Light chain amyloidosis - current findings and future prospects

Native State Stabilization of Amyloidogenic Proteins by Kinetic Stabilizers: Inhibition of Protein Aggregation and Clinical Relevance

Proteinopathies or amyloidoses are a group of life-threatening disorders that result from misfolding of proteins and aggregation into toxic insoluble amyloid aggregates. Amyloid aggregates have low clearance from the body due to the insoluble nature, leading to their deposition in various organs and consequent organ dysfunction. While amyloid deposition in the central nervous system leads to neurodegenerative diseases that mostly cause dementia and difficulty in movement, several other organs, including heart, liver and kidney are also affected by systemic amyloidoses. Regardless of the site of amyloid deposition, misfolding and structural alteration of the precursor proteins play the central role in amyloid formation. Kinetic stabilizers are an emerging class of drugs, which act like pharmacological chaperones to stabilize the native state structure of amyloidogenic proteins and to increase the activation energy barrier that is required for adopting a misfolded structure or conformation, ultimately leading to the inhibition of protein aggregation. In this review, we discuss the kinetic stabilizers that stabilize the native quaternary structure of transthyretin, immunoglobulin light chain and superoxide dismutase 1 that cause transthyretin amyloidoses, light chain amyloidosis and familial amyotrophic lateral sclerosis, respectively.

Amyloidogenic light chains impair plasma cell survival

Systemic light chain amyloidosis (AL) is a clonal plasma cell disorder characterized by the deposition of misfolded immunoglobulin light chains (LC) as insoluble fibrils in organs. The lack of suitable models has hindered the investigation of the disease mechanisms. Our aim was to establish AL LC-producing plasma cell lines and use them to investigate the biology of the amyloidogenic clone. We used lentiviral vectors to generate cell lines expressing LC from patients suffering from AL amyloidosis. The AL LC-producing cell lines showed a significant decrease in proliferation, cell cycle arrest, and an increase in apoptosis and autophagy as compared with the multiple myeloma LC-producing cells. According to the results of RNA sequencing the AL LC-producing lines showed higher mitochondrial oxidative stress, and decreased activity of the Myc and cholesterol pathways. The neoplastic behavior of plasma cells is altered by the constitutive expression of amyloidogenic LC causing intracellular toxicity. This observation may explain the disparity in the malignant behavior of the amyloid clone compared to the myeloma clone. These findings should enable future in vitro studies and help delineate the unique cellular pathways of AL, thus expediting the development of specific treatments for patients with this disorder.

Dissection of the amyloid formation pathway in AL amyloidosis

In antibody light chain (AL) amyloidosis, overproduced light chain (LC) fragments accumulate as fibrils in organs and tissues of patients. In vitro, AL fibril formation is a slow process, characterized by a pronounced lag phase. The events occurring during this lag phase are largely unknown. We have dissected the lag phase of a patient-derived LC truncation and identified structural transitions that precede fibril formation. The process starts with partial unfolding of the V_L domain and the formation of small amounts of dimers. This is a prerequisite for the formation of an ensemble of oligomers, which are the precursors of fibrils. During oligomerization, the hydrophobic core of the LC domain rearranges which leads to changes in solvent accessibility and rigidity. Structural transitions from an anti-parallel to a parallel β-sheet secondary structure occur in the oligomers prior to amyloid formation. Together, our results reveal a rate-limiting multi-step mechanism of structural transitions prior to fibril formation in AL amyloidosis, which offers, in the long run, opportunities for therapeutic intervention.

AL amyloidosis is caused by the accumulation of overproduced light chain (LC) fragments as fibrils in patient organs and it is the most prevalent systemic amyloidosis. Here, the authors combine biochemical and biophysical experiments to characterise the lag phase of a patient-derived truncated LC and they identify structural transitions that precede fibril formation.

Renal Involvement in Systemic Amyloidosis Caused by Monoclonal Immunoglobulins

Kidney involvement in immunoglobulin-related amyloidosis (AIg) is common. Although patients with renal-limited AIg tend not to have the high mortality that patients with cardiac amyloidosis have, they do experience significant morbidity and impact on quality of life. The complexity of the pathogenesis remains incompletely understood. Models have been established to prognosticate and assess for the response to therapy. Patients with advanced renal impairment from immunoglobulin light chain amyloidosis still have poor renal prognosis, and better therapy is needed in order to preserve kidney function. Patients who develop end-stage renal disease can undergo renal replacement therapy with kidney transplantation.

