Lineage tracing clarifies the cellular origin of tissue-resident macrophages in the developing heart

Abstract

Tissue-resident macrophages play essential functions in the maintenance of tissue homeostasis and repair. Recently, the endocardium has been reported as a de novo hemogenic site for the contribution of hematopoietic cells, including cardiac macrophages, during embryogenesis. These observations challenge the current consensus that hematopoiesis originates from the hemogenic endothelium within the yolk sac and dorsal aorta. Whether the developing endocardium has such a hemogenic potential requires further investigation. Here, we generated new genetic tools to trace endocardial cells and reassessed their potential contribution to hematopoietic cells in the developing heart. Fate-mapping analyses revealed that the endocardium contributed minimally to cardiac macrophages and circulating blood cells. Instead, cardiac macrophages were mainly derived from the endothelium during primitive/transient definitive (yolk sac) and definitive (dorsal aorta) hematopoiesis. Our findings refute the concept of endocardial hematopoiesis, suggesting that the developing endocardium gives rise minimally to hematopoietic cells, including cardiac macrophages.

Loss of developmentally derived Irf8+ macrophages promotes hyperinnervation and arrhythmia in the adult zebrafish heart

“…Based on the timing of macrophage seeding at the zebrafish and murine SAN, cardiac macrophages in 4 dpf zebrafish and E16.5 fetal mice are presumed to be derived during primitive or transient definitive hematopoiesis 19,23,24,38 , hereafter referred to as embryonic-derived macrophages. In zebrafish, embryonic-derived macrophages can arise from the zebrafish intermediate cell mass (ICM, murine yolk sac equivalent), AGM, or caudal hematopoietic tissue (CHT, murine fetal liver equivalent) 42 in the first week of development.…”

“…In zebrafish, embryonic-derived macrophages can arise from the zebrafish intermediate cell mass (ICM, murine yolk sac equivalent), AGM, or caudal hematopoietic tissue (CHT, murine fetal liver equivalent) 42 in the first week of development. Considering that embryonic-derived macrophages comprise tissue resident, self-maintaining cardiac macrophage populations 19,23,26 , we asked whether embryonic macrophages could modulate the larval zebrafish heart in vivo . To do this, we employed optogenetics to manipulate macrophage membrane potential at 7 dpf and observed the concomitant effects on heart function.…”

“…Likewise, several reports have identified important novel functions for cardiac macrophages in heart development, homeostasis, electrical modulation, and disease [11][12][13][14][15][16][17][18][19][20][21][22] . The first wave of cardiac macrophages in mice is detectable around embryonic day (E) 10.5 with further expansion occurring through E14.5 19,23,24 .…”

Application of New Lineage Tracing Techniques in Cardiovascular Development and Physiology

“…However, these labeled cardiac ECs did not contribute to hematopoietic cells such as macrophages of the heart or circulating blood cells, suggesting that developing cardiac ECs do not have endogenous hematopoietic potential. 76 As our understanding of cellular heterogeneity deepens, by virtue of single-cell transcriptomics, the identification of cell (sub)types solely by singular markers remains challenging. To specifically label cell (sub)types in vivo, intercellular lineage tracing offers an alternative orthogonal method that allows the tracing of neighboring cell types in a specific anatomic location, providing a more precise mapping approach for cell lineages within a niche even without knowing their specific gene signatures.…”

“…72 Furthermore, specific labeling of cardiac ECs including endocardial cells in the developing heart helps resolve a recent controversy over the hematopoietic potential of the developing endocardium. [73][74][75][76] The cardiac ECs that have contacted with cardiomyocytes can be permanently labeled with a genetic reporter (Figure 4D). However, these labeled cardiac ECs did not contribute to hematopoietic cells such as macrophages of the heart or circulating blood cells, suggesting that developing cardiac ECs do not have endogenous hematopoietic potential.…”