Pulsatile and sustained gonadotropin-releasing hormone (GnRH) receptor signaling: does the Ca2+/NFAT signaling pathway decode GnRH pulse frequency?

ABSTRACT

Gonadotropin-releasing hormone (GnRH) acts via 7 transmembrane region receptors on gonadotrophs to stimulate synthesis and secretion of the luteinizing hormone and follicle-stimulating hormone. It is secreted in pulses, and its effects depend on pulse frequency, but decoding mechanisms are unknown. Here we have used (nuclear factor of activated T-cells 2 (NFAT2)-emerald fluorescent protein) to monitor GnRH signaling. Increasing [Ca(2+)](i) causes calmodulin/calcineurin-dependent nuclear NFAT translocation, a response involving proteins (calmodulins and NFATs) that decode frequency in other systems. Using live cell imaging, pulsatile GnRH caused dose- and frequency-dependent increases in nuclear NFAT2-emerald fluorescent protein, and at low frequency, translocation simply tracked GnRH exposure (albeit with slower kinetics). At high frequency (30-min intervals), failure to return to basal conditions before repeat stimulation caused integrative tracking, illustrating how the relative dynamics of up- and downstream signals can increase efficiency of GnRH action. Mathematical modeling predicted desensitization of GnRH effects on [Ca(2+)](i) and that desensitization would increase with dose, frequency, and receptor number, but no such desensitization was seen in HeLa and/or LbetaT2 cells possibly because pulsatile GnRH did not reduce receptor expression (measured by immunofluorescence). GnRH also caused dose- and frequency-dependent activation of alphaGSU, luteinizing hormone beta, and follicle-stimulating hormone beta luciferase reporters, effects that were blocked by calcineurin inhibition. Pulsatile GnRH also activated an NFAT-responsive luciferase reporter, but this response was directly related to cumulative pulse duration. This together with the lack of desensitization of translocation responses suggests that NFAT may mediate GnRH action but is not a genuine decoder of GnRH pulse frequency.