Age-related changes of germline stem cell activity, niche signaling activity and egg production in Drosophila

Adult stem cells are important in replenishing aged cells to maintain tissue homeostasis. Aging in turn may exert profound effects on stem cell's regenerative potential, but to date the mechanisms of such stem cell aging are poorly understood, and it is not clear to what extent stem cell aging contributes to tissue or organ aging. Here we show in female Drosophila that germline stem cell (GSC) division rate progressively declines with age, which is accompanied by reduced decapentaplegic (dpp) niche signaling pathway activation within GSCs. Egg production also rapidly declines with age, which is accompanied by both decreased stem cell division and increased incidence of cell death of developing eggs, especially in the oldest females. Genetically increasing dpp expression delays GSC activity decline and transiently increases egg production. We conclude that age-related decline of reproduction is caused by both decreased GSC activity and increased incidence of cell death during oogenesis, while decreased GSC activity is attributed to declined signaling from the regulatory niche. We suggest that niche functional decay may be an important mechanism for stem cell aging and system failure.     

Adipocyte lineage cells contribute to the skin stem cell niche to drive hair cycling

In mammalian skin, multiple types of resident cells are required to create a functional tissue and support tissue homeostasis and regeneration. The cells that compose the epithelial stem cell niche for skin homeostasis and regeneration are not well defined. Here, we identify adipose precursor cells within the skin and demonstrate that their dynamic regeneration parallels the activation of skin stem cells. Functional analysis of adipocyte lineage cells in mice with defects in adipogenesis and in transplantation experiments revealed that intradermal adipocyte lineage cells are necessary and sufficient to drive follicular stem cell activation. Furthermore, we implicate PDGF expression by immature adipocyte cells in the regulation of follicular stem cell activity. These data highlight adipogenic cells as skin niche cells that positively regulate skin stem cell activity, and suggest that adipocyte lineage cells may alter epithelial stem cell function clinically.     

Stem cell niches in mammals

Stem cells safeguard tissue homeostasis and guarantee tissue repair throughout life. The decision between self-renewal and differentiation is influenced by a specialized microenvironment called stem cell niche. Physical and molecular interactions with niche cells and orientation of the cleavage plane during stem cell mitosis control the balance between symmetric and asymmetric division of stem cells. Here we highlight recent progress made on the anatomical and molecular characterization of mammalian stem cell niches, focusing particularly on bone marrow, tooth and hair follicle. The knowledge of the regulation of stem cells within their niches in health and disease will be instrumental to develop novel therapies that target stem cell niches to achieve tissue repair and re-establish tissue homeostasis.     

Efficiency of spermatogonial dedifferentiation during aging

Background

Adult stem cells are critical for tissue homeostasis; therefore, the mechanisms utilized to maintain an adequate stem cell pool are important for the survival of an individual. In Drosophila, one mechanism utilized to replace lost germline stem cells (GSCs) is dedifferentiation of early progenitor cells. However, the average number of male GSCs decreases with age, suggesting that stem cell replacement may become compromised in older flies.

Methodology/Principal Findings

Using a temperature sensitive allelic combination of Stat92E to control dedifferentiation, we found that germline dedifferentiation is remarkably efficient in older males; somatic cells are also effectively replaced. Surprisingly, although the number of somatic cyst cells also declines with age, the proliferation rate of early somatic cells, including cyst stem cells (CySCs) increases.

Conclusions

These data indicate that defects in spermatogonial dedifferentiation are not likely to contribute significantly to an aging-related decline in GSCs. In addition, our findings highlight differences in the ways GSCs and CySCs age. Strategies to initiate or enhance the ability of endogenous, differentiating progenitor cells to replace lost stem cells could provide a powerful and novel strategy for maintaining tissue homeostasis and an alternative to tissue replacement therapy in older individuals.     

String (Cdc25) regulates stem cell maintenance, proliferation and aging in Drosophila testis

Tight regulation of stem cell proliferation is fundamental to tissue homeostasis, aging and tumor suppression. Although stem cells are characterized by their high potential to proliferate throughout the life of the organism, the mechanisms that regulate the cell cycle of stem cells remain poorly understood. Here, we show that the Cdc25 homolog String (Stg) is a crucial regulator of germline stem cells (GSCs) and cyst stem cells (CySCs) in Drosophila testis. Through knockdown and overexpression experiments, we show that Stg is required for stem cell maintenance and that a decline in its expression during aging is a critical determinant of age-associated decline in stem cell function. Furthermore, we show that restoration of Stg expression reverses the age-associated decline in stem cell function but leads to late-onset tumors. We propose that Stg/Cdc25 is a crucial regulator of stem cell function during tissue homeostasis and aging.     

