Bài 3: Cơ sở phân tử tính tự làm mới

CƠ SỞ PHÂN TỬ TÍNH TỰ LÀM MỚI TS. Trần Hồng Diễm PTN Nghiên cứu và Ứng dụng Tế bào gốc Trường Đại học KHTN - Đại học Quốc Gia Tp. HCM 10/10/2015 Sự tự làm mới (self-renewal) là quá trình tạo ra những bản sao tế bào gốc thông qua quá trình nguyên phân Ít nhất 1 tế bào chị em vẫn giữ khả năng tự làm mới và biệt hoá •  Phân bào đối xứng (Symmetric cell division) •  Phân bào bất đối xứng (Asymmetric cell division) SELF-RENEWAL tạo ra •  Tạo ra 2 tế bào gốc •  Sự tự làm mới bởi quá trình phân bào đối xứng thường gặp ü  ở các tế bào gốc nhất thời, chúng xuất hiện trong quá trình phát triển của phôi ở GĐ sớm để gia tăng kích thước cơ thể ü  Trong quá trình tái sinh sau tổn thương PHÂN BÀO ĐỐI XỨNG •  Tạo ra 1 tế bào gốc và 1 tế bào biệt hoá hoặc 1 tế bào gốc với khả năng biệt hoá có giới hạn •  Sự tự làm mới bởi quá trình phân bào bất đối xứng có thể thấy ở các tế bào gốc trong phôi ở giai đoạn phát triển muộn và trong cá thể trưởng thành để duy trì cân bằng nội mô PHÂN BÀO BẤT ĐỐI XỨNG observed in mammalian NE cells (Konno et al. 2008). This raises the intriguing possibility that a “parallel” spindle is the result of an elab- orate regulation of spindle orientation, and that a slight tilt in orientation within a small range is actively controlled to dictate symmetric versus asymmetric NE divisions. Interestingly, recent studies found an- other asymmetry during NE cell divisions: Prominin-1, or CD133, localizes to the mid- body ring of dividing NE cells with an interest- ing asymmetry, correlated with the mode of stem cell division. Prominin-1 is expressed in many adult stem cells and cancer stem cells, although its function remains unclear. Before the neurogenic stage of development, when NEcellsareproliferatingbysymmetricdivisions, the Prominin-1-containing midbody ring is excluded from both daughters, being released into the extracellular space (Dubreuil et al. 2007; Farkas and Huttner 2008). Once neurogenesis begins later in development, NE cells start dividing asymmetrically and the Prominin-1-containing ring is inherited by the stem cell (NE cell), whereas the daughter fated to differentiate is devoid of this ring. This strong correlation implies that the Prominin-1-containing midbody ring might be somehow involved in stem cell behavior such as stem cell potential or asymmetric stem A Apical B Apical Neuroectoderm Symmetric Symmetric Asymmetric Symmetric (mutant situation) Asymmetric Figure 3. Spindle orientation and asymmetric division in Drosophila and mouse neuronal stem cells. (A) In Drosophila neuroblast, spindle is oriented perpendicular to the apical crescent (red line) that contains the Baz (Par3)-Par6-aPKC complex, as well as Pins, Insc, and Gai, leading to asymmetric stem cell division. The apical crescent is required for spindle orientation, the basal crescent formation and spindle size asymmetry (i.e., the apical half is larger than the basal half ). The basal crescent (yellow line) contains fate determinants that promote/allow differentiation such as Numb, Miranda, and Prospero. In the mutants that are defective in spindle orientation, the apical and basal crescents are bisected into two daughters, leading to symmetric stem cell division. Neuroectoderm cells (from which neuroblasts are derived) also have the apical complex, except for Insc. They divide symmetrically by orienting mitotic spindle parallel to the apical crescent. Ectopic expression of Insc in these cells result in the recruitment of Pins to the apical cortex, leading to perpendicular spindle orientataion (Yu et al. 2000). (B) In mammalian (mouse) neuroepithelial cells, the mode of cell division shifts from symmetric to asymmetric during development. The stem cell identity is determined by inheritance of a tiny apical cortex containing cadherin (red line), and thus mitotic spindle does not have to tilt significantly to divide asymmetrically. Y.M. Yamashita et al. 10 Cite this article as Cold Spring Harb Perspect Biol 2010;2:a001313 Drosophila mouse Spindle orientation and asymmetric division in neuronal stem cells Asymmetric division of the Drosophila neuroblast is controlled by a closely related mechanism3,17. Moreover, in Drosophila neuroblasts, an evolutionarily conserved cell fate determinant, Numb, is asymmetri- cally localized to daughter cells that are destined to differentiate18. A classic example of an asymmetric division that is controlled by an extrinsic mechanism is provided by the Drosophila germline stem cell, which divides with a reproducible orientation to generate one daughter that remains in the stem-cell niche and retains stem-cell identity, and one daughter that is placed away from the niche and begins to differen- tiate4,19,20. A stem-cell niche is defined as a ‘microenvironment’ that pro- motes stem-cell maintenance (refs 21, 22; see also page 1075). Cells that