Micro-RNA mediated regulation of proliferation,self-renewal and differentiation of mammalian stem cells

Metazoan growth and development is maintained by populations of undifferentiated cells, commonly known as stem cells. Stem cells possess several characteristic properties, including dividing through self-renewing divisions and generating progeny that differentiate to have specialized cell fates. Multiple signaling pathways have been identified which coordinate stem cell proliferation with maintenance and differentiation. Relatively recently, the small, non-protein coding microRNAs (miRNAs) have been identified to function as important regulators in stem cell development. Individual miRNAs are capable of directing the translational repression of many mRNAs targets, generating widespread changes in gene expression. In addition, dysfunction of miRNA expression is commonly associated with cancer development. Cancer stem cells, which are likely responsible for initiating and maintaining tumorigenesis, share many similarities with stem cells and some mechanisms of miRNA function may be in common between these two cell types.     

Capacity for stochastic self-renewal and differentiation in mammalian spermatogonial stem cells

Mammalian spermatogenesis is initiated and sustained by spermatogonial stem cells (SSCs) through self-renewal and differentiation. The basic question of whether SSCs have the potential to specify self-renewal and differentiation in a cell-autonomous manner has yet to be addressed. Here, we show that rat SSCs in ex vivo culture conditions consistently give rise to two distinct types of progeny: new SSCs and differentiating germ cells, even when they have been exposed to virtually identical microenvironments. Quantitative experimental measurements and mathematical modeling indicates that fate decision is stochastic, with constant probability. These results reveal an unexpected ability in a mammalian SSC to specify both self-renewal and differentiation through a self-directed mechanism, and further suggest that this mechanism operates according to stochastic principles. These findings provide an experimental basis for autonomous and stochastic fate choice as an alternative strategy for SSC fate bifurcation, which may also be relevant to other stem cell types.     

The Hippo pathway regulates stem cell proliferation,self-renewal,and differentiation

Stem cells and progenitor cells are the cells of origin for multi-cellular organisms and organs. They play key roles during development and their dysregulation gives rise to human diseases such as cancer. The recent development of induced pluripotent stem cell (iPSC) technology which converts somatic cells to stem-like cells holds great promise for regenerative medicine. Nevertheless, the understanding of proliferation, differentiation, and self-renewal of stem cells and organ-specific progenitor cells is far from clear. Recently, the Hippo pathway was demonstrated to play important roles in these processes. The Hippo pathway is a newly established signaling pathway with critical functions in limiting organ size and suppressing tumorigenesis. This pathway was first found to inhibit cell proliferation and promote apoptosis, therefore regulating cell number and organ size in both Drosophila and mammals. However, in several organs, disturbance of the pathway leads to specific expansion of the progenitor cell compartment and manipulation of the pathway in embryonic stem cells strongly affects their self-renewal and differentiation. In this review, we summarize current observations on roles of the Hippo pathway in different types of stem cells and discuss how these findings changed our view on the Hippo pathway in organ development and tumorigenesis.     

精原干细胞自我更新和分化的调控

精原干细胞（spermatogonial stem cells,SSCs）是体内自然状态下惟一能将遗传信息传至子代的成体干细胞,它们能通过维持自我更新和分化的稳定从而保证雄性生命过程中精子发生的持续进行。了解SSCs自我更新和分化的调节机制有助于阐明精子发生机理,并为探究其他组织中成体干细胞增殖分化的调节机制提供依据。然而目前对于SSCs自我更新和分化的调控机制所知甚少。SSCs的更新与分化遵循特定模式,受以睾丸支持细胞为主要成分的微环境及各种内分泌因素如胶质细胞源神经营养因子（GDNF）、维生素、Ets转录因子ERM/Etv5等的调控。本文评述了SSCs更新与分化的模式以及上述因素对其更新、分化的调控,探讨了其中可能涉及的信号通路,以期为本领域及其他成体干细胞相关研究提供借鉴。     

