Addressing diagnostic dilemmas in eosinophilic esophagitis using esophageal epithelial eosinophil-derived neurotoxin.

OBJECTIVES: Eosinophil-derived neurotoxin (EDN) is a viable marker of eosinophilic esophagitis (EoE) disease activity. We studied the utility of measuring EDN from esophageal epithelial brushings for diagnosing EoE, focusing on two scenarios: (1) cases of exclusive distal eosinophilia and (2) cases of discrepancy between endoscopy and histology. METHODS: Records of patients who underwent esophagogastroduodenoscopy (EGD) with EDN measured via esophageal brushings at Arnold Palmer Hospital for Children in Orlando, Florida from January 2014 to October 2018 were retrospectively reviewed. Demographics, clinical, endoscopic, and histologic data were collected. RESULTS: We reviewed 231 patient records (66.7% male, mean age 10.3 years, range 1-22 years). EDN values correlated with endoscopic reference score (EREFS) and peak eosinophil count (PEC) (Spearman's rho = 0.756 (p < 0.001) and 0.824 (p < 0.001) respectively). Average PEC, EREFS, and EDN concentrations were higher in patients with active EoE than in controls or patients with EoE in remission (inactive). When grouping patients based on esophageal eosinophilia distribution, EDN mirrored PEC, and EREFS. Patients with exclusive distal eosinophilia had lower EDN concentrations than those with eosinophilia in >1 level of the esophagus (23.8 ± 46.1 mcg/mL vs. 171.3 ± 205.8 mcg/mL respectively, p < 0.001). EDN values were more consistent with EREFS in cases of discrepancies between endoscopic findings and pathology (p < 0.001). CONCLUSION: EDN measured in esophageal brushing samples reflects disease activity objectively and accurately. It also offers significant value in cases of exclusive distal esophageal eosinophilia and when discrepancies exist between endoscopy and histology.

Basic amino acid residues of human eosinophil derived neurotoxin essential for glycosaminoglycan binding.

Human eosinophil derived neurotoxin (EDN), a granule protein secreted by activated eosinophils, is a biomarker for asthma in children. EDN belongs to the human RNase A superfamily possessing both ribonucleolytic and antiviral activities. EDN interacts with heparin oligosaccharides and heparin sulfate proteoglycans on bronchial epithelial Beas-2B cells. In this study, we demonstrate that the binding of EDN to cells requires cell surface glycosaminoglycans (GAGs), and the binding strength between EDN and GAGs depends on the sulfation levels of GAGs. Furthermore, in silico computer modeling and in vitro binding assays suggest critical roles for the following basic amino acids located within heparin binding regions (HBRs) of EDN 34QRRCKN39 (HBR1), 65NKTRKN70 (HBR2), and 113NRDQRRD119 (HBR3) and in particular Arg35, Arg36, and Arg38 within HBR1, and Arg114 and Arg117 within HBR3. Our data suggest that sulfated GAGs play a major role in EDN binding, which in turn may be related to the cellular effects of EDN.

An insertion in loop L7 of human eosinophil-derived neurotoxin is crucial for its antiviral activity.

The human eosinophil granule ribonuclease, eosinophil-derived neurotoxin (EDN) has been shown to have antiviral activity against respiratory syncytial virus-B (RSV-B). Other closely related and more active RNases such as RNase A, onconase, and RNase k6 do not have any antiviral activity. A remarkable unique feature of EDN is a nine-residue insertion in its carboxy-terminal loop, L7 which is not present in RNase A, and differs in sequence from the corresponding loop in another eosinophil RNase, eosinophil cationic protein (ECP). ECP has a much lower antiviral activity as compared to EDN. The current study probed the role of loop L7 of EDN in its antiviral activity. Three residues in loop L7, Arg117, Pro120, and Gln122, which diverge between EDN, ECP, and RNase A, were mutated to alanine alone and in combination to generate single, double, and triple mutants. These mutants, despite having RNase activity had decreased antiviral activity towards RSV suggesting the involvement of loop L7 in the interaction of EDN with RSV. It appears that the mutations in loop L7 disrupt the interaction of protein with the viral capsid, thereby inhibiting its entry into the virions. The study demonstrates that besides the RNase activity, loop L7 is another important determinant for the antiviral activity of EDN.

Sequence-specific backbone resonance assignments and microsecond timescale molecular dynamics simulation of human eosinophil-derived neurotoxin.

Eight active canonical members of the pancreatic-like ribonuclease A (RNase A) superfamily have been identified in human. All structural homologs share similar RNA-degrading functions, while also cumulating other various biological activities in different tissues. The functional homologs eosinophil-derived neurotoxin (EDN, or RNase 2) and eosinophil cationic protein (ECP, or RNase 3) are known to be expressed and secreted by eosinophils in response to infection, and have thus been postulated to play an important role in host defense and inflammatory response. We recently initiated the biophysical and dynamical investigation of several vertebrate RNase homologs and observed that clustering residue dynamics appear to be linked with the phylogeny and biological specificity of several members. Here we report the 1H, 13C and 15N backbone resonance assignments of human EDN (RNase 2) and its molecular dynamics simulation on the microsecond timescale, providing means to pursue this comparative atomic-scale functional and dynamical analysis by NMR and computation over multiple time frames.

Molecular recognition of human eosinophil-derived neurotoxin (RNase 2) by placental ribonuclease inhibitor.

