Requirement of matrix metalloproteinase-1 for intestinal homeostasis in the adult Drosophila midgut

Stem cells are tightly regulated by both intrinsic and extrinsic signals as well as the extracellular matrix (ECM) for tissue homeostasis and regenerative capacity. Matrix metalloproteinases (MMPs), proteolytic enzymes, modulate the turnover of numerous substrates, including cytokine precursors, growth factors, and ECM molecules. However, the roles of MMPs in the regulation of adult stem cells are poorly understood. In the present study, we utilize the Drosophila midgut, which is an excellent model system for studying stem cell biology, to show that Mmp1 is involved in the regulation of intestinal stem cells (ISCs). The results showed that Mmp1 is expressed in the adult midgut and that its expression increases with age and with exposure to oxidative stress. Mmp1 knockdown or Timp-overexpressing flies and flies heterozygous for a viable, hypomorphic Mmp1 allele increased ISC proliferation in the gut, as shown by staining with an anti-phospho-histone H3 antibody and BrdU incorporation assays. Reduced Mmp1 levels induced intestinal hyperplasia, and the Mmp1depletion-induced ISC proliferation was rescued by the suppression of the EGFR signaling pathway, suggesting that Mmp1 regulates ISC proliferation through the EGFR signaling pathway. Furthermore, adult gut-specific knockdown and whole-animal heterozygotes of Mmp1 increased additively sensitivity to paraquat-induced oxidative stress and shortened lifespan. Our data suggest that Drosophila Mmp1 is involved in the regulation of ISC proliferation for maintenance of gut homeostasis.     

Intestinal stem cell proliferation and epithelial homeostasis in the adult Drosophila midgut

Adult tissue homeostasis requires a tight balance between the removal of old or damaged cells and the production of new ones. Such processes are usually driven by dedicated stem cells that reside within specific tissue locations or niches.The intestinal epithelium has a remarkable regenerative capacity, which has made it a prime paradigm for the study of stem cell-driven tissue self-renewal. The discovery of the presence of stem cells in the adult midgut of the fruit fly Drosophila melanogaster has significantly impacted our understanding of the role of stem cells in intestinal homeostasis. Here we will review the current knowledge of the main mechanisms involved in the regulation of tissue homeostasis in the adult Drosophila midgut, with a focus on the role of stem cells in this process. We will also discuss processes involving acute or chronic disruption of normal intestinal homeostasis such as damage-induced regeneration and ageing.     

Regulation of adult stem cell behavior by nutrient signaling

Adult stem cells play an essential role throughout life, maintaining tissue and organ function by providing a reservoir of cells for homeostasis and repair. Maintenance and activity of adult stem cells have been the focus of numerous studies that have revealed stem cell-intrinsic factors and signals from the local microenvironment that regulate stem cell behavior. A growing body of work has provided evidence that circulating, systemic factors also contribute to the regulation of stem cell behavior in numerous tissues. We have demonstrated that Drosophila male germline stem cells (GSCs) and intestinal stem cells (ISCs) respond to changes in nutrient availability, specifically amino acids. Furthermore, we have shown that insulin signaling plays an important role in mediating the effects of changes in nutritional conditions. Notably, insulin signaling is cell-autonomously required within male GSCs for maintenance. Here we discuss our data regarding the effects and mechanisms by which changes in systemic nutritional conditions may influence the maintenance and activity of adult stem cells via insulin signaling.Key words: Drosophila, stem cells, nutrition, insulin, niche     

Regulation of adult stem cell behavior by nutrient signaling

Adult stem cells play an essential role?throughout life, maintaining tissue and organ function by providing a reservoir of cells for homeostasis and repair. Maintenance and activity of adult stem cells have been the focus of numerous studies that have revealed stem cell-intrinsic factors and signals from the local microenvironment that regulate stem cell behavior. A growing body of work has provided evidence that circulating, systemic factors also contribute to the regulation of stem cell behavior in numerous tissues. We have demonstrated that Drosophila male germline stem cells (GSCs) and intestinal stem cells (ISCs) respond to changes in nutrient availability, specifically amino acids. Furthermore, we have shown that insulin signaling plays an important role in mediating the effects of changes in nutritional conditions. Notably, insulin signaling is cell-autonomously required within male GSCs for maintenance. Here we discuss our data regarding the effects and mechanisms by which changes in systemic nutritional conditions may influence the maintenance and activity of adult stem cells via insulin signaling.     

Altered modes of stem cell division drive adaptive intestinal growth

Throughout life, adult organs continually adapt to variable environmental factors. Adaptive mechanisms must fundamentally differ from homeostatic maintenance, but little is known about how physiological factors elicit tissue remodeling. Here, we show that specialized stem cell responses underlie the adaptive resizing of a mature organ. In the adult Drosophila midgut, intestinal stem cells interpret a nutrient cue to "break homeostasis" and drive growth when food is abundant. Activated in part by niche production of insulin, stem cells direct a growth program through two altered modes of behavior: accelerated division rates and predominance of symmetric division fates. Together, these altered modes produce a net increase in total intestinal cells, which is reversed upon withdrawal of food. Thus, tissue renewal programs are not committed to maintain cellular equilibrium; stem cells can remodel organs in response to physiological triggers.     

