Understanding Mesangial Pathobiology in AL-Amyloidosis and Monoclonal Ig Light Chain Deposition Disease

Internalisation of immunoglobulin light chains by cardiomyocytes in AL amyloidosis: what can biopsies tell us?

BACKGROUND

Cardiac involvement in systemic light chain amyloidosis (AL) leads to chronic heart failure and is a major prognosis factor. Severe cellular defects are provoked in cardiac cells by tissue-deposited amyloid fibrils of misfolded free immunoglobulin light chains (LCs) and their prefibrillar oligomeric precursors.

OBJECTIVE

Understanding the molecular mechanisms behind cardiac cell cytotoxicity is necessary to progress in therapy and to improve patient management. One key question is how extracellularly deposited molecules exert their toxic action inside cardiac cells. Here we searched for direct evidence of amyloid LC uptake by cardiomyocytes in patient biopsies.

METHODS

We immunolocalized LCs in cardiac biopsies from four AL cardiac amyloidosis patients and analysed histopathological images by high resolution confocal microscopy and 3D image reconstruction.

RESULTS

We show, for the first time directly in patient tissue, the presence of LCs inside cardiomyocytes, and report their proximity to nuclei and to caveolin-3-rich areas. Our observations point to macropinocytosis as a probable mechanism of LC uptake.

CONCLUSIONS

Internalisation of LCs occurs in patient cardiomyocytes. This event could have important consequences for the pathogenesis of the cardiac disease by enabling interactions between amyloid molecules and cellular organelles inducing specific signalling pathways, and might bring new insight regarding treatment.

Management of tracheobronchial amyloidosis: a review of the literature

Introduction

Tracheobronchial amyloidosis is a rare idiopathic disorder characterised by extracellular deposition of misfolded protein fibrils in the tracheobronchial tree. It presents with nonspecific symptoms. Deciding on the best treatment approach can be challenging due to the lack of a treatment guideline. We undertook a review to assess the therapeutic options for tracheobronchial amyloidosis and to highlight gaps within the existing evidence.

Methods

We performed a literature search from 1 January 1990 until 1 March 2022 to identify relevant literature regarding patient characteristics, symptoms, management and prognosis for patients with tracheobronchial amyloidosis.

Results

77 studies consisting of 300 patients were included. We found a great heterogeneity in the management of tracheobronchial amyloidosis patients. Although a fifth of the reported patients were managed with a wait-and-see approach, many different treatments were used as a single intervention, or multiple treatments were combined. An interesting finding is the slightly higher percentage of patients with Sjögren syndrome (n=5, 1.7%) and tracheobronchial amyloidosis compared to the normal population (0.5-1.0%).

Conclusions

There is a great heterogeneity in the management of tracheobronchial amyloidosis patients. The treatment is still based on expert opinion due to the lack of a treatment guideline. Various treatment approaches include a wait-and-see approach, external beam radiotherapy, therapeutic bronchoscopy, immunosuppressive treatment and surgery.

Amyloid deposition through endocytosis in vascular endothelial cells

No mechanistic lead is known for establishing AL amyloid deposits in organs. We here report an electron microscopic (EM) analysis in a case of intestinal AL amyloidosis before initiating treatment for amyloidosis. The dense deposits of amyloid fibrils are concentrated around the small blood vessels in the submucosal area of intestinal tissue. Surprisingly, we observed endothelial cells (ECs) of blood vessels containing plenty of endocytotic (pinocytotic) and transcytotic vesicles at the luminal side and above the basement membrane, indicating the one-way active trafficking of either the immunoglobulin (Ig) light chain or preassembled amyloid fibrils from the luminal side of ECs to the extraluminal area of ECs. Immunoelectron microscopy displayed that the immuno-gold signals were observed in the vascular cavity and the subendothelial area of amyloid deposits. However, there is no sign of an Ig light chain in pinocytotic vesicles. Therefore, the intestinal ECs may actively pump out mainly the preassembled amyloid fibrils (not light chains) from the blood stream into the subendothelial area as a physiologic function.

