An RNA-seq protocol to identify mRNA expression changes in mouse diaphyseal bone: applications in mice with bone property altering Lrp5 mutations

Role of sphingolipid metabolism signaling in a novel mouse model of renal osteodystrophy based on transcriptomic approach

BACKGROUND

Renal osteodystrophy (ROD) is a skeletal pathology associated with chronic kidney disease-mineral and bone disorder (CKD-MBD) that is characterized by aberrant bone mineralization and remodeling. ROD increases the risk of fracture and mortality in CKD patients. The underlying mechanisms of ROD remain elusive, partially due to the absence of an appropriate animal model. To address this gap, we established a stable mouse model of ROD using an optimized adenine-enriched diet and conducted exploratory analyses through ribonucleic acid sequencing (RNA-seq).

METHODS

Male 8-week-old C57BL/6J mice were randomly allocated into three groups: control group (n = 5), adenine and high-phosphate (HP) diet group (n = 20), and the optimized adenine-containing diet group (n = 20) for 12 weeks. We assessed the skeletal characteristics of model mice through blood biochemistry, microcomputed tomography (micro-CT), and bone histomorphometry. RNA-seq was utilized to profile gene expression changes of ROD. We elucidated the functions of differentially expressed genes (DEGs) using gene ontology (GO) analysis, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, and gene set enrichment analysis (GSEA). DEGs were validated via quantitative real-time polymerase chain reaction (qRT-PCR).

RESULTS

By the fifth week, adenine followed by an HP diet induced rapid weight loss and high mortality rates in the mouse group, precluding further model development. Mice with optimized adenine diet-induced ROD displayed significant abnormalities in serum creatinine and blood urea nitrogen levels, accompanied by pronounced hyperparathyroidism and hyperphosphatemia. The femur bone mineral density (BMD) of the model mice was lower than that of control mice, with substantial bone loss and cortical porosity. ROD mice exhibited substantial bone turnover with an increase in osteoblast and osteoclast markers. Transcriptomic profiling revealed 1907 genes with upregulated expression and 723 genes with downregulated expression in the femurs of ROD mice relative to those of control mice. Pathway analyses indicated significant enrichment of upregulated genes in the sphingolipid metabolism pathway. The significant upregulation of alkaline ceramidase 1 (Acer1), alkaline ceramidase 2 (Acer2), prosaposin-like 1 (Psapl1), adenosine A1 receptor (Adora1), and sphingosine-1-phosphate receptor 5 (S1pr5) were successfully validated in mouse femurs by qRT-PCR.

CONCLUSIONS

Optimized adenine diet mouse model may be a valuable proxy for studying ROD. RNA-seq analysis revealed that the sphingolipid metabolism pathway is likely a key player in ROD pathogenesis, thereby providing new avenues for therapeutic intervention.

Swim training induces distinct osseous gene expression patterns in anosteocytic and osteocytic teleost fish

The traditional understanding of bone mechanosensation implicates osteocytes, canaliculi, and the lacunocanalicular network in biomechanical adaptation. However, recent findings challenge this notion, as shown in advanced teleost fish where anosteocytic bone lacking osteocytes are nevertheless responsive to mechanical load. To investigate specific molecular mechanisms involved in bone mechanoadaptation in osteocytic and anosteocytic fish bone, we conducted a 5-min single swim-training experiment with zebrafish and ricefish, respectively. Through RNASeq analysis of fish spines, analyzed at various time points following swim training, we uncovered distinct gene expression patterns in osteocytic and anosteocytic fish bones. Notably, osteocytic fish bone exhibited an early response to mechanical load, contrasting to a delayed response observed in anosteocytic fish bones, both within 8 h following stimulation. We identified an increase in osteoblast differentiation in anosteocytic bone following training, while chordoblast activity was delayed. This temporal response suggests a time-dependent adaptation in anosteocytic bone, indicating the presence of intricate feedback networks within bone that lacks osteocytes.

Controlled mechanical loading affects the osteocyte transcriptome in porcine trabecular bone in situ

INTRODUCTION

Osteocytes modulate bone adaptation in response to mechanical stimuli imparted by the deforming bone tissue in which they are encased by communicating with osteoclasts and osteoblasts as well as other osteocytes in the lacuna-canalicular network through secreted cytokines and chemokines. Understanding the transcriptional response of osteocytes to mechanical stimulation in situ could identify new targets to inhibit bone loss or enhance bone formation in the presence of diseases like osteoporosis or metastatic cancer. We compared the mechanically regulated transcriptional response of osteocytes in trabecular bone following one or three days of controlled mechanical loading.

METHODS

Porcine trabecular bone explants were cultured in a bioreactor for 48 h and subsequently loaded twice a day for one day or 3 days. RNA was isolated and sequenced, and the Tuxedo suite was used to identify differentially expressed genes and pathway analysis was conducted using Ingenuity Pathway Analysis (IPA).

RESULTS

There were about 4000 differentially expressed genes following in situ culture relative to fresh bone. One hundred six genes were differentially expressed between the loaded and non-loaded groups following one day of loading compared to 913 genes after 3 d of loading. Only 45 of these were coincident between the two time points, indicating an evolving transcriptome. Clustering and principal component analysis indicated differences between the loaded and non-loaded groups after 3 d of loading.

DISCUSSION

With sustained loading, there was a nine-fold increase in the number of differentially expressed genes, suggesting that osteocytes respond to loading through sequential activation of downstream genes in the same pathways. The differentially expressed genes were related to osteoarthritis, osteocyte, and chondrocyte signaling pathways. We noted that NFkB and TNF signaling are affected by early loading and this may drive downstream effects on the mechanobiological response. Moreover, these genes may regulate catabolic effects of mechanical disuse through their actions on pre-osteoclasts in the bone marrow niche.

Genetic and Gene Expression Resources for Osteoporosis and Bone Biology Research

PURPOSE OF REVIEW

The integration of data from multiple genomic assays from humans and non-human model organisms is an effective approach to identify genes involved in skeletal fragility and fracture risk due to osteoporosis and other conditions. This review summarizes genome-wide genetic variation and gene expression data resources relevant to the discovery of genes contributing to skeletal fragility and fracture risk.

RECENT FINDINGS

Genome-wide association studies (GWAS) of osteoporosis-related traits are summarized, in addition to gene expression in bone tissues in humans and non-human organisms, with a focus on rodent models related to skeletal fragility and fracture risk. Gene discovery approaches using these genomic data resources are described. We also describe the Musculoskeletal Knowledge Portal (MSKKP) that integrates much of the available genomic data relevant to fracture risk. The available genomic resources provide a wealth of knowledge and can be analyzed to identify genes related to fracture risk. Genomic resources that would fill particular scientific gaps are discussed.

Targeted inhibition of Wnt signaling with a Clostridioides difficile toxin B fragment suppresses breast cancer tumor growth

Wnt signaling pathways are transmitted via 10 homologous frizzled receptors (FZD1-10) in humans. Reagents broadly inhibiting Wnt signaling pathways reduce growth and metastasis of many tumors, but their therapeutic development has been hampered by the side effect. Inhibitors targeting specific Wnt-FZD pair(s) enriched in cancer cells may reduce side effect, but the therapeutic effect of narrow-spectrum Wnt-FZD inhibitors remains to be established in vivo. Here, we developed a fragment of C. difficile toxin B (TcdBFBD), which recognizes and inhibits a subclass of FZDs, FZD1/2/7, and examined whether targeting this FZD subgroup may offer therapeutic benefits for treating breast cancer models in mice. Utilizing 2 basal-like and 1 luminal-like breast cancer models, we found that TcdBFBD reduces tumor-initiating cells and attenuates growth of basal-like mammary tumor organoids and xenografted tumors, without damaging Wnt-sensitive tissues such as bones in vivo. Furthermore, FZD1/2/7-positive cells are enriched in chemotherapy-resistant cells in both basal-like and luminal mammary tumors treated with cisplatin, and TcdBFBD synergizes strongly with cisplatin in inhibiting both tumor types. These data demonstrate the therapeutic value of narrow-spectrum Wnt signaling inhibitor in treating breast cancers.

Comparative study in estrogen-depleted mice identifies skeletal and osteocyte transcriptomic responses to abaloparatide and teriparatide

Osteocytes express parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptors and respond to the PTHrP analog abaloparatide (ABL) and to the PTH 1-34 fragment teriparatide (TPTD), which are used to treat osteoporosis. Several studies indicate overlapping but distinct skeletal responses to ABL or TPTD, but their effects on cortical bone may differ. Little is known about their differential effects on osteocytes. We compared cortical osteocyte and skeletal responses to ABL and TPTD in sham-operated and ovariectomized mice. Administered 7 weeks after ovariectomy for 4 weeks at a dose of 40 μg/kg/d, TPTD and ABL had similar effects on trabecular bone, but ABL showed stronger effects in cortical bone. In cortical osteocytes, both treatments decreased lacunar area, reflecting altered peri-lacunar remodeling favoring matrix accumulation. Osteocyte RNA-Seq revealed that several genes and pathways were altered by ovariectomy and affected similarly by TPTD and ABL. Notwithstanding, several signaling pathways were uniquely regulated by ABL. Thus, in mice, TPTD and ABL induced a positive osteocyte peri-lacunar remodeling balance, but ABL induced stronger cortical responses and affected the osteocyte transcriptome differently. We concluded that ABL affected the cortical osteocyte transcriptome in a manner subtly different from TPTD, resulting in more beneficial remodeling/modeling changes and homeostasis of the cortex.

