Gymnastik- och idrottshögskolan, GIH

Planned and progressive exercise, that is, exercise training, is essential for promoting skeletal muscle adaptation and improvements in athletic performance. However when the training demands exceed the ability to recover, it may lead to a state of persistent maladaptation. A prolonged imbalance between training or competition and recovery can cause extended periods of performance impairment, known as functional or non-functional overreaching, which may progress to overtraining syndrome (OTS). This condition can significantly impair athletic development and, in severe cases, cause athletes to retire earlier than expected. Skeletal muscle is key for physical performance, including the ability to move and utilise energy. Impaired physical performance is a hallmark of OTS, reflecting the central role of skeletal muscle dysfunction in its development and progression. In this review we discuss the potential mechanisms contributing to skeletal muscle impairments in OTS. These include central and peripheral neural fatigue, endocrine dysregulation and altered hormonal responses, impaired mitochondrial bioenergetics and oxidative stress and systemic inflammation. We also draw parallels between maladaptive response in overreaching and OTS and those typically observed in chronic inflammatory and catabolic conditions, as these may offer insights into underlying mechanistic understanding of OTS, as well as future treatment strategies. Because replicating the full clinical picture of OTS in humans in experimental settings is constrained by practical challenges, we further discuss animal models that may allow for controlled investigations of external variables and thus enable enhanced understanding of its complex pathophysiology.

BACKGROUND: Ultra-endurance sports are increasingly popular, yet the long-term physiological consequences of sustained extreme training loads remain poorly understood. In particular, the effects of prolonged ultra-endurance exercise on skeletal muscle structure, function and molecular remodelling are largely unknown. This case study examined a highly experienced ultra-endurance athlete who completed a world-record attempt to run 30 300 km, with extensive phenotyping focusing on skeletal muscle adaptations and recovery.

METHODS: A 49-year-old male athlete (172 cm, 65 kg) ran ~70 km daily for 15 months. Musculoskeletal, cardiac and visceral ultrasonography, leg muscle strength and power measurements were performed before and after the challenge. Muscle biopsies (n = 4) from vastus lateralis were obtained immediately after completion and during 17 months of recovery to assess myosin heavy chain (MHC) composition, mitochondrial electron transport chain (ETC) complexes and proteins involved in mitochondrial turnover, autophagy and inflammation. Body composition, haematological and biochemical markers, and gut microbiota composition were monitored longitudinally.

RESULTS: by ~20%). Muscle fibre composition showed extreme predominance of slow fibres (nearly 100% MHC-I), which persisted during recovery. Most molecular and functional alterations gradually resolved within 10-17 months. Gut microbiota diversity increased during the challenge, with enrichment of Bifidobacterium during running and Akkermansia during recovery.

CONCLUSIONS: Sustaining daily ultrarunning for more than 1 year induces substantial skeletal muscle remodelling, including reduced muscle size, impaired contractile function and mitochondrial maladaptations, despite largely preserved endocrine and haematological stability. These findings highlight skeletal muscle as a primary physiological system challenged during extreme endurance exercise and demonstrate that recovery from such perturbations may require more than one year.

Context Primary hyperparathyroidism leads to hypercalcemia and muscle dysfunction. Muscle weakness is associated with increased morbidity and mortality but is overlooked in surgical treatment guidelines. While parathyroidectomy is the only curative treatment, its effects on skeletal muscle strength and molecular remodeling remain underexplored. Objective Determining functional and molecular changes in skeletal muscle before and after parathyroidectomy. Methods A prospective observational study was conducted in the spring and fall of 2023. Patients underwent surgery at the Endocrine and Sarcoma unit at the Karolinska University Hospital in Stockholm, Sweden. A total of 21 postmenopausal women with primary hyperparathyroidism planned for surgery were included, whereof 15 completed the study protocol. Participants had no disabling comorbidities. Muscle function tests, muscle biopsies, MRI, and biochemical panels were analyzed before and after parathyroidectomy. Muscle composition of m. vastus lateralis was tested with MRI and transcriptomic analysis of muscle biopsies. Leg strength was evaluated with timed stands test and peak torque tests. Activity level was estimated from questionnaires. Results Parathyroidectomy normalized calcium levels (P < .001) and improved muscle strength (P < .005). Muscle volume increased (P = .023) and fat fraction was reduced (P = .013), without changes in physical activity levels. Transcriptomic analysis identified 981 differentially expressed genes postsurgery, enriched in pathways mirroring exercise-induced adaptations. Conclusion These findings highlight the impact of parathyroidectomy on skeletal muscle function and suggest that muscle assessments should be included in surgical referral criteria to address age-related muscle decline and improve long-term outcomes.

