Use of osmium tetroxide staining with microcomputerized tomography to visualize and quantify bone marrow adipose tissue in vivo

Adipocytes reside in discrete, well-defined depots throughout the body. In addition to mature adipocytes, white adipose tissue depots are composed of many cell types, including macrophages, endothelial cells, fibroblasts, and stromal cells, which together are referred to as the stromal vascular fraction (SVF). The SVF also contains adipocyte progenitors that give rise to mature adipocytes in those depots. Marrow adipose tissue (MAT) or marrow fat has long been known to be present in bone marrow (BM) but its origin, development, and function remain largely unknown. Clinically, increased MAT is associated with age, metabolic diseases, drug treatment, and marrow recovery in children receiving radiation and chemotherapy. In contrast to the other depots, MAT is unevenly distributed in the BM of long bones. Conventional quantitation relies on sectioning of the bone to overcome issues with distribution but is time-consuming, resource intensive, inconsistent between laboratories and may be unreliable as it may miss changes in MAT volume. Thus, the inability to quantitate MAT in a rapid, systematic, and reproducible manner has hampered a full understanding of its development and function. In this chapter, we describe a new technique that couples histochemical staining of lipid using osmium tetroxide with microcomputerized tomography to visualize and quantitate MAT within the medullary canal in three dimensions. Imaging of osmium staining provides a high-resolution map of existing and developing MAT in the BM. Because this method is simple, reproducible, and quantitative, we expect it will become a useful tool for the precise characterization of MAT.

Bone marrow adipocytes

Adipocytes were identified in human bone marrow more than a century ago, yet until recently little has been known about their origin, development, function or interactions with other cells in the bone marrow. Little functional significance has been attributed to these cells, a paradigm that still persists today. However, we now know that marrow adipose tissue increases with age and in response to a variety of physiologic induction signals. Bone marrow adipocytes have recently been shown to influence other cell populations within the marrow and can affect whole body metabolism by the secretion of a defined set of adipokines. Recent research shows that marrow adipocytes are distinct from white, brown and beige adipocytes, indicating that the bone marrow is a distinct adipose depot. This review will highlight recent data regarding these areas and the interactions of marrow adipose tissue (MAT) with cells within and outside of the bone marrow.

Early B cell factor 1 regulates adipocyte morphology and lipolysis in white adipose tissue

White adipose tissue (WAT) morphology characterized by hypertrophy (i.e., fewer but larger adipocytes) associates with increased adipose inflammation, lipolysis, insulin resistance, and risk of diabetes. However, the causal relationships and the mechanisms controlling WAT morphology are unclear. Herein, we identified EBF1 as an adipocyte-expressed transcription factor with decreased expression/activity in WAT hypertrophy. In human adipocytes, the regulatory targets of EBF1 were enriched for genes controlling lipolysis and adipocyte morphology/differentiation, and in both humans and murine models, reduced EBF1 levels associated with increased lipolysis and adipose hypertrophy. Although EBF1 did not affect adipose inflammation, TNFα reduced EBF1 gene expression. High-fat diet intervention in Ebf1(+/-) mice resulted in more pronounced WAT hypertrophy and attenuated insulin sensitivity compared with wild-type littermate controls. We conclude that EBF1 is an important regulator of adipose morphology and fat cell lipolysis and may constitute a link between WAT inflammation, altered lipid metabolism, adipose hypertrophy, and insulin resistance.