Light chain amyloidosis induced inflammatory changes in cardiomyocytes and adipose-derived mesenchymal stromal cells

Light chain (AL) amyloidosis is a progressive, degenerative disease characterized by the misfolding and amyloid deposition of immunoglobulin light chain (LC). The amyloid deposits lead to organ failure and death. Our laboratory is specifically interested in cardiac involvement of AL amyloidosis. We have previously shown that the fibrillar aggregates of LC proteins can be cytotoxic and arrest the growth of human RFP-AC16 cardiomyocytes in vitro. We showed that adipose-derived mesenchymal stromal cells (AMSC) can rescue the cardiomyocytes from the fibril-induced growth arrest through contact-dependent mechanisms. In this study, we examined the transcriptome changes of human cardiomyocytes and AMSC in the presence of AL amyloid fibrils. The presence of fibrils causes a 'priming' immune response in AMSC associated with interferon associated genes. Exposure to AL fibrils induced changes in the pathways associated with immune response and extracellular matrix components in cardiomyocytes. We also observed upregulation of innate immune-associated transcripts (chemokines, cytokines, and complement), suggesting that amyloid fibrils initiate an innate immune response on these cells, possibly due to phenotypic transformation. This study corroborates and expands our previous studies and identifies potential new immunologic mechanisms of action for fibril toxicity on human cardiomyocytes and AMSC rescue effect on cardiomyocytes.

Fatal amyloid formation in a patient's antibody light chain is caused by a single point mutation

In systemic light chain amyloidosis, an overexpressed antibody light chain (LC) forms fibrils which deposit in organs and cause their failure. While it is well-established that mutations in the LC’s V_L domain are important prerequisites, the mechanisms which render a patient LC amyloidogenic are ill-defined. In this study, we performed an in-depth analysis of the factors and mutations responsible for the pathogenic transformation of a patient-derived λ LC, by recombinantly expressing variants in E. coli. We show that proteolytic cleavage of the patient LC resulting in an isolated V_L domain is essential for fibril formation. Out of 11 mutations in the patient V_L, only one, a leucine to valine mutation, is responsible for fibril formation. It disrupts a hydrophobic network rendering the C-terminal segment of V_L more dynamic and decreasing domain stability. Thus, the combination of proteolytic cleavage and the destabilizing mutation trigger conformational changes that turn the LC pathogenic.

Amyloid light chain amyloidosis, shortened to AL amyloidosis, is a rare and often fatal disease. It is caused by a disorder of the bone marrow. Usually, cells in the bone marrow produce Y-shaped proteins called antibodies to fight infections. In AL amyloidosis, these cells release too much of the short arm of the antibody, known as its light chain, and the light chains also carry mutations. The antibodies are no longer able to assemble properly, and instead misfold and form structures, known as amyloid fibrils. The fibrils build up outside the cells, gradually causing damage to tissues and organs that can lead to life-threatening organ failure.

Due to the rareness of the disease, diagnosis is often overlooked and delayed. People experience widely varying symptoms, depending on the organs affected. Also, given the diversity of antibodies people make, every person with AL amyloidosis has a variety of mutations implicated in their disease. It is thought that mutations in the antibody light chain make it unstable and prone to misfolding, but it remains unclear which specific mutations trigger a cascade of amyloid fibril formation.

Now, Kazman et al. have pinpointed the exact mechanism in one case of the disease. First, tissue biopsies from a woman with advanced AL amyloidosis were analyzed, and the defunct antibody light chain was isolated. Eleven mutations were identified in the antibody light chain, only one of which was found to be responsible for the formation of the harmful fibrils. The next step was to determine how this one small change was so damaging. The experiments showed that after the antibody light chain was cut in two, a process that happens naturally in the body, this single mutation transforms it into a protein capable of causing disease.

In this ‘bedside to lab bench’ study, Kazman et al. have succeeded in determining the molecular origin of one case of AL amyloidosis. The results have also shown that the instability of antibodies due to mutation does not alone explain the formation of amyloid fibrils in this disease and that the cutting of this protein in two is also important. It is hoped that, in the long run, this work will lead to new diagnostics and treatment options for people with AL amyloidosis.

