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

L-BAIBA Synergizes with Sub-Optimal Mechanical Loading to Promote New Bone Formation

The L-enantiomer of β-aminoisobutyric acid (BAIBA) is secreted by contracted muscle in mice, and exercise increases serum levels in humans. In mice, L-BAIBA reduces bone loss with unloading, but whether it can have a positive effect with loading is unknown. Since synergism can be more easily observed with sub-optimal amounts of factors/stimulation, we sought to determine whether L-BAIBA could potentiate the effects of sub-optimal loading to enhance bone formation. L-BAIBA was provided in drinking water to C57Bl/6 male mice subjected to either 7 N or 8.25 N of sub-optimal unilateral tibial loading for 2 weeks. The combination of 8.25 N and L-BAIBA significantly increased the periosteal mineral apposition rate and bone formation rate compared to loading alone or BAIBA alone. Though L-BAIBA alone had no effect on bone formation, grip strength was increased, suggesting a positive effect on muscle function. Gene expression analysis of the osteocyte-enriched bone showed that the combination of L-BAIBA and 8.25 N induced the expression of loading-responsive genes such as Wnt1, Wnt10b, and the TGFb and BMP signaling pathways. One dramatic change was the downregulation of histone genes in response to sub-optimal loading and/or L-BAIBA. To determine early gene expression, the osteocyte fraction was harvested within 24 hours of loading. A dramatic effect was observed with L-BAIBA and 8.25 N loading as genes were enriched for pathways regulating the extracellular matrix (Chad, Acan, Col9a2), ion channel activity (Scn4b, Scn7a, Cacna1i), and lipid metabolism (Plin1, Plin4, Cidec). Few changes in gene expression were observed with sub-optimal loading or L-BAIBA alone after 24 hours. These results suggest that these signaling pathways are responsible for the synergistic effects between L-BAIBA and sub-optimal loading. Showing that a small muscle factor can enhance the effects of sub-optimal loading of bone may be of relevance for individuals unable to benefit from optimal exercise. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

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.

Activation of Focal Adhesion Kinase Restores Simulated Microgravity-Induced Inhibition of Osteoblast Differentiation via Wnt/Β-Catenin Pathway

Simulated microgravity (SMG) inhibits osteoblast differentiation (OBD) and induces bone loss via the inhibition of the Wnt/β-catenin pathway. However, the mechanism by which SMG alters the Wnt/β-catenin pathway is unknown. We previously demonstrated that SMG altered the focal adhesion kinase (FAK)-regulated mTORC1, AMPK and ERK1/2 pathways, leading to the inhibition of tumor cell proliferation/metastasis and promoting cell apoptosis. To examine whether FAK similarly mediates SMG-dependent changes to Wnt/β-catenin in osteoblasts, we characterized mouse MC3T3-E1 cells cultured under clinostat-modeled SMG (µg) conditions. Compared to cells cultured under ground (1 g) conditions, SMG reduces focal adhesions, alters cytoskeleton structures, and down-regulates FAK, Wnt/β-catenin and Wnt/β-catenin-regulated molecules. Consequently, protein-2 (BMP2), type-1 collagen (COL1), alkaline-phosphatase activity and matrix mineralization are all inhibited. In the mouse hindlimb unloading (HU) model, SMG-affected tibial trabecular bone loss is significantly reduced, according to histological and micro-computed tomography analyses. Interestingly, the FAK activator, cytotoxic necrotizing factor-1 (CNF1), significantly suppresses all of the SMG-induced alterations in MC3T3-E1 cells and the HU model. Therefore, our data demonstrate the critical role of FAK in the SMG-induced inhibition of OBD and bone loss via the Wnt/β-catenin pathway, offering FAK signaling as a new therapeutic target not only for astronauts at risk of OBD inhibition and bone loss, but also osteoporotic patients.

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/β-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).