How B cells influence bone biology in health and disease

It is now well established that important regulatory interactions occur between the cells in the hematopoietic, immune and skeletal systems (osteoimmunology). B lymphocytes (B cells) are responsible for the generation and production of antibodies or immunoglobulins in the body. Together with T cells these lymphocytes comprise the adaptive immune system, which allows an individual to develop specific responses to an infection and retain memory of that infection, allowing for a faster and more robust response if that same infection occurs again. In addition to this immune function, B cells have a close and multifaceted relationship with bone cells. B cells differentiate from hematopoietic stem cells (HSCs) in supportive niches found on endosteal bone surfaces. Cells in the osteoblast lineage support HSC and B cell differentiation in these niches. B cell differentiation is regulated, at least in part, by a series of transcription factors that function in a temporal manner. While these transcription factors are required for B cell differentiation, their loss causes profound changes in the bone phenotype. This is due, in part, to the close relationship between macrophage/osteoclast and B cell differentiation. Cross talk between B cells and bone cells is reciprocal with defects in the RANKL-RANK, OPG signaling axis resulting in altered bone phenotypes. While the role of B cells during normal bone remodeling appears minimal, activated B cells play an important role in many inflammatory diseases with associated bony changes. This review examines the relationship between B cells and bone cells and how that relationship affects the skeleton and hematopoiesis during health and disease.

Ebf1-dependent control of the osteoblast and adipocyte lineages

Ebf1 is a transcription factor essential for B cell fate specification and function and important for the development of olfactory sensory neurons. We show here that Ebf1 also plays an important role in regulating osteoblast and adipocyte development in vivo. Ebf1 mRNA and protein is expressed in MSCs, in OBs at most stages of differentiation, and in adipocytes. Tibiae and femora from Ebf1(-/-) mice had a striking increase in all bone formation parameters examined including the number of OBs, osteoid volume, and bone formation rate. Serum osteocalcin, a marker of bone formation, was significantly elevated in mutant mice. The numbers of osteoclasts in bone were normal in younger (4 week-old) Ebf1(-/-) mice but increased in older (12 week-old) Ebf1(-/-) mice. This correlated well with in vitro osteoclast development from bone marrow cells. In addition to the increased osteoblastogenesis, there was a dramatic increase in adipocyte numbers in the bone marrow of Ebf1(-/-) mice. Increased adiposity was also seen histologically in the liver but not in the spleen of these mice, and accompanied by decreased deposition of adipose to subcutaneous sites. Thus Ebf1-deficient mice appear to be a new model of lipodystrophy. Ebf1 is a rare example of a transcription factor that regulates both the osteoblast and adipocyte lineages similarly.

1,25-Dihydroxyvitamin D3 regulates the expression of low-density lipoprotein receptor-related protein 5 via deoxyribonucleic acid sequence elements located downstream of the start site of transcription

The skeleton is a direct target of vitamin D action, where the hormone modulates the proliferation of osteoblast precursors, their differentiation into mature osteoblasts, and their functional activity. Some of these effects of vitamin D are reminiscent of those orchestrated by the Wnt signaling pathway wherein stimulation of the membrane receptor Frizzled and its coreceptor LRP5 leads to activation of beta-catenin and subsequent transcription-mediated changes in osteoblast biology. Indeed, LRP5 is now known to play a particularly important role in bone formation such that the loss of this component results in a reduction in osteoblast number, a delay in mineralization, and a reduction in peak bone mineral density. Interestingly, we discovered during the course of a vitamin D receptor (VDR) chromatin immunoprecipitation/DNA microarray analysis that 1,25-(OH)2D3 could induce binding of the VDR to sites within the Lrp5 gene locus. VDR and retinoid X receptor binding was evident both in primary osteoblasts as well as in osteoblasts of cell line origin. Importantly, this interaction between 1,25-(OH)2D3-activated VDR and the Lrp5 gene led to both a modification in chromatin structure within the Lrp5 locus and the induction of Lrp5 mRNA transcripts in vivo as well as in vitro. One of these sites within the Lrp5 locus was discovered to confer vitamin D response to a heterologous promoter when introduced into osteoblastic cells, permitting both the identification and characterization of the vitamin D response element located within. Interestingly, additional studies revealed that whereas the regulatory region in the mouse Lrp5 gene was highly conserved in the human genome, the vitamin D response element was not. Our studies show that 1,25-(OH)2D3 can enhance the expression of a critical component of the Wnt signaling pathway that is known to impact osteogenesis.

