A Glutaminyl Cyclase-Catalyzed Alpha-Synuclein Modification Identified In Human Synucleinopathies

Parkinson's disease (PD) is a progressive neurodegenerative disorder that is neuropathologically characterized by degeneration of dopaminergic neurons of the substantia nigra (SN) and formation of Lewy bodies and Lewy neurites composed of aggregated alpha-synuclein. Proteolysis of alpha-synuclein by matrix metalloproteinases was shown to facilitate its aggregation and to affect cell viability. One of the proteolysed fragments, Gln79-alpha-synuclein, possesses a glutamine residue at its N-terminus. We argue that glutaminyl cyclase (QC) may catalyze the pyroglutamate (pGlu)79-alpha-synuclein formation and, thereby, contribute to enhanced aggregation and compromised degradation of alpha-synuclein in human synucleinopathies. Here, the kinetic characteristics of Gln79-alpha-synuclein conversion into the pGlu-form by QC are shown using enzymatic assays and mass spectrometry. Thioflavin T assays and electron microscopy demonstrated a decreased potential of pGlu79-alpha-synuclein to form fibrils. However, size exclusion chromatography and cell viability assays revealed an increased propensity of pGlu79-alpha-synuclein to form oligomeric aggregates with high neurotoxicity. In brains of wild-type mice, QC and alpha-synuclein were co-expressed by dopaminergic SN neurons. Using a specific antibody against the pGlu-modified neo-epitope of alpha-synuclein, pGlu79-alpha-synuclein aggregates were detected in association with QC in brains of two transgenic mouse lines with human alpha-synuclein overexpression. In human brain samples of PD and dementia with Lewy body subjects, pGlu79-alpha-synuclein was shown to be present in SN neurons, in a number of Lewy bodies and in dystrophic neurites. Importantly, there was a spatial co-occurrence of pGlu79-alpha-synuclein with the enzyme QC in the human SN complex and a defined association of QC with neuropathological structures. We conclude that QC catalyzes the formation of oligomer-prone pGlu79-alpha-synuclein in human synucleinopathies, which may-in analogy to pGlu-A beta peptides in Alzheimer's disease-act as a seed for pathogenic protein aggregation.