Neuroinflammation has been identified to play a critical role in several neurodegenerative diseases, including Alzheimer’s disease (AD). The beta-amyloid (Aβ) plaques can induce the inflammatory response of microglia which, in turn, accelerates the progression of AD. Cholinergic anti-inflammatory pathway (CAP) is a neural-immune interaction, in which acetylcholine (ACh), released by efferent neurons, can suppress the systemic inflammatory response by activating alpha 7 nicotinic ACh receptor (α7 nAChR) of the peripheral immune cells. As a mediator of neuroinflammation, the CAP is being proposed to have a functional role in the central nervous system (CNS). In the brain, ACh is synthesized by neurons and released into the synaptic cleft consecutively. Microglia, having the expression of...[ Read more ]

Neuroinflammation has been identified to play a critical role in several neurodegenerative diseases, including Alzheimer’s disease (AD). The beta-amyloid (Aβ) plaques can induce the inflammatory response of microglia which, in turn, accelerates the progression of AD. Cholinergic anti-inflammatory pathway (CAP) is a neural-immune interaction, in which acetylcholine (ACh), released by efferent neurons, can suppress the systemic inflammatory response by activating alpha 7 nicotinic ACh receptor (α7 nAChR) of the peripheral immune cells. As a mediator of neuroinflammation, the CAP is being proposed to have a functional role in the central nervous system (CNS). In the brain, ACh is synthesized by neurons and released into the synaptic cleft consecutively. Microglia, having the expression of α7 nAChR, could be the post-synaptic cell to trigger the ACh signaling. However, acetylcholinesterase (AChE), as the primary enzyme to hydrolyze ACh and the classical target of AD drug development, plays an unclear role in neuroinflammation. Here, we aimed to reveal the regulatory profile of AChE during the inflammatory process of microglial cells and probe the functionality of AChE in CAP at the CNS

Microglia, as the primary resident immune cell in the brain, was applied as the cell model to induce inflammation by treating lipopolysaccharide (LPS). Primary microglial cell from rat and BV2 cell line were used in the study. The key cholinergic molecules were identified in cultured microglial cells, including AChE, α7 nAChR and choline acetyltransferase (ChAT). LPS upregulated the expression of AChE, i.e., the expressions of mRNA and protein, and the enzymatic activity, in dose- and time-dependent manners; this induced expression was mediated by NF-κB and CRE signaling. Application of exogenous ACh inhibited the LPS-induced inflammatory responses by activating α7 nAChR in microglia, which was blocked after elevating the enzymatic activity of AChE. This blockage suggested that AChE regulated LPS-induced inflammation, possibly by controlling the level of ACh. The regulation of AChE in neuroinflammation was further confirmed in vivo. The intraperitoneal injection of LPS induced neuroinflammation in the mouse brain, including the expression of proinflammatory cytokines and microglia activation. The neuroinflammatory responses, induced by the challenge of LPS, were significantly increased in the animals (ACHE/cKI) having overexpression of AChE in both microglia and macrophage, as compared with the wildtypes (ACHE/WT). The transcriptomic analysis of brain tissue revealed the details about the aggravated inflammatory responses in ACHE/cKI group in comparison to that of ACHE/WT group. These findings revealed the functionality of AChE in neuroinflammation, which was upregulated during inflammation, and in turn aggravated the inflammatory responses in the brain.

Berberine and palmatine, two AChE inhibitors, were evaluated as the potential anti-AD drugs here. These alkaloids inhibited the LPS-induced production of proinflammatory cytokines by activating α7 nAChR in microglia and enhanced microglial phagocytosis of Aβ aggregates. Similar to galantamine, berberine and palmatine are alkaloids deriving from Chinese medicinal herbs. Galantamine has been revealed as the allosteric potential ligand of nAChR. Consistently, berberine and palmatine were proposed to act on α7 nAChR here, because its activation on Ca²⁺ influx was blocked by the antagonist of α7 nAChR in microglial cells. Moreover, the combination of these two alkaloids at low concentration showed synergy in anti-inflammatory effect and activation of α7 nAChR. In addition, the combination of alkaloids did not show induction on endoplasmic reticulum (ER)-stress and mitochondrial dysfunction in microglia. In contrast, the combination could even reverse the LPS-induced ER-stress. These results not only suggested the potential of berberine and palmatine as the anti-AD drugs, but provided new insight on the development, or evaluation, of anti-AD drugs, targeting both AChE and α7 nAChR.