You will be presented with the abstract, introduction, and discussion section from a biomedical research article. Your task is to create a summary that a layperson can understand, capturing the essence of the research article. Keep essential scientific terms, but ensure that the language remains clear and the concepts are explained in an uncomplicated manner.

Title: Notch Signaling Mediates the Age-Associated Decrease in Adhesion of Germline Stem Cells to the Niche}
Article:
Abstract:
Stem cells have an innate ability to occupy their stem cell niche, which in turn, is optimized to house stem cells. Organ aging is associated with reduced stem cell occupancy in the niche, but the mechanisms involved are poorly understood. Here, we report that Notch signaling is increased with age in Drosophila female germline stem cells( GSCs), and this results in their removal from the niche. Clonal analysis revealed that GSCs with low levels of Notch signaling exhibit increased adhesiveness to the niche, thereby out-competing their neighbors with higher levels of Notch; adhesiveness is altered through regulation of E-cadherin expression. Experimental enhancement of Notch signaling in GSCs hastens their age-dependent loss from the niche, and such loss is at least partially mediated by Sex lethal. However, disruption of Notch signaling in GSCs does not delay GSC loss during aging, and nor does it affect BMP signaling, which promotes self-renewal of GSCs. Finally, we show that in contrast to GSCs, Notch activation in the niche( which maintains niche integrity, and thus mediates GSC retention) is reduced with age, indicating that Notch signaling regulates GSC niche occupancy both intrinsically and extrinsically. Our findings expose a novel role of Notch signaling in controlling GSC-niche adhesion in response to aging, and are also of relevance to metastatic cancer cells, in which Notch signaling suppresses cell adhesion.
Introduction:
Age-associated depletion of stem cell pools has been reported for mammalian satellite stem cells, Drosophila male and female GSCs, and C. elegans GSCs[1]–[4]; however, the mechanisms underlying such depletion remain unknown. The stem cell niche houses stem cells and maintains their cell identity, by providing physical contact and stemness factors, respectively[5]. In addition to the niche, stem cell-intrinsic factors also regulate stem cell function[6],[7]. These signals are tightly coupled, and regulate stem cells to fit the current needs of the organism. During aging, diminished niche function leads to stem cell loss[1]; on the other hand, it is unknown whether stem cells influence their own attachment to the niche as they age. Moreover, it is also unclear how niche cells coordinate with stem cells in response to aging. Drosophila is a small organism with a short life span; such properties, combined with the availability of powerful genetic approaches, making this organism eminently suitable for investigations into cellular and organismic responses during aging. In addition, the Drosophila ovary houses well-characterized GSCs and their niche( Fig. 1A)[8]. These advantages make the Drosophila ovary an excellent model in which to study the communication of stem cells with themselves and the surrounding environment. One Drosophila ovary is composed of 16 to 20 ovarioles, which are the basic functional unit of egg production[9]. The anterior-most structure of the ovariole is called the germarium; the tip of the germarium contains the GSC niche, which is composed of terminal filament, cap cells, and anterior escort cells[10],[11]. GSCs make direct contact with cap cells, a major niche component, through E-cadherin-mediated cell-cell adhesion[12]; the GSC fusome, an organelle with a membranous-like structure, is juxtaposed to the interface between cap cell and GSC[13]. GSC division gives rise to a cystoblast, which subsequently undergoes four rounds of incomplete division to form a 16-cell cyst, in which the cells are interconnected with branched fusomes[9]. The 16-cell cyst is then surrounded by a layer of follicle cells, and eventually develops into a mature egg. The Notch signaling pathway is highly conserved, and plays critical roles in the regulation of stem cells in different systems[14],[15]. In Drosophila, it controls the maintenance of niche cap cells[16],[17]; however, whether it plays a role in GSCs is not clear. Drosophila has one Notch receptor( encoded by Notch, N), which is a single-pass transmembrane protein. Upon binding to one of its ligands, the cleaved intracellular domain of Notch translocates to the nucleus, where it regulates transcription of gene targets. The extracellular domain of Notch is composed of several epidermal growth factor( EGF)-like repeats, which must be highly glycosylated for proper function of the protein[18]. Fringe( encoded by fng) is a glycosyltransferase that adds N-acetylglucosamine onto this domain, thereby modulating the binding of Notch to its ligands. The insulin/insulin-like growth factor( IGF) signaling pathway is also evolutionarily conserved; this pathway controls various stem cell types in response to nutritional input and aging[8],[19]–[22]. We have previously shown that insulin signaling controls Fringe-mediated Notch activation in the GSC niche, and this is required to maintain niche integrity, thereby supporting the retention of GSCs in the niche[23],[24]. In addition, insulin signaling also directly controls GSC proliferation[25],[26], but it is not clear if Notch signaling is involved in this process. In this article, we demonstrate that insulin and Notch signaling have independent roles in regulating GSCs, in contrast to the earlier observation that insulin signaling regulates Notch activation in niche cells to support their maintenance[23],[24]. While insulin/IGF signaling is required for GSC proliferation and maintenance, Notch signaling underlies the diminished ability of GSCs to occupy the niche during aging. In young ovaries, Notch signaling in GSCs is low, allowing them to persist in the niche; as ovaries age, Notch signaling in GSCs is elevated, resulting in the loss of GSCs from the niche. In contrast to GSCs, Notch signaling in niche cells( which is required for niche integrity) is decreased with age, thus contributing to age-induced GSC loss. We therefore demonstrate that both intrinsic and extrinsic Notch signaling control the niche residency of GSCs during aging.
