Differential trafficking of albumin and IgG facilitated by the neonatal Fc receptor in podocytes in vitro and in vivo

Albumin influences leucocyte FcRn expression in the early days of kidney transplantation

FcRn, a receptor originally known for its involvement in IgG and albumin transcytosis and recycling, is also important in the establishment of the innate and adaptive immune response. Dysregulation of the immune response has been associated with variations in FcRn expression, as observed in cancer. Recently, a link between autophagy and FcRn expression has been demonstrated. Knowing that autophagy is strongly involved in the development of reperfusion injury in kidney transplantation and that albuminemia is transiently decreased in the first 2 weeks after transplantation, we investigated variations in FcRn expression after kidney transplantation. We monitored FcRn levels by flow cytometry in leukocytes from 25 renal transplant patients and considered parameters such as albumin concentrations, estimated glomerular filtration rate, serum creatinine, serum IgG levels, and ischaemia/reperfusion time. Two groups of patients could be distinguished according to their increased or non-increased FcRn expression levels between days 2 and 6 (d2-d6) post-transplantation. Leukocyte FcRn expression at d2-d6 was correlated with albumin concentrations at d0-d2. These results suggest that albumin concentrations at d0-d2 influence FcRn expression at d2-d6, raising new questions about the mechanisms underlying these original observations.

Identification of Four Mouse FcRn Splice Variants and FcRn-Specific Vesicles

Research into the neonatal Fc receptor (FcRn) has increased dramatically ever since Simister and Mostov first purified a rat version of the receptor. Over the years, FcRn has been shown to function not only as a receptor that transfers immunity from mother to fetus but also performs an array of different functions that include transport and recycling of immunoglobulins and albumin in the adult. Due to its important cellular roles, several clinical trials have been designed to either inhibit/enhance FcRn function or develop of non-invasive therapeutic delivery system such as fusion of drugs to IgG Fc or albumin to enhance delivery inside the cells. Here, we report the accidental identification of several FcRn alternatively spliced variants in both mouse and human cells. The four new mouse splice variants are capable of binding immunoglobulins' Fc and Fab portions. In addition, we have identified FcRn-specific vesicles in which immunoglobulins and albumin can be stored and that are involved in the endosomal-lysosomal system. The complexity of FcRn functions offers significant potential to design and develop novel and targeted therapeutics.

The proteasome modulates endocytosis specifically in glomerular cells to promote kidney filtration

Kidney filtration is ensured by the interaction of podocytes, endothelial and mesangial cells. Immunoglobulin accumulation at the filtration barrier is pathognomonic for glomerular injury. The mechanisms that regulate filter permeability are unknown. Here, we identify a pivotal role for the proteasome in a specific cell type. Combining genetic and inhibitor-based human, pig, mouse, and Drosophila models we demonstrate that the proteasome maintains filtration barrier integrity, with podocytes requiring the constitutive and glomerular endothelial cells the immunoproteasomal activity. Endothelial immunoproteasome deficiency as well as proteasome inhibition disrupt the filtration barrier in mice, resulting in pathologic immunoglobulin deposition. Mechanistically, we observe reduced endocytic activity, which leads to altered membrane recycling and endocytic receptor turnover. This work expands the concept of the (immuno)proteasome as a control protease orchestrating protein degradation and antigen presentation and endocytosis, providing new therapeutic targets to treat disease-associated glomerular protein accumulations.

Complement Activation in Nephrotic Glomerular Diseases

The nephrotic syndrome holds significant clinical importance and is characterized by a substantial protein loss in the urine. Damage to the glomerular basement membrane or podocytes frequently underlies renal protein loss. There is an increasing belief in the involvement of the complement system, a part of the innate immune system, in these conditions. Understanding the interactions between the complement system and glomerular structures continually evolves, challenging the traditional view of the blood-urine barrier as a passive filter. Clinical studies suggest that a precise inhibition of the complement system at various points may soon become feasible. However, a thorough understanding of current knowledge is imperative for planning future therapies in nephrotic glomerular diseases such as membranous glomerulopathy, membranoproliferative glomerulonephritis, lupus nephritis, focal segmental glomerulosclerosis, and minimal change disease. This review provides an overview of the complement system, its interactions with glomerular structures, and insights into specific glomerular diseases exhibiting a nephrotic course. Additionally, we explore new diagnostic tools and future therapeutic approaches.

