Tau Antibody Structure Reveals a Molecular Switch Defining a Pathological Conformation of the Tau Protein

Tau antibodies have shown therapeutic potential for Alzheimer's disease and several are in clinical trials. As a microtubule-associated protein, tau relies on dynamic phosphorylation for its normal functions. In tauopathies, it becomes hyperphosphorylated and aggregates into toxic assemblies, which collectively lead to neurodegeneration. Of the phospho-epitopes, the region around Ser396 has received particular attention because of its prominence and stability in tauopathies. Here we report the first structure of a monoclonal tau antibody in complex with the pathologically important phospho-Ser396 residue. Its binding region reveals tau residues Tyr394 to phospho-Ser396 stabilized in a β-strand conformation that is coordinated by a phospho-specific antigen binding site. These details highlight a molecular switch that defines this prominent conformation of tau and ways to target it. Overall, the structure of the antibody-antigen complex clarifies why certain phosphorylation sites in tau are more closely linked to neurodegeneration than others.

Electron Pair Repulsion Responsible for the Peculiar Edge Effects and Surface Chemistry of Black Phosphorus

The electronic and optical properties of black phosphorus (black-P) are significantly modulated by fabricating the edges of this two-dimensional material. Electron lone pairs (ELPs) are ubiquitous in black-P, but their role in creating the edge effects of black-P is poorly understood. Using first-principle calculations, we report ELPs of black-P experience severe Coulomb repulsion and play a central role in creating the edge effects of black-P. We discover the outermost P atoms of the zigzag edges of black-PQDs are free of the Coulomb repulsion, but the P atoms of the armchair edges do experience the Coulomb repulsion. The Coulomb repulsion serves as a new chemical driving force to make electron donor-acceptor bonds with chemical groups bearing vacant orbitals. Our results provide insights into the mechanism responsible for the peculiar edge effects of black-P and highlight the opportunity to use the ELPs of black-P for their damage-free surface functionalization.

[Correlation between Genetic Variants and Polymorphism of Caveolin and Sudden Unexplained Death]

OBJECTIVES

To explore the genetic variation sites of caveolin （CAV） and their correlation with sudden unexplained death （SUD）.

METHODS

The blood samples were collected from SUD group （71 cases）, coronary artery disease （CAD） group （62 cases） and control group （60 cases）, respectively. The genome DNA were extracted and sequencing was performed directly by amplifying gene coding region and exon-intron splicing region of CAV1 and CAV3 using PCR. The type of heritable variation of CVA was confirmed and statistical analysis was performed.

RESULTS

A total of 4 variation sites that maybe significative were identified in SUD group, and two were newfound which were CAV1： c.45C>T （T15T） and CAV1：c.512G>A （R171H）, and two were SNP loci which were CAV1：c.246C>T （rs35242077） and CAV3：c.99C>T （rs1008642） and had significant difference （P<0.05） in allele and genotype frequencies between SUD and control groups. Forementioned variation sites were not found in CAD group.

CONCLUSIONS

The variants of CAV1 and CAV3 may be correlated with a part of SUD group.

[Research Progress of the Correlation between Caveolin and Unexpected Sudden Cardiac Death]

Due to the negative autopsy and without cardiac structural abnormalities, unexpected sudden cardiac death （USCD） is always a tough issue for forensic pathological expertise. USCD may be associated with parts of fatal arrhythmic diseases. These arrhythmic diseases may be caused by disorders of cardiac ion channels or channel-related proteins. Caveolin can combine with multiple myocardial ion channel proteins through its scaffolding regions and plays an important role in maintaining the depolarization and repolarization of cardiac action potential. When the structure and function of caveolin are affected by gene mutations or abnormal protein expression, the functions of the regulated ion channels are correspondingly impaired, which leads to the occurrence of multiple channelopathies, arrhythmia or even sudden cardiac death. It is important to study the effects of caveolin on the functions of ion channels for exploring the mechanisms of malignant arrhythmia and sudden cardiac death.

V1V2 multivalent scaffolds induce focused antibody responses with functional activity, prolonged durability, and modest neutralization potency

The V1V2 region of HIV-1 gp120 is vulnerable to a group of broadly neutralizing monoclonal antibodies (bnMAbs), and RV144 vaccinees developed strongly binding Abs to this region that correlated with a reduced risk of infection. This region of Env is now recognized as an important vaccine target, notwithstanding its relatively poorly immunogenic epitopes. We hypothesized that presenting V1V2 in multivalent scaffolds could improve immunogenicity and structurally constraining the epitopes could induce a more focused immune response in animal models. We designed of a panel of immunogens engrafting the V1V2 domain into trimeric and pentameric scaffolds. Here, we report results of immunogenicity studies in nonhuman primates (NHP) with 2 different V1V2 trimeric scaffolds. Our experimental approach in NHP consisted of 3 DNA and protein co-immunizations and a single protein boost. The DNA components consisted of 50μg of ZM109 gp120 plasmid delivered by gene gun with a simultaneous intramuscular injection of 100 μg of V1V2 scaffold protein immunogens. NHP immune responses were specific for V1V2 conformational epitopes with cross-reactive binding to heterologous gp120s and were functionally active in phagocytosis assays. Notably, V1V2 binding Ab responses were detectable 2 years after immunizations ceased. Neutralizing Abs developed in vaccinated NHP to Tier 1A and 1B, Clade B and Clade C viruses. The V1V2 structurally constrained scaffolds show promise as novel and highly effective at inducing functional antibody responses specifically targeted to the epitopes presented. Our results advance pre-clinical data in support of developing V1V2 vaccine candidates despite the structural complexity of the region.

Funding: P01AI100151-03

Modulation of epitope-specific antibody response by HIV Env immune-complex vaccines

HIV-1 envelope (Env) antigen is an important target for both neutralizing and non-neutralizing antibodies (Ab) against the virus. In the RV144 vaccine trial, correlate for reduced risk of HIV acquisition is the presence of binding but non-neutralizing Ab to V1V2 and V3 loops of HIV Env gp120. However, induction of high titer, durable, and cross-reactive Ab responses against these Env regions by vaccination is still an elusive goal.

Our past studies have shown that V3 immunogenicity can be significantly enhanced by vaccination with gp120/mAb immune complexes (IC) vs gp120 alone. Current study further examined parameters that confer enhanced immunogenicity to IC by testing in mice IC vaccines made with different HIV Env gp120 (clade B vs clade E) and mAb of different specificities (V3, V2, or CD4 binding site (CD4bs)). In addition, IC made of clade C gp140 and CD4bs mAb were also tested. The data showed that Greater titers of cross-reactive V3-binding Ab were induced by IC made of clade B gp120 and mAb to V2 or CD4bs as compare with gp120 alone, although the gp120/CD4bs IC was more potent in eliciting neutralizing V3 Ab. IC made with clade E gp120 or clade C gp140 did not show the same effects.Greater Ab responses to V1V2 were induced by IC made of clade E gp120 vs gp120 alone, regardless of the mAb used for IC, but the Ab were not neutralizing. IC made with clade B gp120 and clade C gp140 did not enhance V1V2 Ab titers.

