Stem cells: keeping BMP signaling local

In stem cell niches, the spatial extent of growth factor signaling needs to be tightly controlled. A new study on the Drosophila testicular germline stem cell niche has revealed that BMP signaling is locally activated through linkage to adherens junctions.     

涝渍害对稻飞虱及其节肢类天敌生态位关系的影响

尝试将生态位理论应用到分析环境条件变异和人类干扰活动对天敌和猎物关系的影响，通过与非涝渍稻田对比分析，发现涝渍害综合作用对稻飞虱及其节肢类天敌的时间生态位，空间生态位以及时间一空间生态位均产生深刻的影响，涝渍害的作用使节肢类天敌和稻飞虱空间生态位宽度普遍缩小，使涝渍地各物种间空间生态位重叠亦普遍减小；使除稻红瓢虫和球腹蛛外的天敌时间生态位宽度扩大；各天敌和中性昆虫与飞虱的空间一时间生态位重叠亦普遍缩小，还讨论了这种影响的机制及改良的生态对策。     

Uncertainty in the niches that maintain haematopoietic stem cells

Haematopoietic stem cell (HSC) niches are specialized microenvironments that contain stem cells and regulate their maintenance. Cells at the interface of bone and the bone marrow (the endosteum) contribute to the creation of HSC niches. It remains uncertain whether this interface itself is a niche, or whether endosteal cells secrete factors that diffuse to nearby niches. Vascular and/or perivascular cells may also create niches as many HSCs are observed around sinusoidal blood vessels, and perivascular cells secrete factors that regulate HSC maintenance. Do endosteal and perivascular cells create distinct niches, or do they contribute to a common niche? We discuss a range of niche models consistent with recent evidence.     

干细胞巢研究进展

干细胞巢即干细胞周围的微环境构成,一般包括干细胞的相邻细胞、粘附分子及基质等,但不同的干细胞有不同的巢结构。干细胞巢通过不同信号途径调控着干细胞的行为,使干细胞的自我更新和分化处于平衡状态。根据近年来有关干细胞巢的研究,本文从果蝇生殖系干细胞巢、哺乳动物造血干细胞巢、肠干细胞巢、毛囊表皮干细胞巢和神经干细胞巢等五个系统分别综述了干细胞巢的构成及其对干细胞的调节作用,探讨了干细胞巢作用于干细胞的内在机制。     

Rap-GEF signaling controls stem cell anchoring to their niche through regulating DE-cadherin-mediated cell adhesion in the Drosophila testis

Stem cells will undergo self-renewal to produce new stem cells if they are maintained in their niches. The regulatory mechanisms that recruit and maintain stem cells in their niches are not well understood. In Drosophila testes, a group of 12 nondividing somatic cells, called the hub, identifies the stem cell niche by producing the growth factor Unpaired (Upd). Here, we show that Rap-GEF/Rap signaling controls stem cell anchoring to the niche through regulating DE-cadherin-mediated cell adhesion. Loss of function of a Drosophila Rap-GEF (Gef26) results in loss of both germline and somatic stem cells. The Gef26 mutation specifically impairs adherens junctions at the hub-stem cell interface, which results in the stem cells "drifting away" from the niche and losing stem cell identity. Thus, the Rap signaling/E-cadherin pathway may represent one mechanism that regulates polarized niche formation and stem cell anchoring.     

High-throughput approaches for the analysis of extrinsic regulators of stem cell fate

The complexity of stem cell niches poses a tremendous challenge to understanding mechanisms of extrinsic regulation of stem cell fate. In order to better understand niche signaling and its effect on stem cell fate choices, in vitro systems are being engineered which recapitulate, in a simplistic but increasingly sophisticated manner, native stem cell niches. New technologies or new combinations of existing technologies allow more systematic ways to probe niche signaling in high-throughput. Systems biology approaches in experimental design, data acquisition and analysis will be necessary to tackle the challenges that lie ahead.     

水稻纹枯病时空生态位施药干扰研究

以施药作为干扰因子,研究干扰条件下水稻纹枯病菌的时间和空间生态位变化.结果表明,各施药处理与对照纹枯病菌的空间生态位存在差异,其中孕穗期施药+齐穗期施药和孕穗期施药+灌浆期施药两处理的空间生态位宽度指数分别为0.520、0.572,与对照的空间生态位宽度指数0.8563相差最大;分蘖盛期一次施药处理的空间生态位宽度指数为0.8577,与对照相差最小.各施药处理和对照的时间生态位宽度无显著差异,表明施药对纹枯病菌的时间生态位宽度影响不大.空间生态位宽度指数、病叶(鞘)率、病情指数及防治效果等计测结果表明,两次施药对病害的控制好于一次施药,其中两次施药中以孕穗期施药+齐穗期施药和孕穗期施药+灌浆期施药对病害的控制作用最强.施药不但压缩了水稻纹枯病菌的空间生态位宽度,限制了其在产量形成较为重要的上部叶位的扩张,而且压缩了水稻纹枯病菌的时间生态位宽度,限制了其在水稻产量形成时期的扩张.但施药只改变了病菌生态位的分布,调整压缩了水稻产量形成期的病害生态位,病菌生态位势的总和不变.     

