Molecular and cellular studies of tryptophanyl-tRNA synthetases using monoclonal antibodies. Remarkable variations in the content of tryptophanyl-tRNA synthetase in the pancreas of different mammals

Tryptophanyl-tRNA Synthetase as a Potential Therapeutic Target

Tryptophanyl-tRNA synthetase (WRS) is an essential enzyme that catalyzes the ligation of tryptophan (Trp) to its cognate tRNA^trp during translation via aminoacylation. Interestingly, WRS also plays physiopathological roles in diseases including sepsis, cancer, and autoimmune and brain diseases and has potential as a pharmacological target and therapeutic. However, WRS is still generally regarded simply as an enzyme that produces Trp in polypeptides; therefore, studies of the pharmacological effects, therapeutic targets, and mechanisms of action of WRS are still at an emerging stage. This review summarizes the involvement of WRS in human diseases. We hope that this will encourage further investigation into WRS as a potential target for drug development in various pathological states including infection, tumorigenesis, and autoimmune and brain diseases.

Non-canonical functions of human cytoplasmic tyrosyl-, tryptophanyl- and other aminoacyl-tRNA synthetases

Aminoacyl-tRNA synthetases catalyze the aminoacylation of their cognate tRNAs. Here we review the accumulated knowledge of non-canonical functions of human cytoplasmic aminoacyl-tRNA synthetases, especially tyrosyl- (TyrRS) and tryptophanyl-tRNA synthetase (TrpRS). Human TyrRS and TrpRS have an extra domain. Two distinct cytokines, i.e., the core catalytic "mini TyrRS" and the extra C-domain, are generated from human TyrRS by proteolytic cleavage. Moreover, the core catalytic domains of human TyrRS and TrpRS function as angiogenic and angiostatic factors, respectively, whereas the full-length forms are inactive for this function. It is also known that many synthetases change their localization in response to a specific signal and subsequently exhibit alternative functions. Furthermore, some synthetases function as sensors for amino acids by changing their protein interactions in an amino acid-dependent manner. Further studies will be necessary to elucidate regulatory mechanisms of non-canonical functions of aminoacyl-tRNA synthetases in particular, by analyzing the effect of their post-translational modifications.

Identification of a residue crucial for the angiostatic activity of human mini tryptophanyl-tRNA synthetase by focusing on its molecular evolution

Human tryptophanyl-tRNA synthetase (TrpRS) exists in two forms: a full-length TrpRS and a mini TrpRS. We previously found that human mini, but not full-length, TrpRS is an angiostatic factor. Moreover, it was shown that the interaction between mini TrpRS and the extracellular domain of vascular endothelial (VE)-cadherin is crucial for its angiostatic activity. However, the molecular mechanism of the angiostatic activity of human mini TrpRS is only partly understood. In the present study, we investigated the effects of truncated (mini) form of TrpRS proteins from human, bovine, or zebrafish on vascular endothelial growth factor (VEGF)-stimulated chemotaxis of human umbilical vein endothelial cells (HUVECs). We show that both human and bovine mini TrpRSs inhibited VEGF-induced endothelial migration, whereas zebrafish mini TrpRS did not. Next, to identify residues crucial for the angiostatic activity of human mini TrpRS, we prepared several site-directed mutants based on amino acid sequence alignments among TrpRSs from various species and demonstrated that a human mini K153Q TrpRS mutant cannot inhibit VEGF-stimulated HUVEC migration and cannot bind to the extracellular domain of VE-cadherin. Taken together, we conclude that the Lys153 residue of human mini TrpRS is a VE-cadherin binding site and is therefore crucial for its angiostatic activity.

Autoantibodies against aminoacyl-tRNA synthetase: novel diagnostic marker for type 1 diabetes mellitus

OBJECTIVES

To investigate whether or not antiaminoacyl-tRNA synthetase (aaRS) autoantibodies could be detected in patients with type 1 diabetes mellitus (DM) and be used as a diagnostic marker for type 1 DM, autoantibodies against aaRSs were measured in the plasma of normal subjects, patients with type 1 DM and patients with type 2 DM.

METHODS

An enzyme-linked immunosorbent assay was performed to detect anti-aaRS autoantibodies in the plasma of normal subjects, and patients with type 1 DM, and patients with type 2 DM.

