Glomerulopathic Light Chains Research Articles

Glomerulopathic Light Chain-Mesangial Cell Interactions: Sortilin-Related Receptor (SORL1) and Signaling

IntroductionDeciphering the intricacies of the interactions of glomerulopathic Ig light chains with mesangial cells is key to delineate signaling events responsible for the mesangial pathologic alterations that ensue.MethodsHuman mesangial cells, caveolin 1 (CAV1), wild type (WT) ,and knockout (KO), were incubated with glomerulopathic light chains purified from the urine of patients with light chain–associated (AL) amyloidosis or light chain deposition disease. Associated signaling events induced by surface interactions of glomerulopathic light chains with caveolins and other membrane proteins, as well as the effect of epigallocatechin-3-gallate (EGCG) on the capacity of mesangial cells to intracellularly process AL light chains were investigated using a variety of techniques, including chemical crosslinking with mass spectroscopy, immunofluorescence, and ultrastructural immunolabeling.ResultsCrosslinking experiments provide evidence suggesting that sortilin-related receptor (SORL1), a transmembrane sorting receptor that regulates cellular trafficking of proteins, is a component of the receptor on mesangial cells for glomerulopathic light chains. Colocalization of glomerulopathic light chains with SORL1 in caveolae and also in lysosomes when light chain internalization occurred, was documented using double immunofluorescence and immunogold labeling ultrastructural techniques. It was found that EGCG directly blocks c-Fos cytoplasmic to nuclei signal translocation after interactions of AL light chains with mesangial cells, resulting in a decrease in amyloid formation.ConclusionOur findings document for the first time a role for SORL1 linked to glomerular pathology and signaling events that take place when certain monoclonal light chains interact with mesangial cells. This finding may lead to novel therapies for treating renal injury caused by glomerulopathic light chains.

Understanding Mesangial Damage and Repair: Insights from an Experimental Model of Immunoglobulin Light Chain-Associated Mesangiopathy

Mesangial injury represents a crucial event in the pathogenesis of light chain-associated glomerulopathies in patients with plasma cell dyscrasias. The glomerulopathic light chains interact with mesangial cells where purported receptors regulate the downstream cellular mechanisms that will be activated and result in glomerular alterations. The physicochemical and conformational characteristics of the abnormal light chains are primarily responsible for the downstream events affecting the mesangial milieu. Different light chains are responsible for two diseases with diametrically opposite mesangial alterations: Light chain deposition disease which results in the expansion of the mesangium due to accumulation of matrix proteins not present in the normal mesangium and AL (light chain-associated) amyloidosis where the native mesangial matrix is replaced by fibrils (amyloid). In both cases there is enhancement of mesangial cell apoptosis and the altered mesangium has a marked decrease of mesangial cells, most undergoing apoptosis. The repair of the damaged mesangium is difficult due to the absence of enough mesangial cells that can participate in the process and also the damage can be so extensive that the intrinsic processes that are available for repair (i.e. recruitment of stem cells from bone marrow and precursor stem cells in renal niches) cannot effectively carry out the recovery. Introducing exogenous mesenchymal stem cells represents a novel therapeutic avenue that has been experimentally tested with promising results. In-vitro, ex-vivo and in-vivo animal models have been created to study these disorders providing excellent platforms to elucidate pathogenesis and provide insightful information that can be translated to the practice of renal pathology, as the in-vitro and in-vivo platforms corroborate each other. The article addresses how mesangial injury occurs in LCDD and AL-amyloidosis as examples of generic mesangiopathies and summarizes existing data regarding the experimental use of exogenous mesenchymal stem cells in mesangial healing.

Extrusion of Amyloid Fibrils to the Extracellular Space in Experimental Mesangial AL-Amyloidosis: Transmission and Scanning Electron Microscopy Studies and Correlation with Renal Biopsy Observations

In vitro studies have provided much information regarding the process of glomerular AL-amyloidogenesis. Research efforts have been successful in deciphering how glomerulopathic light chains interact with mesangial cells. The sequential steps involved in the genesis of amyloid fibrils include interactions with surface caveolae in mesangial cells and internalization of the monoclonal light chains through a clathrin-mediated process followed by trafficking in the mesangial cells to the mature lysosomal compartment where fibrils are formed. This manuscript focuses on how mesangial cells, once amyloid has been formed, deliver the fibrils to the extracellular matrix. The delivery of amyloid fibrils to the outside of the cells is carried out by lysosomes, which abut the mesangial cell membranes and extrude their contents into the extracellular space. This final step responsible for the fibrils to be present predominantly in the extracellular space is well demonstrated with scanning electron microscopy.

Mesenchymal Stem Cells in Mesangial Repair in a Model of Immunoglobulin Light Chain-Mediated Mesangial Injury

Mesangial injury produced by glomerulopathic monoclonal immunoglobulin light chains leads to two diametrically opposed results: matrix loss from mesangiolysis / replacement of mesangial matrix with amyloid (AL-amyloidosis) and increased matrix (light chain deposition disease), depending on the physicochemically abnormal light chains involved. In both situations; however, there is loss of mesangial cells resulting from apoptosis induced by the glomerulopathic light chains. This unique experimental model of mesangial injury resulting in divergent manifestations allows examining the role of mesenchymal stem cells in mesangial glomerular repair in the two typical prototypes of alterations that occur in the mesangium when injured by a variety of injurious agents. In vitro and ex vivo platforms are used to translate information from the in-vitro to the in-vivo arenas. Both platforms highlight the same sequence of events that take place: mesenchymal stem cells identify the site of damage, clear the damaged mesangium by eliminating debris from apoptotic cellular elements and deposited “foreign” (not belonging in the normal mesangium) material and eventually differentiate into mature mesangial cells laying down new matrix. Each of the platforms employed exhibited certain advantages and limitations in visualizing and understanding the processes that occur. When the information generated was combined, a clear view of mechanisms involved and how mesenchymal stem cells participated in mesangial repair emerged.

