Session 5: Environmental Influences as Etiologic Factors in Autism
Emanuel DiCicco-Bloom, MD
UMDNJ-Robert Wood Johnson Medical School
As child neurologists, we frequently evaluate children for abnormal brain development, concerned about attention, learning, behavior, and autism spectrum disorders. Normal brain development depends on interactions among multiple factors including those from genetic, neurochemical, social, and environmental sources. Recent studies indicate that environmental toxicants may injure the developing brain, potentially contributing to cognitive and motor deficits. While negative effects of toxicants on cell migration, differentiation, and survival have been well-characterized, little is known about effects on the generation of neurons (neurogenesis) and underlying mechanisms. Previously, we examined both positive and negative regulators of proliferation in developing brain, defining growth factor and peptide effects in culture and in vivo, in embryonic and postnatal forebrain and hindbrain regions involved in learning, memory and motor functions. Based on extensive studies, we now turn attention to effects of neurotoxicants on neurogenesis, defining mediating mitogenic and cell cycle pathways and designing new model systems.
To define effects of extracellular signals, whether they be endogenous proteins or environmental toxicants, we assess DNA synthesis, cell cycle machinery and cell number of select neuronal precursors. In culture, we use embryonic day 14.5 (E14.5) cerebral cortical precursors, and have found that the endogenous peptide, PACAP, serves as an autocrine anti-mitogenic factor, inhibiting DNA synthesis by reducing the expression of mitogenic growth factor receptors, such as FGF receptor, and by acting directly on intrinsic cell cycle machinery, such as cyclin-dependent kinases and their inhibitors. These effects defined in culture, were also demonstrated in the living embryo in utero.
While major malformations result from exposure to high levels of neurtoxicant in vivo, such as methylmercury (MeHg), adequate surveillance of lower level exposures has suffered from insufficient screening procedures. In our studies of growth factor regulation of postnatal neurogenesis, using cerebellum and hippocampus, we have developed well-characterized biochemical screening procedures, allowing us to perform labor-intensive histological quantification only after optimal dosage and time parameters have been defined. Thus, after acute factor/neurotoxicant treatment, we can detect rapid changes in DNA synthesis in an entire brain region, then in isolated neuronal precursors, and finally confirm effects histologically by mitotic labeling of proliferative zones. Further, acute effects frequently result in longer term changes in neuronal population size: 3 days to 3 weeks later, diminished total cells, defined by quantitative DNA assays, correlates with decreased mitotically labeled precursors as well as absolute neuronal cell number assessed by stereological methods. Using combined cell culture and in vivo approaches, we may now define neurotoxicant effects and underlying pathogenic mechanisms in disorders of brain development.
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