These results demonstrate the capacity of Env/mAb IC vaccines to modulate induction of Ab responses to V1V2 and V3, and this activity is dependent on both Env proteins and mAb used to form IC. Further exploration using a cocktail of IC is proposed to improve Ab repertoires generated upon vaccination.

Differential induction of anti-V3 crown antibodies with cradle- and ladle-binding modes in response to HIV-1 envelope vaccination

The V3 loop in the HIV envelope gp120 is one of the immunogenic sites targeted by Abs. The V3 crown in particular has conserved structural elements recognized by cross-reactive neutralizing Abs, indicating its potential contribution in protection against HIV. Crystallographic analyses of anti-V3 crown mAbs in complex with the V3 peptides have revealed that these mAbs recognize the conserved sites on the V3 crown via two distinct strategies: a cradle-binding mode (V3C) and a ladle-binding (V3L) mode. However, almost all of the anti-V3 crown mAbs studied in the past were isolated from chronically HIV-infected individuals. The extents to which the two types of anti-V3 crown Abs are generated by vaccination are unknown. This study analyzed the prevalence of V3C-type and V3L-type Ab responses in HIV-infected individuals and in HIV envelope-immunized humans and animals using peptide mimotopes that distinguish the two Ab types. The results show that both V3L-type and V3C-type Abs were generated by the vast majority of chronically HIV-infected humans, although the V3L-type were more prevalent. In contrast, only one of the two V3 Ab types was elicited in vaccinated humans or animal models, irrespective of HIV-1 envelope clades, envelope constructs (oligomeric or monomeric), and protocols (DNA plus protein or protein alone) used for vaccinations. The V3C-type Abs were produced by vaccinated humans, macaques, and rabbits, whereas the V3L-type Abs were made by mice. The V3C-type and V3L-type Abs generated by the vaccinations were able to mediate virus neutralization. These data indicate the restricted repertoires and the species-specific differences in the functional V3-specific Ab responses induced by the HIV envelope vaccines. The study implies the need for improving immunogen designs and vaccination strategies to broaden the diversity of Abs in order to target the different conserved epitopes in the V3 loop and, by extension, in the entire HIV envelope.

Antigenic landscape of the HIV-1 envelope and new immunological concepts defined by HIV-1 broadly neutralizing antibodies

The isolation of HIV-1 broadly neutralizing antibodies (bnAbs) has demonstrated the ability of the human immune system to mount effective antibody responses against the virus. To harness this immune potential to elicit similar antibody responses by vaccination, it is important to understand the immunological processes that produce them. Here we review recent advances in crystal structural determinations of HIV-1 bnAb epitopes that directly portray the antigenic landscape of the HIV-1 envelope glycoprotein. We also summarize new immunological concepts implicated in bnAb sequences and their lineage studies.

Structure/Function Studies Involving the V3 Region of the HIV-1 Envelope Delineate Multiple Factors That Affect Neutralization Sensitivity

Antibodies (Abs) specific for the V3 loop of the HIV-1 gp120 envelope neutralize most tier 1 and many tier 2 viruses and are present in essentially all HIV-infected individuals as well as immunized humans and animals. Vaccine-induced V3 Abs are associated with reduced HIV infection rates in humans and affect the nature of transmitted viruses in infected vaccinees, despite the fact that V3 is often occluded in the envelope trimer. Here, we link structural and experimental data showing how conformational alterations of the envelope trimer render viruses exceptionally sensitive to V3 Abs. The experiments interrogated the neutralization sensitivity of pseudoviruses with single amino acid mutations in various regions of gp120 that were predicted to alter packing of the V3 loop in the Env trimer. The results indicate that the V3 loop is metastable in the envelope trimer on the virion surface, flickering between states in which V3 is either occluded or available for binding to chemokine receptors (leading to infection) and to V3 Abs (leading to virus neutralization). The spring-loaded V3 in the envelope trimer is easily released by disruption of the stability of the V3 pocket in the unliganded trimer or disruption of favorable V3/pocket interactions. Formation of the V3 pocket requires appropriate positioning of the V1V2 domain, which is, in turn, dependent on the conformation of the bridging sheet and on the stability of the V1V2 B-C strand-connecting loop.

IMPORTANCE

The levels of antibodies to the third variable region (V3) of the HIV envelope protein correlate with reduced HIV infection rates. Previous studies showed that V3 is often occluded, as it sits in a pocket of the envelope trimer on the surface of virions; however, the trimer is flexible, allowing occluded portions of the envelope (like V3) to flicker into an exposed position that binds antibodies. Here we provide a systematic interrogation of mechanisms by which single amino acid changes in various regions of gp120 (i) render viruses sensitive to neutralization by V3 antibodies, (ii) result in altered packing of the V3 loop, and (iii) activate an open conformation that exposes V3 to the effects of V3 Abs. Taken together, these and previous studies explain how V3 antibodies can protect against HIV-1 infection and why they should be one of the targets of vaccine-induced antibodies.

Vaccine-induced Human Antibodies Specific for the Third Variable Region of HIV-1 gp120 Impose Immune Pressure on Infecting Viruses

To evaluate the role of V3-specific IgG antibodies (Abs) in the RV144 clinical HIV vaccine trial, which reduced HIV-1 infection by 31.2%, the anti-V3 Ab response was assessed. Vaccinees' V3 Abs were highly cross-reactive with cyclic V3 peptides (cV3s) from diverse virus subtypes. Sieve analysis of CRF01_AE breakthrough viruses from 43 vaccine- and 66 placebo-recipients demonstrated an estimated vaccine efficacy of 85% against viruses with amino acids mismatching the vaccine at V3 site 317 (p = 0.004) and 52% against viruses matching the vaccine at V3 site 307 (p = 0.004). This analysis was supported by data showing that vaccinees' plasma Abs were less reactive with I³⁰⁷ when replaced with residues found more often in vaccinees' breakthrough viruses. Simultaneously, viruses with mutations at F³¹⁷ were less infectious, possibly due to the contribution of F³¹⁷ to optimal formation of the V3 hydrophobic core. These data suggest that RV144-induced V3-specific Abs imposed immune pressure on infecting viruses and inform efforts to design an HIV vaccine.

•

The RV144 vaccine reduced infection by viruses with isoleucine in V3 position 307.

•

Many vaccine-induced antibodies are cross-reactive and target an epitope including I³⁰⁷.

•

There was selection for breakthrough viruses carrying F³¹⁷ in V3 (p = 0.004).

•

F³¹⁷ is needed to maintain optimal infectivity.

•

F³¹⁷ is a poor or non-contact residue for vaccine induced V3 antibodies.

Single genome analysis reveals genetic characteristics of Neuroadaptation across HIV-1 envelope

Background

The widespread use of highly effective, combination antiretroviral therapy (cART) has led to a significant reduction in the incidence of HIV-associated dementia (HAD). Despite these advances, the prevalence of HIV-1 associated neurocognitive disorders (HANDs) has been estimated at approximately 40%-50%. In the cART era, the majority of this disease burden is represented by asymptomatic neurocognitive impairment and mild neurocognitive disorder (ANI and MND respectively). Although less severe than HAD, these diagnoses carry with them substantial morbidity.

