Unloading and paralysis of skeletal muscle after spinal cord injury (SCI) results in severe loss of muscle contractile function and deleterious alterations in mitochondrial morphology and performance. One potential mechanism is the de novo appearance of connexin hemichannels (Cx) in paralyzed muscle. We hypothesize Cx destabilize mitochondrial function in paralyzed muscle. Cx43 and 45 appear in the sarcolemma de novo after paralysis and are believed to contribute to muscle atrophy by permitting entry of extracellular calcium (Ca2+) into the muscle fiber. We posit that the systematic study of the role of Cx43 and 45 after SCI using transgenic mouse models will evaluate the potential of blockers of Cx for future drug development. We also propose that protecting mitochondrial function and morphology by a pharmaceutical intervention may be beneficial for muscle health after SCI. SS-31 is a mitochondrial-targeted antioxidant that has reduced muscle loss and protected mitochondrial function in mouse models of muscle wasting. No study has tested whether SS-31 improves muscle health after paralysis. Our overall objective is to find interventions and treatments to increase the well-being of individuals with SCI. We hypothesize that muscle-specific knockout of Cx43 and 45 will protect muscle health after SCI. Additional studies will describe the role of drug or diet interventions in protecting muscle function after paralysis. We believe data from this line of studies will provide insight into multiple novel treatments that protect muscle health after SCI and provide a path to translatable medicine and future clinical investigations. 1.
Specific Aim 1 (Year 1-[3]): Determine whether a muscle-specific knockout of Cx43 and 45 protects mitochondrial morphology and metabolic function in mice paralyzed by SCI Hypothesis: Muscle-specific knockout of Cx43 and 45 will slow the loss of muscle mass and function and protect against the degradation of mitochondrial morphology after contusion SCI. Approach: 4-month old transgenic mice carrying a muscle-specific MyoD-driven Cre-recombinase and floxed Cx43 and 45 will be given a sham or contusion SCI and be compared to a MyoD-Cre-recombinase-only genotype control. Behavioral tests, muscles function tests and biochemical outcomes will be analyzed. 2.
Specific Aim 2 (Year [4-5]): Investigate the efficacy of an SS-31 drug intervention for preserving muscle function and mitochondrial morphology and limiting ROS production after SCI. Hypothesis: Daily injections of SS-31 will protect muscle and mitochondrial morphology after contusion SCI. Approach: 4-month old wild type C57BL/6 female mice will be given a sham or SCI as well as daily injections of SS-31 or vehicle across time. Muscle function will be measured ex vivo; ROS production rate will be measured from isolated muscle mitochondria and whole muscle lysates.
Spinal cord injury (SCI) affects approximately 40,000 Veterans and leads to healthcare costs of $525,000 for the first year after SCI and $80,000 each year thereafter. This provides a strong justification to find treatments which help reduce the financial burden facing Veterans with SCI while also increasing their well-being. Skeletal muscle function and health are severely reduced after SCI and this is associated with negative health factors such as insulin resistance, inflammation and obesity. Protecting mitochondrial health after paralysis may be a way to preserve muscle health after SCI. There are no FDA-approved treatments to preserve mitochondrial health. Investigating whether the appearance of connexin hemichannels in muscle after SCI is linked to mitochondrial health may provide an avenue for future drug design. Testing current drugs for improving mitochondrial health, such as SS-31, can provide essential information regarding how best to improve the health of Veterans with SCI.
