Background In the heart acute injury induces a fibrotic healing response that generates collagen rich scarring that is at first protective but if inappropriately sustained can worsen heart disease. The fibrotic process is initiated by cytokines, neuroendocrine effectors and mechanical strain that promote resident fibroblast differentiation into contractile and extracellular matrix producing myofibroblasts. The mitogen-activated protein kinase (MAPK) p38α (Mapk14 gene) is known to influence the cardiac injury response, but its direct role in orchestrating programmed fibroblast differentiation and fibrosis in vivo is unknown. Methods A conditional Mapk14 allele was used to delete the p38α encoding gene specifically in cardiac fibroblasts or myofibroblasts using 2 different tamoxifen-inducible Cre recombinase expressing gene-targeted mouse lines. Mice were subjected to ischemic injury or chronic neurohumoral stimulation and monitored for survival, cardiac function and fibrotic remodeling. Antithetically, mice with fibroblast-specific transgenic overexpression of activated MAPK kinase 6 (MKK6), a direct inducer of p38, were generated to investigate if this pathway can directly drive myofibroblast formation and the cardiac fibrotic response. Results In mice loss of Mapk14 blocked cardiac fibroblast differentiation into myofibroblasts and ensuing fibrosis in response to ischemic injury or chronic neurohumoral stimulation. A similar inhibition of myofibroblast formation and healing was also observed in a dermal wounding model with deletion of Mapk14. Transgenic mice with fibroblast-specific activation of MKK6-p38 developed interstitial and perivascular fibrosis in the heart, lung and kidney due to enhanced myofibroblast numbers. Mechanistic experiments show that p38 transduces cytokine and mechanical signals into myofibroblast differentiation through the transcription factor serum-response factor (SRF) and the signaling effector calcineurin. Conclusions These findings suggest that signals from diverse modes of injury converge on p38α MAPK within the fibroblast to program the fibrotic response and myofibroblast formation in vivo, suggesting a novel therapeutic approach with p38 inhibitors for future clinical application.

Innate immune cells play important roles in tissue injury and repair following acute myocardial infarction (MI). Although reprogramming of macrophage metabolism has been observed during inflammation and resolution phases, the mechanistic link to macrophage phenotype is not fully understood. In this study, we found myeloid specific deletion of mitochondrial Complex I protein Ndufs4 (mKO) reproduced the pro-inflammatory metabolic profile in macrophages and exaggerated the response to lipopolysacharride. Moreover, mKO mice showed increased mortality, poor scar formation and worsened cardiac function 30 days post-MI. We observed a greater inflammatory response in mKO on day 1 followed by increased cell death of infiltrating macrophages and blunted transition to reparative phase during day 3-7 post-MI. Efferocytosis is impaired in mKO macrophages leading to lower expression of anti-inflammatory cytokine and tissue repair factors, which suppressed the proliferation/activation of myofibroblasts in the infarct area. Mitochondria-targeted ROS scavenging rescued these impairments and improved myofibroblast function in vivo and reduced post-MI mortality in mKO mice. Together these results reveal a critical role of mitochondria in inflammation resolution and tissue repair via modulating efferocytosis and crosstalk with fibroblasts. The findings are significant for post-MI recovery as well as for other inflammatory conditions.

Rationale: Myocardial infarction (MI) causes spatial variation in collagen organization and phenotypic diversity in fibroblasts, which regulate the heart's extracellular matrix (ECM). The relationship between collagen structure and fibroblast phenotype is poorly understood but could provide insights regarding the mechanistic basis for myofibroblast heterogeneity in the injured heart. Objective: To investigate the role of collagen organization in cardiac fibroblast fate determination. Methods and Results: Biomimetic topographies were nanofabricated to recapitulate differential collagen organization in the infarcted mouse heart. Here adult cardiac fibroblasts were freshly isolated and cultured on ECM topographical mimetics for 72 hours. Aligned mimetics caused cardiac fibroblasts to elongate while randomly organized topographies induced circular morphology similar to the disparate myofibroblast morphologies measured in vivo. Alignment cues also induced myofibroblast differentiation, as more than 60% of fibroblasts formed -smooth muscle actin (alphaSMA) stress fibers and expressed myofibroblast-specific ECM genes like periostin. By contrast, random organization caused 38% of cardiac fibroblasts to express alphaSMA albeit with down-regulated myofibroblast-specific ECM genes. Coupling topographical cues with the profibrotic agonist, TGFb, additively upregulated myofibroblast-specific ECM genes independent of topography, but only fibroblasts on flat and randomly oriented mimetics had increased percentages of fibroblasts with alphaSMA stress fibers. Increased tension sensation at focal adhesions induced myofibroblast differentiation on aligned mimetics. These signals were transduced by p38-YAP-TEAD interactions, in which both p38 and YAP-TEAD binding were required for myofibroblast differentiation. By contrast randomly oriented mimetics did not change focal adhesion tension sensation or enrich for p38-YAP-TEAD interactions, which explains the topography-dependent diversity in fibroblast phenotypes observed here. Conclusions: Spatial variations in collagen organization regulate cardiac fibroblast phenotype through mechanical activation of p38-YAP-TEAD signaling, which likely contribute to myofibroblast heterogeneity in the infarcted myocardium.

No abstract

No abstract