CBX2 is required to stabilize the testis pathway by repressing Wnt signaling.

XX and XY fetal gonads are initially bipotential, poised between the ovary and testis fate. Multiple lines of evidence suggest that commitment to testis fate requires the repression of genes associated with ovary fate. It was previously shown that loss of CBX2, the subunit of the Polycomb Repressive Complex 1 (PRC1) that binds H3K27me3 and mediates silencing, leads to ovary development in XY mice and humans. While it had been proposed that CBX2 is an activator of the testis-determining gene Sry, we investigated the alternative possibility that CBX2 has a direct role as a repressor of the antagonistic ovary-promoting pathway. To investigate this possibility, we developed a quantitative genome-wide profile of the repressive histone mark H3K27me3 and its active counterpart H3K4me3 in isolated XY and XX gonadal supporting cells before and after sex determination. We show that testis and ovary sex-determining (SD) genes are bivalent before sex determination, providing insight into how the bipotential state of the gonad is established at the epigenetic level. After sex determination, many SD genes of the alternate pathway remain bivalent, possibly contributing to the ability of these cells to transdifferentiate even in adults. The finding that many genes in the Wnt signaling pathway were targeted for H3K27me3-mediated repression in Sertoli cells led us to test whether deletion of Wnt4 could rescue testis development in Cbx2 mutants. We show that Sry expression and testis development were rescued in XY Cbx2-/-;Wnt4-/- mice. Furthermore, we show that CBX2 directly binds the downstream Wnt signaler Lef1, an ovary-promoting gene that remains bivalent in Sertoli cells. Our results suggest that stabilization of the testis fate requires CBX2-mediated repression of bivalent ovary-determining genes, which would otherwise block testis development.

Gonadal supporting cells acquire sex-specific chromatin landscapes during mammalian sex determination.

Cis-regulatory elements are critical for the precise spatiotemporal regulation of genes during development. However, identifying functional regulatory sites that drive cell differentiation in vivo has been complicated by the high numbers of cells required for whole-genome epigenetic assays. Here, we identified putative regulatory elements during sex determination by performing ATAC-seq and ChIP-seq for H3K27ac in purified XX and XY gonadal supporting cells before and after sex determination in mice. We show that XX and XY supporting cells initiate sex determination with similar chromatin landscapes and acquire sex-specific regulatory elements as they commit to the male or female fate. To validate our approach, we identified a functional gonad-specific enhancer downstream of Bmp2, an ovary-promoting gene. This work increases our understanding of the complex regulatory network underlying mammalian sex determination and provides a powerful resource for identifying non-coding regulatory elements that could harbor mutations that lead to Disorders of Sexual Development.

Intermittent Glucocorticoid Dosing Improves Muscle Repair and Function in Mice with Limb-Girdle Muscular Dystrophy.

The muscular dystrophies are genetically diverse. Shared pathological features among muscular dystrophies include breakdown, or loss of muscle, and accompanying fibrotic replacement. Novel strategies are needed to enhance muscle repair and function and to slow this pathological remodeling. Glucocorticoid steroids, like prednisone, are known to delay loss of ambulation in patients with Duchenne muscular dystrophy but are accompanied by prominent adverse effects. However, less is known about the effects of steroid administration in other types of muscular dystrophies, including limb-girdle muscular dystrophies (LGMDs). LGMD 2B is caused by loss of dysferlin, a membrane repair protein, and LGMD 2C is caused by loss of the dystrophin-associated protein, γ-sarcoglycan. Herein, we assessed the efficacy of steroid dosing on sarcolemmal repair, muscle function, histopathology, and the regenerative capacity of primary muscle cells. We found that in murine models of LGMD 2B and 2C, daily prednisone dosing reduced muscle damage and fibroinflammatory infiltration. However, daily prednisone dosing also correlated with increased muscle adipogenesis and atrophic remodeling. Conversely, intermittent dosing of prednisone, provided once weekly, enhanced muscle repair and did not induce atrophy or adipogenesis, and was associated with improved muscle function. These data indicate that dosing frequency of glucocorticoid steroids affects muscle remodeling in non-Duchenne muscular dystrophies, suggesting a positive outcome associated with intermittent steroid dosing in LGMD 2B and 2C muscle.

Intermittent glucocorticoid treatment enhances skeletal muscle performance through sexually dimorphic mechanisms.

Glucocorticoid steroids are commonly prescribed for many inflammatory conditions, but chronic daily use produces adverse effects, including muscle wasting and weakness. In contrast, shorter glucocorticoid pulses may improve athletic performance, although the mechanisms remain unclear. Muscle is sexually dimorphic and comparatively little is known about how male and female muscles respond to glucocorticoids. We investigated the impact of once-weekly glucocorticoid exposure on skeletal muscle performance comparing male and female mice. One month of once-weekly glucocorticoid dosing improved muscle specific force in both males and females. Transcriptomic profiling of isolated myofibers identified a striking sexually dimorphic response to weekly glucocorticoids. Male myofibers had increased expression of genes in the IGF1/PI3K pathway and calcium handling, while female myofibers had profound upregulation of lipid metabolism genes. Muscles from weekly prednisone-treated males had improved calcium handling, while comparably treated female muscles had reduced intramuscular triglycerides. Consistent with altered lipid metabolism, weekly prednisone-treated female mice had greater endurance relative to controls. Using chromatin immunoprecipitation, we defined a sexually dimorphic chromatin landscape after weekly prednisone. These results demonstrate that weekly glucocorticoid exposure elicits distinct pathways in males versus females, resulting in enhanced performance.

