Structural basis of light chain amyloidogenicity: comparison of the thermodynamic properties, fibrillogenic potential and tertiary structural features of four Vλ6 proteins

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.

Identification of AL proteins from 10 λ-AL amyloidosis patients by mass spectrometry extracted from abdominal fat and heart tissue

BACKGROUND

Systemic AL amyloidosis arises from the misfolding of patient-specific immunoglobulin light chains (LCs). Potential drivers of LC amyloid formation are mutational changes and post-translational modifications (PTMs). However, little information is available on the exact primary structure of the AL proteins and their precursor LCs.

OBJECTIVE

We analyse the exact primary structure of AL proteins extracted from 10 λ AL amyloidosis patients and their corresponding precursor LCs.

MATERIALS AND METHODS

By cDNA sequencing of the precursor LC genes in combination with mass spectrometry of the AL proteins, the exact primary structure and PTMs were determined. This information was used to analyse their biochemical properties.

RESULTS

All AL proteins comprise the V_L and a small part of the C_L with a common C-terminal truncation region. While all AL proteins retain the conserved native disulphide bond of the V_L, we found no evidence for presence of other common PTMs. The analysis of the biochemical properties revealed that the isoelectric point of the V_L is significantly increased due to introduced mutations.

CONCLUSION

Our data imply that mutational changes influence the surface charge properties of the V_L and that common proteolytic processes are involved in the generation of the cleavage sites of AL proteins.

Antibodies gone bad - the molecular mechanism of light chain amyloidosis

Light chain amyloidosis (AL) is a systemic disease in which abnormally proliferating plasma cells secrete large amounts of mutated antibody light chains (LCs) that eventually form fibrils. The fibrils are deposited in various organs, most often in the heart and kidney, and impair their function. The prognosis for patients diagnosed with AL is generally poor. The disease is set apart from other amyloidoses by the huge number of patient-specific mutations in the disease-causing and fibril-forming protein. The molecular mechanisms that drive the aggregation of mutated LCs into fibrils have been enigmatic, which hindered the development of efficient diagnostics and therapies. In this review, we summarize our current knowledge on AL amyloidosis and discuss open issues.

Hiding in plain sight: structure and sequence analysis reveals the importance of the antibody DE loop for antibody-antigen binding

Antibody variable domains contain “complementarity-determining regions” (CDRs), the loops that form the antigen binding site. CDRs1-3 are recognized as the canonical CDRs. However, a fourth loop sits adjacent to CDR1 and CDR2 and joins the D and E strands on the antibody v-type fold. This “DE loop” is usually treated as a framework region, even though mutations in the loop affect the conformation of the CDRs and residues in the DE loop occasionally contact antigen. We analyzed the length, structure, and sequence features of all DE loops in the Protein Data Bank (PDB), as well as millions of sequences from HIV-1 infected and naïve patients. We refer to the DE loop as H4 and L4 in the heavy and light chains, respectively. Clustering the backbone conformations of the most common length of L4 (6 residues) reveals four conformations: two κ-only clusters, one λ-only cluster, and one mixed κ/λ cluster. Most H4 loops are length-8 and exist primarily in one conformation; a secondary conformation represents a small fraction of H4-8 structures. H4 sequence variability exceeds that of the antibody framework in naïve human high-throughput sequences, and both L4 and H4 sequence variability from λ and heavy germline sequences exceed that of germline framework regions. Finally, we identified dozens of structures in the PDB with insertions in the DE loop, all related to broadly neutralizing HIV-1 antibodies (bNabs), as well as antibody sequences from high-throughput sequencing studies of HIV-infected individuals, illuminating a possible role in humoral immunity to HIV-1.

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

Patients with plasma cell dyscrasias produce free abnormal monoclonal Ig light chains that circulate in the blood stream. Some of them, termed glomerulopathic light chains, interact with the mesangial cells and trigger, in a manner dependent of their structural and physicochemical properties, a sequence of pathological events that results in either light chain–derived (AL) amyloidosis (AL-Am) or light chain deposition disease (LCDD). The mesangial cells play a key role in the pathogenesis of both diseases. The interaction with the pathogenic light chain elicits specific cellular processes, which include apoptosis, phenotype transformation, and secretion of extracellular matrix components and metalloproteinases. Monoclonal light chains associated with AL-Am but not those producing LCDD are avidly endocytosed by mesangial cells and delivered to the mature lysosomal compartment where amyloid fibrils are formed. Light chains from patients with LCDD exert their pathogenic signaling effect at the cell surface of mesangial cells. These events are generic mesangial responses to a variety of adverse stimuli, and they are similar to those characterizing other more frequent glomerulopathies responsible for many cases of end-stage renal disease. The pathophysiologic events that have been elucidated allow to propose future therapeutic approaches aimed at preventing, stopping, ameliorating, or reversing the adverse effects resulting from the interactions between glomerulopathic light chains and mesangium.