Role of the mechanisms for antibody repertoire diversification in monoclonal light chain deposition disorders: when a friend becomes foe

The adaptive immune system of jawed vertebrates generates a highly diverse repertoire of antibodies to meet the antigenic challenges of a constantly evolving biological ecosystem. Most of the diversity is generated by two mechanisms: V(D)J gene recombination and somatic hypermutation (SHM). SHM introduces changes in the variable domain of antibodies, mostly in the regions that form the paratope, yielding antibodies with higher antigen binding affinity. However, antigen recognition is only possible if the antibody folds into a stable functional conformation. Therefore, a key force determining the survival of B cell clones undergoing somatic hypermutation is the ability of the mutated heavy and light chains to efficiently fold and assemble into a functional antibody. The antibody is the structural context where the selection of the somatic mutations occurs, and where both the heavy and light chains benefit from protective mechanisms that counteract the potentially deleterious impact of the changes. However, in patients with monoclonal gammopathies, the proliferating plasma cell clone may overproduce the light chain, which is then secreted into the bloodstream. This places the light chain out of the protective context provided by the quaternary structure of the antibody, increasing the risk of misfolding and aggregation due to destabilizing somatic mutations. Light chain-derived (AL) amyloidosis, light chain deposition disease (LCDD), Fanconi syndrome, and myeloma (cast) nephropathy are a diverse group of diseases derived from the pathologic aggregation of light chains, in which somatic mutations are recognized to play a role. In this review, we address the mechanisms by which somatic mutations promote the misfolding and pathological aggregation of the light chains, with an emphasis on AL amyloidosis. We also analyze the contribution of the variable domain (V_L) gene segments and somatic mutations on light chain cytotoxicity, organ tropism, and structure of the AL fibrils. Finally, we analyze the most recent advances in the development of computational algorithms to predict the role of somatic mutations in the cardiotoxicity of amyloidogenic light chains and discuss the challenges and perspectives that this approach faces.

AL(light chain)-amyloidogenesis by mesangial cells involves active participation of lysosomes: An ultrastructural study

Amyloid formation by cells is a stepwise process that occurs in macrophages and cells capable of transforming into a macrophage phenotype. One such cell is the mesangial cell in the kidney. It has been shown that mesangial cells are engaged in AL (light chain associated)- amyloidogenesis after transforming phenotypically from a smooth muscle to a macrophage phenotype. The actual process of amyloid fibril formation has not been dissected. This ultrastructural study which includes the examination of lysosomal gradient specimens addresses this issue by analyzing the sequence of events that takes place as fibrils are formed in endosomes and lysosomes. The findings indicate that fibrillogenesis begins in endosomes but is completed and most pronounced in the lysosomal compartment. As early as 10 min after incubation of human mesangial cells with AL-LCs, amyloid fibrils are formed in endosomes but mostly occurs in the mature lysosomal compartment. This is the first time that fibril formation is demonstrated experimentally occurring inside human mesangial cells and the entire sequence of events taking place is elucidated.

Unfolding and Aggregation Pathways of Variable Domains from Immunoglobulin Light Chains

Light chain amyloidosis is the most common form of systemic amyloidosis. This disease is caused by the formation and deposition of amyloid fibers made from immunoglobulin light chains. Environmental conditions such as pH and temperature can affect protein structure and induce the development of these fibers. Several studies have shed light on the native state, stability, dynamics, and final amyloid state of these proteins; however, the initiation process and the fibril formation pathway remain poorly understood structurally and kinetically. To study this, we analyzed the unfolding and aggregation process of the 6aJL2 protein under acidic conditions, with temperature changes, and upon mutation, using biophysical and computational techniques. Our results suggest that the differences in amyloidogenicity displayed by 6aJL2 under these conditions are caused by traversing different aggregation pathways, including unfolded intermediates and the formation of oligomers.

Macrophages in the reticuloendothelial system inhibit early induction stages of mouse apolipoprotein A-II amyloidosis

Amyloidosis refers to a group of degenerative diseases that are characterized by the deposition of misfolded protein fibrils in various organs. Deposited amyloid may be removed by a phagocyte-dependent innate immune system; however, the precise mechanisms during disease progression remain unclear. We herein investigated the properties of macrophages that contribute to amyloid degradation and disease progression using inducible apolipoprotein A-II amyloidosis model mice. Intravenously injected AApoAII amyloid was efficiently engulfed by reticuloendothelial macrophages in the liver and spleen and disappeared by 24 h. While cultured murine macrophages degraded AApoAII via the endosomal-lysosomal pathway, AApoAII fibrils reduced cell viability and phagocytic capacity. Furthermore, the depletion of reticuloendothelial macrophages before the induction of AApoAII markedly increased hepatic and splenic AApoAII deposition. These results highlight the physiological role of reticuloendothelial macrophages in the early stages of pathogenesis and suggest the maintenance of phagocytic integrity as a therapeutic strategy to inhibit disease progression.

