Fluridone affects quiescent centre division in the Arabidopsis thaliana root stem cell niche

Plants undergo cell division throughout their life in order to maintain their growth. It is well known that root and shoot tip of plants possess meristems, which contain quiescent cells. Fluridone (1-methyl-3-phenyl-5-(3-trifluoromethyl (phenyl))-4-(1H)-pyridinone) is an established inhibitor of both ABA and carotenoid biosynthesis. However, the other functions of fluridone remain undiscovered. In this report, we provide experimental evidence that fluridone plays a role in the division of the quiescent centre of the Arabidopsis root meristem. This study examined the effects of exogenous fluridone and ABA on the development of the stem cell niche in Arabidopsis root. We show that fluridone promoted the division of stem cells in the quiescent centre, whereas exogenous ABA suppressed quiescent centre division. Furthermore, we established a novel regulatory function for fluridone by demonstrating that it plays an important role in postembryonic development.     

Time-course observation of the reconstruction of stem cell niche in the intact root

The stem cell niche (SCN) is critical in maintaining continuous postembryonic growth of the plant root. During their growth in soil, plant roots are often challenged by various biotic or abiotic stresses, resulting in damage to the SCN. This can be repaired by the reconstruction of a functional SCN. Previous studies examining the SCN’s reconstruction often introduce physical damage including laser ablation or surgical excision. In this study, we performed a time-course observation of the SCN reconstruction in pWOX5:icals3m roots, an inducible system that causes non-invasive SCN differentiation upon induction of estradiol on Arabidopsis (Arabidopsis thaliana) root. We found a stage-dependent reconstruction of SCN in pWOX5:icals3m roots, with division-driven anatomic reorganization in the early stage of the SCN recovery, and cell fate specification of new SCN in later stages. During the recovery of the SCN, the local accumulation of auxin was coincident with the cell division pattern, exhibiting a spatial shift in the root tip. In the early stage, division mostly occurred in the neighboring stele to the SCN position, while division in endodermal layers seemed to contribute more in the later stages, when the SCN was specified. The precise re-positioning of SCN seemed to be determined by mutual antagonism between auxin and cytokinin, a conserved mechanism that also regulates damage-induced root regeneration. Our results thus provide time-course information about the reconstruction of SCN in intact Arabidopsis roots, which highlights the stage-dependent re-patterning in response to differentiated quiescent center.

Time course live imaging technique revealed stage-dependent reconstruction patterns of stem cells in the intact Arabidopsis roots in response to differentiated quiescent center.     

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.     

Stem cell niche: Dynamic neighbor of stem cells

Stem cell niche is a specialized and dynamic microenvironment around the stem cells which plays a critical role in maintaining the stemness properties of stem cells. Over the years, advancement in the research activity has revealed the various important aspects of stem cell niche including cell-cell interaction, cell-extracellular matrix interaction, a large number of soluble signaling factors and various biochemical and biophysical cues (such as oxygen tension, flow, and shear and pore size). Stem cells have the potential to be a powerful tool in regenerative medicine due to their self-renewal property and immense differentiation potential. Recent progresses in in vitro culture conditions of embryonic stem cells, adult stem cells and induced pluripotent stem cells have enabled the researchers to investigate and understand the role of the microenvironment in stem cell properties. The engineered artificial stem cell niche has led to a better execution of stem cells in regenerative medicine. Here we elucidate the key components of stem cell niche and their role in niche engineering and stem cell therapeutics.     

Self-maintained escort cells form a germline stem cell differentiation niche

Stem cell self-renewal is controlled by concerted actions of niche signals and intrinsic factors in a variety of systems. In the Drosophila ovary, germline stem cells (GSCs) in the niche continuously self-renew and generate differentiated germ cells that interact physically with escort cells (ECs). It has been proposed that escort stem cells (ESCs), which directly contact GSCs, generate differentiated ECs to maintain the EC population. However, it remains unclear whether the differentiation status of germ cells affects EC behavior and how the interaction between ECs and germ cells is regulated. In this study, we have found that ECs can undergo slow cell turnover regardless of their positions, and the lost cells are replenished by their neighboring ECs via self-duplication rather than via stem cells. ECs extend elaborate cellular processes that exhibit extensive interactions with differentiated germ cells. Interestingly, long cellular processes of ECs are absent when GSC progeny fail to differentiate, suggesting that differentiated germ cells are required for the formation or maintenance of EC cellular processes. Disruption of Rho functions leads to the disruption of long EC cellular processes and the accumulation of ill-differentiated single germ cells by increasing BMP signaling activity outside the GSC niche, and also causes gradual EC loss. Therefore, our findings indicate that ECs interact extensively with differentiated germ cells through their elaborate cellular processes and control proper germ cell differentiation. Here, we propose that ECs form a niche that controls GSC lineage differentiation and is maintained by a non-stem cell mechanism.     

