Diabetic nephropathy induces alterations in the glomerular and tubule lipid profiles

Lipid metabolism disorder in diabetic kidney disease

Diabetic kidney disease (DKD), a significant complication associated with diabetes mellitus, presents limited treatment options. The progression of DKD is marked by substantial lipid disturbances, including alterations in triglycerides, cholesterol, sphingolipids, phospholipids, lipid droplets, and bile acids (BAs). Altered lipid metabolism serves as a crucial pathogenic mechanism in DKD, potentially intertwined with cellular ferroptosis, lipophagy, lipid metabolism reprogramming, and immune modulation of gut microbiota (thus impacting the liver-kidney axis). The elucidation of these mechanisms opens new potential therapeutic pathways for DKD management. This research explores the link between lipid metabolism disruptions and DKD onset.

Lipin-1 deficiency deteriorates defect of fatty acid β-oxidation and lipid-related kidney damage in diabetic kidney disease

Diabetic lipo-toxicity is a fundamental pathophysiologic mechanism in DM and is now increasingly recognized a key determinant of DKD. Targeting lipid metabolic disorders is an important therapeutic strategy for the treatment of DM and its complications, including DKD. This study aimed to explore the molecular mechanism of lipid metabolic regulation in kidney, especially renal PTECs, and elucidate the role of lipid metabolic related molecule lipin-1 in diabetic lipid-related kidney damage. In this study, lipin-1-deficient db/db mouse model and STZ/HFD-induced T2DM mouse model were used to determine the effect of lipin-1 on DKD development. Then RPTCs and LPIN1 knockdown or overexpressed HK-2 cells induced by PA were used to investigate the mechanism. We found that the expression of lipin-1 increased early and then decreased in kidney during the progression of DKD. Glucose and lipid metabolic disorders and renal insufficiency were found in these 2 types of diabetic mouse models. Interestingly, lipin-1 deficiency might be a pathogenic driver of DKD-to-CKD transition, which could further accelerate the imbalance of renal lipid homeostasis, the dysfunction of mitochondrial and energy metabolism in PTECs. Mechanistically, lipin-1 deficiency resulted in aggravated PTECs injury to tubulointerstitial fibrosis in DKD by downregulating FAO via inhibiting PGC-1α/PPARα mediated Cpt1α/HNF4α signaling and upregulating SREBPs to promote fat synthesis. This study provided new insights into the role of lipin-1 as a regulator for maintaining lipid homeostasis in the kidney, especially PTECs, and its deficiency led to the progression of DKD.

Mechanism of peroxidasin inactivation in hyperglycemia: Heme damage by reactive oxygen species

Diabetic complications present a serious health problem. Functional damage to proteins due to post-translational modifications by glycoxidation reactions is a known factor contributing to pathology. Extracellular proteins are especially vulnerable to diabetic damage because robust antioxidant defenses are lacking outside the cell. We investigated glucose-induced inactivation of peroxidasin (PXDN), a heme protein catalyzing sulfilimine crosslinking of collagen IV that reinforce the basement membranes (BM). Experiments using physiological diabetic glucose levels were carried out to exclude several potential mechanisms of PXDN inactivation i.e., direct adduction of glucose, reactive carbonyl damage, steric hindrance, and osmotic stress. Further experiments established that PXDN activity was inhibited via heme degradation by reactive oxygen species. Activity of another extracellular heme protein, myeloperoxidase, was unaffected by glucose because its heme was resistant to glucose-induced oxidative degradation. Our findings point to specific mechanisms which may compromise BM structure and stability in diabetes and suggest potential modes of protection.

Cell and rat serum, urine and tissue metabolomics analysis elucidates the key pathway changes associated with chronic nephropathy and reveals the mechanism of action of rhein

BACKGROUND

Rhein can significantly delay the progression of chronic nephropathy. However, its mechanism of action has not been adequately elaborated, which hinders its extensive clinical application. In this work, the effects of rhein on models of TGF-β-induced NRK-49F cellular fibrosis and rat renal ischemia-reperfusion fibrosis were evaluated using metabolomics and western blotting.

METHODS

The metabolic profiles of NRK-49F cells and rat urine, serum, and kidney tissues in the control, model, and rhein groups were investigated using UPLC-QTOF-MS. The levels of p-P65, p-IKK, p-AKT, p-P38, p-JNK and AP-1 in NRK-49F cells were measured using western blotting and immunofluorescence methods. Molecular docking and network pharmacology methods were employed to explore the relationship between the potential targets of rhein and key proteins in the NF-κB and MAPK signaling pathways.

RESULTS

Various potential metabolites, including sphingolipids, ceramides, phosphatidylcholine, and lysophosphatidylcholine,14-hydroxy-E4-neuroprostane E, and 5-HPETE, were present in the cell, tissue, urine, and serum samples; however, few metabolites matches exactly among the four type of biological samples. These differential metabolites can effectively differentiated between the control, model, and rhein groups. Pathway enrichment analysis of differential metabolites unveiled that sphingolipid metabolism, arachidonic acid metabolism, and glycerophospholipid metabolism were closely related to nephropathy. Phosphorylation levels of AKT, IKK, P65 and AP-1 in NRK-49F cells was reduced by rhein treatment. Network pharmacology and molecular docking showed that the potential targets of rhein might regulated the expression of MAPK and AKT in the NF-κB and MAPK signaling pathways.

CONCLUSION

In brief, rhein might delays the progression of chronic nephropathy via the metabolic pathways, NF-κB and MAPKs signaling pathways, which provides the foundation for its development and clinical application.

Regulation of renal lipid deposition in diabetic nephropathy on morroniside via inhibition of NF-KB/TNF-a/SREBP1c signaling pathway

Morroniside (MOR), a cyclic enol ether terpene glycoside isolated from Cornus officinalis, has been shown to inhibit lipid accumulation, although the mechanism of action is uncertain. The aim of this study was to investigate the potential pathways by which MOR affects renal lipid deposition in diabetic nephropathy (DN). In vitro and in vivo experiments were performed using the PA-induced HK-2 cell model and a KKAy animal model, respectively. Network pharmacological analysis was used to identify potential MOR signaling pathways for DN therapy, with results verified via Western blotting and immunofluorescence experiments. The effect of MOR on lipid metabolism was investigated using BODIPY 493/503 staining. Our results indicate that MOR significantly reduces lipid accumulation both in vitro and in vivo. According to network pharmacology studies, the NF-κB/TNF-α/SREBP1c signaling pathway may be the mechanism of action of MOR in DN. MOR was found to inhibit this pathway by reducing the phosphorylation of NF-κB p65 and the expression of TNF-α and SREBP1c, similar to the effects of Bay11-7082. Additionally, MOR significantly inhibited the expression of lipid factors such as ACC, FAS, and SCD1. In conclusion, MOR can regulate the disruption of lipid metabolism in DN and reduce renal lipid deposition via suppression of the NF-κB/TNF-α/SREBP1c signaling pathway.

Microscopy-Directed Imaging Mass Spectrometry for Rapid High Spatial Resolution Molecular Imaging of Glomeruli

The glomerulus is a multicellular functional tissue unit (FTU) of the nephron that is responsible for blood filtration. Each glomerulus contains multiple substructures and cell types that are crucial for their function. To understand normal aging and disease in kidneys, methods for high spatial resolution molecular imaging within these FTUs across whole slide images is required. Here we demonstrate a workflow using microscopy-driven selected sampling to enable 5 μm pixel size matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) of all glomeruli within whole slide human kidney tissues. Such high spatial resolution imaging entails large numbers of pixels, increasing the data acquisition times. Automating FTU-specific tissue sampling enables high-resolution analysis of critical tissue structures, while concurrently maintaining throughput. Glomeruli were automatically segmented using coregistered autofluorescence microscopy data, and these segmentations were translated into MALDI IMS measurement regions. This allowed high-throughput acquisition of 268 glomeruli from a single whole slide human kidney tissue section. Unsupervised machine learning methods were used to discover molecular profiles of glomerular subregions and differentiate between healthy and diseased glomeruli. Average spectra for each glomerulus were analyzed using Uniform Manifold Approximation and Projection (UMAP) and k-means clustering, yielding 7 distinct groups of differentiated healthy and diseased glomeruli. Pixel-wise k-means clustering was applied to all glomeruli, showing unique molecular profiles localized to subregions within each glomerulus. Automated microscopy-driven, FTU-targeted acquisition for high spatial resolution molecular imaging maintains high-throughput and enables rapid assessment of whole slide images at cellular resolution and identification of tissue features associated with normal aging and disease.

