Essential Role of G Protein-gated Inwardly Rectifying Potassium Channels in Gonadotropin-induced Regulation of GnRH Neuronal Firing and Pulsatile Neurosecretion

Many of the G-protein-coupled receptors for hormones that bind to the cell surface can signal to the interior of the cell through several different classes of G protein. For example, although most of the actions of the prototype beta2-adrenergic receptor are mediated through Gs proteins and the cyclic-AMP-dependent protein kinase (PKA) system, beta-adrenergic receptors can also couple to Gi proteins. Here we investigate the mechanism that controls the specificity of this coupling. We show that in HEK293 cells, stimulation of mitogen-activated protein (MAP) kinase by the beta2-adrenergic receptor is mediated by the betagamma subunits of pertussis-toxin-sensitive G proteins through a pathway involving the non-receptor tyrosine kinase c-Src and the G protein Ras. Activation of this pathway by the beta2-adrenergic receptor requires that the receptor be phosphorylated by PKA because it is blocked by H-89, an inhibitor of PKA. Additionally, a mutant of the receptor, which lacks the sites normally phosphorylated by PKA, can activate adenylyl cyclase, the enzyme that generates cAMP, but not MAP kinase. Our results demonstrate that a mechanism previously shown to mediate uncoupling of the beta2-adrenergic receptor from Gs and thus heterologous desensitization (PKA-mediated receptor phosphorylation), also serves to 'switch' coupling of this receptor from Gs to Gi and initiate a new set of signalling events.

Neurotransmitters acting through G-protein-coupled receptors change the electrical excitability of neurons. Activation of receptors can affect the voltage dependence, the speed of gating, and the probability of opening of various ion channels, thus changing the computational state and outputs of a neuron. Each cell expresses many kinds of receptors, and uses several intracellular signaling pathways to modulate channel function in different ways. It has become possible to dissect these pathways by combining pharmacological and biophysical experiments. Recent patch-clamp work in sympathetic neurons will be summarized to illustrate the mechanisms underlying modulation and its significance.

The luteinizing hormone receptor (LHR) is a member of the subfamily of glycoprotein hormone receptors within the superfamily of G protein-coupled receptor (GPCR)/seven-transmembrane domain receptors. Over the past eight years, major advances have been made in determining the structure and function of the LHR and its gene. The hormone-binding domain has been localized to exons 1-7 in the extracellular (EC) domain/region of the receptor, which contains several leucine-rich repeats. High-affinity binding of LH and human chorionic gonadotrophin (hCG) causes secondary hormone or receptor contacts to be established with regions of the EC loop/transmembrane module that initiate signal transduction. Models of hormone-receptor interaction have been derived from the crystal structures of hCG and of the ribonuclease inhibitor, which also contains leucine-rich repeats. Such models provide a framework for the interpretation of mutational studies and for further experiments. The extracellular domain of the receptor has been overexpressed in vitro, which will facilitate crystallographic resolution of the structure of the receptor-binding site. The transmembrane domain/loop/cytoplasmic module transduces the signal for coupling to G proteins. Several constitutive, activating mutations that cause human disease have been found in helix VI and adjacent structures. These mutations have provided valuable information about mechanisms of signal transfer and G protein coupling. The structure of the LHR gene has been elucidated, and the regulation of its transcription is beginning to be understood. Valuable insights into receptor evolution have been derived from analysis of sequence homologies, the gene structure of glycoprotein hormone receptors and other members of the GPCR family, and the glycoprotein hormone receptor-like precursors identified in several invertebrate species.