The stem cell niche in regenerative medicine

Stem cells are fundamental units for achieving regenerative therapies, which leads naturally to a theoretical and experimental focus on these cells for therapeutic screening and intervention. A growing body of data in many tissue systems indicates that stem cell function is critically influenced by extrinsic signals derived from the microenvironment, or "niche." In this vein, the stem cell niche represents a significant, and largely untapped, entry point for therapeutic modulation of stem cell behavior. This Perspective will discuss how the niche influences stem cells in homeostasis, in the progression of degenerative and malignant diseases, and in therapeutic strategies for tissue repair.     

Anchoring stem cells in the niche by cell adhesion molecules

Adult stem cells generally reside in supporting local micro environments or niches, and intimate stem cell and niche association is critical for their long-term maintenance and function. Recent studies in model organisms especially Drosophila have started to unveil the underlying mechanisms of stem anchorage in the niche at the molecular and cellular level. Two types of cell adhesion molecules are emerging as essential players: cadherin-mediated cell adhesion for keeping stem cells within stromal niches, whereas integrin-mediated cell adhesion for keeping stem cells within epidermal niches. Further understanding stem cell anchorage and release in coupling with environmental changes should provide further insights into homeostasis control in tissues that harbor stem cells.Key words: stem cell, niche, anchorage, cell adhesion, extracellular matrix, cadherin, integrinTissue-specific adult stem cells are characterized by their prolonged self-renewal ability and potentiality to differentiate into one or more types of mature cells. These unique properties make stem cells essential for maintaining tissue homeostasis throughout life. It is generally believed that all adult stem cells reside in specific microenvironments named niches, which provide physical support and produce critical signals to maintain stem cell identity and govern their behavior.^1–4 Consequently, intimate stem cell and niche association is a pre-requisite for stem cell''s long-term maintenance and function. How stem cells are kept within the niche is thus an important issue in stem cell biology. Characterization of a number of stem cell niches in model organisms has led to the classification of niches into two general types: stromal niches where stem cells have direct membrane contact with the niche cells and epidermal niches where stem cells are usually associated with the extracellular matrix (ECM), and do not directly contact any fixed stromal cells.¹ Studies in Drosophila have led to the cellular and functional verification of the stem cell niche theory^5,6 and not surprisingly, have also led to the discovery of the molecular mechanisms anchoring stem cells to the niche. Here I consider recent studies in Drosophila on types of cell adhesions used to anchor stem cells in the niches, and summarize cell adhesion molecules utilized in the most characterized niches in the mammalian tissues, and suggest that cadherin-mediated cell-to-cell adhesion and integrin-mediated cell-to-ECM adhesion are possibly two general mechanisms that function in respective stromal or epidermal niches for stem cell anchorage in diverse organisms.     

Heterocellular molecular contacts in the mammalian stem cell niche

Adult tissue homeostasis and repair relies on prompt and appropriate intervention by tissue-specific adult stem cells (SCs). SCs have the ability to self-renew; upon appropriate stimulation, they proliferate and give rise to specialized cells. An array of environmental signals is important for maintenance of the SC pool and SC survival, behavior, and fate. Within this special microenvironment, commonly known as the stem cell niche (SCN), SC behavior and fate are regulated by soluble molecules and direct molecular contacts via adhesion molecules providing connections to local supporting cells and the extracellular matrix. Besides the extensively discussed array of soluble molecules, the expression of adhesion molecules and molecular contacts is another fundamental mechanism regulating niche occupancy and SC mobilization upon activation. Some adhesion molecules are differentially expressed and have tissue-specific consequences, likely reflecting the structural differences in niche composition and design, especially the presence or absence of a stromal counterpart. However, the distribution and identity of intercellular molecular contacts for adhesion and adhesion-mediated signaling within stromal and non-stromal SCN have not been thoroughly studied. This review highlights common details or significant differences in cell-to-cell contacts within representative stromal and non-stromal niches that could unveil new standpoints for stem cell biology and therapy.     

Stem cell aging is controlled both intrinsically and extrinsically in the Drosophila ovary

It is widely postulated that tissue aging could be, at least partially, caused by reduction of stem cell number, activity, or both. However, the mechanisms of controlling stem cell aging remain largely a mystery. Here, we use Drosophila ovarian germline stem cells (GSCs) as a model to demonstrate that age-dependent decline in the functions of stem cells and their niche contributes to overall stem cell aging. BMP signaling activity from the niche significantly decreases with age, and increasing BMP signaling can prolong GSC life span and promote their proliferation. In addition, the age-dependent E-cadherin decline in the stem cell-niche junction also contributes to stem cell aging. Finally, overexpression of SOD, an enzyme that helps eliminate free oxygen species, in either GSCs or their niche alone can prolong GSC life span and increase GSC proliferation. Therefore, this study demonstrates that stem cell aging is controlled extrinsically and intrinsically in the Drosophila ovary.     

