Numerous signaling pathways and a range of CNS cells have been suggested to be involved in the pathogenesis of neurodegeneration, although they are still far from being completely understood. Neurodegenerative diseases, including PD are commonly associated with neuroinflammation conditions, suggesting that inflammation is not only a result of neurodegeneration but also a crucial player in this process [27]. α-syn protein aggregates which are common pathological phenomenon in neurodegeneration can induce neuroinflammation, which further exacerbates protein aggregation and neurodegeneration. Due to the limited success of traditional treatment methods, intervening inflammatory signaling pathways involved in neurodegeneration are considered to be promising strategies for the therapy of neurodegenerative diseases. In recent years, drugs that suppress neuroinflammation have received increasing amounts of attention. However, current anti-inflammatory medications have drawbacks: aspirin and acetaminophen do not significantly improve symptoms in patients with PD and herbal-based extracts require high drug concentrations for treatment. MK has advantages for use as an anti-inflammatory therapeutic agent for PD. MK is widely used in the clinical treatment of asthma and allergic rhinitis, and its safety has been clinically verified. Moreover, MK can cross the BBB and remain in the brain for 24 hours. Previous studies have reported that leukotriene receptors inhibited by MK are related to microglial activation [11]. Activated microglia synthesize and release leukotrienes. After being released extracellularly, leukotrienes bind to leukotriene receptors on other cells and activate more microglia, which amplifies the neuroinflammatory response. MK, can reduce inflammation by inhibiting leukotriene receptors. Previous studies of MK in the treatment of PD have reported that MK is effective in improving motor deficits in PD or reducing the incidence of PD [12]. However, few studies have assessed the efficacy of MK by evaluating more objective physiologic indices.
P2X7R, an important inflammatory target, is also present in microglia. P2X7R can be activated by extracellular ATP, which are released by damaged neurons. Therefore, P2X7R is considered an ideal imaging target for assessing the role of MK in neuroinflammation. PET, a non-invasive imaging test, has significant advantages in reflecting receptor expression levels in the body. Compared with traditional biochemical analysis, PET not only elucidates the level of inflammation in different regions of the brain, but also reveals its possible role in PD. The ligand GSK1482160, which targets P2X7R, has a half-life of less than 4.5 hours in humans, and [18F]GSK1482160 crosses the BBB successfully [17, 28]. In the present study, whole-brain neuroinflammation was monitored via [18F]GSK1482160 PET, which targets P2X7R, and extensive neuroinflammation was detected in the brains of the PD + NS mice. This result is consistent with previous findings that neuroinflammation occurs in the brains of PD animal models and patients [26, 29]. Moreover, MK-mediated inhibition of neuroinflammation was also revealed by [18F]GSK1482160 PET in this study. This finding suggested that [18F]GSK1482160 PET can serve as an efficacy assessment tool for the MK treatment of PD, which compensates for the lack of methods for assessing the efficacy of anti-inflammatory therapy.
Several brain regions correlated with motor function, suggesting that mice with severe neuroinflammation in these brain regions also exhibited worse motor function. Moreover, different movement disorders corresponded to different neuroinflammatory brain regions (Fig. 7). The brain regions associated with all three motor disorders were AMY, BFS, CTX, HIP, MB, STR, and THA. An additional brain region associated with grip strength was BS, and the additional brain regions associated with endurance deficits were BS, HYT. This might suggest that motor disorders are unequally influenced by neuroinflammation. When different brain regions are affected, it may result in different movement disorders. To the best of our knowledge, this is the first study in which inflammation correlation analysis of specific brain regions was performed in PD animals, revealing the possible roles of different brain regions in different symptoms of PD.
