Comprehensive insight into human aminoacyl-tRNA synthetases as autoantigens in idiopathic inflammatory myopathies.pdf

559 volume 27 number 6 critical reviews in immunology 27 6 559 572 2007 received 9 20 07 accepted 11 14 07 1040 8401 07 35 00 2007 by begell house inc comprehensive insight into human aminoacyl trna synthetases as autoantigens in idiopathic infl ammatory myopathies miroslawa jura 1 2 leszek rychlewski 2 2bioinfobank institute ul limanowskiego 24a 60 744 poznan poland address all correspondence to miroslawa jura msc polish academy of sciences ul z noskowskiego 12 14 61 704 poznan poland tel 61 852 85 03 852 89 19 fax 61 852 05 32 mirka bioinfo pl referee terrance p o hanlon ph d environmental autoimmunity group national institute of environmental health sciences nih bldg 9 room 1w101 msc 0958 9000 rockville pike bethesda md 20892 abstract autoantibodies against aminoacyl trna synthetases anti aars belong to the group of the myositis specifi c autoantibodies msas th eir association with the onset and development of the idiopathic infl ammatory myopathies iim implies their participation in the pathogenesis of the diseases since the appearing of anti jo 1 and other anti aarss is related to characteristic immunogenetic and clinical features they can be considered specifi c markers in diagnosis and classifi cation of patients aff ected by iim here we present an overview of anti aarss their chemoattractant properties their detection methods genetic risks and protective factors key words idiopathic infl ammatory myopathy autoantibodies anti jo 1 anti pl 12 anti pl 7 i introduction idiopathic infl ammatory myopathies iim are systemic connective tissue diseases comprising of a family of rare conditions i e chronic muscle infl ammation and weakness associated with au toimmunity th ese diseases may also aff ect other organs such as lungs gastrointestinal tract and heart arthralgia and arthritis may also occur all in all approximately one person in 100 000 suff ers from iim every year th e most widespread types of iim are polymyositis pm dermatomyosi tis dm and inclusion body myositis ibm which are characterized rather by proximal than distal muscle weakness elevated serum creatine kinase levels electrophysiologic abnormalities and infl ammation on muscle biopsy table 1 th e clinical disease symptoms of dm and pm seem to be very similar and both diseases respond to immunomodulating therapies 1 one of the humoral characteristics of iim is a highly specifi c autoimmune response to compo nents of the translation machinery 2 autoantibod ies against aminoacyl trna synthetases aars belong to the myositis specifi c autoantibodies msa that are indicative for myositis interstitial lung disease and arthritis about 50 of patients suff ering from pm or dm have myositis spe cifi c antibodies other msas are mostly directed against cytoplasmic rna protein complex e g msas against trna synthetases msas against components of the signal recognition particle srp or components of a nucleosome remodeling complex chromodomain helicase dna binding protein 4 mi 2 myositis associated antibodies maa which are shared with other autoimmune 560 critical reviews in immunology diseases are found in 20 50 of the patients th e targets of the maas are the pm scl nucleolar antigen the nuclear ku antigen p70 p80 the small nuclear ribonucleoproteins snrnp and the cytoplasmic ribonucleoproteins rornp 3 5 over the last two decades a number of studies analyzing the autoantibody profi les in sera from patients with iim have been published but there are still many questions that need to be answered we still do not know much about the etiology of these diseases postulations include viral etiol ogy or multiple genetic factors which are most probably important in the etiology and disease progression in this review we will focus on the aarss i e the autoantibodies in iim examine the links between infl ammation and aarss and show the methods of detection of antibod ies against aarss a thorough examination of the msas and an alternative hypothesis of the formation of theses autoantibodies may prove to be helpful in the explanation of pathogenesis of the myositis syndrome ii histidyl trna synthetase histidyl trna synthetase hisrs is the target of the most common group of msas called anti jo 1 autoantibodies against five other aarss are far more seldom anti alanine anti pl 12 and anti threonine trna synthetase anti pl 7 antibodies are found among 3 4 of pm dm patients whereas antibodies