Transthyretin: the servant of many masters

The role of fibrinogen glycation in ATTR: evidence for chaperone activity loss in disease

Transthyretin amyloidosis (ATTR) belongs to a class of disorders caused by protein misfolding and aggregation. ATTR is a disabling disorder of autosomal dominant trait, where transthyretin (TTR) forms amyloid deposits in different organs, causing dysfunction of the peripheral nervous system. We previously discovered that amyloid fibrils from ATTR patients are glycated by methylglyoxal. Even though no consensus has been reached about the actual role of methylglyoxal-derived advanced glycation end-products in amyloid diseases, evidence collected so far points to a role for protein glycation in conformational abnormalities, being ubiquitously found in amyloid deposits in Alzheimer's disease, dialysis-related amyloidosis and Parkinson's diseases. Human fibrinogen, an extracellular chaperone, was reported to specifically interact with a wide spectrum of stressed proteins and suppress their aggregation, being an interacting protein with TTR. Fibrinogen is differentially glycated in ATTR, leading to its chaperone activity loss. Here we show the existence of a proteostasis imbalance in ATTR linked to fibrinogen glycation by methylglyoxal.

Repression of retinal microvascular endothelial cells by transthyretin under simulated diabetic retinopathy conditions

AIM

To investigate biological effects of transthyretin (TTR) on the development of neovascularization under simulated diabetic retinopathy (DR) condition associated with high glucose and hypoxia.

METHODS

Human retinal microvascular endothelial cells (hRECs) were cultured in normal and simulated DR environments with high glucose and hypoxia. The normal serum glucose concentration is approximately 5.5 mmol/L; thus, hyperglycemia was simulated with 25 mmol/L glucose, while hypoxia was induced using 200 µmol/L CoCl2. The influence of TTR on hRECs and human retinal pigment epithelial cells (hRPECs) was determined by incubating the cells with 4 µmol/L TTR in normal and abnormal media. A co-culture system was then employed to evaluate the effects of hRPECs on hRECs.

RESULTS

Decreased hRECs and hRPECs were observed under abnormal conditions, including high-glucose and hypoxic media. In addition, hRECs were significantly inhibited by 4 µmol/L exogenous TTR during hyperglycemic culture. During co-culture, hRPECs inhibited hRECs in both the normal and abnormal environments.

CONCLUSION

hREC growth is inhibited by exogenous TTR under simulated DR environments with high-glucose and hypoxic, particularly in the medium containing 25 mmol/L glucose. hRPECs, which manufacture TTR in the eye, also represses hRECs in the same environment. TTR is predicted to inhibit the proliferation of hRECs and neovascularization.

Mechanism of Action and Clinical Application of Tafamidis in Hereditary Transthyretin Amyloidosis

Transthyretin (TTR) transports the retinol-binding protein-vitamin A complex and is a minor transporter of thyroxine in blood. Its tetrameric structure undergoes rate-limiting dissociation and monomer misfolding, enabling TTR to aggregate or to become amyloidogenic. Mutations in the TTR gene generally destabilize the tetramer and/or accelerate tetramer dissociation, promoting amyloidogenesis. TTR-related amyloidoses are rare, fatal, protein-misfolding disorders, characterized by formation of soluble aggregates of variable structure and tissue deposition of amyloid. The TTR amyloidoses present with a spectrum of manifestations, encompassing progressive neuropathy and/or cardiomyopathy. Until recently, the only accepted treatment to halt progression of hereditary TTR amyloidosis was liver transplantation, which replaces the hepatic source of mutant TTR with the less amyloidogenic wild-type TTR. Tafamidis meglumine is a rationally designed, non-NSAID benzoxazole derivative that binds with high affinity and selectivity to TTR and kinetically stabilizes the tetramer, slowing monomer formation, misfolding, and amyloidogenesis. Tafamidis is the first pharmacotherapy approved to slow the progression of peripheral neurologic impairment in TTR familial amyloid polyneuropathy. Here we describe the mechanism of action of tafamidis and review the clinical data, demonstrating that tafamidis treatment slows neurologic deterioration and preserves nutritional status, as well as quality of life in patients with early-stage Val30Met amyloidosis.

Diagnostic performance of a seven-marker serum protein biosignature for the diagnosis of active TB disease in African primary healthcare clinic attendees with signs and symptoms suggestive of TB

BACKGROUND

User-friendly, rapid, inexpensive yet accurate TB diagnostic tools are urgently needed at points of care in resource-limited settings. We investigated host biomarkers detected in serum samples obtained from adults with signs and symptoms suggestive of TB at primary healthcare clinics in five African countries (Malawi, Namibia, South Africa, The Gambia and Uganda), for the diagnosis of TB disease.

METHODS

We prospectively enrolled individuals presenting with symptoms warranting investigation for pulmonary TB, prior to assessment for TB disease. We evaluated 22 host protein biomarkers in stored serum samples using a multiplex cytokine platform. Using a pre-established diagnostic algorithm comprising of laboratory, clinical and radiological findings, participants were classified as either definite TB, probable TB, questionable TB status or non-pulmonary TB.

RESULTS

Of the 716 participants enrolled, 185 were definite and 29 were probable TB cases, 6 had questionable TB disease status, whereas 487 had no evidence of TB. A seven-marker biosignature of C reactive protein, transthyretin, IFN-γ, complement factor H, apolipoprotein-A1, inducible protein 10 and serum amyloid A identified on a training sample set (n=491), diagnosed TB disease in the test set (n=210) with sensitivity of 93.8% (95% CI 84.0% to 98.0%), specificity of 73.3% (95% CI 65.2% to 80.1%), and positive and negative predictive values of 60.6% (95% CI 50.3% to 70.1%) and 96.4% (95% CI 90.5% to 98.8%), respectively, regardless of HIV infection status or study site.

CONCLUSIONS

We have identified a seven-marker host serum protein biosignature for the diagnosis of TB disease irrespective of HIV infection status or ethnicity in Africa. These results hold promise for the development of a field-friendly point-of-care screening test for pulmonary TB.

Understanding How the Subcommissural Organ and Other Periventricular Secretory Structures Contribute via the Cerebrospinal Fluid to Neurogenesis

The dynamic and molecular composition of the cerebrospinal fluid (CSF) and, consequently, the CSF physiology is much more complex and fascinating than the simplistic view held for decades. Signal molecules either transported from blood to CSF or secreted into the CSF by circumventricular organs and CSF-contacting neurons, use the CSF to reach their targets in the brain, including the pre- and postnatal neurogenic niche. The subcommissural organ (SCO), a highly conserved brain gland present throughout the vertebrate phylum, is one of the sources for signals, as well as the choroid plexus, tanycytes and CSF-contacting neurons. The SCO secretes into the fetal and adult CSF SCO-spondin, transthyretin, and basic fibroblast growth factor. These proteins participate in certain aspects of neurogenesis, such as cell cycle of neural stem cells, neuronal differentiation, and axon pathfinding. Through the CSF, the SCO-secretory proteins may reach virtually any target in the embryonic and adult central nervous system. Since the SCO continues to secrete throughout life span, it seems likely that the neurogenetic property of the SCO compounds would be targeted to the niches where neurogenesis continues in adulthood. This review is aimed to bring into discussion early and new evidence concerning the role(s) of the SCO, and the probable mechanisms by which SCO compounds can readily reach the neurogenic niche of the subventricular zone flowing with the CSF to participate in the regulation of the neurogenic niche. As we unfold the multiples trans-fluid talks between discrete brain domains we will have more tools to influence such talks.

