Nuclear actin and transcriptional activation

Abstract

Differentiated cells do not revert to an embryonic state in normal development. However, the method called nuclear reprogramming enables these differentiated cells to be reversed to an embryonic state. One essential event in the reprogramming process is reactivation of embryonic genes such as Oct4 (also known as Pou5f1). This reprogramming of transcriptional programs can be achieved by transplantation of mammalian somatic nuclei to the giant Xenopus laevis oocyte nucleus, referred to as the germinal vesicle (GV). Factors and mechanisms responsible for this transcriptional reprogramming have not been elucidated. Recently, we have found that a polymerized form of actin is abundantly present in nuclei transplanted into the Xenopus oocyte nucleus and plays an important role in transcriptional reactivation of Oct4. This study emphasizes a significant contribution of nuclear actin in transcriptional activation. Here, we discuss possible roles of nuclear actin in Xenopus oocytes and in other cell types in the context of transcriptional activation.

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As cell differentiation progresses, expression of embryonic genes is silenced. This silenced gene state in a somatic cell can be reprogrammed to be transcriptionally active by several different experimental strategies, including nuclear transfer to an egg/oocyte,1,2 cell fusion with embryonic cells3–5 and induced pluripotency by defined factors.6 Nuclear transfer-mediated reprogramming is believed to be the most efficient route to reprogram somatic cells.7 However, oocyte factors and mechanisms involved in this efficient reprogramming have remained elusive. In order to gain mechanistic insight into the reprogramming by oocytes, we have developed the Xenopus oocyte nuclear transfer system in which hundreds of mammalian nuclei are injected into the germinal vesicle (GV) and reactivation of embryonic genes is detected from the transplanted nuclei without the help of cell division and new protein synthesis.8 The reprogrammed transcripts can be detected in a very sensitive manner and monitored at a single nucleus level.9,10 This simple and quantitative method gave us a unique opportunity to explore oocyte factors and mechanisms involved in the reactivation of silenced embryonic genes, and hence in transcriptional reprogramming. Thus, we have shown requirements for DNA demethylation,11 Tpt112 and oocyte type linker histone B410 for transcriptional reprogramming.

In our recent study we have demonstrated that polymerized actin is present in the GV and transplanted nuclei and plays an important role in transcriptional reprogramming.13 Actin is a major component of the cytoskeleton and continuously changes its polymerized states. Actin in a nucleus (nuclear actin) was found more than 30 years ago although the functional significance of nuclear actin has been questioned for a long time. Nevertheless, a growing body of evidence demonstrates that nuclear actin is involved in many nuclear processes including transcription,14,15 gene movement16,17 and chromatin remodeling.18 Most previous reports used cultured cells and in vitro systems to study functions of nuclear actin. Nuclear actin has also been found in Xenopus oocytes.19,20 The Xenopus germinal vesicle has exceptionally large amounts of nuclear actin because Exportin 6, which exports nuclear actin to the cytoplasm, is not expressed.20 Why does the Xenopus GV need so much nuclear actin? One clear answer to this question is that polymerized nuclear actin supports mechanical integrity of the giant Xenopus GV, which is approximately 100,000 times larger than somatic nuclei.20 This giant oocyte nucleus contributes a stockpile of molecules needed for subsequent early embryonic development. For the efficient production of this maternal stock, the Xenopus oocyte genome shows a unique type of open chromosome structure, referred to as the lampbrush chromosomes.21 The lampbrush chromosomes allow intense transcriptional activity and are found in many animals including amphibians, insects and birds. An early study showed that nuclear actin is involved in transcription from the lampbrush chromosomes in Xenopus.22 Somatic nuclei transplanted into the GV show extensive swelling as if they are remodeled to form a lampbrushlike structure.8 Apart from this morphological change of the transplanted nuclei, our recent study demonstrates that nuclear actin polymerization is required for efficient gene reactivation and efficient transcription from them.13 Taken together, it is tempting to guess that exceptionally large amounts of nuclear actin in the Xenopus GV not only serve as a mechanical support but also help highly active transcription from the oocyte genome. It will be interesting to see whether nuclear actin also plays a role in transcription in oocytes from other organisms, such as the axolotl, that form lampbrush chromosomes.

As mentioned above, involvement of nuclear actin in gene reactivation has been shown using Xenopus oocytes.13 In cultured cells, the importance of nuclear actin in transcriptional activation upon cell differentiation,23,24 retinoic acid treatment25 and inflammatory response26 has also been reported by several different groups. Notably, two recent reports clearly show a requirement for nuclear actin26 and an actin nucleator,24 which regulates actin polymerization, in the context of transcriptional activation and chromatin remodeling. Huang et al. found that coronin 2A, a component of the NCoR complex, mediates interaction with oligomeric nuclear actin and the NCoR complex.26 This binding of nuclear actin to NCoR triggers the clearance of NCoR from gene promoters. Since the NCoR complex represses gene expression, the clearance of NCoR induces gene activation. Thus, nuclear actin is involved in transcriptional activation by de-repressing silenced genes. Taylor et al. showed that the Wiskott-Aldrich syndrome protein (WASp), an actin nucleator, translocates to nuclei during T-cell differentiation and nuclear WASp associates with histone H3K4 trimethyltransferase RBBP5 and H3K9/H3K36 tridemethylase JMJD2A.24 Association of WASp and these enzymes is important for achieving their enzymatic activities on appropriate target genes. Interestingly, polymerized nuclear actin is also recruited to the same target genes as WASp upon gene activation.

Nuclear actin in transcriptional activation is an emerging and fascinating topic for research. We still have many questions to answer in this research field. Xenopus oocytes may be a very suitable material to study functions of nuclear actin since they possess abundant naturally stored nuclear actin, which can be visualized easily as shown in our recent study.13 In the reprogramming field, it might be worth examining whether nuclear actin plays a role in other reprogramming system, such as iPS cells and cell fusion.

Acknowledgments

K.M. is supported by the Japan Society for the Promotion of Science (International Research Fellowship Program). This research is supported by a grant from the Wellcome Trust (Grant RG54943) to J.B.G.

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