Mechanism of depression as a risk factor in the development of Alzheimer’s disease: the function of AQP4 and the glymphatic system

Nov 13, 2016Psychopharmacology

How Depression May Increase Alzheimer's Risk Through Water Channel Function and Brain Waste Clearance

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Abstract

Glymphatic pathway circulation was impaired in mice treated with chronic unpredictable mild stress (CUMS).

CUMS treatment increased accumulation of endogenous and exogenous amyloid beta 42 (Aβ42) in the brains of mice.
Impairment of the glymphatic pathway is associated with suppressed Aβ42 metabolism in CUMS-treated mice.
Treatment with the antidepressant fluoxetine reversed the negative effects of CUMS on the glymphatic system.
In anhedonic mice, expression of the water channel aquaporin 4 was down-regulated in the cortex and hippocampus.
Glymphatic system dysfunction may explain why a history of depression is a strong risk factor for Alzheimer's disease.

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BACKGROUND: Many studies have indicated that a history of depression increases the risk of developing Alzheimer's disease (AD); however, the potential pathogenestic mechanism by which depression functions as a high risk factor for AD remains unknown. Recently, a "cerebral lymphatic system" referred to as "glymphatic system" has been demonstrated to be responsible for neuronal extracellular waste protein clearance via a paravascular pathway. However, the function of glymphatic pathway has not been determined in depressive disorders.

METHODS: The present study used an animal model of chronic unpredictable mild stress (CUMS) to determine the function of glymphatic pathway by using fluorescence tracers. Immunohistochemistry was used to assess the accumulation of endogenous mouse and exogenous human amyloid beta 42 (Aβ42) in CUMS-treated mice with or without treatment with antidepressant fluoxetine.

FINDINGS: Glymphatic pathway circulation was impaired in mice treated with CUMS; moreover, glymphatic pathway dysfunction suppressed Aβ42 metabolism, because the accumulation of endogenous and exogenous Aβ42 was increased in the brains of the CUMS-treated mice. However, treatment with fluoxetine reversed these destructive effects of CUMS on glymphatic system. In anhedonic mice, the expression of the water channel aquaporin 4 (AQP4), a factor in glymphatic pathway dysfunction, was down-regulated in cortex and hippocampus.

CONCLUSION: The dysfunction of glymphatic system suggested why a history of depression may be a strong risk factor for AD in anhedonic mice. We hope our study will contribute to an understanding of the risk mechanism of depressive disorder in the development of AD and the mechanisms of antidepressant therapies in AD.

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Mechanism of depression as a risk factor in the development of Alzheimer’s disease: the function of AQP4 and the glymphatic system

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