Loss of Roundabout Guidance Receptor 2 (Robo2) in Podocytes Protects Adult Mice from Glomerular Injury by Maintaining Podocyte Foot Process Structure

Ramipril therapy in integrin α1-null, autosomal recessive Alport mice triples lifespan: mechanistic clues from RNA-seq analysis

The standard of care for patients with Alport syndrome (AS) is angiotensin-converting enzyme (ACE) inhibitors. In autosomal recessive Alport (ARAS) mice, ACE inhibitors double lifespan. We previously showed that deletion of Itga1 in Alport mice [double-knockout (DKO) mice] increased lifespan by 50%. This effect seemed dependent on the prevention of laminin 211-mediated podocyte injury. Here, we treated DKO mice with vehicle or ramipril starting at 4 weeks of age. Proteinuria and glomerular filtration rates were measured at 5-week intervals. Glomeruli were analyzed for laminin 211 deposition in the glomerular basement membrane (GBM) and GBM ultrastructure was analyzed using transmission electron microscopy (TEM). RNA sequencing (RNA-seq) was performed on isolated glomeruli at all time points and the results were compared with cultured podocytes overlaid (or not) with recombinant laminin 211. Glomerular filtration rate declined in ramipril-treated DKO mice between 30 and 35 weeks. Proteinuria followed these same patterns with normalization of foot process architecture in ramipril-treated DKO mice. RNA-seq revealed a decline in the expression of Foxc2, nephrin (Nphs1), and podocin (Nphs2) mRNAs, which was delayed in the ramipril-treated DKO mice. GBM accumulation of laminin 211 was delayed in ramipril-treated DKO mice, likely due to a role for α1β1 integrin in CDC42 activation in Alport mesangial cells, which is required for mesangial filopodial invasion of the subendothelial spaces of the glomerular capillary loops. Ramipril synergized with Itga1 knockout, tripling lifespan compared with untreated ARAS mice. © 2023 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Ascorbate protects human kidney organoids from damage induced by cell-free hemoglobin

Sepsis-associated acute kidney injury is associated with high morbidity and mortality in critically ill patients. Cell-free hemoglobin (CFH) is released into the circulation of patients with severe sepsis and the levels of CFH are independently associated with mortality. CFH treatment increased cytotoxicity in the human tubular epithelial cell line HK-2. To better model the intact kidney, we cultured human kidney organoids derived from induced pluripotent stem cells. We treated human kidney organoids grown using both three-dimensional and transwell protocols with CFH for 48 h. We found evidence for increased tubular toxicity, oxidative stress, mitochondrial fragmentation, endothelial cell injury and injury-associated transcripts compared to those of the untreated control group. To evaluate the protective effect of clinically available small molecules, we co-treated CFH-injured organoids with ascorbate (vitamin C) or acetaminophen for 48 h. We found significantly decreased toxicity, preservation of endothelial cells and reduced mitochondrial fragmentation in the group receiving ascorbate following CFH treatment. This study provides direct evidence that ascorbate or ascorbic acid protects human kidney cells from CFH-induced damage such as that in sepsis-associated acute kidney injury.

Robo2 promotes osteoblast differentiation and mineralization through autophagy and is activated by parathyroid hormone induction

BACKGROUND

As a systemic skeletal disorder, osteoporosis can increase fracture risk. This study wants to discuss the mechanism of osteoporosis and find possible molecular therapy. Bone morphogenetic protein 2 (BMP2) was utilized to stimulate MC3T3-E1 to establish a cellular osteoporosis model in vitro.

