Pluripotency and reprogramming: meeting report on the Fifth International Meeting of the Stem Cell Network North Rhine Westphalia.

In March 2009, the fifth International Meeting of the Stem Cell Network North Rhine Westphalia took place in Aachen, Germany. Numerous fascinating presentations about reprogramming, stem cells, and therapeutic devices were given. A number of excellent speakers from all over the world were invited to present their work. Over 20 high-profile presentations were given on 2 days under the five different topics: reprogramming, mechanisms regulating stem cells, stem cell differentiation, cancer stem cells, and therapeutic devices. Young researchers had opportunity to present their work in over 120 posters. The meeting started with the main topic: reprogramming. Austin J. Cooney, Baylor College of Medicine, Houston, USA, gave an exiting presentation about alternative pathways to maintain pluripotency. His work is focused on the regulation of Oct4 by nuclear receptors, specifically LRH-1, which regulates directly Oct4 expression and the role of canonical wnt signalling relating to b-catenin, which potentiates reprogramming (Gu et al., 2005). His group showed that ES cells deficient in LRH-1 and b-catenin lose their pluripotency faster than wild-type ES cells. The group established a model for generation of b-catenin = ES cells. In a second model, the effect of LRH-1 was analyzed and the effect of BIO, a GSK3b inhibitor, was evaluated. The results showed that wnt3a induces LRH-1 in a b-catenin-dependent manner, because b-catenin binds directly to TCF elements in the LRH-1 promotor (Lluis and Cosma, 2009; Mullen et al., 2007). Theodore Rasmussen, University of Connecticut, USA, talked about direct reprogramming of somatic cells: from ES cell fusion to iPS. Rasmussen and his working group chose the method of somatic cell nuclear transfer (SCNT) to reprogram differentiated somatic cells to a more pluripotent state. He pointed out that iPS technology is highly promising because it can yield immunocompatible cells by relatively simple, noncontroversial means. Rasmussen’s group used the superb cell biology allowed by SCNT to study rapid and dynamic chromatin remodeling events such as histone replacement, which occur within hours after nuclear transfer with kinetics similar to those of preimplantation development (Chang et al., 2005). FMR reprogramming takes longer than SCNT, but is faster than iPS. FMR is well suited for genetic analyses, because polymorphic differences between the somatic and ES cell fusion partners can be used to trace reprogrammed gene expression (Ambrosi et al., 2007). In addition, genes can be manipulated in ES cells prior to fusion to evaluate their importance for reprogramming. Together, SCNT and FMR offer unique advantages for the investigation of reprogramming mechanisms. The presentation of Sir John B. Gurdon, Cambridge, UK, a pioneer in this field, was focused on nuclear reprogramming by nuclear transfer. John Gurdon and his group try to identify the components of eggs that cause the nuclear reprogramming. One method is to transfer multiple nuclei into the germinal vesicle of growing eggs, the oocytes. These result in lots of mammalian somatic cell nuclei that can be induced to start expressing genes for pluripotency such as Oct4, Nanog, and Sox2. The researchers detect that the following steps are involved in reprogramming: the chromatin becomes highly decondensed, differentiation marks are removed, the provision of transcription factors does not appear to be necessary in reprogramming, and linker histones of the H1 class are exchanged in transplanted nuclei (Gurdon and Melton, 2008). The presentation of Huck-Hui Ng, Genome Institute of Singapore, was focused on the major role of transcriptional regulation in specifying the unique properties of embryonic stem cells. They used chromatin immunoprecipitation coupled to high-throughput sequencing technology to map the binding sites for 13 different transcription factors (Oct4, Sox2, Nanog, Smad1, Stat3, Esrrb, Tcfcp2l1, Klf4, c-Myc, n-Myc, Zfx, E2f1, and Ctcf ) in mouse ES cells (Chen et al., 2008). The data revealed hot spots for transcription factor binding. These hot spots occur in two major clusters, namely, the Oct4-centric clusters and the Myccentric clusters. Dr. Ng also presented data on a new reprogramming factor, Esrrb (Feng et al., 2009). Esrrb is a nuclear orphan receptor that works in conjunction with Oct4 and Sox2 to reprogram mouse fibroblasts to induced pluripotent stem cells. This study demonstrates that one of the core reprogramming factor Klf4, can be substituted by Esrrb. Three outstanding presentations on the mechanisms regulating the stem cell state were given in the second session.