- Email: [email protected]
Differentiation and gene regulation Programming, reprogramming and regeneration
445
Differentiation and gene regulation Programming, reprogramming and regeneration Editorial overview Azim Surani and Austin Smith Current Opinion in Genetics & Development 2003, 13:445–447 0959-437X/$ – see front matter ß 2003 Elsevier Ltd. All rights reserved. DOI 10.1016/j.gde.2003.08.013
Azim Surani Wellcome Trust/Cancer Research UK Institute of Cancer and Developmental Biology, Tennis Court Road, University of Cambridge, Cambridge CB2 1QR, UK e-mail: [email protected]
Azim Surani is the Marshall-Walton Professor of Reproduction at the University of Cambridge. His laboratory studies the specification, development and properties of the mouse germ line, as well as the mechanisms of epigenetic reprogramming of the genome. Austin Smith The Institute for Stem Cell Research, Kings Buildings, University of Edinburgh, West Mains Road, Edinburgh EH9 3JQ, UK e-mail: [email protected] URL: http://www.iscr.ed.ac.uk
Austin Smith is an MRC (Medical Research Council) Research Professor and Head of the Institute for Stem Cell Biology at the University of Edinburgh. He is a stem cell biologist with a primary interest in the selfrenewal and pluripotency of embryonic stem cells.
Abbreviations ES embryonic stem HSC haematopoietic stem cell PcG Polycomb group
As the end of the era of major genome sequencing projects approaches, attention has shifted towards asking how this DNA sequence information is manipulated to create highly sophisticated organisms. Stem cell research and cloning are two complex areas of biological research that have somewhat unexpectedly come to the fore. The multilineage differentiation of embryonic stem (ES) cells illustrates that mobilisation of genetic information can generate a range of cell products in the laboratory. Reports on the identity, potency and alleged plasticity of tissue stem cells have now attracted major headlines and controversy. If we could understand how to program stem cell gene expression and direct differentiation, this would have tremendous potential for medical applications. Mammalian cloning by nuclear transfer of somatic nuclei demonstrates that the epigenetic information that fixes gene expression and maintains cellular diversity can somehow be erased and then reset. If we can elucidate how this reprogramming occurs, new avenues may be opened for stem cell production and tissue regeneration. Articles in this issue of Current Opinion in Genetics & Development capture evolving insights from a range of conceptual and experimental perspectives. The Polycomb group (PcG) proteins are key epigenetic regulators that mediate transcriptional silencing. There have been considerable advances recently towards the understanding of the mechanism by which these complexes work as discussed by Otte and Kwaks. PcG complexes operate in many different circumstances and the nature of the complex is contextdependent. This may be how PcG complexes exhibit diverse functions and act on different target genes. One of their key roles is in germ cells. Setting aside the germ cell lineage from somatic cells is a key decision in development. Germ cells represent investment towards the future of the species whereas somatic cells are of value only to the current individual. This germ cell/soma distinction, first defined by August Weissmann over 100 years ago [1], is best understood in molecular terms in Caenorhabditis elegans, as discussed here by Shin and Mello. In particular, they describe how the Polycomb and NuRD-like chromatin silencing complexes actively prevent germ cells from acquiring a somatic cell fate. The unique biology of germ cells is explored further by Donovan and de Miguel. They recount how mammalian primordial germ cells, which are normally exclusively dedicated to generation of gametes, can be converted into pluripotent stem cells either in tumours or by in vitro manipulation.