Lineage-specific differences and regulatory networks governing human chondrocyte development

To address large gaps in our understanding of the molecular regulation of articular and growth plate cartilage development in humans, we used our directed differentiation approach to generate these distinct cartilage tissues from human embryonic stem cells. The resulting transcriptomic profiles of hESC-derived articular and growth plate chondrocytes were similar to fetal epiphyseal and growth plate chondrocytes, with respect to genes both known and previously unknown to cartilage biology. With the goal to characterize the regulatory landscapes accompanying these respective transcriptomes, we mapped chromatin accessibility in hESC-derived chondrocyte lineages, and mouse embryonic chondrocytes, using ATAC-sequencing. Integration of the expression dataset with the differentially accessible genomic regions revealed lineage-specific gene regulatory networks. We validated functional interactions of two transcription factors (TFs) (RUNX2 in growth plate chondrocytes and RELA in articular chondrocytes) with their predicted genomic targets. The maps we provide thus represent a framework for probing regulatory interactions governing chondrocyte differentiation. This work constitutes a substantial step towards comprehensive and comparative molecular characterizations of distinct chondrogenic lineages and sheds new light on human cartilage development and biology.

The role of Meteorin-like in skeletal development and bone fracture healing

Meteorin-like protein (Metrnl), homologous to the initially identified neurotrophic factor Meteorin, is a secreted, multifunctional protein. Here we used mouse models to investigate Metrnl's role in skeletal development and bone fracture healing. During development Metrnl was expressed in the perichondrium and primary ossification center. In neonates, single cell RNA-seq of diaphyseal bone demonstrated strongest expression of Metrnl transcript by osteoblasts. In vitro, Metrnl was osteoinductive, increasing osteoblast differentiation and mineralization in tissue culture models. In vivo, loss of Metrnl expression resulted in no change in skeletal metrics in utero, at birth, or during postnatal growth. Six-week-old Metrnl-null mice displayed similar body length, body weight, tibial length, femoral length, BV/TV, trabecular number, trabecular thickness, and cortical thickness as littermate controls. In 4-month-old mice, lack of Metrnl expression did not change structural stiffness, ultimate force, or energy to fracture of femora under 3-point-bending. Last, we investigated the role of Metrnl in bone fracture healing. Metrnl expression increased in response to tibial injury, however, loss of Metrnl expression did not affect the amount of bone deposited within the healing tissue nor did it change the structural parameters of healing tissue. This work identifies Metrnl as a dispensable molecule for skeletal development. However, the osteoinductive capabilities of Metrnl may be utilized to modulate osteoblast differentiation in cell-based orthopedic therapies.

The Future of Biomarkers in Veterinary Medicine: Emerging Approaches and Associated Challenges

Simple Summary

In this review we seek to outline the role of new technologies in biomarker discovery, particularly within the veterinary field and with an emphasis on ‘omics’, as well as to examine why many biomarkers-despite much excitement-have not yet made it to clinical practice. Further we emphasise the critical need for close collaboration between clinicians, researchers and funding bodies and the need to set clear goals for biomarker requirements and realistic application in the clinical setting, ensuring that biomarker type, method of detection and clinical utility are compatible, and adequate funding, time and sample size are available for all phases of development.

Abstract

New biomarkers promise to transform veterinary practice through rapid diagnosis of diseases, effective monitoring of animal health and improved welfare and production efficiency. However, the road from biomarker discovery to translation is not always straightforward. This review focuses on molecular biomarkers under development in the veterinary field, introduces the emerging technological approaches transforming this space and the role of ‘omics platforms in novel biomarker discovery. The vast majority of veterinary biomarkers are at preliminary stages of development and not yet ready to be deployed into clinical translation. Hence, we examine the major challenges encountered in the process of biomarker development from discovery, through validation and translation to clinical practice, including the hurdles specific to veterinary practice and to each of the ‘omics platforms–transcriptomics, proteomics, lipidomics and metabolomics. Finally, recommendations are made for the planning and execution of biomarker studies with a view to assisting the success of novel biomarkers in reaching their full potential.

Matrix-bound Cyr61/CCN1 is required to retain the properties of the bone marrow mesenchymal stem cell niche but is depleted with aging

Previously, we showed that extracellular matrices (ECMs), produced ex vivo by various types of stromal cells, direct bone marrow mesenchymal stem cells (BM-MSCs) in a tissue-specific manner and recapitulate physiologic changes characteristic of the aging microenvironment. In particular, BM-MSCs obtained from elderly donors and cultured on ECM produced by young BM stromal cells showed improved quantity, quality and osteogenic differentiation. In the present study, we searched for matrix components that are required for a functional BM-MSC niche by comparing ECMs produced by BM stromal cells from "young" (≤25 y/o) versus "elderly" (≥60 y/o) donors. With increasing donor age, ECM fibrillar organization and mechanical integrity deteriorated, along with the ability to promote BM-MSC proliferation and responsiveness to growth factors. Proteomic analyses revealed that the matricellular protein, Cyr61/CCN1, was present in young, but undetectable in elderly, BM-ECM. To assess the role of Cyr61 in the BM-MSC niche, we used genetic methods to down-regulate the incorporation of Cyr61 during production of young ECM and up-regulate its incorporation in elderly ECM. The results showed that Cyr61-depleted young ECM lost the ability to promote BM-MSC proliferation and growth factor responsiveness. However, up-regulating the incorporation of Cyr61 during synthesis of elderly ECM restored its ability to support BM-MSC responsiveness to osteogenic factors such as BMP-2 and IGF-1. We next examined aging bone and compared bone mineral density and Cyr61 content of L4-L5 vertebral bodies in "young" (9-11 m/o) and "elderly" (21-33 m/o) mice. Our analyses showed that low bone mineral density was associated with decreased amounts of Cyr61 in osseous tissue of elderly versus young mice. Our results strongly demonstrate a novel role for ECM-bound Cyr61 in the BM-MSC niche, where it is responsible for retention of BM-MSC proliferation and growth factor responsiveness, while depletion of Cyr61 from the BM niche contributes to an aging-related dysregulation of BM-MSCs. Our results also suggest new potential therapeutic targets for treating age-related bone loss by restoring specific ECM components to the stem cell niche.

The Interaction Between Intracellular Energy Metabolism and Signaling Pathways During Osteogenesis

Osteoblasts primarily mediate bone formation, maintain bone structure, and regulate bone mineralization, which plays an important role in bone remodeling. In the past decades, the roles of cytokines, signaling proteins, and transcription factors in osteoblasts have been widely studied. However, whether the energy metabolism of cells can be regulated by these factors to affect the differentiation and functioning of osteoblasts has not been explored in depth. In addition, the signaling and energy metabolism pathways are not independent but closely connected. Although energy metabolism is mediated by signaling pathways, some intermediates of energy metabolism can participate in protein post-translational modification. The content of intermediates, such as acetyl coenzyme A (acetyl CoA) and uridine diphosphate N-acetylglucosamine (UDP-N-acetylglucosamine), determines the degree of acetylation and glycosylation in terms of the availability of energy-producing substrates. The utilization of intracellular metabolic resources and cell survival, proliferation, and differentiation are all related to the integration of metabolic and signaling pathways. In this paper, the interaction between the energy metabolism pathway and osteogenic signaling pathway in osteoblasts and bone marrow mesenchymal stem cells (BMSCs) will be discussed.

The Osteocyte Transcriptome: Discovering Messages Buried Within Bone

PURPOSE OF THE REVIEW

Osteocytes are cells embedded within the bone matrix, but their function and specific patterns of gene expression remain only partially defined; this is beginning to change with recent studies using transcriptomics. This unbiased approach can generate large amounts of data and is now being used to identify novel genes and signalling pathways within osteocytes both at baseline conditions and in response to stimuli. This review outlines the methods used to isolate cell populations containing osteocytes, and key recent transcriptomic studies that used osteocyte-containing preparations from bone tissue.

RECENT FINDINGS

Three common methods are used to prepare samples to examine osteocyte gene expression: digestion followed by sorting, laser capture microscopy, and the isolation of cortical bone shafts. All these methods present challenges in interpreting the data generated. Genes previously not known to be expressed by osteocytes have been identified and variations in osteocyte gene expression have been reported with age, sex, anatomical location, mechanical loading, and defects in bone strength. A substantial proportion of newly identified transcripts in osteocytes remain functionally undefined but several have been cross-referenced with functional data. Future work and improved methods (e.g. scRNAseq) likely provide useful resources for the study of osteocytes and important new information on the identity and functions of this unique cell type within the skeleton.