Proteomic analysis of mouse soleus muscles affected by hindlimb unloading and reloading

INTRODUCTION

Disuse muscle atrophy, induced by prolonged space flight, bed rest, or denervation, is a common process with obvious changes in slow-twitch soleus muscles.

METHODS

Proteomic analysis was performed on mouse soleus subjected to hindlimb unloading (HU) and hindlimb reloading (HR) to identify new dysregulated proteins.

RESULTS

Following HU, the mass and cross-sectional area of muscle fibers decreased, but they recovered after HR. Proteomic analyses revealed 9 down-regulated and 7 up-regulated proteins in HU, and 2 down-regulated and 5 up-regulated proteins in HR. The dysregulated proteins were mainly involved in energy metabolism, protein degradation, and cytoskeleton stability. Among the dysregulated proteins were fatty acid binding protein 3, α-B crystalline, and transthyretin.

CONCLUSIONS

These results indicate that muscle atrophy induced by unloading is related to activation of proteolysis, metabolic alterations toward glycolysis, destruction of myofibrillar integrity, and dysregulation of heat shock proteins (HSPs). The dysregulated proteins may play a role in muscle atrophy and the recovery process.

Plasma Transthyretin as a Biomarker of Lean Body Mass and Catabolic States

Plasma transthyretin (TTR) is a plasma protein secreted by the liver that circulates bound to retinol-binding protein 4 (RBP4) and its retinol ligand. TTR is the sole plasma protein that reveals from birth to old age evolutionary patterns that are closely superimposable to those of lean body mass (LBM) and thus works as the best surrogate analyte of LBM. Any alteration in energy-to-protein balance impairs the accretion of LBM reserves and causes early depression of TTR production. In acute inflammatory states, cytokines induce urinary leakage of nitrogenous catabolites, deplete LBM stores, and cause an abrupt decrease in TTR and RBP4 concentrations. As a result, thyroxine and retinol ligands are released in free form, creating a second frontline that strengthens that primarily initiated by cytokines. Malnutrition and inflammation thus keep in check TTR and RBP4 secretion by using distinct and unrelated physiologic pathways, but they operate in concert to downregulate LBM stores. The biomarker complex integrates these opposite mechanisms at any time and thereby constitutes an ideally suited tool to determine residual LBM resources still available for metabolic responses, hence predicting outcomes of the most interwoven disease conditions.

A New Semantic Functional Similarity over Gene Ontology

Identifying functionally similar or closely related genes and gene products has significant impacts on biological and clinical studies as well as drug discovery. In this paper, we propose an effective and practically useful method measuring both gene and gene product similarity by integrating the topology of gene ontology, known functional domains and their functional annotations. The proposed method is comprehensively evaluated through statistical analysis of the similarities derived from sequence, structure and phylogenetic profiles, and clustering analysis of disease genes clusters. Our results show that the proposed method clearly outperforms other conventional methods. Furthermore, literature analysis also reveals that the proposed method is both statistically and biologically promising for identifying functionally similar genes or gene products. In particular, we demonstrate that the proposed functional similarity metric is capable of discoverying new disease related genes or gene products.

Amyloidogenic and non-amyloidogenic transthyretin variants interact differently with human cardiomyocytes: insights into early events of non-fibrillar tissue damage

TTR (transthyretin) amyloidoses are diseases characterized by the aggregation and extracellular deposition of the normally soluble plasma protein TTR. Ex vivo and tissue culture studies suggest that tissue damage precedes TTR fibril deposition, indicating that early events in the amyloidogenic cascade have an impact on disease development. We used a human cardiomyocyte tissue culture model system to define these events. We previously described that the amyloidogenic V122I TTR variant is cytotoxic to human cardiac cells, whereas the naturally occurring, stable and non-amyloidogenic T119M TTR variant is not. We show that most of the V122I TTR interacting with the cells is extracellular and this interaction is mediated by a membrane protein(s). In contrast, most of the non-amyloidogenic T119M TTR associated with the cells is intracellular where it undergoes lysosomal degradation. The TTR internalization process is highly dependent on membrane cholesterol content. Using a fluorescent labelled V122I TTR variant that has the same aggregation and cytotoxic potential as the native V122I TTR, we determined that its association with human cardiomyocytes is saturable with a KD near 650 nM. Only amyloidogenic V122I TTR compete with fluorescent V122I for cell-binding sites. Finally, incubation of the human cardiomyocytes with V122I TTR but not with T119M TTR, generates superoxide species and activates caspase 3/7. In summary, our results show that the interaction of the amyloidogenic V122I TTR is distinct from that of a non-amyloidogenic TTR variant and is characterized by its retention at the cell membrane, where it initiates the cytotoxic cascade.

Modulating inhibitors of transthyretin fibrillogenesis via sulfation: polychlorinated biphenyl sulfates as models

Small molecules that bind with high affinity to thyroxine (T4) binding sites on transthyretin (TTR) kinetically stabilize the protein's tetrameric structure, thereby efficiently decreasing the rate of tetramer dissociation in TTR related amyloidoses. Current research efforts aim to optimize the amyloid inhibiting properties of known inhibitors, such as derivatives of biphenyls, dibenzofurans and benzooxazoles, by chemical modification. In order to test the hypothesis that sulfate group substituents can improve the efficiencies of such inhibitors, we evaluated the potential of six polychlorinated biphenyl sulfates to inhibit TTR amyloid fibril formation in vitro. In addition, we determined their binding orientations and molecular interactions within the T4 binding site by molecular docking simulations. Utilizing this combined experimental and computational approach, we demonstrated that sulfation significantly improves the amyloid inhibiting properties as compared to both parent and hydroxylated PCBs. Importantly, several PCB sulfates were of equal or higher potency than some of the most effective previously described inhibitors.

Assessment of preoperative and postoperative prealbumin in thoracic surgery – a two months experience in a Romanian university hospital / Evaluarea preoperatorie şi postoperatorie a prealbuminei în chirurgia toracică - experiența de 2 luni a unui spital universitar din România

Malnutrition is a frequent and serious finding in surgical departments. Although its consequences include postoperative complications and higher costs, nutritional assessment is not part of the routine preoperative protocols. Nutritional assessment involves clinical and biological parameters and is vital in order to start treatment and improve outcome. Prealbumin is currently recognized as a faithful marker of malnutrition being introduced in practice guidelines. One of the most important aspects about prealbumin is the fact that its variations in time are more valuable than the absolute values. The aim of this study was to assess and compare the perioperative nutritional evolution of patients requiring thoracic surgery, with and without cancer, using prealbumin - preoperative and postoperative - as main marker. Thirty six patients from the Thoracic Surgery Department were assessed prior to surgery by body mass index, Subjective Global Assessment nutrition risk score and routine biochemical parameters. Prealbumin was assessed prior to surgery and 3 days after surgery. The age, length of postoperative stay and the presence was complications was noted. Patients with cancer (n=19) were significantly older than patients without cancer (p=0.007) and were more frequently, but not significantly, evaluated as malnourished through SGA (42.1% compared to 11.6%). Preoperative prealbumin and other parameters did not differ significantly between groups. However, there was a significant postoperative decrease in prealbumin only in patients with cancer. Therefore, prealbumin has been found to be valuable in assessing acute malnutrition in cancer patients, especially if variations are monitored in time, which could be useful in planning nutritional treatment

Cooperative stabilization of transthyretin by clusterin and diflunisal

The circulating protein transthyretin (TTR) can unfold, oligomerize, and form highly structured amyloid fibrils that are deposited in tissues, causing organ damage and disease. This pathogenic process is caused by a heritable TTR point mutation in cases of familial TTR-related amyloidosis or wild-type TTR in cases of age-associated amyloidosis (previously called senile systemic amyloidosis). The TTR amyloid cascade is hypothesized to begin with the dissociation of the TTR native tetrameric structure into folded but unstable monomeric TTR subunits. Unfolding of monomeric TTR initiates an oligomerization process leading to aggregation and fibril formation. Numerous proteostatic mechanisms for regulating the TTR amyloid cascade exist. Extracellular chaperones provide an innate defense against misfolded proteins. Clusterin (CLU), a plasma protein, has the capacity to recognize exposed hydrophobic regions of misfolded proteins, shielding them from aggregation. We have previously demonstrated that CLU is associated with the amyloid fibrils in cardiac tissues from patients with TTR amyloidosis. In this study, we have used tetrameric and monomeric TTR structural variants to determine the ability of CLU to inhibit TTR amyloid fibril formation. Using circular dichroism spectroscopy, we determined that CLU preferentially stabilizes monomeric TTR and generates increasingly stable conformations under acid stress. Moreover, studies using surface plasmon resonance showed a direct interaction of CLU with high-molecular weight TTR oligomers. The interactions of CLU with monomeric and aggregated TTR proceed in a cooperative manner in the presence of diflunisal, a small molecule drug used to stabilize TTR tetramers.

