MED19 and MED26 Are Synergistic Functional Targets of the RE1 Silencing Transcription Factor in Epigenetic Silencing of Neuronal Gene Expression

Abstract

A key hub for the orchestration of epigenetic modifications necessary to restrict neuronal gene expression to the nervous system is the RE1 silencing transcription factor (REST; also known as neuron restrictive silencer factor, NRSF). REST suppresses the nonspecific and premature expression of neuronal genes in non-neuronal and neural progenitor cells, respectively, via recruitment of enzymatically diverse corepressors, including G9a histone methyltransferase (HMTase) that catalyzes di-methylation of histone 3-lysine 9 (H3K9me2). Recently, we identified the RNA polymerase II transcriptional Mediator to be an essential link between RE1-bound REST and G9a in epigenetic suppression of neuronal genes in non-neuronal cells. However, the means by which REST recruits Mediator to facilitate G9a-dependent extra-neuronal gene silencing remains to be elucidated. Here, we identify the MED19 and MED26 subunits in Mediator as direct physical and synergistic functional targets of REST. We show that although REST independently binds to both MED19 and MED26 in isolation, combined depletion of both subunits is required to disrupt the association of REST with Mediator. Furthermore, combined, but not individual, depletion of MED19/MED26 impairs REST-directed recruitment to RE1 elements of Mediator and G9a, leading to a reversal of G9a-dependent H3K9me2 and de-repression of REST-target gene expression. Together, these findings identify MED19/ MED26 as a probable composite REST interface in Mediator and further clarify the mechanistic basis by which Mediator facilitates REST-imposed epigenetic restrictions on neuronal gene expression.

REST and CoREST are transcriptional and epigenetic regulators of seminal neural fate decisions

“…47,48 The Mediator complex is comprised of multiple (~26) subunits with general and specific roles in gene regulation, including activation and repression. The mechanisms by which Mediator affects transcription are still being characterized but include control of both pre-and post-initiation events.…”

“…The core Mediator subunits, Med19 and Med26, interact with REST directly and synergistically, promoting the recruitment of the entire Mediator complex to genomic RE1 sites occupied by REST. 47 In turn, the Mediator complex facilitates the recruitment of G9a to these sites, through interactions between its Med12 subunit and G9a. 50 Notably, Med12 expression is enriched in the nervous system, where it plays a role in regulating neuronal development.…”

The Mediator Complex in Plants: Structure, Phylogeny, and Expression Profiling of Representative Genes in a Dicot (Arabidopsis) and a Monocot (Rice) during Reproduction and Abiotic Stress

“…Med26, a subunit of the Cofactor Required for SP1 Activation complex (Ryu et al, 1999), is a target of a zinc finger TF and is involved in epigenetic silencing of neuronal gene expression along with Med19 (Ding et al, 2009). Our intensive HH study helped us to identify novel Med26 in all land plants; however, they were not detected in the algal group.…”

Epigenetic setting and reprogramming for neural cell fate determination and differentiation

“…The C-terminal repression domain of REST interacts with CoREST [62], which additionally recruits HDACs (HDAC1 and HDAC2), methyl-CpG-binding protein 2, histone H3K4 lysine demethylase, LSD1, histone H3K9 methyltransferases, G9a and SUV39H1, and a component of the SWI/SNF chromatin remodelling complex, BRG1. In addition, REST also associates with a number of other epigenetic and regulatory cofactors that include DNMTs, MBDs and chromatin remodelling enzymes [63]. In this way, REST acts as an adaptable molecular platform to which these factors may all be recruited and promotes dynamic modifications of DNA, histones, nucleosomes and higher order chromatin codes and helps maintain genomic stability.…”