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On the Streets of San Francisco: Highlights from the ISSCR Annual Meeting 2010

2010; Elsevier BV; Volume: 7; Issue: 4 Linguagem: Inglês

10.1016/j.stem.2010.09.002

1934-5909

Andrew G. Elefanty, Robert Blelloch, Emmanuelle Passegué, Marius Wernig, Christine L. Mummery,

Pluripotent Stem Cells Research

The 2010 Annual Meeting of the International Society for Stem Cell Research (ISSCR) was held in San Francisco in June with an exciting program covering a wealth of stem cell research from basic science to clinical research. The 2010 Annual Meeting of the International Society for Stem Cell Research (ISSCR) was held in San Francisco in June with an exciting program covering a wealth of stem cell research from basic science to clinical research. Nearly 4000 scientists from 49 countries filled the vast auditorium of Moscone West in San Francisco for the opening session of the 8th Annual Meeting of the ISSCR. The stellar science presented in 8 plenaries and 15 concurrent sessions by 108 speakers over the ensuing four days led many to conclude that this was the best ISSCR annual meeting to date.The poster boards and exhibit areas were a constant hive of activity.View Large Image Figure ViewerDownload Hi-res image Download (PPT) As promises of new stem cell therapies move closer to reality, ISSCR President Irving Weissman opened the meeting reiterating the society's commitment to champion the responsible clinical translation of stem cell research and highlighting the ISSCR's recent initiative that resulted in the June 2010 launch of a new public education resource, A Closer Look at Stem Cell Treatments (www.closerlookatstemcells.org). In his Keynote Address on neuronal plasticity and diversity, Fred Gage (Salk Institute, USA) led off with a beautiful review of neurogenesis in the adult brain and then focused on the role of long interspersed nuclear element (LINE-1) retrotransposons in modulating gene expression in developing neurons (Singer et al., 2010Singer T. McConnell M.J. Marchetto M.C. Coufal N.G. Gage F.H. Trends Neurosci. 2010; 33: 345-354Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar). Although LINE-1 elements constitute nearly 20% of the mammalian genome, most are truncated copies that are fixed in position. In fact, in the human genome there are approximately 150 full-length potentially mobile LINE-1 elements, and probably only a handful of these are responsible for most retrotransposition activity. New LINE-1 insertions occur predominantly in developing neural cells, both during embryonic neurogenesis and in the adult brain. It is believed that LINE-1 activation, regulated by the Wnt signaling pathway, can affect gene expression and thus cellular phenotype. Gage maintains that LINE-1 induced somatic mosaicism may ultimately influence cognition and behavior—adding a whole new interpretation to the notion of "changing your mind"! The winner of the 2010 ISSCR Outstanding Young Investigator Award, Joanna Wysocka (Stanford University, USA), also discussed developmental plasticity, examining how epigenetic processes influence gene expression in the neural crest. Mutations in the gene encoding chromodomain helicase DNA-binding domain (CHD)7 cause a complex disorder with prominent craniofacial malformations, termed CHARGE syndrome. In order to confirm the long-standing hypothesis that CHARGE syndrome represented a neural crest disorder, Wysocka's laboratory established an in vitro model of neural crest-like cell (hNCLC) formation from hESCs and demonstrated that hNCLCs expressed high levels of CHD7. She showed that siRNA-mediated downregulation of this gene perturbed cell migration and reduced the transcription of genes specifying neural crest migration and specification. Recently, her laboratory discovered the association of CHD7 with another chromatin remodelling protein, PBAF (polybromo- and BRG1-associated factor containing complex) (Bajpai et al., 2010Bajpai R. Chen D.A. Rada-Iglesias A. Zhang J. Xiong Y. Helms J. Chang C.P. Zhao Y. Swigut T. Wysocka J. Nature. 