Gonadal hormone regulation of neurogenesis and therapeutic implications for neurodegenerative diseases

Discovery of adult neurogenesis shed light on the exciting possibility that stimulation of this process can be applied as a novel regenerative strategy for CNS disease intervention. Among the gonadal hormones, estrogens have been extensively studied for their regulatory roles in neurogenesis, and multiple laborotaries have reported its functions in promoting both neural progenitor cell proliferation and survival. The effects of progesterone on neurogenesis, however, were largely undetermined. As such, I undertook a series of studies using cellular, morphological, biochemical, genomic and behavioral approaches to characterize the impact of progesterone on neurogenesis and its underlying mechanism.;In the first part of my thesis, I described the effect of progesterone in regulating neurogenesis and underlying mechanism. My experiments showed that PRA mRNA was not detected in rNPCs, whereas membrane associated PRs, Progesterone Receptor Membrane Components (PGRMC) 1 and 2 mRNA were expressed. Progesterone induced a significant increase in BrdU incorporation that was confirmed by FACS analysis, which indicated that progesterone promoted rNPC exit of G0/G1 phase at 5 hours, followed by an increase in S-phase at 6 hours and M-phase at 8 hours, respectively. Microarray analysis of cell-cycle genes and RT-PCR and Western blot validation revealed that progesterone increased expression of genes that promote mitosis while decreasing expression of genes that repress cell proliferation. Progesterone-induced proliferation was not dependent upon conversion to metabolites and was antagonized by the ERK1/2 inhibitor UO126. Progesterone-induced proliferation was isomer and steroid specific. Computational structural analysis of progesterone and its isomers indicated that the proliferative effect of progesterone is likely to be mediated by PGRMC1/2. These data demonstrate that progesterone can promote NPC proliferation with concomitant regulation in mitotic cell-cycle genes via a PGRMC/Erk pathway mechanism.;In the second part of my thesis, I extended my study to determine the effects of synthetic progetins on neurogenesis. Seven progestins were tested and experiments were carried out to determine their proliferative and neuroprotective effects in vitro. Such effects appeared to be independent of PR binding affinity. Three progestins - Nesterone, Northynodrel and LNG - exhibited both neurogenic and neuroprotective effects, and were selected for animal experiments to determine their in vivo efficacy. Results indicated that among those progestins, Nestorone was most efficacious in promoting CNS plasticity and such effect should be taken into consideration when it comes to therapeutic application.;In the last part of my thesis, I determined the neurogenic efficacy of allopregnanolone, a metabolite of progesterone, under both aging and Alzheimer's disease burden. Previously, we demonstrated that allopregnanolone enhanced the proliferation of neural progenitor cells in vitro and reversed both neurogenic and cognitive deficits in 3-month-old triple transgenic mice of Alzheimer's disease (3xTgAD), an age when no AD pathology is overt. In this study we analyzed the age-dependent effect of APalpha on neurogenic function in 6, 9 and 12-month-old 3xTgAD male mice and compared that to age-matched non-Tg ones. Results indicated that in 3xTgAD mice, APalpha enhanced neurogenesis in an age-dependent manner, suggesting that APalpha could serve as a regenerative agent and restore cognitive performance in brains affected by Alzheimer's disease burden, potentially via promoting functional neurogenesis in the hippocampus.;The discovery that P4 and its structural analogs could promote adult rNPC proliferation highlights the potential use of those molecules as regenerative agents in adult brain. Further, the findings that progestins could differentially regulate neurogenesis demonstrated for the first time that clinical progestins could regulate CNS plasticity and such effects should be considered when using progestins for either hormone replacement therapy or contraceptive therapy. Lastly, allopregnanolone could reverse both neurogenic and cognitive deficit in a mouse model of Alzheimer's disease, suggesting a promising strategy for promoting neurogenesis and restoration of cognitive deficit in the adult brain under both aging and neurodegenerative disease burden. (Abstract shortened by UMI.)...