The Discovery of Leptin

Overview

Leptin is a 167-amino-acid adipokine secreted primarily by adipose tissue. It was identified in 1994 when Jeffrey Friedman and colleagues at Rockefeller University positionally cloned the ob gene in mice, solving a puzzle that had stood since the ob/ob obese mouse strain was described by Ingalls, Dickie, and Snell at the Jackson Laboratory in 1950.

The discovery had an outsized effect on obesity research. For the first time, a specific circulating protein was shown to link body fat stores with central nervous system regulation of appetite and energy expenditure. Mice lacking functional leptin (ob/ob) are obese, hyperphagic, diabetic, and infertile, and all of these phenotypes could be corrected by recombinant leptin administration. A related strain, db/db, proved to have a defect in the leptin receptor, cloned shortly after by Louis Tartaglia and colleagues at Millennium Pharmaceuticals.

Leptin signals to the hypothalamus through its receptor (LEPR, LepR-B), regulating neuropeptide Y, agouti-related peptide, pro-opiomelanocortin, and cocaine- and amphetamine-regulated transcript pathways. It has downstream effects on feeding behavior, metabolism, reproductive function, and immune function.

Key People

• Jeffrey M. Friedman: Rockefeller University geneticist who led the positional cloning of the ob gene.
• Douglas Coleman: Jackson Laboratory scientist whose parabiosis experiments in the 1970s predicted a circulating satiety factor.
• Louis Tartaglia: Led the team that cloned the leptin receptor.
• Stephen O'Rahilly and Sadaf Farooqi: British clinicians who identified human congenital leptin deficiency and treated patients with recombinant leptin.

Timeline

• 1950: The ob/ob mouse is described at Jackson Laboratory.
• 1969–1973: Coleman's parabiosis studies predict a circulating satiety factor.
• 1994: Friedman's group clones the ob gene and identifies leptin.
• 1995: Leptin receptor is cloned; db/db mouse defect is mapped to it.
• 1997: First case reports of human congenital leptin deficiency.
• 1999: Clinical trials of recombinant leptin in common obesity show limited efficacy.
• 2014: Metreleptin (recombinant leptin analog) is approved for generalized lipodystrophy.

Background

Coleman's parabiosis experiments were foundational. By surgically joining the circulations of ob/ob and db/db mice, he showed that the ob/ob mouse lacked a circulating factor that the db/db mouse over-produced but could not respond to. This predicted, decades before the molecules were cloned, that ob/ob and db/db represented hormone and receptor defects respectively.

The discovery of leptin transformed expectations about obesity. Initial hopes that leptin administration might reverse common obesity faded when it became clear that most people with obesity already have high circulating leptin levels and are resistant to its effects. The concept of "leptin resistance," analogous to insulin resistance, now frames much of clinical obesity research.

Modern Relevance

Leptin therapy remains clinically useful in rare conditions, including congenital leptin deficiency and generalized lipodystrophy, where leptin replacement produces dramatic improvements in metabolic and hormonal abnormalities. In more common obesity, leptin's main contribution is conceptual: it firmly established the idea that adipose tissue is an endocrine organ that communicates with the brain.

Research continues on leptin's role in reproductive physiology, immune regulation, bone metabolism, and the interplay with ghrelin, insulin, and GLP-1. Combination strategies — for example, leptin plus amylin — have been explored in clinical trials with mixed results. For more context on adipose-derived signaling, see peptides-in-metabolic-disease.

Related Compounds

• Leptin
• Metreleptin
• Adiponectin
• Neuropeptide Y
• Alpha-MSH