directed against glycine anti ej isoleucine anti oj and asparaginyl anti ks trna synthetases can be found only among several patients about 1 3 6 autoantibodies directed against more than one aars are very rare table 2 anti jo 1 autoantibodies can be detected in 15 30 of all patients with pm in 5 10 table 2 autoantibodies against human aminoacyl trna synthetases table 1 features of idiopathic infl ammatory diseases generalcirculatory system respiratory system digestive systemendocrine system jointother tiredness weight loss cardiomyopathy hypertension athelosklerosis conduction disturbances interstitial fi brosis cough aspiration atelectasis peristalsis abnormalities ischemic necrosis chyme standstill thyroidistisarthralgias arthritis refl ux dysphagia uncontrolled swallowing humoral abnormalities 561 volume 27 number 6 of patients with dm and in up to 70 of all patients with myositis with concurrent interstitial pulmonary fi brosis th ese antibodies are found seldom in children with pm or dm and in other connective tissue diseases 4 7 8 th e human histidyl trna synthetase gene encodes an enzyme that catalyzes the esterifi cation of histidine to its cognate trna as the fi rst step in protein biosynthesis human cytosolic hisrs is a homodimeric class ii enzyme of aminoacyl trna synthetases enzymes in this class share an antiparallel beta sheet formation fl anked by alpha helices and are mostly dimeric or multi meric human cytosolic hisrs consists of 509 amino acid residues and is formed by two subunits of 57 4kda 9 10 unlike other class ii enzymes hisrs requires only two mg2 ions to trigger the activation reaction 11 12 th is is due to the presence of a unique arg259 th e guanidino group of the arginine reacts with the phosphate of atp and thereby replaces the mg2 ion 13 th e human histidyl trna synthetase is en coded by a gene with a bidirectional promotor th e promotor controls the transcription of both hisrs 2 kb and an unknown ho3 2 5 kb gene th ese genes are oriented head to head th e longest orf within ho3 1518 bp is identical in sequence to hisrs in 75 th e gene product of ho3 shares a 72 amino acid sequence iden tity with hisrs fig 1 ho3 also possesses the class ii aarss motifs signifi cantly ho3 and hisrs diff er from each other in fi rst two exons resulting in diff erent n terminal domains in contrast to the ho3 the n terminal domain of hisrs has a coiled coil structure th e ho3 n terminal domain contains a mitochondrial targeting sequence th e steady state of ho3 and hisrs mrnas levels vary among tissues due to the similarity between hisrs and ho3 one can expect ho3 to take part in protein synthesis or at least to retain some enzymatic functions just as atp his or trnahis binding 14 although the function of the ho3 polypeptide is still unknown it is worth checking if this peptide also possesses the epitopes targeted by autoantigens it is pos sible that the ho3 peptide may take part in the pathogenesis of the iim one of the characteristics of some mammalian aarss is the presence of n terminal domain extensions containing 60 amino acids th is do main has alpha helical structure and is enriched with basis amino acids it is postulated that the n terminal domain of hisrs interacts with the trna molecule stabilizes complex formation increases the catalytic effi ciency of the enzyme and is involved in trna cycling during translation 15 16 th is sequence motif shared among several classes of synthetases is found only in eukaryotes 17 19 a hisrs as an antigen in iim there have been many attempts to identify epitopes responsible for the interaction between hisrs and autoantibodies 10 hisrs possesses two coiled coil structures fig 2 one of them is situated within the n terminal domain and contains amino acids 1 46 th e other one includes amino acids 387 428 20 coiled coil structures are found far more frequently in autoantigens than in other proteins which suggests that coiled coil structure itself might be an important epitope signifi cantly a truncated human hisrs which lacks the fi rst 60 amino acids is inactive both as an antigen and an enzyme 21 within most patients the immunodominant epitope of hisrs is found in the n terminus approximately 37 of autoantigens in a variety of figure 1 schematic illustration of cdna fragment encoding hrs and ho3 with bidirectional promotor chromosome 5 location 5q31 3 the direction of transcription of each gene arrows is illustrated above their respective translational start codons atg the ancestral gene for class