MPO/HOCl Facilitates Apoptosis and Ferroptosis in the SOD1G93A Motor Neuron of Amyotrophic Lateral Sclerosis

Background: Oxidative stress and Reactive Oxygen Species (ROS) are important in the pathogenesis of amyotrophic lateral sclerosis (ALS). Hypochlorous acid (HOCl) is a powerful oxidant of the Reactive Oxygen Species (ROS) family. HOCl's role in the progress of ALS remains unclear due to the lack of an effective HOCl detection method. Cumulative evidence supports oxidative damage incurred by mutant hSOD1 contributing to motor neuron death; however, whether HOCl as well as its catalytic Enzyme myeloperoxidase (MPO) function in the cell death of SOD1^G93A ALS remains elusive.

Methods: The hSOD1^WT and hSOD1^G93A NSC-34 cell and SOD1^G93A ALS mouse models were employed. With a novel fluorescent HOCl probe, HKOCl-3, we detected the expressions of HOCl and its catalytic Enzyme, MPO, in the above models in vitro and in vivo. The regulation of MPO/HOCl by hSOD1^G93A mutation and cell deaths by MPO/HOCl were also assayed, including Apoptosis, Ferroptosis, and Autophagy.

Results: Our results showed that hSOD1^G93A mutation promoted the activation of the MPO/HOCl pathway in SOD1^G93A ALS cell models. The activation of MPO/HOCl pathways facilitated Apoptosis and Ferroptosis through increasing the Bax/Bcl-2 ratio and expression of Caspase-3 or inhibiting the expressions of GPX4 and NQO1 and thus leading to irreversible lipid peroxidation. Overexpressed FSP1, a glutathione-independent suppressor, could ameliorate Ferroptosis. In vivo, we demonstrated that the activation of the MPO/HOCl pathway occurred differently in motor neurons of the motor cortices, brain stems, and spinal cords in male and female SOD1^G93A transgenic mice. In addition, inhibiting MPO improved the motor performance of SOD1^G93A transgenic mice, as demonstrated by the rotarod test.

Conclusions: We concluded that aggregation of mutant hSOD1 proteins contributed to activation of the MPO/HOCl pathway, triggering Apoptosis and Ferroptosis in motor neuronal deaths and exerting impaired motor performance.