Muscle Wasting in Burns: Pivotal Role of Akt/PKB

热损伤后骨骼肌的重要功能变化是肌肉无力，这与肌肉量减少（萎缩）有关，导致通气不足、难以脱下呼吸器和活动减少。加速肌肉蛋白分解的公认途径是泛素-蛋白体系统的激活。细胞凋亡或程序性细胞死亡是最近被描述的一种实质组织丧失的机制，它可以由许多因素引发，包括生长因子戒断。最近的研究证实了烧伤后骨骼肌细胞凋亡的变化。胰岛素是一种关键的生长因子，通过下游Akt/PKB信号通路，在蛋白质合成、线粒体功能、抗凋亡等方面发挥关键作用。胰岛素抵抗伴随着PI3-K/AktJPKB通路信号的减少，是烧伤的伴随特征。经检验的假设是，生长因子（胰岛素）信号的减少，特别是通过Akt/PKB导致肌肉细胞凋亡的变化，线粒体起着核心作用。也有假设认为，烧伤导致诱导型一氧化氮合酶（iNOS）的激活，并释放活性一氧化氮（NO），从而通过翻译后修饰改变Akt/PKB。因此，拟议的研究，使用肌肉细胞培养，和热损伤的啮齿动物，将批判性地评估

The important functional change in skeletal muscle following thermal injury is muscle weakness, which is associated with loss of muscle mass (atrophy) resulting in hypoventilation, difficulty in weaning off respirators, and decreased mobilization. A recognized pathway for accelerated muscle protein breakdown is the activation of ubiquitin-proteosome system. Apoptosis or programmed cell death is a relatively recently described mechanism of loss of parenchymal tissue and can be initiated by many factors, including growth factor withdrawal. Recent studies following burn injury have confirmed apoptotic changes in skeletal muscles. Insulin is a key growth factor, which, via downstream Akt/PKB signaling pathway, plays a key role in protein synthesis, mitochondrial function, and anti-apoptosis. Insulin resistance with decreased signaling via PI3-K/AktJPKB pathway, is a concomitant feature of burns. The hypothesis tested is that the decreased growth factor (insulin) signaling, specifically via Akt/PKB leads to apoptotic changes in muscle with mitochondria playing a central role. It is also postulated that the burn injury results in the activation of inducible nitric oxide synthase (iNOS) with release of reactive nitric oxide (NO), which alters Akt/PKB by post translational modifications. Thus, the proposed studies, using muscle cell cultures, and rodents with thermal injury, will critically evaluate the role of NO in decreased Akt/PKB activation, mitochondrial function, and in the apoptotic changes seen in muscle. The hypothesis that burn-induced mitochondrial, apoptotic and functional (tension) changes in muscle can be attenuated by either infection/over-expression of constitutively active Akt/PKB, highly specific iNOS inhibitors, or by inhibition of pro-apoptotic caspases will also be tested. The proposed studies, using morphologic, biochemical and molecular pharmacological approaches, together with functional (tension studies), and protein kinetic analyses would characterize the effects of burn injury on local and distant muscle apoptosis, and their relationship to neuromuscular dysfunction. Our studies on the effectiveness of iNOS inhibitor, and use of iNOS knockout mice, and the use of caspase inhibitors will thus provide significant insights into the pathogenesis of muscle wasting, and furnish novel therapeutic armamentaria and/or strategies to treat burn-induced neuromuscular dysfunction in humans. Information obtained from these mechanistic studies will provide a scientific basis and rationale for therapeutic maneuvers to prevent and/or rectify neuromuscular complications of burns in humans.