Protein profiling of isolated uterine AA amyloidosis causing fetal death in goats

Pathologic amyloid accumulates in the CNS or in peripheral organs, yet the mechanism underlying the targeting of systemic amyloid deposits is unclear. Serum amyloid A (SAA) 1 and 2 are produced predominantly by the liver and form amyloid most commonly in the spleen, liver, and kidney. In contrast, SAA3 is produced primarily extrahepatically and has no causal link to amyloid formation. Here, we identified 8 amyloidosis cases with amyloid composed of SAA3 expanding the uterine wall of goats with near-term fetuses. Uterine amyloid accumulated in the endometrium, only at the site of placental attachment, compromising maternal-fetal gas and nutrient exchange and leading to fetal ischemia and death. No other organ contained amyloid. SAA3 mRNA levels in the uterine endometrium were as high as SAA2 in the liver, yet mass spectrometry of the insoluble uterine peptides identified SAA3 as the predominant protein, and not SAA1 or SAA2. These findings suggest that high local SAA3 production led to deposition at this unusual site. Although amyloid A (AA) amyloid deposits typically consist of an N-terminal fragment of SAA1 or SAA2, here, abundant C-terminal peptides indicated that the uterine amyloid was largely composed of full-length SAA3. The exclusive deposition of SAA3 amyloid in the uterus, together with elevated uterine SAA3 transcripts, suggests that the uterine amyloid deposits were due to locally produced SAA3. This is the first report of SAA3 as a cause of amyloidosis and of AA amyloid deposited exclusively in the uterus.

Quantification of quaternary structure stability in aggregation-prone proteins under physiological conditions: the transthyretin case

The quaternary structure stability of proteins is typically studied under conditions that accelerate their aggregation/unfolding processes on convenient laboratory time scales. Such conditions include high temperature or pressure, chaotrope-mediated unfolding, or low or high pH. These approaches have the limitation of being nonphysiological and that the concentration of the protein in solution is changing as the reactions proceed. We describe a methodology to define the quaternary structure stability of the amyloidogenic homotetrameric protein transthyretin (TTR) under physiological conditions. This methodology expands from a described approach based on the measurement of the rate of subunit exchange of TTR with a tandem flag-tagged (FT₂) TTR counterpart. We demonstrate that subunit exchange of TTR with FT₂·TTR can be analyzed and quantified using a semi-native polyacrylamide gel electrophoresis technique. In addition, we biophysically characterized two FT₂·TTR variants derived from wild-type and the amyloidogenic variant Val122Ile TTR, both of which are associated with cardiac amyloid deposition late in life. The FT₂·TTR variants have similar amyloidogenic potential and similar thermodynamic and kinetic stabilities compared to those of their nontagged counterparts. We utilized the methodology to study the potential of the small molecule SOM0226, a repurposed drug under clinical development for the prevention and treatment of the TTR amyloidoses, to stabilize TTR. The results enabled us to characterize the binding energetics of SOM0226 to TTR. The described technique is well-suited to study the quaternary structure of other human aggregation-prone proteins under physiological conditions.

Amyloid-clearing proteins and their epigenetic regulation as a therapeutic target in Alzheimer's disease

Abnormal elevation of amyloid β-peptide (Aβ) levels in the brain is the primary trigger for neuronal cell death specific to Alzheimer’s disease (AD). It is now evident that Aβ levels in the brain are manipulable due to a dynamic equilibrium between its production from the amyloid precursor protein (APP) and removal by amyloid clearance proteins. Clearance can be either enzymic or non-enzymic (binding/transport proteins). Intriguingly several of the main amyloid-degrading enzymes (ADEs) are members of the M13 peptidase family (neprilysin (NEP), NEP2 and the endothelin converting enzymes (ECE-1 and -2)). A distinct metallopeptidase, insulin-degrading enzyme (IDE), also contributes to Aβ degradation in the brain. The ADE family currently embraces more than 20 members, both membrane-bound and soluble, and of differing cellular locations. NEP plays an important role in brain function terminating neuropeptide signals. Its decrease in specific brain areas with age or after hypoxia, ischaemia or stroke contribute significantly to the development of AD pathology. The recently discovered mechanism of epigenetic regulation of NEP (and other genes) by the APP intracellular domain (AICD) and its dependence on the cell type and APP isoform expression suggest possibilities for selective manipulation of NEP gene expression in neuronal cells. We have also observed that another amyloid-clearing protein, namely transthyretin (TTR), is also regulated in the neuronal cell by a mechanism similar to NEP. Dependence of amyloid clearance proteins on histone deacetylases and the ability of HDAC inhibitors to up-regulate their expression in the brain opens new avenues for developing preventive strategies in AD.

Transthyretin as a potential biomarker for the differential diagnosis between lung cancer and lung infection

Satisfactory biomarkers for screening and early diagnosis of lung cancer remain scarce and require further investigation. The aim of the present study was to examine the changes of the biochemical and protein composition in the serum and pleural effusion from lung cancer and lung infection (bacterial pneumonia) patients. A total of 92 patients with lung cancer, 38 with bacterial pneumonia and 42 healthy controls were enrolled in the study. The serum levels of cholesterol, apolipoprotein A and transthyretin (TTR) in the lung cancer patients were higher than that of the lung infection patients (P<0.05). The levels of TTR were higher, whereas the activity of adenosine deaminase (ADA) was lower in the pleural effusion from the lung cancer patients compared to the lung infection patients (P<0.05). Furthermore, the pleural effusion/serum TTR ratios in the lung cancer patients were higher, whereas the ratios of ADA were lower (P<0.05). By matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis, four major peaks corresponding to native TTR, Sul-TTR, Cys-TTR and Cysgly-TTR were observed in the serum of the lung cancer and lung infection patients. A significant increase was found in the proportion of Cysgly-TTR in the pleural effusion from the patients with lung cancer. The data indicated that a combination of pleural effusion/serum TTR ratios and modified TTR may be beneficial for the differential diagnosis between lung cancer and lung infection.

