Motor neuron degeneration in the cortex, brain stem, and spinal cord is a prominent feature of ALS, a fatal neurodegenerative disease characterized by progressive muscle paralysis [25]. In the present study, low-dose Cu exposure exacerbated motor dysfunction in SOD1G93A mice, accelerated motor neuron degeneration in the spinal cord, and increased muscle atrophy and fibrosis. The underlying mechanism involved the impairment of mitochondrial function, particularly inhibition of mitophagy. Administration of UA improved motor function in SOD1G93A mice exposed to Cu by activating mitophagy, highlighting the crucial role of mitophagy in Cu-mediated aggravation of ALS and exploring a promising treatment strategy for ALS.
In ALS, alterations in Cu homeostasis may contribute to the disease's development. Spinal cord tissue from sporadic ALS patients has shown a notable increase in Cu concentration [26]. Elevated blood Cu levels have also been identified as potential risk factors for ALS [27]. Disruptions in Cu balance can compromise the functionality of enzymes, receptors, and transporter structures, leading to oxidative stress, alpha-synuclein aggregation, fiber formation, and activation of microglia cells [28, 29]. In this study, low-dose Cu exposure exacerbated motor decline and associated pathological changes in SOD1G93A mice, a model of ALS. Furthermore, metascope analysis in Cu-exposed SOD1G93A mice revealed abnormal expression of proteins related to mitochondrial processes, such as inflammatory immune response and mitochondrial electron respiratory transport chain. Decreased activity of citrate synthase and respiratory chain complexes I + III, II + III, and IV in the spinal cord tissue of ALS patients post-mortem has been illuminated [30–32]. This may be attributed to selective loss of mitochondria or increased mitochondrial DNA damage in the ALS spinal cord [33].
Maintenance of healthy mitochondria through the process of mitophagy is pivotal in various neurodegenerative diseases such as Alzheimer's, Parkinson's, ALS, frontotemporal dementia, and Huntington's disease. Insufficient mitophagy leads to the accumulation of damaged mitochondria, resulting in increased oxidative stress and reduced ATP levels, leading to cellular damage and apoptosis [34–36]. Excessive accumulation of Cu induces tissue damage by promoting apoptosis and inhibiting mitophagy, along with the down-regulation of autophagy-related proteins, such as Atg5, Beclin1, Pink1, Parkin, P62, and LC3B [37]. In the present study, this disruption was observed in SOD1G93A mice exposed to Cu, where a decrease in crucial proteins involved in mitophagy was noted, further exacerbated ATP reduction and oxidative stress. Evidence suggests that Cu exposure disrupts the autophagy-lysosomal pathway in ATP7B-deficient hepatocytes [38], and chronic Cu exposure may induce pathological damage by interfering with the mitophagy and subsequent apoptosis [39]. Notably, the activation of mitophagy may serve as an initial response to stress, the subsequent oxidative stress due to mitochondrial dysfunction is a common factor in ALS and other neurodegenerative diseases. During the ALS process, energy metabolism disturbances contribute to activation of astrocyte and microglia, triggering damage in motor neurons mediated by pathways like NF-κB or TGFB through [40–42]. In the context of ALS, exposure to Cu induced mitochondrial dysfunction by hindering mitophagy, resulting in neuron loss in SOD1G93A mice.
In order to determine whether mitophagy plays a decisive role in ALS exacerbated by Cu exposure, UA, a mitophagy activator was administrated and showed beneficial effects by improving the motor function, alleviating muscle atrophy and fibrosis, reducing motor neuron loss, and mitigating neuroinflammation. The therapeutic effect of UA was mediated by activation of autophagy and mitophagy which manifested with increasing the expression of PINK1, Parkin, and LAMP1 in the spinal cord. Previous studies have indicated that UA enhances ATP and NAD + levels by up-regulating Sirtuin 1 and peroxisome proliferator-activated receptor gamma coactivator 1-α, thereby improving skeletal muscle and mitochondrial function [15]. Additionally, in LPS-stimulated J774.1 mice macrophages, UA has been found to inhibit pro-inflammatory M1 macrophage polarization and the subsequent release of pro-inflammatory cytokines by increasing autophagy flux which prevents nuclear translocation and activation of the AKT/mTOR signaling pathway [16]. This study marks the first to demonstrate that UA enhances mitochondrial function through mitophagy activation, suppresses inflammation, and delays functional deterioration in Cu-exposed SOD1G93A mice. These findings offer crucial insights into ALS mechanisms and pave the way for novel ALS treatment approaches.