Research Interests Most recently my research has focused on elucidating the role of the serotonin type 3A receptor (5-HT3AR) in the pituitary gland. Currently my laboratory is investigating this receptor's involvement in regulating luteinizing hormone (LH) release from pituitary gonadotropes and on characterizing the cis-acting promoter elements that regulate its transcription. Thus, we are conducting physiological studies to decipher the role of the receptor in vivo in conjunction with molecular studies aimed at understanding how it is regulated at the genetic level. Maintaining these two complimentary lines of research enhances our overall understanding of the biological role of the 5-HT3AR and increases the number of potential funding mechanisms available for supporting this research.
The 5-HT3AR is a multisubunit serotonin-gated cation channel that is expressed in discrete regions of the nervous system and in the pituitary gland. Although its presence in the pituitary gland has been documented, its function in this tissue has not been established. Studies in my laboratory have demonstrated that blocking the 5-HT3AR decreases LH release from gonadotrope-derived LbT2 cells, and that overexpression of the 5-HT3AR increases LH release and upregulates LHb promoter activity. Further, these studies show that the 5-HT3AR gene itself is regulated by gonadotropin releasing hormone (GnRH). Taken together, these data suggest that the 5-HT3AR is an integral component of the hypothalamic-pituitary-gonadal (HPG) axis. However, additional analyses are necessary to determine its precise regulatory role. Toward this end, we have initiated studies to characterize the 5-HT3AR gene promoter in LbT2 cells and to generate transgenic mice that overexpress the 5-HT3AR selectively in pituitary gonadotropes. These studies will determine how the receptor is regulated in gonadotropes and will provide insight into physiological role of the 5-HT3AR in the pituitary gland.
Luteinizing hormone is a fundamental component of the HPG axis as it plays a pivotal role in controlling reproductive function in mammals by stimulating steroidogenesis and gametogenesis. In addition, increased LH has been implicated as a contributor to the neurodegenerative processes associated with Alzheimer's disease. Thus, defining the mechanisms that control LH synthesis and secretion is of paramount importance for a complete understanding of HPG axis function in the context of reproductive fitness and aging. While it is well known that gonadotropin hormone releasing hormone (GnRH) binding to receptors on pituitary gonadotropes stimulates the synthesis and release of LH, it has been shown that GnRH by itself does not fully account for basal LH transcription and exocytosis. Thus, other mechanisms within the gonadotrope must act to maintain normal levels of LH, and we have identified 5-HT3AR activation as one potential mechanism.
An immediate goal of our molecular studies is to identify the cis-acting elements in 5-HT3AR promoter that allow for basal transcriptional activity and that confer GnRH sensitivity in LbT2 cells. To do this we have constructed a number of 5' deletions of the murine 5-HT3AR promoter linked to a luciferase reporter gene and have begun to characterize their constituent regulatory elements in LbT2 cells and At-T20 cells, which represent pituitary corticotropes. Although there are over 3500 published papers on the 5-HT3AR, and it is involved with numerous physiological processes, including maintenance of heart rate, nociception, and digestive function, little is known about how its promoter is regulated. Our studies will provide novel information about the mechanisms that govern 5-HT3AR gene expression that will be of interest to a wide cross section of physiologists and neuroscientists.
The overall goal of this research to fully define the role of the 5-HT3AR in the HPG axis. This goal will be achieved by using converging methods to achieve an overall understanding of the physiological processes that are affected by this receptor. In addition to our molecular approach we will utilize transgenic technology to generate lines of mice that overexpress the 5-HT3AR selectively in pituitary gonadotropes. Based on our in vitro work, we expect 5-HT3AR overexpressing mice to hypersecrete LH. These mice will then be used to assess the effects of LH hypersecretion. Hypersecretion of gonadotropins has clear implications for reproductive fitness and has recently been identified as a potential facilitator of the neurodegenerative disease processes in humans. I have recently been awarded an Indiana University Medical School Biomedical Research Grant to produce these mice and I expect to have potential founders within the next several weeks. My initial studies with these mice will focus on determining the extent to which 5-HT3AR overexpression affects LH secretion and the affect of elevated LH on beta-amyloid precursor protein (ABeta). Recent studies have suggested that LH increases insoluble ABeta accumulation and that the observed neuroprotective effects of estrogen may actually be indirect via a negative feedback mechanism that inhibits LH synthesis and secretion in gonadotropes. If 5-HT3AR overexpression produces sustained elevations in LH, this mouse model will be a valuable tool with which to investigate this possibility. With my background in molecular neuroscience and neuropharmacology I am well prepared to investigate both these avenues. This research will provide novel information about HPG axis regulation, neurodegenerative processes and basic mammalian physiology.
These lines of research will add to our existing knowledge of HPG axis regulation and may provide a novel mouse model of Alzheimer's disease.