Protein interactomics based on direct molecular fishing on paramagnetic particles: practical realization and further SPR validation

There is increasing evidence that proteins function in the cell as integrated stable or temporally formed protein complexes, interactomes. Previously, using model systems we demonstrated applicability of direct molecular fishing on paramagnetic particles for protein interactomics (Ershov et al. Proteomics, 2012, 12, 3295). In the present study, we have used a combination of affinity-based molecular fishing and subsequent MS for investigation of human liver proteins involved in interactions with immobilized microsomal cytochrome b5 (CYB5A), and also transthyretin and BSA as alternative affinity ligands (baits). The LC-MS/MS identification of prey proteins fished on these baits revealed three sets of proteins: 98, 120, and 220, respectively. Comparison analysis of these sets revealed only three proteins common for all the baits. In the case of paired analysis, the number of common proteins varied from 2 to 9. The binding capacity of some identified proteins has been validated by a SPR-based biosensor. All the investigated proteins effectively interacted with the immobilized CYB5A (Kd values ranged from 0.07 to 1.1 μM). Results of this study suggest that direct molecular fishing is applicable for analysis of protein-protein interactions (PPI) under normal and pathological conditions, in which altered PPIs are especially important.

Urine retinol-binding protein 4: a functional biomarker of the proximal renal tubule

Measurement of retinol-binding protein 4 in urine (uRBP4) is arguably the most sensitive biomarker for loss of function of the human proximal renal tubule. Megalin- and cubilin-receptor-mediated endocytosis normally absorbs > 99% of the approximately 1.5 g/24 h of protein filtered by the renal glomerulus. When this fails there is "tubular proteinuria," comprising uRBP4, albumin, and many other proteins and peptides. This tubular proteinuria is a consistent feature of the renal Fanconi syndrome (FS) and measurement of uRBP4 appears to be an excellent screening test for FS. FS occurs in rare inherited renal diseases including cystinosis, Dent disease, Lowe syndrome, and autosomal dominant FS. Acquired FS occurs in paraproteinemias, tubulointerstitial renal disease, oncogenic osteomalacia, Chinese herbs nephropathy, and Balkan endemic nephropathy. Though poorly understood, FS may be associated with HIV disease and antiretroviral treatment; cadmium poisoning may cause FS. In addition to FS, uRBP4 measurement has a different role: the early detection of acute kidney injury. Urine RBP4 comprises several isoforms, including intact plasma RBP4, MW 21.07 kDa, and C-terminal truncated forms, des-L- and des-LL-RBP4, also probably plasma derived. In FS, uRBP4 levels are about 104-fold above the upper limit of normal and small increments are frequently seen in carriers of some inherited forms of FS and in acquired disease. The very high levels in disease, frequent assay nonlinearity, lack of defined calibrants, and multiple uRBP4 isoforms make accurate assay challenging; top-down mass spectrometry has brought advances. Assays for uRBP4 with defined molecular targets allowing good interlaboratory comparisons are needed.

Physiological IgM class catalytic antibodies selective for transthyretin amyloid

Peptide bond-hydrolyzing catalytic antibodies (catabodies) could degrade toxic proteins, but acquired immunity principles have not provided evidence for beneficial catabodies. Transthyretin (TTR) forms misfolded β-sheet aggregates responsible for age-associated amyloidosis. We describe nucleophilic catabodies from healthy humans without amyloidosis that degraded misfolded TTR (misTTR) without reactivity to the physiological tetrameric TTR (phyTTR). IgM class B cell receptors specifically recognized the electrophilic analog of misTTR but not phyTTR. IgM but not IgG class antibodies hydrolyzed the particulate and soluble misTTR species. No misTTR-IgM binding was detected. The IgMs accounted for essentially all of the misTTR hydrolytic activity of unfractionated human serum. The IgMs did not degrade non-amyloidogenic, non-superantigenic proteins. Individual monoclonal IgMs (mIgMs) expressed variable misTTR hydrolytic rates and differing oligoreactivity directed to amyloid β peptide and microbial superantigen proteins. A subset of the mIgMs was monoreactive for misTTR. Excess misTTR was dissolved by a hydrolytic mIgM. The studies reveal a novel antibody property, the innate ability of IgMs to selectively degrade and dissolve toxic misTTR species as a first line immune function.

Low prealbumin is a significant predictor of medical complications in severe anorexia nervosa

OBJECTIVE

Prealbumin levels have been proven to correlate with hospital length of stay, wound healing, infection rates, and mortality in adults hospitalized for medical or surgical purposes, or those who have chronic illnesses. Little is known about the utility of prealbumin evaluation in adults with severe anorexia nervosa (AN).

METHOD

We retrospectively evaluated prealbumin levels, along with numerous other clinical parameters relevant to illness acuity and early refeeding outcomes, in 132 adults with AN admitted for definitive inpatient medical stabilization from October 1, 2008 to December 31, 2012. Per clinical protocol, prealbumin was checked on admission and approximately weekly thereafter until discharge.

RESULTS

Patients had a median age of 28 years old, a mean admission body mass index (BMI) of 12.9 kg/m(2) (S.D. 6.1), and 89% of patients were women. A total of 47% of patients had a low prealbumin at the time of admission. By discharge, 77% of patients had normalized their prealbumin levels. Patients with low admission prealbumin levels had a threefold increased risk of refeeding hypophosphatemia and a twofold increase in hypoglycemia compared with patients who had a normal admission prealbumin, independent of admission BMI.

DISCUSSION

A low serum prealbumin level appeared concurrent with other markers of serious medical compromise, and was associated with two potentially life threatening complications of early refeeding: hypophosphatemia and hypoglycemia. The cause of low prealbumin remains elusive. Prealbumin should be checked in patients with severe AN prior to initiating weight restoration, as low levels may be an important harbinger of early refeeding complications.

Transthyretin suppresses the toxicity of oligomers formed by misfolded proteins in vitro

Although human transthyretin (TTR) is associated with systemic amyloidoses, an anti-amyloidogenic effect that prevents Aβ fibril formation in vitro and in animal models has been observed. Here we studied the ability of three different types of TTR, namely human tetramers (hTTR), mouse tetramers (muTTR) and an engineered monomer of the human protein (M-TTR), to suppress the toxicity of oligomers formed by two different amyloidogenic peptides/proteins (HypF-N and Aβ42). muTTR is the most stable homotetramer, hTTR can dissociate into partially unfolded monomers, whereas M-TTR maintains a monomeric state. Preformed toxic HypF-N and Aβ42 oligomers were incubated in the presence of each TTR then added to cell culture media. hTTR, and to a greater extent M-TTR, were found to protect human neuroblastoma cells and rat primary neurons against oligomer-induced toxicity, whereas muTTR had no protective effect. The thioflavin T assay and site-directed labeling experiments using pyrene ruled out disaggregation and structural reorganization within the discrete oligomers following incubation with TTRs, while confocal microscopy, SDS-PAGE, and intrinsic fluorescence measurements indicated tight binding between oligomers and hTTR, particularly M-TTR. Moreover, atomic force microscopy (AFM), light scattering and turbidimetry analyses indicated that larger assemblies of oligomers are formed in the presence of M-TTR and, to a lesser extent, with hTTR. Overall, the data suggest a generic capacity of TTR to efficiently neutralize the toxicity of oligomers formed by misfolded proteins and reveal that such neutralization occurs through a mechanism of TTR-mediated assembly of protein oligomers into larger species, with an efficiency that correlates inversely with TTR tetramer stability.

Transthyretin proteins regulate angiogenesis by conferring different molecular identities to endothelial cells

Familial amyloidotic polyneuropathy (FAP) has a high prevalence in Portugal, and the most common form of hereditary amyloidosis is caused by an amyloidogenic variant of transthyretin (TTR) with a substitution of methionine for valine at position 30 (V30M). Until now, the available efficient therapy is liver transplantation, when performed in an early phase of the onset of the disease symptoms. However, transplanted FAP patients have a significantly higher incidence of early hepatic artery thrombosis compared with non-FAP transplanted patients. Because FAP was described as an independent risk factor for early hepatic artery thrombosis, more studies to understand the underlying mechanisms involved in this outcome are of the utmost importance. Knowing that the liver is the major site for TTR production, we investigated the biological effects of TTR proteins in the vasculature and on angiogenesis. In this study, we identified genes differentially expressed in endothelial cells exposed to the WT or V30M tetramer. We found that endothelial cells may acquire different molecular identities when exposed to these proteins, and consequently TTR could regulate angiogenesis. Moreover, we show that V30M decreases endothelial survival by inducing apoptosis, and it inhibits migration. These findings provide new knowledge that may have critical implications in the prevention of early hepatic artery thrombosis in FAP patients after liver transplantation.

