Choroid plexus dysfunction impairs beta-amyloid clearance in a triple transgenic mouse model of Alzheimer's disease

Abstract

Compromised secretory function of choroid plexus (CP) and defective cerebrospinal fluid (CSF) production, along with accumulation of beta-amyloid (Aβ) peptides at the blood-CSF barrier (BCSFB), contribute to complications of Alzheimer's disease (AD). The AD triple transgenic mouse model (3xTg-AD) at 16 month-old mimics critical hallmarks of the human disease: β-amyloid (Aβ) plaques and neurofibrillary tangles (NFT) with a temporal- and regional- specific profile. Currently, little is known about transport and metabolic responses by CP to the disrupted homeostasis of CNS Aβ in AD. This study analyzed the effects of highly-expressed AD-linked human transgenes (APP, PS1 and tau) on lateral ventricle CP function. Confocal imaging and immunohistochemistry revealed an increase only of Aβ42 isoform in epithelial cytosol and in stroma surrounding choroidal capillaries; this buildup may reflect insufficient clearance transport from CSF to blood. Still, there was increased expression, presumably compensatory, of the choroidal Aβ transporters: the low density lipoprotein receptor-related protein 1 (LRP1) and the receptor for advanced glycation end product (RAGE). A thickening of the epithelial basal membrane and greater collagen-IV deposition occurred around capillaries in CP, probably curtailing solute exchanges. Moreover, there was attenuated expression of epithelial aquaporin-1 and transthyretin (TTR) protein compared to Non-Tg mice. Collectively these findings indicate CP dysfunction hypothetically linked to increasing Aβ burden resulting in less efficient ion transport, concurrently with reduced production of CSF (less sink action on brain Aβ) and diminished secretion of TTR (less neuroprotection against cortical Aβ toxicity). The putative effects of a disabled CP-CSF system on CNS functions are discussed in the context of AD.

Keywords: 3xTg-AD mice; Alzheimer disease; amyloid-β; aquaporin-1; choroid plexus; collagen-IV; dysfunction; transthyretin.

Figure 1

Choroid plexus (CP) confocal microscopy images immunolabeled from Non-Tg (A,C) and 3xTg-AD mice (B,D) stained with anti-Aβ40 (A,B) and anti-Aβ42 (C,D). The asterisks show the blood vessels staining and arrowheads show the cytoplasmic staining of CP epithelial cells (CP-EP) of lateral ventricle (LV). According to figure (see also the results in text) in the left bottom (E) is also shown a scheme about the distribution of Aβ40 and Aβ42 in vessels (V), stroma (ST) and epithelial cells (EP) of 3xTg-AD compared to Non-Tg mice. On the right bottom (F) are presented values of stain intensitites for Aβ40 and Aβ42 as the means ± SEM; n = eight animals per group; * p < 0.01 3xTg-AD vs. Non-Tg mice. Scale bars, 30 μm.

Figure 2

CP confocal microscopy images immunolabeled from Non-Tg (A,C) and 3xTg-AD mice (B,D) stained with anti-LRP1 (A,B) and anti-RAGE (C,D). An increase in cytoplasmic expression of LRP1 and RAGE in epithelial cells of lateral ventricle (LV) of 3xTg-AD compared to Non-Tg mice is shown. On the left bottom (E) are represented values of stain intensitites for LRP1 and RAGE as the means ± SEM; n = eight animals per group; * p < 0.01 3xTg-AD vs. Non-Tg mice. Scale bars, 30 μm.

Figure 3

CP immunohistochemistry images from Non-Tg (A,B) and 3xTg-AD mice (C,D) show CP basement membrane and basal membrane of CP vessels (V) immunostained with anti-collagen-IV (arrows). (B,D) are enlarged images of (A,C) respectively. An increased collagen-IV expression in CP of 3xTg-AD (C,D) compared to Non-Tg mice (A,B) is shown. On the left bottom are represented values of stain intensities for collagen-IV (E) as the means ± SEM; LV: lateral ventricle; n = eight animals per group; * p < 0.01 3xTg-AD vs. Non-Tg mice. Scale bars, 60 μm.

Figure 4

CP confocal images immunolabeled from Non-Tg (A,C) and 3xTg-AD mice (B,D) stained with anti-TTR (A,B) and anti-AQP-1 (C,D). A reduction in cytoplasmic expression of TTR (A,B) as well as in AQP-1 expression in the apical membrane of epithelial cells (C,D) is shown in 3xTg-AD when compared to Non-Tg mice (arrowheads). The arrows in (D) indicate areas where AQP1 was not detected on the apical membrane of CP epithelium. On the left bottom side are represented values of stain intensitites for TTR and AQP-1 (E) as the means ± SEM; LV: lateral ventricle; n = eight animals per group; * p < 0.01 3xTg-AD vs. Non- Tg mice. Scale bars, 30 μm.