Gelsolin domain 2 Ca2+ affinity determines susceptibility to furin proteolysis and familial amyloidosis of finnish type

A novel GSN variant outside the G2 calcium-binding domain associated with Amyloidosis of the Finnish type

Gelsolin (GSN) variants have been implicated in amyloidosis of the Finnish type. This case series reports a novel GSN:c.1477T>C,p.(Trp493Arg) variant in a family with ocular and systemic features consistent with Finnish Amyloidosis. Exome sequencing performed on affected individuals from two families manifesting cutis laxa and polymorphic corneal stromal opacities demonstrated the classic GSN:c.654G>A,p.Asp214Asn variant in single affected individual from one family, and a previously undocumented GSN:c.1477T>C variant in three affected first-degree relatives from a separate family. Immunohistochemical studies on corneal tissue from a proband with the c.1477T>C variant identified gelsolin protein within histologically defined corneal amyloid deposits. This study reports a novel association between the predicted pathogenic GSN:c.1477T>C variant and amyloidosis of the Finnish type, and is the first to provide functional evidence of a pathological GSN variant at a locus distant to the critical G2 calcium-binding region, resulting in the phenotype of amyloidosis of the Finnish type.

The structure of N184K amyloidogenic variant of gelsolin highlights the role of the H-bond network for protein stability and aggregation properties

Mutations in the gelsolin protein are responsible for a rare conformational disease known as AGel amyloidosis. Four of these mutations are hosted by the second domain of the protein (G2): D187N/Y, G167R and N184K. The impact of the latter has been so far evaluated only by studies on the isolated G2. Here we report the characterization of full-length gelsolin carrying the N184K mutation and compare the findings with those obtained on the wild type and the other variants. The crystallographic structure of the N184K variant in the Ca²⁺-free conformation shows remarkable similarities with the wild type protein. Only minimal local rearrangements can be observed and the mutant is as efficient as the wild type in severing filamentous actin. However, the thermal stability of the pathological variant is compromised in the Ca²⁺-free conditions. These data suggest that the N to K substitution causes a local disruption of the H-bond network in the core of the G2 domain. Such a subtle rearrangement of the connections does not lead to significant conformational changes but severely affects the stability of the protein.

High-resolution crystal structure of gelsolin domain 2 in complex with the physiological calcium ion

The second domain of gelsolin (G2) hosts mutations responsible for a hereditary form of amyloidosis. The active form of gelsolin is Ca²⁺-bound; it is also a dynamic protein, hence structural biologists often rely on the study of the isolated G2. However, the wild type G2 structure that have been used so far in comparative studies is bound to a crystallographic Cd²⁺, in lieu of the physiological calcium. Here, we report the wild type structure of G2 in complex with Ca²⁺ highlighting subtle ion-dependent differences. Previous findings on different G2 mutations are also briefly revised in light of these results.

Molecular basis of a novel renal amyloidosis due to N184K gelsolin variant

Mutations in gelsolin are responsible for a systemic amyloidosis first described in 1969. Until recently, the disease was associated with two substitutions of the same residue, leading to the loss of the calcium binding site. Novel interest arose in 2014 when the N184K variant of the protein was identified as the etiological agent of a novel kidney-localized amyloidosis. Here we provide a first rationale for N184K pathogenicity. We show that the mutation induces a destabilization of gelsolin second domain, without compromising its calcium binding capacity. X-ray data combined with molecular dynamics simulations demonstrates that the primary source of the destabilization is a loss of connectivity in proximity of the metal. Such rearrangement of the H-bond network does not have a major impact on the overall fold of the domain, nevertheless, it increases the flexibility of a stretch of the protein, which is consequently processed by furin protease. Overall our data suggest that the N184K variant is subjected to the same aberrant proteolytic events responsible for the formation of amyloidogenic fragments in the previously characterized mutants. At the same time our data suggest that a broader number of mutations, unrelated to the metal binding site, can lead to a pathogenic phenotype.