RESULTS

From the 65 (normal), 58 (type 1 DM) and 57 (type 2 DM) subjects, anti-aaRS autoantibodies were found in 37.9% of patients with type 1 DM compared with 1.54% of the non-diabetic controls, and 5.26% of the patients with type 2 DM (p <0.0001). In addition, anti-aaRS autoantibodies were identified in 30% of patients with type 1 DM without classical type 1 DM autoantibodies.

CONCLUSION

Anti-aaRS autoantibodies were identified in 37.9% of patients with type 1 DM. The results of this study demonstrate for the first time that autoantibodies against aaRSs are specifically associated with type 1 DM.

Tryptamine induces tryptophanyl-tRNA synthetase-mediated neurodegeneration with neurofibrillary tangles in human cell and mouse models

The neuropathological hallmarks of Alzheimer's disease (AD) and other taupathies include neurofibrillary tangles and plaques. Despite the fact that only 2-10% of AD cases are associated with genetic mutations, no nontransgenic or metabolic models have been generated to date. The findings of tryptophanyl-tRNA synthetase (TrpRS) in plaques of the AD brain were reported recently by the authors. Here it is shown that expression of cytoplasmic-TrpRS is inversely correlated with neurofibrillary degeneration, whereas a nonionic detergent-insoluble presumably aggregated TrpRS is simultaneously accumulated in human cells treated by tryptamine, a metabolic tryptophan analog that acts as a competitive inhibitor of TrpRS. TrpRSN- terminal peptide self-assembles in double-helical fibrils in vitro. Herein, tryptamine causes neuropathy characterized by motor and behavioral deficits, hippocampal neuronal loss, neurofibrillary tangles, amyloidosis, and glucose decrease in mice. Tryptamine induced the formation of helical fibrillary tangles in both hippocampal neurons and glia. Taken together with the authors' previous findings of tryptamine-induced nephrotoxicity and filamentous tangle formation in kidney cells, the authors' data indicates a general role of tryptamine in cell degeneration and loss. It is concluded that tryptamine as a component of a normal diet can induce neurodegeneration at the concentrations, which might be consumed along with food. Tryptophan-dependent tRNAtrp aminoacylation catalyzed by TrpRS can be inhibited by its substrate tryptophan at physiological concentrations was demonstrated. These findings indicate that the dietary supplementation with tryptophan as a tryptamine competitor may not counteract the deleterious influence of tryptamine. The pivotal role of TrpRS in protecting against neurodegeneration is suggested, providing an insight into the pathogenesis and a possible treatment of neurodegenerative diseases.

Hormonal activity of AIMP1/p43 for glucose homeostasis

AIMP1/p43 is known as a cytokine working in the control of angiogenesis, inflammation, and wound healing. Here we report its enrichment in pancreatic alpha cells and glucagon-like hormonal activity. AIMP1 is secreted from the pancreas upon glucose starvation. Exogenous infusion of AIMP1 increased plasma levels of glucose, glucagon, and fatty acid, and AIMP1-deficient mice showed reduced plasma glucose levels compared with the wild-type mice under fasting conditions. Thus, AIMP1 plays a glucagon-like role in glucose homeostasis.

RNA-protein interactions at the initial and terminal stages of protein biosynthesis as investigated by Lev Kisselev (on the occasion of his 70th anniversary)

This review highlights studies by Lev L. Kisselev and his colleagues on the initial and terminal stages of protein biosynthesis, which cover the period of the last 45 years (1961-2006). They investigated spatial structure of tRNAs, structure and functions of aminoacyl-tRNA-synthetases of higher organisms, and the final step of protein synthesis, termination of translation. L. Kisselev and his team have made three major contributions to these fields of molecular biology; (i) they proposed the hypothesis on the role of anticodon triplet of tRNA in recognition by cognate aminoacyl-tRNA synthetase, which has been experimentally confirmed and is now included in textbooks; (ii) identified primary structures and functions of two eukaryotic protein factors (eRF1 and eRF3) playing a pivotal role in translation termination; (iii) characterized a structural basis for stop codon recognition by eRF1 within the ribosome and discovered the negative structural elements of eRF1, limiting its recognition of one or two stop-codons.

