NDUFS4 regulates cristae remodeling in diabetic kidney disease.

The mitochondrial electron transport chain (ETC) is a highly adaptive process to meet metabolic demands of the cell, and its dysregulation has been associated with diverse clinical pathologies. However, the role and nature of impaired ETC in kidney diseases remains poorly understood. Here, we generate diabetic mice with podocyte-specific overexpression of Ndufs4, an accessory subunit of mitochondrial complex I, as a model investigate the role of ETC integrity in diabetic kidney disease (DKD). We find that conditional male mice with genetic overexpression of Ndufs4 exhibit significant improvements in cristae morphology, mitochondrial dynamics, and albuminuria. By coupling proximity labeling with super-resolution imaging, we also identify the role of cristae shaping protein STOML2 in linking NDUFS4 with improved cristae morphology. Together, we provide the evidence on the central role of NDUFS4 as a regulator of cristae remodeling and mitochondrial function in kidney podocytes. We propose that targeting NDUFS4 represents a promising approach to slow the progression of DKD.

The transcription factor ChREBP links mitochondrial lipidomes to mitochondrial morphology and progression of diabetic kidney disease.

A substantial body of evidence has established the contributions of both mitochondrial dynamics and lipid metabolism to the pathogenesis of diabetic kidney disease (DKD). However, the precise interplay between these two key metabolic regulators of DKD is not fully understood. Here, we uncover a link between mitochondrial dynamics and lipid metabolism by investigating the role of carbohydrate-response element-binding protein (ChREBP), a glucose-responsive transcription factor and a master regulator of lipogenesis, in kidney podocytes. We find that inducible podocyte-specific knockdown of ChREBP in diabetic db/db mice improves key biochemical and histological features of DKD in addition to significantly reducing mitochondrial fragmentation. Because of the critical role of ChREBP in lipid metabolism, we interrogated whether and how mitochondrial lipidomes play a role in ChREBP-mediated mitochondrial fission. Our findings suggest a key role for a family of ether phospholipids in ChREBP-induced mitochondrial remodeling. We find that overexpression of glyceronephosphate O-acyltransferase, a critical enzyme in the biosynthesis of plasmalogens, reverses the protective phenotype of ChREBP deficiency on mitochondrial fragmentation. Finally, our data also points to Gnpat as a direct transcriptional target of ChREBP. Taken together, our results uncover a distinct mitochondrial lipid signature as the link between ChREBP-induced mitochondrial dynamics and progression of DKD.

NDUFS4 Regulates Cristae Remodeling in Diabetic Kidney Disease.

The mitochondrial electron transport chain (ETC) is a highly adaptive process to meet metabolic demands of the cell, and its dysregulation has been associated with diverse clinical pathologies. However, the role and nature of impaired ETC in kidney diseases remains poorly understood. Here, we generated diabetic mice with podocyte-specific overexpression of Ndufs4, an accessory subunit of mitochondrial complex I, as a model to investigate the role of ETC integrity in diabetic kidney disease (DKD). We find that these conditional mice exhibit significant improvements in cristae morphology, mitochondrial dynamics, and albuminuria. By coupling proximity labeling with super-resolution imaging, we also identify the role of cristae shaping proteins in linking NDUFS4 with improved cristae morphology. Taken together, we discover the central role of NDUFS4 as a powerful regulator of cristae remodeling, respiratory supercomplexes assembly, and mitochondrial ultrastructure in vitro and in vivo. We propose that targeting NDUFS4 represents a promising approach to slow the progression of DKD.