The Antibody Light-Chain Linker Regulates Domain Orientation and Amyloidogenicity

The antibody light chain (LC) consists of two domains and is essential for antigen binding in mature immunoglobulins. The two domains are connected by a highly conserved linker that comprises the structurally important Arg108 residue. In antibody light chain (AL) amyloidosis, a severe protein amyloid disease, the LC and its N-terminal variable domain (V_L) convert to fibrils deposited in the tissues causing organ failure. Understanding the factors shaping the architecture of the LC is important for basic science, biotechnology and for deciphering the principles that lead to fibril formation. In this study, we examined the structure and properties of LC variants with a mutated or extended linker. We show that under destabilizing conditions, the linker modulates the amyloidogenicity of the LC. The fibril formation propensity of LC linker variants and their susceptibility to proteolysis directly correlate implying an interplay between the two LC domains. Using NMR and residual dipolar coupling-based simulations, we found that the linker residue Arg108 is a key factor regulating the relative orientation of the V_L and C_L domains, keeping them in a bent and dense, but still flexible conformation. Thus, inter-domain contacts and the relative orientation of V_L and C_L to each other are of major importance for maintaining the structural integrity of the full-length LC.

Identification of two principal amyloid-driving segments in variable domains of Ig light chains in systemic light-chain amyloidosis

Systemic light-chain amyloidosis (AL) is a human disease caused by overexpression of monoclonal immunoglobulin light chains that form pathogenic amyloid fibrils. These amyloid fibrils deposit in tissues and cause organ failure. Proteins form amyloid fibrils when they partly or fully unfold and expose segments capable of stacking into β-sheets that pair and thereby form a tight, dehydrated interface. These structures, termed steric zippers, constitute the spines of amyloid fibrils. Here, using a combination of computational (with ZipperDB and Boston University ALBase), mutational, biochemical, and protein structural analyses, we identified segments within the variable domains of Ig light chains that drive the assembly of amyloid fibrils in AL. We demonstrate that there are at least two such segments and that each one can drive amyloid fibril assembly independently of the other. Our analysis revealed that peptides derived from these segments form steric zippers featuring a typical dry interface with high-surface complementarity and occupy the same spatial location of the Greek-key immunoglobulin fold in both λ and κ variable domains. Of note, some predicted steric-zipper segments did not form amyloid fibrils or assembled into fibrils only when removed from the whole protein. We conclude that steric-zipper propensity must be experimentally validated and that the two segments identified here may represent therapeutic targets. In addition to elucidating the molecular pathogenesis of AL, these findings also provide an experimental approach for identifying segments that drive fibril formation in other amyloid diseases.

Integrative Transcriptomic and microRNAomic Profiling Reveals Immune Mechanism for the Resilience to Soybean Meal Stress in Fish Gut and Liver

In aquafeeds, fish-meal has been commonly replaced with plant protein, which often causes enteritis. Currently, foodborne enteritis has few solutions in regards to prevention or cures. The recovery mechanism from enteritis in herbivorous fish may further help understand prevention or therapy. However, few reports could be found regarding the recovery or resilience to fish foodborne enteritis. In this study, grass carp was used as an animal model for soybean meal induced enteritis and it was found that the fish could adapt to the soybean meal at a moderate level of substitution. Resilience to soybean meal stress was found in the 40% soybean meal group for juvenile fish at growth performance, morphological and gene expression levels, after a 7-week feeding trial. Furthermore, the intestinal transcriptomic data, including transcriptome and miRNAome, was applied to demonstrate resilience mechanisms. The result of this study revealed that in juvenile grass carp after a 7-week feeding cycle with 40% soybean meal, the intestine recovered via enhancing both an immune tolerance and wound healing, the liver gradually adapted via re-balancing immune responses, such as phagosome and complement cascades. Also, many immune factors in the gut and liver were systemically revealed among stages of on-setting, remising, and recovering (or relief). In addition, miRNA regulation played a key role in switching immune states. Thus, the present data systemically demonstrated that the molecular adaptation mechanism of fish gut-liver immunity is involved in the resilience to soybean meal stress.

Convergent mechanisms favor fast amyloid formation in two lambda 6a Ig light chain mutants

Extracellular deposition as amyloids of immunoglobulin light chains causes light chain amyloidosis. Among the light chain families, lambda 6a is one of the most frequent in light chain amyloidosis patients. Its germline protein, 6aJL2, and point mutants, R24G and P7S, are good models to study fibrillogenesis, because their stability and fibril formation characteristics have been described. Both mutations make the germline protein unstable and speed up its ability to aggregate. To date, there is no molecular mechanism that explains how these differences in amyloidogenesis can arise from a single mutation. To look into the structural and dynamical differences in the native state of these proteins, we carried out molecular dynamics simulations at room temperature. Despite the structural similarity of the germline protein and the mutants, we found differences in their dynamical signatures that explain the mutants' increased tendency to form amyloids. The contact network alterations caused by the mutations, though different, converge in affecting two anti-aggregation motifs present in light chain variable domains, suggesting a different starting point for aggregation in lambda chains compared to kappa chains.

