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.

Osteocyte Estrogen Receptor ß (Ot-ERß) Regulates Bone Turnover and Skeletal Adaptive Response to Mechanical Loading Differently in Male and Female Growing and Adult Mice.

Age-related bone loss is a failure of balanced bone turnover and diminished skeletal mechanoadaptation. Estrogen receptors, ERα and ERß, play critical roles in osteoprotective regulation activated by estrogen and mechanical signals. Previous studies mainly focused on ERα and showed that osteocyte-ERα (Ot-ERα) regulated trabecular, but not cortical bone, and played a minor role in load-induced cortical adaptation. However, the role of Ot-ERß in bone mass regulation remains unrevealed. To address this issue, we characterized bone (re)modeling and gene expression in male and female mice with Ot-ERß deletion (ERß-dOT) and littermate control (LC) at 10 weeks (young) or 28 weeks (adult) of age, as well as their responses to in vivo tibial compressive loading. Increased cancellous bone mass appeared in the L4 vertebral body of young male ERß-dOT mice. At the same time, femoral cortical bone gene expression showed signs consistent with elevated osteoblast and osteoclast activities (type-I collagen, Cat K, RANKL). Upregulated androgen receptor (AR) expression was observed in young male ERß-dOT mice relative to LC, suggesting a compensatory effect of testosterone on male bone protection. In contrast, bone mass in L4 decreased in adult male ERß-dOT mice, attributed to potentially increased bone resorption activity (Cat K) with no change in bone formation. There was no effect of ERß-dOT on bone mass or gene expression in female mice. Sex-dependent regulation of Ot-ERß also appeared in load-induced cortical responsiveness. Young female ERß-dOT mice showed an enhanced tibial cortical anabolic adaptation compared with LC. In contrast, an attenuated cortical anabolic response presented at the proximal tibia in male ERß-dOT mice at both ages. For the first time, our findings suggest that Ot-ERß regulates bone (re)modeling and the response to mechanical signals through different mechanisms in males and females. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Notum Deletion From Late-Stage Skeletal Cells Increases Cortical Bone Formation and Potentiates Skeletal Effects of Sclerostin Inhibition.

Wnt signaling plays a vital role in the cell biology of skeletal patterning, differentiation, and maintenance. Notum is a secreted member of the α/ß-hydrolase superfamily that hydrolyzes the palmitoleoylate modification on Wnt proteins, thereby disrupting Wnt signaling. As a secreted inhibitor of Wnt, Notum presents an attractive molecular target for improving skeletal health. To determine the cell type of action for Notum's effect on the skeleton, we generated mice with Notum deficiency globally (Notum-/- ) and selectively (Notumf/f ) in limb bud mesenchyme (Prx1-Cre) and late osteoblasts/osteocytes (Dmp1-Cre). Late-stage deletion induced increased cortical bone properties, similar to global mutants. Notum expression was enhanced in response to sclerostin inhibition, so dual inhibition (Notum/sclerostin) was also investigated using a combined genetic and pharmacologic approach. Co-suppression increased cortical properties beyond either factor alone. Notum suppressed Wnt signaling in cell reporter assays, but surprisingly also enhanced Shh signaling independent of effects on Wnt. Notum is an osteocyte-active suppressor of cortical bone formation that is likely involved in multiple signaling pathways important for bone homeostasis © 2021 American Society for Bone and Mineral Research (ASBMR).

Improving Bone Health by Optimizing the Anabolic Action of Wnt Inhibitor Multitargeting.

Sclerostin antibody (romosozumab) was recently approved for clinical use in the United States to treat osteoporosis. We and others have explored Wnt-based combination therapy to disproportionately improve the anabolic effects of sclerostin inhibition, including cotreatment with sclerostin antibody (Scl-mAb) and Dkk1 antibody (Dkk1-mAb). To determine the optimal ratio of Scl-mAb and Dkk1-mAb for producing maximal anabolic action, the proportion of Scl-mAb and Dkk1-mAb were systematically varied while holding the total antibody dose constant. A 3:1 mixture of Scl-mAb to Dkk1-mAb produced two to three times as much cancellous bone mass as an equivalent dose of Scl-mAb alone. Further, a 75% reduction in the dose of the 3:1 mixture was equally efficacious to a full dose of Scl-mAb in the distal femur metaphysis. The Scl-mAb/Dkk1-mAb combination approach was highly efficacious in the cancellous bone mass, but the cortical compartment was much more subtly affected. The osteoanabolic effects of Wnt pathway targeting can be made more efficient if multiple antagonists are simultaneously targeted. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Cancellous Bone May Have a Greater Adaptive Strain Threshold Than Cortical Bone.

