High-Resolution Transcriptomic Profiling of the Heart During Chronic Stress Reveals Cellular Drivers of Cardiac Fibrosis and Hypertrophy.

BACKGROUND: Cardiac fibrosis is a key antecedent to many types of cardiac dysfunction including heart failure. Physiological factors leading to cardiac fibrosis have been recognized for decades. However, the specific cellular and molecular mediators that drive cardiac fibrosis, and the relative effect of disparate cell populations on cardiac fibrosis, remain unclear. METHODS: We developed a novel cardiac single-cell transcriptomic strategy to characterize the cardiac cellulome, the network of cells that forms the heart. This method was used to profile the cardiac cellular ecosystem in response to 2 weeks of continuous administration of angiotensin II, a profibrotic stimulus that drives pathological cardiac remodeling. RESULTS: Our analysis provides a comprehensive map of the cardiac cellular landscape uncovering multiple cell populations that contribute to pathological remodeling of the extracellular matrix of the heart. Two phenotypically distinct fibroblast populations, Fibroblast-Cilp and Fibroblast-Thbs4, emerged after induction of tissue stress to promote fibrosis in the absence of smooth muscle actin-expressing myofibroblasts, a key profibrotic cell population. After angiotensin II treatment, Fibroblast-Cilp develops as the most abundant fibroblast subpopulation and the predominant fibrogenic cell type. Mapping intercellular communication networks within the heart, we identified key intercellular trophic relationships and shifts in cellular communication after angiotensin II treatment that promote the development of a profibrotic cellular microenvironment. Furthermore, the cellular responses to angiotensin II and the relative abundance of fibrogenic cells were sexually dimorphic. CONCLUSIONS: These results offer a valuable resource for exploring the cardiac cellular landscape in health and after chronic cardiovascular stress. These data provide insights into the cellular and molecular mechanisms that promote pathological remodeling of the mammalian heart, highlighting early transcriptional changes that precede chronic cardiac fibrosis.

Endocytosis Assays Using Cleavable Fluorescent Dyes.

Endocytosis mediates the entry of surface and extracellular cargoes into the cell. In this chapter, we describe assays to quantitively measure the endocytosis of both soluble and transmembrane cargo proteins, taking advantage of cleavable fluorescent dyes labeling cargo proteins or antibodies recognizing cargo proteins. After removing surface-bound fluorescent dye, internalized cargoes are measured using confocal imaging and flow cytometry. We also describe strategies to determine the role of clathrin-mediated endocytosis (CME) in the internalization of a cargo by using a small molecule inhibitor of CME and knockout (KO) of the AAGAB gene, which encodes an essential regulator of CME.

Cardiac cellularity is dependent upon biological sex and is regulated by gonadal hormones.

AIMS: Sex differences have been consistently identified in cardiac physiology and incidence of cardiac disease. However, the underlying biological causes for the differences remain unclear. We sought to characterize the cardiac non-myocyte cellular landscape in female and male hearts to determine whether cellular proportion of the heart is sex-dependent and whether endocrine factors modulate the cardiac cell proportions. METHODS AND RESULTS: Utilizing high-dimensional flow cytometry and immunofluorescence imaging, we found significant sex-specific differences in cellular composition of the heart in adult and juvenile mice, that develops postnatally. Removal of systemic gonadal hormones by gonadectomy results in rapid sex-specific changes in cardiac non-myocyte cellular proportions including alteration in resident mesenchymal cell and leucocyte populations, indicating gonadal hormones and their downstream targets regulate cardiac cellular composition. The ectopic reintroduction of oestrogen and testosterone to female and male mice, respectively, reverses many of these gonadectomy-induced compositional changes. CONCLUSION: This work shows that the constituent cell types of the mouse heart are hormone-dependent and that the cardiac cellular landscapes are distinct in females and males, remain plastic, and can be rapidly modulated by endocrine factors. These observations have implications for strategies aiming to therapeutically alter cardiac cellular heterogeneity and underscore the importance of considering biological sex for studies examining cardiac physiology and stress responses.

Single-Cell Transcriptional Profiling Reveals Cellular Diversity and Intercommunication in the Mouse Heart.

Characterization of the cardiac cellulome, the network of cells that form the heart, is essential for understanding cardiac development and normal organ function and for formulating precise therapeutic strategies to combat heart disease. Recent studies have reshaped our understanding of cardiac cellular composition and highlighted important functional roles for non-myocyte cell types. In this study, we characterized single-cell transcriptional profiles of the murine non-myocyte cardiac cellular landscape using single-cell RNA sequencing (scRNA-seq). Detailed molecular analyses revealed the diversity of the cardiac cellulome and facilitated the development of techniques to isolate understudied cardiac cell populations, such as mural cells and glia. Our analyses also revealed extensive networks of intercellular communication and suggested prevalent sexual dimorphism in gene expression in the heart. This study offers insights into the structure and function of the mammalian cardiac cellulome and provides an important resource that will stimulate studies in cardiac cell biology.