Mass Spectrometry of Minimal Change Disease and Focal Segmental Glomerulosclerosis.

INTRODUCTION: The relation between minimal change disease (MCD) and primary focal segmental glomerulosclerosis (FSGS) (pFSGS) remains debatable, and the recent discovery of antinephrin and antislit antibodies, supports the hypothesis of a continuum disease. However, the overall expression of podocyte proteins in these disease states is not known. METHODS: In this observational proteomic study, we used laser microdissection and mass spectrometry (LMD/MS) to determine the proteomic profile of podocyte proteins in kidney biopsies of adult patients with MCD (n = 6), pFSGS (n = 7), secondary FSGS (sFSGS) (n = 10), and genetic FSGS (gFSGS) (n = 6). Time-zero kidney transplant (T0) (n = 6), IgA nephropathy (IgAN) (n = 5) and membranous nephropathy (MN) (n = 8) served as nonproteinuric and proteinuric controls, respectively. MS results were expressed as total spectrum representing the relative abundance of the specific protein. In addition, immunofluorescence (IF) staining assessing nephrin (NPHS1) and podocin (NPHS2) were performed. RESULTS: LMD/MS show moderate baseline total spectrum of podocyte proteins, NPHS1, NPHS2, CD2-associated protein, alpha-actinin-2, inverted formin-2 (INF2), and dystroglycan 1 (DAG1) in T0 and MN controls. In contrast, there was a significant loss of all 6 podocyte proteins in MCD, pFSGS, sFSGS, and gFSGS but not in MN cases. IF staining confirmed podocyte loss of NPHS1 and NPHS2 in MCD, pFSGS, sFSGS, and gFSGS but not in T0 or MN. CONCLUSION: LMD/MS and IF staining show significant and comparable loss of podocyte proteins involving the slit diaphragm and actin cytoskeleton in MCD, pFSGS, sFSGS, and gFSGS suggesting a common final pathway of podocyte injury. Surprisingly, despite similar degrees of proteinuria, MN was not associated with significant loss of podocyte proteins.