Sites of Cre-recombinase activity in mouse lines targeting skeletal cells

In vivo dynamics of hard tissue-forming cell origins: Insights from Cre/loxP-based cell lineage tracing studies

Bone tissue provides structural support for our bodies, with the inner bone marrow (BM) acting as a hematopoietic organ. Within the BM tissue, two types of stem cells play crucial roles: mesenchymal stem cells (MSCs) (or skeletal stem cells) and hematopoietic stem cells (HSCs). These stem cells are intricately connected, where BM-MSCs give rise to bone-forming osteoblasts and serve as essential components in the BM microenvironment for sustaining HSCs. Despite the mid-20th century proposal of BM-MSCs, their in vivo identification remained elusive owing to a lack of tools for analyzing stemness, specifically self-renewal and multipotency. To address this challenge, Cre/loxP-based cell lineage tracing analyses are being employed. This technology facilitated the in vivo labeling of specific cells, enabling the tracking of their lineage, determining their stemness, and providing a deeper understanding of the in vivo dynamics governing stem cell populations responsible for maintaining hard tissues. This review delves into cell lineage tracing studies conducted using commonly employed genetically modified mice expressing Cre under the influence of LepR, Gli1, and Axin2 genes. These studies focus on research fields spanning long bones and oral/maxillofacial hard tissues, offering insights into the in vivo dynamics of stem cell populations crucial for hard tissue homeostasis.

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.

MyD88 in osteoclast and osteoblast lineages differentially controls bone remodeling in homeostasis and malaria

Chronic bone loss is an under-recognized complication of malaria, the underlying mechanism of which remains incompletely understood. We have previously shown that persistent accumulation of Plasmodium products in the bone marrow leads to chronic inflammation in osteoblast (OB) and osteoclast (OC) precursors causing bone loss through MyD88, an adaptor molecule for diverse inflammatory signals. However, the specific contribution of MyD88 signaling in OB or OC precursors in malaria-induced bone loss remains elusive. To assess the direct cell-intrinsic role of MyD88 signaling in adult bone metabolism under physiological and infection conditions, we used the Lox-Cre system to specifically deplete MyD88 in the OB or OC lineages. Mice lacking MyD88 primarily in the maturing OBs showed a comparable decrease in trabecular bone density by microcomputed tomography to that of controls after Plasmodium yoelii non-lethal infection. In contrast, mice lacking MyD88 in OC precursors showed significantly less trabecular bone loss than controls, suggesting that malaria-mediated inflammatory mediators are primarily controlled by MyD88 in the OC lineage. Surprisingly, however, depletion of MyD88 in OB, but not in OC, precursors resulted in reduced bone mass with decreased bone formation rates in the trabecular areas of femurs under physiological conditions. Notably, insulin-like growth factor-1, a key molecule for OB differentiation, was significantly lower locally and systemically when MyD88 was depleted in OBs. Thus, our data demonstrate an indispensable intrinsic role for MyD88 signaling in OB differentiation and bone formation, while MyD88 signaling in OC lineages plays a partial role in controlling malaria-induced inflammatory mediators and following bone pathology. These findings may lead to the identification of novel targets for specific intervention of bone pathologies, particularly in malaria-endemic regions.

Evaluation of Potential Roles of Zinc Finger Homeobox 3 (Zfhx3) Expressed in Chondrocytes and Osteoblasts on Skeletal Growth in Mice

Bone formation is tightly modulated by genetically encoded molecular proteins that interact to regulate cellular differentiation and secretion of bony matrix. Many transcription factors are known to coordinate these events by controlling gene transcription within networks. However, not all factors involved are known. Here, we identified a novel function for Zinc Finger Homeobox 3 (Zfhx3), a gene encoding a transcription factor, as a regulator of bone metabolism. We knocked out Zfhx3 conditionally in mice in either chondrocytes or osteoblasts and characterized their bones by micro-CT in 12-week-old mice. We observed a negative effect in linear bone growth in both knockout mice but reduced bone mass only in mice with Zfhx3 deleted in osteoblasts. Loss of Zfhx3 expression in osteoblasts affected trabecular bone mass in femurs and vertebrae in both sexes but influenced cortical bone volume fraction only in females. Moreover, transcriptional analysis of femoral bones in osteoblast Zfhx3 conditional knockout mice revealed a reduced expression of osteoblast genes, and histological evaluation of trabecular bones suggests that Zfhx3 causes changes in bone formation and not resorption. The loss of Zfhx3 causes reductions in trabecular bone area and osteoid volume, but no changes in the expression of osteoclast differentiation markers or number of TRAP stained osteoclasts. These studies introduce Zfhx3 as a relevant factor toward understanding gene regulatory networks that control bone formation and development of peak bone mass.

