Linkage of alternative exon assembly in Drosophila TrpA1 transcripts

Abstract

Drosophila TrpA1 (transient receptor potential ankyrin 1) transcripts are alternatively spliced at two distinct sites each with a choice of mutually exclusive exons. The first site determines exon1 encoding the amino terminus to produce either nucleophile-, electrophile- and noxious temperature-gated TRPA1(A) or electrophile- and innocuous warmth-gated TRPA1(B). The second site selects for exon10, resulting in TrpA1 variants with either exon10a or exon10b encoding a domain between the N-terminal ankyrin repeats and the transmembrane segments. Although unbiased assembly would generate TRPA1 with four different domain combinations, the functional impact of these alternative domains remains to be thoroughly examined. Here, we find that there is a relatively strong linkage in mRNA splicing between the two sites in case of TrpA1(B), but not TrpA1(A), transcripts. Our semi-quantitative assay, consisting of reverse transcription polymerase chain reaction (RT-PCR) and Sanger sequencing, revealed that exon10b is little coupled with TrpA1(B) transcripts, suggesting that only three isoforms, TRPA1(A)-exon10a [denoted as TRPA1(A)], TRPA1(A)-exon10b [TRPA1(A)10b] and TRPA1(B)-exon10a [TRPA1(B)], are present at detectable levels using our method. Interestingly, heterologously expressed TRPA1(A)10b showed elevated sensitivity to low concentrations of N-methyl maleimide (NMM), a cysteine-modifying electrophile, compared with other isoforms. Equivalent isoforms in malaria-transmitting Anopheles gambiae displayed a similar pattern of isoform-dependent NMM dose dependences, suggesting that the chemosensory regulation by selective domain assembly is conserved in insect TRPA1s. Thus, alternative RNA splicing of exon10 is coordinated in conjunction with the first exons, regulating chemical sensitivity of insect TRPA1s.