CD44 isoform expression mediated by alternative splicing: tissue-specific regulation in mice

CD44 is a widely distributed cell surface glycoproteln which shows heterogeneity in molecular expression as a result of post-translatlonal modification as well as alternative splicing of CD44 mRNA. Functional studies have indicated that CD44 plays a role as an adhesion molecule and that different CD44-expresslng cells differ in their capacities for CD44-dependent llgand binding. These observations have raised the possibility that structural modifications of CD44, including those resulting from alternatively spliced mRNA isoforms, are involved in the functional heterogeneity of CD44. To assess the expression of CO44 isoforms in the mouse, we examined CD44 cDNA by reverse transcription polymerase chain reaction (RT-PCR). Southern blotting of PCR products with a CD44 cDNA probe or with internal ollgonucleotldes revealed the expression in mouse tumor cell lines and normal tissues of multiple CD44 mRNA products which are larger than that observed in the absence of variable exon expression. Interestingly, different mouse tissues, Including lymphold cells, showed unique patterns of alternative CD44 mRNA in Southern blotting analysis. The use of exon-speciflc primers allowed detection of multiple alternatively spliced mRNA species involving expression of at least seven variable exons. Cloning and sequencing of these PCR products revealed sequence Identity with recently identified genomlc CD44 sequences and confirmed that the PCR products correspond to mature mRNA expressing alternatively spliced CD44 exons. Taken together, these findings demonstrate that the mouse expresses multiple variably spliced CD44 isoforms and that expression Is regulated in a tissue- and cell-type specific manner.