Extracellular Alpha-Synuclein Promotes a Neuroinhibitory Secretory Phenotype in Astrocytes

Abstract

Multiple system atrophy (MSA) and dementia with Lewy bodies (DLB) are α-synucleinopathies that exhibit widespread astrogliosis as a component of the neuroinflammatory response. Munc18, a protein critical to vesicle exocytosis, was previously found to strongly mark morphologically activated astrocytes in brain tissue of MSA patients. Immunofluorescence of MSA, DLB and normal brain tissue sections was combined with cell culture and co-culture experiments to investigate the relationship between extracellular α-synuclein and the transition to a secretory astrocyte phenotype. Increased Munc18-positive vesicles were resolved in activated astrocytes in MSA and DLB tissue compared to controls, and they were also significantly upregulated in the human 1321N1 astrocytoma cell line upon treatment with α-synuclein, with parallel increases in GFAP expression and IL-6 secretion. In co-culture experiments, rat primary astrocytes pretreated with α-synuclein inhibited the growth of neurites of co-cultured primary rat neurons and upregulated chondroitin sulphate proteoglycan. Taken together, these results indicate that the secretory machinery is significantly upregulated in the astrocyte response to extracellular α-synuclein and may participate in the release of neuroinhibitory and proinflammatory factors in α-synucleinopathies.

Regulated exocytosis from astrocytes: a matter of vesicles?

“…In the last 10 years, only a few research papers have specifically characterized exocytosis of synaptic-like microvesicles from astrocytes (Cali et al, 2014;Stenovec et al, 2016Stenovec et al, , 2018Stenovec et al, , 2020Lasič et al, 2017;Sobieski et al, 2017;Wolfes et al, 2017;Chowdhury et al, 2018;Eersapah et al, 2019;Rituper et al, 2022), all from in vitro models. None of these works has shown any EM micrograph, and only two are showing some fluorescence immunohistochemistry with markers for vesicular markers in situ (Plá et al, 2017;Di Marco Vieira et al, 2020). Looking at in vivo studies, two recent papers characterizing neuro-glia interplay at synaptic level, suggest the involvement of SLMVs exocytosis in the process.…”

Astrocytic modulation of neuronal signalling

“…The misfolded a -synuclein released by neurons is taken up by astrocytes via endocytosis and triggers a pro-inflammatory response ( Lee et al, 2010a ; Rannikko et al, 2015 ), including the secretion of pro-inflammatory and neuroinhibitory factors ( Kekesi et al, 2019 ). Astrocytic response to a -synuclein is correlated to elevated levels of the mammalian homologue of UNC-18 (Munc18) -a protein essential for vesicle exocytosis, accompanied by reactive astrocytic morphology and increased expression of IL-6 ( Di Marco Vieira et al, 2020 ). Indeed, Cavaliere and colleagues demonstrated that Lewy body fractions containing human α-synuclein are taken up more efficiently by astrocytes rather than neurons and induces high expression of endogenous α-syn while the transport of α-synuclein takes place bi-directionally between both cell types and causes astrogliosis ( Cavaliere et al, 2017 ).…”

Neurons and Glia Interplay in α-Synucleinopathies

“…Given that astrocytes produce pro-inflammatory cytokines and chemokines as a response to various stimuli, it has been proposed that such astrocytes may mediate the microglial activation detected in aSyn-related brain diseases [ 605 , 606 ]. Various WT or mutant aSyn conformations have been shown to trigger the up-regulation of pro-inflammatory modulators in astrocytes, such as ICAM-1, IL-6 and TNF-α, leading to microglial activation, neuroinflammation and neurotoxic events during PD progression [ 496 , 587 , 607 ]. Specifically, transgenic mice inducibly overexpressing the PD-related A53T mutant aSyn selectively in astrocytes exhibited reactive astrogliosis accompanied by increased inflammatory responses and microglial activation in brain regions with significant neuronal loss [ 593 ].…”