Involvement of Protein Kinase D1 in Signal Transduction from the Protein Kinase C Pathway to the Tyrosine Kinase Pathway in Response to Gonadotropin-releasing Hormone

Receptor-proximal effectors mediating GnRH actions in the goldfish pituitary: Involvement of G protein subunits and GRKs

In goldfish (Carassius auratus), two endogenous isoforms of gonadotropin-releasing hormone (GnRH) stimulate luteinizing hormone (LH) and growth hormone (GH) secretion. These isoforms, GnRH2 and GnRH3, act on a shared population of cell-surface GnRH receptors (GnRHRs) expressed on both gonadotrophs and somatotrophs, and can signal through unique, yet partially overlapping, suites of intracellular effectors, in a phenomenon known as functional selectivity or biased signalling. In this study, G-protein alpha (Gα) subunits were targeted with two inhibitors, YM-254890 and BIM-46187, to ascertain the contribution of specific G-protein subunits in GnRH signalling. Results with the Gα_q/11-specific inhibitor YM-254890 on primary cultures of goldfish pituitary cells revealed the use of these subunits in GnRH control of both LH and GH release, as well as GnRH-induced elevations in phospho-ERK levels. Results with the pan-Gα inhibitor BIM-46187 matched those using YM-254890 in LH release but GH responses differed, indicating additional, non-Gα_q/11 subunits may be involved in somatotrophs. BIM-46187 also elevated unstimulated LH and GH release suggesting that Gα subunits regulate basal hormone secretion. Furthermore, G-protein-coupled receptor kinase (GRK2/3) inhibition reduced LH responses to GnRH2 and GnRH3, and selectively enhanced GnRH2-stimulated GH release, indicating differential use of GRK2/3 in GnRH actions on gonadotrophs and somatotrophs. These findings in a primary untransformed system provide the first direct evidence to establish Gα_q/11 as an obligate driver of GnRH signalling in goldfish pituitary cells, and additionally describe the differential agonist- and cell type-selective involvement of GRK2/3 in this system.

Roles of Pyk2 in signal transduction after gonadotropin-releasing hormone receptor stimulation

The receptor for gonadotropin-releasing hormone (GnRH) is highly expressed in hypothalamic neurons. It has been reported that GnRH treatment of cultured GnRH neurons (GT1-7 cells) activated proline-rich tyrosine kinase 2 (Pyk2), and Pyk2 was involved in the activation of extracellular signal-regulated protein kinase 1 (ERK1) and ERK2 (ERK1/2). In the present study, we first examined the possibility that GnRH treatment might activate epidermal growth factor receptor (EGFR). We found that activation of EGFR after GnRH treatment for 5 min was much less than after EGF or heparin-binding EGF treatment. Next, we examined whether or not Pyk2 bound to growth factor receptor-binding protein 2 (Grb2). We overexpressed FLAG-fused Pyk2 in GT1-7 cells, and immunoprecipitated Pyk2 using an anti-FLAG antibody. The binding of Pyk2 to Grb2 was detected only after GnRH treatment. In contrast, a site-directed mutant of Pyk2 wherein tyrosine 881 was mutated to phenylalanine did not bind to Grb2. Studies with small interfering RNA and inhibitors indicated that the activation of Grb2/Ras/Raf/MEK was a major pathway to ERK1/2 activation after the short-term treatment of GT1-7 cells with GnRH.

Rational design of a heterotrimeric G protein α subunit with artificial inhibitor sensitivity

Transmembrane signals initiated by a range of extracellular stimuli converge on members of the Gq family of heterotrimeric G proteins, which relay these signals in target cells. Gq family G proteins comprise Gq, G11, G14, and G16, which upon activation mediate their cellular effects via inositol lipid–dependent and –independent signaling to control fundamental processes in mammalian physiology. To date, highly specific inhibition of Gq/11/14 signaling can be achieved only with FR900359 (FR) and YM-254890 (YM), two naturally occurring cyclic depsipeptides. To further development of FR or YM mimics for other Gα subunits, we here set out to rationally design Gα16 proteins with artificial FR/YM sensitivity by introducing an engineered depsipeptide-binding site. Thereby we permit control of G16 function through ligands that are inactive on the WT protein. Using CRISPR/Cas9-generated Gαq/Gα11-null cells and loss- and gain-of-function mutagenesis along with label-free whole-cell biosensing, we determined the molecular coordinates for FR/YM inhibition of Gq and transplanted these to FR/YM-insensitive G16. Intriguingly, despite having close structural similarity, FR and YM yielded biologically distinct activities: it was more difficult to perturb Gq inhibition by FR and easier to install FR inhibition onto G16 than perturb or install inhibition with YM. A unique hydrophobic network utilized by FR accounted for these unexpected discrepancies. Our results suggest that non-Gq/11/14 proteins should be amenable to inhibition by FR scaffold–based inhibitors, provided that these inhibitors mimic the interaction of FR with Gα proteins harboring engineered FR-binding sites.