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.

Site-Specific Interactions with Copper Promote Amyloid Fibril Formation for λ6aJL2-R24G

Light-chain amyloidosis (AL) is one of the most common systemic amyloidoses, and it is characterized by the deposition of immunoglobulin light chain (LC) variable domains as insoluble amyloid fibers in vital organs and tissues. The recombinant protein 6aJL2-R24G contains λ6a and JL2 germline genes and also contains the Arg24 by Gly substitution. This mutation is present in 25% of all amyloid-associated λ6 LC cases, reduces protein stability, and increases the propensity to form amyloid fibers. In this study, it was found that the interaction of 6aJL2-R24G with Cu(II) decreases the thermal stability of the protein and accelerates the amyloid fibril formation, as observed by fluorescence spectroscopy. Isothermal calorimetry titration showed that Cu(II) binds to the protein with micromolar affinity. His99 may be one of the main Cu(II) interaction sites, as observed by nuclear magnetic resonance spectroscopy. The binding of Cu(II) to His99 induces larger fluctuations of the CDR1 and loop C″, as shown by molecular dynamics simulations. Thus, Cu(II) binding may be inducing the loss of interactions between CDR3 and CDR1, making the protein less stable and more prone to form amyloid fibers. This study provides insights into the mechanism of metal-induced aggregation of the 6aJL2-R24G protein and sheds light on the bio-inorganic understanding of AL disease.

Crystal structure of 6aJL2-R24G light chain variable domain: Does crystal packing explain amyloid fibril formation?

Light chain amyloidosis is one of the most common systemic amyloidosis, characterized by the deposition of immunoglobulin light variable domain as insoluble amyloid fibrils in vital organs, leading to the death of patients. Germline λ6a is closely related with this disease and has been reported that 25% of proteins encoded by this germline have a change at position 24 where an Arg is replaced by a Gly (R24G). This germline variant reduces protein stability and increases the propensity to form amyloid fibrils. In this work, the crystal structure of 6aJL2-R24G has been determined to 2.0 Å resolution by molecular replacement. Crystal belongs to space group I2₁2₁2₁ (PDB ID 5JPJ) and there are two molecules in the asymmetric unit. This 6aJL2-R24G structure as several related in PDB (PDB entries: 5C9K, 2W0K, 5IR3 and 1PW3) presents by crystal packing the formation of an octameric assembly in a helicoidal arrangement, which has been proposed as an important early stage in amyloid fibril aggregation. However, other structures of other protein variants in PDB (PDB entries: 3B5G, 3BDX, 2W0L, 1CD0 and 2CD0) do not make the octameric assembly, regardless their capacity to form fibers in vitro or in vivo. The analysis presented here shows that the ability to form the octameric assembly in a helicoidal arrangement in crystallized light chain immunoglobulin proteins is not required for amyloid fibril formation in vitro. In addition, the fundamental role of partially folded states in the amyloid fibril formation in vitro, is not described in any crystallographic structure published or analyzed here, being those structures, in any case examples of proteins in their native states. Those partially folded states have been recently described by cryo-EM studies, showing the necessity of structural changes in the variants before the amyloid fiber formation process starts.

New clues into the self-assembly of Vmh2, a basidiomycota class I hydrophobin

Hydrophobins are fungal proteins that can self-assemble into amphiphilic films at hydrophobic-hydrophilic interfaces. Class I hydrophobin aggregates resemble amyloid fibrils, sharing some features with them. Here, five site-directed mutants of Vmh2, a member of basidiomycota class I hydrophobins, were designed and characterized to elucidate the molecular determinants playing a key role in class I hydrophobin self-assembly. The mechanism of fibril formation proposed for Vmh2 foresees that the triggering event is the destabilization of a specific loop (L1), leading to the formation of a β-hairpin, which in turn generates the β-spine of the amyloid fibril.

Immunoglobulin light chain amyloid aggregation

Light chain (AL) amyloidosis is a devastating, complex, and incurable protein misfolding disease. It is characterized by an abnormal proliferation of plasma cells (fully differentiated B cells) producing an excess of monoclonal immunoglobulin light chains that are secreted into circulation, where the light chains misfold, aggregate as amyloid fibrils in target organs, and cause organ dysfunction, organ failure, and death. In this article, we will review the factors that contribute to AL amyloidosis complexity, the findings by our laboratory from the last 16 years and the work from other laboratories on understanding the structural, kinetics, and thermodynamic contributions that drive immunoglobulin light chain-associated amyloidosis. We will discuss the role of cofactors and the mechanism of cellular damage. Last, we will review our recent findings on the high resolution structure of AL amyloid fibrils. AL amyloidosis is the best example of protein sequence diversity in misfolding diseases, as each patient has a unique combination of germline donor sequences and multiple amino acid mutations in the protein that forms the amyloid fibril.

