α-Synuclein: Multiple pathogenic roles in trafficking and proteostasis pathways in Parkinson's disease

Abstract

Parkinson's disease (PD) is a common age-related neurodegenerative disorder characterized by the loss of dopaminergic neurons in the midbrain. A hallmark of both familial and sporadic PD is the presence of Lewy body inclusions composed mainly of aggregated α-synuclein (α-syn), a presynaptic protein encoded by the SNCA gene. The mechanisms driving the relationship between α-syn accumulation and neurodegeneration are not completely understood, although recent evidence indicates that multiple branches of the proteostasis pathway are simultaneously perturbed when α-syn aberrantly accumulates within neurons. Studies from patient-derived midbrain cultures that develop α-syn pathology through the endogenous expression of PD-causing mutations show that proteostasis disruption occurs at the level of synthesis/folding in the endoplasmic reticulum (ER), downstream ER-Golgi trafficking, and autophagic-lysosomal clearance. Here, we review the fundamentals of protein transport, highlighting the specific steps where α-syn accumulation may intervene and the downstream effects on proteostasis. Current therapeutic efforts are focused on targeting single pathways or proteins, but the multifaceted pathogenic role of α-syn throughout the proteostasis pathway suggests that manipulating several targets simultaneously will provide more effective disease-modifying therapies for PD and other synucleinopathies.

Keywords: Parkinson’s disease; autophagy; protein trafficking; synuclein.

Figure 1.

Unfolded protein response (UPR). The three major UPR signaling pathways (1) protein kinase-like ER kinase (PERK), (2) inositol-requiring enzyme 1 (IRE1), and (3) activating transcription factor 6 (ATF6). Each pathway is inhibited by direct binding of GRP78. Under endoplasmic reticulum (ER) stress, GRP78 will dissociate and assist with misfolded proteins inside the ER lumen, resulting in UPR activation. (1) PERK activation results in downstream phosphorylation of eukaryotic initiation factor 2 (elF2α). Inhibition of elF2α decreases the translation of new proteins, thereby assisting in reducing burden to the ER. Further, activating transcription factor 4 (ATF4) mRNA is selectively translated because of PERK activation. ATF4 then translocates to the nucleus, where it assists in the regulation of adaptive stress response genes. (2) IRE1 activation results in X-box-binding protein 1 (XBP1) mRNA splicing and subsequent translation of XBP1s. XBP1 protein translocates to the nucleus and assists in the activation of genes involved in ER-associated degradation (ERAD) and protein folding. (3) Activation of ATF6 allows it to translocate to the Golgi. It is then cleaved (ATF6-N) and serves as a transcriptional activator for ERAD machinery and other chaperones important for maintaining ER homeostasis. Adapted from Walter and Ron 2011. Created with BioRender.com.

Figure 2.

Alternate handling of GCase in the endoplasmic reticulum (ER). (A) GCase is first translocated into the ER to be folded. (B) Under homeostatic conditions, properly folded GCase undergoes lysosomal integral membrane protein 2 (LIMP-2)–mediated vesicular transport to the Golgi and eventually to lysosomes. (C) If GCase misfolding occurs, ER chaperones (CANX/CALR) will assist in refolding. (D) If it remains misfolded, GCase will exit the folding cycle and is typically degraded by the proteasome via ER associated degradation (ERAD). (E) If ER stress persists, the unfolded protein response (UPR) is initiated. (F) In Parkinson’s disease patient neurons that harbor a wild-type SNCA triplication mutation, endogenous α-syn accumulation causes wild-type GCase to accumulate and form insoluble aggregates in the ER. Toxic GCase aggregation is insufficient to trigger the UPR in these neurons, despite their ability to respond to chemical UPR inducers. This suggests a deficiency in their ability to recognize and handle misfolded proteins in the ER. Further, misfolded wild-type GCase is not engaging with ERAD in these neurons. (G) ER-phagy is an additional quality control mechanism that selectively degrades regions of the ER via the autophagic pathway. Ongoing work aims to determine the contributions of ER-phagy in degrading ERAD-resistant cargo. Created with BioRender.com.

Figure 3.

Potential α-synuclein (α-syn) interference mechanisms of ER-phagy. (A) α-Syn has been reported to associate with endoplasmic reticulum (ER) chaperones that assist in ER quality control and ER-phagy (CANX, GRP78, GRP94). This may result in reduced chaperone output during ER stress and impair selective ER turnover. (B) Given its affinity for charged, curved membranes, it is possible that α-syn disrupts ER-phagy receptor-autophagic machinery interactions via LC3-interacting regions (LIRs) on the receptor and LC3 molecules on the growing phagophore. This could disrupt the proper turnover of select cargo and the ER membrane upstream of autophagosome-lysosome fusion. (C) α-Syn impairs late-stage autophagy through blockage of autophagosome-lysosome fusion. This occurs via aberrant interaction with the SNARE protein ykt6. Created with Biorender.com.

Figure 4.

Intracellular protein trafficking phenotypes in α-synuclein (α-syn) accumulation Parkinson’s disease models. Accumulation of α-syn intracellularly is associated with proteostatic change at multiple stages of secretory trafficking. This includes endoplasmic reticulum (ER) function, morphology, and quality control mechanisms; ER-Golgi trafficking; Golgi and trans-Golgi network function and morphology; hydrolase trafficking and lysosomal function; the endolysosomal pathway; and the autophagic-lysosomal pathway. Created with BioRender.com.