create stem-cell niches include cap cells in the Drosophila ovary19 and hub cells in the Drosophila testis23,24. In the ovary, cap cells synthesize ligands called Decapentaplegic (DPP) and Glass bottom boat (GBB) that activate bone morphogenetic protein (BMP) signalling in germ- line stem cells, thereby repressing the gene bag-of-marbles25,26, which encodes a protein that promotes differentiation27. In the testis, hub cells synthesize a ligand called Unpaired that activates the JAK–STAT (Janus kinase and signal transducer and activator of transcription) signalling pathway in germline stem cells to prevent differentiation, presumably by controlling target genes that remain to be identified4,23,24. Special- ized junctions at the interface between the niche and germline stem cells anchor the stem cell to the niche28,29. The mechanism controlling orientation of the mitotic spindle relies on centrosomal components in spermatogonial stem cells29. More importantly for our discussion here, the orientation of these asymmetric stem-cell divisions controls the location of daughter cells and thus their access to extrinsic signals that regulate stem-cell identity. It is important to note that asymmetric divisions can be governed by both intrinsic partitioning of fate regulators and asymmetric expo- sure to extrinsic cues. Sperm entry initiates asymmetry of the C. elegans zygote30,31, and signalling from the neural epithelium orients divisions of Drosophila neuroblasts32 (Fig. 2c). Furthermore, Numb modifies the response to Notch signalling of the daughter cell that inherits it, indicat- ing that cell fate determinants can function by altering the response to external cues33. By contrast, asymmetric division of Drosophila germline stem cells does not seem to rely on partitioning of cell fate determi- nants. Although each germline stem cell is marked by a cytoplasmic organelle called the ‘spectrosome’, the function of this asymmetrically distributed organelle remains uncertain. In addition, daughters of the germline stem-cell division seem to be equivalent in developmental potential, as we discuss below. Thus, these divisions seem to be intrinsi- cally symmetric in terms of developmental potential, but seem to achieve an asymmetric outcome through the distinct positions of the daughter cells relative to the niche. Some mammalian stem-cell divisions possess hallmarks of asymme- try and seem to be controlled by evolutionarily conserved mechanisms. An example is the division of neural progenitors. Undifferentiated neu- ral progenitors in the developing rodent cortex distribute Numb asym- metrically to precursors destined for neurogenesis34–36. The inhibition of Notch signalling by Numb is crucial for neurogenesis in flies, and it also seems to be involved in the regulation of mammalian asymmetric division33,37,38. Numb is also asymmetrically distributed to progeny of cultured satellite muscle cells, where it promotes myogenic differen- tiation of one daughter cell39. Thus, asymmetric segregation of Numb may be a common mode of control. A second example is the regulated orientation of mitotic spindles, which has been found in both mamma- lian basal epidermal progenitors7 and cortical ventricular zone neural progenitors40. Indeed, spindle orientation relies on the cortical locali- zation of the conserved PAR–aPKC complex7, a mechanism that also controls the asymmetric division of Drosophila neuroblasts41,42. Thus, mammalian progenitors are likely to use some of the mechanisms of invertebrate progenitors to divide asymmetrically. Symmetric divisions can expand stem-cell number Symmetric stem-cell divisions have been observed during the devel- opment of both invertebrates and vertebrates. Symmetric stem-cell divisions are also common during wound healing and regeneration. A hallmark of all three processes is an increase in the number of stem cells. This increase cannot be explained by a strategy restricted to asymmetric cell division in which only one daughter cell maintains stem-cell identity. A classic example of symmetric stem-cell division during develop- ment occurs in the C. elegans germ line. The larval nematode hatches from its eggshell with only two germline stem cells but, during sub- sequent larval development, these germ cells proliferate to produce roughly 2,000 descendants in the adult gonad, including one pool of undifferentiated germ cells and another pool of differentiating gam- etes5,43 (Fig. 3a). During larval development and in adults, C. elegans germline stem cells are maintained by signalling from a niche formed by the ‘distal tip cell’5. In contrast to the Drosophila niches, which rely on BMP and JAK–STAT signalling23,24,44,45, the distal tip cell niche uses Notch signalling to control C. elegans germline stem cells throughout development and in adults43. a Self-renewal Generation of