PGC1α is required for the renoprotective effect of lncRNA Tug1 in vivo and links Tug1 with urea cycle metabolites.

lncRNA taurine-upregulated gene 1 (Tug1) is a promising therapeutic target in the progression of diabetic nephropathy (DN), but the molecular basis of its protection remains poorly understood. Here, we generate a triple-mutant diabetic mouse model coupled with metabolomic profiling data to interrogate whether Tug1 interaction with peroxisome proliferator-activated receptor gamma coactivator 1α (PGC1α) is required for mitochondrial remodeling and progression of DN in vivo. We find that, compared with diabetic conditional deletion of Pgc1α in podocytes alone (db/db; Pgc1αPod-f/f), diabetic Pgc1α knockout combined with podocyte-specific Tug1 overexpression (db/db; TugPodTg; Pgc1αPod-f/f) reverses the protective phenotype of Tug1 overexpression, suggesting that PGC1α is required for the renoprotective effect of Tug1. Using unbiased metabolomic profiling, we find that altered urea cycle metabolites and mitochondrial arginase 2 play an important role in Tug1/PGC1α-induced mitochondrial remodeling. Our work identifies a functional role of the Tug1/PGC1α axis on mitochondrial metabolic homeostasis and urea cycle metabolites in experimental models of diabetes.

Long Noncoding RNAs and Their Therapeutic Promise in Diabetic Nephropathy.

Recent advances in large-scale RNA sequencing and genome-wide profiling projects have unraveled a heterogeneous group of RNAs, collectively known as long noncoding RNAs (lncRNAs), which play central roles in many diverse biological processes. Importantly, an association between aberrant expression of lncRNAs and diverse human pathologies has been reported, including in a variety of kidney diseases. These observations have raised the possibility that lncRNAs may represent unexploited potential therapeutic targets for kidney diseases. Several important questions regarding the functionality of lncRNAs and their impact in kidney diseases, however, remain to be carefully addressed. Here, we provide an overview of the main functions and mechanisms of actions of lncRNAs, and their promise as therapeutic targets in kidney diseases, emphasizing on the role of some of the best-characterized lncRNAs implicated in the pathogenesis and progression of diabetic nephropathy.

Role for carbohydrate response element-binding protein (ChREBP) in high glucose-mediated repression of long noncoding RNA Tug1.

Long noncoding RNAs (lncRNAs) have been shown to play key roles in a variety of biological activities of the cell. However, less is known about how lncRNAs respond to environmental cues and what transcriptional mechanisms regulate their expression. Studies from our laboratory have shown that the lncRNA Tug1 (taurine upregulated gene 1) is crucial for the progression of diabetic kidney disease, a major microvascular complication of diabetes. Using a combination of proximity labeling with the engineered soybean ascorbate peroxidase (APEX2), ChIP-qPCR, biotin-labeled oligonucleotide pulldown, and classical promoter luciferase assays in kidney podocytes, we extend our initial observations in the current study and now provide a detailed analysis on a how high-glucose milieu downregulates Tug1 expression in podocytes. Our results revealed an essential role for the transcription factor carbohydrate response element binding protein (ChREBP) in controlling Tug1 transcription in the podocytes in response to increased glucose levels. Along with ChREBP, other coregulators, including MAX dimerization protein (MLX), MAX dimerization protein 1 (MXD1), and histone deacetylase 1 (HDAC1), were enriched at the Tug1 promoter under high-glucose conditions. These observations provide the first characterization of the mouse Tug1 promoter's response to the high-glucose milieu. Our findings illustrate a molecular mechanism by which ChREBP can coordinate glucose homeostasis with the expression of the lncRNA Tug1 and further our understanding of dynamic transcriptional regulation of lncRNAs in a disease state.

MTHFD2 links RNA methylation to metabolic reprogramming in renal cell carcinoma.