Results

In this cross-sectional study, single genome amplification (SGA) was used to sequence 717 full-length HIV-1 envelope (env) clade B variants from the paired cerebrospinal fluid (CSF) and blood plasma samples of fifteen chronically infected HIV-positive individuals with normal neurocognitive performance (NCN), ANI and MND. Various degrees of compartmentalization were found across disease states and history of cART utilization. In individuals with compartmentalized virus, mean HIV-1 env population diversity was lower in the CSF than plasma-derived variants. Overall, mean V1V2 loop length was shorter in CSF-derived quasispecies when compared to contemporaneous plasma populations, and this was found to correlate with a lower mean number of N-linked glycosylation sites in this region. A number of discrete amino acid positions that correlate strongly with compartmentalization in the CSF were identified in both variable and constant regions of gp120 as well as in gp41. Correlated mutation analyses further identified that a subset of amino acid residues in these compartmentalization “hot spot” positions were strongly correlated with one another, suggesting they may play an important, definable role in the adaptation of viral variants to the CSF. Analysis of these hot spots in the context of a well-supported crystal structure of HIV-1 gp120 suggests mechanisms through which amino acid differences at the identified residues might contribute to viral compartmentalization in the CSF.

Conclusions

The detailed analyses of SGA-derived full length HIV-1 env from subjects with both normal neurocognitive performance and the most common HAND diagnoses in the cART era allow us to identify novel and confirm previously described HIV-1 env genetic determinants of neuroadaptation and relate potential motifs to HIV-1 env structure and function.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-014-0065-0) contains supplementary material, which is available to authorized users.

Vaccine focusing to cross-subtype HIV-1 gp120 variable loop epitopes

We designed synthetic, epitope-focused immunogens that preferentially display individual neutralization epitopes targeted by cross-subtype anti-HIV V3 loop neutralizing monoclonal antibodies (mAbs). Vaccination of rabbits with these immunogens resulted in the elicitation of distinct polyclonal serum Abs that exhibit cross-subtype neutralization specificities mimicking the mAbs that guided the design. Our results prove the principle that a predictable range of epitope-specific polyclonal cross-subtype HIV-1 neutralizing Abs can be intentionally elicited in mammals by vaccination. The precise boundaries of the epitopes and conformational flexibility in the presentation of the epitopes in the immunogen appeared to be important for successful elicitation. This work may serve as a starting point for translating the activities of human broadly neutralizing anti-HIV-1 monoclonal antibodies (bNAbs) into matched immunogens that can contribute to an efficacious HIV-1 vaccine.

Thermodynamic signatures of the antigen binding site of mAb 447-52D targeting the third variable region of HIV-1 gp120

The third variable region (V3) of HIV-1 gp120 plays a key role in viral entry into host cells; thus, it is a potential target for vaccine design. Human monoclonal antibody (mAb) 447-52D is one of the most broadly and potently neutralizing anti-V3 mAbs. We further characterized the 447-52D epitope by determining a high-resolution crystal structure of the Fab fragment in complex with a cyclic V3 and interrogated the antigen-antibody interaction by a combination of site-specific mutagenesis, isothermal titration calorimetry (ITC) and neutralization assays. We found that 447-52D's neutralization capability is correlated with its binding affinity and at 25 °C the Gibbs free binding energy is composed of a large enthalpic component and a small favorable entropic component. The large enthalpic contribution is due to (i) an extensive hydrogen bond network, (ii) a π-cation sandwiching the V3 crown apex residue Arg(315), and (iii) a salt bridge between the 447-52D heavy chain residue Asp(H95) and Arg(315). Arg(315) is often harbored by clade B viruses; thus, our data explained why 447-52D preferentially neutralizes clade B viruses. Interrogation of the thermodynamic signatures of residues at the antigen binding interface gives key insights into their contributions in the antigen-antibody interaction.

Epitope mapping of conformational V2-specific anti-HIV human monoclonal antibodies reveals an immunodominant site in V2

In the case-control study of the RV144 vaccine trial, the levels of antibodies to the V1V2 region of the gp120 envelope glycoprotein were found to correlate inversely with risk of HIV infection. This recent demonstration of the potential role of V1V2 as a vaccine target has catapulted this region into the focus of HIV-1 research. We previously described seven human monoclonal antibodies (mAbs) derived from HIV-infected individuals that are directed against conformational epitopes in the V1V2 domain. In this study, using lysates of SF162 pseudoviruses carrying V1V2 mutations, we mapped the epitopes of these seven mAbs. All tested mAbs demonstrated a similar binding pattern in which three mutations (F176A, Y177T, and D180L) abrogated binding of at least six of the seven mAbs to ≤15% of SF162 wildtype binding. Binding of six or all of the mAbs was reduced to ≤50% of wildtype by single substitutions at seven positions (168, 180, 181, 183, 184, 191, and 193), while one change, V181I, increased the binding of all mAbs. When mapped onto a model of V2, our results suggest that the epitope of the conformational V2 mAbs is located mostly in the disordered region of the available crystal structure of V1V2, overlapping and surrounding the α4β7 binding site on V2.

Immune exposure to a specific sequence of antibody escape variants could program long-term potential for neutralization breadth in subtype A HIV-1 infection (P6161)

Approximately one quarter of HIV-1 infected individuals will generate broadly neutralizing antibodies, but the exact mechanisms for triggering and maturation of these responses are presently undefined. We consequently investigated the roots of neutralization breadth. In a subtype A HIV-1 infected Rwandan seroconverter, we pinpointed the primary neutralizing antibody target and the successive routes of viral escape using envelope glycoproteins from the transmitted/founder virus and longitudinal escape variants along with autologous plasma and monoclonal antibodies. Initially, a single mutation at one of three clustered residues proximal to gp120’s third hypervariable loop conferred viral escape. This putative epitope subsequently elicited at least two somatically related monoclonal antibodies, which bound and neutralized the established escape mutations. Resistance to this secondary wave of immune pressure then arose in later viral envelopes through introduction of two glycans that obscured this consistently targeted space. At 16-months post-infection, what had been a narrow, regional response evolved to force recognition and neutralization of distinct envelope portions, which resulted in moderate cross-clade humoral breadth. Our data suggest that unveiling a certain chain of envelope mutations could drive B cells toward the production of broadly neutralizing antibodies. Appreciating this knowledge during immunogen construction could positively impact HIV-1 vaccine design.

