An important functional change in skeletal muscle following burn injury is muscle weakness, which is associated with loss of muscle mass (atrophy). This results in hypoventilation, difficulty in weaning from respirators, and decreased mobilization. A recognized pathway for accelerated muscle protein breakdown is the activation of the ubiquitin-proteosome system. Apoptosis or programmed cell death is a relatively recently described mechanism for the 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 muscle. 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 PI 3-K/Akt/PKB pathway is a concomitant feature of burns.

Hypothesis Guiding the Research

It is hypothesized that the decreased growth factor (insulin) signaling, specifically via Akt/PKB leads to and/or exacerbates apoptotic changes in muscle with mitochondria playing a central role. It is also proposed that the inflammatory response mediated by burn injury results in the activation of inducible nitric oxide synthase (iNOS) with release of reactive nitrogen species [nitrosonium ion (NO) equivalent, peroxynitrite], which alter Akt/PKB and mitochondrial function. Using muscle cell cultures and a model system with thermal injury, this project critically evaluates the role of NO in decreased Akt/PKB activation, mitochondrial function, and in the apoptotic changes seen in muscle. The studies in Project 4 are complemented by NMR studies of mitochondrial function and other metabolic studies in humans and model systems. The potential to pharmacologically modulate Akt/PKB function and apoptotic changes, and therefore improve muscle mass and muscle function, is also tested.

Specific Aims

Specific Aim 1 tests the hypothesis that decreased insulin (growth factor) signaling via Akt/PKB results in mitochondrial dysfunction, increased activation of pro-apoptotic molecules, and attenuation of anti-apoptotic signals leading to apoptosis in skeletal muscle cell.

Specific Aim 2 uses a model system in vivo to test the hypothesis that apoptotic changes in skeletal muscle following burns go pari passu with decreased signal transduction via key anti- apoptotic molecule, Akt/PKB, and that iNOS plays an important role in these changes.

Specific Aim 3 tests 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, or by inhibition of pro-apoptotic caspases.

Innovation

Three decades of intense research defining the hypermetabolic response have carefully dissected the post-injury changes in carbohydrate, fat and protein metabolism following severe injury. Further definition of the substrate alterations in intermediary metabolism is currently underway within this Center. This project now explores the consequences of these metabolic changes in skeletal muscle as it presents as severe muscle wasting and muscle weakness. Few investigators have considered this topic and the project PIs' laboratory is one of the first labs to demonstrate that skeletal muscle apoptosis does occur in response to severe injury. Injured patients often suffer many functional and clinical adverse consequences of muscle wasting and weakness following injury. Modern nutritional intervention still does not provide sufficient nitrogen replacement to such patients. Eventually, muscle mass erodes so severely that even with vigorous nitrogen and calorie supplementation, basic nitrogen synthetic processes fail. This outcome may explain the profound metabolic failure experienced as the terminal course of the hypercatabolic SIRS syndrome. Agents that can reverse these responses promise to improve the outcome in severely injured patients. Potential therapeutic opportunities in other clinical processes, such as aging, might exist.

For more information about this project, please contact Dr. Jeeva Martyn or Dr. Masao Kaneki.

[top]