A characterisation of mononuclear phagocyte dynamics in the healthy and regenerating zebrafish heart

Abstract

The inability of mammals to regenerate damaged tissue following myocardial infarction results in extensive scarring and dysfunction of the heart. The adult zebrafish undergoes a similar scarring response, but is subsequently able to regenerate fully functional cardiac muscle and concomitantly resolve scar tissue. Mononuclear phagocytes (MNPs), comprising monocytes, macrophages and dendritic cells, play key roles in orchestrating both of these responses. In mammals, tissue resident macrophages have been shown to exert a beneficial influence on healing, yet specific subpopulations of pro-inflammatory, recruited macrophages have been attributed deleterious roles in mediating adverse scarring. How zebrafish MNP populations differ to promote a regenerative outcome is not well understood, largely due to the paucity of cell-specific markers that can segregate populations. In an attempt to better delineate MNPs, the co-expression of classical macrophage markers, mpeg1.1 and csf1ra, was investigated in adult zebrafish hearts. Surprisingly, this identified a discrete population of mpeg1.1+csf1ra- B cells and NKL cells, but also identified a transient population of injury-responsive csf1ra+mpeg1.1- monocytes, indicating that mpeg1.1 expression can stratify populations of monocytes and macrophages. Establishment of an ex vivo imaging platform also enabled the live imaging of cardiac macrophage populations, and, alongside detailed three-dimensional imaging, revealed new insights into their motility, migration and cellular interactions. Analysis of csf1ra mutant fish also revealed a dramatic deficiency of both resident, and injury-associated macrophages deep within the myocardium, highlighting the requirement of csf1ra for normal cardiac MNP migration. This deficiency also appeared to alter scar dynamics following cardiac injury. Similarly, the absence of il1b signalling, which is mediated by MNP populations and enhances recruitment of pro-inflammatory leukocytes, also appeared to affect collagen I resolution. Collectively, this has expanded our knowledge of the interplay between cardiac MNP dynamics and scarring, and has built a foundation that will facilitate the future study of these populations.

Date of Award	27 Sept 2022
Original language	English
Awarding Institution	University of Bristol
Sponsors	Wellcome Trust
Supervisor	Beck J Richardson (Supervisor) & Paul B Martin (Supervisor)