Cooperative cell-cell actin network remodeling to perform Gap junction endocytosis.

BACKGROUND: The endocytosis of Gap junction plaques (GJP) requires cytoskeletal forces to internalize such large membranous structures. Actin, which partners the connexin proteins constituting Gap junctions and is located close to Annular Gap Junctions (AGJ), could be actively involved in this physiological process. RESULTS: Electron Microscopy and Light Microscopy images, associated with time-lapse analysis and 3D reconstruction, used at high resolution and enhanced using ImageJ based software analysis, revealed that: i) actin cables, originating from Donor cells, insert on the edge of GJP and contribute to their invagination, giving rise to AGJ, whereas actin cables on the Acceptor cell side of the plaque are not modified; ii) actin cables from the Donor cell are continuous with the actin network present over the entire GJP surface. These actin cables fuse at a single point distant from the plaque, which then detaches itself from the membrane, condensing to form an actin mass during the final internalization process; iii) the Acceptor cell participates in the last step of the endocytic invagination process by forming an annular actin structure known as an actin ring. CONCLUSIONS: Together, these data suggest that the endocytosis of GJP is an example of a unique cooperative mechanism between the Donor (the traction of its actin cables) and the Acceptor cells (forming the actin ring).

RéSUMé: CONTEXTE: L'endocytose des plaques de jonctions communicantes ou jonctions gap (GJP) nécessite les forces du cytosquelette pour internaliser ces grandes structures membranaires. L'actine, partenaire des connexines, proteins constitutives des jonctions gap (Gj), localisée proche des jonctions gap annulaires (GJA), pourrait être impliquée dans ce processus physiologique. RéSULTATS: L' imagerie par microscopie optique et électronique, associées avec des analyses vidéo et des reconstructions en relief/3D, examinées à haute résolution et améliorées après traitement par des logiciels développés sous ImageJ, montrent que: i) des câbles d'actine, originaires des cellules donneuses, s'insèrent sur le bord des plaques jonctionnelles et facilitent leur invagination pour former les GJA tandis que les câbles d'actine des cellules receveuses ne sont pas modifies; ii) les câbles d'actine des cellules donneuses sont en continuité avec le réseau d'actine qui couvre la totalité de la surface de la plaque. De plus, ces câbles fusionnent en un point unique, à distance de la plaque, qui se détache de la région membranaire pour former une masse d'actine à la fin du processus d'endocytose; iii) la cellule receveuse participe à l'étape ultime du processus d'endocytose de la plaque en formant un anneau d'actine. CONCLUSIONS: L'ensemble de nos résultats montrent que l'endocytose des plaques jonctionnelles est un exemple de coopération unique entre la cellule donneuse (grâce à la traction des câbles d'actine) et la cellule receveuse (anneau d'actine).

PI4P and BLOC-1 remodel endosomal membranes into tubules.

Intracellular trafficking is mediated by transport carriers that originate by membrane remodeling from donor organelles. Tubular carriers contribute to the flux of membrane lipids and proteins to acceptor organelles, but how lipids and proteins impose a tubular geometry on the carriers is incompletely understood. Using imaging approaches on cells and in vitro membrane systems, we show that phosphatidylinositol-4-phosphate (PI4P) and biogenesis of lysosome-related organelles complex 1 (BLOC-1) govern the formation, stability, and functions of recycling endosomal tubules. In vitro, BLOC-1 binds and tubulates negatively charged membranes, including those containing PI4P. In cells, endosomal PI4P production by type II PI4-kinases is needed to form and stabilize BLOC-1-dependent recycling endosomal tubules. Decreased PI4KIIs expression impairs the recycling of endosomal cargoes and the life cycles of intracellular pathogens such as Chlamydia bacteria and influenza virus that exploit the membrane dynamics of recycling endosomes. This study demonstrates how a phospholipid and a protein complex coordinate the remodeling of cellular membranes into functional tubules.

A role for Dynlt3 in melanosome movement, distribution, acidity and transfer.

Skin pigmentation is dependent on cellular processes including melanosome biogenesis, transport, maturation and transfer to keratinocytes. However, how the cells finely control these processes in space and time to ensure proper pigmentation remains unclear. Here, we show that a component of the cytoplasmic dynein complex, Dynlt3, is required for efficient melanosome transport, acidity and transfer. In Mus musculus melanocytes with decreased levels of Dynlt3, pigmented melanosomes undergo a more directional motion, leading to their peripheral location in the cell. Stage IV melanosomes are more acidic, but still heavily pigmented, resulting in a less efficient melanosome transfer. Finally, the level of Dynlt3 is dependent on ß-catenin activity, revealing a function of the Wnt/ß-catenin signalling pathway during melanocyte and skin pigmentation, by coupling the transport, positioning and acidity of melanosomes required for their transfer.