Porosome: the secretory NanoMachine in cells

Dynamics of plasma membrane surface related to the release of extracellular vesicles by mesenchymal stem cells in culture

Extracellular vesicles (exosomes and shedding vesicles) released by mesenchymal stem cells (MSCs) are regarded as a storable, cell-free alternative with comparable therapeutic potential to their parent cells. Shedding vesicles originate as bulges on the cell surface but little is known about their turnover or how their formation can be stimulated. We have used atomic force microscopy (AFM) to follow the formation dynamics of bulges in living adipose tissue-derived MSCs. AFM images showed that, in general, MSCs present hundreds of nanosized protrusions on their surface with life spans of 10-20 min. Scanning electron microscopy confirmed those images and showed that bulges are also formed on filamentous processes. Extracellular vesicles deposited on the culture surface have comparable sizes to those of bulges showing up on the cell surface. The amount of protrusions on cells treated with progesterone or PDGF-BB, two treatments that stimulate the secretion of extracellular vesicles in MSCs, was evaluated by AFM. Measurements of the cross-area at 50 nm over the cell surface provided estimates of the amount of protrusions and showed that these values increased with the stimulating treatments. Our study suggests that shedding vesicles constitute a large population of the extracellular vesicle pool.

Fine-Tuning of CD8(+) T Cell Mitochondrial Metabolism by the Respiratory Chain Repressor MCJ Dictates Protection to Influenza Virus

Mitochondrial respiration is regulated in CD8(+) T cells during the transition from naive to effector and memory cells, but mechanisms controlling this process have not been defined. Here we show that MCJ (methylation-controlled J protein) acted as an endogenous break for mitochondrial respiration in CD8(+) T cells by interfering with the formation of electron transport chain respiratory supercomplexes. Metabolic profiling revealed enhanced mitochondrial metabolism in MCJ-deficient CD8(+) T cells. Increased oxidative phosphorylation and subcellular ATP accumulation caused by MCJ deficiency selectively increased the secretion, but not expression, of interferon-γ. MCJ also adapted effector CD8(+) T cell metabolism during the contraction phase. Consequently, memory CD8(+) T cells lacking MCJ provided superior protection against influenza virus infection. Thus, MCJ offers a mechanism for fine-tuning CD8(+) T cell mitochondrial metabolism as an alternative to modulating mitochondrial mass, an energetically expensive process. MCJ could be a therapeutic target to enhance CD8(+) T cell responses.

Function Suggests Nano-Structure: Quantitative Structural Support for SNARE-Mediated Pore Formation

Granule secretory content is released in either basal or calcium-activated complete exocytosis mode. A vital element in these processes is the establishment of a fusion pore between the granule membrane and the plasma membrane, initiated by the formation of a circular rosette docking arrangement of SNARE protein complexes. The controversially disputed number of SNARE complexes needed for granule priming leading to the formation of the fusion pore, is granule-size dependent and varies between secretion modes. Resorting to a statistical mechanics approach that views SNARE complexes and Ca(2+) ions as interacting particles, we have developed a relationship that links secretion rate to SNARE rosette size, Ca(2+) concentration and Ca(2+) ion cooperativity. Data are presented and discussed which suggest this SNARE-dependent generalization of existing narrow-range biophysical models that correlate secretion rate with Ca(2+) concentration and maximal Ca(2+) ion cooperativity. Evidence from dozens of examples in the literature advocate for this relation, which holds through the entire biological range. The coalescence of so many areas of diverse research methodologies has greatly augmented our understanding of so many different sequences of granule life cycle. Accordingly, these new tools may become valuable in a variety of electrophysiological experiments.

Porosome: a membrane microdomain acting as the universal secretory portal in exocytosis

Most, if not all, cells in the organism, at least in some period of their lifetime, secrete materials that are produced within the cell. Cell secretion is a phenomenon requiring membrane fusion at a specialized plasma membrane structure called the 'porosome,' which allows the material stored within secretory vesicles to be delivered to the cell's exterior environment. This is achieved when the secretory vesicles fuse at the base of the porosome complex, establishing a fusion pore or fluid continuity between the vesicle interior and the cell's exterior. Besides cell secretion, membrane fusion is necessary for intracellular membrane traffic and vesicular transport from one endomembrane bound structure to another. In addition to cell secretion, membrane fusion is necessary for intracellular membrane trafficking and vesicle transport from one intracellular membrane to another. We suggest that the debate about whether to use the term 'porosome' or 'fusion pore' to describe this process is unnecessary, since both of these terms are useful in describing aspects of the last event of cell secretion, namely exocytosis. In this review, we will summarize the information related to the discovery of the porosome, a universal secretory portal for exocytosis, and discuss porosome molecular organization and function. Finally, we will develop the notion that the porosome is a specialized plasma membrane microdomain.

New insights into roles of acidocalcisomes and contractile vacuole complex in osmoregulation in protists

While free-living protists are usually subjected to hyposmotic environments, parasitic protists are also in contact with hyperosmotic habitats. Recent work in one of these parasites, Trypanosoma cruzi, has revealed that its contractile vacuole complex, which usually collects and expels excess water as a mechanism of regulatory volume decrease after hyposmotic stress, has also a role in cell shrinking when the cells are submitted to hyperosmotic stress. Trypanosomes also have an acidic calcium store rich in polyphosphate (polyP), named the acidocalcisome, which is involved in their response to osmotic stress. Here, we review newly emerging insights on the role of acidocalcisomes and the contractile vacuole complex in the cellular response to hyposmotic and hyperosmotic stresses. We also review the current state of knowledge on the composition of these organelles and their other roles in calcium homeostasis and protein trafficking.