When contactin antibodies hit the podocyte: a new neurorenal syndrome

Neuronal Proteins as Antigenic Targets in Membranous Nephropathy

CONTEXT

The discovery of new target antigens in membranous nephropathy (MN) has revealed new disease phenotypes and, in some cases, has suggested mechanisms of disease shared by two concurrent autoimmune diseases. Subject of Review: Several recent reports and an accompanying editorial describe the association of anti-contactin-1 (CNTN1) autoantibodies of the IgG4 subclass with a novel subtype of MN that co-occurs with a form of chronic inflammatory demyelinating polyradiculoneuropathy caused by anti-CNTN1 antibodies. CNTN1, the cellular source of which is still undetermined, is identified as the target antigen in the kidney since it is present within glomerular subepithelial deposits and anti-CNTN1 IgG4 antibodies can be eluted from the corresponding kidney biopsy tissue. Second Opinion: These new reports reinforce recent findings that many proteins targeted in several other types of primary and secondary MN are proteins whose expression is shared by podocytes and neurons. While complement-mediated podocyte damage represents a well-established paradigm in the pathogenesis of MN, interference with the normal functions of these shared proteins by autoantibodies should be considered as another potential mechanism of glomerular injury to be explored in future research.

Update on New Antigens in the Pathogenesis of Membranous Nephropathy

Previously, membranous nephropathies were divided into primary and secondary categories when the exact mechanism or pathogenetic factor were unknown. Approximately 70% accounted for primary membranous nephropathies. The remaining 30% were called secondary because they developed due to well-known diseases such as autoimmune diseases, tumours, infections, or drug assumptions. The discoveries of the M-type phospholipase A2 receptor and of thrombospondin type 1 domain containing 7A as causative antigens in a part of the so-called primary membranous nephropathies opened new knowledge on the effective causes of a large part of these diseases. The availability of novel techniques such as laser micro-dissection and tandem mass spectrometry, as well as immunochemistry with antibodies directed against novel proteins, allowed the confirmation of new antigens involved. The use of confocal microscopy and Western blot allowed detection of the new antigen on glomerular membrane, and the same antigen and relative antibodies have been detected in serum samples. Through these techniques, four new antigens were first detected, including neural epidermal growth factor 1 and semaphorin 3B in the so-called primary membranous nephropathy, and exostosin 1 and 2 and neural cell adhesion molecule 1 in lupus membranous nephropathy. The aim of this study is to describe the characteristics of the new antigens discovered and their association with other diseases. In addition, new antigens are on the horizon, and the story of primary membranous nephropathy is still to be completely written and understood.

Traditional Chinese Medicine in Treating Primary Podocytosis: From Fundamental Science to Clinical Research

Podocytes form a key component of the glomerular filtration barrier. Damage to podocytes is referred to as “podocyte disease.” There are many causes of podocyte injury, including primary injury, secondary injury, and gene mutations. Primary podocytosis mostly manifests as nephrotic syndrome. At present, first-line treatment is based on glucocorticoid administration combined with immunosuppressive therapy, but some patients still progress to end-stage renal disease. In Asia, especially in China, traditional Chinese medicine (TCM) still plays an important role in the treatment of kidney diseases. This study summarizes the potential mechanism of TCM and its active components in protecting podocytes, such as repairing podocyte injury, inhibiting podocyte proliferation, reducing podocyte apoptosis and excretion, maintaining podocyte skeleton structure, and upregulating podocyte-related protein expression. At the same time, the clinical efficacy of TCM in the treatment of primary podocytosis (including idiopathic membranous nephropathy, minimal change disease, and focal segmental glomerulosclerosis) is summarized to support the development of new treatment strategies for primary podocytosis.

Contactin 1, a Potential New Antigen Target in Membranous Nephropathy: A Case Report

Several novel antigens have recently been characterized in membranous nephropathy (MN), but those involved in the rare cases of MN associated with inflammatory neuropathies remain elusive. Although several antibodies have been identified in the serum, there is no evidence so far for their deposition in glomeruli. We report the case of a 73-year-old woman who was referred because of subacute onset of proximal asymmetric lower limb weakness together with ataxic gait. She was diagnosed with inflammatory neuropathy. Testing showed an estimated glomerular filtration rate of 73mL/min/1.73m², hypoalbuminemia (2.89g/dL), and proteinuria (3.6g/d). Autoantibodies (antinuclear antibody, anti-extractable nuclear antigen antibody, anti-double stranded DNA antibody, lupus anticoagulant, anticardiolipin antibody, antineutrophil cytoplasmic antibody) were undetectable. Serum immunoglobulin and complement levels were normal. A kidney biopsy with electron microscopy examination showed a classical picture of MN. Testing for antibodies to phospholipase A₂ receptor (PLA₂R) gave negative results in the serum, and PLA₂R and THSD7A antigens were not detected in kidney tissue. Anti-contactin 1 (CNTN1) antibody was detected by enzyme-linked immunosorbent assay at a 1:100 dilution of serum and shown to be mostly of IgG4 subclass by Western blot. CNTN1 antigen was colocalized with IgG4 within immune deposits by confocal microscopy. This observation suggests a pathophysiological link between inflammatory neuropathies and MN. CNTN1 should be considered as a potential candidate antigen involved in MN and tested in PLA₂R-negative forms associated with inflammatory neuropathies.

New antigens involved in membranous nephropathy beyond phospholipase A2 receptor

When the physiopathology of membranous nephropathy was first described, almost 30% of cases were recognized to be secondary to well-known diseases such as autoimmune diseases, tumors or infections. The remaining 70% cases were called primary membranous nephropathy as the exact mechanism or pathogenic factor involved was unknown. The discovery of the M type phospholipase A2 receptor and thrombospondin type 1 domain containing 7A as causative antigens in these “so called” primary membranous nephropathies provided new insights into the effective causes of a large proportion of these cases. Novel techniques such as laser microdissection and tandem mass spectrometry as well as immunochemistry with antibodies directed against novel proteins allowed the confirmation of new involved antigens. Finally, using confocal microscopy to localize these new antigens and immunoglobulin G and Western blot analysis of serum samples, these new antigens were detected on the glomerular membrane, and the related antibodies were detected in serum samples. The same antigens have been recognized in some cases of secondary membranous disease due to autoimmune diseases, tumors and infections. This has allowed examination of the relationship between antigens in primary membranous nephropathy and their presence in some secondary nephropathies. The aim of this study is to describe the characteristics of the new antigens discovered and their association with other diseases.