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.

[Uromodulin gene polymorphisms in patients with cast nephropathy in multiple myeloma]

AIM

To investigate the nature of mutations in exons 4 and 5 of the uromodulin (UM) gene, including in the area encoding the domain of 8 cysteines (D8C), in patients with multiple myeloma (MM) with the secretion of monoclonal light chains (LC) in cast nephropathy (CN) and without kidney injury.

SUBJECTS AND METHODS

The investigation enrolled 24 patients in MM remission, who were observed to have monoclonal LC secretion at onset. Group 1 included 14 patients with CN; Group 2 consisted of 10 patients with normal renal function (a comparison group). The compared groups did not differ in the number of serum and urinary monoclonal LCs. Genomic DNA was extracted from the peripheral blood samples of patients. The nucleotide sequence of exons 4 and 5 of the UM gene was determined by the Sanger method.

RESULTS

No differences were found in the frequency of polymorphisms depending on the severity of kidney injury. The missense mutation p.142R>R/Q in the UM gene, which had not been previously described, was discovered.

CONCLUSION

The patients with MM were not found to have statistically significant differences in the frequency and nature of polymorphisms of exons 4 and 5 in the UM gene, including in the area encoding D8C, in CN without kidney injury.

Differential recruitment efficacy of patient-derived amyloidogenic and myeloma light chain proteins by synthetic fibrils-A metric for predicting amyloid propensity

BACKGROUND

Monoclonal free light chain (LC) proteins are present in the circulation of patients with immunoproliferative disorders such as light chain (AL) amyloidosis and multiple myeloma (MM). Light chain-associated amyloid is a complex pathology composed of proteinaceous fibrils and extracellular matrix proteins found in all patients with AL and in ~10-30% of patients who presented with MM. Amyloid deposits systemically in multiple organs and tissues leading to dysfunction and ultimately death. The overall survival of patients with amyloidosis is worse than for those with early stage MM.

METHODS AND FINDINGS

We have developed a sensitive binding assay quantifying the recruitment of full length, patient-derived LC proteins by synthetic amyloid fibrils, as a method for studying their amyloidogenic potential. In a survey of eight urinary LC, both AL and MM-associated proteins were recruited by synthetic amyloid fibrils; however, AL-associated LC bound significantly more efficiently (p < 0.05) than did MM LCs. The LC proteins used in this study were isolated from urine and presumed to represent a surrogate of serum free light chains.

CONCLUSION

The binding of LC to synthetic fibrils in this assay accurately differentiated LC with amyloidogenic propensity from MM LC that were not associated with clinical amyloid disease. Notably, the LC from a MM patient who subsequently developed amyloid behaved as an AL-associated protein in the assay, indicating the possibility for identifying MM patients at risk for developing amyloidosis based on the light chain recruitment efficacy. With this information, at risk patients can be monitored more closely for the development of amyloidosis, allowing timely administration of novel, amyloid-directed immunotherapies-this approach may improve the prognosis for these patients.

Epigallocatechin-3-gallate preferentially induces aggregation of amyloidogenic immunoglobulin light chains

Antibody light chain amyloidosis is a rare disease caused by fibril formation of secreted immunoglobulin light chains (LCs). The huge variety of antibody sequences puts a serious challenge to drug discovery. The green tea polyphenol epigallocatechin-3-gallate (EGCG) is known to interfere with fibril formation in general. Here we present solution- and solid-state NMR studies as well as MD simulations to characterise the interaction of EGCG with LC variable domains. We identified two distinct EGCG binding sites, both of which include a proline as an important recognition element. The binding sites were confirmed by site-directed mutagenesis and solid-state NMR analysis. The EGCG-induced protein complexes are unstructured. We propose a general mechanistic model for EGCG binding to a conserved site in LCs. We find that EGCG reacts selectively with amyloidogenic mutants. This makes this compound a promising lead structure, that can handle the immense sequence variability of antibody LCs.

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.

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.

