AUTHOR=Chung Lucia Yi-Ru , Lin Yi-Ting , Liu Chi , Tai Yi-Cheng , Lin Han-Yi , Lin Chin-Hsien , Chen Ching-Chow 

TITLE=Neuroinflammation Upregulated Neuronal Toll-Like Receptors 2 and 4 to Drive Synucleinopathy in Neurodegeneration

JOURNAL=Frontiers in Pharmacology

VOLUME=Volume 13 - 2022

YEAR=2022

URL=https://www.frontiersin.org/journals/pharmacology/articles/10.3389/fphar.2022.845930

DOI=10.3389/fphar.2022.845930

ISSN=1663-9812

ABSTRACT=BACKGROUND: Parkinson’s disease (PD) is characterized by intraneuronal α-synuclein aggregation called Lewy bodies and progressive dopaminergic neurodegeneration. Toll-like receptor (TLR) signaling is a major pathway mediating inflammation. The molecular link how neuroinflammation upregulates neuronal TLRs and induces accumulation of α-synuclein aggregates to drive synucleinopathy remains to be determined. 
OBJECTIVE: Despite conditioned medium from microglia and TLR agonists were utilized to study their effects on neuronal cells, a transwell co-culture system comprising lipopolysaccharide-activated microglia on top and retinoic acid-differentiated SH-SY5Y cells on bottom more mimicking in vivo neuroinflammation was employed to elucidate the mechanism of activated microglia on neuronal cells. 
METHODS: Genetic variants of TLRs in PD patients were genotyped as well as assessed the multiplex cytokines, sRAGE and HMGB1. Co-culture system was employed to measure α-synuclein aggregates and neurite shortening by confocal microscope. The expression of TLR2/4 and autophagy flux were detected by western blot and immunofluorescence.
RESULTS: PD patients showed higher plasma levels of proinflammatory cytokines and genetic TLR4 variant, c.896 A>G (p. D299G). Elevated proinflammatory cytokines in co-culture medium was also seen. Phosphorylation and aggregation of α-synuclein, shortening of neurite, upregulation of TLR2/4 expression, activation of downstream p38 and JNK, and dampening autophagic flux were seen in SH-SY5Y cells co-cultured with activated microglia. Those were prevented by inhibiting TLR2/4 and p38/JNK signaling.
CONCLUSIONS: Activated microglia-derived neuroinflammation induced neuronal TLR2/4-p38/JNK activation to perturb autophagy, causing accumulation of α-synuclein aggregates and neurite shortening. Targeting neuronal TLR2/4 pathway might be a mechanistic-based therapy for neurodegenerative disease, such as PD.