Regulatory role of the second gelsolin-like domain of Caenorhabditis elegans gelsolin-like protein 1 (GSNL-1) in its calcium-dependent conformation and actin-regulatory activities

Caenorhabditis elegans gelsolin-like protein-1 (GSNL-1) is an unconventional member of the gelsolin family of actin-regulatory proteins. Unlike typical gelsolin-related proteins with three or six G domains, GSNL-1 has four gelsolin-like (G) domains (G1-G4) and exhibits calcium-dependent actin filament severing and capping activities. The first G domain (G1) of GSNL-1 is necessary for its actin-regulatory activities. However, how other domains in GSNL-1 participate in regulation of its functions is not understood. Here, we report biochemical evidence that the second G domain (G2) of GSNL-1 has a regulatory role in its calcium-dependent conformation and actin-regulatory activities. Comparison of the sequences of gelsolin-related proteins from various species indicates that sequences of G2 are highly conserved. Among the conserved residues in G2, we focused on D162 of GSNL-1, since equivalent residues in gelsolin and severin are part of the calcium-binding sites and is a pathogenic mutation site in human gelsolin causing familial amyloidosis, Finish-type. The D162N mutation does not alter the inactive and fully calcium-activated states of GSNL-1 for actin filament severing (at 20 nM GSNL-1) and capping activities (at 50 nM GSNL-1). However, under these conditions, the mutant shows reduced calcium sensitivity for activation. By contrast, the D162N mutation strongly enhances susceptibility of GSNL-1 to chymotrypsin digestion only at high calcium concentrations but not at low calcium concentrations. The mutation also reduces affinity of GSNL-1 with actin monomers. These results suggest that G2 of GSNL-1 functions as a regulatory domain for its calcium-dependent actin-regulatory activities by mediating conformational changes of the GSNL-1 molecule.

N-terminal region of gelsolin induces apoptosis of activated hepatic stellate cells by a caspase-dependent mechanism

Activated hepatic stellate cells (HSCs) are the major source for alteration of extracellular matrix in fibrosis and cirrhosis. Conditioned medium (CM) collected from immortalized human hepatocytes (IHH) have earlier been shown to be responsible for apoptosis of HSCs. In this study, we have shown that antibodies raised against a peptide derived from a linear B-cell epitope in the N-terminal region of gelsolin identified a gelsolin fragment in IHH CM. Analysis of activated stellate cell death by CM collected from Huh7 cells transfected with plasmids encoding gelsolin deletion mutants suggested that the N-terminal half of gelsolin contained sequences which were responsible for stellate cell death. Further analysis determined that this activity was restricted to a region encompassing amino acids 1–70 in the gelsolin sequence; antibody directed to an epitope within this region was able to neutralize stellate cell death. Gelsolin modulation of cell death using this fragment involved upregulation of TRAIL-R1 and TRAIL-R2, and involved caspase 3 activation by extrinsic pathway. The apoptotic activity of N-terminal gelsolin fragments was restricted to activated but not quiescent stellate cells indicating its potential application in therapeutic use as an anti-fibrotic agent. Gelsolin fragments encompassing N-terminal regions in polypeptides of different molecular sizes were detected by N-terminal peptide specific antiserum in IHH CM immunoprecipitated with chronically HCV infected patient sera, suggesting the presence of autoantibodies generated against N-terminal gelsolin fragments in patients with chronic liver disease.

Familial amyloidotic polyneuropathy type IV--gelsolin amyloidosis

Familial amyloidotic polyneuropathy type IV, or Gelsolin amyloidosis (GA), is a rare condition caused by G654A or G654T mutation in gelsolin gene at 9q32-34. Gelsolin seems essential in many processes, including inflammation, cell motility, neural recovery, apoptosis and even carcinogenesis. So far reported from many European countries, USA, Japan, Iran and Brazil, GA is probably still underdiagnosed. The typical diagnostic triad includes corneal lattice dystrophy, progressive bilateral facial paralysis and cutis laxa. Patients present with progressive cranial and peripheral neuropathy, eye symptoms, usually mild proteinuria, and cardiac conductive disturbances with age. Central nervous system symptoms are rare. Gelsolin amyloid collection in tissues is widespread. To date, treatment is symptomatic. Regular check-ups with ophthalmologist are recommended. Plastic surgery relieves the functional symptoms caused by facial paralysis and loose, hanging facial skin.

