolic response include deletions of: proteoglycan 4 (Prg4, -9.7-fold), low-density lipoprotein receptor-related protein 6 (Lrp6, 1.3-fold), and low-density lipoprotein receptor-related protein 5 (Lrp5, -1.0-fold). Anabolic actions of iPTH were broadly affected via multiple and diverse genes. This data provides critical insight for future research and development, as well as application to human therapeutics. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

NMP4 regulates the innate immune response to influenza A virus infection

Severe influenza A virus infection typically triggers excessive and detrimental lung inflammation with massive cell infiltration and hyper-production of cytokines and chemokines. We identified a novel function for nuclear matrix protein 4 (NMP4), a zinc-finger-containing transcription factor playing roles in bone formation and spermatogenesis, in regulating antiviral immune response and immunopathology. Nmp4-deficient mice are protected from H1N1 influenza infection, losing only 5% body weight compared to a 20% weight loss in wild type mice. While having no effects on viral clearance or CD8/CD4 T cell or humoral responses, deficiency of Nmp4 in either lung structural cells or hematopoietic cells significantly reduces the recruitment of monocytes and neutrophils to the lungs. Consistent with fewer innate cells in the airways, influenza-infected Nmp4-deficient mice have significantly decreased expression of chemokine genes Ccl2, Ccl7 and Cxcl1 as well as pro-inflammatory cytokine genes Il1b and Il6. Furthermore, NMP4 binds to the promoters and/or conserved non-coding sequences of the chemokine genes and regulates their expression in mouse lung epithelial cells and macrophages. Our data suggest that NMP4 functions to promote monocyte- and neutrophil-attracting chemokine expression upon influenza A infection, resulting in exaggerated innate inflammation and lung tissue damage.

eIF2α signaling regulates autophagy of osteoblasts and the development of osteoclasts in OVX mice

Bone loss in postmenopausal osteoporosis is induced chiefly by an imbalance of bone-forming osteoblasts and bone-resorbing osteoclasts. Salubrinal is a synthetic compound that inhibits de-phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α). Phosphorylation of eIF2α alleviates endoplasmic reticulum (ER) stress, which may activate autophagy. We hypothesized that eIF2α signaling regulates bone homeostasis by promoting autophagy in osteoblasts and inhibiting osteoclast development. To test the hypothesis, we employed salubrinal to elevate the phosphorylation of eIF2α in an ovariectomized (OVX) mouse model and cell cultures. In the OVX model, salubrinal prevented abnormal expansion of rough ER and decreased the number of acidic vesiculars. It regulated ER stress-associated signaling molecules such as Bip, p-eIF2α, ATF4 and CHOP, and promoted autophagy of osteoblasts via regulation of eIF2α, Atg7, LC3, and p62. Salubrinal markedly alleviated OVX-induced symptoms such as reduction of bone mineral density and bone volume fraction. In primary bone-marrow-derived cells, salubrinal increased the differentiation of osteoblasts, and decreased the formation of osteoclasts by inhibiting nuclear factor of activated T-cells cytoplasmic 1 (NFATc1). Live cell imaging and RNA interference demonstrated that suppression of osteoclastogenesis is in part mediated by Rac1 GTPase. Collectively, this study demonstrates that ER stress-autophagy axis plays an important role in OVX mice. Bone-forming osteoblasts are restored by maintaining phosphorylation of eIF2α, and bone-resorbing osteoclasts are regulated by inhibiting NFATc1 and Rac1 GTPase.

Loss of Nmp4 optimizes osteogenic metabolism and secretion to enhance bone quality

