Kelly E. Mayo
Research in the laboratory is focused on cellular signaling and gene regulation in the mammalian neuroendocrine system. We have characterized several genes whose protein products act as peptide hormones in the brain and endocrine system, or as receptors for these hormones. Our goals include elucidating the molecular and cellular mechanisms by which these genes are regulated, and understanding how these hormones and receptors act as modulators of critical physiological processes such as growth and reproduction.
To better understand the regulation of growth, we are studying growth hormone-releasing hormone (GHRH), a neuropeptide that regulates pituitary growth hormone secretion. GHRH is synthesized predominantly in neurosecretory cells within the arcuate nuclei of the hypothalamus. Early studies from our laboratory utilizing transgenic mice overexpressing GHRH indicated that GHRH has profound stimulatory effects on pituitary somatotroph proliferation and on growth hormone secretion. These transgenic mice therefore have a gigantic phenotype, and mimic in some respects human diseases of growth hormone hypersecretion resulting from ectopic production of GHRH by endocrine tumors. GHRH has also been implicated as a paracrine regulatory factor in the placenta, ovary and testis. In the brain and placenta, unique promoters and alternative RNA processing are utilized to generate tissue-specific forms of the GHRH mRNA and allow for tissue-specific regulation of GHRH production.
We recently identified a pituitary receptor for GHRH, and are currently investigating signal transduction by this receptor, analyzing the regulation of receptor synthesis, and exploring the potential involvement of the GHRH receptor in disorders of growth. We found that the GHRH receptor is mutated in the little mouse, an animal model for human isolated growth hormone deficiency. A missense mutation in which aspartic acid 60 within the amino-terminal extracellular domain of the receptor is changed to glycine results in a receptor that is unable to bind the ligand GHRH, leading to somatotroph hypoplasia and diminished growth hormone secretion in these mice.
A second area of interest within the laboratory focuses on the expression of the reproductive hormones inhibin and activin, dimeric ovarian proteins that control the secretion of follicle-stimulating hormone from the pituitary and also have important roles as intraovarian regulatory factors. We are investigating the expression of the inhibin and activin alpha, beta-A and beta-B subunit genes in reproductive tissues, using cell culture, cell transfection, and transgenic animal models. Much of this work has involved an examination of cAMP-dependent signal transduction pathways within ovarian granulosa cells. Inhibin subunit gene expression in ovarian granulosa cells is stimulated by the pituitary gonadotropin follicle-stimulating hormone. FSH stimulation is mediated by cAMP-dependent pathways, and we have found that there is a rapid and transient phosphorylation of the transcription factor CREB in these cells, leading to its activation. At least two of the inhibin subunit genes have CREB binding sites in their promoter regions, and these appear to be critical for FSH or cAMP-induced gene expression. In the ovary, inhibin subunit gene expression is repressed by luteinizing hormone, which also acts through cAMP-dependent pathways. We have observed a rapid and transient expression of ICER, a cAMP-induced repressor of transcription derived from the CREM gene, in the LH-stimulated ovary, suggesting that this factor might be an important negative regulator of inhibin subunit gene expression during the rodent estrous cycle.
Although the alpha and beta chains of inhibin and activin are structurally very similar, and they have homologous sequences, these hormones have distinct dimerization capacities and opposing biological activities. To determine what regions of these proteins encode specificity for binding to the appropriate receptor and for activating subsequent signaling pathways, we have generated alpha-beta chimeric proteins. We have found that most of these chimeric hormone precursors fail to be appropriately processed by cells, and the mature chimeric hormones are therefore not secreted. However, several of these chimeras exhibit strong "dominant-negative" characteristics, and can act to inhibit the production of wild-type activin. Most recently, we have generated many different single amino acid changes in the inhibin and activin beta-A subunit, and these are currently being tested for their dimerization potential and for biological activity. Experiments are also in progress using transgenic animal models to explore the pathophysiology and tissue-specific expression of the inhibin and activin genes. We have generated and characterized a line of transgenic mice that express an inhibin a subunit transgene and as a consequence exhibit reduced fertility and eventually develop large ovarian cysts. We are attempting to establish how the inhibin-activin-FSH axis is altered in these mice, and are trying to determine if they might represent a useful animal model for human reproductive disorders that might involve altered production of the inhibins or activins.