Mitofusin 2 plays a critical role in maintaining the functional integrity of the neuromuscular-skeletal axis

PURPOSE

Mitofusin 2 (Mfn2) is one of two mitofusins involved in regulating mitochondrial size, shape and function, including mitophagy, an important cellular mechanism to limit oxidative stress. Reduced expression of Mfn2 has been associated with impaired osteoblast differentiation and function and a reduction in the number of viable osteocytes in bone. We hypothesized that the genetic absence of Mfn2 in these cells would increase their susceptibility to aging-associated metabolic stress, leading to a progressive impairment in skeletal homeostasis over time.

METHODS

Mfn2 was selectively deleted in vivo at three different stages of osteoblast lineage commitment by crossing mice in which the Mfn2 gene was floxed with transgenic mice expressing Cre under the control of the promoter for Osterix (OSX), collagen1a1, or DMP1 (Dentin Matrix Acidic Phosphoprotein 1).

RESULTS

Mice in which Mfn2 was deleted using DMP1-cre demonstrated a progressive and dramatic decline in bone mineral density (BMD) beginning at 10 weeks of age (n = 5 for each sex and each genotype from age 10 to 20 weeks). By 15 weeks, there was evidence for a functional decline in muscle performance as assessed using a rotarod apparatus (n = 3; 2 males/ 1 female for each genotype), accompanied by a decline in lean body mass. A marked reduction in trabecular bone mass was evident on bone histomorphometry, and biomechanical testing at 25 weeks (k/o: 2 male/1 female, control 2 male/2 female) revealed severely impaired femur strength. Extensive regional myofiber atrophy and degeneration was observed on skeletal muscle histology. Electron microscopy showed progressive disruption of cellular architecture, with disorganized sarcomeres and a bloated mitochondrial reticulum. There was also evidence of neurodegeneration within the ventral horn and roots of the lumbar spinal cord, which was accompanied by myelin loss and myofiber atrophy. Deletion of Mfn2 using OSX-cre or Col1a1-cre did not result in a musculoskeletal phenotype. Where possible, male and female animals were analyzed separately, but small numbers of animals in each group limited statistical power. For other outcomes, where sex was not considered, small sample sizes might still limit the strength of the observation.

CONCLUSION

Despite known functional overlap of Mfn1 and Mfn2 in some tissues, and their co-expression in bone, muscle and spinal cord, deletion of Mfn2 using the 8 kB DMP1 promoter uncovered an important non-redundant role for Mfn2 in maintaining the neuromuscular/bone axis.

Conditional loss of CaMKK2 in Osterix-positive osteoprogenitors enhances osteoblast function in a sex-divergent manner

Ca2+/calmodulin-dependent protein kinase kinase 2 (CaMKK2) is a multi-functional, serine/threonine protein kinase with predominant roles in inflammation, systemic energy metabolism, and bone remodeling. We previously reported that global ablation of CaMKK2 or its systemic pharmacological inhibition led to bone mass accrual in mice by stimulating osteoblasts and inhibiting osteoclasts. However, a direct, cell-intrinsic role for the kinase in the osteoblast lineage has not been established. Here we report that conditional deletion of CaMKK2 from osteoprogenitors, using the Osterix 1 (Osx1) - GFP::Cre (tetracycline-off) mouse line, resulted in increased trabecular bone mass due to an acute stimulation of osteoblast function in male and female mice. The acute simulation of osteoblasts and bone formation following conditional ablation of osteoprogenitor-derived CaMKK2 was sustained only in female mice. Periosteal bone formation at the cortical bone was enhanced only in male conditional knockout mice without altering cortical bone mass or strength. Prolonged deletion of CaMKK2 in early osteoblasts was accompanied by a stimulation of osteoclasts in both sexes, indicating a coupling effect. Notably, alterations in trabecular and cortical bone mass were absent in the doxycycline-removed "Cre-only" Osx1-GFP::Cre mice. Thus, the increase in osteoblast function at the trabecular and cortical bone surfaces following the conditional deletion of CaMKK2 in osteoprogenitors is indicative of a direct but sex-divergent role for the kinase in osteoblasts.

Metabolic regulation of skeletal cell fate and function

Bone development and bone remodelling during adult life are highly anabolic processes requiring an adequate supply of oxygen and nutrients. Bone-forming osteoblasts and bone-resorbing osteoclasts interact closely to preserve bone mass and architecture and are often located close to blood vessels. Chondrocytes within the developing growth plate ensure that bone lengthening occurs before puberty, but these cells function in an avascular environment. With ageing, numerous bone marrow adipocytes appear, often with negative effects on bone properties. Many studies have now indicated that skeletal cells have specific metabolic profiles that correspond to the nutritional microenvironment and their stage-specific functions. These metabolic networks provide not only skeletal cells with sufficient energy, but also biosynthetic intermediates that are necessary for proliferation and extracellular matrix synthesis. Moreover, these metabolic pathways control redox homeostasis to avoid oxidative stress and safeguard cell survival. Finally, several intracellular metabolites regulate the activity of epigenetic enzymes and thus control the fate and function of skeletal cells. The metabolic profile of skeletal cells therefore not only reflects their cellular state, but can also drive cellular activity. Insight into skeletal cell metabolism will thus not only advance our understanding of skeletal development and homeostasis, but also of skeletal disorders, such as osteoarthritis, diabetic bone disease and bone malignancies.

