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.

Abstract 79: Sex-dependent Role of Macrophage-derived Insulin-like Growth Factor-1 in Body Growth, Cardiac Homeostasis and Responses to Cardiac Tissue Stress

Biological sex is a key factor influencing the development and progression of cardiovascular disease. While sexual dimorphism has been identified in cardiac physiology and sex-based differences are observed in clinical contexts, the molecular and cellular bases for these differences are not understood.

Recently, we found for the first time that female and male hearts are different in their cellular composition. Female hearts comprise significantly greater numbers of resident mesenchymal cells, including cardiac fibroblasts, compared to male hearts, and female hearts consist of differing proportions of multiple leukocyte subsets. We found that these differences are regulated by gonadal hormones.

Extending on our previous work, this study investigates sexual dimorphism in cell-specific IGF-1 production and its impact on cardiac inflammatory responses. IGF-1 is a growth factor that is essential for development and injury responses, and that has been previously shown to function in a sex-dependent manner.

Using genetic, transcriptomic and high-dimensional flow cytometry approaches, we found that macrophages and fibroblasts are the principle IGF-1 producing cells in the heart, with macrophages from female mouse hearts producing twice as much IGF-1 as in males. This contributed to a ~20% greater level of IGF-1 found in the hearts of females compared to males. Furthermore, constitutive genetic ablation of macrophage-derived IGF-1 resulted in ~30% reduction in total IGF-1 in hearts of female mice, whereas no reduction was observed in male hearts. Surprisingly, we also found that ablation of macrophage-derived IGF-1 reduces body weight in female mice by ~10%. Finally, we found that macrophage-derived IGF-1 regulates cardiac inflammatory responses in a sex-dependent manner.

To our knowledge, this is the first study to demonstrate that a macrophage-derived growth factor is essential for normal body growth and plays a sex-dependent role in cardiac inflammatory responses. Our observations provide a cellular and molecular basis for the sexual dimorphism observed in cardiac physiology and pathology, and underscore the importance of considering biological sex in experimental models of cardiac development and disease.

Revisiting Cardiac Cellular Composition

RATIONALE

Accurate knowledge of the cellular composition of the heart is essential to fully understand the changes that occur during pathogenesis and to devise strategies for tissue engineering and regeneration.

OBJECTIVE

To examine the relative frequency of cardiac endothelial cells, hematopoietic-derived cells, and fibroblasts in the mouse and human heart.

METHODS AND RESULTS

Using a combination of genetic tools and cellular markers, we examined the occurrence of the most prominent cell types in the adult mouse heart. Immunohistochemistry revealed that endothelial cells constitute >60%, hematopoietic-derived cells 5% to 10%, and fibroblasts <20% of the nonmyocytes in the heart. A refined cell isolation protocol and an improved flow cytometry approach provided an independent means of determining the relative abundance of nonmyocytes. High-dimensional analysis and unsupervised clustering of cell populations confirmed that endothelial cells are the most abundant cell population. Interestingly, fibroblast numbers are smaller than previously estimated, and 2 commonly assigned fibroblast markers, Sca-1 and CD90, under-represent fibroblast numbers. We also describe an alternative fibroblast surface marker that more accurately identifies the resident cardiac fibroblast population.

CONCLUSIONS

This new perspective on the abundance of different cell types in the heart demonstrates that fibroblasts comprise a relatively minor population. By contrast, endothelial cells constitute the majority of noncardiomyocytes and are likely to play a greater role in physiological function and response to injury than previously appreciated.

Age-related changes in tissue macrophages precede cardiac functional impairment

Cardiac tissue macrophages (cTMs) are abundant in the murine heart but the extent to which the cTM phenotype changes with age is unknown. This study characterizes aging-dependent phenotypic changes in cTM subsets. Using the Cx₃cr1^GFP/+ mouse reporter line where GFP marks cTMs, and the tissue macrophage marker Mrc1, we show that two major cardiac tissue macrophage subsets, Mrc1⁻GFP^hi and Mrc1⁺GFP^hi cTMs, are present in the young (<10 week old) mouse heart, and a third subset, Mrc1⁺GFP^lo, comprises ~50% of total Mrc1⁺ cTMs from 30 weeks of age. Immunostaining and functional assays show that Mrc1⁺ cTMs are the principal myeloid sentinels in the mouse heart and that they retain proliferative capacity throughout life. Gene expression profiles of the two Mrc1⁺ subsets also reveal that Mrc1⁺GFP^lo cTMs have a decreased number of immune response genes (Cx₃cr1, Lpar6, CD9, Cxcr4, Itga6 and Tgfβr1), and an increased number of fibrogenic genes (Ltc4s, Retnla, Fgfr1, Mmp9 and Ccl24), consistent with a potential role for cTMs in cardiac fibrosis. These findings identify early age-dependent gene expression changes in cTMs, with significant implications for cardiac tissue injury responses and aging-associated cardiac fibrosis.