Targeted deletion of a distant transcriptional enhancer of the receptor activator of nuclear factor-kappaB ligand gene reduces bone remodeling and increases bone mass

Receptor activator of nuclear factor-kappaB ligand (RANKL) is essential for osteoclast differentiation, and hormones and cytokines that stimulate bone resorption increase RANKL expression in stromal/osteoblastic cells. We have previously shown that PTH and 1,25-dihydroxyvitamin D(3) control murine RANKL gene expression in vitro, in part, via an evolutionarily conserved transcriptional enhancer, designated the distal control region (DCR), located 76 kb upstream from the transcription start site. Herein we describe the phenotype of mice lacking this enhancer. Deletion of the DCR reduced PTH and 1,25-dihydroxyvitamin D(3) stimulation of RANKL mRNA and osteoclast formation in primary bone marrow cultures as well as stimulation of RANKL mRNA in bone. DCR deletion also reduced basal RANKL mRNA levels in bone, thymus, and spleen. Moreover, mice lacking the DCR exhibited increased bone mass and strength. The increase in bone mass was due to reduced osteoclast and osteoblast formation leading to a low rate of bone remodeling similar to that observed in humans and mice with hypoparathyroidism. These findings demonstrate that hormonal control of RANKL expression via the DCR is a critical determinant of the rate of bone remodeling.

Perspectives on mechanisms of gene regulation by 1,25-dihydroxyvitamin D3 and its receptor

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) functions as a systemic signal in vertebrate organisms to control the expression of genes whose products are vital to the maintenance of calcium and phosphorus homeostasis. This regulatory capability is mediated by the vitamin D receptor (VDR) which localizes at DNA sites adjacent to the promoter regions of target genes and initiates the complex events necessary for transcriptional modulation. Recent investigations using chromatin immunoprecipitation techniques combined with various gene scanning methodologies have revealed new insights into the location, structure and function of these regulatory regions. In the studies reported here, we utilized the above techniques to identify key enhancer regions that mediate the actions of vitamin D on the calcium ion channel gene TRPV6, the catabolic bone calcium-mobilizing factor gene RankL and the bone anabolic Wnt signaling pathway co-receptor gene LRP5. We also resolve the mechanism whereby 1,25(OH)(2)D(3) autoregulates the expression of its own receptor. The results identify new features of vitamin D-regulated enhancers, including their locations at gene loci, the structure of the VDR binding sites located within, their modular nature and their functional activity. Our studies suggest that vitamin D enhancers regulate the expression of key target genes by facilitating the recruitment of both the basal transcriptional machinery as well as the protein complexes necessary for altered gene expression.

Altered metabolism and lipodystrophy in the early B-cell factor 1-deficient mouse

We previously reported that mice deficient for the transcription factor early B-cell factor (Ebf1) exhibit markedly increased numbers of osteoblasts, bone formation rate, and serum osteocalcin, but the bone marrow of Ebf1(-/-) mice is also striking in its increased marrow adiposity. The purpose of this work was to analyze the metabolic phenotype that accompanies the altered bone morphology of Ebf1(-/-) mice. Whereas marrow adiposity was increased, deposition of white adipose tissue in other regions of the body was severely reduced (sc 40-50%, abdominally 80-85%). Brown adipose exhibited decreased lipid deposition. Subcutaneous and perigonadal white adipose tissue showed a decrease in mRNA transcripts for peroxisomal proliferator-activated receptor-gamma2 and CCAAT/enhancer-binding protein-beta in Ebf1(-/-) tissue compared with wild type. Circulating levels of leptin were decreased in Ebf1(-/-) animals compared with their littermate controls (down 65-95%), whereas adiponectin remained comparable after 2 wk of age. Serum analysis also found the Ebf1(-/-) animals were hypoglycemic and hypotriglyceridemic. After ip injection of insulin, the serum glucose levels in Ebf1(-/-) mice took longer to recover, and after a glucose challenge the Ebf1(-/-) animals reached serum glucose levels almost twice that of their wild-type counterparts. Measurement of circulating pancreatic hormones revealed normal or reduced insulin levels in the Ebf1(-/-) mice, whereas glucagon was significantly increased (up 1.7- to 8.5-fold). Metabolically the Ebf1(-/-) mice had increased O(2) consumption, CO(2) production, food and water intake, and activity. Markers for gluconeogenesis, however, were decreased in the Ebf1(-/-) mice compared with controls. In conclusion, the Ebf1-deficient animals exhibit defects in adipose tissue deposition with increased marrow adiposity and impaired glucose mobilization.