LRP5-Mediated Lipid Uptake Modulates Osteogenic Differentiation of Bone Marrow Mesenchymal Stromal Cells

Nutritional microenvironment determines the specification of progenitor cells, and lipid availability was found to modulate osteogenesis in skeletal progenitors. Here, we investigated the implications of lipid scarcity in the osteogenic differentiation of bone marrow mesenchymal stromal cells (BMSCs) and the role of low-density lipoprotein receptor-related protein 5 (LRP5), a co-receptor transducing canonical Wnt/beta-catenin signals, in BMSC lipid uptake during osteogenesis. The osteogenic differentiation of murine BMSCs was suppressed by lipid scarcity and partially rescued by additional fatty acid treatment with oleate. The enhancement of osteogenesis by oleate was found to be dosage-dependent, along with the enhanced activation of beta-catenin and Wnt target genes. Conditional knockout (CKO) of Lrp5 gene in murine mesenchymal lineage using Lrp5^fl/fl;Prrx1-cre mice led to decreased bone quality and altered fat distribution in vivo. After Lrp5 ablation using adenoviral Cre-recombinase, the accumulation of lipid droplets in BMSC cytoplasm was significantly reduced, and the osteogenesis of BMSCs was suppressed. Moreover, the impaired osteogenesis due to either lipid scarcity or Lrp5 ablation could be rescued by recombinant Wnt3a protein, indicating that the osteogenesis induced by Wnt/beta-catenin signaling was independent of LRP5-mediated lipid uptake. In conclusion, lipid scarcity suppresses BMSC osteogenic differentiation. LRP5 plays a role in the uptake of lipids in BMSCs and therefore mediates osteogenic specification.

RNA-seq Analysis of Peri-Implant Tissue Shows Differences in Immune, Notch, Wnt, and Angiogenesis Pathways in Aged Versus Young Mice

The number of total joint replacements (TJRs) in the United States is increasing annually. Cementless implants are intended to improve upon traditional cemented implants by allowing bone growth directly on the surface to improve implant longevity. One major complication of TJR is implant loosening, which is related to deficient osseointegration in cementless TJRs. Although poor osseointegration in aged patients is typically attributed to decreased basal bone mass, little is known about the molecular pathways that compromise the growth of bone onto porous titanium implants. To identify the pathways important for osseointegration that are compromised by aging, we developed an approach for transcriptomic profiling of peri-implant tissue in young and aged mice using our murine model of osseointegration. Based on previous findings of changes of bone quality associated with aging, we hypothesized that aged mice have impaired activation of bone anabolic pathways at the bone-implant interface. We found that pathways most significantly downregulated in aged mice relative to young mice are related to angiogenic, Notch, and Wnt signaling. Downregulation of these pathways is associated with markedly increased expression of inflammatory and immune genes at the bone-implant interface in aged mice. These results identify osseointegration pathways affected by aging and suggest that an increased inflammatory response in aged mice may compromise peri-implant bone healing. Targeting the Notch and Wnt pathways, promoting angiogenesis, or modulating the immune response at the peri-implant site may enhance osseointegration and improve the outcome of joint replacement in older patients. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

RNAseq and RNA molecular barcoding reveal differential gene expression in cortical bone following hindlimb unloading in female mice

Disuse-induced bone loss is seen following spinal cord injury, prolonged bed rest, and exposure to microgravity. We performed whole transcriptomic profiling of cortical bone using RNA sequencing (RNAseq) and RNA molecular barcoding (NanoString) on a hindlimb unloading (HLU) mouse model to identify genes whose mRNA transcript abundances change in response to disuse. Eleven-week old female C57BL/6 mice were exposed to ambulatory loading or HLU for 7 days (n = 8/group). Total RNA from marrow-flushed femoral cortical bone was analyzed on HiSeq and NanoString platforms. The expression of several previously reported genes associated with Wnt signaling and metabolism was altered by HLU. Furthermore, the increased abundance of transcripts, such as Pfkfb3 and Mss51, after HLU imply these genes also have roles in the cortical bone’s response to altered mechanical loading. Our study demonstrates that an unbiased approach to assess the whole transcriptomic profile of cortical bone can reveal previously unidentified mechanosensitive genes and may eventually lead to novel targets to prevent disuse-induced osteoporosis.

Functional interaction between Wnt and Bmp signaling in periosteal bone growth

Wnt and Bmp proteins are well known to regulate bone development and homeostasis. Although both signals are extensively studied, their potential interaction in vivo is less well understood. Previous studies have shown that deletion of Bmpr1a, a type I receptor for Bmp signaling, results in excessive trabecular bone formation while diminishing periosteal bone growth. Moreover, forced-expression of the Wnt antagonist Sost suppresses the overgrowth of trabecular bone caused by Bmpr1a deletion, thus implicating hyperactive Wnt signaling in the excessive trabecular bone formation. However, it remains uncertain whether Wnt and Bmp signaling interacts in regulating the periosteal bone growth. Here we show that multiple Wnt genes are markedly suppressed in the cortical bone without Bmpr1a. Importantly, overexpression of Wnt7b fully rescues periosteal bone growth in the Bmpr1a-deficient mice. Thus, pharmacological activation of Wnt signaling can restore normal bone size without intact Bmp signaling.

Osteocyte transcriptome mapping identifies a molecular landscape controlling skeletal homeostasis and susceptibility to skeletal disease

Osteocytes are master regulators of the skeleton. We mapped the transcriptome of osteocytes from different skeletal sites, across age and sexes in mice to reveal genes and molecular programs that control this complex cellular-network. We define an osteocyte transcriptome signature of 1239 genes that distinguishes osteocytes from other cells. 77% have no previously known role in the skeleton and are enriched for genes regulating neuronal network formation, suggesting this programme is important in osteocyte communication. We evaluated 19 skeletal parameters in 733 knockout mouse lines and reveal 26 osteocyte transcriptome signature genes that control bone structure and function. We showed osteocyte transcriptome signature genes are enriched for human orthologs that cause monogenic skeletal disorders (P = 2.4 × 10-22) and are associated with the polygenic diseases osteoporosis (P = 1.8 × 10-13) and osteoarthritis (P = 1.6 × 10-7). Thus, we reveal the molecular landscape that regulates osteocyte network formation and function and establish the importance of osteocytes in human skeletal disease.

Biallelic mutations in LAMA5 disrupts a skeletal noncanonical focal adhesion pathway and produces a distinct bent bone dysplasia

Background

Beyond its structural role in the skeleton, the extracellular matrix (ECM), particularly basement membrane proteins, facilitates communication with intracellular signaling pathways and cell to cell interactions to control differentiation, proliferation, migration and survival. Alterations in extracellular proteins cause a number of skeletal disorders, yet the consequences of an abnormal ECM on cellular communication remains less well understood

Methods

Clinical and radiographic examinations defined the phenotype in this unappreciated bent bone skeletal disorder. Exome analysis identified the genetic alteration, confirmed by Sanger sequencing. Quantitative PCR, western blot analyses, immunohistochemistry, luciferase assay for WNT signaling were employed to determine RNA, proteins levels and localization, and dissect out the underlying cell signaling abnormalities.  Migration and wound healing assays examined cell migration properties.

Findings

This bent bone dysplasia resulted from biallelic mutations in LAMA5, the gene encoding the alpha-5 laminin basement membrane protein. This finding uncovered a mechanism of disease driven by ECM-cell interactions between alpha-5-containing laminins, and integrin-mediated focal adhesion signaling, particularly in cartilage. Loss of LAMA5 altered β1 integrin signaling through the non-canonical kinase PYK2 and the skeletal enriched SRC kinase, FYN. Loss of LAMA5 negatively impacted the actin cytoskeleton, vinculin localization, and WNT signaling.

Interpretation

This newly described mechanism revealed a LAMA5-β1 Integrin-PYK2-FYN focal adhesion complex that regulates skeletogenesis, impacted WNT signaling and, when dysregulated, produced a distinct skeletal disorder.

Funding

Supported by NIH awards R01 AR066124, R01 DE019567, R01 HD070394, and U54HG006493, and Czech Republic grants INTER-ACTION LTAUSA19030, V18-08-00567 and GA19-20123S.

The Role of Omega-3 Polyunsaturated Fatty Acids from Different Sources in Bone Development

N-3 polyunsaturated fatty acids (PUFAs) are essential nutrients that must be obtained from the diet. We have previously showed that endogenous n-3 PUFAs contribute to skeletal development and bone quality in fat-1 mice. Unlike other mammals, these transgenic mice, carry the n-3 desaturase gene and thus can convert n-6 to n-3 PUFAs endogenously. Since this model does not mimic dietary exposure to n-3 PUFAs, diets rich in fish and flaxseed oils were used to further elucidate the role of n-3 PUFAs in bone development. Our investigation reveals that dietary n-3 PUFAs decrease fat accumulation in the liver, lower serum fat levels, and alter fatty acid (FA) content in liver and serum. Bone analyses show that n-3 PUFAs improve mechanical properties, which were measured using a three-point bending test, but exert complex effects on bone structure that vary according to its source. In a micro-CT analysis, we found that the flaxseed oil diet improves trabecular bone micro-architecture, whereas the fish oil diet promotes higher bone mineral density (BMD) with no effect on trabecular bone. The transcriptome characterization of bone by RNA-seq identified regulatory mechanisms of n-3 PUFAs via modulation of the cell cycle and peripheral circadian rhythm genes. These results extend our knowledge and provide insights into the molecular mechanisms of bone remodeling regulation induced by different sources of dietary n-3 PUFAs.

Transcriptome profile of highly osteoblastic/cementoblastic periodontal ligament cell clones

Heterogeneous cell populations of osteo/cementoblastic (O/C) or fibroblastic phenotypes constitute the periodontal dental ligament (PDL). A better understanding of these PDL cell subpopulations is essential to propose regenerative approaches based on a sound biological rationale.

