Investigating the role of macrophage-deposited collagen in the injured mouse heart

After a heart attack a scar forms to prevent the heart from rupturing. This scar is thought to arise from a single cell type called myofibroblasts, which
become activated by incoming immune cells and then deposit scar-forming proteins such as collagens. We have shown that the immune cells themselves
(macrophages) can contribute collagens directly to the scar and that this can happen in situations where the heart regenerates (zebrafish, neonatal mice)
and when there is a permanent scar without tissue restoration (adult mice; human patients). The fact that scar forms during both settings suggest it is different in composition, yet this has not been investigated.

We propose to analyse the scars in both neonatal and adult mice using electron microscopy and to determine which cells communicate with the scar in regenerating and non-regenerating mouse hearts following injury. We will determine which macrophages produce collagen and whether different collagens may be laid-down in regenerative versus non-regenerative hearts. Finally, we will knock-down collagens in macrophages to determine the function of this novel source of scar.

This study will provide a comprehensive understanding of scar formation after heart attack and may enable us to modify scarring as a treatment for human patients.

Following myocardial infarction (MI) acute wound healing and long-term tissue remodelling are mediated by fibrosis and scarring. This serves initially to
prevent ventricular rupture but can progress to heart failure. The current dogma is that the scar consists of extracellular matrix proteins, predominantly
collagens, produced by activated fibroblasts (myofibroblasts). In a recent study comparing regenerative and non-regenerative zebrafish and mouse
models, we revealed that macrophages also deposit collagen within the forming scar following heart injury. Whilst this study implicated macrophages as
direct contributors to fibrosis during heart repair, the question as to why macrophages produce collagen remains unanswered. Here we propose to
investigate the role of macrophage-collagen during neonatal mouse heart injury in transition from regeneration to fibrosis and in the adult infarcted mouse heart. We will combine comprehensive ECM proteomics and cryo-Electron Microscopy (EM) to fully characterise scar content, ultrastructure and cell-matrix
interactions across stages. We will use SC-RNA-Seq transcriptomics and spatial proteomics to identify which subsets of macrophages produce collagen as
well as macrophage-tracking and conditional targeting of collagen isoforms to determine function. A comprehensive understanding of cardiac fibrosis,
including the potential for targeting macrophage-induced pro-fibrotic pathways, will assist in the development of therapies for patients suffering from
acute MI.