Gelsolin amyloidosis: genetics, biochemistry, pathology and possible strategies for therapeutic intervention

Protein misassembly into aggregate structures, including cross-β-sheet amyloid fibrils, is linked to diseases characterized by the degeneration of post-mitotic tissue. While amyloid fibril deposition in the extracellular space certainly disrupts cellular and tissue architecture late in the course of amyloid diseases, strong genetic, pathological and pharmacologic evidence suggests that the process of amyloid fibril formation itself, known as amyloidogenesis, likely causes these maladies. It seems that the formation of oligomeric aggregates during the amyloidogenesis process causes the proteotoxicity and cytotoxicity characteristic of these disorders. Herein, we review what is known about the genetics, biochemistry and pathology of familial amyloidosis of Finnish type (FAF) or gelsolin amyloidosis. Briefly, autosomal dominant D187N or D187Y mutations compromise Ca(2+) binding in domain 2 of gelsolin, allowing domain 2 to sample unfolded conformations. When domain 2 is unfolded, gelsolin is subject to aberrant furin endoproteolysis as it passes through the Golgi on its way to the extracellular space. The resulting C-terminal 68 kDa fragment (C68) is susceptible to extracellular endoproteolytic events, possibly mediated by a matrix metalloprotease, affording 8 and 5 kDa amyloidogenic fragments of gelsolin. These amyloidogenic fragments deposit systemically, causing a variety of symptoms including corneal lattice dystrophy and neurodegeneration. The first murine model of the disease recapitulates the aberrant processing of mutant plasma gelsolin, amyloid deposition, and the degenerative phenotype. We use what we have learned from our biochemical studies, as well as insight from mouse and human pathology to propose therapeutic strategies that may halt the progression of FAF.

A molecular history of the amyloidoses

The molecular investigation of the amyloidoses began in the mid-19th century with the observation of areas in human tissues obtained at autopsy that were homogeneous and eosinophilic with conventional stains but became blue when exposed to mixtures of iodine and sulfuric acid. The foci corresponded to regions formerly identified as "waxy" or lardaceous. Subsequent identification of the characteristic staining of the same tissues with metachromatic dyes such as crystal violet or with the cotton dye Congo red (particularly under polarized light) and thioflavins allowed the pathological classification of those tissues as belonging to a set of disorders known as the amyloidoses. Not unexpectedly, progress has reflected evolving technology and parallel advances in all fields of biological science. Investigation using contemporary methods has expanded our notions of amyloid proteins from being simply agents or manifestations of systemic, largely extracellular diseases to include "protein-only infection," the concept that "normal" functional amyloids might exist in eukaryotes and prokaryotes and that aggregatability may be an intrinsic structural price to be paid for some functional protein domains. We now distinguish between the amyloidoses, that is, diseases caused by the deposition of amyloid fibrils and amyloid proteins (i.e., purified or recombinant proteins that form amyloid fibrils in vitro), which may or may not be associated with disease in vivo.

Heparin binds 8 kDa gelsolin cross-β-sheet oligomers and accelerates amyloidogenesis by hastening fibril extension

Glycosaminoglycans (GAGs) are highly sulfated linear polysaccharides prevalent in the extracellular matrix, and they associate with virtually all amyloid deposits in vivo. GAGs accelerate the aggregation of many amyloidogenic peptides in vitro, but little mechanistic evidence is available to explain why. Herein, spectroscopic methods demonstrate that GAGs do not affect the secondary structure of the monomeric 8 kDa amyloidogenic fragment of human plasma gelsolin. Moreover, monomerized 8 kDa gelsolin does not bind to heparin under physiological conditions. In contrast, 8 kDa gelsolin cross-β-sheet oligomers and amyloid fibrils bind strongly to heparin, apparently because of electrostatic interactions between the negatively charged polysaccharide and a positively charged region of the 8 kDa gelsolin assemblies. Our observations are consistent with a scaffolding mechanism whereby cross-β-sheet oligomers, upon formation, bind to GAGs, accelerating the fibril extension phase of amyloidogenesis, possibly by concentrating and orienting the oligomers to more efficiently form amyloid fibrils. Notably, heparin decreases the 8 kDa gelsolin concentration necessary for amyloid fibril formation, likely a consequence of fibril stabilization through heparin binding. Because GAG overexpression, which is common in amyloidosis, may represent a strategy for minimizing cross-β-sheet oligomer toxicity by transforming them into amyloid fibrils, the mechanism described herein for GAG-mediated acceleration of 8 kDa gelsolin amyloidogenesis provides a starting point for therapeutic strategy development. The addition of GAG mimetics, small molecule sulfonates shown to reduce the amyloid load in animal models of amyloidosis, to a heparin-accelerated 8 kDa gelsolin aggregation reaction mixture neither significantly alters the rate of amyloidogenesis nor prevents oligomers from binding to GAGs, calling into question their commonly accepted mechanism.