A goal of osteoporosis therapy is to restore lost bone with structurally sound tissue. Mice lacking the transcription factor nuclear matrix protein 4 (Nmp4, Zfp384, Ciz, ZNF384) respond to several classes of osteoporosis drugs with enhanced bone formation compared with wild-type (WT) animals. Nmp4-/- mesenchymal stem/progenitor cells (MSPCs) exhibit an accelerated and enhanced mineralization during osteoblast differentiation. To address the mechanisms underlying this hyperanabolic phenotype, we carried out RNA-sequencing and molecular and cellular analyses of WT and Nmp4-/- MSPCs during osteogenesis to define pathways and mechanisms associated with elevated matrix production. We determined that Nmp4 has a broad impact on the transcriptome during osteogenic differentiation, contributing to the expression of over 5,000 genes. Phenotypic anchoring of transcriptional data was performed for the hypothesis-testing arm through analysis of cell metabolism, protein synthesis and secretion, and bone material properties. Mechanistic studies confirmed that Nmp4-/- MSPCs exhibited an enhanced capacity for glycolytic conversion: a key step in bone anabolism. Nmp4-/- cells showed elevated collagen translation and secretion. The expression of matrix genes that contribute to bone material-level mechanical properties was elevated in Nmp4-/- cells, an observation that was supported by biomechanical testing of bone samples from Nmp4-/- and WT mice. We conclude that loss of Nmp4 increases the magnitude of glycolysis upon the metabolic switch, which fuels the conversion of the osteoblast into a super-secretor of matrix resulting in more bone with improvements in intrinsic quality.

Wnt3a involved in the mechanical loading on improvement of bone remodeling and angiogenesis in a postmenopausal osteoporosis mouse model

Osteoporosis is a major health problem, making bones fragile and susceptible to fracture. Previous works showed that mechanical loading stimulated bone formation and accelerated fracture healing. Focusing on the role of Wnt3a (wingless/integrated 3a), this study was aimed to assess effects of mechanical loading to the spine, using ovariectomized (OVX) mice as a model of osteoporosis. Two-week daily application of this novel loading (4 N, 10 Hz, 5 min/d) altered bone remodeling with an increase in Wnt3a. Spinal loading promoted osteoblast differentiation, endothelial progenitor cell migration, and tube formation and inhibited osteoclast formation, migration, and adhesion. A transient silencing of Wnt3a altered the observed loading effects. Spinal loading significantly increased bone mineral density, bone mineral content, and bone area per tissue area. The loaded OVX group showed a significant increase in the number of osteoblasts and reduction in osteoclast surface/bone surface. Though expression of osteoblastic genes was increased, the levels of osteoclastic genes were decreased by loading. Spinal loading elevated a microvascular volume as well as VEGF expression. Collectively, this study supports the notion that Wnt3a-mediated signaling involves in the effect of spinal loading on stimulating bone formation, inhibiting bone resorption, and promoting angiogenesis in OVX mice. It also suggests that Wnt3a might be a potential therapeutic target for osteoporosis treatment.-Li, X., Liu, D., Li, J., Yang, S., Xu, J., Yokota, H., Zhang, P. Wnt3a involved in the mechanical loading on improvement of bone remodeling and angiogenesis in a postmenopausal osteoporosis mouse model.

Improving Combination Osteoporosis Therapy in a Preclinical Model of Heightened Osteoanabolism

Combining anticatabolic agents with parathyroid hormone (PTH) to enhance bone mass has yielded mixed results in osteoporosis patients. Toward the goal of enhancing the efficacy of these regimens, we tested their utility in combination with loss of the transcription factor Nmp4 because disabling this gene amplifies PTH-induced increases in trabecular bone in mice by boosting osteoblast secretory activity. We addressed whether combining a sustained anabolic response with an anticatabolic results in superior bone acquisition compared with PTH monotherapy. Additionally, we inquired whether Nmp4 interferes with anticatabolic efficacy. Wild-type and Nmp4-/- mice were ovariectomized at 12 weeks of age, followed by therapy regimens, administered from 16 to 24 weeks, and included individually or combined PTH, alendronate (ALN), zoledronate (ZOL), and raloxifene (RAL). Anabolic therapeutic efficacy generally corresponded with PTH + RAL = PTH + ZOL > PTH + ALN = PTH > vehicle control. Loss of Nmp4 enhanced femoral trabecular bone increases under PTH + RAL and PTH + ZOL. RAL and ZOL promoted bone restoration, but unexpectedly, loss of Nmp4 boosted RAL-induced increases in femoral trabecular bone. The combination of PTH, RAL, and loss of Nmp4 significantly increased bone marrow osteoprogenitor number, but did not affect adipogenesis or osteoclastogenesis. RAL, but not ZOL, increased osteoprogenitors in both genotypes. Nmp4 status did not influence bone serum marker responses to treatments, but Nmp4-/- mice as a group showed elevated levels of the bone formation marker osteocalcin. We conclude that the heightened osteoanabolism of the Nmp4-/- skeleton enhances the effectiveness of diverse osteoporosis treatments, in part by increasing hyperanabolic osteoprogenitors. Nmp4 provides a promising target pathway for identifying barriers to pharmacologically induced bone formation.

