Cellular Interactions with Thrombospondin

Division Of Clinical Sciences - Nci

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Abstract

CD47 is a marker of self and a signaling receptor for thrombospondin-1 that is also an enriched membrane component of extracellular vesicles (EVs) released by various cell types. Previous studies identified CD47-dependent functional effects of T cell EVs on target cells, mediated by delivery of their RNA contents, and enrichment of specific subsets of coding and noncoding RNAs in CD47+ EVs. Here, transcriptomic analyses of EVs released by human and murine T cells revealed a global CD47-dependent enrichment of 5'-7-methylguanosine-capped microRNAs and mRNAs in EVs relative to levels in the cells. Knockdown or loss of CD47 in wild type Jurkat T cells or treatment with thrombospondin-1 enhanced levels of specific capped-RNAs released in EVs, and re-expressing CD47 in null cells decreased their levels. Mass spectrometry and co-immunoprecipitation identified specific interactions of CD47 with components of the exportin-1/Ran nuclear export complex and its known cargo proteins and between the CD47 cytoplasmic adapter ubiquilin-1 and the exportin-1/Ran complex. Interaction with CD47 was inhibited following alkylation of exportin-1 at Cys528 by leptomycin B. Leptomycin B modification of exportin-1 increased levels of cap-dependent RNAs and their association with exportin-1 in EVs released from wild type but not CD47-deficient cells. Therefore, CD47 regulates the trafficking of cap-dependent RNAs to EVs through physical interactions with the exportin-1/Ran transport complex. Targeted gene disruption in mice has provided valuable insights into the functions of matricellular proteins. Apart from missense and loss of function mutations that have been associated with inherited diseases, however, their functions in humans remain unclear. The availability of deep exome sequencing data from over 140,000 individuals in the Genome Aggregation Database provided an opportunity to examine intolerance to loss of function and missense mutations in human matricellular genes. The probability of loss-of-function intolerance (pLI) differed widely within members of the thrombospondin, CYR61/CTGF/NOV (CCN), tenascin, small integrin-binding ligand N-linked glycoproteins (SIBLING), and secreted protein, acidic and rich in cysteine (SPARC) gene families. Notably, pLI values in humans had limited correlation with viability of the corresponding homozygous null mice. Among the thrombospondins, only THBS1 was highly loss-intolerant (pLI = 1). In contrast, Thbs1 is not essential for viability in mice. Several known thrombospondin-1 receptors were similarly loss-intolerant, although thrombospondin-1 is not the exclusive ligand for some of these receptors. The frequencies of missense mutations in THBS1 and the gene encoding its signaling receptor CD47 indicated conservation of some residues implicated in specific receptor binding. Deficits in missense mutations were also observed for other thrombospondin genes and for SPARC, SPOCK1, SPOCK2, TNR, and DSPP. The intolerance of THBS1 to loss of function in humans and elevated pLI values for THBS2, SPARC, SPOCK1, TNR, and CCN1 support important functions for these matricellular protein genes in humans, some of which may relate to functions in reproduction or responding to environmental stresses.