NOGO-A induction and localization during chick brain development indicate a role disparate from neurite outgrowth inhibition.

BACKGROUND: Nogo-A, a myelin-associated protein, inhibits neurite outgrowth and abates regeneration in the adult vertebrate central nervous system (CNS) and may play a role in maintaining neural pathways once established. However, the presence of Nogo-A during early CNS development is counterintuitive and hints at an additional role for Nogo-A beyond neurite inhibition. RESULTS: We isolated chicken NOGO-A and determined its sequence. A multiple alignment of the amino acid sequence across divergent species, identified five previously undescribed, Nogo-A specific conserved regions that may be relevant for development. NOGO gene transcripts (NOGO-A, NOGO-B and NOGO-C) were differentially expressed in the CNS during development and a second NOGO-A splice variant was identified. We further localized NOGO-A expression during key phases of CNS development by in situ hybridization. CNS-associated NOGO-A was induced coincident with neural plate formation and up-regulated by FGF in the transformation of non-neural ectoderm into neural precursors. NOGO-A expression was diffuse in the neuroectoderm during the early proliferative phase of development, and migration, but localized to large projection neurons of the optic tectum and tectal-associated nuclei during architectural differentiation, lamination and network establishment. CONCLUSION: These data suggest Nogo-A plays a functional role in the determination of neural identity and/or differentiation and also appears to play a later role in the networking of large projection neurons during neurite formation and synaptogenesis. These data indicate that Nogo-A is a multifunctional protein with additional roles during CNS development that are disparate from its later role of neurite outgrowth inhibition in the adult CNS.

Chick embryos exposed to trichloroethylene in an ex ovo culture model show selective defects in early endocardial cushion tissue formation.

BACKGROUND: Formation of the primitive heart is a critical step for establishing a competent circulatory system necessary for continued morphogenesis, and as such has significant potential as a target for environmental insult. The goal of this study was to identify the initial cellular events that precede more superficially observable abnormalities resulting from exposing early chick embryos to trichloroethylene (TCE). METHODS: A whole embryo culture method was used to assess the susceptibility of endocardial epithelial-mesenchymal transformation in the early chick heart to TCE. This method has the benefits of maintaining the anatomical relationships of developing tissues and organs, instantaneously exposing precisely staged embryos to quantifiable levels of TCE in a protein-free medium, and the ability to directly monitor developmental morphology. RESULTS: A minority of embryos (Hamburger and Hamilton [HH] stage 13-14) exposed to TCE (10-80 ppm) were not viable after 24 hr in culture and exhibited a variety of gross malformations in a dose-dependent fashion. However, the majority of treated embryos remained viable and developed into HH stage 17 embryos that were superficially indistinguishable from vehicle-treated controls. Further analysis of the hearts of these superficially normal embryos by whole-mount confocal microscopy revealed selective reduction in the number of atrioventricular canal mesenchymal cells. Additionally, those mesenchymal cells that did develop migrated abnormally as long thin cords of adherent cells. CONCLUSIONS: The regional selectivity of these effects in the chick heart suggests a critical window of susceptibility to TCE in the epithelial-mesenchymal transformation of atrioventricular canal endocardium.

Neurocan in the embryonic avian heart and vasculature.

The chondroitin sulfate proteoglycan (CSPG) neurocan was previously considered to be nervous-system specific. However, we have found neurocan in the embryonic heart and vasculature. In stage 11 quail embryos, neurocan was prominently expressed in the myocardium, dorsal mesocardium, heart-forming fields, splanchnic mesoderm, and vicinity of the extraembryonic vaculature, and at lower levels in the endocardium. A comparison of neurocan staining with QH1 staining of vascular endothelial cells demonstrates that neurocan is frequently expressed by cells adjacent to endothelial cells, rather than by endothelial cells themselves. In some cases, a dispersed subset of cells are neurocan-positive in a field of cells that otherwise appear uniform in morphology. Later in development, neurocan expression becomes relatively limited to the nervous system. However, even in 10-day embryos, neurocan is expressed in the chorio-allantoic membrane in the tissue that separates closely packed, small-diameter blood vessels. In summary, our results suggest that neurocan may function as a barrier that regulates vascular patterning during development.