Exploiting the neonatal crystallizable fragment receptor to treat kidney disease

The neonatal Fc receptor (FcRn) was initially discovered as the receptor that allowed passive immunity in newborns by transporting maternal IgG through the placenta and enterocytes. Since its initial discovery, FcRn has been found to exist throughout all stages of life and in many different cell types. Beyond passive immunity, FcRn is necessary for intrinsic albumin and IgG recycling and is important for antigen processing and presentation. Given its multiple important roles, FcRn has been utilized in many disease treatments including a new class of agents that were developed to inhibit FcRn for treatment of a variety of autoimmune diseases. Certain cell populations within the kidney also express high levels of this receptor. Specifically, podocytes, proximal tubule epithelial cells, and vascular endothelial cells have been found to utilize FcRn. In this review, we summarize what is known about FcRn and its function within the kidney. We also discuss how FcRn has been used for therapeutic benefit, including how newer FcRn inhibiting agents are being used to treat autoimmune diseases. Lastly, we will discuss what renal diseases may respond to FcRn inhibitors and how further work studying FcRn within the kidney may lead to therapies for kidney diseases.

New insights into the immune functions of podocytes: the role of complement

Podocytes are differentiated epithelial cells which play an essential role to ensure a normal function of the glomerular filtration barrier (GFB). In addition to their adhesive properties in maintaining the integrity of the filtration barrier, they have other functions, such as synthesis of components of the glomerular basement membrane (GBM), production of vascular endothelial growth factor (VEGF), release of inflammatory proteins, and expression of complement components. They also participate in the glomerular crosstalk through multiple signalling pathways, including endothelin-1, VEGF, transforming growth factor β (TGFβ), bone morphogenetic protein 7 (BMP-7), latent transforming growth factor β-binding protein 1 (LTBP1), and extracellular vesicles.Growing literature suggests that podocytes share many properties of innate and adaptive immunity, supporting a multifunctional role ensuring a healthy glomerulus. As consequence, the "immune podocyte" dysfunction is thought to be involved in the pathogenesis of several glomerular diseases, referred to as "podocytopathies." Multiple factors like mechanical, oxidative, and/or immunologic stressors can induce cell injury. The complement system, as part of both innate and adaptive immunity, can also define podocyte damage by several mechanisms, such as reactive oxygen species (ROS) generation, cytokine production, and endoplasmic reticulum stress, ultimately affecting the integrity of the cytoskeleton, with subsequent podocyte detachment from the GBM and onset of proteinuria.Interestingly, podocytes are found to be both source and target of complement-mediated injury. Podocytes express complement proteins which contribute to local complement activation. At the same time, they rely on several protective mechanisms to escape this damage. Podocytes express complement factor H (CFH), one of the main regulators of the complement cascade, as well as membrane-bound complement regulators like CD46 or membrane cofactor protein (MCP), CD55 or decay-accelerating factor (DAF), and CD59 or defensin. Further mechanisms, like autophagy or actin-based endocytosis, are also involved to ensure podocyte homeostasis and protection against injury.This review will provide an overview of the immune functions of podocytes and their response to immune-mediated injury, focusing on the pathogenic link between complement and podocyte damage.

Single-Cell Transcriptomic Analysis Reveals Macrophage–Tumor Crosstalk in Hepatocellular Carcinoma

As one of the most malignant cancer, hepatocellular carcinoma (HCC) has a complex ecosystem featured by high heterogeneity. Cell crosstalk is demonstrated to be critical for HCC development. However, the cell communication orchestration in HCC remains largely unknown. Here, by analyzing the single-cell transcriptomes of the primary tumor tissues (n = 10) and tumor-adjacent tissues (n = 8) derived from 10 patients with HCC, we found that the proportions of plasmacytoid dendritic cells (pDCs) and natural killer (NK) cells were reduced and that the proportion of macrophages was increased in the immune component of the primary tumor, compared with those in the tumor-adjacent tissue. Furthermore, we found widespread communication between macrophage populations and other cell types, and this communication was remarkably strengthened in the primary tumor, especially with HCC malignant cells. In addition, the SPP1–CD44 axis was identified as a unique interaction between macrophages and HCC malignant cells. Our comprehensive portrait of cell communication patterns over the HCC ecosystem reveals further insights into immune infiltration.

