Fibroblasts are a dynamic cell type that achieve selective differentiated states to mediate acute wound healing and long-term tissue remodeling with scarring. With myocardial infarction injury, cardiomyocytes are replaced by secreted extracellular matrix proteins produced by proliferating and differentiating fibroblasts. Here, we employed 3 different mouse lineage-tracing models and stage-specific gene profiling to phenotypically analyze and classify resident cardiac fibroblast dynamics during myocardial infarction injury and stable scar formation. Fibroblasts were activated and highly proliferative, reaching a maximum rate within 2 to 4 days after infarction injury, at which point they expanded 3.5-fold and were maintained long term. By 3 to 7 days, these cells differentiated into myofibroblasts that secreted abundant extracellular matrix proteins and expressed smooth muscle α-actin to structurally support the necrotic area. By 7 to 10 days, myofibroblasts lost proliferative ability and smooth muscle α-actin expression as the collagen-containing extracellular matrix and scar fully matured. However, these same lineage-traced initial fibroblasts persisted within the scar, achieving a new molecular and stable differentiated state referred to as a matrifibrocyte, which was also observed in the scars of human hearts. These cells express common and unique extracellular matrix and tendon genes that are more specialized to support the mature scar.

Abstract-The cardiac extracellular matrix is a dynamic structural support network that is both influenced by, and a regulator of, pathological remodeling and hypertrophic growth. In response to pathologic insults, the adult heart reexpresses the secreted extracellular matrix protein periostin (Pn). Here we show that Pn is critically involved in regulating the cardiac hypertrophic response, interstitial fibrosis, and ventricular remodeling following long-term pressure overload stimulation and myocardial infarction. Mice lacking the gene encoding Pn (Postn) were more prone to ventricular rupture in the first 10 days after a myocardial infarction, but surviving mice showed less fibrosis and better ventricular performance. Pn Ϫ/Ϫ mice also showed less fibrosis and hypertrophy following long-term pressure overload, suggesting an intimate relationship between Pn and the regulation of cardiac remodeling. In contrast, inducible overexpression of Pn in the heart protected mice from rupture following myocardial infarction and induced spontaneous hypertrophy with aging. With respect to a mechanism underlying these alterations, Pn Ϫ/Ϫ hearts showed an altered molecular program in fibroblast function. Indeed, fibroblasts isolated from Pn Ϫ/Ϫ hearts were less effective in adherence to cardiac myocytes and were characterized by a dramatic alteration in global gene expression (7% of all genes). These are the first genetic data detailing the function of Pn in the adult heart as a regulator of cardiac remodeling and hypertrophy. (Circ Res. 2007;101:313-321.)

SUMMARY In the heart, augmented Ca2+ fluxing drives contractility and ATP generation through mitochondrial Ca2+ loading. Pathologic mitochondrial Ca2+ overload with ischemic injury triggers mitochondrial permeability transition pore (MPTP) opening and cardiomyocyte death. Mitochondrial Ca2+ uptake is primarily mediated by the mitochondrial Ca2+ uniporter (MCU). Here we generated mice with adult and cardiomyocyte-specific deletion of Mcu, which produced mitochondria refractory to acute Ca2+ uptake, augmented ATP production and MPTP opening upon acute Ca2+ challenge. Mice lacking Mcu in the adult heart were also protected from acute ischemia-reperfusion injury. However, resting/basal mitochondrial Ca2+ levels were normal in hearts of Mcu-deleted mice and mitochondria lacking MCU eventually loaded with Ca2+ after stress stimulation. Indeed, Mcu-deleted mice were unable to immediately sprint on a treadmill unless warmed-up for 30 minutes. Hence, MCU is a dedicated regulator of short-term mitochondrial Ca2+ loading underlying a “fight-or-flight” response that acutely matches cardiac workload with ATP production.