Mapping and molecular characterization of novel monoclonal antibodies to conformational epitopes on NH2 and COOH termini of mammalian tryptophanyl-tRNA synthetase reveal link of the epitopes to aggregation and Alzheimer's disease

Tryptophanyl-tRNA synthetase (TrpRS) is an interferon-induced phosphoprotein with autoantigenic and cytokine activities detected in addition to its canonical function in tRNA aminoacylation. The availability of monoclonal antibodies (mAbs) specific for TrpRS is important for development of tools for TrpRS monitoring. A molecular characterization of two mAbs raised in mice, using purified, enzymatically active bovine TrpRS as the inoculating antigen, is presented in this report. These IgG1 antibodies are specific for bovine, human and rabbit but not E. coli TrpRS. Immunoreactivity and specificity of mAbs were verified with purified recombinant hTrpRS expressed in E. coli and TrpRS-derived synthetic peptides. One of the mAbs, 9D7 is able to disaggregate fibrils formed by Ser32-Tyr50 TrpRS-peptide. Epitope mapping revealed that disaggregation ability correlates with binding of 9D7 to this peptide in ELISA and immunocytochemistry. This epitope covers a significant part of N-terminal extension that suggested to be proteolytically deleted in vivo from the full-length TrpRS whereas remaining COOH-fragment possesses a cytokine activity. For epitope mapping of mAb 6C10, the affinity selected phage-displayed peptides were used as a database for prediction of conformational discontinuous epitopes within hTrpRS crystal structure. Using computer algorithm, this epitope is attributed to COOH-terminal residues Asp409-Met425. In immunoblotting, the 6C10 mAb reacts preferably with (i) oligomer than monomer, and (ii) bound than free TrpRS forms. The hTrpRS expression was shown to correlate with growth rates of neuroblastoma and pancreatic cancer cells. Immunohistochemically both mAbs revealed extracellular plaque-like aggregates in hippocampus of Alzheimer's disease brain.

A new gamma-interferon-inducible promoter and splice variants of an anti-angiogenic human tRNA synthetase

Two forms of human tryptophanyl-tRNA synthetase (TrpRS) are produced in vivo through alternative mRNA splicing. The two forms, full-length TrpRS and mini TrpRS, are catalytically active, but are distinguished by the striking anti-proliferative and anti-angiogenic activity specific to mini TrpRS. Here we describe two new splice variants of human TrpRS mRNA. Their production was strongly regulated by gamma-interferon (IFN-gamma), an anti-proliferative cytokine known to stimulate the expression of other anti-angiogenic factors. A new IFN-gamma-sensitive promoter was demonstrated to drive production of these splice variants. In human endothelial cells, both the newly discovered and a previously reported promoter were shown to respond specifically to IFN-gamma and not to other cytokines such as tumor necrosis factor-alpha, transforming growth factor-beta, interleukin-4 or erythropoietin. In addition, both promoters were stimulated by the 'downstream' interferon regulatory factor 1 that, in turn, is known to be regulated by the 'upstream' signal transducer and activator of transcription 1alpha subunit. Thus, the tandem promoters provide a dual system to regulate expression and alternative splicing of human TrpRS in vivo.

A short peptide insertion crucial for angiostatic activity of human tryptophanyl-tRNA synthetase

Human tryptophanyl-tRNA synthetase (TrpRS) is secreted into the extracellular region of vascular endothelial cells. The splice variant form (mini TrpRS) functions in vascular endothelial cell apoptosis as an angiostatic cytokine. In contrast, the closely related human tyrosyl-tRNA synthetase (TyrRS) functions as an angiogenic cytokine in its truncated form (mini TyrRS). Here, we determined the crystal structure of human mini TrpRS at a resolution of 2.3 A and compared the structure with those of prokaryotic TrpRS and human mini TyrRS. Deletion of the tRNA anticodon-binding (TAB) domain insertion, consisting of eight residues in the human TrpRS, abolished the enzyme's apoptotic activity for endothelial cells, whereas its translational catalysis and cell-binding activities remained unchanged. Thus, we have identified the inserted peptide motif that activates the angiostatic signaling.

A human aminoacyl-tRNA synthetase as a regulator of angiogenesis

Aminoacyl-tRNA synthetases catalyze the first step of protein synthesis. It was shown recently that human tyrosyl-tRNA synthetase (TyrRS) can be split into two fragments having distinct cytokine activities, thereby linking protein synthesis to cytokine signaling pathways. Tryptophanyl-tRNA synthetase (TrpRS) is a close homologue of TyrRS. A natural fragment, herein designated as mini TrpRS, was shown by others to be produced by alternative splicing. Production of this fragment is reported to be stimulated by IFN-gamma, a cytokine that also stimulates production of angiostatic factors. Mini TrpRS is shown here to be angiostatic in a mammalian cell culture system, the chicken embryo, and two independent angiogenesis assays in the mouse. The full-length enzyme is inactive in the same assays. Thus, protein synthesis may be linked to the regulation of angiogenesis by a natural fragment of TrpRS.