Lysophosphatidic Acid Induces Podocyte Pyroptosis in Diabetic Nephropathy by an Increase of Egr1 Expression via Downregulation of EzH2

Podocyte damage and renal inflammation are the main features and pathogenesis of diabetic nephropathy (DN). Inhibition of lysophosphatidic acid (LPA) receptor 1 (LPAR1) suppresses glomerular inflammation and improves DN. Herein, we investigated LPA-induced podocyte damage and its underlying mechanisms in DN. We investigated the effects of AM095, a specific LPAR1 inhibitor, on podocytes from streptozotocin (STZ)-induced diabetic mice. E11 cells were treated with LPA in the presence or absence of AM095, and the expression of NLRP3 inflammasome factors and pyroptosis were measured. A chromatin immunoprecipitation assay and Western blotting were performed to elucidate underlying molecular mechanisms. Gene knockdown by transfecting small interfering RNA was used to determine the role of the transcription factor Egr1 (early growth response protein 1) and histone methyltransferase EzH2 (Enhancer of Zeste Homolog 2) in LPA-induced podocyte injury. AM095 administration inhibited podocyte loss, NLRP3 inflammasome factor expression, and cell death in STZ-induced diabetic mice. In E11 cells, LPA increased NLRP3 inflammasome activation and pyroptosis via LPAR1. Egr1 mediated NLRP3 inflammasome activation and pyroptosis in LPA-treated E11 cells. LPA decreased H3K27me3 enrichment at the Egr1 promoter in E11 cells by downregulating EzH2 expression. EzH2 knockdown further increased LPA-induced Egr1 expression. In podocytes from STZ-induced diabetic mice, AM095 suppressed Egr1 expression increase and EzH2/H3K27me3 expression reduction. Collectively, these results demonstrate that LPA induces NLRP3 inflammasome activation by downregulating EzH2/H3K27me3 and upregulating Egr1 expression, resulting in podocyte damage and pyroptosis, which may be a potential mechanism of DN progression.

Unraveling the Crosstalk between Lipids and NADPH Oxidases in Diabetic Kidney Disease

Diabetic kidney disease (DKD) is a serious complication of diabetes mellitus and a leading cause of end-stage renal disease. Abnormal lipid metabolism and intrarenal accumulation of lipids have been shown to be strongly correlated with the development and progression of diabetic kidney disease (DKD). Cholesterol, phospholipids, triglycerides, fatty acids, and sphingolipids are among the lipids that are altered in DKD, and their renal accumulation has been linked to the pathogenesis of the disease. In addition, NADPH oxidase-induced production of reactive oxygen species (ROS) plays a critical role in the development of DKD. Several types of lipids have been found to be tightly linked to NADPH oxidase-induced ROS production. This review aims to explore the interplay between lipids and NADPH oxidases in order to provide new insights into the pathogenesis of DKD and identify more effective targeted therapies for the disease.

Phosphatidylinositol metabolism of the renal proximal tubule S3 segment is disturbed in response to diabetes

Diabetes is a main risk factor for kidney disease, causing diabetic nephropathy in close to half of all patients with diabetes. Metabolism has recently been identified to be decisive in cell fate decisions and repair. Here we used mass spectrometry imaging (MSI) to identify tissue specific metabolic dysregulation, in order to better understand early diabetes-induced metabolic changes of renal cell types. In our experimental diabetes mouse model, early glomerular glycocalyx barrier loss and systemic metabolic changes were observed. In addition, MSI targeted at small molecule metabolites and glycero(phospho)lipids exposed distinct changes upon diabetes in downstream nephron segments. Interestingly, the outer stripe of the outer medullar proximal tubular segment (PT_S3) demonstrated the most distinct response compared to other segments. Furthermore, phosphatidylinositol lipid metabolism was altered specifically in PT_S3, with one of the phosphatidylinositol fatty acid tails being exchanged from longer unsaturated fatty acids to shorter, more saturated fatty acids. In acute kidney injury, the PT_S3 segment and its metabolism are already recognized as important factors in kidney repair processes. The current study exposes early diabetes-induced changes in membrane lipid composition in this PT_S3 segment as a hitherto unrecognized culprit in the early renal response to diabetes.

Integrative phosphatidylcholine metabolism through phospholipase A2 in rats with chronic kidney disease

Dysregulation in lipid metabolism is the leading cause of chronic kidney disease (CKD) and also the important risk factors for high morbidity and mortality. Although lipid abnormalities were identified in CKD, integral metabolic pathways for specific individual lipid species remain to be clarified. We conducted ultra-high-performance liquid chromatography-high-definition mass spectrometry-based lipidomics and identified plasma lipid species and therapeutic effects of Rheum officinale in CKD rats. Adenine-induced CKD rats were administered Rheum officinale. Urine, blood and kidney tissues were collected for analyses. We showed that exogenous adenine consumption led to declining kidney function in rats. Compared with control rats, a panel of differential plasma lipid species in CKD rats was identified in both positive and negative ion modes. Among the 50 lipid species, phosphatidylcholine (PC), lysophosphatidylcholine (LysoPC) and lysophosphatidic acid (LysoPA) accounted for the largest number of identified metabolites. We revealed that six PCs had integral metabolic pathways, in which PC was hydrolysed into LysoPC, and then converted to LysoPA, which was associated with increased cytosolic phospholipase A2 protein expression in CKD rats. The lower levels of six PCs and their corresponding metabolites could discriminate CKD rats from control rats. Receiver operating characteristic curves showed that each individual lipid species had high values of area under curve, sensitivity and specificity. Administration of Rheum officinale significantly improved impaired kidney function and aberrant PC metabolism in CKD rats. Taken together, this study demonstrates that CKD leads to PC metabolism disorders and that the dysregulation of PC metabolism is involved in CKD pathology.

Mapping glycation and glycoxidation sites in collagen I of human cortical bone

Accumulation of advanced glycation end products (AGEs), particularly in long-lived extracellular matrix proteins, has been implicated in pathogenesis of diabetic complications and in aging. Knowledge about specific locations of AGEs and their precursors within protein primary structure is critical for understanding their physiological and pathophysiological impact. However, the information on specific AGE sites is lacking. Here, we identified sequence positions of four major AGEs, carboxymethyllysine, carboxyethyllysine, 5-hydro-5-methyl imidazolone, and 5-hydro-imidazolone, and an AGE precursor fructosyllysine within the triple helical region of collagen I from cortical bone of human femurs. The presented map provides a basis for site-specific quantitation of AGEs and other non-enzymatic post-translational modifications and identification of those sites affected by aging, diabetes, and other diseases such as osteoporosis; it can also help in guiding future studies of AGE impact on structure and function of collagen I in bone.

Redox phospholipidomics analysis reveals specific oxidized phospholipids and regions in the diabetic mouse kidney

While it is generally accepted that oxidative stress impacts the diabetic kidney and contributes to pathogenesis, there is a substantial lack of knowledge about the molecular entity and anatomic location of a variety of reactive species. Here we provide a novel “oxidative stress map” of the diabetic kidney – the first of its kind, and identify specific, oxidized and other reactive lipids and their location. We used the db/db mouse model and Desorption Electrospray Ionization (DESI) mass spectrometry combined with heatmap image analysis. We analyzed a comprehensive array of phospholipid peroxide species in normal (db/m) and diabetic (db/db) kidneys using DESI imaging. Oxilipidomics heatmaps of the kidneys were generated focusing on phospholipids and their potential peroxidized products. We identified those lipids that undergo peroxidation in diabetic nephropathy. Several phospholipid peroxides and their spatial distribution were identified that were specific to the diabetic kidney, with significant enrichment in oxygenated phosphatidylethanolamines (PE) and lysophosphatidylethanolamine. Beyond qualitative and semi-quantitative information about the targets, the approach also reveals the anatomic location and the extent of lipid peroxide signal propagation across the kidney. Our approach provides novel, in-depth information of the location and molecular entity of reactive lipids in an organ with a very heterogeneous landscape. Many of these reactive lipids have been previously linked to programmed cell death mechanisms. Thus, the findings may be relevant to understand what impact phospholipid peroxidation has on cell and mitochondria membrane integrity and redox lipid signaling in diabetic nephropathy.