Light Chain Amyloid Fibrils Cause Metabolic Dysfunction in Human Cardiomyocytes

Light chain (AL) amyloidosis is the most common form of systemic amyloid disease, and cardiomyopathy is a dire consequence, resulting in an extremely poor prognosis. AL is characterized by the production of monoclonal free light chains that deposit as amyloid fibrils principally in the heart, liver, and kidneys causing organ dysfunction. We have studied the effects of amyloid fibrils, produced from recombinant λ6 light chain variable domains, on metabolic activity of human cardiomyocytes. The data indicate that fibrils at 0.1 μM, but not monomer, significantly decrease the enzymatic activity of cellular NAD(P)H-dependent oxidoreductase, without causing significant cell death. The presence of amyloid fibrils did not affect ATP levels; however, oxygen consumption was increased and reactive oxygen species were detected. Confocal fluorescence microscopy showed that fibrils bound to and remained at the cell surface with little fibril internalization. These data indicate that AL amyloid fibrils severely impair cardiomyocyte metabolism in a dose dependent manner. These data suggest that effective therapeutic intervention for these patients should include methods for removing potentially toxic amyloid fibrils.

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.

Inhibition of Light Chain 6aJL2-R24G Amyloid Fiber Formation Associated with Light Chain Amyloidosis

Light chain amyloidosis (AL) is a deadly disease characterized by the deposition of monoclonal immunoglobulin light chains as insoluble amyloid fibrils in different organs and tissues. Germ line λ VI has been closely related to this condition; moreover, the R24G mutation is present in 25% of the proteins of this germ line in AL patients. In this work, five small molecules were tested as inhibitors of the formation of amyloid fibrils from the 6aJL2-R24G protein. We have found by thioflavin T fluorescence and transmission electron microscopy that EGCG inhibits 6aJL2-R24G fibrillogenesis. Furthermore, using nuclear magnetic resonance spectroscopy, dynamic light scattering, and isothermal titration calorimetry, we have determined that the inhibition is due to binding to the protein in its native state, interacting mainly with aromatic residues.

Differential effects on light chain amyloid formation depend on mutations and type of glycosaminoglycans

Amyloid light chain (AL) amyloidosis is a protein misfolding disease where immunoglobulin light chains sample partially folded states that lead to misfolding and amyloid formation, resulting in organ dysfunction and death. In vivo, amyloid deposits are found in the extracellular space and involve a variety of accessory molecules, such as glycosaminoglycans, one of the main components of the extracellular matrix. Glycosaminoglycans are a group of negatively charged heteropolysaccharides composed of repeating disaccharide units. In this study, we investigated the effect of glycosaminoglycans on the kinetics of amyloid fibril formation of three AL cardiac amyloidosis light chains. These proteins have similar thermodynamic stability but exhibit different kinetics of fibril formation. We also studied single restorative and reciprocal mutants and wild type germ line control protein. We found that the type of glycosaminoglycan has a different effect on the kinetics of fibril formation, and this effect seems to be associated with the natural propensity of each AL protein to form fibrils. Heparan sulfate accelerated AL-12, AL-09, κI Y87H, and AL-103 H92D fibril formation; delayed fibril formation for AL-103; and did not promote any fibril formation for AL-12 R65S, AL-103 delP95aIns, or κI O18/O8. Chondroitin sulfate A, on the other hand, showed a strong fibril formation inhibition for all proteins. We propose that heparan sulfate facilitates the formation of transient amyloidogenic conformations of AL light chains, thereby promoting amyloid formation, whereas chondroitin sulfate A kinetically traps partially unfolded intermediates, and further fibril elongation into fibrils is inhibited, resulting in formation/accumulation of oligomeric/protofibrillar aggregates.

Site-directed mutagenesis reveals regions implicated in the stability and fiber formation of human λ3r light chains

Light chain amyloidosis (AL) is a disease that affects vital organs by the fibrillar aggregation of monoclonal light chains. λ3r germ line is significantly implicated in this disease. In this work, we contrasted the thermodynamic stability and aggregation propensity of 3mJL2 (nonamyloidogenic) and 3rJL2 (amyloidogenic) λ3 germ lines. Because of an inherent limitation (extremely low expression), Cys at position 34 of the 3r germ line was replaced by Tyr reaching a good expression yield. A second substitution (W91A) was introduced in 3r to obtain a better template to incorporate additional mutations. Although the single mutant (C34Y) was not fibrillogenic, the second mutation located at CDR3 (W91A) induced fibrillogenesis. We propose, for the first time, that CDR3 (position 91) affects the stability and fiber formation of human λ3r light chains. Using the double mutant (3rJL2/YA) as template, other variants were constructed to evaluate the importance of those substitutions into the stability and aggregation propensity of λ3 light chains. A change in position 7 (P7D) boosted 3rJL2/YA fibrillogenic properties. Modification of position 48 (I48M) partially reverted 3rJL2/YA fibril aggregation. Finally, changes at positions 8 (P8S) or 40 (P40S) completely reverted fibril formation. These results confirm the influential roles of N-terminal region (positions 7 and 8) and the loop 40-60 (positions 40 and 48) on AL. X-ray crystallography revealed that the three-dimensional topology of the single and double λ3r mutants was not significantly altered. This mutagenic approach helped to identify key regions implicated in λ3 AL.

