Sulfur Dioxide Contributes To Memory Deficits And The Neuroprotection Of MAGL Inhibition

As the small particulate matter of 2.5 microns or less in diameter (PM2.5) and Nitrogen dioxide (NO2), Sulfur dioxide (SO2) is a major air pollutant worldwide, especially in most developing countries. High levels of sulfur compounds are continually emitted into the environment from the combustion of fossil fuels for industrial energy and of solid-fuel for domestic heating, mainly the combustion of coal. In China, the coal accounts for about 70% of the total energy consumption, and the emissions of SO2, which are still more than dust, nitrogen oxides and other atmospheric pollutants, contribute 85% of the air pollution from burning coal. So atmospheric SO2 pollution is widespread over the mining areas and industrial resident areas, the harmful effects of SO2 pollution on public health are of more great importance and have arrested more attention of public.Traditionally, the relevant researches are often paying more attention onto the effects of SO2 air pollution on respiratory system, and evidences link it with an array of adverse respiratory effects including bronchoconstriction and increased asthma symptoms. In recent years, an increasing body of researches reports that SO2 is responsible for oxidative damages and DNA breaks in several organs through its derivatives, suggesting that SO2 is a systemic toxicant. Several epidemiological studies reported that SO2 air pollution is not only involved in the increase of risk for hospitalization and mortality of neurological dysfunction, but also linked with many neurological disorders such as migraine headaches, stroke and brain cancer, suggesting SO2 air pollution is an important incentive for neurological diseases. However, the existing experimental studies lack evidences as to the relevance between neuroinflammation and synaptic damage even neurological dysfunction induced by SO2 exposure. Particularly, the remarkable features of air pollution exposure are lifelong duration and at low concentration; the effects of chronically exposure of SO2 at low concentration on neurocognitive function and neuropathological mechanism were not well characterized, especially the crosstalk of neuronal function and synaptic plasticity and neuroinflammation.In this thesis, effects of different concentration and duration time of SO2 exposure on nervous system of Wistar rats were characterized in detail at the levels of behavior, tissue pathology and molecular biology. The spatial learning and memory deficit was measured through Morris water maze, synaptic ultrastructure was observed by transmission electron microscopy (TEM), the glial activation reaction was determined by immunohistochemistry, the mRNA level and protein expression was detected by qRT-PCR and Western blot; inflammatory cytokines expression was examined via ELISA kits. Besides, the anti-inflammatory and neuroprotective roles of Monoacylglycerol lipase (MAGL) inactivation by JZL184, a potent inhibiter for MAGL, were illustrated detailedly during the process of SO2 contributing to pathogenesis of neuropathology in mice.The results showed that, acute exposure of SO2 at higher concentration could increase presynaptic vesicles and thicken/expand postsynaptic density in rat hippocampus; the expression of presynaptic marker proteins synaptophysin (SYP) and postsynaptic density protein 95 (PSD-95) was significantly increased, the gene transcription level of AMPA and NMDA receptor subunits and the activity-regulated cytoskeletal associated gene(Arc), an immediate early gene for synaptic plasticity, was reduced as well as the expression of protein kinases such as protein kinase A (PKA), protein kinase C (PKC) and calcium/calmodulin-dependent protein kinase II a(CaMK II a). The results suggested that SO2 inhalation disturbed synaptic signaling and synaptic plasticity.As to the subchronic exposure model, SO2 inhalation at lower concentration could reduced the number of target zone crossings and time spent in the target quadrant during the probe session in the spatial memory retention of the Morris water maze (MWM); reduced the transcription level of Arc and glutamate receptor subunits, and attenuated the expression of protein kinases as well as the marker proteins SYP and PSD95, the electron microscopy showed that SO2 inhalation reduced presynaptic vesicles and thinned postsynaptic density. Also, SO2 inhalation increased FJC positive neurons and activated astrocytes and microglia in both the cortex and hippocampus, accompanied with upregulation of inflammatory cytokines release in cortex, including tumor necrosis factor-a (TNF-a), interleukin-1β (IL-1β), cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS). The results suggested that SO2 inhalation could lead to synaptic injury and neurocognitive dysfunction via neuroinflammation. Similarly, chronic exposure of SO2 also caused injuries in spatial learning and memory and synaptic plasticity through inflammation in hippocampus. Taken together, the results suggested that long-term exposure to SO2 air pollution impaired spatial learning and memory, and indicated a close link between the neurobiological changes and the neurocognitive dysfunction.It is reported that endogenous 2-arachidonoylglycerol (2-AG) possesses significant anti-inflammatory and neuroprotective effects in response to harmful insults in the brain, and 85% of brain 2-AG is hydrolyzed by MAGL. Thus a reasonable inference would be made that MAGL inactivation might be a promising molecular target for elevating 2-AG in brain to keep SO2 exposure provoked destructive response in check and to maintain brain function. Here C57BL/6j mice were exposed to SO2 directly in the absence and presence of JZL184 pretreatment, a potent MAGL inhibitor. The results showed that SO2 exposure also damaged spatial msemory retention in MWM, and caused neuroinflammatory and synaptic injury in mice. However, MAGL inactivation by JZL184 could decrease FJC positive neurons and prevent astrocytes and microglia reactive in both the cortex and hippocampus when mice were under SO2 stress, and as an advantageous result, the integrity of synaptic structural was maintained and the spatial memory was improved in MWM. The results suggested that MAGL inactivation prevented SO2 exposure provoked neuroinflammation and cognitive impairment, and indicated MAGL inactivation was likely a promising molecular target for preventing SO2 induced pathology and maintaining brain function.In conclusion, the results in this thesis revealed the effects of different concentrations and duration time of SO2 exposure on central nervous system (CNS), and indicated the underlying mechanisms had a close link with neuroinflammation and synaptic plasticity. This study provided a series of theoretical and experimental basis for evaluating the influences of acute SO2 exposure in emergencies and chronic SO2 exposure in air pollution on CNS. This research further explored and elaborated on that MAGL inactivation played important anti-inflammatory and neuroprotective roles to prevent SO2 induced neuropathology in mice, which would provide a new idea of ameliorating and therapeutic approaches for preventing brain injuries resulting from SO2 pollution incidents.