DALIS: proteolytic centers in dendritic cells

Abstract

In conditions of impaired proteolysis, proteasomes and ubiquitinated proteins aggregate, forming structures dubbed ‘aggresomes’. These are located usually at the nuclear periphery around the centrosome and are often surrounded by a vimentin cage. The reason why aggresomes form is unclear. They could serve either to localize otherwise toxic aggregates or to deliver them to an autophagic pathway. It is also possible that proteasome-dependent protein degradation is selectively enhanced or concentrated in this region of the cell (the ‘proteolytic center’ hypothesis). Whatever the reason for aggresome formation, it seems to be associated with several neurodegenerative disorders – aggresome-like inclusion bodies are usually found in affected neurons.Now, Pierre and colleagues [1xTransient aggregation of ubiquitinated proteins during dendritic cell maturation. Lelouard, H. et al. Nature. 2002; 417: 177–182CrossRef | PubMed | Scopus (118)See all References][1] have provided evidence for discrete foci of ubiquitinated proteins that are apparently unrelated to the pathological aggresomes, having instead a physiological function. Mature dendritic (Langerhans) cells both in vivo and in vitro display aggregation of ubiquitinated proteins in response to inflammatory stimulation. Under those conditions, dendritic cells form from 1 to 4 rounded aggregates per cell, which are up to 4 μm in diameter. However, in contrast to the ‘aggresomes’, their formation does not depend on microtubular transport, they do not form around centrosomes and they do not have the vimentin cage. Therefore, they have been nicknamed ‘dendritic-cell aggresome-like inducible structures’ (DALIS). The authors point out the transient nature of DALIS in contrast to the aggresomes as they arise 4 h after lipopolysaccharide stimulation and disappear 24–36 h later. Pathological aggresomes are indeed stable; however, aggregates of ubiquitinated proteins arising after a short period of proteasome inhibition also are cleared from the cells 48 h later, probably by means of autophagy. Dendritic cells discriminate different subpopulations of ubiquitinated proteins as DALIS do not contain heat-shock proteins or the proteasome substrate IκBα, which must therefore be degraded in another location. However, co-treatment with a proteasome inhibitor induces the appearance of those proteins in DALIS. Most of the components of DALIS are newly synthesized proteins, most likely ‘defective ribosomal products’ (DRiPs), as judged by the effects of cycloheximide and canavanin on DALIS formation, radioactive labeling, as well as by the fact that ribosomes are concentrated in the cytoplasm surrounding DALIS.Altogether, the discovery of the DALIS reveals a physiological function for an aggregate of ubiquitinated proteins in dendritic cells. Many questions remain unanswered. It remains to be proven exactly how DALIS are involved in antigen presentation. Although it seems at first glance that DALIS are different from the ‘aggresomes’, they might actually represent a very early stage of this structure, arising at the proteolytic center of the cell – that is, at an area of the cytoplasm dedicated to the proteasome-mediated degradation of proteins. It remains to be tested whether proteasomes and other components of the ubiquitin pathway are also enriched at DALIS as well as whether autophagy is involved in clearing DALIS away.