Hyperthyroidism-driven bone loss depends on BMP receptor Bmpr1a expression in osteoblasts

Hyperthyroidism is a well-known trigger of high bone turnover that can lead to the development of secondary osteoporosis. Previously, we have shown that blocking bone morphogenetic protein (BMP) signaling systemically with BMPR1A-Fc can prevent bone loss in hyperthyroid mice. To distinguish between bone cell type-specific effects, conditional knockout mice lacking Bmpr1a in either osteoclast precursors (LysM-Cre) or osteoprogenitors (Osx-Cre) were rendered hyperthyroid and their bone microarchitecture, strength and turnover were analyzed. While hyperthyroidism in osteoclast precursor-specific Bmpr1a knockout mice accelerated bone resorption leading to bone loss just as in wildtype mice, osteoprogenitor-specific Bmpr1a deletion prevented an increase of bone resorption and thus osteoporosis with hyperthyroidism. In vitro, wildtype but not Bmpr1a-deficient osteoblasts responded to thyroid hormone (TH) treatment with increased differentiation and activity. Furthermore, we found an elevated Rankl/Opg ratio with TH excess in osteoblasts and bone tissue from wildtype mice, but not in Bmpr1a knockouts. In line, expression of osteoclast marker genes increased when osteoclasts were treated with supernatants from TH-stimulated wildtype osteoblasts, in contrast to Bmpr1a-deficient cells. In conclusion, we identified the osteoblastic BMP receptor BMPR1A as a main driver of osteoporosis in hyperthyroid mice promoting TH-induced osteoblast activity and potentially its coupling to high osteoclastic resorption.

Refining the identity of mesenchymal cell types associated with murine periosteal and endosteal bone

Single-cell RNA-seq has led to novel designations for mesenchymal cells associated with bone as well as multiple designations for what appear to be the same cell type. The main goals of this study were to increase the amount of single-cell RNA sequence data for osteoblasts and osteocytes, to compare cells from the periosteum to those inside bone, and to clarify the major categories of cell types associated with murine bone. We created an atlas of murine bone-associated cells by harmonizing published datasets with in-house data from cells targeted by Osx1-Cre and Dmp1-Cre driver strains. Cells from periosteal bone were analyzed separately from those isolated from the endosteum and trabecular bone. Over 100,000 mesenchymal cells were mapped to reveal 11 major clusters designated fibro-1, fibro-2, chondrocytes, articular chondrocytes, tenocytes, adipo-Cxcl12 abundant reticular (CAR), osteo-CAR, preosteoblasts, osteoblasts, osteocytes, and osteo-X, the latter defined in part by periostin expression. Osteo-X, osteo-CAR, and preosteoblasts were closely associated with osteoblasts at the trabecular bone surface. Wnt16 was expressed in multiple cell types from the periosteum but not in cells from endocortical or cancellous bone. Fibro-2 cells, which express markers of stem cells, localized to the periosteum but not trabecular bone in adult mice. Suppressing bone remodeling eliminated osteoblasts and altered gene expression in preosteoblasts but did not change the abundance or location of osteo-X or osteo-CAR cells. These results provide a framework for identifying bone cell types in murine single-cell RNA-seq datasets and suggest that osteoblast progenitors reside near the surface of remodeling bone.

Minimally invasive longitudinal intravital imaging of cellular dynamics in intact long bone

Intravital two-photon microscopy enables deep-tissue imaging at high temporospatial resolution in live animals. However, the endosteal bone compartment and underlying bone marrow pose unique challenges to optical imaging as light is absorbed, scattered and dispersed by thick mineralized bone matrix and the adipose-rich bone marrow. Early bone intravital imaging methods exploited gaps in the cranial sutures to bypass the need to penetrate through cortical bone. More recently, investigators have developed invasive methods to thin the cortical bone or implant imaging windows to image cellular dynamics in weight-bearing long bones. Here, we provide a step-by-step procedure for the preparation of animals for minimally invasive, nondestructive, longitudinal intravital imaging of the murine tibia. This method involves the use of mixed bone marrow radiation chimeras to unambiguously double-label osteoclasts and osteomorphs. The tibia is exposed by a simple skin incision and an imaging chamber constructed using thermoconductive T-putty. Imaging sessions up to 12 h long can be repeated over multiple timepoints to provide a longitudinal time window into the endosteal and marrow niches. The approach can be used to investigate cellular dynamics in bone remodeling, cancer cell life cycle and hematopoiesis, as well as long-lived humoral and cellular immunity. The procedure requires an hour to complete and is suitable for users with minimal prior expertise in small animal surgery.