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Oxidative stress is one of the major players in the pathogenesis of diabetic nephropathy.

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Here, using an innovative mass spectrometry approach (DESI), we provide a novel oxilipidomics map of the diabetic kidney.

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Our analysis revealed enrichment of specific oxygenated phospholipid species at specific regions of the diabetic kidney.

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Our findings will likely further our understanding in how and where lipid peroxidation impacts the diabetic kidney.

Causative or associative: A critical review of the role of advanced glycation end-products in bone fragility

The accumulation of advanced glycation end-products (AGEs) in the organic matrix of bone with aging and chronic disease such as diabetes is thought to increase fracture risk independently of bone mass. However, to date, there has not been a clinical trial to determine whether inhibiting the accumulation of AGEs is effective in preventing low-energy, fragility fractures. Moreover, unlike with cardiovascular or kidney disease, there are also no pre-clinical studies demonstrating that AGE inhibitors or breakers can prevent the age- or diabetes-related decrease in the ability of bone to resist fracture. In this review, we critically examine the case for a long-standing hypothesis that AGE accumulation in bone tissue degrades the toughening mechanisms by which bone resists fracture. Prior research into the role of AGEs in bone has primarily measured pentosidine, an AGE crosslink, or bulk fluorescence of hydrolysates of bone. While significant correlations exist between these measurements and mechanical properties of bone, multiple AGEs are both non-fluorescent and non-crosslinking. Since clinical studies are equivocal on whether circulating pentosidine is an indicator of elevated fracture risk, there needs to be a more complete understanding of the different types of AGEs including non-crosslinking adducts and multiple non-enzymatic crosslinks in bone extracellular matrix and their specific contributions to hindering fracture resistance (biophysical and biological). By doing so, effective strategies to target AGE accumulation in bone with minimal side effects could be investigated in pre-clinical and clinical studies that aim to prevent fragility fractures in conditions that bone mass is not the underlying culprit.

Effect of BBT-877, a novel inhibitor of ATX, on a mouse model of type 1 diabetic nephropathy

Diabetic nephropathy (DN) is one of the common microvascular complications of diabetes. Autotaxin (ATX) is an enzyme with lysophospholipase D activity, producing lysophosphatidic acid (LPA). LPA signaling has been implicated in renal fibrosis, thereby inducing renal dysfunction. BBT-877 is an orally administered small molecule inhibitor of ATX. However, its effect on DN has not been studied so far. In this study, we investigated the effect of BBT-877, a novel inhibitor of ATX, on the pathogenesis of DN in a mouse model of streptozotocin (STZ)-induced diabetes. BBT-877 treatment significantly reduced albuminuria, albumin-to-creatinine ratio (ACR), neutrophil gelatinase-associated lipocalin (NGAL), and glomerular volume compared to the STZ-vehicle group. Interestingly, BBT-877 treatment attenuated hyperglycemia and dyslipidemia in STZ-induced diabetes mice. In the liver, the expression levels of β-oxidation-related genes such as PPAR α and CPT1 were significantly decreased in STZ-induced diabetic mice. However, this effect was reversed by BBT-877 treatment. BBT-877 treatment also suppressed mRNA levels of pro-inflammatory cytokines IL-6, MCP-1, and TNF-α and protein levels of fibrotic factors (TGF-β, fibronectin, CTGF, and collagen type Ι alpha Ι (COL1A1)) in the kidneys of STZ-induced diabetic mice. In conclusion, our results indicate that BBT-877 is effective in preventing the pathogenesis of DN by reducing systemic blood glucose levels and inhibiting inflammation and fibrosis in the renal tissue of diabetes mice. These novel findings suggest that inhibition of ATX may be a potential therapeutic target for DN.

Spatial localization of β-unsaturated aldehyde markers in murine diabetic kidney tissue by mass spectrometry imaging

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease. Limitations in current diagnosis and screening methods have sparked a search for more specific and conclusive biomarkers. Hyperglycemic conditions generate a plethora of harmful molecules in circulation and within tissues. Oxidative stress generates reactive α-dicarbonyls and β-unsaturated hydroxyhexenals, which react with proteins to form advanced glycation end products. Mass spectrometry imaging (MSI) enables the detection and spatial localization of molecules in biological tissue sections. Here, for the first time, the localization and semiquantitative analysis of “reactive aldehydes” (RAs) 4-hydroxyhexenal (4-HHE), 4-hydroxynonenal (4-HNE), and 4-oxo-2-nonenal (4-ONE) in the kidney tissues of a diabetic mouse model is presented. Ionization efficiency was enhanced through on-tissue chemical derivatization (OTCD) using Girard’s reagent T (GT), forming positively charged hydrazone derivatives. MSI analysis was performed using matrix-assisted laser desorption ionization (MALDI) coupled with Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR). RA levels were elevated in diabetic kidney tissues compared to lean controls and localized throughout the kidney sections at a spatial resolution of 100 µm. This was confirmed by liquid extraction surface analysis–MSI (LESA-MSI) and liquid chromatography–mass spectrometry (LC–MS). This method identified β-unsaturated aldehydes as “potential” biomarkers of DN and demonstrated the capability of OTCD-MSI for detection and localization of poorly ionizable molecules by adapting existing chemical derivatization methods. Untargeted exploratory distribution analysis of some precursor lipids was also assessed using MALDI-FT-ICR-MSI.

Decreased GM3 correlates with proteinuria in minimal change nephrotic syndrome and focal segmental glomerulosclerosis

BACKGROUND

Glycolipids on cell membrane rafts play various roles by interacting with glycoproteins. Recently, it was reported that the glycolipid GM3 is expressed in podocytes and may play a role in podocyte protection. In this report, we describe the correlation between changes in GM3 expression in glomeruli and proteinuria in minimal change nephrotic syndrome (MCNS) and focal segmental glomerulosclerosis (FSGS) patients.

METHODS

We performed a case-control study of the correlation between nephrin/GM3 expression levels and proteinuria in MCNS and FSGS patients who underwent renal biopsy at our institution between 2009 and 2014. Normal renal tissue sites were used from patients who had undergone nephrectomy at our institution and gave informed consent.

RESULTS

Both MCNS and FSGS had decreased GM3 and Nephrin expression compared with the normal (normal vs. MCNS, FSGS; all p < 0.01). Furthermore, in both MCNS and FSGS, GM3 expression was negatively correlated with proteinuria (MCNS: r = - 0.61, p < 0.01, FSGS: r = - 0.56, p < 0.05). However, nephrin expression had a trend to correlate with proteinuria in FSGS (MCNS: r = 0.19, p = 0.58, FSGS: r = - 0.48, p = 0.06). Furthermore, in a simple linear regression analysis, GM3 expression also correlated with proteinuric change after 12 months of treatment (MCNS: r = 0.40, p = 0.38, FSGS: r = 0. 68, p < 0.05).

CONCLUSION

We showed for the first time that decreased GM3 expression correlates with proteinuria in MCNS and FSGS patients. Further studies are needed on the podocyte-protective effects of GM3.

Lysophosphatidic Acid Is a Proinflammatory Stimulus of Renal Tubular Epithelial Cells

Chronic kidney disease (CKD) refers to a spectrum of diseases defined by renal fibrosis, permanent alterations in kidney structure, and low glomerular-filtration rate. Prolonged epithelial-tubular damage involves a series of changes that eventually lead to CKD, highlighting the importance of tubular epithelial cells in this process. Lysophosphatidic acid (LPA) is a bioactive lipid that signals mainly through its six cognate LPA receptors and is implicated in several chronic inflammatory pathological conditions. In this report, we have stimulated human proximal tubular epithelial cells (HKC-8) with LPA and 175 other possibly pathological stimuli, and simultaneously detected the levels of 27 intracellular phosphoproteins and 32 extracellular secreted molecules with multiplex ELISA. This quantification revealed a large amount of information concerning the signaling and the physiology of HKC-8 cells that can be extrapolated to other proximal tubular epithelial cells. LPA responses clustered with pro-inflammatory stimuli such as TNF and IL-1, promoting the phosphorylation of important inflammatory signaling hubs, including CREB1, ERK1, JUN, IκΒα, and MEK1, as well as the secretion of inflammatory factors of clinical relevance, including CCL2, CCL3, CXCL10, ICAM1, IL-6, and IL-8, most of them shown for the first time in proximal tubular epithelial cells. The identified LPA-induced signal-transduction pathways, which were pharmacologically validated, and the secretion of the inflammatory factors offer novel insights into the possible role of LPA in CKD pathogenesis.

