Biologic Mechanisms for Pain Variation After Physical Activity in Osteoarthritis

Osteoarthritis (OA) in the knee is characterized by chronic inflammatory pain that is not necessarily associated with the amount of joint damage.1 Clinical practice guidelines recommend physical activity (PA) for osteoarthritis pain,2 but uptake of PA among adults with OA is very low.3 One reason for this is that while PA can reduce pain among adults with lower extremity OA,4,5 it does so differentially, decreasing pain sensitivity for about half of adults with OA but actually increasing pain sensitivity for the other half.6 Further, a recent meta-analysis revealed that engaging in a single type of PA (e.g. aerobic exercise or resistance training) reduces OA knee pain, but there was large heterogeneity in the results which could not be explained by age, sex, BMI, alignment in the knee, disease severity, or baseline pain.5 One of the goals of developing individualized PA interventions for adults with OA is to elucidate the mechanisms by which PA reduces OA pain and for whom PA most effectively diminishes the pain. Aerobic physical activity, such as walking, increases cellular capacity for energy generation (ATP production) via oxidative phosphorylation up to 2-fold by stimulating mitochondrial biogenesis.7,8 This phenomenon occurs not only in skeletal muscle,7 but also in brain cells,9,10 liver cells,9,11,12 adipose tissue,13 kidney cells,12 and leukocytes14 indicating that PA likely increases metabolic demand systemically. Moreover, PA is thought to create adaptive changes in the activity and/or abundance of proteins involved in processes related to mitochondrial function.8 Mitochondrial function, including energy generation through oxidative phosphorylation; inflammation; and mitochondrial related protein expression are key features in osteoarthritis15,16 and chronic inflammatory pain.17,18 Animal models of inflammatory pain demonstrate a cellular metabolic shift from oxidative phosphorylation to glycolysis in chronic inflammatory states via the pyruvate dehydrogenase kinase 2/4 (PKD2/4)-pyruvate dehydrogenase (PDH)-lactic acid axis.19 This results in an increase in lactic acid production in the affected area. The ensuing acidic microenvironment amplifies the nociceptive response via recruitment of additional pro-algesic proinflammatory cytokines which "activate nociceptors and spinal glia to cause peripheral and central sensitizations, respectively".19 Thus, improvement in the capacity to generate ATP through oxidative phosphorylation, and associated reduction of glycolysis, may reduce pain sensitivity. However, while a large body of animal and correlational data supports a strong link between oxidative potential and pain outcomes, experimental evidence of cause and effect remains sparse, especially in humans.8 The investigators are hypothesizing that individual differences in systemic cellular bioenergetic function, inflammation, and protein expression influence the effect of PA to reduce pain sensitivity in adults with knee OA. The purpose of this quasi-experimental pilot study is to test mitochondrial bioenergetics (oxidative phosphorylation, mitochondrial content) in platelets, inflammation (cytokines) and protein expression as mechanisms for variation in pain sensitivity immediately after 30 minutes of walking (i.e. "acute") and after six weeks of walking three times a week for 30 minutes (i.e. "long-term") in adults with knee osteoarthritis. The investigators will address the following specific aims and hypotheses in a sample of 40 adults with radiologic evidence of hip or knee OA and 20 age/gender matched healthy controls: Aim 1: To examine the effects of a six week (three days/week) walking program on pain thresholds in adults with knee OA as compared to healthy controls H1.1: Pain sensitivity (Quantitative Sensory Testing) will increase in approximately 50% of adults with OA and decrease in approximately 50% of adults with OA after acute and long-term PA. H1.2: Pain sensitivity will decrease in healthy controls after acute and long-term PA. Aim2: To test the role of mitochondrial bioenergetics (oxidative phosphorylation, mitochondrial content) as a mechanism for variation in pain sensitivity after PA in older adults with knee OA. H2.1: Pain sensitivity is negatively associated with mitochondrial function (oxidative phosphorylation, mitochondrial content) in platelets at baseline, after acute PA and long-term PA H2.2: Healthy controls will have higher capacity for oxidative phosphorylation in platelets than OA participants. Aim3: To test the role of inflammation as a mechanism for variation in pain sensitivity after physical activity in older adults with knee OA. H3.1: Pain sensitivity is positively associated with increased circulating proinflammatory cytokines (c-reactive protein, interleukin (IL)-1, IL-1β, IL-6, IL-10, tumor necrosis factor (TNF)-α, PGES) at baseline, after acute and long-term PA. Aim 4: To generate hypotheses regarding the role of proteomics in variation in pain sensitivity after physical activity (immediacy following and after six weeks of walking program) Changes in protein expression will depend on the half-life of the protein being expressed which can range from minutes to days.8 Thus, it is important to examine adaptive changes in protein expression in both the short (minutes/day post PA) and long term (days/weeks between bouts of physical activity).