Neuronal circuitry mechanism regulating adult quiescent neural stem-cell fate decision

Adult neurogenesis arises from neural stem cells within specialized niches. Neuronal activity and experience, presumably acting on this local niche, regulate multiple stages of adult neurogenesis, from neural progenitor proliferation to new neuron maturation, synaptic integration and survival. It is unknown whether local neuronal circuitry has a direct impact on adult neural stem cells. Here we show that, in the adult mouse hippocampus, nestin-expressing radial glia-like quiescent neural stem cells (RGLs) respond tonically to the neurotransmitter γ-aminobutyric acid (GABA) by means of γ2-subunit-containing GABAA receptors. Clonal analysis of individual RGLs revealed a rapid exit from quiescence and enhanced symmetrical self-renewal after conditional deletion of γ2. RGLs are in close proximity to terminals expressing 67-kDa glutamic acid decarboxylase (GAD67) of parvalbumin-expressing (PV+) interneurons and respond tonically to GABA released from these neurons. Functionally, optogenetic control of the activity of dentate PV+ interneurons, but not that of somatostatin-expressing or vasoactive intestinal polypeptide (VIP)-expressing interneurons, can dictate the RGL choice between quiescence and activation. Furthermore, PV+ interneuron activation restores RGL quiescence after social isolation, an experience that induces RGL activation and symmetrical division. Our study identifies a niche cell–signal–receptor trio and a local circuitry mechanism that control the activation and self-renewal mode of quiescent adult neural stem cells in response to neuronal activity and experience.     

The Lkb1 metabolic sensor maintains haematopoietic stem cell survival

Haematopoietic stem cells (HSCs) can convert between growth states that have marked differences in bioenergetic needs. Although often quiescent in adults, these cells become proliferative upon physiological demand. Balancing HSC energetics in response to nutrient availability and growth state is poorly understood, yet essential for the dynamism of the haematopoietic system. Here we show that the Lkb1 tumour suppressor is critical for the maintenance of energy homeostasis in haematopoietic cells. Lkb1 inactivation in adult mice causes loss of HSC quiescence followed by rapid depletion of all haematopoietic subpopulations. Lkb1-deficient bone marrow cells exhibit mitochondrial defects, alterations in lipid and nucleotide metabolism, and depletion of cellular ATP. The haematopoietic effects are largely independent of Lkb1 regulation of AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) signalling. Instead, these data define a central role for Lkb1 in restricting HSC entry into cell cycle and in broadly maintaining energy homeostasis in haematopoietic cells through a novel metabolic checkpoint.     

Stem cell division is regulated by the microRNA pathway

One of the key characteristics of stem cells is their capacity to divide for long periods of time in an environment where most of the cells are quiescent. Therefore, a critical question in stem cell biology is how stem cells escape cell division stop signals. Here, we report the necessity of the microRNA (miRNA) pathway for proper control of germline stem cell (GSC) division in Drosophila melanogaster. Analysis of GSCs mutant for dicer-1 (dcr-1), the double-stranded RNaseIII essential for miRNA biogenesis, revealed a marked reduction in the rate of germline cyst production. These dcr-1 mutant GSCs exhibit normal identity but are defective in cell cycle control. On the basis of cell cycle markers and genetic interactions, we conclude that dcr-1 mutant GSCs are delayed in the G1 to S transition, which is dependent on the cyclin-dependent kinase inhibitor Dacapo, suggesting that miRNAs are required for stem cells to bypass the normal G1/S checkpoint. Hence, the miRNA pathway might be part of a mechanism that makes stem cells insensitive to environmental signals that normally stop the cell cycle at the G1/S transition.     

A skin microRNA promotes differentiation by repressing 'stemness'

In stratified epithelial tissues, homeostasis relies on the self-renewing capacity of stem cells located within the innermost basal layer. As basal cells become suprabasal, they lose proliferative potential and embark on a terminal differentiation programme. Here, we show that microRNA-203 is induced in the skin concomitantly with stratification and differentiation. By altering miR-203's spatiotemporal expression in vivo, we show that miR-203 promotes epidermal differentiation by restricting proliferative potential and inducing cell-cycle exit. We identify p63 as one of the conserved targets of miR-203 across vertebrates. Notably, p63 is an essential regulator of stem-cell maintenance in stratified epithelial tissues. We show that miR-203 directly represses the expression of p63: it fails to switch off suprabasally when either Dicer1 or miR-203 is absent and it becomes repressed basally when miR-203 is prematurely expressed. Our findings suggest that miR-203 defines a molecular boundary between proliferative basal progenitors and terminally differentiating suprabasal cells, ensuring proper identity of neighbouring layers.     