Kinetic control in protein folding for light chain amyloidosis and the differential effects of somatic mutations

Light chain amyloidosis is a devastating disease where immunoglobulin light chains form amyloid fibrils, resulting in organ dysfunction and death. Previous studies have shown a direct correlation between the protein thermodynamic stability and the propensity for amyloid formation for some proteins involved in light chain amyloidosis. Here we investigate the effect of somatic mutations on protein stability and in vitro fibril formation of single and double restorative mutants of the protein AL-103 compared to the wild-type germline control protein. A scan rate dependence and hysteresis in the thermal unfolding and refolding was observed for all proteins. This indicates that the unfolding/refolding reaction is kinetically determined with different kinetic constants for unfolding and refolding even though the process remains experimentally reversible. Our structural analysis of AL-103 and AL-103 delP95aIns suggests a kinetic coupling of the unfolding/refolding process with cis-trans prolyl isomerization. Our data reveal that the deletion of proline 95a (AL-103 delP95aIns), which removes the trans-cis di-proline motif present in the patient protein AL-103, results in a dramatic increment in the thermodynamic stability and a significant delay in fibril formation kinetics with respect to AL-103. Fibril formation is pH dependent; all proteins form fibrils at pH2; reactions become slower and more stochastic as the pH increases up to pH7. Based on these results, we propose that, in addition to thermodynamic stability, kinetic stability (possibly influenced by the presence of cis proline 95a) plays a major role in the AL-103 amyloid fibril formation process.

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.

Systemic amyloidoses

The amyloidoses are a group of protein misfolding diseases in which the precursor protein undergoes a conformational change that triggers the formation of amyloid fibrils in different tissues and organs, causing cell death and organ failure. Amyloidoses can be either localized or systemic. In localized amyloidosis, amyloid deposits form at the site of precursor protein synthesis, whereas in systemic amyloidosis, amyloid deposition occurs distant from the site of precursor protein secretion. We review the type of proteins and cells involved and what is known about the complex pathophysiology of these diseases. We focus on light chain amyloidosis to illustrate how biochemical and biophysical studies have led to a deeper understanding of the pathogenesis of this devastating disease. We also review current cellular, tissue, and animal models and discuss the challenges and opportunities for future studies of the systemic amyloidoses.

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.

Current perspectives on cardiac amyloidosis

Amyloidosis represents a group of diseases in which proteins undergo misfolding to form insoluble fibrils with subsequent tissue deposition. While almost all deposited amyloid fibers share a common nonbranched morphology, the affected end organs, clinical presentation, treatment strategies, and prognosis vary greatly among this group of diseases and are largely dependent on the specific amyloid precursor protein. To date, at least 27 precursor proteins have been identified to result in either local tissue or systemic amyloidosis, with nine of them manifesting in cardiac deposition and resulting in a syndrome termed "cardiac amyloidosis" or "amyloid cardiomyopathy." Although cardiac amyloidosis has been traditionally considered to be a rare disorder, as clinical appreciation and understanding continues to grow, so too has the prevalence, suggesting that this disease may be greatly underdiagnosed. The most common form of cardiac amyloidosis is associated with circulating amyloidogenic monoclonal immunoglobulin light chain proteins. Other major cardiac amyloidoses result from a misfolding of products of mutated or wild-type transthyretin protein. While the various cardiac amyloidoses share a common functional consequence, namely, an infiltrative cardiomyopathy with restrictive pathophysiology leading to progressive heart failure, the underlying pathophysiology and clinical syndrome varies with each precursor protein. Herein, we aim to provide an up-to-date overview of cardiac amyloidosis from nomenclature to molecular mechanisms and treatment options, with a particular focus on amyloidogenic immunoglobulin light chain protein cardiac amyloidosis.

[Cutaneous amyloidosis]

Amyloids are common protein aggregates in nature. Some amyloids fulfill important biological tasks while others are known to cause diseases. Despite the fact that the ultrastructure of amyloid is highly conserved, the mechanism of amyloidogenesis remains a challenging research topic. In humans, amyloidoses may develop in the skin or lead to skin signs due to secondary cutaneous involvement. An accurate diagnostic procedure is crucial for planning the therapy of this heterogeneous group of diseases. Therefore, the aim of this paper is to give an overview on the different kinds of amyloidoses as well as on diagnostic and therapeutic approaches. Furthermore, the discrimination between functional and disease-causing amyloid is briefly presented.

