Targeting of SUMOylation leads to cBAF complex stabilization and disruption of the SS18::SSX transcriptome in Synovial Sarcoma.

Synovial Sarcoma (SS) is driven by the SS18::SSX fusion oncoprotein. and is ultimately refractory to therapeutic approaches. SS18::SSX alters ATP-dependent chromatin remodeling BAF (mammalian SWI/SNF) complexes, leading to the degradation of canonical (cBAF) complex and amplified presence of an SS18::SSX-containing non-canonical BAF (ncBAF or GBAF) that drives an SS-specific transcription program and tumorigenesis. We demonstrate that SS18::SSX activates the SUMOylation program and SSs are sensitive to the small molecule SAE1/2 inhibitor, TAK-981. Mechanistically, TAK-981 de-SUMOylates the cBAF subunit SMARCE1, stabilizing and restoring cBAF on chromatin, shifting away from SS18::SSX-ncBAF-driven transcription, associated with DNA damage and cell death and resulting in tumor inhibition across both human and mouse SS tumor models. TAK-981 synergized with cytotoxic chemotherapy through increased DNA damage, leading to tumor regression. Targeting the SUMOylation pathway in SS restores cBAF complexes and blocks the SS18::SSX-ncBAF transcriptome, identifying a therapeutic vulnerability in SS, positioning the in-clinic TAK-981 to treat SS.

Reambulation following hindlimb unloading attenuates disuse-induced changes in murine fracture healing.

Patients with bone and muscle loss from prolonged disuse have higher risk of falls and subsequent fragility fractures. In addition, fracture patients with continued disuse and/or delayed physical rehabilitation have worse clinical outcomes compared to individuals with immediate weight-bearing activity following diaphyseal fracture. However, the effects of prior disuse followed by physical reambulation on fracture healing cellular processes and adjacent bone and skeletal muscle recovery post-injury remains poorly defined. To bridge this knowledge gap and inform future treatment and rehabilitation strategies for fractures, a preclinical model of fracture healing with a history of prior unloading with and without reambulation was employed. First, skeletally mature male and female C57BL/6J mice (18 weeks) underwent hindlimb unloading by tail suspension (HLU) for 3 weeks to induce significant bone and muscle loss modeling enhanced bone fragility. Next, mice had their right femur fractured by open surgical dissection (stabilized with 24-gauge pin). Then, mice were randomly assigned to continued HLU or allowed normal weight-bearing reambulation (HLU + R). Mice given normal cage activity throughout the experiment served as healthy age-matched controls. All mice were sacrificed 4-days (DPF4) or 14-days (DPF14) following fracture to assess healing and uninjured hindlimb musculoskeletal properties (6-10 mice per treatment group/biological sex/timepoint). We found that continued disuse following fracture led to severely diminished uninjured hindlimb skeletal muscle mass (gastrocnemius and soleus) and femoral bone volume adjacent to the fracture site compared to healthy age-matched controls across mouse sexes. Furthermore, HLU led to significantly decreased periosteal expansion (DPF4) and osteochondral tissue formation by DPF14, and trends in increased osteoclastogenesis (DPF14) and decreased woven bone vascular area (DPF14). In contrast, immediate reambulation for 2 weeks after fracture, even following a period of prolonged disuse, was able to increase hindlimb skeletal tissue mass and increase osteochondral tissue formation, albeit not to healthy control levels, in both mouse sexes. Furthermore, reambulation attenuated osteoclast formation seen in woven bone tissue undergoing disuse. Our results suggest that weight-bearing skeletal loading in both sexes immediately following fracture may improve callus healing and prevent further fall risk by stimulating skeletal muscle anabolism and decreasing callus resorption compared to minimal or delayed rehabilitation regimens.

A bone-specific adipogenesis pathway in fat-free mice defines key origins and adaptations of bone marrow adipocytes with age and disease.

Bone marrow adipocytes accumulate with age and in diverse disease states. However, their origins and adaptations in these conditions remain unclear, impairing our understanding of their context-specific endocrine functions and relationship with surrounding tissues. In this study, by analyzing bone and adipose tissues in the lipodystrophic 'fat-free' mouse, we define a novel, secondary adipogenesis pathway that relies on the recruitment of adiponectin-negative stromal progenitors. This pathway is unique to the bone marrow and is activated with age and in states of metabolic stress in the fat-free mouse model, resulting in the expansion of bone marrow adipocytes specialized for lipid storage with compromised lipid mobilization and cytokine expression within regions traditionally devoted to hematopoiesis. This finding further distinguishes bone marrow from peripheral adipocytes and contributes to our understanding of bone marrow adipocyte origins, adaptations, and relationships with surrounding tissues with age and disease.

A Neuroskeletal Atlas: Spatial Mapping and Contextualization of Axon Subtypes Innervating the Long Bones of C3H and B6 Mice.

