Abstract
Cytoplasmic structures showing a selective concentration of both polyubiquitinated proteins and proteasome have been described in various epithelial, hematopoietic, mesenchymal and neural cells in vitro or in fetal tissues, as well as in chronically-infected, mutated preneoplastic and neoplastic tissues. These cytoplasmic structures differ from other ubiquitin-reactive cytoplasmic bodies, like sequestosomes, aggresome-like-induced structures in dendritic cells (DALIS)/non-dendritic cells (ALIS) and aggresomes in showing distinctive ultrastructural organization (particle-rich cytoplasmic structure or PaCS), a cytochemical pattern and a functional profile. Their formation can be induced in vitro in dendritic or natural killer cells by trophic factors and interleukin treatment. They originate in close connection with ribosomes, while, as a result of their growth, the cytoskeleton and other surrounding organelles are usually dislocated outside their core. Interestingly, these particulate cytoplasmic structures are often found to fill cytoplasmic blebs forming proteasome- and polyubiquitinated protein-discharging vesicles, called ectosomes, which are found to detach from the cell and freely float in the extracellular space. To clearly point out the importance of the polyubiquitinated proteins and proteasome containing cytoplasmic structures, their role in cell biology and pathology has been carefully analyzed.
Highlights
“aggresomes” accumulating mutated proteins [2], the “aggresome-like-induced structures” in dendritic cells (DALIS) [3,4] or in non-dendritic cells (ALIS) [5], the amorphous to fibrillary “sequestosomes” [6], the yeast “juxtanuclear quality control compartment” (JUNQ) and the perivacuolar “insoluble protein deposit” (IPOD) [7] and the “particle-rich cytoplasmic structure” (PaCS) [8] accumulating ubiquitin proteasome system (UPS) components are among such bodies
Recent correlative electron/confocal microscopy investigations and direct ultrastructural immunogold observations [6,14,15] have allowed for the characterization in mammalian cells of at least four types of structurally- and cytochemically-different bodies with apparently different roles in cell biology and pathology: sequestosomes, DALIS/ALIS, pericentriolar aggresomes and PaCSs
Under transmission electron microscopy (TEM) of aldehyde-osmium fixed, resin-embedded sections (Figure 1), PaCS appears as a collection of barrel-like particles in a relatively clear cytoplasmic area void of cytoskeleton fibrils and usually surrounded by ribosomes, with or without rough endoplasmic reticulum (RER) cisternae
Summary
Various ubiquitin-storing cytoplasmic bodies or structures, with or without proteasome colocalization, have been reported in cell lines under stress conditions, leading to the aggregation of misfolded proteins. The “proteolytic centers” developing in cells under proteasome function inhibitors [1], the pericentriolar “aggresomes” accumulating mutated proteins [2], the “aggresome-like-induced structures” in dendritic cells (DALIS) [3,4] or in non-dendritic cells (ALIS) [5], the amorphous to fibrillary “sequestosomes” [6], the yeast “juxtanuclear quality control compartment” (JUNQ) and the perivacuolar “insoluble protein deposit” (IPOD) [7] and the “particle-rich cytoplasmic structure” (PaCS) [8] accumulating ubiquitin proteasome system (UPS) components are among such bodies.
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