Late endosomes

C1C-6 is a late endosomal chloride transporter. Its disruption in mice led to lysosomal storage disease. C1C-7 is expressed in late endosomes and lysosomes. It needs Ostml as (3-subunit [3]. The disruption of either C1C-7 or Ostml in mice and man leads to severe osteopetrosis, retinal degeneration, and a severe lysosomal storage disease. ClC-7/Ostml is highly expressed in osteoclasts. In these cells, it is inserted together with the proton pump into the specialized plasma membrane ( ruffled border ) that faces the reabsorption lacuna. Osteoclasts are still present in C1C-7 knockout... 

Chen, W, Sun, Y, Welch, C, Gorelik, A, Leventhal, AR, Tabas, I, and Tall, AR, 2001. Preferential ATP-binding cassette transporter A1-mediated cholesterol efflux from late endosomes/lysosomes. J Biol Chem 276, 43564-43569. 

Alpy, F., Stoeckel, M. E., Dierich, A. et al. 2001. The steroidogenic acute regulatory protein homolog MLN64, a late endosomal cholesterol-binding protein. J. Biol. Chem., 276(6) 4261-4269. 

The retrieval mechanism for the M6P receptor resembles the one previously described for the H/KDEL receptor [53]. Optimal binding of M6P receptor to M6P occurs at pH 6.5-6.7, the pH found in the TGN. When transport vesicles arrive at late endosomes, the pH is lowered by the action of H+ pumps. The affinity of the M6P receptor for its ligands is reduced at acid pHs, resulting in M6P receptor releasing the M6P in the late endosome. As a result, transport of lysosomal hydrolases occurs unidirectionally. Once the M6P receptor releases M6P-bearing hydrolases, the receptor can be returned to the TGN for reuse. Transport of the M6P receptor to either TGN or late endosome relies on signal peptides on the cytoplasmic tail region of the M6P receptor. 

Under some circumstances, lysosomal hydrolases may fail to be properly packaged in the TGN, so they enter the default pathway to the cell surface, where they are secreted. Although these hydrolases do little harm at the nearly neutral pH of most extracellular fluids, they can also be returned to lysosomes by a pathway known as receptor-mediated endocytosis. In this pathway, M6P receptors are sent to the plasma membrane, where they bind escaped lysosomal hydrolases and bring them back to lysosomes through the early and late endosomes. Receptor-mediated endocytosis is a major component of the endocytic pathways for trafficking of membrane proteins and merit more detailed consideration. 

The more acidic late endosomes (pH ca. 5.5 or less) are sometimes referred to as pre-lysosomes. Digestion of the food material or other macromolecules... 

Shortly after formation of coated vesicles, the clathrin coat is removed and the vesicles are referred to as endosomes. Endosomes are roughly 300 400 nm in diameter when fully mature. Antibodies to earlier and late endosomes are available from antibodies-online GmbH, Aachen, Germany (http //www.antikoerper-online. de/). Early endosomal antigen 1 (EEA1) is a 162 kDa membrane-bound protein component specific to the early endosomes and is essential for their fusion with early endocytic vesicles for subsequent redistribution of extracellular compounds to... 

In endocytosis, vesicles are formed at the plasma membrane and then transported to an endosome. (More precisely, endosomes should at least be classified into early endosomes and late endosomes, but this fact is ignored here.) The endocytic pathway also includes the following routes from the endosome to the lysosome, from the endosome to the plasma... 

Early endosomes are the main sorting station in the endocytic pathway. In their acidic interior (pH 5.9-6.0), the receptor and its ligand can be released. The receptor may be recycled to the surface by vesicles that fuse with the plasma membrane. Material that cannot escape from the early endosomes is further transported via multivesicular bodies to late endosomes and digesting lysosomes that contain a broad spectrum of peptidases and hydrolases in an acidic surrounding [for reviews on endocytosis see Refs. (10-12), for review on clathrin uptake see Refs. (9,13)]. 

