Reprogramming the pluripotent cell cycle: Restoration of an abbreviated G1 phase in human induced pluripotent stem (iPS) cells

TLDR

Induced pluripotent stem cells reprogrammed from fibroblasts acquire stem‑cell‑like properties, but it is unclear whether they replicate the abbreviated G1 phase and rapid subnuclear organization seen in human embryonic stem cells. The study aimed to compare cell‑cycle characteristics of iPS cells with those of human embryonic stem cells. The authors measured BrdU incorporation, performed FACS and Ki67 staining, and used immunofluorescence microscopy and FISH to analyze histone locus body organization in iPS cells. Human iPS cells exhibit a 2.5‑hour G1 phase, a 16‑18‑hour cell cycle, and rapid HLB reorganization, mirroring the key properties of hES cells and supporting a functional link between abbreviated cycle and pluripotency.

Abstract

Induced pluripotent stem (iPS) cells derived from terminally differentiated human fibroblasts are reprogrammed to possess stem cell like properties. However, the extent to which iPS cells exhibit unique properties of the human embryonic stem (hES) cell cycle remains to be established. hES cells are characterized by an abbreviated G1 phase (∼ 2.5 h) and accelerated organization of subnuclear domains that mediate the assembly of regulatory machinery for histone gene expression [i.e., histone locus bodies (HLBs)]. We therefore examined cell cycle parameters of iPS cells in comparison to hES cells. Analysis of DNA synthesis [5-bromo-2'-deoxy-uridine (BrdU) incorporation], cell cycle distribution (FACS analysis and Ki67 staining) and subnuclear organization of HLBs [immunofluorescence microscopy and fluorescence in situ hybridization (FISH)] revealed that human iPS cells have a short G1 phase (∼ 2.5 h) and an abbreviated cell cycle (16-18 h). Furthermore, HLBs are formed and reorganized rapidly after mitosis (within 1.5-2 h). Thus, reprogrammed iPS cells have cell cycle kinetics and dynamic subnuclear organization of regulatory machinery that are principal properties of pluripotent hES cells. Our findings support the concept that the abbreviated cell cycle of hES and iPS cells is functionally linked to pluripotency.