Latent TGFβ binding proteins (LTBPs) bind to inactive TGFβ in the extracellular matrix. In mice, muscular dystrophy symptoms are intensified by a genetic polymorphism that changes the hinge region of LTBP, leading to increased proteolytic susceptibility and TGFβ release. We have found that the hinge region of human LTBP4 was also readily proteolyzed, and that proteolysis could be blocked by an antibody to the hinge region. Transgenic mice were generated to carry a bacterial artificial chromosome encoding the human LTBP4 gene. These transgenic mice displayed larger myofibers, increased damage after muscle injury, and enhanced TGFβ signaling. In the mdx mouse model of Duchenne muscular dystrophy, the human LTBP4 transgene exacerbated muscular dystrophy symptoms and resulted in weaker muscles with an increased inflammatory infiltrate and greater LTBP4 cleavage in vivo. Blocking LTBP4 cleavage may be a therapeutic strategy to reduce TGFβ release and activity and decrease inflammation and muscle damage in muscular dystrophy.

Objective S100A12 and fibroblast growth factor 23 (FGF23) are biomarkers of cardiovascular morbidity and mortality in patients with chronic kidney disease (CKD). We tested the hypothesis that human S100/calgranulin would accelerate cardiovascular disease in mice subjected to CKD. Approach and Results A bacterial artificial chromosome of the human S100/calgranulin gene cluster containing the genes and regulatory elements for S100A8, S100A9 and S100A12 was expressed in C57BL/6J mouse (hBAC-S100) to generate a novel humanized mouse model. CKD was induced by ureteral ligation and hBAC-S100 mice and WT mice were studied after 10 weeks of chronic uremia. hBAC-S100 mice with CKD showed increased FGF 23 in the hearts, left ventricular hypertrophy (LVH), diastolic dysfunction, focal cartilaginous metaplasia and calcification of the mitral and aortic valve annulus together with aortic valve sclerosis. This phenotype was not observed in WT mice with CKD or in hBAC-S100 mice lacking the receptor for advanced glycation endproducts (RAGE) with CKD, suggesting that the inflammatory milieu mediated by S100/RAGE promotes pathological cardiac hypertrophy in CKD. In vitro, inflammatory stimuli including IL-6, TNFα, LPS, or serum from hBAC-S100 mice up regulated FGF23 mRNA and protein in primary murine neonatal and adult cardiac fibroblasts. Conclusions Myeloid-derived human S100/calgranulin is associated with the development of cardiac hypertrophy and ectopic cardiac calcification in a RAGE dependent manner in a mouse model of CKD. We speculate that FGF23 produced by cardiac fibroblasts in response to cytokines may act in a paracrine manner to accelerate LVH and diastolic dysfunction in hBAC-S100 mice with CKD.

Latent TGFβ binding proteins (LTBPs) regulate the extracellular availability of latent TGFβ. LTBP4 was identified as a genetic modifier of muscular dystrophy in mice and humans. An in-frame insertion polymorphism in the murine Ltbp4 gene associates with partial protection against muscular dystrophy. In humans, nonsynonymous single nucleotide polymorphisms in LTBP4 associate with prolonged ambulation in Duchenne muscular dystrophy. To better understand LTBP4 and its role in modifying muscular dystrophy, we created transgenic mice overexpressing the protective murine allele of LTBP4 specifically in mature myofibers using the human skeletal actin promoter. Overexpression of LTBP4 protein was associated with increased muscle mass and proportionally increased strength compared to age-matched controls. In order to assess the effects of LTBP4 in muscular dystrophy, LTBP4 overexpressing mice were bred to mdx mice, a model of Duchenne muscular dystrophy. In this model, increased LTBP4 led to greater muscle mass with proportionally increased strength, and decreased fibrosis. The increase in muscle mass and reduction in fibrosis were similar to what occurs when myostatin, a related TGFβ family member and negative regulator of muscle mass, was deleted in mdx mice. Supporting this, we found that myostatin forms a complex with LTBP4 and that overexpression of LTBP4 led to a decrease in myostatin levels. LTBP4 also interacted with TGFβ and GDF11, a protein highly related to myostatin. These data identify LTBP4 as a multi-TGFβ family ligand binding protein with the capacity to modify muscle disease through overexpression.

Disruption of the dystrophin complex causes muscle injury, dysfunction, cell death and fibrosis. Excess transforming growth factor (TGF) β signaling has been described in human muscular dystrophy and animal models, where it is thought to relate to the progressive fibrosis that characterizes dystrophic muscle. We now found that canonical TGFβ signaling acutely increases when dystrophic muscle is stimulated to contract. Muscle lacking the dystrophin-associated protein γ-sarcoglycan (Sgcg null) was subjected to a lengthening protocol to produce maximal muscle injury, which produced rapid accumulation of nuclear phosphorylated SMAD2/3. To test whether reducing SMAD signaling improves muscular dystrophy in mice, we introduced a heterozygous mutation of SMAD4 (S4) into Sgcg mice to reduce but not ablate SMAD4. Sgcg/S4 mice had improved body mass compared with Sgcg mice, which normally show a wasting phenotype similar to human muscular dystrophy patients. Sgcg/S4 mice had improved cardiac function as well as improved twitch and tetanic force in skeletal muscle. Functional enhancement in Sgcg/S4 muscle occurred without a reduction in fibrosis, suggesting that intracellular SMAD4 targets may be important. An assessment of genes differentially expressed in Sgcg muscle focused on those encoding calcium-handling proteins and responsive to TGFβ since this pathway is a target for mediating improvement in muscular dystrophy. These data demonstrate that excessive TGFβ signaling alters cardiac and muscle performance through the intracellular SMAD pathway.

Anterior shoulder pathology involving the subscapularis is often associated with the biceps tendon because both anatomic structures intersect the lesser tuberosity. Standard procedures for such pathology often involve simultaneous subscapularis repairs and biceps tenodesis. Single anterior portal subscapularis repairs have been emerging in the past 5 years because of cost-effectiveness and efficiency. Biceps tenodesis is a common procedure performed both open and arthroscopically. This technique takes advantage of the close relation between the long head of the biceps tendon and subscapularis tendon to restore the functional length-tension relation and preserve function through fixation using a single portal and a single knotless suture anchor.