Viral escape from neutralizing antibodies in early subtype A HIV-1 infection drives an increase in autologous neutralization breadth

Antibodies that neutralize (nAbs) genetically diverse HIV-1 strains have been recovered from a subset of HIV-1 infected subjects during chronic infection. Exact mechanisms that expand the otherwise narrow neutralization capacity observed during early infection are, however, currently undefined. Here we characterized the earliest nAb responses in a subtype A HIV-1 infected Rwandan seroconverter who later developed moderate cross-clade nAb breadth, using (i) envelope (Env) glycoproteins from the transmitted/founder virus and twenty longitudinal nAb escape variants, (ii) longitudinal autologous plasma, and (iii) autologous monoclonal antibodies (mAbs). Initially, nAbs targeted a single region of gp120, which flanked the V3 domain and involved the alpha2 helix. A single amino acid change at one of three positions in this region conferred early escape. One immunoglobulin heavy chain and two light chains recovered from autologous B cells comprised two mAbs, 19.3H-L1 and 19.3H-L3, which neutralized the founder Env along with one or three of the early escape variants carrying these mutations, respectively. Neither mAb neutralized later nAb escape or heterologous Envs. Crystal structures of the antigen-binding fragments (Fabs) revealed flat epitope contact surfaces, where minimal light chain mutation in 19.3H-L3 allowed for additional antigenic interactions. Resistance to mAb neutralization arose in later Envs through alteration of two glycans spatially adjacent to the initial escape signatures. The cross-neutralizing nAbs that ultimately developed failed to target any of the defined V3-proximal changes generated during the first year of infection in this subject. Our data demonstrate that this subject's first recognized nAb epitope elicited strain-specific mAbs, which incrementally acquired autologous breadth, and directed later B cell responses to target distinct portions of Env. This immune re-focusing could have triggered the evolution of cross-clade antibodies and suggests that exposure to a specific sequence of immune escape variants might promote broad humoral responses during HIV-1 infection.

Efficiency of bridging-sheet recruitment explains HIV-1 R5 envelope glycoprotein sensitivity to soluble CD4 and macrophage tropism

HIV-1 R5 viruses vary extensively in their capacity to infect macrophages. R5 viruses that confer efficient infection of macrophages are able to exploit low levels of CD4 for infection and predominate in brain tissue, where macrophages are a major target for infection. HIV-1 R5 founder viruses that are transmitted were reported to be non-macrophage-tropic. Here, we investigated the sensitivities of macrophage-tropic and non-macrophage-tropic R5 envelopes to neutralizing antibodies. We observed striking differences in the sensitivities of Env(+) pseudovirions to soluble CD4 (sCD4) and to neutralizing monoclonal antibodies (MAbs) that target the CD4 binding site. Macrophage-tropic R5 Envs were sensitive to sCD4, while non-macrophage-tropic Envs were significantly more resistant. In contrast, all Envs were sensitive to VRC01 regardless of tropism, while MAb b12 conferred an intermediate neutralization pattern where all the macrophage-tropic and about half of the non-macrophage-tropic Envs were sensitive. CD4, b12, and VRC01 share binding specificities on the outer domain of gp120. However, these antibodies differ in their ability to induce conformational changes on the trimeric envelope and in specificity for residues on the V1V2 loop stem and β20-21 junction that are targets for CD4 in recruiting the bridging sheet. These distinct specificities of CD4, b12, and VRC01 likely explain the observed differences in Env sensitivity to inhibition by these reagents and provide an insight into the envelope mechanisms that control macrophage tropism. We present a model where the efficiency of bridging-sheet recruitment by CD4 is a major determinant of HIV-1 R5 envelope sensitivity to soluble CD4 and macrophage tropism.

Skin tattooing as a novel approach for DNA vaccine delivery

Nucleic acid-based vaccination is a topic of growing interest, especially plasmid DNA (pDNA) encoding immunologically important antigens. After the engineered pDNA is administered to the vaccines, it is transcribed and translated into immunogen proteins that can elicit responses from the immune system. Many ways of delivering DNA vaccines have been investigated; however each delivery route has its own advantages and pitfalls. Skin tattooing is a novel technique that is safe, cost-effective, and convenient. In addition, the punctures inflicted by the needle could also serve as a potent adjuvant. Here, we a) demonstrate the intradermal delivery of plasmid DNA encoding enhanced green fluorescent protein (pCX-EGFP) in a mouse model using a tattooing device and b) confirm the effective expression of EGFP in the skin cells using confocal microscopy.

Structure of the pentameric ligand-gated ion channel GLIC bound with anesthetic ketamine

Pentameric ligand-gated ion channels (pLGICs) are targets of general anesthetics, but a structural understanding of anesthetic action on pLGICs remains elusive. GLIC, a prokaryotic pLGIC, can be inhibited by anesthetics, including ketamine. The ketamine concentration leading to half-maximal inhibition of GLIC (58 μM) is comparable to that on neuronal nicotinic acetylcholine receptors. A 2.99 Å resolution X-ray structure of GLIC bound with ketamine revealed ketamine binding to an intersubunit cavity that partially overlaps with the homologous antagonist-binding site in pLGICs. The functional relevance of the identified ketamine site was highlighted by profound changes in GLIC activation upon cysteine substitution of the cavity-lining residue N152. The relevance is also evidenced by changes in ketamine inhibition upon the subsequent chemical labeling of N152C. The results provide structural insight into the molecular recognition of ketamine and are valuable for understanding the actions of anesthetics and other allosteric modulators on pLGICs.

[Spectrum-active relation research on Typha angustifolia before and after carbonized]

OBJECTIVE

To study the components of Typha angustifolia change before and after carbonized and the correlation with its effective.

METHODS

The chemical constitutions between the pollen and its carbonized product were compared by UPLC-MASS and their thrombin activity was tested.

RESULTS

The change of components was significant especially the flavonoids. The content of flavonoid glycoside was reduced obviously and the thrombin activity showed the main difference was quercetin and isorhamnetin.

CONCLUSION

The flavonoids in pollen typha may be the main factors in their thrombin activity.

Functional and immunochemical cross-reactivity of V2-specific monoclonal antibodies from HIV-1-infected individuals

The recent analysis of the first successful RV144 vaccine trial revealed that a high titer of plasma anti-V2 antibodies (Abs) correlated with a decreased risk of HIV-1 infection in vaccine recipients. To understand the mechanism of immune correlates, we studied seven anti-V2 monoclonal Abs (mAbs) developed from HIV-1 infected individuals. The V2 mAbs target conserved epitopes, including the binding site for α4β7 integrin, and are broadly cross-reactive with various gp120 proteins. Preferential usage of the VH1-69 gene by V2 mAbs may depend on selection by the same antigenic structure. Six of seven V2 mAbs weakly neutralized four to eight of the 41 pseudoviruses tested and resistance to neutralization was correlated with longer V2 domains. The data suggest the presence of shared, conserved structural elements in the V2 loop, and these can be used in the design of vaccine immunogens inducing broadly reactive Abs with anti-viral activities.

Structure of the pentameric ligand-gated ion channel ELIC cocrystallized with its competitive antagonist acetylcholine

ELIC, the pentameric ligand-gated ion channel from Erwinia chrysanthemi, is a prototype for Cys-loop receptors. Here we show that acetylcholine is a competitive antagonist for ELIC. We determine the acetylcholine–ELIC cocrystal structure to a 2.9-Å resolution and find that acetylcholine binding to an aromatic cage at the subunit interface induces a significant contraction of loop C and other structural rearrangements in the extracellular domain. The side chain of the pore-lining residue F247 reorients and the pore size consequently enlarges, but the channel remains closed. We attribute the inability of acetylcholine to activate ELIC primarily to weak cation-π and electrostatic interactions in the pocket, because an acetylcholine derivative with a simple quaternary-to-tertiary ammonium substitution activates the channel. This study presents a compelling case for understanding the structural underpinning of the functional relationship between agonism and competitive antagonism in the Cys-loop receptors, providing a new framework for developing novel therapeutic drugs.

