成体干细胞的可塑性:横向分化还是细胞融合?

近年来研究显示成体干细胞(adult stem cells)具有可塑性(plasticity),不仅可以生成它们所在组织的成熟细胞,而且在特定环境下能分化成其他组织类型细胞,这种跨系或跨胚层分化现象称为横向分化或转分化(transdifferentiation)。横向分化已为成体干细胞的研究和临床应用包括组织器官损伤的修复提供了新的思路和应用前景。然而,最近的一些研究进展又引出不同的解释,即成体干细胞的可塑性是由于细胞融合(cellfusion)的结果。在此,就成体干细胞的可塑性、横向分化、细胞融合等方面研究作一综述。     

干细胞与心肌细胞替代治疗

胚胎干细胞及来源于骨髓、骨骼肌、血管、肝脏、皮肤、脂肪等组织器官的成体干细胞均有多向分化潜能。胚胎干细胞可分化为3个胚层的所有组织细胞。成体干细胞具有可塑性和转分化的潜能。在一定条件下,这些干细胞可被诱导分化为心肌细胞。成年心脏可能存在心肌干细胞,具有增殖和分化为包括跳动性心肌细胞的多种细胞的潜能。因此,干细胞可用于心肌细胞替代治疗,以替代死亡的心肌细胞,改善心脏功能,防治心肌梗塞后心衰、减少心肌重构等症状。本文对干细胞治疗心肌梗塞有关进展及问题作一综述。     

脑损伤后星形胶质细胞的重编程及意义

脑损伤后神经元的死亡及凋亡使脑组织功能受损,是患者出现肢体、语言功能障碍等后遗症的主要原因。因此,修复受损脑组织的神经元是治疗的关键。近年研究表明,星形胶质细胞能发生重编程转化为神经元,其重编程的方式有去分化和转分化两种。去分化主要在体外诱导星形胶质细胞形成神经球,但这种神经球移植回体内后并不能产生神经元。转分化方式,包括直接转分化和间接转分化。间接转分化过程产生新生神经元的周期较长,且存在形成肿瘤的风险;直接转分化尤其是体内的直接转分化方式既避免了细胞移植的复杂过程,又能避免间接转分化方式形成肿瘤的风险,是脑损伤后新生神经元最安全有效的方法。该文就正常星形胶质细胞与脑损伤后反应性星形胶质细胞的重编程的机制和意义进行综述。     

胰岛beta细胞再生研究进展

胰岛beta细胞分泌胰岛素调控体内血糖水平,胰岛beta细胞数量减少会导致糖尿病的发生。胰岛移植是目前治疗糖尿病的有效方法,但是目前仍然面临供体短缺等巨大障碍,因此研究胰岛beta细胞再生对于糖尿病的临床治疗具有深远意义。beta细胞的再生来源主要包括内源性beta细胞增殖、多能干细胞分化和其他非beta细胞的转分化。成体是否存在内源性胰腺干细胞依然是领域内亟待解决的重要科学问题之一。本文总结了与胰岛beta细胞再生相关的研究发现与进展,并讨论了内源性胰岛beta细胞增殖、诱导多能干细胞分化、非胰岛beta细胞重编程等方法在糖尿病治疗中需要注意的问题和潜在应用前景。     

胰腺干细胞生物学特性研究进展

胰腺干细胞是一类来源于胎儿或成年胰腺组织中的成体干细胞.多数研究认为胰腺干细胞体外培养时,贴壁生长,呈多角形上皮样,具有较高的扩增性;表达胰腺发育过程中的一些决定因子Pdx1、HNF3β、Ngn3、Th及Isll等,还共表达胰腺终末细胞释放的激素glucagon、insulin等,甚至共表达来源于其它胚层细胞的表达物CK19、nestin等;具有多分化潜能,其特点是在自然分化过程中,优先分化为胰腺组织细胞,在特定的条件下,也可转分化为其它组织细胞.胰腺干细胞生物学的不断深入研究,为分离、克隆胰腺干细胞及定向诱导其分化为功能性胰岛移植治疗人类糖尿病奠定了基础.     

