Intracellular particle transport

Polarized intracellular particle transport saltatory movements and cytoplasmic streaming. Intern. Rev. Cytol., in press. 

Whether these principles apply to other types of particles is unknown, nor is it clear whether actin also is important in intracellular particle transport. Brody et al. (148) concluded that central transport of chrysotile was actin mediated, and Gehr et al. (154), using cytomagnetometric measurements of magnetite particles in alveolar macrophages, were of the opinion that both actin and microtubules played a role. 

Niacin (vitamin B3) has broad applications in the treatment of lipid disorders when used at higher doses than those used as a nutritional supplement. Niacin inhibits fatty acid release from adipose tissue and inhibits fatty acid and triglyceride production in liver cells. This results in an increased intracellular degradation of apolipoprotein B, and in turn, a reduction in the number of VLDL particles secreted (Fig. 9-4). The lower VLDL levels and the lower triglyceride content in these particles leads to an overall reduction in LDL cholesterol as well as a decrease in the number of small, dense LDL particles. Niacin also reduces the uptake of HDL-apolipoprotein A1 particles and increases uptake of cholesterol esters by the liver, thus improving the efficiency of reverse cholesterol transport between HDL particles and vascular tissue (Fig. 9-4). Niacin is indicated for patients with elevated triglycerides, low HDL cholesterol, and elevated LDL cholesterol.3... 

These examples indicate that sugars also act as necessary modulators, and help in establishing a conformation that is needed for membrane transport. Indeed, the ability of nonglycosylated G proteins of VSV to be transported intracellularly under certain conditions, and to form virus particles (as mentioned in Section IV,2, on conformation), is correlated with their physicochemical properties. 

Mitochondria are intracellular centers for aerobic metabolism. They are cell organelles that are identified by well-defined structural and biochemical properties. In morphological terms, mitochondria are relatively large particles that are characterized by the presence of two membranes, a smooth outer membrane that is permeable to most important metabolites and an inner membrane that has unique transport properties. The inner membrane is highly folded, which serves to increase its surface area. Figure E10.1, which shows the structure of a typical mitochondrion, divides the organelle into four major components inner membrane, outer membrane, intermembrane space, and the matrix. These regions are associated with different and... 

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]. 

This is in contrast to viruses, where the virus particles also show active transport when present in the cytosol after fusion with the plasma membrane or endosomal membrane [60-62], This is due to the ability of specific proteins of the virus particle to bind motor proteins. Single-particle tracking reveals that the quantitative intracellular transport properties of internalized non-viral gene vectors (e.g., polyplexes) are similar to that of viral vectors (e.g., adenovirus) [63]. Suk et al. showed that over 80% of polyplexes and adenoviruses in neurons are subdiffusive and 11-13% are actively transported. However, their trafficking pathways are substantially different. Polyplexes colocalized with endosomal compartments whereas adenovirus particles quickly escaped endosomes after endocytosis. Nevertheless, both exploit the intracellular transport machinery to be actively transported. 

Suh J, Wirtz D, Hanes J (2004) Real-time intracellular transport of gene nanocarriers studied by multiple particle tracking. Biotechnol Prog 20 598-602... 

Viruses are exclusively intracellular organisms and therefore depend on the cells to multiply. A complete viral particle, or virion, consists of one nucleic acid molecule (RNA or DNA) covered by a protein layer (nucleo-capsid). Some virions have a lipid cover with a glycoprotein envelope. The main function of the virion is to transport the viral genome to the interior of the host cell to be replicated and amplified. 

The endoplasmic reticulum (ER) is composed of flattened sacs and tubes of membranous bilayers that extend throughout the cytoplasm enclosing a large intracellular space. The luminal space (Fig. 1-5) is continuous with the outer membrane of the nuclear envelope (Fig. 1-10). It is involved in the synthesis of proteins and their transport to the cytoplasmic membrane (via vesicles, small spherical particles with an outer bilayer membrane). The rough ER (RER) has flattened stacks of membrane that are studded on the outer (cytoplasmic) face with ribosomes (discussed later in this section) that... 

This is indicated by a linear or saturating behavior of as a function of time. In this phase, the particles entrapped in intracellular vesicles diffuse in the cytosol, waiting for an encounter with a motor protein as a microtubule-dependent transporting system. Phase III started with a dramatic increase in particle velocity and directed movement over long distances (several micrometer) within the cell along microtubules. The directed movement is demonstrated by the quadratic dependence of as a function of time with transport velocities up to 4 j,m... 

Lichens have been used as good bioaccumulators of atmospherically transported heavy metals by different researchers (47-53). Lichens have neither roots, nor other adsorptive structures or a waxy cuticle and for mineral nutrition are largely dependent on atmospheric depositions. Lichens act as bioaccumulators via their capacity to adsorb metals from the environment. Three mechanisms have been proposed (a) intracellular uptake via an exchange process (b) intracellular accumulation and (c) trapping of metal-rich particles (54). In spite of that, there are still some gaps in the understanding of the overall process responsible for the metal uptake and accumulation in lichens. 

---