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 f alpha 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 Rb-269 (gln --> pro) and Ob-Ra-269 (gln --> pro) have decreased cell-surface expression and decreased leptin binding affinity. Ob-Rb269 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 ObRb(269) (gln --> pro) was observed for activation of egr-1-luc but only under conditions when JAK2 was coexpressed with Ob-Rb269 (gln --> pro). Fi nally, Ob-Ra-269 (gln --> pro) increased ability to internalize leptin but is less efficient at degrading leptin, as compared with Ob-Ra. In conclusion, both Ob-Ra269 (gln --> pro) and Ob-Rb-269 (gln --> pro) have multiple functional defects.