Prophylactic pamidronate partially protects from glucocorticoid-induced bone loss in the mdx mouse model of Duchenne muscular dystrophy

Extracellular matrix: Dystroglycan interactions-Roles for the dystrophin-associated glycoprotein complex in skeletal tissue dynamics

Contributions made by the dystrophin-associated glycoprotein complex (DGC) to cell-cell and cell-extracellular matrix (ECM) interactions are vital in development, homeostasis and pathobiology. This review explores how DGC functions may extend to skeletal pathophysiology by appraising the known roles of its major ECM ligands, and likely associated DGC signalling pathways, in regulating cartilage and bone cell behaviour and emergent skeletal phenotypes. These considerations will be contextualised by highlighting the potential of studies into the role of the DGC in isolated chondrocytes, osteoblasts and osteoclasts, and by fuller deliberation of skeletal phenotypes that may emerge in very young mice lacking vital, yet diverse core elements of the DGC. Our review points to roles for individual DGC components-including the glycosylation of dystroglycan itself-beyond the establishment of membrane stability which clearly accounts for severe muscle phenotypes in muscular dystrophy. It implies that the short stature, low bone mineral density, poor bone health and greater fracture risk in these patients, which has been attributed due to primary deficiencies in muscle-evoked skeletal loading, may instead arise due to primary roles for the DGC in controlling skeletal tissue (re)modelling.

Combined growth hormone and insulin-like growth factor-1 rescues growth retardation in glucocorticoid-treated mdxmice but does not prevent osteopenia

Short stature and osteoporosis are common in Duchenne muscular dystrophy (DMD) and its pathophysiology may include an abnormality of the growth hormone/insulin-like growth factor-1 (GH/IGF-1) axis, which is further exacerbated by long-term glucocorticoid (GC) treatment. Hence, an agent that has anabolic properties and may improve linear growth would be beneficial in this setting and therefore requires further exploration. A 5-week-old x-linked muscular dystrophy (mdx) mice were used as a model of DMD. They were treated with prednisolone ± GH + IGF-1 for 4 weeks and then compared to control mdx mice to allow the study of both growth and skeletal structure. GC reduced cortical bone area, bone fraction, tissue area and volume and cortical bone volume, as assessed by micro computed tomography (CT) In addition, GC caused somatic and skeletal growth retardation but improved grip strength. The addition of GH + IGF-1 therapy rescued the somatic growth retardation and induced additional improvements in grip strength (16.9% increase, P < 0.05 compared to control). There was no improvement in bone microarchitecture (assessed by micro-CT and static histomorphometry) or biomechanical properties (assessed by three-point bending). Serum bone turnover markers (Serum procollagen 1 intact N-terminal propeptide (P1NP), alpha C-terminal telopeptide (αCTX)) also remained unaffected. Further work is needed to maximise these gains before proceeding to clinical trials in boys with DMD.

Trabecular Bone Volume Is Reduced, With Deteriorated Microstructure, With Aging in a Rat Model of Duchenne Muscular Dystrophy

We aimed to clarify the effect of aging on trabecular bone volume and trabecular bone microstructure in a rat model of Duchenne muscular dystrophy (DMD). Six rats each of wild type (WT) and DMD model at 15 weeks of age, and 4 rats each at 30 weeks of age, were analyzed by dual energy X-ray absorptiometry and by micro-CT for analysis of trabecular and cortical bone of the femur. Bone mineral density was significantly lower in the DMD group than in the WT group at both 15 and 30 weeks of age. Micro-CT showed that trabecular bone volume and number were not significantly different between the two groups at 15 weeks, but at 30 weeks both were significantly lower in the DMD group than in the WT group. Connectivity density and structure model index were not significantly different between the two groups at 15 weeks, but at 30 weeks they differed significantly. No significant differences between the WT and DMD groups in cortical thickness and cortical area were evident at both 15 and 30 weeks. In conclusion, trabecular bone volume is significantly reduced, with deteriorated microstructure, with aging in a rat model of DMD.

Growth Hormone Increases Bone Toughness and Decreases Muscle Inflammation in Glucocorticoid-Treated Mdx Mice, Model of Duchenne Muscular Dystrophy

The absence of functional dystrophin with mutations of the dystrophin-encoding gene in Duchenne muscular dystrophy (DMD) results in muscle inflammation and degeneration, as well as bone fragility. Long-term glucocorticoid therapy delays the muscular disease progression but suppresses growth hormone secretion, resulting in short stature and further deleterious effects on bone strength. This study evaluated the therapeutic potential of daily growth hormone therapy in growing mdx mice as a model of DMD. Growth hormone treatment on its own or in combination with glucocorticoids significantly improved muscle histology and function and decreased markers of inflammation in mdx mice. Glucocorticoid treatment thinned cortical bone and decreased bone strength and toughness. Despite the minimal effects of growth hormone on bone microarchitecture, it significantly improved biomechanical properties of femurs and vertebrae, even in the presence of glucocorticoid treatment. Together these studies suggest that the use of growth hormone in DMD should be considered for improvements to muscle and bone health. © 2019 American Society for Bone and Mineral Research.