Inhibition of ChREBP ubiquitination via the ROS/Akt-dependent downregulation of Smurf2 contributes to lysophosphatidic acid-induced fibrosis in renal mesangial cells

Background

Mesangial cell fibrosis, a typical symptom of diabetic nephropathy (DN), is a major contributor to glomerulosclerosis. We previously reported that the pharmacological blockade of lysophosphatidic acid (LPA) signaling improves DN. Although LPA signaling is implicated in diabetic renal fibrosis, the underlying molecular mechanisms remain unclear. Here, the role of carbohydrate-responsive element-binding protein (ChREBP) in LPA-induced renal fibrosis and the underlying mechanisms were investigated.

Methods

Eight-week-old wild-type and db/db mice were intraperitoneally injected with the vehicle or an LPAR1/3 antagonist, ki16425 (10 mg/kg), for 8 weeks on a daily basis, following which the mice were sacrificed and renal protein expression was analyzed. SV40 MES13 cells were treated with LPA in the presence or absence of ki16425, and the expression of ChREBP and fibrotic factors, including fibronectin, TGF-β, and IL-1β, was examined. The role of ChREBP in the LPA-induced fibrotic response was investigated by ChREBP overexpression or knockdown. The involvement of Smad ubiquitination regulatory factor-2 (Smurf2), an E3 ligase, in LPA-induced expression of ChREBP and fibrotic factors was investigated by Smurf2 overexpression or knockdown. To identify signaling molecules regulating Smurf2 expression by LPA, pharmacological inhibitors such as A6370 (Akt1/2 kinase inhibitor) and Ly 294002 (PI3K inhibitor) were used.

Results

The renal expression of ChREBP increased in diabetic db/db mice, and was reduced following treatment with the ki16425. Treatment with LPA induced the expression of ChREBP and fibrotic factors, including fibronectin, TGF-β, and IL-1β, in SV40 MES13 cells, which were positively correlated. The LPA-induced expression of fibrotic factors increased or decreased following ChREBP overexpression and knockdown, respectively. The production of reactive oxygen species (ROS) mediated the LPA-induced expression of ChREBP and fibrotic factors, and LPA decreased Smurf2 expression via Traf4-mediated ubiquitination. The LPA-induced expression of ubiquitinated-ChREBP increased or decreased following Smurf2 overexpression and knockdown, respectively. Additionally, Smurf2 knockdown significantly increased the expression of ChREBP and fibrotic factors. The pharmacological inhibition of Akt signaling suppressed the LPA-induced alterations in the expression of ChREBP and Smurf2.

Conclusion

Collectively, the results demonstrated that the ROS/Akt-dependent downregulation of Smurf2 and the subsequent increase in ChREBP expression might be one of the mechanisms by which LPA induces mesangial cell fibrosis in DN.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12929-022-00814-1.

Modified lipid metabolism and cytosolic phospholipase A2 activation in mesangial cells under pro-inflammatory conditions

Diabetic kidney disease is a consequence of hyperglycemia and other complex events driven by early glomerular hemodynamic changes and a progressive expansion of the mesangium. The molecular mechanisms behind the pathophysiological alterations of the mesangium are yet to be elucidated. This study aimed at investigating whether lipid signaling might be the missing link. Stimulation of human mesangial cells with high glucose primed the inflammasome-driven interleukin 1 beta (IL-1β) secretion, which in turn stimulated platelet-derived growth factor (PDGF-BB) release. Finally, PDGF-BB increased IL-1β secretion synergistically. Both IL-1β and PDGF-BB stimulation triggered the formation of phosphorylated sphingoid bases, as shown by lipidomics, and activated cytosolic phospholipase cPLA2, sphingosine kinase 1, cyclooxygenase 2, and autotaxin. This led to the release of arachidonic acid and lysophosphatidylcholine, activating the secretion of vasodilatory prostaglandins and proliferative lysophosphatidic acids. Blocking cPLA2 release of arachidonic acid reduced mesangial cells proliferation and prostaglandin secretion. Validation was performed in silico using the Nephroseq database and a glomerular transcriptomic database. In conclusion, hyperglycemia primes glomerular inflammatory and proliferative stimuli triggering lipid metabolism modifications in human mesangial cells. The upregulation of cPLA2 was critical in this setting. Its inhibition reduced mesangial secretion of prostaglandins and proliferation, making it a potential therapeutical target.

High-Resolution Tandem Mass Spectrometry Identifies a Particular Ganglioside Pattern in Early Diabetic Kidney Disease of Type 2 Diabetes Mellitus Patients

Considering the valuable information provided by glycosphingolipids as molecular markers and the limited data available for their detection and characterization in patients suffering from Type 2 diabetic kidney disease (DKD), we developed and implemented a superior method based on high-resolution (HR) mass spectrometry (MS) and tandem MS (MS/MS) for the determination of gangliosides in the urine of DKD patients. This study was focused on: (i) testing of the HR MS and MS/MS feasibility and performances in mapping and sequencing of renal gangliosides in Type 2 DM patients; (ii) determination of the changes in the urine gangliosidome of DKD patients in different stages of the disease-normo-, micro-, and macroalbuminuria-in a comparative assay with healthy controls. Due to the high resolution and mass accuracy, the comparative MS screening revealed that the sialylation status of the ganglioside components; their modification by O-acetyl, CH3COO-, O-fucosyl, and O-GalNAc; as well as the composition of the ceramide represent possible markers for early DKD detection, the assessment of disease progression, and follow-up treatment. Moreover, structural investigation by MS/MS demonstrated that GQ1d(d18:1/18:0), GT1α(d18:1/18:0) and GT1b(d18:1/18:0) isomers are associated with macroalbuminuria, meriting further investigation in relation to their role in DKD.

Uncovering Molecular Heterogeneity in the Kidney With Spatially Targeted Mass Spectrometry

The kidney functions through the coordination of approximately one million multifunctional nephrons in 3-dimensional space. Molecular understanding of the kidney has relied on transcriptomic, proteomic, and metabolomic analyses of kidney homogenate, but these approaches do not resolve cellular identity and spatial context. Mass spectrometry analysis of isolated cells retains cellular identity but not information regarding its cellular neighborhood and extracellular matrix. Spatially targeted mass spectrometry is uniquely suited to molecularly characterize kidney tissue while retaining in situ cellular context. This review summarizes advances in methodology and technology for spatially targeted mass spectrometry analysis of kidney tissue. Profiling technologies such as laser capture microdissection (LCM) coupled to liquid chromatography tandem mass spectrometry provide deep molecular coverage of specific tissue regions, while imaging technologies such as matrix assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) molecularly profile regularly spaced tissue regions with greater spatial resolution. These technologies individually have furthered our understanding of heterogeneity in nephron regions such as glomeruli and proximal tubules, and their combination is expected to profoundly expand our knowledge of the kidney in health and disease.

Rho GTPases in kidney physiology and diseases

Rho family GTPases are molecular switches best known for their pivotal role in dynamic regulation of the actin cytoskeleton, but also of cellular morphology, motility, adhesion and proliferation. The prototypic members of this family (RhoA, Rac1 and Cdc42) also contribute to the normal kidney function and play important roles in the structure and function of various kidney cells including tubular epithelial cells, mesangial cells and podocytes. The kidney's vital filtration function depends on the structural integrity of the glomerulus, the proximal portion of the nephron. Within the glomerulus, the architecturally actin-based cytoskeleton podocyte forms the final cellular barrier to filtration. The glomerulus appears as a highly dynamic signalling hub that is capable of integrating intracellular cues from its individual structural components. Dynamic regulation of the podocyte cytoskeleton is required for efficient barrier function of the kidney. As master regulators of actin cytoskeletal dynamics, Rho GTPases are therefore of critical importance for sustained kidney barrier function. Dysregulated activities of the Rho GTPases and of their effectors are implicated in the pathogenesis of both hereditary and idiopathic forms of kidney diseases. Diabetic nephropathy is a progressive kidney disease that is caused by injury to kidney glomeruli. High glucose activates RhoA/Rho-kinase in mesangial cells, leading to excessive extracellular matrix production (glomerulosclerosis). This RhoA/Rho-kinase pathway also seems involved in the post-transplant hypertension frequently observed during treatment with calcineurin inhibitors, whereas Rac1 activation was observed in post-transplant ischaemic acute kidney injury.