Neural stem cells: balancing self-renewal with differentiation

Stem cells are captivating because they have the potential to make multiple cell types yet maintain their undifferentiated state. Recent studies of Drosophila and mammalian neural stem cells have shed light on how stem cells regulate self-renewal versus differentiation and have revealed the proteins, processes and pathways that all converge to regulate neural progenitor self-renewal. If we can better understand how stem cells balance self-renewal versus differentiation, we will significantly advance our knowledge of embryogenesis, cancer biology and brain evolution, as well as the use of stem cells for therapeutic purposes.     

Epigenetic modifications and self-renewal regulation of mouse germline stem cells

Germline stem (GS) cells were established from gonocytes and spermatogonia of postnatal mouse testes. GS cells proliferate in the presence of several kinds of cytokines, and a small percentage of GS cells also show spermatogonial stem cell (SSC) activity, i.e., they differentiate into sperm after being transplanted into infertile mouse testes without endogenous spermatogenesis. Interestingly, in GS cell culture, we also found that pluripotent stem cells (multipotent germline stem cells (mGS cells)) could be derived and these mGS cells do not have normal androgenetic genomic imprinting marks that are shown in GS cells, e.g., H19 hypermethylation. A new culture system for fetal male germ cells (embryonic GS (eGS) cells) has also been recently developed. Although these cells exhibited SSC potential, the offspring from cultured cells showed heritable imprinting defects in their DNA methylation patterns. In an attempt to understand the self-renewal machinery in SSCs, we transfected H-Ras and cylin D2 into GS cells, and successfully reconstructed the SSC self-renewal ability without using exogenous cytokines. Although these cells showed SSC activity in germ cell transplantation assays, we also found development of seminomatous tumors, possibly induced by excessive self-renewing signal. These stem cell culture systems are useful tools not only for understanding the mechanisms of self-renewal or epigenetic reprogramming but also for clarifying the mechanism of germ cell tumor development.     

Interleukin-15 regulates proliferation and self-renewal of adult neural stem cells

The impact of inflammation is crucial for the regulation of the biology of neural stem cells (NSCs). Interleukin-15 (IL-15) appears as a likely candidate for regulating neurogenesis, based on its well-known mitogenic properties. We show here that NSCs of the subventricular zone (SVZ) express IL-15, which regulates NSC proliferation, as evidenced by the study of IL-15-/- mice and the effects of acute IL-15 administration, coupled to 5-bromo-2'-deoxyuridine/5-ethynyl-2'-deoxyuridine dual-pulse labeling. Moreover, IL-15 regulates NSC differentiation, its deficiency leading to an impaired generation of neuroblasts in the SVZ-rostral migratory stream axis, recoverable through the action of exogenous IL-15. IL-15 expressed in cultured NSCs is linked to self-renewal, proliferation, and differentiation. IL-15-/- NSCs presented deficient proliferation and self-renewal, as evidenced in proliferation and colony-forming assays and the analysis of cell cycle-regulatory proteins. Moreover, IL-15-deficient NSCs were more prone to differentiate than wild-type NSCs, not affecting the cell population balance. Lack of IL-15 led to a defective activation of the JAK/STAT and ERK pathways, key for the regulation of proliferation and differentiation of NSCs. The results show that IL-15 is a key regulator of neurogenesis in the adult and is essential to understanding diseases with an inflammatory component.     

