RESUMO
Cultures of endogenous GnRH neurons and the GT1 GnRH neuronal cell line release GnRH in pulses (intrinsic pulsatile release) with an interpulse frequency similar to that seen in castrated animals. In both GT1 cells and transgenic rats, lowering cAMP levels by expression of a phosphodiesterase decreased the frequency of intrinsic GnRH pulsatility. We asked whether the cyclic nucleotide-gated cation (CNG) channels expressed in GT1 cells participated in cAMP modulation of intrinsic GnRH pulsatility. Because expression of the CNGA2 subunit is essential for formation of functional CNG channels, we developed an adenovirus (Ad) vector expressing a short interference RNA (siRNA) to the CNGA2 subunit (Ad-CNG-siRNA) or as an infection control, to the coding region of luciferase (Ad-Luc-siRNA). Infection with the Ad-CNG-siRNA of COS cells transfected with a CNGA2 expression vector significantly inhibited CNGA2 protein levels by 74% by Western blot. Infection of GT1-1 cells with Ad-CNG-siRNA resulted in a 68% decrease in the levels of CNGA2 mRNA, a 44% decrease in protein levels, and a clear decrease in immunostaining with an antibody to CNGA2. Infection of GT1-1 cells with Ad-CNG-siRNA decreased spontaneous Ca2+ oscillations compared with Ad-Luc-siRNA-infected or uninfected cells by 71%. Furthermore infection with Ad-CNG-siRNA resulted in a 2-fold increase in the interpulse interval in GnRH secretion (49.4+/-9.1 min) compared with uninfected cells (25.9+/-2.5 min) or Ad-Luc-siRNA (29.3+/-2.8 min)-infected cells. These data provide the first direct evidence that the CNG channel is a downstream signaling molecule in the regulation of the frequency of intrinsic GnRH pulsatility by cAMP.
Assuntos
Hormônio Liberador de Gonadotropina/metabolismo , Canais Iônicos/metabolismo , Adenoviridae/genética , Animais , Western Blotting , Células COS , Cálcio/metabolismo , Linhagem Celular , Chlorocebus aethiops , Colforsina/farmacologia , AMP Cíclico/metabolismo , Canais de Cátion Regulados por Nucleotídeos Cíclicos , Expressão Gênica/efeitos dos fármacos , Vetores Genéticos/genética , Humanos , Canais Iônicos/genética , Canais Iônicos/fisiologia , Modelos Biológicos , Neurônios/citologia , Neurônios/efeitos dos fármacos , Neurônios/metabolismo , RNA Interferente Pequeno/genética , Ratos , Reação em Cadeia da Polimerase Via Transcriptase Reversa , Transdução de Sinais/fisiologia , TransfecçãoRESUMO
We studied the signaling pathways coupling gonadotropin-releasing hormone (GnRH) secretion to elevations in cAMP levels in the GT1 GnRH-secreting neuronal cell line. We hypothesized that increased cAMP could be acting directly by means of cyclic nucleotide-gated (CNG) cation channels or indirectly by means of activation of cAMP-dependent protein kinase (PKA). We showed that GT1 cells express the three CNG subunits present in olfactory neurons (CNG2, -4.3, and -5) and exhibit functional cAMP-gated cation channels. Activation of PKA does not appear to be necessary for the stimulation of GnRH release by increased levels of cAMP. In fact, pharmacological inhibition of PKA activity caused an increase in the basal secretion of GnRH. Consistent with this observation activation PKA inhibited adenylyl cyclase activity, presumably by inhibiting adenylyl cyclase V expressed in the cells. Therefore, the stimulation of GnRH release by elevations in cAMP appears to be the result of depolarization of the neurons initiated by increased cation conductance by cAMP-gated cation channels. Activation of PKA may constitute a negative-feedback mechanisms for lowering cAMP levels. We hypothesize that these mechanisms could result in oscillations in cAMP levels, providing a biochemical basis for timing the pulsatile release of GnRH.