2010; 463: 958-962Crossref PubMed Scopus (429) Google Scholar). This research has broad implications, providing an example for the synergistic control of distal enhancers by complexes of chromatin remodeling proteins. This year, the Anne McLaren Memorial Lecture was delivered by Brigid Hogan (Duke University, USA), who discussed epithelial stem and progenitor cells in lung development, homeostasis, and repair—a timely topic given that 2010 is the Year of the Lung. Highlighting important differences in size and structure between mouse and human lung, Hogan pointed out that undifferentiated basal cells, which express the transcription factor Trp-63 (p63) and cytokeratins 5 (Krt5) and 14 (Krt14), are restricted to the trachea in mice but are found throughout the small airways in the human lung. Lineage-tracing studies in the mouse demonstrated that tracheal basal cells gave rise to ciliated and secretory cells and that viable basal cells could be isolated from tracheal epithelium by virtue of their expression of the nerve growth factor receptor (Ngfr, p75). Growth in a clonogenic assay in vitro revealed that basal cells could renew and differentiate in the absence of stroma. Translation of this work to human lung epithelium revealed similar clonogenic properties in human lung basal cells purified on the basis of their combined expression of NGFR and ITGA6 (Rock et al., 2009Rock J.R. Onaitis M.W. Rawlins E.L. Lu Y. Clark C.P. Xue Y. Randell S.H. Hogan B.L. Proc. Natl. Acad. Sci. USA. 2009; 106: 12771-12775Crossref PubMed Scopus (1015) Google Scholar). Currently, her laboratory is dissecting the pathways that bias basal cell differentiation toward either secretory or ciliated epithelium. A session devoted to cell therapy opened with an update of the much-publicized clinical trial to evaluate the role of human embryonic stem cell (hESC)-derived oligodendrocytes in the treatment of acute thoracic spinal cord damage. Joseph Gold (Geron Corporation, USA) explained that stored oligodendrocytes will be injected into the spinal cord lesion in patients within 2 weeks of injury. On the basis of preclinical studies in rats, it is hypothesized that the oligodendrocytes might have a beneficial effect by restoring the damaged myelin sheath around nerve bundles. The trial was temporarily put on hold after the identification of microscopic epithelial cysts in the spinal cord of some recipient rats. It appears that these were benign, composed of endothelial cells, allowing the clinical hold to be lifted on July 30, 2010 and the trial to proceed. Despite significant advances in the generation of cardiomyocytes from human pluripotent stem cells, several speakers indicated that using these cells for therapy still seemed remote. Joseph Gold, Gordon Keller (McEwen Centre for Regenerative Medicine, Canada), Kenneth Chien (Massachusetts General Hospital, USA) and Christine Mummery (Leiden University Medical Center, Netherlands) described high yields of cardiomyocytes from hESC and human induced pluripotent stem cells (hiPSCs), with cultures under defined growth factor conditions reproducibly generating over 50% cardiomyocytes. Although the spectacular beating sheets of immature cells should be well-suited to transplantation, there remained significant additional hurdles to overcome. In addition to hazards posed by xenoreagents, residual undifferentiated pluripotent cells and immune rejection, cardiac cell transplantation presents risks of inducing arrhythmias and difficulties in achieving proper cell alignment. However, both Keller and Chien presented potential tissue engineering solutions to the latter problems. Geron's cardiomyocyte studies are now being extended to the guinea pig and pig—larger animal models in which it would be easier to detect adverse effects and to meet challenges related to cell preparation, administration, and safety associated with the larger cell doses required for human cell therapy. In addition to improving the efficiency of stem cell differentiation, for many applications it would also be useful to identify cell-surface markers that could be used to enrich for viable progenitors or differentiated cells. Ali Nsair (University of California, Los Angeles, USA) proposed that tripotent cardiac progenitors expressing the transcription factor Isl1, usually identified by expression of the VEGF2-receptor flk1, could be more readily identified with a combination of antibodies directed against the tyrosine kinase receptors flt1 and flt4. Cells