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Thrombospondins (Thbs) are a family of five secreted matricellular glycoproteins in vertebrates that broadly affect cell-matrix interaction. While Thbs4 is known to protect striated muscle from disease by enhancing sarcolemmal stability through increased integrin and dystroglycan attachment complexes, here we show that Thbs3 antithetically promotes sarcolemmal destabilization by reducing integrin function, augmenting disease-induced decompensation. Deletion of Thbs3 in mice enhances integrin membrane expression and membrane stability, protecting the heart from disease stimuli. Transgene-mediated overexpression of α7β1D integrin in the heart ameliorates the disease predisposing effects of Thbs3 by augmenting sarcolemmal stability. Mechanistically, we show that mutating Thbs3 to contain the conserved RGD integrin binding domain normally found in Thbs4 and Thbs5 now rescues the defective expression of integrins on the sarcolemma. Thus, Thbs proteins mediate the intracellular processing of integrin plasma membrane attachment complexes to regulate the dynamics of cellular remodeling and membrane stability.

The master cytokine TGF-[beta] mediates tissue fibrosis associated with inflammation and tissue injury. TGF-[beta] induces fibroblast activation and differentiation into myofibroblasts that secrete extracellular matrix proteins. Canonical TGF-[beta] signaling mobilizes Smad2 and Smad3 transcription factors that control fibrosis by promoting gene expression. However, the importance of TGF-[beta]-Smad2/3 signaling in fibroblast-mediated cardiac fibrosis has not been directly evaluated in vivo. Here, we examined pressure overload-induced cardiac fibrosis in fibroblast- and myofibroblast-specific inducible Creexpressing mouse lines with selective deletion of the TGF-[beta] receptors Tgfbr1/2, Smad2, or Smad3. Fibroblast-specific deletion of Tgfbr1/2 or Smad3, but not Smad2, markedly reduced the pressure overload-induced fibrotic response as well as fibrosis mediated by a heart-specific, latency-resistant TGF-[beta] mutant transgene. Interestingly, cardiac fibroblast-specific deletion of Tgfbr1/2, but not Smad2/3, attenuated the cardiac hypertrophic response to pressure overload stimulation. Mechanistically, loss of Smad2/3 from tissue-resident fibroblasts attenuated injury-induced cellular expansion within the heart and the expression of fibrosis-mediating genes. Deletion of Smad2/3 or Tgfbr1/2 from cardiac fibroblasts similarly inhibited the gene program for fibrosis and extracellular matrix remodeling, although deletion of Tgfbr1/2 uniquely altered expression of an array of regulatory genes involved in cardiomyocyte homeostasis and disease compensation. These findings implicate TGF-[beta]Smad2/3 signaling in activated tissue-resident cardiac fibroblasts as principal mediators of the fibrotic response., Introduction Heart failure is an increasingly prevalent disease characterized by reduced cardiac output resulting from a progressive loss of cardiomyocytes, ventricular chamber remodeling, and accumulation of interstitial fibrosis (1). Excessive [...]

If and how the heart regenerates after an injury event is highly debated. c-kit-expressing cardiac progenitor cells have been reported as the primary source for generation of new myocardium after injury. Here we generated two genetic approaches in mice to examine whether endogenous [c-kit.sup.+] cells contribute differentiated cardiomyocytes to the heart during development, with ageing or after injury in adulthood. A complementary DNA encoding either Cre recombinase or a tamoxifen-inducible MerCreMer chimaeric protein was targeted to the Kit locus in mice and then bred with reporter lines to permanently mark cell lineage. Endogenous [c-kit.sup.+] cells did produce new cardiomyocytes within the heart, although at a percentage of approximately 0.03 or less, and if a preponderance towards cellular fusion is considered, the percentage falls to below approximately 0.008. By contrast, [c-kit.sup.+] cells amply generated cardiac endothelial cells. Thus, endogenous [c-kit.sup.+] cells can generate cardiomyocytes within the heart, although probably at a functionally insignificant level., The adult mammalian heart was originally proposed to be essentially incapable of renewal after injury or with ageing; although some recent studies have shown that the heart is capable of [...]

Cyclophilin D (which is encoded by the Ppif gene) is a mitochondrial matrix peptidyl-prolyl isomerase known to modulate opening of the mitochondrial permeability transition pore (MPTP). Apart from regulating necrotic cell death, the physiologic function of the MPTP is largely unknown. Here we have shown that [Ppif.sup.[-/-]] mice exhibit substantially greater cardiac hypertrophy, fibrosis, and reduction in myocardial function in response to pressure overload stimulation than control mice. In addition, [Ppif.sup.[-/-]] mice showed greater hypertrophy and lung edema as well as reduced survival in response to sustained exercise stimulation. Cardiomyocyte-specific transgene expression of cyclophilin D in [Ppif.sup.[-/-]] mice rescued the enhanced hypertrophy, reduction in cardiac function, and rapid onset of heart failure following pressure overload stimulation. Mechanistically, the maladaptive phenotype in the hearts of [Ppif.sup.[-/-]] mice was associated with an alteration in MPTP-mediated [Ca.sup.2+] efflux resulting in elevated levels of mitochondrial matrix [Ca.sup.2+] and enhanced activation of [Ca.sup.2+]-dependent dehydrogenases. Elevated matrix [Ca.sup.2+] led to increased glucose oxidation relative to fatty acids, thereby limiting the metabolic flexibility of the heart that is critically involved in compensation during stress. These findings suggest that the MPTP maintains homeostatic mitochondrial [Ca.sup.2+] levels to match metabolism with alterations in myocardial workload, thereby suggesting a physiologic function for the MPTP., Introduction Opening of the mitochondrial permeability transition pore (MPTP) is critically involved in regulating cell death by inducing a sustained and irreversible loss of inner mitochondrial membrane potential, coinciding with [...]

Author(s): Jop H. van Berlo [1]; Onur Kanisicak [2]; Marjorie Maillet [2]; Ronald J. Vagnozzi [2]; Jason Karch [2]; Suh-Chin J. Lin [2]; Ryan C. Middleton [3]; Eduardo Marbn [3]; [...]