Polarity in Stem Cell Division: Asymmetric Stem Cell Division in Tissue Homeostasis

Many adult stem cells divide asymmetrically to balance self-renewal and differentiation, thereby maintaining tissue homeostasis. Asymmetric stem cell divisions depend on asymmetric cell architecture (i.e., cell polarity) within the cell and/or the cellular environment. In particular, as residents of the tissues they sustain, stem cells are inevitably placed in the context of the tissue architecture. Indeed, many stem cells are polarized within their microenvironment, or the stem cell niche, and their asymmetric division relies on their relationship with the microenvironment. Here, we review asymmetric stem cell divisions in the context of the stem cell niche with a focus on Drosophila germ line stem cells, where the nature of niche-dependent asymmetric stem cell division is well characterized.Asymmetric cell division allows stem cells to self-renew and produce another cell that undergoes differentiation, thus providing a simple method for tissue homeostasis. Stem cell self-renewal refers to the daughter(s) of stem cell division maintaining all stem cell characteristics, including proliferation capacity, maintenance of the undifferentiated state, and the capability to produce daughter cells that undergo differentiation. A failure to maintain the correct stem cell number has been speculated to lead to tumorigenesis/tissue hyperplasia via stem cell hyperproliferation or tissue degeneration/aging via a reduction in stem cell number or activity (Morrison and Kimble 2006; Rando 2006). This necessity changes during development. The stem cell pool requires expansion earlier in development, whereas maintenance is needed later to sustain tissue homeostasis.There are two major mechanisms to sustain a fixed number of adult stem cells: stem cell niche and asymmetric stem cell division, which are not mutually exclusive. Stem cell niche is a microenvironment in which stem cells reside, and provides essential signals required for stem cell identity (Fig. 1A). Physical limitation of niche “space” can therefore define stem cell number within a tissue. Within such a niche, many stem cells divide asymmetrically, giving rise to one stem cell and one differentiating cell, by placing one daughter inside and another outside of the niche, respectively (Fig. 1A). Nevertheless, some stem cells divide asymmetrically, apparently without the niche. For example, in Drosophila neuroblasts, cell-intrinsic fate determinants are polarized within a dividing cell, and subsequent partitioning of such fate determinants into daughter cells in an asymmetric manner results in asymmetric stem cell division (Fig. 1B) (see Fig. 3A and Prehoda 2009).Open in a separate windowFigure 1.Mechanisms of asymmetric stem cell division. (A) Asymmetric stem cell division by extrinsic fate determinants (i.e., the stem cell niche). The two daughters of stem cell division will be placed in distinct cellular environments either inside or outside the stem cell niche, leading to asymmetric fate choice. (B) Asymmetric stem cell division by intrinsic fate determinants. Fate determinants are polarized in the dividing stem cells, which are subsequently partitioned into two daughter cells unequally, thus making the division asymmetrical. Self-renewing (red line) and/or differentiation promoting (green line) factors may be involved.In this review, we focus primarily on asymmetric stem cell divisions in the Drosophila germ line as the most intensively studied examples of niche-dependent asymmetric stem cell division. We also discuss some examples of stem cell division outside Drosophila, where stem cells are known to divide asymmetrically or in a niche-dependent manner.     

The Drosophila ovary: an active stem cell community

Only a small number of cells in adult tissues (the stem cells) possess the ability to self-renew at every cell division,while producing differentiating daughter cells to maintain tissue homeostasis for an organism's lifetime.The Drosophilaovary harbors three different types of stem cell populations (germline stem cell (GSC),somatic stem cell (SSC) andescort stem cell (ESC)) located in a simple anatomical structure known as germarium,rendering it one of the best modelsystems for studying stem cell biology due to reliable stem cell identification and available sophisticated genetic toolsfor manipulating gene functions.Particularly,the niche for the GSC is among the first and best studied ones,and studieson the GSC and its niche have made many unique contributions to a better understanding of relationships between stemcells and their niche.So far,both the GSC and the SSC have been shown to be regulated by extrinsic factors originatingfrom their niche and intrinsic factors functioning within.Multiple signaling pathways are required for controlling GSCand SSC self-renewal and differentiation,which provide unique opportunities to investigate how multiple signals fromthe niche are interpreted in the stem cell.Since the Drosophila ovary contains three types of stem cells,it also providesoutstanding opportunities to study how multiple stem cells in a given tissue work collaboratively to contribute to tissuefunction and maintenance.This review highlights recent major advances in studying Drosophila ovarian stem cells andalso discusses future directions and challenges.     