A root-specific iron-regulated gene of tomato encodes a lysyl-tRNA-synthetase-like protein

The tomato mutant chloronerva exhibits a defect in iron-uptake regulation. Despite high apoplastic and symplastic iron concentrations, the mutant shows characteristic symptoms of iron deficiency. Using a subtractive-hybridisation approach, we have screened for cDNA clones specific for genes with altered expression in wild-type versus mutant root tissue. Based on this clone collection, we have isolated and characterised a 2075-bp full-length cDNA encoding a lysyl-tRNA-synthetase-like protein. The corresponding gene is localised as a single copy on chromosome 10. Its expression is strongly induced by changes in the iron status of the plant. This iron-dependent regulation is superimposed upon a strict root specificity of gene expression. Possible functions of the gene product other than in protein biosynthesis will be discussed.

Tryptophanyl-tRNA synthetase as a human autoantigen

Autoantibodies to highly purified tryptophanyl-tRNA synthetase, consisting of two approximately 60-kDa subunits (6.1.1.2, TrpRS), were detected in some sera of donors and patients with various diagnosis using the newly developed 125I-TrpRS-radiodot, 125I-TrpRS-radioblot, ELISA and Western immunoblotting. The percentage of positive sera appears to be dependent upon the method of sera testing. The autoimmune sera recognized both the native and denatured TrpRS forms. The binding of the human serum to the 60-kDa band of tissue extract was demonstrable by the 125I-TrpRS-blot as well as Western blot techniques. The possible role of infections in the induction of anti-TrpRS antibodies and maintenance of the autoimmune response is discussed.

Interferon induction of human tryptophanyl-tRNA synthetase safeguards the synthesis of tryptophan-rich immune-system proteins: a hypothesis

Ever since the discovery that the human tryptophanyl-tRNA synthetase (TrpRS)-encoding gene is induced by interferon (IFN) [J. Fleckner et al., Proc. Natl. Acad. Sci. USA 88 (1991) 11520-11524] and contains IFN-response regulatory elements [Frolova et al., Gene 128 (1993) 237-245], the biological rationale for this induction has remained unresolved. A survey of immune system proteins in this study reveals that the human major histocompatibility complex (MHC) antigens, beta-2-microglobulin (beta MG) and complement factor B, which are known to be induced by IFN, together with immunoglobulins (Ig) are all exceptionally enriched in Trp residues, as compared to human proteins in general. It also reveals the conservation of a sequence motif, CX10-17 WX26-62C, in Ig domains. The conservation of this sequence motif and the utility of Trp residues within antigen-binding sites clearly contribute to the Trp enrichment in Ig. These observations suggest a biological rationale for the induction of TrpRS by IFN in safeguarding Trp incorporation for the IFN-enhanced synthesis of immunological molecules.

Nucleoside triphosphatase activity associated with the N-terminal domain of mammalian tryptophanyl-tRNA synthetase

Bovine tryptophanyl-tRNA synthetase (EC 6.1.1.2) deprived of Zn2+ by chelation with the phosphonate analog of Ap4A hydrolyzed ATP(GTP) to ADP(GDP) although its ability to form tryptophanyl adenylate was impaired. This hydrolytic activity is stimulated by Mg2+ and Mn2+ ions and inhibited by Zn2+. Monoclonal antibody Am1 against the N-terminal domain of the enzyme completely abolished ATP(GTP)ase activity. The core peptide generated after proteolytic splitting of the N-domain lacks this activity. We suggest that the nucleotide binding site(s) different from ATP sites involved in aminoacylation reaction reside(s) at the N-terminal domain(s) of the enzyme.