mRNA increased transiently, followed by increased ribosomal DNA transcription, leading to elevated total RNA levels. Skeletal muscle-specific ribosomal protein paralog RPL3L was replaced by the ubiquitously expressed RPL3 during the initial phases of recovery, but this shift was reversed by day 28. A substantial transcriptomic response was observed in human muscle injury, with heavy emphasis on MYC-induced anabolism and inflammation. This supports a model in which MYC-driven changes in ribosomal content and composition form a core anabolic module in skeletal muscle repair, potentially representing a targetable axis to enhance recovery after muscle injury.

OBJECTIVE: Rheumatoid arthritis (RA) is a common chronic systemic inflammatory disease that causes musculoskeletal impairments and fatigue. Physical activity is recommended for individuals with RA, and health-enhancing physical activity (HEPA) has been shown to improve health perception and physical fitness in this group. However, the molecular adaptations of skeletal muscle in response to an exercise intervention are still unexplored in individuals with RA. This study aimed to assess the skeletal muscle response to a 2-year HEPA intervention in individuals with RA.

METHODS: Thirteen individuals with RA (65 ± 2 years old, 13 ± 2 years disease duration) participated. The 2-year HEPA intervention involved 150 min of weekly moderately intense aerobic activity and twice-weekly circuit training. Practical and theoretical physiotherapist support was available the first year, but not the second year. Skeletal muscle biopsies, functional assessments, and mass spectrometry-based proteomics analysis were conducted.

RESULTS: Compliance was high the first year but dropped significantly the second year. Functional improvements in strength, endurance, and lower extremity muscle function (TST) were observed after year 1. Proteomics analysis revealed significant enrichment of mitochondrial proteins including COX8A, citrate synthase, M2OM, NDUFA6, NDUFS2, and VDAC3 after year 1, indicating positive muscle adaptations. However, these changes regressed to baseline levels by year 2.

CONCLUSION: HEPA can induce beneficial mitochondrial adaptations in skeletal muscle of individuals with RA. However, insufficient compliance and progression in HEPA exercise load led to a reversal of these adaptations. Continuous support and motivation are crucial for maintaining and progressing exercise levels and muscle health in individuals with RA. Key points • Health-enhancing physical activity (HEPA) can induce beneficial mitochondrial adaptations in the skeletal muscle proteome of individuals with RA. • Positive effects on mitochondrial protein levels aligned with the participants compliance to the HEPA intervention. • Results emphasizes that sustaining and progressing exercise regimen is crucial to maintain beneficial adaptations for individuals with RA.

Catalase, a canonical antioxidant enzyme, has been shown to prevent maladaptive oxidative damage and protect against aging-associated deterioration of cellular functions. In cardiac tissue, catalase protects cardiac myocytes against a variety of pathological changes by maintaining normal contractile function. Here we elucidated how catalase can protect cardiomyocyte excitation-contraction (EC) coupling we focused on exploring the subcellular location and specific effects of catalase in cardiomyocytes. To determine the effects of catalase on cardiomyocyte EC-coupling, we used adenoviral overexpression and pharmacological inhibition of catalase with 3AT in cultured adult mouse ventricular myocytes combined with detailed characterization of Ca²⁺ signaling. We found that endogenous catalase co-localizes with RyRs in the subspace between the junctional sarcoplasmic reticulum (SR) and t-tubules. When virally overexpressed, catalase increases the ROS scavenging capacity of myocytes and enhances Ca²⁺-release by increasing synchronization of local Ca²⁺-release. To improve Ca²⁺-release, catalase acts to prevent membrane peroxidation and prevents degradation of t-tubule structures. Both the functional and structural effects of catalase were reversed by catalase inhibition with 3AT. The effect of catalase on Ca²⁺-release was more pronounced in cardiomyocytes isolated from transverse aortic constriction-operated, hypertrophied mouse hearts. Overall, our data suggest that catalase located to the subspace has a specific role in stabilizing and protecting Ca²⁺-release structures from degradation in conditions associated with increased oxidative stress.

