A strategy for synthesis of pathogenic human immunoglobulin free light chains in E. coli

Biophysical characterization of human-cell-expressed, full-length κI O18/O8, AL-09, λ6a, and Wil immunoglobulin light chains

Immunoglobulin light chain (AL) amyloidosis involves the deposition of insoluble monoclonal AL protein fibrils in the extracellular space of different organs leading to dysfunction and death. Development of methods to efficiently express and purify AL proteins with acceptable standards of homogeneity and structural integrity has become critical to understand the in vitro and in vivo aspects of AL protein aggregation, and thus the disease progression. In this study, we report the biophysical characterization of His-tagged and untagged versions of AL full-length (FL) κI and λ6 subgroup proteins and their mutants expressed from the Expi293F human cell line. We used an array of biophysical and biochemical methods to analyze the structure and stability of the monomers, oligomerization states, and thermodynamic characteristics of the purified FL proteins and how they compare with the bacterially expressed FL proteins. Our results demonstrate that the tagged and untagged versions of FL proteins have comparable stability to proteins expressed in bacterial cells but exhibit multiple unfolding transitions and reversibility. Non-reducing SDS-PAGE and analytical ultracentrifugation analysis showed presence of monomers and dimers, with an insignificant amount of higher-order oligomers, in the purified fraction of all proteins. Overall, the FL proteins were expressed with sufficient yields for biophysical studies and can replace bacterial expression systems.

Cu(II) Binding Increases the Soluble Toxicity of Amyloidogenic Light Chains

Light chain amyloidosis (AL) is caused by the aberrant overproduction of immunoglobulin light chains (LCs). The resulting abnormally high LC concentrations in blood lead to deposit formation in the heart and other target organs. Organ damage is caused not only by the accumulation of bulky amyloid deposits, but extensive clinical data indicate that circulating soluble LCs also exert cardiotoxic effects. The nematode C. elegans has been validated to recapitulate LC soluble toxicity in vivo, and in such a model a role for copper ions in increasing LC soluble toxicity has been reported. Here, we applied microscale thermophoresis, isothermal calorimetry and thermal melting to demonstrate the specific binding of Cu²⁺ to the variable domain of amyloidogenic H7 with a sub-micromolar affinity. Histidine residues present in the LC sequence are not involved in the binding, and yet their mutation to Ala reduces the soluble toxicity of H7. Copper ions bind to and destabilize the variable domains and induce a limited stabilization in this domain. In summary, the data reported here, elucidate the biochemical bases of the Cu²⁺-induced toxicity; moreover, they also show that copper binding is just one of the several biochemical traits contributing to LC soluble in vivo toxicity.

Dissecting the Molecular Features of Systemic Light Chain (AL) Amyloidosis: Contributions from Proteomics

Amyloidoses are characterized by aggregation of proteins into highly ordered amyloid fibrils, which deposit in the extracellular space of tissues, leading to organ dysfunction. In AL (amyloid light chain) amyloidosis, the most common form in Western countries, the amyloidogenic precursor is a misfolding-prone immunoglobulin light chain (LC), which, in the systemic form, is produced in excess by a plasma cell clone and transported to target organs though blood. Due to the primary role that proteins play in the pathogenesis of amyloidoses, mass spectrometry (MS)-based proteomic studies have gained an established position in the clinical management and research of these diseases. In AL amyloidosis, in particular, proteomics has provided important contributions for characterizing the precursor light chain, the composition of the amyloid deposits and the mechanisms of proteotoxicity in target organ cells and experimental models of disease. This review will provide an overview of the major achievements of proteomic studies in AL amyloidosis, with a presentation of the most recent acquisitions and a critical discussion of open issues and ongoing trends.