Differential lipidomics of HK-2 cells and exosomes under high glucose stimulation

Abnormal cellular lipid metabolism has a very important role in the occurrence and progression of diabetic kidney disease (DKD). However, the lipid composition and differential expression by high glucose stimulation of renal tubular cells and their exosomes, which is a vital part of the development of DKD, are largely unknown. In this study, based on targeted lipid analysis by isotope labeling and tandem mass spectrometry, a total of 421 and 218 lipid species were quantified in HK-2 cells and exosomes, respectively. More importantly, results showed that GM3 d18:1/22:0, GM3 d18:1/16:0, GM3 d18:0/16:0, GM3 d18:1/22:1 were significantly increased, while LPE18:1, LPE, CL66:4 (16:1), BMP36:3, CL70:7 (16:1), CL74:8 (16:1) were significantly decreased in high glucose-stimulated HK-2 cells. Also, PG36:1, FFA22:5, PC38:3, SM d18:1/16:1, CE-16:1, CE-18:3, CE-20:5, and CE-22:6 were significantly increased, while GM3 d18:1/24:1, GM3 were significantly decreased in exosomes secreted by high glucose-stimulated HK-2 cells. Furthermore, TAG, PC, CL were decreased significantly in the exosomes comparing with the HK-2 cells, and LPA18:2, LPI22:5, PG32:2, FFA16:1, GM3 d18:1/18:1, GM3 d18:1/20:1, GM3 d18:0/20:0, PC40:6p, TAG52:1(18:1), TAG52:0(18:0), CE-20:5, CE-20:4, CE-22:6 were only found in exosomes. In addition, the expression of PI4P in HK-2 cells decreased under a high glucose state. These data may be useful to provide new targets for exploring the mechanisms of DKD.

The Updates of Podocyte Lipid Metabolism in Proteinuric Kidney Disease

BACKGROUND

Podocytes, functionally specialized and terminally differentiated glomerular visceral epithelial cells, are critical for maintaining the structure and function of the glomerular filtration barrier. Podocyte injury is considered as the most important early event contributing to proteinuric kidney diseases such as obesity-related renal disease, diabetic kidney disease, focal segmental glomerulosclerosis, membranous nephropathy, and minimal change disease. Although considerable advances have been made in the understanding of mechanisms that trigger podocyte injury, cell-specific and effective treatments are not clinically available.

SUMMARY

Emerging evidence has indicated that the disorder of podocyte lipid metabolism is closely associated with various proteinuric kidney diseases. Excessive lipid accumulation in podocytes leads to cellular dysfunction which is defined as lipotoxicity, a phenomenon characterized by mitochondrial oxidative stress, actin cytoskeleton remodeling, insulin resistance, and inflammatory response that can eventually result in podocyte hypertrophy, detachment, and death. In this review, we summarize recent advances in the understanding of lipids in podocyte biological function and the regulatory mechanisms leading to podocyte lipid accumulation in proteinuric kidney disease.

KEY MESSAGES

Targeting podocyte lipid metabolism may represent a novel therapeutic strategy for patients with proteinuric kidney disease.

Sodium-Glucose Cotransporter 2 Inhibitors and the Kidney

Diabetic kidney disease (DKD) accounts for about half of individuals entering end-stage renal disease programs. Patients with DKD frequently have associated microvascular complications and are at very high risk for developing macrovascular complications. Comprehensive treatment involves slowing or preventing the decline in glomerular filtration rate (GFR) and preventing macrovascular and further microvascular complications. Maintaining an A1C <6.5% represents primary prevention; in established DKD, tight blood pressure control is essential. ACE inhibitors/angiotensin receptor blockers (ARBs) and sodium-glucose cotransporter 2 (SGLT2) inhibitors can be used in combination to slow the rate of decline in GFR. This article reviews the general approach to DKD treatment and summarizes renal outcomes in four cardiovascular outcomes trials of SGLT2 inhibitors. Together, these trials provide conclusive evidence that SGLT2 inhibitors, added to an ACE inhibitor or ARB, slow the progression of DKD.

Urinary 2-Hydroxyglutarate Enantiomers Are Markedly Elevated in a Murine Model of Type 2 Diabetic Kidney Disease

Metabolic reprogramming is a hallmark of diabetic kidney disease (DKD); nutrient overload leads to increased production of metabolic byproducts that may become toxic at high levels. One metabolic byproduct may be 2-hydroxyglutarate (2-HG), a metabolite with many regulatory functions that exists in both enantiomeric forms physiologically. We quantitatively determined the levels of L and D-2HG enantiomers in the urine, plasma, and kidney cortex of db/db mice, a pathophysiologically relevant murine model of type 2 diabetes and DKD. We found increased fractional excretion of both L and D-2HG enantiomers, suggesting increased tubular secretion and/or production of the two metabolites in DKD. Quantitation of TCA cycle metabolites in db/db cortex suggests that TCA cycle overload and an increase in 2-HG precursor substrate, α-ketoglutarate, drive the increased L and D-2HG production in DKD. In conclusion, we demonstrated increased 2-HG enantiomer production and urinary excretion in murine type 2 DKD, which may contribute to metabolic reprogramming and progression of diabetic kidney disease.

Renin-angiotensin system inhibition reverses the altered triacylglycerol metabolic network in diabetic kidney disease

OBJECTIVE

Dyslipidemia is a significant risk factor for progression of diabetic kidney disease (DKD). Determining the changes in individual lipids and lipid networks across a spectrum of DKD severity may identify lipids that are pathogenic to DKD progression.

METHODS

We performed untargeted lipidomic analysis of kidney cortex tissue from diabetic db/db and db/db eNOS^-/- mice along with non-diabetic littermate controls. A subset of mice were treated with the renin-angiotensin system (RAS) inhibitors, lisinopril and losartan, which improves the DKD phenotype in the db/db eNOS^-/- mouse model.

RESULTS

Of the three independent variables in this study, diabetes had the largest impact on overall lipid levels in the kidney cortex, while eNOS expression and RAS inhibition had smaller impacts on kidney lipid levels. Kidney lipid network architecture, particularly of networks involving glycerolipids such as triacylglycerols, was substantially disrupted by worsening kidney disease in the db/db eNOS^-/- mice compared to the db/db mice, a feature that was reversed with RAS inhibition. This was associated with decreased expression of the stearoyl-CoA desaturases, Scd1 and Scd2, with RAS inhibition.

CONCLUSIONS

In addition to the known salutary effect of RAS inhibition on DKD progression, our results suggest a previously unrecognized role for RAS inhibition on the kidney triacylglycerol lipid metabolic network.

High-Resolution Human Kidney Molecular Histology by Imaging Mass Spectrometry of Lipids

For many years, traditional histology has been the gold standard for the diagnosis of many diseases. However, alternative and powerful techniques have appeared in recent years that complement the information extracted from a tissue section. One of the most promising techniques is imaging mass spectrometry applied to lipidomics. Here, we demonstrate the capabilities of this technique to highlight the architectural features of the human kidney at a spatial resolution of 10 μm. Our data demonstrate that up to seven different segments of the nephron and the interstitial tissue can be readily identified in the sections according to their characteristic lipid fingerprints and that such fingerprints are maintained among different individuals (n = 32). These results set the foundation for further studies on the metabolic bases of the diseases affecting the human kidney.