Multi-peptide nLC-PC-IDMS-SRM-based assay for the quantification of biomarkers in the chicken ovarian cancer model

A novel form of ovomacroglobulin/ovostatin (OVOS2) predicted from EST data was previously identified in the chicken ovarian cancer model using a mass spectrometry-based shotgun label-free proteomics strategy. The quantitative label-free data from plasma showed a significant increase over time with the spontaneous onset and progression of ovarian cancer making it a potential protein biomarker for further study. Two other proteins of interest identified from this initial study included vitellogenin-1 (Vit-1), a lipid-transport protein tied to egg production, and transthyretin (TTR), a retinol binding transport protein currently used in the clinical management of ovarian cancer. A multiplexed protein cleavage isotope dilution mass spectrometry (PC-IDMS) assay was developed to quantify OVOS2, Vit-1, and TTR by selected reaction monitoring (SRM). A total of 6 stable isotope labeled (SIL) peptide standards were used in the assay with three tryptic peptides from OVOS2, one for Vit-1, and two for TTR. The assay was developed for use with un-depleted raw plasma combined with the filter assisted sample preparation (FASP) method and its use was also demonstrated for matched ovary tissue samples. The PC-IDMS data for the two TTR peptides did not correlate with each other with more than a 10-fold difference in concentration for all 5 time points measured. The PC-IDMS data from the longitudinal plasma samples correlated well for OVOS2 and Vit-1 whereas TTR was inconclusive. Interestingly, the absolute amount for one of the OVOS2 SIL peptides was 2-fold less compared with the other two SIL peptides. These data illustrate the successes and challenges of qualifying quantitative levels of proteins from an in-gel digestion sample preparation followed by LC-MS/MS (GeLC) label-free discovery-based approach to a targeted SRM-based quantitative assay in plasma and tissues.

Age-related oxidative modifications of transthyretin modulate its amyloidogenicity

The transthyretin amyloidoses are diseases of protein misfolding characterized by the extracellular deposition of fibrils and other aggregates of the homotetrameric protein transthyretin (TTR) in peripheral nerves, heart, and other tissues. Age is the major risk factor for the development of these diseases. We hypothesized that an age-associated increase in the level of protein oxidation could be involved in the onset of the senile forms of the TTR amyloidoses. To test this hypothesis, we have produced and characterized relevant age-related oxidative modifications of the wild type (WT) and the Val122Ile (V122I) TTR variant, both involved in cardiac TTR deposition in the elderly. Our studies show that methionine/cysteine-oxidized TTR and carbonylated TTR from either the WT or the V122I variant are thermodynamically less stable than their nonoxidized counterparts. Moreover, carbonylated WT and carbonylated V122I TTR have a stronger propensity to form aggregates and fibrils than WT and V122I TTR, respectively, at physiologically attainable pH values. It is well-known that TTR tetramer dissociation, the limiting step for aggregation and amyloid fibril formation, can be prevented by small molecules that bind the TTR tetramer interface. Here, we report that carbonylated WT TTR is less amenable to resveratrol-mediated tetramer stabilization than WT TTR. All the oxidized forms of TTR tested are cytotoxic to a human cardiomyocyte cell line known to be a target for cardiac-specific TTR variants. Overall, these studies demonstrate that age-related oxidative modifications of TTR can contribute to the onset of the senile forms of the TTR amyloidoses.

Mechanism-based testing strategy using in vitro approaches for identification of thyroid hormone disrupting chemicals

The thyroid hormone (TH) system is involved in several important physiological processes, including regulation of energy metabolism, growth and differentiation, development and maintenance of brain function, thermo-regulation, osmo-regulation, and axis of regulation of other endocrine systems, sexual behaviour and fertility and cardiovascular function. Therefore, concern about TH disruption (THD) has resulted in strategies being developed to identify THD chemicals (THDCs). Information on potential of chemicals causing THD is typically derived from animal studies. For the majority of chemicals, however, this information is either limited or unavailable. It is also unlikely that animal experiments will be performed for all THD relevant chemicals in the near future for ethical, financial and practical reasons. In addition, typical animal experiments often do not provide information on the mechanism of action of THDC, making it harder to extrapolate results across species. Relevant effects may not be identified in animal studies when the effects are delayed, life stage specific, not assessed by the experimental paradigm (e.g., behaviour) or only occur when an organism has to adapt to environmental factors by modulating TH levels. Therefore, in vitro and in silico alternatives to identify THDC and quantify their potency are needed. THDC have many potential mechanisms of action, including altered hormone production, transport, metabolism, receptor activation and disruption of several feed-back mechanisms. In vitro assays are available for many of these endpoints, and the application of modern '-omics' technologies, applicable for in vivo studies can help to reveal relevant and possibly new endpoints for inclusion in a targeted THDC in vitro test battery. Within the framework of the ASAT initiative (Assuring Safety without Animal Testing), an international group consisting of experts in the areas of thyroid endocrinology, toxicology of endocrine disruption, neurotoxicology, high-throughput screening, computational biology, and regulatory affairs has reviewed the state of science for (1) known mechanisms for THD plus examples of THDC; (2) in vitro THD tests currently available or under development related to these mechanisms; and (3) in silico methods for estimating the blood levels of THDC. Based on this scientific review, the panel has recommended a battery of test methods to be able to classify chemicals as of less or high concern for further hazard and risk assessment for THD. In addition, research gaps and needs are identified to be able to optimize and validate the targeted THD in vitro test battery for a mechanism-based strategy for a decision to opt out or to proceed with further testing for THD.

Increased expression of transthyretin in leptin-deficient ob/ob mice is not causative for their major phenotypic abnormalities

The hormone leptin is a critical regulator of adipogenesis and energy metabolism. Similarly, leptin-deficient ob/ob mice display various metabolic abnormalities, including not only obesity and insulin resistance, but also hypogonadism and high bone mass. By genome-wide expression analysis using hypothalamus RNA from wild-type and ob/ob mice, we observed the increased expression of the gene for transthyretin (Ttr) in the latter, as confirmed by quantitative real-time-polymerase chain reaction. Because Ttr encodes a carrier protein for retinol transport, and because we further found increased retinol levels in the serum of ob/ob mice, we investigated whether the additional absence of Ttr would influence the ob/ob phenotype. It was found that Ttr-deficient ob/ob mice were indistinguishable from ob/ob littermates in terms of body weight, as well as serum glucose, insulin and cholesterol levels. Although all of these parameters were identical to wild-type controls in Ttr-deficient mice, we found that the sole deletion of Ttr caused a significant increase of trabecular bone mass, bone marrow adiposity and mean adipocyte area in white adipose tissue. Interestingly, all these latter parameters were highest in Ttr-deficient ob/ob mice, and only in these mice did we observe a full penetrance of liver steatosis at 24 weeks of age. Taken together, our data demonstrate that the increased expression of Ttr in ob/ob mice does not cause (but rather attenuates) their phenotypic abnormalities.