Clinical trials using adult stem cells to regenerate damaged heart tissue continue to this day 1,2 despite ongoing questions of efficacy and a lack of mechanistic understanding of the underlying biologic effect 3. The rationale for these cell therapy trials is derived from animal studies that show a modest but reproducible improvement in cardiac function in models of cardiac ischemic injury 4,5. Here we examined the mechanistic basis for cell therapy in mice after ischemia/ reperfusion (I/R) injury, and while heart function was enhanced, it was not associated with new cardiomyocyte production. Cell therapy improved heart function through an acute sterile immune response characterized by the temporal and regional induction of CCR2 + and CX3CR1 + macrophages. Intra-cardiac injection of 2 distinct types of adult stem cells, freeze/thaw-killed cells or a chemical inducer of the innate immune response similarly induced regional CCR2 + and CX3CR1 + macrophage accumulation and provided functional rejuvenation to the I/R-injured heart. This selective macrophage response altered cardiac fibroblast activity, reduced border zone extracellular matrix (ECM) content, and enhanced the mechanical properties of the injured area. Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:

Muscular dystrophies comprise a diverse group of genetic disorders that lead to muscle wasting and, in many instances, premature death 1 . Many mutations that cause muscular dystrophy compromise the support network that connects myofilament proteins within the cell to the basal lamina outside the cell, rendering the sarcolemma more permeable or leaky. Here we show that deletion of the gene encoding cyclophilin D (Ppif) rendered mitochondria largely insensitive to the calcium overloadinduced swelling associated with a defective sarcolemma, thus reducing myofiber necrosis in two distinct models of muscular dystrophy. Mice lacking δ-sarcoglycan (Scgd −/− mice) showed markedly less dystrophic disease in both skeletal muscle and heart in the absence of Ppif. Moreover, the premature lethality associated with deletion of Lama2, encoding the α-2 chain of laminin-2, was rescued, as were other indices of dystrophic disease. Treatment with the cyclophilin inhibitor Debio-025 similarly reduced mitochondrial swelling and necrotic disease manifestations in mdx mice, a model of Duchenne muscular dystrophy, and in Scgd −/− mice. Thus, mitochondrial-dependent necrosis represents a prominent disease mechanism in muscular dystrophy, suggesting that inhibition of cyclophilin D could provide a new pharmacologic treatment strategy for these diseases.The muscular dystrophies are inherited disorders that mostly affect striated muscle tissue, resulting in progressive muscle weakness, wasting and, in many instances, premature death 1 . Many characterized mutations in humans that result in muscular dystrophy cause alterations either in structural proteins that attach the underlying contractile proteins to the basal lamina, which provides rigidity to the skeletal muscle cell membrane (sarcolemma), or in proteins that HHMI Author ManuscriptHHMI Author Manuscript HHMI Author Manuscript directly stabilize or repair the cell membrane 1-3 . For example, loss of dystrophin or other components of the dystrophin-glycoprotein complex leads to a fundamental alteration in the physical properties of the sarcolemma, permitting contraction-induced microtears and the unregulated exchange of ions, increasing the influx of calcium 2-4 . Moreover, total intracellular calcium or subsarcolemmal calcium levels have been found to be elevated in skeletal muscle cells or fibers from dystrophic mice 5,6 .In response to sustained increases in intracellular calcium concentrations, such as after ischemic injury, mitochondria undergo a so-called 'permeability transition' 7 . This process results in the regulated formation of a large pore complex that spans the outer and inner mitochondrial membranes, leading to an irreversible loss of matrix and intermembrane contents and swelling of the mitochondria. If this transition state is not reversed in a timely manner, the mitochondria can rupture, causing necrotic and/or apoptotic cell death. Cyclophilin D is a mitochondrial matrix prolyl cis-trans isomerase that directly regulates calcium-and reactive oxygen specie...