ii aminoacyl trna synthetases had undergone inverted duplication and translocation resulting in two genes in opposite orientation 562 critical reviews in immunology autoimmune diseases contain a similar coiled coil structure it has been put forward that coiled coil structure facilitates the uptake of the protein by antigen presenting cells or it might be capable of binding to nucleic acid and causing uptake or display it is possible that the coiled coil structure is digested within the cell to enable the interac tion with the human leukocyte antigens hla 22 some reports confi rm that antibodies against asparaginyl trna synthetase asnrs recognize a coiled coil structure 23 b anti jo 1 antibodies anti jo 1 antibodies can be divided into two classes conformational dependent and inde pendent th ey are polyclonal and can recognize epitopes that vary among the myositis patients 24 25 heterogeneity among anti jo 1 antibodies is probably not triggered by initial immune response against the hisrs but by the secondary response against fragmented hisrs which is released from a damaged muscle 26 both conformational depen dent and independent epitopes are interspersed along the length of the molecule it appears that there is no obvious relationship between confor mational dependent or independent autoepitopes and catalytic site on the hisrs enzyme 24 residues 2 44 and 286 509 of the human hisrs were identifi ed as epitopes by western blot analyses with sera from patients aff ected by myositis igg which is specifi c for these epitopes does not cause the inhibition of hisrs it was set forth that the anti jo 1 antibodies need at least a third probably nonlinear epitope to recognize hisrs 27 th e sera from anti jo 1 positive patients in some cases also contain antibodies directed against the trnahis th e major epitope of the trna molecule is conformational dependent and com posed of the d and t loops fig 3 ribonuclease protection assays revealed that the g residues in the regions are not proteolytically cleaved when trnahis is bound to igg igg1 was the most fre quently appearing isotype of immunoglobulins 17 18 th ere was also was a small amount of igg3 and igm 28 29 anti trnahis are highly specifi c a base substitutions or modifi cations within d and or t loops interfere with anti trnahis activity th e most convincing explanation of the appearance of anti trnahis seems to be the concept of epitope spreading development of an immune response to epitopes distinct and non cross reactive with the disease causing epitope it is supported by the fact that these antigens were found only in anti jo 1 positive sera 30 some antibodies bind close to the sites of aarss in the place where atp and histidine usually do 31 th is indicates that these antibodies are competitive inhibitors of the aminoacylation reaction with respect to atp and histidine other antibodies called noncompetitive inhibitors bind free trnahis and trnahis bound to hisrs 32 th e antibodies exhibit a stronger inhibition of the hisrs trnahis complex than in case of the free enzyme experiments conducted on mice figure 2 the coiled coil structure of protein a two strands of amino acids that are identical wrap around each other b coiled coil protein as a repeated consensus substrings of the form a b c d e f g 563 volume 27 number 6 showed that hisrs alone does not generate myositis 33 a noteworthy co occurrence of the anti ro52 and anti jo 1 in the sera of patients with idiopathic infl ammatory myopathies has been observed 34 th e presence of autoantibodies against the ro52 protein in these sera is very high 58 no cross reactivity between these antibodies was detected however the occurrence of anti ro52 antibodies is not limited to sera with anti jo 1 antibodies 35 th ere is a likelihood that the tissue specifi c alteration in muscle and or lung leads to genera tion of unique forms of the targeted autoantigens it may be caused by the changes in conformation and susceptibility to cleavage by grb as well as alteration of the reaction catalyzed by the enzyme by dissociation of proteins complex which results in cellular stress and activation of proteases iii other aminoacyl trna synthetases as the antigens anti pl 12 sera react with both alanyl trna synthetase alars and the trnaala 36 39 anti trnaala antibodies bind to the fully mature trnaala and the epitope is located at a 9 base region of the anticodon stem loop that includes igc anticodon pl 12 antigen 38 th ere is a possibility that n1 methylinosine which is