In vitro and in vivo studies on transthyretin oligomerization

The Journey of Human Transthyretin: Synthesis, Structure Stability, and Catabolism

Transthyretin (TTR) is a homotetrameric protein mainly synthesised by the liver and the choroid plexus whose function is to carry the thyroid hormone thyroxine and the retinol-binding protein bound to retinol in plasma and cerebrospinal fluid. When the stability of the tetrameric structure is lost, it breaks down, paving the way for the aggregation of TTR monomers into insoluble fibrils leading to transthyretin (ATTR) amyloidosis, a progressive disorder mainly affecting the heart and nervous system. Several TTR gene mutations have been characterised as destabilisers of TTR structure and are associated with hereditary forms of ATTR amyloidosis. The reason why also the wild-type TTR is intrinsically amyloidogenic in some subjects is largely unknown. The aim of the review is to give an overview of the TTR biological life cycle which is largely unknown. For this purpose, the current knowledge on TTR physiological metabolism, from its synthesis to its catabolism, is described. Furthermore, a large section of the review is dedicated to examining in depth the role of mutations and physiological ligands on the stability of TTR tetramers.

TGFβ Impairs HNF1α Functional Activity in Epithelial-to-Mesenchymal Transition Interfering With the Recruitment of CBP/p300 Acetyltransferases

The cytokine transforming growth factor β (TGFβ) plays a crucial role in the induction of both epithelial-to-mesenchymal transition (EMT) program and fibro-cirrhotic process in the liver, where it contributes also to organ inflammation following several chronic injuries. All these pathological situations greatly increase the risk of hepatocellular carcinoma (HCC) and contribute to tumor progression. In particular, late-stage HCCs are characterized by constitutive activation of TGFβ pathway and by an EMT molecular signature leading to the acquisition of invasive and metastatic properties. In these pathological conditions, the cytokine has been shown to induce the transcriptional downregulation of HNF1α, a master regulator of the epithelial/hepatocyte differentiation and of the EMT reverse process, the mesenchymal-to-epithelial transition (MET). Therefore, the restoration of HNF1α expression/activity has been proposed as targeted therapeutic strategy for liver fibro-cirrhosis and late-stage HCCs. In this study, TGFβ is found to trigger an early functional inactivation of HNF1α during EMT process that anticipates the effects of the transcriptional downregulation of its own gene. Mechanistically, the cytokine, while not affecting the HNF1α DNA-binding capacity, impaired its ability to recruit CBP/p300 acetyltransferases on target gene promoters and, consequently, its transactivating function. The loss of HNF1α capacity to bind to CBP/p300 and HNF1α functional inactivation have been found to correlate with a change of its posttranslational modification profile. Collectively, the results obtained in this work unveil a new level of HNF1α functional inactivation by TGFβ and contribute to shed light on the early events triggering EMT in hepatocytes. Moreover, these data suggest that the use of HNF1α as anti-EMT tool in a TGFβ-containing microenvironment may require the design of new therapeutic strategies overcoming the TGFβ-induced HNF1α inactivation.

TGFbeta Induces Binucleation/Polyploidization in Hepatocytes through a Src-Dependent Cytokinesis Failure

In all mammals, the adult liver shows binucleated as well as mononucleated polyploid hepatocytes. The hepatic polyploidization starts after birth with an extensive hepatocyte binucleation and generates hepatocytes of several ploidy classes. While the functional significance of hepatocyte polyploidy is becoming clearer, how it is triggered and maintained needs to be clarified. Aim of this study was to identify a major inducer of hepatocyte binucleation/polyploidization and the cellular and molecular mechanisms involved. We found that, among several cytokines analyzed, known to be involved in early liver development and/or mass control, TGFbeta1 was capable to induce, together with the expected morphological changes, binucleation in hepatocytes in culture. Most importantly, the pharmacological inhibition of TGFbeta signaling in healthy mice during weaning, when the physiological binucleation occurs, induced a significant decrease of hepatocyte binucleation rate, without affecting cell proliferation and hepatic index. The TGFbeta-induced hepatocyte binucleation resulted from a cytokinesis failure, as assessed by video microscopy, and is associated with a delocalization of the cytokinesis regulator RhoA-GTPase from the mid-body of dividing cells. The use of specific chemical inhibitors demonstrated that the observed events are Src-dependent. Finally, the restoration of a fully epithelial phenotype by TGFbeta withdrawal gave rise to a cell progeny capable to maintain the polyploid state. In conclusion, we identified TGFbeta as a major inducer of hepatocyte binucleation both in vitro and in vivo, thus ascribing a novel role to this pleiotropic cytokine. The production of binucleated/tetraploid hepatocytes is due to a cytokinesis failure controlled by the molecular axis TGFbeta/Src/RhoA.

