Recent studies identified cyclase-associated proteins (CAPs) as important regulators of actin dynamics that control assembly and disassembly of actin filaments (F-actin). While these studies significantly advanced our knowledge of their molecular functions, the physiological relevance of CAPs largely remained elusive. Gene targeting in mice implicated CAP2 in heart physiology and skeletal muscle development. Heart defects in CAP2 mutant mice were associated with altered activity of serum response factor (SRF), a transcription factor involved in multiple biological processes including heart function, but also skeletal muscle development. By exploiting mouse embryonic fibroblasts (MEFs) from CAP2 mutant mice, we aimed at deciphering the CAP2-dependent mechanism relevant for SRF activity. Reporter assays and mRNA quantification by qPCR revealed reduced SRF-dependent gene expression in mutant MEFs. Reduced SRF activity was associated with moderately increased G-actin levels and reduced nuclear levels of myocardin-related transcription factor A (MRTF-A), a transcriptional SRF coactivator that is shuttled out of the nucleus and, hence, inhibited upon G-actin binding. Moreover, pharmacological actin manipulation with jasplakinolide (JASP) restored MRTF-A distribution in mutant MEFs. Our data are in line with a model in which CAP2 controls the MRTF-SRF pathway in an actindependent manner. While MRTF-A localization and SRF activity was impaired under basal conditions, serum stimulation induced nuclear MRTF-A translocation and SRF activity in mutant MEFs similar to controls. In summary, our data revealed that in MEFs CAP2 controls basal MRTF-A localization and SRF activity, while it was dispensable for serum-induced nuclear MRTF-A translocation and SRF stimulation.

中文翻译：

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环化酶相关蛋白2（CAP2）控制小鼠胚胎成纤维细胞中的MRTF-A定位和SRF活性

最近的研究确定环化酶相关蛋白（CAPs）是肌动蛋白动力学的重要调节剂，可控制肌动蛋白丝（F-actin）的组装和拆卸。尽管这些研究极大地提高了我们对其分子功能的了解，但CAP的生理相关性仍然难以捉摸。小鼠基因靶向在心脏生理和骨骼肌发育中涉及CAP2。CAP2突变小鼠的心脏缺陷与血清反应因子（SRF）的活性改变有关，血清反应因子（SRF）是涉及多种生物学过程（包括心脏功能）以及骨骼肌发育的转录因子。通过利用CAP2突变小鼠的小鼠胚胎成纤维细胞（MEF），我们旨在破译与SRF活性相关的CAP2依赖性机制。通过qPCR进行的记者测定和mRNA定量显示，突变MEF中SRF依赖性基因表达降低。SRF活性降低与G-肌动蛋白水平适度增加和心肌相关转录因子A（MRTF-A）的核水平降低有关，MRTF-A是一种转录SRF共激活因子，它从核中穿梭出来，因此受G-肌动蛋白结合的抑制。此外，药理肌动蛋白操纵雅斯普利奈德（JASP）恢复突变MEFs中的MRTF-A分布。我们的数据与CAP2以肌动蛋白独立方式控制MRTF-SRF途径的模型相符。虽然在基础条件下MRTF-A的定位和SRF活性受到损害，但血清刺激在类似于对照的突变型MEF中诱导了核MRTF-A的移位和SRF活性。综上所述，