Up-regulation of DUSP5 and DUSP6 by gonadotropin-releasing hormone in cultured hypothalamic neurons, GT1-7 cells

Gonadotropin-releasing hormone (GnRH) is secreted from hypothalamic neurons (GnRH neurons) and stimulates anterior pituitary gonadotrophs to synthesize and secrete gonadotropins. In addition to gonadotrophs, GnRH neurons also express GnRH receptors, and the autocrine action of GnRH is reportedly involved in the regulation of functions of GnRH neurons. There is accumulating evidence that extracellular signal-regulated kinase (ERK), one of mitogen-activated protein kinases (MAPKs), is activated by GnRH and involved in various effects of GnRH in GnRH neurons. In the present study, we performed microarray analysis to examine the types of genes whose expression was regulated by GnRH in immortalized mouse GnRH neurons (GT1-7 cells). We found that 257 genes among 55,681 genes examined were up-regulated after 30-min treatment of GT1-7 cells with GnRH. These up-regulated genes included four dual-specificity MAPK phosphatases (DUSPs), DUSP1, DUSP2, DUSP5, and DUSP6. Reverse transcription-polymerase chain reaction analysis confirmed that the mRNA levels of DUSP5 and DUSP6 were robustly increased within 30 min. U0126, an inhibitor of ERK activation, completely inhibited the increases in the mRNA levels of DUSP5 and DUSP6. Immunoblotting analysis revealed that ERK activation peaked at 5 min and declined steeply at 60 min, whereas DUSP5 and DUSP6 proteins were increased from 60 min. It was notable that down-regulation of DUSP6 augmented GnRH-induced ERK activation approximately 1.7-fold at 60 min. These results suggested that the up-regulation of DUSP6 regulates the duration of ERK activation at least in part.

Nasal delivery of nerve growth factor rescue hypogonadism by up-regulating GnRH and testosterone in aging male mice

BACKGROUND

Nerve growth factor (NGF) plays essential roles in regulating the development and maintenance of central sympathetic and sensory neurons. However, the effects of NGF on hypogonadism remain unexplored.

METHODS

To assess the effects of NGF on hypogonadism, we established a convenient and noninvasive way to deliver NGF to the hypothalamus by spraying liposome-encapsulated NGF into the nasal cavity. The ten-month-old aging male senescence accelerate mouse P8 (SAMP8) mice with age-related hypogonadotrophic hypogonadism were used to study the role of NGF in hypogonadism. The age-matched accelerated senescence-resistant mouse R1 (SAMR1) served as a control. The ten-month-old SAMP8 mice were treated with NGF twice per week for 12 weeks. Sexual hormones, sexual behaviors, and fertility were analyzed after NGF treatment. And the mechanisms of NGF in sex hormones sexual function were also studied.

FINDINGS

NGF could enhance the sexual function, improve the quality of the sperm, and restore the fertility of aging male SAMP8 mice with age-related hypogonadism by activating gonadotropin-releasing hormone (GnRH) neuron and regulating secretion of GnRH. And NGF regulated the GnRH release through the PKC/p-ERK1/2/p-CREB signal pathway.

INTERPRETATION

These results suggest that NGF treatment could alleviate various age-related hypogonadism symptoms in male SAMP8 and may be usefulness for age-related hypogonadotrophic hypogonadism and its related subfertility. FUND: National Natural Science Foundation of China, Natural Science Foundation of Guangdong Province, the Science and Technology Plan Project of Guangzhou, Wenzhou Science & Technology Bureau, Guangdong Province Pearl River Scholar Fund, Guangdong province science and technology innovation leading Scholar Fund.

Lordosis facilitated by GPER-1 receptor activation involves GnRH-1, progestin and estrogen receptors in estrogen-primed rats

The present study assessed the participation of membrane G-protein coupled estrogen receptor 1 (GPER-1) and gonadotropin releasing hormone 1 (GnRH-1) receptor in the display of lordosis induced by intracerebroventricular (icv) administration of G1, a GPER-1 agonist, and by unesterified 17β-estradiol (free E₂). In addition, we assessed the participation of both estrogen and progestin receptors in the lordosis behavior induced by G1 in ovariectomized (OVX), E₂-benzoate (EB)-primed rats. In Experiment 1, icv injection of G1 induced lordosis behavior at 120 and 240min. In Experiment 2, icv injection of the GPER-1 antagonist G15 significantly reduced lordosis behavior induced by either G1 or free E₂. In addition, Antide, a GnRH-1 receptor antagonist, significantly depressed G1 facilitation of lordosis behavior in OVX, EB-primed rats. Similarly, icv injection of Antide blocked the stimulatory effect of E₂ on lordosis behavior. In Experiment 3, systemic injection of either tamoxifen or RU486 significantly reduced lordosis behavior induced by icv administration of G1 in OVX, EB-primed rats. The results suggest that GnRH release activates both estrogen and progestin receptors and that this activation is important in the chain of events leading to the display of lordosis behavior in response to activation of GPER-1 in estrogen-primed rats.