Multifunctional roles of gelsolin in health and diseases

Gelsolin, a Ca(2+) -regulated actin filament severing, capping, and nucleating protein, is an ubiquitous, multifunctional regulator of cell structure and metabolism. More recent data show that gelsolin can act as a transcriptional cofactor in signal transduction and its own expression and function can be influenced by epigenetic changes. Here, we review the functions of the plasma and cytoplasmic forms of gelsolin, and their manifold impacts on cancer, apoptosis, infection and inflammation, cardiac injury, pulmonary diseases, and aging. An improved understanding of the functions and regulatory mechanisms of gelsolin may lead to new considerations of this protein as a potential biomarker and/or therapeutic target.

The 8 and 5 kDa fragments of plasma gelsolin form amyloid fibrils by a nucleated polymerization mechanism, while the 68 kDa fragment is not amyloidogenic

Familial amyloidosis of Finnish type (FAF), or gelsolin amyloidosis, is a systemic amyloid disease caused by a mutation (D187N/Y) in domain 2 of human plasma gelsolin, resulting in domain 2 misfolding within the secretory pathway. When D187N/Y gelsolin passes through the Golgi, furin endoproteolysis within domain 2 occurs as a consequence of the abnormal conformations that enable furin to bind and cleave, resulting in the secretion of a 68 kDa C-terminal fragment (amino acids 173-755, C68). The C68 fragment is cleaved upon secretion from the cell by membrane type 1 matrix metalloprotease (MT1-MMP), affording the 8 and 5 kDa fragments (amino acids 173-242 and 173-225, respectively) comprising the amyloid fibrils in FAF patients. Herein, we show that the 8 and 5 kDa gelsolin fragments form amyloid fibrils by a nucleated polymerization mechanism. In addition to demonstrating the expected concentration dependence of a nucleated polymerization reaction, the addition of preformed amyloid fibrils, or "seeds", was shown to bypass the requirement for the formation of a high-energy nucleus, accelerating 8 and 5 kDa D187N gelsolin amyloidogenesis. The C68 fragment can form small oligomers, but not amyloid fibrils, even when seeded with preformed 8 kDa fragment plasma gelsolin fibrils. Because the 68 kDa fragment of gelsolin does not form amyloid fibrils in vitro or in a recently published transgenic mouse model of FAF, we propose that administration of an MT1-MMP inhibitor could be an effective strategy for the treatment of FAF.

Secretion of amyloidogenic gelsolin progressively compromises protein homeostasis leading to the intracellular aggregation of proteins

Familial amyloidosis of Finnish type (FAF) is a systemic amyloid disease associated with the deposition of proteolytic fragments of mutant (D187N/Y) plasma gelsolin. We report a mouse model of FAF featuring a muscle-specific promoter to drive D187N gelsolin synthesis. This model recapitulates the aberrant endoproteolytic cascade and the aging-associated extracellular amyloid deposition of FAF. Amyloidogenesis is observed only in tissues synthesizing human D187N gelsolin, despite the presence of full-length D187N gelsolin and its 68-kDa cleavage product in blood-demonstrating the importance of local synthesis in FAF. Loss of muscle strength was progressive in homozygous D187N gelsolin mice. The presence of misfolding-prone D187N gelsolin appears to exacerbate the age-associated decline in cellular protein homeostasis (proteostasis), reflected by the intracellular deposition of numerous proteins, a characteristic of the most common degenerative muscle disease of aging humans, sporadic inclusion body myositis.

Hacking the code of amyloid formation: the amyloid stretch hypothesis

Many research efforts in the last years have been directed towards understanding the factors determining protein misfolding and amyloid formation. Protein stability and amino acid composition have been identified as the two major factors in vitro. The research of our group has been focused on understanding the relationship between amino acid sequence and amyloid formation. Our approach has been the design of simple model systems that reproduce the biophysical properties of natural amyloids. An amyloid sequence pattern was extracted that can be used to detect amyloidogenic hexapeptide stretches in proteins. We have added evidence supporting that these amyloidogenic stretches can trigger amyloid formation by nonamyloidogenic proteins. Some experimental results in other amyloid proteins will be analyzed under the conclusions obtained in these studies. Our conclusions together with evidences from other groups suggest that amyloid formation is the result of the interplay between a decrease of protein stability, and the presence of highly amyloidogenic regions in proteins. As many of these results have been obtained in vitro, the challenge for the next years will be to demonstrate their validity in in vivo systems.

