Adiponectin Reduces Glomerular Endothelial Glycocalyx Disruption and Restores Glomerular Barrier Function in a Mouse Model of Type 2 Diabetes

Adiponectin has vascular anti-inflammatory and protective effects. Although adiponectin protects against the development of albuminuria, historically, the focus has been on podocyte protection within the glomerular filtration barrier (GFB). The first barrier to albumin in the GFB is the endothelial glycocalyx (eGlx), a surface gel-like barrier covering glomerular endothelial cells (GEnCs). In diabetes, eGlx dysfunction occurs before podocyte damage; hence, we hypothesized that adiponectin could protect from eGlx damage to prevent early vascular damage in diabetic kidney disease (DKD). Globular adiponectin (gAd) activated AMPK signaling in human GEnCs through AdipoR1. It significantly reduced eGlx shedding and the tumor necrosis factor-α (TNF-α)-mediated increase in syndecan-4 (SDC4) and MMP2 mRNA expression in GEnCs in vitro. It protected against increased TNF-α mRNA expression in glomeruli isolated from db/db mice and against expression of genes associated with glycocalyx shedding (namely, SDC4, MMP2, and MMP9). In addition, gAd protected against increased glomerular albumin permeability (Ps'alb) in glomeruli isolated from db/db mice when administered intraperitoneally and when applied directly to glomeruli (ex vivo). Ps'alb was inversely correlated with eGlx depth in vivo. In summary, adiponectin restored eGlx depth, which was correlated with improved glomerular barrier function, in diabetes.

ARTICLE HIGHLIGHTS

Imaging Reveals Importance of Shape and Flexibility for Glomerular Filtration of Biologics

Advances in antibody engineering have enabled the construction of novel molecular formats in diverse shapes and sizes, providing new opportunities for cancer immunotherapeutic drug discovery while also revealing limitations in knowledge of structure-activity relationships. The current understanding of renal filtration originates largely from data reported for dextrans, IgG, albumin, and selected globular proteins. For a one-armed IgG-based T-cell imaging agent, we observed higher renal signal than typically observed for bivalent IgGs, prompting us to explore the factors governing renal filtration of biologics. We constructed a small representative library of IgG-like formats with varied shapes and hinge flexibilities falling broadly into two categories: branched molecules including bivalent IgG and (scFv)2Fc, and nonbranched molecules including one-armed IgG, one-armed IgG with stacked Fab, and one-armed IgG with a rigid IgA2 hinge. Transmission electron microscopy revealed Y-shaped structures for the branched molecules and pseudo-linear structures for the nonbranched molecules. Single-photon emission CT imaging, autoradiography, and tissue harvest studies demonstrated higher renal uptake and catabolism for nonbranched molecules relative to branched molecules. Among the nonbranched molecules, the one-armed IgG with rigid IgA2 hinge molecule demonstrated higher kidney uptake and decreased systemic exposure relative to molecules with a more flexible hinge. Our results show that differences in shape and hinge flexibility drive the increased glomerular filtration of one-armed relative to bivalent antibodies and highlight the practical advantages of using imaging to assess renal filtration properties. These findings are particularly relevant for T-cell-dependent bispecific molecules, many of which have nonstandard antibody structures.

Janus kinase 2/signal transducer and activator of transcription 3 inhibitors attenuate the effect of cardiotrophin-like cytokine factor 1 and human focal segmental glomerulosclerosis serum on glomerular filtration barrier

Recurrence of idiopathic focal segmental glomerulosclerosis (FSGS) after renal transplantation is believed to be caused by a circulating factor(s). We detected cardiotrophin-like cytokine factor 1 (CLCF1), a member of the interleukin 6 family, in the plasma from patients with recurrent FSGS. We hypothesized that CLCF1 contributes to the effect of FSGS serum on the glomerular filtration barrier in vitro. Presently, we studied the effect of CLCF1 on isolated rat glomeruli using an in vitro assay of albumin permeability (P(alb)). CLCF1 (0.05-100 ng/mL) increased P(alb) and caused maximal effect at 5-10 ng/mL (P < 0.001). The increase in Palb was analogous to the effect of FSGS serum. Anti-CLCF1 monoclonal antibody blocked the CLCF1-induced increase in P(alb) and significantly attenuated the effect of FSGS serum (P < 0.001). The heterodimer composed of CLCF1 and cosecreted molecule cytokine receptor-like factor 1 (CRLF1) attenuated the increase in P(alb) caused by CLCF1 or FSGS serum. Western blot analysis showed that CLCF1 upregulated phosphorylation of signal transducer and activator of transcription 3 (STAT3) (Tyr705) in glomeruli. This effect was diminished by the heterodimer CLCF1-CRLF1. Janus kinase 2 (JAK2) inhibitor BMS-1119543 or STAT3 inhibitor Stattic significantly blocked the effect of CLCF1 or FSGS serum on P(alb) (P < 0.001). These novel findings suggest that when monomeric CLCF1 increases P(alb), the heterodimer CLCF1-CRLF1 may protect the glomerular filtration barrier. We speculate that albuminuria in FSGS is related to qualitative or quantitative changes in the CLCF1-CRLF1 complex, and that JAK2 or STAT3 inhibitors may be novel therapeutic agents to treat FSGS.

DPP4 inhibition attenuates filtration barrier injury and oxidant stress in the zucker obese rat

OBJECTIVE

Obesity-related glomerulopathy is characterized initially by glomerular hyperfiltration with hypertrophy and then development of proteinuria. Putative mechanisms include endothelial dysfunction and filtration barrier injury due to oxidant stress and immune activation. There has been recent interest in targeting dipeptidyl peptidase 4 (DPP4) enzyme due to increasing role in non-enzymatic cellular processes.