Hydralazine as a Versatile and Universal Matrix for High-Molecular Coverage and Dual-Polarity Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging

Few matrices have the potential to be universally applicable for imaging vast endogenous compounds ranging from micro to macromolecules. In this article, we present hydralazine (HZN) as a versatile and universal matrix for matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) of a wide range of endogenous compounds between 50.0 and 20,000.0 Da. HZN was prepared from its hydrochloride by alkalizing HZN·HCl with ammonia to enhance the optical absorptivity at the preferred MALDI UV laser wavelength. To further improve its performance for MALDI MS, HZN was doped with NH₄OH or TFA, resulting in matrix superior performance for imaging biologically relevant compounds in the negative and positive-ion modes, respectively. The analyte-matrix interaction was also enhanced by the optimized matrix solvent and the deposition amount. Compared with conventional matrices such as 2,5-dihydroxybenzoic acid, α-cyano-4-hydroxycinnamic acid, and 9-aminoacridine (9-AA), the HZN matrix provided higher sensitivity, broader molecular coverage, and improved signal intensities. Its broad acquisition range makes it versatile for imaging small molecular metabolites and lipids, as well as proteins. In addition, HZN was applied successfully for the visualization of tissue-specific distributions and changes of small molecules, lipids, and proteins in the kidney and liver sections of obese ob/ob and diabetic db/db mice. The use of the HZN matrix shows great potential application in the field of pathological research.

Pathophysiology of diabetic kidney disease: impact of SGLT2 inhibitors

Diabetic kidney disease is the leading cause of kidney failure worldwide; in the USA, it accounts for over 50% of individuals entering dialysis or transplant programmes. Unlike other complications of diabetes, the prevalence of diabetic kidney disease has failed to decline over the past 30 years. Hyperglycaemia is the primary aetiological factor responsible for the development of diabetic kidney disease. Once hyperglycaemia becomes established, multiple pathophysiological disturbances, including hypertension, altered tubuloglomerular feedback, renal hypoxia, lipotoxicity, podocyte injury, inflammation, mitochondrial dysfunction, impaired autophagy and increased activity of the sodium-hydrogen exchanger, contribute to progressive glomerular sclerosis and the decline in glomerular filtration rate. The quantitative contribution of each of these abnormalities to the progression of diabetic kidney disease, as well as their role in type 1 and type 2 diabetes mellitus, remains to be determined. Sodium-glucose co-transporter 2 (SGLT2) inhibitors have a beneficial impact on many of these pathophysiological abnormalities; however, as several pathophysiological disturbances contribute to the onset and progression of diabetic kidney disease, multiple agents used in combination will likely be required to slow the progression of disease effectively.

Gangliosides in Podocyte Biology and Disease

Gangliosides constitute a subgroup of glycosphingolipids characterized by the presence of sialic acid residues in their structure. As constituents of cellular membranes, in particular of raft microdomains, they exert multiple functions, some of them capital in cell homeostasis. Their presence in cells is tightly regulated by a balanced expression and function of the enzymes responsible for their biosynthesis, ganglioside synthases, and their degradation, glycosidases. The dysregulation of their abundance results in rare and common diseases. In this review, we make a point on the relevance of gangliosides and some of their metabolic precursors, such as ceramides, in the function of podocytes, the main cellular component of the glomerular filtration barrier, as well as their implications in podocytopathies. The results presented in this review suggest the pertinence of clinical lipidomic studies targeting these metabolites.

Increased Levels of Renal Lysophosphatidic Acid in Rodent Models with Renal Disease

Lysophosphatidic acid (LPA) is a bioactive lipid mediator that has been implicated in the pathophysiology of kidney disease. However, few studies have attempted to measure changes in the levels of various LPA species in the kidney after the development of renal disease. The present study measured the renal LPA levels during the development of kidney disease in rat models of hypertension, diabetes, and obstructive nephropathy using liquid chromatography/mass spectrometry/mass spectrometry. LPA levels (sum of 16:0, 18:0, 18:1, 18:2, and 20:4 LPA) were higher in the renal cortex of hypertensive Dahl salt-sensitive (Dahl S) rats fed a high-salt diet than those in normotensive rats fed a low-salt diet (296.6 ± 22.9 vs. 196.3 ± 8.5 nmol/g protein). LPA levels were elevated in the outer medulla of the kidney of streptozotocin-induced type 1 diabetic Dahl S rats compared with control rats (624.6 ± 129.5 vs. 318.8 ± 17.1 nmol/g protein). LPA levels were also higher in the renal cortex of 18-month-old, type 2 diabetic nephropathy (T2DN) rats with more severe renal injury than in 6-month-old T2DN rats (184.9 ± 20.9 vs. 116.9 ± 6.0 nmol/g protein). LPA levels also paralleled the progression of renal fibrosis in the renal cortex of Sprague-Dawley rats after unilateral ureteral obstruction (UUO). Administration of an LPA receptor antagonist, Ki16425, reduced the degree of renal fibrosis in UUO rats. These results suggest that the production of renal LPA increases during the development of renal injury and contributes to renal fibrosis. SIGNIFICANCE STATEMENT: The present study reveals that the lysophosphatidic acid (LPA) levels increase in the kidney in rat models of hypertension, diabetes, and obstructive nephropathy, and administration of an LPA receptor antagonist attenuates renal fibrosis. Therapeutic approaches that target the formation or actions of renal LPA might be renoprotective and have therapeutic potential.

Storage Conditions of Human Kidney Tissue Sections Affect Spatial Lipidomics Analysis Reproducibility

Lipids often are labile, unstable, and tend to degrade overtime, so it is of the upmost importance to study these molecules in their most native state. We sought to understand the optimal storage conditions for spatial lipidomic analysis of human kidney tissue sections. Specifically, we evaluated human kidney tissue sections on several different days throughout the span of a week using our established protocol for elucidating lipids using high mass resolution matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). We studied kidney tissue sections stored under five different conditions: open stored at -80 °C, vacuumed sealed and stored at -80 °C, with matrix preapplied before storage at -80 °C, under a nitrogen atmosphere and stored at -80 °C, and at room temperature in a desiccator. Results were compared to data obtained from kidney tissue sections that were prepared and analyzed immediately after cryosectioning. Data was processed using METASPACE. After a week of storage, the sections stored at room temperature showed the largest amount of lipid degradation, while sections stored under nitrogen and at -80 °C retained the greatest number of overlapping annotations in relation to freshly cut tissue. Overall, we found that molecular degradation of the tissue sections was unavoidable over time, regardless of storage conditions, but storing tissue sections in an inert gas at low temperatures can curtail molecular degradation within tissue sections.

The roles of melatonin on kidney injury in obese and diabetic conditions

Obesity is a common and complex health problem worldwide and can induce the development of Type 2 diabetes. Chronic kidney disease (CKD) is a complication occurring as a result of obesity and diabetic conditions that lead to an increased mortality rate. There are several mechanisms and pathways contributing to kidney injury in obese and diabetic conditions. The expansion of adipocytes triggers proinflammatory cytokines release into blood circulation and bind with the receptors at the cell membranes of renal tissues leading to kidney injury. Obesity-mediated inflammation, oxidative stress, apoptosis, and mitochondrial dysfunction are the important causes and progression of CKD. Melatonin (N-acetyl-5-methoxytryptamine) is a neuronal hormone that is synthesized by the pineal gland and plays an essential role in regulating several physiological functions in the human body. Moreover, melatonin has pleiotropic effects such as antioxidant, anti-inflammation, antiapoptosis. In this review, the relationship between obesity, diabetic condition, and kidney injury and the renoprotective effect of melatonin in obese and diabetic conditions from in vitro and in vivo studies have been summarized and discussed.

Comprehensive Lipidome Profiling of the Kidney in Early-Stage Diabetic Nephropathy

Metabolic changes associated with diabetes are reported to lead to the onset of early-stage diabetic nephropathy (DN). Furthermore, lipotoxicity is implicated in renal dysfunction. Most studies of DN have focused on a single or limited number of lipids, and the lipidome of the kidney during early-stage DN remains to be elucidated. In the present study, we aimed to comprehensively identify lipid abnormalities during early-stage DN; to this end, we established an early-stage DN rat model by feeding a high-sucrose and high-fat diet combined with administration of low-dose streptozotocin. Using a high-coverage, targeted lipidomic approach, we established the lipid profile, comprising 437 lipid species and 25 lipid classes, of the kidney cortex in normal rats and the DN rat model. Our findings additionally confirmed that the DN rat model had been successfully established. We observed distinct lipidomic signatures in the DN kidney, with characteristic alterations in side chain composition and degree of unsaturation. Glyceride lipids, especially cholesteryl esters, showed a significant increase in the DN kidney cortex. The levels of most phospholipids exhibited a decline, except those of phospholipids with side chain of 36:1. Furthermore, the levels of lyso-phospholipids and sphingolipids, including ceramide and its derivatives, were dramatically elevated in the present DN rat model. Our findings, which provide a comprehensive lipidome of the kidney cortex in rats with DN, are expected to be useful for the identification of pathologically relevant lipid species in DN. Furthermore, the results represent novel insights into the mechanistic basis of DN.