www.current-opinion.com
Current Opinion in Genetics & Development 2003, 13:445–447
446 Differentiation and gene regulation
This alteration of cell fate is a rare example of direct cellular reprogramming. A second is transdetermination of Drosophila imaginal discs, a process where changes in cell fate are well documented. Maves and Schubiger describe experiments that have indicated that response of selector genes to intercellular signalling molecules is at the centre of such transdetermination events. Now the challenge is to determine how signalling pathways act to generate an altered heritable chromatin structure that governs expression of selector genes. Cell differentiation status and potency can also be altered by cell fusion, which places a nucleus in a foreign cytoplasmic environment. Excluding fusion has become important when considering reports of ‘stem cell plasticity’, the apparent transdifferentiation of tissue stem cells in mammals. Vassilopoulos and Russell posit that fusion events between host and donor cells can explain some of the reported plasticity phenomena, albeit that the frequency of such fusion events is very low. The most dramatic example of reprogramming follows transfer of somatic nuclei into oocytes. Here the somatic nucleus can be fully reprogrammed to attain totipotency. There is a quest to unravel precisely how the specific chromatin state associated with a differentiated phenotype can be erased and an appropriate new state established to confer totipotency. Jouneau and Renard also remind us that many cloned embryos show placental defects. They suggest that because trophectoderm is the first specialised cell lineage that must be elaborated in a reprogrammed embryo, this may also be the most susceptible to errors. Moving away from a mammal-centric view, we see breathtaking instances of tissue regeneration in some species, prompting the question as to why stem cells in adult mammals are so reluctant to show such versatility. An immense capacity for regeneration in planarians is described by Agata. These worms can regenerate an entire individual from small fragments by recruiting pluripotent stem cells. This simple organism may provide important insights into the identity and functions of genes involved in the regeneration process. Some vertebrates also retain great capacity for regeneration such as those urodeles that can regenerate tails and limbs. Amputation results in production of new progenitor cells that are used to rebuild an entire anatomically correct limb. This is a remarkable example of tissue plasticity, apparently involving both dedifferentiation and transdetermination, as discussed in this issue by Tanaka. Better understanding of this system may bring us closer to unlocking a potential hidden regenerative capacity in adult cells in mammals. A more familiar and widely used model amphibian is Xenopus laevis. Okabayashi and Asashima describe use of the now classic Xenopus animal cap assay to follow differentiation events in response to signalling molecules. This Current Opinion in Genetics & Development 2003, 13:445–447
approach is somewhat analogous to current efforts to direct differentiation of mammalian pluripotent ES cells in vitro. The animal cap studies have provided important insights into the role of factors such as activin, and may present prototypes for generating organ systems in vitro and for delineating mechanisms of organogenesis. The longest studied and best understood of adult mammalian stem cells is the haematopoietic stem cell (HSC), discussed by Ema and Nakauchi. However, it is only relatively recently that successful purification of HSCs has been achieved enabling a direct examination of their molecular properties. The recent finding that the PcG protein Bmi-1 has an important role in self-renewal of HSCs is intriguing. At the same time, greater understanding of the control of differentiation of the cells in this lineage is emerging. Johnson and Calame discuss differentiation of B-lymphocytes using tools that should provide information on regulatory genetic networks. Neural crest cells have long been known to exhibit multipotency, most notably giving rise to both neural and nonneural cell types. Like stem cells, their fate is dictated by external cues, including cytokines and cell–cell interactions, that determine expression of transcription factors. Le Douarin and Dupin also discuss how differentiated neural crest cells can revert to multipotent precursors under certain conditions and subsequently undergo transdifferentiation. This may provide another opportunity to examine epigenetic reprogramming. A mesodermal derivative that appears to show multipotency is the mesoangioblast, as reported by Cossu and Bianco. These cells are found close to the developing aorta, can generate differentiated mesodermal cell types such as skeletal muscle in vitro, and retain the capacity for self-renewal. Delineating the origin of these cells may give insight into the relationship between vascular and extravascular mesoderm. Another area receiving considerable attention at present is neural stem cell biology. Doetsch describes the niche for stem cells in the adult mammalian brain. Defining how niches operate is central to understanding the maintenance of adult stem cells and their recruitment into differentiation. Signalling molecules, extracellular matrix and other cues appear to act in combination. Ongoing studies may uncover how intrinsic epigenetic programming of the stem cells is influenced by external cues. Neural tube development is probably one of the best understood systems for specification of alternative cell fates within a vertebrate tissue. Sonic hedgehog signalling upstream of the Gli transcriptional factors has been shown to play a critical role as discussed by Ruiz I Altaba, Nguyeˆ n and Palma. These studies reveal how Sonic hedgehog acts as a mitogen, morphogen and a survival factor. Detailed analyses are providing a paradigm for how the numbers of progenitors and their patterning are regulated during the development of complex structures. www.current-opinion.com
Editorial overview Surani and Smith 447
Finally, Byrne, Kidner and Martienssen discuss stem cells in plants where not only can they persist for years, but it is also possible to derive stem cells directly from somatic cells. Again, signalling pathways dictate cell-fate decisions, which are then propagated by epigenetic mechanisms. It is their evident versatility that has enabled plants to make up nearly 90% of the biomass on earth. Lessons that we can learn from plants and other organisms provide insights into gene regulation and how diverse cellular and organismal phenotypes can be generated. We
www.current-opinion.com
could also gain a greater understanding of the evolutionary forces that have shaped diverse systems in the living world. If we can master the ability to regulate stem cell gene expression and cellular differentiation, we will create exciting opportunities for regenerative applications in human medicine and perhaps also be better able to protect other species.
Reference 1.
Weissmann A: Die Kontinuitaet des Keimplasmas als Grundlage einer Theorie der Vererbung. Fisher-Verlag: Jena; 1885.
