Role of Keratin 18 for Stress-induced Adaptive Strength Gains

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PROJECT SUMMARY Sarcopenia, loss of muscle strength and mass with age, is a major component of frailty. Muscle weakness appears to be critical as low strength is associated with more markers of frailty than chronological age. Geriatric and frail individuals are at greater risk of functional impairments due to an inability of skeletal muscle to adapt to physical stressors. Strength adaptations in response to physical stress or resistance training have been linked to loss of muscle quality, particularly excitation contraction (EC) coupling failure. Identifying mechanisms that counteract contraction-induced EC coupling failure and promote skeletal muscle adaptations are therefore essential to combating sarcopenia and frailty. Recent data suggest that keratin 18 (Krt18), an intermediate filament protein that associates with the dystrophin-glycoprotein complex, may be an important mediator of stress-induced adaptive strength gains that decreases with age. Specifically, we observed that the Krt18 gene was the most highly upregulated (7-fold) in muscle from young mice that gained strength after eccentric exercise. We also observed that increased expression of the Krt18 protein and strength gains were associated with increased expression of membrane-associated proteins. In muscle of old mice, expression of Krt18 was blunted and strength gains were dramatically lower when compared to young mice. Thus, our overarching hypothesis is that Krt18 facilitates skeletal muscle adaptation to exercise by increasing plasmalemmal stability to maintain EC coupling processes that become disrupted by repeated bouts of eccentric contractions. Ultimately, we posit Krt18 may play a role in increasing skeletal muscle resiliency and attenuating development and progression of sarcopenia and frailty. In Aim 1 we will determine if adaptive strength gains following repeated bouts of eccentric contractions are mediated by Krt18 in young mice by examining young (3-5-month-old) male and female wildtype (WT) and Krt18 knockout mice. In Aim 2 we will determine if age-related loss of Krt18 expression in muscle after repeated bouts of eccentric contractions is associated with blunted strength gains. To integrate age-related muscle adaptation (or lack thereof) with Krt18 expression, we will use young (3-5 months), adult (20-22 months) and old (27-29 months) male and female WT mice. All mice will perform repeated bouts of eccentric contractions in vivo and changes in isometric torque and plasmalemmal excitability will be tracked. Following the last contractile test, ex vivo physiology will be utilized to indirectly assess EC coupling failure. Markers of muscle damage will also be assessed and content of Krt18 and Krt18-intereacting proteins (e.g., dystrophin) will be measured. We predict that the ability of skeletal muscle remodel and adapt to repeated bouts of physical stress will correlate directly with the level of expression of Krt18, consistent with the idea, that the loss of Krt18 with age is an important mediator sarcopenia and frailty.