An amuse-bouche of stem cell regulation: Underlying principles and mechanisms from adult Drosophila intestinal stem cells

Stem cells have essential functions in the development and maintenance of our organs. Improper regulation of adult stem cells and tissue homeostasis can result in cancers and age-dependent decline. Therefore, understanding how tissue-specific stem cells can accurately renew tissues is an important aim of regenerative medicine. The Drosophila midgut harbors multipotent adult stem cells that are essential to renew the gut in homeostatic conditions and upon stress-induced regeneration. It is now a widely used model system to decipher regulatory mechanisms of stem cell biology. Here, we review recent findings on how adult intestinal stem cells differentiate, interact with their environment, and change during aging.     

The stem cell niche: lessons from the Drosophila testis

In metazoans, tissue maintenance and regeneration depend on adult stem cells, which are characterized by their ability to self-renew and generate differentiating progeny in response to the needs of the tissues in which they reside. In the Drosophila testis, germline and somatic stem cells are housed together in a common niche, where they are regulated by local signals, epigenetic mechanisms and systemic factors. These stem cell populations in the Drosophila testis have the unique advantage of being easy to identify and manipulate, and hence much progress has been made in understanding how this niche operates. Here, we summarize recent work on stem cells in the adult Drosophila testis and discuss the remarkable ability of these stem cells to respond to change within the niche.     

Gbb/Bmp signaling is essential for maintaining germline stem cells and for repressing bam transcription in the Drosophila testis

Stem cells are responsible for replacing damaged or dying cells in various adult tissues throughout a lifetime. They possess great potential for future regenerative medicine and gene therapy. However, the mechanisms governing stem cell regulation are poorly understood. Germline stem cells (GSCs) in the Drosophila testis have been shown to reside in niches, and thus these represent an excellent system for studying relationships between niches and stem cells. Here we show that Bmp signals from somatic cells are essential for maintaining GSCs in the Drosophila testis. Somatic cyst cells and hub cells express two Bmp molecules, Gbb and Dpp. Our genetic analysis indicates that gbb functions cooperatively with dpp to maintain male GSCs, although gbb alone is essential for GSC maintenance. Furthermore, mutant clonal analysis shows that Bmp signals directly act on GSCs and control their maintenance. In GSCs defective in Bmp signaling, expression of bam is upregulated, whereas forced bam expression in GSCs causes the GSCs to be lost. This study demonstrates that Bmp signals from the somatic cells maintain GSCs, at least in part, by repressing bam expression in the Drosophila testis. dpp signaling is known to be essential for maintaining GSCs in the Drosophila ovary. This study further suggests that both Drosophila male and female GSCs use Bmp signals to maintain GSCs.     

Maintaining tissue homeostasis: dynamic control of somatic stem cell activity

Long-term maintenance of tissue homeostasis relies on the accurate regulation of somatic stem cell activity. Somatic stem cells have to respond to tissue damage and proliferate according to tissue requirements while avoiding overproliferation. The regulatory mechanisms involved in these responses are now being unraveled in the intestinal epithelium of Drosophila, providing new insight into strategies and mechanisms of stem cell regulation in barrier epithelia. Here, we review these studies and highlight recent findings in vertebrate epithelia that indicate significant conservation of regenerative strategies between vertebrate and fly epithelia.     

Universal patterns of stem cell fate in cycling adult tissues

In cycling tissues that exhibit high turnover, tissue maintenance and repair are coordinated by stem cells. But, how frequently stem cells are replaced following differentiation, aging or injury remains unclear. By drawing together the results of recent lineage-tracing studies, we propose that tissue stem cells are routinely lost and replaced in a stochastic manner. We show that stem cell replacement leads to neutral competition between clones, resulting in two characteristic and recurring patterns of clone fate dynamics, which provide a unifying framework for interpreting clone fate data and for measuring rates of stem cell loss and replacement in vivo. Thus, we challenge the concept of the stem cell as an immortal, slow-cycling, asymmetrically dividing cell.     

Pathogenic stimulation of intestinal stem cell response in drosophila

Stem cell‐mediated tissue repair is a promising approach for many diseases. Mammalian intestine is an actively regenerating tissue such that epithelial cells are constantly shedding and underlying precursor cells are constantly replenishing the loss of cells. An imbalance of these processes will lead to intestinal diseases including inflammation and cancer. Mammalian intestinal stem cells (ISCs) are located in bases of crypts but at least two groups of cells have been cited as stem cells. Moreover, precursor cells in the transit amplifying zone can also proliferate. The involvement of multiple cell types makes it more difficult to examine tissue damage response in mammalian intestine. In adult Drosophila midgut, the ISCs are the only cells that can go through mitosis. By feeding pathogenic bacteria and stress inducing chemicals to adult flies, we demonstrate that Drosophila ISCs in the midgut can respond by increasing their division. The resulting enteroblasts, precursor cells for enterocytes and enteroendocrine cells, also differentiate faster to become cells resembling enterocyte lineage. These results are consistent with the idea that Drosophila midgut stem cells can respond to tissue damage induced by pathogens and initiate tissue repair. This system should allow molecular and genetic analyses of stem cell‐mediated tissue repair. J. Cell. Physiol. 220: 664–671, 2009. © 2009 Wiley‐Liss, Inc.     