Single-Cell RNA Sequencing of Calvarial and Long-Bone Endocortical Cells

Single-cell RNA sequencing (scRNA-Seq) is emerging as a powerful technology to examine transcriptomes of individual cells. We determined whether scRNA-Seq could be used to detect the effect of environmental and pharmacologic perturbations on osteoblasts. We began with a commonly used in vitro system in which freshly isolated neonatal mouse calvarial cells are expanded and induced to produce a mineralized matrix. We used scRNA-Seq to compare the relative cell type abundances and the transcriptomes of freshly isolated cells to those that had been cultured for 12 days in vitro. We observed that the percentage of macrophage-like cells increased from 6% in freshly isolated calvarial cells to 34% in cultured cells. We also found that Bglap transcripts were abundant in freshly isolated osteoblasts but nearly undetectable in the cultured calvarial cells. Thus, scRNA-Seq revealed significant differences between heterogeneity of cells in vivo and in vitro. We next performed scRNA-Seq on freshly recovered long bone endocortical cells from mice that received either vehicle or sclerostin-neutralizing antibody for 1 week. We were unable to detect significant changes in bone anabolism-associated transcripts in immature and mature osteoblasts recovered from mice treated with sclerostin-neutralizing antibody; this might be a consequence of being underpowered to detect modest changes in gene expression, because only 7% of the sequenced endocortical cells were osteoblasts and a limited portion of their transcriptomes were sampled. We conclude that scRNA-Seq can detect changes in cell abundance, identity, and gene expression in skeletally derived cells. In order to detect modest changes in osteoblast gene expression at the single-cell level in the appendicular skeleton, larger numbers of osteoblasts from endocortical bone are required. © 2020 American Society for Bone and Mineral Research.

New and Old Osteocytic Cell Lines and 3D Models

The aim of this review was to compile a list of tools currently available to study bone cells and in particular osteocytes. As the interest (and importance) in osteocyte biology has greatly expanded over the past decade, new tools and techniques have become available to study these elusive cells, RECENT FINDINGS: Osteocytes are the main orchestrators of bone remodeling. They control both osteoblasts and osteoclast activities via cell-to cell communication or through secreted factors. Osteocytes are also the mechanosensors of the bone and they orchestrate skeletal adaptation to loads. Recent discoveries have greatly expanded our knowledge and understanding of these cells and new models are now available to further uncover the functions of osteocytes. Novel osteocytic cell lines, primary cultures, and 3D scaffolds are now available to investigators to further unravel the functions and roles of these cells.

Old Mice Have Less Transcriptional Activation But Similar Periosteal Cell Proliferation Compared to Young-Adult Mice in Response to in vivo Mechanical Loading

Mechanical loading is a potent strategy to induce bone formation, but with aging, the bone formation response to the same mechanical stimulus diminishes. Our main objectives were to (i) discover the potential transcriptional differences and (ii) compare the periosteal cell proliferation between tibias of young-adult and old mice in response to strain-matched mechanical loading. First, to discover potential age-related transcriptional differences, we performed RNA sequencing (RNA-seq) to compare the loading responses between tibias of young-adult (5-month) and old (22-month) C57BL/6N female mice following 1, 3, or 5 days of axial loading (loaded versus non-loaded). Compared to young-adult mice, old mice had less transcriptional activation following loading at each time point, as measured by the number of differentially expressed genes (DEGs) and the fold-changes of the DEGs. Old mice engaged fewer pathways and gene ontology (GO) processes, showing less activation of processes related to proliferation and differentiation. In tibias of young-adult mice, we observed prominent Wnt signaling, extracellular matrix (ECM), and neuronal responses, which were diminished with aging. Additionally, we identified several targets that may be effective in restoring the mechanoresponsiveness of aged bone, including nerve growth factor (NGF), Notum, prostaglandin signaling, Nell-1, and the AP-1 family. Second, to directly test the extent to which periosteal cell proliferation was diminished in old mice, we used bromodeoxyuridine (BrdU) in a separate cohort of mice to label cells that divided during the 5-day loading interval. Young-adult and old mice had an average of 15.5 and 16.7 BrdU+ surface cells/mm, respectively, suggesting that impaired proliferation in the first 5 days of loading does not explain the diminished bone formation response with aging. We conclude that old mice have diminished transcriptional activation following mechanical loading, but periosteal proliferation in the first 5 days of loading does not differ between tibias of young-adult and old mice. © 2020 American Society for Bone and Mineral Research.

Transcriptomics: a Solution for Renal Osteodystrophy?

PURPOSE OF REVIEW

The molecular mechanisms of the bone disease associated with chronic kidney disease (CKD), called renal osteodystrophy (ROD), are poorly understood. New transcriptomics technologies may provide clinically relevant insights into the pathogenesis of ROD. This review summarizes current progress and limitations in the study and treatment of ROD, and in transcriptomics analyses of skeletal tissues.

RECENT FINDINGS

ROD is characterized by poor bone quality and strength leading to increased risk of fracture. Recent studies indicate permanent alterations in bone cell populations during ROD. Single-cell transcriptomics analyses, successful at identifying specialized cell subpopulations in bone, have not yet been performed in ROD. ROD is a widespread poorly understood bone disease with limited treatment options. Transcriptomics analyses of bone are needed to identify the bone cell subtypes and their role in the pathogenesis of ROD, and to develop adequate diagnosis and treatment strategies.

Transformation of Mature Osteoblasts into Bone Lining Cells and RNA Sequencing-Based Transcriptome Profiling of Mouse Bone during Mechanical Unloading

BACKGROUND

We investigated RNA sequencing-based transcriptome profiling and the transformation of mature osteoblasts into bone lining cells (BLCs) through a lineage tracing study to better understand the effect of mechanical unloading on bone loss.

METHODS

Dmp1-CreERt2(+):Rosa26R mice were injected with 1 mg of 4-hydroxy-tamoxifen three times a week starting at postnatal week 7, and subjected to a combination of botulinum toxin injection with left hindlimb tenotomy starting at postnatal week 8 to 10. The animals were euthanized at postnatal weeks 8, 9, 10, and 12. We quantified the number and thickness of X-gal(+) cells on the periosteum of the right and left femoral bones at each time point.

RESULTS

Two weeks after unloading, a significant decrease in the number and a subtle change in the thickness of X-gal(+) cells were observed in the left hindlimbs compared with the right hindlimbs. At 4 weeks after unloading, the decrease in the thickness was accelerated in the left hindlimbs, although the number of labeled cells was comparable. RNA sequencing analysis showed downregulation of 315 genes in the left hindlimbs at 2 and 4 weeks after unloading. Of these, Xirp2, AMPD1, Mettl11b, NEXN, CYP2E1, Bche, Ppp1r3c, Tceal7, and Gadl1 were upregulated during osteoblastogenic/osteocytic and myogenic differentiation in vitro.

CONCLUSION

These findings demonstrate that mechanical unloading can accelerate the transformation of mature osteoblasts into BLCs in the early stages of bone loss in vivo. Furthermore, some of the genes involved in this process may have a pleiotropic effect on both bone and muscle.

Male mice with elevated C-type natriuretic peptide-dependent guanylyl cyclase-B activity have increased osteoblasts, bone mass and bone strength

C-type natriuretic peptide (CNP) activation of guanylyl cyclase (GC)-B, also known as NPR2, stimulates cGMP synthesis and bone elongation. CNP activation requires the phosphorylation of multiple GC-B residues and dephosphorylation inactivates the receptor. GC-B7E/7E knockin mice, expressing a glutamate-substituted, "pseudophosphorylated," form of GC-B, exhibit increased CNP-dependent GC activity. Since mutations that constitutively activate GC-B in the absence of CNP result in low bone mineral density in humans, we determined the skeletal phenotype of 9-week old male GC-B7E/7E mice. Unexpectedly, GC-B7E/7E mice have significantly greater tibial and L5 vertebral trabecular bone volume fraction, tibial trabecular number, and tibial bone mineral density. Cortical cross-sectional area, cortical thickness, periosteal diameter and cortical cross-sectional moment of inertia were also significantly increased in GC-B7E/7E tibiae. Three-point bending measurements demonstrated that the mutant tibias and femurs had greater ultimate load, stiffness, energy to ultimate load, and energy to failure. No differences in microhardness indicated similar bone quality at the tissue level between the mutant and wildtype bones. Procollagen 1 N-terminal propeptide and osteocalcin were elevated in serum, and osteoblast number per bone perimeter and osteoid width per bone perimeter were elevated in tibias from the mutant mice. In contrast to mutations that constitutively activate GC-B, we report that mutations that enhance GC-B activity only in the presence of its natural ligand, increase bone mass, bone strength, and the number of active osteoblasts at the bone surface.