It was also observed by [18F]L-DOPA PET that the function of DA neurons was also partially restored after MK treatment, suggesting that damage to DA neurons can be effectively ameliorated by suppressing microglia-associated neuroinflammation alone, which further confirms that inflammation is a key factor in PD. (Fig. 6). Previous studies have shown that inflammation may lead to reduce DA synthesis and/or impaired DA packaging or release, all of which may impair DA function, leading to reduced motivation and motor slowing in patients [30]. MK significantly reversed the decrease in DA uptake by DA neurons, implying that MK may fill the gap of drug therapy in this area.
Correlation analysis also revealed that motor functions were positively correlated with DA neurons function, a result that is similar to that of previous studies. Previous studies have shown that DA neurons function in the STR and substantia nigra (SN) is positively correlated with motor function in PD patients [31]. However, in addition to the five common dopamine pathways, DA-uptake neurons also exist in other brain regions [32]. In our study, a decrease in DA uptake in other brain regions was found, and DA uptake in other brain regions was also closely related to PD symptoms. It has been shown that not only striatal and SN DA neurons are associated with PD symptoms. These brain regions, other than the striatum and SN, also play important roles in the development of PD. These issues have received little attention in previous clinical diagnosis and disease assessment. The primary motor cortex is primarily associated with motor planning and execution in PD, and direct modulation of this cortex activity facilitates the improvement of motor function in PD patients [33]. Dysfunction of this cortex depends on the severity and/or duration of DA depletion [33], which is consistent with our experimental results. In the present study, it was observed that DA uptake in the cortex was positively correlated with muscular endurance. Our study suggested that the attention given to DA-injured neurons in PD patients should not be limited to the STR and SN but should also be integrated into other brain regions.
Quantification of neuroinflammation facilitates clinical disease assessment. In the early stages of PD, patients with mild DA neuronal damage may be diagnosed with mild PD; however, this initial grading cannot be representative of their prognosis. Some patients with mild PD may suffer from significantly activated neuroinflammation. These patients may experience faster deterioration and worse PD symptoms, even if DA neurons damage is not significant at the time of examination. Early DA repletion therapy may fail to slow the persistent damage to DA neurons. Thus, the quantification of inflammation allows for earlier monitoring of disease conditions and enables earlier intervention.
In addition, previous research has shown that PD treatment cannot be directed only to DA neurons injury and that drug treatments targeting DA neurons have both a honeymoon effect and do not significantly improve the non-motor symptoms of PD [8]. Non-DA neurons (such as those in the noradrenergic system, serotonergic system, and cholinergic system) also play important roles in the progression of PD, especially in patients with non-motor symptoms; therefore, damage to these neurons should be emphasized [8]. However, in practical clinical diagnosis, DA neurons are usually given more attention, and the assessments of other neuronal injuries are neglected. PET imaging of DA neurons, such as [18F]L-DOPA, is a routine test for PD diagnosis and is unable to suggest other types of neuronal damage. Moreover, clinicians cannot perform multiple PET scans targeting different neurons without clarifying exactly what kind of non-DA neurons damage is present. Therefore, in vivo imaging of neuroinflammation, which is activated at the time of cell damage and can be recognized by [18F]GSK1482160, is advantageous. In vivo neuroimaging of neuroinflammation further suggested that other brain regions may be damaged in patients with PD, thus aiding in the diagnosis and treatment of PD.
Nevertheless, there are several limitations to our study. First, transgenic mice do not perfectly mimic the progression of human PD. Many previous studies have shown that the vast majority of typical PD cases fit the Braak staging scheme, which means that α-syn is distributed in the peripheral nervous system in early PD and then spreads to the advanced neocortex as the disease progresses [34]. However, in A53T α-syn transgenic mouse brains, the appearance of α-syn does not follow Braak staging, which limits the observation of the pathological features of early PD. Moreover, although neuroinflammation can cause potential damage to non-DA neurons in PD patients, the specific neuronal damage involved has still not been determined. Current imaging of neuroinflammation suggests only the presence of neuronal damage or death, but the specific types of damaged neurons still need to be visualized in combination with other probes targeting different neurons.