A kinetic coupling between protein unfolding and aggregation controls time-dependent solubility of the human myeloma antibody light chain

Protein aggregation is one of the most critical processes affecting protein solubility in various contexts-from protein therapeutics formulation to protein diseases. In general, time-dependent changes in protein solubility are complex kinetically driven processes that often involve a triggering event that consists of a protein unfolding/misfolding followed by the assembling of aggregation-competent protein species. In this study, we have examined the relation between stability and time-dependent solubility of the recombinant human antibody light chain, hLC, which was found to form renal tubular casts in the multiple myeloma patient. To analyze the aggregation quantitatively, the hLC stability and protein solubility assays were performed in vitro at elevated temperatures. A differential acceleration of the processes at high temperatures enabled us to dissect observed kinetics of irreversible hLC unfolding and aggregation. We find that for hLC these processes have different molecularity and activation energy barriers. While the irreversible unfolding of hLC is a unimolecular step with a substantial activation energy barrier of 260 kJ/mol, the aggregation is rate-limited by the bimolecular reaction, which is characterized by a lower activation energy barrier of 40 kJ/mol. By the combination of experimental assays at different temperatures, different protein concentrations and kinetic modeling using ordinary differential equations, we were able to extrapolate time-dependent protein solubility to temperatures where both unfolding and aggregation processes are strongly kinetically coupled. Our study enables mechanism-based evaluation and interpretation of different physico-chemical factors contributing to the hLC unfolding and aggregation and their effect on the formation of extracellular protein deposits.

Structural basis for the stabilization of amyloidogenic immunoglobulin light chains by hydantoins

Misfolding and aggregation of immunoglobulin light chains (LCs) leads to the degeneration of post-mitotic tissue in the disease immunoglobulin LC amyloidosis (AL). We previously reported the discovery of small molecule kinetic stabilizers of the native dimeric structure of full-length LCs, which slow or stop the LC aggregation cascade at the outset. A predominant structural category of kinetic stabilizers emerging from the high-throughput screen are coumarins substituted at the 7-position, which bind at the interface between the two variable domains of the light chain dimer. Here, we report the binding mode of another, more polar, LC kinetic stabilizer chemotype, 3,5-substituted hydantoins. Computational docking, solution nuclear magnetic resonance experiments, and x-ray crystallography show that the aromatic substructure emerging from the hydantoin 3-position occupies the same LC binding site as the coumarin ring. Notably, the hydantoin ring extends beyond the binding site mapped out by the coumarin hits. The hydantoin ring makes hydrogen bonds with both LC monomers simultaneously. The alkyl substructure at the hydantoin 5-position partially occupies a novel binding pocket proximal to the pocket occupied by the coumarin substructure. Overall, the hydantoin structural data suggest that a larger area of the LC variable-domain-variable-domain dimer interface is amenable to small molecule binding than previously demonstrated, which should facilitate development of more potent full-length LC kinetic stabilizers.

Cryo-EM structure of cardiac amyloid fibrils from an immunoglobulin light chain AL amyloidosis patient

Systemic light chain amyloidosis (AL)  is a life-threatening disease caused by aggregation and deposition of monoclonal immunoglobulin light chains (LC) in target organs. Severity of heart involvement is the most important factor determining prognosis. Here, we report the 4.0 Å resolution cryo-electron microscopy map and molecular model of amyloid fibrils extracted from the heart of an AL amyloidosis patient with severe amyloid cardiomyopathy. The helical fibrils are composed of a single protofilament, showing typical 4.9 Å stacking and cross-β architecture. Two distinct polypeptide stretches (total of 77 residues) from the LC variable domain (V_l) fit the fibril density. Despite V_l high sequence variability, residues stabilizing the fibril core are conserved through different cardiotoxic V_l, highlighting structural motifs that may be common to misfolding-prone LCs. Our data shed light on the architecture of LC amyloids, correlate amino acid sequences with fibril assembly, providing the grounds for development of innovative medicines.