Discussion:
Notch signaling plays critical roles in stem cell maintenance, differentiation, and aging[15]; however, the mechanisms by which Notch signaling controls stem cell aging remain unclear. It has been shown that Notch signaling controls myogenesis and neurogenesis via the control of skeleton muscle stem cells and neural progenitors; however, a reduction in Notch signaling impairs these stem cell functions during aging[41],[42]. Unlike muscle stem cells and neural progenitors, we found that Notch signaling is up-regulated in GSCs with age, and this suppresses GSC adhesion to the niche. Increased Notch signaling activity is also observed in mesenchymal stem cells( MSCs) of patients with Hutchinson-Gilford Progeria Syndrome( HGPS), a premature-aging disease; this increase is caused by abnormal activation of Lamin A, which induces nuclear defects and DNA damage[43],[44]. Increased Notch activation in MSCs forces their differentiation toward osteoblasts, but prevents their differentiation into adipocytes. It is possible that GSCs and MSCs share similar gene expression profiles, which are required for activation of Notch signaling in response to aging; this is supported by the detection of germinal transcripts in MSCs[45]. The number of stem cells gradually decline during aging[32], suggesting that the remaining GSCs in the niche are more competitive than those that are lost. It has been proposed that under normal conditions, cycling tissue stem cells behave as an equipotent population, in which the balance between differentiation and proliferation is achieved by frequent and stochastic stem cell loss and replacement( a process termed neutral competition), thereby maintaining tissue homeostasis[46],[47]. However, this balance is disrupted during aging, and, assuming that stem cell loss rate is higher than the speed of replacement, this results in stem cell loss. Here, we argue that aging induces the differences among stem cells in the niche which affect niche residency. For example, aged GSCs in the niche accumulate different levels of DNA damage[48], which may result from the accumulation of excessive reactive oxygen species( ROS); this may ultimately lead to GSC loss[32]. In addition, we also observed that GSCs in the niche exhibited various levels of enhanced Notch signaling( S11 Fig.); this suggests that GSCs with higher levels of Notch signaling activity compared to their counterparts in the niche will be lost earlier, because of decreased GSC-niche attachment. Intriguingly, ROS have the ability to activate Notch signaling via the activation of metalloprotease ADAM17, which triggers the release of NICD[49]. Taken together, Notch signaling might serve as a quality control to removal “poor” GSCs from the niche. Notch signaling is used for cell-fate determination and appropriate stem cell function in several different contexts, while the manner in which it is regulated is different between the two sexes. This arises because Sxl, a female-specific RNA-binding protein, negatively regulates Notch signaling during larval and follicular epithelium development, by binding to N mRNA to control Notch translation[36]. In this study, we show that Sxl maintains low levels of Notch signaling in young female GSCs, and elimination of Sxl in GSCs elevates Notch signaling, triggering early GSC loss. In addition, the pattern of Sxl expression and Notch signaling activity in the germ line is reciprocal( see Fig. 6G and S1A Fig.); Sxl is strongly expressed in GSCs and cytoblasts, but weakly in differentiating germ cell cysts, while Notch signaling activity is low in GSCs and cystoblasts, but high in germ cell cysts. These results suggest that Sxl may regulate Notch signaling in female GSCs through a post-transcriptional process present in other cell types. Notably, Notch signaling activities were similar between male GSCs and their progeny( S12 Fig.); this raises the question, ‘how does Sxl modulation of Notch signaling shape the differences between male and female GSCs? ’ In addition, it is not clear how Sxl expression is decreased in cells that require higher Notch signaling, such as aged GSCs. It has been shown that transcription of sxl is directly regulated by JAK/STAT signaling during embryogenesis[50]. However, unlike Sxl, JAK/STAT signaling is not required for female GSC maintenance[2],[3]. In a study by Vied et al.