Spatiotemporal regulation of polyploidy in zebrafish cardiac tissue regeneration

Summary
Adult zebrafish have a remarkable capacity to regenerate the heart with minimal scarring. Understanding the
underlying cellular and molecular mechanisms will help addressing the regenerative deficiency in the adult
mammalian heart. We recently found that the zebrafish epicardium (the outermost layer of vertebrate hearts)
regenerates after injury by the creation of a leader region of polyploid cells (having two or more copies of the
genome). Polyploidy has been observed in many mammalian organs following injury and recently has been
invoked in mechanisms of tissue repair. However, the functional significance of polyploidy, as well as its
underlying mechanisms in tissue repair, remains elusive, representing a major knowledge gap in harnessing the
advantages of polyploidy in tissue repair. We found that, through collective cell migration, these leader epicardial
cells guide a trailing population of much smaller, dividing follower cells to repopulate the wound. The leader cell
population is established and maintained by endoreplication and is eliminated through apoptosis upon
completion of regeneration, indicating a transient role. The elevated cellular tension in the leader cells drives
endoreplication. This coordinated behavior of leader and follower cells facilitates robust regeneration of the
epicardium. Also, we found that the polyploid epicardial cells are a major source of paracrine secretion for heart
regeneration. The overall objective of our proposal is to understand the mechanisms that regulate spatiotemporal
cell behavior of the epicardium and how defects in this behavior impact heart regeneration. Through single-cell
RNA sequencing, reporter assays, and pharmacological treatments, we have discovered a novel signaling
pathway together with Yap signaling that participate in the spatiotemporal polyploidization in the epicardium. We
will 1) characterize the signaling cascade that involves mechanical cues, Yap, and the new pathway in regulating
spatiotemporal polyploidization during epicardial regeneration, 2) define the leader signals that drive leader-
follower coordination in epicardial regeneration, and 3) investigate the functional significance of epicardial
polyploidy in heart regeneration. The proposed research will define a new signaling paradigm in guiding cell
cycle decisions for efficient heart regeneration. Moreover, polyploid cells are present in normal tissues such as
the mammalian cardiomyocytes, as well as in pathological processes such as lung injury, acute kidney injury,
and cancer. Results from our study will unearth conceptual innovations concerning the regulation of cell cycle
decisions to mediate physiological and pathological polyploidization and robust tissue regeneration.

Grant Number: 5R01HL166518-03
NIH Institute/Center: NIH
Principal Investigator: Jingli Cao