Molecular and pathogenic effects of endoplasmic reticulum aminopeptidases ERAP1 and ERAP2 in MHC-I-associated inflammatory disorders: Towards a unifying view

The inflammatory diseases that are most strongly associated with major histocompatibility Complex class I (MHC-I) alleles are also influenced by endoplasmic reticulum aminopeptidase (ERAP) 1 and/or 2, often in epistasis with the susceptibility MHC-I allele. This review will focus on the four major MHC-I-associated inflammatory disorders: ankylosing spondylitis, birdshot chorioretinopathy, Behçet’s disease and psoriasis. The genetics of ERAP1/ERAP2 association and the alterations induced by polymorphism of these enzymes on the risk MHC-I allotypes will be examined. A pattern emerges of analogous effects on peptide length, sequence and affinity of disparate peptidomes, suggesting that similar peptide-mediated mechanisms underlie the pathogenesis and the joint contribution of ERAP1/ERAP2 and MHC-I to distinct inflammatory diseases. Processing of specific antigens, peptide-dependent changes in global properties of the MHC-I molecules, such as folding and stability, or both may be pathogenic.     

Crystal structures of the endoplasmic reticulum aminopeptidase-1 (ERAP1) reveal the molecular basis for N-terminal peptide trimming

Endoplasmatic reticulum aminopeptidase 1 (ERAP1) is a multifunctional enzyme involved in trimming of peptides to an optimal length for presentation by major histocompatibility complex (MHC) class I molecules. Polymorphisms in ERAP1 have been associated with chronic inflammatory diseases, including ankylosing spondylitis (AS) and psoriasis, and subsequent in vitro enzyme studies suggest distinct catalytic properties of ERAP1 variants. To understand structure-activity relationships of this enzyme we determined crystal structures in open and closed states of human ERAP1, which provide the first snapshots along a catalytic path. ERAP1 is a zinc-metallopeptidase with typical H-E-X-X-H-(X)(18)-E zinc binding and G-A-M-E-N motifs characteristic for members of the gluzincin protease family. The structures reveal extensive domain movements, including an active site closure as well as three different open conformations, thus providing insights into the catalytic cycle. A K(528)R mutant strongly associated with AS in GWAS studies shows significantly altered peptide processing characteristics, which are possibly related to impaired interdomain interactions.     

ERAP1 shapes just part of the immunopeptidome

ERAP1 is an aminopeptidase involved in trimming long peptides to the lengths required for presentation by MHC class I. ERAP1 substrate preference is for peptides with hydrophobic or aliphatic N-terminal amino acids, with lower efficacy with charged and small hydrophilic amino acids and almost complete inefficiency with proline. Since ERAP1 efficiently trims peptides to eight amino acids or even shorter, and many MHC-I allotypes can only bind peptides that are eight or nine amino acids or longer, ERAP1 both produces and destroys potential ligands of these alleles. The observation that ERAP1 modulates the levels of presentation for only a subset of the immunopeptidome conflicts with the common assumption that most MHC-I peptides are derived from longer peptides that are produced by the proteasome, transported into the endoplasmic reticulum (ER) by the Transporter Associated Peptide Presentation (TAP) and then trimmed by ERAP1. A more likely mechanism is that cellular protein degradation produces surplus amounts of peptides that fit perfectly and are rapidly loaded onto the MHC, with only a minority of peptides requiring trimming within the ER before loading. Alternatively, ERAP1 may not be present in all ER compartments or vesicles where peptide processing and loading take place and thus affects just a subset of the immunopeptidome.     

The genetics,structure and function of the M1 aminopeptidase oxytocinase subfamily and their therapeutic potential in immune-mediated disease

The oxytocinase subfamily of M1 aminopeptidases plays an important role in processing and trimming of peptides for presentation on major histocompatibility (MHC) Class I molecules. Several large-scale genomic studies have identified association of members of this family of enzymes, most notably ERAP1 and ERAP2, with immune-mediated diseases including ankylosing spondylitis, psoriasis and birdshot chorioretinopathy. Much is now known about the genetics of these enzymes and how genetic variants alter their function, but how these variants contribute to disease remains largely unresolved. Here we discuss what is known about their structure and function and highlight some of the knowledge gaps that affect development of drugs targeting these enzymes.     

ERAP1基因与银屑病的相关性研究进展

银屑病是一种免疫介导的慢性炎症性皮肤病,遗传因素在发病中起重要作用。ERAP1基因现已被证实为银屑病的易感基因,目前研究发现ERAP1基因分子的功能与银屑病的发病相关,ERAP1基因与其他银屑病易感基因、环境因素之间的交互作用可能参与银屑病发病,其基因型可能影响银屑病患者的临床表型(发病年龄、发病严重程度、皮损类型、伴发疾病、家族史等)。     

Endoplasmic Reticulum Aminopeptidase 2, a common immunological link to adverse pregnancy outcomes and cancer clearance?