selected from differentiating hESCs, hiPSCs, and human fetal heart with this strategy were enriched for cardiomyocyte, endothelial, and smooth muscle differentiation potential. Diseases affecting the skin and the cornea have also been targeted for stem cell therapies. Daniel Miller (University of Washington, USA) presented a combined cell (keratinocyte) and gene therapy approach to a severe blistering skin disease, epidermolysis bullosa simplex, caused by a dominantly inherited mutation in KRT5 or KRT14. Miller used an adeno-associated virus vector to target and repair the mutation in human keratinocytes and showed that these genetically repaired cells, grown on a matrix scaffold, organized into a normal skin epithelium, which could be successfully grafted to athymic mice (Petek et al., 2010Petek L.M. Fleckman P. Miller D.G. Mol. Ther. 2010; 18: 1624-1632Abstract Full Text Full Text PDF PubMed Scopus (41) Google Scholar). Impressive progress in stem cell therapy has also been made for some eye diseases. Graziella Pellegrini (University of Modena and Reggio Emilia, Italy) described the use of stem cells from the corneal limbus in the treatment of corneal disease. Limbal stem cells cultured from the contralateral healthy eye were expanded in vitro and transplanted to repair the damaged cornea. Long-term follow-up revealed that successful ongoing corneal regeneration was dependent upon the frequency of high proliferative potential, p63-expressing "holoclones" within the transplanted population (Rama et al., 2010Rama P. Matuska S. Paganoni G. Spinelli A. De Luca M. Pellegrini G. N. Engl. J. Med. 2010; 363: 147-155Crossref PubMed Scopus (820) Google Scholar). This avenue of research is particularly promising because there are options to use either autologous or allogeneic limbal stem cells, given that the cornea is not sensitive to immune rejection. Alexandra Capela, from StemCells, Inc., USA, discussed the use of human fetal central nervous system stem (hCNS-SCns) cells for the treatment of age related macular degeneration. hCNS-SCns cells have been shown to be neuroprotective in a mouse model of infantile neuronal ceroid lipofuscinosis (Tamaki et al., 2009Tamaki S.J. Jacobs Y. Dohse M. Capela A. Cooper J.D. Reitsma M. He D. Tushinski R. Belichenko P.V. Salehi A. et al.Cell Stem Cell. 2009; 5: 310-319Abstract Full Text Full Text PDF PubMed Scopus (111) Google Scholar) and are now under evaluation for the treatment of fatal neurodegenerative disorders in children. Capela argued that retinal degenerative diseases might similarly benefit from neuroprotective strategies to reduce photoreceptor loss. In rats predisposed to postnatal retinal degeneration, injection of hCNS-SCns cells successfully preserved photoreceptors and maintained near-normal visual acuity. The opportunity to test this therapy in humans is eagerly awaited since hCNS-SCns cells have already been banked for use in clinical trials for neurodegenerative diseases. The generation of human pancreatic beta cells as cell therapy for type 1 diabetes is an area of intense research interest for groups around the world. Kevin D'Amour (ViaCyte, Inc., USA; formerly Novocell, Inc.) described their recent progress in the differentiation of hESCs to pancreatic progenitors. Previously, they published that hESCs differentiated to pancreatic endoderm reversed type 1 diabetes in a mouse model following a 3 month in vivo maturation stage (Kroon et al., 2008Kroon E. Martinson L.A. Kadoya K. Bang A.G. Kelly O.G. Eliazer S. Young H. Richardson M. Smart N.G. Cunningham J. et al.Nat. Biotechnol. 2008; 26: 443-452Crossref PubMed Scopus (1402) Google Scholar). D'Amour described hESC differentiation in cellular aggregates, rather than as a monolayer, in order to enable upscaling to the cell numbers required for clinical use. The differentiated product was high in purity for pancreatic precursors, could be cryopreserved, and differentiated efficiently in vivo to yield structures that were very similar to pancreatic islets. D'Amour also described an encapsulation device that would address issues of patient safety, by simultaneously protecting the graft from immunological attack and blocking egress of any unwanted proliferating cells, while still enabling graft endocrine function. Two plenary sessions were