Multiple enhancer regions located at significant distances upstream of the transcriptional start site mediate RANKL gene expression in response to 1,25-dihydroxyvitamin D3

One of the primary regulators of receptor activator of NF-kappaB ligand (RANKL) is 1,25-dihydoxyvitamin D(3) (1,25(OH)(2)D(3)). To elucidate the mechanism whereby 1,25(OH)(2)D(3) activates RANKL expression we screened some 300kb of the RANKL gene locus using a ChIP on chip analysis and identified five potential regulatory regions lying significant distances upstream of the transcription start site (TSS), the farthest over 70kb from the TSS. A direct ChIP analysis confirmed the presence of the VDR/RXR heterodimer at these sites. The binding of the VDR was associated with histone modification and enhanced entry of RNA polymerase II, indicating an important functional consequence to the localization of these transcription factors in response to 1,25(OH)(2)D(3). The region -76kb upstream from the TSS, termed D5, was capable of mediating VDR-dependent transcriptional output in response to 1,25(OH)(2)D(3) in luciferase assays. The identified VDRE in this region was able to confer dramatic 1,25(OH)(2)D(3) sensitivity to heterologous promoters. This region was highly evolutionarily conserved and functionally active in the human RANKL gene as well. We propose that the RANKL gene is regulated via multiple enhancers that while located at significant distances from the TSS, likely form a chromatin hub centered on the RankL promoter.

Regulation of aryl hydrocarbon receptor function by selective estrogen receptor modulators

Selective estrogen receptor modulators (SERMs), such as tamoxifen (TAM), have been used extensively for the treatment and prevention of breast cancer and other pathologies associated with aberrant estrogen receptor (ER) signaling. These compounds exhibit cell-selective agonist/antagonist activities as a consequence of their ability to induce different conformational changes in ER, thereby enabling it to recruit functionally distinct transcriptional coregulators. However, the observation that SERMs can also regulate aspects of calcium signaling and apoptosis in an ER-independent manner in some systems suggests that some of the activity of drugs within this class may also arise as a consequence of their ability to interact with targets other than ER. In this study, we demonstrate that 4-hydroxy-TAM (4OHT), an active metabolite of TAM, directly binds to and modulates the transcriptional activity of the aryl hydrocarbon receptor (AHR). Of specific interest was the observation, that in the absence of ER, 4OHT can induce the expression of AHR target genes involved in estradiol metabolism, cellular proliferation, and metastasis in cellular models of breast cancer. The potential role for AHR in SERM pharmacology was further underscored by the ability of 4OHT to suppress osteoclast differentiation in vitro in part through AHR. Cumulatively, these findings provide evidence that it is necessary to reevaluate the relative roles of ER and AHR in manifesting the pharmacological actions and therapeutic efficacy of TAM and other SERMs.

Receptor activator of nuclear factor-kappaB ligand-induced nuclear factor of activated T cells (C1) autoregulates its own expression in osteoclasts and mediates the up-regulation of tartrate-resistant acid phosphatase

