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无力，其与肌肉质量损失（萎缩）相关，导致通气不足、难以脱离呼吸机和活动减少。一个公认的加速肌肉蛋白质分解的途径是激活泛素-蛋白体系统。细胞凋亡或程序性细胞死亡是最近描述的实质组织损失的机制，并且可以由许多因素引发，包括生长因子撤回。最近的研究证实，烧伤后骨骼肌细胞凋亡的变化。胰岛素是一种重要的生长因子，通过下游的Akt/PKB信号通路，在蛋白质合成、线粒体功能和抗凋亡等方面发挥重要作用。胰岛素抵抗伴随着通过PI 3-K/AktJPKB途径的信号传导减少，是烧伤的伴随特征。所检验的假设是生长因子（胰岛素）信号传导（特别是通过Akt/PKB）的降低导致肌肉中的细胞凋亡变化，其中线粒体起核心作用。也有推测认为烧伤导致诱导型一氧化氮合酶（iNOS）的激活和反应性一氧化氮（NO）的释放，其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，和啮齿动物与热损伤，将严格评估 NO在降低Akt/PKB活化、线粒体功能和在肌肉中观察到的凋亡变化中的作用。烧伤诱导的线粒体，细胞凋亡和功能（张力）的肌肉变化可以减弱的假设，无论是感染/过度表达的组成型活性Akt/PKB，高度特异性的iNOS抑制剂，或通过抑制促凋亡半胱天冬酶。拟议的研究，使用形态学，生物化学和分子药理学的方法，连同功能（张力研究），和蛋白质动力学分析将表征烧伤对局部和远端肌肉凋亡的影响，以及它们与神经肌肉功能障碍的关系。因此，我们对iNOS抑制剂的有效性、iNOS敲除小鼠的使用以及半胱天冬酶抑制剂的使用的研究将为肌肉萎缩的发病机制提供重要的见解，并为治疗烧伤诱导的神经肌肉萎缩提供新的治疗手段和/或策略。 人类的功能障碍从这些机制研究中获得的信息将为预防和/或纠正人类烧伤神经肌肉并发症的治疗策略提供科学依据和原理。

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.