The impact of type III sodium-dependent phosphate transporters (Pit 1 and Pit 2) on podocyte and kidney function

Development and validation of preeclampsia predictive models using key genes from bioinformatics and machine learning approaches

Background

Preeclampsia (PE) poses significant diagnostic and therapeutic challenges. This study aims to identify novel genes for potential diagnostic and therapeutic targets, illuminating the immune mechanisms involved.

Methods

Three GEO datasets were analyzed, merging two for training set, and using the third for external validation. Intersection analysis of differentially expressed genes (DEGs) and WGCNA highlighted candidate genes. These were further refined through LASSO, SVM-RFE, and RF algorithms to identify diagnostic hub genes. Diagnostic efficacy was assessed using ROC curves. A predictive nomogram and fully Connected Neural Network (FCNN) were developed for PE prediction. ssGSEA and correlation analysis were employed to investigate the immune landscape. Further validation was provided by qRT-PCR on human placental samples.

Result

Five biomarkers were identified with validation AUCs: CGB5 (0.663, 95% CI: 0.577-0.750), LEP (0.850, 95% CI: 0.792-0.908), LRRC1 (0.797, 95% CI: 0.728-0.867), PAPPA2 (0.839, 95% CI: 0.775-0.902), and SLC20A1 (0.811, 95% CI: 0.742-0.880), all of which are involved in key biological processes. The nomogram showed strong predictive power (C-index 0.873), while FCNN achieved an optimal AUC of 0.911 (95% CI: 0.732-1.000) in five-fold cross-validation. Immune infiltration analysis revealed the importance of T cell subsets, neutrophils, and NK cells in PE, linking these genes to immune mechanisms underlying PE pathogenesis.

Conclusion

CGB5, LEP, LRRC1, PAPPA2, and SLC20A1 are validated as key diagnostic biomarkers for PE. Nomogram and FCNN could credibly predict PE. Their association with immune infiltration underscores the crucial role of immune responses in PE pathogenesis.

Pit 1 transporter (SLC20A1) as a key factor in the NPP1-mediated inhibition of insulin signaling in human podocytes

Podocytes are crucially involved in blood filtration in the glomerulus. Their proper function relies on efficient insulin responsiveness. The insulin resistance of podocytes, defined as a reduction of cell sensitivity to this hormone, is the earliest pathomechanism of microalbuminuria that is observed in metabolic syndrome and diabetic nephropathy. In many tissues, this alteration is mediated by the phosphate homeostasis-controlling enzyme nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1). By binding to the insulin receptor (IR), NPP1 inhibits downstream cellular signaling. Our previous research found that hyperglycemic conditions affect another protein that is involved in phosphate balance, type III sodium-dependent phosphate transporter 1 (Pit 1). In the present study, we evaluated the insulin resistance of podocytes after 24 h of incubation under hyperinsulinemic conditions. Thereafter, insulin signaling was inhibited. The formation of NPP1/IR complexes was observed at that time. A novel finding in the present study was our observation of an interaction between NPP1 and Pit 1 after the 24 h stimulation of podocytes with insulin. After downregulation of the SLC20A1 gene, which encodes Pit 1, we established insulin resistance in podocytes that were cultured under native conditions, manifested as a lack of intracellular insulin signaling and the inhibition of glucose uptake via the glucose transporter type 4. These findings suggest that Pit 1 might be a major factor that participates in the NPP1-mediated inhibition of insulin signaling.

Hyperglycemic environment disrupts phosphate transporter function and promotes calcification processes in podocytes and isolated glomeruli

Soft tissue calcification is a pathological phenomenon that often occurs in end-stage chronic kidney disease (CKD), which is caused by diabetic nephropathy, among other factors. Hyperphosphatemia present during course of CKD contributes to impairments in kidney function, particularly damages in the glomerular filtration barrier (GFB). Essential elements of the GFB include glomerular epithelial cells, called podocytes. In the present study, we found that human immortalized podocytes express messenger RNA and protein of phosphate transporters, including NaPi 2c (SLC34A3), Pit 1 (SLC20A1), and Pit 2 (SLC20A2), which are sodium-dependent and mediate intracellular phosphate (Pi) transport, and XPR1, which is responsible for extracellular Pi transport. We found that cells that were grown in a medium with a high glucose (HG) concentration (30 mM) expressed less Pit 1 and Pit 2 protein than podocytes that were cultured in a standard glucose medium (11 mM). We found that exposure of the analyzed transporters in the cell membrane of the podocyte is altered by HG conditions. We also found that the activity of tissue nonspecific alkaline phosphatase increased in HG, causing a rise in Pi generation. Additionally, HG led to a reduction of the amount of ectonucleotide pyrophosphatase/phosphodiesterase 1 in the cell membrane of podocytes. The extracellular concentration of pyrophosphate also decreased under HG conditions. These data suggest that a hyperglycemic environment enhances the production of Pi in podocytes and its retention in the extracellular space, which may induce glomerular calcification.