Isolation and purification of recombinant immunoglobulin light chain variable domains from the periplasmic space of Escherichia coli

Immunoglobulin light chain amyloidosis is the most common form of systemic amyloidosis. However, very little is known about the underlying mechanisms that initiate and modulate the associated protein aggregation and deposition. Model systems have been established to investigate these disease-associated processes. One of these systems comprises two 114 amino acid light-chain variable domains of the kappa 4 IgG family, SMA and LEN. Despite high sequence identity (93%), SMA is amyloidogenic in vivo, but LEN adopts a stable dimer, displaying amyloidogenic properties only under destabilising conditions in vitro. We present here a refined and reproducible periplasmic expression and purification protocol for SMA and LEN that improves on existing methods and provides high yields of pure protein (10-50mg/L), particularly suitable for structural studies that demand highly concentrated and purified proteins. We confirm that recombinant SMA and LEN proteins have structure and dimerization capabilities consistent with the native proteins and employ fluorescence to probe internalization and cellular localization within cardiomyocytes. We propose periplasmic expression and simplified chromatographic steps outlined here as an optimized method for production of these and other variable light chain domains to investigate the underlying mechanisms of light chain amyloidosis. We show that SMA and LEN can be internalised within cardiomyocytes and were observed to localise to the perinuclear area, assessed by confocal microscopy as a possible mechanism for underlying cytotoxicity and pathogenesis associated with amyloidosis.

Incomplete Refolding of Antibody Light Chains to Non-Native, Protease-Sensitive Conformations Leads to Aggregation: A Mechanism of Amyloidogenesis in Patients?

Genetic, biochemical, and pharmacologic evidence supports the hypothesis that conformationally altered or misfolded protein states enable aggregation and cytotoxicity in the systemic amyloid diseases. Reversible structural fluctuations of natively folded proteins are involved in the aggregation of many degenerative disease associated proteins. Herein, we use antibody light chains (LCs) that form amyloid fibrils in AL amyloidosis to consider an alternative hypothesis of amyloidogenesis: that transient unfolding and incomplete extracellular refolding of secreted proteins can lead to metastable, alternatively folded states that are more susceptible to aggregation or to endoproteolysis that can release aggregation-prone fragments. Refolding of full-length λ6a LC dimers comprising an interchain disulfide bond from heat- or chaotrope-denatured ensembles in buffers yields the native dimeric state as well as alternatively folded dimers and aggregates. LC variants lacking an interchain disulfide bond appear to refold fully reversibly to the native state. The conformation of a backbone peptidyl-proline amide in the LC constant domain, which is cis in the native state, may determine whether the LC refolds back to the native state. A proline to alanine (P147A) LC variant, which cannot form the native cis-amide conformation, forms amyloid fibrils from the alternatively folded ensemble, whereas all the full-length λ6a LCs we have studied to date do not form amyloid under analogous conditions. P147A LC variants are susceptible to endoproteolysis by thrombin, enabling amyloidogenesis of the fragments released. Thus, non-native LC structural ensembles containing a tyrosine 146-proline 147 trans-amide bond can initiate and propagate amyloid formation, either directly or after aberrant endoproteolysis.

Concurrent structural and biophysical traits link with immunoglobulin light chains amyloid propensity

Light chain amyloidosis (AL), the most common systemic amyloidosis, is caused by the overproduction and the aggregation of monoclonal immunoglobulin light chains (LC) in target organs. Due to genetic rearrangement and somatic hypermutation, virtually, each AL patient presents a different amyloidogenic LC. Because of such complexity, the fine molecular determinants of LC aggregation propensity and proteotoxicity are, to date, unclear; significantly, their decoding requires investigating large sets of cases. Aiming to achieve generalizable observations, we systematically characterised a pool of thirteen sequence-diverse full length LCs. Eight amyloidogenic LCs were selected as responsible for severe cardiac symptoms in patients; five non-amyloidogenic LCs were isolated from patients affected by multiple myeloma. Our comprehensive approach (consisting of spectroscopic techniques, limited proteolysis, and X-ray crystallography) shows that low fold stability and high protein dynamics correlate with amyloidogenic LCs, while hydrophobicity, structural rearrangements and nature of the LC dimeric association interface (as observed in seven crystal structures here presented) do not appear to play a significant role in defining amyloid propensity. Based on the structural and biophysical data, our results highlight shared properties driving LC amyloid propensity, and these data will be instrumental for the design of synthetic inhibitors of LC aggregation.

