alpha-synuclein and Ochronosis

Alkaptonuria (AKU) is an ultra-rare inborn error of metabolism characterized by homogentisic acid (HGA) accumulation due to a deficient activity of the homogentisate 1.2-dioxygenase (HGD) enzyme. This leads to the production of dark pigments that are deposited onto connective tissues, a condition named 'ochronosis' and whose mechanisms are not completely clear. Recently, the potential role of hitherto unidentified proteins in the ochronotic process was hypothesized, and the presence of Serum Amyloid A (SAA) in alkaptonuric tissues was reported, allowing the classification of AKU as a novel secondary amyloidosis.. Gel electrophoresis, Western Blot, Congo Red-based assays and electron microscopy were used to investigate the effects of HGA on the aggregation and fibrillation propensity of amyloidogenic proteins and peptides [Aβ(1-42), transthyretin, atrial natriuretic peptide, α-synuclein and SAA]. LC/MS and in silico analyses were undertaken to identify possible binding sites for HGA (or its oxidative metabolite, a benzoquinone acetate or BQA) in SAA.. We found that HGA might act as an amyloid aggregation enhancer in vitro for all the tested proteins and peptides in a time- and dose- dependent fashion, and identified a small crevice at the interface between two HGD subunits as a candidate binding site for HGA/BQA.. HGA might be an important amyloid co- component playing significant roles in AKU amyloidosis.. Our results provide a possible explanation for the clinically verified onset of amyloidotic processes in AKU and might lay the basis to setup proper pharmacological approaches to alkaptonuric ochronosis, which are still lacking.

Topics: Alkaptonuria; alpha-Synuclein; Amyloid beta-Peptides; Amyloidogenic Proteins; Amyloidosis; Atrial Natriuretic Factor; Binding Sites; Connective Tissue; Homogentisate 1,2-Dioxygenase; Homogentisic Acid; Humans; Ochronosis; Oxidation-Reduction; Prealbumin; Protein Aggregation, Pathological; Serum Amyloid A Protein