Nerves in bone play well-established roles in pain and vasoregulation and have been associated with progression of skeletal disorders, including osteoporosis, fracture, arthritis, and tumor metastasis. However, isolation of the region-specific mechanisms underlying these relationships is limited by our lack of quantitative methods for neuroskeletal analysis and precise maps of skeletal innervation. To overcome these limitations, we developed an optimized workflow for imaging and quantitative analysis of axons in and around the bone, including validation of Baf53b-Cre in concert with R26R-tdTomato (Ai9) as a robust pan-neuronal reporter system for use in musculoskeletal tissues. In addition, we created comprehensive maps of sympathetic adrenergic and sensory peptidergic axons within and around the full length of the femur and tibia in two strains of mice (B6 and C3H). In the periosteum, these maps were related to the surrounding musculature, including entheses and myotendinous attachments to bone. Three distinct patterns of periosteal innervation (termed type I, II, III) were defined at sites that are important for bone pain, bone repair, and skeletal homeostasis. For the first time, our results establish a gradient of bone marrow axon density that increases from proximal to distal along the length of the tibia and define key regions of interest for neuroskeletal studies. Lastly, this information was related to major nerve branches and local maps of specialized mechanoreceptors. This detailed mapping and contextualization of the axonal subtypes innervating the skeleton is intended to serve as a guide during the design, implementation, and interpretation of future neuroskeletal studies and was compiled as a resource for the field as part of the NIH SPARC consortium. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR)..

Immunostaining of Skeletal Tissues.

Immunostaining is the process of identifying proteins in tissue sections by incubating the sample with antibodies specific to the protein of interest, then visualizing the bound antibody using a chromogen (immunohistochemistry or IHC) or fluorescence (immunofluorescence or IF). Unlike in situ hybridization, which identifies gene transcripts in cells, immunostaining identifies the products themselves and provides information about their localization within cells (nuclear, cytoplasmic, or membrane) or extracellular matrix. This can be particularly important in the context of bone and cartilage because they contain many cell types as well as matrix components, each with distinct protein expression patterns. As the number of antibodies continues to grow, this technique has become vital for research laboratories studying the skeleton. Here, we describe a detailed protocol for antibody-based in situ analysis of bone and associated tissues, addressing specific issues associated with staining of hard and matrix-rich tissues.

Bone marrow adipose tissue does not express UCP1 during development or adrenergic-induced remodeling.

Adipocytes within the skeleton are collectively termed bone marrow adipose tissue (BMAT). BMAT contributes to peripheral and local metabolism, however, its capacity for cell-autonomous expression of uncoupling protein 1 (UCP1), a biomarker of beige and brown adipogenesis, remains unclear. To overcome this, Ucp1-Cre was used to drive diphtheria toxin expression in cells expressing UCP1 (Ucp1Cre+/DTA+). Despite loss of brown adipose tissue, BMAT volume was not reduced in Ucp1Cre+/DTA+ mice. Comparably, in mTmG reporter mice (Ucp1Cre+/mTmG+), Ucp1-Cre expression was absent from BMAT in young (3-weeks) and mature (16-weeks) male and female mice. Further, ß3-agonist stimulation failed to induce Ucp1-Cre expression in BMAT. This demonstrates that BMAT adipocytes are not UCP1-expressing beige/brown adipocytes. Thus, to identify novel and emerging roles for BMAT adipocytes in skeletal and whole-body homeostasis, we performed gene enrichment analysis of microarray data from adipose tissues of adult rabbits. Pathway analysis revealed genetic evidence for differences in BMAT including insulin resistance, decreased fatty acid metabolism, and enhanced contributions to local processes including bone mineral density through candidate genes such as osteopontin. In sum, this supports a paradigm by which BMAT adipocytes are a unique subpopulation that is specialized to support cells within the skeletal and hematopoietic niche.

Shared Autonomic Pathways Connect Bone Marrow and Peripheral Adipose Tissues Across the Central Neuraxis.

Bone marrow adipose tissue (BMAT) is increased in both obesity and anorexia. This is unique relative to white adipose tissue (WAT), which is generally more attuned to metabolic demand. It suggests that there may be regulatory pathways that are common to both BMAT and WAT and also those that are specific to BMAT alone. The central nervous system (CNS) is a key mediator of adipose tissue function through sympathetic adrenergic neurons. Thus, we hypothesized that central autonomic pathways may be involved in BMAT regulation. To test this, we first quantified the innervation of BMAT by tyrosine hydroxylase (TH) positive nerves within the metaphysis and diaphysis of the tibia of B6 and C3H mice. We found that many of the TH+ axons were concentrated around central blood vessels in the bone marrow. However, there were also areas of free nerve endings which terminated in regions of BMAT adipocytes. Overall, the proportion of nerve-associated BMAT adipocytes increased from proximal to distal along the length of the tibia (from ~3-5 to ~14-24%), regardless of mouse strain. To identify the central pathways involved in BMAT innervation and compare to peripheral WAT, we then performed retrograde viral tract tracing with an attenuated pseudorabies virus (PRV) to infect efferent nerves from the tibial metaphysis (inclusive of BMAT) and inguinal WAT (iWAT) of C3H mice. PRV positive neurons were identified consistently from both injection sites in the intermediolateral horn of the spinal cord, reticular formation, rostroventral medulla, solitary tract, periaqueductal gray, locus coeruleus, subcoeruleus, Barrington's nucleus, and hypothalamus. We also observed dual-PRV infected neurons within the majority of these regions. Similar tracings were observed in pons, midbrain, and hypothalamic regions from B6 femur and tibia, demonstrating that these results persist across mouse strains and between skeletal sites. Altogether, this is the first quantitative report of BMAT autonomic innervation and reveals common central neuroanatomic pathways, including putative "command" neurons, involved in coordinating multiple aspects of sympathetic output and facilitation of parallel processing between bone marrow/BMAT and peripheral adipose tissue.

Corrigendum: Shared Autonomic Pathways Connect Bone Marrow and Peripheral Adipose Tissues Across the Central Neuraxis.

[This corrects the article DOI: 10.3389/fendo.2019.00668.].