Multi-omics analysis in developmental bone biology

Single-cell omics and multi-omics have revolutionized our understanding of molecular and cellular biological processes at a single-cell level. In bone biology, the combination of single-cell RNA-sequencing analyses and in vivo lineage-tracing approaches has successfully identified multi-cellular diversity and dynamics of skeletal cells. This established a new concept that bone growth and regeneration are regulated by concerted actions of multiple types of skeletal stem cells, which reside in spatiotemporally distinct niches. One important subtype is endosteal stem cells that are particularly abundant in young bone marrow. The discovery of this new skeletal stem cell type has been facilitated by single-cell multi-omics, which simultaneously measures gene expression and chromatin accessibility. Using single-cell omics, it is now possible to computationally predict the immediate future state of individual cells and their differentiation potential. In vivo validation using histological approaches is the key to interpret the computational prediction. The emerging spatial omics, such as spatial transcriptomics and epigenomics, have major advantage in retaining the location of individual cells within highly complex tissue architecture. Spatial omics can be integrated with other omics to further obtain in-depth insights. Single-cell multi-omics are now becoming an essential tool to unravel intricate multicellular dynamics and intercellular interactions of skeletal cells.

A framework for defining mesenchymal cell types associated with murine periosteal and endosteal bone

Single-cell RNA sequencing has led to numerous novel designations for mesenchymal cell types associated with bone. Consequently, there are now multiple designations for what appear to be the same cell type. In addition, existing datasets contain relatively small numbers of mature osteoblasts and osteocytes and there has been no comparison of periosteal bone cells to those at the endosteum and trabecular bone. The main goals of this study were to increase the amount of single cell RNA sequence data for osteoblasts and osteocytes, to compare cells from the periosteum to those inside bone, and to clarify the major categories of cell types associated with murine bone. To do this, we created an atlas of murine bone-associated cells by harmonizing published datasets with in-house data from cells targeted by Osx1-Cre and Dmp1-Cre driver strains. Cells from periosteal bone were analyzed separately from those isolated from the endosteum and trabecular bone. Over 100,000 mesenchymal cells were mapped to reveal 11 major clusters designated fibro-1, fibro-2, chondrocytes, articular chondrocytes, tenocytes, adipo-CAR, osteo-CAR, pre-osteoblasts, osteoblasts, osteocytes, and osteo-X, the latter defined in part by Postn expression. Osteo-X, osteo-CAR, and pre-osteoblasts were closely associated with osteoblasts at the trabecular bone surface. Wnt16 was expressed in multiple cell types from the periosteum but not in any cells from endocortical or cancellous bone. Fibro-2 cells, which express markers of skeletal stem cells, localized to the periosteum but not trabecular bone in adult mice. Suppressing bone remodeling eliminated osteoblasts and altered gene expression in pre-osteoblasts but did not change the abundance or location of osteo-X or osteo-CAR cells. These results provide a framework for identifying bone cell types in murine single cell RNA sequencing datasets and suggest that osteoblast progenitors reside near the surface of remodeling bone.

A review of tamoxifen administration regimen optimization for Cre/loxp system in mouse bone study

Gene knockout is a technique routinely used in basic experimental research, particularly in mouse skeletal and developmental studies. Tamoxifen-induced Cre/loxp system is known for its temporal and spatial precision and commonly utilized by researchers. However, tamoxifen has been shown its side effects on affecting the phenotype of mouse bone directly. This review aimed to optimize tamoxifen administration regimens including its dosage and duration, to identify an optimal induction strategy that minimizes potential side effects while maintaining recombination efficacy. This study will help researchers in designing gene knockout experiments in bone when using tamoxifen.

The Past, Present, and Future of Genetically Engineered Mouse Models for Skeletal Biology

The ability to create genetically engineered mouse models (GEMMs) has exponentially increased our understanding of many areas of biology. Musculoskeletal biology is no exception. In this review, we will first discuss the historical development of GEMMs and how these developments have influenced musculoskeletal disease research. This review will also update our 2008 review that appeared in BONEKey, a journal that is no longer readily available online. We will first review the historical development of GEMMs in general, followed by a particular emphasis on the ability to perform tissue-specific (conditional) knockouts focusing on musculoskeletal tissues. We will then discuss how the development of CRISPR/Cas-based technologies during the last decade has revolutionized the generation of GEMMs.

Why Animal Experiments Are Still Indispensable in Bone Research: A Statement by the European Calcified Tissue Society

Major achievements in bone research have always relied on animal models and in vitro systems derived from patient and animal material. However, the use of animals in research has drawn intense ethical debate and the complete abolition of animal experimentation is demanded by fractions of the population. This phenomenon is enhanced by the reproducibility crisis in science and the advance of in vitro and in silico techniques. 3D culture, organ-on-a-chip, and computer models have improved enormously over the last few years. Nevertheless, the overall complexity of bone tissue cross-talk and the systemic and local regulation of bone physiology can often only be addressed in entire vertebrates. Powerful genetic methods such as conditional mutagenesis, lineage tracing, and modeling of the diseases enhanced the understanding of the entire skeletal system. In this review endorsed by the European Calcified Tissue Society (ECTS), a working group of investigators from Europe and the US provides an overview of the strengths and limitations of experimental animal models, including rodents, fish, and large animals, as well the potential and shortcomings of in vitro and in silico technologies in skeletal research. We propose that the proper combination of the right animal model for a specific hypothesis and state-of-the-art in vitro and/or in silico technology is essential to solving remaining important questions in bone research. This is crucial for executing most efficiently the 3R principles to reduce, refine, and replace animal experimentation, for enhancing our knowledge of skeletal biology, and for the treatment of bone diseases that affect a large part of society. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Cellular dynamics of distinct skeletal cells and the development of osteosarcoma

