A NOVEL SINGLE‑MOLECULE ARRAY ASSAY IDENTIFIES INCREASED PLASMA ECTO‑GPR37 IN PARKINSON’S DISEASE

Parkinson’s disease (PD) lacks robust, minimally invasive biomarkers that reflect disease-relevant pathophysiology and suitable for repeated sampling. GPR37 is an orphan G protein-coupled receptor highly expressed in the central nervous system that undergoes proteolytic cleavage, releasing its extracellular N-terminal ectodomain (ecto-GPR37). We previously identified ecto-GPR37 as a cerebrospinal fluid biomarker associated with PD and disease progression, but the clinical applicability of CSF measurements is limited by the invasive nature of lumbar puncture. Here, we developed and analytically validated a custom ultrasensitive digital immunoassay based on single-molecule array (SiMoA) technology to quantify ecto-GPR37 in human plasma. The assay employs paired antibodies targeting non-overlapping N-terminal epitopes and achieves reliable quantification at picogram range level, below the detection limits of conventional ELISAs. Applying this assay to plasma samples from patients with PD and age-matched neurological controls (NC), we show for the first time that ecto-GPR37 is detectable in plasma of patients. Interestingly, these findings are consistent with previous CSF-based observations and support the biological relevance of GPR37 processing in PD context. Our results provide proof-of-concept that an ultrasensitive digital immunoassay can reliably quantify ecto-GPR37 in human plasma. Accordingly, plasma ecto-GPR37 emerges as a promising candidate that might be included in multimodal biomarker panels for PD, with potential utility for early diagnosis, prognostic assessment, patient stratification, or longitudinal monitoring.