The Skeletal Cellular and Molecular Underpinning of the Murine Hindlimb Unloading Model

Bone adaptation to spaceflight results in bone loss at weight bearing sites following the absence of the stimulus represented by ground force. The rodent hindlimb unloading model was designed to mimic the loss of mechanical loading experienced by astronauts in spaceflight to better understand the mechanisms causing this disuse-induced bone loss. The model has also been largely adopted to study disuse osteopenia and therefore to test drugs for its treatment. Loss of trabecular and cortical bone is observed in long bones of hindlimbs in tail-suspended rodents. Over the years, osteocytes have been shown to play a key role in sensing mechanical stress/stimulus via the ECM-integrin-cytoskeletal axis and to respond to it by regulating different cytokines such as SOST and RANKL. Colder experimental environments (~20-22°C) below thermoneutral temperatures (~28-32°C) exacerbate bone loss. Hence, it is important to consider the role of environmental temperatures on the experimental outcomes. We provide insights into the cellular and molecular pathways that have been shown to play a role in the hindlimb unloading and recommendations to minimize the effects of conditions that we refer to as confounding factors.

The Diminishing Returns of Mechanical Loading and Potential Mechanisms that Desensitize Osteocytes

Adaptation to mechanical loading is critical to maintaining bone mass and offers therapeutic potential to preventing age-related bone loss and osteoporosis. However, increasing the duration of loading is met with "diminishing returns" as the anabolic response quickly becomes saturated. As a result, the anabolic response to daily activities and repetitive bouts of loading is limited by the underlying mechanisms that desensitize and render bone unresponsive at the cellular level. Osteocytes are the primary cells that respond to skeletal loading and facilitate the overall anabolic response. Although many of osteocytes' signaling mechanisms activated in response to loading are considered anabolic in nature, several of them can also render osteocytes insensitive to further stimuli and thereby creating a negative feedback loop that limits osteocytes' overall response. The purpose of this review is to examine the potential mechanisms that may contribute to the loss of mechanosensitivity. In particular, we examined the inactivation/desensitization of ion channels and signaling molecules along with the potential role of endocytosis and cytoskeletal reorganization. The significance in defining the negative feedback loop is the potential to identify unique targets for enabling osteocytes to maintain their sensitivity. In doing so, we can begin to cultivate new strategies that capitalize on the anabolic nature of daily activities that repeatedly load the skeleton.

Botulinum Toxin A and Osteosarcopenia in Experimental Animals: A Scoping Review

We conducted a scoping review to investigate the effects of intramuscular injection of Botulinum Toxin A (BoNT-A) on bone morphology. We investigated if the muscle atrophy associated with Injection of BoNT-A had effects on the neighboring bone. We used the search terms: osteopenia, bone atrophy, Botulinum Toxin A, Micro-CT, mice or rat. The following databases were searched: Medline, Embase, PubMed and the Cochrane Library, between 1990 and 2020. After removal of duplicates, 228 abstracts were identified of which 49 studies satisfied our inclusion and exclusion criteria. The majority of studies (41/49) reported a quantitative reduction in at least one measure of bone architecture based on Micro-CT. The reduction in the ratio of bone volume to tissue volume varied from 11% to 81% (mean 43%) according to the experimental set up and study time points. While longer term studies showed muscle recovery, no study showed complete recovery of all bone properties at the termination of the study. In experimental animals, intramuscular injection of BoNT-A resulted in acute muscle atrophy and acute degradation of the neighboring bone segment. These findings may have implications for clinical protocols in the use of Botulinum Toxin in children with cerebral palsy, with restraint recommended in injection protocols and consideration for monitoring bone density. Clinical studies in children with cerebral palsy receiving injections of Botulinum are indicated.