The pentameric ligand gated ion channel from Erwinia chrysanthemi (ELIC) is similar in structure to the nicotinic acetylcholine receptor, a member of the Cys-loop receptor family. This study reports the crystal structure of ELIC bound to acetylcholine and shows that acetylcholine is a competitive antagonist of ELIC.

Structural analysis of human and macaque mAbs 2909 and 2.5B: implications for the configuration of the quaternary neutralizing epitope of HIV-1 gp120

The quaternary neutralizing epitope (QNE) of HIV-1 gp120 is preferentially expressed on the trimeric envelope spikes of intact HIV virions, and QNE-specific monoclonal antibodies (mAbs) potently neutralize HIV-1. Here, we present the crystal structures of the Fabs of human mAb 2909 and macaque mAb 2.5B. Both mAbs have long beta hairpin CDR H3 regions >20 Å in length that are each situated at the center of their respective antigen-binding sites. Computational analysis showed that the paratopes include the whole CDR H3, while additional CDR residues form shallow binding pockets. Structural modeling suggests a way to understand the configuration of QNEs and the antigen-antibody interaction for QNE mAbs. Our data will be useful in designing immunogens that may elicit potent neutralizing QNE Abs.

Structure-guided design and immunological characterization of immunogens presenting the HIV-1 gp120 V3 loop on a CTB scaffold

V3 loop is a major neutralizing determinant of the HIV-1 gp120. Using 3D structures of cholera toxin B subunit (CTB), complete V3 in the gp120 context, and V3 bound to a monoclonal antibody (mAb), we designed two V3-scaffold immunogen constructs (V3-CTB). The full-length V3-CTB presenting the complete V3 in a structural context mimicking gp120 was recognized by the large majority of our panel of 24 mAbs. The short V3-CTB presenting a V3 fragment in the conformation observed in the complex with the 447-52D Fab, exhibited high-affinity binding to this mAb. The immunogens were evaluated in rabbits using DNA-prime/protein-boost protocol. Boosting with the full-length V3-CTB induced high anti-V3 titers in sera that potently neutralize multiple HIV virus strains. The short V3-CTB was ineffective. The results suggest that very narrow antigenic profile of an immunogen is associated with poor Ab response. An immunogen with broader antigenic activity elicits robust Ab response.

Conserved structural elements in the V3 crown of HIV-1 gp120

Binding of the third variable region (V3) of the HIV-1 envelope glycoprotein gp120 to the cell-surface coreceptors CCR5 or CXCR4 during viral entry suggests that there are conserved structural elements in this sequence-variable region. These conserved elements could serve as epitopes to be targeted by a vaccine against HIV-1. Here we perform a systematic structural analysis of representative human anti-V3 monoclonal antibodies in complex with V3 peptides, revealing that the crown of V3 has four conserved structural elements: an arch, a band, a hydrophobic core and the peptide backbone. These are either unaffected by or are subject to minimal sequence variation. As these regions are targeted by cross-clade neutralizing human antibodies, they provide a blueprint for the design of vaccine immunogens that could elicit broadly cross-reactive protective antibodies.

Structural conservation predominates over sequence variability in the crown of HIV type 1's V3 loop

The diversity of HIV-1 is a confounding problem for vaccine design, as the human immune response appears to favor poor or strain-specific responses to any given HIV-1 virus strain. A significant portion of this diversity is manifested as sequence variability in the loops of HIV-1's surface envelope glycoprotein. Here we show that the most variable sequence positions in the third variable (V3) loop crown cluster to a small zone on the surface of one face of the V3 loop ss-hairpin conformation. These results provide a novel visualization of the gp120 V3 loop, specifically demonstrating a surprising preponderance of conserved three-dimensional structure in a highly sequence-variable region. From a structural point of view, there appears to be less diversity in this region of the HIV-1 "principle neutralizing domain" than previously appreciated.

Structural and functional studies of Aspergillus oryzae cutinase: enhanced thermostability and hydrolytic activity of synthetic ester and polyester degradation

Cutinases are responsible for hydrolysis of the protective cutin lipid polyester matrix in plants and thus have been exploited for hydrolysis of small molecule esters and polyesters. Here we explore the reactivity, stability, and structure of Aspergillus oryzae cutinase and compare it to the well-studied enzyme from Fusarium solani. Two critical differences are highlighted in the crystallographic analysis of the A. oryzae structure: (i) an additional disulfide bond and (ii) a topologically favored catalytic triad with a continuous and deep groove. These structural features of A. oryzae cutinase are proposed to result in an improved hydrolytic activity and altered substrate specificity profile, enhanced thermostability, and remarkable reactivity toward the degradation of the synthetic polyester polycaprolactone. The results presented here provide insight into engineering new cutinase-inspired biocatalysts with tailor-made properties.

A new activity of anti-HIV and anti-tumor protein GAP31: DNA adenosine glycosidase--structural and modeling insight into its functions

We report here the high-resolution atomic structures of GAP31 crystallized in the presence of HIV-LTR DNA oligonucleotides systematically designed to examine the adenosine glycosidase activity of this anti-HIV and anti-tumor plant protein. Structural analysis and molecular modeling lead to several novel findings. First, adenine is bound at the active site in the crystal structures of GAP31 to HIV-LTR duplex DNA with 5' overhanging adenosine ends, such as the 3'-processed HIV-LTR DNA but not to DNA duplex with blunt ends. Second, the active site pocket of GAP31 is ideally suited to accommodate the 5' overhanging adenosine of the 3'-processed HIV-LTR DNA and the active site residues are positioned to perform the adenosine glycosidase activity. Third, GAP31 also removes the 5'-end adenine from single-stranded HIV-LTR DNA oligonucleotide as well as any exposed adenosine, including that of single nucleotide dAMP but not from AMP. Fourth, GAP31 does not de-purinate guanosine from di-nucleotide GT. These results suggest that GAP31 has DNA adenosine glycosidase activity against accessible adenosine. This activity is distinct from the generally known RNA N-glycosidase activity toward the 28S rRNA. It may be an alternative function that contributes to the antiviral and anti-tumor activities of GAP31. These results provide molecular insights consistent with the anti-HIV mechanisms of GAP31 in its inhibition on the integration of viral DNA into the host genome by HIV-integrase as well as irreversible topological relaxation of the supercoiled viral DNA.

Structural basis of the cross-reactivity of genetically related human anti-HIV-1 mAbs: implications for design of V3-based immunogens

Human monoclonal antibodies 447-52D and 537-10D, both coded by the VH3 gene and specific for the third variable region (V3) of the HIV-1 gp120, were found to share antigen-binding structural elements including an elongated CDR H3 forming main-chain interactions with the N terminus of the V3 crown. However, water-mediated hydrogen bonds and a unique cation-pi sandwich stacking allow 447-52D to be broadly reactive with V3 containing both the GPGR and GPGQ crown motifs, while the deeper binding pocket and a buried Glu in the binding site of 537-10D limit its reactivity to only V3 containing the GPGR motif. Our results suggest that the design of immunogens for anti-V3 antibodies should avoid the Arg at the V3 crown, as GPGR-containing epitopes appear to select for B cells making antibodies of narrower specificity than V3 that carry Gln at this position.