Mesenchymal stromal cells protect human cardiomyocytes from amyloid fibril damage

BACKGROUND AIMS

Light chain (AL) amyloidosis is a protein misfolding disease characterized by extracellular deposition of immunoglobulin light chains (LC) as amyloid fibrils. Patients with LC amyloid involvement of the heart have the worst morbidity and mortality. Current treatments target the plasma cells to reduce further production of amyloid proteins. There is dire need to understand the mechanisms of cardiac tissue damage from amyloid to develop novel therapies. We recently reported that LC soluble and fibrillar species cause apoptosis and inhibit cell growth in human cardiomyocytes. Mesenchymal stromal cells (MSCs) can promote wound healing and tissue remodeling. The objective of this study was to evaluate MSCs to protect cardiomyocytes affected by AL amyloid fibrils.

METHODS

We used live cell imaging and proteomics to analyze the effect of MSCs in the growth arrest caused by AL amyloid fibrils.

RESULTS

We evaluated the growth of human cardiomyocytes (RFP-AC16 cells) in the presence of cytotoxic LC amyloid fibrils. MSCs reversed the cell growth arrest caused by LC fibrils. We also demonstrated that this effect requires cell contact and may be mediated through paracrine factors modulating cell adhesion and extracellular matrix remodeling. To our knowledge, this is the first report of MSC protection of human cardiomyocytes in amyloid disease.

CONCLUSIONS

This important proof of concept study will inform future rational development of MSC therapy in cardiac LC amyloid.

Effect of amino acid mutations on the conformational dynamics of amyloidogenic immunoglobulin light-chains: A combined NMR and in silico study

The conformational dynamics of a pathogenic κ4 human immunoglobulin light-chain variable domain, SMA, associated with AL amyloidosis, were investigated by ¹⁵N relaxation dispersion NMR spectroscopy. Compared to a homologous light-chain, LEN, which differs from SMA at eight positions but is non-amyloidogenic in vivo, we find that multiple residues in SMA clustered around the N-terminus and CDR loops experience considerable conformational exchange broadening caused by millisecond timescale protein motions, consistent with a destabilized dimer interface. To evaluate the contribution of each amino acid substitution to shaping the dynamic conformational landscape of SMA, NMR studies were performed for each SMA-like point mutant of LEN followed by in silico analysis for a subset of these proteins. These studies show that a combination of only three mutations located within or directly adjacent to CDR3 loop at the dimer interface, which remarkably include both destabilizing (Q89H and Y96Q) and stabilizing (T94H) mutations, largely accounts for the differences in conformational flexibility between LEN and SMA. Collectively, our studies indicate that a correct combination of stabilizing and destabilizing mutations is key for immunoglobulin light-chains populating unfolded intermediates that result in amyloid formation, and underscore the complex nature of correlations between light-chain conformational flexibility, thermodynamic stability and amyloidogenicity.

Solid-state NMR chemical shift assignments for AL-09 VL immunoglobulin light chain fibrils

Light chain (AL) amyloidosis is a systemic disease characterized by the formation of immunoglobulin light-chain fibrils in critical organs of the body. The light-chain protein AL-09 presents one severe case of cardiac AL amyloidosis, which contains seven mutations in the variable domain (V_L) relative to its germline counterpart, κI O18/O8 V_L. Three of these mutations are non-conservative-Y87H, N34I, and K42Q-and previous work has shown that they are responsible for significantly reducing the protein's thermodynamic stability, allowing fibril formation to occur with fast kinetics and across a wide-range of pH conditions. Currently, however, there is extremely limited structural information available which explicitly describes the residues that are involved in supporting the misfolded fibril structure. Here, we assign the site-specific ¹⁵N and ¹³C chemical shifts of the rigid residues of AL-09 V_L fibrils by solid-state NMR, reporting on the regions of the protein involved in the fibril as well as the extent of secondary structure.

