Intracellular protein trafficking and secretion of proteins into the extracellular environment are sequential and tightly regulated processes in eukaryotic cells. Conventionally, proteins that are bound for secretion harbor an N-terminal signal peptide that guides their movement from the endoplasmic reticulum (ER) and Golgi apparatus to the exterior of the cell. However, some proteins can bypass this system using unconventional mechanisms, including direct translocation across the plasma membrane, transporter-mediated secretion, and intracellular vesicle-mediated exocytosis. Unconventionally secreted proteins have been implicated in inflammation, neurodegeneration, and cancer. Understanding the mechanisms that drive unconventional protein secretion can, therefore, reveal novel therapeutic targets and approaches.
Autophagy, traditionally known for degrading and recycling cytoplasmic components to maintain cellular homeostasis, has recently emerged as a novel route for the unconventional secretion of leaderless proteins. In a previous study researchers from Doshisha University, Japan, revealed that PARK7/DJ-1 – a Parkinson-associated protein renowned for its antioxidative function and mitochondrial protection – utilizes an autophagy-based mechanism for stress-induced secretion. Despite this breakthrough, the molecular events and regulatory mechanisms governing this unconventional secretion pathway remained largely undefined. To address these gaps, the same research team has uncovered critical insights into the intracellular trafficking and extracellular release of PARK7.
Their latest work delineates how this multifunctional protein is directed to the extracellular environment in response to cellular stress. The researchers found that 6-OHDA treatment in human cervical carcinoma cells induced a dose-dependent increase in PARK7 secretion. Notably, this increase was unaffected by blocking the conventional ER-to-Golgi trafficking protein pathway and the exosomal release pathway, thus confirming that PARK7 was released via an unconventional mechanism. Additionally, 6-OHDA treatment also led to a dose-dependent increase in the autophagosomal marker (LC3B) and a corresponding decrease in the autophagic substrate (p62 SQSTM1), indicating activation of autophagy. Notably, blocking the early stage of autophagy significantly reduced 6-OHDA-induced LC3B-II formation and PARK7 secretion.
The decreasing effect on p62 is particularly important for some cellular defence mechanisms against oidative stress. P62, indeed can bind to Keap1, a natural suppressor of the Nrf-2 trasncription factor, either directly or indirectly and disrupt the interaction between Keap1 and Nrf-2. This disrupts the normal regulation of Nrf-2 and leads to its stabilization and activation in a non-canonical manner. Nrf-2 drives the expression of antioxidant proteins (e.g. HO-1, TxR1, SOD2) that protects cells against programmed cell death under stressful stimuli. This mechanism could be responsible for the activation of protective mechanisms in neuronal cells (also in substrantia nigra neurons) upon exposure to dopaminergic toxins like manganese, chlorurated solvents and the same 6-OHDA used to replicate in vitro what happens in vivo.
Additionally, treatment with an antioxidant agent nullified 6-OHDA treatment-induced oxidative stress, thereby, reducing autophagy induction and PARK7 secretion. Conversely, treatment with rapamycin, an inducer of autophagy, increased LC3B-II levels and enhanced PARK7 secretion, indicating that autophagy induction was essential for PARK7 secretion. Notably, inhibiting other protein degradation pathways, such as the ubiquitin–proteasome system, did not affect PARK7 secretion. Delving deeper, the researchers found that blocking autophagosome-lysosome fusion abrogated 6-OHDA-induced PARK7 secretion; and that the SNARE complex, a group of membrane-bound proteins that mediate vesicle fusion in endocytic and secretory pathways, was required for the 6-OHDA-induced extracellular secretion of PARK7.
The researchers also identified specific sequence motifs in PARK7 that were selectively recognized by chaperones, thereby recruiting PARK7 to intact lysosomes, which ultimately fused with autophagosomes to form “secretory autolysosomes.” Overall, these findings provide novel insights into unconventional mechanisms that drive protein secretion. The study opens avenues for developing targeted therapies that can regulate the levels of PARK7 in Parkinson disease and related conditions. Furthermore, PARK7 and related proteins hold promise as biomarkers for early disease detection, enabling timely interventions before symptoms progress.
- Edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
Scientific references
Dash BK et al. PNAS USA 2025; 122(19):e2414790122.
Dash BK et al. Redox Exp Med. 2022; 2022:R96–R115.
Hou X, Watzalavik J et al. J Mol Biol. 2020; 432(8):2651.
