Vesicular traffic in cell navigation

Biogenesis and secretion of exosomes

Although observed for several decades, the release of membrane-enclosed vesicles by cells into their surrounding environment has been the subject of increasing interest in the past few years, which led to the creation, in 2012, of a scientific society dedicated to the subject: the International Society for Extracellular Vesicles. Convincing evidence that vesicles allow exchange of complex information fuelled this rise in interest. But it has also become clear that different types of secreted vesicles co-exist, with different intracellular origins and modes of formation, and thus probably different compositions and functions. Exosomes are one sub-type of secreted vesicles. They form inside eukaryotic cells in multivesicular compartments, and are secreted when these compartments fuse with the plasma membrane. Interestingly, different families of molecules have been shown to allow intracellular formation of exosomes and their subsequent secretion, which suggests that even among exosomes different sub-types exist.

Cholesterol regulates Syntaxin 6 trafficking at trans-Golgi network endosomal boundaries

Inhibition of cholesterol export from late endosomes causes cellular cholesterol imbalance, including cholesterol depletion in the trans-Golgi network (TGN). Here, using Chinese hamster ovary (CHO) Niemann-Pick type C1 (NPC1) mutant cell lines and human NPC1 mutant fibroblasts, we show that altered cholesterol levels at the TGN/endosome boundaries trigger Syntaxin 6 (Stx6) accumulation into VAMP3, transferrin, and Rab11-positive recycling endosomes (REs). This increases Stx6/VAMP3 interaction and interferes with the recycling of αVβ3 and α5β1 integrins and cell migration, possibly in a Stx6-dependent manner. In NPC1 mutant cells, restoration of cholesterol levels in the TGN, but not inhibition of VAMP3, restores the steady-state localization of Stx6 in the TGN. Furthermore, elevation of RE cholesterol is associated with increased amounts of Stx6 in RE. Hence, the fine-tuning of cholesterol levels at the TGN-RE boundaries together with a subset of cholesterol-sensitive SNARE proteins may play a regulatory role in cell migration and invasion.

Expression of α-taxilin in the murine gastrointestinal tract: potential implication in cell proliferation

α-Taxilin, a binding partner of the syntaxin family, is a candidate tumor marker. To gain insight into the physiological role of α-taxilin in normal tissues, we examined α-taxilin expression by Western blot and performed immunochemical analysis in the murine gastrointestinal tract where cell renewal vigorously occurs. α-Taxilin was expressed in the majority of the gastrointestinal tract and was prominently expressed in epithelial cells positive for Ki-67, a marker of actively proliferating cells. In the small intestine, α-taxilin was expressed in transient-amplifying cells and crypt base columnar cells intercalated among Paneth cells. In the corpus and antrum of the stomach, α-taxilin was expressed in cells localized in the lower pit and at the gland, respectively, but not in parietal or zymogenic cells. During development of the small intestine, α-taxilin was expressed in Ki-67-positive regions. Inhibition of cell proliferation by suppression of the Notch cascade using a γ-secretase inhibitor led to a decrease in α-taxilin- and Ki-67-positive cells in the stomach. These results suggest that expression of α-taxilin is regulated in parallel with cell proliferation in the murine gastrointestinal tract.

Toxoplasma gondii Syntaxin 6 is required for vesicular transport between endosomal-like compartments and the Golgi complex

Apicomplexans are obligate intracellular parasites that invade the host cell in an active process that relies on unique secretory organelles (micronemes, rhoptries and dense granules) localized at the apical tip of these highly polarized eukaryotes. In order for the contents of these specialized organelles to reach their final destination, these proteins are sorted post-Golgi and it has been speculated that they pass through endosomal-like compartments (ELCs), where they undergo maturation. Here, we characterize a Toxoplasma gondii homologue of Syntaxin 6 (TgStx6), a well-established marker for the early endosomes and trans Golgi network (TGN) in diverse eukaryotes. Indeed, TgStx6 appears to have a role in the retrograde transport between ELCs, the TGN and the Golgi, because overexpression of TgStx6 results in the development of abnormally shaped parasites with expanded ELCs, a fragmented Golgi and a defect in inner membrane complex maturation. Interestingly, other organelles such as the micronemes, rhoptries and the apicoplast are not affected, establishing the TGN as a major sorting compartment where several transport pathways intersect. It therefore appears that Toxoplasma has retained a plant-like secretory pathway.

Goliath family E3 ligases regulate the recycling endosome pathway via VAMP3 ubiquitylation

Diverse cellular processes depend on endocytosis, intracellular vesicle trafficking, sorting and exocytosis, processes regulated post-transcriptionally by modifications such as phosphorylation and ubiquitylation. In addition to sorting to the lysosome, cargo is recycled to the plasma membrane via recycling endosomes. Here, we describe a role of the goliath gene family of protease-associated (PA) domain E3 ligases in regulating recycling endosome trafficking. The two Drosophila members of this family--Goliath and Godzilla(CG10277)--are located on endosomes, and both ectopic expression and loss-of-function lead to the accumulation of Rab5-positive giant endosomes. Furthermore, the human homologue RNF167 exhibits similar behaviour. We show that the soluble N-ethylmaleimide-sensitive fusion attachment protein receptor (SNARE) protein VAMP3 is a target of these ubiquitin ligases, and that recycling endosome trafficking is abrogated in response to their activity. Furthermore, mutation of the Godzilla ubiquitylation target lysines on VAMP3 abrogates the formation of enlarged endosomes induced by either Godzilla or RNF167. Thus, Goliath ubiquitin ligases play a novel role in regulating recycling endosome trafficking via ubiquitylation of the VAMP3 SNARE protein.

Intracellular signaling and membrane trafficking control bidirectional growth cone guidance

The formation of precise neuronal networks is critically dependent on the motility of axonal growth cones. Extracellular gradients of guidance cues evoke localized Ca(2+) elevations to attract or repel the growth cone. Recent studies strongly suggest that the polarity of growth cone guidance, with respect to the localization of Ca(2+) signals, is determined by Ca(2+) release from the endoplasmic reticulum (ER) in the following manner: Ca(2+) signals containing ER Ca(2+) release cause growth cone attraction, while Ca(2+) signals without ER Ca(2+) release cause growth cone repulsion. Recent studies have also shown that exocytic and endocytic membrane trafficking can drive growth cone attraction and repulsion, respectively, downstream of Ca(2+) signals. Most likely, these two mechanisms underlie cue-induced axon guidance, in which a localized imbalance between exocytosis and endocytosis dictates bidirectional growth cone steering. In this Update Article, I summarize recent advances in growth cone research and propose that polarized membrane trafficking plays an instructive role to spatially localize steering machineries, such as cytoskeletal components and adhesion molecules.