Targeted Central Nervous System Irradiation with Proton Microbeam Induces Mitochondrial Changes in Caenorhabditis elegans

SIMPLE SUMMARY: Radiotherapy is a common treatment for cancer and is used for approximately half of cancer patients around the globe. In recent years, significant advancements in technology and imaging have allowed for more accurate targeting of tumor cells using protons while minimizing damage to healthy tissues. Despite these advancements, the complete eradication of treatment-related complications for patients remains an ongoing challenge. In this context, research studies are being conducted on the biological mechanisms involved in the initiation and progression of these side-effects to quantify their risk of occurrence and to offer new therapies for treating them. Using the nematode Caenorhabditis elegans biological model, the consequences of targeted central nervous system proton irradiation were studied. C. elegans were micro-irradiated with 220 Gy of protons (4 MeV) in the central nervous system and the mitochondrial function was assessed. Our results indicate that proton irradiation induced the instant loss of mitochondrial membrane potential in the targeted area with oxidative stress and an increase in the mitochondrial DNA copy number 24 h after irradiation. Furthermore, proton irradiation induced autophagy in the targeted region. This study shows the global mitochondrial damage in the central nervous system area following proton exposure. These results highlight the important role of mitochondria in radiation-induced damage in healthy tissues. ABSTRACT: Fifty percent of all patients with cancer worldwide require radiotherapy. In the case of brain tumors, despite the improvement in the precision of radiation delivery with proton therapy, studies have shown structural and functional changes in the brains of treated patients with protons. The molecular pathways involved in generating these effects are not completely understood. In this context, we analyzed the impact of proton exposure in the central nervous system area of Caenorhabditis elegans with a focus on mitochondrial function, which is potentially implicated in the occurrence of radiation-induced damage. To achieve this objective, the nematode C. elegans were micro-irradiated with 220 Gy of protons (4 MeV) in the nerve ring (head region) using the proton microbeam, MIRCOM. Our results show that protons induce mitochondrial dysfunction, characterized by an immediate dose-dependent loss of the mitochondrial membrane potential (ΔΨm) associated with oxidative stress 24 h after irradiation, which is itself characterized by the induction of the antioxidant proteins in the targeted region, observed using SOD-1::GFP and SOD-3::GFP strains. Moreover, we demonstrated a two-fold increase in the mtDNA copy number in the targeted region 24 h after irradiation. In addition, using the GFP::LGG-1 strain, an induction of autophagy in the irradiated region was observed 6 h following the irradiation, which is associated with the up-regulation of the gene expression of pink-1 (PTEN-induced kinase) and pdr-1 (C. elegans parkin homolog). Furthermore, our data showed that micro-irradiation of the nerve ring region did not impact the whole-body oxygen consumption 24 h following the irradiation. These results indicate a global mitochondrial dysfunction in the irradiated region following proton exposure. This provides a better understanding of the molecular pathways involved in radiation-induced side effects and may help in finding new therapies.