Characterization of C3 in C3 glomerulopathy

Area of Science:

• Nephrology
• Complement System Biology
• Mass Spectrometry in Proteomics

Background:

• C3 glomerulopathy (C3G) is characterized by complement alternative pathway overactivity and C3 deposition in glomeruli.
• C3G subtypes include C3 glomerulonephritis (C3GN) and dense deposit disease (DDD), with C3 as the primary glomerular complement protein.
• The precise composition of C3 breakdown products in C3G has remained largely unknown.

Purpose of the Study:

• To elucidate the composition of C3 deposits in C3G by analyzing C3 breakdown products.
• To compare the C3 fragment profiles between C3GN and DDD subtypes.

Main Methods:

• Analysis of six cases each of C3GN and DDD using laser microdissection and mass spectrometry.
• Determination of relative abundance of C3 fragments by analyzing amino acid sequences of C3 spectra.

Main Results:

• C3dg was identified as the predominant C3 cleavage product in both C3GN and DDD, showing the highest amino acid coverage.
• Amino acid sequences mapping to C3α (C3α1 and α2) and C3β were detected, representing iC3b.
• Absence of amino acids mapping to C3a and C3f, with similar C3 spectra observed in both C3GN and DDD.

Conclusions:

• The abundance of C3dg suggests active C3b deposition in the glomerulus, likely via thioester binding to the glomerular basement membrane and endothelial glycocalyx.
• Regulatory protein-mediated inactivation of C3b leads to iC3b generation, with subsequent cleavages predominantly leaving C3dg.
• These findings provide new insights into the complement cascade's role in C3G pathogenesis.