The glymphatic system, a fluid-clearance pathway involved in brain waste clearance, is known to be impaired in neurological disorders, including Alzheimer’s disease (AD). For this reason, it is important to understand the specific mechanisms and factors controlling glymphatic function. This pathway enables the flow of cerebrospinal fluid (CSF) into the brain and subsequently the brain interstitium, supported by aquaporins (AQPs). Continuous CSF transport through the brain parenchyma is critical for the effective transport and drainage of waste solutes, such as toxic proteins, through the glymphatic system. However, a balance between CSF production and secretion from the choroid plexus, through AQP regulation, is also needed. Thus, any condition that affects CSF homeostasis will also interfere with effective waste removal through the clearance glymphatic pathway and the subsequent processes of neurodegeneration. In this review, we highlight the role of AQPs in the choroid plexus in the modulation of CSF homeostasis and, consequently, the glymphatic clearance pathway, with a special focus on AD.

Impaired brain clearance mechanisms may result in the accumulation of aberrant proteins that define Alzheimer’s disease (AD). The water channel protein astrocytic aquaporin 4 (AQP4) is essential for brain amyloid-β clearance, but it is known to be abnormally expressed in AD brains. The expression of AQPs is differentially regulated during diverse brain injuries, but, whereas AQP4 expression and function have been studied in AD, less is known about AQP5. AQP5 functions include not only water transport but also cell migration mediated by cytoskeleton regulation. Moreover, AQP5 has been reported to be expressed in astrocytes, which are regulated after ischemic and traumatic injury. Additionally, AQP5 is particularly abundant in the salivary glands suggesting that it may be a crucial factor in gland dysfunction associated with AD. Herein, we aim to determine whether AQP5 expression in submandibular glands and the brain was altered in AD. First, we demonstrated impaired AQP5 expression in submandibular glands in APP/PS1 mice and AD patients. Subsequently, we observed that AQP5 expression was upregulated in APP/PS1 cerebral cortex and confirmed its expression both in astrocytes and neurons. Our findings propose AQP5 as a significant role player in AD pathology, in addition to AQP4, representing a potential target for the treatment of AD.

Alzheimer’s disease (AD), the most prevalent form of dementia, is a neurodegenerative disorder characterized by different pathological symptomatology, including disrupted circadian rhythm. The regulation of circadian rhythm depends on the light information that is projected from the retina to the suprachiasmatic nucleus in the hypothalamus. Studies of AD patients and AD transgenic mice have revealed AD retinal pathology, including amyloid-β (Aβ) accumulation that can directly interfere with the regulation of the circadian cycle. Although the cause of AD pathology is poorly understood, one of the main risk factors for AD is female gender. Here, we found that female APP/PS1 mice at 6- and 12-months old display severe circadian rhythm disturbances and retinal pathological hallmarks, including Aβ deposits in retinal layers. Since brain Aβ transport is facilitated by aquaporin (AQP)4, the expression of AQPs were also explored in APP/PS1 retina to investigate a potential correlation between retinal Aβ deposits and AQPs expression. Important reductions in AQP1, AQP4, and AQP5 were detected in the retinal tissue of these transgenic mice, mainly at 6-months of age. Taken together, our findings suggest that abnormal transport of Aβ, mediated by impaired AQPs expression, contributes to the retinal degeneration in the early stages of AD.

Recent evidence has highlighted the role of the oral microbiome in the inflammatory response and pathogenesis of Alzheimer's disease (AD). The oral pathogen Porphyromonas gingivalis (P. gingivalis) and its gingipain virulence factors may be involved in the development of AD. Concurrently, a decline in the antimicrobial protein lactoferrin has been observed in AD saliva. In this study, we analyzed whether the presence of P. gingivalis and gingipain were associated with salivary lactoferrin and inflammatory markers in patients with AD, mild cognitive impairment (MCI) and age-matched controls. Salivary presence of P. gingivalis was higher in MCI and AD patients and exhibited a negative correlation with global cognitive function. We further showed a marked reduction in salivary lactoferrin with AD progression. Salivary/plasma levels of pro-inflammatory and anti-inflammatory cytokines were significantly higher and lower, respectively, in MCI and AD patients compared to controls. Specifically, salivary levels of Interleukin-6 (IL-6) and IL-23 were higher in MCI and AD patients compared to controls, while the IL-1 receptor antagonist (IL-1ra) and IP-10 were reduced in both saliva and plasma in AD patients. Moreover, stromal cell derived factor 1α (SDF1α)/CXCL12, macrophage inflammatory protein-1β (MIP-1β), and vascular endothelial growth factor (VEGF) salivary and plasma levels were reduced in AD patients. Correlation analyses revealed an inverse relationship between salivary lactoferrin and bacterial load and cytokine concentrations. Our findings suggest that AD is associated with alterations to the oral environment, as revealed by higher P. gingivalis presence, lower lactoferrin levels and elevated pro-inflammatory cytokines.

Lactoferrin is present in brain cells and is known to protect against injury through anti-inflammatory and immunomodulatory processes. Furthermore, it has also been found within amyloid plaques in Alzheimer’s disease (AD), leading to the hypothesis that its presence in the AD brain may be detrimental. However, the identity of the brain cells that produce lactoferrin and how its expression changes in AD remain unclear. In this study, we examined lactoferrin expression and localization in 3- and 12-month-old male and female APP/PS1 mice. Using a microglial enrichment protocol and confocal imaging, we identified microglia as an important source of brain lactoferrin, and demonstrated that its expression is modulated by age, sex, and amyloid burden. In female mice, microglial lactoferrin expression was increased at both the mRNA and protein levels in 3-month-old APP/PS1 mice compared with age-matched wild-type controls. Conversely, lactoferrin transcript in the enrichment fraction, and protein localization were also reduced in microglia from 12-month-old female APP/PS1 mice, coinciding with increased amyloid burden. We further demonstrated that gene expression of lactoferrin follows daily variations under a light–dark cycle, similar to established microglial markers such as Iba1 and P2ry12, with alterations in this rhythmic pattern in APP/PS1 mice occurring predominantly in females. In addition, our findings suggest that neurons may internalize lactoferrin via LRP1, whose expression is downregulated by amyloid burden. This Aβ-dependent relationship was further corroborated in both microglial and neuronal in vitro models. Collectively, these findings identify microglia as an important source of lactoferrin in the brain. We suggest that sex- and age-dependent alterations in microglial lactoferrin expression may contribute to the sexual dimorphism observed in AD, possibly through alterations in LRP1-mediated neuronal uptake.