Marina Cefis

Objective: Neuromuscular electrical stimulation (NMES) is increasingly used to preserve or restore muscle function in patients unable to exercise, and acute NMES improves cognitive performance and increases hippocampal mature brain-derived neurotrophic factor (BDNF). However, the consequences of repeated sessions for muscle-to-brain signaling remain poorly understood. We examined whether seven consecutive days of quadriceps NMES would reproduce this acute response. Methods: Male Wistar rats received daily 30-min quadriceps NMES or SHAM sessions under isoflurane for seven consecutive days. Blood was collected immediately after the final session and 24 h later, when muscle and hippocampal tissues were also collected. An independent cohort underwent behavioral testing. Results: Repeated NMES increased serum creatine kinase activity and quadriceps fibrillar collagen deposition without altering muscle mass, myofiber size, or degeneration-regeneration markers. These changes were accompanied by low-grade systemic inflammation and corticosterone elevation persisting 24 h after the final session. In the hippocampus, NMES increased inflammatory cytokine expression, astrocytic and microglial markers, and glucocorticoid receptor phosphorylation. Mature BDNF, TrkB phosphorylation and ERK1/2 signaling decreased, whereas proBDNF, p75NTR and JNK signaling increased; Akt and mTOR phosphorylation were unchanged. Synaptophysin, GAP43 and PSD95 also decreased. NMES increased marble burying and forced-swim immobility, without detectable changes in locomotion, recognition memory, or neuronal profile density. Conclusions: Repeated daily NMES engaged a distinct, maladaptive mode of muscle-to-brain signaling rather than reproducing the benefits reported after acute stimulation. Stimulation parameters and recovery intervals that avoid this response need to be defined before repeated NMES can be recommended as an exercise substitute. Graphical

L’exercice physique (EX) est reconnu comme la stratégie non pharmacologique la plus efficace pour améliorer la santé cérébrale. Les études menées chez l’Homme et l’animal s’accordent pour impliquer le brain-derived neurotrophic factor (BDNF), une neurotrophine dont les taux cérébraux augmentent en réponse à l’EX et qui est unanimement reconnue comme une molécule de signalisation cruciale de la neuroplasticité. Principalement exprimé par les neurones, le BDNF est également très exprimé par la cellule endothéliale et la cellule musculaire. Très largement sollicités lors d’un effort physique, l’endothélium et le muscle pourraient intervenir dans les effets positifs induits par l’EX. Bien qu’il existe aujourd’hui un consensus sur l’implication du BDNF dans les effets cérébraux de l’EX, il n’en existe pas concernant les modalités d’EX à pratiquer pour optimiser de manière efficace la plasticité cérébrale. Dans ce contexte, les objectifs de ces travaux étaient de déterminer l’impact des modalités de l’EX sur les expressions protéiques de BDNF dans différents territoires (cerveau, endothélium, muscle) et d’étudier les mécanismes à l’origine de l’augmentation de BDNF en réponse à l’EX.Nos résultats montrent que 1) l’expression du BDNF dans des vaisseaux périphériques de même territoire vasculaire (diamètre interne différent) est similaire en réponse à l’EX et majoritairement d’origine endothéliale, 2) l’augmentation de l’expression cérébrale de BDNF en réponse à l’EX dépend de l’intensité de l’EX, mais pas du type de contraction (excentrique/concentrique), 3) la mémoire est restaurée par un EX de forte intensité, 4) l’EX n’impacte pas l’expression musculaire de BDNF, mais augmente l’expression du précurseur de l’irisine (FNDC5), 5) l’expression du BDNF dépend de la composition du muscle en fibres musculaires, 6) les effets cérébraux de l’intensité de l’EX ne semblent pas être reliés à la surexpression de l’irisine musculaire.En conclusion, nos données démontrent que l’EX impacte positivement l’expression endothéliale, cérébrale mais pas musculaire de BDNF. Les résultats mettent en évidence l’importance du paramètre intensité de l’EX sur les taux cérébraux de BDNF. Enfin, selon nos données obtenues, l’irisine et le BDNF musculaires ne semblent pas être impliqués dans l’augmentation cérébrale de BDNF en fonction de l’intensité de l’EX.