One-carbon metabolism plays a central role in a broad array of metabolic processes required for the survival and growth of tumor cells. However, the molecular basis of how one-carbon metabolism may influence RNA methylation and tumorigenesis remains largely unknown. Here we show MTHFD2, a mitochondrial enzyme involved in one-carbon metabolism, contributes to the progression of renal cell carcinoma (RCC) via a novel epitranscriptomic mechanism that involves HIF-2α. We found that expression of MTHFD2 was significantly elevated in human RCC tissues, and MTHFD2 knockdown strongly reduced xenograft tumor growth. Mechanistically, using an unbiased methylated RNA immunoprecipitation sequencing (meRIP-Seq) approach, we found that MTHFD2 plays a critical role in controlling global N6-methyladenosine (m6A) methylation levels, including the m6A methylation of HIF-2α mRNA, which results in enhanced translation of HIF-2α. Enhanced HIF-2α translation, in turn, promotes the aerobic glycolysis, linking one-carbon metabolism to HIF-2α-dependent metabolic reprogramming through RNA methylation. Our findings also suggest that MTHFD2 and HIF-2α form a positive feedforward loop in RCC, promoting metabolic reprograming and tumor growth. Taken together, our results suggest that MTHFD2 links RNA methylation status to the metabolic state of tumor cells in RCC.

Drp1S600 phosphorylation regulates mitochondrial fission and progression of nephropathy in diabetic mice.

Phosphorylation of Dynamin-related protein1 (Drp1) represents an important regulatory mechanism for mitochondrial fission. Here we established the role of Drp1 Serine 600 (S600) phosphorylation on mitochondrial fission in vivo, and assessed the functional consequences of targeted elimination of the Drp1S600 phosphorylation site in progression of diabetic nephropathy (DN). We generated a knockin mouse in which S600 was mutated to alanine (Drp1S600A). We found that diabetic Drp1S600A mice exhibited improved biochemical and histological features of DN along with reduced mitochondrial fission and diminished mitochondrial ROS in vivo. Importantly, we observed that the effect of Drp1S600 phosphorylation on mitochondrial fission in the diabetic milieu was stimulus- but not cell type-dependent. Mechanistically, we showed that mitochondrial fission in high glucose conditions occurs through concomitant binding of phospho-Drp1S600 with mitochondrial fission factor (Mff) and actin-related protein 3 (Arp3), ultimately leading to accumulation of F-actin and Drp1 on the mitochondria. Taken together, these findings establish that a single phosphorylation site in Drp1 can regulate mitochondrial fission and progression of DN in vivo, and highlight the stimulus-specific consequences of Drp1S600 phosphorylation on mitochondrial dynamics.

Real-time in vivo mitochondrial redox assessment confirms enhanced mitochondrial reactive oxygen species in diabetic nephropathy.

While increased mitochondrial reactive oxygen species have been commonly implicated in a variety of disease states, their in vivo role in the pathogenesis of diabetic nephropathy remains controversial. Using a two-photon imaging approach with a genetically encoded redox biosensor, we monitored mitochondrial redox state in the kidneys of experimental models of diabetes in real-time in vivo. Diabetic (db/db) mice that express a redox-sensitive Green Fluorescent Protein biosensor (roGFP) specifically in the mitochondrial matrix (db/dbmt-roGFP) were generated, allowing dynamic monitoring of redox changes in the kidneys. These db/dbmt-roGFP mice exhibited a marked increase in mitochondrial reactive oxygen species in the kidneys. Yeast NADH-dehydrogenase, a mammalian Complex I homolog, was ectopically expressed in cultured podocytes, and this forced expression in roGFP-expressing podocytes prevented high glucose-induced increases in mitochondrial reactive oxygen species. Thus, in vivo monitoring of mitochondrial roGFP in diabetic mice confirms increased production of mitochondrial reactive oxygen species in the kidneys.

Long noncoding RNA Tug1 regulates mitochondrial bioenergetics in diabetic nephropathy.