After membrane ruffling and formation of the so-called pseudopodia, the material is engulfed by the cell and is further transported to vesicles (phagosomes/macropinosomes) that have the ability to become acidified. These vesicles fuse rapidly with late endosomes and/or lysosomes, exposing their contents to the hydrophilic enzymes. 

Several aspects of intracellular trafficking should be kept in mind in the intracellular trafficking section. The first is the dependence of acidification of endosomes on the uptake of liposomes. This aspect is sometimes discussed when analyzing clathrin uptake. However, several other pathways are also in need of acidic compartments as a destination of uptake so, we list this factor as an individual aspect. Other aspects of intracellular trafficking that are of interest are the transport from early endosomes to late endosomes, the dependence of actin filaments and dynamin, and/or microtubules. Furthermore, the energy dependence of liposome uptake is discussed. 

It is reported that the transport from early to late endosomes can be blocked by incubating cells at 16°C (76,88). 

Starvation and incubation with labeled dextran For this method, cells are incubated under starvation [Earle s balanced salt solution (BBSS) medium] conditions for two hours. To label early endocytic compartments, cells are subsequently incubated with 0.5 mg/mL dextran tetramethylrhoda-mine for five minutes. To label late endosomes, cells are washed and further incubated for an additional 10 minutes (130). 

Monoclonal) antibodies Other characteristics of late endosomes are unique lipids or proteins associated with the different compartments. These can be used as a target for (monoclonal) antibodies. Often, also the recombinant GFP-fusion proteins are available (such as GFP-Rab5 and GFP-RhoB). 

Lysobisphosphatidic acid (LBPA) also distinguishes late endosomes. LBPA is shaped like an inverted cone it has a much larger head than tail and enters highly curved membrane regions. The lipid may help in the accumulation of molecules like cholesterol by specific lipid-protein interactions (131). 

For more detailed information on late endosomes and lysosome biogenesis, refer to the articles by and Piper et al. and Lnzio et al. (132-134). 

Nocodazole (1.5 pg/mL, one hour) depolymerizes microtubules, which prevents the transport vesicle from fusing with the prelysosomal compartments (late endosomes) and protects the contents of the transport vesicles from lysosomal degradation (94). 

Kobayashi T. Late endosomal membranes rich in lysobisphosphatidic acid (LBPA) regulate cholesterol transport. Nat Cell Biol 1999 1 113-118. 

Piper RC, Luzio JP. Late endosomes sorting and partitioning in multivesicular bodies. Traffic 2001 2(9) 612-621. 

Figure 1 The mode of action for bacterial AB-type exotoxins. AB-toxins are enzymes that modify specific substrate molecules in the cytosol of eukaryotic cells. Besides the enzyme domain (A-domain), AB-toxins have a binding/translocation domain (B-domain) that specifically interacts with a cell-surface receptor and facilitates internalization of the toxin into cellular transport vesicles, such as endosomes. In many cases, the B-domain mediates translocation of the A-domain into the cytosol by pore formation in cellular membranes. By following receptor-mediated endocytosis, AB-type toxins exploit normal vesicle traffic pathways into cells. One type of toxin escapes from early acidified endosomes (EE) into the cytosol, thus they are referred to as short-trip-toxins . In contrast, the long-trip-toxins take a retrograde route from early endosomes (EE) through late endosomes (LE), trans-Golgi network (TGN), and Golgi apparatus into the endoplasmic reticulum (ER) from where the A-domains translocate into the cytosol to modify specific substrates.

Protein prenylation leads to an increased hydrophobicity of proteins, typically resulting in an increased affinity for membranes. In 2004 studies on the cellular location of prenylated RhoB proteins showed that RhoB can undergo farnesylation (RhoB-F) as well as geranylgeranylation (RhoB-GG). With the aid of specific prenyl transferase inhibitors, it was revealed that RhoB-GG is localized to multivesicular late endosomes. 

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