Osteoclasts are large multinucleated, bone-resorbing cells derived from hematopoietic precursors in response to receptor activator of nuclear factor-kappaB ligand (RANKL). RANKL activates a number of signal transduction pathways, which stimulate, in turn, a series of specific transcription factors that initiate the process of osteoclastogenesis. Perhaps the most important of these is nuclear factor of activated T cells cytoplasmic 1 (NFATc1), a DNA-binding protein that upon activation translocates to the nucleus where it stimulates transcription. The objective of this study was to explore the process whereby RANKL induces NFATc1 and to assess the role of this factor in the activation of an additional key osteoclast target gene. We found that whereas several NFAT members are expressed in RAW264.7 cells, soluble RANKL-induced up-regulation is limited to NFATc1 through a mechanism that is largely autoregulatory. Thus, although we observed the presence of resident NFAT members at the inducible Nfatc1 P1 promoter at very early times after RANKL treatment, a selective and time-dependent increase in the binding of up-regulated NFATc1 to Nfatc1 was observed beginning at 12 h. Several additional factors that are activated by soluble RANKL and also participate in NFATc1 up-regulation include c-Fos and RNA polymerase II. Chromatin immunoprecipitation analysis also revealed a similar, time-dependent accumulation of NFATc1 at multiple sites on the Acp5 promoter, thereby highlighting a central contributing role for NFATc1 in the activation of this gene as well. Our studies provide additional molecular detail regarding the mechanisms through which RANKL induces NFATc1 in osteoclast precursors and into mechanisms by which NFATc1 induces the expression of at least one gene responsible for the osteoclast phenotype.

1,25-Dihydroxyvitamin D3 induces expression of the Wnt signaling co-regulator LRP5 via regulatory elements located significantly downstream of the gene's transcriptional start site

Canonical Wnt signaling is essential for bone formation. Activation involves binding of secreted members of the Wnt family of proteins with a membrane receptor Frizzled on osteoblasts, an interaction that is facilitated by LRP5/LRP6 co-receptors. LRP5 is known to play a particularly important role in bone formation such that the loss of this protein results in a reduction in osteoblast number, a delay in mineralization and a reduction in peak BMD. During the course of a VDR ChIP-chip analysis we found that 1,25(OH)(2)D(3) could induce binding of the VDR to sites within the Lrp5 gene locus. Importantly, this interaction between 1,25(OH)(2)D(3)-activated VDR and the Lrp5 gene led to both a modification in chromatin structure within the Lrp5 locus and the induction of LRP5 mRNA transcripts in vivo as well as in vitro. One site within Lrp5 was discovered to confer 1,25(OH)(2)D(3) response to a heterologous promoter in osteoblastic cells, permitting both the identification and characterization of the component VDRE. While the regulatory region in Lrp5 was highly conserved in the human genome, the VDRE was not. Our studies show that 1,25(OH)(2)D(3) can enhance the expression of a critical component of the Wnt signaling pathway which is known to impact osteogenesis.

IL-1β Drives Production of FGF-23 at the Onset of Chronic Kidney Disease in Mice

FGF-23 has arisen as an early biomarker of renal dysfunction, but at the onset of chronic kidney disease (CKD), data suggest that FGF-23 may be produced independently of the parathyroid hormone (PTH), 1,25(OH)₂ -vitamin D₃ signaling axis. Iron status is inversely correlated to the level of circulating FGF-23, and improvement in iron bioavailability within patients correlates with a decrease in FGF-23. Alternately, recent evidence also supports a regulatory role of inflammatory cytokines in the modulation of FGF-23 expression. To determine the identity of the signal from the kidney-inducing upregulation of osteocytic FGF-23 at the onset of CKD, we utilized a mouse model of congenital CKD that fails to properly mature the glomerular capillary tuft. We profiled the sequential presentation of indicators of renal dysfunction, phosphate imbalance, and iron bioavailability and transport to identify the events that initiate osteocytic production of FGF-23 during the onset of CKD. We report here that elevations in circulating intact-FGF-23 coincide with the earliest indicators of renal dysfunction (P14), and precede changes in serum phosphate or iron homeostasis. Serum PTH was also not changed within the first month. Instead, production of the inflammatory protein IL-1β from the kidney and systemic elevation of it in the circulation matched the induction of FGF-23. IL-1β's ability to induce FGF-23 was confirmed on bone chips in culture and within mice in vivo. Furthermore, neutralizing antibody to IL-1β blocked FGF-23 expression in both our congenital model of CKD and a second nephrotoxic serum-mediated model. We conclude that early CKD resembles a situation of primary FGF-23 excess mediated by inflammation. These findings do not preclude that altered mineral availability or anemia can later modulate FGF-23 levels but find that in early CKD they are not the driving stimulus for the initial upregulation of FGF-23. © 2020 American Society for Bone and Mineral Research.