Uropathogenic E. coli adhesin-induced host cell receptor conformational changes: implications in transmembrane signaling transduction

Urinary tract infection is the second most common infectious disease and is caused predominantly by type 1-fimbriated uropathogenic Escherichia coli (UPEC). UPEC initiates infection by attaching to uroplakin (UP) Ia, its urothelial surface receptor, via the FimH adhesins capping the distal end of its fimbriae. UP Ia, together with UP Ib, UP II, and UP IIIa, forms a 16-nm receptor complex that is assembled into hexagonally packed, two-dimensional crystals (urothelial plaques) covering >90% of the urothelial apical surface. Recent studies indicate that FimH is the invasin of UPEC as its attachment to the urothelial surface can induce cellular signaling events including calcium elevation and the phosphorylation of the UP IIIa cytoplasmic tail, leading to cytoskeletal rearrangements and bacterial invasion. However, it remains unknown how the binding of FimH to the UP receptor triggers a signal that can be transmitted through the highly impermeable urothelial apical membrane. We show here by cryo-electron microscopy that FimH binding to the extracellular domain of UP Ia induces global conformational changes in the entire UP receptor complex, including a coordinated movement of the tightly bundled transmembrane helices. This movement of the transmembrane helix bundles can cause a corresponding lateral translocation of the UP cytoplasmic tails, which can be sufficient to trigger downstream signaling events. Our results suggest a novel pathogen-induced transmembrane signal transduction mechanism that plays a key role in the initial stages of UPEC invasion and receptor-mediated bacterial invasion in general.

Enhanced susceptibility of Cpt1c knockout mice to glucose intolerance induced by a high-fat diet involves elevated hepatic gluconeogenesis and decreased skeletal muscle glucose uptake

AIMS/HYPOTHESIS

Carnitine palmitoyltransferase-1 (CPT1)c is a novel isoform in the CPT1 family and is found specifically in the brain. Cpt1c knockout (KO) mice are more susceptible to high-fat diet (HFD)-induced obesity. However, the underlying mechanism of this phenotype and the question of whether CPT1c is involved in the pathogenesis of diet-induced insulin resistance are unclear.

METHODS

To assess the potential role of CPT1c in the regulation of whole-body glucose homeostasis, we generated Cpt1c KO mice and challenged them with HFD or standard chow. Glucose homeostasis of each group was assessed weekly.

RESULTS

After 8 weeks of HFD feeding, Cpt1c KO mice developed a phenotype of more severe insulin resistance than that in wild-type controls. The increased susceptibility of Cpt1c KO mice to HFD-induced insulin resistance was independent of obesity. Impaired glucose tolerance in Cpt1c KO mice was attributable to elevated hepatic gluconeogenesis and decreased glucose uptake in skeletal muscle. These effects correlated with decreased hepatic and intramuscular fatty acid oxidation and expression of oxidative genes as well as with elevated triacylglycerol content in these tissues. Interestingly, Cpt1c deletion caused a specific elevation of hypothalamic CPT1a and CPT1b isoform expression and activity. We demonstrated that elevated plasma NEFA concentration is one mechanism via which this compensatory effect is induced.

CONCLUSIONS/INTERPRETATION

These results further establish the role of CPT1c in controlling whole-body glucose homeostasis and in the regulation of hypothalamic Cpt1 isoform expression. We identify changes in hepatic and skeletal muscle glucose metabolism as important mechanisms determining the phenotype of Cpt1c KO mice.

Structure of a sliding clamp on DNA

The structure of the E. coli beta clamp polymerase processivity factor has been solved in complex with primed DNA. Interestingly, the clamp directly binds the DNA duplex and also forms a crystal contact with the ssDNA template strand, which binds into the protein-binding pocket of the clamp. We demonstrate that these clamp-DNA interactions function in clamp loading, perhaps by inducing the ring to close around DNA. Clamp binding to template ssDNA may also serve to hold the clamp at a primed site after loading or during switching of multiple factors on the clamp. Remarkably, the DNA is highly tilted as it passes through the beta ring. The pronounced 22 degrees angle of DNA through beta may enable DNA to switch between multiple factors bound to a single clamp simply by alternating from one protomer of the ring to the other.

Characteristics of the phagocytic cup induced by uropathogenic Escherichia coli

Uropathogenic Escherichia coli invade the urothelial umbrella cells by using the zipper mechanism. However, the details of the early events of this invasion, such as the formation of the phagocytic cup, are not yet well understood. We show here, using thin section electron microscopy and immunogold labeling, that the plasma membrane curves around the bacterial surface in the phagocytic cup. There exists a uniform gap between the bacterium and the urothelial membrane, and actin filaments are present in the phagocytic cup. We suggest that the action-reaction between the mechanical forces generated by pilus retraction of the bacterium and the actin polymerization in the urothelial cell plays a role in maintaining the phagocytic cup. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

A pachygyria-causing alpha-tubulin mutation results in inefficient cycling with CCT and a deficient interaction with TBCB

The agyria (lissencephaly)/pachygyria phenotypes are catastrophic developmental diseases characterized by abnormal folds on the surface of the brain and disorganized cortical layering. In addition to mutations in at least four genes--LIS1, DCX, ARX and RELN--mutations in a human alpha-tubulin gene, TUBA1A, have recently been identified that cause these diseases. Here, we show that one such mutation, R264C, leads to a diminished capacity of de novo tubulin heterodimer formation. We identify the mechanisms that contribute to this defect. First, there is a reduced efficiency whereby quasinative alpha-tubulin folding intermediates are generated via ATP-dependent interaction with the cytosolic chaperonin CCT. Second, there is a failure of CCT-generated folding intermediates to stably interact with TBCB, one of the five tubulin chaperones (TBCA-E) that participate in the pathway leading to the de novo assembly of the tubulin heterodimer. We describe the behavior of the R264C mutation in terms of its effect on the structural integrity of alpha-tubulin and its interaction with TBCB. In spite of its compromised folding efficiency, R264C molecules that do productively assemble into heterodimers are capable of copolymerizing into dynamic microtubules in vivo. The diminished production of TUBA1A tubulin in R264C individuals is consistent with haploinsufficiency as a cause of the disease phenotype.

Atomic force microscopy of Mammalian urothelial surface

The mammalian urothelium apical surface plays important roles in bladder physiology and diseases, and it provides a unique morphology for ultrastructural studies. Atomic force microscopy (AFM) is an emerging tool for studying the architecture and dynamic properties of biomolecular structures under near-physiological conditions. However, AFM imaging of soft tissues remains a challenge because of the lack of efficient methods for sample stabilization. Using a porous nitrocellulose membrane as the support, we were able to immobilize large pieces of soft mouse bladder tissue, thus enabling us to carry out the first AFM investigation of the mouse urothelial surface. The submicrometer-resolution AFM images revealed many details of the surface features, including the geometry of the urothelial plaques that cover the entire surface and the membrane interdigitation at the cell borders. This interdigitation creates a membrane zipper, likely contributing to the barrier function of the urothelium. In addition, we were able to image the intracellular bacterial communities of type 1-fimbriated bacteria grown between the intermediate filament bundles of the umbrella cells, shedding light on the bacterial colonization of the urothelium.