devoted to metabolic regulation and stress response in stem cells, which attested to the strong interest in basic stem cell biology. Celeste Simon (University of Pennsylvania, USA) described her ongoing work on oxygen deprivation and hypoxia-inducible factors (HIFs) in stem cells and cancer. The HIFs and their interacting partner, ARNT, modulate several essential stem cell effector pathways, including Notch, Wnt/β-catenin, and Oct4 that influence stem cell proliferation, differentiation and pluripotency in low oxygen concentrations. She highlighted the importance of these regulatory mechanisms for neuronal and hematopoietic stem cell (HSC) maintenance in their respective hypoxic niches. Ricardo Pardal (Instituto de Biomedicina de Sevilla, Spain) described hypoxia-induced postnatal neurogenesis in the carotid body, the organ detecting oxygen tension in the arterial blood, through changes in the ratio of quiescent and proliferative glial-like stem cells. Marc Van Gilst (Fred Hutchinson Cancer Research Center, USA) and Yukiko Yamashita (University of Michigan, USA) showed that nutrient availability regulates stem cell numbers and overall tissue architecture. Using the nematode, C. elegans, Van Gilst illustrated that regulation of fat expenditure and lipid synthesis controlled the biological activity of germline stem cells and the reproductive status of the worm through the NHR49 pathway (Angelo and Van Gilst, 2009Angelo G. Van Gilst M.R. Science. 2009; 326: 954-958Crossref PubMed Scopus (191) Google Scholar). Yamashita explained that nutrients provided to the fruit fly D. melanogaster impacted on centrosome orientation and dictated the rate of cell division in the male germline stem cells through the insulin pathway. Pier Giuseppe Pelicci (University of Milan, Italy) delivered the first in a series of talks describing the DNA damage response of several stem cell populations that highlighted the prominent roles and contrasting functions of the p53 and p21 tumor suppressor genes (Cicalese et al., 2009Cicalese A. Bonizzi G. Pasi C.E. Faretta M. Ronzoni S. Giulini B. Brisken C. Minucci S. Di Fiore P.P. Pelicci P.G. Cell. 2009; 138: 1083-1095Abstract Full Text Full Text PDF PubMed Scopus (571) Google Scholar, Viale et al., 2009Viale A. De Franco F. Orleth A. Cambiaghi V. Giuliani V. Bossi D. Ronchini C. Ronzoni S. Muradore I. Monestiroli S. et al.Nature. 2009; 457: 51-56Crossref PubMed Scopus (255) Google Scholar). Using X-rays to illustrate genotoxic stress, he discussed the importance of the cell cycle inhibitor p21 in maintaining the stem cell pool in normal and malignant HSCs and mammary stem cells. Lenhard Rudolph (Ulm University, Germany) discussed the ameliorating effects of p21 deletion on the shortened lifespan of telomere dysfunctional mice and expanded on the role of p53-induced senescence and apoptosis as a response to telomere dysfunction. p53 played a unique role in protecting adult stem cells against the accumulation of mutations with consequences for aging and cancer development (Begus-Nahrmann et al., 2009Begus-Nahrmann Y. Lechel A. Obenauf A.C. Nalapareddy K. Peit E. Hoffmann E. Schlaudraff F. Liss B. Schirmacher P. Kestler H. et al.Nat. Genet. 2009; 41: 1138-1143Crossref PubMed Scopus (87) Google Scholar). Emmanuelle Passegué (University of California, San Francisco, USA) showed that enhanced prosurvival gene expression and activation of p53-mediated DNA damage response ensured the survival of HSCs in response to ionizing radiation. She also presented provocative data indicating that the prevalent DNA repair mechanism active in quiescent HSCs (nonhomologous end-joining mediated repair) is prone to generating mutations in this long-maintained self-renewing population (Mohrin et al., 2010Mohrin M. Bourke E. Alexander D. Warr M.R. Barry-Holson K. Le Beau M.M. Morrison C.G. Passegué E. Cell Stem Cell. 2010; 7: 174-185Abstract Full Text Full Text PDF PubMed Scopus (457) Google Scholar). Cedric Blanpain (Université Libre de Bruxelles, Belgium) reported similar findings in hair follicle bulge stem cells (Sotiropoulou et al., 2010Sotiropoulou P.A. Candi A. Mascré G. De Clercq S. Youssef K.K. Lapouge G. Dahl E. Semeraro C. Denecker G. Marine J.C. Blanpain C. Nat. Cell Biol. 2010; 12: 572-582Crossref PubMed Scopus (197) Google