Immunoglobulin Light Chains Form an Extensive and Highly Ordered Fibril Involving the N- and C-Termini

Light-chain (AL)-associated amyloidosis is a systemic disorder involving the formation and deposition of immunoglobulin AL fibrils in various bodily organs. One severe instance of AL disease is exhibited by the patient-derived variable domain (V_L) of the light chain AL-09, a 108 amino acid residue protein containing seven mutations relative to the corresponding germline protein, κI O18/O8 V_L. Previous work has demonstrated that the thermodynamic stability of native AL-09 V_L is greatly lowered by two of these mutations, Y87H and N34I, whereas a third mutation, K42Q, further increases the kinetics of fibril formation. However, detailed knowledge regarding the residues that are responsible for stabilizing the misfolded fibril structure is lacking. In this study, using solid-state NMR spectroscopy, we show that the majority of the AL-09 V_L sequence is immobilized in the fibrils and that the N- and C-terminal portions of the sequence are particularly well-structured. Thus, AL-09 V_L forms an extensively ordered and β-strand-rich fibril structure. Furthermore, we demonstrate that the predominant β-sheet secondary structure and rigidity observed for in vitro prepared AL-09 V_L fibrils are qualitatively similar to those observed for AL fibrils extracted from postmortem human spleen tissue, suggesting that this conformation may be representative of a common feature of AL fibrils.

Cell Damage in Light Chain Amyloidosis: FIBRIL INTERNALIZATION, TOXICITY AND CELL-MEDIATED SEEDING

Light chain (AL) amyloidosis is an incurable human disease characterized by the misfolding, aggregation, and systemic deposition of amyloid composed of immunoglobulin light chains (LC). This work describes our studies on potential mechanisms of AL cytotoxicity. We have studied the internalization of AL soluble proteins and amyloid fibrils into human AC16 cardiomyocytes by using real time live cell image analysis. Our results show how external amyloid aggregates rapidly surround the cells and act as a recruitment point for soluble protein, triggering the amyloid fibril elongation. Soluble protein and external aggregates are internalized into AC16 cells via macropinocytosis. AL amyloid fibrils are shown to be highly cytotoxic at low concentrations. Additionally, caspase assays revealed soluble protein induces apoptosis, demonstrating different cytotoxic mechanisms between soluble protein and amyloid aggregates. This study emphasizes the complex immunoglobulin light chain-cell interactions that result in fibril internalization, protein recruitment, and cytotoxicity that may occur in AL amyloidosis.

Probing the role of λ6 immunoglobulin light chain dimerization in amyloid formation

Light chain amyloidosis (AL) is a lethal disease associated with the deposition of misfolded immunoglobulin light chains (LC) as amyloid fibrils in the extracellular space of vital organs. The exact mechanisms of LC self-assembly and the molecular basis leading to cellular and organ failure still remain poorly understood. In this study, we investigated the relationship between the quaternary structure, the stability and the amyloidogenecity of LC variable domain (VL) from the λ6 germline. We observed that the amyloidogenic λ6 Wil and its non-amyloidogenic counterpart Jto dimerize in a concentration-dependent manner and that the dimer affinity is considerably decreased in the presence of a high ionic strength. Our results showed that the dimeric state delays the structural conversion associated with amyloid formation and that the monomer is critical to initiate amyloidogenesis. Thermal and chemical unfolding studies revealed that the dimeric state of VL λ6 has an equivalent stability to the monomer. This indicates that the protective effect of dimerization is not related to thermodynamic stability but, most likely, resides in specific structural features. The toxicity of monomeric Jto and Wil as well as fibrillar aggregates was evaluated on cardiomyoblasts and ThT-negative proteospecies reduced cellular viability when employed at high concentration. This study provides novel insights into the complex process of LC amyloidogenesis and suggests that dimer stabilization constitutes a promising strategy to prevent self-assembly and amyloid deposition.

Mutations can cause light chains to be too stable or too unstable to form amyloid fibrils

Light chain (AL) amyloidosis is an incurable human disease, where the amyloid precursor is a misfolding-prone immunoglobulin light-chain. Here, we identify the role of somatic mutations in the structure, stability and in vitro fibril formation for an amyloidogenic AL-12 protein by restoring four nonconservative mutations to their germline (wild-type) sequence. The single restorative mutations do not affect significantly the native structure, the unfolding pathway, and the reversibility of the protein. However, certain mutations either decrease (H32Y and H70D) or increase (R65S and Q96Y) the protein thermal stability. Interestingly, the most and the least stable mutants, Q96Y and H32Y, do not form amyloid fibrils under physiological conditions. Thus, Q96 and H32 are key residues for AL-12 stability and fibril formation and restoring them to the wild-type residues preclude amyloid formation. The mutants whose equilibrium is shifted to either the native or unfolded states barely sample transient partially folded states, and therefore do not form fibrils. These results agree with previous observations by our laboratory and others that amyloid formation occurs because of the sampling of partially folded states found within the unfolding transition (Blancas-Mejia and Ramirez-Alvarado, Ann Rev Biochem 2013;82:745-774). Here we provide a new insight on the AL amyloidosis mechanism by demonstrating that AL-12 does not follow the established thermodynamic hypothesis of amyloid formation. In this hypothesis, thermodynamically unstable proteins are more prone to amyloid formation. Here we show that within a thermal stability range, the most stable protein in this study is the most amyloidogenic protein.

