Surviving Acute Organ Failure: Cell Polyploidization and Progenitor Proliferation

Polyploidy: A Biological Force From Cells to Ecosystems

Polyploidy, resulting from the duplication of the entire genome of an organism or cell, greatly affects genes and genomes, cells and tissues, organisms, and even entire ecosystems. Despite the wide-reaching importance of polyploidy, communication across disciplinary boundaries to identify common themes at different scales has been almost nonexistent. However, a critical need remains to understand commonalities that derive from shared polyploid cellular processes across organismal diversity, levels of biological organization, and fields of inquiry - from biodiversity and biocomplexity to medicine and agriculture. Here, we review the current understanding of polyploidy at the organismal and suborganismal levels, identify shared research themes and elements, and propose new directions to integrate research on polyploidy toward confronting interdisciplinary grand challenges of the 21^st century.

An abundant quiescent stem cell population in Drosophila Malpighian tubules protects principal cells from kidney stones

Adult Drosophila Malpighian tubules have low rates of cell turnover but are vulnerable to damage caused by stones, like their mammalian counterparts, kidneys. We show that Drosophilarenal stem cells (RSCs) in the ureter and lower tubules comprise a unique, unipotent regenerative compartment. RSCs respond only to loss of nearby principal cells (PCs), cells critical for maintaining ionic balance. Large polyploid PCs are outnumbered by RSCs, which replace each lost cell with multiple PCs of lower ploidy. Notably, RSCs do not replenish principal cells or stellate cells in the upper tubules. RSCs generate daughters by asymmetric Notch signaling, yet RSCs remain quiescent (cell cycle-arrested) without damage. Nevertheless, the capacity for RSC-mediated repair extends the lifespan of flies carrying kidney stones. We propose that abundant, RSC-like stem cells exist in other tissues with low rates of turnover where they may have been mistaken for differentiated tissue cells.

Residual Diploidy in Polyploid Tissues: A Cellular State with Enhanced Proliferative Capacity for Tissue Regeneration?

A major objective of modern biomedical research aims to promote tissue self-regeneration after injury, obviating the need for whole organ transplantation and avoiding mortality due to organ failure. Identifying the population of cells capable of regeneration, alongside understanding the molecular mechanisms that activate that population to re-enter the cell cycle, are two important steps to advancing the field of endogenous tissue regeneration toward the clinic. In recent years, an emerging trend has been observed, whereby polyploidy of relevant parenchymal cells, arising from alternative cell cycles as part of a normal developmental process, is linked to restricted proliferative capacity of those cells. An accompanying hypothesis, therefore, is that a residual subpopulation of diploid parenchymal cells retains proliferative competence and is the major driver for any detected postnatal cell turnover. In this perspective review, we examine the emerging literature on residual diploid parenchymal cells and the possible link of this population to endogenous tissue regeneration, in the context of both heart and liver. We speculate on additional cell types that may play a similar role in their respective tissues and discuss outstanding questions for the field.

Molecular Mechanisms of the Acute Kidney Injury to Chronic Kidney Disease Transition: An Updated View

Increasing evidence has demonstrated the bidirectional link between acute kidney injury (AKI) and chronic kidney disease (CKD) such that, in the clinical setting, the new concept of a unified syndrome has been proposed. The pathophysiological reasons, along with the cellular and molecular mechanisms, behind the ability of a single, acute, apparently self-limiting event to drive chronic kidney disease progression are yet to be explained. This acute injury could promote progression to chronic disease through different pathways involving the endothelium, the inflammatory response and the development of fibrosis. The interplay among endothelial cells, macrophages and other immune cells, pericytes and fibroblasts often converge in the tubular epithelial cells that play a central role. Recent evidence has strengthened this concept by demonstrating that injured tubules respond to acute tubular necrosis through two main mechanisms: The polyploidization of tubular cells and the proliferation of a small population of self-renewing renal progenitors. This alternative pathophysiological interpretation could better characterize functional recovery after AKI.

Cardiomyocyte Polyploidy and Implications for Heart Regeneration

In mammals, most cardiomyocytes (CMs) become polyploid (they have more than two complete sets of chromosomes). The purpose of this review is to evaluate assumptions about CM ploidy that are commonly discussed, even if not experimentally demonstrated, and to highlight key issues that are still to be resolved. Topics discussed here include (a) technical and conceptual difficulties in defining a polyploid CM, (b) the candidate role of reactive oxygen as a proximal trigger for the onset of polyploidy, (c) the relationship between polyploidization and other aspects of CM maturation, (d) recent insights related to the regenerative role of the subpopulation of CMs that are not polyploid, and (e) speculations as to why CMs become polyploid at all. New approaches to experimentally manipulate CM ploidy may resolve some of these long-standing and fundamental questions.

Wound-induced polyploidization is driven by Myc and supports tissue repair in the presence of DNA damage

Tissue repair usually requires either polyploid cell growth or cell division, but the molecular mechanism promoting polyploidy and limiting cell division remains poorly understood. Here, we find that injury to the adult Drosophila epithelium causes cells to enter the endocycle through the activation of Yorkie-dependent genes (Myc and E2f1). Myc is even sufficient to induce the endocycle in the uninjured post-mitotic epithelium. As result, epithelial cells enter S phase but mitosis is blocked by inhibition of mitotic gene expression. The mitotic cell cycle program can be activated by simultaneously expressing the Cdc25-like phosphatase String (stg), while genetically depleting APC/C E3 ligase fizzy-related (fzr). However, forcing cells to undergo mitosis is detrimental to wound repair as the adult fly epithelium accumulates DNA damage, and mitotic errors ensue when cells are forced to proliferate. In conclusion, we find that wound-induced polyploidization enables tissue repair when cell division is not a viable option.