Metabolic pathways and immunometabolism in rare kidney diseases

Abstract

Objectives: To characterise renal tissue metabolic pathway gene expression in different forms of glomerulonephritis.

Methods: Patients with nephrotic syndrome (NS), antineutrophil cytoplasmic antibody-associated vasculitis (AAV), systemic lupus erythematosus (SLE) and healthy living donors (LD) were studied. Clinically indicated renal biopsies were obtained at time of diagnosis and microdissected into glomerular and tubulointerstitial compartments. Microarray-derived differential gene expression of 88 genes representing critical enzymes of metabolic pathways and 25 genes related to immune cell markers was compared between disease groups. Correlation analyses measured relationships between metabolic pathways, kidney function and cytokine production.

Results: Reduced steady state levels of mRNA species were enriched in pathways of oxidative phosphorylation and increased in the pentose phosphate pathway (PPP) with maximal perturbation in AAV and SLE followed by NS, and least in LD. Transcript regulation was isozymes specific with robust regulation in hexokinases, enolases and glucose transporters. Intercorrelation networks were observed between enzymes of the PPP (eg, transketolase) and macrophage markers (eg, CD68) (r=0.49, p<0.01). Increased PPP transcript levels were associated with reduced glomerular filtration rate in the glomerular (r=-0.49, p<0.01) and tubulointerstitial (r=-0.41, p<0.01) compartments. PPP expression and tumour necrosis factor activation were tightly co-expressed (r=0.70, p<0.01).

Conclusion: This study demonstrated concordant alterations of the renal transcriptome consistent with metabolic reprogramming across different forms of glomerulonephritis. Activation of the PPP was tightly linked with intrarenal macrophage marker expression, reduced kidney function and increased production of cytokines. Modulation of glucose metabolism may offer novel immune-modulatory therapeutic approaches in rare kidney diseases.

Keywords: granulomatosis with polyangiitis; lupus nephritis; systemic vasculitis.

Figure 1

Bar graphs of metabolic pathways showing disease-specific differential gene expression within the glomerular compartment in the discovery cohort (A). Bar graph of pentose phosphate pathway gene expression within the tubulointerstitial compartment in the discovery cohort (B). Differences in composite gene expression were compared in the discovery cohort between hypoxia-induced factor 1-alpha (HIF-1α) glycolytic targets and HIF-1α independent glycolytic enzymes in the glomerular compartment (C). *p<0.05; **p<0.01; ***p<0.005; ****p<0.001, NS, not significant. AAV, antineutrophil cytoplasmic antibody-associated vasculitis; TCA, tricarboxylic acid.

Figure 2

Comparison of differences in glomerular gene expression (log2 mRNA levels) among selected metabolic isozymes in patients with nephrotic syndrome, antineutrophil cytoplasmic antibody-associated vasculitis and healthy donors in the discovery cohort. *p<0.05; **p<0.01; ***p<0.005; ****p<0.001, NS, not significant.

Figure 3

Pentose phosphate pathway (PPP) gene expression is differentially regulated in a variety of glomerulonephritis and is associated with impaired kidney function. PPP gene expression differences among groups collected within the discovery cohort in the glomerular (A) and tubulointerstitial (B) compartments. Comparison of PPP gene expression within the glomerular compartment in the discovery cohort between patients with antineutrophil cytoplasmic antibody-associated vasculitis (AAV) or systemic lupus erythematosus (SLE) categorised by glucocorticoid use at the time of biopsy (C). Validation of PPP gene expression differences within the glomerular compartment in the validation cohort (D). Correlation between PPP expression and glomerular filtration rate in the glomerular (E) and tubulointerstitial (F) compartments in the discovery cohort. *p<0.05; **p<0.01; ***p<0.005; ****p<0.001, NS, not significant. FSGS, focal segmental glomerulosclerosis; MCD, minimal change disease; MGN, membranous glomerulonephritis.

Figure 4

Myeloid cells are likely a major source of activated pentose phosphate pathway (PPP) gene expression. Subset prediction from enrichment correlation (SPEC) predicts that renal tubule and monocyte/macrophages are the likely source of PPP in the discovery cohort with the intensity of the red bar indicates degree of confidence in the bar plot correlation of cell types and PPP expression (A). The monocyte/macrophage enrichment score correlates with the PPP enrichment score in antineutrophil cytoplasmic antibody-associated vasculitis (AAV), while the tubule enrichment score correlates with the PPP enrichment score in healthy living donor and nephrotic syndrome samples (B). Transketolase (TKT), a key regulatory enzyme within the PPP, correlates with CD14, a marker for monocyte/macrophages in the glomerular compartment in the discovery cohort (C). Tissue immunofluorescence demonstrates localisation of TKT and HIF-1α within monocytes/macrophages (CD14) in the glomerular compartment (D). The PPP gene expression score is strongly associated with increased expression of a tumour necrosis factor (TNF) activation score within the glomerular compartment (E).