Formation of amyloid fibers by monomeric light chain variable domains

Systemic light chain amyloidosis is a lethal disease characterized by excess immunoglobulin light chains and light chain fragments composed of variable domains, which aggregate into amyloid fibers. These fibers accumulate and damage organs. Some light chains induce formation of amyloid fibers, whereas others do not, making it unclear what distinguishes amyloid formers from non-formers. One mechanism by which sequence variation may reduce propensity to form amyloid fibers is by shifting the equilibrium toward an amyloid-resistant quaternary structure. Here we identify the monomeric form of the Mcg immunoglobulin light chain variable domain as the quaternary unit required for amyloid fiber assembly. Dimers of Mcg variable domains remain stable and soluble, yet become prone to assemble into amyloid fibers upon disassociation into monomers.

Amyloid formation in light chain amyloidosis

Light chain amyloidosis is one of the unique examples within amyloid diseases where the amyloidogenic precursor is a protein that escapes the quality control machinery and is secreted from the cells to be circulated in the bloodstream. The immunoglobulin light chains are produced by an abnormally proliferative monoclonal population of plasma cells that under normal conditions produce immunoglobulin molecules such as IgG, IgM or IgA. Once the light chains are in circulation, the proteins misfold and deposit as amyloid fibrils in numerous tissues and organs, causing organ failure and death. While there is a correlation between the thermodynamic stability of the protein and the kinetics of amyloid formation, we have recently found that this correlation applies within a thermodynamic range, and it is only a helpful correlation when comparing mutants from the same protein. Light chain amyloidosis poses unique challenges because each patient has a unique protein sequence as a result of the selection of a germline gene and the incorporation of somatic mutations. The exact location of the misfolding process is unknown as well as the full characterization of all of the toxic species populated during the amyloid formation process in light chain amyloidosis.

IgG Fab fragments forming bivalent complexes by a conformational mechanism that is reversible by osmolytes

Generated by proteolytic cleavage of immunoglobulin, Fab fragments possess great promise as blocking reagents, able to bind receptors or other targets without inducing cross-linking. However, aggregation of Fab preparations is a common occurrence, which generates intrinsic stimulatory capacity and thwarts signal blockade strategies. Using a panel of biochemical approaches, including size exclusion chromatography, SDS-PAGE, mass spectrometry, and cell stimulation followed by flow cytometry, we have measured the oligomerization and acquisition of stimulatory capacity that occurs in four monoclonal IgG Fabs specific for TCR/CD3. Unexpectedly, we observed that all Fabs spontaneously formed complexes that were precisely bivalent, and these bivalent complexes possessed most of the stimulatory activity of each Fab preparation. Fabs composing bivalent complexes were more susceptible to proteolysis than monovalent Fabs, indicating a difference in conformation between the Fabs involved in these two different states of valency. Because osmolytes represent a class of compounds that stabilize protein folding and conformation, we sought to determine the extent to which the amino acid osmolyte l-proline might impact bivalent Fab complexation. We found that l-proline (i) inhibited the adoption of the conformation associated with bivalent complexation, (ii) preserved Fab monovalency, (iii) reversed the conformation of preformed bivalent Fabs to that of monovalent Fabs, and (iv) separated a significant percentage of preformed bivalent complexes into monovalent species. Thus, Fab fragments can adopt a conformation that is compatible with folding or packing of a bivalent complex in a process that can be inhibited by osmolytes.

Tyrosine residues mediate fibril formation in a dynamic light chain dimer interface

Light chain amyloidosis is an incurable protein misfolding disease where monoclonal immunoglobulin light chains misfold and deposit as amyloid fibrils, causing organ failure and death. Previously, we determined that amyloidogenic light chains AL-09 and AL-103 do not form fibrils at pH 10 (tyrosine pK(a)). There are three tyrosine residues (32, 91, and 96) clustered in the dimer interface, interacting differently in the two light chain proteins due to their two different dimer conformations. These tyrosines may be ionized at pH 10, causing repulsion and inhibiting fibril formation. Here, we characterize single and double Tyr-to-Phe mutations in AL-09 and AL-103. All AL-09 Tyr-to-Phe mutants form fibrils at pH 10, whereas none of the AL-103 mutants form fibrils at pH 10. NMR studies suggest that although both AL-09 and AL-103 present conformational heterogeneity, only AL-09 favors dimer conformations where tyrosine residues mediate crucial interactions for amyloid formation.

