Pro- and anti-mitotic actions of PACAP in the developing cortex
Creators: name: Yan, Yan
Publisher: name: No Publisher Supplied
Year: 2011

description: The multi-functional neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) has been associated with different physiological processes. The actions of PACAP signaling in neurogenetic regulation are complex: the peptide functions in cell cycle progression, differentiation and precursor survival. In this thesis, I focused on its functions in neurogenesis, including proliferation, differentiation, cell death and different signaling pathways in which it is involved. PACAP has been well established to act as an anti-mitogenic signal in neuronal precursors at embryonic day (E) 13.5 onward. Furthermore, PACAP exhibits trophic effects in cerebellum and hippocampus postnatally. PACAP is present at the onset of neurogenesis (E9.5 rats) (Zhou et al., 1999), PACAP well-defined proliferation inhibitory effects seem inconsistent with the very robust proliferation. Since the majority of the cells are proliferating in the environment that is rich in mitogens in early neurogenesis. Due to the extremely tiny embryos (~3000μm in length at E10 rats) and lack of young cortical precursor culture models, the roles of PACAP in early corticogenesis has not been studied thoroughly and therefore are unclear. Defining the role of PACAP in brain development is important because recent studies suggest PACAP signaling abnormalities may contribute to schizophrenia (Hashimoto et al., 200&amp;) and post-traumatic stress disorder (PTSD) (Ressler et al., 2011). Thus I decided to define the function of PACAP during early neurogenesis in the developing cerebral cortex.    PAC1 receptors, especially hop and short isoforms, differentially couple to and activate distinct pathways that produce pro- or anti- mitogenic actions in neuronal precursors, respectively. Previously we found that the anti-mitogenic activity in older cortical precursors, both in culture and in vivo, is caused by activated cAMP signaling through the short isoform. Hop isoform, however, is linked to pro-mitogenic activity through phospholipase C (PLC)/protein kinase C (PKC) pathway. Furthermore, ectopic over-expression of hop isoform transformed the anti-mitotic effects of PACAP in E14 cortical precursors into a pro-mitogenic signal (Nicot and DiCicco-Bloom, 2001). These results suggest that expression of distinct PAC1 isoforms is essential for regulating neural precursor mitosis.     E9.5 PACAP-/- mice exhibited a decrease in BrdU labeling index, suggesting that PACAP normally promotes proliferation at this time. I established a culture model of young precursors (E10.5 rat; E9.5 mouse) viable for up to 48h, overcoming previous limitations of studying this early age precursors in regular media. In contrast to older precursors, I found in this model at 24h that PACAP exhibits mitogenic effects stimulating S-phase entry and proliferation of E10.5 precursors without affecting cell survival, evokes intracellular calcium fluxes and increases phospho-PKC levels. The opposing effects of PACAP during development are due to age-dependent receptor isoform (hop to short) switch. Significantly, I found the expression of the hop isoform is 24-fold greater than short at E10.5, a ratio that is reversed at E14.5 when short isoform expression is 15-fold greater. In addition, both the hop expression and PACAP mitogenic activity are maintained by retinoic acid exposure.     These observations suggest that PACAP elicits temporally specific effects on cortical neurogenesis determined by developmentally regulated expression of PAC1 receptor isoforms.