The unprecedented extension in human life expectancy has precipitated a surge in age-associated neurodegenerative disorders (1). Alzheimer's disease (AD), the most prevalent cause of dementia, currently affects tens of millions of people worldwide, posing a tremendous burden on healthcare systems and society (Livingston et al., 2020; Scheltens et al., 2021). Despite decades of intensive research, effective disease-modifying therapies remain unavailable. This has prompted a strategic shift in focus toward discovering novel therapeutic targets beyond the classical amyloid and tau pathways (Long and Holtzman, 2019).

AD is pathologically characterized by two hallmark lesions: extracellular accumulation of amyloid-β (Aβ) plaques and intracellular neurofibrillary tangles composed of hyperphosphorylated tau (p-tau). These pathologies collectively drive synaptic failure, neuronal loss, and cognitive decline (Bloom, 2014; Scheltens et al., 2021). While neuronal dysfunction has long been central to AD research, growing evidence now highlights the pivotal role of astrocytes in disease pathogenesis (Arranz and De Strooper, 2019). In the physiological conditions, astrocytes are essential for maintaining homeostasis, supporting synaptic structural preservation, and providing neurotrophic support(Chung et al., 2015). In response to AD-associated pathological stimuli, astrocytes enter a state of reactive astrogliosis, characterized by morphological hypertrophy and profound functional changes(Escartin et al., 2021). Frequently, these reactive astrocytes adopt a neurotoxic phenotype (A1-like), releasing pro-inflammatory cytokines such as IL-1β and TNF-α, which exacerbate neuroinflammation and accelerate neuronal death(Frost and Li, 2017; Habbas et al., 2015; Habib et al., 2020). Consequently, the key molecular regulators driving this pathogenic shift represent promising therapeutic targets (Reichenbach et al., 2019).

Sialylation, a major cell-surface glycosylation pathway, critically regulates cell-cell interactions and signal transduction in the central nervous system (Schnaar et al., 2014). This modification is catalyzed by sialyltransferases, which attach sialic acid to glycoproteins and glycolipids, thereby modulating their function (Harduin-Lepers et al., 2005). Of particular interest is ST6 N-acetylgalactosaminide alpha-2,6-sialyltransferase 5 (St6galnac5), an enzyme implicated in modify cell surface properties (Li et al., 2023). Although St6galnac5 has been previously implicated in mediating the interaction between cells and the blood-brain barrier in the context of metastasis(Bos et al., 2009), its function within the neurodegenerative microenvironment remains largely unexplored. Recent transcriptomic studies have hinted at aberrant glycosylation patterns in AD astrocytes(Ma et al., 2022; Tang et al., 2023), yet whether St6galnac5 acts as a driver of the reactive astrocytic phenotype in AD is unknown.

Emerging evidence suggests that hyper-sialylation may alter the secretory profile of astrocytes and impair their ability to support neuronal connectivity (Allen and Eroglu, 2017; Schjoldager et al., 2020). Indeed, aberrant sialylation patterns have been increasingly recognized to modulate glial inflammatory responses and disrupt astrocyte-synapse crosstalk(Haukedal and Freude, 2020; Pluvinage et al., 2019). Thus, we hypothesized that the upregulation of St6galnac5 in AD astrocytes might drive the transition toward a pro-inflammatory state, thereby contributing to the loss of synaptic integrity. Given the limited efficacy of broad anti-inflammatory strategies (Thakur et al., 2023), interventions aimed at upstream regulators of astrocytic identity present a more targeted therapeutic avenue (Liddelow et al., 2017).

In this study, we aimed to elucidate the role of St6galnac5 in AD pathogenesis and evaluate its potential as a therapeutic target. We first observed significant upregulation of St6galnac5 in hippocampal astrocytes of AD model mice, where it was associated with a specific pro-inflammatory subpopulation(Habib et al., 2020). To determine the functional consequences of this upregulation, we utilized an astrocyte-specific adeno-associated virus (AAV) approach to silence St6galnac5 in the hippocampus of 3xTg-AD mice(Lee et al., 2008). We performed a comprehensive analysis combining behavioral assessments, histopathology, and single-nucleus RNA sequencing (snRNA-seq) to dissect the underlying mechanisms. Our results demonstrate that specific knockdown of astrocytic St6galnac5 promotes an A2-like astrocyte profile in vitro, reduces Aβ and tau pathology in vivo with increased astrocytic AQP4, preserves synaptic integrity, and improves spatial memory and fear of open places. These findings uncover a novel sialylation-dependent mechanism of astrocytic regulation and highlight St6galnac5 as a potent target for intervening in the progression of Alzheimer's disease.