Age-related changes in polycomb gene regulation disrupt lineage fidelity in intestinal stem cells.

Tissue homeostasis requires long-term lineage fidelity of somatic stem cells. Whether and how age-related changes in somatic stem cells impact the faithful execution of lineage decisions remains largely unknown. Here, we address this question using genome-wide chromatin accessibility and transcriptome analysis as well as single-cell RNA-seq to explore stem-cell-intrinsic changes in the aging Drosophila intestine. These studies indicate that in stem cells of old flies, promoters of Polycomb (Pc) target genes become differentially accessible, resulting in the increased expression of enteroendocrine (EE) cell specification genes. Consistently, we find age-related changes in the composition of the EE progenitor cell population in aging intestines, as well as a significant increase in the proportion of EE-specified intestinal stem cells (ISCs) and progenitors in aging flies. We further confirm that Pc-mediated chromatin regulation is a critical determinant of EE cell specification in the Drosophila intestine. Pc is required to maintain expression of stem cell genes while ensuring repression of differentiation and specification genes. Our results identify Pc group proteins as central regulators of lineage identity in the intestinal epithelium and highlight the impact of age-related decline in chromatin regulation on tissue homeostasis.

Hallmarks of aging Drosophila intestinal stem cells.

The age-associated decline of regenerative capacity in many tissues is a consequence of stem cell intrinsic and extrinsic perturbations that are only beginning to be understood. To gain insight into mechanisms of this age-related decline, a comprehensive understanding of these perturbations is necessary. Drosophila intestinal stem cells (ISCs) have served as a prime model in which to explore these age-related changes, and in which to identify intervention strategies to improve regenerative capacity and extend lifespan. In this review, we summarize and discuss important work that has contributed to our understanding of how aging impacts ISC regulation in relation to well-described "hallmarks" of aging.

Isolating intestinal stem cells from adult Drosophila midguts by FACS to study stem cell behavior during aging.

Aging tissue is characterized by a continuous decline in functional ability. Adult stem cells are crucial in maintaining tissue homeostasis particularly in tissues that have a high turnover rate such as the intestinal epithelium. However, adult stem cells are also subject to aging processes and the concomitant decline in function. The Drosophila midgut has emerged as an ideal model system to study molecular mechanisms that interfere with the intestinal stem cells' (ISCs) ability to function in tissue homeostasis. Although adult ISCs can be easily identified and isolated from midguts of young flies, it has been a major challenge to study endogenous molecular changes of ISCs during aging. This is due to the lack of a combination of molecular markers suitable to isolate ISCs from aged intestines. Here we propose a method that allows for successful dissociation of midgut tissue into living cells that can subsequently be separated into distinct populations by FACS. By using dissociated cells from the esg-Gal4, UAS-GFP fly line, in which both ISCs and the enteroblast (EB) progenitor cells express GFP, two populations of cells are distinguished based on different GFP intensities. These differences in GFP expression correlate with differences in cell size and granularity and represent enriched populations of ISCs and EBs. Intriguingly, the two GFP-positive cell populations remain distinctly separated during aging, presenting a novel technique for identifying and isolating cell populations enriched for either ISCs or EBs at any time point during aging. The further analysis, for example transcriptome analysis, of these particular cell populations at various time points during aging is now possible and this will facilitate the examination of endogenous molecular changes that occur in these cells during aging.

A role for Drosophila Wnt-4 in heart development.

In vertebrates, different Wnt-signaling pathways are required in a temporally coordinated manner to promote cardiogenesis. In Drosophila, wingless holds an essential role in heart development. Among the known Drosophila Wnts is DWnt4, the function of which has been studied in various developmental processes except for heart development. We re-evaluated the expression pattern of DWnt4 during embryogenesis and show that transcripts are not restricted to the dorsal ectoderm but are also present in the cardiogenic mesoderm. Moreover, we detect DWnt4 mRNA transcripts in myocardial cells by stage 16. The heart phenotype in DWnt4 mutant embryos is characterized by various degrees of disrupted expression of different cardiac markers. Overexpression of Dwnt4 also affects heart marker expression, which can be partially rescued by simultaneous inhibition of PKC. Our data reveal a role for DWnt4 in cardiogenesis; however, integration of DWnt4 with other known signaling pathways that function in heart development still awaits further investigation.