Trends in Genetics
Volume 33, Issue 1, January 2017, Pages 16-33
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Review
Linking Telomere Regulation to Stem Cell Pluripotency

https://doi.org/10.1016/j.tig.2016.10.007Get rights and content

Trends

Telomere maintenance and homeostasis is required for the unlimited self-renewal and pluripotency of stem cells.

In addition to telomerase, epigenetic modification by histone modifications (e.g., H3K9me3) and DNA demethylation by Ten-eleven Translocation (Tet) enzymes regulates telomere maintenance.

Telomere rejuvenation is an important part of reprogramming to pluripotency. H3K9me3/2 represses, whereas histone acetylation facilitates telomere elongation and reprogramming. Rif1 stabilizes H3K9me3/2 and may also interact with Nr0b1 to suppress two-cell genes, including Zscan4, to maintain telomere homeostasis.

H3.3/ATRX/DAXX/CAF-1 complexes are required for telomere integrity.

Understanding of telomere reprogramming and maintenance may improve iPSCs. Overexpression of Tet1, Tet2, Zscan4, or T-box 3 transcription factor (Tbx3), or addition of 2i, or histone deacetylase (HDAC) or DNA methylation inhibitors can elongate telomeres and facilitate reprogramming.

Embryonic stem cells (ESCs), somatic cell nuclear transfer ESCs, and induced pluripotent stem cells (iPSCs) represent the most studied group of PSCs. Unlimited self-renewal without incurring chromosomal instability and pluripotency are essential for the potential use of PSCs in regenerative therapy. Telomere length maintenance is critical for the unlimited self-renewal, pluripotency, and chromosomal stability of PSCs. While telomerase has a primary role in telomere maintenance, alternative lengthening of telomere pathways involving recombination and epigenetic modifications are also required for telomere length regulation, notably in mouse PSCs. Telomere rejuvenation is part of epigenetic reprogramming to pluripotency. Insights into telomere reprogramming and maintenance in PSCs may have implications for understanding of aging and tumorigenesis. Here, I discuss the link between telomere elongation and homeostasis to the acquisition and maintenance of stem cell pluripotency, and their regulatory mechanisms by epigenetic modifications.

Section snippets

Pluripotent Stem Cells

PSCs (see Glossary) including ESCs, somatic cell nuclear transfer ESCs (scntESCs), and iPSCs have a unique competence for unlimited self-renewal and pluripotency. Unlimited self-renewal is essential for expanding cell cultures to achieve a large number of cells sufficient for clinical applications in stem cell-based therapy. The pluripotency characteristic is equally important in that these cells have the capacity to differentiate into all cell types that comprise an individual. Therefore, they

Role of Telomerase in Telomere Maintenance

Telomerase is activated in stem cells and germ cells during human embryonic development and differentiation, and in cancers. Telomerase also is strongly expressed and required for telomere maintenance and the long-term self-renewal and pluripotency of mouse and human PSCs (Box 2) 9, 10, 11, 12, 13, 14, 15. Moreover, mRtel1 is required for DNA replication, recombination, and repair, and for the efficient elongation of telomeres by telomerase in mESCs 16, 17. Mouse ESCs (mESCs) and/or iPSCs with

Epigenetic Regulation of Telomere Length Maintenance and Homeostasis

Compared with differentiated somatic cells, the heterochromatic telomeric chromatin of ESCs is in a less compact state with relatively lower levels of H3K9me3 and H4K20me3 21, 47. Original findings suggest that DNA methylation negatively regulates telomere lengths [48]. mESCs deficient for the DNA methyltransferases DNMT1, or both DNMT3a and DNMT3b, exhibit dramatically elongated telomeres compared with wildtype controls, associated with increased telomeric recombination, as indicated by TSCE

Remarkable Telomere Elongation in miPSCs

Telomere shortening leads to cell senescence and organism aging. Reprogramming of telomerase and telomeres during induction of iPSCs occurs gradually, but before activation of endogenous pluripotent genes [22]. Telomeres continue to elongate during passages with acquisition of full reprogramming to pluripotency.

Pluripotency transcription factors can activate telomerase genes. The Terc promoter is bound and activated by OCT3/4 and NANOG [108]. Tert upregulation occurs simultaneously with the

Telomere Reprogramming Links to Somatic Cell Reprogramming and Pluripotency

Somatic cell nuclear transfer (SCNT) using oocytes was exploited to generate nuclear transfer embryos from which pluripotent scntESCs are then derived. If we can discover the secrets of how eggs can efficiently reprogram somatic cell nuclei by nuclear transfer, some of these egg molecules or mechanisms could be incorporated into the iPS route to improve the efficiency and cell number production of therapeutically useful cells [119]. Nuclear transfer allows somatic cells to undergo extensive

Insights Into Telomere Reprogramming by SCNT for Improving iPSC Induction and Quality

iPSC quality can be improved using reprogramming factors in early embryos. Zscan4 addition into Yamanaka factors enhances telomere reprogramming and the elongation and genomic stability of miPSCs [139]. Consistently, small molecules that activate Zscan4 expression increase the generation of high quality iPSCs with preservation of genomic integrity [140]. Coincidently, these molecules (e.g., SB431542, PD0325901, and AZA) reportedly enhance somatic cell reprogramming and iPSC induction and

Concluding Remarks

Sufficient telomere lengths and genomic stability are critical for the unlimited self-renewal and pluripotency of PSCs. Telomere lengths and homeostasis in ESCs and/or iPSCs are regulated by epigenetic modifications and their modulators, in addition to telomerase activity. Histone acetylation, and the Tbx3 and Tet enzymes act as positive regulators and Rif1, BRCA1, H3.3/ATRX/DAXX/DEK, CAF-1, and Nr0b1 act as negative regulators of telomere length via epigenetic modifications. Moreover, ESC

Acknowledgments

I am grateful to all of my laboratory members, past and present, and to my collaborators and colleagues in the field for helping with the critical experiments and fruitful discussion that led to this review. I apologize for being unable to explicitly discuss other important contributions due to space limitations and my knowledge. I also would like to thank the funding from the China Ministry of Science and Technology Major Research Program, Program of International S&T Cooperation, and the

Glossary

Chimera production test
an in vivo assay of developmental pluripotency of PSCs by injection of the cells into recipient blastocysts, or into or aggregation with early cleavage embryos from different origins to create chimeras by combining embryos of two or more genotypes, demonstrating the competence of PSCs to contribute to all cell lineages of a mouse, including the germ line.
Embryonic stem cells (ESCs)
these classical PSCs are isolated from early cleavage embryos or the inner cell mass (ICM)

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