Alzheimer’s disease (AD) is an irreversible, progressive neurodegenerative disease characterized by memory loss and cognitive dysfunction. The neuropathological hallmark of AD consists of senile plaques, which are mainly composed of the amyloid-β (Aβ) form of the aggregated peptide that is predominantly localized in the cortex and hippocampus. The underlying mechanism of Aβ is postulated to be altering neuronal Ca2+ homeostasis, which disrupts membrane Ca2+ permeability through interaction with Ca2+-permeable ion channels/ receptors, lipid rafts destabilization, and forming Ca2+-permeable pores (Wang and Zheng, 2019). Consistent with this proposed mechanism, the application of Aβ to hippocampal tissues or cultured neurons evokes dysregulated Ca2+ influx into cells (De Felice et al., 2007, Tyurikova et al., 2018). Therefore, the interaction between Aβ and intracellular Ca2+ is a crucial factor in the AD-related pathophysiologies that cause cognitive deficits, cell death, and neuronal degeneration.

Ca2+/ calmodulin-dependent protein kinases (CaMKs) are widely distributed in neurons and glial cells, such as astrocytes and microglia of the mammalian brain, and play critical roles in cortical functions, including cognition, attention, and memory (Moini and Piran, 2020). The CaMK family is mainly comprises CaMKI, CaMKII, and CaMKIV. These proteins undergo conformational changes, and specific Ca2+/calmodulin complexes interact with a broad range of target proteins (Takemoto-Kimura et al., 2017). CaMKIIα is predominantly expressed in the dendritic spine of excitatory pyramidal neurons in cortex and hippocampus, and it plays an essential role in memory formation and induction of synaptic plasticity (Fukunaga et al., 1995, Yasuda et al., 2022). Furthermore, CaMKIIα phosphorylates the GluA1 subunit of α-amino-3-hydroxy-5-methylisozazole-4-propionate receptor at Ser-831 residue, which contributes to enhanced synaptic strength following long-term potentiation (LTP) induction (Barria et al., 1997). By contrast, CaMKIV is predominantly localized to the nuclei of neurons in the cortex and hippocampus (Jensen et al., 1991, Nakamura et al., 1995). In addition, CaMKIV transcriptionally regulates the gene expression of brain-derived neurotrophic factor (BDNF) through phosphorylation of the cAMP response element-binding protein (CREB) at the Ser-133 residue, which in turn modulates the induction of adult neurogenesis and synaptic plasticity (Tao et al., 1998, Ribar et al., 2000). Like the CaMKs, protein kinase C (PKC) is essential for LTP induction in the hippocampus (Collingridge et al., 2004). Specifically, PKCα is enriched in the postsynaptic region (Kamphuis et al., 1995) and phosphorylates the Ser-896 residue of N-methyl-D-aspartate receptor (NMDAR) NR1 subunit (Sanchez-Perez and Felipo, 2005).

Brazilian green propolis (propolis) is a natural resinous substance produced by Apis mellifera honey bees from the buds of Baccharis dracunculifolia DC (Asteraceae). Propolis mainly consists of caffeoylquinic acid and multiple derivatives of cinnamic acid, including artepillin C, p-coumaric acid, baccharin, and drupanin (da Silva Filho et al., 2008). Several studies have reported that propolis is effective in the treatment of cognitive dysfunction, depressed mood, anxiety, and pain (Paulino et al., 2006, Reis et al., 2014, Zhu et al., 2018). However, the precise mechanism underlying the improvement in these symptoms remains unclear.

In this study, we investigated whether propolis ameliorates cognitive deficits in APP-KI mice. We demonstrated that propolis induces transients of intracellular Ca2+ ([Ca2+]i) in Neuro-2A (N2A) cells and antagonizes the elevation of [Ca2+]i transient by pretreatment with Aβ1−42 oligomers. Propolis at 300–1000 mg/kg p.o. once daily for 8 weeks significantly ameliorated cognitive deficit in APP-KI mice at 4 months of age via upregulation of CaMKIIα, PKCα, and CaMKIV activities in CA1. However, propolis did not exhibit beneficial effects in APP-KI mice at 12 months of age, which resulted in elevated Aβ levels in the brain.