植物根尖分生组织干细胞调控模式

静止中心(QC)形成和干细胞区特化是植物根尖分生组织确立的标志。静止中心位于根尖分生组织中心,干细胞围绕在静止中心细胞周围。依赖于生长素的PLT途径和不依赖于生长素的SCR/SHR途径共同发挥维持静止中心细胞稳定的作用。静止中心和干细胞区的柱干细胞之间存在类似于WUS/CLV3的WOX5/ACR4/CLE40的反馈抑制调节途径,该调节途径维持着静止中心细胞和柱干细胞之间的平衡。静止中心和其他类型干细胞之间也可能存在类似的反馈抑制调节途径。生长素、细胞分裂素、赤霉素等植物激素信号在根干细胞功能发挥方面也起到重要作用,与各种基因一起组成根分生组织干细胞调控网络。     

PML targeting eradicates quiescent leukaemia-initiating cells

The existence of a small population of 'cancer-initiating cells' responsible for tumour maintenance has been firmly demonstrated in leukaemia. This concept is currently being tested in solid tumours. Leukaemia-initiating cells, particularly those that are in a quiescent state, are thought to be resistant to chemotherapy and targeted therapies, resulting in disease relapse. Chronic myeloid leukaemia is a paradigmatic haematopoietic stem cell disease in which the leukaemia-initiating-cell pool is not eradicated by current therapy, leading to disease relapse on drug discontinuation. Here we define the critical role of the promyelocytic leukaemia protein (PML) tumour suppressor in haematopoietic stem cell maintenance, and present a new therapeutic approach for targeting quiescent leukaemia-initiating cells and possibly cancer-initiating cells by pharmacological inhibition of PML.     

Self-renewal and expansion of single transplanted muscle stem cells

Adult muscle satellite cells have a principal role in postnatal skeletal muscle growth and regeneration. Satellite cells reside as quiescent cells underneath the basal lamina that surrounds muscle fibres and respond to damage by giving rise to transient amplifying cells (progenitors) and myoblasts that fuse with myofibres. Recent experiments showed that, in contrast to cultured myoblasts, satellite cells freshly isolated or satellite cells derived from the transplantation of one intact myofibre contribute robustly to muscle repair. However, because satellite cells are known to be heterogeneous, clonal analysis is required to demonstrate stem cell function. Here we show that when a single luciferase-expressing muscle stem cell is transplanted into the muscle of mice it is capable of extensive proliferation, contributes to muscle fibres, and Pax7(+)luciferase(+) mononucleated cells can be readily re-isolated, providing evidence of muscle stem cell self-renewal. In addition, we show using in vivo bioluminescence imaging that the dynamics of muscle stem cell behaviour during muscle repair can be followed in a manner not possible using traditional retrospective histological analyses. By imaging luciferase activity, real-time quantitative and kinetic analyses show that donor-derived muscle stem cells proliferate and engraft rapidly after injection until homeostasis is reached. On injury, donor-derived mononucleated cells generate massive waves of cell proliferation. Together, these results show that the progeny of a single luciferase-expressing muscle stem cell can both self-renew and differentiate after transplantation in mice, providing new evidence at the clonal level that self-renewal is an autonomous property of a single adult muscle stem cell.     

