The Spectrum of Minimal Change Disease/Focal Segmental Glomerulosclerosis: From Pathogenesis to Proteomic Biomarker Research

Abstract

Podocyte injury plays a central role in both focal segmental glomerulosclerosis (FSGS) and minimal change disease (MCD). Pathogenic mechanisms are diverse and incompletely understood, partially overlap between FSGS and MCD, and are not reflected by kidney biopsy. In order to optimize the current variable response to treatment, personalized management should rely on pathogenesis. One promising approach involves identifying biomarkers associated with specific pathogenic pathways. With the advancement of technology, proteomic studies could be a valuable tool to improve knowledge in this area and define valid biomarkers, as they have in other areas of glomerular disease. This work attempts to cover and discuss the main mechanisms of podocyte injury, followed by a review of the recent literature on proteomic biomarker studies in podocytopathies. Most of these studies have been conducted on biofluids, while tissue proteomic studies applied to podocytopathies remain limited. While we recognize the importance of non-invasive biofluid biomarkers, we propose a sequential approach for their development: tissue proteomics could first identify proteins with increased expression that may reflect underlying disease mechanisms; subsequently, the validation of these proteins in urine or plasma could pave the way to a diagnostic and prognostic biomarker-based approach.

Keywords: biomarkers; focal segmental glomerulosclerosis; minimal change disease; podocytopathies; proteomics.

Figure 1

Schematic representation of podocyte pathogenic genes. Gene variants are grouped according to cellular structure. PLCE1, phospholipase C epsilon 1; TRPC 5/6, transient receptor potential channel 5/6; NPHS1, nephrin; NPHS2, podocin; CD2AP, CD2-associated protein; NEPH1, Kin of IRRE-like protein 1; MYO1E, myosin 1E; SYNPO, synaptopodin; INF2, inverted-formin2; ACTN4, alpha actinin 4; ITGB4, integrin ß4; ITGA3, integrin α3; ARGHA24, Rho-GTPase-activating protein 24; ARGHDIA, Rho-GDP-dissociation inhibitor 1; DLC1, Rho GTPase-activating protein 7; KANK 1/4/5, KN motif and ankyrin repeat domains 1/4/5; LAMB2, laminin subunit beta 2; COL4A3/4/5, collagen type IV aplha 3/4/5 chain; APOL1, Apolipoprotein A1; EMP2, epithelial membrane protein2; COQ 2/6/8, coenzime Q2/6/8; PDSS2, decaprenyl-diphosphate synthase subunit 2; MTTL1, mitochondrially encoded tRNA leucine 1; ADCK4, coenzyme Q8B; SGLP1, sphingosine-1-phosphate lyase 1; ALG1, chitobiosyldiphosphodolichol beta-mannosyltransferase; SCARB2, lysosomal integral membrane protein-2; SMARCLA1, SWI/SNF-related matrix-associated actin-dependent regulator of chromatin subfamily A-like protein 1; LMX1B, LIM homeobox transcription factor 1-beta; NXF5, nuclear RNA Export Factor 5; WT1, Wilms tumor 1; LMNA, lamin A/C; PAX2, paired box gene 2; MAFB, transcriptor factor MafB. This figure was created with BioRender.com.

Figure 2

Schematic representation of immune-mediated podocyte injury. Podocytes interact with immune cells through major histocompatibility complex type I (MHC I) and II (MHC II) but also possess co-stimulatory molecules for B lymphocytes (CD86) and for T lymphocytes (CD80). Imbalance between effector and regulatory T lymphocytes, interleukin/cytokine synthesis, and B cell-mediated antibody synthesis against podocyte structures can occur due to podocyte interaction with immune cells. Toll-like receptors (TLRs) can detect signals like pathogen-related molecular patterns (PAMPs) and damage-related molecular pattern (DAMPs) both from outside (e.g., lipopolysaccharide, lipids, and proteins) and within the cell (e.g., free nucleic acids). Upon binding TLRs, increased transcription of inflammatory genes (NF-kß, IRF3, IRF7, and AP1) and inflammasome activation are mediated through intracellular signaling proteins such as TIRAP, MyD88 and MAPK, ultimately resulting in cytoskeleton rearrangement and apoptosis. Interleukin receptors (IL Rs) and interferon receptors (INF Rs) also increase the transcription of inflammatory and profibrotic factors through the JAK/STAT signaling pathway. Complement-mediated podocyte injury can occur due to sub-lytic amounts of membrane attack complex (MAC). The activation of the NF-kß pathway and a rapid increase in intracellular Ca²⁺ induce NFk-ß-mediated inflammatory response, mitochondrial stress, oxidative stress, and endoplasmic reticulum stress. The lack of inhibitor factors such as CD55 or complement factor H (FH) can trigger and intensify MAC production. This figure was created with BioRender.com.

Figure 3

Schematic representation of mechanical stress-related podocyte injury. Angiotensin II receptor 1 (AT1 R) upon stimulation causes increased intracellular calcium influx through transient receptor potential channel 6 (TRPC6), followed by increased calcineurin activity and increased degradation of synaptopodin, an actin-binding protein. Cytoskeletal rearrangements are controlled by Rho GTPases: cell division control protein 42 (CDC42), Rac family small GTPase 1 (RAC1), and Ras homolog family member A (RhoA). Mechanical stress is sensed and transmitted by B integrins to erythrocyte membrane protein band 4.1 like 5 (EPB41L5). EPB41L5 activates intracellular pathways which regulate the translation of proteins that influence RhoA activity and cytoskeletal rearrangement. Yes-associated protein (YAP) is a transcription co-regulator factor stimulated by EPBB41L5 activity. YAP nuclear translocation results in the translation of proteins that regulate podocyte Rho GTPase activity. Membrane proteins like TRPC5/6 also can directly regulate the activity of Rho GTPases, other membrane proteins may be also involved in Rho GTPase regulation (dashed arrows). Permeability factors (soluble urokinase plasminogen activator receptor, suPAR; hemopexin, HPX; and cardiotrophin-like cytokine factor 1, CLCF1) interfere with FA; in particular, suPAR binds and activates αvß3 B integrin, further modulating cytoskeleton rearrangements via CDC42 and RAC1. APOL1 expression increases the linking of suPAR. This figure was created with BioRender.com.

Figure 4

Differentially expressed proteomic signatures of patients with FSGS, patients with MCD, and healthy controls. Reprinted with permission from Bărar et al. [76].

Figure 5

Proposed approach for biomarker development.