SRF'ing and SAP'ing - the role of MRTF proteins in cell migration

Abstract

Actin-based cell migration is a fundamental cellular activity that plays a crucial role in a wide range of physiological and pathological processes. An essential feature of the remodeling of actin cytoskeleton during cell motility is the de novo synthesis of factors involved in the regulation of the actin cytoskeleton and cell adhesion in response to growth-factor signaling, and this aspect of cell migration is critically regulated by serum-response factor (SRF)-mediated gene transcription. Myocardin-related transcription factors (MRTFs) are key coactivators of SRF that link actin dynamics to SRF-mediated gene transcription. In this Review, we provide a comprehensive overview of the role of MRTF in both normal and cancer cell migration by discussing its canonical SRF-dependent as well as its recently emerged SRF-independent functions, exerted through its SAP domain, in the context of cell migration. We conclude by highlighting outstanding questions for future research in this field.

Keywords: Actin; Cancer; Cell migration; MRTF; SAP; SRF.

Fig. 1.

Main structural features of MRTF. Shown are the main domains of MRTFs. Located N-terminally are the three RPEL domains (R1, R2 and R3; dark gray) separated by spacer regions (light gray), all of which are bound in a pentavalent complex to actin (blue ovals). The extended basic region comprising boxes B3 and B2 contains a bipartite nuclear localization signal (NLS); a second basic region (B1) with another NLS and an SRF-binding site; a glutamine (Q)-rich domain that promotes nuclear export and regulates SRF binding; a putative DNA-binding motif – the SAP domain – that allows MRTF to transcribe genes in SRF-independent manner; a leucine-zipper (LZ) that facilitates homo- and hetero-dimerization of the protein; and the transcriptional activation domain (TAD). Notice that myocardin has similar structural motifs.

Fig. 2.

Regulation of MRTF by actin and phosphorylation. Binding of growth factors to growth factor receptors (GFRs) activates Rho-GTPase or Ras-MAPK signaling pathways, leading to activation of SRF and its subsequent association with the MRTF or TCF families of coactivators, respectively. Activation of Rho-GTPase initiates actin polymerization and promotes the dissociation of MRTF from monomeric actin (G-actin). MRTF then translocates to the nucleus where, together with SRF, it promotes the expression of SRF target genes. Dissociation of MRTF from G-actin is also facilitated by sequestration of actin by other actin-binding proteins (ABPs) through competitive inhibition. Nuclear import and export of MRTF is regulated by either repressive or activating phosphorylation. MRTF-SRF signaling is also promoted through polymerization of nuclear actin or through interaction of the ABP filamin with MRTF, thereby inhibiting MRTF phosphorylation (i.e. repression) and attenuating its interaction with actin.

Fig. 3.

Dual regulation of MRTF through cell–cell adhesion and TGFβ signaling. MRTF function can be positively or negatively regulated by loss of cell–cell adhesion and SMAD3. Binding of SMAD3 to MRTF promotes GSK3β-mediated phosphorylation of MRTF, leading to MRTF ubiquitylation and degradation. The inhibitory effect of SMAD3 on MRTF can be relieved by β-catenin-mediated sequestration of SMAD3 upon its dissociation from E-cadherin following the disruption of cell–cell junctions. This process is further reinforced by MRTF-SRF-mediated stimulation of TAZ expression, a negative regulator of SMAD3 gene expression. There is also a mutual dependence between MRTF-SRF and TAZ signaling. Disruption of cell–cell adhesion also triggers MRTF-SRF activation through the activation of a Rac–PAK–p38MAPK signaling axis. p38MAPK, which is also activated by TGFβ signaling, promotes MRTF-SRF activity through MRTF-dependent NOX-mediated phosphorylation and activation of MRTF in a SMAD3-independent manner.

Fig. 4.

Illustration of the different pathways linking MRTF to cell motility. MRTF potentially regulates all major aspects of cell migration, including EMT, dynamic control of actin polymerization and membrane protrusion, contractility, cell–cell adhesion and cell–ECM adhesion, cell polarity, cell deformability, microtubule acetylation, membrane-cytoskeletal linkage and ECM proteolysis. Shown here are some of the prominent protein-coding and microRNA genes that are important for cell motility and are regulated by MRTF- and SRF-dependent transcription, as well as those that are independent of SRF and mediated by the SAP domain of MRTF and cooperative interactions with other transcription factors, for example SMAD3. See main text for detailed discussion. ATAT1, tubulin acetyltransferase 1; PIP5K, phosphatidylinositol-4-phosphate-5-kinase. The arrow indicates that externalization of ABPs can also lead to altered actin dynamics and protrusion.