Proteotoxicity in cardiac amyloidosis: amyloidogenic light chains affect the levels of intracellular proteins in human heart cells

AL amyloidosis is characterized by widespread deposition of immunoglobulin light chains (LCs) as amyloid fibrils. Cardiac involvement is frequent and leads to life-threatening cardiomyopathy. Besides the tissue alteration caused by fibrils, clinical and experimental evidence indicates that cardiac damage is also caused by proteotoxicity of prefibrillar amyloidogenic species. As in other amyloidoses, the damage mechanisms at cellular level are complex and largely undefined. We have characterized the molecular changes in primary human cardiac fibroblasts (hCFs) exposed in vitro to soluble amyloidogenic cardiotoxic LCs from AL cardiomyopathy patients. To evaluate proteome alterations caused by a representative cardiotropic LC, we combined gel-based with label-free shotgun analysis and performed bioinformatics and data validation studies. To assess the generalizability of our results we explored the effects of multiple LCs on hCF viability and on levels of a subset of cellular proteins. Our results indicate that exposure of hCFs to cardiotropic LCs translates into proteome remodeling, associated with apoptosis activation and oxidative stress. The proteome alterations affect proteins involved in cytoskeletal organization, protein synthesis and quality control, mitochondrial activity and metabolism, signal transduction and molecular trafficking. These results support and expand the concept that soluble amyloidogenic cardiotropic LCs exert toxic effects on cardiac cells.

Cardiac Light Chain Amyloidosis: The Role of Metal Ions in Oxidative Stress and Mitochondrial Damage

AIMS

The knowledge of the mechanism underlying the cardiac damage in immunoglobulin light chain (LC) amyloidosis (AL) is essential to develop novel therapies and improve patients' outcome. Although an active role of reactive oxygen species (ROS) in LC-induced cardiotoxicity has already been envisaged, the actual mechanisms behind their generation remain elusive. This study was aimed at further dissecting the action of ROS generated by cardiotoxic LC in vivo and investigating whether transition metal ions are involved in this process. In the absence of reliable vertebrate model of AL, we used the nematode Caenorhabditis elegans, whose pharynx is an "ancestral heart."

RESULTS

LC purified from patients with severe cardiac involvement intrinsically generated high levels of ROS and when administered to C. elegans induced ROS production, activation of the DAF-16/forkhead transcription factor (FOXO) pathway, and expression of proteins involved in stress resistance and survival. Profound functional and structural ROS-mediated mitochondrial damage, similar to that observed in amyloid-affected hearts from AL patients, was observed. All these effects were entirely dependent on the presence of metal ions since addition of metal chelator or metal-binding 8-hydroxyquinoline compounds (chelex, PBT2, and clioquinol) permanently blocked the ROS production and prevented the cardiotoxic effects of amyloid LC. Innovation and Conclusion: Our findings identify the key role of metal ions in driving the ROS-mediated toxic effects of LC. This is a novel conceptual advance that paves the way for new pharmacological strategies aimed at not only counteracting but also totally inhibiting the vicious cycle of redox damage. Antioxid. Redox Signal. 27, 567-582.

