Caplan Syndrome: Dell'Angelica EC

Dell'Angelica EC, Mullins C, Caplan S, Bonifacino JS. · Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892-5430, USA. · FASEB J. · Pubmed #10877819 links to  free full text

Abstract: Lysosomes are membrane-bound cytoplasmic organelles involved in intracellular protein degradation. They contain an assortment of soluble acid-dependent hydrolases and a set of highly glycosylated integral membrane proteins. Most of the properties of lysosomes are shared with a group of cell type-specific compartments referred to as 'lysosome-related organelles', which include melanosomes, lytic granules, MHC class II compartments, platelet-dense granules, basophil granules, azurophil granules, and Drosophila pigment granules. In addition to lysosomal proteins, these organelles contain cell type-specific components that are responsible for their specialized functions. Abnormalities in both lysosomes and lysosome-related organelles have been observed in human genetic diseases such as the Chediak-Higashi and Hermansky-Pudlak syndromes, further demonstrating the close relationship between these organelles. Identification of genes mutated in these human diseases, as well as in mouse and Drosophila: pigmentation mutants, is beginning to shed light on the molecular machinery involved in the biogenesis of lysosomes and lysosome-related organelles.

Caplan S, Dell'Angelica EC, Gahl WA, Bonifacino JS. · Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, Building 18T Room 101, 18 Library Dr. MSC 5430, 20892-5430, Bethesda, MD, USA. · Immunol Lett. · Pubmed #10841946 No free full text.

Abstract: The major histocompatibility complex class II subunits (MHC-II) alpha and beta assemble with the invariant chain (Ii) in the endoplasmic reticulum and are transported to endosomal-lysosomal organelles known as MHC class II compartments (MIICs). Although it has been shown that two dileucine-based signals in the cytosolic tail of Ii, as well as a dileucine-based signal in the tail of the beta chain mediate sorting to MIICs, the molecular mechanisms by which alphabetaIi complexes are sorted have yet to be resolved fully. The AP-3 adaptor complex stands out as a particularly good candidate for mediating this targeting because: (i) it has a proven role in the trafficking of membrane proteins to lysosome-related organelles; and (ii) it has the ability to interact with dileucine-based signals in vitro. To investigate the potential role of AP-3 in transport of MHC-II to MIICs, we have examined MHC-II trafficking in human B-lymphoblast lines from patients with Hermansky-Pudlak syndrome type 2 (HPS-2), which are deficient in the AP-3 complex. Pulse-chase analyses revealed no significant alteration in the kinetics of synthesis and degradation of either MHC-II subunits or Ii. Moreover, we observed neither impairment of the formation of compact SDS-resistant alphabeta dimers, nor delay in the appearance of a conformational epitope indicative of a mature, Ii-free alphabeta dimer. Finally, we demonstrated that in HPS-2 patients' cells, there was no delay in the expression of the alphabeta dimers on the cell surface. Thus, AP-3 does not seem to be essential for normal trafficking of MHC-II. These findings have important implications for HPS-2 patients, because they suggest that the recurrent bacterial infections suffered by these patients are not likely due to impaired antigen processing and presentation by MHC-II.