Autism spectrum disorders
Creators: name: Connacher, Robert James
Publisher: name: No Publisher Supplied
Year: 2017

description: Cortical Neurogenesis is a highly regulated process which requires neural precursor population expansion followed by migration and subsequent differentiation.  Early cortical overgrowth has been observed in individuals with autism spectrum disorder (ASD) and may underpin observed cases of macrocephaly seen within ASD subgroups.  To define mechanisms by which this process could be altered, studies have identified environmental and genetic ASD risk factors, including Valproic acid (VPA) and the copy number variant 16p11.2 (16P), respectively.   Valproic acid is a neurotheraputic medicine intended to treat epilepsy, migraines, and bipolar disorder but poses ASD risk to prenatally exposed fetuses when mothers are taking this drug.  Prenatal VPA exposure in rodents studies have replicated behavioral and cellular deficits seen in ASD, including alterations in neurogenesis, but convergence of mechanisms remain undefined.  Studies suggest VPA may impact development through Histone Deacetylase (HDAC) inhibition, though this enzymatic inhibition has not been directly confirmed in primary neural cells.  The function of HDACs is to epigenetically regulate acetylation sites which can change gene expression.  Therefore, I characterized HDAC message and protein in the developing rodent brain, and confirmed that VPA can enzymatically inhibit these proteins.  Previously, we found that VPA exposure promotes G1 to S phase transition through rapid increases of G1 cyclins and acetylated Histone H3, suggesting epigenetic regulation of this process.  Therefore, I measured mRNA of G1 cyclins after acute VPA exposure, and found upregulation in transcription of these genes, supporting the notion that post-translational modifications of histones may promote proliferation through G1-S phase transition.  Additionally, our prior studies found that the increased proliferation resulted in a bigger brain by P21 with more upper layer neurons.  Others have also indicated prenatal VPA exposure promotes neurogenesis of upper layer neurons.  Therefore with these findings, I characterized the percentage of Pax6 and Tbr2 progenitor cells in S phase, to determine specificity of VPA effect.  Interestingly, only the Tbr2 population had more cells in S phase, providing explanation for increased upper layer neurons at P21.    Altered neurogenesis is also observed in the copy number variant 16p11.2, a genetic risk factor for ASD.  In the 16p11.2 chromosomal deletion, one copy of 27 genes is missing, including MAPK3, which encodes ERK1.  This gene is a central component in the ERK signaling pathway, which is important for regulating cellular growth and proliferation.  Therefore, I investigated proliferative changes and signaling alterations in Neural Precursor Cells (NPCs) derived from control and 16p11.2 patients iPSCs.  I found that 16p11.2 NPCs had increased DNA synthesis under control media but exhibited reduced responses to mitogenic stimulation with FGF, a developmental extracellular factor known to activate the ERK pathway.  Further characterization of this pathway under control conditions revealed equivalent if not elevated phosphorylation of ERK1, but approximately 50% less Total ERK1.  Additionally, as may be predicted, there were elevations in cyclin D1 and P-S6, suggesting mechanisms by which DNA synthesis is increased in these cells. In aggregate, these studies identify cortical neurogenesis as a common target of risk factors that contribute to neuropsychiatric diseases.