An osteocalcin-deficient mouse strain without endocrine abnormalities

Osteocalcin (OCN), the most abundant noncollagenous protein in the bone matrix, is reported to be a bone-derived endocrine hormone with wide-ranging effects on many aspects of physiology, including glucose metabolism and male fertility. Many of these observations were made using an OCN-deficient mouse allele (Osc^–) in which the 2 OCN-encoding genes in mice, Bglap and Bglap2, were deleted in ES cells by homologous recombination. Here we describe mice with a new Bglap and Bglap2 double-knockout (dko) allele (Bglap/2^{p.Pro25fs17Ter}) that was generated by CRISPR/Cas9-mediated gene editing. Mice homozygous for this new allele do not express full-length Bglap or Bglap2 mRNA and have no immunodetectable OCN in their serum. FTIR imaging of cortical bone in these homozygous knockout animals finds alterations in the collagen maturity and carbonate to phosphate ratio in the cortical bone, compared with wild-type littermates. However, μCT and 3-point bending tests do not find differences from wild-type littermates with respect to bone mass and strength. In contrast to the previously reported OCN-deficient mice with the Osc⁻allele, serum glucose levels and male fertility in the OCN-deficient mice with the Bglap/2^{pPro25fs17Ter} allele did not have significant differences from wild-type littermates. We cannot explain the absence of endocrine effects in mice with this new knockout allele. Possible explanations include the effects of each mutated allele on the transcription of neighboring genes, or differences in genetic background and environment. So that our findings can be confirmed and extended by other interested investigators, we are donating this new Bglap and Bglap2 double-knockout strain to the Jackson Laboratories for academic distribution.

Cells that make and maintain bone express proteins that function either locally or systemically. The former proteins, such as type 1 collagen, affect the material properties of the skeleton, while the latter, such as fibroblast growth factor 23, enable the skeleton to communicate with other organ systems. Mutations that affect the functions of most bone-cell-expressed proteins cause diseases that have similar features in humans and other mammals such as mice, for example, brittle bone diseases for type 1 collagen mutations and hypophosphatemic rickets for mutations in fibroblast growth factor 23. Our study focuses on another bone-cell-expressed protein, osteocalcin, which has been suggested to function locally to affect bone strength and systemically as a hormone. Studies using osteocalcin knockout mice led other investigators to suggest endocrine roles for osteocalcin in regulating blood glucose, male fertility, muscle mass, brain development, behavior, and cognition. We therefore decided to generate a new strain of osteocalcin knockout mice that could also be used to investigate these nonskeletal effects. To our surprise, the osteocalcin knockout mice we created did not significantly differ from wild-type mice for the three phenotypes we examined: bone strength, blood glucose, and male fertility. Our data are consistent with findings from osteocalcin knockout rats but are inconsistent with data from the original osteocalcin knockout mice. Because we do not know why our new strain fails to recapitulate the phenotypes previously reported for another knockout mouse stain, we have donated our mice to a public repository so that they can be easily obtained and studied in other academic laboratories.

Proteolysis and cartilage development are activated in the synovium after surgical induction of post traumatic osteoarthritis

Anterior cruciate ligament (ACL) transection surgery in the minipig induces post-traumatic osteoarthritis (PTOA) in a pattern similar to that seen in human patients after ACL injury. Prior studies have reported the presence of cartilage matrix-degrading proteases, such as Matrix metalloproteinase-1 (MMP-1) and A disintegrin and metalloproteinase with thrombospondin motifs 4 (ADAMTS-4), in the synovial fluid of injured or arthritic joints; however, the tissue origin of these proteases is unknown. The objective of this study was to identify transcriptional processes activated in the synovium after surgical induction of PTOA with ACL transection, and to determine if processes associated with proteolysis were enriched in the synovium after ACL transection. Unilateral ACL transection was performed in adolescent Yucatan minipigs and synovium samples were collected at 1, 5, 9, and 14 days post-injury. Transcriptome-wide gene expression levels were determined using bulk RNA-Sequencing in the surgical animals and control animals with healthy knees. The greatest number of transcripts with significant changes was observed 1 day after injury. These changes were primarily associated with cellular proliferation, consistent with measurements of increased cellularity of the synovium at the two-week time point. At five to 14 days, the expression of transcripts relating to proteolysis and cartilage development was significantly enriched. While protease inhibitor-encoding transcripts (TIMP2, TIMP3) represented the largest fraction of protease-associated transcripts in the uninjured synovium, protease-encoding transcripts (including MMP1, MMP2, ADAMTS4) predominated after surgery. Cartilage development-associated transcripts that are typically not expressed by synovial cells, such as ACAN and COMP, were enriched in the synovium following ACL-transection. The upregulation in both catabolic processes (proteolysis) and anabolic processes (cartilage development) suggests that the synovium plays a complex, balancing role in the early response to PTOA induction.

Perlecan/Hspg2 deficiency impairs bone's calcium signaling and associated transcriptome in response to mechanical loading

Perlecan, a heparan sulfate proteoglycan, acts as a mechanical sensor for bone to detect external loading. Deficiency of perlecan increases the risk of osteoporosis in patients with Schwartz-Jampel Syndrome (SJS) and attenuates loading-induced bone formation in perlecan deficient mice (Hypo). Considering that intracellular calcium [Ca²⁺]_i is an ubiquitous messenger controlling numerous cellular processes including mechanotransduction, we hypothesized that perlecan deficiency impairs bone's calcium signaling in response to loading. To test this, we performed real-time [Ca²⁺]_i imaging on in situ osteocytes of adult murine tibiae under cyclic loading (8N). Relative to wild type (WT), Hypo osteocytes showed decreases in the overall [Ca²⁺]_i response rate (-58%), calcium peaks (-33%), cells with multiple peaks (-53%), peak magnitude (-6.8%), and recovery speed to baseline (-23%). RNA sequencing and pathway analysis of tibiae from mice subjected to one or seven days of unilateral loading demonstrated that perlecan deficiency significantly suppressed the calcium signaling, ECM-receptor interaction, and focal adhesion pathways following repetitive loading. Defects in the endoplasmic reticulum (ER) calcium cycling regulators such as Ryr1/ryanodine receptors and Atp2a1/Serca1 calcium pumps were identified in Hypo bones. Taken together, impaired calcium signaling may contribute to bone's reduced anabolic response to loading, underlying the osteoporosis risk for the SJS patients.

Frizzled-4 is required for normal bone acquisition despite compensation by Frizzled-8

The activation of the Wnt/β-catenin signaling pathway is critical for skeletal development but surprisingly little is known about the requirements for the specific frizzled (Fzd) receptors that recognize Wnt ligands. To define the contributions of individual Fzd proteins to osteoblast function, we profiled the expression of all 10 mammalian receptors during calvarial osteoblast differentiation. Expression of Fzd4 was highly upregulated during in vitro differentiation and therefore targeted for further study. Mice lacking Fzd4 in mature osteoblasts had normal cortical bone structure but reduced cortical tissue mineral density and also exhibited an impairment in the femoral trabecular bone acquisition that was secondary to a defect in the mineralization process. Consistent with this observation, matrix mineralization, markers of osteoblastic differentiation, and the ability of Wnt3a to stimulate the accumulation of β-catenin were reduced in cultures of calvarial osteoblasts deficient for Fzd4. Interestingly, Fzd4-deficient osteoblasts exhibited an increase in the expression of Fzd8 both in vitro and in vivo, which suggests that the two receptors may exhibit overlapping functions. Indeed, ablating a single Fzd8 allele in osteoblast-specific Fzd4 mutants produced a more severe effect on bone acquisition. Taken together, our data indicate that Fzd4 is required for normal bone development and mineralization despite compensation from Fzd8.

Identification of mRNAs Related to Tibial Cartilage Development of Yorkshire Piglets

Cartilage dysplasia is one of the important reasons for the weakness of pig limbs and hooves. Porcine rickets with weak limbs and hooves bring huge economic losses to the pig industry. However, research on the development of pig cartilage is lacking. This study investigated the key genes and molecular mechanisms involved in cartilage development via an RNA-seq technique. Samples of proximal tibia cartilage were collected from three normal piglets with 1 day, 14 days, and 28 days of age, respectively, and then these samples were divided into two comparison groups (1-day vs. 14-day group, 14-day vs. 28-day group). Through the transcriptome analysis, 108 differentially expressed genes (DEGs), such as FORL2, were obtained from 1-day vs. 14-day comparison group, and 3602 DEGs were obtained from 14-day vs. 28-day comparison group, including SOX9, BMP6, and MMP13. The gene ontology (GO) functional and KEGG pathway enrichment revealed that many functions of DEGs were related to bone development. The pathways of DEGs from Day 1 vs. Day 14 were mainly enriched in mineral absorption, but the DEGs of Day 14 vs. Day 28 were enriched in osteoclast differentiation. Then, the expression patterns of six candidate genes were verified via qPCR. In conclusion, candidate genes affecting cartilage development in Yorkshire pigs were obtained by transcriptome analysis, and the clues showed that Day 14 to Day 28 is a more active and extensive period in cartilage developments, which played a key role in revealing the molecular mechanism of pig cartilage development basis, also compensating for vacancies in cartilage research.

