Abstract
BackgroundNuclear receptors have crucial roles in all metazoan animals as regulators of gene transcription. A wide range of studies have elucidated molecular and biological significance of nuclear receptors but there are still a large number of animals where the knowledge is very limited. In the present study we have identified an RXR type of nuclear receptor in the salmon louse (Lepeophtheirus salmonis) (i.e. LsRXR). RXR is one of the two partners of the Ecdysteroid receptor in arthropods, the receptor for the main molting hormone 20-hydroxyecdysone (E20) with a wide array of effects in arthropods.ResultsFive different LsRXR transcripts were identified by RACE showing large differences in domain structure. The largest isoforms contained complete DNA binding domain (DBD) and ligand binding domain (LBD), whereas some variants had incomplete or no DBD. LsRXR is transcribed in several tissues in the salmon louse including ovary, subcuticular tissue, intestine and glands. By using Q-PCR it is evident that the LsRXR mRNA levels vary throughout the L. salmonis life cycle. We also show that the truncated LsRXR transcript comprise about 50% in all examined samples. We used RNAi to knock-down the transcription in adult reproducing female lice. This resulted in close to zero viable offspring. We also assessed the LsRXR RNAi effects using a L. salmonis microarray and saw significant effects on transcription in the female lice. Transcription of the major yolk proteins was strongly reduced by knock-down of LsRXR. Genes involved in lipid metabolism and transport were also down regulated. Furthermore, different types of growth processes were up regulated and many cuticle proteins were present in this group.ConclusionsThe present study demonstrates the significance of LsRXR in adult female L. salmonis and discusses the functional aspects in relation to other arthropods. LsRXR has a unique structure that should be elucidated in the future.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-1277-y) contains supplementary material, which is available to authorized users.
Highlights
Nuclear receptors have crucial roles in all metazoan animals as regulators of gene transcription
Cloning and sequence analysis of LsRXR During our Expressed Sequence Tags (ESTs)-sequencing effort we identified one clone with significant Blast hits with Retinoid X Receptors (RXRs)/USP nuclear receptors from invertebrates
We examined the dataset from that study [18] and found that ten of the regulated transcripts are regulated after RNAi knock-down of LsRXR
Summary
Nuclear receptors have crucial roles in all metazoan animals as regulators of gene transcription. Nuclear receptors (NR) are ligand dependent transcription factors that regulate a range of different biological processes and they have been identified in all main groups of metazoan animals. Retinoid X Receptors (RXRs), commonly known as ultraspiracle (USP) in insects [1], belong to the nuclear receptor superfamily and targets multiple signaling pathways. RXRs can activate transcription either as homodimers or as obligate heterodimeric partners with other nuclear receptors. A critical element in regulating gene expression is the conformational change in RXRs induced by ligand binding. This change results in increased ligand affinity and transcription machinery assembly. Whereas the ligand for RXR is Eichner et al BMC Genomics (2015) 16:81 known to be 9-cis retinoic acid [2] the ligands of USPs remain unknown
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