The regulatory roles of long noncoding RNAs (lncRNAs) in transcriptional coactivators are still largely unknown. Here, we have shown that the peroxisome proliferator-activated receptor γ (PPARγ) coactivator α (PGC-1α, encoded by Ppargc1a) is functionally regulated by the lncRNA taurine-upregulated gene 1 (Tug1). Further, we have described a role for Tug1 in the regulation of mitochondrial function in podocytes. Using a murine model of diabetic nephropathy (DN), we performed an unbiased RNA-sequencing (RNA-seq) analysis of kidney glomeruli and identified Tug1 as a differentially expressed lncRNA in the diabetic milieu. Podocyte-specific overexpression (OE) of Tug1 in diabetic mice improved the biochemical and histological features associated with DN. Unexpectedly, we found that Tug1 OE rescued the expression of PGC-1α and its transcriptional targets. Tug1 OE was also associated with improvements in mitochondrial bioenergetics in the podocytes of diabetic mice. Mechanistically, we found that the interaction between Tug1 and PGC-1α promotes the binding of PGC-1α to its own promoter. We identified a Tug1-binding element (TBE) upstream of the Ppargc1a gene and showed that Tug1 binds with the TBE to enhance Ppargc1a promoter activity. These findings indicate that a direct interaction between PGC-1α and Tug1 modulates mitochondrial bioenergetics in podocytes in the diabetic milieu.

SUMO Modification Reverses Inhibitory Effects of Smad Nuclear Interacting Protein-1 in TGF-ß Responses.

SNIP1 (Smad nuclear interacting protein 1) is a transcription repressor for the TGF-ß and NF-κB signaling pathways through disrupting the recruitment of co-activator p300. However, it is unclear how the functions of SNIP1 in the TGF-ß signaling pathway are controlled. Our present studies show that SNIP1 is covalently modified by small ubiquitin-like modifier (SUMO) in vitro and in vivo at three lysine sites: Lys5, Lys30, and Lys108, with Lys30 being the major SUMO modification site. SUMOylation of SNIP1 is enhanced by SUMO E3 ligase PIAS proteins and inhibited by SUMO proteases SENP1/2. Furthermore, we find that SUMOylation of SNIP1 attenuates its inhibitory effect in TGF-ß signaling because the SUMO-conjugated form of SNIP1 exhibits impaired ability to disrupt the formation of Smad complex and the interaction between p300 and Smads. Subsequently, SUMOylation of SNIP1 leads to the loss of SNIP1-mediated inhibition on expression of the TGF-ß target genes PAI-1 and MMP2 and eventually enhances TGF-ß-regulated cell migration and invasion.

miR-93 regulates Msk2-mediated chromatin remodelling in diabetic nephropathy.

How the kidney responds to the metabolic cues from the environment remains a central question in kidney research. This question is particularly relevant to the pathogenesis of diabetic nephropathy (DN) in which evidence suggests that metabolic events in podocytes regulate chromatin structure. Here, we show that miR-93 is a critical metabolic/epigenetic switch in the diabetic milieu linking the metabolic state to chromatin remodelling. Mice with inducible overexpression of a miR-93 transgene exclusively in podocytes exhibit significant improvements in key features of DN. We identify miR-93 as a regulator of nucleosomal dynamics in podocytes. miR-93 has a critical role in chromatin reorganization and progression of DN by modulating its target Msk2, a histone kinase, and its substrate H3S10. These findings implicate a central role for miR-93 in high glucose-induced chromatin remodelling in the kidney, and provide evidence for a previously unrecognized role for Msk2 as a target for DN therapy.

MicroRNA-22 is a master regulator of bone morphogenetic protein-7/6 homeostasis in the kidney.

