Cleavage of the N-terminal signal sequence and C-terminal glycophosphatidylinositol (GPI)-anchor sequence results in a mature peptide of 27 residues in mouse and 32 residues in humans (Kay et al., 1991). CD24 is translated as a precursor protein of 76 or 80 residues in mice and humans, respectively. Furthermore, the expression of CD24 correlated with some but not all of its known ligands in a tissues-specific manner, suggesting that novel ligands have yet to be identified and that cell-specific ligand expression can influence CD24 function.ĬD24, also known as the Heat Stable Antigen or nectadrin, was first identified in mice nearly forty years ago (Springer et al., 1978). The expression of CD24 is similarly complex, with immune cells showing dynamic changes in mRNA levels during development, while immunologically privileged and developing tissues show a high, static expression level that decreases in mature tissues. Of note, we found no obvious criteria distinguishing CD24 genes from those annotated as CD24-like. The CD24 genomic structure is diverse between and within species, with varying numbers of exons, introns, and the presence of untranslated regions. CD24 arose prior to the reptilian–avian divergence and is conserved across many mammalian species, although we were unable to identify CD24 in marsupials or monotremes.
We further examined the mRNA expression of CD24 and its known ligands in Mus musculus in immune cells, immunologically privileged tissues, developing brain, and developing and regenerating liver. We analyzed the organization and evolution of the CD24 gene from 56 mammalian, avian and reptilian species.
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