UBR5 is an E3-ubiquitin-ligase positively associated with anabolism, hypertrophy and recovery from atrophy in skeletal muscle. The precise mechanisms underpinning UBR5's role in the regulation of skeletal muscle mass remains unknown. The present study aimed to elucidate these mechanisms by silencing the UBR5 gene in-vivo. To achieve this aim, we electroporated a UBR5-RNAi plasmid into mouse tibialis anterior muscle to investigate the impact of reduced UBR5 on mechano-transduction signalling MEK/ERK/p90RSK and Akt/GSK3β/p70S6K/4E-BP1/rpS6 pathways. Seven days post UBR5 RNAi electroporation, while reductions in overall muscle mass were not detected, mean CSA of GFP-positive fibers was reduced (-9.5%) and the number of large fibers was lower versus the control. Importantly, UBR5-RNAi significantly reduced total RNA, muscle protein synthesis, ERK1/2, Akt and GSK3β activity. Whilst p90RSK phosphorylation significantly increased, total p90RSK protein levels demonstrated a 45% reduction with UBR5-RNAi. Finally, these early events after 7 days of UBR5 knockdown culminated in significant reductions in muscle mass (-4.6%) and larger reductions in fiber CSA (-18.5%) after 30 days. This was associated with increased levels of the phosphatase PP2Ac, and inappropriate chronic elevation of p70S6K and rpS6 between 7 and 30 days, and corresponding reductions in eIF4e. This study demonstrates UBR5 plays an important role in anabolism/hypertrophy, whereby knockdown of UBR5 culminates in skeletal muscle atrophy.

中文翻译：

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E3泛素连接酶UBR5的敲低及其在骨骼肌合成代谢中的作用

UBR5是一种E3泛素连接酶，与骨骼肌的合成代谢，肥大和从萎缩中恢复正相关。支撑UBR5在骨骼肌质量调节中作用的确切机制尚不清楚。本研究旨在通过在体内沉默UBR5基因来阐明这些机制。为实现此目标，我们将UBR5-RNAi质粒电穿孔到小鼠胫前肌中，以研究还原的UBR5对机械转导信号MEK / ERK / p90RSK和Akt /GSK3β/ p70S6K / 4E-BP1 / rpS6途径的影响。UBR5 RNAi电穿孔后7天，虽然未检测到总体肌肉质量降低，但GFP阳性纤维的平均CSA降低了（-9.5％），大纤维的数量比对照组少。重要的是，UBR5-RNAi显着降低了总RNA，肌肉蛋白质合成，ERK1 / 2，Akt和GSK3β活性。虽然p90RSK的磷酸化显着增加，但总的p90RSK蛋白水平显示出UBR5-RNAi降低了45％。最后，UBR5敲低7天后的这些早期事件最终导致30天后肌肉质量显着减少（-4.6％）和纤维CSA较大减少（-18.5％）。这与磷酸酶PP2Ac的水平升高，p70S6K和rpS6在7至30天之间不适当的慢性升高以及eIF4e的相应降低有关。这项研究表明，UBR5在合成代谢/肥大中起重要作用，因此，敲低UBR5最终会导致骨骼肌萎缩。UBR5敲低7天后，这些早期事件最终导致30天后肌肉质量显着减少（-4.6％）和纤维CSA大幅减少（-18.5％）。这与磷酸酶PP2Ac的水平升高，p70S6K和rpS6在7至30天之间的不适当的慢性升高以及eIF4e的相应降低有关。这项研究表明，UBR5在合成代谢/肥大中起重要作用，因此，敲低UBR5最终会导致骨骼肌萎缩。UBR5敲低7天后的这些早期事件最终导致30天后肌肉质量显着减少（-4.6％）和纤维CSA大幅减少（-18.5％）。这与磷酸酶PP2Ac的水平升高，p70S6K和rpS6在7至30天之间的不适当的慢性升高以及eIF4e的相应降低有关。这项研究表明，UBR5在合成代谢/肥大中起重要作用，因此，敲低UBR5最终会导致骨骼肌萎缩。