Kevin Catt, MD, PhD, Chief
The Endocrinology and Reproduction Research Branch (ERRB) investigates the molecular and cellular mechanisms of reproductive hormone action, the structure-function properties and signaling pathways of peptide hormone receptors, and the roles of phosphoinositides, phosphorylation, and sulfonation in signal transduction and metabolic regulation.
Tamas Balla’s Section on Molecular Signal Transduction has identified type-III PI4Kalpha as the enzyme that replenishes the agonist-sensitive plasma membrane pool of phosphoinositides. He and his colleagues used a combination of techniques to inhibit or downregulate individual PI4K isoforms and to monitor PI(4)P and PI(4,5)P2 production in cell populations and single cells. In a collaborative effort, they are currently developing several PI3K inhibitors with the ability to discriminate between PI4K isoforms, and they have identified an inhibitor that specifically inhibits PI4KIIIbeta. In separate studies, they demonstrated that the ability of selected pleckstrin-homology domains to interact with membranes depends on both the domains’ protein-lipid and protein-protein interactions.
Kevin Catt’s Section on Hormonal Regulation investigates the molecular mechanisms of activation, signaling, and function of G protein–coupled receptors for angiotensin II (AT1R) and gonadotropin-releasing hormone (GnRHR). Most recently, the Section identified the mechanisms of AT1R- and GnRHR-mediated transactivation of EGFR and ERK phosphorylation in hepatic and neuronal cells and the specific proteins associated with agonist-activated AT1R. Agonist-induced and caveolin-dependent molecular complex formation between AT1R and EGFR caused phosphorylation of AT1R and of EGFR. In GnRH neurons, individual serotonin receptor subtypes mediated distinct actions via specific G proteins to control signaling pathways, neuronal firing, and pulsatile GnRH secretion. In addition, the Section found that Gi-regulated potassium (GIRK) channels mediate the inhibitory regulation of GnRH neuronal activity and pulsatile neurosecretion.
Maria Dufau, who heads the Section on Molecular Endocrinology, and members of her group found that histone modification is a dominant factor in the silencing and activation of luteinizing hormone receptor (LHR) gene expression and that DNA methylation and demethylation are operative under the architecture defined by histone modifications. The results point to a mechanism distinct from that derived by epigenetic studies on tumor suppressor genes. The Section also identified two novel gonadotropin-regulated genes: GRTH/Ddx25 is expressed exclusively in testicular cell compartments and is essential for the completion of gametogenesis while GR-LACS with acyl-CoA synthetase activity is expressed in the adrenal, Leydig cells, ovarian follicles undergoing atresia, and brain. Studies on the human prolactin receptor revealed that hormone-independent homodimerization and heterodimerization between the long activating receptor and two short inhibitory forms at the cell membrane account for the ability of the short forms to inhibit the hormone-activated function of the long receptor.
Kuo-Ping Huang’s Section on Metabolic Regulation studies synaptic plasticity in neurogranin (Ng) knockout mice, which exhibit deficits in learning and memory (L&M) in cognitive tasks and long-term potentiation (LTP). Hippocampal Ng concentrations correlate highly with L&M performance, and enriched environments increase hippocampal Ng content, behavioral test performance, and expression of LTP. Ng promotes rises in neuronal free Ca2+ that enhance synaptic responses and favor induction of LTP over long-term depression. Ng may regulate neuronal signaling and enhance synaptic plasticity because, at higher Ng concentration, the formation of Ng/CaM complexes effectively raises [Ca2+]i at any given level of Ca2+ influx. The consequent signal amplification enhances synaptic plasticity as well as L&M. Furthermore, Ng is a potent reductant and may function as an antioxidant to protect neurons from damage during oxidative stress.
Stanko Stojilkovic’s Section on Cellular Signaling investigates calcium signaling and cellular regulation. Work on ATP-gated calcium-conducting purinergic receptor channels focused on molecular determinants of the agonist binding and gating domains. The Section also studied adenylyl cyclase subtypes critical for the reciprocal modulation of cyclic nucleotides and voltage-gated calcium influx in spontaneously firing pituitary lactotrophs. Experiments with endothelins revealed a dual mode of control of the secretory pathway in lactotrophs, through inhibition of voltage-gated calcium influx and downstream of voltage-gated calcium influx. The Section also found that a Gz protein–mediated pathway effectively blocks hormone secretion distal to calcium entry.
Charles Strott’s Section on Steroid Regulation investigates molecular mechanisms and biologic implications of modifying substances by sulfonation. He and his coworkers clone sulfotransferase genes in order to examine gene products, tissue expression, and transcriptional regulation. They are currently focusing on human SULT2B1 isoforms that sulfonate steroids/sterols, studying the enzymes’ roles in normal physiology and disease processes. SULT21B1a produces pregnenolone sulfate, an important neurosteroid. SULT2B1b, on the one hand, creates an active molecule, cholesterol sulfate, that plays an important role in epidermal development (barrier formation), platelet aggregation (hemostasis), and sperm capacitation (reproduction). Conversely, SULT2B1b acts to detoxify metabolites such as 7-ketocholesterol, which induces retinal degeneration (leading to blindness) and formation of macrophage foam cells (leading to atherosclerosis).