Myc and SAGA rewire an alternative splicing network during early somatic cell reprogramming

Abstract

Embryonic stem cells are maintained in a self-renewing and pluripotent state by multiple regulatory pathways. Pluripotent-specific transcriptional networks are sequentially reactivated as somatic cells reprogram to achieve pluripotency. How epigenetic regulators modulate this process and contribute to somatic cell reprogramming is not clear. Here we performed a functional RNAi screen to identify the earliest epigenetic regulators required for reprogramming. We identified components of the SAGA histone acetyltransferase complex, in particular Gcn5, as critical regulators of reprogramming initiation. Furthermore, we showed in mouse pluripotent stem cells that Gcn5 strongly associates with Myc and that, upon initiation of somatic reprogramming, Gcn5 and Myc form a positive feed-forward loop that activates a distinct alternative splicing network and the early acquisition of pluripotency-associated splicing events. These studies expose a Myc-SAGA pathway that drives expression of an essential alternative splicing regulatory network during somatic cell reprogramming.

Enzymatic modules of the SAGA chromatin-modifying complex play distinct roles in Drosophila gene expression and development

“…In yeast, DUB module subunits Ubp8 and Sgf73 contribute to centromere stability, while the HAT module subunits Gcn5 and Ada3 do not (Canzonetta et al 2015). An RNAi screen found that mouse GCN5 regulates reprogramming initiation in embryonic stem cells, whereas the DUB module appears to play no role in this process (Hirsch et al 2015). In contrast, loss of the Drosophila Non-stop, Sgf11, and Ada2b causes similar defects in axon guidance in the eye imaginal disc .…”

Resetting the epigenome for heart regeneration.

“…cMyc can increase transcription by stimulating histone acetylation and by upregulating HATs (such as general control of amino acid synthesis protein 5 (Gcn5)), which deposit H3K27ac marks at promoter sites [114]. Interestingly, Myc and Gcn5 are only required during the initial wave of reprogramming and are important for pluripotency acquisition but not pluripotency maintenance [114], which further supports the pre-programming of chromatin prior to the establishment of cell identity. Interestingly, Myc and Gcn5 are not simply global amplifiers of transcription but localize to specific genomic regions [114].…”

“…Interestingly, Myc and Gcn5 are only required during the initial wave of reprogramming and are important for pluripotency acquisition but not pluripotency maintenance [114], which further supports the pre-programming of chromatin prior to the establishment of cell identity. Interestingly, Myc and Gcn5 are not simply global amplifiers of transcription but localize to specific genomic regions [114]. In corroboration of a mechanistic link between cMyc and histone acetylation, use of valproic acid, a small molecule that inhibits HDAC2, can substitute for cMyc in reprogramming cocktails [115].…”

Unmasking the U2AF homology motif family: a bona fide protein–protein interaction motif in disguise

“…Despite many exposés, whether new confessions of ULM extensions, phosphorylation-sensitivity, and cooperative binding will emerge as recurring dramas among UHM-ULM partners for now remain tantalizing family secrets. In the longer term, understanding the interplay among UHMs and ULMs may offer new therapeutic avenues for targeting the spliceosome (for review, see Bonnal et al 2012), which has been highlighted recently for MYCdriven malignancies (Hirsch et al 2015;Hsu et al 2015;Koh et al 2015) and already is promising for compounds that target the ULM-containing protein SF3b155 (for review, see Salton and Misteli 2016).…”