Distinct regulatory functions of calpain 1 and 2 during neural stem cell self-renewal and differentiation

Calpains are calcium regulated cysteine proteases that have been described in a wide range of cellular processes, including apoptosis, migration and cell cycle regulation. In addition, calpains have been implicated in differentiation, but their impact on neural differentiation requires further investigation. Here, we addressed the role of calpain 1 and calpain 2 in neural stem cell (NSC) self-renewal and differentiation. We found that calpain inhibition using either the chemical inhibitor calpeptin or the endogenous calpain inhibitor calpastatin favored differentiation of NSCs. This effect was associated with significant changes in cell cycle-related proteins and may be regulated by calcium. Interestingly, calpain 1 and calpain 2 were found to play distinct roles in NSC fate decision. Calpain 1 expression levels were higher in self-renewing NSC and decreased with differentiation, while calpain 2 increased throughout differentiation. In addition, calpain 1 silencing resulted in increased levels of both neuronal and glial markers, β-III Tubulin and glial fibrillary acidic protein (GFAP). Calpain 2 silencing elicited decreased levels of GFAP. These results support a role for calpain 1 in repressing differentiation, thus maintaining a proliferative NSC pool, and suggest that calpain 2 is involved in glial differentiation.     

Mechanisms that mediate stem cell self-renewal and differentiation

Stem cells have two common properties: the capacity for self-renewal and the potential to differentiate into one or more specialized cell types. In general, stem cells can be divided into two broad categories: adult (somatic) stem cells and embryonic stem cells. Recent evidence suggested that tumors may contain "cancer stem cells" with indefinite potential for self-renewal. In this review, we will focus on the molecular mechanisms regulating embryonic stem cell self-renewal and differentiation, and discuss how these mechanisms may be relevant in cancer cells.     

Niche-mediated control of human embryonic stem cell self-renewal and differentiation

Complexity in the spatial organization of human embryonic stem cell (hESC) cultures creates heterogeneous microenvironments (niches) that influence hESC fate. This study demonstrates that the rate and trajectory of hESC differentiation can be controlled by engineering hESC niche properties. Niche size and composition regulate the balance between differentiation-inducing and -inhibiting factors. Mechanistically, a niche size-dependent spatial gradient of Smad1 signaling is generated as a result of antagonistic interactions between hESCs and hESC-derived extra-embryonic endoderm (ExE). These interactions are mediated by the localized secretion of bone morphogenetic protein-2 (BMP2) by ExE and its antagonist, growth differentiation factor-3 (GDF3) by hESCs. Micropatterning of hESCs treated with small interfering (si) RNA against GDF3, BMP2 and Smad1, as well treatments with a Rho-associated kinase (ROCK) inhibitor demonstrate that independent control of Smad1 activation can rescue the colony size-dependent differentiation of hESCs. Our results illustrate, for the first time, a role for Smad1 in the integration of spatial information and in the niche-size-dependent control of hESC self-renewal and differentiation.     

Redox homeostasis: the linchpin in stem cell self-renewal and differentiation

Stem cells are characterized by their unique ability of self-renewal to maintain the so-called stem cell pool. Over the past decades, reactive oxygen species (ROS) have been recognized as toxic aerobic metabolism byproducts that are harmful to stem cells, leading to DNA damage, senescence or cell death. Recently, a growing body of literature has shown that stem cells reside in redox niches with low ROS levels. The balance of Redox homeostasis facilitates stem cell self-renewal by an intricate network. Thus, to fully decipher the underlying molecular mechanisms involved in the maintenance of stem cell self-renewal, it is critical to address the important role of redox homeostasis in the regulation of self-renewal and differentiation of stem cells. In this regard, we will discuss the regulatory mechanisms involved in the subtly orchestrated balance of redox status in stem cells by scavenger antioxidant enzyme systems that are well monitored by the hypoxia niches and crucial redox regulators including forkhead homeobox type O family (FoxOs), apurinic/apyrimidinic (AP) endonuclease1/redox factor-1 (APE1/Ref-1), nuclear factor erythroid-2-related factor 2 (Nrf2) and ataxia telangiectasia mutated (ATM). We will also introduce several pivotal ROS-sensitive molecules, such as hypoxia-inducible factors, p38 mitogen-activated protein kinase (p38) and p53, involved in the redox-regulated stem cell self-renewal. Specifically, all the aforementioned molecules can act as ‘redox sensors'' by virtue of redox modifications of their cysteine residues, which are critically important in the control of protein function. Given the importance of redox homeostasis in the regulation of stem cell self-renewal, understanding the underlying molecular mechanisms involved will provide important new insights into stem cell biology.     

miRNAs在干细胞自我更新和分化中的调控作用

干细胞与microRNAs(miRNAs)均为近年来研究的热点问题。干细胞是一类具有自我更新与多项分化潜能的细胞, 因与生物发育和癌症发生的密切联系而越来越受到人们的重视。miRNAs是一类长约22nt的小分子非编码RNA, 具有高度的种间保守性和时空特异性, 在转录后水平调节靶基因的表达, 是细胞内基因表达的基本调控机制之一。最近的一些研究表明, miRNAs在干细胞的自我更新和分化过程中具有重要的调控作用。这些研究主要采用两种策略: (1)缺失/突变干细胞中miRNAs合成途径必需酶(包括Dicer1、Loqs、DGCR8、Argnaute蛋白等), 通过细胞特性变化来研究其功能; (2)直接筛选干细胞中的特异性miRNAs并研究其功能。针对干细胞中miRNAs的研究对深入了解干细胞自我更新和分化的机制以及干细胞的鉴定具有重要的意义。文章基于近年来的研究对干细胞相关的miRNAs进行了综述。     

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.     

Erk2 signaling and early embryo stem cell self-renewal

Differentiation of the mammalian blastocyst generates two distinct cell lineages: the trophectoderm, which contributes to the trophoblast layers of the placenta, and the inner cell mass, which forms the embryo. We and others recently demonstrated that the MAP kinase ERK2 is essential for trophoblast development. Erk2 mutant embryos fail to form extra-embryonic ectoderm and the ectoplacental cone, suggesting a role for ERK2 activation in the proliferation of trophoblast stem (TS) cells. Previous studies have documented that ERK1/2 activity is dispensable for proliferation of embryonic stem (ES) cells and rather interferes with self-renewal. Thus, signaling by the ERK1/2 MAP kinase pathway appears to be critical for the regulation of self-renewal and propagation of early embryo stem cell populations.