The ability to “reprogram” differentiated adult cells to a state that resembles embryonic stem cells has created wide-ranging opportunities for development of relevant in vitro disease models and patient-specific cell-replenishment therapies.
Initial efforts to generate human induced pluripotent stem cells (iPS cells) required simultaneous co-infection of cells with four separate retroviral expression vectors (Oct-4, Klf4, Sox-2, and c-Myc). Each vector carried one transcription factor, which resulted in a high number of genomic integrations.
Alternative approaches to iPS generation have included use of plasmids and nonintegrating adenovirus vectors to deliver the transcription factors. The rates at which cells convert to pluripotency using these methods, however, are far lower than those obtained using retroviral vectors.
Generation of human and mouse iPS cells is now routinely accomplished using a single, excisable polycistronic lentiviral vector that delivers all four Yamanaka transcription factors. EMD Millipore's STEMCCA™ lentiviral-based reprogramming yields cells that form multilayered tightly packed colonies with well-defined borders. These cells stain positive for alkaline phosphatase, express embryonic stem cell markers, form embryoid bodies, and differentiate into all three germ layers.
Use of a single vector significantly reduces the number of viral integrations required—in some cases, iPS clones possessing only a single viral integrant can be isolated.
Even with the use of a single vector, however, reprogramming human somatic cells remains a highly inefficient and time-consuming process. Small molecules targeting specific signaling pathways are being investigated for their ability to enhance reprogramming and/or replace the transcription factors required for reprogramming.
In this study, chemical compounds were screened for their effects on increasing the ratio of fully reprogrammed SSEA4+TRA-1-60+ Hoechst dim iPS cells versus reprogramming intermediates, increasing colony numbers, and reducing the time to establishment of full reprogrammed iPS cell colonies.
Improving Reprogramming Efficiency
Human foreskin fibroblasts were seeded at a density of 10,000/well and transduced with the Human STEMCCA Constitutive Polycistronic Lentivirus Reprogramming Kit (EMD Millipore). Various combinations of small molecules involved in TGF, Wnt, and MAPK signaling pathways along with epigenetic modifiers were screened.
A total of 25 small molecule boost cocktails were evaluated; boost cocktails were added at day six after replating the fibroblasts onto a feeder layout of inactive mouse embryonic fibroblasts. Reprogramming without chemical treatment was used as a control for all experiments.
Small molecule boost cocktails were identified that increased the number of iPS colonies by up to two- to threefold (Figure 1). The colonies possessed the distinctive 2-D morphology that is reminiscent of human embryonic stem cells and could be easily passaged in contrast to the untreated control that exhibited 3-D morphology and were difficult to passage.
In addition, some treatments yielded colonies with proliferation kinetics similar to human embryonic stem cells starting at the first passage rather than passage three as seen with untreated controls.
Chemically treated human iPS cells possessed fast proliferation kinetics; early passages from P0 to P3 were shortened to five to six days per passage period—a timeframe that is similar to the proliferation rate of normal human embryonic stem cells.
Treatment 2 was selected on the basis that it significantly improved both the quality of colonies formed and the efficiency of reprogramming. Treatment 2 is herein referred to as Human iPS Cell Boost Supplement.
In the presence of the Human iPS Cell Boost Supplement, the number of colonies formed increased threefold (Figure 2A) when used in combination with the mouse STEMCCA lentivirus kits and 15-fold when used in combination with the human STEMCCA lentivirus kits. Colonies stained positive for both human ESC markers, SSEA-4 (Figure 2F), and TRA-1-60 (Figure 2G). TRA-1-60+ colonies were not observed in the untreated control cultures (Figure 2D).