[4], Hh signaling was observed to suppress the function of Sxl in the female germline; however, Hh signaling activity is decreased during aging[51]. These results imply that Sxl expression in GSCs is controlled by neither JAK/STAT nor Hh signaling. Although Sxl is a female-specific protein, sxl transcripts with the third exon are present in males; these transcripts encode a non-functional Sxl protein with an early stop codon[52]. During development of females, sxl is maintained in the on-state via an auto-regulatory feedback loop[53]. In this loop, female Sxl proteins promote their own expression by directing the splicing machinery to skip the third exon of transcripts derived from the constitutive sxl late promoter, which is active in both males and females. Aging is frequently associated with down-regulation of pre-mRNA processing factors that are required for alternative splicing[54], raising the possibility that the decrease in Sxl expression in GSCs with age is due to reduced efficiency of splicing. Nevertheless, further investigation is required to pinpoint the mechanism behind this phenomenon. We observed that Notch signaling in the niche and GSCs was affected by age in different ways; during aging, Notch signaling was decreased in the niche, but increased in GSCs. A subset of niche cap cells make direct contact with GSCs[24], raising the possibility that Notch signaling in niche cells may affect Notch signaling in GSCs, and vice versa. However, disruption of Notch signaling in GSCs did not affect Notch signaling in niche cap cells( see S2 Fig.), and knock down of N or Dl in niche cap cells did not affect Notch signaling in GSCs( S13 Fig.). In addition, our mosaic clonal analysis revealed that within Nloss of function heterozygous mutant niches, Nloss of function homozygous mutant GSCs( lacking GFP) exhibit higher levels of Notch signaling as compared to their neighboring control GSCs( most of which will be heterozygous for Nloss of function alleles, as the recombination efficiency between two in trans FRT19 loci is less than 30%), indicating that the increase of Notch signaling in GSCs occurs in a cell-autonomous manner. Similar conclusions were reached through GSC clonal analysis using the Ngain of function allele. Further, induction of Notch signaling in GSCs enhances GSC loss, and based on our microarray analysis of bam mutant GSCs distal from the niche; we conclude that the regulation of E-cadherin by Notch signaling in GSCs is performed in a cell-autonomous manner. These results also suggest that the mechanisms that regulate Notch signaling in niche cap cells and GSCs are independent of one another. Indeed, while Notch signaling in GSCs is controlled by Sxl, knock down of sxl expression in niche cap cells did not affect Notch activation( S13 Fig.). Furthermore, we previously demonstrated that Fringe( which adds sugar to Notch) is required for Notch activation in niche cap cells[55]), while excessive fng( induced by Foxo under insulin insufficiency) suppresses Notch activation. Here, we observed that N and fng mutant GSCs exhibit similar behavior; however, Notch signaling is not decreased in insulin signaling-defective GSCs( S14 Fig.), suggesting that Notch signaling is differentially regulated between GSCs and their niche.
Aging is frequently associated with a decline in the size of stem cell pools , but little is known regarding the molecular mechanisms underlying this process . Here , we report that Notch signaling is increased in GSCs as they age , and this promotes their removal from the niche in an E-cadherin dependent manner . In contrast to GSCs , niche cells exhibit decreased Notch signaling with age; Notch signaling in these cells controls niche integrity , and consequently GSC retention . While Notch signaling in the niche is regulated by insulin signaling , Notch signaling in GSCs is controlled by Sex lethal , an RNA-binding protein . These results imply that Notch signaling is regulated in a cell-type-dependent manner , and coordination between GSCs and their niche facilitates the removal of cells from the niche during the aging process .