Centromeres attach chromosomes to the spindle during mitosis, thereby ensuring the equal distribution of chromosomes into daughter cells. Transcriptionally silent heterochromatin of unknown function is associated with centromeres in many organisms. In the fission yeast Schizosaccharomyces pombe, the silent mating-type loci, centromeres, and telomeres are assembled into silent heterochromatin-like domains. The Swi6 chromodomain protein affects this silencing, and now it is shown that Swi6p localizes with these three chromosomal regions. In cells lacking Swi6p, centromeres lag on the spindle during anaphase and chromosomes are lost at high rates. Thus, Swi6p is located at fission yeast centromeres and is required for their proper function.

Tissue mast cells (MC) are recognized as key effector cells of immediate-type allergic reactions releasing inf lammatory mediators and cytokines on stimulation with antigen, but they also might be involved in IgE-independent inf lammatory and tissue repair processes. The mechanism of human MC regulation in tissue is not fully understood. Here, we show that IL-4, in synergy with stem cell factor (SCF), regulates the function of purified human MC isolated from intestinal tissue. Whereas SCF induced only marginal proliferation of MC cultured in vitro up to 4 weeks, addition of IL-4 and SCF strongly increased the proliferation rate. Moreover, IL-4, which by itself had no visible effect on human MC, enhanced the release of histamine, leukotriene C 4 , and IL-5 in MC triggered by IgE receptor crosslinking. The IL-4 effects occurred in a dose-dependent fashion (ED 50 ‫؍‬ 100 pg͞ml) and could be totally blocked by a competitive IL-4 receptor antagonist. Our data indicate that IL-4 is an important regulator of human MC function and suggest that mature MC retain the capacity to proliferate in a particular tissue environment.

Mast cells are recognized as a new type of immunoregulatory cells capable of producing different cytokines. So far, little is known about the cytokine profile of mature human mast cells isolated from intestinal tissue and cultured in the presence of stem cell factor (SCF). We observed that these cells express the proinflammatory cytokines TNF-α, IL-1β, IL-6, IL-8, IL-16, and IL-18 without further stimulation. Both IgE-dependent and IgE-independent agonists (e.g., Gram-negative bacteria) enhanced expression of TNF-α. Another set of cytokines consisting of IL-3, IL-5, IL-9, and IL-13 was expressed following activation by IgE receptor cross-linking. If mast cells were cultured in the presence of IL-4 and SCF, the production and release of IL-3, IL-5, and IL-13 was increased up to 4-fold compared with mast cells cultured with SCF alone. By contrast, IL-6 expression was completely blocked in response to culture with IL-4. In summary, our data show that mature human mast cells produce proinflammatory cytokines that may be up-regulated following triggering with IgE-independent agonists such as bacteria, whereas activation by IgE receptor cross-linking results in the expression of Th2-type cytokines. IL-4 enhances the expression of Th2-type cytokines but does not affect or even down-regulates proinflammatory cytokines.

Background-Several inflammatory disorders of the intestine are characterised by enhanced expression of tumour necrosis factor (TNF-). Monocytes and macrophages have been suggested as a major cellular source of TNF-in human gut, whereas mast cells, although known to be capable of producing TNF-, have been poorly examined in this respect. Aims-To investigate whether human intestinal mast cells can produce TNF-, and which factors regulate TNF-production in these cells. (Gut 1999;44:643-652) Methods-Mast

Mediator release from mast cells (MC) is a crucial step in allergic and non-allergic inflammatory disorders. However, the final events in response to activation leading to membrane fusion and thereby facilitating degranulation have hitherto not been analyzed in human MC. Soluble N-ethyl-maleimide-sensitive factor attachment protein receptors (SNARE) represent a highly conserved family of proteins that have been shown to mediate intracellular membrane fusion events. Here, we show that mature MC isolated from human intestinal tissue express soluble N-ethylmaleide sensitive factor attachment protein (SNAP)-23, Syntaxin (STX)-1B, STX-2, STX-3, STX-4, and STX-6 but not SNAP-25. Furthermore, we found that primary human MC express substantial amounts of vesicle associated membrane protein (VAMP)-3, VAMP-7 and VAMP-8 and, in contrast to previous reports about rodent MC, only low levels of VAMP-2. Furthermore, VAMP-7 and VAMP-8 were found to translocate to the plasma membrane and interact with SNAP-23 and STX-4 upon activation. Inhibition of SNAP-23, STX-4, VAMP-7 or VAMP-8, but not VAMP-2 or VAMP-3, resulted in a markedly reduced highaffinity IgE receptor-mediated histamine release. In summary, our data show that mature human MC express a specific pattern of SNARE and that VAMP-7 and VAMP-8, but not VAMP-2, are required for rapid degranulation.Supporting Information for this article is available at http://www.wiley-vch.de/contents/jc_2040/2008/37634_s.pdf IntroductionMast cells (MC) are well known to play a key role in allergic and a number of non-allergic inflammatory conditions. They are capable of releasing a number of pre-stored or de novo synthesized proinflammatory ** The first two authors contributed equally to this work. The role of SNARE-proteins in human MC is still largely elusive. However, experiments conducted in rodent basophilic leukemia cells (RBL-2H3), a widely used MC cell line, have provided solid data, that members of the SNARE-family are indeed specifically expressed in different compartments of these cells and that they are crucially involved in mediator secretion [16][17][18][19][20]. These reports attribute a key role in exocytotic events in MC to the t-SNARE SNAP-23 [21]. SNAP-23 represents the ubiquitously expressed counterpart of its neuronal isoform and is found in tissues throughout the body [21,23,24]. Furthermore, SNAP-23 has been demonstrated to mediate exocytosis in other immune cells as well [12,25,26]. It forms a stable SNARE complex together with a syntaxin (STX), another t-SNARE. Various STX-isoforms have been identified in myeloid cells [12,27,28]. Finally, docking and fusion events at the plasma membrane require v-SNARE, namely vesicle-associated membrane proteins (VAMP). Different VAMP isoforms seem to be specifically involved in mediating the various intracellular trafficking events. VAMP-1 and VAMP-2, also called synaptobrevins, have been implicated in regulated exocytosis in neurons [29] and non-neuronal cells [30], while VAMP-3, or cellubrevin, was attributed a role in consti...