Ancient Origins and Evolutionary Conservation of Intracellular and Neural Signaling Pathways Engaged by the Leptin Receptor

Abstract

In mammals, leptin acts on leptin receptor (LepR) -expressing neurons in the brain to suppress food intake and stimulate whole-body metabolism. A similar action of leptin on food intake has been reported in the frog Xenopus laevis and in several bony fishes. However, the intracellular signaling and neural pathways by which leptin regulates energy balance have not been investigated outside of mammals. Using reporter assays and site-directed mutagenesis we show that the frog LepR signals via signal transducer and activator of transcription (STAT) 3 and STAT5 through evolutionarily conserved tyrosine residues in the LepR cytoplasmic domain. In situ hybridization histochemistry for LepR mRNA in brain and pituitary showed strong expression in the magno- and parvocellular divisions of the anterior preoptic area (homologous to the mammalian paraventricular nucleus), the suprachiasmatic nucleus, ventral hypothalamus, and pars intermedia and pars distalis of the anterior pituitary. Leptin injection increased phosphorylated STAT3 immunoreactivity in LepR mRNA-positive cells, and induced socs3 and pomc mRNAs. Microarray analysis of preoptic area/hypothalamus/pituitary 2 hours after leptin injection identified leptin-regulated genes that included c-fos, a known leptin-activated gene; pituitary follicle-stimulating hormone subunit β, suggesting an important role for leptin in the reproductive axis of frogs; and B-cell translocation factor 2, which has important functions in neurogenesis. Our findings support that the intracellular signaling pathways and neural substrates that mediate leptin actions on energy balance were present in the common ancestor of modern amphibians and amniotes and have been conserved over 350 million years of evolutionary time.

Acute anorexigenic action of leptin in rainbow trout is mediated by the hypothalamic Pi3k pathway

“…The present data also agree with studies using peripheral Lep treatment of rainbow trout and Atlantic salmon, indicating that the anorexigenic action of Lep in salmonids is mediated through the Pomc pathway (Murashita et al 2008(Murashita et al , 2011. These data are also in agreement with data on frog Xenopus laevis, where ICV Lep administration increases pomc transcription (Cui et al 2014). Also in mammals, LEP primarily acts on the POMC/CART neurons in the ARC (Varela & Horvath 2012), while in fish, the NLT of the hypothalamus is considered to be homologous to the mammalian ARC (Biran et al 2012).…”

“…This is further supported by the increased phosphor-Akt-ir cells in this hypothalamic area following a central Lep injection, as the change in intracellular signaling must be induced through the functional Lep receptor. Thus, the LepR L distribution in rainbow trout appears to be similar to that in rat, where the full-length LEPRb is predominantly found within the arcuate, dorsomedial, and ventromedial nuclei (Elmquist et al 1998), as well as in Xenopus, where the LepR is found within the ventral hypothalamus (Cui et al 2014).…”

“…Similar response variability has been noted in other studies on early vertebrates. The ICV Lep administration transiently increased npy mRNA levels in frog brain (Cui et al 2014). In the goldfish Carassius auratus that had been acclimated to a feeding schedule, ICV Lep administration did not affect npy expression in fed fish at 2 h post-feeding, but suppressed fasting-induced npy expression in unfed fish (Volkoff et al 2003).…”

“…The current understanding of Lep intracellular signaling pathways and effects in nonmammalian vertebrates is limited. The classic LepR-Stat3 signal pathway is evolutionarily conserved in teleosts and frogs (Gong & Björnsson 2014, Cui et al 2014. The central action of Lep on glucosensing system can be related to the Jak-Stat and Pi3k-mTor pathway in rainbow trout (Aguilar et al 2010).…”

Molecular cloning, characterization and evolutionary analysis of leptin gene in Chinese giant salamander, Andrias davidianus

“…Phylogenetic reconstruction revealed that the topologies of vertebrate leptins are consistent with the general understanding of the evolution of vertebrates, corresponding to clades response [64], and to be associated with growth rate [65][66][67][68][69] in different vertebrates. The JAK/STAT pathway is found to be conserved between frogs and mammals [70], suggesting conservation of the leptin signaling mechanism across vertebrate groups. More diverse physiological roles have been characterized in teleosts than those found in mammals [39].…”