Aging-related muscle atrophy and weakness contribute to loss of mobility, falls and disability. Mitochondrial dysfunction is widely considered a key contributing mechanism to muscle aging. However, mounting evidence position physical activity as a confounding factor, making unclear whether muscle mitochondria accumulate bona fide defects with aging. To disentangle aging from physical activity-related mitochondrial adaptations, we functionally profiled skeletal muscle mitochondria in 51 inactive and 88 active men aged 20-93. Physical activity status conferred partial protection against age-related decline in physical performance. A trend for reduced muscle mitochondrial respiration with aging was observed in inactive but not in active participants, indicating that aging per se does not alter mitochondrial respiratory capacity. Mitochondrial reactive oxygen species (ROS) production was unaffected by aging and active participants displayed higher ROS production. In contrast, mitochondrial calcium retention capacity decreased with aging regardless of physical activity status and correlated with muscle mass, performance and the stress-responsive metabokine GDF15. Targeting mitochondrial calcium handling may hold promise for treating aging-related muscle impairments.

Aim: Sarcopenia, the aging‐related loss of muscle mass and function, is a debilitating process negatively impacting the quality of life of affected individuals. Although the mechanisms underlying sarcopenia are incompletely understood, impairments in mitochondrial dynamics, including mitochondrial fusion, have been proposed as a contributing factor. However, the potential of upregulating mitochondrial fusion proteins to alleviate the effects of aging on skeletal muscles remains unexplored. We therefore hypothesized that overexpressing Mitofusin 2 (MFN2) in skeletal muscle in vivo would mitigate the effects of aging on muscle mass and improve mitochondrial function.Methods: MFN2 was overexpressed in young (7 mo) and old (24 mo) male mice for 4 months through intramuscular injections of an adeno‐associated viruses. The impacts of MFN2 overexpression on muscle mass and fiber size (histology), mitochondrial respiration, and H₂O₂ emission (Oroboros fluororespirometry), and various signaling pathways (qPCR and western blotting) were investigated.Results: MFN2 overexpression increased muscle mass and fiber size in both young and old mice. No sign of fibrosis, necrosis, or inflammation was found upon MFN2 overexpression, indicating that the hypertrophy triggered by MFN2 overexpression was not pathological. MFN2 overexpression even reduced the proportion of fibers with central nuclei in old muscles. Importantly, MFN2 overexpression had no impact on muscle mitochondrial respiration and H₂O₂ emission in both young and old mice. MFN2 overexpression attenuated the increase in markers of impaired autophagy in old muscles.Conclusion: MFN2 overexpression may be a viable approach to mitigate aging‐related muscle atrophy and may have applications for other muscle disorders.

Septic patients frequently develop skeletal muscle wasting and weakness, resulting in severe clinical consequences and adverse outcomes. Autophagy is a stress-induced degradative process essential to cell survival. Recent studies have demonstrated that sepsis triggers sustained induction of autophagy in skeletal muscles, although the impact of this enhanced autophagy on sepsis-induced muscle dysfunction remains unclear. Atg7 is an autophagy gene that plays a major role in autophagosome formation. Using an inducible and muscle-specific Atg7 knockout mouse model (Atg7 ^iSkM-KO ), we investigated the functional importance of skeletal muscle autophagy in sepsis. Sepsis was induced using cecal ligation and perforation (CLP) with a sham operation serving as a control. Atg7 ^iSkM-KO mice exhibited a more severe phenotype in response to sepsis, marked by severe muscle wasting and contractile dysfunction, hypoglycemia, higher ketone levels and a decreased in survival as compared to mice with intact Atg7 . Several genes that encode 26S proteasome subunits were upregulated, suggesting that activation of the ubiquitin-proteasome system is responsible for the severe muscle atrophy that was seen in these mice. Sepsis and Atg7 deletion resulted in the accumulation of mitochondrial dysfunction, although sepsis did not further worsen mitochondrial dysfunction in Atg7 ^iSkM-KO mice. Overall, our study demonstrates that autophagy inactivation in skeletal muscles triggers significant worsening of sepsis-induced contractile and metabolic dysfunctions and negatively impacts survival. Induction of autophagy in skeletal muscles in response to sepsis thus represents a protective mechanism.