成体干细胞多能性研究进展

成体干细胞是存在于机体组织的一类原始状态细胞,它们能够进行自我复制和特异分化,用于维持新陈代谢和创伤修复,年珲来越来越多的实验表明成体干细胞多向分化潜能,一种组织的干细胞可以分化成其他组织类型的细胞。作者介绍了国际上对成体干细胞概念的新看法,讨论了成体干细胞多能性的调控机理及与之相关的研究方法,还简要概括了成体干细胞在理论和临床应用上的重要意义。     

成体干细胞在1型糖尿病中的应用研究及展望

成体干细胞存在于机体已分化的组织中,可在一定条件下分化为特定类型的细胞。成体干细胞来源广,移植后不存在免疫排斥反应,在1型糖尿病治疗领域有广阔的应用前景,可以作为受损伤或无功能的β细胞的替代细胞。该文主要概述了近年来成体干细胞在1型糖尿病中的应用研究及面临的问题。     

成体干细胞向肝细胞分化

成体干细胞跨越胚层限制分化为其他胚层来源的细胞,对揭示不同胚层细胞间相互分化的生物学意义和机制具有重要学术价值,并可以为临床细胞移植治疗开辟新的途径,从而成为当前研究的热点之一。综述了近年来肝源性卵圆细胞、成肝细胞、骨髓源干细胞和其他成体干细胞跨越分化为肝细胞的研究现状与进展,以及卵圆细胞、成肝细胞等的分离鉴定,表面标志、生物学特征和跨越分化机制,并对成体干细胞在肝脏疾病细胞治疗上的应用前景作了展望。     

干细胞概述

干细胞是存在于胚胎和成体中的一类特殊细胞,它能长期地自我更新,在特定的条件下具有分化形成多种终末细胞的能力,不同来源的干细胞分化潜能各异。从早期胚胎内细胞团分离的胚胎干细胞能分化形成个体所有的细胞类型,并具有在体外无限增殖的能力,是最具有临床应用前景和研究价值的干细胞之一。在成体各种组织和器官中也存在成体干细胞,用于维持机体结构和功能的稳态。近期有关成体干细胞可塑性的研究和成体组织中多能干细胞存在的证据扩大了人们对成体干细胞分化潜能的认识。干细胞具有的多向分化潜能和自我更新能力使其成为未来再生医学的重要种子细胞,并成为研究人类早期胚层特化和器官形成、药物筛选以及基因治疗的最佳工具。     

上皮细胞转分化及其表观遗传学调控

上皮细胞转分化现象及其与疾病发生发展的关系,近年已成为细胞生物学、免疫学等多学科关注的聚焦点。转分化作为细胞分化发育的基本生物学现象,存在于机体诸多生理病理过程,也受表观遗传学的调控。相对于经典遗传学而言,表观遗传学作为一门新兴学科,其为生物体的基因表达调控及遗传现象提供了新的理论阐释。现知,DNA甲基化、组蛋白修饰及非编码RNA等均可导致上皮细胞基因发生表观遗传改变,与上皮细胞转分化的发生发展密切相关,并在该过程中发挥重要的调控作用。进一步阐明细胞转分化的分子基础及其表观遗传学调控机制,将有助于认识生命现象基本过程,并可为炎症性疾病、自身免疫病、器官纤维化,以及肿瘤发生与转移等机制的研究与防治,提供新的思路和应对策略。对上皮细胞转分化与表观遗传学调控关系作一简述。     

Transdifferentiation of adipogenically differentiated cells into osteogenically or chondrogenically differentiated cells: Phenotype switching via dedifferentiation

Reprogramming is a new wave in cellular therapies to achieve the vital goals of regenerative medicine. Transdifferentiation, whereas the differentiated state of cells could be reprogrammed into other cell types, meaning cells are no more locked in their differentiated circle. Hence, cells of choice from abundant and easily available sources such as fibroblast and adipose tissue could be converted into cells of demand, to restore the diseased tissues. Before diverting this new approach into effective clinical use, transdifferentiation could not be simply overlooked, as it challenges the normal paradigms of biological laws, where mature cells transdifferentiate not only within same germ layers, but even across the lineage boundaries. How unipotent differentiated cells reprogram into another, and whether transdifferentiation proceeds via a direct cell-to-cell conversion or needs dedifferentiation. To address such questions, MSC were adipogenically differentiated followed by direct transdifferentiation, and subsequently examined by histology, immunohistochemistry, qPCR and single cell analysis. Direct cellular conversion of adipogenic lineage cells into osteogenic or chondrogenic resulted in mixed culture of both lineage cells (adipogenic and new acquiring osteogenic/chondrogenic phenotypes). On molecular level, such conversion was confirmed by significantly upregulated expression of PPARG, FABP4, SPP1 and RUNX2. Chondrogenic transdifferentiation was verified by significantly upregulated expression of PPARG, FABP4, SOX9 and COL2A1. Single cell analysis did not support the direct cell-to-cell conversion, rather described the involvement of dedifferentiation. Moreover, some differentiated single cells did not change their phenotype and were resistant to transdifferentiation, suggesting that differentiated cells behave differently during cellular conversion. An obvious characterization of differentiated cells could be helpful to understand the process of transdifferentiation.     

