Interactions of decay-accelerating factor (DAF) with haemagglutinating human enteroviruses: utilizing variation in primate DAF to map virus binding sites

Complement protective epitopes and CD55-microtubule complexes facilitate the invasion and intracellular persistence of uropathogenic Escherichia coli

BACKGROUND

 Escherichia coli-bearing Dr-adhesins (Dr+ E. coli) cause chronic pyelonephritis in pregnant women and animal models. This chronic renal infection correlates with the capacity of bacteria to invade epithelial cells expressing CD55. The mechanism of infection remains unknown.

METHODS

 CD55 amino acids in the vicinity of binding pocket-Ser155 for Dr-adhesin were mutated to alanine and subjected to temporal gentamicin-invasion/gentamicin-survival assay in Chinese hamster ovary cells. CD55/microtubule (MT) responses were studied using confocal/electron microscopy, and 3-dimensional structure analysis.

RESULTS

 Mutant analysis revealed that complement-protective CD55-Ser165 and CD55-Phe154 epitopes control E. coli invasion by coregulating CD55-MT complex expression. Single-point CD55 mutations changed E. coli to either a minimally invasive (Ser165Ala) or a hypervirulent pathogen (Phe154Ala). Thus, single amino acid modifications with no impact on CD55 structure and bacterial attachment can have a profound impact on E. coli virulence. While CD55-Ser165Ala decreased E. coli invasion and led to dormant intracellular persistence, intracellular E. coli in CD55-Phe154Ala developed elongated forms (multiplying within vacuoles), upregulated CD55-MT complexes, acquired CD55 coat, and escaped phagolysosomal fusion.

CONCLUSIONS

 E. coli target complement-protective CD55 epitopes for invasion and exploit CD55-MT complexes to escape phagolysosomal fusion, leading to a nondestructive parasitism that allows bacteria to persist intracellularly.

Interaction of decay-accelerating factor with echovirus 7

Echovirus 7 (EV7) belongs to the Enterovirus genus within the family Picornaviridae. Many picornaviruses use IgG-like receptors that bind in the viral canyon and are required to initiate viral uncoating during infection. However, in addition, some of the enteroviruses use an alternative or additional receptor that binds outside the canyon. Decay-accelerating factor (DAF) has been identified as a cellular receptor for EV7. The crystal structure of EV7 has been determined to 3.1-Å resolution and used to interpret the 7.2-Å-resolution cryo-electron microscopy reconstruction of EV7 complexed with DAF. Each DAF binding site on EV7 is near a 2-fold icosahedral symmetry axis, which differs from the binding site of DAF on the surface of coxsackievirus B3, indicating that there are independent evolutionary processes by which DAF was selected as a picornavirus accessory receptor. This suggests that there is an advantage for these viruses to recognize DAF during the initial process of infection.

Interaction of decay-accelerating factor with coxsackievirus B3

Many entero-, parecho-, and rhinoviruses use immunoglobulin (Ig)-like receptors that bind into the viral canyon and are required to initiate viral uncoating during infection. However, some of these viruses use an alternative or additional receptor that binds outside the canyon. Both the coxsackievirus-adenovirus receptor (CAR), an Ig-like molecule that binds into the viral canyon, and decay-accelerating factor (DAF) have been identified as cellular receptors for coxsackievirus B3 (CVB3). A cryoelectron microscopy reconstruction of a variant of CVB3 complexed with DAF shows full occupancy of the DAF receptor in each of 60 binding sites. The DAF molecule bridges the canyon, blocking the CAR binding site and causing the two receptors to compete with one another. The binding site of DAF on CVB3 differs from the binding site of DAF on the surface of echoviruses, suggesting independent evolutionary processes.

CD55 in rat male reproductive tissue: Differential expression in testis and expression of a unique truncated isoform on spermatozoa