[AL amyloidosis]

AL amyloidosis is a systemic disease characterised by pathogenetic proteins produced by malignant plasma cells and the deposition of them in different organs of the body. Amyloidogenic protein is the light chain of the monoclonal immunoglobulin, which becomes water insoluble, precipitates and deposites in the extracellular space resulting damage of organ function. AL amyloidosis belongs to plasma cell dyscrasias or it can associate to other monoclonal B-cell diseases. Diagnosis - such as in case of other types of amyloidosis - is based on histology. Identification of the amyloidogenic protein often needs special examinations. The goal of the therapy is the eradication of the malignant cell clone. Therapeutical armamentarium has been largely flared in the past few decades, several drugs with new mechanisms of action are available (thalidomide, lenalidomide, bortezomib). The standard treatment is high dose chemotherapy followed by autologous stem cell transplantation in case of eligible patients. Transplantation uneligible patients can be treated with a low dose alkylating agent with or without dexamethasone, or with the new agents. The therapeutical decision must be preceded by very thorough risk assessment. Early diagnosis and the prompt beginning of the treatment has great significance because the evolving functional abnormalities of parenchymal organs (mainly cardiac failure) prevents the effectivity of the treatment. Amyloidosis is an orphan disease, special centers play a significant role in the field of clinical trials.

How to manage primary amyloidosis

Immunoglobulin light chain amyloidosis is a protein deposition disorder where the precursor protein represents a monoclonal immunoglobulin light or heavy chain. Deposition in viscera results in restrictive cardiomyopathy, nephrotic range proteinuria, demyelinating peripheral neuropathy, hepatomegaly and malabsorption syndrome. Diagnosis requires biopsy with Congo red staining. Invasive biopsies are not required generally. It is essential that after a histologic diagnosis is obtained, the tissue is validated to have an immunoglobulin light chain composition so patients are spared unnecessary chemotherapy. The disease prognosis and patient monitoring are linked to serialized measurement of cardiac biomarkers and immunoglobulin-free light chains. Most patients require cytotoxic chemotherapy. For some patients, this therapy involves stem cell collection and myeloablative chemotherapy; for others, chemotherapy includes an alkylator and a corticosteroid; and for some, it involves addition of a novel agent in the form of an immunomodulatory drug or a proteasome inhibitor. Delays in diagnosis continue to be an obstacle to initiating effective therapy. Early mortality rates remain high. Effective chemotherapy can result in reversal of organ dysfunction and recovery. Reductions in light chain production translate to improved survival.

Glycosaminoglycans promote fibril formation by amyloidogenic immunoglobulin light chains through a transient interaction

Amyloid formation occurs when a precursor protein misfolds and aggregates, forming a fibril nucleus that serves as a template for fibril growth. Glycosaminoglycans are highly charged polymers known to associate with tissue amyloid deposits that have been shown to accelerate amyloidogenesis in vitro. We studied two immunoglobulin light chain variable domains from light chain amyloidosis patients with 90% sequence identity, analyzing their fibril formation kinetics and binding properties with different glycosaminoglycan molecules. We find that the less amyloidogenic of the proteins shows a weak dependence on glycosaminoglycan size and charge, while the more amyloidogenic protein responds only minimally to changes in the glycosaminoglycan. These glycosaminoglycan effects on fibril formation do not depend on a stable interaction between the two species but still show characteristic traits of an interaction-dependent mechanism. We propose that transient, predominantly electrostatic interactions between glycosaminoglycans and the precursor proteins mediate the acceleration of fibril formation in vitro.