Stem-cell-based approaches for regenerative medicine

Recent success in transplantation of islets raises the hopes of diabetic patients that replacement therapies may be a feasible treatment of their disease. Although several lines of evidence suggest that stem cells exist in the pancreas, it is still technically hard for us to isolate or maintain the stem cells in vitro. The establishment of human embryonic stem (ES) cells has excited scientists regarding their potential medical use in tissue replacement therapy. When applied with appropriate signals, ES cells can be directed to differentiate into a specific cell lineage. Therefore, ES cells are no doubt an excellent source not only for regenerative medicine but also for studies of early events of pancreatic development, and to portray the pancreatic progenitor cells. Despite many attempts that have been tried, the efficiency of differentiation of ES cells into islets is still very low. This low efficiency reflects our lack of understanding of the intrinsic and extrinsic signals which regulate the developmental processes of the pancreas. In this review, I present a summary of recent works on ES cells, the identification of pancreatic progenitor cells from the adult pancreas, and refer to the possibilities of transdifferentiation from adult stem cells derived from other tissues.     

Cyclin-dependent kinase 4 signaling acts as a molecular switch between syngenic differentiation and neural transdifferentiation in human mesenchymal stem cells

Multipotent mesenchymal stem/stromal cells (MSCs) are capable of differentiating into a variety of cell types from different germ layers. However, the molecular and biochemical mechanisms underlying the transdifferentiation of MSCs into specific cell types still need to be elucidated. In this study, we unexpectedly found that treatment of human adipose- and bone marrow-derived MSCs with cyclin-dependent kinase (CDK) inhibitor, in particular CDK4 inhibitor, selectively led to transdifferentiation into neural cells with a high frequency. Specifically, targeted inhibition of CDK4 expression using recombinant adenovial shRNA induced the neural transdifferentiation of human MSCs. However, the inhibition of CDK4 activity attenuated the syngenic differentiation of human adipose-derived MSCs. Importantly, the forced regulation of CDK4 activity showed reciprocal reversibility between neural differentiation and dedifferentiation of human MSCs. Together, these results provide novel molecular evidence underlying the neural transdifferentiation of human MSCs; in addition, CDK4 signaling appears to act as a molecular switch from syngenic differentiation to neural transdifferentiation of human MSCs.     

Cnidarians as a model system for understanding evolution and regeneration

Hydra and Podocolyne are two cnidarian animals which provide complementary advantages for analysing developmental mechanisms possibly reflecting the basic developmental processes shared by most bilaterians. Interestingly, these mechanisms remain accessible all along the life of these animals, which bud and regenerate, whatever their age. The Hydra polyp permits a direct study of the molecular cascades linking amputation to regeneration. Podocoryne displays a complete life cycle, polyp and medusa stages with a fast and inducible sexual cycle and an unparalleled In vitro transdifferentiation potential. In both cases, a large number of evolutionarily conserved molecular markers are available, and analysis of their regulation highlights the molecular mechanisms which underly pattern formation in these two species.     

