Sigma-1 receptor (Sig1R) is an endoplasmic reticulum (ER) chaperone protein involved in regulating ER function, cellular stress responses, and autophagy, and disturbed Sig1R function has been associated with neurodegenerative and neuropsychiatric disorders, including Alzheimer's disease and amyotrophic lateral sclerosis. Although Sig1R has been implicated in secretory pathways and detected extracellularly, its direct presence in isolated extracellular vesicles (EVs) has not been clearly established. In this study, we aimed to confirm Sig1R in isolated EVs by co-expressing it alongside tetraspanin EV markers. Expression of fluorescently tagged Sig1R together with fluorescent protein-labeled tetraspanins CD9, CD63, and CD81 in both cells and isolated EVs was verified using live-cell imaging, emission spectrum measurements, and Western blotting. Efficient expression of these proteins in cells was achieved by using the MultiBacMam expression system and their presence in EVs was detected through advanced TIRF-based multi-well single-particle EV analysis. We observed significantly higher levels of Sig1R in CD63- and CD9-labeled EVs in comparison with CD81-labeled EVs. The obtained data suggest that Sig1R may be released into the extracellular space via exocytotic pathways linked to exosome secretion. The presence of Sig1R in exosomes underscores its potential as a biomarker in neurological disorders, emphasizing the need for further exploration of its diagnostic and other applications.
Vavers et al. (Wed,) studied this question.