The critical role of the constant region in thermal stability and aggregation of amyloidogenic immunoglobulin light chain

Light chain (LC) amyloidosis (AL) is a fatal disease in which immunoglobulin LC deposit as fibrils. Although the LC amyloid-forming propensity is attributed primarily to the variable region, fibrils also contain full-length LC comprised of variable-joining (V(L)) and constant (C(L)) regions. To assess the role of C(L) in fibrillogenesis, we compared the thermal stability of full-length LC and corresponding V(L) and C(L) fragments. Protein unfolding and aggregation were monitored by circular dichroism and light scattering. A full-length λ6 LC purified from urine of a patient with AL amyloidosis showed irreversible unfolding coupled to aggregation. The transition temperature decreased at slower heating rates, indicating kinetic effects. Next, we studied five recombinant λ6 proteins: full-length amyloidogenic LC, its V(L), germline LC, germline V(L), and C(L). Amyloidogenic and germline proteins showed similar rank order of stability, V(L) < LC < C(L); hence, in the full-length LC, V(L) destabilizes C(L). Amyloidogenic proteins were less stable than their germline counterparts, suggesting that reduction in V(L) stability destabilizes the full-length LC. Thermal unfolding of the full-length amyloidogenic and germline LC required high activation energy and involved irreversible aggregation, yet the unfolding of the isolated V(L) and C(L) fragments was partially reversible. Therefore, compared to their fragments, full-length LCs are more likely to initiate aggregation during unfolding and provide a template for the V(L) deposition. The kinetic barrier for this aggregation is regulated by the stability of the V(L) region. This represents a paradigm shift in AL fibrillogenesis and suggests C(L) region as a potential therapeutic target.

Inhibition of pathologic immunoglobulin-free light chain production by small interfering RNA molecules

OBJECTIVE

Morbidity and mortality occurring in patients with multiple myeloma, AL amyloidosis, and light chain deposition disease can result from the pathologic deposition of monoclonal immunoglobulin light chains (LCs) in kidneys and other organs. To reduce synthesis of such components, therapy for these disorders typically has involved antiplasma cell agents; however, this approach is not always effective and can have adverse consequences. We have investigated another means to achieve this objective; namely, RNA interference.

MATERIALS AND METHODS

SP2/O mouse myeloma cells were stably transfected with a construct encoding a λ6 LC (Wil) under control of the cytomegalovirus promoter, while λ2-producing myeloma cell line RPMI 8226 was purchased from the American Type Culture Collection (Manassas, VA, USA). Both were treated with small interfering RNA directed specifically to the V, J, or C portions of the molecules and then analyzed by enzyme-linked immunosorbent assay, flow cytometry, and real-time polymerase chain reaction.

RESULTS

Transfected cells were found to constitutively express detectable quantities of messenger RNA and protein Wil and, after exposure to small interfering RNAs, an ∼ 40% reduction in messenger RNA and LC production was evidenced at 48 hours. An even greater effect was seen with the 8226 cells.

CONCLUSIONS

Our results have shown that RNA interference can markedly reduce LC synthesis and provide the basis for testing the therapeutic potential of this strategy using in vivo experimental models of multiple myeloma.

Backbone and side-chain (1)H, (13)C and (15)N resonance assignments of LEN, a human immunoglobulin kappaIV light-chain variable domain

(1)H, (13)C and (15)N resonance assignments are presented for a recombinant 114 amino acid human immunoglobulin (Ig) kappaIV light-chain variable domain (VL) LEN, which displays a high degree of sequence identity with another human Ig kappaIV VL, SMA. While SMA is highly amyloidogenic in vivo and in vitro and has been linked to the pathogenesis of light-chain amyloidosis, LEN is non-amyloidogenic in vivo and can be converted to the amyloid state only in vitro under destabilizing conditions. Measurements of longitudinal and transverse amide (15)N relaxation rates confirm that, as expected, LEN is a dimer at physiological pH and typical concentrations used for NMR studies, and the analysis of secondary chemical shifts indicates that the protein has a high beta-sheet content. These findings are consistent with previously published biophysical data and the high-resolution X-ray structure of LEN.