The neural stem cell niche

The neural stem cell niche defines a zone in which stem cells are retained after embryonic development for the production of new cells of the nervous system. This continual supply of new neurons and glia then provides the postnatal and adult brain with an added capacity for cellular plasticity, albeit one that is restricted to a few specific zones within the brain. Critical to the maintenance of the stem cell niche are microenvironmental cues and cell-cell interactions that act to balance stem cell quiescence with proliferation and to direct neurogenesis versus gliogenesis lineage decisions. Ultimately, based on the location of the niche, stem cells of the adult brain support regeneration in the dentate gyrus of the hippocampus and the olfactory bulb through neuron replacement. Here, we provide a summary of the current understanding of the organization and control mechanisms of the neural stem cell niche.     

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。     

Haematopoietic stem cell niche in Drosophila

Development and homeostasis of the haematopoietic system is dependent upon stem cells that have the unique ability to both self-renew and to differentiate in all cell lineages of the blood. The crucial decision between haematopoietic stem cell (HSC) self-renewal and differentiation must be tightly controlled. Ultimately, this choice is regulated by the integration of intrinsic signals together with extrinsic cues provided by an exclusive microenvironment, the so-called haematopoietic niche. Although the haematopoietic system of vertebrates has been studied extensively for many decades, the specification of the HSC niche and its signals involved are poorly understood. Much of our current knowledge of how niches regulate long-term maintenance of stem cells is derived from studies on Drosophila germ cells. Now, two recently published studies by Mandal et al.1 and Krezmien et al.2 describe the Drosophila haematopoietic niche and signal transduction pathways that are involved in the maintenance of haematopoietic precursors. Both reports emphasize several features that are important for controlling stem cell behavior and show parallels to both the vertebrate haematopoietic niche as well as the Drosophila germline stem cell niches in ovary and testis. The findings of both papers shed new light on the specific interactions between haematopoietic progenitors and their microenvironment.     

A stem cell niche dominance theorem

Background  

Multilevelness is a defining characteristic of complex systems. For example, in the intestinal tissue the epithelial lining is organized into crypts that are maintained by a niche of stem cells. The behavior of the system 'as a whole' is considered to emerge from the functioning and interactions of its parts. What we are seeking here is a conceptual framework to demonstrate how the "fate" of intestinal crypts is an emergent property that inherently arises from the complex yet robust underlying biology of stem cells.     

Regulation of hematopoietic stem cells in the niche

Hematopoiesis provides a suitable model for understanding adult stem cells and their niche. Hematopoietic stem cells(HSCs) continuously produce blood cells through orchestrated proliferation, self-renewal, and differentiation in the bone marrow(BM). Within the BM exists a highly organized microenvironment termed "niche" where stem cells reside and are maintained. HSC niche is the first evidence that a microenvironment contributes to protecting stem cell integrity and functionality in mammals. Although multiple models exist, recent progress has principally elucidated the cellular complexity of the HSC niche that maintains and regulates HSCs in BM. Here we introduce the development and summarize the achievements of HSC niche studies.     

Nonmyelinating Schwann cells maintain hematopoietic stem cell hibernation in the bone marrow niche

Hematopoietic stem cells (HSCs) reside and self-renew in the bone marrow (BM) niche. Overall, the signaling that regulates stem cell dormancy in the HSC niche remains controversial. Here, we demonstrate that TGF-β type II receptor-deficient HSCs show low-level Smad activation and impaired long-term repopulating activity, underlining the critical role of TGF-β/Smad signaling in HSC maintenance. TGF-β is produced as a latent form by a variety of cells, so we searched for those that express activator molecules for latent TGF-β. Nonmyelinating Schwann cells in BM proved responsible for activation. These glial cells ensheathed autonomic nerves, expressed HSC niche factor genes, and were in contact with a substantial proportion of HSCs. Autonomic nerve denervation reduced the number of these active TGF-β-producing cells and led to rapid loss of HSCs from BM. We propose that glial cells are components of a BM niche and maintain HSC hibernation by regulating activation of latent TGF-β.     

EphB signaling controls lineage plasticity of adult neural stem cell niche cells

Stem cells remain in specialized niches over the lifespan of the organism in many organs to ensure tissue homeostasis and enable regeneration. How the niche is maintained is not understood, but is probably as important as intrinsic stem cell self-renewal capacity for tissue integrity. We here demonstrate a high degree of phenotypic plasticity of the two main niche cell types, ependymal cells and astrocytes, in the neurogenic lateral ventricle walls in the adult mouse brain. In response to a lesion, astrocytes give rise to ependymal cells and ependymal cells give rise to niche astrocytes. We identify EphB2 forward signaling as a key pathway regulating niche cell plasticity. EphB2 acts downstream of Notch and is required for the maintenance of ependymal cell characteristics, thereby inhibiting the transition from ependymal cell to astrocyte. Our results show that niche cell identity is actively maintained and that niche cells retain a high level of plasticity.