The Wingless/Int-1 (WNT) signaling network is essential to orchestrate central physiological processes such as embryonic development and tissue homeostasis. In the currently held tenet, WNT/beta-catenin signaling is initiated by WNT-induced recruitment of Frizzleds (FZDs) and LRP5/6 followed by the formation of a multiprotein signalosome complex. Here, we use bioluminescence resonance energy transfer (BRET) to show that different WNT paralogs dynamically trigger FZD-LRP6 association. While WNT-induced receptor interaction was independent of C-terminal LRP6 phosphorylation, it was allosterically modulated by binding of the phosphoprotein Dishevelled (DVL) to FZD. WNT-16B emerged as a ligand of particular interest, as it efficiently promoted FZD-LRP6 association but, unlike WNT-3A, did not lead to WNT/beta-catenin signaling. Transcriptomic analysis further revealed distinct transcriptional fingerprints of WNT-3A and WNT-16B stimulation in HEK293 cells. Additionally, single-molecule tracking demonstrated that, despite increasing FZD5 and LRP6 confinement, WNT-16B stimulation did not result in formation of higher-order receptor clusters, in contrast to WNT-3A. Our results suggest that FZD-WNT-LRP5/6 complex formation alone is not sufficient for the initiation of WNT/beta-catenin signaling. Instead, we propose a two-step model, where initial ligand-induced FZD-LRP6 association must be followed by receptor clustering into higher-order complexes and subsequent phosphorylation of LRP6 for efficient activation of WNT/beta-catenin signaling.

Several metabolites have been shown to have independent and at times unexpected biological effects outside of their metabolic pathways. These include succinate, lactate, fumarate, and 2-hydroxyglutarate. 2-Hydroxybutyrate (2HB) is a byproduct of endogenous cysteine synthesis, produced during periods of cellular stress. 2HB rises acutely after exercise; it also rises during infection and is also chronically increased in a number of metabolic disorders. We show here that 2HB inhibits branched-chain aminotransferase enzymes, which in turn triggers a SIRT4-dependent shift in the compartmental abundance of protein ADP-ribosylation. The 2HB-induced decrease in nuclear protein ADP-ribosylation leads to a C/EBP beta-mediated transcriptional response in the branched-chain amino acid degradation pathway. This response to 2HB exposure leads to an improved oxidative capacity in vitro. We found that repeated injection with 2HB can replicate the improvement to oxidative capacity that occurs following exercise training. Together, we show that 2-HB regulates fundamental aspects of skeletal muscle metabolism.

The Frizzled family (FZD1-10) of G protein-coupled receptors regulates WNT signaling mediating proliferative input. Dysregulation of FZD7 and exaggerated WNT/beta-catenin signaling is frequently observed in intestinal cancers. Therefore, it is attractive to develop therapeutics targeting FZD7 for cancer treatment. Structure-based virtual screening has identified compound 28, which inhibited WNT/beta-catenin signaling based on the luciferase-based reporter gene TOPFlash assay. However, upon pharmacological validation, compound 28 rather acts as a potent Firefly luciferase (Fluc) inhibitor (IC50 = 30 nM), matching the reported IC50 for compound 28-mediated inhibition in the TOPFlash assay. Moreover, we employed Fluc-independent assays, a FZD7-focused bioluminescence resonance energy transfer biosensor and quantitative PCR, to emphasize the inability of compound 28 to inhibit the WNT-3A-induced conformational dynamics in FZD7 and transcription of Axin2, a WNT target gene. Thus, we underline the importance of counter screens to validate compounds that interfere with the detection technology used for compound screening.