Selective enrichment and fractionation of phosphopeptides from peptide mixtures by isoelectric focusing after methyl esterification

We have developed a new strategy to enrich and fractionate phosphopeptides from peptide mixtures based on the difference in their isoelectric points (pIs) after methyl esterification. After isoelectric focusing (IEF) of a methylated tryptic digest of a mixture of alpha-S-casein and beta-casein, phosphopeptides were selectively enriched at acidic and neutral pHs while nonphosphopeptides left the focusing gel because their pIs are higher than the upper limit of the immobilized pH gradient. We wrote a web-based program, pIMethylation, to predict the pIs for peptides with and without methyl esterification. Theoretical calculations using pIMethylation indicated that methylated phosphopeptides and non-phosphopeptides can be grouped on the basis of the number of phosphate groups and basic residues in each peptide. Our IEF results were consistent with theoretical pIs of methylated peptides calculated by pIMethylation. We also showed that 2,6-dihydroxy-acetophenone is superior to 2,5-dihydroxybenzoic acid as a matrix for MALDI Q-TOF MS of methylated phosphopeptides in both positive and negative ion modes.

Integrity of all four transmembrane domains of the tetraspanin uroplakin Ib is required for its exit from the ER

The surface of the mammalian urinary bladder is covered by a crystalline, asymmetric unit membrane (AUM) structure that contains the four major uroplakins (UPs): Ia, Ib, II and IIIa. UPIa and UPIb belong to the family of tetraspanins. Although UPIa and UPIb are structurally conserved, only UPIb could exit from the endoplasmic reticulum (ER) and reach the cell surface when expressed alone in 293T cells. Modifications of the large extracellular loop of UPIb, such as mutation of the N-glycosylation site or the cysteines involved in the formation of three disulfide bridges, or exchanging the large luminal loop of UPIb with that of UPIa did not affect the ability of UPIb to reach the cell surface. However, modifications of any of the four transmembrane domains of UPIb led to ER retention, suggesting that the proper formation of helical bundles consisting of the tetraspanin transmembrane domains is a prerequisite for UPIb to exit from the ER. Results of sedimentation analysis suggested that aggregate formation is a mechanism for ER retention.

An arsenite-inducible 19S regulatory particle-associated protein adapts proteasomes to proteotoxicity

Protein misfolding caused by exposure to arsenite is associated with transcriptional activation of the AIRAP gene. We report here that AIRAP is an arsenite-inducible subunit of the proteasome's 19S cap that binds near PSMD2 at the 19S base. Compared to the wild-type, knockout mouse cells or C. elegans lacking AIRAP accumulate more polyubiquitylated proteins and exhibit higher levels of stress when exposed to arsenite, and proteasomes isolated from arsenite-treated AIRAP knockout cells are relatively impaired in substrate degradation in vitro. AIRAP's association with the 19S cap reverses the stabilizing affect of ATP on the 26S proteasome during particle purification, and AIRAP-containing proteasomes, though constituted of 19S and 20S subunits, acquire features of hybrid proteasomes with both 19S and 11S regulatory caps. These features include enhanced cleavage of peptide substrates and suggest that AIRAP adapts the cell's core protein degradation machinery to counteract proteotoxicity induced by an environmental toxin.

Structural basis for tetraspanin functions as revealed by the cryo-EM structure of uroplakin complexes at 6-A resolution

Tetraspanin uroplakins (UPs) Ia and Ib, together with their single-spanning transmembrane protein partners UP II and IIIa, form a unique crystalline 2D array of 16-nm particles covering almost the entire urothelial surface. A 6 Å–resolution cryo-EM structure of the UP particle revealed that the UP tetraspanins have a rod-shaped structure consisting of four closely packed transmembrane helices that extend into the extracellular loops, capped by a disulfide-stabilized head domain. The UP tetraspanins form the primary complexes with their partners through tight interactions of the transmembrane domains as well as the extracellular domains, so that the head domains of their tall partners can bridge each other at the top of the heterotetramer. The secondary interactions between the primary complexes and the tertiary interaction between the 16-nm particles contribute to the formation of the UP tetraspanin network. The rod-shaped tetraspanin structure allows it to serve as stable pilings in the lipid sea, ideal for docking partner proteins to form structural/signaling networks.

pH-dependent conformational switch activates the inhibitor of transcription elongation

Gfh1, a transcription factor from Thermus thermophilus, inhibits all catalytic activities of RNA polymerase (RNAP). We characterized the Gfh1 structure, function and possible mechanism of action and regulation. Gfh1 inhibits RNAP by competing with NTPs for coordinating the active site Mg2+ ion. This coordination requires at least two aspartates at the tip of the Gfh1 N-terminal coiled-coil domain (NTD). The overall structure of Gfh1 is similar to that of the Escherichia coli transcript cleavage factor GreA, except for the flipped orientation of the C-terminal domain (CTD). We show that depending on pH, Gfh1-CTD exists in two alternative orientations. At pH above 7, it assumes an inactive 'flipped' orientation seen in the structure, which prevents Gfh1 from binding to RNAP. At lower pH, Gfh1-CTD switches to an active 'Gre-like' orientation, which enables Gfh1 to bind to and inhibit RNAP.

Distinct glycan structures of uroplakins Ia and Ib: structural basis for the selective binding of FimH adhesin to uroplakin Ia

Although it has been shown that mouse uroplakin (UP) Ia, a major glycoprotein of urothelial apical surface, can serve as the receptor for the FimH lectin adhesin of type 1-fimbriated Escherichia coli, the organism that causes a great majority of urinary tract infections, the glycan structure of this native receptor was unknown. Using a sensitive approach that combines in-gel glycosidase and protease digestions, permethylation of released glycans, and mass spectrometry, we have elucidated for the first time the native glycoform structures of the mouse UPIa receptor and those of its non-binding homolog, UPIb, and have determined the glycosylation site occupancy. UPIa presents a high level of terminally exposed mannose residues (located on Man(6)GlcNAc(2) to Man(9)GlcNAc(2)) that are capable of specifically interacting with FimH. We have shown that this property is conserved not only in the mouse uroplakins but also in cattle and, even more importantly, in human UPIa, thus establishing the concept that UPIa is a major urothelial receptor in humans and other mammals for the mannose-specific FimH variant. In contrast, our results indicate that most terminally exposed glycans of mouse UPIb are non-mannose residues, thus explaining the failure of FimH to bind to this UPIb. In cattle, on the other hand, complex carbohydrates constituted only about 20% of the UPIb N-linked glycans. Human UPIa contained exclusively high mannose glycans, and human UPIb contained only complex glycans. The drastically different carbohydrate processing of the UPIa and UPIb proteins, two closely related members of the tetraspanin family, may reflect differences in their folding and masking due to their interactions with their associated proteins, UPII and UPIIIa, respectively. Results from this study shed light on the molecular pathogenesis of urinary tract infections and may aid in the design of glyco-mimetic inhibitors for preventing and treating this disease.