Stem cell niches

The nature of the stem cell niche and its interaction with stem cells is one of fundamental problems in the biology of stem cells. Stem cell niches are formed during ontogeny. A niche can remain vacant and exist independently of stem cells; however, stem cell self-renewal cannot be maintained for long periods outside of the niche except for particular conditions, e.g., in vitro. A vacant niche can be occupied by excessive or transplanted stem cells and can provide for their functioning. A niche size allows a definite number of stem cells to be maintained. Excessive stem cells either differentiate in the presence of specific signal(s) or undergo apoptosis in the absence of such signal. Thus, the niches control the number of stem cells in the body and protect it from excessive stem cell proliferation. Under particular conditions, stem cells can leave and return to their niches. Stem cells are retained in the niche by cell-to-cell interactions and adhesion to the extracellular matrix. Both the niches and stem cells arise at a particular ontogenetic stage and are capable of long self-renewal. The development can be described in terms of the formation of stem cells and their niches.     

Location, location, location: the cancer stem cell niche

The existence of a stem cell niche, or physiological microenvironment, consisting of specialized cells that directly and indirectly participate in stem cell regulation has been verified for mammalian adult stem cells in the intestinal, neural, epidermal, and hematopoietic systems. In light of these findings, it has been proposed that a "cancer stem cell niche" also exists and that interactions with this tumor niche may specify a self-renewing population of tumor cells. We discuss emerging data that support the idea of a veritable cancer stem cell niche and propose several models for the relationship between cancer cells and their niches.     

Temporal variability of ecological niches: a study on intertidal macrobenthic fauna

The determination of temporal niche dynamics under field conditions is an important component of a species’ ecology. Recent developments in niche mapping, and the possibility to account for spatial autocorrelation in species distributions, hold promise for the statistical approach explored here. Using species counts from a landscape‐scale benthic monitoring programme in the western Dutch Wadden Sea during 1997–2005 in combination with sediment characteristics and tidal height as explanatory variables, we statistically derive realised niches for two bivalves, two crustaceans and three polychaetes, encompassing predators, suspension and bottom feeding functional groups. Unsurprisingly, realized niches varied considerably between species. Intraspecific temporal variation was assessed as overlap between the year‐specific niche and the overall mean niche, and this analysis revealed considerable variation between years. The main functional groups represented by these species showed idiosyncratic and wide variability through the study period. There were no strong associations between niche characteristics and mean abundance or body size. Our assessment of intraspecific niche variability has ramifications for species distribution models in general and offers advances from previous methods. 1) By assessing species’ realized niches in the multivariate environmental space, analyses are independent from the relative availability of particular environments. Predicted realized niches present differences between years, rather than annual differences in environmental conditions. 2) Using spatially explicit models to predict species habitat preferences provide more precise and unbiased estimates of species–environment relationships. 3) Current niche models assume constant niches, whereas we illustrate how much these can vary over only a few generations. This emphasizes the potentially limited scope of global change studies with forecasts based on single‐time species distribution snapshots.     

An epithelial niche in the Drosophila ovary undergoes long-range stem cell replacement

Adult epithelial stem cells are thought to reside in specific niches, where they are maintained by adhesion to stromal cells and by intercellular signals. In niches that harbor multiple adjacent stem cells, such as those maintaining Drosophila germ cells, lost stem cells are replaced by division of neighboring stem cells or reversion of transit cells. We have characterized the Drosophila follicle stem cell (FSC) niche as a model of the epithelial niche to learn whether nonneighboring cells can also generate stem cell replacements. Exactly two stroma-free FSC niches holding single FSCs are located in fixed locations on opposite edges of the Drosophila ovariole. FSC daughters regularly migrate across the width of the ovariole to the other niche before proliferating and contributing to the follicle cell monolayer. Crossmigrating FSC daughters compete with the resident FSC for niche occupancy and are the source of replacement FSCs. The ability of stem cell daughters to target a distant niche and displace its resident stem cell suggests that precancerous mutations might spread from niche to niche within stem cell-based tissues.     

