Potential functions of embryonic cardiac macrophages in angiogenesis, lymphangiogenesis and extracellular matrix remodeling

Abstract

The role of cardiac tissue macrophages (cTMs) during pre- and postnatal developmental stages remains in many aspects unknown. We aimed to characterize cTM populations and their potential functions based on surface markers. Our in situ studies of immunostained cardiac tissue specimens of murine fetuses (from E11to E17) revealed that a significant number of embryonic cTMs (phenotyped by CD45, CD68, CD64, F4/80, CD11b, CD206, Lyve-1) resided mostly in the subepicardial space, not in the entire myocardial wall, as observed in adult individuals. cTMs accompanied newly developed blood and lymphatic vessels adhering to vessel walls by cellular processes. A subpopulation of CD68-positive cells was found to form accumulations in areas of massive apoptosis during the outflow tract remodeling and shortening. Flow cytometry analysis at E14 and E17 stages revealed newly defined three subpopulations:CD64low, CD64highCD206-and CD64highCD206+. The levels of mRNA expression for genes related to regulation of angiogenesis (VEGFa, VEGFb, VEGFc, bFGF), lymphangiogenesis (VEGFc) and extracellular matrix (ECM) remodeling (MMP13, Arg1, Ym1/Chil3, Retlna/FIZZ1) differed among the selected populations and/or embryonic stages. Our results demonstrate a diversity of embryonic cTMs and their tissue-specific locations, suggesting their various potential roles in regulating angiogenesis, lymphangiogenesis and ECM remodeling.

The Evolving Roles of Cardiac Macrophages in Homeostasis, Regeneration, and Repair

“…Compared to microglia and spleen monocytes/macrophages, cardiac-resident macrophages have enriched expression of signature genes like the folate receptor 2 (Folr2); the lymphatic vessel endothelial hyaluronan receptor 1 (Lyve1); the scavenging receptor for the hemoglobin–haptoglobin complex (CD163); the insulin-like growth factor 1 (Igf1); the mannose receptor 1 (Mrc1 or CD206); resistin-like alpha (Retnla); and the LPS co-receptor CD14, which resemble an alternative activation phenotype that is associated with anti-inflammatory properties and wound resolution [ 53 ]. However, some genes have unique functions which are important in the context of cardiac homeostasis and development, such as Lyve1, implicated in adipose tissue angiogenesis [ 54 ] and lymphangiogenesis in the heart [ 55 , 56 ]; or Igf1, involved in coronary development and maturation during cardiac development [ 48 ]. Furthermore, the high expression of the scavenging receptors Mertk, CD206, CD163, and CD14 suggest a highly phagocytic capacity, which is fundamental to maintain a clean extracellular environment for proper myocardial performance [ 51 ].…”

“…Ccr2− cardiac-resident macrophages are located within the myocardium, near the epicardium and in close relation with newly forming capillaries, veins, and lymphatic vessels at early stages of embryogenesis (E14.5) [ 48 , 55 ]. The mouse genetic line Csf1 op/op , which lacks tissue-resident macrophages derived from yolk sac progenitors, has an aberrant pattern of vasculature within the heart, characterized by an increased number of small diameter capillaries (<12 μm 2 ), but fewer large-diameter capillaries (>12 μm 2 ).…”

“…In addition to coronary development, cardiac-resident macrophages are essential for proper lymphangiogenesis and lymphatic vessel maturation in the heart. Cardiac-resident macrophages derived from yolk-sac precursors colocalize with lymphatic endothelial cells (LEC) [ 55 , 56 ] and function as chaperone cells, guiding the LEC to another nearby LEC in order to form lymphatic junctions, extending the lymphatic network of the heart [ 56 ]. Macrophage depletion during early embryogenesis prevents cardiac-resident macrophage seeding resulting in impaired lymphangiogenesis [ 56 ].…”

“…Inhibition of hyaluronan by hyaluronidase impedes sprouting and bridging between LECs [ 56 ]. Interestingly, only cardiac-resident macrophages expressing Lyve1 contribute to lymphangiogenesis [ 55 ], which are cardiac-resident macrophages developed during primitive hematopoiesis in the yolk-sac [ 10 , 17 , 55 , 56 ]. Modulation of lymphangiogenesis by cardiac-resident macrophages is important not only from a developmental perspective, but also in the context of ischemic cardiac remodeling since lymphangiogenesis is a mechanism that regulates adverse remodeling and facilitates circulation of infiltrating immune cells and clearance of dead material from the ischemic heart after injury [ 63 , 64 , 65 ].…”

In focus in HCB

“…However, more recently, mounting evidence indicates that macrophage phenotypes are tissue-specific, and the origin of tissue macrophages has become a topic of wide-spread interest in cell biology (Epelman et al 2014). Gula et al (2021) have now performed a series of experiments designed to characterize the phenotypes and functions of cardiac macrophages during development of the murine heart. They utilized a variety of experimental techniques including multi-label immunofluorescence for cell phenotype determination on frozen sections and tissue whole-mount preparations imaged by both conventional wide-field and confocal microscopy, flow cytometry on isolated embryonic cardiac cells, and RT-PCR on sorted single cell suspensions.…”