Negative cell-cycle regulators cooperatively control self-renewal and differentiation of haematopoietic stem cells

Haematopoietic stem cells (HSCs) are capable of shifting from a state of relative quiescence under homeostatic conditions to rapid proliferation under conditions of stress. The mechanisms that regulate the relative quiescence of stem cells and its association with self-renewal are unclear, as is the contribution of molecular regulators of the cell cycle to these decisions. Understanding the mechanisms that govern these transitions will provide important insights into cell-cycle regulation of HSCs and possible therapeutic approaches to expand HSCs. We have investigated the role of two negative regulators of the cell cycle, p27(Kip1) and MAD1, in controlling this transition. Here we show that Mad1(-/-)p27(Kip1-/-) bone marrow has a 5.7-fold increase in the frequency of stem cells, and surprisingly, an expanded pool of quiescent HSCs. However, Mad1(-/-)p27(Kip1-/-) stem cells exhibit an enhanced proliferative response under conditions of stress, such as cytokine stimulation in vitro and regeneration of the haematopoietic system after ablation in vivo. Together these data demonstrate that the MYC-antagonist MAD1 and cyclin-dependent kinase inhibitor p27(Kip1) cooperate to regulate the self-renewal and differentiation of HSCs in a context-dependent manner.     

猕猴神经干细胞的诱导分化、干性维持及同种脑内移植

为探索猕猴神经干细胞分化及特性维持,推进神经干细胞临床应用研究,该实验以绿色荧光蛋白(green fluorescence protein,GFP)为标记探讨猕猴胚胎干细胞向玫瑰花环(rosettes)结构神经干细胞的分化及其碱性成纤维细胞生长因子(basic fibroblast growth factor,bFGF)和表皮生长因子(epidermal growth factor,EGF)的扩增培养。结果表明:1)建立了稳定高效的猕猴神经干细胞分化体系,在该分化体系下,GFP标记猕猴胚胎干细胞在分化的第12天时,95%以上的细胞分化为神经干细胞;2)分化得到的Rosettes结构神经干细胞经bFGF/EGF扩增后,能够较好地维持其Rosettes结构;3)经bFGF/EGF扩增后的rosettes结构神经干细胞移植到猕猴脑内后能够较好的存活并向神经元分化,即bFGF/EGF扩增培养能较好地维持Rosettes结构的神经干细胞,且移植到猕猴脑内的该细胞亦能够较好地存活并向神经元分化,该结果为神经干细胞应用于临床提供了基础理论依据。     

Angiocrine factors from Akt-activated endothelial cells balance self-renewal and differentiation of haematopoietic stem cells

Endothelial cells establish an instructive vascular niche that reconstitutes haematopoietic stem and progenitor cells (HSPCs) through release of specific paracrine growth factors, known as angiocrine factors. However, the mechanism by which endothelial cells balance the rate of proliferation and lineage-specific differentiation of HSPCs is unknown. Here, we demonstrate that Akt activation in endothelial cells, through recruitment of mTOR, but not the FoxO pathway, upregulates specific angiocrine factors that support expansion of CD34(-)Flt3(-) KLS HSPCs with long-term haematopoietic stem cell (LT-HSC) repopulation capacity. Conversely, co-activation of Akt-stimulated endothelial cells with p42/44 MAPK shifts the balance towards maintenance and differentiation of the HSPCs. Selective activation of Akt1 in the endothelial cells of adult mice increased the number of colony forming units in the spleen and CD34(-)Flt3(-) KLS HSPCs with LT-HSC activity in the bone marrow, accelerating haematopoietic recovery. Therefore, the activation state of endothelial cells modulates reconstitution of HSPCs through the modulation of angiocrine factors, with Akt-mTOR-activated endothelial cells supporting the self-renewal of LT-HSCs and expansion of HSPCs, whereas MAPK co-activation favours maintenance and lineage-specific differentiation of HSPCs.     

Wnt信号在胚胎干细胞自我更新和分化中的作用

胚胎干细胞分离自胚泡内细胞团,具有无限自我更新和多向分化潜能,有很大的医学应用前景。Wnt家族是一类分泌型的细胞信号传导蛋白,可以通过复杂的信号传递通路调控胚胎的早期发育,对细胞的分化、增殖及生长具有重要的调节作用。该文就Wnt信号通路调节胚胎干细胞的自我更新和分化作一综述。