Expression of a Degradation-Resistant β-Catenin Mutant in Osteocytes Protects the Skeleton From Mechanodeprivation-Induced Bone Wasting

Mechanical stimulation is a key regulator of bone mass, maintenance, and turnover. Wnt signaling is a key regulator of mechanotransduction in bone, but the role of β-catenin-an intracellular signaling node in the canonical Wnt pathway-in disuse mechanotransduction is not defined. Using the β-catenin exon 3 flox (constitutively active [CA]) mouse model, in conjunction with a tamoxifen-inducible, osteocyte-selective Cre driver, we evaluated the effects of degradation-resistant β-catenin on bone properties during disuse. We hypothesized that if β-catenin plays an important role in Wnt-mediated osteoprotection, then artificial stabilization of β-catenin in osteocytes would protect the limbs from disuse-induced bone wasting. Two disuse models were tested: tail suspension, which models fluid shift, and botulinum-toxin (botox)-induced muscle paralysis, which models loss of muscle force. Tail suspension was associated with a significant loss of tibial bone mass and density, reduced architectural properties, and decreased bone formation indices in uninduced (control) mice, as assessed by dual-energy X-ray absorptiometry (DXA), micro-computed tomography (µCT), and histomorphometry. Activation of the βcatCA allele in tail-suspended mice resulted in little to no change in those properties; ie, these mice were protected from bone loss. Similar protective effects were observed among botox-treated mice when the βcatCA was activated. RNAseq analysis of altered gene regulation in tail-suspended mice yielded 35 genes, including Wnt11, Gli1, Nell1, Gdf5, and Pgf, which were significantly differentially regulated between tail-suspended β-catenin stabilized mice and tail-suspended nonstabilized mice. Our findings indicate that selectively targeting/blocking of β-catenin degradation in bone cells could have therapeutic implications in mechanically induced bone disease. © 2019 American Society for Bone and Mineral Research.

Using mouse genetics to understand human skeletal disease

Technological advances have enabled the study of the human genome in incredible detail with relative ease. However, our ability to interpret the functional significance of the millions of genetic variants present within each individual is limited. As a result, the confident assignment of disease-causing variant calls remains a significant challenge. Here we explore how mouse genetics can help address this deficit in functional genomic understanding. Underpinned by marked genetic correspondence, skeletal biology shows inter-species similarities which provide important opportunities to use data from mouse models to direct research into the genetic basis of skeletal pathophysiology. In this article we outline critical resources that may be used to establish genotype/phenotype relationships in skeletal tissue, identify genes with established skeletal effects and define the transcriptome of critical skeletal cell types. Finally, we outline how these mouse resources might be utilized to progress from a list of human sequence variants toward plausible gene candidates that contribute to skeletal disease.

RNA-seq in Skeletal Biology

PURPOSE OF REVIEW

The goal of this paper is to review state-of-the-art transcriptome profiling methods and their recent applications in the field of skeletal biology.

RECENT FINDINGS

Next-generation sequencing of mRNA (RNA-seq) methods have been established and routinely used in skeletal biology research. RNA-seq has led to the identification of novel genes and transcription factors involved in skeletal development and disease, through its application in small and large animal models, as well as human tissue and cells. With the availability of advanced techniques such as single-cell RNA-seq, novel cell types in skeletal tissues are being identified. As the sequencing technologies are rapidly evolving, the exciting discoveries supported by transcriptomics will continue to emerge and improve our understanding of the biology of the skeleton.

Targeted genetic screening in mice through haploid embryonic stem cells identifies critical genes in bone development

Mutagenic screening is powerful for identifying key genes involved in developmental processes. However, such screens are successful only in lower organisms. Here, we develop a targeted genetic screening approach in mice through combining androgenetic haploid embryonic stem cells (AG-haESCs) and clustered regularly interspaced palindromic repeats/CRISPR-associated protein 9 (CRISPR-Cas9) technology. We produced a mutant semi-cloned (SC) mice pool by oocyte injection of AG-haESCs carrying constitutively expressed Cas9 and an single guide RNA (sgRNA) library targeting 72 preselected genes in one step and screened for bone-development-related genes through skeletal analysis at birth. This yielded 4 genes: Zic1 and Clec11a, which are required for bone development, and Rln1 and Irx5, which had not been previously considered. Whereas Rln1-/- mice exhibited small skeletal size only at birth, Irx5-/- mice showed skeletal abnormalities both in postnatal and adult phases due to decreased bone mass and increased bone marrow adipogenesis. Mechanistically, iroquois homeobox 5 (IRX5) promotes osteoblastogenesis and inhibits adipogenesis by suppressing peroxisome proliferator activated receptor γ (PPARγ) activation. Thus, AG-haESC-mediated functional mutagenic screening opens new avenues for genetic interrogation of developmental processes in mice.

Regulation of Osteoblast Metabolism by Wnt Signaling

Wnt/β-catenin signaling plays a critical role in the achievement of peak bone mass, affecting the commitment of mesenchymal progenitors to the osteoblast lineage and the anabolic capacity of osteoblasts depositing bone matrix. Recent studies suggest that this evolutionarily-conserved, developmental pathway exerts its anabolic effects in part by coordinating osteoblast activity with intermediary metabolism. These findings are compatible with the cloning of the gene encoding the low-density lipoprotein related receptor-5 (LRP5) Wnt co-receptor from a diabetes-susceptibility locus and the now well-established linkage between Wnt signaling and metabolism. In this article, we provide an overview of the role of Wnt signaling in whole-body metabolism and review the literature regarding the impact of Wnt signaling on the osteoblast's utilization of three different energy sources: fatty acids, glucose, and glutamine. Special attention is devoted to the net effect of nutrient utilization and the mode of regulation by Wnt signaling. Mechanistic studies indicate that the utilization of each substrate is governed by a unique mechanism of control with β-catenin-dependent signaling regulating fatty acid β-oxidation, while glucose and glutamine utilization are β-catenin-independent and downstream of mammalian target of rapamycin complex 2 (mTORC2) and mammalian target of rapamycin complex 1 (mTORC1) activation, respectively. The emergence of these data has provided a new context for the mechanisms by which Wnt signaling influences bone development.

CYR61/CCN1 Regulates Sclerostin Levels and Bone Maintenance

CYR61/CCN1 is a matricellular protein that resides in the extracellular matrix, but serves regulatory rather than structural roles. CYR61/CCN1 is found in mineralized tissues and has been shown to influence bone healing in vivo and osteogenic differentiation in vitro. In this study we generated Cyr61 bone-specific knockout mice to examine the physiological role of CYR61/CCN1 in bone development and maintenance in vivo. Extensive analysis of Cyr61 conditional knockout mice showed a significant decrease in both trabecular and cortical bone mass as compared to WT littermates. Our data suggest that CYR61/CCN1 exerts its effects on mature osteoblast/osteocyte function to modulate bone mass. Specifically, changes were observed in osteocyte/osteoblast expression of RankL, VegfA, and Sost. The increase in RankL expression was correlated with a significant increase in osteoclast number; decreased VegfA expression was correlated with a significant decrease in bone vasculature; increased Sost expression was associated with decreased Wnt signaling, as revealed by decreased Axin2 expression and increased adiposity in the bone marrow. Although the decreased number of vascular elements in bone likely contributes to the low bone mass phenotype in Cyr61 conditional knockout mice, this cannot explain the observed increase in osteoclasts and the decrease in Wnt signaling. We conducted in vitro assays using UMR-106 osteosarcoma cells to explore the role CYR61/CCN1 plays in modulating Sost mRNA and protein expression in osteocytes and osteoblasts. Overexpression of CYR61/CCN1 can suppress Sost expression in both control and Cyr61 knockout cells, and blocking Sost with siRNA can rescue Wnt responsiveness in Cyr61 knockout cells in vitro. Overall, our data suggest that CYR61/CCN1 modulates mature osteoblast and osteocyte function to regulate bone mass through angiogenic effects as well as by modulating Wnt signaling, at least in part through the Wnt antagonist Sost. © 2018 American Society for Bone and Mineral Research.

Sclerostin Antibody Augments the Anabolic Bone Formation Response in a Mouse Model of Mechanical Tibial Loading

Decreased activity or expression of sclerostin, an endogenous inhibitor of Wnt/β-catenin signaling, results in increased bone formation and mass. Antibodies targeting and neutralizing sclerostin (Scl-Ab) have been shown to increase bone mass and reduce fracture risk. Sclerostin is also important in modulating the response of bone to changes in its biomechanical environment. However, the effects of Scl-Ab on mechanotransduction are unclear, and it was speculated that the loading response may be altered for individuals receiving Scl-Ab therapy. To address this, we carried out a 2-week study of tibial cyclic compressive loading on C57Bl/6 mice treated with vehicle or 100 mg/kg/wk Scl-Ab. Increases in bone volume, density, and dynamic bone formation were found with loading, and the anabolic response was further increased by the combination of load and Scl-Ab. To investigate the underlying mechanism, gene profiling by RNA sequencing (RNAseq) was performed on tibias isolated from mice from all four experimental groups. Major alterations in Wnt/β-catenin gene expression were found with tibial loading, however not with Scl-Ab treatment alone. Notably, the combination of load and Scl-Ab elicited a synergistic response from a number of specific Wnt-related and mechanotransduction factors. An unexpected finding was significant upregulation of factors in the Rho GTPase signaling pathway with combination treatment. In summary, combination therapy had a more profound anabolic response than either Scl-Ab or loading treatment alone. The Wnt/β-catenin and Rho GTPase pathways were implicated within bone mechanotransduction and support the concept that bone mechanotransduction is likely to encompass a number of interconnected signaling pathways. © 2017 American Society for Bone and Mineral Research.