differentiated cells c d Stem-cell population b Stem-cell population Figure 1 | Stem-cell strategies. a, Stem cells (orange) must accomplish the dual task of self-renewal and generation of differentiated cells (green). b–d, Possible stem-cell strategies that maintain a balance of stem cells and differentiated progeny. b, Asymmetric cell division: each stem cell generates one daughter stem cell and one daughter destined to differentiate. c, d, Population strategies. A population strategy provides dynamic control over the balance between stem cells and differentiated cells — a capacity that is necessary for repair after injury or disease. In this scheme, stem cells are defined by their ‘potential’ to generate both stem cells and differentiated daughters, rather than their actual production of a stem cell and a differentiated cell at each division. c, Symmetric cell division: each stem cell can divide symmetrically to generate either two daughter stem cells or two differentiated cells. d, Combination of cell divisions: each stem cell can divide either symmetrically or asymmetrically. b ca Figure 2 | Controls of asymmetric stem-cell division. Three simple mechanisms are shown, but others are plausible. For molecular details, see recent reviews1–4,12,17,31. a, Asymmetric localization of cell polarity regulators (red) initiates the asymmetric division. Shown is asymmetric assembly of the PAR–aPKC complex at one end of the dividing cell. Stem cells are orange, differentiated cells are green. b, Cell fate determinants (red) can be segregated to the cytoplasm of one daughter cell, as shown here, or they can be associated with the membrane, centrosome or another cellular constituent that is differentially distributed to the daughters. c, Regulated orientation of the mitotic spindle retains only one daughter in the stem-cell niche (red), such that only that daughter cell has access to extrinsic signals necessary for maintaining stem-cell identity. This mechanism achieves an asymmetric outcome, even though the division itself is intrinsically symmetric. In an alternative but similar model, the daughter cell placed away from the niche is exposed to signals that induce differentiation. 1069 NATURE|Vol 441|29 June 2006 INSIGHT REVIEW Morrison.indd 1069 16/6/06 3:15:07 pm Nature Publishing Group ©2006 Tế bào gốc Asymmetric division of the Drosophila neuroblast is controlled by a closely related mechanism3,17. Moreover, in Drosophila neuroblasts, an evolutionarily conserved c ll fate determinant, Numb, is asymmetri- ally oca iz to daughter cells that ar destined to differentiate18. A classic example of an asymmetric div sio that is controlled by an extrinsic mechanism is provided by th Drosophila germline stem cell, which divid s with a reproducible o entat on to generate one daughter that remains in the stem-cell niche and retains stem-cell id n ity, and one daughter tha is placed away fr m the niche and begins to ifferen- iate4,19,20. A tem-cell niche is defin d as a ‘microenvironment’ that pro- motes stem-cell maintenance (refs 21, 22; see also page 1075). Cells that create stem-c ll niches include cap cells in the Drosophila ovary19 and hub cells in the Drosophila testis23,24. In th ovary, cap cells synthesize ligand called Decap ntaplegic (DPP) and Glass b ttom bo t (GBB) that activate bone m rphog netic protein (BMP) signal ing i g rm- ne stem cells, thereby re r ssing the ge e bag-of-marbles25,26, which encodes a protei that promotes differentiation27. In the testis, hub cells synthesize a ligand called Unpaired t at activates the JAK–STAT (Janus kinase and signal ransducer and activator of transcription) signalling pathway in germline stem cells to prevent diff rentiation, presumably by controllin t rget genes that remain to be identified4,23,24. Spec al- ized junctions at th interface betw en the niche and germline stem cells anchor the stem c ll to t e niche28,29. The mechanism controll ng orientation of the mito ic spindle relies on centrosomal components in spermatogonial stem cells29. Mor importantly for our discuss on here, he rien ation of these asymmetric stem-cell divisions c trol the location of daughter ll and thus their access t extrinsic signals that regulate stem-cell identity. It is important to note that asymme ric divisions can be gover ed by both intrinsic partit oning of fate regulators and asymmetric expo- sure to extrinsic cues. Sp rm entr initiates asymmetry of th C. elegans z gote30,31, and signalling from the neural epithelium orients divisions of Dr sophila neuroblasts32 (Fig. 2c). Furthermore, Numb modifi s the response to Notch signalling of t e daughter cell that inherits it, nd cat- ing that cell fate determinants can function by altering the respon e to external cues33. By contrast, asymmetric division of Drosophila germline stem cells does no seem to rely on partitioning of cell