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Background

Studies dating back to 1980 anticipated that the adult mammalian heart contains several cell populations other than cardiomyocytes, including macrophages [1]. However, thanks to the development of state-of-the-art tools (e.g. genetic fate mapping, advanced flow cytometry, single-cell RNA-sequencing), we are now becoming conscious of the abundance, functional heterogeneity, and diverse ontogeny of cardiac macrophages.

A line of research of the Lavine lab is devoted to describe the diversity, ontogeny and function of cardiac resident macrophages (CRMs). This lab has contributed with seminal papers to uncover the functions of CRMs, such as facilitation of electrical conduction, coronary development and maturation, and cell junction formation. In this occasion, Wong et al. shed light on the pathophysiological role of CRMs in a mouse model of human dilated cardiomyopathy (Tnnt2ΔK210/ ΔK210 mouse) and delineated the molecular mechanism by which macrophages detect mechanical strain and promote beneficial cardiac remodeling and angiogenesis.

Key findings

The main findings of the current study can be summarized as follows:

1.    The authors developed two elegant strategies to assess monocyte contribution to each cardiac macrophage population in the context of heart failure: (1) using CCR2gfp knock-in mice that allow visualization of CCR2+ cells and simultaneously generates CCR2 KO mice, and (2) by means of genetic lineage tracing (Flt3Cre-Rosa26-tdTomato mice). Thus, they demonstrate that CCR2- macrophages are a mixed population of embryonic- and definitive hematopoiesis-derived macrophages, which are maintained by self-renewal independently of blood monocytes.

2.    Interestingly, the disparate origin of CCR2- macrophages (definitive or extra-embryonic hematopoiesis) does not seem to endow them with distinct functions. In contrast, transcriptomic analysis shows different functions for CCR2- and CCR2+ macrophages, with CCR2- macrophages involved in cell migration, and cell adhesion.

3.    Selective CCR2- macrophage deletion with the CD169-DTR mouse model dramatically accelerates mortality in mice with chronic heart failure, indicating a beneficial role of CCR2- macrophages in this context. Despite no alteration in cardiac contractility, CCR2- macrophage depletion attenuates cardiac remodeling and blunts expansion of the coronary vasculature in chronically failing hearts.

4.    Cardiac tissue immunostaining, electron microscopy, ex vivo two-photon microscopy, and in vitro experiments revealed that, while CCR2- macrophages establish physical contact with adjacent cardiomyocytes through focal adhesion complexes, CCR2- macrophages do not directly contact cardiomyocytes.

5.    In vitro mechanical stretching experiments showed that macrophages are involved in the detection of mechanical strain via the Transient Receptor Potential Cation Channel TRPV4, which translates in enhanced expression of pro-angiogenic and remodeling factors (i.e. IGF1, HBEGF, and CYR61).

6.    Finally, the authors assess the functional relevance of TRPV4 in vivo through chemical activation or inhibition in chronically failing hearts. These experiments demonstrate that TRPV4 regulates expression of the pro-angiogenic factor IGF1 in cardiac macrophages. In addition, inhibition of TRPV4 in chronically failing hearts mimics the phenotype described for mice with CCR2- macrophage depletion (reduced LV dilation and impaired vascular network expansion), suggesting that TRPV4 action associates to these macrophages.

In conclusion, Wong et al. propose that, in the failing heart, CCR2- macrophages sense mechanical stretch thanks to their interactions with neighboring cardiomyocytes, get activated via TRPV4, and secrete growth factors that promote adaptive left ventricular remodeling and coronary angiogenesis. Thus, they ultimately contribute to the survival of the chronically failing heart.

Questions for the authors

1.    The authors compared physiological versus maladaptive cardiac remodeling, highlighting that transcriptional activation patterns differ from one to another. Do they hypothesize that TRPV4 activation in CRMs is exclusive for chronic heart failure and would not be seen, for example, in exercise-conditioned cardiac hypertrophy?

2.    Cardiac fibrosis is central to the pathology of heart failure, particularly heart failure with preserved ejection fraction [2, 3]. How inconvenient could be the lack of interstitial fibrosis in Tnnt2ΔK210/ΔK210 mice for the current study when compared to other chronic heart failure models that induce myocardial fibrosis (e.g. transverse aortic constriction (TAC) or isoproterenol infusion)?

3.    Tnnt2ΔK210/ΔK210 KO mice are already born with impaired myocardial contractility and heart function. Have the authors considered the additional use of an inducible model of cardiomyopathy to corroborate the findings?

4.    Why have the authors chosen to perform a microarray instead of a RNA-seq, which will be more informative? Transcriptional analysis reveals alternative functions for CCR2- and CCR2+ macrophage populations. Have the authors performed functional in vitro experiments to further support this finding?

References

1.        Nag, A.C., Study of non-muscle cells of the adult mammalian heart: a fine structural analysis and distribution. Cytobios, 1980. 28(109): p. 41-61.

2.        Sweeney, M., B. Corden, and S.A. Cook, Targeting cardiac fibrosis in heart failure with preserved ejection fraction: mirage or miracle? 2020. 12(10): p. e10865.

3.        González, A., et al., Myocardial Interstitial Fibrosis in Heart Failure: Biological and Translational Perspectives. J Am Coll Cardiol, 2018. 71(15): p. 1696-1706.