Required Enhancer: Matrin-3 Network Interactions for Pit1 Homeodomain Transcription Programs

Abstract

Homeodomain proteins, described 30 years ago ^{1, 2} , exert essential roles in development as regulators of target gene expression ^{3, 4} , however the molecular mechanism underlying transcriptional activity of homeodomain factors remains poorly understood. Here, investigation of a developmentally-required POU-homeodomain transcription factor, Pit1/Pou1f1, has revealed that, unexpectedly, binding of Pit1-occupied enhancers ⁵ to a nuclear matrin-3-rich network/architecture ^{6, 7} is a key event in effective activation of the Pit1-regulated enhancer/coding gene transcriptional program. Pit1 association with Satb1 ⁸ and β-catenin is required for this tethering event. A naturally-occurring, dominant negative, point mutation in human Pit1 (R271W), causing combined pituitary hormone deficiency (CPDH) ⁹ , results in loss of Pit1 association with β-catenin and Satb1 and therefore the matrin-3-rich network, blocking Pit1-dependent enhancer/coding target gene activation. This defective activation can be rescued by artificial tethering of the mutant R271W Pit1 protein to the matrin-3 network, bypassing the prerequisite association with β-catenin and Satb1 otherwise required. The matrin-3 network-tethered R271W Pit1 mutant, but not the untethered protein, restores Pit1-dependent activation of the enhancers and recruitment of co-activators, exemplified by p300, causing both eRNA transcription and target gene activation. These studies have thus revealed an unanticipated homeodomain factor/β-catenin/Satb1-dependent localization of target gene regulatory enhancer regions to a subnuclear architectural structure that serves as an underlying mechanism by which an enhancer-bound homeodomain factor effectively activates developmental gene transcriptional programs.