Intermittent PTH treatment improves bone and muscle in glucocorticoid treated Mdx mice: A model of Duchenne Muscular Dystrophy

Duchenne Muscular Dystrophy (DMD) is a progressive muscle disorder caused by genetic mutations of the dystrophin encoding gene. In the absence of functional dystrophin, DMD patients suffer from muscle inflammation and wasting, as well as compromised bone health with increased risk of fracture. The use of high dose glucocorticoids (GC) as the standard therapy also contributes to bone fragility. This study examined the effects of intermittent, daily administered parathyroid hormone (iPTH), an approved bone anabolic therapy, on growing bone and dystrophic muscle in the presence and absence of prednisone treatment using the Mdx mouse model of DMD. Five-weeks of prednisone treatment in Mdx mice decreased cortical bone thickness and area (p < 0.001), with a large increase in endocortical osteoclasts that were significantly improved by PTH treatment (p < 0.001). GC-induced decreases in cortical bone toughness and modulus were improved with iPTH therapy (p < 0.05). Mdx mice showed significantly less bone mass in trabecular compartments of lumbar vertebrae and iPTH treatment, with or without glucocorticoids, significantly improved structural and material properties of this bone. Prednisone improved grip strength and endurance of treadmill running, which were maintained and further improved, respectively, in co-treated Mdx mice. Altogether, our study demonstrates that iPTH therapy significantly ameliorated GC-induced bone loss and maintained or further enhanced the positive effects of GCs on dystrophic muscle function. These findings give insight into the potential for use of teriparatide to treat growing bone in children with DMD.

Pre-treatment with Pamidronate Improves Bone Mechanical Properties in Mdx Mice Treated with Glucocorticoids

Duchenne muscular dystrophy (DMD) is an X-linked disease of progressive muscle deterioration and weakness. Patients with DMD have poor bone health which is partly due to treatment with glucocorticoids, a standard therapy to prolong muscle function that also induces bone loss. Bisphosphonates are used to treat adults at risk of glucocorticoid-induced osteoporosis but are not currently used in DMD patients until after they sustain fractures. In this study, C57BL/10ScSn-mdx mice, a commonly used DMD animal model, received continuous glucocorticoid, prednisone treatment (0.083 mg/day) from 5 to 10 weeks of age. Pre-treatment with the bisphosphonate pamidronate started at 4 weeks of age over a period of 2 weeks or 6 weeks (cumulative dose 8 mg/kg for both) to assess the effectiveness of the two dosing regimens in ameliorating glucocorticoid-induced bone loss. Mdx mice treated with prednisone had improved muscle function that was not changed by pamidronate treatment. Glucocorticoid treatment caused cortical bone loss and decreased cortical bone strength. Both 2 and 6 week pamidronate treatment increased cortical thickness and bone area compared to prednisone-treated Mdx mice, however, only 2 week pamidronate treatment improved the strength of cortical bone compared to that of glucocorticoid-treated Mdx mice. In the trabecular bone, both pamidronate treatments significantly increased the amount of bone, and increased the ultimate load but not the energy to fail. These results highlight the importance of when and how much bisphosphonate is administered prior to glucocorticoid exposure.

Targeting the Muscle-Bone Unit: Filling Two Needs with One Deed in the Treatment of Duchenne Muscular Dystrophy

PURPOSE OF REVIEW

In Duchenne muscular dystrophy (DMD), the progressive skeletal and cardiac muscle dysfunction and degeneration is accompanied by low bone mineral density and bone fragility. Glucocorticoids, which remain the standard of care for patients with DMD, increase the risk of developing osteoporosis. The scope of this review emphasizes the mutual cohesion and common signaling pathways between bone and skeletal muscle in DMD.

RECENT FINDINGS

The muscle-bone interactions involve bone-derived osteokines, muscle-derived myokines, and dual-origin cytokines that trigger common signaling pathways leading to fibrosis, inflammation, or protein synthesis/degradation. In particular, the triad RANK/RANKL/OPG including receptor activator of NF-kB (RANK), its ligand (RANKL), along with osteoprotegerin (OPG), regulates bone matrix modeling and remodeling pathways and contributes to muscle pathophysiology in DMD. This review discusses the importance of the muscle-bone unit in DMD and covers recent research aimed at determining the muscle-bone interactions that may eventually lead to the development of multifunctional and effective drugs for treating muscle and bone disorders regardless of the underlying genetic mutations in DMD.

Effect of 25-HydroxyVitamin D Deficiency and Its Interaction with Prednisone Treatment on Musculoskeletal Health in Growing Mdx Mice

Duchenne muscular dystrophy (DMD) results from genetic mutations of the gene encoding dystrophin, leading to muscle inflammation and degeneration that is typically treated with glucocorticoids. DMD and its treatment with glucocorticoids result in poor bone health and high risk of fractures. Insufficient levels of 25-hydroxyvitamin D (25-hydroxy D) that may contribute to weakened bone are routinely found in DMD patients. To determine the effect of 25-hydroxy D deficiency, this study examined the effects of low vitamin D dietary intake with and without glucocorticoids on the musculoskeletal system of the Mdx mouse model of DMD. At 10 weeks of age, Mdx mice on control diet had low trabecular bone mineral density of distal femurs and lumbar vertebrae with increased osteoclast numbers compared to wild-type mice. Low vitamin D intake resulted in 25-hydroxy D deficiency but had no effect on trabecular or cortical bone. Cortical bone loss and bone weakness were induced by glucocorticoids while they improved muscle grip strength in Mdx mice. 25-hydroxy D deficiency did not result in any significant effects on growing bone or muscle in the Mdx mice. In combination with glucocorticoid treatment, low 25-hydroxy D resulted in no change in cortical bone mineral density but bone ductility was significantly increased suggesting lower bone mineralization.