Gelsolin and diseases

Gelsolin is a calcium-activated actin filament severing and capping protein found in many cell types and as a secreted form in the plasma of vertebrates. Mutant mice for gelsolin as well as clinical studies have shown that gelsolin is linked to a number of pathological conditions such as inflammation, cancer and amyloidosis. The tight regulation of gelsolin by calcium is crucial for its physiological role and constitutive activation leads to apoptosis. In the following we will give an overview on how gelsolin is regulated by calcium, and which clinical conditions have been linked to lack or misregulation of gelsolin.

The integration of cell and chemical biology in protein folding

Eukaryotic cells are specialized, interdependent functional units of complex tissues that are composed of metabolically integrated systems defined by chemically distinct organelles that operate as reaction vessels. It is now clear that the small-molecule and polymer-based composition of these organelles plays a crucial role in generating and maintaining protein folds and functions through the systems chemistry of the local environments.

Proteome profiling of corneal epithelium and identification of marker proteins for keratoconus, a pilot study

The purpose of this study is to identify corneal proteins differentially expressed between keratoconus and normal epithelial samples. Proteins from the corneal epithelium were isolated from 6 keratoconus and 6 myopia patients (controls) and separated by 2D-gel electrophoresis. Six % and 12% SDS-PAGE gels were used to separate low and high molecular weight proteins. Gels were silver stained and protein spots were defined by Melanie II software. The proteins that were most altered in expression comparing keratoconus and controls were extracted, trypsin-digested, and identified by mass spectroscopy. Approximately 200-500 protein spots were detected on each gel. Nineteen spots were identified as differentially expressed between keratoconus and reference epithelium including cytokeratin 3 (< 7.8 fold), gelsolin (1.6 fold), S100A4 (1.9 fold), and enolase 1 (0.72 fold). Another identified protein found at very high levels was cytokeratin 12. Gelsolin, cytokeratin 3, and cytokeratin 12 have previously been described to be involved in other corneal diseases. Three proteins, gelsolin, alpha enolase, and S100A4 were identified to be differentially expressed in keratoconus compared to reference epithelium and thus may be involved in the pathogenesis.

Ca2+ binding protects against gelsolin amyloidosis

Amyloid diseases occur when native or mutant polypeptides misfold and aggregate to form deposits in the extracellular space. There are at least 20 proteins associated with amyloid diseases, including the well-known amyloid-beta peptide that is the causative agent for Alzheimer's disease (AD). This review describes familial amyloidosis of Finnish type (FAF), an amyloid disease caused by mutations in plasma gelsolin, a secreted protein that contains multiple Ca2+-binding domains. The FAF mutations result in a loss of the Ca2+-binding site in domain 2 of plasma gelsolin. The resulting decreased stability gives rise to susceptibility to the protease furin in the Golgi. Furin cleavage generates a secreted fragment that undergoes a second proteolytic event in the extracellular matrix to produce a peptide that self-assembles into amyloid plaques. Thus, Ca2+ binding in native plasma gelsolin protects against amyloid disease.

Structure of the N-terminal half of gelsolin bound to actin: roles in severing, apoptosis and FAF

The actin filament-severing functionality of gelsolin resides in its N-terminal three domains (G1-G3). We have determined the structure of this fragment in complex with an actin monomer. The structure reveals the dramatic domain rearrangements that activate G1-G3, which include the replacement of interdomain interactions observed in the inactive, calcium-free protein by new contacts to actin, and by a novel G2-G3 interface. Together, these conformational changes are critical for actin filament severing, and we suggest that their absence leads to the disease Finnish-type familial amyloidosis. Furthermore, we propose that association with actin drives the calcium-independent activation of isolated G1-G3 during apoptosis, and that a similar mechanism operates to activate native gelsolin at micromolar levels of calcium. This is the first structure of a filament-binding protein bound to actin and it sets stringent, high-resolution limitations on the arrangement of actin protomers within the filament.

Pathological and functional amyloid formation orchestrated by the secretory pathway

Amyloidogenesis has historically been associated with pathology in a class of neurodegenerative diseases known as amyloid diseases. Recent studies have shown that proteolysis by furin during secretion initiates both variant gelsolin amyloidogenesis, associated with the disease familial amyloidosis of Finnish type, and Pmel17 fiber formation, which is necessary for the functional biogenesis of melanosomes. Proteolysis combined with organelle-dependent environment changes orchestrate amyloidogenesis associated with both pathological processes and a functional pathway.