Abstracts

Malformations of Cortical Development (MCD) underlie many forms of pediatric epilepsy. To study the disturbed mechanisms of neurodevelopment that ultimately result in a malformed brain, we used an injury-induced rodent model for Cortical Dysplasia (CD) i.e., prenatal MAM (methylazoxymethanol)-exposure in the rat. These animals exhibit anatomical and physiological aspects of seizure syndromes normally associated with MCD in humans. Here we focused on studying early embryonic disruption of the marginal zone (MZ), a structure established prior to teratogen exposure. Understanding the developmental processes that are disrupted in the abnormal brain can provide further insight into these malformation disorders and potentially offer other avenues for intervention or treatment. Brain tissue from MAM-exposed or control rats were perfused and fixed with paraformaldehyde (PFA), cryoprotected, and cut. For immunohistochemistry, sections were incubated with the appropriate primary antibody, followed by either flourescent secondary antibody or biotinylated secondary antibody for DAB labeling. For single or double [italic]in situ [/italic]hybridization, tissue was incubated with the appropriate DIG-labeled and/or FITC-labeled RNA probes, followed by either chromagen-based or TSA-fluorescent-based staining. For slice cultures, MAM or untreated E20 rat pup brains were embedded and cut into 250[mu]m sections that were cultured under various pharmocological treatments for 48 h. The MZ in embryonic slice cultures from MAM-exposed rats was thicker, with dispersion of reelin-positive cells away from the pial surface. These dispersed cells were also positive for calretinin and p73, markers for Cajal-Retzius (C-R) cells. A soluble factor from control slice cultures reduced the number of dispersed cells in the MZ of slices from MAM animals. Further studies suggest that the factor involved is SDF-1: (i) the expression of SDF-1 is decreased at the meninges of MAM animals; (ii) SDF-1 receptor antagonists effectively block the rescue by conditioned medium or co-culture; (iii) SDF-1 supplied in the medim rescued the dispersion of reelin-positive cells; and (iv) SDF-1 antagonists induced MZ disorganization (a phenocopy of in vivo MAM exposure) in control slice cultures. The principal findings of this study are: (1) Exposure to a prenatal insult, such as MAM, can disrupt brain development by affecting ongoing morphogenesis of the cortex, but may also affect previously established structures, like the MZ; (2) Global insults by teratogens can result in specific molecular alterations, such as the loss of SDF-1 in the meninges; (3) SDF-1 produced by the meninges is required for retention of C-R cells in the marginal zone; and (4) The marginal zone is a dynamic structure requiring maintenance to preserve proper cortical architecture. (Supported by NIH R01 NS040272-05 (S.C.B.) [amp] Epilepsy Foundation of America (M.P.).)