MKL1-actin pathway restricts chromatin accessibility and prevents mature pluripotency activation

Abstract

Actin cytoskeleton is well-known for providing structural/mechanical support, but whether and how it regulates chromatin and cell fate reprogramming is far less clear. Here, we report that MKL1, the key transcriptional co-activator of many actin cytoskeletal genes, regulates genomic accessibility and cell fate reprogramming. The MKL1-actin pathway weakens during somatic cell reprogramming by pluripotency transcription factors. Cells that reprogram efficiently display low endogenous MKL1 and inhibition of actin polymerization promotes mature pluripotency activation. Sustained MKL1 expression at a level seen in typical fibroblasts yields excessive actin cytoskeleton, decreases nuclear volume and reduces global chromatin accessibility, stalling cells on their trajectory toward mature pluripotency. In addition, the MKL1-actin imposed block of pluripotency can be bypassed, at least partially, when the Sun2-containing linker of the nucleoskeleton and cytoskeleton (LINC) complex is inhibited. Thus, we unveil a previously unappreciated aspect of control on chromatin and cell fate reprogramming exerted by the MKL1-actin pathway.

Reprogramming progressive cells display low CAG promoter activity

“…Indeed, when we FACS sorted cells based on forward scatter alone, a crude measure of cell size, and compared their reprogramming efficiency, smaller cells indeed displayed higher reprogramming activity (Figures 4B-C). Such observation is consistent with our previous finding that reprogramming requires reduced MKL1/SRF activity (Hu et al, 2019).…”

“…We examined whether CAG:H2B-GFP intensity decreases during early reprogramming to reflect the decreasing MKL1/SRF activity (Hu et al, 2019). Shortly after Dox addition, there was a marked down-regulation of CAG:H2B-GFP intensity within the mCherry+ population ( Figures 1B-C), consistent with reduced activity of MKL1/SRF.…”

“…These transcription factors are now known to antagonize Yamanka reprogramming, or established pluripotency by redefining the enhancer-Pol II relationship (Chronis et al, 2017;Hamilton et al, 2019;Liu et al, 2015). We have recently reported that MKL1/SRF activity potently prevents the activation of mature pluripotency by hindering chromatin accessibility via an actin-LINC-dependent process (Hu et al, 2019). Taken together, these evidences suggest that the transcriptional activity of the ubiquitously expressed SRF could potentially serve as an indicator for how likely a somatic cell could progress into pluripotency.…”

Cell cycle dynamics in the reprogramming of cellular identity

“…demonstrated that low actin cytoskeleton is critical for the maturation of reprogramming fibroblasts. Elevated actin cytoskeleton by sustained MKL1 activity potently restricts nuclear dynamics, reduces chromatin accessibility, and inhibits pluripotency activation, partly through constricting the nuclei via the linker of nucleoskeleton and cytoskeleton complex . This work depicts a novel perspective on how cell cycle could regulate cell fate and demonstrates how a ubiquitous structural network could regulate nuclear dynamics and control cell fate.…”

“…In the fibroblast reprogramming model, dramatic downregulation of the actin cytoskeleton occurs as cells undergo this fate transition [100,101]. The systematic reduction of the actin cytoskeletal network is caused by reduced activity of the transcription factors megakaryoblastic leukemia-1/serum response factor (MKL1/SRF) [101]. Hu et al demonstrated that low actin cytoskeleton is critical for the maturation of reprogramming fibroblasts.…”