Early B Cell Factor 1 (EBF1) Regulates Glomerular Development by Controlling Mesangial Maturation and Consequently COX-2 Expression

BACKGROUND

We recently showed the transcription factor Early B cell factor 1 (EBF1) is essential for the last stages of metanephric development, and that mice globally deficient in EBF1 display impaired maturation of peripheral glomeruli. EBF1 is present within multiple glomerular cell types, including the glomerular mesangium and podocytes.

METHODS

To identify which cell type is driving the glomerular developmental defects in the global EBF1 knockout mice, we deleted EBF1 from the mesangium/pericytes (Foxd1-cre) or podocytes (Podocin-cre) in mice.

RESULTS

Deletion of EBF1 from Foxd1 lineage cells resulted in hypoplastic kidneys, poorly differentiated peripheral glomeruli, and decreased proximal tubular mass in the outer cortex. Renal insufficiency was apparent at P21 when proteinuria presents, fibrosis of both the glomeruli and interstitium rapidly progresses, microthrombi appear, and hematuria develops. Approximately half of the Foxd1⁺, Ebf1 ^fl/fl mice die before they are 3 months old. Mice with podocyte-targeted deletion of EBF1 exhibited no developmental abnormalities. Mice with Ebf1 deficiency in Foxd1 lineage cells shared characteristics with Ptgs2/COX-2-insufficient models, and mechanistic investigation revealed impaired calcineurin/NFATc1 activation and decreased COX-2 expression. Deletion of COX-2 from the interstitial/mesangial lineage displayed a less severe phenotype than EBF1 deficiency in mice. Overexpressing COX-2 in the EBF1-deficient mice, however, partially restored glomerular development.

CONCLUSIONS

The results suggest that EBF1 regulates metanephric development at the last stages of glomerular maturation through its actions in the stromal progenitor (Foxd1⁺) lineage where it mediates proper regulation of calcineurin/NFAT signaling and COX-2 expression.

"Small" Intestinal Immunopathology Plays a "Big" Role in Lethal Cytokine Release Syndrome, and Its Modulation by Interferon-γ, IL-17A, and a Janus Kinase Inhibitor

Chimeric antigen receptor T cell (CART) therapy, administration of certain T cell-agonistic antibodies, immune check point inhibitors, coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome–coronavirus 2 (SARS-CoV-2) and Toxic shock syndrome (TSS) caused by streptococcal as well as staphylococcal superantigens share one common complication, that is T cell-driven cytokine release syndrome (CRS) accompanied by multiple organ dysfunction (MOD). It is not understood whether the failure of a particular organ contributes more significantly to the severity of CRS. Also not known is whether a specific cytokine or signaling pathway plays a more pathogenic role in precipitating MOD compared to others. As a result, there is no specific treatment available to date for CRS, and it is managed only symptomatically to support the deteriorating organ functions and maintain the blood pressure. Therefore, we used the superantigen-induced CRS model in HLA-DR3 transgenic mice, that closely mimics human CRS, to delineate the immunopathogenesis of CRS as well as to validate a novel treatment for CRS. Using this model, we demonstrate that (i) CRS is characterized by a rapid rise in systemic levels of several Th1/Th2/Th17/Th22 type cytokines within a few hours, followed by a quick decline. (ii) Even though multiple organs are affected, small intestinal immunopathology is the major contributor to mortality in CRS. (iii) IFN-γ deficiency significantly protected from lethal CRS by attenuating small bowel pathology, whereas IL-17A deficiency significantly increased mortality by augmenting small bowel pathology. (iv) RNA sequencing of small intestinal tissues indicated that IFN-γ-STAT1-driven inflammatory pathways combined with enhanced expression of pro-apoptotic molecules as well as extracellular matrix degradation contributed to small bowel pathology in CRS. These pathways were further enhanced by IL-17A deficiency and significantly down-regulated in mice lacking IFN-γ. (v) Ruxolitinib, a selective JAK-1/2 inhibitor, attenuated SAg-induced T cell activation, cytokine production, and small bowel pathology, thereby completely protecting from lethal CRS in both WT and IL-17A deficient HLA-DR3 mice. Overall, IFN-γ-JAK-STAT-driven pathways contribute to lethal small intestinal immunopathology in T cell-driven CRS.