fat determi- nants. Although each germline st m ell i marked by a cytoplasmic organel e call d the ‘spectr some’, the function of this asymmetrically distributed organ lle remains uncertain. In addition, daughters of the germline stem-cell division seem to be equivalent in developmental potential, as we discuss below. Thus, hese divisions seem to be intrinsi- cally symmetric in terms of dev lopmental potential, but s em to achieve an asymmetric out ome throug the di tinct positions of the daughter e s relativ to the niche. Some ammalian stem-cell divisions possess hallmarks of asymm - try and seem to be controlled by evolutionarily conserved mechanisms. An example is the division of neural progenitor . Undifferenti ted neu ral progenitors in the developing r dent co tex distribute Numb asy - metrically to precursors destined for neurogenesis34–36. The nhibition of Notch s gnalling by Numb is c ucial for n urogenesis in flies, and it also seems t be involved in the regulation of mammalian asymmetric division33,37,38. Num is also asymmetrically distributed to proge y of cultur d sa ellite muscle cells, where it promotes yogenic differen- tiat on of one da ghter cell39. Thus, asymmetric segrega ion f Numb may b a common mod of control. A second example is the regulated orientation of mitotic spindles, which has be n found in both mamma- lian bas l epidermal progenitors7 and cortical ventricular zon neural p ogenit rs40. Indeed, spindle orientation r lies o the c rtical locali z tion of the conserved PAR–aPKC complex7, a me hanism that also controls the asymmetric divisi n of Dr sophila neuroblasts41,42. Thus, mammalian prog nitors are likely to use some of the mechanisms of inve tebrat progenitors to d vide asymmetric lly. Symmetric divisions can expand stem-cel number Symmetric stem-cell divisions have been observed during the devel- op ent of both invertebrat s and vertebrat s. Sy m tric stem-cell divisions are also common during wou d healing and re en ration. A hallmark of all three processes is an increase in the number of stem cell . This increase cannot be explained by strategy restricted to asym et ic cel division in which o ly one daug ter cell maintains stem-cell identity. A classic example of symmetric stem-cell division during develop- ment occurs in he C. elegans germ line. The larval nematode hatches from it eggshell with only two germline stem cells b t, during sub sequent la val development, these germ cells prolif rate to produce roughly 2,000 descenda ts in the adult gonad, inc uding one pool of undifferenti te germ cells and another pool of dif entiating gam- etes5,43 (Fig. 3a). During larval d velopme t and in adults, C. elegans germline stem cells are maintained by signalling from a iche formed by the ‘distal tip cell’5. In contrast to the Drosophila niches, which rely on BMP and JAK–STAT sig alling23,24,44,45, the distal tip cell niche uses No ch sign lling to control C. elegans germline stem cells throughout development and in adults43. a Self-renewal Generation of differentiated cells c d Stem-cell population b Stem-cell population Figure 1 | Stem-cell strategies. a, Stem cells (orange) must accomplish the dual task of self-renewal and generation of differentiated cells (green). b–d, Possible stem-cell strategies that maintain a balance of stem cells and differentiated progeny. b, Asymmetric cell division: each stem cell generates one daughter ste cell and one daughter destined to differentiate. c, d, Population strategies. A population strategy provides dynamic control over the balance between stem cells and differentiated cells — a capacity that is necessary for repair after injury or disease. In this scheme, stem cells are defined by their ‘potential’ to generate both stem cells and differentiated daughters, rather than their actual production of a stem cell and a differentiated cell at each division. c, Symmetric cell division: each stem cell can divide symmetrically to generate either two daughter stem cells or two differentiated cells. d, Combination of cell divisions: each stem cell can divide either sym etrically or asymmetrically. b ca Figure 2 | Controls of asymmetric stem-cell division. Three simple mechanisms are shown, but others are plausible. For molecular details, see recent reviews1–4,12,17,31. a, Asymmetric localization of cell polarity regulators (red) initiates the asymmetric division. Shown is asy metric assembly of the PAR–aPKC complex at one end of the dividing cell. Stem cells are orange, differentiated cells are green. b, Cell fate deter inants (red) can be segregated to the cytoplasm of one daughter cell, as shown here, or they can be associated with the membrane, centrosome or another cellular constituent that is differentially distributed to the daughters. c, Regulated orientation of the mitotic spindle retains only one daughter in the stem-cell