ABSTRACT
CD1d is a major histocompatibility complex (MHC) class I-related molecule that functions in glycolipid antigen presentation to distinct subsets of T cells that express natural killer receptors and an invariant T-cell receptor-alpha chain (invariant NKT cells). The acquisition of glycolipid antigens by CD1d occurs, in part, in endosomes through the function of resident lipid transfer proteins, namely saposins. Here we show that microsomal triglyceride transfer protein (MTP), a protein that resides in the endoplasmic reticulum of hepatocytes and intestinal epithelial cells (IECs) and is essential for lipidation of apolipoprotein B, associates with CD1d in hepatocytes. Hepatocytes from animals in which Mttp (the gene encoding MTP) has been conditionally deleted, and IECs in which Mttp gene products have been silenced, are unable to activate invariant NKT cells. Conditional deletion of the Mttp gene in hepatocytes is associated with a redistribution of CD1d expression, and Mttp-deleted mice are resistant to immunopathologies associated with invariant NKT cell-mediated hepatitis and colitis. These studies indicate that the CD1d-regulating function of MTP in the endoplasmic reticulum is complementary to that of the saposins in endosomes in vivo.
Subject(s)
Antigens, CD1/physiology , Carrier Proteins/physiology , Abetalipoproteinemia/genetics , Abetalipoproteinemia/pathology , Abetalipoproteinemia/physiopathology , Animals , Antigens, CD1d , Base Sequence , Carrier Proteins/genetics , DNA, Complementary/genetics , Endoplasmic Reticulum/physiology , Gene Silencing , Hepatocytes/immunology , Hepatocytes/physiology , Killer Cells, Natural/immunology , Liver/pathology , Mice , Mice, Inbred C57BL , Mice, Mutant Strains , T-Lymphocyte Subsets/immunologyABSTRACT
Adjacent mitochondrial open reading frames orf355 and orf77 are associated with S cytoplasmic male sterility (CMS-S) in maize, but the mechanisms leading to collapse of developing CMS-S pollen are unknown. Sequence similarity between orf77 and the mitochondrial ATP synthase subunit 9 (atp9) locus led us to examine RNA editing in orf77 and atp9 transcripts of pre-collapse CMS-S microspores. Editing of atp9 was not influenced by the presence of orf77 transcripts. Sequence analysis of cDNA clones demonstrated that atp9 transcripts are fully edited in CMS-S microspores. Orf77 nucleotides corresponding to edited nucleotides in atp9 were either not edited or edited inefficiently within the context of orf77, perhaps due to limited conservation of flanking sequences between orf77 and atp9. However, eight of ten orf77 cDNA clones carried an unexpected terminating edit that truncated orf77 to predict a peptide of 17 amino acids (ORF17) sharing significant identity with the C-terminal transmembrane domain of the ATP9 protein.
