Background: ME/CFS is a chronic inflammatory condition characterized by severe muscle fatigue, pain, and cognitive impairment. The most reliable pathological symptom of ME/CFS is post-exertional malaise (PEM), which is persistent muscle fatigue and pain after treadmill exercise. Upon treadmill exercise, patients are often reported to have a “crash” or “collapse” with severe and prolonged mental and physical tiredness. However, the molecular mechanism of PEM is poorly understood.

The mammalian target of Rapamycin or mTOR is a kinase that is historically involved in the pathogenesis of many chronic inflammatory diseases starting from cancer to diabetes and neuroinflammation to neurodegenerative diseases. Its profound role in immunology and energy metabolism can never be underestimated. One of the critical metabolic events controlled by mTOR is autophagy.  Early autophagy protein ATG13 is phosphorylated by mTOR causing its abortion from the autophagy complex. Subsequently, the autophagy impairment may cause immune dysregulation. 

Results: Upon chronic activation of mTOR by pharmacological intervention (shown in video) and a transgenic ATG13 repressor mouse model with suppressed ATG13 expression displayed a strong inhibition of autophagy in the muscle tissue, infiltration of inflamed macrophages in the vasculature of muscle tissue, and demyelinating response in nerve fibers and spinal cord. Subsequent behavioral analyses including grip strength measurement and gross movement analyses revealed that these mice exhibited fatigue and exacerbated exhaustion upon treadmill exercise. While recording the electromyogram (EMG) in the biceps muscle, we observed myokymic discharge characterized by low-frequency waves with periodic bursts of high amplitude waves, known as “ marching soldier “pattern of waves suggesting the potential inflammatory demyelination in muscle-serving spinal nerves. Our immunohistochemical studies further revealed that the integrity of myelin layer in muscle-serving nerves was severely compromised after pharmacological intervention of mTOR agonist or the mutation in atg13 gene (Tg-ATG13 mice).  Next we performed a single-session treadmill exercise followed by muscle fatigue analysis to explore a PEM-relevant behavioral deficit. Interestingly, after long-term oral administration of mTOR agonist ( 2 weeks ; alternate day; at a dose of 5 mg/kg bwt) followed by a single-session treadmill exercise( 15 minutes at a speed of 14 rpm) severely impaired the movement in 6-8 weeks old female B6 mice. These exhaustion sustained for 1-2 weeks and then these animals recovered. Interestingly, a similar treadmill experiment left 6-8 weeks’ old female Tg-ATG13 mice paralyzed for 5-10 minutes similar to “crush”.

Highlights: 1. The potential role of mTOR activation in post-exertional fatigue is highlighted.

2. As a molecular mechanism, mTOR activation augments autophagy impairment via ATG13 inactivation.
3. Autophagy impairment induces IL-6 and RANTES via STAT3, demyelinates nerves in the muscle and spinal cord.
4. ATG13 repressor mice (Tg-ATG13) displayed inflammatory demyelination and post-treadmill fatigue.

 Future Direction: It will be interesting to see if the muscle-specific mutation of ATG13 can generate similar phenotypes. We are currently conducting a clinical trial of mTOR inhibitor rapamycin in ME/CFS, that shows a significant recovery from PEM in almost 60% of a mid-sized cohort (~180) of ME/CFS patients. For more read https://rdcu.be/d5yaB