Stem cell dynamics in response to nutrient availability

When nutrient availability becomes limited, animals must actively adjust their metabolism to allocate limited resources and maintain tissue homeostasis. However, it is poorly understood how tissues maintained by adult stem cells respond to chronic changes in metabolism. To begin to address this question, we fed flies a diet lacking protein (protein starvation) and assayed both germline and intestinal stem cells. Our results revealed a decrease in stem cell proliferation and a reduction in stem cell number; however, a small pool of active stem cells remained. Upon refeeding, stem cell number increased dramatically, indicating that the remaining stem cells are competent to respond quickly to changes in nutritional status. Stem cell maintenance is critically dependent upon intrinsic and extrinsic factors that act to regulate stem cell behavior. Activation of the insulin/IGF signaling pathway in stem cells and adjacent support cells in the germline was sufficient to suppress stem cell loss during starvation. Therefore, our data indicate that stem cells can directly sense changes in the systemic environment to coordinate their behavior with the nutritional status of the animal, providing a paradigm for maintaining tissue homeostasis under metabolic stress.     

Age-dependent changes in the subcallosal zone neurogenesis of mice

There are several known neurogenic areas including subventricular zone and subgranular layer in the dentate gyrus of the hippocampus. Both germinal centers exhibit an age-dependent decline in cell proliferation and neurogenesis, which may be associated with age-related decline in brain function. We recently identified the subcallosal zone (SCZ) as a novel neural stem cell niche with a potential to spontaneously produce new neuroblasts. We examined whether SCZ neurogenesis is also regulated by the age of mice. The number of newly generated neuroblasts was reduced in the SCZ with age, and only marginal number of DCX-labeled neuroblasts was found in 6-month-old SCZ, which is most likely due to reduced proliferation of progenitor cells and loss of neural stem cells (NSCs). This age-dependent changes in the SCZ occurred earlier than that of other neurogenic brain regions. The neurosphere assay in vitro confirmed the depletion of NSCs within the SCZ of young adults. However, marked induction of neuroblast production in the SCZ was seen in 6-month-old mice after traumatic brain injury. Taken together, these results indicate that a rapid decline in SCZ neurogenesis in mice is due to depletion of NSCs and reduced capacity to produce neuroblasts.     

The Fate of Spermatogonial Stem Cells in the Cryptorchid Testes of RXFP2 Deficient Mice

The environmental niche of the spermatogonial stem cell pool is critical to ensure the continued generation of the germ cell population. To study the consequences of an aberrant testicular environment in cryptorchidism we used a mouse model with a deletion of Rxfp2 gene resulting in a high intra-abdominal testicular position. Mutant males were infertile with the gross morphology of the cryptorchid testis progressively deteriorating with age. Few spermatogonia were identifiable in 12 month old cryptorchid testes. Gene expression analysis showed no difference between mutant and control testes at postnatal day 10. In three month old males a decrease in expression of spermatogonial stem cell (SSC) markers Id4, Nanos2, and Ret was shown. The direct counting of ID4+ cells supported a significant decrease of SSCs. In contrast, the expression of Plzf, a marker for undifferentiated and differentiating spermatogonia was not reduced, and the number of PLZF+ cells in the cryptorchid testis was higher in three month old testes, but equal to control in six month old mutants. The PLZF+ cells did not show a higher rate of apoptosis in cryptorchid testis. The expression of the Sertoli cell FGF2 gene required for SSC maintenance was significantly reduced in mutant testis. Based on these findings we propose that the deregulation of somatic and germ cell genes in the cryptorchid testis, directs the SSCs towards the differentiation pathway. This leads to a depletion of the SSC pool and an increase in the number of PLZF+ spermatogonial cells, which too, eventually decreases with the exhaustion of the stem cell pool. Such a dynamic suggests that an early correction of cryptorchidism is critical for the retention of the SSC pool.     

