title: Cellular localization and dendritic function of rat isoforms of the SRF coactivator MKL1 in cortical neurons.
abstract: The ability of megakaryoblastic leukemia 1 (MKL1) to function as a serum response factor (SRF) coactivator is regulated through its association with G-actin. In the cytoplasm, MKL1 binds to G-actin through RPXXXEL (RPEL) motifs. However, dissociation of MKL1 from G-actin triggers its translocation into the nucleus where it stimulates SRF-mediated gene expression. Previous characterization of rat MKL1 gene products has identified several isoforms: full-length MKL1, basic, SAP, and coiled-coil domain (BSAC), MKL1-elongated derivative of yield (MELODY), and MKL1met. In this study, we have investigated whether these MKL1 isoforms, which contain different numbers of RPEL motifs, differ in their subcellular localization, transcriptional activity, and effect on dendritic number and axonal length. Immunofluorescent staining of cultured cortical neurons expressing individual FLAG-tagged MKL1 isoforms indicated that all MKL1 isoforms are present in both the cytoplasm and the nucleus. However, MKL1met, which contains two RPEL motifs, shows enhanced nuclear staining compared with the other three isoforms, full-length MKL1, basic, SAP, and coiled-coil domain, and MKL1-elongated derivative of yield, which contain three RPEL motifs. Consistent with its preferential nuclear localization, overexpression of MKL1met, but not other isoforms, increases SRF-mediated transcriptional responses and reduces the number of dendrites. In contrast to the inhibitory effect of MKL1met on dendritic number, axonal length is not affected by overexpression of any of the MKL1 isoforms. These findings suggest that the subcellular localization of MKL1 isoforms, which is mediated by the number of actin-binding RPEL motifs, regulates their effect on SRF-mediated gene expression and dendritic morphology.
authors.affiliation: aLaboratory of Molecular Neurobiology, Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, Toyama bDepartment of Neurobiology and Behavior, Gunma University, Graduate School of Medicine, Maebashi cDivision of Pharmacology, Biological Safety Research Center, National Institute of Health Sciences, Tokyo, Japan dSolomon H. Snyder Department of Neuroscience, Johns Hopkins University, School of Medicine, Baltimore, Maryland, USA.
authors.forename: Mitsuru
authors.initials: M
authors.lastname: Ishikawa
authors.forename: Jun
authors.initials: J
authors.lastname: Shiota
authors.forename: Yuta
authors.initials: Y
authors.lastname: Ishibashi
authors.forename: Tomoyuki
authors.initials: T
authors.lastname: Hakamata
authors.forename: Shizuku
authors.initials: S
authors.lastname: Shoji
authors.forename: Mamoru
authors.initials: M
authors.lastname: Fukuchi
authors.forename: Masaaki
authors.initials: M
authors.lastname: Tsuda
authors.forename: Tomoaki
authors.initials: T
authors.lastname: Shirao
authors.forename: Yuko
authors.initials: Y
authors.lastname: Sekino
authors.forename: Jay M
authors.initials: JM
authors.lastname: Baraban
authors.forename: Akiko
authors.initials: A
authors.lastname: Tabuchi
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000166:cytology*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D003712:Dendrites / Q000378:metabolism*; D004622:Embryo, Mammalian; D005260:Female; D009474:Neurons / Q000166:cytology*; D011247:Pregnancy; D020033:Protein Isoforms / Q000378:metabolism; D017207:Rats, Sprague-Dawley; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism*; D014157:Transcription Factors; D014162:Transfection
chemical_list: D004268:DNA-Binding Proteins; C000594702:Mrtfa protein, rat; D020033:Protein Isoforms; D015534:Trans-Activators; D014157:Transcription Factors