Investigating the role of extracellular matrix proteins in peripheral skeletal muscle adaptation in COPD patients

Kritikaki, Efpraxia (2024) Investigating the role of extracellular matrix proteins in peripheral skeletal muscle adaptation in COPD patients. Doctoral thesis, Northumbria University.

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Abstract

Background: Cachexia and muscle wasting are prevalent extrapulmonary comorbidities observed in patients with chronic obstructive pulmonary disease (COPD) associated with decreased quality of life and increased mortality rates. While exercise training is the primary therapeutic strategy that attenuates the detrimental effects of muscle wasting in COPD, cachectic patients do not present the same degree of muscle remodelling capacity compared to non-cachectic COPD patients. Extracellular matrix (ECM) represents a dynamic and complex environment that is highly responsive to external stimuli. ECM plays a pivotal role in maintaining the structural integrity of skeletal muscle and orchestrating essential physiological processes during growth, regeneration, and remodelling. Imbalance in ECM composition, which can occur during aging or in the presence of pathological muscular conditions have profound effects on muscle homeostasis, function, and overall structure. Although various biological mechanisms contributing to COPD-related muscle wasting have been identified, the involvement of ECM in this condition remains largely unknown.
Hypothesis: It was hypothesised that COPD patients would exhibit distinct alterations in the composition of skeletal muscle ECM compared to healthy individuals. It was also hypothesised that the completion of an exercise training intervention would induce differential ECM expression adaptations in cachectic COPD patients compared to non-cachectic and healthy controls. Given that cigarette smoking (CS) is the most significant contributing factor to the development of COPD, it was expected to alter the expression of ECM components in skeletal muscle.
Objectives: The current study aimed to explore, for the first time, the composition of ECM in the skeletal muscle of COPD patients. It aimed also to identify whether the effect of exercise training induces altered adaptive responses in ECM composition among cachectic, non-cachectic COPD patients and healthy controls and to examine the molecular pathways involved. In addition, the study sought to screen potential molecular pathways involved in these ECM adaptations to understand the mechanisms involved in the restricted adaptive response seen in cachectic muscle after exercise. Furthermore, another aim of this study was to explore the effect of exposure to CS and acute hypoxia on the expression of skeletal muscle ECM components. Moreover, the effect of a tobacco compound on myogenic differentiation was investigated to further understand the influence of CS on muscle ECM dynamics.
Methods: The study investigated the gene and protein expression of selected ECM molecules in the vastus lateralis biopsies of 29 COPD patients [19 non-cachectic patients
(FFMI >17 kg•m-2), and 10 cachectic patients (FFMI <17 kg•m-2)], as well as 14 healthy controls, before and after participation in a training intervention. Additionally, the impact of exposure to CS and acute hypoxia on ECM gene expression was examined in the gastrocnemius muscle of guinea pigs. Furthermore, in vitro differentiation of C2C12 myotubes was utilized to assess the effects of cotinine treatment on ECM, myogenic, and differentiation markers. The assessment of gene expression was performed using quantitative real-time polymerase chain reaction (qRT-PCR), while protein expression was determined through enzyme-linked immunosorbent assay (ELISA) techniques.
Results: At baseline, COPD patients demonstrated distinct changes in the transcription and translation of key ECM molecules linked to muscle structure, regeneration, and repair. Baseline differences between non-cachectic and cachectic COPD patients were observed only at the transcriptional level, with cachectic patients displaying decreased transcriptional activity of ECM molecules. Upon exercise training, contrasting adaptations were observed between cachectic COPD patients and the cohorts with preserved muscle mass at both the expression of ECM and myogenic markers. Conversely, non-cachectic COPD patients and healthy individuals demonstrated similar responses to exercise training. Phospho-kinase analysis revealed that activation of p38 MAPK signalling pathway could be a potential pathway involved in the limited remodelling capacity seen in cachectic COPD patients after exercise training. In addition, exposure to CS directly affected the skeletal muscle ECM composition, while acute hypoxia also induced changes but at a smaller magnitude. In vitro findings showed that presence of cotinine, a tobacco by-product, hindered the myogenic and differentiative capacity of C2C12 myoblasts, as well as ECM transcription.
Conclusions: This study provides novel insights into the differential basal ECM composition of skeletal muscle between COPD patients and healthy individuals. In addition, our findings highlight that exercise training induces altered response in the expression of ECM molecules in cachectic COPD patients compared to non-cachectic and healthy individuals, concomitant with the enhancement of a glycolytic phenotype. These results demonstrate the potential involvement of an imbalanced ECM composition in the observed limited remodelling capacity seen in cachectic COPD patients after exercise training. Importantly, targeting ECM molecules may hold therapeutic significance for combatting COPD muscle wasting.

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