Targeting glycosphingolipid metabolism as a potential therapeutic approach for treating disease in female MRL/lpr lupus mice

Glycosphingolipids (GSLs) hexosylceramides and lactosylceramides are elevated in lupus mice and human patients with nephritis. Whereas other renal diseases characterized by increased GSL levels are thought to be a result of upregulated GSL synthesis, our results suggest elevated hexosylceramides and lactosylceramides in lupus nephritis is a result of increased catabolism of ganglioside GM3 due to significantly increased neuraminidase (NEU) activity. Thus, we hypothesized GM3 would be decreased in lupus nephritis kidneys and blocking NEU activity would reduce GSLs and improve disease in lupus mice. Female MRL/lpr lupus mice were treated with water or the NEU inhibitor oseltamivir phosphate at the onset of proteinuria to block GSL catabolism. Age-matched (non-nephritic) female MRL/MpJ lupus mice served as controls. Renal GM3 levels were significantly higher in the nephritic MRL/lpr water-treated mice compared to non-nephritic MRL/MpJ mice, despite significantly increased renal NEU activity. Blocking GSL catabolism increased, rather than decreased, renal and urine GSL levels and disease was not significantly impacted. A pilot study treating MRL/lpr females with GlcCer synthase inhibitor Genz-667161 to block GSL synthesis resulted in a strong significant negative correlation between Genz-667161 dose and renal GSL hexosylceramide and GM3 levels. Splenomegaly was negatively correlated and serum IgG levels were marginally correlated with increasing Genz-667161 dose. These results suggest accumulation of renal GM3 may be due to dysregulation of one or more of the GSL ganglioside pathways and inhibiting GSL synthesis, but not catabolism, may be a therapeutic approach for treating lupus nephritis.

DESI-MSI and METASPACE indicates lipid abnormalities and altered mitochondrial membrane components in diabetic renal proximal tubules

INTRODUCTION

Diabetic kidney disease (DKD) is the most prevalent complication in diabetic patients, which contributes to high morbidity and mortality. Urine and plasma metabolomics studies have been demonstrated to provide valuable insights for DKD. However, limited information on spatial distributions of metabolites in kidney tissues have been reported.

OBJECTIVES

In this work, we employed an ambient desorption electrospray ionization-mass spectrometry imaging (DESI-MSI) coupled to a novel bioinformatics platform (METASPACE) to characterize the metabolome in a mouse model of DKD.

METHODS

DESI-MSI was performed for spatial untargeted metabolomics analysis in kidneys of mouse models (F1 C57BL/6J-Ins2Akita male mice at 17 weeks of age) of type 1 diabetes (T1D, n = 5) and heathy controls (n = 6).

RESULTS

Multivariate analyses (i.e., PCA and PLS-DA (a 2000 permutation test: P < 0.001)) showed clearly separated clusters for the two groups of mice on the basis of 878 measured m/z's in kidney cortical tissues. Specifically, mice with T1D had increased relative abundances of pseudouridine, accumulation of free polyunsaturated fatty acids (PUFAs), and decreased relative abundances of cardiolipins in cortical proximal tubules when compared with healthy controls.

CONCLUSION

Results from the current study support potential key roles of pseudouridine and cardiolipins for maintaining normal RNA structure and normal mitochondrial function, respectively, in cortical proximal tubules with DKD. DESI-MSI technology coupled with METASPACE could serve as powerful new tools to provide insight on fundamental pathways in DKD.

Rotten to the Cortex: Ceramide-Mediated Lipotoxicity in Diabetic Kidney Disease

Diabetic kidney disease (DKD) is a prevalent and progressive comorbidity of diabetes mellitus that increases one's risk of developing renal failure. Progress toward development of better DKD therapeutics is limited by an incomplete understanding of forces driving and connecting the various features of DKD, which include renal steatosis, fibrosis, and microvascular dysfunction. Herein we review the literature supporting roles for bioactive ceramides as inducers of local and systemic DKD pathology. In rodent models of DKD, renal ceramides are elevated, and genetic and pharmacological ceramide-lowering interventions improve kidney function and ameliorate DKD histopathology. In humans, circulating sphingolipid profiles distinguish human DKD patients from diabetic controls. These studies highlight the potential for ceramide to serve as a central and therapeutically tractable lipid mediator of DKD.

Renal Lipid Metabolism and Lipotoxicity in Diabetes

The pathogenesis of diabetic kidney disease is a complex process caused by both glucotoxicity and lipotoxicity due to lipid accumulation. In cases of diabetic animals, lipid deposition is found in both tubular and glomerular portions of the kidneys, which are the major sites of diabetic nephropathy lesions. The aim of this review was to provide insights into the mechanisms that lead to the development of renal lipid accumulation and the effects of renal lipotoxicity in the diabetic condition. An increased number of lipogenic genes and a decreased number of lipid oxidation genes are also detected in diabetic kidneys, both of which lead to lipid accumulation. The induction of oxidative stress, inflammation, fibrosis and apoptosis caused by lipid accumulation and lipid metabolites is called lipotoxicity. Renal lipotoxicity due to derangement in lipid metabolism may be a pathogenic mechanism leading to diabetic nephropathy and renal dysfunction.

Increased lipogenesis and impaired β-oxidation predict type 2 diabetic kidney disease progression in American Indians

BACKGROUNDIn this study, we identified the lipidomic predictors of early type 2 diabetic kidney disease (DKD) progression, which are currently undefined.METHODSThis longitudinal study included 92 American Indians with type 2 diabetes. Serum lipids (406 from 18 classes) were quantified using mass spectrometry from baseline samples when iothalamate-based glomerular filtration rate (GFR) was at least 90 mL/min. Affymetrix GeneChip Array was used to measure renal transcript expression. DKD progression was defined as at least 40% decline in GFR during follow-up.RESULTSParticipants had a mean age of 45 ± 9 years and median urine albumin/creatinine ratio of 43 (interquartile range 11-144). The 32 progressors had significantly higher relative abundance of polyunsaturated triacylglycerols (TAGs) and a lower abundance of C16-C20 acylcarnitines (ACs) (P < 0.001). In a Cox regression model, the main effect terms of unsaturated free fatty acids and phosphatidylethanolamines and the interaction terms of C16-C20 ACs and short-low-double-bond TAGs by categories of albuminuria independently predicted DKD progression. Renal expression of acetyl-CoA carboxylase-encoding gene (ACACA) correlated with serum diacylglycerols in the glomerular compartment (r = 0.36, and P = 0.006) and with low-double-bond TAGs in the tubulointerstitial compartment (r = 0.52, and P < 0.001).CONCLUSIONCollectively, the findings reveal a previously unrecognized link between lipid markers of impaired mitochondrial β-oxidation and enhanced lipogenesis and DKD progression in individuals with preserved GFR. Renal acetyl-CoA carboxylase activation accompanies these lipidomic changes and suggests that it may be the underlying mechanism linking lipid abnormalities to DKD progression.TRIAL REGISTRATIONClinicalTrials.gov, NCT00340678.FUNDINGNIH R24DK082841, K08DK106523, R03DK121941, P30DK089503, P30DK081943, and P30DK020572.

Two Specific Sulfatide Species Are Dysregulated during Renal Development in a Mouse Model of Alport Syndrome

Alport syndrome is caused by mutations in collagen IV that alter the morphology of renal glomerular basement membrane. Mutations result in proteinuria, tubulointerstitial fibrosis, and renal failure but the pathogenic mechanisms are not fully understood. Using imaging mass spectrometry, we aimed to determine whether the spatial and/or temporal patterns of renal lipids are perturbed during the development of Alport syndrome in the mouse model. Our results show that most sulfatides are present at similar levels in both the wild-type (WT) and the Alport kidneys, with the exception of two specific sulfatide species, SulfoHex-Cer(d18:2/24:0) and SulfoHex-Cer(d18:2/16:0). In the Alport but not in WT kidneys, the levels of these species mirror the previously described abnormal laminin expression in Alport syndrome. The presence of these sulfatides in renal tubules but not in glomeruli suggests that this specific aberrant lipid pattern may be related to the development of tubulointerstitial fibrosis in Alport disease.