Mechanisms and Clinical Applications of Glucocorticoid Steroids in Muscular Dystrophy.

Glucocorticoid steroids are widely used as immunomodulatory agents in acute and chronic conditions. Glucocorticoid steroids such as prednisone and deflazacort are recommended for treating Duchenne Muscular Dystrophy where their use prolongs ambulation and life expectancy. Despite this benefit, glucocorticoid use in Duchenne Muscular Dystrophy is also associated with significant adverse consequences including adrenal suppression, growth impairment, poor bone health and metabolic syndrome. For other forms of muscular dystrophy like the limb girdle dystrophies, glucocorticoids are not typically used. Here we review the experimental evidence supporting multiple mechanisms of glucocorticoid action in dystrophic muscle including their role in dampening inflammation and myofiber injury. We also discuss alternative dosing strategies as well as novel steroid agents that are in development and testing, with the goal to reduce adverse consequences of prolonged glucocorticoid exposure while maximizing beneficial outcomes.

Anti-latent TGFß binding protein 4 antibody improves muscle function and reduces muscle fibrosis in muscular dystrophy.

Duchenne muscular dystrophy, like other muscular dystrophies, is a progressive disorder hallmarked by muscle degeneration, inflammation, and fibrosis. Latent transforming growth factor ß (TGFß) binding protein 4 (LTBP4) is an extracellular matrix protein found in muscle. LTBP4 sequesters and inhibits a precursor form of TGFß. LTBP4 was originally identified from a genome-wide search for genetic modifiers of muscular dystrophy in mice, where there are two different alleles. The protective form of LTBP4, which contains an insertion of 12 amino acids in the protein's hinge region, was linked to increased sequestration of latent TGFß, enhanced muscle membrane stability, and reduced muscle fibrosis. The deleterious form of LTBP4 protein, lacking 12 amino acids, was more susceptible to proteolysis and promoted release of latent TGF-ß, and together, these data underscored the functional role of LTBP4's hinge. Here, we generated a monoclonal human anti-LTBP4 antibody directed toward LTBP4's hinge region. In vitro, anti-LTBP4 bound LTBP4 protein and reduced LTBP4 proteolytic cleavage. In isolated myofibers, the LTBP4 antibody stabilized the sarcolemma from injury. In vivo, anti-LTBP4 treatment of dystrophic mice protected muscle against force loss induced by eccentric contraction. Anti-LTBP4 treatment also reduced muscle fibrosis and enhanced muscle force production, including in the diaphragm muscle, where respiratory function was improved. Moreover, the anti-LTBP4 in combination with prednisone, a standard of care for Duchenne muscular dystrophy, further enhanced muscle function and protected against injury in mdx mice. These data demonstrate the potential of anti-LTBP4 antibodies to treat muscular dystrophy.

Moderate exercise improves function and increases adiponectin in the mdx mouse model of muscular dystrophy.

The loss of dystrophin produces a mechanically fragile sarcolemma, causing muscle membrane disruption and muscle loss. The degree to which exercise alters muscular dystrophy has been evaluated in humans with Duchenne Muscular Dystrophy (DMD) and in mouse models including the mdx mouse but with inconsistent findings. We now examined two different levels of exercise, moderate and low intensity, in the mdx mouse model in the DBA2J background. mdx mice at 4-5 months of age were subjected to two different doses of exercise. We found a dose-dependent benefit for low and moderate exercise, defined as 4 m/min or 8 m/min, for 30 minutes three times a week. After six months, exercised mdx mice showed improved tetanic and specific force compared to the sedentary group. We also observed increased respiratory capacity manifesting as greater minute volume, as well as enhanced cardiac function mitigating the decline of fractional shortening that is normally seen. Exercised mdx mice also showed a dose-dependent increase in serum adiponectin with a concomitant reduced adipocyte cross sectional area. These findings identify moderate intensity exercise as a means to improve muscle performance in the mdx DBA2J mice and suggest serum adiponectin as a biomarker for beneficial exercise effect in DMD.

Sex reversal following deletion of a single distal enhancer of Sox9.

Cell fate decisions require appropriate regulation of key genes. Sox9, a direct target of SRY, is pivotal in mammalian sex determination. In vivo high-throughput chromatin accessibility techniques, transgenic assays, and genome editing revealed several novel gonadal regulatory elements in the 2-megabase gene desert upstream of Sox9 Although others are redundant, enhancer 13 (Enh13), a 557-base pair element located 565 kilobases 5' from the transcriptional start site, is essential to initiate mouse testis development; its deletion results in XY females with Sox9 transcript levels equivalent to those in XX gonads. Our data are consistent with the time-sensitive activity of SRY and indicate a strict order of enhancer usage. Enh13 is conserved and embedded within a 32.5-kilobase region whose deletion in humans is associated with XY sex reversal, suggesting that it is also critical in humans.