Transcriptional profiling of synovium in a porcine model of early post-traumatic osteoarthritis

To determine the transcriptional profile of synovium during the molecular phase of post-traumatic osteoarthritis, anterior cruciate ligament transections (ACL) were performed in 36 Yucatan minipigs. Equal numbers were randomly assigned to no further treatment, ACL reconstruction or repair. Perimeniscal synovium for histopathology and RNA-sequencing was harvested at 1 and 4 weeks post-operatively and from six healthy control animals. Microscopic synovitis scores significantly worsened at 1 (p < 0.001) and 4 weeks (p = 0.003) post-surgery relative to controls, and were driven by intimal hyperplasia and increased stromal cellularity without inflammatory infiltrates. Synovitis scores were similar between no treatment, reconstruction, and repair groups (p ≥ 0.668). Relative to no treatment at 1 week, 88 and 367 genes were differentially expressed in the reconstruction and repair groups, respectively (227 and 277 at 4 weeks). Relative to controls and with the treatment groups pooled, 1,683 transcripts were concordantly differentially expressed throughout the post-surgery time-course. Affected pathways included, proteolysis_connective tissue degradation (including upregulations of protease-encoding MMP1, MMP13, and ADAMTS4), and development_cartilage development (including upregulations of ACAN, SOX9, and RUNX2), among others. Using linear regression, significant associations of post-surgery synovial expression levels of 20 genes with the articular cartilage glycosaminoglycan loss were identified. These genes were predominantly related to embryonic skeletal system development and included RUNX2. In conclusion, this study confirmed an increased synovial expression of genes that may serve as targets to prevent cartilage degradation, including MMP1, MMP13, and ADAMTS4, in knees with microscopic synovitis and cartilage proteoglycan loss. Attractive novel targets include regulators of embryonic developmental processes in synovium. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Spina bifida-predisposing heterozygous mutations in Planar Cell Polarity genes and Zic2 reduce bone mass in young mice

Fractures are a common comorbidity in children with the neural tube defect (NTD) spina bifida. Mutations in the Wnt/planar cell polarity (PCP) pathway contribute to NTDs in humans and mice, but whether this pathway independently determines bone mass is poorly understood. Here, we first confirmed that core Wnt/PCP components are expressed in osteoblasts and osteoclasts in vitro. In vivo, we performed detailed µCT comparisons of bone structure in tibiae from young male mice heterozygous for NTD-associated mutations versus WT littermates. PCP signalling disruption caused by Vangl2 (Vangl2Lp/+) or Celsr1 (Celsr1Crsh/+) mutations significantly reduced trabecular bone mass and distal tibial cortical thickness. NTD-associated mutations in non-PCP transcription factors were also investigated. Pax3 mutation (Pax3Sp2H/+) had minimal effects on bone mass. Zic2 mutation (Zic2Ku/+) significantly altered the position of the tibia/fibula junction and diminished cortical bone in the proximal tibia. Beyond these genes, we bioinformatically documented the known extent of shared genetic networks between NTDs and bone properties. 46 genes involved in neural tube closure are annotated with bone-related ontologies. These findings document shared genetic networks between spina bifida risk and bone structure, including PCP components and Zic2. Genetic variants which predispose to spina bifida may therefore independently diminish bone mass.

Bone marrow mechanotransduction in porcine explants alters kinase activation and enhances trabecular bone formation in the absence of osteocyte signaling

Bone is a dynamic tissue that can adapt its architecture in response to mechanical signals under the control of osteocytes, which sense mechanical deformation of the mineralized bone. However, cells in the marrow are also mechanosensitive and may contribute to load-induced bone adaptation, as marrow is subjected to mechanical stress during bone deformation. We investigated the contribution of mechanotransduction in marrow cells to trabecular bone formation by applying low magnitude mechanical stimulation (LMMS) to porcine vertebral trabecular bone explants in an in situ bioreactor. The bone formation rate was higher in stimulated explants compared to unloaded controls which represent a disuse condition (CNT). However, sclerostin protein expression in osteocytes was not different between groups, nor was expression of osteocytic mechanoregulatory genes SOST, IGF-1, CTGF, and Cyr61, suggesting the mechanoregulatory program of osteocytes was unaffected by the loading regime. In contrast, c-Fos, a gene indicative of mechanical stimulation, was upregulated in the marrow cells of mechanically stimulated explants, while the level of activated c-Jun decreased by 25%. The activator protein 1 (AP-1) transcription factor is a heterodimer of c-Fos and c-Jun, which led us to investigate the expression of the downstream target gene cyclin-D1, a gene associated with cell cycle progression and osteogenesis. Cyclin-D1 gene expression in the stimulated marrow was approximately double that of the controls. The level of phosphorylated PYK2, a purported inhibitor of osteoblast differentiation, also decreased in marrow cells from stimulated explants. Taken together, mechanotransduction in marrow cells induced trabecular bone formation independent of osteocyte signaling. Identifying the specific cells and signaling pathways involved, and verifying them with inhibition of specific signaling molecules, could lead to potential therapeutic targets for diseases characterized by bone loss.

β-Catenin Directs Long-Chain Fatty Acid Catabolism in the Osteoblasts of Male Mice

Wnt-initiated signaling through a frizzled receptor and the low-density lipoprotein-related receptor-5 coreceptor instructs key anabolic events during skeletal development, homeostasis, and repair. Recent studies indicate that Wnt signaling also regulates the intermediary metabolism of osteoblastic cells, inducing glucose consumption in osteoprogenitors and fatty acid utilization in mature osteoblasts. In this study, we examined the role of the canonical Wnt-signaling target, β-catenin, in the control of osteoblast metabolism. In vitro, Wnt ligands and agonists that stimulated β-catenin activation in osteoblasts enhanced fatty acid catabolism, whereas genetic ablation of β-catenin dramatically reduced oleate oxidation concomitant with reduced osteoblast maturation and increased glycolytic metabolism. Temporal ablation of β-catenin expression in osteoblasts in vivo produced the expected low-bone-mass phenotype and also led to an increase in white adipose tissue mass, dyslipidemia, and impaired insulin sensitivity. Because the expression levels of enzymatic mediators of fatty acid β-oxidation are reduced in the skeleton of β-catenin mutants, these results further confirm the role of the osteoblast in lipid metabolism and indicate that the influence of Wnt signaling on fatty acid utilization proceeds via its canonical signaling pathway.

Bone healing in an aged murine fracture model is characterized by sustained callus inflammation and decreased cell proliferation

Geriatric fractures take longer to heal and heal with more complications than those of younger patients; however, the mechanistic basis for this difference in healing is not well understood. To improve this understanding, we investigated cell and molecular differences in fracture healing between 5-month-old (young adult) and 25-month-old (geriatric) mice healing utilizing high-throughput analysis of gene expression. Mice underwent bilateral tibial fractures and fracture calluses were harvested at 5, 10, and 20 days post-fracture (DPF) for analysis. Global gene expression analysis was performed using Affymetrix MoGene 1.0 ST microarrays. After normalization, data were compared using ANOVA and evaluated using Principal Component Analysis (PCA), CTen, heatmap, and Incromaps analysis. PCA and cross-sectional heatmap analysis demonstrated that DPF followed by age had pronounced effects on changes in gene expression. Both un-fractured and 20 DPF aged mice showed increased expression of immune-associated genes (CXCL8, CCL8, and CCL5) and at 10 DPF, aged mice showed increased expression of matrix-associated genes, (Matn1, Ucma, Scube1, Col9a1, and Col9a3). Cten analysis suggested an enrichment of CD8+ cells and macrophages in old mice relative to young adult mice and, conversely, a greater prevalence of mast cells in young adult mice relative to old. Finally, consistent with the PCA data, the classic bone healing pathways of BMP, Indian Hedgehog, Notch and Wnt clustered according to the time post-fracture first and age second.

CLINICAL SIGNIFICANCE

Greater understanding of age-dependent molecular changes with healing will help form a mechanistic basis for therapies to improve patient outcomes. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:149-158, 2018.

Transcriptional profiling of articular cartilage in a porcine model of early post-traumatic osteoarthritis

To identify the molecular pathophysiology present in early post-traumatic osteoarthritis (PTOA), the transcriptional profile of articular cartilage and its response to surgical PTOA induction were determined. Thirty six Yucatan minipigs underwent anterior cruciate ligament (ACL) transection and were randomly assigned in equal numbers to no further treatment, reconstruction or ligament repair. Cartilage was harvested at 1 and 4 weeks post-operatively and histology and RNA-sequencing were performed and compared to controls. Microscopic cartilage scores significantly worsened at 1 (p = 0.028) and 4 weeks (p = 0.001) post-surgery relative to controls, but did not differ between untreated, reconstruction or repair groups. Gene expression after ACL reconstruction and ACL transection were similar, with only 0.03% (including SERPINB7 and CR2) and 0.2% of transcripts (including INHBA) differentially expressed at 1 and 4 weeks respectively. COL2A1, COMP, SPARC, CHAD, and EF1ALPHA were the most highly expressed non ribosomal, non mitochondrial genes in the controls and remained abundant after surgery. A total of 1,275 genes were differentially expressed between 1 and 4 weeks post-surgery. With the treatment groups pooled, 682 genes were differentially expressed at both time-points, with the most significant changes observed in MMP1, COCH, POSTN, CYTL1, and PTGFR. This study confirmed the development of a microscopic PTOA stage after ACL surgery in the porcine model. Upregulation of multiple proteases (including MMP1 and ADAMTS4) were found; however, the level of expression remained orders of magnitude below that of extracellular matrix protein-coding genes (including COL2A1 and ACAN). In summary, genes with established roles in PTOA as well as novel targets for specific intervention were identified. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:318-329, 2018.