The aims of the present study were to investigate in brain of adult rats (1) whether exercise-induced activation of brain-derived neurotrophic factor (BDNF)/tropomyosin-related kinase B (TrkB) pathway is dependent on exercise intensity modality and (2) whether exercise-induced improvement of memory is proportional to this pathway activation. Wistar rats were subjected to low (12 m/min) or high (18 m/min) exercise intensity on horizontal treadmill (30 min/day, 7 consecutive days) that corresponds to ~ 40 and 70% of maximal aerobic speed, respectively. Animals treated with scopolamine to induce memory impairment were subjected to novel object recognition test to assess potential improvement in cognitive function. Expressions of BDNF, phosphorylated TrkB receptors, synaptophysin (a marker of synaptogenesis), c-fos (a neuronal activity marker) and phosphorylated endothelial nitric oxide synthase (a cerebral blood flow marker) were measured in prefrontal cortex and hippocampus of different groups of rats. In terms of cognition, our data reported that only the most intense exercise improves memory performance. Our data also revealed that BDNF pathway is dependent on intensity modality of exercise with a gradual effect in hippocampus whereas only the highest intensity leads to this pathway activation in prefrontal cortex. Our study revealed that memory improvement through BDNF pathway activation is dependent on exercise intensity. While reporting that our protocol is sufficient to improve cognition in animals with impaired memory, our data suggest that prefrontal cortex is possibly a more suitable structure than hippocampus when neuroplastic markers are used to mirror potential improvement in memory performance.

Introduction: The elevation of brain-derived neurotrophic factor (BDNF) levels in the brain and the subsequent phosphorylation of its cognate tropomyosin-related kinase B (TrkB) receptors at tyrosine 816 (pTrkB^tyr816) are largely involved in the positive effect of aerobic exercise on brain functioning. Although BDNF levels were reported to increase in proportion with exercise intensity, the effect of the type of contraction is unknown. Therefore, the cerebral BDNF/TrkB pathway was investigated after uphill and downhill treadmill activities at equivalent intensity to preferentially induce eccentric and concentric contractions, respectively. Methods: A treadmill activity (30 min·d⁻¹ for seven consecutive days) either in a horizontal position at two different speeds to modulate intensity (experiment 1) or at three different inclinations (null, −10%, and +5%) but at equivalent intensity to modulate the type of contraction (experiment 2) was induced in rats. Both experiments included sedentary rats. Levels of BDNF, pTrkB^tyr816, synaptophysin (marker of synaptogenesis), endothelial nitric oxide synthase phosphorylated at serine 1177 (peNOS^ser1177), and c-fos levels (indicators of elevation in blood flow in the cerebrovasculature and neuronal activity, respectively) were measured in motor- and cognition-related brain regions using Western blotting analysis. Results: Experiment 1 indicated that treadmill activity induces an intensity-dependent increase in peNOS^ser1177, c-fos, and BDNF levels. Experiment 2 showed that intensity of exercise as well as activation of the cerebral BDNF pathway, and synaptogenesis did not differ among horizontal, uphill, and downhill treadmill activities. Conclusion: The cerebral response of the BDNF pathway to a treadmill activity is dependent on exercise intensity, but not on the type of contraction (eccentric vs concentric).