A general diffusion model for analyzing the efficacy of synaptic input to threshold neurons

We describe a general diffusion model for analyzing the efficacy of individual synaptic inputs to threshold neurons. A formal expression is obtained for the system propagator which, when given an arbitrary initial state for the cell, yields the conditional probability distribution for the state at all later times. The propagator for a cell with a finite threshold is written as a series expansion, such that each term in the series depends only on the infinite threshold propagator, which in the diffusion limit reduces to a Gaussian form. This procedure admits a graphical representation in terms of an infinite sequence of diagrams. To connect the theory to experiment, we construct an analytical expression for the primary correlation kernel (PCK) which profiles the change in the instantaneous firing rate produced by a single postsynaptic potential (PSP). Explicit solutions are obtained in the diffusion limit to first order in perturbation theory. Our approximate expression resembles the PCK obtained by computer simulation, with the accuracy depending strongly on the mode of firing. The theory is most accurate when the synaptic input drives the membrane potential to a mean level more than one standard deviation below the firing threshold, making such cells highly sensitive to synchronous synaptic input.     

Functional insights from proteome-wide structural modeling of <Emphasis Type="Italic">Treponema pallidum</Emphasis> subspecies <Emphasis Type="Italic">pallidum</Emphasis>, the causative agent of syphilis

Background

Syphilis continues to be a major global health threat with 11 million new infections each year, and a global burden of 36 million cases. The causative agent of syphilis, Treponema pallidum subspecies pallidum, is a highly virulent bacterium, however the molecular mechanisms underlying T. pallidum pathogenesis remain to be definitively identified. This is due to the fact that T. pallidum is currently uncultivatable, inherently fragile and thus difficult to work with, and phylogenetically distinct with no conventional virulence factor homologs found in other pathogens. In fact, approximately 30% of its predicted protein-coding genes have no known orthologs or assigned functions. Here we employed a structural bioinformatics approach using Phyre2-based tertiary structure modeling to improve our understanding of T. pallidum protein function on a proteome-wide scale.

Results

Phyre2-based tertiary structure modeling generated high-confidence predictions for 80% of the T. pallidum proteome (780/978 predicted proteins). Tertiary structure modeling also inferred the same function as primary structure-based annotations from genome sequencing pipelines for 525/605 proteins (87%), which represents 54% (525/978) of all T. pallidum proteins. Of the 175 T. pallidum proteins modeled with high confidence that were not assigned functions in the previously annotated published proteome, 167 (95%) were able to be assigned predicted functions. Twenty-one of the 175 hypothetical proteins modeled with high confidence were also predicted to exhibit significant structural similarity with proteins experimentally confirmed to be required for virulence in other pathogens.

Conclusions

Phyre2-based structural modeling is a powerful bioinformatics tool that has provided insight into the potential structure and function of the majority of T. pallidum proteins and helped validate the primary structure-based annotation of more than 50% of all T. pallidum proteins with high confidence. This work represents the first T. pallidum proteome-wide structural modeling study and is one of few studies to apply this approach for the functional annotation of a whole proteome.

     

Epstein-Barr Virus Uses Different Complexes of Glycoproteins gH and gL To Infect B Lymphocytes and Epithelial Cells