Strain magnitude has a controlling influence on bone adaptive response. However, questions remain as to how and if cancellous and cortical bone tissues respond differently to varied strain magnitudes, particularly at a molecular level. The goal of this study was to characterize the time-dependent gene expression, bone formation, and structural response of the cancellous and cortical bone of female C57Bl/6 mice to mechanical loading by applying varying load levels (low: -3.5 N; medium: -5.2 N; high: -7 N) to the skeleton using a mouse tibia loading model. The loading experiment showed that cortical bone mass at the tibial midshaft was significantly enhanced following all load levels examined and bone formation activities were particularly elevated at the medium and high loads applied. In contrast, for the proximal metaphyseal cancellous bone, only the high load led to significant increases in bone mass and bone formation indices. Similarly, expression of genes associated with inhibition of bone formation (e.g., Sost) was altered in the diaphyseal cortical bone at all load levels, but in the metaphyseal cortico-cancellous bone only by the high load. Finite element analysis determined that the peak tensile or compressive strains that were osteogenic for the proximal cancellous bone under the high load were significantly greater than those that were osteogenic for the midshaft cortical tissues under the low load. These results suggest that the magnitude of the strain stimulus regulating structural, cellular, and molecular responses of bone to loading may be greater for the cancellous tissues than for the cortical tissues. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Co-deletion of Lrp5 and Lrp6 in the skeleton severely diminishes bone gain from sclerostin antibody administration.

The cysteine knot protein sclerostin is an osteocyte-derived secreted inhibitor of the Wnt co-receptors LRP5 and LRP6. LRP5 plays a dominant role in bone homeostasis, but we previously reported that Sost/sclerostin suppression significantly increased osteogenesis regardless of Lrp5 presence or absence. Those observations suggested that the bone forming effects of sclerostin inhibition can occur through Lrp6 (when Lrp5 is suppressed), or through other yet undiscovered mechanisms independent of Lrp5/6. To distinguish between these two possibilities, we generated mice with compound deletion of Lrp5 and Lrp6 selectively in bone, and treated them with sclerostin monoclonal antibody (Scl-mAb). All mice were homozygous flox for both Lrp5 and Lrp6 (Lrp5f/f; Lrp6f/f), and varied only in whether or not they carried the Dmp1-Cre transgene. Positive (Cre+) and negative (Cre-) mice were injected with Scl-mAb or vehicle from 4.5 to 14 weeks of age. Vehicle-treated Cre+ mice exhibited significantly reduced skeletal properties compared to vehicle-treated Cre- mice, as assessed by DXA, µCT, pQCT, and histology, indicating that Lrp5/6 deletions were effective and efficient. Scl-mAb treatment improved nearly every bone-related parameter among Cre- mice, but the same treatment in Cre+ mice resulted in little to no improvement in skeletal properties. For the few endpoints where Cre+ mice responded to Scl-mAb, it is likely that antibody-induced promotion of Wnt signaling occurred in cell types earlier in the mesenchymal/osteoblast differentiation pathway than the Dmp1-expressing stage. This latter conclusion was supported by changes in some histomorphometric parameters. In conclusion, unlike with the deletion of Lrp5 alone, the bone-selective late-stage co-deletion of Lrp5 and Lrp6 significantly impairs or completely nullifies the osteogenic action of Scl-mAb, and highlights a major role for both Lrp5 and Lrp6 in the mechanism of action for the bone-building effects of sclerostin antibody.

The mTORC2 Component Rictor Is Required for Load-Induced Bone Formation in Late-Stage Skeletal Cells.

