Cloning and genomic characterization of sytdep, a new synaptotagmin XIV-related gene

SYT14L, especially its C2 domain, is involved in regulating melanocyte differentiation

BACKGROUND

The formation of dendrites by melanocytes is highly analogous to that process in neural cells. We previously reported that a C2 domain-containing protein, copine-1, is involved in the extension of dendrites by neural cells. However, the effect of C2 domain-containing proteins in dendrite formation by melanocytes has not yet been elucidated.

OBJECTIVE

The aim of this study was to screen novel C2 domain-containing proteins related to dendrite outgrowth in melanocytes and to investigate their precise roles in melanocyte dendrite formation during differentiation.

METHODS

We transduced mouse melan-a melanocytes with a recombinant adenovirus expressing a C2 domain library. Dendrite elongation, melanin content, tyrosinase activity and Western blot analyses were conducted to elucidate the possible underlying mechanisms of action in melanocytes.

RESULTS

Sixteen sets of C2 domain-containing proteins were identified whose over-expression resulted in dendrite lengthening. Among those, we focused on the C2 domain of SYT14L (truncated mutant of SYT14L) in this study. Forced expression of full length SYT14L or the C2 domain of SYT14L induced a significant elongation of dendrite length accompanied by the induction of melanocyte differentiation-related markers, including melanin synthesis, tyrosinase catalytic activity and the expression of tyrosinase (TYR), tyrosinase related protein-1 (TRP-1) and TRP-2. In addition, over-expression of either the C2 domain or the full length form of SYT14L significantly increased the phosphorylation of ERK and CREB.

CONCLUSION

These results suggest that SYT14L, especially its C2 domain, may play an important role in regulating melanocyte differentiation through the modulation of ERK and (or) CREB signaling.

Complexin activates exocytosis of distinct secretory vesicles controlled by different synaptotagmins

Complexins are SNARE-complex binding proteins essential for the Ca(2+)-triggered exocytosis mediated by synaptotagmin-1, -2, -7, or -9, but the possible role of complexins in other types of exocytosis controlled by other synaptotagmin isoforms remains unclear. Here we show that, in mouse olfactory bulb neurons, synaptotagmin-1 localizes to synaptic vesicles and to large dense-core secretory vesicles as reported previously, whereas synaptotagmin-10 localizes to a distinct class of peptidergic secretory vesicles containing IGF-1. Both synaptotagmin-1-dependent synaptic vesicle exocytosis and synaptotagmin-10-dependent IGF-1 exocytosis were severely impaired by knockdown of complexins, demonstrating that complexin acts as a cofactor for both synaptotagmin-1 and synaptotagmin-10 despite the functional differences between these synaptotagmins. Rescue experiments revealed that only the activating but not the clamping function of complexins was required for IGF-1 exocytosis controlled by synaptotagmin-10. Thus, our data indicate that complexins are essential for activation of multiple types of Ca(2+)-induced exocytosis that are regulated by different synaptotagmin isoforms. These results suggest that different types of regulated exocytosis are mediated by similar synaptotagmin-dependent fusion mechanisms, that particular synaptotagmin isoforms confer specificity onto different types of regulated exocytosis, and that complexins serve as universal synaptotagmin adaptors for all of these types of exocytosis independent of which synaptotagmin isoform is involved.

Rab27a regulates exocytosis of tertiary and specific granules in human neutrophils

The correct mobilization of cytoplasmic granules is essential for the proper functioning of human neutrophils in host defense and inflammation. In this study, we have found that human peripheral blood neutrophils expressed high levels of Rab27a, whereas Rab27b expression was much lower. This indicates that Rab27a is the predominant Rab27 isoform present in human neutrophils. Rab27a was up-regulated during neutrophil differentiation of HL-60 cells. Subcellular fractionation and immunoelectron microscopy studies of resting human neutrophils showed that Rab27a was mainly located in the membranes of specific and gelatinase-enriched tertiary granules, with a minor localization in azurophil granules. Rab27a was largely absent from CD35-enriched secretory vesicles. Tertiary and specific granule-located Rab27a population was translocated to the cell surface upon neutrophil activation with PMA that induced exocytosis of both tertiary and specific granules. Specific Abs against Rab27a inhibited Ca(2+) and GTP-gamma-S activation and PMA-induced exocytosis of CD66b-enriched tertiary and specific granules in electropermeabilized neutrophils, whereas secretion of CD63-enriched azurophil granules was scarcely affected. Human neutrophils lacked or expressed low levels of most Slp/Slac2 proteins, putative Rab27 effectors, suggesting that additional proteins should act as Rab27a effectors in human neutrophils. Our data indicate that Rab27a is a major component of the exocytic machinery of human neutrophils, modulating the secretion of tertiary and specific granules that are readily mobilized upon neutrophil activation.

Involvement of Munc18 isoforms in the regulation of granule exocytosis in neutrophils

Human neutrophil granule exocytosis mobilizes a complex set of secretory granules. This involves different combinations of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins to facilitate membrane fusion. The control mechanisms governing the late fusion steps are still poorly understood. Here, we have analyzed SNARE-interacting Sec1/Munc18 (SM) family members. We found that human neutrophils express Munc18-2 and Munc18-3 isoforms and that Munc18-2 interacts with the target-SNARE syntaxin 3. Munc18-2 was associated preferentially with primary granules but could also be found with secondary and tertiary granules, while Munc18-3 was majorily associated with secondary and tertiary granules. Ultrastructural analysis showed that both Munc18-2 and Munc18-3 were often located in close proximity to their respective SNARE-binding partners syntaxin 3 and syntaxin 4. Both isoforms were also found in plasma membrane fractions and in the cytosol, where they associate with cytoskeletal elements. Upon stimulation, Munc18-2 and Munc18-3 redistributed and became enriched on granules and in the plasma membrane. Munc18-2 primary granule exocytosis can be blocked by introduction of Munc18-2-specific antibodies indicating a crucial role in primary granule fusion. Our results suggest that Munc18-2 acts as a regulator of primary granule exocytosis, while Munc18-3 may preferentially regulate the fusion of secondary granules.