CD55 is a key regulator of complement activation, expressed on most tissues and cells in man and other mammals. In the rat, alternative splicing in the gene encoding CD55 yields GPI-anchored (GPI-CD55) and transmembrane (TM-CD55) forms. Published Northern blot analysis indicated that while GPI-CD55 was broadly expressed, TM-CD55 was primarily expressed in the testis, although the precise site of expression was not identified. To clarify the distribution of CD55 isoforms in rat reproductive tissues, we first performed immunohistochemistry and Western blot analysis with an anti-rat CD55 mAb that recognized all reported CD55 isoforms, and a polyclonal immunoglobulin specific for TM-CD55. CD55 was absent in testis prior to puberty. Post-puberty, CD55 was expressed at high levels on all spermiogenic cells from step 6 spermatid onward, and on mature spermatozoa focussed on the acrosome, but was absent from support cells and early progenitors. Enzymatic digestion revealed that GPI-CD55 was predominant in testis and spermatozoa. Staining for TM-CD55 with specific immunoglobulin confirmed its absence from mature sperm and expression on spermatids only between steps 11 and 14 of development. GPI-CD55 on spermatozoa was of lower molecular weight than that in testis and other tissues; sequencing from spermatozoal mRNA identified a unique isoform of GPI-CD55 missing short consensus repeat 4. The predominant acrosome expression and presence of a unique, truncated isoform of CD55 on spermatozoa provides further support for the hypothesis that the acrosome is a highly specialized region in which closely regulated complement activation may contribute to reproductive function.

Structural and Functional Insights into the Interaction of Echoviruses and Decay-accelerating Factor

Many enteroviruses bind to the complement control protein decay-accelerating factor (DAF) to facilitate cell entry. We present here a structure for echovirus (EV) type 12 bound to DAF using cryo-negative stain transmission electron microscopy and three-dimensional image reconstruction to 16-A resolution, which we interpreted using the atomic structures of EV11 and DAF. DAF binds to a hypervariable region of the capsid close to the 2-fold symmetry axes in an interaction that involves mostly the short consensus repeat 3 domain of DAF and the capsid protein VP2. A bulge in the density for the short consensus repeat 3 domain suggests that a loop at residues 174-180 rearranges to prevent steric collision between closely packed molecules at the 2-fold symmetry axes. Detailed analysis of receptor interactions between a variety of echoviruses and DAF using surface plasmon resonance and comparison of this structure (and our previous work; Bhella, D., Goodfellow, I. G., Roversi, P., Pettigrew, D., Chaudhry, Y., Evans, D. J., and Lea, S. M. (2004) J. Biol. Chem. 279, 8325-8332) with reconstructions published for EV7 bound to DAF support major differences in receptor recognition among these viruses. However, comparison of the electron density for the two virus.receptor complexes (rather than comparisons of the pseudo-atomic models derived from fitting the coordinates into these densities) suggests that the dramatic differences in interaction affinities/specificities may arise from relatively subtle structural differences rather than from large-scale repositioning of the receptor with respect to the virus surface.

Pathogenesis of Afa/Dr diffusely adhering Escherichia coli

Over the last few years, dramatic increases in our knowledge about diffusely adhering Escherichia coli (DAEC) pathogenesis have taken place. The typical class of DAEC includes E. coli strains harboring AfaE-I, AfaE-II, AfaE-III, AfaE-V, Dr, Dr-II, F1845, and NFA-I adhesins (Afa/Dr DAEC); these strains (i) have an identical genetic organization and (ii) allow binding to human decay-accelerating factor (DAF) (Afa/Dr(DAF) subclass) or carcinoembryonic antigen (CEA) (Afa/Dr(CEA) subclass). The atypical class of DAEC includes two subclasses of strains; the atypical subclass 1 includes E. coli strains that express AfaE-VII, AfaE-VIII, AAF-I, AAF-II, and AAF-III adhesins, which (i) have an identical genetic organization and (ii) do not bind to human DAF, and the atypical subclass 2 includes E. coli strains that harbor Afa/Dr adhesins or others adhesins promoting diffuse adhesion, together with pathogenicity islands such as the LEE pathogenicity island (DA-EPEC). In this review, the focus is on Afa/Dr DAEC strains that have been found to be associated with urinary tract infections and with enteric infection. The review aims to provide a broad overview and update of the virulence aspects of these intriguing pathogens. Epidemiological studies, diagnostic techniques, characteristic molecular features of Afa/Dr operons, and the respective role of Afa/Dr adhesins and invasins in pathogenesis are described. Following the recognition of membrane-bound receptors, including type IV collagen, DAF, CEACAM1, CEA, and CEACAM6, by Afa/Dr adhesins, activation of signal transduction pathways leads to structural and functional injuries at brush border and junctional domains and to proinflammatory responses in polarized intestinal cells. In addition, uropathogenic Afa/Dr DAEC strains, following recognition of beta(1) integrin as a receptor, enter epithelial cells by a zipper-like, raft- and microtubule-dependent mechanism. Finally, the presence of other, unknown virulence factors and the way that an Afa/Dr DAEC strain emerges from the human intestinal microbiota as a "silent pathogen" are discussed.