MSTN is a key mediator for low-intensity pulsed ultrasound preventing bone loss in hindlimb-suspended rats

Low-intensity pulsed ultrasound (LIPUS) has been used to accelerate bone fracture healing. However, the issue whether LIPUS is effective in preventing osteoporosis has not been clarified, and if so, what possible mechanisms might be responsible. Myostatin (MSTN) is a negative regulator of muscle growth, and its absence will trigger a positive response to bone. In this study, we examined the effects of LIPUS on bone micro-structure, mechanical properties and damage healing of hindlimb-suspended rats, and investigated whether the inhibition of MSTN plays a role in this process. The rats were randomly divided into four groups: Normal control group (NC), Hind limb suspension group (HLS), Hind limb suspension and 80 mW/cm² LIPUS irradiation group (HLS+ 80 mW/cm²), Hind limb suspension and 30 mW/cm² LIPUS irradiation group (HLS+ 30 mW/cm²). The HLS+ 80 mW/cm² rats were treated with LIPUS (1 MHz, 80 mW/cm²) and the HLS+ 30 mW/cm² rats were treated with LIPUS (1 MHz, 30 mW/cm²) on the femur for 20 min/day for 28 days. MC3T3-E1 cells were respectively cultured with the serum of wild type mouse and MSTN knockout mouse at 1% concentration for 7 days. After 28 days, LIPUS effectively prevented the destruction of bone microstructure and the decline of mechanical properties, and promoted bone defect healing in the tail-suspended rats. In addition, LIPUS effectively reduced the MSTN content in the quadriceps and serum of the tail-suspended rats, inhibited its receptor and downstream signaling molecules and activated the Wnt signaling pathway in femurs. Growth of MC-3T3-E1 cell cultured with the serum of MSTN knockout mice was superior to that with wild mice serum on day 7. These results indicate that MSTN is a key mediator in LIPUS preventing bone loss caused by hindlimb-suspension.

An Activating Variant in CTNNB1 is Associated with a Sclerosing Bone Dysplasia and Adrenocortical Neoplasia

CONTEXT

The WNT/β-catenin pathway is central to the pathogenesis of various human diseases including those affecting bone development and tumor progression.

OBJECTIVE

To evaluate the role of a gain-of-function variant in CTNNB1 in a child with a sclerosing bone dysplasia and an adrenocortical adenoma.

DESIGN

Whole exome sequencing with corroborative biochemical analyses.

PATIENTS

We recruited a child with a sclerosing bone dysplasia and an adrenocortical adenoma together with her unaffected parents.

INTERVENTION

Whole exome sequencing and performance of immunoblotting and luciferase-based assays to assess the cellular consequences of a de novo variant in CTNNB1.

MAIN OUTCOME MEASURE(S)/RESULT

A de novo variant in CTNNB1 (c.131C>T; p.[Pro44Leu]) was identified in a patient with a sclerosing bone dysplasia and an adrenocortical adenoma. A luciferase-based transcriptional assay of WNT signaling activity verified that the activity of β-catenin was increased in the cells transfected with a CTNNB1p.Pro44Leu construct (P = 4.00 × 10-5). The β-catenin p.Pro44Leu variant was also associated with a decrease in phosphorylation at Ser45 and Ser33/Ser37/Thr41 in comparison to a wild-type (WT) CTNNB1 construct (P = 2.16 × 10-3, P = 9.34 × 10-8 respectively).

CONCLUSION

Increased β-catenin activity associated with a de novo gain-of-function CTNNB1 variant is associated with osteosclerotic phenotype and adrenocortical neoplasia.

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 (Lrp4^KI), based on a published mutation in patients with high bone mass (HBM). Lrp4^KI 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 Lrp4^KI mice. Conversely, sclerostin inhibition had blunted osteoanabolic effects in Lrp4^KI mice. In a disuse-induced bone wasting model, Lrp4^KI 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.

•

Missense mutation in the third beta-propeller of Lrp4 improve bone properties

•

The R1170W mutation in Lrp4 interferes with sclerostin inhibition in vivo

•

The R1170W Lrp4 mutation alters the bone wasting effects of mechanical disuse