Endocrine Abstracts

GnRH is secreted in pulses and its effects on pituitary gonadotropes depend on pulse frequency. This is crucial for physiological control and therapeutic manipulation of the system (in IVF and treatment of hormone-dependent cancers) but GnRH pulse frequency decoding mechanisms are unknown. The simplest form of frequency dependence is a linear relationship between integrated inputs and outputs but such ‘integrative tracking’ cannot explain the bell-shaped frequency-response relationships seen for many GnRH effects. GnRH acts via Gq/11 coupled GPCRs to activate effectors including ERKs, which mediate many transcriptional effects of GnRH but little is known about ERK signaling with pulsatile stimulation so we have explored this with automated fluorescence microscopy in HeLa cells transduced with adenovirus expressing ERK2-GFP (1). Five minute GnRH pulses caused rapid, transient and reproducible ERK2-GFP activation (nuclear translocation) at varied pulse concentrations (0.01–100 nM) and frequencies (0.25–2/h). Using an Egr1 luciferase reporter, increasing pulse frequency increased the transcriptional response (to ERK activation) but increasing pulse duration had a less pronounced effect (i.e. Egr-1 luciferase was approximately doubled by doubling frequency from 0.5 to 1 pulse/h, but a similar effect required a 10× increase in pulse duration from 1 to 10 min). Exploring activation of endogenous ERKs revealed that ppERK1/2 levels continue to rise for at least 3 min after a 1 min GnRH pulse, which may explain the unexpectedly high transcriptional response to very brief stimulation. Thus, varying pulse frequency implies that the ERK pathway is a simple integrative tracker of GnRH pulse frequency, but varying pulse duration reveals it is not. ERK activation is more sensitive to pulse frequency than it is to pulse duration (in this short time-frame) and this may have adaptive advantages for use of the ERK pathway in frequency-encoded signaling system.

Declaration of funding: The work was funded by the Medical Research Council (Grant 93477) and Wellcome Trust (Project and Equipment Grants 076557 and 078407).