Er et al. 2009). Cleavage is followed by polymerization into physiological PMEL

Er et al. 2009). Cleavage is followed by polymerization into physiological PMEL amyloid fibrils within the MVE/premelanosome. This process is reminiscent of the proposed mechanism of intravesicular amyloid-formation. The viral oncogene latent membrane protein LMP1 is another protein that relies on CD63-dependent sorting into a subtype of ILVs that are characterized by low cholesterol and are exosomally secreted (Verweij et al. 2011). Exosomes can either be secreted in a constitutive or regulated process. An increase in intracellular calcium can trigger MVE fusion and exosome release in various cell types, including neurons, via a mechanism similar to that described for secretory lysosomes (Faure et al. 2006; Savina et al. 2003). The latter process requires synaptotagmin VII, rab27, Munc13-4, AP3 and VAMP7 (Lakkaraju and Rodriguez-Boulan 2008). However, whether these molecules are also involved in MVE fusion and subsequent exosome release is unclear. The secretion of exosomes involves tethering, docking and fusion of the MVE at the plasma membrane. Several regulatory factors of this machinery have been identified, including rab11, the rhoAeffector citron kinase, rab27 and rab35 (Loomis et al. 2006; Savina et al. 2002; Ostrowski et al. 2010; Hsu et al. 2010). Calcium enhances exosome release probably by stimulating the fusion of MVEs with the plasma cell membrane in a VATPase V0-subunit-dependent manner (Liegeois et al. 2006; Marshansky and Futai 2008). Changes in intracellular ion concentrations after the P2X7-receptor-induced activation of the ATP-gated ion channel have been described to trigger the release of exosomes in immune cells (Qu and Dubyak 2009). Other stimulatory factors, such as DNA damage and (oxidative) stress also promote exosome release, consistent with a role for exosomes in the removal of toxic molecules from the cell (Lespagnol et al.Enoblituzumab 2008). Microparticles shed from the plasma membrane are dependent on the calcium-induced reorganization of the cytoskeleton and membrane lipid asymmetry. The outer membrane leaflet of microparticles is enriched in aminophospholipids such as phoshatidylserine (PS) and phosphatidylethanolamine (PE) and the asymmetric distribution of these lipids has been proposed as a mechanism to trigger membrane bending because of their conical shape (Basse et al. 1993; Wehman et al. 2011). Lipid asymmetry is, among other factors, created by the enzymatic activity of scramblase, which translocates and enriches PS and PE from the inner to the outer membrane leaflet (Contreras et al. 2010). This is illustrated by the deficiency of procoagulatory platelet microvesiculation observed in Scott’s syndrome in which the lipid asymmetry of the outer plasma membrane is dysregulated and PE and PS are mainly restricted to the inner leaflet of the bilayer (Lhermusier et al.Chlorpheniramine maleate 2011).PMID:22664133 Recently, the transmembrane flippase TAT-5 has been shown, in Caenorhabditis elegans, selectively to enrich PE within the inner leaflet without affecting PS asymmetry (Wehman et al. 2011). A deficiency in TAT-5 results in PE enrichment within the outer leaflet and vesicle shedding, whereas TAT-1 mutations, which lead to the accumulation of PS within the outer leaflet, have no impact on vesicle release. In addition, Wehman et al. (2011) have identified rab11 and the ESCRT complex as promoting microvesicle formation. Whether the conical shape of PE mediates the outward bending or whether the relative decrease of PE at the inner leaflet shifts.