Scholar), which suggests that vulnerability to mutagenesis might be a general property of quiescent stem cell populations either normal or cancerous. Craig Jordan (University of Rochester, USA) postulated that killing cancer stem cells requires combinatorial drug therapies that will more effectively kill all tumor cells by inhibiting developmental pathways and antagonizing protective mechanisms that are active in stem and progenitor cells. As examples of the cancer stem cell targets that new therapies might address, he listed induction of oxidative stress linked with concomitant inhibition of the NFκB-mediated survival pathway, the redox balancing system, heat-shock proteins, and anti-oxidant protective mechanisms. The intense interest in cellular reprogramming since the initial presentation of iPSCs at the 4th ISSCR Annual Meeting in 2006 continued during this meeting. Shinya Yamanaka (Kyoto University, Japan and Gladstone Institutes, USA) discussed the requirement for Myc genes in reprogramming somatic cells to pluripotency. He concluded that c-Myc increased the frequency of iPSC generation but that this was associated with reactivation of the c-Myc virus and tumor formation in mice generated with these cells. However, Yamanaka reported reprogramming fibroblasts by using a cocktail of factors in which L-Myc, a weakly transforming family member of the Myc family, was substituted for c-Myc. The inclusion of L-Myc led to more efficient generation of iPSC clones that infrequently led to tumors in chimeric mice (Nakagawa et al., 2010Nakagawa M. Takizawa N. Narita M. Ichisaka T. Yamanaka S. Proc. Natl. Acad. Sci. USA. 2010; 107: 14152-14157Crossref PubMed Scopus (309) Google Scholar). Work by several laboratories employed different approaches to compare ESCs and iPSCs. George Daley (Children's Hospital Boston, USA) described global DNA methylation analysis that revealed a significant number of differentially methylated regions between ESCs and iPSCs, consistent with the concept that iPSCs retain an "epigenetic memory" that reflected their cell type of origin. Perhaps as a consequence of these epigenetic differences, Daley noted that iPSCs tended to differentiate more efficiently toward cell types related to their cell of origin (Kim et al., 2010Kim K. Doi A. Wen B. Ng K. Zhao R. Cahan P. Kim J. Aryee M.J. Ji H. Ehrlich L.I. et al.Nature. 2010; (in press. Published online July 19, 2010)https://doi.org/10.1038/nature09342Crossref Scopus (1718) Google Scholar). In an elegant series of studies, Konrad Hochedlinger (Massachusetts General Hospital, USA) profiled genetically matched mouse iPSCs that differed in their capacity to produce high contribution chimeras and "all-iPSC" mice by tetraploid complementation. Surprisingly, he uncovered a single locus on chromosome 12, containing a few mRNAs and microRNAs (miRNAs), whose expression correlated with the developmental potential (Stadtfeld et al., 2010Stadtfeld M. Apostolou E. Akutsu H. Fukuda A. Follett P. Natesan S. Kono T. Shioda T. Hochedlinger K. Nature. 2010; 465: 175-181Crossref PubMed Scopus (653) Google Scholar). In most iPSC clones the locus was silenced, and the iPSCs produced low contribution chimeras. In rare iPSC clones in which the locus was expressed, the cells made high contribution chimeras and even all-iPSC mice. Treatment of the silenced clones with a histone deacetylase inhibitor reactivated expression from the locus and induced full developmental potential to the iPSCs. Christa Buecker (Massachusetts General Hospital, USA) contrasted the distinct leukemia inhibitory factor (LIF)-dependent ESC and fibroblast growth factor (FGF)-dependent epiblast stem cell (EpiSCs) "states" that can be reversibly adopted by mouse pluripotent cells. Human ESCs more closely resemble mouse EpiSCs in their growth rates, factor requirements, and reluctance to passage as single cells. To derive human cell lines that were more similar to mESCs and therefore more amenable to genetic modification, Buecker and colleagues introduced inducible reprogramming factors into human fibroblasts that they cultured in the presence of LIF to derive colonies of cells (denoted hLR5 cells) that morphologically and immunophenotypically resembled mESCs (Buecker et al., 2010Buecker C. Chen H.H. Polo J.M. Daheron L. Bu