Functional analysis of misfolded transthyretin extracted from abnormal vitreous with high myopia related ocular pathologies

BACKGROUND

In the development of high myopia, some secondary ocular diseases such as macular detachment (MD) and macular hole (MH) may occur owing to the elongation of the eyeball. Higher concentrations of misfolded transthyretin (TTR) had been detected in abnormal vitreous humor, but the mechanisms are still unclear.

METHOD

TTRs of high myopia and healthy vitreous were purified with TTR polyclonal antibody-Sepharose. Gel exclusion chromatography, cleavage activity assay and a fluorescent probe were employed for the functional comparison of natural and abnormal TTRs.

RESULTS

Compared with natural transthyretin, MH TTR showed lower retinol-binding protein (RBP) binding ability; and MD TTR could not bind with RBP at all. Additionally, MH and MD TTR did not reveal cleavage activity against apolipoprotein AI (apoA-I). Furthermore, the kinetic parameters of the interactions between abnormal TTRs and a thyroxine-like fluorescent probe were quite different from those of natural TTR.

CONCLUSIONS

The results suggested that misfolded TTRs in MD and MH patients' vitreous completely or partially lost natural bio-functions; and this should be associated with abnormal high TTR levels.

A vitamin for the brain

In the central nervous system (CNS) the function of retinoic acid, the active metabolite of vitamin A, is best understood from its action in guiding embryonic development; as development comes to completion, retinoic acid signaling declines. However, it is increasingly recognized that this signaling mechanism does not disappear in the adult brain but becomes more regionally focused and takes on new roles. These functions are often tied to processes of neural plasticity whether in the hippocampus, through homeostatic neural plasticity, the olfactory bulb or the hypothalamus. The role of retinoic acid in the control of plastic processes has led to suggestions of its involvement in neural disorders, both degenerative and psychiatric. This review presents a snapshot of developments in these areas over recent years.

Human placental transthyretin in fetal growth restriction in combination with preeclampsia and the HELLP syndrome

Fetal growth restriction is a serious, still poorly understood pregnancy-related pathology often associated with preeclampsia. Recent studies speculate on the role of human transthyretin, a carrier protein for thyroxin and retinol binding protein, in the etiology of both pregnancy pathologies. Objective was to investigate the localization and abundance of transthyretin (TTR) in placentas of pregnancies suffering from fetal growth restriction with and without preeclampsia and HELLP. This was a retrospective case control study on human paraffin-embedded placentas from pregnancies with a gestational age at delivery between the 24th and 34th week of gestation. 16 placentas were included in this study, 11 cases and 5 from normotensive pregnancies as controls. Cases were divided into three groups: four from early onset idiopathic intrauterine growth restriction (IUGR), four from early-onset severe preeclampsia (PE), and three from early-onset IUGR with preeclampsia plus HELLP syndrome. Distribution and abundance of TTR were investigated by means of immunohistochemistry. Semi quantitative analysis of TTR staining of placental sections revealed that TTR was mostly expressed in the villous trophoblast covering placental villi. Only weak staining of TTR in villous stroma could be detected. The comparison of placentas revealed that in pure IUGR and severe PE there is a much stronger TTR reactivity compared to controls and cases with IUGR + PE + HELLP. Concluding, the study showed that TTR is dysregulated in cases of IUGR and severe early onset preeclampsia. Interestingly, TTR expression is not affected in cases with HELLP syndrome that reveal the same staining intensities as age-matched controls.

Proteomic evaluation of sheep serum proteins

BACKGROUND

The applications of proteomic strategies to ovine medicine remain limited. The definition of serum proteome may be a good tool to identify useful protein biomarkers for recognising sub-clinical conditions and overt disease in sheep. Findings from bovine species are often directly translated for use in ovine medicine. In order to characterize normal protein patterns and improve knowledge of molecular species-specific characteristics, we generated a two-dimensional reference map of sheep serum. The possible application of this approach was tested by analysing serum protein patterns in ewes with mild broncho-pulmonary disease, which is very common in sheep and in the peripartum period which is a stressful time, with a high incidence of infectious and parasitic diseases.

RESULTS

This study generated the first reference 2-DE maps of sheep serum. Overall, 250 protein spots were analyzed, and 138 identified.Compared with healthy sheep, serum protein profiles of animals with rhino-tracheo-bronchitis showed a significant decrease in protein spots identified as transthyretin, apolipoprotein A1 and a significant increase in spots identified as haptoglobin, endopin 1b and alpha1B glycoprotein.In the peripartum period, haptoglobin, alpha-1-acid glycoprotein, apolipoprotein A1 levels rose, while transthyretin content dropped.

CONCLUSIONS

This study describes applications of proteomics in putative biomarker discovery for early diagnosis as well as for monitoring the physiological and metabolic situations critical for ovine welfare.

Specific disruption of transthyretin(105-115) fibrilization using "stabilizing" inhibitors of transthyretin amyloidogenesis

Transthyretin (TTR) is a cerebrospinal fluid and serum protein that undergoes ordered aggregation (amyloidogenesis) in familial amyloidotic polyneuropathy (FAP) and senile systemic amyloidosis (SSA). It is now widely accepted that dissociation of the native TTR tetramer is a precondition for amyloidogenesis; thus, molecules that stabilize the tetramer have received much attention as potential TTR amyloidosis inhibitors. Many of these inhibitors bind to the thyroxine (T(4)) binding pocket and interact specifically with a section of the TTR sequence, corresponding to residues 105-115, that is implicated in amyloidogenic propensity. In this work, we study the effects of "stabilizing" inhibitors on ordered aggregation of TTR(105-115) peptide. We show that molecules known to bind full-length TTR at the T(4) site are potent, specific inhibitors of ordered aggregation, while molecules that do not interact with TTR exhibit milder, nonspecific disruption through a "hyperbundling" effect. Our results suggest that, in addition to annealing the native tetramer, "stabilizing" inhibitors may also directly disrupt amyloidogenic aggregation of TTR monomers through specific interactions with the exposed TTR(105-115) sequence.

Bioconcentration and metabolism of decabromodiphenyl ether (BDE-209) result in thyroid endocrine disruption in zebrafish larvae

Polybrominated diphenyl ethers (PBDEs) have the potential to disturb the thyroid endocrine system, but little is known of such effects or underlying mechanisms of BDE-209 in fish. In the present study, bioconcentration and metabolism of BDE-209 were investigated in zebrafish embryos exposed at concentrations of 0, 0.08, 0.38 and 1.92 mg/L in water until 14 days post-fertilization (dpf). Chemical analysis revealed that BDE-209 was accumulated in zebrafish larvae, while also metabolic products were detected, including octa- and nona-BDEs, with nona-BDEs being predominant. The exposure resulted in alterations of both triiodothyronine (T3) and thyroxine (T4) levels, indicating thyroid endocrine disruption. Gene transcription in the hypothalamic-pituitary-thyroid (HPT) axis was further examined, and the results showed that the genes encoding corticotrophin-releasing hormone (CRH) and thyroid-stimulating hormone (TSHβ) were transcriptionally significantly up-regulated. Genes involved in thyroid development (Pax8 and Nkx2.1) and synthesis (sodium/iodide symporter, NIS, thyroglobulin, TG) were also transcriptionally up-regulated. Up-regulation of mRNA for thyronine deiodinase (Dio1 and Dio2) and thyroid hormone receptors (TRα and TRβ) was also observed. However, the genes encoding proteins involved in TH transport (transthyretin, TTR) and metabolism (uridinediphosphate-glucuronosyl-transferase, UGT1ab) were transcriptionally significantly down-regulated. Furthermore, protein synthesis of TG was significantly up-regulated, while that of TTR was significantly reduced. These results suggest that the hypothalamic-pituitary-thyroid axis can be evaluated to determine thyroid endocrine disruption by BDE-209 in developing zebrafish larvae.

