Nielsen, Julie Charlotte; (2024) MRTF-A/B signalling and control of cell proliferation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Myocardin related transcription factors A/B (MRTF-A/B) are transcriptional co-activators of serum response factor (SRF). MRTF-SRF signalling is activated upon actin polymerisation that allows nuclear accumulation of MRTF. In turn, MRTF-SRF signalling induces transcription of genes encoding components and regulators of the actin cytoskeleton. Hence, MRTF-SRF signalling have been shown to play a key role in cytoskeletal processes including cell shape, adhesion, and migration. In this project I have investigated the role of MRTF-SRF signalling in proliferation. Inactivation of MRTF-A/B or SRF results in proliferation arrest accompanied by positive SA--gal staining, an increase in p27 and TLR2 protein levels and a decrease in CDK1 and CKS2 protein levels. In addition, proliferation arrest induced by MRTF-A/B inactivation was associated with a SASP-like expression profile. MRTF-A/B dKO MEFs propagated the cell cycle arrest to neighbouring cells, suggesting a senescence-like phenotype. Intriguingly, both the MRTF-A/B inactivation-induced proliferation arrest and cytoskeletal defects were reversible upon re-expression of exogenous MRTF-A. Moreover, the proliferation arrest and cell cycle markers of MRTF-A/B dKO MEFs were phenocopied by long-term serum deprivation. This raises questions as to whether the MRTF-A/B inactivation-induced cell cycle arrest is similar to that seen in quiescent cells. Deletion of p27 was not sufficient to prevent induction of the proliferation arrest in MRTF-A/B dKO MEFs, suggesting that p27 either plays a minimal role in this setting or that there is redundancy from other cell cycle inhibitors. A CRISPR-KO screen was performed to identify regulators in an unbiased manner; however, no hits could be confirmed. ROCK inhibition induced a proliferation arrest associated with similar cell cycle markers as those seen upon MRTF-A/B inactivation. Taken together with the fact that MRTF-A/B are major regulators of the actin cytoskeleton, these results suggest that an impaired cytoskeleton may underlie the proliferation arrest upon inactivation of MRTF-A/B.

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