Bone contributes to the maintenance of vital biological activities. At the cellular level, multiple types of skeletal cells, including skeletal stem and progenitor cells (SSPCs), osteoblasts, chondrocytes, marrow stromal cells, and adipocytes, orchestrate skeletal events such as development, aging, regeneration, and tumorigenesis. Osteosarcoma (OS) is a primary malignant tumor and the main form of bone cancer. Although it has been proposed that the cellular origins of OS are in osteogenesis-related skeletal lineage cells with cancer suppressor gene mutations, its origins have not yet been fully elucidated because of a poor understanding of whole skeletal cell diversity and dynamics. Over the past decade, the advent and development of single-cell RNA sequencing analyses and mouse lineage-tracing approaches have revealed the diversity of skeletal stem and its lineage cells. Skeletal stem cells (SSCs) in the bone marrow endoskeletal region have now been found to efficiently generate OS and to be robust cells of origin under p53 deletion conditions. The identification of SSCs may lead to a more limited redefinition of bone marrow mesenchymal stem/stromal cells (BM-MSCs), and this population has been thought to contain cells from which OS originates. In this mini-review, we discuss the cellular diversity and dynamics of multiple skeletal cell types and the origin of OS in the native in vivo environment in mice. We also discuss future challenges in the study of skeletal cells and OS.

Targeting osteocytes vs osteoblasts

Although osteoblasts and osteocytes are descended from the same lineage, they each have unique and essential roles in bone. Targeting gene deletion to osteoblasts and osteocytes using the Cre/loxP system has greatly increased our current understanding of how these cells function. Additionally, the use of the Cre/loxP system in conjunction with cell-specific reporters has enabled lineage tracing of these bone cells both in vivo and ex vivo. However, concerns have been raised regarding the specificity of the promoters used and the resulting off-target effects on cells within and outside of the bone. In this review, we have summarized the main mouse models that have been used to determine the functions of specific genes in osteoblasts and osteocytes. We discuss the expression patterns and specificity of the different promoter fragments during osteoblast to osteocyte differentiation in vivo. We also highlight how their expression in non-skeletal tissues may complicate the interpretation of study results. A thorough understanding of when and where these promoters are activated will enable improved study design and greater confidence in data interpretation.

Perspective: The current state of Cre driver mouse lines in skeletal research: Challenges and opportunities

The Cre/Lox system has revolutionized the ability of biomedical researchers to ask very specific questions about the function of individual genes in specific cell types at specific times during development and/or disease progression in a variety of animal models. This is true in the skeletal biology field, and numerous Cre driver lines have been created to foster conditional gene manipulation in specific subpopulations of bone cells. However, as our ability to scrutinize these models increases, an increasing number of issues have been identified with most driver lines. All existing skeletal Cre mouse models exhibit problems in one or more of the following three areas: (1) cell type specificity-avoiding Cre expression in unintended cell types; (2) Cre inducibility-improving the dynamic range for Cre in inducible models (negligible Cre activity before induction and high Cre activity after induction); and (3) Cre toxicity-reducing the unwanted biological effects of Cre (beyond loxP recombination) on cellular processes and tissue health. These issues are hampering progress in understanding the biology of skeletal disease and aging, and consequently, identification of reliable therapeutic opportunities. Skeletal Cre models have not advanced technologically in decades despite the availability of improved tools, including multi-promoter-driven expression of permissive or fragmented recombinases, new dimerization systems, and alternative forms of recombinases and DNA sequence targets. We review the current state of skeletal Cre driver lines, and highlight some of the successes, failures, and opportunities to improve fidelity in the skeleton, based on successes pioneered in other areas of biomedical science.