Current Opinion in Genetics & Development 2003, 13:445–447
Differentiation and gene regulation Programming, reprogramming and regeneration Editorial overview Azim Surani and Austin Smith Current Opinion in Genetics & Development 2003, 13:445–447 0959-437X/$ – see front matter ß 2003 Elsevier Ltd. All rights reserved. DOI 10.1016/j.gde.2003.08.013
Azim Surani Wellcome Trust/Cancer Research UK Institute of Cancer and Developmental Biology, Tennis Court Road, University of Cambridge, Cambridge CB2 1QR, UK e-mail: [email protected]
Azim Surani is the Marshall-Walton Professor of Reproduction at the University of Cambridge. His laboratory studies the specification, development and properties of the mouse germ line, as well as the mechanisms of epigenetic reprogramming of the genome. Austin Smith The Institute for Stem Cell Research, Kings Buildings, University of Edinburgh, West Mains Road, Edinburgh EH9 3JQ, UK e-mail: [email protected] URL: http://www.iscr.ed.ac.uk
Austin Smith is an MRC (Medical Research Council) Research Professor and Head of the Institute for Stem Cell Biology at the University of Edinburgh. He is a stem cell biologist with a primary interest in the selfrenewal and pluripotency of embryonic stem cells.
Abbreviations ES embryonic stem HSC haematopoietic stem cell PcG Polycomb group
As the end of the era of major genome sequencing projects approaches, attention has shifted towards asking how this DNA sequence information is manipulated to create highly sophisticated organisms. Stem cell research and cloning are two complex areas of biological research that have somewhat unexpectedly come to the fore. The multilineage differentiation of embryonic stem (ES) cells illustrates that mobilisation of genetic information can generate a range of cell products in the laboratory. Reports on the identity, potency and alleged plasticity of tissue stem cells have now attracted major headlines and controversy. If we could understand how to program stem cell gene expression and direct differentiation, this would have tremendous potential for medical applications. Mammalian cloning by nuclear transfer of somatic nuclei demonstrates that the epigenetic information that fixes gene expression and maintains cellular diversity can somehow be erased and then reset. If we can elucidate how this reprogramming occurs, new avenues may be opened for stem cell production and tissue regeneration. Articles in this issue of Current Opinion in Genetics & Development capture evolving insights from a range of conceptual and experimental perspectives. The Polycomb group (PcG) proteins are key epigenetic regulators that mediate transcriptional silencing. There have been considerable advances recently towards the understanding of the mechanism by which these complexes work as discussed by Otte and Kwaks. PcG complexes operate in many different circumstances and the nature of the complex is contextdependent. This may be how PcG complexes exhibit diverse functions and act on different target genes. One of their key roles is in germ cells. Setting aside the germ cell lineage from somatic cells is a key decision in development. Germ cells represent investment towards the future of the species whereas somatic cells are of value only to the current individual. This germ cell/soma distinction, first defined by August Weissmann over 100 years ago [1], is best understood in molecular terms in Caenorhabditis elegans, as discussed here by Shin and Mello. In particular, they describe how the Polycomb and NuRD-like chromatin silencing complexes actively prevent germ cells from acquiring a somatic cell fate. The unique biology of germ cells is explored further by Donovan and de Miguel. They recount how mammalian primordial germ cells, which are normally exclusively dedicated to generation of gametes, can be converted into pluripotent stem cells either in tumours or by in vitro manipulation.