The Epstein-Barr virus (EBV) gH-gL complex includes a third glycoprotein, gp42. gp42 binds to HLA class II on the surfaces of B lymphocytes, and this interaction is essential for infection of the B cell. We report here that, in contrast, gp42 is dispensable for infection of epithelial cell line SVKCR2. A soluble form of gp42, gp42.Fc, can, however, inhibit infection of both cell types. Soluble gp42 can interact with EBV gH and gL and can rescue the ability of virus lacking gp42 to transform B cells, suggesting that a gH-gL-gp42.Fc complex can be formed by extrinsic addition of the soluble protein. Truncated forms of gp42.Fc that retain the ability to bind HLA class II but that cannot interact with gH and gL still inhibit B-cell infection by wild-type virus but cannot inhibit infection of SVKCR2 cells or rescue the ability of recombinant gp42-negative virus to transform B cells. An analysis of wild-type virions indicates the presence of more gH and gL than gp42. To explain these results, we describe a model in which wild-type EBV virions are proposed to contain two types of gH-gL complexes, one that includes gp42 and one that does not. We further propose that these two forms of the complex have mutually exclusive abilities to mediate the infection of B cells and epithelial cells. Conversion of one to the other concurrently alters the ability of virus to infect each cell type. The model also suggests that epithelial cells may express a molecule that serves the same cofactor function for this cell type as HLA class II does for B cells and that the gH-gL complex interacts directly with this putative epithelial cofactor.All herpesviruses examined to date encode a complex of two glycoproteins, gH and gL, that appear to be necessary, if not sufficient, for virus penetration. Glycoprotein gH is generally thought to be the major player in virus cell fusion (5, 6, 8, 14, 20, 25, 26), while the role of gL is to serve as a chaperone, essential for folding and transport of functional gH (3, 11, 13, 20, 21, 28, 29). The Epstein-Barr virus (EBV) gH-gL complex follows this pattern. Glycoprotein gp85, the gH homolog, is retained in the endoplasmic reticulum in the absence of gp25, the EBV gL (38), and virosomes made from EBV proteins depleted of the gH-gL complex bind to cells but fail to fuse (9). The EBV gH-gL complex, however, includes a third glycoprotein, gp42, which is the product of the BZLF2 open reading frame (ORF) (18). This third component has also proven to be essential for penetration of the major target cell of EBV, the B lymphocyte. Several lines of evidence indicate that gp42 is a ligand for HLA class II and, further, that HLA class II functions as a cell surface cofactor for EBV entry into this cell type. Glycoprotein gp42 interacts with the β₁ domain of HLA class II protein HLA-DR (30), and a monoclonal antibody (MAb) to gp42 called F-2-1 interferes with this interaction (17). MAb F-2-1 has no effect on EBV attachment via glycoprotein gp350/220 to its primary receptor, complement receptor type 2 (CR2; CD21) but inhibits the fusion of the virus with the B-cell membrane (22). Similarly, a MAb to HLA-DR or a soluble form of gp42 blocks B-cell transformation. Finally, B-cell lines which lack expression of HLA class II are not susceptible to superinfection with EBV unless expression of class II is restored (17). Most recently, we derived a recombinant virus with gp42 expression deleted and confirmed that loss of the glycoprotein resulted in a virus that attached to the B-cell surface but that failed to penetrate unless it was treated with the fusogenic agent polyethylene glycol (36).Although most is known about the early interactions of EBV with B lymphocytes in vitro since these cells are readily available and easy to culture, infection is not restricted to this cell type in vivo. During our initial analysis of the biology of gp42 we had therefore examined its potential role in infection of a then newly derived model epithelial cell line, SVKCR2. SVKCR2 cells are transformed with simian virus 40 and stably transfected with B-cell receptor CR2 (19). They are poorly infectable with many strains of EBV, but in excess of 30% of the cells can be infected with the Akata strain of virus as judged by the expression of EBV latent protein EBNA 1 (18, 19). We found that MAb F-2-1 had no effect on the infection of SVKCR2 cells. At the same time, a second MAb, E1D1, which reacts with an epitope that can be formed by the coexpression of gH and gL in the absence of gp42, neutralized infection of SVKCR2 cells, but had no effect on the infection of lymphocytes. These data strongly suggested that the involvement of the gH-gL complex in the internalization of virus into the two cell types was different. We hypothesized that just as EBV has evolved a glycoprotein, gp350/220, which is uniquely adapted for attachment to B lymphocytes, so it has evolved a second glycoprotein, gp42, uniquely adapted for penetration into the same cell type (18). The implication was that gp42 might be dispensable for infection of epithelial cells.Since we made our initial observations with SVKCR2 cells, several novel reagents, including the Akata strain virus with the expression of gp42 deleted, have become available. The recent insights into the role of HLA class II in B-cell infection also provided new impetus to reexamine the involvement of the gH-gL complex in epithelial cell infection. We report here that gp42 is not required for infection of SVKCR2 cells despite the fact that the soluble form of the protein that inhibits B-cell infection can also neutralize infection of SVKCR2 cells. To explain these apparently anomalous results, we describe a model which proposes that wild-type EBV virions contain two types of gH-gL complexes, one that includes gp42 and one that does not. We further propose that the tripartite “B-cell complexes” are not functional for infection of epithelial cells, just as the bipartite “epithelial cell complexes” are unable to mediate infection of the B lymphocyte.     

Ambitious new CD-ROM on endangered and extinct species

(This article is a US Government work and, as such, is in the public domain in the United States of America.)