LGALS3 (galectin 3) mediates an unconventional secretion of SNCA/α-synuclein in response to lysosomal membrane damage by the autophagic-lysosomal pathway in human midbrain dopamine neurons

Abstract

Numerous lines of evidence support the premise that the misfolding and subsequent accumulation of SNCA/α-synuclein (synuclein alpha) is responsible for the underlying neuronal pathology observed in Parkinson disease (PD) and other synucleinopathies. Moreover, the cell-to-cell transfer of these misfolded SNCA species is thought to be responsible for disease progression and the spread of cellular pathology throughout the brain. Previous work has shown that when exogenous, misfolded SNCA fibrils enter cells through endocytosis, they can damage and rupture the membranes of their endocytotic vesicles in which they are trafficked. Rupture of these vesicular membranes exposes intralumenal glycans leading to galectin protein binding, subsequent autophagic protein recruitment, and, ultimately, their introduction into the autophagic-lysosomal pathway. Increasing evidence indicates that both pathological and non-pathological SNCA species undergo autophagy-dependent unconventional secretion. While other proteins have also been shown to be secreted from cells by autophagy, what triggers this release process and how these specific proteins are recruited to a secretory autophagic pathway is largely unknown. Here, we use a human midbrain dopamine (mDA) neuronal culture model to provide evidence in support of a cellular mechanism that explains the cell-to-cell transfer of pathological forms of SNCA that are observed in PD. We demonstrate that LGALS3 (galectin 3) mediates the release of SNCA following vesicular damage. SNCA release is also dependent on TRIM16 (tripartite motif containing 16) and ATG16L1 (autophagy related 16 like 1), providing evidence that secretion of SNCA is mediated by an autophagic secretory pathway.

Acidic nanoparticles protect against α‐synuclein‐induced neurodegeneration through the restoration of lysosomal function

“…Compared with control cells, α-syn overexpression triggered LMP as evidenced by a 3 to 5-fold increase in the cytosolic activity of CatD (Figure 1g), β-hexosaminidase (Figure 1h), and ACP2 (Figure 1i) in M17-WTsyn cells, which were all rescued by aNPs treatment as measured in lysosome-free cytosolic fractions (Figure 1g-i), suggesting a direct beneficial effect of aNPs on lysosomal membrane integrity in M17-WTsyn cells. Further supporting the occurrence of LMP in M17-WTsyn cells (Burbidge et al, 2021) and the impact of aNPs, we measured galectin puncta at leaky lysosomes as a highly sensitive assay for LMP. Accordingly, M17-WTsyn cells exhibited increased Gal3-positive vesicles per cell, using a mCherry-…”

Autophagy-Related Pathways in Vesicular Unconventional Protein Secretion

“…TRIM16 is known to interact with GABARAP yet not with sequestosome-1/p62 ( Mandell et al, 2014 ), a classical degradative autophagy receptor ( Bjørkøy et al, 2005 ), which suggests that certain receptor/adaptor proteins potentially guide cargos to the secretory route avoiding disposal mechanism. A recent study additionally shows that the galectin3-mediated UPS of α-synuclein is dependent on TRIM16 ( Burbidge et al, 2021 ).…”

“…Additionally, Atg5, Atg7, Atg8 and Atg12 have been shown to be required for Acb1 secretion in S. cerevisiae ( Duran et al, 2010 ). ATG16L1, interacting with the ATG12-ATG5 conjugate, also participates in the UPS of IL-1β, PARK7/DJ-1, and α-synuclein in mammalian cells ( Kimura et al, 2017 ; Urano et al, 2018 ; Burbidge et al, 2021 ). The respective secretions of chemokine C-X-C motif ligand 8 (CXCL8), leukemia inhibitory factor (LIF), and family with sequence similarity 3 member C (FAM3C) were each enhanced when low-autophagy melanoma cells were treated with the autophagy-inducing tat-BECN1 peptide and reduced when ATG7 was silenced in high-autophagy cells ( Kraya et al, 2015 ).…”

Propagation of Parkinson's disease by extracellular vesicle production and secretion

“…The connection between the inhibition of the autophagy-lysosomal pathway (ALP) and higher levels of α-synuclein cargo in EVs has also been shown [ 92 ]. Endocytosis, of α-synuclein fibrils can induce lysosomal stress and induce autophagic responses that lead to the release of α-synuclein from the cell [ 93 ]. Furthermore, it has been found that the secretion of α-synuclein in the context of impeded lysosomal function is mediated by LGALS3, which codes for the protein galectin 3.…”

“…Furthermore, it has been found that the secretion of α-synuclein in the context of impeded lysosomal function is mediated by LGALS3, which codes for the protein galectin 3. Following treatment with lysosomal acidification inhibitors, Baf-A1, and Chloroquine, EVs were found to contain α-synuclein as well as galectin 3, a process that is increased when disturbance of the ALP occurs [ 93 ]. Treatment with α-synuclein fibrils resulted in observations of endogenously expressed SNCA and exogenous SNCA fibrils colocalising with galectin-positive intracellular vesicles and, ultimately, in EVs released from cells.…”