Cardiac myosin binding protein C (cMyBP-C) has three phosphorylatable serines at its N terminus (Ser-273, Ser-282, and Ser-302), and the residues' phosphorylation states may alter thick filament structure and function. To examine the effects of cMyBP-C phosphorylation, we generated transgenic mice with cardiac-specific expression of a cMyBP-C in which the three phosphorylation sites were mutated to aspartic acid, mimicking constitutive phosphorylation (cMyBP-C AllP؉ ). The allele was bred into a cMyBP-C null background (cMyBP-C (t/t) ) to ensure the absence of endogenous dephosphorylated cMyBP-C. cMyBP-C AllP؉ was incorporated normally into the cardiac sarcomere and restored normal cardiac function in the null background. However, subtle changes in sarcomere ultrastructure, characterized by increased distances between the thick filaments, indicated that phosphomimetic cMyBP-C affects thick-thin filament relationships, and yeast two-hybrid data and pull-down studies both showed that charged residues in these positions effectively prevented interaction with the myosin heavy chain. Confirming the physiological relevance of these data, the cMyBP-C AllP؉:(t/t) hearts were resistant to ischemia-reperfusion injury. These data demonstrate that cMyBP-C phosphorylation functions in maintaining thick filament spacing and structure and can help protect the myocardium from ischemic injury.heart ͉ ischemia C ardiac myosin binding protein C (cMyBP-C) is localized to the sarcomere's thick filaments where it has structural and regulatory functions. MYBPC3 mutations account for 20-30% of all mutations linked to familial hypertrophic cardiomyopathy (1). cMyBP-C belongs to the intracellular Ig superfamily and is composed of Ig and fibronectin type-3 repeating domains (Fig. 1A). It is present not only in cardiac muscle, but also in skeletal muscle before the skeletal muscle-type isoforms are expressed, suggesting that the cardiac isoform is functional in early myofibrillogenesis and regenerating muscle (2, 3). cMyBP-C may modulate myosin assembly (4) and stabilize thick filaments (5). It binds titin via domains C8-C10 (6) and actin in the Pro-Alarich sequences between the C0 and C1 domains (7), which appear to be important for the precise arrangement of the actin-myosin filaments. Compared with the two skeletal muscle isoforms, the cardiac isoform contains an extra Ig domain at the N terminus (C0), an insertion of 28 residues within the C5 domain, and three potential phosphorylation sites that are substrates for cAMP-dependent PKA, Ca 2ϩ -calmodulin-activated kinase, and PKC (8). This region is located between the C1 and C2 domains of the N terminus, which binds to the subfragment 2 (S2) segment of myosin close to the lever arm domain (9-11), and this interaction may be dynamically regulated by the differential phosphorylation of cMyBP-C (12). cMyBP-C is the only thick filament protein that is differentially phosphorylated at multiple sites by the enzymes PKA, PKC, and Ca 2ϩ -calmodulin-activated kinase (13). Reconstitution studie...

SUMMARY Thrombospondin (Thbs) proteins are induced in sites of tissue damage or active remodeling. The endoplasmic reticulum (ER) stress response is also prominently induced with disease where it regulates protein production and resolution of misfolded proteins. Here we describe a novel function for Thbs’ as ER resident effectors of an adaptive ER stress response. Thbs4 cardiac-specific transgenic mice were protected from myocardial injury while Thbs4−/− mice were sensitized to cardiac maladaptation. Thbs induction produced a unique profile of adaptive ER stress response factors and expansion of the ER and downstream vesicles. The type-3 repeat domain in Thbs’ bind the ER luminal domain of activating transcription factor 6α (Atf6α) to promote its nuclear shuttling. Thbs4−/−mice failed to show activation of Atf6α and other ER stress response factors with injury, and Thbs4-mediated protection was lost when Atf6α was deleted. Hence, Thbs’ can function inside the cell during disease/remodeling to augment ER function and protect through a mechanism involving regulation of Atf6α.

A critical event in ischemia-based cell death is the opening of the mitochondrial permeability transition pore (MPTP). However, the molecular identity of the components of the MPTP remains unknown. Here, we determined that the Bcl-2 family members Bax and Bak, which are central regulators of apoptotic cell death, are also required for mitochondrial pore-dependent necrotic cell death by facilitating outer membrane permeability of the MPTP. Loss of Bax/Bak reduced outer mitochondrial membrane permeability and conductance without altering inner membrane MPTP function, resulting in resistance to mitochondrial calcium overload and necrotic cell death. Reconstitution with mutants of Bax that cannot oligomerize and form apoptotic pores, but still enhance outer membrane permeability, permitted MPTP-dependent mitochondrial swelling and restored necrotic cell death. Our data predict that the MPTP is an inner membrane regulated process, although in the absence of Bax/Bak the outer membrane resists swelling and prevents organelle rupture to prevent cell death.DOI: http://dx.doi.org/10.7554/eLife.00772.001