Border-zone cardiomyocytes and macrophages contribute to remodeling of the extracellular matrix to promote cardiomyocyte invasion during zebrafish cardiac regeneration.

Despite numerous advances in our understanding of zebrafish cardiac regeneration, an aspect that remains less studied is how regenerating cardiomyocytes invade, and eventually replace, the collagen-containing fibrotic tissue following injury. Here, we provide an in-depth analysis of the process of cardiomyocyte invasion using live-imaging and histological approaches. We observed close interactions between protruding cardiomyocytes and macrophages at the wound border zone, and macrophage-deficient irf8 mutant zebrafish exhibited defects in extracellular matrix (ECM) remodeling and cardiomyocyte protrusion into the injured area. Using a resident macrophage ablation model, we show that defects in ECM remodeling at the border zone and subsequent cardiomyocyte protrusion can be partly attributed to a population of resident macrophages. Single-cell RNA-sequencing analysis of cells at the wound border revealed a population of cardiomyocytes and macrophages with fibroblast-like gene expression signatures, including the expression of genes encoding ECM structural proteins and ECM-remodeling proteins. The expression of mmp14b , which encodes a membrane-anchored matrix metalloproteinase, was restricted to cells in the border zone, including cardiomyocytes, macrophages, fibroblasts, and endocardial/endothelial cells. Genetic deletion of mmp14b led to a decrease in 1) macrophage recruitment to the border zone, 2) collagen degradation at the border zone, and 3) subsequent cardiomyocyte invasion. Furthermore, cardiomyocyte-specific overexpression of mmp14b was sufficient to enhance cardiomyocyte invasion into the injured tissue and along the apical surface of the wound. Altogether, our data shed important insights into the process of cardiomyocyte invasion of the collagen-containing injured tissue during cardiac regeneration. They further suggest that cardiomyocytes and resident macrophages contribute to ECM remodeling at the border zone to promote cardiomyocyte replenishment of the fibrotic injured tissue.

Comparative single-cell profiling reveals distinct cardiac resident macrophages essential for zebrafish heart regeneration.

Zebrafish exhibit a robust ability to regenerate their hearts following injury, and the immune system plays a key role in this process. We previously showed that delaying macrophage recruitment by clodronate liposome (-1d_CL, macrophage-delayed model) impairs neutrophil resolution and heart regeneration, even when the infiltrating macrophage number was restored within the first week post injury (Lai et al., 2017). It is thus intriguing to learn the regenerative macrophage property by comparing these late macrophages vs. control macrophages during cardiac repair. Here, we further investigate the mechanistic insights of heart regeneration by comparing the non-regenerative macrophage-delayed model with regenerative controls. Temporal RNAseq analyses revealed that -1d_CL treatment led to disrupted inflammatory resolution, reactive oxygen species homeostasis, and energy metabolism during cardiac repair. Comparative single-cell RNAseq profiling of inflammatory cells from regenerative vs. non-regenerative hearts further identified heterogeneous macrophages and neutrophils, showing alternative activation and cellular crosstalk leading to neutrophil retention and chronic inflammation. Among macrophages, two residential subpopulations (hbaa+ Mac and timp4.3+ Mac 3) were enriched only in regenerative hearts and barely recovered after +1d_CL treatment. To deplete the resident macrophage without delaying the circulating macrophage recruitment, we established the resident macrophage-deficient model by administrating CL earlier at 8 d (-8d_CL) before cryoinjury. Strikingly, resident macrophage-deficient zebrafish still exhibited defects in revascularization, cardiomyocyte survival, debris clearance, and extracellular matrix remodeling/scar resolution without functional compensation from the circulating/monocyte-derived macrophages. Our results characterized the diverse function and interaction between inflammatory cells and identified unique resident macrophages prerequisite for zebrafish heart regeneration.

Heparan sulfate glycosaminoglycans modulate migration and survival in zebrafish primordial germ cells.

Early in embryonic development, primordial germ cells (PGCs) are specified and migrate from the site of their origin to where the gonad develops, following a specific route. Heparan sulfate glycosaminoglycans (HS-GAGs) are ubiquitous in extracellular matrix and the cell surface and have long been speculated to play a role during the migration of PGCs. In line with this speculation, whole-mount immunohistochemistry revealed the existence of HS-GAGs in the vicinity of migrating PGCs in early zebrafish embryos. To examine the roles of HS-GAGs during PGC migration, zebrafish heparanase 1 (hpse1), which degrades HS-GAGs, was cloned and overexpressed specifically in PGCs. The guidance signal for the migration of PGCs was disrupted with the overexpression of hpse1, as cluster formation and marginal localization at the blastoderm were significantly perturbed at 6 hours postfertilization. Furthermore, the number of PGCs was significantly decreased with the lack of vicinal HS-GAGs, as observed in the whole-mount in situ hybridization and quantitative PCR of the PGC marker gene vasa. Terminal deoxynucleotidyl transferase dUTP nick-end labeling indicated significantly increased apoptosis in PGCs overexpressing hpse1, suggesting that HS-GAGs contribute to the maintenance of PGC survival. In conclusion, HS-GAGs play multifaceted roles in PGCs during migration and are required both for guidance signals and multiplication of PGCs.