Molecular insights into the heterogeneity of telomere reprogramming in induced pluripotent stem cells

Abstract

Rejuvenation of telomeres with various lengths has been found in induced pluripotent stem cells (iPSCs). Mechanisms of telomere length regulation during induction and proliferation of iPSCs remain elusive. We show that telomere dynamics are variable in mouse iPSCs during reprogramming and passage, and suggest that these differences likely result from multiple potential factors, including the telomerase machinery, telomerase-independent mechanisms and clonal influences including reexpression of exogenous reprogramming factors. Using a genetic model of telomerase-deficient (Terc −/− and Terc +/− ) cells for derivation and passages of iPSCs, we found that telomerase plays a critical role in reprogramming and self-renewal of iPSCs. Further, telomerase maintenance of telomeres is necessary for induction of true pluripotency while the alternative pathway of elongation and maintenance by recombination is also required, but not sufficient. Together, several aspects of telomere biology may account for the variable telomere dynamics in iPSCs. Notably, the mechanisms employed to maintain telomeres during iPSC reprogramming are very similar to those of embryonic stem cells. These findings may also relate to the cloning field where these mechanisms could be responsible for telomere heterogeneity after nuclear reprogramming by somatic cell nuclear transfer.

Zscan4 promotes genomic stability during reprogramming and dramatically improves the quality of iPS cells as demonstrated by tetraploid complementation

“…In contrast, during somatic reprogramming induced by the Yamanaka factors, telomere elongation is primarily mediated by telomerase [59,60]. Endogenous Zscan4 is not activated during the early stages of reprogramming induced by OKSM or OSK, as shown in our study (Supplementary information, Figure S1F) and in other report [38], further supporting the hypothesis that telomerase activity is the primary mechanism of telomere re-elongation regulation during OSKM-or OSK-mediated reprogramming. However, telomere extension by telomerase is a slow process and requires a number of cell divisions to restore a normal average telomere length [59,61].…”

“…We anticipated that the quality of OSKMZ-or OS-KZ-induced iPS cells might be improved significantly. We measured telomere length in iPS cells generated with OSKM and OSKMZ using two independent techniques, Southern telomere restriction analysis (TRF) [41] and Q-FISH on metaphase spreads [38]. As expected, telomeres were longer in OSKMZ-induced iPS cells compared to the corresponding OSKM-induced iPS cells, as determined by both TRF ( Figure 6A) and Q-FISH analysis ( Figure 6B), indicating that Zscan4 contributes to the generation of iPS cells with longer telomeres.…”

“…We examined relative telomere length by quantitative telomere FISH (Q-FISH) [38] in OSKM-and OSKMZ-infected MEFs on days 3 and 6 post-infection. Zscan4 promoted telomere elongation starting as early as day 3 following retrovirus infection ( Figure 4A).…”

The 2013 special issue on stem cell biology

“…Along the way, the field of stem cell biology has naturally become an important research area covered by papers published in Cell Research [2]. For instance, in the past year of 2012, Cell Research has published a number of important papers related to the stem cell field, covering diverse aspects and topics such as induced pluripotent stem (iPS) cells [3][4][5][6][7], mechanistic studies of pluripotency and differentiation [8][9][10][11], modeling of human diseases using stem cell-based systems [6,12], direct reprogramming of somatic cells to other cell types without passing through an pluripotent intermediate [13][14][15][16], as well as neural crest stem cells [17] and cancer stem cells [18][19][20].…”

The role of telomeres and telomerase reverse transcriptase isoforms in pluripotency induction and maintenance

“…27,28 Similarly, human and murine induced pluripotent stem cells (iPSCs) are capable of carrying out re-elongation of telomeres in at least some clones, although the telomere length and activity characteristics appear to be clonally unique. [28][29][30][31][32][33] Conversely, the process of differentiation triggers down-regulation of telomerase and initiates telomeric attrition. 34,35 Following the extended cellular division required to form adult tissues, terminally differentiated cell types typically exhibit low or undetectable levels of telomerase activity and shortened telomeres.…”