Accumulating evidence suggests that microRNAs (miRNAs) contribute to a myriad of kidney diseases. However, the regulatory role of miRNAs on the key molecules implicated in kidney fibrosis remains poorly understood. Bone morphogenetic protein-7 (BMP-7) and its related BMP-6 have recently emerged as key regulators of kidney fibrosis. Using the established unilateral ureteral obstruction (UUO) model of kidney fibrosis as our experimental model, we examined the regulatory role of miRNAs on BMP-7/6 signaling. By analyzing the potential miRNAs that target BMP-7/6 in silica, we identified miR-22 as a potent miRNA targeting BMP-7/6. We found that expression levels of BMP-7/6 were significantly elevated in the kidneys of the miR-22 null mouse. Importantly, mice with targeted deletion of miR-22 exhibited attenuated renal fibrosis in the UUO model. Consistent with these in vivo observations, primary renal fibroblast isolated from miR-22-deficient UUO mice demonstrated a significant increase in BMP-7/6 expression and their downstream targets. This phenotype could be rescued when cells were transfected with miR-22 mimics. Interestingly, we found that miR-22 and BMP-7/6 are in a regulatory feedback circuit, whereby not only miR-22 inhibits BMP-7/6, but miR-22 by itself is induced by BMP-7/6. Finally, we identified two BMP-responsive elements in the proximal region of miR-22 promoter. These findings identify miR-22 as a critical miRNA that contributes to renal fibrosis on the basis of its pivotal role on BMP signaling cascade.

Mitochondrial fission triggered by hyperglycemia is mediated by ROCK1 activation in podocytes and endothelial cells.

Several lines of evidence suggest that mitochondrial dysfunction plays a critical role in the pathogenesis of microvascular complications of diabetes, including diabetic nephropathy. However, the signaling pathways by which hyperglycemia leads to mitochondrial dysfunction are not fully understood. Here we examined the role of Rho-associated coiled coil-containing protein kinase 1 (ROCK1) on mitochondrial dynamics by generating two diabetic mouse models with targeted deletions of ROCK1 and an inducible podocyte-specific knockin mouse expressing a constitutively active (cA) mutant of ROCK1. Our findings suggest that ROCK1 mediates hyperglycemia-induced mitochondrial fission by promoting dynamin-related protein-1 (Drp1) recruitment to the mitochondria. Deletion of ROCK1 in diabetic mice prevented mitochondrial fission, whereas podocyte-specific cA-ROCK1 mice exhibited increased mitochondrial fission. Importantly, we found that ROCK1 triggers mitochondrial fission by phosphorylating Drp1 at serine 600 residue. These findings provide insights into the unexpected role of ROCK1 in a signaling cascade that regulates mitochondrial dynamics.

MicroRNA-29c is a signature microRNA under high glucose conditions that targets Sprouty homolog 1, and its in vivo knockdown prevents progression of diabetic nephropathy.

Although several recent publications have suggested that microRNAs contribute to the pathogenesis of diabetic nephropathy, the role of miRNAs in vivo still remains poorly understood. Using an integrated in vitro and in vivo comparative miRNA expression array, we identified miR-29c as a signature miRNA in the diabetic environment. We validated our profiling array data by examining miR-29c expression in the kidney glomeruli obtained from db/db mice in vivo and in kidney microvascular endothelial cells and podocytes treated with high glucose in vitro. Functionally, we found that miR-29c induces cell apoptosis and increases extracellular matrix protein accumulation. Indeed, forced expression of miR-29c strongly induced podocyte apoptosis. Conversely, knockdown of miR-29c prevented high glucose-induced cell apoptosis. We also identified Sprouty homolog 1 (Spry1) as a direct target of miR-29c with a nearly perfect complementarity between miR-29c and the 3'-untranslated region (UTR) of mouse Spry1. Expression of miR-29c decreased the luciferase activity of Spry1 when co-transfected with the mouse Spry1 3'-UTR reporter construct. Overexpression of miR-29c decreased the levels of Spry1 protein and promoted activation of Rho kinase. Importantly, knockdown of miR-29c by a specific antisense oligonucleotide significantly reduced albuminuria and kidney mesangial matrix accumulation in the db/db mice model in vivo. These findings identify miR-29c as a novel target in diabetic nephropathy and provide new insights into the role of miR-29c in a previously unrecognized signaling cascade involving Spry1 and Rho kinase activation.