Proteostasis strategies for restoring alpha1-antitrypsin deficiency

The function of the human proteome is defined by the proteostasis network (PN) (Science 2008;319:916; Science 2010;329:766), a biological system that generates, protects, and, where necessary, degrades a protein to optimize the cell, tissue, and organismal response to diet, stress, and aging. Numerous human diseases result from the failure of proteins to fold properly in response to mutation, disrupting the proteome. In the case of the exocytic pathway, this includes proteostasis components that direct folding, and export of proteins from the endoplasmic reticulum (ER). Included here are serpin deficiencies, a class of related diseases that result in a significant reduction of secretion of serine proteinase inhibitors from the liver into serum. In response to misfolding, variants of the serine protease α(1)-antitrypsin (α1AT) fail to exit the ER and are targeted for either ER-associated degradation or autophagic pathways. The challenge for developing α1AT deficiency therapeutics is to understand the PN pathways involved in folding and export. Herein, we review the role of the PN in managing the protein fold and function during synthesis in the ER and trafficking to the cell surface or extracellular space. We highlight the role of the proteostasis boundary to define the operation of the proteome (Annu Rev Biochem 2009;78:959). We discuss how manipulation of folding energetics or the PN by pharmacological intervention could provide multiple routes for restoration of variant α1AT function to the benefit of human health.

A single mutation promotes amyloidogenicity through a highly promiscuous dimer interface

Light chain amyloidosis is a devastating protein misfolding disease characterized by the accumulation of amyloid fibrils that causes tissue damage and organ failure. These fibrils are composed of monoclonal light chain protein secreted from an abnormal proliferation of bone marrow plasma cells. We previously reported that amyloidogenic light chain protein AL-09 adopts an altered dimer while its germline protein (kappaI O18/O8) forms a canonical dimer observed in other light chain crystal structures. In solution, conformational heterogeneity obscures all NMR signals at the AL-09 and kappaI O18/O8 dimer interfaces, so we solved the nuclear magnetic resonance structure of two related mutants. AL-09 H87Y adopts the normal dimer interface, but the kappaI Y87H solution structure presents an altered interface rotated 180 degrees relative to the canonical dimer interface and 90 degrees from the AL-09 arrangement. Our results suggest that promiscuity in the light chain dimer interface may promote new intermolecular contacts that may contribute to amyloid fibril structure.

Amyloidosis and POEMS syndrome

IMPORTANCE OF THE FIELD

Treatment options for amyloidosis and polyneuropathy, organomegaly, endocrinopathy, M protein, and skin changes (POEMS) syndrome have rapidly increased in the past years, but many patients are diagnosed late in the disease course and do not receive state-of-the art therapy.

AREAS COVERED IN THIS REVIEW

Stem-cell transplantation and novel agents have widened the chemotherapy alternatives available in these disorders and combinations of novel agents with high-dose therapy further improve treatment options. This review covers the main areas of debate in the optimal treatment amyloidosis and POEMS patients, focusing on the implications for everyday clinical practice and management strategies published in the past 36 months.

WHAT THE READER WILL GAIN

Insights into treatment strategies are provided in the review. Keys to early recognition of the syndromes are reviewed.

TAKE-HOME MESSAGE

With early diagnosis most patients are therapy candidates. New agents and new application of stem-cell transplantation have dramatically improved outcomes for these previously uniformly poor prognosis disorders.

Partially folded aggregation intermediates of human gammaD-, gammaC-, and gammaS-crystallin are recognized and bound by human alphaB-crystallin chaperone

Human gamma-crystallins are long-lived, unusually stable proteins of the eye lens exhibiting duplicated, double Greek key domains. The lens also contains high concentrations of the small heat shock chaperone alpha-crystallin, which suppresses aggregation of model substrates in vitro. Mature-onset cataract is believed to represent an aggregated state of partially unfolded and covalently damaged crystallins. Nonetheless, the lack of cell or tissue culture for anucleate lens fibers and the insoluble state of cataract proteins have made it difficult to identify the conformation of the human gamma-crystallin substrate species recognized by human alpha-crystallin. The three major human lens monomeric gamma-crystallins, gammaD, gammaC, and gammaS, all refold in vitro in the absence of chaperones, on dilution from denaturant into buffer. However, off-pathway aggregation of the partially folded intermediates competes with productive refolding. Incubation with human alphaB-crystallin chaperone during refolding suppressed the aggregation pathways of the three human gamma-crystallin proteins. The chaperone did not dissociate or refold the aggregated chains under these conditions. The alphaB-crystallin oligomers formed long-lived stable complexes with their gammaD-crystallin substrates. Using alpha-crystallin chaperone variants lacking tryptophans, we obtained fluorescence spectra of the chaperone-substrate complex. Binding of substrate gamma-crystallins with two or three of the four buried tryptophans replaced by phenylalanines showed that the bound substrate remained in a partially folded state with neither domain native-like. These in vitro results provide support for protein unfolding/protein aggregation models for cataract, with alpha-crystallin suppressing aggregation of damaged or unfolded proteins through early adulthood but becoming saturated with advancing age.