B cells regulate murine gammaherpesvirus 68 latency

The dynamics of the establishment of, and reactivation from, gammaherpesviruses latency has not been quantitatively analyzed in the natural host. Gammaherpesvirus 68 (gammaHV68) is a murine gammaherpesvirus genetically related to primate gammaherpesviruses that establishes a latent infection in infected mice. We used limiting dilution reactivation (frequency of cells reactivating gammaHV68 in vitro) and limiting dilution PCR (frequency of cells carrying gammaHV68 genome) assays to compare gammaHV68 latency in normal (C57BL/6) and B-cell-deficient (MuMT) mice. After intraperitoneal (i.p.) inoculation, latent gammaHV68 was detected in the spleen, bone marrow, and peritoneal cells. Both B-cell-deficient and C57BL/6 mice established latent infection in peritoneal cells after either i.p. or intranasal (i.n.) inoculation. In contrast, establishment of splenic latency was less efficient in B-cell-deficient than in C57BL/6 mice after i.n. inoculation. Analysis of reactivation efficiency (reactivation frequency compared to frequency of cells carrying gammaHV68 genome) revealed that (i) regardless of route or mouse strain, splenic cells reactivated gammaHV68 less efficiently than peritoneal cells, (ii) the frequency of cells carrying gammaHV68 genome was generally comparable over the course of infection between C57BL/6 and B-cell-deficient mice, (iii) between 28 and 250 days after infection, cells from B-cell-deficient mice reactivated gammaHV68 10- to 100-fold more efficiently than cells from C57BL/6 mice, (iv) at 7 weeks postinfection, B-cell-deficient mice had more genome-positive peritoneal cells than C57BL/6 mice, and (v) mixing cells (ratio of 3 to 1) that reactivated inefficiently with cells that reactivated efficiently did not significantly decrease reactivation efficiency. Consistent with a failure to normally regulate chronic gammaHV68 infection, the majority of infected B-cell-deficient mice died between 100 and 200 days postinfection. We conclude that (i) B cells are not required for establishment of gammaHV68 latency, (ii) there are organ-specific differences in the efficiency of gammaHV68 reactivation, (iii) B cells play a crucial role in regulating reactivation of gammaHV68 from latency, and (iv) B cells are important for controlling chronic gammaHV68 infection.     

Phospho-NHE3 forms membrane patches and interacts with beta-actin to sense and maintain constant direction during cell migration

The Na(+)/H(+) exchanger NHE3 colocalizes with beta-actin at the leading edge of directionally migrating cells. Using human osteosarcoma cells (SaOS-2), rat osteoblasts (calvaria), and human embryonic kidney (HEK) cells, we identified a novel role for NHE3 via beta-actin in anode and cathode directed motility, during electrotaxis. NHE3 knockdown by RNAi revealed that NHE3 expression is required to achieve constant directionality and polarity in migrating cells. Phosphorylated NHE3 (pNHE3) and beta-actin complex formation was impaired by the NHE3 inhibitor S3226 (IC50 0.02 µM). Fluorescence cross-correlation spectroscopy (FCCS) revealed that the molecular interactions between NHE3 and beta-actin in membrane protrusions increased 1.7-fold in the presence of a directional cue and decreased 3.3-fold in the presence of cytochalasin D. Data from flow cytometric analysis showed that membrane potential of cells (V_mem) decreases in directionally migrating, NHE3-deficient osteoblasts and osteosarcoma cells whereas only V_mem of wild type osteoblasts is affected during directional migration. These findings suggest that pNHE3 has a mechanical function via beta-actin that is dependent on its physiological activity and V_mem. Furthermore, phosphatidylinositol 3,4,5-trisphosphate (PIP3) levels increase while PIP2 remains stable when cells have persistent directionality. Both PI3 kinase (PI3K) and Akt expression levels change proportionally to NHE3 levels. Interestingly, however, the content of pNHE3 level does not change when PI3K/Akt is inhibited. Therefore, we conclude that NHE3 can act as a direction sensor for cells and that NHE3 phosphorylation in persistent directional cell migration does not involve PI3K/Akt during electrotaxis.     

Linkage disequilibrium and natural selection for mimicry in the Batesian mimic Hypolimnas misippus (L.) (Lepidoptera: Nymphalidae) in the Afrotropics

On two occasions, on opposite sides of the African continent (Cape Coast, Ghana, and Dar es Salaam, Tanzania), high adult population densities in the polymorphic butterfly Hypolimnas misippus (a presumed mimic of Danaus chrysippus) were followed by linkage disequilibrium in combinations of fore‐ and hindwing colour patterns. On both occasions, disequilibrium was caused by significant changes in morph frequencies favouring rarer and more mimetic forms. Recaptures were too few for analysis at Dar, although the changes there took place within a single generation and must have been the result of differential survival. Recapture rate data and survival rate estimates at Cape Coast support the hypothesis that selective predation was responsible, as does the observation of synchronous linkage disequilibrium at Dar in the model D. chrysippus, indicating parasitic mimicry. There was clear selection for the perfection of mimicry for forewings at Dar and for hindwings at Cape Coast. Disequilibrium is also reported for two other sites, Legon (Ghana) and Boksburg (South Africa) and, in all four sites, it was associated with an increase in the most mimetic forms. New chemical evidence is presented to support the contention that D. chrysippus is a defended model. Although all the evidence leads to the conclusion that H. misippus is a Batesian mimic of D. chrysippus, many questions remain, particularly with regard to the identity of predators, the episodic nature of selective predation events, and their apparent lack of lasting and significant impact on overall gene frequencies. We conclude that H. misippus presents both challenges and opportunities for studies on mimicry, and we suggest that linkage disequilibrium can be a useful generic indicator for Gestalt predation on polymorphic prey. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 180–194.     

Urban Place: Reconnecting with the Natural World

Urban Place: Reconnecting with the Natural World. Peggy F. Barlett, ed. Cambridge, MA: MIT Press, 2005. 330 pp.