Subset of the periodontal ligament expressed leptin receptor contributes to part of hard tissue-forming cells

The lineage of periodontal ligament (PDL) stem cells contributes to alveolar bone (AB) and cementum formation, which are essential for tooth-jawbone attachment. Leptin receptor (LepR), a skeletal stem cell marker, is expressed in PDL; however, the stem cell capacity of LepR⁺ PDL cells remains unclear. We used a Cre/LoxP-based approach and detected LepR-cre-labeled cells in the perivascular around the root apex; their number increased with age. In the juvenile stage, LepR⁺ PDL cells differentiated into AB-embedded osteocytes rather than cementocytes, but their contribution to both increased with age. The frequency of LepR⁺ PDL cell-derived lineages in hard tissue was < 20% per total cells at 1-year-old. Similarly, LepR⁺ PDL cells differentiated into osteocytes following tooth extraction, but their frequency was < 9%. Additionally, both LepR⁺ and LepR^- PDL cells demonstrated spheroid-forming capacity, which is an indicator of self-renewal. These results indicate that both LepR⁺ and LepR^- PDL populations contributed to hard tissue formation. LepR^- PDL cells increased the expression of LepR during spheroid formation, suggesting that the LepR^- PDL cells may hierarchically sit upstream of LepR⁺ PDL cells. Collectively, the origin of hard tissue-forming cells in the PDL is heterogeneous, some of which express LepR.

Visualizing Cathepsin K-Cre Expression at the Single-Cell Level with GFP Reporters

The Cre/lox system is a fundamental tool for functional genomic studies, and a number of Cre lines have been generated to target genes of interest spatially and temporally in defined cells or tissues; this approach has greatly expanded our knowledge of gene functions. However, the limitations of this system have recently been recognized, and we must address the challenge of so-called nonspecific/off-target effects when a Cre line is utilized to investigate a gene of interest. For example, cathepsin K (Ctsk) has been used as a specific osteoclast marker, and Cre driven by its promoter is widely utilized for osteoclast investigations. However, Ctsk-Cre expression has recently been identified in other cell types, such as osteocytes, periosteal stem cells, and tenocytes. To better understand Ctsk-Cre expression and ensure appropriate use of this Cre line, we performed a comprehensive analysis of Ctsk-Cre expression at the single-cell level in major organs and tissues using two green fluorescent protein (GFP) reporters (ROSA nT-nG and ROSA tdT) and a tissue clearing technique in young and aging mice. The expression profile was further verified by immunofluorescence staining and droplet digital RT-PCR. The results demonstrate that Ctsk-Cre is expressed not only in osteoclasts but also at various levels in osteoblast lineage cells and other major organs/tissues, particularly in the brain, kidney, pancreas, and blood vessels. Furthermore, Ctsk-Cre expression increases markedly in the bone marrow, skeletal muscle, and intervertebral discs in aging mice. These data will be valuable for accurately interpreting data obtained from in vivo studies using Ctsk-Cre mice to avoid potentially misleading conclusions. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Erythropoietin Receptor (EPOR) Signaling in the Osteoclast Lineage Contributes to EPO-Induced Bone Loss in Mice

Erythropoietin (EPO) is a pleiotropic cytokine that classically drives erythropoiesis but can also induce bone loss by decreasing bone formation and increasing resorption. Deletion of the EPO receptor (EPOR) on osteoblasts or B cells partially mitigates the skeletal effects of EPO, thereby implicating a contribution by EPOR on other cell lineages. This study was designed to define the role of monocyte EPOR in EPO-mediated bone loss, by using two mouse lines with conditional deletion of EPOR in the monocytic lineage. Low-dose EPO attenuated the reduction in bone volume (BV/TV) in Cx3cr1^Cre EPOR^f/f female mice (27.05%) compared to controls (39.26%), but the difference was not statistically significant. To validate these findings, we increased the EPO dose in LysM^Cre model mice, a model more commonly used to target preosteoclasts. There was a significant reduction in both the increase in the proportion of bone marrow preosteoclasts (CD115⁺) observed following high-dose EPO administration and the resulting bone loss in LysM^Cre EPOR^f/f female mice (44.46% reduction in BV/TV) as compared to controls (77.28%), without interference with the erythropoietic activity. Our data suggest that EPOR in the monocytic lineage is at least partially responsible for driving the effect of EPO on bone mass.

Osteoblast Lineage Cell-derived Sema3A Regulates Bone Homeostasis Independently of Androgens

Semaphorin 3A (Sema3A) coordinates bone resorption and formation under the control of estrogen signaling. However, the contribution of osteoblast lineage cell-derived Sema3A to vertebral homeostasis has remained unclear. Moreover, it is unknown whether androgen signaling is involved in Sema3A expression in osteoblast lineage cells. In this study, we show that osteoblast lineage cell-derived Sema3A plays a key role in bone homeostasis independent of androgen signaling. Sema3a deletion with Sp7-Cre did not alter the trabecular bone mass in lumbar vertebrae, along with there being no significant difference in Sema3a mRNA expression. In contrast, osteoblast lineage cell-specific deletion of Sema3A with BGLAP-Cre led to decreased bone volume in both long bones and lumbar vertebrae. In addition, osteoblast lineage cell-derived Sema3A was not involved in orchidectomy-induced bone loss because androgen deficiency did not affect Sema3A protein expression. Thus, these results indicate that Sema3A derived from osteoblast lineage cells acts as an osteoprotective factor, even in vertebrae, and its expression is controlled in an androgen-independent manner.