METHODS

The Zucker obese (ZO) rat (aged 8 weeks) fed a normal chow or diet containing the DPP4 inhibitor linagliptin for 8 weeks (83 mg/kg rat chow) was utilized.

RESULTS

Compared to lean controls, there were increases in plasma DPP4 activity along with proteinuria in ZO rats. ZO rats further displayed increases in glomerular size and podocyte foot process effacement. These findings occurred in parallel with decreased endothelial stromal-derived factor-1α (SDF-1α), increased oxidant markers, and tyrosine phosphorylation of nephrin and serine phosphorylation of the mammalian target of rapamycin (mTOR). DPP4 inhibition improved proteinuria along with filtration barrier remodeling, circulating and kidney tissue DPP4 activity, increased active glucagon like peptide-1 (GLP-1) as well as SDF-1α, and improved oxidant markers and the podocyte-specific protein nephrin.

CONCLUSIONS

These data support a role for DPP4 in glomerular filtration function and targeting DPP4 with inhibition improves oxidant stress-related glomerulopathy and associated proteinuria.

Impact of high glucose and transforming growth factor-β on bioenergetic profiles in podocytes

Diabetic nephropathy is the most common cause of chronic renal failure in industrialized countries. Depletion of podocytes plays an important role in the progression of diabetic glomerulopathy. Various factors in the diabetic milieu lead to serious podocyte stress driving the cells toward cell cycle arrest (p27(Kip1)), hypertrophy, detachment, and apoptosis. Mitochondria are responsible for oxidative phosphorylation and energy supply in podocytes. Recent studies indicated that mitochondrial dysfunction is a key factor in diabetic nephropathy. In the present study, we investigated metabolic profiles of podocytes under diabetic conditions. We examined oxygen consumption rates (OCRs) and oxidative phosphorylation complex activities in murine podocytes. Cells were exposed to high glucose for 48 hours, cultured for 10 passages under high-glucose conditions (30 mmol/L), or incubated with transforming growth factor-β (5 ng/mL) for 24 hours. After prolonged exposure to high glucose, podocytes showed a significantly increased OCR at baseline and also a higher OCR after addition of oligomycin, indicating significant changes in mitochondrial energy metabolism. Higher OCRs after inhibition of respiration by rotenone also indicated changes in nonmitochondrial respiration. Podocytes stimulated with a proapoptotic concentration of transforming growth factor-β displayed similar bioenergetic profiles, even with decreased citrate synthase activity. In all tested conditions, we found a higher cellular nicotinamide adenine dinucleotide content and changes in activities of respiratory chain complexes. In summary, we provide for the first time evidence that key factors of the diabetic milieu induce changes in glucose metabolism and mitochondrial function in podocytes.

Hydrogen peroxide induces dimerization of protein kinase G type Iα subunits and increases albumin permeability in cultured rat podocytes

Podocytes help regulate filtration barrier permeability in the kidneys. They express contractile proteins that are characteristic of smooth muscle cells as well as receptors for vasoactive factors such as angiotensin II and atrial natriuretic peptide (ANP). The later one generates intracellular cGMP, with subsequent activation of cGMP-dependent protein kinase; PKG (isoform PKGIα and PKGIβ). In this study, we asked whether hydrogen peroxide (H(2)O(2)), a physiological vasorelaxing factor, affected podocyte permeability and the podoctye PKGIα signaling pathway. Expression of PKGIα was confirmed in cultured rat podocytes using RT-PCR, immunofluorescence, and Western blotting. Exposure of podocytes to exogenous H(2)O(2) (100 µM) in non-reducing conditions increased the formation of PKGIα interprotein disulfide bonds, affected the phosphorylation of PKG target proteins, namely MYPT1 (maximal increase of about 57% at 30 min) and MLC (maximal decrease of about 62% at 10 min). Furthermore, H(2)O(2) increased the permeability of a layer of podocytes to albumin: Transmembrane flux for albumin increased five-fold (106.6 ± 5.2 µg/ml vs. 20.2 ± 2.5 µg/ml, P < 0.05, n = 5), and the PKG inhibitor Rp-8-Br-cGMPS (100 µM) prevented the flux increase. These data suggest that oxidative modulation of PKGIα in podocytes plays an important

Attenuation of glomerular filtration barrier damage in adriamycin-induced nephropathic rats with bufalin: an antiproteinuric agent

Proteinuria is an important risk factor for the progression and prognosis of chronic kidney disease. Bufalin, a cardiotonic steroid, has been shown to posses a variety of biological activities including cardiotonic, anaesthetic and antineoplastic activities, and regulate the immune response. This study investigated the effects of bufalin against proteinuria and glomerular filtration barrier damage in rats with adriamycin (ADR)-induced nephropathy. We compared the blood and urine biochemical indices and the histologic and ultrastructure of the glomerulus in ADR rats with and without the intervention of bufalin or prednisone. The transcription, expression and distribution of the podocyte-associated molecules were compared utilising RT-PCR, western blotting and immunohistochemical staining. We found that bufalin reduced the urinary protein excretion and optimised the lipidaemia of the ADR rats. Bufalin alleviated the removal of podocyte foot processes and attenuated the changes in nephrin, podocin and integrin-linked kinase (ILK) stainings in the glomerulus of the ADR rats. Bufalin notably decreased the expression of nephrin and ILK but inhibited the down-regulation of podocin in protein levels on the renal cortex of the ADR rats. Additionally, bufalin inhibited the up-regulation of podocin and ILK in mRNA levels but did not affect nephrin mRNA levels. These results suggest that bufalin could alleviate ADR-induced proteinuria by protecting the glomerular filtration barrier and may be a novel potential therapeutic agent for proteinuria-associated kidney disease.