Bone relies on mechanical cues to build and maintain tissue composition and architecture. Our understanding of bone cell mechanotransduction continues to evolve, with a few key signaling pathways emerging as vital. Wnt/ß-catenin, for example, is essential for proper anabolic response to mechanical stimulation. One key complex that regulates ß-catenin activity is the mammalian target of rapamycin complex 2 (mTORc2). mTORc2 is critical for actin cytoskeletal reorganization, an indispensable component in mechanotransduction in certain cell types. In this study, we probed the impact of the mTORc2 signaling pathway in osteocyte mechanotransduction by conditionally deleting the mTORc2 subunit Rictor in Dmp1-expressing cells of C57BL/6 mice. Conditional deletion of the Rictor was achieved using the Dmp1-Cre driver to recombine Rictor floxed alleles. Rictor mutants exhibited a decrease in skeletal properties, as measured by DXA, µCT, and mechanical testing, compared with Cre-negative floxed littermate controls. in vivo axial tibia loading conducted in adult mice revealed a deficiency in the osteogenic response to loading among Rictor mutants. Histological measurements of osteocyte morphology indicated fewer, shorter cell processes in Rictor mutants, which might explain the compromised response to mechanical stimulation. In summary, inhibition of the mTORc2 pathway in late osteoblasts/osteocytes leads to decreased bone mass and mechanically induced bone formation. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Pten deletion in Dmp1-expressing cells does not rescue the osteopenic effects of Wnt/ß-catenin suppression.

Skeletal homeostasis is sensitive to perturbations in Wnt signaling. Beyond its role in the bone, Wnt is a major target for pharmaceutical inhibition in a wide range of diseases, most notably cancers. Numerous clinical trials for Wnt-based candidates are currently underway, and Wnt inhibitors will likely soon be approved for clinical use. Given the bone-suppressive effects accompanying Wnt inhibition, there is a need to expose alternate pathways/molecules that can be targeted to counter the deleterious effects of Wnt inhibition on bone properties. Activation of the Pi3k/Akt pathway via Pten deletion is one possible osteoanabolic pathway to exploit. We investigated whether the osteopenic effects of ß-catenin deletion from bone cells could be rescued by Pten deletion in the same cells. Mice carrying floxed alleles for Pten and ß-catenin were bred to Dmp1-Cre mice to delete Pten alone, ß-catenin alone, or both genes from the late-stage osteoblast/osteocyte population. The mice were assessed for bone mass, density, strength, and formation parameters to evaluate the potential rescue effect of Pten deletion in Wnt-impaired mice. Pten deletion resulted in high bone mass and ß-catenin deletion resulted in low bone mass. Compound mutants had bone properties similar to ß-catenin mutant mice, or surprisingly in some assays, were further compromised beyond ß-catenin mutants. Pten inhibition, or one of its downstream nodes, is unlikely to protect against the bone-wasting effects of Wnt/ßcat inhibition. Other avenues for preserving bone mass in the presence of Wnt inhibition should be explored to alleviate the skeletal side effects of Wnt inhibitor-based therapies.

Lrp4 Mediates Bone Homeostasis and Mechanotransduction through Interaction with Sclerostin In Vivo.

Wnt signaling plays a key role in regulating bone remodeling. In vitro studies suggest that sclerostin's inhibitory action on Lrp5 is facilitated by the membrane-associated receptor Lrp4. We generated an Lrp4 R1170W knockin mouse model (Lrp4KI), based on a published mutation in patients with high bone mass (HBM). Lrp4KI mice have an HBM phenotype (assessed radiographically), including increased bone strength and formation. Overexpression of a Sost transgene had osteopenic effects in Lrp4-WT but not Lrp4KI mice. Conversely, sclerostin inhibition had blunted osteoanabolic effects in Lrp4KI mice. In a disuse-induced bone wasting model, Lrp4KI mice exhibit significantly less bone loss than wild-type (WT) mice. In summary, mice harboring the Lrp4-R1170W missense mutation recapitulate the human HBM phenotype, are less sensitive to altered sclerostin levels, and are protected from disuse-induced bone loss. Lrp4 is an attractive target for pharmacological targeting aimed at increasing bone mass and preventing bone loss due to disuse.

Twist1 Inactivation in Dmp1-Expressing Cells Increases Bone Mass but Does Not Affect the Anabolic Response to Sclerostin Neutralization.