METHODS

Initially, the viability of BMP2-induced MC3T3-E1 was assessed with a Cell counting kit-8 (CCK-8) assay. By real-time quantitative PCR (RT-qPCR) and western blot, Robo2 expression after roundabout (Robo) silencing or overexpression was estimated. Besides, alkaline phosphatase (ALP) expression, mineralization level and LC3II green fluorescent protein (GFP) expression were evaluated using ALP assay, Alizarin red staining and immunofluorescence staining, separately. Additionally, the expression of proteins related to osteoblast differentiation and autophagy was analyzed by RT-qPCR and western blot. Then, following autophagy inhibitor 3-methyladenine (3-MA) treatment, osteoblast differentiation and mineralization were measured again.

RESULTS

MC3T3-E1 cells were differentiated into osteoblasts under BMP2 induction and Robo2 expression was greatly ascended. After Robo2 silencing, Robo2 expression was markedly diminished. ALP activity and mineralization level in BMP2-induced MC3T3-E1 cells were declined after depleting Robo2. Robo2 expression was conspicuously enhanced after overexpressing Robo2. Robo2 overexpression promoted the differentiation and mineralization of BMP2-induced MC3T3-E1 cells. Rescue experiments revealed that Robo2 silence and its overexpression could regulate the autophagy of BMP2-stimulated MC3T3-E1 cells. After 3-MA treatment, the increased ALP activity and mineralization level of BMP2-induced MC3T3-E1 cells with Robo2 upregulation were reduced. Furthermore, parathyroid hormone 1-34 (PTH1-34) treatment enhanced the expression of ALP, Robo2, LC3II and Beclin-1 and reduced the levels of LC3I and p62 of MC3T3-E1 cells concentration-dependently.

CONCLUSION

Collectively, Robo2, which was activated by PTH1-34, promoted osteoblast differentiation and mineralization through autophagy.

Novel Treatment Paradigms: Focal Segmental Glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is a histologic pattern of injury defined by the presence of sclerosis in some (segmental) of certain glomeruli (focal). On electron microscopy, it is characterized by a variable degree of podocyte foot process effacement and gaps in the coverage of the glomerular basement membrane. The pattern of injury occurs when podocytes, highly differentiated cells with limited regenerative capacity, are reduced in number. The heterogeneity in underlying causes of podocyte loss results in equally variable clinical phenotypes. Recent work acknowledging advances in defining the genetic and immunologic basis of disease has redefined the classification of FSGS. Unprecedented clinical trial activity and efficacy of repurposed agents presents hope for improved therapeutic options. This minireview summarizes recent advances with a focus on novel treatment paradigms in FSGS.

Roles of glomerular endothelial hyaluronan in the development of proteinuria

Vascular endothelial cells are covered with glycocalyx comprising heparan sulfate, hyaluronan, chondroitin sulfate, and associated proteins. Glomerular endothelial glycocalyx is involved in protecting against induction of proteinuria and structural damage, but the specific components in glycocalyx that represent therapeutic targets remain unclear. Anti-vascular endothelial growth factor (VEGF) therapy is associated with an increased risk of glomerular endothelial injury. This study investigated whether hyaluronan could provide a therapeutic target to protect against proteinuria. We conducted ex vivo and in vivo experiments to explore the effects of degrading glomerular hyaluronan by administering hyaluronidase and of supplementation with hyaluronan. We investigated hyaluronan expression using biotin-labeled hyaluronan-binding protein (HABP) in human kidney specimens or serum hyaluronan in endothelial injuries under inhibition of VEGF signaling. We directly demonstrated hyaluronan in glomerular endothelial layers using HABP staining. Ex vivo and in vivo experiments showed the development of proteinuria after digestion of hyaluronan in glomerular capillaries. Supplementation with hyaluronan after hyaluronidase treatment suppressed proteinuria. Mice in the in vivo study developed albuminuria after intraperitoneal injection of hyaluronidase with decreased glomerular hyaluronan and increased serum hyaluronan. In human kidneys with endothelial cell dysfunction and proteinuria due to inhibition of VEGF, glomerular expression of hyaluronan was reduced even in normal-appearing glomeruli. Serum hyaluronan levels were elevated in patients with pre-eclampsia with VEGF signaling inhibition. Our data suggest that hyaluronan itself plays crucial roles in preventing proteinuria and preserving the integrity of endothelial cells. Hyaluronan could provide a therapeutic target for preventing glomerular endothelial glycocalyx damage, including VEGF signaling inhibition.

