ALTERNATIVELY SPLICED, GERMLINE J-ALPHA-11-2C-ALPHA MESSENGER-RNAS ARE THE PREDOMINANT T-CELL RECEPTOR-ALPHA TRANSCRIPTS IN MOUSE KIDNEY

We recently reported the expression of a truncated T cell receptor (TC R) alpha mRNA in kidney and brain of normal mice. In the kidney, the t runcated TCR alpha transcript was expressed by bone marrow-dependent, non-T large interstitial cells located predominantly in the medulla. H ere, we report the molecular characterization of the truncated TCR alp ha transcript from kidney. Using a modified anchored-PCR (A-PCR) techn ique and directional cloning, 37 cDNA clones extending 5' of the C alp ha region were generated, cDNA sequencing showed that 29 of the clones (78%) originated in the J alpha 11-2 region. Of these clones, 17 star ted upstream or in the J alpha 11-2 exon and contained the entire J al pha 11-2 sequence correctly spliced to the first C alpha exon. Analysi s of the sequence revealed the presence of multiple stop codons in all three reading frames. The other 12 clones originated further upstream of the J alpha 11-2 exon and did not include the J alpha 11-2 exon, b ut rather arose from the joining of a cryptic splice donor signal to t he usual TCR alpha C splice acceptor. This alternatively spliced trans cript contained an open reading frame extending from the upstream J al pha 11 region to 82 nucleotides downstream of the beginning of the TCR C or region, and potentially encoded a 36 amino acid polypeptide. The remaining eight clones all contained the J alpha 11 TA61 region corre ctly spliced to C alpha With two of these extending upstream of the J alpha TA61 exon. The predominance of J alpha 11-2-C alpha containing c lones was confirmed by RNase protection assay using total RNA from kid ney and spleen of scid mice. The 3' region of the transcript contained a fully conserved, correctly spliced TCR alpha C region which was pol yadenylated at the 3' end. The truncated TCR a mRNA could be detected in preparations of cytoplasmic RNA, indicating that this transcript fo llows a normal RNA processing pathway. Our results demonstrate that th e truncated TCR alpha mRNA expressed in normal mouse kidney is a germl ine J-C transcript resulting from transcription initiated predominantl y upstream of the J alpha 11-2 region. This germline transcript in the kidney is undergoing alternative splicing leading to the appearance o f an open reading frame coding for a short polypeptide. These results suggest that the product of this transcript may be functionally releva nt.