MicroRNA-30a Targets Notch1 to Alleviate Podocyte Injury in Lupus Nephritis

The microRNA miR-30a has been reported to mitigate podocyte damage and resist injurious factors in lupus nephritis (LN), but the precise molecular mechanisms underlying these effects remain elusive. We hypothesized that miR-30a can ameliorate podocyte injury by downregulating the Notch1 signaling pathway and investigated the role of miR-30a in the pathogenesis of podocyte-treated with Immunoglobulin G from patients with LN (IgG-LN). The study enrolled 30 patients from new-onset systemic lupus erythematosus and 28 healthy individuals, then evaluated the levels of their serum miR-30a using RT-qPCR. Additionally, MPC5 cells were transfected with NICD-vector to overexpress Notch1, then with miR-30a mimics or inhibitors to determine miR-30a effects on Notch1. Analysis of function and regulatory mechanisms were performed with RT-qPCR, Western blotting, and CCK8 assays. Furthermore, we verified the candidate sequence targeted by miR-30a using a luciferase reporter gene assay. We observed a significant decrease in the serum miR-30a levels in patients with LN. Also, in IgG-LN-treated podocytes, miR-30a decreased and Notch1 expression was elevated. Bioinformatic analysis and transfection experiments revealed that Notch1 is a direct target of miR-30a. Further supporting this finding, miR-30a upregulation appeared to alleviate IgG-LN-treated podocyte injury, and Notch1 overexpression reversed this effect. To conclude, miR-30a can ameliorate podocyte injury via suppression of the Notch1 signaling pathway.

Activation of FcRn Mediates a Primary Resistance Response to Sorafenib in Hepatocellular Carcinoma by Single-Cell RNA Sequencing

Sorafenib is the first-line therapeutic option for advanced hepatocellular carcinoma (HCC). Many patients exhibit a primary resistance (PR) response after initial treatment. In previous studies, compared to acquired resistance, the mechanism of PR is unclear. The present study aimed to evaluate the response of patient samples to sorafenib by patient-derived xenograft (PDX) models, and the differences at the transcriptome level between the sorafenib PR group and the sorafenib sensitive group were analyzed by single-cell sequencing technology. A specific cell cluster may be differentiated by the liver bud hepatic cells, and the JUN transcription factors in this cell cluster were highly activated. The albumin is secreted by other cell clusters, and the cluster stimulates the FcRn complex receptor to activate the HIF pathway and cell proliferation, resulting in a poor response to sorafenib. These findings are validated by both cell communication analysis and experiments. Thus, the current studies provided a novel approach for the treatment of sorafenib-resistant HCC.

Delivery of Orally Administered Digestible Antibodies Using Nanoparticles

Oral administration of medications is highly preferred in healthcare owing to its simplicity and convenience; however, problems of drug membrane permeability can arise with any administration method in drug discovery and development. In particular, commonly used monoclonal antibody (mAb) drugs are directly injected through intravenous or subcutaneous routes across physical barriers such as the cell membrane, including the epithelium and endothelium. However, intravenous administration has disadvantages such as pain, discomfort, and stress. Oral administration is an ideal route for mAbs. Nonetheless, proteolysis and denaturation, in addition to membrane impermeability, pose serious challenges in delivering peroral mAbs to the systemic circulation, biologically, through enzymatic and acidic blocks and, physically, through the small intestinal epithelium barrier. A number of clinical trials have been performed using oral mAbs for the local treatment of gastrointestinal diseases, some of which have adopted capsules or tablets as formulations. Surprisingly, no oral mAbs have been approved clinically. An enteric nanodelivery system can protect cargos from proteolysis and denaturation. Moreover, mAb cargos released in the small intestine may be delivered to the systemic circulation across the intestinal epithelium through receptor-mediated transcytosis. Oral Abs in milk are transported by neonatal Fc receptors to the systemic circulation in neonates. Thus, well-designed approaches can establish oral mAb delivery. In this review, I will introduce the implementation and possibility of delivering orally administered mAbs with or without nanoparticles not only to the local gastrointestinal tract but also to the systemic circulation.

Stop, think SCORTCH: rethinking the traditional 'TORCH' screen in an era of re-emerging syphilis

BACKGROUND

The epidemiology of congenital infections is ever changing, with a recent resurgence in syphilis infection rates seen in the UK. Identification of congenital infection is often delayed; early recognition and management of congenital infections is important. Testing modalities and investigations are often limited, leading to missed diagnostic opportunities.

