Amyloidogenicity of Immunoglobulin Light Chains

Abstract

Systemic light chain amyloidosis (AL) is a rare protein aggregation disease. It usually strikes in the wake of myeloma, which affects plasma cells in the adaptive immune system. During plasma cell development, the immunoglobulin light chain (LC) genes undergo several rearrangements that leave each clone with a unique protein sequence. The produced monoclonal light chains (LC) are deposited as amyloid in AL but not in Multiple Myeloma (MM) patients. We aim to elucidate the biophysical basis of this difference. In both diseases large amounts of soluble LC are secreted into circulation and excreted with urine. Hence we hypothesize that amyloidogenicity depends on the amyloid formation propensity of the individual LC sequences rather than being a result of different LC concentrations being present in both diseases, which may also alter susceptibilities to the green tea phenol Epigallocatechin-3-gallate (EGCG).To test this hypothesis we used a simple diafiltration approach to isolate LC from AL and MM patient¯s urine, including only cases with albuminuria less than 5% of total proteinuria. We monitored their aggregation under physiological conditions in presence and absence of EGCG over a time course of three weeks in a Thioflavin T assay and compared the aggregate sizes at different time points by semi-denaturing SDS-PAGE and filter retardation assay. We probed stabilities of native and of aggregated LC by Guanidine and thermal denaturation and imaged aggregate morphologies by atomic force microscopy (AFM).Each individual LC displayed unique characteristic aggregation kinetics. However, there were no systematic differences between proteins from MM and AL patients. EGCG treatment accelerated the formation of large aggregates that are partially stable against SDS denaturation. By determining the sequence of the LC protein via MS/MS we hope to establish a correlation between sequences, aggregation propensities and clinical parameters in AL and MM.