Aged vervet monkeys developing transthyretin amyloidosis with the human disease-causing Ile122 allele: a valid pathological model of the human disease

Mutant forms of transthyretin (TTR) cause the most common type of autosomal-dominant hereditary systemic amyloidosis. In addition, wild-type TTR causes senile systemic amyloidosis, a sporadic disease seen in the elderly. Although spontaneous development of TTR amyloidosis had not been reported in animals other than humans, we recently determined that two aged vervet monkeys (Chlorocebus pygerythrus) spontaneously developed systemic TTR amyloidosis. In this study here, we first determined that aged vervet monkeys developed TTR amyloidosis and showed cardiac dysfunction but other primates did not. We also found that vervet monkeys had the TTR Ile122 allele, which is well known as a frequent mutation-causing human TTR amyloidosis. Furthermore, we generated recombinant monkey TTRs and determined that the vervet monkey TTR had lower tetrameric stability and formed more amyloid fibrils than did cynomolgus monkey TTR, which had the Val122 allele. We thus propose that the Ile122 allele has an important role in TTR amyloidosis in the aged vervet monkey and that this monkey can serve as a valid pathological model of the human disease. Finally, from the viewpoint of molecular evolution of TTR in primates, we determined that human TTR mutations causing the leptomeningeal phenotype of TTR amyloidosis tended to occur in amino acid residues that showed no diversity throughout primate evolution. Those findings may be valuable for understanding the genotype-phenotype correlation in this inherited human disease.

Induction of immediate early genes in the mouse auditory cortex after auditory cued fear conditioning to complex sounds

Immediate early genes (IEGs) are widely used as markers to delineate neuronal circuits because they show fast and transient expression induced by various behavioral paradigms. In this study, we investigated the expression of the IEGs c-fos and Arc in the auditory cortex of the mouse after auditory cued fear conditioning using quantitative polymerase chain reaction and microarray analysis. To test for the specificity of the IEG induction, we included several control groups that allowed us to test for factors other than associative learning to sounds that could lead to an induction of IEGs. We found that both c-fos and Arc showed strong and robust induction after auditory fear conditioning. However, we also observed increased expression of both genes in any control paradigm that involved shocks, even when no sounds were presented. Using mRNA microarrays and comparing the effect of the various behavioral paradigms on mRNA expression levels, we did not find genes being selectively upregulated in the auditory fear conditioned group. In summary, our results indicate that the use of IEGs to identify neuronal circuits involved specifically in processing of sound cues in the fear conditioning paradigm can be limited by the effects of the aversive unconditional stimulus and that activity levels in a particular primary sensory cortical area can be strongly influenced by stimuli mediated by other modalities.

Signaling by vitamin A and retinol-binding protein in regulation of insulin responses and lipid homeostasis

Vitamin A, retinol, circulates in blood bound to serum retinol binding protein (RBP) and is transported into cells by a membrane protein termed stimulated by retinoic acid 6 (STRA6). It was reported that serum levels of RBP are elevated in obese rodents and humans, and that increased level of RBP in blood causes insulin resistance. A molecular mechanism by which RBP can exert such an effect is suggested by the recent discovery that STRA6 is not only a vitamin A transporter but also functions as a surface signaling receptor. Binding of RBP-ROH to STRA6 induces the phosphorylation of a tyrosine residue in the receptor C-terminus, thereby activating a JAK/STAT signaling cascade. Consequently, in STRA6-expressing cells such as adipocytes, RBP-ROH induces the expression of STAT target genes, including SOCS3, which suppresses insulin signaling, and PPARγ, which enhances lipid accumulation. RBP-retinol thus joins the myriad of cytokines, growth factors and hormones which regulate gene transcription by activating cell surface receptors that signal through activation of Janus kinases and their associated transcription factors STATs. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

The relative amounts of plasma transthyretin forms in familial transthyretin amyloidosis: a quantitative analysis by Fourier transform ion-cyclotron resonance mass spectrometry

Familial transthyretin amyloidosis (ATTR) is a fatal autosomal dominant disease characterized by the formation of amyloid fibers, mainly composed of transthyretin (TTR). Protein aggregation and amyloid fiber formation are considered concentration dependent processes and since most ATTR patients are heterozygous it is crucial to determine the ratio between mutant and non-mutant TTR forms in human plasma. Using a high resolution mass spectrometry based approach we determined the ratio of TTR forms in ATTR patients, V30M mutation carriers, symptomatic and asymptomatic ones, as well as ATTR patients that received a wild type cadaveric liver transplant. Domino transplanted patients that received a liver from an ATTR patient were also investigated. We found that although wild type TTR is diminished in the plasma of non-transplanted ATTR patients comparatively to healthy subjects, the relationship with the V30M variant does not change with illness progression. Those who received a wild type liver showed no mutant protein while domino transplanted patients presented the same relative amount of V30M as found in asymptomatic and symptomatic individuals. The V30M to wild type TTR ratio in plasma is the same for all ATTR patients studied, showing no variation with disease clinical progression. Our results point to the involvement of additional non-genetic factors on the pathogenesis of this disease.

Transthyretin and the brain re-visited: is neuronal synthesis of transthyretin protective in Alzheimer's disease?

Since the mid-1990's a trickle of publications from scattered independent laboratories have presented data suggesting that the systemic amyloid precursor transthyretin (TTR) could interact with the amyloidogenic β-amyloid (Aβ) peptide of Alzheimer's disease (AD). The notion that one amyloid precursor could actually inhibit amyloid fibril formation by another seemed quite far-fetched. Further it seemed clear that within the CNS, TTR was only produced in choroid plexus epithelial cells, not in neurons. The most enthusiastic of the authors proclaimed that TTR sequestered Aβ in vivo resulting in a lowered TTR level in the cerebrospinal fluid (CSF) of AD patients and that the relationship was salutary. More circumspect investigators merely showed in vitro interaction between the two molecules. A single in vivo study in Caenorhabditis elegans suggested that wild type human TTR could suppress the abnormalities seen when Aβ was expressed in the muscle cells of the worm. Subsequent studies in human Aβ transgenic mice, including those from our laboratory, also suggested that the interaction reduced the Aβ deposition phenotype. We have reviewed the literature analyzing the relationship including recent data examining potential mechanisms that could explain the effect. We have proposed a model which is consistent with most of the published data and current notions of AD pathogenesis and can serve as a hypothesis which can be tested.

Neuronal production of transthyretin in human and murine Alzheimer's disease: is it protective?

Transthyretin (TTR), a systemic amyloid precursor in the human TTR amyloidoses, interacts with β-amyloid (Aβ) in vitro, inhibits Aβ fibril formation, and suppresses the Alzheimer's disease (AD) phenotype in APP23 mice bearing a human APP gene containing the Swedish autosomal dominant AD mutation. In the present study, we show that TTR is a neuronal product upregulated in AD. Immunohistochemical analysis reveals that, in contrast to brains from non-demented age-matched individuals and control mice, the majority of hippocampal neurons from human AD and all those from the APP23 mouse brains contain TTR. Quantitative PCR for TTR mRNA and Western blot analysis show that primary neurons from APP23 mice transcribe TTR mRNA, and the cells synthesize and secrete TTR protein. TTR mRNA abundance is greatly increased in cultured cortical and hippocampal embryonic neurons and cortical lysates from adult APP23 mice. Antibodies specific for TTR and Aβ pulled down TTR/Aβ complexes from cerebral cortical extracts of APP23 mice and some human AD patients but not from control brains. In complementary tissue culture experiments, recombinant human TTR suppressed the cytotoxicity of soluble Aβ aggregates added to mouse neurons and differentiated human SH-SY5Y neuroblastoma cells. The findings that production of Aβ, its precursor, or its related peptides induces neuronal TTR transcription and synthesis and the presence of Aβ/TTR complexes in vivo suggest that increased TTR production coupled with interaction between TTR and Aβ and/or its related peptides may play a role in natural resistance to human AD.