Wnt signaling plays a major role in bone metabolism. Advances in our understanding of secreted regulators of Wnt have yielded several therapeutic targets to stimulate osteoanabolism-the most promising of which is the Wnt inhibitor sclerostin. Sclerostin antibody recently gained approval for clinical use to treat osteoporosis, but the biology surrounding sclerostin antagonism is still incompletely understood. Numerous factors regulate the efficacy of sclerostin inhibition on bone formation, a process known as self-regulation. In previous communications we reported that the basic helix-loop-helix transcription factor Twist1-a gene know to regulate skeletal development-is highly upregulated among the osteocyte cell population in mice treated with sclerostin antibody. In this communication, we tested the hypothesis that preventing Twist1 upregulation by deletion of Twist1 from late-stage osteoblasts and osteocytes would increase the efficacy of sclerostin antibody treatment, since Twist1 is known to restrain osteoblast activity in many models. Twist1-floxed loss-of-function mice were crossed to the Dmp1-Cre driver to delete Twist1 in Dmp1-expressing cells. Conditional Twist1 deletion was associated with a mild but significant increase in bone mass, as assessed by dual energy x-ray absorptiometry (DXA) and microCT (µCT) for many endpoints in both male and female mice. Biomechanical properties of the femur were not affected by conditional mutation of Twist1. Sclerostin antibody improved all bone properties significantly, regardless of Twist1 status, sex, or endpoint examined. No interactions were detected when Twist1 status and antibody treatment were examined together, suggesting that Twist1 upregulation in the osteocyte population is not an endogenous mechanism that restrains the osteoanabolic effect of sclerostin antibody treatment. In summary, Twist1 inhibition in the late-stage osteoblast/osteocyte increases bone mass but does not affect the anabolic response to sclerostin neutralization.

Expression of a Degradation-Resistant ß-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 ß-catenin-an intracellular signaling node in the canonical Wnt pathway-in disuse mechanotransduction is not defined. Using the ß-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 ß-catenin on bone properties during disuse. We hypothesized that if ß-catenin plays an important role in Wnt-mediated osteoprotection, then artificial stabilization of ß-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 ßcatCA 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 ßcatCA 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 ß-catenin stabilized mice and tail-suspended nonstabilized mice. Our findings indicate that selectively targeting/blocking of ß-catenin degradation in bone cells could have therapeutic implications in mechanically induced bone disease. © 2019 American Society for Bone and Mineral Research.

Osteocytes and mechanical loading: The Wnt connection.

Bone adapts to the mechanical forces that it experiences. Orthodontic tooth movement harnesses the cell- and tissue-level properties of mechanotransduction to achieve alignment and reorganization of the dentition. However, the mechanisms of action that permit bone resorption and formation in response to loads placed on the teeth are incompletely elucidated, though several mechanisms have been identified. Wnt/Lrp5 signalling in osteocytes is a key pathway that modulates bone tissue's response to load. Numerous mouse models that harbour knock-in, knockout and transgenic/overexpression alleles targeting genes related to Wnt signalling point to the necessity of Wnt/Lrp5, and its localization to osteocytes, for proper mechanotransduction in bone. Alveolar bone is rich in osteocytes and is a highly mechanoresponsive tissue in which components of the canonical Wnt signalling cascade have been identified. As Wnt-based agents become clinically available in the next several years, the major challenge that lies ahead will be to gain a more complete understanding of Wnt biology in alveolar bone so that improved/expedited tooth movement becomes a possibility.

Induction of Lrp5 HBM-causing mutations in Cathepsin-K expressing cells alters bone metabolism.

High-bone-mass (HBM)-causing missense mutations in the low density lipoprotein receptor-related protein-5 (Lrp5) are associated with increased osteoanabolic action and protection from disuse- and ovariectomy-induced osteopenia. These mutations (e.g., A214V and G171V) confer resistance to endogenous secreted Lrp5/6 inhibitors, such as sclerostin (SOST) and Dickkopf homolog-1 (DKK1). Cells in the osteoblast lineage are responsive to canonical Wnt stimulation, but recent work has indicated that osteoclasts exhibit both indirect and direct responsiveness to canonical Wnt. Whether Lrp5-HBM receptors, expressed in osteoclasts, might alter osteoclast differentiation, activity, and consequent net bone balance in the skeleton, is not known. To address this, we bred mice harboring heterozygous Lrp5 HBM-causing conditional knock-in alleles to Ctsk-Cre transgenic mice and studied the phenotype using DXA, µCT, histomorphometry, serum assays, and primary cell culture. Mice with HBM alleles induced in Ctsk-expressing cells (TG) exhibited higher bone mass and architectural properties compared to non-transgenic (NTG) counterparts. In vivo and in vitro measurements of osteoclast activity, population density, and differentiation yielded significant reductions in osteoclast-related parameters in female but not male TG mice. Droplet digital PCR performed on osteocyte enriched cortical bone tubes from TG and NTG mice revealed that ~8-17% of the osteocyte population (depending on sex) underwent recombination of the conditional Lrp5 allele in the presence of Ctsk-Cre. Further, bone formation parameters in the midshaft femur cortex show a small but significant increase in anabolic action on the endocortical but not periosteal surface. These findings suggest that Wnt/Lrp5 signaling in osteoclasts affects osteoclastogenesis and activity in female mice, but also that some of the changes in bone mass in TG mice might be due to Cre expression in the osteocyte population.