Placental ribonuclease inhibitor (RI) binds diverse mammalian RNases with dissociation constants that are in the femtomolar range. Previous studies on the complexes of RI with RNase A and angiogenin revealed that RI utilises largely distinctive interactions to achieve high affinity for these two ligands. Here we report a 2.0 angstroms resolution crystal structure of RI in complex with a third ligand, eosinophil-derived neurotoxin (EDN), and a mutational analysis based on this structure. The RI-EDN interface is more extensive than those of the other two complexes and contains a considerably larger set of interactions. Few of the contacts present in the RI-angiogenin complex are replicated; the correspondence to the RI-RNase A complex is somewhat greater, but still modest. The energetic contributions of various interface regions differ strikingly from those in the earlier complexes. These findings provide insight into the structural basis for the unusual combination of high avidity and relaxed stringency that RI displays.

Role of catalytic and non-catalytic subsite residues in ribonuclease activity of human eosinophil-derived neurotoxin.

Human eosinophil-derived neurotoxin (EDN), a secretory protein from eosinophils, is a member of the RNase A superfamily. The ribonucleolytic activity of EDN is central to its biological activities. EDN binds RNA in a cationic cleft, and the interaction between EDN and RNA substrate extends beyond the scissile bond. Based on its homology with RNase A, putative substrate binding subsites have been identified in EDN. The B1 and B2 subsites interact specifically with bases, whereas P0, P1, and P2 subsites interact with phosphoryl groups. In this study, we evaluated the role of putative residues of these subsites in the ribonucleolytic activity of EDN. We demonstrate that of the two base binding subsites, B1 is critical for the catalytic activity of EDN, as the substrate cleavage was dramatically reduced upon substitution of B1 subsite residues. Among the phosphate-binding subsites, P1 is the most crucial as mutations of its constituting residues totally abolished the catalytic activity of EDN. Mutation of P0 and P2 subsite residues only affected the catalytic activity on the homopolymer Poly(U). Our study demonstrates that P1 and B1 subsites of EDN are critical for its catalytic activity and that the other phosphate-binding subsites are involved in the activity on long homopolymeric substrates.

Eosinophil-derived neurotoxin / RNase 2: connecting the past, the present and the future.

The eosinophil-derived neurotoxin (EDN, also known as eosinophil protein-X) is best-known as one of the four major proteins found in the large specific granules of human eosinophilic leukocytes. Although it was named for its discovery and initial characterization as a neurotoxin, it is also expressed constitutively in human liver tissue and its expression can be induced in macrophages by proinflammatory stimuli. EDN and its divergent orthologs in rodents have ribonuclease activity, and are members of the extensive RNase A superfamily, although the relationship between the characterized physiologic functions and enzymatic activity remains poorly understood. Recent explorations into potential physiologic functions for EDN have provided us with some insights into its role in antiviral host defense, as a chemoattractant for human dendritic cells, and most recently, as an endogenous ligand for toll-like receptor (TLR)2.

Human eosinophil-derived neurotoxin: involvement of a putative non-catalytic phosphate-binding subsite in its catalysis.

Human eosinophil-derived neurotoxin (EDN) or RNase 2, found in the non-core matrix of eosinophils is a ribonuclease belonging to the Ribonuclease A superfamily. EDN manifests a number of bioactions including neurotoxic and antiviral activities, which are dependent on its ribonuclease activity. The core of the catalytic site of EDN contains various base and phosphate-binding subsites. Unlike many members of the RNase A superfamily, EDN contains an additional non-catalytic phosphate-binding subsite, P(-1). Although RNase A also contains a P(-1) subsite, the composition of the site in EDN and RNase A is different. In the current study we have generated site-specific mutants to study the role of P(-1) subsite residues Arg(36), Asn(39), and Gln(40) of EDN in its catalytic activity. The individual mutation of Arg(36), Asn (39), and Gln(40) resulted in a reduction in the catalytic activity of EDN on poly(U) and poly(C). However, there was no change in the activities on yeast tRNA and dinucleotide substrates. The study shows that the P(-1) subsite is crucial for the ribonucleolytic activity of EDN on polymeric RNA substrates.

Crystal structures of eosinophil-derived neurotoxin (EDN) in complex with the inhibitors 5'-ATP, Ap3A, Ap4A, and Ap5A.

Eosinophil-derived neurotoxin (EDN) is a catalytically proficient member of the pancreatic ribonuclease superfamily secreted along with other eosinophil granule proteins during innate host defense responses and various eosinophil-related inflammatory and allergic diseases. The ribonucleolytic activity of EDN is central to its antiviral and neurotoxic activities and possibly to other facets of its biological activity. To probe the importance of this enzymatic activity further, specific inhibitors will be of great aid. Derivatives of 5'-ADP are among the most potent inhibitors currently known. Here, we use X-ray crystallography to investigate the binding of four natural nucleotides containing this moiety. 5'-ATP binds in two alternative orientations, one occupying the B2 subsite in a conventional manner and one being a retro orientation with no ordered adenosine moiety. Diadenosine triphosphate (Ap3A) and diadenosine tetraphosphate (Ap4A) bind with one adenine positioned at the B2 subsite, the polyphosphate chain extending across the P1 subsite in an ill-defined conformation, and a disordered second adenosine moiety. Diadenosine pentaphosphate (Ap5A), the most avid inhibitor of this series, binds in a completely ordered fashion with one adenine positioned conventionally at the B2 subsite, the polyphosphate chain occupying the P1 and putative P(-1) subsites, and the other adenine bound in a retro-like manner at the edge of the B1 subsite. The binding mode of each of these inhibitors has features seen in previously determined structures of adenosine diphosphates. We examine the structure-affinity relationships of these inhibitors and discuss the implications for the design of improved inhibitors.