EGFR/Ras/MAPK signaling mediates adult midgut epithelial homeostasis and regeneration in Drosophila

Many tissues in higher animals undergo dynamic homeostatic growth, wherein damaged or aged cells are replaced by the progeny of resident stem cells. To maintain homeostasis, stem cells must respond to tissue needs. Here we show that in response to damage or stress in the intestinal (midgut) epithelium of adult Drosophila, multiple EGFR ligands and rhomboids (intramembrane proteases that activate some EGFR ligands) are induced, leading to the activation of EGFR signaling in intestinal stem cells (ISCs). Activation of EGFR signaling promotes ISC division and midgut epithelium regeneration, thereby maintaining tissue homeostasis. ISCs defective in EGFR signaling cannot grow or divide, are poorly maintained, and cannot support midgut epithelium regeneration after enteric infection by the bacterium Pseudomonas entomophila. Furthermore, ISC proliferation induced by Jak/Stat signaling is dependent upon EGFR signaling. Thus the EGFR/Ras/MAPK signaling pathway plays central, essential roles in ISC maintenance and the feedback system that mediates intestinal homeostasis.     

Stem cell self-renewal in intestinal crypt

As a rapidly cycling tissue capable of fast repair and regeneration, the intestinal epithelium has emerged as a favored model system to explore the principles of adult stem cell biology. However, until recently, the identity and characteristics of the stem cell population in both the small intestine and colon has remained the subject of debate. Recent studies based on targeted lineage tracing strategies, combined with the development of an organotypic culture system, have identified the crypt base columnar cell as the intestinal stem cell, and have unveiled the strategy by which the balance between proliferation and differentiation is maintained. These results show that intestinal stem cells operate in a dynamic environment in which frequent and stochastic stem cell loss is compensated by the proliferation of neighboring stem cells. We review the basis of these experimental findings and the insights they offer into the mechanisms of homeostatic stem cell regulation.     

Dynamics of the male germline stem cell population during aging of Drosophila melanogaster

Drosophila melanogaster has emerged as an important model system for the study of both stem cell biology and aging. Much is known about how molecular signals from the somatic niche regulate adult stem cells in the germline, and a variety of environmental factors as well as single point mutations have been shown to affect lifespan. Relatively little is known, however, about how aging affects specific populations of cells, particularly adult stem cells that may be susceptible to aging-related damage. Here we show that male germline stem cells (GSCs) are lost from the stem cell niche during aging, but are efficiently replaced to maintain overall stem cell number. We also find that the division rate of GSCs slows significantly during aging, and that this slowing correlates with a reduction in the number of somatic hub cells that contribute to the stem cell niche. Interestingly, slowing of stem cell division rate was not observed in long-lived methuselah mutant flies. We finally investigated whether two mechanisms that are thought to be used in other adult stem cell types to minimize the effects of aging were operative in this system. First, in many adult tissues stem cells exhibit markedly fewer cell cycles relative to transit-amplifying cells, presumably protecting the stem cell pool from replication-associated damage. Second, at any given time not all stem cells actively cycle, leading to 'clonal succession' from the reserve pool of initially quiescent stem cells. We find that neither of these mechanisms is used in Drosophila male GSCs.     

Amputation induces stem cell mobilization to sites of injury during planarian regeneration

How adult stem cell populations are recruited for tissue renewal and repair is a fundamental question of biology. Mobilization of stem cells out of their niches followed by correct migration and differentiation at a site of tissue turnover or injury are important requirements for proper tissue maintenance and regeneration. However, we understand little about the mechanisms that control this process, possibly because the best studied vertebrate adult stem cell systems are not readily amenable to in vivo observation. Furthermore, few clear examples of the recruitment of fully potent stem cells, compared with limited progenitors, are known. Here, we show that planarian stem cells directionally migrate to amputation sites during regeneration. We also show that during tissue homeostasis they are stationary. Our study not only uncovers the existence of specific recruitment mechanisms elicited by amputation, but also sets the stage for the systematic characterization of evolutionarily conserved stem cell regulatory processes likely to inform stem cell function and dysfunction in higher organisms, including humans.     

New advances in stem cell biology: A perspective from gametogenesis

Regeneration of adult tissues relies on a small population of adult stem cells located in a specialized microenvironment. The adult stem cells divide continuously to produce new stem cells, as well as differentiated daughter cells to replenish lost cells due to damage or aging. The molecular mechanisms controlling their ability to divide, self-renew and differentiate remain largely undiscovered. The Drosophila reproductive systems have proven to be excellent models to understand the basic mechanisms regulating stem cell function. This report summarizes some of the recent advances in this field that were presented at the 49th Drosophila Research Conference held in San Diego in April 2008.