Enhanced transthyretin tetramer stability following expression of an amyloid disease transsuppressor variant in mammalian cells

BACKGROUND

The transthyretin (TTR) amyloidosis is an incurable fatal inherited disease that is characterized by progressive peripheral and autonomic neuropathy. It is caused by missense amyloidogenic mutations in the TTR gene that destabilize the native tetrameric state and lead to the cytotoxic misfolded monomeric state. One interesting variant (T119M) stabilizes heterotetramers with amyloidogenic TTR and, in the reported heterozygous individuals, protects the carriers from disease. In the present study, we characterize in vitro and in vivo the ectopic expression of the human T119M mutant, termed a transsuppressor for TTR amyloid disease.

METHODS

Lentiviral vectors encoding wild or mutant forms of human TTR were constructed and transduced to the human hepatocellular carcinoma cell line, HepG2, or mice. Heterooligomerization between T119M TTR and amyloidogenic variants was analysed by immunoprecipitation following western blotting.

RESULTS

T119M TTR was stably expressed in transduced HepG2 cells and was secreted as an oligomer that can interact with its native partner, retinol-binding protein. Importantly, the T119M TTR formed secreted heterooligomers with amyloidogenic TTR variants, V30M, L55P and V122I, in HepG2 cells that were more stable than the homooligomers of the same amyloidogenic TTR variants. Human T119M TTR also formed heterooligomers with V30M TTR in transduced mice.

CONCLUSIONS

The results obtained in the present study demonstrate the stabilization of heterotetramers by T119M TTR in human cells and suggest that gene transfer of T119M TTR may have potential as a gene therapy for TTR amyloidosis.

Role of STAT3 in liver regeneration: survival, DNA synthesis, inflammatory reaction and liver mass recovery

The hepatoprotective effect of interleukin-6 (IL-6)/signal transducer and activator of transcription 3 (STAT3) has been well documented. However, reports on the role of IL-6/STAT3 in liver regeneration are conflicting probably due to the fact that the model of Stat3 knockout mice were complicated with obesity and fatty liver, which may cause some secondary effects on liver regeneration. To study the direct role of STAT3 and to circumvent the problems of obesity and fatty liver in liver regeneration, we generated conditional STAT3 knockout in the liver (L-Stat3(-/-)) using a transthyretin-driven Cre-lox method. The L-Stat3(-/-) mice were born with the expected Mendelian frequency and showed no obesity or other obvious phenotype. After partial hepatectomy, mortality in the L-Stat3(-/-) mice was significantly higher than the littermate Stat3(f/+) controls in the early time points (<24 h). Hepatocyte DNA synthesis in the survived L-Stat3(-/-) mice slightly decreased as compared with Stat3(f/+) mice at 40 h after partial hepatectomy, whereas similar hepatocyte DNA synthesis was found at other time points and liver mass could be completely recovered in the L-Stat3(-/-) mice. In another model of liver regeneration induced by subcutaneous injection of carbon tetrachloride (CCl(4)), hepatocyte DNA synthesis in the CCl(4)-treated L-Stat3(-/-) mice also decreased as compared with Stat3(f/+) mice at 40 h after injection but not at other time points. In addition, infiltration of neutrophils and monocyte increased in the liver of CCl(4)-treated L-Stat3(-/-) mice compared to wild-type mice. In conclusion, STAT3 is required for survival in the acute stage after 70% hepatectomy and plays a role in inflammatory reaction after hepatocyte necrosis. However, the hepatocytic STAT3 may have limited role in liver mass recovery although DNA synthesis may be impaired.

