Alterations of Astrocytic Aquaporin 4 expression and distribution in Down Syndrome with Alzheimer’s Disease onset

Abstract

AbstractBackgroundAstrocytic Aquaporin 4 (AQP4) is a water channel that is typically expressed in astrocytic endfeet, associated with the blood brain barrier (BBB). This association is thought to facilitate the removal of waste products (e.g. beta amyloid (Aβ)) via the ‘glymphatic system’. With Aβ plaque deposition in Alzheimer’s Disease (AD), AQP4 becomes dysregulated observed as a shift in distribution from astrocytic endfeet to astrocytic processes, resulting in inefficient Aβ removal. Here, AQP4 expression and distribution within astrocytes was characterized via immunohistochemistry in human brain tissue from people with Down Syndrome without AD (DS) and with AD (DSAD). We hypothesized that AQP4 protein level would be higher and distributed along astrocytic processes as opposed to the BBB in DSAD relative to DS.MethodAQP4 was evaluated by immunohistochemistry and quantified using QuPath (% loads, positive pixel count) for both frontal (FC) and occipital cortex (OC) in the following groups: DS (n = 17, age range = 0‐51) and DSAD (n = 20, age range = 46‐70). In addition, AQP4 distribution was classified by a blinded observer who annotated distinct AQP4 positive astrocytic morphologies, listed as follows: In WM, ‘Star,’ ‘Webbed,’ and ‘Stippled,’ and in GM, ‘Bushy,’ ‘Branched,’ ‘Globular,’ ‘Endfoot/Vascular Associated.’ResultWe show that AQP4 protein staining was higher in GM DS with AD neuropathology than in DS without AD neuropathology for both FC and OC (FC p = 0.0007; OC p = 0.0034). In addition, we observed a shift from ‘star’ shaped distribution within DS astrocytes to more ‘webbed’ and ‘stippled’ shapes in DSAD; this was not observed in OC. We found a shift in GM morphologies from ‘bushy’ morphology of AQP4 immunostaining in DS to more ‘Branched and Globular’ shapes in DSAD in OC whereas there is a shift from a ‘Bushy’ morphology in DS to more ‘Endfoot/Vascular Associated’ morphologies in the DSAD FC.ConclusionAQP4 expression is higher in DSAD than DS and appears to show a spectrum of astrocyte morphologies with distinct patterns based upon spatial location (FC/OC and GM/WM). Taken together, AQP4 dysfunction and shift to abnormal spatial location may reduce Aβ clearance through the glymphatic system, leading to enhanced plaque formation in DSAD.