The involvement of branched-chain amino acids (BCAAs) in aromatic trihalogenated DBP exposure-induced kidney damage in mice

Disinfection by-products (DBPs) are inevitably generated in the process of disinfection. Among them, aromatic halogenated DBPs, such as 2,4,6-trichlorophenol (TCP), 2,4,6-tribromophenol (TBP) and 2,4,6-triiodophenol (TIP), have attracted considerable interest for their high toxicity. A systematic nephrotoxicity evaluation of 2,4,6-trihalophenols is still lacking. In this study, mice were exposed to TCP, TBP and TIP ranging from environmental-related low concentration to high concentration that commonly used in animal study (0.5-200 μg/L). Kidney histopathology, urine protein detection and urine metabolomics were performed. Remarkable changes including kidney damage, proteinuria and glomerular mesangial cell proliferation were observed after three 2,4,6-trihalophenol exposure, even at low concentration of 0.5 μg/L. The nephrotoxicity rank order was TIP > TBP > TCP. Additionally, in vivo exposure to 2,4,6-trihalophenols also led to apparent changes in urinary metabolic profiles. Biosynthesis pathways of branched-chain amino acids (BCAAs, containing valine, leucine and isoleucine) were disturbed even at the early stage of exposure (4 weeks). Intriguingly, it has been reported that BCAAs could promote the proliferation of glomerular mesangial cells. Thus, in vitro cell experiments were further performed on mouse glomerular mesangial cell line MES-13. Consistently with in vivo results, cell proliferation was observed in MES-13 cells after exposure to 2,4,6-trihalophenols, especially to TBP and TIP. Meanwhile, TCP at high concentration, TBP and TIP at not only high concentration but also low concentration, induced BCAAs accumulation in glomerular mesangial cells, which was completely commensurate to that observed in cell proliferation assay. Then the proliferation of MES-13 cells induced by 2,4,6-trihalophenols was remarkably inhibited after BCAAs interference. Here we provide direct link between disturbed BCAAs and the nephrotoxicity of 2,4,6-trihalophenols. 2,4,6-trihalophenols could induce excess BCAAs, which further led to proliferation of glomerular mesangial cells and renal injury. This study revealed the nephrotoxicity of aromatic trihalogenated DBPs and provided new insights into the potential toxic mechanisms.

Dynamic protein structures in normal function and pathologic misfolding in systemic amyloidosis

Dynamic and disordered regions in native proteins are often critical for their function, particularly in ligand binding and signaling. In certain proteins, however, such regions can contribute to misfolding and pathologic deposition as amyloid fibrils in vivo. For example, dynamic and disordered regions can promote amyloid formation by destabilizing the native structure, by directly triggering the aggregation, by promoting protein condensation, or by acting as sites of early proteolytic cleavage that favor a release of aggregation-prone fragments or facilitate fibril maturation. At the same time, enhanced dynamics in the native protein state accelerates proteolytic degradation that counteracts amyloid accumulation in vivo. Therefore, the functional need for dynamic protein regions must be balanced against their inherently labile nature. How exactly this balance is achieved and how is it shifted upon amyloidogenic mutations or post-translational modifications? To illustrate possible scenarios, here we review the beneficial and pathologic roles of dynamic and disordered regions in the native states of three families of human plasma proteins that form amyloid precursors in systemic amyloidoses: immunoglobulin light chain, apolipoproteins, and serum amyloid A. Analysis of structure, stability and local dynamics of these diverse proteins and their amyloidogenic variants exemplifies how disordered/dynamic regions can provide a functional advantage as well as an Achilles heel in pathologic amyloid formation.

