ApoE4 Accelerates The Progression Of Alzheimer’s Disease By Impairing The Glymphatic System

Background: As the global population ages,the prevalence of Alzheimer’s disease(AD)continues to rise,imposing a significant economic burden on society and families.AD is characterized by pathological and functional changes,including the accumulation of amyloid proteins,the formation of neurofibrillary tangles,neuroinflammation,and cognitive impairment.Currently,there is a lack of effective drugs for treating AD,and once the disease progresses,reversal and recovery become challenging.Consequently,early diagnosis and prevention of AD are crucial.Apolipoprotein E(APOE)is a vital apolipoprotein found in plasma,contributing to lipid metabolism and maintaining cholesterol balance.It plays a pivotal role in the normal growth,development,and repair of the nervous system.Moreover,APOE affects the deposition and clearance of amyloid proteins,immune regulation,and inhibition of tumor cell proliferation.APOE exists in three distinct isoforms,namely APOE2,APOE3,and APOE4,which are encoded by the ε2,ε3,and ε4 alleles of the APOE gene.APOE4,structurally and functionally less stable than APOE2 and APOE3,significantly increases the risk of developing AD and reduces the average age of onset in a dose-dependent manner.Despite numerous studies on the association between APOE4 and accelerated AD development,the precise underlying mechanism remains to be fully understood.The glymphatic system,a crucial component of cerebrospinal fluid circulation,facilitates the clearance of metabolic waste,including amyloid proteins,immune cells,inflammatory factors,and cellular debris from the brain.This waste removal process occurs through drainage into the peripheral system via the deep cervical lymph nodes.Dysfunction of the glymphatic system is closely linked to the progression and pathological processes of AD.However,there is currently no research available on the potential impact of APOE4 on glymphatic system function.Objective: The objective of this study is to investigate the potential impact of APOE4 on glymphatic system function and its association with glymphatic system dysfunction.Additionally,we aim to explore the interaction between APOE4 and other pathways and pathological mechanisms related to Alzheimer’s disease(AD),with the goal of fully elucidating the role of APOE4 in AD development.Methods: Firstly,we examined the presence of glymphatic dysfunction and AD-associated pathologies in aged C57BL/6J mice with targeted human APOE4 replacement.APOE4 mice aged 17-18 months and age-matched wild-type(WT)mice were utilized.Behavioral tests were conducted to assess cognitive dysfunction in elderly APOE4 mice compared to elderly WT mice.Multi-photon in vivo imaging was employed to observe the lymphatic-like pathway and blood-brain barrier(BBB)integrity in aged APOE4 mice.Fluorescent labeling of amyloid beta(Aβ)was utilized to investigate whether APOE4 carriers exhibited amyloid deposition in the brain.Fluorescent labeling of neurons,microglia,and astrocytes was employed to assess neuronal loss and brain inflammation in APOE4 carriers.Elderly APOE4 mice displayed evident glymphatic system dysfunction and AD-related pathological changes compared to age-matched WT mice.To further investigate the early effects and mechanisms of APOE4,we examined whether glymphatic system impairment occurred in early-stage APOE4 carriers.Young APOE4 mice on a C57BL/6J background aged 2-3 months and age-matched WT mice were utilized.Multi-photon imaging was employed to assess the effects of APOE4 on the lymphatic-like pathway and BBB integrity.Fluorescein 5-isothiocyanate,a molecule of similar molecular weight to Aβ,was injected into the brain parenchyma to observe its clearance efficiency.Immunofluorescence staining and Western blotting were conducted to detect molecular pathological characteristics in young APOE4 carrier mice.The study revealed that APOE4 impairs glymphatic system function in the early stages by reducing the polar distribution of AQP4.To further explore whether APOE4 exacerbates neurological damage caused by chronic systemic inflammation through glymphatic system impairment,2-3 month-old normal C57BL/6J APOE4 mice and WT mice were used.Chronic systemic inflammation was induced by intraperitoneal injection of lipopolysaccharide(LPS),while the control group received normal saline injections.Mice were divided into the following groups: WT Saline,WT LPS,APOE4 Saline,and APOE4 LPS.Multi-photon imaging was employed to observe lymphatic drainage in APOE4 mice with chronic systemic inflammation,and deep cervical lymph nodes were imaged to assess drainage.Histopathological staining was conducted to observe chronic systemic inflammation-related neuronal injury and inflammatory response in the brains of APOE4 carriers.The study revealed that APOE4 can exacerbate nerve injury caused by chronic systemic inflammation through glymphatic system impairment.Results: Aged APOE4 mice exhibited significant cognitive impairment compared to age-matched WT mice.Moreover,glymphatic system imaging and vascular imaging revealed dysfunction of the glymphatic system and loss of blood-brain barrier(BBB)integrity in aged APOE4 mice.Histopathological analysis showed neuronal loss in the cortex and hippocampus,amyloid deposition in the brain parenchyma,and activation of astrocytes and microglia in APOE4 aged mice.In contrast,there was no significant difference in cognitive function between young APOE4 mice and age-matched WT mice.Notably,evident glymphatic system dysfunction was observed in young APOE4 mice,primarily affecting the brain interstitial fluid diffusion pathway.The BBB remained intact in young APOE4 mice,with no loss of vascular endothelial cells or deposition of Aβ1-42 in the brain parenchyma.However,the clearance of brain parenchyma was significantly slower in young APOE4 mice compared to WT mice.Furthermore,there was a loss of polar aquaporin-4(AQP4)expression and decreased expression of read-through extended AQP4(AQP4X)in the cortex of APOE4 young mice,without significant differences in the expression of AQP4 subtypes M1 and M23 or the M23:M1 ratio.Intervention with apigenin significantly increased AQP4 X expression in the lateral ventricle of young APOE4 mice,accelerating glymphatic system clearance.Chronic systemic inflammation exacerbated glymphatic system damage in APOE4 mice,resulting in reduced drainage of deep cervical lymph nodes,increased inflammation,and neuronal loss in the brain.Conclusions: APOE4 plays a promotive role in the pathological progression of Alzheimer’s disease by impairing glymphatic system function in the early stages.Under normal conditions,brain solute clearance is reduced.In the context of chronic systemic inflammation,APOE4 exacerbates nerve damage caused by inflammation.Additionally,with aging,APOE4 leads to the loss of BBB integrity,amyloid deposition in the brain parenchyma,neuronal loss,cognitive decline,and other typical dysfunctions and pathological changes associated with Alzheimer’s disease.