Short-term intermittent administration of parathyroid hormone facilitates osteogenesis by different mechanisms in cancellous and cortical bone

Intermittent administration of human parathyroid hormone (1–34)[hPTH(1–34)] induces anabolic action on the bones. To understand the mechanism underlying the early phase of hPTH(1–34)-induced anabolic action, we investigated the expression profiles of osterix and sclerostin after short-term intermittent administration of hPTH(1–34) using immunohistochemistry in adult rats. In the cancellous bone, hPTH(1–34) administration greatly increased the number of osterix-positive cells in the bone marrow on day 1, but the cells gradually decreased on days 3 and 5. Injections of hPTH(1–34) induced no significant changes in the number of sclerostin-positive osteocytes in the cancellous bone. In the cortical bone, intermittent administration of hPTH(1–34) significantly reduced the number of sclerostin-positive osteocytes. The serum sclerostin level was downregulated and the osteocalcin level was upregulated on day 5 after intermittent administration of hPTH(1–34). Intermittent hPTH(1–34) injections increased osteoblast surface, osteoid thickness, and osteoid surface in cancellous bone, but not in cortical bone. This study suggested that the increase in osterix-positive osteoprogenitors in cancellous bone and the decrease in sclerostin-positive osteocytes in cortical bone play important roles in anabolic action on osteogenesis induced by short-term administration of hPTH(1–34).

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We analyzed the effects of hPTH(1–34) injection into rats at early phase.

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hPTH(1–34) injection increased the osterix-positive cells in bone marrow.

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hPTH(1–34) injection decreased sclerostin-positive cells in cortical bone.

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hPTH(1–34) exerts different effects in cancellous and cortical bone.

Genome-Wide Mapping and Interrogation of the Nmp4 Antianabolic Bone Axis

PTH is an osteoanabolic for treating osteoporosis but its potency wanes. Disabling the transcription factor nuclear matrix protein 4 (Nmp4) in healthy, ovary-intact mice enhances bone response to PTH and bone morphogenetic protein 2 and protects from unloading-induced osteopenia. These Nmp4(-/-) mice exhibit expanded bone marrow populations of osteoprogenitors and supporting CD8(+) T cells. To determine whether the Nmp4(-/-) phenotype persists in an osteoporosis model we compared PTH response in ovariectomized (ovx) wild-type (WT) and Nmp4(-/-) mice. To identify potential Nmp4 target genes, we performed bioinformatic/pathway profiling on Nmp4 chromatin immunoprecipitation sequencing (ChIP-seq) data. Mice (12 w) were ovx or sham operated 4 weeks before the initiation of PTH therapy. Skeletal phenotype analysis included microcomputed tomography, histomorphometry, serum profiles, fluorescence-activated cell sorting and the growth/mineralization of cultured WT and Nmp4(-/-) bone marrow mesenchymal stem progenitor cells (MSPCs). ChIP-seq data were derived using MC3T3-E1 preosteoblasts, murine embryonic stem cells, and 2 blood cell lines. Ovx Nmp4(-/-) mice exhibited an improved response to PTH coupled with elevated numbers of osteoprogenitors and CD8(+) T cells, but were not protected from ovx-induced bone loss. Cultured Nmp4(-/-) MSPCs displayed enhanced proliferation and accelerated mineralization. ChIP-seq/gene ontology analyses identified target genes likely under Nmp4 control as enriched for negative regulators of biosynthetic processes. Interrogation of mRNA transcripts in nondifferentiating and osteogenic differentiating WT and Nmp4(-/-) MSPCs was performed on 90 Nmp4 target genes and differentiation markers. These data suggest that Nmp4 suppresses bone anabolism, in part, by regulating IGF-binding protein expression. Changes in Nmp4 status may lead to improvements in osteoprogenitor response to therapeutic cues.

