TLR2 and α-synuclein mediated pathology in human neuronal cell models

Abstract

Parkinson’s disease (PD) is a progressively debilitating neurodegenerative disorder with the formation and development of Lewy bodies (LBs) and Lewy neurites (LNs) as its pathological characteristics. The most prominent and well-studied component of LBs and LNs is α-synuclein, which is thought to propagate through PD brain and contribute to neural dysfunction and clinical symptoms. The α-synuclein protein invades vulnerable neurons in the PD brain in a predictable, staged pattern. Recent evidence suggests that this propagation has some characteristics similar to the propagation of the prion protein, with distinct toxic α-synuclein species triggering the pathological conversion of normal endogenous α-synuclein in neighboring cells. However, the mechanism by which α-synuclein is taken up, accumulated and transferred within neurons remains to be fully defined. In this study I have used two different models of PD pathology, α-synuclein pre-formed fibrils (PFFs) and TLR2 activation with Pam3CSK4, to induce α-synuclein accumulation in differentiated SH-SY5Y cells and primary induced pluripotent stem cells (IPSCs)-derived neurons. Both cell models resulted in the accumulation of endogenous α-synuclein which peaked at 4-6 days post treatment. Such accumulation was absent in α-synuclein knockout SH-SY5Y cells, indicating the essentiality of endogenous α-synuclein for PD pathology. The PFF- or Pam3CSK4- triggered α-synuclein aggregation was associated with a temporal block in autophagy, as indicated by an increase in the selective autophagy markers p62/SQSTM1, LC3 and LAMP2, suggesting α-synuclein may be increased due to impaired macroautophagy / chaperone-mediated autophagy (CMA) clearance. Moreover, the accumulation of α-synuclein could be ameliorated by promoting autophagy with rapamycin or activators of the 5’ AMP-activated protein kinase (AMPK). TLR2 has recently been suggested as a receptor for endocytosis of α-synuclein. I generated TLR2 KO cells and observed an attenuation of α-synuclein accumulation as well as a suppression of inflammatory responses post Pam3CSK4 treatment in SH-SY5Y cells. Finally, a number of small molecule inhibitors targeting the TLR2 pathway were identified to ameliorate the accumulation of α-synuclein in neural cells, albiet less significantly than the AMPK agonists. These observations increase understanding of the mechanisms involved in PD pathology and provide a convenient model for the screening of potential therapeutics to prevent α-synuclein accumulation. These results suggested that targeting the AKT-mTORC1 and/or TLR2 signaling pathways might be potential therapeutic options for preventing α-synuclein accumulation in PD.