A Phase 1 first-in-human study of the safety, tolerability, and pharmacokinetics of the ROBO2 fusion protein PF-06730512 in healthy participants

Proteinuria associated with podocyte effacement is a hallmark of focal segmental glomerulosclerosis (FSGS). Preclinical studies implicated ROBO2/SLIT2 signaling in the regulation of podocyte adhesion, and inhibition of this pathway is a novel target to slow FSGS disease progression. This first-in-human dose-escalation study evaluated the safety, tolerability, pharmacokinetics, and immunogenicity of PF-06730512, an Fc fusion protein that targets the ROBO2/SLIT2 pathway, in healthy adults. In this Phase 1, double-blind, sponsor-open study, single ascending dose (SAD) cohorts were randomized to receive up to 1000 mg or placebo intravenously (IV); multiple ascending dose (MAD) cohorts were randomized to receive up to 400 mg subcutaneous (SC) doses, 1000 mg IV dose, or matching placebo. Safety evaluations were performed up to 71 (SAD) and 113 (MAD) days after dosing; blood samples were collected to measure serum PF-06730512 concentrations and antidrug antibodies (ADA) to PF-06730512. Seventy-nine participants (SAD, 47; MAD, 32) were enrolled. There were 108 mild (SAD, 46; MAD, 62) and 21 moderate (SAD, 13; MAD, 8) treatment-emergent adverse events (TEAEs); no deaths, treatment-related serious AEs, severe TEAEs, or infusion reactions were reported. PF-06730512 exposure generally increased in an approximately dose-proportional manner; mean t1/2 ranged from 12-15 days across 50-1000 mg doses. Immunogenicity incidence was low (SAD, 0 ADA+; MAD, 2 ADA+). In conclusion, single IV doses of PF-06730512 up to 1000 mg and multiple IV and SC dosing up to 1000 and 400 mg, respectively, were safe and well tolerated in healthy participants. Further trials in patients with FSGS are warranted. Clinical trial registration: Clinicaltrials.gov: NCT03146065.

The glomerular filtration barrier: a structural target for novel kidney therapies

Loss of normal kidney function affects more than 10% of the population and contributes to morbidity and mortality. Kidney diseases are currently treated with immunosuppressive agents, antihypertensives and diuretics with partial but limited success. Most kidney disease is characterized by breakdown of the glomerular filtration barrier (GFB). Specialized podocyte cells maintain the GFB, and structure-function experiments and studies of intercellular communication between the podocytes and other GFB cells, combined with advances from genetics and genomics, have laid the groundwork for a new generation of therapies that directly intervene at the GFB. These include inhibitors of apolipoprotein L1 (APOL1), short transient receptor potential channels (TRPCs), soluble fms-like tyrosine kinase 1 (sFLT1; also known as soluble vascular endothelial growth factor receptor 1), roundabout homologue 2 (ROBO2), endothelin receptor A, soluble urokinase plasminogen activator surface receptor (suPAR) and substrate intermediates for coenzyme Q10 (CoQ₁₀). These molecular targets converge on two key components of GFB biology: mitochondrial function and the actin-myosin contractile machinery. This Review discusses therapies and developments focused on maintaining GFB integrity, and the emerging questions in this evolving field.