Mammalian tryptophanyl-tRNA synthetases

Aminoacyl-tRNA synthetases of higher organisms are far less studied compared to their prokaryotic and unicellular eukaryotic counterparts. However, many aminoacyl-tRNA synthetases from multi-cellular organisms exhibit certain features not yet described for the same enzymes of bacteria or yeast. Tryptophanyl-tRNA synthetases (TrpRS) are among the most thoroughly studied mammalian enzymes of this group. TrpRS are Zn(2+)-dependent, dimeric, class I aminoacyl-tRNA synthetases with known amino acid sequence for four different mammalian orders. TrpRS is not associated in a stable multi-synthetase complex, although it exhibits a long N-terminal extension absent from bacterial TrpRS. The human gene encoding TrpRS belongs to the interferon-responsive gene family and TrpRS activity drastically increases after interferon gamma induction. For unknown reasons TrpRS is overproduced in pancreas of Ruminantia. Other data on TrpRS available so far are summarized and briefly discussed here.

Carbohydrates in mammalian tryptophanyl-tRNA synthetase

Homogeneous preparations of bovine tryptophanyl-tRNA synthetase (EC 6.1.1.2) contain monosaccharides (mannose, fucose, galactose, N-acetylglucosamine) as revealed by liquid chromatography. Their content comprises 2.5-3.0% (w/w) of the enzyme composed of two subunits (60 kDa x 2). The same set of sugars was detected in elastase and CNBr-generated fragments (with molecular masses of approx. 40 kDa and 30 kDa, respectively). It is concluded that bovine tryptophanyl-tRNA synthetase, in addition to being a metallo- and phosphoprotein, is also a glycoprotein.

Cloning and nucleotide sequence of the structural gene encoding for human tryptophanyl-tRNA synthetase

A structural gene encoding bovine (b) tryptophanyl-tRNA synthetase (WRS) has recently been cloned and sequenced [Garret et al., Biochemistry 30 (1991) 7809-7817]. Using part of this sequence as a hybridisation probe we have cloned and sequenced a structural gene encoding human polypeptide highly homologous with two mammalian proteins, bWRS [Garret et al., Biochemistry 30 (1991) 7809-7817; EMBL accession No. X52113] and rabbit peptide chain release factor [Lee et al., Proc. Natl. Acad. Sci. USA 87 (1990) 3508-3512]. Identification of the sequence encoding a human WRS is based on (i) the presence of 'HIGH' and 'KMSKS' structural motifs typical for class-I aminoacyl-tRNA synthetases [Eriani et al., Nature 347 (1990) 203-206]; (ii) coincidence of the number of SH groups per subunit estimated experimentally [Muench et al., Science 187 (1975) 1089-1091] and deduced from the cDNA sequence (six in both cases); (iii) close resemblance of two WRS polypeptides sequenced earlier [Muench et al., Science 187 (1975) 1089-1091] and the predicted structure in two different regions.

Immuno-chemical non-cross-reactivity between eukaryotic and prokaryotic seryl-tRNA synthetases

Monospecific polyclonal antibodies (pAbs) against highly purified bovine seryl-tRNA synthetase (SerRS, EC 6.1.1.1) were prepared and their specificity tested. The interactions of pAbs with SerRS from different organisms were investigated by protein immunoblotting and ELISA methods. pAbs inhibit eukaryotic SerRS aminoacylating activity and exert no effect on SerRS activity from prokaryotes. It is proposed that prokaryotic and eukaryotic SerRS evolve from different ancestor genes.

A mammalian tryptophanyl-tRNA synthetase shows little homology to prokaryotic synthetases but near identity with mammalian peptide chain release factor

Determination of the amino acid sequence of beef pancreas tryptophanyl-tRNA synthetase was undertaken through both cDNA and direct peptide sequencing. A full-length cDNA clone containing a 475 amino acid open reading frame was obtained. The molecular mass of the corresponding peptide chain, 53,728 Da, was in agreement with that of beef tryptophanyl-tRNA synthetase, as determined by physicochemical methods (54 kDa). Expression of this clone in Escherichia coli led to tryptophanyl-tRNA synthetase activity in cell extracts. The open reading frame included two sequences analogous to the consensus sequences, HIGH and KMSKS, found in class I aminoacyl-tRNA synthetases. The homology with prokaryotic and yeast mitochondrial tryptophanyl-tRNA synthetases was low and was limited to the regions of the consensus sequences. However, a 90% homology was observed with the recently described rabbit peptide chain release factor (eRF) [Lee et al. (1990) Proc. Natl. Acad. Sci. 87, 3508-3512]. Such a strong homology may reveal a new group of genes deriving from a common ancestor, the products of which could be involved in tRNA aminoacylation (tryptophanyl-tRNA synthetase) or translation termination (eRF).