Nuclear transport of the serum response factor coactivator MRTF-A is downregulated at tensional homeostasis.

The serum response factor (SRF) coactivator myocardin-related transcription factor A (MAL/MKL1/MRTF-A), the nuclear transport and activity of which is regulated by monomeric actin, has been implicated in tension-based regulation of SRF-mediated transcriptional activity. However, the mechanisms involved remain unclear. We used fibroblasts grown within collagen matrices to explore whether MRTF-A transport is regulated by tissue tension. We show that MRTF-A nuclear accumulation following stimulation with serum, actin drugs or acute mechanical stress is prevented within mechanically loaded, anchored matrices at tensional homeostasis. This is accompanied by a higher G/F actin ratio, defective nuclear import and increased cofilin expression. We propose that tension regulates MRTF-A/SRF activity through cofilin-mediated modulation of actin dynamics.