Water Channel Protein 4 Mediates Astrocyte Response To Endogenous And Exogenous β-amyloid

Background: Recent studies have shown that astrocytes play an important role in the pathological process of Alzheimer’s disease(AD). Aquaporin-4(AQP4), an important glial water transport protein, regulates astrocyte biology and function. Our previous study found that AQP4 knockout affects the toxicity of β-amyloid(Aβ) to cultured astrocytes. However, the roles of AQP4 on the astrocyte pathology in the AD process warrant further study.Objective: The present dissertation was designed to study and clarify regulatory roles of AQP4 in the pathological process of reactive astrocytes in response to endogenous and exogenous Aβ, which may provide the theoretical and experimental basis for exploring the potential treatment of AD via targeting on AQP4.Methods and contents:(1) In order to clarify the role of AQP4 in reactive astrogliosis in response to exogenous Aβ, adult AQP4 knockout mice and wild-type mice received microinjection of Aβ1-42 into the cerebral cortex and perfusion 48 h later, then astrocyte activation and neuronal apoptosis were evaluated by HE staining and immunohistochemistry for TUNEL, glial fibrillary acidic protein(GFAP), AQP4 and low-density lipoprotein receptor-related protein-1(LRP-1), respectively.(2) In order to clarify whether AQP4 is involved in astrocyte reactivity during the Aβ aggregation process, APP/PS1 transgenic mice and wild-type littermates reared 3, 6 and 11 months. Aβ plaque burden on different age mouse brains was detected by Thioflavin S fluorescent staining. GFAP, AQP4 or LRP-1 immunohistochemistry combined with Congo Red staining was performed to observe astrocyte activation, AQP4 and Aβ transporter expression, repsectively.(3) In order to further clarify the contribution of AQP4 to reactive astrogliosis in AD brain, AQP4 knockout-APP/PS1 transgenic mice were established and raised to 3 and 6 months of age by crossing AQP4 knockout mice with APP/PS1 transgenic littermates. Thioflavin S staining and GFAP, AQP4, LRP-1, ionized calcium binding adaptor molecule 1(Iba-1) or synaptophysin(SYN) immunohistochemical staining combined with Congo Red staining were carried out to observe plaque burden, reactive astrogliosis, microglial reactivity and synaptic protein loss in these mice.Results:(1) Compared with wild-type mice, AQP4 knockout mice showed significantly reduced astrocyte activation, and increased brain tissue damage and neuronal apoptosis induced by exogenous Aβ1-42, accompanied with AQP4 and LRP-1 upregulation in the cerebral injection site, suggesting that AQP4 gene deletion reduces astrocyte activation and clear of exogenous Aβ, thus exacerbating Aβ-induced neuronal damage.(2) Aβ plaques occasionally occurred at 3-month-old APP/PS1 transgenic mouse brain, but Aβ plaque deposition significantly increased from the age of 6 months onwards. Astrocytes were activated surrounding plaque, accompanied by significantly increased AQP4 and LRP-1 expression, suggesting that AQP4 participates in the pathological process of reactive astrocytes associated with brain Aβ clearance.(3) Compared with APP/PS1 mice, AQP4 gene deletion in APP/PS1 mice causes less astrocyte activation at 3 months of age, but higher reactive astrogliosis at six months of age with reduced Aβ plaque deposition but no changes in LRP-1 expression, microglial activation and SYN protein loss, suggesting that AQP4 gene deletion alters the astrocyte pathology during the Aβ aggregation process in AD mouse brain.Conclusion: These results confirm that AQP4 mediates reactive changes of astrocytes in response to endogenous and exogenous Aβ, thus its deletion delays the pathology process of reactive astrocytes induced by Aβ.