Ecrg4 expression and its product augurin in the choroid plexus: impact on fetal brain development, cerebrospinal fluid homeostasis and neuroprogenitor cell response to CNS injury

The content & composition of cerebrospinal fluid (CSF) is determined in large part by the choroid plexus (CP) & specifically, a specialized epithelial cell (CPe) layer that responds to, synthesizes, & transports peptide hormones into & out of CSF. Together with ventricular ependymal cells, these CPe relay homeostatic signals throughout the central nervous system (CNS) & regulate CSF hydrodynamics. One new candidate signal is augurin <deleted – space limit>…explore whether Ecrg4 & its product augurin, can be implicated in CNS development & the response to CNS injury. Ecrg4 gene expression in CNS & peripheral tissues was studied by in situ hybridization & quantitative RT-PCR. Augurin, the protein encoded by Ecrg4, was detected by immunoblotting, immunohistochemistry & ELISA. The biological consequence of augurin over-expression was studied in a cortical stab model of rat CNS injury by intra-cerebro-ventricular injection of an adenovirus vector containing the Ecrg4 cDNA. The biological consequences of reduced augurin expression were evaluated by characterizing the CNS phenotype caused by Ecrg4 gene knockdown in developing zebrafish embryos. Gene expression & immunohistochemical analyses revealed that, the CP is a major source of Ecrg4 in the CNS & that Ecrg4 mRNA is predominantly localized to choroid plexus epithelial (CPe), ventricular & central canal cells of the spinal cord. After a stab injury into the brain however, both augurin staining & Ecrg4 gene expression decreased precipitously. If the loss of augurin was circumvented by over-expressing Ecrg4 in vivo, BrdU incorporation by cells in the subependymal zone decreased. Inversely, gene knockdown of Ecrg4 in developing zebrafish embryos caused increased proliferation of GFAP-positive cells & induced a dose-dependent hydrocephalus-like phenotype that could be rescued by co-injection of antisense morpholinos with Ecrg4 mRNA. An unusually elevated expression of the Ecrg4 gene in the CP implies that its product, augurin, plays a role in CP-CSF-CNS function. The results are all consistent with a model whereby an injury-induced decrease in augurin dysinhibits target cells at the ependymal-subependymal interface. We speculate that the ability of CP & ependymal epithelium to alter the progenitor cell response to CNS injury may be mediated, in part by Ecrg4. If so, the canonic control of its promoter by DNA methylation may implicate epigenetic mechanisms in neuroprogenitor fate & function in the CNS.