Targeting Oxidative Stress to Prevent Vascular and Skeletal Muscle Dysfunction During Disuse

Overview

Prolonged periods of reduced activity are associated with decreased vascular function and muscle atrophy. Physical inactivity due to acute hospitalization is also associated with impaired recovery, hospital readmission, and increased mortality. Older adults are a particularly vulnerable population as functional (vascular and skeletal muscle mitochondrial dysfunction) and structural deficits (loss in muscle mass leading to a reduction in strength) are a consequence of the aging process. The combination of inactivity and aging poses an added health threat to these individuals by accelerating the negative impact on vascular and skeletal muscle function and dysfunction. The underlying factors leading to vascular and skeletal muscle dysfunction are unknown, but have been linked to increases in oxidative stress. Additionally, there is a lack of understanding of how vascular function is impacted by inactivity in humans and how these changes are related to skeletal muscle function. It is our goal to investigate the mechanisms that contribute to disuse muscle atrophy and vascular dysfunction in order to diminish their negative impact, and preserve vascular and skeletal muscle function across all the lifespan.

Disuse following injury or during acute hospitalization is associated with a host of negative outcomes including functional deficiencies, hospital readmission, disability, and increased mortality. Older adults are a particularly vulnerable population as functional (vascular and skeletal muscle dysfunction) and structural deficits (loss in muscle mass leading to a reduction in strength) are present as a consequence of the aging process. Any additional and accelerated insult caused by disuse poses a serious health threat to these older individuals by depleting their already diminished physiological and functional reserve and hastening the onset of disability. Current strategies aimed at preserving function during disuse have focused on preserving skeletal muscle mass and strength while the critical role of the vasculature has been largely ignored. Moreover, the underlying cause of dysfunction has not been adequately addressed in humans. This disintegrated and myopic approach likely contributes to the fact that interventions capable of preserving health during disuse do not exist. The vascular and skeletal muscle systems are inextricably linked to optimal mobility through oxygen and nutrient delivery, thus, vascular dysfunction likely contributes to and exacerbates skeletal muscle deficiencies that occur during disuse. To fully understand the impact of disuse on health and mobility and develop effective countermeasures it is our contention that both the vascular and musculoskeletal systems must be examined and the root cause of the problem must be addressed. While the underlying factors leading to these accelerated losses during disuse are unknown, they appear to be mechanistically linked to oxidative stress. The long term goal is to minimize losses in vascular and skeletal muscle function that occur during disuse in order to maintain functional reserve and avoid serious adverse events. The objective here, which is the next step in pursuit of this goal, is to determine how oxidative stress contributes to disuse-induced vascular and skeletal muscle dysfunction. It is our central hypothesis that oxidative stress triggers the accelerated declines in vascular and skeletal muscle function during disuse. To test this hypothesis and provide compelling evidence that oxidative stress is the trigger of dysfunction the investigators will utilize two novel and fundamentally distinct strategies to improve redox balance during disuse. In Aim 1, the mitochondrial targeted antioxidant (MITO-AO) mitoquinone will be administered during disuse to improve free radical scavenging at the level of the mitochondria. In Aim 2, activation of Nuclear Factor Erythroid-2-like 2 (Nrf2) the "master regulator of antioxidant enzymes" will be accomplished with PB125 (a novel naturally occurring Nrf2 activator) to augment endogenous antioxidant defense systems. The impact of these interventions on measures of isolated and integrated vascular and skeletal muscle function before and after disuse will be examined. The central hypothesis is supported by preliminary data reporting substantial losses in vascular and skeletal muscle function and concomitant increases in oxidative stress following 5 days of bed rest. Importantly, MITO-AO prevents disuse-induced losses in muscle mass and restores age-related deficits in vascular function in aged animals and humans (preliminary data). Additionally, PB125 activates the Nrf2 pathway at multiple control points resulting in prolonged and amplified activation and subsequent gene expression of key antioxidant enzymes leading to a decrease in oxidative stress in humans (preliminary data).