L. Barakat T.S. Okwieka P. Porter A. Gribnau J. Hochedlinger K. Geijsen N. Cell Stem Cell. 2010; 6: 535-546Abstract Full Text Full Text PDF PubMed Scopus (176) Google Scholar). Interestingly, levels of endogenous OCT4, NANOG, SOX2, and MYC remained low in hLR5 cells and they remained dependent upon the expression of exogenous reprogramming factors. However, Buecker observed that the hLR5 cells displayed a similar facility for genetic modification to mESCs, generating over 100-fold more stable transfectants than an equivalent number of hESCs. The ability of pigmented retinal epithelium in the chick to develop into lens cells was observed over 30 years ago, and this switch in cellular differentiation was termed "transdifferentiation" (Eguchi and Kodama, 1993Eguchi G. Kodama R. Curr. Opin. Cell Biol. 1993; 5: 1023-1028Crossref PubMed Scopus (193) Google Scholar). In this case, it was clear that the cells "dedifferentiated" to a stem or progenitor cell state first. Infrequently, the mis-expression of a single gene can convert one cell type into another, without apparent dedifferentiation, as was the case for muscle switching of C3H10T1/2 fibroblasts by MyoD (Davis et al., 1987Davis R.L. Weintraub H. Lassar A.B. Cell. 1987; 51: 987-1000Abstract Full Text PDF PubMed Scopus (2473) Google Scholar). The advent of somatic cell reprogramming fueled renewed interest in direct cellular fate conversion using multiple genes, thus circumventing a pluripotent intermediate. Following on the heels of the successful conversion of adult pancreatic exocrine cells to beta cells in vivo reported by Douglas Melton (Harvard University, USA) at the ISSCR 6th Annual Meeting in Philadelphia, several speakers presented the results of direct reprogramming studies. Marius Wernig (Stanford University, USA) described experiments that aimed to generate neural lineages in a single step. He transduced murine fibroblasts with a combination of 19 candidate genes and eventually identified three factors that efficiently reprogrammed fibroblasts to induced neural cells, with a predominantly excitatory cortical neuron phenotype (Vierbuchen et al., 2010Vierbuchen T. Ostermeier A. Pang Z.P. Kokubu Y. Südhof T.C. Wernig M. Nature. 2010; 463: 1035-1041Crossref PubMed Scopus (2266) Google Scholar). It remains to be seen whether the reprogramming factors can be dispensed with and how readily neurons of different subtypes can be produced. Kevin Eggan (Harvard University, USA) explored the reprogramming of fibroblasts directly to motor neurons, with a view to developing new treatments for amyotrophic lateral sclerosis. He reprogrammed fibroblasts harboring a motor neuron-specific GFP reporter with various combinations of factors. Success eluded him, however, until he also added the three factors used by Wernig. In the final talk on this theme, Deepak Srivastava (Gladstone Institutes, USA) discussed the reprogramming of cardiac-derived "fibroblasts" to cardiomyocytes (Ieda et al., 2010Ieda M. Fu J.-D. Delgado-Olguin P. Vedantham V. Hayashi Y. Bruneau B.G. Srivastava D. Cell. 2010; 142: 375-386Abstract Full Text Full Text PDF PubMed Scopus (1888) Google Scholar). He used a combination of 14 transcription factors to reprogram the fibroblasts to a cardiac fate. Srivastava refined the list of reprogramming genes, leading to the conclusion that a three factor combination would successfully reprogram fibroblasts to beating, electrically active cells with a phenotype similar to ventricular cardiomyocytes. Importantly, using a lineage tracing strategy, his group showed that the fibroblasts were directly induced into cardiomyocytes, bypassing a progenitor intermediate. Primordial germ cells (PGCs) hold a unique place in the stem cell hierarchy given that it is these cells that give rise to gametes and represent a literal genetic link to the next generation. Several talks were related to the biology of PGCs derived from mammals and nonvertebrate species. Crucial to PGC formation in females is the reactivation of the somatically silenced X chromosome as the PGCs migrate to the urogenital ridge. Kathrin Plath (University of California, Los Angeles, USA) pointed out that, unlike female mESCs, most female hESC lines carry one inactive X chromosome. Recent work from the group of Jaenisch and