An Acidic Environment Induces APOL1-Associated Mitochondrial Fragmentation

IFI16 Is Indispensable for Promoting HIF-1α-Mediated APOL1 Expression in Human Podocytes under Hypoxic Conditions

Genetic variants in the protein-coding regions of APOL1 are associated with an increased risk and progression of chronic kidney disease (CKD) in African Americans. Hypoxia exacerbates CKD progression by stabilizing HIF-1α, which induces APOL1 transcription in kidney podocytes. However, the contribution of additional mediators to regulating APOL1 expression under hypoxia in podocytes is unknown. Here, we report that a transient accumulation of HIF-1α in hypoxia is sufficient to upregulate APOL1 expression in podocytes through a cGAS/STING/IRF3-independent pathway. Notably, IFI16 ablation impedes hypoxia-driven APOL1 expression despite the nuclear accumulation of HIF-1α. Co-immunoprecipitation assays indicate no direct interaction between IFI16 and HIF-1α. Our studies identify hypoxia response elements (HREs) in the APOL1 gene enhancer/promoter region, showing increased HIF-1α binding to HREs located in the APOL1 gene enhancer. Luciferase reporter assays confirm the role of these HREs in transcriptional activation. Chromatin immunoprecipitation (ChIP)-qPCR assays demonstrate that IFI16 is not recruited to HREs, and IFI16 deletion reduces HIF-1α binding to APOL1 HREs. RT-qPCR analysis indicates that IFI16 selectively affects APOL1 expression, with a negligible impact on other hypoxia-responsive genes in podocytes. These findings highlight the unique contribution of IFI16 to hypoxia-driven APOL1 gene expression and suggest alternative IFI16-dependent mechanisms regulating APOL1 gene expression under hypoxic conditions.

APOL1-mediated monovalent cation transport contributes to APOL1-mediated podocytopathy in kidney disease

Two coding variants of apolipoprotein L1 (APOL1), called G1 and G2, explain much of the excess risk of kidney disease in African Americans. While various cytotoxic phenotypes have been reported in experimental models, the proximal mechanism by which G1 and G2 cause kidney disease is poorly understood. Here, we leveraged 3 experimental models and a recently reported small molecule blocker of APOL1 protein, VX-147, to identify the upstream mechanism of G1-induced cytotoxicity. In HEK293 cells, we demonstrated that G1-mediated Na+ import/K+ efflux triggered activation of GPCR/IP3-mediated calcium release from the ER, impaired mitochondrial ATP production, and impaired translation, which were all reversed by VX-147. In human urine-derived podocyte-like epithelial cells (HUPECs), we demonstrated that G1 caused cytotoxicity that was again reversible by VX-147. Finally, in podocytes isolated from APOL1 G1 transgenic mice, we showed that IFN-γ-mediated induction of G1 caused K+ efflux, activation of GPCR/IP3 signaling, and inhibition of translation, podocyte injury, and proteinuria, all reversed by VX-147. Together, these results establish APOL1-mediated Na+/K+ transport as the proximal driver of APOL1-mediated kidney disease.

Long-term cadmium exposure impairs cognitive function by activating lnc-Gm10532/m6A/FIS1 axis-mediated mitochondrial fission and dysfunction

Cadmium (Cd), a ubiquitous environmental contaminant, is deemed a possible aetiological cause of cognitive disorders in humans. Nevertheless, the exact mechanism by which chronic exposure to Cd causes neurotoxicity is not fully understood. In this study, mouse neuroblastoma cells (Neuro-2a cells) and primary hippocampal neurons were exposed to low-dose (1, 2, and 4 μM for Neuro-2a cells or 0.5, 1, and 1.5 μM for hippocampal neurons) cadmium chloride (CdCl₂) for 72 h (h), and male mice (C57BL/6J, 8 weeks) were orally administered CdCl₂ (0.6 mg/L, approximately equal to 2.58 μg/kg·bw/d) for 6 months to investigate the effects and mechanism of chronic Cd-induced neurotoxicity. Here, chronic exposure to Cd impaired mitochondrial function by promoting excess reactive oxygen species (ROS) production, altering mitochondrial membrane potential (Δψm) and reducing adenosine triphosphate (ATP) content, contributing to neuronal cell death. Specifically, microarray analysis revealed that the long noncoding RNA Gm10532 (lnc-Gm10532) was most highly expressed in Neuro-2a cells exposed to 4 μM CdCl₂ for 72 h compared with controls, and inhibition of lnc-Gm10532 significantly antagonized CdCl₂-induced mitochondrial dysfunction and neurotoxicity. Mechanistically, lnc-Gm10532 increased Fission 1 (FIS1) expression and mitochondrial fission by recruiting the m6A writer methyltransferase-like 14 (METTL14) and enhancing m6A modification of Fis1 mRNA. Moreover, lnc-Gm10532 was also required for chronic Cd-induced mitochondrial dysfunction and memory deficits in a rodent model. Therefore, data of this study reveal a new epigenetic mechanism of chronic Cd neurotoxicity.

A glomerular transcriptomic landscape of apolipoprotein L1 in Black patients with focal segmental glomerulosclerosis

Apolipoprotein L1 (APOL1)-associated focal segmental glomerulosclerosis (FSGS) is the dominant form of FSGS in Black individuals. There are no targeted therapies for this condition, in part because the molecular mechanisms underlying APOL1's pathogenic contribution to FSGS are incompletely understood. Studying the transcriptomic landscape of APOL1 FSGS in patient kidneys is an important way to discover genes and molecular behaviors that are unique or most relevant to the human disease. With the hypothesis that the pathology driven by the high-risk APOL1 genotype is reflected in alteration of gene expression across the glomerular transcriptome, we compared expression and co-expression profiles of 15,703 genes in 16 Black patients with FSGS at high-risk vs 14 Black patients with a low-risk APOL1 genotype. Expression data from APOL1-inducible HEK293 cells and normal human glomeruli were used to pursue genes and molecular pathways uncovered in these studies. We discovered increased expression of APOL1 and nine other significant differentially expressed genes in high-risk patients. This included stanniocalcin, which has a role in mitochondrial and calcium-related processes along with differential correlations between high- and low-risk APOL1 and metabolism pathway genes. There were similar correlations with extracellular matrix- and immune-related genes, but significant loss of co-expression of mitochondrial genes in high-risk FSGS, and an NF-κB-down regulating gene, NKIRAS1, as the most significant hub gene with strong differential correlations with NDUF family (mitochondrial respiratory genes) and immune-related (JAK-STAT) genes. Thus, differences in mitochondrial gene regulation appear to underlie many differences observed between high- and low-risk Black patients with FSGS.