Leptin ( ob gene) of the South African clawed frog Xenopus laevis

doi:10.1073/pnas.0507519103

Leptin (ob gene) of the South African clawed frog Xenopus laevis

Department of Molecular, Cellular, and Developmental Biology, University of Michigan, 830 North University Avenue, Ann Arbor, MI 48109-1048

Edited by Jeffrey M. Friedman, The Rockefeller University, New York, NY, and approved May 12, 2006 (received for review August 29, 2005)

Abstract

Leptin, the protein product of the obese (ob) gene, is a type-I cytokine hormone secreted by fat that is integral to food intake regulation and influences almost every physiological system in juvenile and adult mammals. Since the identification of leptin in the mouse in 1994, biologists have searched for orthologous genes in other species with limited success. In this article, we report the identification and functional characterization of leptin and leptin receptor (LR) in Xenopus. Despite low amino acid sequence similarity to mammalian leptins (≈35%) the frog protein has a nearly identical predicted tertiary structure and can activate the frog and mouse LRs in vitro. We showed that recombinant frog leptin (rxLeptin) is a potent anorexigen in frogs, as it is in mammals, but this response does not develop until midprometamorphosis. However, during early prometamorphosis, exogenous rxLeptin induced growth and development of the hind limb, where LR mRNA is expressed. The rxLeptin also stimulated cell proliferation in cultured hind limbs from early prometamorphic tadpoles, as measured by [³H]thymidine uptake. These findings are evidence that leptin can influence limb growth and differentiation during early development. Furthermore, the isolation and characterization of leptin and its receptor in a nonamniote provides an essential foundation for elucidating the structural and functional evolution of this important hormone.

Footnotes

^†To whom correspondence should be addressed. E-mail: rdenver{at}umich.edu
↵*Present address: Department of Biology, Vassar College, 124 Raymond Avenue, Poughkeepsie, NY 12604.
Author contributions: E.J.C. and R.J.D. designed research, performed research, analyzed data, and wrote the paper.
Conflict of interest statement: No conflicts declared.
This paper was submitted directly (Track II) to the PNAS office.
Data deposition: The sequences reported in this paper have been deposited in the GenBank database [accession nos. AY884210 (X. laevis obese mRNA), DQ149644 (X. laevis leptin receptor partial cDNA), and DQ401069 (X. tropicalis leptin receptor mRNA)].
Abbreviations:

Abbreviations:

LRs,

leptin receptors;

STAT-3,

signal transducer and activator of transcription 3;

LRb,

the long form of the LR;

rxLeptin,

recombinant frog leptin;

RTqPCR,

quantitative RT-PCR;

NF,

Nieuwkoop–Faber.