Relationship between novel isoforms, functionally important domains, and subcellular distribution of CD164/endolyn

Functional analyses have indicated that the human CD164 sialomucin may play a key role in hematopoiesis by facilitating the adhesion of human CD34(+) cells to the stroma and by negatively regulating CD34(+)CD38(lo/-) cell pro liferation. We have identified three novel human CD164 variants derived by alternative splicing of bona fide exons from a single genomic transcription unit. The predominant CD164(E1-6) isoform, encoded by six exons, is a type I transmembrane protein containing two extracellular mucin domains (I and II) interrupted by a cysteine-rich non-mucin domain, The 103B2/9E10 and 105 A5 epitopes, which specify ligand binding characteristics, are located on t he exon 1-encoded mucin domain I. Three human CD164(E1-6) mRNA species, exh ibiting differential polyadenylation site usage, are differentially express ed in hematopoietic and non-hematopoietic tissues. This study provides addi tional evidence that human CD164(E1-6) represents the ortholog of murine MG C-24v and rat endolyn. Comparative analysis of murine MGC-24v/CD164(E1-6) w ith human CD164(E1-6) revealed two potential splice variants and a similar genomic structure. Whereas the human CD164 gene is located on chromosome 6q 21, the mouse gene occurs in a syntenic region on chromosome 10B1-B2. By co nfocal microscopy, human CD164 in CD34(+)CD38(+) hematopoietic progenitor ( KG1B) and epithelial cell lines appears to be localized primarily in endoso mes and lysosomes, with low concentrations at the cell surface. However, in a minority of KG1B cells, CD164 is more prominently expressed at the plasm a membrane and in the recycling endosomes, suggesting that its distribution is regulated in cells of hematopoietic origin.