The Kinetic Stability of a Full-Length Antibody Light Chain Dimer Determines whether Endoproteolysis Can Release Amyloidogenic Variable Domains

Light chain (LC) amyloidosis (AL amyloidosis) appears to be caused by the misfolding, or misfolding and aggregation of an antibody LC or fragment thereof and is fatal if untreated. LCs are secreted from clonally expanded plasma cells, generally as disulfide-linked dimers, with each monomer comprising one constant and one variable domain. The energetic contribution of each domain and the role of endoproteolysis in AL amyloidosis remain unclear. To investigate why only some LCs form amyloid and cause organ toxicity, we measured the aggregation propensity and kinetic stability of LC dimers and their associated variable domains from AL amyloidosis patients and non-patients. All the variable domains studied readily form amyloid fibrils, whereas none of the full-length LC dimers, even those from AL amyloidosis patients, are amyloidogenic. Kinetic stability-that is, the free energy difference between the native state and the unfolding transition state-dictates the LC's unfolding rate. Full-length LC dimers derived from AL amyloidosis patients unfold more rapidly than other full-length LC dimers and can be readily cleaved into their component domains by proteases, whereas non-amyloidogenic LC dimers are more kinetically stable and resistant to endoproteolysis. Our data suggest that amyloidogenic LC dimers are kinetically unstable (unfold faster) and are thus susceptible to endoproteolysis that results in the release amyloidogenic LC fragments, whereas other LCs are not as amenable to unfolding and endoproteolysis and are therefore aggregation resistant. Pharmacologic kinetic stabilization of the full-length LC dimer could be a useful strategy to treat AL amyloidosis.

In situ characterization of protein aggregates in human tissues affected by light chain amyloidosis: a FTIR microspectroscopy study

Light chain (AL) amyloidosis, caused by deposition of amyloidogenic immunoglobulin light chains (LCs), is the most common systemic form in industrialized countries. Still open questions, and premises for developing targeted therapies, concern the mechanisms of amyloid formation in vivo and the bases of organ targeting and dysfunction. Investigating amyloid material in its natural environment is crucial to obtain new insights on the molecular features of fibrillar deposits at individual level. To this aim, we used Fourier transform infrared (FTIR) microspectroscopy for studying in situ unfixed tissues (heart and subcutaneous abdominal fat) from patients affected by AL amyloidosis. We compared the infrared response of affected tissues with that of ex vivo and in vitro fibrils obtained from the pathogenic LC derived from one patient, as well as with that of non amyloid-affected tissues. We demonstrated that the IR marker band of intermolecular β-sheets, typical of protein aggregates, can be detected in situ in LC amyloid-affected tissues, and that FTIR microspectroscopy allows exploring the inter- and intra-sample heterogeneity. We extended the infrared analysis to the characterization of other biomolecules embedded within the amyloid deposits, finding an IR pattern that discloses a possible role of lipids, collagen and glycosaminoglycans in amyloid deposition in vivo.

Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to detect monoclonal immunoglobulin light chains in serum and urine

RATIONALE

Use of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) to monitor serum and urine samples for endogenous monoclonal immunoglobulins. MALDI-TOFMS is faster, fully automatable, and provides superior specificity compared to protein gel electrophoresis (PEL).

METHODS

Samples were enriched for immunoglobulins in 5 min using Melon Gel™ followed by reduction with dithiothreitol for 15 min to separate immunoglobulin light chains and heavy chains. Samples were then desalted using C4 ZipTips, mixed with sinapinic acid matrix, and analyzed on a Bruker Biflex III MALDI-TOF mass spectrometer.

RESULTS

Monoclonal immunoglobulin light chains were identified in serum and urine samples from patients with a known monoclonal gammopathy using MALDI-TOFMS with minimal sample preparation.

CONCLUSIONS

MALDI-TOFMS can identify a monoclonal immunoglobulin in serum and urine samples. The molecular mass of the monoclonal immunoglobulin light chain is obtained providing unprecedented specificity compared to PEL. In addition, the methodology can be automated, making it a practical alternative to PEL.

