Healing of myocardial infarction (MI) requires monocytes/macrophages These mononuclear phagocytes likely degrade released macromolecules and aid in scavenging of dead cardiomyocytes, while mediating aspects of granulation tissue formation and remodeling The mechanisms that orchestrate such divergent functions remain unknown In view of the heightened appreciation of the heterogeneity of circulating monocytes, we investigated whether distinct monocyte subsets contribute in specific ways to myocardial ischemic injury in mouse MI We identify two distinct phases of monocyte participation after MI and propose a model that reconciles the divergent properties of these cells in healing Infarcted hearts modulate their chemokine expression profile over time, and they sequentially and actively recruit Ly-6Chi and -6Clo monocytes via CCR2 and CX3CR1, respectively Ly-6Chi monocytes dominate early (phase I) and exhibit phagocytic, proteolytic, and inflammatory functions Ly-6Clo monocytes dominate later (phase II), have attenuated inflammatory properties, and express vascular–endothelial growth factor Consequently, Ly-6Chi monocytes digest damaged tissue, whereas Ly-6Clo monocytes promote healing via myofibroblast accumulation, angiogenesis, and deposition of collagen MI in atherosclerotic mice with chronic Ly-6Chi monocytosis results in impaired healing, underscoring the need for a balanced and coordinated response These observations provide novel mechanistic insights into the cellular and molecular events that regulate the response to ischemic injury and identify new therapeutic targets that can influence healing and ventricular remodeling after MI

https://rupress.org/jem/article-pdf/204/12/3037/1160364/jem_20070885.pdf

The healing myocardium sequentially mobilizes two monocyte subsets with divergent and complementary functions

Background —Although patients with heart failure express elevated circulating levels of tumor necrosis factor-α (TNF-α) in their peripheral circulation, the structural and functional effects of circulating levels of pathophysiologically relevant concentrations of TNF-α on the heart are not known. Methods and Results —Osmotic infusion pumps containing either diluent or TNF-α were implanted into the peritoneal cavity of rats. The rate of TNF-α infusion was titrated to obtain systemic levels of biologically active TNF-α comparable to those reported in patients with heart failure (≈80 to 100 U/mL), and the animals were examined serially for 15 days. Two-dimensional echocardiography was used to assess changes in left ventricular (LV) structure (remodeling) and LV function. Video edge detection was used to assess isolated cell mechanics, and standard histological techniques were used to assess changes in the volume composition of LV cardiac myocytes and the extracellular matrix. The reversibility of cytokine-induced effects was determined either by removal of the osmotic infusion pumps on day 15 or by treatment of the animals with a soluble TNF-α antagonist (TNFR:Fc). The results of this study show that a continuous infusion of TNF-α led to a time-dependent depression in LV function, cardiac myocyte shortening, and LV dilation that were at least partially reversible by removal of the osmotic infusion pumps or treatment of the animals with TNFR:Fc. Conclusions —These studies suggest that pathophysiologically relevant concentrations of TNF-α are sufficient to mimic certain aspects of the phenotype observed in experimental and clinical models of heart failure.

Pathophysiologically relevant concentrations of tumor necrosis factor- α promote progressive left ventricular dysfunction and remodeling in rats

Rationale: The developing heart requires both mechanical load and vascularization to reach its proper size, yet the regulation of human heart growth by these processes is poorly understood.

Objective: We seek to elucidate the responses of immature human myocardium to mechanical load and vascularization using tissue engineering approaches.

