Teleost leptins

Teleost leptins are a family of peptide hormones found in fish (teleostei) that are orthologs of the mammalian hormone leptin. The teleost and mammalian leptins appear to have similar functions, namely, regulation of energy intake and expenditure.

The leptin (LEP) hormone was long thought to be specific to mammals, but in recent years the gene (lep) has been found in amphibia such as the tiger salamander (Ambystoma tigrinum), and the African clawed frog (Xenopus laevi). The discovery of lep in puffer fish (Takifugu rubripes) demonstrates the ancient ancestry of this hormone.

There are two closely related lep paralogues in Atlantic salmon (Salmo salar). A single lep gene has been documented for green-spotted pufferfish (Tetraodon nigroviridis), rainbow trout (Oncorhynchus mykiss), Arctic charr (Salvelinus alpinus), silver carp (Hypophthalmichthys molitrix), and grass carp (Ctenopharyngodon idellus). In other species there are reports of two closely related lep paralogues, including common carp (Cyprinus carpio) and Atlantic salmon. More distantly related lep genes have been found in medaka (Oryzias latipes) and zebrafish (Danio rerio). At least 2 leptin genes (lepa and lepb) exist in the crown-clade (Fig. 1). Early findings have shown that lepa and lepb share low interspecies aa identity, and are argued to have arisen through whole genome duplication, which occurred early in the teleost lineage. The duplicity of genes has been described for Atlantic salmon, Japanese medaka, common carp and zebrafish. Both lep paralogues cluster with lepa, and therefore suggest that at least one or more form (lepb) may exist in this species, since it is tetraploid. However, previous attempts using genomic synteny have only found the putative genomic duplicates in medaka and zebrafish paralogue. Currently it remains unclear, whether lepb exists in other teleosts due to the degenerative nature of this paralogue.

The large differences among endothermic (warm-blooded) mammalian and ectothermic (cold-blooded) teleost leptins raised the question of whether the energy homeostatic functions of the teleost leptins are conserved. Initial phylogenetic analysis has revealed that amino acid conservation with other vertebrate Lep orthologues is low, with only 13.2% sequence identity between torafugu and human LEP. Subsequent investigations have confirmed the low amino acid identity of teleost leps compared to mammalian LEP.

The three-dimensional homology modeling predicts strong conservation of the tertiary structure between Atlantic salmon and other teleost Leps compared to their mammalian orthologues (Fig. 2).

Both lepa1 and lepa2 have two characteristic cysteine residues which predict the formation of a disulfide bond in Lep, which is a pre-equisite for this 3D configuration and bioactivity of human LEP. The models suggest that the bonding of lepa2 might be different from lepa1. There are several differences between the 3D structures of lepa1 and lepa2; e.g. α-helix 5 is considerable shorter in lepa1 than lepa2. Furthermore, α-helix 1 for lepa2 appears to be split by a short-disordered region, and may therefore have a poorer affinity. However, considering that it is a predicted model based upon the structure mask of human LEP, the significance of these putative conformational adjustments remains to be tested.

The importance of the conserved tertiary structure of Lep is most likely explained by requirements for specific LepR-binding affinity and is constrained by the structure of the receptor-binding pocket. This might also explain some of the results from studies on teleost using heterologous mammalian Lep. E.g. treatment with the mammalian hormone caused an anorexic effect in goldfish (Carassius auratus) and green sunfish (Lepomis cyanellus), but not in Coho salmon (Oncorhynchus kisutch), channel catfish (Ictalurus punctatus) and green sunfish. These contradicting results have been explained by the relatively large differences in amino acid sequences observed between mammals and fish.

Rønnestad and colleagues recently detected five isoforms of the leptin receptor (lepr) that have differences in 3'-end of the mRNA sequence. Of these, only the longest form conserved all functionally important domains (such as three fibronectin type III domains, the Ig C2-like domain, a pair of WSXWS motifs, two JAK2-binding motif boxes, and a STAT-binding domain), while the other four forms have only the intra-cellular region. The long form of mammalian LepR has a function for full signal transduction through the JAK/STAT pathways, whereas the shorter forms exhibit partial or no signaling capabilities. The biological importance of long form LepR via the JAK/STAT pathway in maintaining body weight and energy homeostasis has been demonstrated. Previous studies in teleosts have only identified a single lepr. Rønnestad et al., is the first to report that plural LepR transcripts in any ectotherm species. When looking at the available motif for lepr, the model suggests that it would bind easily to lepa1 and not lepa2 (Fig. 2). Furthermore, the relatively ubiquitous expression of lepr in salmon tissues supports diverse roles of lep in teleosts.