Deletion of the transcription factor EBF1 in perivascular stroma disrupts skeletal homeostasis and precipitates premature aging of the marrow microenvironment

Early B cell factor 1 (EBF1) is a transcription factor expressed by multiple lineages of stromal cells within the bone marrow. While cultures of Ebf1-deficient cells have been demonstrated to have impaired differentiation into either the osteoblast or adipogenic lineage in vitro by several groups, in vivo there has been a nominal consequence of the loss of EBF1 on skeletal development. In this study we used Prx-cre driven deletion of Ebf1 to eliminate EBF1 from the entire mesenchymal lineage of the skeleton and resolve this discrepancy. We report here that EBF1 is expressed primarily in the Mesenchymal Stem and Progenitor Cell (MSPC)-Adipo, MSPC-Osteo, and the Early Mesenchymal Progenitors, and that loss of EBF1 has a plethora of consequences to maintenance of the skeleton throughout adulthood. Stroma from the Prx-cre;Ebf1fl/fl bones had impaired osteogenic differentiation, an age-dependent loss of CFU-F, and elevated senescence accompanying Ebf1-deletion. New bone formation was reduced after 3 months, and resulted in a quiescent bone environment with fewer osteoblasts and an accompanied reduction in osteoclast-mediated remodeling. Consequently, bones were less ductile at a younger age, and deletion of EBF1 dramatically impaired fracture repair. Disruption of EBF1 in perivascular populations also rearranged the vascular network within these bones and disrupted cytokine signaling from key hematopoietic niches resulting in anemia, reductions in B cells, and myeloid skewing of marrow hematopoietic lineages. Mechanistically we observed disrupted BMP signaling within Ebf1-deficient progenitors with reduced SMAD1-phosphorylation, and elevated secretion of the soluble BMP-inhibitor Gremlin from the MSPC-Adipo cells. Ebf1-deficient progenitors also exhibited posttranslational suppression of glucocorticoid receptor expression. Together, these results suggest that EBF1 signaling is required for mesenchymal progenitor mobilization to maintain the adult skeleton, and that the primary action of EBF1 in the early mesenchymal lineage is to promote proliferation, and differentiation of these perivascular cells to sustain a healthy tissue.

Optimized Methyl methacrylate embedding of small and large undecalcified bones

Methyl methacrylate (MMA) plastic embedding has been long established as a technique for the processing and histological assessment of bones. It provides the added benefit over paraffin in that it does not require decalcification of the tissue in order visualize the cellular detail, thus preserving vital information about the amount of unmineralized osteoid present in addition to the degree of mineralization in the bone. It also allows for the incorporation of dynamic histomorphometric analysis through the retention of fluorescent labels incorporated into the bone. Efficient infiltration of hard tissue is essential to the processing of bones and producing quality slides suitable for achieving usable quantifiable histology out the other end. This technique:•Updates previously published MMA embedding protocols to reflect utilization of stabilized acrylamides (over the unstabilized reagents of the past)•Outlines the techniques that are important for embedding both small (mus), medium (rattus), and large (porcine, lagomorph, human) histological samples.•Updates the clearing and infiltration processes utilized and validates quality of the sample preparation though histological staining to confirm preservation of cellular detail, mineralization information, and enzymatic activity.