Serotonin signaling is associated with lower amyloid-β levels and plaques in transgenic mice and humans

Abstract

Aggregation of amyloid-β (Aβ) as toxic oligomers and amyloid plaques within the brain appears to be the pathogenic event that initiates Alzheimer's disease (AD) lesions. One therapeutic strategy has been to reduce Aβ levels to limit its accumulation. Activation of certain neurotransmitter receptors can regulate Aβ metabolism. We assessed the ability of serotonin signaling to alter brain Aβ levels and plaques in a mouse model of AD and in humans. In mice, brain interstitial fluid (ISF) Aβ levels were decreased by 25% following administration of several selective serotonin reuptake inhibitor (SSRI) antidepressant drugs. Similarly, direct infusion of serotonin into the hippocampus reduced ISF Aβ levels. Serotonin-dependent reductions in Aβ were reversed if mice were pretreated with inhibitors of the extracellular regulated kinase (ERK) signaling cascade. Chronic treatment with an SSRI, citalopram, caused a 50% reduction in brain plaque load in mice. To test whether serotonin signaling could impact Aβ plaques in humans, we retrospectively compared brain amyloid load in cognitively normal elderly participants who were exposed to antidepressant drugs within the past 5 y to participants who were not. Antidepressant-treated participants had significantly less amyloid load as quantified by positron emission tomography (PET) imaging with Pittsburgh Compound B (PIB). Cumulative time of antidepressant use within the 5-y period preceding the scan correlated with less plaque load. These data suggest that serotonin signaling was associated with less Aβ accumulation in cognitively normal individuals.

Fig. 1.

SSRIs reduce ISF Aβ levels in vivo. Two- to 3-mo-old PS1APP hemizygous mice were administered vehicle (PBS) or one of several antidepressants by i.p. injection: fluoxetine 10 mg/kg, desvenlafaxine 30 mg/kg, citalopram 5 mg/kg and 10 mg/kg, tianeptine 20 mg/kg (n = 5–8 per group). (A) As assessed by in vivo microdialysis, SSRIs reduced ISF Aβ_x-40 levels significantly beginning between 10 and 14 h after administration. (B) Twenty-four hours after administration, fluoxetine and desvenlafaxine reduced Aβ_x-40 levels to 73.4 ± 2.0% (P < 0.0001; n = 6) and 71.6 ± 1.2% (P = 0.001; n = 6), respectively, compared with baseline levels in each mouse. Doses of 10 mg/kg and 5 mg/kg citalopram reduced ISF Aβ_x-40 to 74.0 ± 5.4% (P = 0.004; n = 8) and 83.5 ± 4.2% (P = 0.02; n = 6) of baseline levels, respectively. ISF Aβ levels did not change significantly in tianeptine and vehicle-treated mice (n = 5 per group). (C) PS1APP mice treated with 10 mg/kg citalopram had similar reductions in ISF Aβ_x-40 and Aβ_x-42 levels; 71.9 ± 9.4% and 66.7 ± 16.3% of baseline levels, respectively, by 24 h posttreatment (n = 5). (D) PS1APP mice were treated with vehicle (artificial CSF, aCSF) or 2 mM serotonin directly to the hippocampus by reverse microdialysis for 14 h. By 8 h of administration, serotonin reduced ISF Aβ_x-40 levels to 66.7 ± 7.2% of baseline (P = 0.003, n = 5 per group). After 8 h, mice were administered a γ-secretase inhibitor, Compound E (20 mg/kg i.p.) to assess Aβ_x-40 half-life. Data presented as mean ± SEM.

Fig. 2.

ERK-dependent changes in Aβ metabolism. (A) Young PS1APP mice were administered vehicle (aCSF), a MEK inhibitor (PD98059, 100 μM), or an ERK inhibitor (FR180204, 100 μM) by reverse microdialysis (n = 6–8 per group). At 24 h from the beginning of treatment, the MEK and ERK inhibitors alone significantly increased ISF Aβ levels by 37.7 ± 3.9% (P = 0.001) and 39.4 ± 1.6% (P < 0.0001) compared with baseline levels. Coadministration of citalopram (10 mg/kg i.p.) with either of these inhibitors blocked the SSRI-dependent reduction in ISF Aβ. ISF Aβ levels were not significantly different between inhibitor-treated and inhibitor plus citalopram-treated mice. (B) Citalopram increased α-secretase enzymatic activity by 25 ± 4.9% (P = 0.01) within the hippocampus with no change in β-secretase activity (n = 8 per group). Data presented as mean ± SEM.

Fig. 3.

Chronic SSRI administration reduces plaque load in PS1APP transgenic mice. Beginning at 3 mo of age, PS1APP hemizygous mice were treated with water or citalopram (8 mg/kg/day) in drinking water for 4 mo (n = 10 per group). Representative images of cortex and hippocampus stained for Aβ plaques in (A) water and (B) citalopram-treated mice. (C) Quantification of plaque load in the hippocampus and cortex was performed blinded. Surface area covered by plaques was reduced in citalopram-treated mice to 57.9 ± 6.1% (P = 0.03) and 49.5 ± 6.2% (P = 0.004) of mean levels in water-treated mice. (D) Guanidine-extracted Aβ_x-40 and Aβ_x-42 in the hippocampus and cortex was significantly reduced in citalopram-treated mice. (E) Aβ_x-40 and Aβ_x-42 levels within the CSF of citalopram-treated mice were reduced to 71.4 ± 7.5% (P = 0.02) and 49.5 ± 6.2% (P < 0.0001) compared with mean levels in water-treated mice. (F) α-Secretase enzymatic activity was significantly increased in chronic citalopram mice (P < 0.001); however, β-secretase activity was unchanged. (G) Quantitative PCR showed that mRNA levels of ADAM10 did not change significantly but that memapsin-2 (P = 0.01) and two components of the γ-secretase complex, presenilin-1 and nicastrin, were significantly reduced in citalopram-treated mice (P = 0.02 and P = 0.01; n = 9–10 per group). APH-1B, neprilysin, and LRP1 mRNA levels did not change following citalopram treatment. Values normalized to mean level in water-treated mice. Data presented as mean ± SEM.

Fig. 4.

Antidepressant use is associated with less cortical amyloid in human participants. (A) Mean cortical binding potential of PIB (MCBP) in participants who had taken antidepressants within the past 5 y “treated” versus those who had not been treated “untreated.” Mean exposure time for the treated group = 34.5 mo (SD 23.6). MCBP in untreated participants was 0.13 ± 0.22 (n = 134; mean ± SD) versus 0.06 ± 0.20 (n = 43; P = 0.01). (B) Normalized Aβ PET subtraction image of untreated participants, minus treated participants, showing the pattern of cortical rim Aβ plaque accumulation typical of AD.

Fig. 5.

Model of serotonergic regulation of Aβ metabolism. Serotonin receptors are activated on the cell surface, which initiates a signaling cascade that leads to ERK phosphorylation and activation. ERK appears to increase α-secretase cleavage and may reduce γ-secretase cleavage of APP. The particular serotonin receptor subtypes responsible for ERK activation in this paradigm are unknown. Not all receptors or ligands that activate ERK will alter APP processing, however similar to serotonin, NMDA receptors can also modulate APP processing through the ERK pathway.