Alzheimer's disease (AD) is a prevalent chronic neurodegenerative disorder and the leading cause of dementia, characterized by cognitive impairments such as memory loss, disorientation, problem-solving deficits, and difficulty with daily tasks (Huma et al., 2022). Beta-amyloid (Aβ) deposition, leading to the formation of senile plaques between neurons, is one of the main pathological features in the brain of AD patients (Zhang et al., 2023). Among the various hypotheses concerning the onset of AD, the Aβ toxicity hypothesis has been extensively studied and considered to play a central role in the occurrence and development of AD. However, the failure of clinical trials targeting β-secretase or γ-secretase inhibitors, which inhibit Aβ production, has raised some doubts about the Aβ hypothesis. Nevertheless, regardless of the cause of excessive Aβ accumulation in the brain, it is undeniable that the accumulation of Aβ results in a variety of toxic effects on neurons. Reducing Aβ accumulation in the brain is beneficial for alleviating its toxicity and protecting neural cells, as well as in the prevention/treatment of AD, which remains the consensus among the majority. Studies have found that in sporadic AD patients (constituting over 95% of all AD cases), Aβ accumulation in the brain is not primarily due to increased production but rather impaired clearance of Aβ(Mawuenyega et al., 2010). Recently, Leqembi™, a monoclonal antibody drug targeting Aβ, received full approval from the U.S. FDA, showing significant Aβ clearance and improvement of cognitive impairment in a Phase III clinical trial of AD. The approval of Leqembi™ once again supports the Aβ hypothesis and suggests that clearing accumulated Aβ in the brain is an effective therapeutic strategy for prevention and treatment of AD.

The glymphatic system, a network of tunnel-like perivascular spaces that promotes directional, bulk fluid movement mediated by aquaporin-4 (AQP4) at the astrocyte endfeet, has recently been discovered to be widely distributed throughout the brain (Xie et al., 2013; Iliff et al., 2012). It is one of the most important pathways for the brain to remove metabolites, soluble proteins, and foreign substances (Iliff et al., 2014; Lundgaard et al., 2017). Aβ in the brain is transported through the glymphatic system to the meninges and deep cervical lymph nodes (dcLNs), where it is eventually reabsorbed into the systemic circulation for metabolism (Aspelund et al., 2015; Louveau et al., 2015). AQP4 is the most prevalent type of aquaporin in the brain (Verkman and Song, 2006). It has been determined that AQP4 is primarily expressed at the astrocyte endfeet surrounding the cerebral vasculature (this expression pattern is defined as the AQP4 polarity) and facilitates the entry of cerebrospinal fluid (CSF) from the perivascular space into the interstitial space of brain tissue and flushes the brain parenchyma, a process closely related to the function of the glymphatic system (Iliff et al., 2012; Yang et al., 2011). Peng et al. found that both influx and tracer clearance were significantly reduced in the CSF of APP/PS1 transgenic mice, indicating glymphatic system dysfunction. The research team observed impaired glymphatic system function in aged APP/PS1 transgenic mice (12–13 months old) with significant Aβ plaques as well as in young mice (2–3 months old) without significant Aβ plaques, suggesting that impaired glymphatic system function may occur prior to Aβ deposition. This inability of the glymphatic system could be a major contributor to Aβ deposition (Peng et al., 2016; Feng et al., 2020).

Despite the huge research and development funds invested by global pharmaceutical companies, only four anti-AD medications have been approved by the FDA as of 2021. These drugs only temporarily improve symptoms by increasing neurotransmitters in the brain but do not alleviate pathological changes or slow disease progression (Gauthier et al., 2021; Mi et al., 2021). Our previous work found that administration of OAB-14, a novel small molecular compound modified by bexarotene whose structure and mechanism are distinct from the anti-AD drugs that have failed in recent clinical trials, can improve cognitive impairments in a variety of mouse models of AD within 15 days. Unlike anti-AD drugs that have failed in clinical trials and being used clinically today, OAB-14 exerts an anti-neuroinflammatory effect and reduces Aβ deposition through mechanisms other than inhibiting the Aβ production enzyme (Yuan et al., 2019). Additionally, experiments revealed that OAB-14 significantly up-regulated the expression of AQP1 and AQP4 in cultured astrocytes in vitro, suggesting that OAB-14 may play an essential role in improving glymphatic function.

The present study aims to investigate whether OAB-14 improves AD symptoms by enhancing the clearance of Aβ from the brain via the glymphatic system. To achieve this, experiments were conducted on APPswe/PS1dE9 (APP/PS1) transgenic mice and AQP4 gene knockout mice with glymphatic system dysfunction. TGN-020, a specific AQP4 inhibitor, and dcLNs ligation surgery were utilized to verify the relationship between increased Aβ clearance through the glymphatic system and improved cognitive impairments. Fluorescently labeled Aβ1-42 and the central inert substance Dextran were used as CSF markers, the influx and efflux of CSF tracers into and out of the brain parenchyma and distribution in the dcLNs were examined to assess the improvement in glymphatic system function by OAB-14. The findings of this study will help to elucidate the targets of OAB-14 against AD and provide a theoretical basis for the development of anti-AD medications.