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

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.