α-Synuclein ER Stress Antibody Sampler Kit #26791

Secretory and transmembrane proteins are synthesized on polysomes and translocate into the endoplasmic reticulum (ER) where they are often modified by the formation of disulfide bonds, amino-linked glycosylation and folding. The ER contains a pool of molecular chaperone proteins, including calnexin and BiP. Calnexin is an ER membrane, calcium-binding protein that retains newly synthesized glycoproteins inside the ER to ensure proper folding and quality control (1,2). Irregular protein folding within the ER increases BiP synthesis, which binds misfolded proteins to prevent them from forming aggregates and to assist them to refold properly (3). ER homeostasis disruptions lead to the accumulation of unfolded proteins. The ER has developed an adaptive mechanism called the unfolded protein response (UPR) to counteract compromised protein folding (4). The protein kinase-like endoplasmic reticulum kinase (PERK) eIF2α kinase is an ER resident transmembrane protein that couples ER stress signals to translation inhibition. ER stress increases PERK activity, which phosphorylates eIF2α to reduce protein translation (5,6). During ER stress, the level of CHOP expression is also elevated and CHOP functions to mediate programmed cell death (7).

Parkinson’s disease (PD), the second most common neurodegenerative disease after Alzheimer’s disease (AD), is a progressive movement disorder characterized by rigidity, tremors, and postural instability. The pathological hallmark of PD is progressive loss of dopaminergic neurons in the substantia nigra of the ventral midbrain and the presence of intracellular Lewy bodies in surviving neurons of the brain stem (8). α-Synuclein, a 140 amino acid protein expressed abundantly in the brain, is a major component of aggregates found in Lewy bodies (9). Recent evidence suggests that aggregation of α-Synuclein induces ER stress while also reducing the ability of neurons to respond to protein misfolding through activation of the UPR. This increases ER fragmentation, impairs ER-to-Golgi trafficking and the maturation of proteins, and can induce lysosomal dysfunction downstream (10). β-glucocerebrosidase (GCase) is a lysosomal enzyme that catalyzes the hydrolysis of glucocerebroside into free ceramide and glucose (11). Lysosomal breakdown of glucocerebroside is required for complex lipid cellular metabolism and proper cellular membrane turnover (12). Immature and misfolded forms of GCase can accumulate in the ER as a result of α-Synuclein aggregation, while the GCase that does reach the lysosome exhibits reduced enzymatic activity (10).

Mutations in GBA, the gene that encodes GCase, are the most common genetic risk factor for PD and another synucleinopathy, dementia with Lewy bodies (DLB) (10,13). These mutations can inhibit chaperone-mediated autophagy (CMA), a pathway that contributes to lysosomal function and ER homeostasis (14). Autophagy is a catabolic process for the autophagosomic-lysosomal degradation of bulk cytoplasmic contents (15,16). There are three classes of autophagy in mammalian cells: CMA, macroautophagy, and microautophagy. Mislocalization of GCase due to PD-linked mutations has been shown to increase accumulation of proteins that are degraded by CMA, including α-Synuclein, inducing a positive feedback loop that promotes further aggregation (14,17).

The blockage of the CMA pathway can lead to a compensatory increase in macroautophagy. Macroautophagy is classified by the formation of an autophagosome, which targets cargo for degradation through fusion with a lysosome (17,18). Formation of the autophagosome involves a ubiquitin-like conjugation system in which Atg12 is covalently bound to Atg5 and targeted to autophagosome vesicles (19-21).