CAWS administration increases the expression of interferon γ and complement factors that lead to severe vasculitis in DBA/2 mice

Background

Candida albicans water-soluble fraction (CAWS), a mannoprotein-β-glucan complex obtained from the culture supernatant of C. albicans NBRC1385, causes CAWS-mediated vasculitis (CAWS-vasculitis) in B6 and DBA/2 mice with mild and lethal symptoms, respectively. Why CAWS is lethal only in DBA/2 mice remains unknown.

Results

We performed DNA microarray analyses using mRNA obtained from peripheral blood mononuclear cells (PBMCs) of B6 and DBA/2 mice and compared their respective transcriptomes. We found that the mRNA levels of interferon-γ (Ifng) and several genes that regulate the complement system, such as C3, C4, Cfb, Cfh, and Fcna, were increased dramatically only in DBA/2 mice at 4 and 8 weeks after CAWS administration. The dramatic increase was confirmed by quantitative real-time polymerase chain reactions (qRT-PCR). Moreover, mRNA levels of immune-related genes, such as Irf1, Irf7, Irf9, Cebpb, Ccl4, Itgam, Icam1, and IL-12rb1, whose expression levels are known to be increased by Ifng, were also increased, but only in DBA/2 mice. By contrast, the mRNA level of Dectin-2, the critical receptor for the α-mannans of CAWS, was increased slightly and similarly in both B6 and DBA/2 mice after CAWS administration.

Conclusions

Taken together, our results suggest that CAWS administration induces Dectin-2 mediated CAWS-vasculitis in both B6 and DBA/2 mice and the expression of Ifng, but only in DBA/2 mice, which led to increased expression of C3, C4, Cfb, Cfh, and Fcna and an associated increase in lethality in these mice. This model may contribute to our understanding of the pathogenesis of severe human vasculitis.

Functional impairment of bone formation in the pathogenesis of osteoporosis: the bone marrow regenerative competence

The skeleton is a high-renewal organ that undergoes ongoing cycles of remodeling. The regenerative bone formation arm ultimately declines in the aging, postmenopausal skeleton, but current therapies do not adequately address this deficit. Bone marrow is the primary source of the skeletal anabolic response and the mesenchymal stem cells (MSCs), which give rise to bone matrix-producing osteoblasts. The identity of these stem cells is emerging, but it now appears that the term 'MSC' has often been misapplied to the bone marrow stromal cell (BMSC), a progeny of the MSC. Nevertheless, the changes in BMSC phenotype associated with age and estrogen depletion likely contribute to the attenuated regenerative competence of the marrow and may reflect alterations in MSC phenotype. Here we summarize current concepts in bone marrow MSC identity, and within this context, review recent observations on changes in bone marrow population dynamics associated with aging and menopause.

Nmp4/CIZ closes the parathyroid hormone anabolic window

Chronic degenerative diseases are increasing with the aging U.S. population. One consequence of this phenomenon is the need for long-term osteoporosis therapies. Parathyroid hormone (PTH), the only FDA-approved treatment that adds bone to the aged skeleton, loses its potency within two years of initial treatment but the mechanism regulating its limited "anabolic window" is unknown. We have discovered that disabling the nucleocytoplasmic shuttling transcription factor nuclear matrix protein 4/cas interacting zinc finger protein (Nmp4/CIZ) in mice extends the PTH bone-forming capacity. Nmp4 was discovered during our search for nuclear matrix transcription factors that couple this hormone's impact on osteoblast cytoskeletal and nuclear organization with its anabolic capacity. CIZ was independently discovered as a protein that associates with the focal adhesion-associated mechanosensor p130Cas. The Nmp4/CIZ-knockout (KO) skeletal phenotype exhibits a modestly enhanced bone mineral density but manifests an exaggerated response to both PTH and to BMP2 and is resistant to disuse-induced bone loss. The cellular basis of the global Nmp4/CIZ-KO skeletal phenotype remains to be elucidated but may involve an expansion of the bone marrow osteoprogenitor population along with modestly enhanced osteoblast and osteoclast activities supporting anabolic bone turnover. As a shuttling Cys(2)His(2) zinc finger protein, Nmp4/CIZ acts as a repressive transcription factor perhaps associated with epigenetic remodeling complexes, but the functional significance of its interaction with p130Cas is not known. Despite numerous remaining questions, Nmp4/CIZ provides insights into how the anabolic window is regulated, and itself may provide an adjuvant therapy target for the treatment of osteoporosis by extending PTH anabolic efficacy.