The folding pathway of the antibody V(L) domain

Antibodies are modular proteins consisting of domains that exhibit a beta-sandwich structure, the so-called immunoglobulin fold. Despite structural similarity, differences in folding and stability exist between different domains. In particular, the variable domain of the light chain V(L) is unusual as it is associated with misfolding diseases, including the pathologic assembly of the protein into fibrillar structures. Here, we have analysed the folding pathway of a V(L) domain with a view to determine features that may influence the relationship between productive folding and fibril formation. The V(L) domain from MAK33 (murine monoclonal antibody of the subtype kappa/IgG1) has not previously been associated with fibrillisation but is shown here to be capable of forming fibrils. The folding pathway of this V(L) domain is complex, involving two intermediates in different pathways. An obligatory early molten globule-like intermediate with secondary structure but only loose tertiary interactions is inferred. The native state can then be formed directly from this intermediate in a phase that can be accelerated by the addition of prolyl isomerases. However, an alternative pathway involving a second, more native-like intermediate is also significantly populated. Thus, the protein can reach the native state via two distinct folding pathways. Comparisons to the folding pathways of other antibody domains reveal similarities in the folding pathways; however, in detail, the folding of the V(L) domain is striking, with two intermediates populated on different branches of the folding pathway, one of which could provide an entry point for molecules diverted into the amyloid pathway.

How I treat amyloidosis

Amyloidosis is an uncommon disorder in which proteins change conformation, aggregate, and form fibrils that infiltrate tissues, leading to organ failure and death. The most frequent types are light-chain (AL) derived from monoclonal B-cell disorders producing amyloidogenic immunoglobulin light chains, and the hereditary and "senile systemic" (ATTR) variants from mutant and wild-type transthyretin (TTR). Diagnosis requires tissue biopsy. AL is more frequent and causes more organ disease than ATTR. Although both can cause cardiomyopathy and heart failure, AL progresses more quickly, so survival depends on timely diagnosis. Typing is usually based on clinical and laboratory findings with monoclonal gammopathy evaluation and, if indicated, TTR gene testing. Direct tissue typing is required when one patient has 2 potential amyloid-forming proteins. In coming years, widespread use of definitive proteomics will improve typing. New therapies are in testing for ATTR, whereas those for AL have followed multiple myeloma, leading to improved survival. Challenges of diagnosing and caring for patients with amyloidosis include determination of type, counseling, and delivery of prompt therapy often while managing multisystem disease. Recent advances grew from clinical research and advocacy in many countries, and global husbandry of such efforts will reap future benefits for families and patients with amyloidosis.

Structural alterations within native amyloidogenic immunoglobulin light chains

Amyloid diseases are characterized by the misfolding of a precursor protein that leads to amyloid fibril formation. Despite the fact that there are different precursors, some commonalities in the misfolding mechanism are thought to exist. In light chain amyloidosis (AL), the immunoglobulin light chain forms amyloid fibrils that deposit in the extracellular space of vital organs. AL proteins are thermodynamically destabilized compared to non-amyloidogenic proteins and some studies have linked this instability to increased fibril formation rates. Here we present the crystal structures of two highly homologous AL proteins, AL-12 and AL-103. This structural study shows that these proteins retain the canonical germ line dimer interface. We highlight important structural alterations in two loops flanking the dimer interface and correlate these results with the somatic mutations present in AL-12 and AL-103. We suggest that these alterations are informative structural features that are likely contributing to protein instability that leads to conformational changes involved in the initial events of amyloid formation.

Thermodynamic and kinetic characterization of a germ line human lambda6 light-chain protein: the relation between unfolding and fibrillogenesis

Proteins encoded by the gene segment 6a of the lambda variable light-chain repertoire are strongly associated with amyloid deposition. 6aJL2 is a model protein constructed with the predicted sequences encoded by the 6a and JL2 germ line genes. In this work, we characterized the urea- and temperature-induced unfolding of 6aJL2. In the short time scale, spectroscopic, hydrodynamic and calorimetric experiments were compatible with a two-state transition. Furthermore, DeltaG, m and the midpoint urea concentration obtained from equilibrium experiments were compatible with those obtained from kinetic experiments. Since fibril formation is a slow process, samples were also incubated for longer times. After incubation for several hours at 37 degrees C, spectroscopic, hydrodynamic and calorimetric experiments revealed the presence of a partially unfolded off-pathway intermediate around the midpoint urea concentration (1.5-3.0 M urea). In vitro fibrillogenesis assays show that the maximum growth rate for fibril formation and the minimum lag time were obtained at urea concentrations where the partially unfolded state was populated (2.5 M urea at 37 degrees C). This indicates that this partially unfolded state is critical for in vitro fibril formation. Concentration-dependent kinetics and hydrodynamic properties of the intermediate were consistent with a soluble oligomeric state. The intermediate is formed around the midpoint urea concentration, where the native and unfolded states are equally populated and their rate of interconversion is the slowest. This situation may promote the slow accumulation of an intermediate state that is prone to aggregate.

