On November 24th, the team led by Liu Xingguo, Pei Duanqing, and Chen Keshi from the Guangzhou Institutes of Biomedicine and Health, Chinese Academy of Sciences published the latest research results on the journal Cell Death & Differentiation by Nature, titled "MAP2K6 Remodels Chromatin and Facilitates Reprogramming by Activating Gatad2b-Phosphorylation Dependent Heterochromatin Loosening". The study found that the secondary kinase MAP2K6 in the MAPK signaling pathway can open heterochromatin by phosphorylating Gatad2b and increase histone acetylation modifications in the promoter regions of pluripotency genes, ultimately increasing the expression of pluripotency genes and promoting the acquisition of pluripotency. This work not only reveals the role of MAP2K phosphorylation-mediated chromatin state in cell fate regulation, but also discovers a new mode of "progressive modification" signal transduction where kinase-initiated phosphorylation-acetylation of proteins occurs.

In the cell nucleus, chromatin is divided into euchromatin and heterochromatin. Euchromatin is in an extended state and has transcriptional activity, while heterochromatin is in a condensed state and has no transcriptional activity. The interconversion between the two is crucial in cell fate transitions. The process of somatic cell reprogramming to induce pluripotent stem cells (iPSCs) must undergo significant chromatin remodeling at the epigenetic level, making it an ideal model for studying epigenetic regulation.

A series of novel modes of epigenetic regulation in the cell nucleus, such as cytoplasmic signal regulation of DNA methylation, histone methylation, histone acetylation, and lactation, have been discovered through this model. However, elucidating how kinase-mediated phosphorylation signals remodel chromatin structure remains a critical unresolved scientific problem.

The MAPK signaling pathway is a three-tiered cascade kinase system that responds to various extracellular stimuli in a switch-like manner. It participates in and regulates a variety of cellular physiological and pathological activities such as cell differentiation, apoptosis, transformation, and aging. MKK proteins are the central kinases in the MAPK cascade system, with the dual characteristics of being activated by specific upstream kinases and activating specific downstream kinases to ensure the accuracy of signal transduction. Among the 7 secondary kinases of MAP2K1-7, only MAP2K3 and MAP2K6 can dissociate heterochromatin and efficiently promote reprogramming. These two kinases have similar structures and functions, sharing the common downstream kinase P38, but their heterochromatin dissociation function does not depend on the activation of P38. Phosphoproteomic analysis identified the chromatin factor Gatad2b as a novel MAP2K6 substrate, offering insight into its heterochromatin remodeling function.

Gatad2b is an important component of the ATP-dependent chromatin remodeling complex NuRD, which can participate in nucleosome remodeling and histone acetylation modifications. MAP2K6 phosphorylates Gatad2b at the S487 and T490 sites to dissociate heterochromatin and promote histone acetylation modifications. After the dissociation of heterochromatin, MAP2K6 promotes the binding and transcriptional activation of reprogramming factors Sox2 and Klf4 to their downstream targets, thereby promoting the acquisition of pluripotency. 

In the classical kinase signaling pathway, signals are transmitted through protein phosphorylation. However, in our latest research, we have found that signals can be progressively transmitted from protein phosphorylation to protein acetylation. We found that the secondary kinase family members MAP2K3/6 can regulate histone acetylation through the phosphorylation of Gatad2b. This indicates a new mode of "progressive modification" signal transduction where different protein post-translational modifications are linked, and this plays an important role in the regulation of pluripotent stem cell fate.

The MAPK pathway responds to various extracellular stimuli and plays a key role in multiple life processes such as cell differentiation, apoptosis, transformation, and aging. Our work shows that this pathway regulates gene expression through histone modifications, revealing a new signaling pathway between extracellular stimuli and intracellular gene expression that has broad physiological and pathological significance.

This study was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Chinese Academy of Sciences, Guangdong Province, and Guangzhou City.