Structural basis of urothelial permeability barrier function as revealed by Cryo-EM studies of the 16 nm uroplakin particle

The apical surface of terminally differentiated mammalian urothelial umbrella cells is covered by numerous plaques consisting of two-dimensional (2D) crystals of hexagonally packed 16 nm uroplakin particles, and functions as a remarkable permeability barrier. To determine the structural basis of this barrier function, we generated, by electron cryo microscopy, a projection map of the isolated mouse urothelial plaques at 7 A and a 3D structure at 10 A resolution. Our results indicate that each 16 nm particle has a central 6 nm lipid-filled 'hole' surrounded by 6 inverted U-shaped subunits, each consisting of an inner and an outer subdomain connected via a distal joint. The transmembrane portion of each subdomain can fit about 5 helices. This finding, coupled with our STEM and EM data, suggests that uroplakin pairs Ia/II and Ib/III are associated with the inner and outer subdomains, respectively. Since the inner subdomains interconnect to form a ring, which can potentially segregate the lipids of the central hole from those outside, the 2D crystalline uroplakin network may impose an organized state and a severely restricted freedom of movement on the lipid components, thus reducing membrane fluidity and contributing to the barrier function of urothelial plaques. Our finding that distinct uroplakin substructures are in contact with the cytoplasmic and exoplasmic leaflets of the plaque suggests that the two leaflets may have different lipid composition and contribute asymmetrically to the barrier function. We propose that the crystalline lattice structure of uroplakin, through its interactions with specialized lipids, plays a major role in the remarkable permeability barrier function of urothelial apical surface. Our results also have implications for the transmembrane signal transduction in urothelial cells as induced by the binding of uropathogenic E. coli to its uroplakin receptor.

Localization of uroplakin Ia, the urothelial receptor for bacterial adhesin FimH, on the six inner domains of the 16 nm urothelial plaque particle

The binding of uropathogenic Escherichia coli to the urothelial surface is a critical initial event for establishing urinary tract infection, because it prevents the bacteria from being removed by micturition and it triggers bacterial invasion as well as host cell defense. This binding is mediated by the FimH adhesin located at the tip of the bacterial type 1-fimbrium and its urothelial receptor, uroplakin Ia (UPIa). To localize the UPIa receptor on the 16 nm particles that form two-dimensional crystals of asymmetric unit membrane (AUM) covering >90 % of the apical urothelial surface, we constructed a 15 A resolution 3-D model of the mouse 16 nm AUM particle by negative staining and electron crystallography. Similar to previous lower-resolution models of bovine and pig AUM particles, the mouse 16 nm AUM particle consists of six inner and six outer domains that are interconnected to form a twisted ribbon-like structure. Treatment of urothelial plaques with 0.02-0.1 % (v/v) Triton X-100 allowed the stain to penetrate into the membrane, revealing parts of the uroplakin transmembrane moiety with an overall diameter of 14 nm, which was much bigger than the 11 nm value determined earlier by quick-freeze deep-etch. Atomic force microscopy of native, unfixed mouse and bovine urothelial plaques confirmed the overall structure of the luminal 16 nm AUM particle that was raised by 6.5 nm above the luminal membrane surface and, in addition, revealed a circular, 0.5 nm high, cytoplasmic protrusion of approximately 14 nm diameter. Finally, a difference map calculated from the mouse urothelial plaque images collected in the presence and absence of recombinant bacterial FimH/FimC complex revealed the selective binding of FimH to the six inner domains of the 16 nm AUM particle. These results indicate that the 16 nm AUM particle is anchored by a approximately 14 nm diameter transmembrane stalk, and suggest that bacterial binding to UPIa that resides within the six inner domains of the 16 nm AUM particle may preferentially trigger transmembrane signaling involved in bacterial invasion and host cell defense.

Uroplakin Ia is the urothelial receptor for uropathogenicEscherichia coli: evidence from in vitro FimH binding

The binding of uropathogenic Escherichia coli to the urothelial surface is a crucial initial event for establishing urinary tract infection because it allows the bacteria to gain a foothold on the urothelial surface, thus preventing them from being removed by micturition. In addition, it triggers bacterial invasion as well as host urothelial defense. This binding is mediated by the FimH adhesin located at the tip of the bacterial type 1-fimbrium, a filamentous attachment apparatus, and its urothelial receptor. We have prepared a biotinylated, recombinant FimH-FimC adhesin:chaperone complex and used it to identify its mouse urothelial receptor. The FimH-FimC complex binds specifically to a single 24 kDa major mouse urothelial plaque protein, which we identified as uroplakin Ia by mass spectrometry, cDNA cloning and immunoreactivity. The terminal mannosyl moieties on Asn-169 of uroplakin Ia are responsible for FimH as well as concanavalin A binding. Although FimH binds to uroplakin Ia with only moderate strength (Kd ∼100 nM between pH 4 and 9), the binding between multiple fimbriae of a bacterium and the crystalline array of polymerized uroplakin receptors should achieve high avidity and stable bacterial attachment. The FimH-FimC complex binds preferentially to the mouse urothelial umbrella cells in a pattern similar to uroplakin staining. Our results indicate that the structurally related uroplakins Ia and Ib are glycosylated differently, that uroplakin Ia serves as the urothelial receptor for the type 1-fimbriated E. coli, and that the binding of uropathogenic bacteria to uroplakin Ia may play a key role in mediating the urothelial responses to bacterial attachment.

Organization of uroplakin subunits: transmembrane topology, pair formation and plaque composition

The apical surfaces of urothelial cells are almost entirely covered with plaques consisting of crystalline, hexagonal arrays of 16 nm uroplakin particles. Although all four uroplakins, when SDS-denatured, can be digested by chymotrypsin, most uroplakin domains in native urothelial plaques are resistant to the enzyme, suggesting a tightly packed structure. The only exception is the C-terminal, cytoplasmic tail of UPIII (UPIII) which is highly susceptible to proteolysis, suggesting a loose configuration. When uroplakins are solubilized with 2% octylglucoside and fractionated with ion exchangers, UPIa and UPII were bound as a complex by a cation exchanger, whereas UPIb and UPIII were bound by an anion exchanger. This result is consistent with the fact that UPIa and UPIb are cross-linked to UPII and UPIII, respectively, and suggests that the four uroplakins form two pairs consisting of UPIa/II and UPIb/III. Immunogold labelling using a new mouse monoclonal antibody, AU1, revealed that UPIII is present in all urothelial plaques, indicating that the two uroplakin pairs are not segregated into two different types of urothelial plaque and that all plaques must have a similar uroplakin composition. Taken together, these results indicate that uroplakins form a tightly packed structure, that the four uroplakins interact specifically forming two pairs, and that both uroplakin pairs are required for normal urothelial plaque formation.