Different telomere-length dynamics at the inner cell mass versus established embryonic stem (ES) cells

Area of Science:

• Cell Biology
• Genetics
• Developmental Biology

Background:

• Murine embryonic stem (ES) cells possess significantly longer telomeres compared to cells in embryonic tissues.
• The origin of these hyper-long telomeres in ES cells—whether inherent to pluripotent stem cells or acquired during in vitro culture—remains unclear.

Purpose of the Study:

• To investigate whether hyper-long telomeres are a natural characteristic of pluripotent stem cells, specifically the inner cell mass (ICM) at the blastocyst stage.
• To determine if telomere elongation is acquired during the in vitro derivation and expansion of ES cell lines.

Main Methods:

• In vivo analysis of morula-injected ES cells at the blastocyst stage.
• Analysis of telomere length and heterochromatic marks during in vitro derivation of ES cell lines.
• Quantification of telomere repeat binding factor 1 (TRF1) levels in cultured ICM cells.

Main Results:

• Inner cell mass (ICM) cells exhibit telomere elongation during the in vitro derivation process for establishing ES cell lines.
• In vivo studies confirm that ES cells injected into morulae maintain hyper-long telomeres at the blastocyst stage, exceeding those of the native blastocyst ICM.
• Telomere lengthening during ES cell derivation correlates with reduced heterochromatic marks at telomeres.
• Elevated levels of the telomere-capping protein TRF1 are observed in cultured ICM cells prior to telomere elongation, coinciding with pluripotency marker expression.

Conclusions:

• Hyper-long telomeres in ES cells are likely acquired during continuous telomere lengthening of proliferating ICM cells maintained in a pluripotent epigenetic state.
• High TRF1 levels in pluripotent cells appear crucial for proficient capping of newly synthesized telomeres.
• A distinction exists between ICM cells within the blastocyst and ES cells in culture, with ES cells exhibiting abnormally long telomeres due to continuous lengthening during derivation.