Identification of alternative transcripts of rat CD9 expressed by tumorigenic neural cell lines and in normal tissues

Sonja Wolfahrt,Georg Sauer,Helmut Deissler,Sandra Herman,Claus-Jürgen Scholz

doi:10.1590/s1415-47572013000200019

Abstract

CD9 is the best-studied member of the tetraspanin family of transmembrane proteins. It is involved in various fundamental cellular processes and its altered expression is a characteristic of malignant cells of different origins. Despite numerous investigations confirming its fundamental role, the heterogeneity of CD9 or other tetraspanin proteins was considered only to be caused by posttranslational modification, rather than alternative splicing. Here we describe the first identification of CD9 transcript variants expressed by cell lines derived from fetal rat brain cells. Variant mRNA-B lacks a potential translation initiation codon in the alternative exon 1 and seems to be characteristic of the tumorigenic BT cell lines. In contrast, variant mRNA-C can be translated from a functional initiation codon located in its extended exon 2, and substantial amounts of this form detected in various tissues suggest a contribution to CD9 functions. From the alternative sequence of variant C, a different membrane topology (5 transmembrane domains) and a deviating spectrum of functions can be expected.

Highlights

CD9 is the prototype member of the tetraspanin protein family consisting of two extracellular loops and short intracellular ends that are linked through four transmembrane domains
The presence of CD9 transcripts was confirmed by room temperature (RT)-PCR, but specific PCR products were not observed when one of the primers was located in exon 1 of the CD9 gene, suggesting transcripts lacking exon 1 or containing an alternative exon 1 (Supplementary Material Figure S1)
Such aberrant variants have not been recognized previously, most likely due to dominant expression of variant A. mRNA-B seems to be a specific characteristic of the tumorigenic BT cell lines derived by in vitro carcinogenesis from immature rat brain cells (Laerum et al, 1977)

Summary

Introduction

CD9 is the prototype member of the tetraspanin protein family consisting of two extracellular loops and short intracellular ends that are linked through four transmembrane domains. In the fetal and adult rat brain, CD9 was predominantly detected in distinct regions It is a component of plasma membranes of all types of mature and premature glial cells with a pronounced expression by oligodendrocytes and, in the peripheral nervous system (PNS), Schwann cells (Tole and Patterson, 1993; Anton et al, 1995; Kaprielian et al, 1995; Deissler et al., 1996; Terada et al, 2002). Most malignant neural rat cell lines established from fetal brain cells after in vivo-exposure to ethylnitrosourea strongly expressed CD9 Two of these cell lines (BT8Ca and BT12Ca) were CD9-negative or showed minimal CD9specific immunoreactivity assessed by flow cytometry (Deissler et al, 1996). The deduced amino acid sequence, beginning with a functional translation initiation codon, suggested altered membrane topology and functions of this CD9 variant

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Conclusion