1. Summary BACKGROUND: Duchenne muscular dystrophy (DMD) is an X-linked neuromuscular disease characterized by progressive muscle wasting and early death. Inflammation, muscle necrosis and fibrosis form the main pathological manifestations, but impaired mitophagy is also linked with DMD. Molecular therapies are entering the clinic, yet are not able to cure patients. Current treatment involves glucocorticoids, however, severe side-effects associated with these necessitates the need for new treatment options. This study investigates ectoine as a potential supportive treatment option to reduce the required glucocorticoid dose, with focus on its effect on mitophagy. METHODS: Muscle tissue was obtained from 6-week-old healthy controls, saline-treated mdx mice, mdx mice treated with ectoine via intraperitoneal injections, and mdx mice receiving ectoine through drinking water. Quantitative analysis was performed by western blotting of musculus soleus samples, while qualitative analysis was conducted via double immunofluorescence staining on musculus tibialis anterior sections. RESULTS: No significant differences in mitophagy markers were observed between the different groups. However, a trend of increased mitophagy could be noted after ectoine treatment in soleus muscle, suggesting a potential role of ectoine in ameliorating disease progression. Contrary, in tibialis anterior muscles altered distribution of Parkin and LC3II was observed after treatment with ectoine. CONCLUSIONS: The study showed effects of ectoine treatment, which could be of benefit to DMD. Further research is however needed in order to fully understand the complexity of autophagy processes in DMD, taking into account the age and muscle-specific expression patterns of autophagy genes, to elucidate the therapeutic potential of ectoine in DMD. 2. Layman summary with societal impact Duchenne muscular dystrophy is a severe disease leading to a gradual decline in muscle strength, leading to patients needing a wheelchair and assisted ventilation. Ultimately, this result in early death, typically around the age of 40. Current therapy includes approaches to partially restore the dystrophin protein defect and glucocorticoids as anti-inflammatory agents, yet these have many severe side effects, necessitating the need for new treatment options. This underscores the importance and urgency of research in this topic. Many patients take supplements like vitamins, but most cannot be recommended as insights in the effect of these supplements and studies of their clinical benefit are lacking. This study investigates ectoine as a nutritional supplement and potential supportive therapy for Duchenne muscular dystrophy. The standard mouse model of the disease was used to investigate the impact of ectoine on mitophagy, as impairment of this process is shown to be an early event in the disease. Mitophagy is the process by which dysfunctional or damaged mitochondria are removed. These are the energy suppliers of our bodies and therefore are important for the optimal functioning of our body, especially high energy-consuming tissues such as our muscles. No significant differences in mitophagy markers could be shown between healthy, diseased and treated mice. However, a positive trend was noted after ectoine treatment, especially when it was administered via drinking water. These results suggest that ectoine may potentially aid muscle tissue recovery, thereby improving the quality of life of these patients and prolonging their life expectations. Further research is, however, needed to fully explore the therapeutic potential of ectoine in Duchenne muscular dystrophy.