Hemoglobin-α-synuclein complex exhibited age-dependent alterations in the human striatum and peripheral RBCs

Hemoglobin (Hb) is the main component of red blood cells, and the majority of alpha-synuclein in the blood is present inside them. In Parkinson's disease and aging human brains, the levels of mitochondrial neuronal hemoglobin are significantly decreased in neurons where α-Syn has accumulated, suggesting a relationship between these two proteins. In the present study, we demonstrate that a complex comprising α-Syn and Hb exists in both peripheral RBCs and brain tissue in aging humans using Co-Immunoprecipitation and a newly established ELISA assay. The nHb^α-Syn levels in the mitochondrial fraction of the human striatum reduced in an age-dependent manner which was negatively correlated with the nHb^α-Syn complex levels in the cytoplasmic extraction. In contrast, no age-related alteration in the nHb^α-Syn complex was found in the above subcellular fractions of the human cerebellum. Furthermore, the nHb^α-Syn complex levels in RBCs increased with age, which agrees with the changes observed in the cytoplasm of the human striatum. Meanwhile, sequential window acquisition of all theoretical mass spectra-MS (SWATH-MS) was used to confirm the presence of the Hb^α-Syn complex in RBCs and also found that nHb^α-Syn levels increased notably in PD patients compared to healthy controls (HC). Receiver operating characteristic (ROC) curves indicated that Hb^α-Syn in PD patients was distinct from HC, with areas under the curve (AUCs) of 0.875. The above results suggest that the alteration in nHb^α-Syn levels in RBCs reflects those in select regions of the human brain, thereby providing a possibility of using Hb^α-Syn as a biomarker to diagnose PD.