A specialized vascular niche for adult neural stem cells

Stem cells reside in specialized niches that regulate their self-renewal and differentiation. The vasculature is emerging as an important component of stem cell niches. Here, we show that the adult subventricular zone (SVZ) neural stem cell niche contains an extensive planar vascular plexus that has specialized properties. Dividing stem cells and their transit-amplifying progeny are tightly apposed to SVZ blood vessels both during homeostasis and regeneration. They frequently contact the vasculature at sites that lack astrocyte endfeet and pericyte coverage, a modification of the blood-brain barrier unique to the SVZ. Moreover, regeneration often occurs at these sites. Finally, we find that circulating small molecules in the blood enter the SVZ. Thus, the vasculature is a key component of the adult SVZ neural stem cell niche, with SVZ stem cells and transit-amplifying cells uniquely poised to receive spatial cues and regulatory signals from diverse elements of the vascular system.     

新疆麦蚜主要天敌的生态位

利用生态位理论,对新疆塔城、伊犁两地区麦田麦蚜主要天敌类群瓢虫类、食蚜蝇类、斑腹蝇类、蚜茧蜂类和蚜小蜂类的生态位宽度和重叠度进行了计算和分析。瓢虫类在塔城和伊犁小麦田的时间生态位宽度均较长。塔城食蚜蝇类和瓢虫类的时间生态位重叠度最大。伊犁麦田食蚜蝇类空间生态位最宽,瓢虫类时-空生态位宽度最大,斑腹蝇类和瓢虫类时间生态位的重叠度最大,蚜小蜂和斑腹蝇空间生态位以及时-空生态位的重叠度最大。对伊犁大麦田和燕麦田蚜虫的天敌生态位也进行了计算分析。     

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.     

A family business: stem cell progeny join the niche to regulate homeostasis

Stem cell niches, the discrete microenvironments in which the stem cells reside, play a dominant part in regulating stem cell activity and behaviours. Recent studies suggest that committed stem cell progeny become indispensable components of the niche in a wide range of stem cell systems. These unexpected niche inhabitants provide versatile feedback signals to their stem cell parents. Together with other heterologous cell types that constitute the niche, they contribute to the dynamics of the microenvironment. As progeny are often located in close proximity to stem cell niches, similar feedback regulations may be the underlying principles shared by different stem cell systems.     

美洲斑潜蝇及其寄生性天敌的生态位

通过调查美洲斑潜蝇及其主要寄生性天敌冈崎姬小蜂Neochrysoclaris okazakii Kamijo,芙新姬小蜂Neochrysoclaris formosa Westood和攀金姬小蜂Chrysocharis petheus Walker的时间和空间生态位,测定出生态位宽度,重叠和比例相似性,在生态位测定的基础上,按生态位的理论观点,探讨了寄生性天敌在美洲斑潜蝇综合治理中的控制作用。     

Extracellular matrix and the neural stem cell niche

Basal lamina is present in many stem cell niches, but we still have a poor understanding of the role of this and other extracellular matrix (ECM) components. Here, we review current knowledge regarding ECM expression and function in the neural stem cell niche, focusing on the subependymal zone of the adult CNS. An increasing complexity of ECM molecules has been described, and a number of receptors expressed on the stem cells identified. Experiments perturbing the niche using genetics or cytotoxic ablation of the rapidly dividing precursors, or using explant culture models to examine specific growth factors, have been influential in showing how changes in these ECM receptors might regulate neural stem cell behavior. However the role of changes in the matrix itself remains to be determined. The answers will be important, as they will point to the molecules required to engineer niches ex-vivo so as to provide tools for regenerative neuroscience.     

The stem cell potential of glia: lessons from reactive gliosis

Astrocyte-like cells, which act as stem cells in the adult brain, reside in a few restricted stem cell niches. However, following brain injury, glia outside these niches acquire or reactivate stem cell potential as part of reactive gliosis. Recent studies have begun to uncover the molecular pathways involved in this process. A comparison of molecular pathways activated after injury with those involved in the normal neural stem cell niches highlights strategies that could overcome the inhibition of neurogenesis outside the stem cell niche and instruct parenchymal glia towards a neurogenic fate. This new view on reactive glia therefore suggests a widespread endogenous source of cells with stem cell potential, which might potentially be harnessed for local repair strategies.     

The Drosophila ovarian and testis stem cell niches: similar somatic stem cells and signals

The stem cell niches at the apex of Drosophila ovaries and testes have been viewed as distinct in two major respects. While both contain germline stem cells, the testis niche also contains "cyst progenitor" stem cells, which divide to produce somatic cells that encase developing germ cells. Moreover, while both niches utilize BMP signaling, the testis niche requires a key JAK/STAT signal. We now show, by lineage marking, that the ovarian niche also contains a second type of stem cell. These "escort stem cells" morphologically resemble testis cyst progenitor cells and their daughters encase developing cysts before undergoing apoptosis at the time of follicle formation. In addition, we show that JAK/STAT signaling also plays a critical role in ovarian niche function, and acts within escort cells. These observations reveal striking similarities in the stem cell niches of male and female gonads, and suggest that they are largely governed by common mechanisms.