Age, Sex Hormones, and Circadian Rhythm Regulate the Expression of Amyloid-Beta Scavengers at the Choroid Plexus

Abstract

Accumulation of amyloid-beta (Aβ) in the brain is thought to derive from the impairment of Aβ clearance mechanisms rather than from its overproduction, which consequently contributes to the development of Alzheimer's disease. The choroid plexus epithelial cells constitute an important clearance route for Aβ, either by facilitating its transport from the cerebrospinal fluid to the blood, or by synthesizing and secreting various proteins involved in Aβ degradation. Impaired choroid plexus synthesis, secretion, and transport of these Aβ-metabolizing enzymes have been therefore associated with the disruption of Aβ homeostasis and amyloid load. Factors such as aging, female gender, and circadian rhythm disturbances are related to the decline of choroid plexus functions that may be involved in the modulation of Aβ-clearance mechanisms. In this study, we investigated the impact of age, sex hormones, and circadian rhythm on the expression of Aβ scavengers such as apolipoprotein J, gelsolin, and transthyretin at the rat choroid plexus. Our results demonstrated that mRNA expression and both intracellular and secreted protein levels of the studied Aβ scavengers are age-, sex-, and circadian-dependent. These data suggest that the Aβ-degradation and clearance pathways at the choroid plexus, mediated by the presence of Aβ scavengers, might be compromised as a consequence of aging and circadian disturbances. These are important findings that enhance the understanding of Aβ-clearance-regulating mechanisms at the blood-cerebrospinal fluid barrier.

Keywords: age; amyloid-beta scavengers; blood–cerebrospinal fluid barrier; choroid plexus; circadian rhythm; sex hormones.

Figure 1

Effects of age on the expression of apolipoprotein J (APOJ), gelsolin (GLS), and transthyretin (TTR) in the choroid plexus (CP). mRNA expression, intracellular protein and secreted protein levels of APOJ (A), GLS (B), and TTR (C) in CP explants and conditioned media of newborn (Nb), young (Yng), and adult (Ad) rats was analyzed by real time RT-PCR and Western blot, respectively. Bar graphs represent mean ± SEM (N ≥ 3; * p < 0.05, ** p < 0.01, *** p < 0.001; one-way ANOVA followed by Bonferroni’s post hoc test).

Figure 2

Effects of sex on the expression of apolipoprotein J (APOJ), gelsolin (GLS), and transthyretin (TTR) in the choroid plexus (CP). mRNA expression, intracellular protein, and secreted protein levels of APOJ (A), GLS (B), and TTR (C) in CP explants and conditioned media of male and female from young (Yng) and adult (Ad) groups was analyzed by real time RT-PCR and Western blot, respectively. Bar graphs represent mean ± SEM (N ≥ 3; ** p < 0.01, *** p < 0.001; One-way ANOVA followed by Bonferroni’s post hoc test).

Figure 3

Circadian profile of choroid plexus (CP) Aβ scavengers. ApoJ, Gls, and Ttr expression was analyzed every 6 h during a 24-h period in intact females (orange), intact males (blue), sham-operated (sham; green), and ovariectomized animals (OVX, pink). Graphs represent the relative mRNA expression levels of ApoJ (A), Gls (B), and Ttr (C) in the CP. Each data point represents the mean ± SEM (n = 4–5). CircWave analysis (right side of the panel) shows the corresponding statistical analysis for rhythmicity (statistical analysis is shown in Table 1). No curve could be fitted for Gls circadian expression. The center of gravity (phase) is represented by the vertical line intersecting each curve. White and grey backgrounds represent light and dark phases, respectively. Data from ZT1 and ZT25 are double-plotted.