PODO: Trial Design: Phase 2 Study of PF-06730512 in Focal Segmental Glomerulosclerosis

Introduction

Focal segmental glomerulosclerosis (FSGS) is characterized by proteinuria and a histologic pattern of glomerular lesions of diverse etiology that share features including glomerular scarring and podocyte foot process effacement. Roundabout guidance receptor 2 (ROBO2)/slit guidance ligand 2 (SLIT2) signaling destabilizes the slit diaphragm and reduces podocyte adhesion to the glomerular basement membrane (GBM). Preclinical studies suggest that inhibition of glomerular ROBO2/SLIT2 signaling can stabilize podocyte adhesion and reduce proteinuria. This clinical trial evaluates the preliminary efficacy and safety of ROBO2/SLIT2 inhibition with the ROBO2 fusion protein PF-06730512 in patients with FSGS.

Methods

The Study to Evaluate PF-06730512 in Adults With FSGS (PODO; ClinicalTrials.gov identifier NCT03448692), an open-label, phase 2a, multicenter trial in adults with FSGS, will enroll patients into 2 cohorts (n = 22 per cohort) to receive either high- or low-dose PF-06730512 (intravenous) every 2 weeks for 12 weeks. Key inclusion criteria include a confirmed biopsy diagnosis of FSGS, an estimated glomerular filtration rate (eGFR) ≥45 ml/min/1.73 m² based on the Chronic Kidney Disease Epidemiology Collaboration formula (30–45 with a recent biopsy), and urinary protein-to-creatinine ratio (UPCR) >1.5 g/g. Key exclusion criteria include collapsing FSGS, serious/active infection, ≥50% tubulointerstitial fibrosis on biopsy, and organ transplantation. The primary endpoint is change from baseline to week 13 in UPCR; secondary endpoints include safety, changes in eGFR, and PF-06730512 serum concentration.

Results

This ongoing trial will report the efficacy, safety, pharmacokinetics, and biomarker results of PF-06730512 for patients with FSGS.

Conclusion

Findings from this proof-of-concept study may support further development and evaluation of PF-06730512 to treat FSGS and warrant assessment in phase 3 clinical trials.

Control of Podocyte and Glomerular Capillary Wall Structure and Elasticity by WNK1 Kinase

Cytoskeletal structure and its regulation are essential for maintenance of the differentiated state of specific types of cells and their adaptation to physiologic and pathophysiologic conditions. Renal glomerular capillaries, composed of podocytes, endothelial cells, and the glomerular basement membrane, have distinct structural and biophysical properties and are the site of injury in many glomerular diseases. Calcineurin inhibitors, immunosuppressant drugs used for organ transplantation and auto-immune diseases, can protect podocytes and glomerular capillaries from injury by preserving podocyte cytoskeletal structure. These drugs cause complications including hypertension and hyperkalemia which are mediated by WNK (With No Lysine) kinases as well as vasculopathy with glomerulopathy. WNK kinases and their target kinases oxidative stress-responsive kinase 1 (OSR1) and SPS1-related proline/alanine-rich kinase (SPAK) have fundamental roles in angiogenesis and are activated by calcineurin inhibitors, but the actions of these agents on kidney vasculature, and glomerular capillaries are not fully understood. We investigated WNK1 expression in cultured podocytes and isolated mouse glomerular capillaries to determine if WNK1 contributes to calcineurin inhibitor-induced preservation of podocyte and glomerular structure. WNK1 and OSR1/SPAK are expressed in podocytes, and in a pattern similar to podocyte synaptopodin in glomerular capillaries. Calcineurin inhibitors increased active OSR1/SPAK in glomerular capillaries, the Young’s modulus (E) of glomeruli, and the F/G actin ratio, effects all blocked by WNK inhibition. In glomeruli, WNK inhibition caused reduced and irregular synaptopodin-staining, abnormal capillary and foot process structures, and increased deformability. In cultured podocytes, FK506 activated OSR1/SPAK, increased lamellipodia, accelerated cell migration, and promoted traction force. These actions of FK506 were reduced by depletion of WNK1. Collectively, these results demonstrate the importance of WNK1 in regulation of the podocyte actin cytoskeleton, biophysical properties of glomerular capillaries, and slit diaphragm structure, all of which are essential to normal kidney function.