Mass spectrometric immunoassay for quantitative determination of transthyretin and its variants

Transthyretin (TTR, or prealbumin) is a tetrameric protein found in plasma and cerebrospinal fluid. Its major role is to transport thyroid hormones (thyroxin-T4) and retinol (through association with retinol-binding protein). TTR has been studied extensively due to the great number of point mutations that result in sequence heterogeneity. Many of these variants are associated with pathological conditions that result in extracellular deposition of amyloid fibers in tissues. In this work, we have developed a rapid mass spectrometric immunoassay for determination and quantification of TTR and its variants from human serum and plasma samples. The assay was fully characterized in terms of its precision, linearity and recovery characteristics. The new assay was also compared with a conventional TTR ELISA. Furthermore, we have applied the optimized method to analyze TTR and its modifications in 44 human plasma samples, and in the process optimized a method for TTR proteolytic digestion and identification of point mutations.

Mechanisms of transthyretin cardiomyocyte toxicity inhibition by resveratrol analogs

The transthyretin amyloidoses are a subset of protein misfolding diseases characterized by the extracellular deposition of aggregates derived from the plasma homotetrameric protein transthyretin (TTR) in peripheral nerves and the heart. We have established a robust disease-relevant human cardiac tissue culture system to explore the cytotoxic effects of amyloidogenic TTR variants. We have employed this cardiac amyloidosis tissue culture model to screen 23 resveratrol analogs as inhibitors of amyloidogenic TTR-induced cytotoxicity and to investigate their mechanisms of protection. Resveratrol and its analogs kinetically stabilize the native tetramer preventing the formation of cytotoxic species. In addition, we demonstrate that resveratrol can accelerate the formation of soluble non-toxic aggregates and that the resveratrol analogs tested can bring together monomeric TTR subunits to form non-toxic native tetrameric TTR.

Heparan sulfate/heparin promotes transthyretin fibrillization through selective binding to a basic motif in the protein

Transthyretin (TTR) is a homotetrameric protein that transports thyroxine and retinol. Tetramer destabilization and misfolding of the released monomers result in TTR aggregation, leading to its deposition as amyloid primarily in the heart and peripheral nervous system. Over 100 mutations of TTR have been linked to familial forms of TTR amyloidosis. Considerable effort has been devoted to the study of TTR aggregation of these mutants, although the majority of TTR-related amyloidosis is represented by sporadic cases due to the aggregation and deposition of the otherwise stable wild-type (WT) protein. Heparan sulfate (HS) has been found as a pertinent component in a number of amyloid deposits, suggesting its participation in amyloidogenesis. This study aimed to investigate possible roles of HS in TTR aggregation. Examination of heart tissue from an elderly cardiomyopathic patient revealed substantial accumulation of HS associated with the TTR amyloid deposits. Studies demonstrated that heparin/HS promoted TTR fibrillization through selective interaction with a basic motif of TTR. The importance of HS for TTR fibrillization was illustrated in a cell model; TTR incubated with WT Chinese hamster ovary cells resulted in fibrillization of the protein, but not with HS-deficient cells (pgsD-677). The effect of heparin on TTR fibril formation was further demonstrated in a Drosophila model that overexpresses TTR. Heparin was colocalized with TTR deposits in the head of the flies reared on heparin-supplemented medium, whereas no heparin was detected in the nontreated flies. Heparin of low molecular weight (Klexane) did not demonstrate this effect.

Sulfated glycosaminoglycans accelerate transthyretin amyloidogenesis by quaternary structural conversion

Glycosaminoglycans (GAGs), which are found in association with all extracellular amyloid deposits in humans, are known to accelerate the aggregation of various amyloidogenic proteins in vitro. However, the precise molecular mechanism(s) by which GAGs accelerate amyloidogenesis remains elusive. Herein, we show that sulfated GAGs, especially heparin, accelerate transthyretin (TTR) amyloidogenesis by quaternary structural conversion. The clustering of sulfate groups on heparin and its polymeric nature are essential features for accelerating TTR amyloidogenesis. Heparin does not influence TTR tetramer stability or TTR dissociation kinetics, nor does it alter the folded monomer-misfolded monomer equilibrium directly. Instead, heparin accelerates the conversion of preformed TTR oligomers into larger aggregates. The more rapid disappearance of monomeric TTR in the presence of heparin likely reflects the fact that the monomer-misfolded amyloidogenic monomer-oligomer-TTR fibril equilibria are all linked, a hypothesis that is strongly supported by the light scattering data. TTR aggregates prepared in the presence of heparin exhibit a higher resistance to trypsin and proteinase K proteolysis and a lower exposure of hydrophobic side chains comprising hydrophobic clusters, suggesting an active role for heparin in amyloidogenesis. Our data suggest that heparin accelerates TTR aggregation by a scaffold-based mechanism, in which the sulfate groups comprising GAGs interact primarily with TTR oligomers through electrostatic interactions, concentrating and orienting the oligomers, facilitating the formation of higher molecular weight aggregates. This model raises the possibility that GAGs may play a protective role in human amyloid diseases by interacting with proteotoxic oligomers and promoting their association into less toxic amyloid fibrils.

Identification and validation of novel cerebrospinal fluid biomarkers for staging early Alzheimer's disease

BACKGROUND

Ideally, disease modifying therapies for Alzheimer disease (AD) will be applied during the 'preclinical' stage (pathology present with cognition intact) before severe neuronal damage occurs, or upon recognizing very mild cognitive impairment. Developing and judiciously administering such therapies will require biomarker panels to identify early AD pathology, classify disease stage, monitor pathological progression, and predict cognitive decline. To discover such biomarkers, we measured AD-associated changes in the cerebrospinal fluid (CSF) proteome.

METHODS AND FINDINGS

CSF samples from individuals with mild AD (Clinical Dementia Rating [CDR] 1) (n = 24) and cognitively normal controls (CDR 0) (n = 24) were subjected to two-dimensional difference-in-gel electrophoresis. Within 119 differentially-abundant gel features, mass spectrometry (LC-MS/MS) identified 47 proteins. For validation, eleven proteins were re-evaluated by enzyme-linked immunosorbent assays (ELISA). Six of these assays (NrCAM, YKL-40, chromogranin A, carnosinase I, transthyretin, cystatin C) distinguished CDR 1 and CDR 0 groups and were subsequently applied (with tau, p-tau181 and Aβ42 ELISAs) to a larger independent cohort (n = 292) that included individuals with very mild dementia (CDR 0.5). Receiver-operating characteristic curve analyses using stepwise logistic regression yielded optimal biomarker combinations to distinguish CDR 0 from CDR>0 (tau, YKL-40, NrCAM) and CDR 1 from CDR<1 (tau, chromogranin A, carnosinase I) with areas under the curve of 0.90 (0.85-0.94 95% confidence interval [CI]) and 0.88 (0.81-0.94 CI), respectively.

CONCLUSIONS

Four novel CSF biomarkers for AD (NrCAM, YKL-40, chromogranin A, carnosinase I) can improve the diagnostic accuracy of Aβ42 and tau. Together, these six markers describe six clinicopathological stages from cognitive normalcy to mild dementia, including stages defined by increased risk of cognitive decline. Such a panel might improve clinical trial efficiency by guiding subject enrollment and monitoring disease progression. Further studies will be required to validate this panel and evaluate its potential for distinguishing AD from other dementing conditions.