char acteristic for trnaala and one or more modifi ed nucleosides within the anticodon loop create the epitopes or that they assist in the recognition by the antibody 40 41 alars and trnaala antigens can co occur in the same serum similar observations were made with the anti jo 1 sera th is fi nding supports the epitope spreading concept of devel opment of the antibodies 42 th e conformational epitope of alars was localized outside the catalytic region within amino acids 730 951 close to the c terminal end 43 th is was confi rmed by the observation that the heated antigen was inactive and the sera were negative by immunoblotting 44 autoantibodies against asparaginyl trna synthetase anti ks react only with the enzyme trna complex th ey recognize a conformational independent epitope in the n terminal region of the enzyme and conformational dependent epitope in the catalytic domain it was shown that anti ks do not interfere with the fi rst step of the aminoacylation reaction that is amino figure 3 secondary structure of human trnahis and proposed antigenic determinant in three dimensional model of trnahis 564 critical reviews in immunology acid activation as a result of the accession of the anti ks to the synthetase the reaction is prevented by the increase of the affi nity of the enzyme to its cognate trna th e autoantibody probably induces conformational changes of the asnrs and hinders the transfer of the activated amino acid to the acceptor stem of the trna in contrast to anti jo 1 it does not interfere with atp and asparagine binding 23 anti oj antibodies are directed against iso leucyl trna synthetase which is an element of a multienzyme complex th e sera that contain anti oj also precipitate other components of the multienzyme complex but anti oj primarily reacts with isoleucyl trna synthetase 8 45 th is is the only exception in which serum from one patient reacts with more then one synthetase anti ej is the antibody directed against glyrs th e ability of the autoantibodies to inhibit the functional activity of glyrs suggests that the epitope is located within a functional domain autoantibody does not react with the trna binding site of the enzyme because autoantibody immunoprecipitates trna without binding to it directly 46 47 anti pl 7 was found to recognize the con formational epitope 3 aarss represent a group of enzymes that have a similar function and structure neverthe less antibodies to diff erent synthetases in general do not cross react 43 th e mechanism of blocking aarss by auto antibodies is not fully understood although some data indicate that autoantibodies might disrupt protein folding and or conformational changes associated with substrate binding anti jo 1 and anti pl 12 can directly pre cipitate trna independent of the corresponding aarss 23 one might suspect the mechanisms underlying aminoacylation inhibition to be diverse for the diff erent antisynthetase antibodies iv chemoattractant properties of aarss th e idiopathic infl ammatory myopathies repre sent autoimmune disorders in which cytolysis of muscle cells is induced by antigen specifi c t cells th ose cells are directed against jo 1 and other aminoacyl trna synthetases like pl 12 and pl 7 th ese antigens elicit both humoral and cell mediated immune response presence of t cells specifi c against jo 1 in the peripheral blood and the chemotactic properties of jo 1 to immature dendritic cells idcs and t lymphocytes indicates that jo 1 leads to the breakdown of tolerance and plays a direct role in initiation and development of the autoimmune disease 2 48 some aarss have the ability to induce leukocyte migration hisrs and asnrs are able to bring about migration of cd4 and cd8 t cells interleukin il 2 activated monocytes and idcs in vitro interestingly also a fragment of hisrs n terminal domain amino acids 1 48 is similar to hisrs chemotactic for lymphocyte and activates monocyte in vitro although it is believed that cd8 cells participate in the pathogenesis of muscle damage the evidence is still not proved 5 hisrs activates ccr5 and asnrs ccr3 chemokine receptors which are expressed on idcs table 3 the immune system cells which migration is caused by aminoacyl trna synthetases 565 volume 27 number 6 hisrs and ccl5 have the same receptor which suggests that hisrs in the same way as ccl5 may respectively interact with other receptors e g ccr1 ccr3 table 3 49 seryl trna synthetase induces migration of ccr3 transfected cells but not idcs signifi cant ly aspartyl and lysyl trna s