Role of translational research advancing the understanding of the pathogenesis of light chain-mediated glomerulopathies

Glomerulopathic light chains engage in pathological interactions with mesangial cells resulting in alterations in glomerular homeostasis. The crucial pathological events are centered in the mesangium and, therefore, research dealing with pathogenesis of these disorders is focused on this glomerular compartment. Particular physicochemical characteristics of these light chains are responsible for their ability to alter mesangial milieu leading to glomerular damage. An in vitro model has been used to dissect the processes involved. This model has been instrumental in providing a solid platform from which to observe in a dynamic fashion how mesangial cells handle pathogenic light chains and the sequential steps that are involved in the progressive glomerular damage. Key steps amenable to possible modulation have been defined and should provide a solid platform to design and test therapeutic interventions. In the past significant difficulties have been encountered in the development of animal models of light chain-induced glomerular damage. However, in the last few years a new generation of animal models has emerged to address whether what has been documented in vitro retains significance in vivo. Preliminary observations appear to substantiate this.

Different types of glomerulopathic light chains interact with mesangial cells using a common receptor but exhibit different intracellular trafficking patterns

Patients with plasma cell dyscrasias may have circulating light chains (LCs), some of which are nephrotoxic. Nephrotoxic LCs can affect the various renal compartments. Some of these LCs may produce predominantly proximal tubular damage, while others are associated with distal nephron obstruction (the so-called ‘myeloma kidney'). Both these are considered tubulopathic (T) LCs. A receptor has been found in proximal tubular cells (cubilin/megalin complex), which mediates the absorption of LCs and is involved in the pathogenesis of tubulopathies that occurs in these patients. Another group of nephrotoxic LCs is associated with glomerular damage and these are considered as glomerulopathic (G). These patients with G-LCs may develop AL-amyloidosis (AL-Am) or LC deposition disease (LCDD). Recent evidence indicates that the physicochemical characteristics (amino-acid composition and conformation of the variable region) of a given nephrotoxic LC may be the most significant factor in determining the type and location of renal damage within the nephron. Other factors may also be involved, including yet uncharacterized host factors that may include genetic polymorphism, among others. Interestingly, the amount of LC production by the clone of plasma cells does not correlate directly with the severity of the renal alterations. Understanding the nature of the interactions between G-LCs and mesangial cells (MCs) is crucial to define key steps that may be targeted for therapeutic purposes. Experimental studies have delineated important aspects pertaining to interactions between G-LCs and MCs, indicating that these interactions are receptor mediated. The data presented in the current study support a single receptor present on MCs for both LCDD and AL-LCs, as clearly demonstrated with competition and colocalization immunofluorescence (IF) studies. This receptor resides in caveolae present on the plasma membrane of HMCs and is overexpressed when HMCs are incubated with G-LCs but not TLCs. Caveolae play a fundamental role in receptor-mediated endocytosis, a crucial process in the internalization of AL-LCs and amyloidogenesis. LC internalization is clathrin mediated. The data also indicate that intracellular trafficking in MCs is different for AL-LCs and LCDD-LCs. AL-LCs are delivered to the mature lysosomal compartment where amyloid formation occurs. LCDD-LCs alter mesangial function and phenotype by interacting with the MC surface membranes through similar receptors as the AL-LCs. The data also demonstrated that cubilin and megalin were absent on MCs, so the receptor involved is different from the one already characterized in the proximal tubules.

Glomerulopathic Light Chain-Mesangial Cell Interactions Modulate in Vitro Extracellular Matrix Remodeling and Reproduce Mesangiopathic Findings Documented in Vivo

Glomerulopathic light chains (LCs) are associated with two distinct mesangiopathies: AL (light-chain-related) amyloidosis and light-chain deposition disease (LCDD) with immunomorphologic features that are well documented in the literature. Even though both conditions are caused by monoclonal LCs, these entities differ dramatically in their morphologic expressions. In AL amyloidosis the mesangial matrix is replaced by amyloid fibrils, while in LCDD the matrix increases as a consequence of deposition of excess extracellular matrix (ECM). The immunomorphologic mesangial alterations observed in biopsy material are closely reproduced in vitro when mesangial cells grown on an artificial matrix are incubated with monoclonal light chains obtained from the urine of patients with either condition. This article summarizes previously reported data, reports new findings, and focuses on integrating all the available information on the subject. When mesangial cells are incubated with LCDD-LCs, production of ECM proteins (collagen IV, laminin, fibronectin, and tenascin) is increased, with maximum effect at 72 hours post LC treatment. A concomitant decrease in collagenase IV activity further accentuates the accumulation of mesangial matrix. These effects are mediated through transforming growth factor-beta (TGF-beta) activation. In contrast, when mesangial cells are incubated with Am-LCs, a decrease in ECM protein production and a stimulatory effect on collagenase IV is observed, which results in matrix degradation and facilitates amyloid deposition. The decreased TGF-beta documented in the literature in this setting precludes adequate matrix repair. These findings substantiate the morphologic alterations observed in renal biopsy specimens and in the in vitro model. Using this in vitro model, it is then possible to delineate the LC interactions with putative receptors at the mesangial cell surface that regulate mesangial cell pathobiologic responses and mesangial matrix homeostasis.