Intracellular vesicle transport

In eukaryotes, soluble N-ethylmaleimide-sensitive factor (NSF) adaptor proteins (SNAPs) receptors (SNAREs) are known to be required for docking and fusion of intracellular transport vesicles with acceptor/target membranes. The fusion of vesicles in the secretory pathway involves target-SNAREs (t-SNAREs) on the target membrane and vesicle-SNAREs (v-SNAREs) on vesicle membranes that recognize each other and assemble into trans-SNARE complexes (Sollner et al., 1993). 

At the same time, recent MD simulations have shown the capability of dealing with system sizes and curvatures that are comparable with those found in intracellular transport vesicles and that have been traditionally addressed solely using continuum methods. ... 

TRANSPORT VESICLES ARE KEY PLAYERS IN INTRACELLULAR PROTEIN TRAFFIC... 

Vesicles lie at the heart of intracellular transport of many proteins. Recently, significant progress has been made in understanding the events involved in vesicle formation and transport. This has transpired because of the use of a number of approaches. These include establishment of cell-free systems with which to study vesicle formation. For instance, it is possible to observe, by electron microscopy, budding of vesicles from Golgi preparations incubated with cytosol and ATP. The development of genetic approaches for studying vesicles in yeast has also been crucial. The piemre is complex, with its own nomenclamre (Table 46-7), and involves a variety of cytosolic and membrane proteins, GTP, ATP, and accessory factors. 

A highly stable and shielded polyplex should circulate in the blood stream without undesired interactions until it reaches the target cell. At that location, specific interactions with the cell surface should trigger intracellular uptake. While lipid membrane interaction is undesired at the cell surface, it should happen subsequently within the endosomal vesicle and mediate polyplex delivery into the cytosol. During or after intracellular transport to the site of action, the polyplex stability should be weakened to an extent that the nucleic acid is accessible to exert its function. 

There are also monomeric G-proteins. Just like the trimeric G-pro-teins, they are involved as signal relays and timers. The Ras superfamily relays signals from receptor tyrosine kinases to downstream elements that eventually regulate transcription. Rho and Rac relay signals from cell-surface receptors to the cytoskeleton, while Rab regulates intracellular transport of vesicles. Regardless of what they do, they use the timer mechanism provided by the G-protein. Three-letter acronyms (TLA), such as Ras, Rho, and Rab, are difficult to remember, sometimes even when you know what the letters stand for. Unfortunately, there s nothing you can do about this except to memorize them. 

Lipids are transported between membranes. As indicated above, lipids are often biosynthesized in one intracellular membrane and must be transported to other intracellular compartments for membrane biogenesis. Because lipids are insoluble in water, special mechanisms must exist for the inter- and intracellular transport of membrane lipids. Vesicular trafficking, cytoplasmic transfer-exchange proteins and direct transfer across membrane contacts can transport lipids from one membrane to another. The best understood of such mechanisms is vesicular transport, wherein the lipid molecules are sorted into membrane vesicles that bud out from the donor membrane and travel to and then fuse with the recipient membrane. The well characterized transport of plasma cholesterol into cells via receptor-mediated endocytosis is a useful model of this type of lipid transport. [9, 20]. A brain specific transporter for cholesterol has been identified (see Chapter 5). It is believed that transport of cholesterol from the endoplasmic reticulum to other membranes and of glycolipids from the Golgi bodies to the plasma membrane is mediated by similar mechanisms. The transport of phosphoglycerides is less clearly understood. Recent evidence suggests that net phospholipid movement between subcellular membranes may occur via specialized zones of apposition, as characterized for transfer of PtdSer between mitochondria and the endoplasmic reticulum [21]. 

Neurons constitute the most striking example of membrane polarization. A single neuron typically maintains thousands of discrete, functional microdomains, each with a distinctive protein complement, location and lifetime. Synaptic terminals are highly specialized for the vesicle cycling that underlies neurotransmitter release and neurotrophin uptake. The intracellular trafficking of a specialized type of transport vesicles in the presynaptic terminal, known as synaptic vesicles, underlies the ability of neurons to receive, process and transmit information. The axonal plasma membrane is specialized for transmission of the action potential, whereas the plasma... 

Figure 2. The confocal microscopy results show the clustering of CD95 upon stimulation. Clustered CD95 co-localizes with ASM in the cell membrane. ASM is transported to the cell membrane most likely with intracellular storage vesicles, which fuse with the cell membrane upon CD95 stimulation. Unstimulated cells show a homogenous distribution of CD95 in the cell membrane. Cells were stimulated via CD95 for 2 min or left unstimulated, fixed, permeabilized, stained with a FlTC-labeled anti-CD95 and a Texas Red anti-ASM antibody and analyzed by confocal microscopy. The right pictures show the overlay.

Phospholipids travel to their intracellular destinations via transport vesicles or specific proteins. 

As gene carriers are internalized by endocytosis, the motion characteristics inside the cell resembles the movement of the endosomal compartments within the cell and the formed vesicles are transported along the microtubule network [38]. Suh et al. [41] quantified the transport of individual internalized polyplexes by multiple-particle tracking and showed that the intracellular transport characteristics of polyplexes depend on spatial location and time posttransfection. Within 30 min, polyplexes accumulated around the nucleus. An average of the transport modes over a 22.5 h period after transfection showed that the largest fraction of polyplexes with active transport was found in the peripheral region of the cells whereas polyplexes close to the nucleus were largely diffusive and subdiffusive. Disruption of the microtubule network by nocodazole completely inhibits active transport and also the perinuclear accumulation of polyplexes [37, 40, 47]. 

AP-3 is one of four adaptor protein complexes, each of which forms a coat on the cytosolic surface of certain intracellular membranes for the purpose of forming a transport vesicle with selected proteins needed elsewhere in the cell (see Bonifacino and Glick, 2004, Robinson, 2004, and Newell-Litwa et al.,... 

Protein transport between intracellular compartments is mediated by a mechanism that is well-conserved among all eukaryotes, from yeast to man. The transport mechanism involves carrier vesicles that bud from one organelle and fuse selectively to another. Specialized proteins are required for vesicle transport, docking, and fusion, and they have been generically named SNAREs (an acronym for soluble N-ethylma-leimide-sensitive fusion attachment protein receptor). SNAREs have been divided into those associated with the vesicle (termed v-SNAREs), and those associated with the target (termed t-SNAREs). The key protein, which led to the discovery of SNAREs was NSF, an ATPase found ubiquitously in all cells, and involved in numerous intracellular transport events. The subsequent identification of soluble proteins stably bound to NSF, the so-called SNARE complex, led to the formulation of the SNARE hypothesis, which posits that all intracellular fusion events are mediated by SNAREs (Rothman, 2002). 

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