Nuclear actin tunes ERK activity to promote somatic cell reprogramming into pluripotency
Yamanaka reprogramming is a stochastic process resulting in only a small fraction of somatic cells successfully converting into iPSCs. The molecular and cellular basis underlying this stochasticity remains elusive. Here we demonstrate that this stochasticity can be eliminated when ERK activity is tuned within a narrow range. This tuning can be accomplished by one tenth the concentration of MEK inhibitor in the 2i media. In the absence of pharmacologic ERK inhibition, cells fine tune ERK by morphological changes, growing taller by allocating more actin into their nucleus. A minimal cell height of 10 ?m is required for pluripotency, a cell geometric feature underlying the “dome-shaped” colony morphology of naïve pluripotent stem cells. Nuclear actin tunes ERK activity by binding to and immobilizing TFII-I?, a transcription factor that binds and mediates ERK's nuclear activation. This work uncovers a mechanistic link for how cell morphology controls cell identity. Insights into these fundamentally coupled processes provide effective approaches to overcome the stochasticity in reprogramming into pluripotency. Overall design: We performed bulk RNA-seq of control (EV), Actin overexpressed (Actin)  and NLS-actin OE (NLA-Actin) in mouse MEF and day 4 reporgramming cells. We also performed bulk RNA-seq of green positive or red positive reprogramming cells with or without NLS-actin under day 10 and day 14.