METHODS

The SCORTCH (syphilis, cytomegalovirus (CMV), 'other', rubella, toxoplasmosis, chickenpox, herpes simplex virus (HSV) and blood-borne viruses) acronym increases the awareness of clinicians to the increased risk of congenital syphilis, while considering other infectious aetiologies including: zika, malaria, chagas disease, parvovirus, enterovirus, HIV, hepatitis B and C, and human T-lymphotropic virus 1, in addition to the classic congenital infections recognised in the 'TORCH screen' (toxoplasmosis, 'other', rubella, CMV, HSV). The SCORTCH diagnostic approach describes common signs present in infants with congenital infection, details serological testing for mother and infant and important direct diagnostics of the infant. Direct diagnostic investigations include: radiology, ophthalmology, audiology, microbiological and PCR testing for both the infant and placental tissue, the latter also warrants histopathology.

CONCLUSION

The traditional 'TORCH screen' focuses on serology-specific investigations, often omits important direct diagnostic testing of the infant, and fails to consider emerging and re-emerging congenital infections. In recognition of syphilis as a re-emerging pathogen and the overlapping clinical presentations of various infectious aetiologies, we advocate for a broader outlook using the SCORTCH diagnostic approach.

Podocyte-specific knockout of the neonatal Fc receptor (FcRn) results in differential protection depending on the model of glomerulonephritis

Podocytes have been proposed to be antigen presenting cells (APCs). In traditional APCs, the neonatal Fc receptor (FcRn) is required for antigen presentation and global knockout of FcRn protects against glomerulonephritis. Since podocytes express FcRn, we sought to determine whether the absence of podocyte FcRn ameliorates immune-mediated disease. We examined MHCII and costimulatory markers expression in cultured wild type (WT) and FcRn knockout (KO) podocytes. Interferon gamma (IFNγ) induced MHCII expression in both WT and KO podocytes but did not change CD80 expression. Neither WT nor KO expressed CD86 or inducible costimulatory ligand (ICOSL) at baseline or with IFNγ. Using an antigen presentation assay, WT podocytes but not KO treated with immune complexes induced a modest increase in IL-2. Induction of the anti-glomerular basement membrane (anti-GBM) model resulted in a significant decrease in glomerular crescents in podocyte-specific FcRn knockout mouse (podFcRn KO) versus controls but the overall percentage of crescents was low. To examine the effects of the podocyte-specific FcRn knockout in a model with a longer autologous phase, we used the nephrotoxic serum nephritis (NTS) model. We found that the podFcRn KO mice had significantly reduced crescent formation and glomerulosclerosis compared to control mice. This study demonstrates that lack of podocyte FcRn is protective in immune mediated kidney disease that is dependent on an autologous phase. This study also highlights the difference between the anti-GBM model and NTS model of disease.

Novel Fanconi renotubular syndromes provide insights in proximal tubule pathophysiology

The various forms of Fanconi renotubular syndromes (FRTS) offer significant challenges for clinicians and present unique opportunities for scientists who study proximal tubule physiology. This review will describe the clinical characteristics, genetic underpinnings, and underlying pathophysiology of the major forms of FRST. Although the classic forms of FRTS will be presented (e.g., Dent disease or Lowe syndrome), particular attention will be paid to five of the most recently discovered FRTS subtypes caused by mutations in the genes encoding for L-arginine:glycine amidinotransferase (GATM), solute carrier family 34 (type Ii sodium/phosphate cotransporter), member 1 (SLC34A1), enoyl-CoAhydratase/3-hydroxyacyl CoA dehydrogenase (EHHADH), hepatocyte nuclear factor 4A (HNF4A), or NADH dehydrogenase complex I, assembly factor 6 (NDUFAF6). We will explore how mutations in these genes revealed unexpected mechanisms that led to compromised proximal tubule functions. We will also describe the inherent challenges associated with gene discovery studies based on findings derived from small, single-family studies by focusing the story of FRTS type 2 (SLC34A1). Finally, we will explain how extensive alternative splicing of HNF4A has resulted in confusion with mutation nomenclature for FRTS type 4.

