Deficiency Of Aquaporin4Results In Low Level Of TGF-beta In Midbrain And Severe Parkinson’s Disease

Parkinson’s disease (PD) is the most common movement disorder and second,only to Alzheimer’s disease as a cause of age-linked neurodegeneration. About2%ofthe population over the age of60is affected. Prominent clinical features are motorsymptoms (bradykinesia, tremor, rigidity, and postural instability) and nonmotor-related symptoms (olfactory deficits, autonomic dysfunction, depression, cognitivedeficits, and sleep disorders). Neuropathological hallmarks are intracellular inclusionscontaining a-synuclein called Lewy bodies and Lewy neurites and the loss ofdopaminergic neurons in the substantia nigra of the midbrain and in other brainregions as well. Neuroinflammatory processes play a significant role in thepathogenesis of Parkinson’s disease (PD). Epidemiologic, animal, human, andtherapeutic studies all support the presence of a neuroinflammatory cascade in disease.The inflammatory process in PD is characterized by activation of resident microglia,with few reactive astrocytes. Activated microglia affect neuronal injury and deaththrough production of pro-inflammatory factors, reactive oxygen species and bymobilization of adaptive immune responses and cell chemotaxis leading totransendothelial migration of immunocytes across the blood–brain barrier andperpetuation of neural damage. Reactive microglia have been identified close to Lewybodies (LB) in the SN of both idiopathic and genetic cases of PD and PD dementia, aswell as in animal models of the disease. However, the role of alpha-synuclein (amajor component of Lewy bodies that can cause neurodegeneration when aggregated) in microglial activation seems to be prominent, as damaged nigral neurons releaseaggregated alpha-synuclein into SN, which activates microglia with production ofproinflammatory mediators, thereby leading to persistent and progressive nigralneurodegeneration in PD. Several in vitro studies reveal that damaged DA neuronsrelease several factors that seem to activate microglia and are implicated in neuronaldegeneration in Parkinson’s disease. Matrix metalloproteinase3,(MMP3),α-synuclein and neuromelanin are released by damaged DA neurons and induce ROSproduction by overactivated microglia, in addition to other mechanism. Current workis therefore beginning to focus on the stimuli necessary to initiate deleteriousmicroglial function, where studies have revealed several triggers ofinflammation-mediated neurodegeneration are present in the environment.Aquaporins (AQPs) are integral membrane proteins that serve as channels in thetransfer of water, and in some cases, small solutes across the membrane. AQPs playimportant roles in regulating water homeostasis under physiological and pathologicalconditions. So far, at least13aquaporin isoforms (AQP0-AQP12) have beenidentified in mammalian species. Aquaporin-4(AQP4) is mainly expressed inastrocytes. One literature has reported that AQP4was expressed in reactivatedmicroglia induced by lipopolysaccharide. In the previous studies, we found thatAQP4knockout increased the sensitivity of dopaminergic neurons to MPTPneurotoxicity, suggesting that AQP4plays an important role in the PD process bymodulating astroglial response. Our previous study also showed that administration ofthe neurotoxin MPTP, there were remarkably greater losses of TH+neurons in theSNpc and more robust microgliosis response in AQP4-/-mice compared to WT mice.microglia is the most important immune cell which mediate immune inflammation inparkinson’s disease. Why did AQP4deficient mice show severer hyper-microgliosisand more neuron damage after MPTP treatment? We think there are two possibilities.First different microgliosis resulted from microglia itself? Second differentmicrogliosis resulted from any other factors which were different between the brainsin AQP4+/+and AQP4-/-mice? So, we intend to analysis the mechanisms underlyingthe hyperactive microglial responses and more severe PD clinical symptoms after administration of MPTP in AQP4-deficient mice.The main results we got are as follows:1) Primary microglial cells from AQP4+/+or AQP4-/-mice showed similar responsesunder LPS stimulation. The phenotype of the primary microglial cells from AQP4+/+and AQP4-/-mice showed no significant difference and the level of certainLPS-induced cytokines was similar in either AQP4+/+or AQP4-/-microglia cells.2）Brain homogenate from MPTP-treated AQP4deficient mice induced strongeractivation in BV-2cells. Brain homogenate from MPTP-treated AQP4-/-micestimulating BV-2cells: higher expression of CD80, CD40(surface molecules) andpro-inflammatory cytokines expression; but lower level of TGF-β (anti-inflammatorycytokine). Brain homogenates from AQP4+/+and AQP4-/-mice, which is responsiblefor different level of BV-2activation after MPTP treatment.3）Midbrain from AQP4-/-mice expressed higher level of a-syn with or without acuteor chronic MPTP treatment. In basal and MPTP condition, AQP4-/-mice expressedvery high level of a-syn; while a-syn level was very low in AQP4+/+mice.4）AQP4-/-neurons expressed higher level of a-syn with or without mpp+treatment;Neither AQP4+/+nor AQP4-/-astrocytes showed significant a-syn protein expression5）MPTP treatment failed to increase TGF-beta production in the midbrain andperipheral blood of AQP4-/-mice. MPP+treatment failed to increase TGF-betaproduction in AQP4-/-microglia.6) Injection of TGF-beta in the striatum significantly reduced the nerve damage andactivation of astrocyte and microglia in MPTP-treated AQP4-/-mice. Brainhomogenate from MPTP-treated AQP4-/-mice with TGF-beta deactivated BV-2cellsin vitro. Brain homogenate from MPTP-treated AQP4+/+mice with anti-TGF-betaaggregate BV-2cells activation.In summary, our findings demonstrates that the factor(s) in brain homogenatesfrom AQP4+/+and AQP4-/-mice, which is responsible for different level ofmicrogliosis and neuron damage after MPTP treatment and it is irrelevant tomicroglia itself. a-syn may not be sufficient to inducing neuron pathology in mouse model of PD. MPTP treatment failed to increase TGF-beta expression in AQP4-/-mice,and injection of TGF-beta in the midbrain significantly reduced the nerve damage anddeactivated astrocyte and microglia in MPTP-treated AQP4-/-mice. Results suggestthat the therapeutic strategies target to TGF-beta modulation with AQP4may offer agreat potential approach for the development of new strategies for neurodegenerativediseases or other immune-associated diseases treatment.