Secretion without membrane fusion: porocytosis

Mast cell mediators: their differential release and the secretory pathways involved

Mast cells (MC) are widely distributed throughout the body and are common at mucosal surfaces, a major host-environment interface. MC are functionally and phenotypically heterogeneous depending on the microenvironment in which they mature. Although MC have been classically viewed as effector cells of IgE-mediated allergic diseases, they are also recognized as important in host defense, innate and acquired immunity, homeostatic responses, and immunoregulation. MC activation can induce release of pre-formed mediators such as histamine from their granules, as well as release of de novo synthesized lipid mediators, cytokines, and chemokines that play diverse roles, not only in allergic reactions but also in numerous physiological and pathophysiological responses. Indeed, MC release their mediators in a discriminating and chronological manner, depending upon the stimuli involved and their signaling cascades (e.g., IgE-mediated or Toll-like receptor-mediated). However, the precise mechanisms underlying differential mediator release in response to these stimuli are poorly known. This review summarizes our knowledge of MC mediators and will focus on what is known about the discriminatory release of these mediators dependent upon diverse stimuli, MC phenotypes, and species of origin, as well as on the intracellular synthesis, storage, and secretory processes involved.

Using enrichment index for quality control of secretory protein sample and identification of secretory proteins

Analysis of secretory proteins is an important area in proteomic research. We propose that a good secretory protein sample should be enriched with known secretory proteins, and a secretory protein should be enriched in the secretory protein sample compared with its corresponding soluble cell lysate. Positive identifications of proteins were subjected to quantitation of spectral counts, which reflect relative protein abundance. Enrichment index of the sample (EIS) and the enrichment index for protein (EIP) were obtained by comparing proteins identified in the secretory protein sample and those in the soluble cell lysate sample. The quality of the secretory protein sample can be represented by EIS. EIP was used to identify the secretory proteins.The secretory proteins from mouse dendritic cell sarcoma (DCS) were analyzed by MS. The EISs of two samples were 75.4 and 84.65, respectively. 72 proteins were significantly enriched in secretory protein samples, of which 42 proteins were either annotated in Swiss-Prot and/or predicted by signal peptides to be secretory. In the remaining 30 proteins, 12 and 15 proteins were positively predicted by SecretomeP and ProP, respectively, and 5 proteins were positive by both methods. Furthermore, 11 proteins were found to be present in exosome in other studies that involved mice dendritic cell lines. We suggest that this assessment method is helpful for systemic research of secretory proteins and biomarker discovery for diseases such as cancer.

Getting a G--RRP on regulated exocytosis in the heart

A study by Rybkin et al. (see p. 527) substantially advances our understanding of regulated exocytois by specialized secretory cells, such as atrial myocytes. A second member of the Ras-related protein family, RRP17, was identified and shown to participate in regulating the secretion of the cardiac-derived peptide hormone, atrial natriuretic peptide. In addition to the heart, RRP17 was shown to be expressed in neuronal, pancreatic, and skeletal muscle cells, suggesting a widespread role in regulated secretion for this new protein.

Antiallergic drugs, azelastine hydrochloride and epinastine hydrochloride, inhibit ongoing IgE secretion of rat IgE-producing hybridoma FE-3 cells

We asked whether or not antiallergic drugs, azelastine hydrochloride and epinastine hydrochloride, inhibit IgE secretion from IgE-producing hybridoma FE-3 cells. FE-3 cells were cultured in the presence of azelastine or epinastine for 24 h, washed in phosphate-buffered saline , and then recultured in the medium in the absence of the antiallergic drugs. IgE levels in the cultured medium as well as those in the cytoplasm of FE-3 cells were measured by enzyme-linked immunosorbent assay. mRNA levels of Cepsilon, activation-induced cytidine deaminase (AID), XBP-1, and Bip were estimated by northern blot or reverse transcriptase polymerase chain reaction analysis. The activities of nuclear factor-kappa B (NF-kappaB) were analyzed by electrophoretic mobility shift assay (EMSA). Phosphorylation of I kappa B alpha (IkappaBalpha) was analyzed by immunoprecipitation followed by western blot analysis. IgE levels in the cultured medium and in the microsome fraction were lower on the treatment with 10(-5) M azelastine or epinastine than those on the treatment with vehicle. Cepsilon and AID mRNA levels were lower on the treatment with 10(-5) M azelastine than those on the treatment with vehicle, but were not decreased on the treatment with 10(-5) M epinastine. XBP-1 and Bip mRNA levels were not altered following treatment of the antiallergic drugs. Azelastine at 10(-5) M, but not epinastine, reduced DNA binding activity of NF-kappaB and also diminished IkappaBalpha phosphorylation, leading to sustaining IkappaBalpha protein levels. These findings suggest that azelastine exerts its inhibitory effect on the IgE secretion from FE-3 cells through the inhibition of Cepsilon mRNA expression, and that the inhibitory effect of epinastine is, at least in part, due to suppression of IgE synthesis at the post-transcriptional level.

Fine ultrastructure of chromaffin granules in rat adrenal medulla indicative of a vesicle-mediated secretory process

Observation by transmission electron microscopy, coupled with morphometric analysis and estimation procedure, revealed unique ultrastructural features in 25.94% of noradrenaline (NA)-containing granules and 16.85% of adrenaline (A)-containing granules in the rat adrenal medulla. These consisted of evaginations of the granule limiting membrane to form budding structures having different morphology and extension. In 14.8% of NA granules and 12.0% of A granules, outpouches were relatively short, looked like small blebs emerging from the granule surface and generally contained electron-dense material. A proportion of 11.2% of NA granules and 4.9% of A granules revealed the most striking ultrastructural features. These secretory organelles presented thin, elongated, tail-like or stem-like appendages, which were variably filled by chromaffin substance and terminated with spherical expansions of different electron density. A cohort of vesicles of variable size (30-150 nm in diameter) and content was found either close to them or in the intergranular cytosol. Examination of adrenal medullary cells fixed by zinc iodide-osmium tetroxide (ZIO) revealed fine electron dense precipitates in chromaffin granules, budding structures as well as cytoplasmic vesicles. These data indicate that a common constituent is revealed by the ZIO histochemical reaction in chromaffin cells. As catecholic compounds are the main tissue targets of ZIO complexes, catecholamines are good candidates to be responsible for the observed ZIO reactivity. This study adds further to the hypothesis that release of secretory material from chromaffin granules may be accomplished by a vesiclular transport mechanism typical of piecemeal degranulation.