Isolation, purification, and conditional immortalization of murine glomerular endothelial cells of microvascular phenotype

Glomerular endothelial cells (GEnC) are a specialized microvascular subset of endothelial cells that, when injured, result in many types of diseases within the kidney. Thus, techniques to study GEnC in a cell culture system are important to investigate mechanisms of GEnC injury. Studies of endothelial cell function in culture have predominately relied on using macrovascular endothelial cells from vascular areas other than the glomerulus. Over the last 15 years, glomerular endothelial cells lines have been created but were isolated by targeting cells expressing CD31. Some studies identified endothelial cells isolated from the microvasculature do not express CD31 and some suggest that CD31+ cells are phenotypically different than endothelial cells found in capillaries. Here we detail our method of isolation, purification, and conditional immortalization of mouse glomerular endothelial cells targeting endothelial cells that do not express CD31.•

This method allows for isolation, purification, and conditional immortalization of glomerular endothelial cells for continued passage of GEnCs beyond that of primary cell culture.

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This method can be used in genetically modified mice to investigate how a modification of a specific gene or protein affects the glomerular endothelium at the cellular level.

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Remember that the abstract is what readers see first in electronic abstracting & indexing services.

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Knockout of the neonatal Fc receptor in cultured podocytes alters IL-6 signaling and the actin cytoskeleton

The neonatal Fc receptor (FcRn) has been shown to be required for antigen presentation in dendritic cells, and global knockout of FcRn attenuates immune-mediated kidney disease. Podocytes express interleukin-6 (IL-6) receptor and produce IL-6 under proinflammatory conditions. Here we examined the role of FcRn in the IL-6-mediated inflammatory response in podocytes. We examined IL-6 production by ELISA and expression by qPCR in wild type (WT) and FcRn knockout (KO) podocytes after treatment with proinflammatory stimuli as well as IL-6-mediated signaling via the JAK/STAT pathway. We also examined podocyte motility in cultured WT and KO podocytes after a proinflammatory challenge. We found that FcRn KO podocytes produced minimal amount of IL-6 after treatment with albumin, IgG, or immune complexes whereas WT podocytes had a robust response. FcRn KO podocytes also had minimal expression of IL-6 compared with WT. By Western blotting, there was significantly less phosphorylated STAT3 in KO podocytes after treatment with IFNγ or immune complexes. In a scratch assay, FcRn KO podocytes showed increased motility comparted KO, suggesting a defect in actin dynamics. Cultured FcRn KO podocytes also demonstrated abnormal stress fibers compared with WT and the defect could be rescued by IL-6 treatment. This study shows that in podocytes, FcRn modulates the IL-6 mediated response to proinflammatory stimuli and regulates podocytes actin structure, motility and synaptopodin expression.

Neonatal Fc Receptor (FcRn) Enhances Tissue Distribution and Prevents Excretion of nab-Paclitaxel

nab-Paclitaxel ( nab-P), an albumin-bound formulation of paclitaxel, was developed to improve the tolerability and antitumor activity of taxanes. The neonatal Fc receptor (FcRn) is a transport protein that can bind to albumin and regulate the homeostasis of circulating albumin. Therefore, the pharmacokinetics and pharmacodynamics of nab-P may be impacted by FcRn expression. This study aimed to investigate the effects of FcRn on nab-P elimination and distribution to targeted tissues. Wild-type and FcRn-knockout (FcRn-KO) mice were treated with nab-P, mouse-specific nab-P (distribution experiments only), and solvent-based paclitaxel (pac-T). Blood and tissue samples were collected for distribution analyses. Organ, urine, and fecal samples were collected for elimination analyses. The nab-P tissue penetration in the pancreas, fat pad, and kidney of wild-type mice, as reflected by the ratio of tissue/plasma concentration, was significantly higher (ranging from 5 to 80 fold) than that of FcRn-KO mice. In contrast, the tissue penetration of pac-T in these organs of FcRn-KO mice was similar to that of wild-type mice. More importantly, the excretion of nab-P in feces of FcRn-KO mice (45-68%) was significantly higher than that of wild-type mice (26-46%) from 8 to 48 h post treatment. In comparison, the difference of excretion of pac-T in feces between FcRn-KO mice and wild-type mice was smaller than that of nab-P. Furthermore, greater tissue penetration and fecal excretion were observed with nab-P than pac-T in both FcRn-KO and wild-type mice. These findings suggest that FcRn enhances the tissue distribution and penetration of nab-P in the targeted organs, while FcRn prevents excretion of nab-P to feces in the intestinal lumen. The findings support the notion that albumin nanoparticle delivery alters drug distribution and elimination through an FcRn-mediated process to impact drug efficacy and toxicity.