Fat cells reactivate quiescent neuroblasts via TOR and glial insulin relays in Drosophila

Many stem, progenitor and cancer cells undergo periods of mitotic quiescence from which they can be reactivated. The signals triggering entry into and exit from this reversible dormant state are not well understood. In the developing Drosophila central nervous system, multipotent self-renewing progenitors called neuroblasts undergo quiescence in a stereotypical spatiotemporal pattern. Entry into quiescence is regulated by Hox proteins and an internal neuroblast timer. Exit from quiescence (reactivation) is subject to a nutritional checkpoint requiring dietary amino acids. Organ co-cultures also implicate an unidentified signal from an adipose/hepatic-like tissue called the fat body. Here we provide in vivo evidence that Slimfast amino-acid sensing and Target of rapamycin (TOR) signalling activate a fat-body-derived signal (FDS) required for neuroblast reactivation. Downstream of this signal, Insulin-like receptor signalling and the Phosphatidylinositol 3-kinase (PI3K)/TOR network are required in neuroblasts for exit from quiescence. We demonstrate that nutritionally regulated glial cells provide the source of Insulin-like peptides (ILPs) relevant for timely neuroblast reactivation but not for overall larval growth. Conversely, ILPs secreted into the haemolymph by median neurosecretory cells systemically control organismal size but do not reactivate neuroblasts. Drosophila thus contains two segregated ILP pools, one regulating proliferation within the central nervous system and the other controlling tissue growth systemically. Our findings support a model in which amino acids trigger the cell cycle re-entry of neural progenitors via a fat-body-glia-neuroblasts relay. This mechanism indicates that dietary nutrients and remote organs, as well as local niches, are key regulators of transitions in stem-cell behaviour.     

mTORC1 in the Paneth cell niche couples intestinal stem-cell function to calorie intake

How adult tissue stem and niche cells respond to the nutritional state of an organism is not well understood. Here we find that Paneth cells, a key constituent of the mammalian intestinal stem-cell (ISC) niche, augment stem-cell function in response to calorie restriction. Calorie restriction acts by reducing mechanistic target of rapamycin complex 1 (mTORC1) signalling in Paneth cells, and the ISC-enhancing effects of calorie restriction can be mimicked by rapamycin. Calorie intake regulates mTORC1 in Paneth cells, but not ISCs, and forced activation of mTORC1 in Paneth cells during calorie restriction abolishes the ISC-augmenting effects of the niche. Finally, increased expression of bone stromal antigen 1 (Bst1) in Paneth cells—an ectoenzyme that produces the paracrine factor cyclic ADP ribose—mediates the effects of calorie restriction and rapamycin on ISC function. Our findings establish that mTORC1 non-cell-autonomously regulates stem-cell self-renewal, and highlight a significant role of the mammalian intestinal niche in coupling stem-cell function to organismal physiology.     

The stem-cell niche as an entity of action

Stem-cell populations are established in 'niches'--specific anatomic locations that regulate how they participate in tissue generation, maintenance and repair. The niche saves stem cells from depletion, while protecting the host from over-exuberant stem-cell proliferation. It constitutes a basic unit of tissue physiology, integrating signals that mediate the balanced response of stem cells to the needs of organisms. Yet the niche may also induce pathologies by imposing aberrant function on stem cells or other targets. The interplay between stem cells and their niche creates the dynamic system necessary for sustaining tissues, and for the ultimate design of stem-cell therapeutics.     

Imbalance between pSmad3 and Notch induces CDK inhibitors in old muscle stem cells

Adult skeletal muscle robustly regenerates throughout an organism's life, but as the muscle ages, its ability to repair diminishes and eventually fails. Previous work suggests that the regenerative potential of muscle stem cells (satellite cells) is not triggered in the old muscle because of a decline in Notch activation, and that it can be rejuvenated by forced local activation of Notch. Here we report that, in addition to the loss of Notch activation, old muscle produces excessive transforming growth factor (TGF)-beta (but not myostatin), which induces unusually high levels of TGF-beta pSmad3 in resident satellite cells and interferes with their regenerative capacity. Importantly, endogenous Notch and pSmad3 antagonize each other in the control of satellite-cell proliferation, such that activation of Notch blocks the TGF-beta-dependent upregulation of the cyclin-dependent kinase (CDK) inhibitors p15, p16, p21 and p27, whereas inhibition of Notch induces them. Furthermore, in muscle stem cells, Notch activity determines the binding of pSmad3 to the promoters of these negative regulators of cell-cycle progression. Attenuation of TGF-beta/pSmad3 in old, injured muscle restores regeneration to satellite cells in vivo. Thus a balance between endogenous pSmad3 and active Notch controls the regenerative competence of muscle stem cells, and deregulation of this balance in the old muscle microniche interferes with regeneration.     