Role of mutations and post-translational modifications in systemic AL amyloidosis studied by cryo-EM

Systemic AL amyloidosis is a rare disease that is caused by the misfolding of immunoglobulin light chains (LCs). Potential drivers of amyloid formation in this disease are post-translational modifications (PTMs) and the mutational changes that are inserted into the LCs by somatic hypermutation. Here we present the cryo electron microscopy (cryo-EM) structure of an ex vivo λ1-AL amyloid fibril whose deposits disrupt the ordered cardiomyocyte structure in the heart. The fibril protein contains six mutational changes compared to the germ line and three PTMs (disulfide bond, N-glycosylation and pyroglutamylation). Our data imply that the disulfide bond, glycosylation and mutational changes contribute to determining the fibril protein fold and help to generate a fibril morphology that is able to withstand proteolytic degradation inside the body.

Renal amyloidosis: Pathogenesis

For many years amyloidosis was considered an extremely rare, somewhat mysterious disease. However, in the last 2-3 decades its pathogenesis, particularly that of renal amyloidosis has been carefully dissected in the research laboratory using in-vitro and, to a lesser extent, in-vivo models. These have provided a molecular understanding of sequential events that take place in the renal mesangium leading to the formation of amyloid fibrils and eventual extrusion into the mesangial matrix, which itself becomes seriously damaged and, in due time, replaced by the fibrillary material. Amyloid, once considered to be an "inert" substance, has been proven to be involved in crucial biological processes that result in the destruction and eventual replacement of normal renal constituents. This review centers on mechanisms involved in the renal glomerular amyloidosis to understand its pathogenesis.

Barriers to Small Molecule Drug Discovery for Systemic Amyloidosis

Inhibition of amyloid fibril formation could benefit patients with systemic amyloidosis. In this group of diseases, deposition of amyloid fibrils derived from normally soluble proteins leads to progressive tissue damage and organ failure. Amyloid formation is a complex process, where several individual steps could be targeted. Several small molecules have been proposed as inhibitors of amyloid formation. However, the exact mechanism of action for a molecule is often not known, which impedes medicinal chemistry efforts to develop more potent molecules. Furthermore, commonly used assays are prone to artifacts that must be controlled for. Here, potential mechanisms by which small molecules could inhibit aggregation of immunoglobulin light-chain dimers, the precursor proteins for amyloid light-chain (AL) amyloidosis, are studied in assays that recapitulate different aspects of amyloidogenesis in vitro. One molecule reduced unfolding-coupled proteolysis of light chains, but no molecules inhibited aggregation of light chains or disrupted pre-formed amyloid fibrils. This work demonstrates the challenges associated with drug development for amyloidosis, but also highlights the potential to combine therapies that target different aspects of amyloidosis.

A Fresh Perspective on Monoclonal Gammopathies of Renal Significance

Monoclonal gammopathies of renal significance (MGRS) encompass a remarkable variety of kidney diseases that result from intrinsic nephrotoxic properties of certain monoclonal Igs or their subunits. Effective disease-modifying treatments rely on the targeting of a malignant B-cell clone that may be demonstrable but often is quite hypothetical. Hence, convincing arguments for the genuine monoclonal character of the causative mono-isotypic Ig tissue deposits is needed for design of appropriate treatment strategies. The purpose of this article was to critically analyze distinct situations of suspected MGRS that occur in the practice of pathologists, nephrologists, hematologists, and immunologists. A particular focus of interest is the group of conditions known as proliferative glomerulonephritis with mono-isotypic immunoglobulin deposits (PGNMIDs), which illustrates the difficulties and ambiguities surrounding a definitive assignment of MGRS status.

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

INTRODUCTION

Deciphering 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.

METHODS

Human 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.

RESULTS

Crosslinking 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.

CONCLUSION

Our 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.

Randall-Type Monoclonal Immunoglobulin Deposition Disease: New Insights into the Pathogenesis, Diagnosis and Management

Randall-type monoclonal immunoglobulin deposition disease (MIDD) is a rare disease that belongs to the spectrum of monoclonal gammopathy of renal significance (MGRS). Renal involvement is prominent in MIDD, but extra-renal manifestations can be present and may affect global prognosis. Recent data highlighted the central role of molecular characteristics of nephrotoxic monoclonal immunoglobulins in the pathophysiology of MIDD, and the importance of serum free light chain monitoring in the diagnosis and follow-up disease. Clone-targeted therapy is required to improve the overall and renal survival, and the achievement of a rapid and deep hematological response is the goal of therapy. This review will focus on the recent progress in the pathogenesis and management of this rare disease.