www.current-opinion.com
Current Opinion in Genetics & Development 2003, 13:445–447
446 Differentiation and gene regulation
This alteration of cell fate is a rare example of direct cellular reprogramming. A second is transdetermination of Drosophila imaginal discs, a process where changes in cell fate are well documented. Maves and Schubiger describe experiments that have indicated that response of selector genes to intercellular signalling molecules is at the centre of such transdetermination events. Now the challenge is to determine how signalling pathways act to generate an altered heritable chromatin structure that governs expression of selector genes. Cell differentiation status and potency can also be altered by cell fusion, which places a nucleus in a foreign cytoplasmic environment. Excluding fusion has become important when considering reports of ‘stem cell plasticity’, the apparent transdifferentiation of tissue stem cells in mammals. Vassilopoulos and Russell posit that fusion events between host and donor cells can explain some of the reported plasticity phenomena, albeit that the frequency of such fusion events is very low. The most dramatic example of reprogramming follows transfer of somatic nuclei into oocytes. Here the somatic nucleus can be fully reprogrammed to attain totipotency. There is a quest to unravel precisely how the specific chromatin state associated with a differentiated phenotype can be erased and an appropriate new state established to confer totipotency. Jouneau and Renard also remind us that many cloned embryos show placental defects. They suggest that because trophectoderm is the first specialised cell lineage that must be elaborated in a reprogrammed embryo, this may also be the most susceptible to errors. Moving away from a mammal-centric view, we see breathtaking instances of tissue regeneration in some species, prompting the question as to why stem cells in adult mammals are so reluctant to show such versatility. An immense capacity for regeneration in planarians is described by Agata. These worms can regenerate an entire individual from small fragments by recruiting pluripotent stem cells. This simple organism may provide important insights into the identity and functions of genes involved in the regeneration process. Some vertebrates also retain great capacity for regeneration such as those urodeles that can regenerate tails and limbs. Amputation results in production of new progenitor cells that are used to rebuild an entire anatomically correct limb. This is a remarkable example of tissue plasticity, apparently involving both dedifferentiation and transdetermination, as discussed in this issue by Tanaka. Better understanding of this system may bring us closer to unlocking a potential hidden regenerative capacity in adult cells in mammals. A more familiar and widely used model amphibian is Xenopus laevis. Okabayashi and Asashima describe use of the now classic Xenopus animal cap assay to follow differentiation events in response to signalling molecules. This Current Opinion in Genetics & Development 2003, 13:445–447
approach is somewhat analogous to current efforts to direct differentiation of mammalian pluripotent ES cells in vitro. The animal cap studies have provided important insights into the role of factors such as activin, and may present prototypes for generating organ systems in vitro and for delineating mechanisms of organogenesis. The longest studied and best understood of adult mammalian stem cells is the haematopoietic stem cell (HSC), discussed by Ema and Nakauchi. However, it is only relatively recently that successful purification of HSCs has been achieved enabling a direct examination of their molecular properties. The recent finding that the PcG protein Bmi-1 has an important role in self-renewal of HSCs is intriguing. At the same time, greater understanding of the control of differentiation of the cells in this lineage is emerging. Johnson and Calame discuss differentiation of B-lymphocytes using tools that should provide information on regulatory genetic networks. Neural crest cells have long been known to exhibit multipotency, most notably giving rise to both neural and nonneural cell types. Like stem cells, their fate is dictated by external cues, including cytokines and cell–cell interactions, that determine expression of transcription factors. Le Douarin and Dupin also discuss how differentiated neural crest cells can revert to multipotent precursors under certain conditions and subsequently undergo transdifferentiation. This may provide another opportunity to examine epigenetic reprogramming. A mesodermal derivative that appears to show multipotency is the mesoangioblast, as reported by Cossu and Bianco. These cells are found close to the developing aorta, can generate differentiated mesodermal cell types such as skeletal muscle in vitro, and retain the capacity for self-renewal. Delineating the origin of these cells may give insight into the relationship between vascular and extravascular mesoderm. Another area receiving considerable attention at present is neural stem cell biology. Doetsch describes the niche for stem cells in the adult mammalian brain. Defining how niches operate is central to understanding the maintenance of adult stem cells and their recruitment into differentiation. Signalling molecules, extracellular matrix and other cues appear to act in combination. Ongoing studies may uncover how intrinsic epigenetic programming of the stem cells is influenced by external cues. Neural tube development is probably one of the best understood systems for specification of alternative cell fates within a vertebrate tissue. Sonic hedgehog signalling upstream of the Gli transcriptional factors has been shown to play a critical role as discussed by Ruiz I Altaba, Nguyeˆ n and Palma. These studies reveal how Sonic hedgehog acts as a mitogen, morphogen and a survival factor. Detailed analyses are providing a paradigm for how the numbers of progenitors and their patterning are regulated during the development of complex structures. www.current-opinion.com
Editorial overview Surani and Smith 447
Finally, Byrne, Kidner and Martienssen discuss stem cells in plants where not only can they persist for years, but it is also possible to derive stem cells directly from somatic cells. Again, signalling pathways dictate cell-fate decisions, which are then propagated by epigenetic mechanisms. It is their evident versatility that has enabled plants to make up nearly 90% of the biomass on earth. Lessons that we can learn from plants and other organisms provide insights into gene regulation and how diverse cellular and organismal phenotypes can be generated. We
www.current-opinion.com
could also gain a greater understanding of the evolutionary forces that have shaped diverse systems in the living world. If we can master the ability to regulate stem cell gene expression and cellular differentiation, we will create exciting opportunities for regenerative applications in human medicine and perhaps also be better able to protect other species.
Reference 1.
Weissmann A: Die Kontinuitaet des Keimplasmas als Grundlage einer Theorie der Vererbung. Fisher-Verlag: Jena; 1885.
Current Opinion in Genetics & Development 2003, 13:445–447