Complex effects of the gastropod Assiminea globulus on benthic community structure in a marine-dominated lagoon

Grazing is a key factor structuring aquatic communities, and can have both positive and negative effects. Here, we report on the effects of an intertidal estuarine gastropod, Assiminea globulus, on sediment bacteria, microphytobenthos and meiofauna, based on field observations and manipulative experiments. Field observations showed inconsistent patterns of chlorophyll-a concentrations in zones occupied by A. globulus relative to zones lacking this species, and meiofaunal density increased downshore. Bacterial density, however, was always consistently and significantly elevated in zones of A. globulus across all sites sampled, suggesting promotion of bacterial density by this snail. Field experiments were more conclusive, showing a significant negative effect of A. globulus on sediment chlorophyll-a levels at both the surface and at a sediment depth of 2 cm, but positive effects on bacterial density at both depths. A. globulus density did not affect overall meiofaunal density, but did affect meiofaunal community structure, generally by depressing the density of harpacticoid copepods but promoting platyhelminthes, and enhancing taxonomic diversity at high density. The complex effects of A. globulus probably reflect the interactive effects of bioturbation, fertilisation, grazing and pelletisation, all of which need to be considered when understanding grazer effects on soft-sediment communities.     

T-cell recognition of glycolipids presented by CD1 proteins

The most well-known molecular paradigm of antigen recognition by T cells involves partial digestion of proteins to generate small peptides, which bind to major histocompatibility complex (MHC) proteins. Recent studies of CD1, an MHC class I homolog encoded outside the MHC, have revealed that it presents diverse glycolipids to T cells. The molecular mechanism for lipid antigen recognition involves insertion of the lipid portion of antigens into a hydrophobic groove to form CD1-lipid complexes, which contact T-cell receptors (TCRs). Here, we examine the known antigen structures presented by CD1, the majority of which have sugar moieties that are capable of interacting with TCRs. Recognition of carbohydrate epitopes is precise, and lipid-reactive T cells alter systemic immune responses in models of infectious and autoimmune disease. These findings provide a previously unrecognized mechanism by which the cellular immune system can recognize alterations in many types of carbohydrate structures.     

Identification and characterization of five-transmembrane isoforms of human vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide receptors

The seven-transmembrane (7TM) G-protein-coupled neuroendocrine receptors VPAC1 (HGNC approved gene symbol VIPR1) and VPAC2 (HGNC approved gene symbol VIPR2) are expressed in different tissues and involved in the regulation of important biological functions. We now report the identification and characterization of novel five-transmembrane(5TM) forms of both human VPAC1 and human VPAC2. These alternatively spliced variant mRNAs result from the skipping of exons 10/11, spanning the third intracellular loop, the fourth extracellular loop, and the transmembrane regions 6 and 7, producing in-frame 5TM receptors predicted to lack a G-protein-binding motif. RT-PCR showed that these 5TM receptors are differentially expressed in transformed and normal cells. Translation of the 5TM protein was demonstrated by transfection and expression in CHO cells. Following agonist stimulation, differential signaling of the 7TM versus 5TM forms was shown both for the activation of adenylate cyclase and for tyrosine phosphorylation. The identification of these splice variants in various cells and their expression and differential signal transduction compared to the 7TM form suggest that these novel receptors have biological relevance.     

Disruption of the A-kinase anchoring domain in flagellar radial spoke protein 3 results in unregulated axonemal cAMP-dependent protein kinase activity and abnormal flagellar motility

Biochemical studies of Chlamydomonas flagellar axonemes revealed that radial spoke protein (RSP) 3 is an A-kinase anchoring protein (AKAP). To determine the physiological role of PKA anchoring in the axoneme, an RSP3 mutant, pf14, was transformed with an RSP3 gene containing a mutation in the PKA-binding domain. Analysis of several independent transformants revealed that the transformed cells exhibit an unusual phenotype: a fraction of the cells swim normally; the remainder of the cells twitch feebly or are paralyzed. The abnormal/paralyzed motility is not due to an obvious deficiency of radial spoke assembly, and the phenotype cosegregates with the mutant RSP3. We postulated that paralysis was due to failure in targeting and regulation of axonemal cAMP-dependent protein kinase (PKA). To test this, reactivation experiments of demembranated cells were performed in the absence or presence of PKA inhibitors. Importantly, motility in reactivated cell models mimicked the live cell phenotype with nearly equal fractions of motile and paralyzed cells. PKA inhibitors resulted in a twofold increase in the number of motile cells, rescuing paralysis. These results confirm that flagellar RSP3 is an AKAP and reveal that a mutation in the PKA binding domain results in unregulated axonemal PKA activity and inhibition of normal motility.