Synthesis of transthyretin by the ependymal cells of the subcommissural organ

Transthyretin (TTR) is a protein involved in the transport of thyroid hormones in blood and cerebrospinal fluid (CSF). The only known source of brain-produced TTR is the choroid plexus. In the present investigation, we have identified the subcommissural organ (SCO) as a new source of brain TTR. The SCO is an ependymal gland that secretes glycoproteins into the CSF, where they aggregate to form Reissner's fibre (RF). Evidence exists that the SCO also secretes proteins that remain soluble in the CSF. To investigate the CSF-soluble compounds secreted by the SCO further, antibodies were raised against polypeptides partially purified from fetal bovine CSF. One of these antibodies (against a 14-kDa compound) reacted with secretory granules in cells of fetal and adult bovine SCO, organ-cultured bovine SCO and the choroid plexus of several mammalian species but not with RF. Western blot analyses with this antibody revealed two polypeptides of 14 kDa and 40 kDa in the bovine SCO, in the conditioned medium of SCO explants, and in fetal and adult bovine CSF. Since the monomeric and tetrameric forms of TTR migrate as bands of 14 kDa and 40 kDa by SDS-polyacrylamide gel electrophoresis, a commercial preparation of human TTR was run, with both bands being reactive with this antibody. Bovine SCO was also shown to synthesise mRNA encoding TTR under in vivo and in vitro conditions. We conclude that the SCO synthesises TTR and secretes it into the CSF. Colocalisation studies demonstrated that the SCO possessed two populations of secretory cells, one secreting both RF glycoproteins and TTR and the other secreting only the former. TTR was also detected in the SCO of bovine embryos suggesting that this ependymal gland is an important source of TTR during brain development.

Hepatic synthesis, maturation and complex formation between retinol-binding protein and transthyretin

The retinol/retinol-binding protein/transthyretin complex, that carries and delivers hydrophobic retinol molecules to target cells, is assembled in the hepatocyte endoplasmic reticulum. In this paper, we review data related to events that lead to the formation of this complex, including transthyretin oligomerization and retinol-binding protein secretion. Our studies on transthyretin oligomerization have demonstrated that cleavage of signal peptide and the environment of endoplasmic reticulum influence transthyretin oligomerization. In vitro, mutated transthyretin without signal sequence fails to form dimers, while wild-type transthyretin is translocated into the microsomes where it forms dimers and small amounts of tetramers. In vivo, tetramers were detected in HepG2 cells but not in transfected Cos cells, suggesting that tissue-specific factors affect tetramer stability. In vitamin A deficiency, retinol-binding protein secretion is blocked and the protein accumulates in the endoplasmic reticulum, from where it is promptly released after retinol repletion. We use MMH cells to identify factors involved in complex formation, retention and secretion, the crucial steps to understand the molecular mechanisms underlying vitamin A homeostasis. In parallel, studies on vitamin A transport in fish are in progress; retinol-binding protein and transthyretin have already been characterized in different fish species.

Evolution of structure, ontogeny of gene expression, and function of Xenopus laevis transthyretin

Xenopus laevis transthyretin (xTTR) cDNA was cloned and sequenced. The derived amino acid sequence was very similar to those of other vertebrate transthyretins (TTR). TTR gene expression was observed during metamorphosis in X. laevis tadpole liver but not in tadpole brain nor adult liver. Recombinant xTTR was synthesized in Pichia pastoris and identified by amino acid sequence, subunit molecular mass, tetramer formation, and binding to retinol-binding protein. Contrary to mammalian xTTRs, the affinity of xTTR was higher for L-triiodothyronine than for L-thyroxine. The regions of the TTR genes coding for the NH(2)-terminal sections of the polypeptide chains of TTR seem to have evolved by stepwise shifts of mRNA splicing sites between exons 1 and 2, resulting in shorter and more hydrophilic NH(2) termini. This may be one molecular mechanism of positive Darwinian evolution. Open reading frames with xTTR-like sequences in the genomes of C. elegans and several microorganisms suggested evolution of the TTR gene from ancestor TTR gene-like "DNA modules." Increasing preference for binding of L-thyroxine over L-triiodothyronine may be associated with evolving tissue-specific regulation of thyroid hormone action by deiodination.

MMH cells: An in vitro model for the study of retinol-binding protein secretion regulated by retinol

The untransformed stable cell line Met murine hepatocytes (MMH), generated from liver explants of transgenic mice expressing a constitutively active truncated form of the human hepatocyte growth factor receptor (cyto-Met), represents an innovative tool for in vitro studies of liver function. In the present report, we show that the MMH-D3 line isolated from the liver of a 3-day-old mouse is a useful model to investigate the regulation of the synthesis and secretion of retinol-binding protein (RBP) by retinol (vitamin A alcohol). Experiments with Northern blot hybridization, metabolic labeling of cellular proteins followed by immunoprecipitation, and Western blot analysis demonstrated that, similarly to the in vivo situation, in MMH-D3 cells the presence of retinol does not affect transcriptional and translational rate of the RBP gene but is essential for regulating the secretion rate of the protein. Unlike HepG2 human hepatocarcinoma cells used thus far in studies of retinoid metabolism, including the synthesis and secretion of RBP, vitamin A deficiency causes, in MMH-D3 cells, the inhibiton of RBP secretion and the protein accumulation in the cell, whereas retinol repletion promptly results in RBP secretion. This model will be very useful in future studies on vitamin A distribution in the organism.