Structural insights into the role of mutations in amyloidogenesis

Mechanisms of amyloidogenesis are not well understood, including potential structural contributions of mutations in the process. Our previous research indicated that the dimer interface of amyloidogenic immunoglobulin light chain protein AL-09 is twisted 90 degrees relative to the protein from its germline sequence, kappaI O18/O8. Here we report a systematic restoration of AL-09 to its germline sequence by mutating the non-conservative somatic mutations located in the light chain dimer interface. Among these mutants, we find a correlation between increased thermodynamic stability and an increase in the lag time for fibril formation. The restorative mutant AL-09 H87Y completes the trifecta and restores the dimer interface observed in kappaI O18/O8, emphasizing the potential importance of the structural integrity of these proteins to protect against amyloidogenicity. We also find that adding amyloidogenic mutations into the germline protein illustrates mutational cooperativity in promoting amyloidogenesis.

Biologic and genetic characterization of the novel amyloidogenic lambda light chain-secreting human cell lines, ALMC-1 and ALMC-2

Primary systemic amyloidosis (AL) is a rare monoclonal plasma cell (PC) disorder characterized by the deposition of misfolded immunoglobulin (Ig) light chains (LC) in vital organs throughout the body. To our knowledge, no cell lines have ever been established from AL patients. Here we describe the establishment of the ALMC-1 and ALMC-2 cell lines from an AL patient. Both cell lines exhibit a PC phenotype and display cytokine-dependent growth. Using a comprehensive genetic approach, we established the genetic relationship between the cell lines and the primary patient cells, and we were also able to identify new genetic changes accompanying tumor progression that may explain the natural history of this patient's disease. Importantly, we demonstrate that free lambda LC secreted by both cell lines contained a beta structure and formed amyloid fibrils. Despite absolute Ig LC variable gene sequence identity, the proteins show differences in amyloid formation kinetics that are abolished by the presence of Na(2)SO(4). The formation of amyloid fibrils from these naturally secreting human LC cell lines is unprecedented. Moreover, these cell lines will provide an invaluable tool to better understand AL, from the combined perspectives of amyloidogenic protein structure and amyloid formation, genetics, and cell biology.

Altered dimer interface decreases stability in an amyloidogenic protein

Amyloidoses are devastating and currently incurable diseases in which the process of amyloid formation causes fatal cellular and organ damage. The molecular mechanisms underlying amyloidoses are not well known. In this study, we address the structural basis of immunoglobulin light chain amyloidosis, which results from deposition of light chains produced by clonal plasma cells. We compare light chain amyloidosis protein AL-09 to its wild-type counterpart, the kappaI O18/O8 light chain germline. Crystallographic studies indicate that both proteins form dimers. However, AL-09 has an altered dimer interface that is rotated 90 degrees from the kappaI O18/O8 dimer interface. The three non-conservative mutations in AL-09 are located within the dimer interface, consistent with their role in the decreased stability of this amyloidogenic protein. Moreover, AL-09 forms amyloid fibrils more quickly than kappaI O18/O8 in vitro. These results support the notion that the increased stability of the monomer and delayed fibril formation, together with a properly formed dimer, may be protective against amyloidogenesis. This could open a new direction into rational drug design for amyloidogenic proteins.

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.

Localization of a conformational epitope common to non-native and fibrillar immunoglobulin light chains

Amyloid fibrils and partially unfolded intermediates may be distinguished serologically from native amyloidogenic precursor proteins or peptides. In this regard, we had previously reported that the IgG1 mAb 11-1F4, generated by immunizing mice with a thermally denatured variable region fragment of the human Igkappa4 Bence Jones protein Len, reacted specifically with light chain (LC) fibrils, irrespective of kappa or lambda isotype but, notably, did not with native molecules (Hrncic, R. et al. (2000) Am. J. Pathol. 157, 1239-1246). To elucidate the molecular basis of this specificity, we have used a europium-linked fluorescent immunoassay, where it was demonstrated through epitope mapping that mAb 11-1F4 recognizes a conformational determinant contained within the first (N-terminal) 18 amino acids of misfolded LCs. The nature of this epitope was evidenced in competition studies where the peptide Len (1-18), but not the intact protein or other LCs, inhibited the binding of the antibody to fibrils. This unique reactivity was dependent on the structural integrity of this portion of the molecule, particularly the presence of a highly conserved prolyl residue at position 8. On the basis of our experimental data, we posit that the mAb 11-1F4 binding site found on partially denatured and fibrillar LCs involves an inducible N-terminal main chain reversal that results in the formation of a proline anchored beta-turn. Our delineation of this LC fibril-associated epitope provides the rationale for the design of novel amyloid-reactive antibodies with diagnostic and therapeutic potential for patients with LC-associated and other forms of amyloidosis.