Globo-series glycosphingolipids enhance Toll-like receptor 4-mediated inflammation and play a pathophysiological role in diabetic nephropathy

Alteration of glycosphingolipid (GSL) expression plays key roles in the pathogenesis and pathophysiology of many important human diseases, including cancer, diabetes and glycosphingolipidosis. Inflammatory processes are involved in development and progression of diabetic nephropathy, a major complication of type 2 diabetes mellitus. GSLs are known to play roles in inflammatory responses in various diseases, and levels of renal GSLs are elevated in mouse models of diabetic nephropathy; however, little is known regarding the pathophysiological role of these GSLs in this disease process. We studied proinflammatory activity of GSLs in diabetic nephropathy using spontaneously diabetic mouse strain KK. Mice were fed a high-fat diet (HFD) (60% kcal from fat) or normal diet (ND) (4.6% kcal from fat) for a period of 8 wk. HFD-feeding resulted in quantitative and qualitative changes of renal globo-series GSLs (particularly Gb3Cer), upregulation of TNF-α, and induction of renal inflammation. Gb3Cer/Gb4Cer treatment enhanced inflammatory responses via TLR4 in TLR4/MD-2 complex expressing cells, including HEK293T, mouse bone marrow-derived macrophages (BMDMs) and human monocytes. Our findings suggest that HFD-induced increase of Gb3Cer/Gb4Cer positively modulate TLR4-mediated inflammatory response, and that such GSLs play an important pathophysiological role in diabetic nephropathy.

Metabolic Imaging at the Single-Cell Scale: Recent Advances in Mass Spectrometry Imaging

There is an increasing appreciation that every cell, even of the same type, is different. This complexity, when additionally combined with the variety of different cell types in tissue, is driving the need for spatially resolved omics at the single-cell scale. Rapid advances are being made in genomics and transcriptomics, but progress in metabolomics lags. This is partly because amplification and tagging strategies are not suited to dynamically created metabolite molecules. Mass spectrometry imaging has excellent potential for metabolic imaging. This review summarizes the recent advances in two of these techniques: matrix-assisted laser desorption ionization (MALDI) and secondary ion mass spectrometry (SIMS) and their convergence in subcellular spatial resolution and molecular information. The barriers that have held back progress such as lack of sensitivity and the breakthroughs that have been made including laser-postionization are highlighted as well as the future challenges and opportunities for metabolic imaging at the single-cell scale.

Lysophosphatidic Acid Signaling in Diabetic Nephropathy

Lysophosphatidic acid (LPA) is a bioactive phospholipid present in most tissues and body fluids. LPA acts through specific LPA receptors (LPAR1 to LPAR6) coupled with G protein. LPA binds to receptors and activates multiple cellular signaling pathways, subsequently exerting various biological functions, such as cell proliferation, migration, and apoptosis. LPA also induces cell damage through complex overlapping pathways, including the generation of reactive oxygen species, inflammatory cytokines, and fibrosis. Several reports indicate that the LPA–LPAR axis plays an important role in various diseases, including kidney disease, lung fibrosis, and cancer. Diabetic nephropathy (DN) is one of the most common diabetic complications and the main risk factor for chronic kidney diseases, which mostly progress to end-stage renal disease. There is also growing evidence indicating that the LPA–LPAR axis also plays an important role in inducing pathological alterations of cell structure and function in the kidneys. In this review, we will discuss key mediators or signaling pathways activated by LPA and summarize recent research findings associated with DN.

Metabolite aberrations in early diabetes detected in rat kidney using mass spectrometry imaging

Diabetic kidney disease is a serious complication of diabetes that can ultimately lead to end-stage renal disease. The pathogenesis of diabetic kidney disease is complex, and fundamental research is still required to provide a better understanding of the driving forces behind it. We report regional metabolic aberrations from an untargeted mass spectrometry imaging study of kidney tissue using an insulinopenic rat model of diabetes. Diabetes was induced by intravenous injection of streptozotocin, and kidneys were harvested 2 weeks thereafter. Imaging was performed using nanospray desorption electrospray ionization connected to a high-mass-resolving mass spectrometer. No histopathological changes were observed in the kidney sections; however, mass spectrometry imaging revealed a significant increase in several 18-carbon unsaturated non-esterified fatty acid species and monoacylglycerols. Notably, these 18-carbon acyl chains were also constituents of several increased diacylglycerol species. In addition, a number of short- and long-chain acylcarnitines were found to be accumulated while several amino acids were depleted. This study presents unique regional metabolic data indicating a dysregulated energy metabolism in renal mitochondria as an early response to streptozotocin-induced type I diabetes. Graphical abstract.

Long-Term Intensive Lifestyle Intervention Promotes Improvement of Stage III Diabetic Nephropathy

BACKGROUND Diabetic nephropathy (DN) is a potentially fatal complication of diabetes mellitus. While lifestyle changes can reduce diabetes risk, it is unclear whether improved lifestyle can slow or reverse DN progression. This study evaluated whether an intensive lifestyle intervention (IL-I) targeting weight loss and inflammation through increased physical activity and reduced caloric intake can delay DN progression. MATERIAL AND METHODS Patients were randomly divided into 2 groups. Both groups received diet and exercise guidelines, but one (IL-I) received more frequent external support than the other (control). We compared markers of metabolic and cardiovascular health, redox status, inflammation, and renal function between groups at 3 and 6 months. Metabolic and cardiovascular metrics included BMI, blood pressure, blood glycosylated hemoglobin (HbA1c), and serum HDL-cholesterol. Redox status was evaluated by serum superoxide dismutase (SOD) and the lipid oxidation product malondialdehyde (MDA), while inflammation was assessed by serum concentrations of IL-6 and TNF-alpha. Renal function was assessed by urine/serum 8-OHdG, albumin: creatinine ratio (ACR), and the renal fibrosis marker TGF-ß1. RESULTS Both groups demonstrated initial BMI reduction, lower HbA1c, and higher HDL-cholesterol, but changes were significantly larger in the IL-I group at 6 months. Blood pressure at 6 months was reduced only in the IL-I group. The IL-I group also achieved a greater sustained SOD increase and MDA reduction. Finally, only the IL-I group demonstrated significant reductions in urine ACR, serum/urine 8-OHdG, and plasma TGF-ß1. These indicators deteriorated after IL-I was stopped. CONCLUSIONS Lifestyle changes including exercise and diet can delay renal damage and promote improvement from DN.

Glycosylated sphingolipids and progression to kidney dysfunction in type 1 diabetes

BACKGROUND

Glycosphingolipids are important components of cell membranes, modulators of cell-cell interactions and cell recognition, and have recently emerged as bioactive molecules and important players in nearly all cell biological processes. We previously have shown that decreased plasma levels of long and very long species of ceramides were able to predict the development of macroalbuminuria (MA) in type 1 diabetes.

OBJECTIVE

This study proposed to examine whether plasma glycosphingolipids could predict development of diabetic nephropathy, assessed as MA or chronic kidney disease (CKD).

METHODS

Measurement of plasma hexosylceramides (H) and lactosylceramides (L) were conducted in the Lipidomics Core Facility of our Institution in a subcohort of 432 patients from the DCCT/Epidemiology of Diabetes Interventions and Complications cohort in plasma collected at entry into the study. Inverse probability weighted Cox proportional hazards regression models were used to assess the effect of glycosphingolipids levels on the risk of developing MA (albumin excretion rate ≥300 mg/24 hours) or CKD (glomerular filtration rate <60 mL/min) over a period of 21 to 28 years.

RESULTS

Decreases of several long and very long chain lactosylceramides were significantly associated with increased risk of progression to MA but not CKD. Among the hexosylceramides, the only significant association observed was between one of its minor species C18:1-H and CKD.

CONCLUSION

Our findings showed that decreased levels of long and very long lactosylceramides were able to predict the development of MA in type 1 diabetes. This finding is similar to previous findings showing that low levels of long and very long ceramides were also able to predict development of MA in the same cohort. Further studies are needed to determine the changes in sphingolipid metabolism leading to the development of complications.