Novel mitochondrial protein interactors of immunoglobulin light chains causing heart amyloidosis

In immunoglobulin (Ig) light-chain (LC) (AL) amyloidosis, AL deposition translates into life-threatening cardiomyopathy. Clinical and experimental evidence indicates that soluble cardiotoxic LCs are themselves harmful for cells, by which they are internalized. Hypothesizing that interaction of soluble cardiotoxic LCs with cellular proteins contributes to damage, we characterized their interactome in cardiac cells. LCs were purified from patients with AL amyloidosis cardiomyopathy or multiple myeloma without amyloidosis (the nonamyloidogenic/noncardiotoxic LCs served as controls) and employed at concentrations in the range observed in AL patients' sera. A functional proteomic approach, based on direct and inverse coimmunoprecipitation and mass spectrometry, allowed identifying LC-protein complexes. Findings were validated by colocalization, fluorescence lifetime imaging microscopy (FLIM)-fluorescence resonance energy transfer (FRET), and ultrastructural studies, using human primary cardiac fibroblasts (hCFs) and stem cell-derived cardiomyocytes. Amyloidogenic cardiotoxic LCs interact in vitro with specific intracellular proteins involved in viability and metabolism. Imaging confirmed that, especially in hCFs, cardiotoxic LCs (not controls) colocalize with mitochondria and spatially associate with selected interactors: mitochondrial optic atrophy 1-like protein and peroxisomal acyl-coenzyme A oxidase 1 (FLIM-FRET efficiencies 11 and 6%, respectively). Cardiotoxic LC-treated hCFs display mitochondrial ultrastructural changes, supporting mitochondrial involvement. We show that cardiotoxic LCs establish nonphysiologic protein-protein contacts in human cardiac cells, offering new clues on the pathogenesis of AL cardiomyopathy.

Investigating heart-specific toxicity of amyloidogenic immunoglobulin light chains: A lesson from C. elegans

Abnormalities in protein folding are involved in many localized and systemic diseases, all of which are characterized by insoluble amyloid formation and deposition. In immunoglobulin light chain (LC) amyloidosis, the most frequent systemic form of amyloidosis, the amyloid involvement of the heart dictates the prognosis and the elucidation of the mechanism of heart targeting and toxicity is essential for designing and testing new effective treatments. To this end, the availability of an appropriate animal model is crucial. We recently described the use of C. elegans as an innovative experimental system to investigate in vivo the pathogenic effects of monoclonal LC. This idea stems from the knowledge that the worm's pharynx is an "ancestral heart" with the additional ability to recognize stressor compounds. The feeding of worms with LC purified from patients suffering from cardiomyopathy, selectively and permanently impaired the pharyngeal function. This irreversible damage resulted in time, in a significant reduction in the lifespan of worms. We also reported that the ability of LC to generate reactive oxygen species was associated with their toxic effects and was counteracted by anti-oxidant compounds. This new nematode-based assay represents a promising model for elucidating the heart-specific toxicity of LC and for a rapid screening of new therapeutic strategies.

A Caenorhabditis elegans-based assay recognizes immunoglobulin light chains causing heart amyloidosis

Poor prognosis and limited therapeutic options characterize immunoglobulin light-chain (AL) amyloidosis with major heart involvement. Reliable experimental models are needed to study light-chain (LC)/heart interactions and to explore strategies for prevention of cardiac damage. We have exploited the nematode Caenorhabditis elegans as a novel tool, because its pharynx is evolutionarily related to the vertebrate heart. Our data demonstrate that the pharyngeal pumping of C elegans is significantly and selectively reduced by LCs from AL patients suffering from cardiomyopathy, but not by amyloid LCs with different organ tropism or nonamyloidogenic LCs from multiple myeloma. This functional alteration is dependent on the LC concentration and results in persistent pharyngeal dysfunction and in a significant reduction of the worms' lifespan. These manifestations are paralleled by an increase of mitochondrial reactive oxygen species and can be prevented by treatment with antioxidant agents. In conclusion, these data indicate that this nematode-based assay is a promising surrogate model for investigating the heart-specific toxicity of amyloidogenic LCs and for a rapid screening of new therapeutic strategies.