Induction of a NOTCH3 Lehman syndrome mutation in osteocytes causes osteopenia in male C57BL/6J mice

Lateral Meningocele or Lehman Syndrome (LMS) is associated with NOTCH3 mutations causing deletions of the PEST domain and a gain-of-NOTCH3 function. We demonstrated that Notch3em1Ecan mice harboring Notch3 mutations analogous to those found in LMS are osteopenic because of enhanced bone resorption. To determine the contribution of specific cell lineages to the phenotype, we created a conditional-by-inversion (Notch3COIN) model termed Notch3em2Ecan in which Cre recombination generates a Notch3INV allele expressing a NOTCH3 mutant lacking the PEST domain. Germ line Notch3COIN inversion caused osteopenia and phenocopied the Notch3em1Ecan mutant, validating the model. To induce the mutation in osteocytes, smooth muscle and endothelial cells, Notch3COIN mice were bred with mice expressing Cre from the Dmp1, Sm22a and Cdh5 promoters, respectively, creating experimental mice harboring Notch3INV alleles in Cre-expressing cells and control littermates harboring Notch3COIN alleles. Notch3COIN inversion in osteocytes led to femoral and vertebral cancellous bone osteopenia, whereas Notch3COIN inversion in mural Sm22a or endothelial Cdh5-expressing cells did not result in a skeletal phenotype. In conclusion, introduction of the LMS mutation in osteocytes but not in vascular cells causes osteopenia and phenocopies Notch3em1Ecan global mutant mice.

Sexually Dimorphic Increases in Bone Mass Following Tissue-specific Overexpression of Runx1 in Osteoclast Precursors

Many metabolic bone diseases arise as a result excessive osteoclastic bone resorption, which has motivated efforts to identify new molecular targets that can inhibit the formation or activity of these bone-resorbing cells. Mounting evidence indicates that the transcription factor Runx1 acts as a transcriptional repressor of osteoclast formation. Prior studies using a conditional knockout approach suggested that Runx1 in osteoclast precursors acts as an inhibitor of osteoclastogenesis; however, the effects of upregulation of Runx1 on osteoclast formation remain unknown. In this study, we investigated the skeletal effects of conditional overexpression of Runx1 in preosteoclasts by crossing novel Runx1 gain-of-function mice (Rosa26-LSL-Runx1) with LysM-Cre transgenic mice. We observed a sex-dependent effect whereby overexpression of Runx1 in female mice increased trabecular bone microarchitectural indices and improved torsion biomechanical properties. These effects were likely mediated by delayed osteoclastogenesis and decreased bone resorption. Transcriptomics analyses during osteoclastogenesis revealed a distinct transcriptomic profile in the Runx1-overexpressing cells, with enrichment of genes related to redox signaling, apoptosis, osteoclast differentiation, and bone remodeling. These data further confirm the antiosteoclastogenic activities of Runx1 and provide new insight into the molecular targets that may mediate these effects.

Ablation of the miRNA cluster 24 in cartilage and osteoblasts impairs bone remodeling

MicroRNAs (miRNAs) post-transcriptionally regulate cartilage and bone development and function, however, only few miRNAs have been described to play a role for cartilage to bone transition in vivo. Previously, we showed that cartilage-specific deletion of the Mirc24 cluster in newborn male mice leads to impaired growth plate cartilage development due to increased RAF/MEK/ERK signaling and affects the stability of the cartilage extracellular matrix on account of decreased SOX6 and SOX9 and increased MMP13 levels. Here, we studied how Mirc24 cluster inactivation in cartilage and osteoblasts leads to an increased bone density associated with defects in collagen remodeling in trabecular bone. No changes in osteoblast distribution were observed, whereas the number of osteoclasts was reduced and TRAP activity in osteoclasts decreased. Surprisingly, an increased level of cluster-encoded miR-322 or miR-503 raises Rankl gene expression and inactivation of the cluster in chondrocytes reduces Rankl expression. These results suggest that the Mirc24 cluster regulates Rankl expression in chondrocytes at the chondro-osseous border, where the cluster is mainly expressed to modulate osteoclast formation, bone remodeling and bone integrity.

Skeletal and gene-regulatory functions of nuclear sex steroid hormone receptors

The wide variety of sex hormone actions underlie bone growth and health, and their actions mediate gene regulation by the cognate nuclear receptors. Nuclear androgen and estrogen receptors (AR, and ERα/ERβ) are hormone-dependent and DNA binding- transcription regulatory factors, and gene regulation by sex hormones often accompany with chromatin remodeling under aid of a number of co-regulators. As sex hormone biosynthesis is under highly regulated systemic and local regulations, the skeletal actions of sex hormones could be inferred from only the phenotypic abnormalities in skeleton in mouse genetic models deficient of nuclear receptors selectively in specific types of bone cells as well as at specific cell differentiation stages. Anabolic androgen actions and anti-bone resorptive estrogen actions are discussed here from the phenotypic abnormalities in such model mice. Though rapid gene regulation by sex hormones may not require chromatin reorganization, dynamic chromatin reconfiguration looks to facilitate profound and long-term hormonal actions. In this review, we focus the recent findings in gene regulation at a chromatin level, particularly of the function of enhancer RNAs transcribed from strong enhancers, and in the role of liquid-liquid phase separation state in transcription initiation through chromatin reconfiguration.

