RESUMEN
Vertebrate nonmuscle cells express two actin isoforms: cytoplasmic ß- and γ-actin. Because of the presence and localized translation of ß-actin at the leading edge, this isoform is generally accepted to specifically generate protrusive forces for cell migration. Recent evidence also implicates ß-actin in gene regulation. Cell migration without ß-actin has remained unstudied until recently and it is unclear whether other actin isoforms can compensate for this cytoplasmic function and/or for its nuclear role. Primary mouse embryonic fibroblasts lacking ß-actin display compensatory expression of other actin isoforms. Consistent with this preservation of polymerization capacity, ß-actin knockout cells have unchanged lamellipodial protrusion rates despite a severe migration defect. To solve this paradox we applied quantitative proteomics revealing a broad genetic reprogramming of ß-actin knockout cells. This also explains why reintroducing ß-actin in knockout cells does not restore the affected cell migration. Pathway analysis suggested increased Rho-ROCK signaling, consistent with observed phenotypic changes. We therefore developed and tested a model explaining the phenotypes in ß-actin knockout cells based on increased Rho-ROCK signaling and increased TGFß production resulting in increased adhesion and contractility in the knockout cells. Inhibiting ROCK or myosin restores migration of ß-actin knockout cells indicating that other actins compensate for ß-actin in this process. Consequently, isoactins act redundantly in providing propulsive forces for cell migration, but ß-actin has a unique nuclear function, regulating expression on transcriptional and post-translational levels, thereby preventing myogenic differentiation.
Asunto(s)
Actinas/metabolismo , Movimiento Celular/fisiología , Fibroblastos/metabolismo , Proteómica/métodos , Actinas/genética , Amidas/farmacología , Animales , Western Blotting , Adhesión Celular/efectos de los fármacos , Adhesión Celular/genética , Adhesión Celular/fisiología , Movimiento Celular/efectos de los fármacos , Movimiento Celular/genética , Células Cultivadas , Embrión de Mamíferos/citología , Embrión de Mamíferos/embriología , Embrión de Mamíferos/metabolismo , Fibroblastos/citología , Regulación del Desarrollo de la Expresión Génica , Ratones , Ratones Noqueados , Isoformas de Proteínas/genética , Isoformas de Proteínas/metabolismo , Seudópodos/genética , Seudópodos/metabolismo , Seudópodos/fisiología , Piridinas/farmacología , Reacción en Cadena de la Polimerasa de Transcriptasa Inversa , Transducción de Señal/efectos de los fármacos , Transducción de Señal/genética , Transducción de Señal/fisiología , Factor de Crecimiento Transformador beta/genética , Factor de Crecimiento Transformador beta/metabolismo , Quinasas Asociadas a rho/antagonistas & inhibidores , Quinasas Asociadas a rho/genética , Quinasas Asociadas a rho/metabolismoRESUMEN
Generation of gain-of-function transgenic mice by targeting the Rosa26 locus has been established as an alternative to classical transgenic mice produced by pronuclear microinjection. However, targeting transgenes to the endogenous Rosa26 promoter results in moderate ubiquitous expression and is not suitable for high expression levels. Therefore, we now generated a modified Rosa26 (modRosa26) locus that combines efficient targeted transgenesis using recombinase-mediated cassette exchange (RMCE) by Flipase (Flp-RMCE) or Cre recombinase (Cre-RMCE) with transgene expression from exogenous promoters. We silenced the endogenous Rosa26 promoter and characterized several ubiquitous (pCAG, EF1α and CMV) and tissue-specific (VeCad, αSMA) promoters in the modRosa26 locus in vivo. We demonstrate that the ubiquitous pCAG promoter in the modRosa26 locus now offers high transgene expression. While tissue-specific promoters were all active in their cognate tissues they additionally led to rare ectopic expression. To achieve high expression levels in a tissue-specific manner, we therefore combined Flp-RMCE for rapid ES cell targeting, the pCAG promoter for high transgene levels and Cre/LoxP conditional transgene activation using well-characterized Cre lines. Using this approach we generated a Cre/LoxP-inducible reporter mouse line with high EGFP expression levels that enables cell tracing in live cells. A second reporter line expressing luciferase permits efficient monitoring of Cre activity in live animals. Thus, targeting the modRosa26 locus by RMCE minimizes the effort required to target ES cells and generates a tool for the use exogenous promoters in combination with single-copy transgenes for predictable expression in mice.
Asunto(s)
Sitios Genéticos/genética , Integrasas/metabolismo , Mutagénesis Insercional/métodos , Regiones Promotoras Genéticas/genética , Proteínas/genética , Transgenes/genética , Animales , Sitios de Ligazón Microbiológica/genética , Células Madre Embrionarias/metabolismo , Genes Reporteros/genética , Proteínas Fluorescentes Verdes/metabolismo , Luciferasas/metabolismo , Ratones , Ratones Endogámicos BALB C , Ratones Transgénicos , Especificidad de Órganos/genética , ARN no TraducidoRESUMEN
Conventional approaches to produce transgenic mice recurrently yield unpredictable patterns and levels of transgene expression, a situation calling for the development of new techniques to overcome these drawbacks in the context of overexpression studies. Here we present an efficient method for rapid and reproducible transgenesis using the recombinase mediated cassette exchange (RMCE) (Bouhassira et al.: Blood 90:3332-3344, 1997) procedure. A lox511-EGFP-TK/neo-loxP cassette was placed under the control of the endogenous mouse beta-actin promoter. Heterozygous mice revealed strong and ubiquitous EGFP expression throughout embryogenesis and adulthood. Reproducibly, the same expression pattern was obtained with RMCE when it was used to replace the EGFP-harboring cassette by ECFP or placental alkaline phosphatase (PLAP) reporter genes (DePrimo et al.: Transgenic Res 5:459-466, 1996). Furthermore, the RMCE procedure proved efficient as well in embryonic stem (ES) cells as directly in zygotes. Our results demonstrate ubiquitous expression of floxed transgenes in the endogenous beta-actin locus and they support the general use of the beta-actin locus for targeted transgenesis.