Methods and Results: Using human embryonic stem cell and human induced pluripotent stem cell–derived cardiomyocytes in a 3-dimensional collagen matrix, we show that uniaxial mechanical stress conditioning promotes 2-fold increases in cardiomyocyte and matrix fiber alignment and enhances myofibrillogenesis and sarcomeric banding. Furthermore, cyclic stress conditioning markedly increases cardiomyocyte hypertrophy (2.2-fold) and proliferation rates (21%) versus unconditioned constructs. Addition of endothelial cells enhances cardiomyocyte proliferation under all stress conditions (14% to 19%), and addition of stromal supporting cells enhances formation of vessel-like structures by ≈10-fold. Furthermore, these optimized human cardiac tissue constructs generate Starling curves, increasing their active force in response to increased resting length. When transplanted onto hearts of athymic rats, the human myocardium survives and forms grafts closely apposed to host myocardium. The grafts contain human microvessels that are perfused by the host coronary circulation.

Conclusions: Our results indicate that both mechanical load and vascular cell coculture control cardiomyocyte proliferation, and that mechanical load further controls the hypertrophy and architecture of engineered human myocardium. Such constructs may be useful for studying human cardiac development as well as for regenerative therapy.

# Novelty and Significance {#article-title-70}

Growth of Engineered Human Myocardium With Mechanical Loading and Vascular Coculture

Periostin is predominantly expressed in collagen-rich fibrous connective tissues that are subjected to constant mechanical stresses including: heart valves, tendons, perichondrium, cornea, and the periodontal ligament (PDL) Based on these data we hypothesize that periostin can regulate collagen I fibrillogenesis and thereby affect the biomechanical properties of connective tissues Immunoprecipitation and immunogold transmission electron microscopy experiments demonstrate that periostin is capable of directly interacting with collagen I To analyze the potential role of periostin in collagen I fibrillogenesis, gene targeted mice were generated Transmission electron microscopy and morphometric analyses demonstrated reduced collagen fibril diameters in skin dermis of periostin knockout mice, an indication of aberrant collagen I fibrillogenesis In addition, differential scanning calorimetry (DSC) demonstrated a lower collagen denaturing temperature in periostin knockout mice, reflecting a reduced level of collagen cross-linking Functional biomechanical properties of periostin null skin specimens and atrioventricular (AV) valve explant experiments provided direct evidence of the role that periostin plays in regulating the viscoelastic properties of connective tissues Collectively, these data demonstrate for the first time that periostin can regulate collagen I fibrillogenesis and thereby serves as an important mediator of the biomechanical properties of fibrous connective tissues

/pdf/periostin-regulates-collagen-fibrillogenesis-and-the-1x6asxlmxh.pdf

Periostin regulates collagen fibrillogenesis and the biomechanical properties of connective tissues

https://www.incor.usp.br/news/artigos/prognostic-significance-myocardial-fibrosis.pdf

Prognostic Significance of Myocardial Fibrosis Quantification by Histopathology and Magnetic Resonance Imaging in Patients With Severe Aortic Valve Disease

Collagen plays a major role in the structural organization of the heart and therefore direct visualization of collagen fibers is a crucial component of cardiac analysis. Although linearly polarized light has proven an effective tool for the examination of myocardial collagen in histologic sections, the use of circularly polarized light may offer advantages and additional possibilities. We examined the potential enhancement of collagen analysis using circularly polarized light in two ways. We first measured the brightness, and hence indirectly assessed the birefringence, of collagen fibers in scars examined at different times after myocardial infarction. Secondly, we measured collagen content in myocardial tissue and compared results obtained from brightfield analysis of trichrome stained sections with those obtained from circularly polarized light analysis of picrosirius red stained sections. We observed a progressive increase in the maximum brightness of collagen fibers in the scar with time, and a time-dependent shift in the relative distribution of collagen fiber brightness from lower to higher levels. We found consistently lower values of collagen content in trichrome stained versus picrosirius red stained tissue, and concluded that trichrome staining underestimated collagen content. The information provided by these studies could not be obtained by brightfield analysis and could be only partially obtained from linearly polarized light analysis. Thus, analysis using circularly polarized light has the ability to enhance histologic assessment of tissue and can provide additional insights into the composition and structure of myocardial collagen.

J. G. Pickering

Papers

Quantitative assessment of myocardial collagen with picrosirius red staining and circularly polarized light.