Functional properties of leptin receptor isoforms containing the gln-->pro extracellular domain mutation of the fatty rat.

Mutations of the leptin receptor have been found to cause obesity in rodents. The fa mutation that is responsible for obesity in Zucker rats is a missense mutation (269 gln-->pro) in the extracellular domain of the leptin receptor. We have characterized the effects of this mutation on the two major isoforms of the leptin receptor, Ob-Rb and Ob-Ra, by studying cell-surface expression, leptin binding affinity, signaling capacity, and receptor-mediated internalization and degradation of leptin in transfected mammalian cell lines. Both Ob-Rb(269 gln-->pro) and Ob-Ra(269 gln-->pro) have decreased cell-surface expression and decreased leptin binding affinity. Ob-Rb(269 gln-->pro) was shown to have defective signaling to the JAK-STAT pathway and markedly diminished ability to activate transcription of the egr-1 promoter. Constitutive ligand-independent activation of Ob-Rb(269 gln-->pro) was observed for activation of egr-1-luc but only under conditions when JAK2 was coexpressed with Ob-Rb(269 gln-->pro), Finally, Ob-Ra(269 gln-->pro) has an increased ability to internalize leptin but is less efficient at degrading leptin, as compared with Ob-Ra. In conclusion, both Ob-Ra(269 gln-->pro) and Ob-Rb(269 gln-->pro) have multiple functional defects.