Mitalipova showed that this may be due to oxidative stress and that female hESCs derived under 5% oxygen conditions retained two active X chromosomes that were randomly inactivated upon further differentiation (Lengner et al., 2010Lengner C.J. Gimelbrant A.A. Erwin J.A. Cheng A.W. Guenther M.G. Welstead G.G. Alagappan R. Frampton G.M. Xu P. Muffat J. et al.Cell. 2010; 141: 872-883Abstract Full Text Full Text PDF PubMed Scopus (302) Google Scholar). Interestingly, Plath showed that, like the hESCs, hiPSCs had only one active X chromosome. However, unlike the derivation of hESCs, reprogramming under hypoxic conditions did not support X reactivation (Tchieu et al., 2010Tchieu J. Kuoy E. Chin M.H. Trinh H. Patterson M. Sherman S.P. Aimiuwu O. Lindgren A. Hakimian S. Zack J.A. et al.Cell Stem Cell. 2010; 7: 329-342Abstract Full Text Full Text PDF PubMed Scopus (216) Google Scholar). Plath made the important observation that because the inactivated X is retained during reprogramming and differentiation, iPSC clones were well positioned for the study of X-linked diseases (such as Duchenne Muscular Dystrophy) because it would be possible to get clonal lines either expressing the wild-type or mutant allele from the same female patient. Studies in nonvertebrate species reported examples of gene mutations affecting germ cell development that also affect humans. As an example, Shuyi Chen (Stowers Institute for Medical Research, USA) discussed the role of Lis1, which is required for maintaining BMP signaling and the balance between self-renewal and differentiation in Drosophila germ cells. A further highlight was the presentation by Mitinori Saitou (RIKEN Center for Developmental Biology, Japan), who studied germ cell specification in mice and the role of the transcription factors Blimp1 (Prdm1) and Prdm14, which are coexpressed in the PGCs (Ohinata et al., 2009Ohinata Y. Ohta H. Shigeta M. Yamanaka K. Wakayama T. Saitou M. Cell. 2009; 137: 571-584Abstract Full Text Full Text PDF PubMed Scopus (384) Google Scholar). Saitou showed that extraembryonic signals were key regulators of germ cell induction. Under defined culture conditions, in the presence of BMP4, most of the isolated epiblast cells in wild-type mouse embryos became Blimp1- and alkaline phosphatase-positive PGCs. These culture-induced PGCs developed into sperm upon injection into the testes of aspermic mice and gave rise to viable offspring. Similar signaling pathways appear to be active in mESCs and data presented also suggested that the dynamics of gene expression were similar whether the PGCs were derived from the epiblast or ESCs. Robert Blelloch (University of California, San Francisco, USA) discussed the roles for miRNAs in regulating the switch between self-renewal and differentiation in embryonic stem cells (Melton et al., 2010Melton C. Judson R.L. Blelloch R. Nature. 2010; 463: 621-626Crossref PubMed Scopus (557) Google Scholar). Blelloch's laboratory discovered antagonistic roles for two families of miRNAs, the ESCC and let-7 miRNAs, which are highly expressed in ESCs and differentiated tissues, respectively. The ESCC miRNAs enhance self-renewal of ESCs and promote the dedifferentiation of somatic cells to iPSCs. In contrast, the let-7 miRNAs promote the differentiation of ESCs and inhibit the dedifferentiation of somatic cells to iPSCs. He also discussed surprising findings that all miRNA function is suppressed in oocytes and preimplantation embryos, postulating that this may be essential for the massive reprogramming that occurs in the early embryo (Suh et al., 2010Suh N. Baehner L. Moltzahn F. Melton C. Shenoy A. Chen J. Blelloch R. Curr. Biol. 2010; 20: 271-277Abstract Full Text Full Text PDF PubMed Scopus (328) Google Scholar). Narry Kim (Seoul National University, Republic of Korea) presented elegant work from her laboratory on the posttranscriptional regulation of let-7 (Heo et al., 2009Heo I. Joo C. Kim Y.K. Ha M. Yoon M.J. Cho J. Yeom K.H. Han J. Kim V.N. Cell. 2009; 138: 696-708Abstract Full Text Full Text PDF PubMed Scopus (644) Google Scholar). Like all canonical miRNAs, let-7 is transcribed as a long pri-miRNA. The pri-miRNA is processed first by an RNase, Drosha, to a pre-miRNA and then by another RNase, Dicer, to a mature miRNA. Kim and others have shown that the