Neural tube expression of pituitary adenylate cyclase-activating peptide (PACAP) and receptor: Potential role in patterning and neurogenesis

Abstract

Neural tube patterning in vertebrates is controlled in part by locally secreted factors that act in a paracrine manner on nearby cells to regulate proliferation and gene expression. We show here by in situ hybridization that genes for the neuropeptide pituitary adenylate cyclaseactivating peptide (PACAP) and one of its high-affinity receptors (PAC 1 ) are widely expressed in the mouse neural tube on embryonic day (E) 10.5. Transcripts for the ligand are present in differentiating neurons in much of the neural tube, whereas the receptor gene is expressed in the underlying ventricular zone, most prominently in the alar region and f loor plate. PACAP potently increased cAMP levels more than 20-fold in cultured E10.5 hindbrain neuroepithelial cells, suggesting that PACAP activates protein kinase A (PKA) in the neural tube and might act in the process of patterning. Consistent with this possibility, PACAP down-regulated expression of the sonic hedgehog-and PKA-dependent target gene gli-1 in cultured neuroepithelial cells, concomitant with a decrease in DNA synthesis. PACAP is thus an early inducer of cAMP levels in the embryo and may act in the neural tube during patterning to control cell proliferation and gene expression.Recent studies suggest that phenotypic determination in the developing nervous system results from interactions of patterning genes conserved through evolution (1). For example, sonic hedgehog (shh) is one of three mammalian homologs to the segment polarity gene hedgehog (hh). shh has been implicated as a notochord-and floor plate-secreted factor that controls dorsal͞ventral patterning in the vertebrate neural tube (1-4). Abundant genetic and molecular evidence in flies (5), fish (6), and mice (7) indicates that hh and its homologs act by antagonizing cAMP-dependent protein kinase A (PKA) signaling. Although shh may act in the ventral tube by blocking constitutive PKA activity, it is possible that physiological activators of the cAMP͞PKA pathway are important in patterning.We considered that pituitary adenylate cyclase-activating peptide (PACAP) might be involved in patterning for several reasons. First, although PACAP originally was discovered as a hypothalamic factor that potently increased cAMP in the pituitary through G protein-coupled receptors (8), peptide expression later was localized to many central and peripheral neuronal populations as well as the developing embryo (9). Second, the 27-aa form of the peptide, PACAP-27, is conserved 100% in species ranging from fish to humans. Finally, our tissue culture studies indicated that PACAP and a closely related peptide vasoactive intestinal peptide stimulate cAMP production, regulating proliferation, differentiation, and͞or survival of multiple neuronal precursors (10-14). The current studies indicate that a functional PACAP ligand͞receptor cAMP signaling system is expressed in the neural tube at the onset of neurogenesis, raising the possibility that PACAP might be involved in neural tube patterning. MATERIALS AND METHODS In Situ Hybridi...

Regulation of neuroblast mitosis is determined by PACAP receptor isoform expression

“…Ligands for G protein-coupled receptors (GPCR) may play significant roles in neurogenesis in a regionally or developmentally specific fashion (6)(7)(8)(9). The neuropeptide PACAP (pituitary adenylate cyclase activating polypeptide ) exhibits either positive or negative mitogenic effects depending on the neuroblast (10)(11)(12). The basis of these opposite effects is undefined, but may be due to differential expression of PACAP receptor isoforms and consequent activation of distinct signal transduction pathways.…”

“…In primary neuroblasts, we previously defined region-specific mitogenic effects of PACAP, which may be attributed to receptor isoforms (17). For example, in embryonic hindbrain (11) and cortical neuroblasts (10,17), which express the short variant, PACAP stimulates AC only and elicits mitotic inhibition. In contrast, in sympathetic neuroblasts, which express the hop variant, PACAP activates both AC and PLC pathways and stimulates precursor mitosis (12,17).…”

Expression and Function of cGMP-dependent Protein Kinase Type I during Medaka Fish Embryogenesis

“…It has been proposed that atrial natriuretic peptides are major ligands in the cGMP-signaling pathway in the neural tube of the chick explant system (27). On the other hand, the expressions of neuronal nitric-oxide synthase and PKG I have been detected in the floor plate and dorsal root ganglia during embryogenesis (9, 10), whereas two upstream molecules of the PKA signaling pathway, pituitary adenylyl cyclase-activating peptide (PACAP) and its receptor (PAC 1 ), have been detected within the ventricular zone, most prominently in the dorsal region and the floor plate (35). Moreover, the soluble GC-and PKG I-knockdown embryos displayed similar phenotypes, as described above.…”

Identification of pituitary adenylate cyclase-activating polypeptide1-38-binding factor in human plasma, as ceruloplasmin

“…Although the functional role of PACAP is not yet fully elucidated, there is evidence that the peptide is a hypothalamic regulatory factor influencing hormonal secretion from the anterior pituitary lobe [18][19][20][21]. A neurotrophic role during injury repair [22] as well as a function in early neurogenesis has also been suggested [23]. Furthermore, a hormonal role for the peptide is likely.…”