Microarray analyses of transdifferentiated mesenchymal stem cells

The molecular events associated with the age-related gain of fatty tissue in human bone marrow are still largely unknown. Besides enhanced adipogenic differentiation of mesenchymal stem cells (MSCs), transdifferentiation of osteoblast progenitors may contribute to bone-related diseases like osteopenia. Transdifferentiation of MSC-derived osteoblast progenitors into adipocytes and vice versa has previously been proven feasible in our cell culture system. Here, we focus on mRNA species that are regulated during transdifferentiation and represent possible control factors for the initiation of transdifferentiation. Microarray analyses comparing transdifferentiated cells with normally differentiated cells exhibited large numbers of reproducibly regulated genes for both, adipogenic and osteogenic transdifferentiation. To evaluate the relevance of individual genes, we designed a scoring scheme to rank genes according to reproducibility, regulation level, and reciprocity between the different transdifferentiation directions. Thereby, members of several signaling pathways like FGF, IGF, and Wnt signaling showed explicitly differential expression patterns. Additional bioinformatic analysis of microarray analyses allowed us to identify potential key factors associated with transdifferentiation of adipocytes and osteoblasts, respectively. Fibroblast growth factor 1 (FGF1) was scored as one of several lead candidate gene products to modulate the transdifferentiation process and is shown here to exert inhibitory effects on adipogenic commitment and differentiation.     

Transdifferentiation--fact or artifact

Normal development appears to involve a progressive restriction in developmental potential. However, recent evidence suggests that this progressive restriction is not irreversible and can be altered to reveal novel phenotypic potentials of stem, progenitor, and even differentiated cells. While some of these results can be explained by the presence of contaminating cell populations, persistence of pluripotent stem cells, cell fusion, etc., several examples exist that are difficult to explain as anything other than "true transdifferentiation" and/or dedifferentiation. These examples of transdifferentiation are best explained by understanding how the normal process of progressive cell fate restriction occurs during development. We suggest that subversion of epigenetic controls regulating cell type specific gene expression likely underlies the process of transdifferentiation and it may be possible to identify specific factors to control the transdifferentiation process. We predict, however, that transdifferentiation will not be reliable or reproducible and will probably require complex manipulations.     

MicroRNAs and regeneration: Let-7 members as potential regulators of dedifferentiation in lens and inner ear hair cell regeneration of the adult newt

MicroRNAs are known to regulate the expression of many mRNAs by binding to complementary target sequences at the 3'UTRs. Because of such properties, miRNAs may regulate tissue-specific mRNAs as a cell undergoes transdifferentiation during regeneration. We have tested this hypothesis during lens and hair cell regeneration in newts using microarray analysis. We found that distinct sets of miRNAs are associated with lens and hair cell regeneration. Members of the let-7 family are expressed in both events and they are regulated in a similar fashion. All the let-7 members are down regulated during the initiation of regeneration, which is characterized by dedifferentiation of terminally differentiated cells. This is the first report to correlate expression of miRNAs as novel regulators of vertebrate regeneration, alluding to a novel mechanism whereby transdifferentiation occurs.     

ITE promotes hypoxia-induced transdifferentiation of human pulmonary arterial endothelial cells possibly by activating transforming growth factor-β/Smads and MAPK/ERK pathways

This study aimed to investigate the transdifferentiation of human pulmonary arterial endothelial cells (HPAECs) into smooth muscle like (SM-like) cells under hypoxic conditions and reveal the role of endogenous small molecular compound 2-(1′H-indole-3′-carbonyl)-thiazole-4-carboxylicacid methyl ester (ITE) in this process. HPAECs were treated by hypoxia and hypoxia + ITE with different durations. The endothelial markers (CD31 and VE-cad) and smooth muscle markers (α-SMA, SM22α, and OPN) were investigated by immunofluorescence double staining, and their expressions, along with the differentiation regulators transforming growth factor-β (TGF-β) ligands and downstream signals including TGF-β1, bone morphogenetic protein (BMP2), BMP9, Samd2/3, ERK, and p38 MAPK, were determined by Western blot analysis. The viability and proliferation of HPAECs were detected by Cell Counting Kit-8 (CCK-8) method and bromodeoxyuridine (BrdU) assays. As a result, hypoxia induced HPAECs transdifferentiation from paving-stone-like into polygonal or spindle cells, whose number increased greatly after additional ITE stimulation for 7 days. Compared with the normoxic HPAECs, the expression of endothelial markers reduced and smooth muscle markers were enhanced with the extension of hypoxia + ITE treatment, and meanwhile the cell viability increased significantly. Hypoxia could promote expression of TGF-β1 protein rather than BMP2 and BMP9, and regulate phosphorylation levels of Samd2/3, ERK and p38 MAPK in different manners. In conclusion, ITE can promote the hypoxia-induced transdifferentiation of HPAECs into SM-like cells via TGF-β/Smads and MAPK/ERK pathways.