Sclerostin neutralization unleashes the osteoanabolic effects of Dkk1 inhibition.

The WNT pathway has become an attractive target for skeletal therapies. High-bone-mass phenotypes in patients with loss-of-function mutations in the LRP5/6 inhibitor Sost (sclerosteosis), or in its downstream enhancer region (van Buchem disease), highlight the utility of targeting Sost/sclerostin to improve bone properties. Sclerostin-neutralizing antibody is highly osteoanabolic in animal models and in human clinical trials, but antibody-based inhibition of another potent LRP5/6 antagonist, Dkk1, is largely inefficacious for building bone in the unperturbed adult skeleton. Here, we show that conditional deletion of Dkk1 from bone also has negligible effects on bone mass. Dkk1 inhibition increases Sost expression, suggesting a potential compensatory mechanism that might explain why Dkk1 suppression lacks anabolic action. To test this concept, we deleted Sost from osteocytes in, or administered sclerostin neutralizing antibody to, mice with a Dkk1-deficient skeleton. A robust anabolic response to Dkk1 deletion was manifest only when Sost/sclerostin was impaired. Whole-body DXA scans, µCT measurements of the femur and spine, histomorphometric measures of femoral bone formation rates, and biomechanical properties of whole bones confirmed the anabolic potential of Dkk1 inhibition in the absence of sclerostin. Further, combined administration of sclerostin and Dkk1 antibody in WT mice produced a synergistic effect on bone gain that greatly exceeded individual or additive effects of the therapies, confirming the therapeutic potential of inhibiting multiple WNT antagonists for skeletal health. In conclusion, the osteoanabolic effects of Dkk1 inhibition can be realized if sclerostin upregulation is prevented. Anabolic therapies for patients with low bone mass might benefit from a strategy that accounts for the compensatory milieu of WNT inhibitors in bone tissue.

WNT-mediated Modulation of Bone Metabolism: Implications for WNT Targeting to Treat Extraskeletal Disorders.

The WNT-signaling pathway is involved in cellular and tissue functions that control such diverse processes as body axis patterning, cellular proliferation, differentiation, and life span. The long list of molecules that can participate or modify WNT signaling makes this pathway one of the most complex in cell biology. In bone tissues, WNT signaling is required for proper skeletal development, and human mutations in various components of the cascade revealed insights into pharmacologic targeting that can be harnessed to improve skeletal health. In particular, mutations in genes that code for the WNT-signaling inhibitor sclerostin or the WNT coreceptor lipoprotein receptor-related protein 5 have highlighted the potential therapeutic value of recapitulating those effects in patients with low bone mass. A constant challenge in this area is selectively modifying WNT components in the tissue of interest, as WNT has manifold effects in nearly every tissue.

Sost, independent of the non-coding enhancer ECR5, is required for bone mechanoadaptation.

Sclerostin (Sost) is a negative regulator of bone formation that acts upon the Wnt signaling pathway. Sost is mechanically regulated at both mRNA and protein level such that loading represses and unloading enhances Sost expression, in osteocytes and in circulation. The non-coding evolutionarily conserved enhancer ECR5 has been previously reported as a transcriptional regulatory element required for modulating Sost expression in osteocytes. Here we explored the mechanisms by which ECR5, or several other putative transcriptional enhancers regulate Sost expression, in response to mechanical stimulation. We found that in vivo ulna loading is equally osteoanabolic in wildtype and Sost-/- mice, although Sost is required for proper distribution of load-induced bone formation to regions of high strain. Using Luciferase reporters carrying the ECR5 non-coding enhancer and heterologous or homologous hSOST promoters, we found that ECR5 is mechanosensitive in vitro and that ECR5-driven Luciferase activity decreases in osteoblasts exposed to oscillatory fluid flow. Yet, ECR5-/- mice showed similar magnitude of load-induced bone formation and similar periosteal distribution of bone formation to high-strain regions compared to wildtype mice. Further, we found that in contrast to Sost-/- mice, which are resistant to disuse-induced bone loss, ECR5-/- mice lose bone upon unloading to a degree similar to wildtype control mice. ECR5 deletion did not abrogate positive effects of unloading on Sost, suggesting that additional transcriptional regulators and regulatory elements contribute to load-induced regulation of Sost.