Isolation and characterization of the polymeric Ig receptor gene from the cold adapted teleost Trematomus bernacchi

The polymeric immunoglobulin receptor (pIgR) plays a pivotal role in vertebrate immunity, as it mediates the transport of mucosal antibodies across epithelial layers into the external secretions. In recent years, much attention has been devoted to the function of pIgR in teleost fish, however, information on its gene structure remains still limited. Not even data are available on pIgR from teleost species living under extreme conditions, such as Notothenioidei (Perciform suborder), the dominant group of fish inhabiting the extremely cold environment of Antarctica. To enhance the current knowledge in this field, we characterized the structure of the pIgR gene of the Antarctic teleost Trematomus bernacchii, through a comparative analysis built on genomic and transcriptomic databases available for multiple species belonging to five perciform suborders. We identified unexpected modifications in the Antarctic pIgR genes, e.g., intron lengthening, transposable element content and additional regulatory elements. Furthermore, the full-length cDNA and deduced amino acid sequence were obtained. Multiple sequence alignment highlighted that several amino acid substitutions were exclusive to Antarctic pIgRs. Among notable changes, some led to a gain of N-glycosylation sites, strongly suggesting a putative role in the adaptive response to cold temperatures. Expression analysis through q-PCR and in situ hybridization showed that pIgR transcripts were constitutively expressed in the mucosal tissues and liver. Overall, our work unraveled specific features of the pIgR gene from cold adapted teleost species, providing additional information regarding the structure and organization of pIgR genes in teleost fish.