Claude D. Pepper Older Americans Independence Center (OAIC); Skeletal Muscle Apoptosis and Physical Performance; Oxidative RNA/DNA Damage and Repair in Aged Human Muscle

Specific Aims:

Specific Aim 1. To determine the extent of muscle apoptosis during aging in young healthy controls and in high-functioning and low-functioning older subjects. The investigators hypothesize that the observed level of muscle cell apoptosis is associated with physical disability. The investigators will recruit a cohort of ten young control subjects (age 20-35). The investigators will also recruit and stratify a cohort of forty elderly (age 70-99 years) into two distinct categories by set criteria based on their level of physical performance. Subjects with a summary performance score ≥ 11 at the Short Physical Performance Battery (SPPB) (66) will be considered high-functioning. Conversely, subjects with a summary performance score ≤ 7 will be considered low-functioning. In addition to the SPPB, the investigators will also perform the hand grip strength test and the isokinetic knee extensor test, and will implement 3D magnetic resonance imaging (MRI) to precisely quantify the quadriceps contractile area. The investigators will obtain a muscle biopsy for each subject to determine the severity of apoptosis using biochemical and histological analyses (see Methods). Disability will be assessed using a self-report questionnaire. Hence, the investigators will determine the extent muscle apoptosis and the investigators will be able to correlate this to the level of physical disability and muscle mass.

Specific Aim 2. To elucidate the role of Poly(ADP-ribose) polymerase 1 (PARP-1) and apoptosis-inducing factor (AIF) in the induction of apoptosis in human skeletal muscle. The investigators hypothesize that the observed cell loss due to apoptosis is mainly related to a caspase-independent mechanism. The investigators have recently shown that activation of mitochondrial caspase-independent pathway of apoptosis may play a more important role than the caspase-dependent pathway in contributing to nuclear cell death (67). Furthermore, the investigators hypothesize that the PARP-1/AIF pathway of apoptosis might be involved in the age-related muscle wasting process. The investigators will quantitatively assess PARP-1 and AIF and correlate these measures with the extent of muscle apoptosis.

Specific Aim 3. To assess the role of mitochondria dysfunction and energy failure in muscle apoptosis, sarcopenia, and disability. The investigators hypothesize that mitochondrial dysfunction and the resulting reduced production of adenosine triphosphate (ATP) is causally related to sarcopenia, possibly by triggering the apoptosis program. It has been estimated that the mean ATP production in the quadriceps muscle of old human subjects is approximately 50% of that of younger subjects (68), and it has been postulated that energy failure might trigger apoptosis (58). Consistently with these findings, the investigators have recently found that in rats' gastrocnemius muscle, ATP content and the rate of ATP production declined by ~50% with age (59). The investigators will quantitatively assess myocytes' ATP content along with key mitochondrial metabolic enzymes, namely citrate synthase, cytochrome-c-oxidase (COX), ATPase, and aconitase. The investigators will correlate these parameters with the level of apoptosis and with measures of physical performance, disability, and muscle mass.

Specific Aim 4. To correlate changes in physical function and muscle mass with muscle apoptosis and biochemical parameters at follow-up (longitudinal study). After 4 years subjects in the high-performance group will be reevaluated for muscle apoptosis rate, PARP-1 and AIF activity, and energy production. The investigators will correlate these parameters with measures of physical performance and muscle mass assessed by MRI, as well as with self-reported disability. Hence, the investigators will be able to directly correlate an eventual decline in physical performance, muscle mass and functional status at this very old age with changes in muscle apoptosis and in biochemical parameters.