Bone mass and adaptation to mechanical loading are sexually dimorphic in adult osteoblast-specific ERα knockout mice

Estrogen receptor-alpha (ERα) regulates bone mass and is implicated in bone tissue's response to mechanical loading. The effects of ERα deletion in mice depend on sex, anatomical location, and the cellular stage at which ERα is removed. Few studies have investigated the effect of age on the role of ERα in skeletal maintenance and functional adaptation. We previously demonstrated that bone mass and adaptation to loading were altered in growing 10-week-old female and male mice lacking ERα in mature osteoblasts and osteocytes (pOC-ERαKO). Here our goal was to determine the effects of ERα and mechanical loading in skeletally-mature adult mice. We subjected 26-week-old skeletally-mature adult pOC-ERαKO and littermate control (LC) mice of both sexes to two weeks of in vivo cyclic tibial loading. ERα deletion in male mice did not alter bone mass or the response to loading. Adult female pOC-ERαKO mice had reduced cancellous and cortical bone mass and increased adaptation to high-magnitude mechanical loading compared to LC mice. Thus, ERα deletion from mature osteoblasts reduced the bone mass and increased the mechanoadaptation of adult female but not male mice. Additionally, compared to our previous work in young mice, adult female mice had greatly reduced mechanoadaptation and adult male mice retained most of their mechanoadaptation with age.

Anxiety and Cognition in Cre- Collagen Type II Sirt1 K/O Male Mice

Introduction

Using transgenic collagen type II-specific Sirt1 knockout (CKO) mice we studied the role of Sirt1 in nutritional induced catch up growth (CUG) and we found that these mice have a less organized growth plate and reduced efficiency of CUG. In addition, we noted that they weigh more than control (CTL) mice. Studying the reason for the increased weigh, we found differences in activity and brain function.

Methods

Several tests for behavior and activity were used: open field; elevated plus maze, Morris water maze, and home cage running wheels. The level of Glu- osteocalcin, known to connect bone and brain function, was measured by Elisa; brain Sirt1 was analyzed by western blot.

Results

We found that CKO mice had increased anxiety, with less spatial memory, learning capabilities and reduced activity in their home cages. No significant differences were found between CKO and CTL mice in Glu- osteocalcin levels; nor in the level of brain SIRT1.

Discussion/Conclusion

Using transgenic collagen type II-specific Sirt1 knockout (CKO) mice we found a close connection between linear growth and brain function. Using a collagen type II derived system we affected a central regulatory mechanism leading to hypo activity, increased anxiety, and slower learning, without affecting circadian period. As children with idiopathic short stature are more likely to have lower IQ, with substantial deficits in working memory than healthy controls, the results of the current study suggest that SIRT1 may be the underlying factor connecting growth and brain function.

Myeloid Lineage Ablation of Phlpp1 Regulates M-CSF Signaling and Tempers Bone Resorption in Female Mice

Prior work demonstrated that Phlpp1 deficiency alters trabecular bone mass and enhances M-CSF responsiveness, but the cell types and requirement of Phlpp1 for this effect were unclear. To understand the function of Phlpp1 within myeloid lineage cells, we crossed Phlpp1 floxed mice with mice harboring LysM-Cre. Micro-computed tomography of the distal femur of 12-week-old mice revealed a 30% increase in bone volume per total volume of Phlpp1 female conditional knockouts, but we did not observe significant changes within male Phlpp1 cKOLysM mice. Bone histomorphmetry of the proximal tibia further revealed that Phlpp1 cKOLysM females exhibited elevated osteoclast numbers, but conversely had reduced levels of serum markers of bone resorption as compared to littermate controls. Osteoblast number and serum markers of bone formation were unchanged. In vitro assays confirmed that Phlpp1 ablation enhanced osteoclast number and area, but limited bone resorption. Additionally, reconstitution with exogenous Phlpp1 suppressed osteoclast numbers. Dose response assays demonstrated that Phlpp1-/- cells are more responsive to M-CSF, but reconstitution with Phlpp1 abrogated this effect. Furthermore, small molecule-mediated Phlpp inhibition enhanced osteoclast numbers and size. Enhanced phosphorylation of Phlpp substrates-including Akt, ERK1/2, and PKCζ-accompanied these observations. In contrast, actin cytoskeleton disruption occurred within Phlpp inhibitor treated osteoclasts. Moreover, Phlpp inhibition reduced resorption of cells cultured on bovine bone slices in vitro. Our results demonstrate that Phlpp1 deficiency within myeloid lineage cells enhances bone mass by limiting bone resorption while leaving osteoclast numbers intact; moreover, we show that Phlpp1 represses osteoclastogenesis and controls responses to M-CSF.