精原干细胞niche的研究进展

精原干细胞(spermatogonial stem cells,SSCs)是指睾丸内位于曲精细管基膜上既能自我更新维持自身适量恒定,又能定向分化产生精母细胞的一类原始精原细胞。随着干细胞深入的研究,人们发现了一种控制着干细胞可塑性与命运的微环境,此微环境被称为干细胞niche,干细胞niche由niche细胞、细胞外基质、细胞因子等构成。精原干细胞niche是由黏附因子、生长因子、支持细胞、间质细胞以及小管周肌肉细胞组成。大量的研究表明支持细胞在睾丸中是主要的成体细胞,通过分泌可溶性的因子来影响精原干细胞niche的结构与功能,同时支持细胞还能够间接的影响其他的成体细胞。随着年龄的增长使得精原干细胞niche的功能下降。精原干细胞数量以及精原干细胞niche为我们研究组织特异性干细胞生物学以及保持再生组织平衡提供了很宝贵的线索,精原干细胞对于保持组织的自我更新具有很重要的作用,并且受到人们大量的关注,然而精原干细胞niche也起到很重要的作用,它为治疗一些疾病提供新途径.本文将综述精原干细胞niche及其变化对精原干细胞功能调节的相关研究进展。     

Autophagy in stem cell aging

Aging is responsible for changes in mammalian tissues that result in an imbalance to tissue homeostasis and a decline in the regeneration capacity of organs due to stem cell exhaustion. Autophagy is a constitutive pathway necessary to degrade damaged organelles and protein aggregates. Autophagy is one of the hallmarks of aging, which involves a decline in the number and functionality of stem cells. Recent studies show that stem cells require autophagy to get rid of cellular waste produced during the quiescent stage. In particular, two independent studies in muscle and hematopoietic stem cells demonstrate the relevance of the autophagy impairment for stem cell exhaustion and aging. In this review, we summarize the main results of these works, which helped to elucidate the impact of autophagy in stem cell activity as well as in age‐associated diseases.     

Cell–cell contact and signaling in the muscle stem cell niche

Muscle stem cells (also called satellite cells or SCs) rely on their local niche for regulatory signals during homeostasis and regeneration. While a number of cell types communicate indirectly through secreted factors, here we focus on the significance of direct contact between SCs and their neighbors. During quiescence, SCs reside under a basal lamina and receive quiescence-promoting signals from their adjacent skeletal myofibers. Upon injury, the composition of the niche changes substantially, enabling the formation of new contacts that mediate proliferation, self-renewal, and differentiation. In this review, we summarize the latest work in understanding cell–cell contact within the satellite cell niche and highlight areas of open questions for future studies.     

Why Adult Stem Cell Functionality Declines with Age? Studies from the Fruit Fly Drosophila Melanogaster Model Organism

Highly regenerative adult tissues are supported by rare populations of stem cells that continuously divide to self-renew and generate differentiated progeny. This process is tightly regulated by signals emanating from surrounding cells to fulfill the dynamic demands of the tissue. One of the hallmarks of aging is slow and aberrant tissue regeneration due to deteriorated function of stem and supporting cells. Several Drosophila regenerative tissues are unique in that they provide exact identification of stem and neighboring cells in whole-tissue anatomy. This allows for precise tracking of age-related changes as well as their targeted manipulation within the tissue. In this review we present the stem cell niche of Drosophila testis, ovary and intestine and describe the major changes and phenotypes that occur in the course of aging. Specifically we discuss changes in both intrinsic properties of stem cells and their microenvironment that contribute to the decline in tissue functionality. Understanding these mechanisms in adult Drosophila tissues will likely provide new paradigms in the field of aging.     

Aging shifts mitochondrial dynamics toward fission to promote germline stem cell loss

Changes in mitochondrial dynamics (fusion and fission) are known to occur during stem cell differentiation; however, the role of this phenomenon in tissue aging remains unclear. Here, we report that mitochondrial dynamics are shifted toward fission during aging of Drosophila ovarian germline stem cells (GSCs), and this shift contributes to aging‐related GSC loss. We found that as GSCs age, mitochondrial fragmentation and expression of the mitochondrial fission regulator, Dynamin‐related protein (Drp1), are both increased, while mitochondrial membrane potential is reduced. Moreover, preventing mitochondrial fusion in GSCs results in highly fragmented depolarized mitochondria, decreased BMP stemness signaling, impaired fatty acid metabolism, and GSC loss. Conversely, forcing mitochondrial elongation promotes GSC attachment to the niche. Importantly, maintenance of aging GSCs can be enhanced by suppressing Drp1 expression to prevent mitochondrial fission or treating with rapamycin, which is known to promote autophagy via TOR inhibition. Overall, our results show that mitochondrial dynamics are altered during physiological aging, affecting stem cell homeostasis via coordinated changes in stemness signaling, niche contact, and cellular metabolism. Such effects may also be highly relevant to other stem cell types and aging‐induced tissue degeneration.