藻蓝色素蛋白通过调控miR-642a-5p抑制LTEP-a2细胞的增殖和迁移

研究藻蓝色素蛋白抑制非小细胞肺癌LTEP-a2体外增殖迁移的机制.以LTEP-a2细胞为模型,采用高通量miRNA转录组学对藻蓝蛋白处理后细胞中差异表达的miRNA进行了筛选;对筛选出的差异miRNA,利用体外转染mimics的方法对细胞的增殖、迁移能力进行验证.研究结果显示,藻蓝蛋白处理LTEP-a2细胞后能够显著增加细胞内miR-642a-5p的表达水平;过表达miR-642a-5p能够显著抑制细胞的体外增殖、迁移能力;此外,藻蓝蛋白可以通过上调miR-642a-5p表达抑制核受体NF-κB信号通路,降低蛋白磷酸化水平,进而抑制LTEP-a2细胞的增殖迁移能力.研究能够为藻蓝色素蛋白的应用以及非小细胞肺癌的治疗提供一定的理论基础.      

Asymmetric and symmetric stem-cell divisions in development and cancer

Much has been made of the idea that asymmetric cell division is a defining characteristic of stem cells that enables them to simultaneously perpetuate themselves (self-renew) and generate differentiated progeny. Yet many stem cells can divide symmetrically, particularly when they are expanding in number during development or after injury. Thus, asymmetric division is not necessary for stem-cell identity but rather is a tool that stem cells can use to maintain appropriate numbers of progeny. The facultative use of symmetric or asymmetric divisions by stem cells may be a key adaptation that is crucial for adult regenerative capacity.     

Adult pancreatic beta-cells are formed by self-duplication rather than stem-cell differentiation

How tissues generate and maintain the correct number of cells is a fundamental problem in biology. In principle, tissue turnover can occur by the differentiation of stem cells, as is well documented for blood, skin and intestine, or by the duplication of existing differentiated cells. Recent work on adult stem cells has highlighted their potential contribution to organ maintenance and repair. However, the extent to which stem cells actually participate in these processes in vivo is not clear. Here we introduce a method for genetic lineage tracing to determine the contribution of stem cells to a tissue of interest. We focus on pancreatic beta-cells, whose postnatal origins remain controversial. Our analysis shows that pre-existing beta-cells, rather than pluripotent stem cells, are the major source of new beta-cells during adult life and after pancreatectomy in mice. These results suggest that terminally differentiated beta-cells retain a significant proliferative capacity in vivo and cast doubt on the idea that adult stem cells have a significant role in beta-cell replenishment.     

Microenvironmental regulation of stem cells in intestinal homeostasis and cancer

The identification of intestinal stem cells as well as their malignant counterparts, colon cancer stem cells, has undergone rapid development in recent years. Under physiological conditions, intestinal homeostasis is a carefully balanced and efficient interplay between stem cells, their progeny and the microenvironment. These interactions regulate the astonishingly rapid renewal of the intestinal epithelial layer, which consequently puts us at serious risk of developing cancer. Here we highlight the microenvironment-derived signals that regulate stem-cell fate and epithelial differentiation. As our understanding of normal intestinal crypt homeostasis grows, these developments may point towards new insights into the origin of cancer and the maintenance and regulation of cancer stem cells.     

The let-7-Imp axis regulates ageing of the Drosophila testis stem-cell niche

Adult stem cells support tissue homeostasis and repair throughout the life of an individual. During ageing, numerous intrinsic and extrinsic changes occur that result in altered stem-cell behaviour and reduced tissue maintenance and regeneration. In the Drosophila testis, ageing results in a marked decrease in the self-renewal factor Unpaired (Upd), leading to a concomitant loss of germline stem cells. Here we demonstrate that IGF-II messenger RNA binding protein (Imp) counteracts endogenous small interfering RNAs to stabilize upd (also known as os) RNA. However, similar to upd, Imp expression decreases in the hub cells of older males, which is due to the targeting of Imp by the heterochronic microRNA let-7. In the absence of Imp, upd mRNA therefore becomes unprotected and susceptible to degradation. Understanding the mechanistic basis for ageing-related changes in stem-cell behaviour will lead to the development of strategies to treat age-onset diseases and facilitate stem-cell-based therapies in older individuals.