Endoplasmic Reticulum Aminopeptidase 2 (ERAP2) trims HLA class I-binding peptides, determining the peptide repertoire presented for immune recognition. Variation in the ERAP2 amino acid sequence could affect the ability of some fetuses and tumors to achieve immune evasion. For example, homozygosity for an ERAP2 variant that has increased trimming efficiency for hydrophobic molecules has never been detected in mothers and fetuses. Thus, it is possible that this single nucleotide polymorphism (SNP) in the ERAP2 gene has been selected against in order to prevent alteration of the immune privileged uterine environment, and to allow tumors to escape immune recognition. Currently, there are no immunological treatments or prophylactic approaches to ensure a healthy pregnancy outcome, and the success of cancer immunotherapies is variable. Understanding the role of ERAP2 in immune evasion mechanisms in pregnancy and cancer may improve fetal survival and tumor clearance. This review summarizes current knowledge about ERAP2 and its N392 variant, and their relationship to pregnancy outcomes and cancer immune evasion/recognition.     

Differences between disease-associated endoplasmic reticulum aminopeptidase 1 (ERAP1) isoforms in cellular expression,interactions with tumour necrosis factor receptor 1 (TNF-R1) and regulation by cytokines

Endoplasmic reticulum aminopeptidase 1 (ERAP1) processes peptides for major histocompatibility complex (MHC) class I presentation and promotes cytokine receptor ectodomain shedding. These known functions of ERAP1 may explain its genetic association with several autoimmune inflammatory diseases. In this study, we identified four novel alternatively spliced variants of ERAP1 mRNA, designated as ΔExon-11, ΔExon-13, ΔExon-14 and ΔExon-15. We also observed a rapid and differential modulation of ERAP1 mRNA levels and spliced variants in different cell types pretreated with lipopolysaccharide (LPS). We have studied three full-length allelic forms of ERAP1 (R127-K528, P127-K528, P127-R528) and one spliced variant (ΔExon-11) and assessed their interactions with tumour necrosis factor receptor 1 (TNF-R1) in transfected cells. We observed variation in cellular expression of different ERAP1 isoforms, with R127-K528 being expressed at a much lower level. Furthermore, the cellular expression of full-length P127-K528 and ΔExon-11 spliced variant was enhanced significantly when co-transfected with TNF-R1. Isoforms P127-K528, P127-R528 and ΔExon-11 spliced variant associated with TNF-R1, and this interaction occurred in a region within the first 10 exons of ERAP1. Supernatant-derived vesicles from transfected cells contained the full-length and ectodomain form of soluble TNF-R1, as well as carrying the full-length ERAP1 isoforms. We observed marginal differences between TNF-R1 ectodomain levels when co-expressed with individual ERAP1 isoforms, and treatment of transfected cells with tumour necrosis factor (TNF), interleukin (IL)-1β and IL-10 exerted variable effects on TNF-R1 ectodomain cleavage. Our data suggest that ERAP1 isoforms may exhibit differential biological properties and inflammatory mediators could play critical roles in modulating ERAP1 expression, leading to altered functional activities of this enzyme.     

Crystal structure of a polypeptide’s C-terminus in complex with the regulatory domain of ER aminopeptidase 1

Endoplasmic reticulum aminopeptidase 1 (ERAP1) is involved in the final processing of peptide precursors to generate the N-termini of MHC class I-restricted epitopes. ERAP1 thus influences immunodominance and cytotoxic immune responses by controlling the peptide repertoire available for cell surface presentation by MHC molecules. To enable this critical role in antigen processing, ERAP1 trims peptides by a unique molecular ruler mechanism that turns on/off hydrolysis activity in a peptide-length and −sequence dependent manner. Thus unlike other aminopeptidases, ERAP1 could recognize both the N- and C-termini of peptides in order to read the substrate’s length. To exemplify and validate this molecular ruler mechanism, we have carried out crystallographic studies on molecular recognition of antigenic peptide’s C-terminus by ERAP1. In this report, we have determined a 2.8 Å-resolution crystal structure of an intermolecular complex between the ERAP1 regulatory domain and a natural epitope’s C-terminus displayed in a fusion protein. It reveals the structural details of peptide’s C-termini recognition by ERAP1. ERAP1 uses specificity pockets on the regulatory domain to bind the peptide’s carboxyl end and side chain of the C-terminal anchoring residue. At the same time, flexibility in length and sequence at the middle of peptides is accommodated by a kink with minimal interactions with ERAP1.     

ERAP1基因与山东汉族人群强直性脊柱炎的相关性

目的利用基于群体的病例对照关联分析方法,探讨ERAP1基因是否为北方汉族人群的易感基因。方法从山东省立医院收集了311例临床确诊的强直性脊柱炎病例及320例正常对照资料,利用Taqman探针法对ERAP1基因内的两个单核苷酸多态性位点rs7711564和rs27434分型,利用PLINK软件行Hardy-Weinberg遗传平衡检验、等位基因及基因型频率分布分析。样本统计学效能采用CaTS软件计算。结果两个位点在病例组及对照组中均达到遗传平衡,其中rs7711564位点罕见等位基因频率在病例组及对照组分别为0.430 9和0.489 1(OR=1.26,P=0.04),rs27434位点罕见等位基因频率在病例组和对照组分别为0.483 9与0.421 9(OR=0.78,P=0.03)。结论 ERAP1基因内的遗传变异与北方汉族人群强直性脊柱炎相关,进一步证实了该基因是强直性脊柱炎的易感基因。