The temporal expression of adipokines during spinal fusion

BACKGROUND CONTEXT

Adipokines are secreted by white adipose tissue and have been associated with fracture healing. Our goal was to report the temporal expression of adipokines during spinal fusion in an established rabbit model.

PURPOSE

Our goal was to report the temporal expression of adipokines during spinal fusion in an established rabbit model.

STUDY DESIGN

The study design included a laboratory animal model.

METHODS

New Zealand white rabbits were assigned to either sham surgery (n=2), unilateral posterior spinal fusion (n=14), or bilateral posterior spinal fusion (n=14). Rabbits were euthanized 1-6 and 10 weeks out from surgery. Fusion was evaluated by radiographs, manual palpation, and histology. Reverse transcription-polymerase chain reaction on the bone fusion mass catalogued the gene expression of leptin, adiponectin, resistin, and vascular endothelial growth factor (VEGF) at each time point. Results were normalized to the internal control gene, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (2^ΔCt), and control bone sites (2^ΔΔCt). Quantitative data were analyzed by two-factor analysis of variance (p<.05).

RESULTS

Manual palpation scores, radiograph scores, and histologic findings showed progression of boney fusion over time (p<.0003). The frequency of fusion by palpation after 4 weeks was 68.75%. Leptin expression in decortication and bone graft sites peaked at 5 weeks after the fusion procedure (p=.0143), adiponectin expression was greatest 1 week after surgery (p<.001), VEGF expression peaked at 4 weeks just after initial increases in leptin expression (p<.001), and resistin decreased precipitously 1 week after the fusion procedure (p<.001).

CONCLUSIONS

Leptin expression is likely associated with the maturation phase of bone fusion. Adiponectin and resistin may play a role early on during the fusion process. Our results suggest that leptin expression may be upstream of VEGF expression during spinal fusion, and both appear to play an important role in bone spinal fusion.

Bone mechanobiology in mice: toward single-cell in vivo mechanomics

Mechanically driven bone (re)modeling is a multiscale process mediated through complex interactions between multiple cell types and their microenvironments. However, the underlying mechanisms of how cells respond to mechanical signals are still unclear and are at the focus of the field of bone mechanobiology. Traditionally, this complex process has been addressed by reducing the system to single scales and cell types. It is only recently that more integrative approaches have been established to study bone mechanobiology across multiple scales in which mechanical load at the organ level is related to molecular responses at the cellular level. The availability of mouse loading models and imaging techniques with improved spatial and temporal resolution has made it possible to track dynamic bone (re)modeling at the tissue and cellular level in vivo. Coupled with advanced computational models, the (re)modeling activities at the tissue scale can be associated with the mechanical microenvironment. However, methods are lacking to link the molecular responses of different cell types to their local mechanical microenvironment and bone (re)modeling activities occurring at the tissue scale. With recent improvements in "omics" technologies and single-cell molecular biology, it is now possible to sequence the complete genome and transcriptome of single cells. These technologies offer unique opportunities to comprehensively investigate the cellular transcriptional profiles within their specific microenvironment. By combining single-cell "omics" technologies with well-established tissue-scale models of bone mechanobiology, we propose a mechanomics approach to locally analyze the transcriptome of single cells with respect to their local 3D mechanical in vivo environment.

A Memory of Early Life Physical Activity Is Retained in Bone Marrow of Male Rats Fed a High-Fat Diet

Studies have reported opposing effects of high-fat (HF) diet and mechanical stimulation on lineage commitment of the bone marrow stem cells. Yet, how bone marrow modulates its gene expression in response to the combined effects of mechanical loading and a HF diet has not been addressed. We investigated whether early-life (before onset of sexual maturity at 6 weeks of age) voluntary physical activity can modulate the effects of a HF diet on male Sprague Dawley rats. In the bone marrow, early-life HF diet resulted in adipocyte hypertrophy and a pro-inflammatory and pro-adipogenic gene expression profile. The bone marrow of the rats that undertook wheel exercise while on a HF diet retained a memory of the early-life exercise. This memory lasted at least 60 days after the cessation of the voluntary exercise. Our results are consistent with the marrow adipose tissue having a unique response to HF feeding in the presence or absence of exercise.

Immediate Administration of Intraarticular Triamcinolone Acetonide After Joint Injury Modulates Molecular Outcomes Associated With Early Synovitis

OBJECTIVE

To test whether intraarticular corticosteroid injection mitigates injury-induced synovitis and collagen degradation after anterior cruciate ligament transection (ACLT) and to characterize the synovial response using a functional genomics approach in a preclinical model of posttraumatic osteoarthritis.

METHODS

Yorkshire pigs underwent unilateral ACLT without subsequent corticosteroid injection (the ACLT group; n = 6) or ACLT with immediate injection of 20 mg triamcinolone acetonide (the steroid group; n = 6). A control group of pigs (the intact group; n = 6) did not undergo surgery. Total synovial membrane cellularity and synovial fluid concentration of C1,2C neoepitope-bearing collagen fragments 14 days after injury were primary end points and were compared between the ACLT, steroid, and intact groups. Cells were differentiated by histologic phenotype and counted, while RNA sequencing was used to quantify transcriptome-wide gene expression and monocyte, macrophage, and lymphocyte markers.

RESULTS

In the intact group, total cellularity was 13% (95% confidence interval [95% CI] 9-16) and the C1,2C concentration was 0.24 μg/ml (95% CI 0.08-0.39). In the ACLT group, significant increases were observed in total cellularity (to 21% [95% CI 16-27]) and C1,2C concentration (to 0.49 μg/ml [95% CI 0.39-0.59]). Compared to values in the ACLT group, total cellularity in the steroid group was nonsignificantly decreased to 17% (95% CI 15-18) (P = 0.26) and C1,2C concentration in the steroid group was significantly decreased to 0.29 μg/ml (95% CI 0.23-0.35) (P = 0.04). A total of 255 protein-coding transcripts were differentially expressed between the ACLT group and the intact group. These genes mainly enriched pathways related to cellular immune response, proteolysis, and angiogenesis. Mononuclear leukocytes were the dominant cell type in cell-dense areas. MARCO, SOCS3, CCR1, IL4R, and MMP2 expression was significantly associated with C1,2C levels.

CONCLUSION

Early intraarticular immunosuppression mitigated injury-induced increases in collagen fragments, an outcome better predicted by specific marker expression than by histologic measures of synovitis.

Clec11a/osteolectin is an osteogenic growth factor that promotes the maintenance of the adult skeleton

Bone marrow stromal cells maintain the adult skeleton by forming osteoblasts throughout life that regenerate bone and repair fractures. We discovered that subsets of these stromal cells, osteoblasts, osteocytes, and hypertrophic chondrocytes secrete a C-type lectin domain protein, Clec11a, which promotes osteogenesis. Clec11a-deficient mice appeared developmentally normal and had normal hematopoiesis but reduced limb and vertebral bone. Clec11a-deficient mice exhibited accelerated bone loss during aging, reduced bone strength, and delayed fracture healing. Bone marrow stromal cells from Clec11a-deficient mice showed impaired osteogenic differentiation, but normal adipogenic and chondrogenic differentiation. Recombinant Clec11a promoted osteogenesis by stromal cells in culture and increased bone mass in osteoporotic mice in vivo. Recombinant human Clec11a promoted osteogenesis by human bone marrow stromal cells in culture and in vivo. Clec11a thus maintains the adult skeleton by promoting the differentiation of mesenchymal progenitors into mature osteoblasts. In light of this, we propose to call this factor Osteolectin.

DOI:http://dx.doi.org/10.7554/eLife.18782.001

Transcriptional profiling of cortical versus cancellous bone from mechanically-loaded murine tibiae reveals differential gene expression

Mechanical loading is an anabolic stimulus that increases bone mass, and thus a promising method to counteract osteoporosis-related bone loss. The mechanism of this anabolism remains unclear, and needs to be established for both cortical and cancellous envelopes individually. We hypothesized that cortical and cancellous bone display different gene expression profiles at baseline and in response to mechanical loading. To test this hypothesis, the left tibiae of 10-week-old female C57Bl/6 mice were subjected to one session of axial tibial compression (9N, 1200cycles, 4Hz triangle waveform) and euthanized 3 and 24h following loading. The right limb served as the contralateral control. We performed RNA-seq on marrow-free metaphyseal samples from the cortical shell and the cancellous core to determine differential gene expression at baseline (control limb) and in response to load. Differential expression was verified with qPCR. Cortical and cancellous bone exhibited distinctly different transcriptional profiles basally and in response to mechanical loading. More genes were differentially expressed with loading at 24h with more genes downregulated at 24h than at 3h in both tissues. Enhanced Wnt signaling dominated the response in cortical bone at 3 and 24h, but in cancellous bone only at 3h. In cancellous bone at 24h many muscle-related genes were downregulated. These findings reveal key differences between cortical and cancellous genetic regulation in response to mechanical loading. Future studies at different time points and multiple loading sessions will add to our knowledge of cortical and cancellous mechanotransduction with the potential to identify new targets for mouse genetic knockout studies and drugs to treat osteoporosis.