Angela S Howard

Johns Hopkins University, Baltimore, Maryland, United States

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Publications (4)15.2 Total impact

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    ABSTRACT: A primary role of acetylcholinesterase (AChE) is regulation of cholinergic neurotransmission by hydrolysis of synaptic acetylcholine. In the developing nervous system, however, AChE also functions as a morphogenic factor to promote axonal growth. This raises the question of whether organophosphorus pesticides (OPs) that are known to selectively bind to and inactivate the enzymatic function of AChE also interfere with its morphogenic function to perturb axonogenesis. To test this hypothesis, we exposed primary cultures of sensory neurons derived from embryonic rat dorsal root ganglia (DRG) to chlorpyrifos (CPF) or its oxon metabolite (CPFO). Both OPs significantly decreased axonal length at concentrations that had no effect on cell viability, protein synthesis or the enzymatic activity of AChE. Comparative analyses of the effects of CPF and CPFO on axonal growth in DRG neurons cultured from AChE nullizygous (AChE -/-) versus wild type (AChE +/+) mice indicated that while these OPs inhibited axonal growth in AChE+/+ DRG neurons, they had no effect on axonal growth in AChE -/- DRG neurons. However, transfection of AChE -/- DRG neurons with cDNA encoding full-length AChE restored the wild type response to the axon inhibitory effects of OPs. These data indicate that inhibition of axonal growth by OPs requires AChE, but the mechanism involves inhibition of the morphogenic rather than enzymatic activity of AChE. These findings suggest a novel mechanism for explaining not only the functional deficits observed in children and animals following developmental exposure to OPs, but also the increased vulnerability of the developing nervous system to OPs.
    Toxicology and Applied Pharmacology 05/2008; 228(1):32-41. DOI:10.1016/j.taap.2007.11.005 · 3.71 Impact Factor
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    ABSTRACT: Evidence that children are widely exposed to organophosphorus pesticides (OPs) and that OPs cause developmental neurotoxicity in animal models raises significant concerns about the risks these compounds pose to the developing human nervous system. Critical to assessing this risk is identifying specific neurodevelopmental events targeted by OPs. Observations that OPs alter brain morphometry in developing rodents and inhibit neurite outgrowth in neural cell lines suggest that OPs perturb neuronal morphogenesis. However, an important question yet to be answered is whether the dysmorphogenic effect of OPs reflects perturbation of axonal or dendritic growth. We addressed this question by quantifying axonal and dendritic growth in primary cultures of embryonic rat sympathetic neurons derived from superior cervical ganglia (SCG) following in vitro exposure to chlorpyrifos (CPF) or its metabolites CPF-oxon (CPFO) and trichloropyridinol (TCP). Axon outgrowth was significantly inhibited by CPF or CPFO, but not TCP, at concentrations > or =0.001 microM or 0.001 nM, respectively. In contrast, all three compounds enhanced BMP-induced dendritic growth. Acetylcholinesterase was inhibited only by the highest concentrations of CPF (> or =1 microM) and CPFO (> or =1 nM); TCP had no effect on this parameter. In summary, these compounds perturb neuronal morphogenesis via opposing effects on axonal and dendritic growth, and both effects are independent of acetylcholinesterase inhibition. These findings have important implications for current risk assessment practices of using acetylcholinesterase inhibition as a biomarker of OP neurotoxicity and suggest that OPs may disrupt normal patterns of neuronal connectivity in the developing nervous system.
    Toxicology and Applied Pharmacology 09/2005; 207(2):112-24. DOI:10.1016/j.taap.2004.12.008 · 3.71 Impact Factor
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    ABSTRACT: It has been suggested that pesticide exposure may be a contributing factor underlying the increased incidence of asthma in the United States and other industrialized nations. To test this hypothesis, airway hyperreactivity was measured in guinea pigs exposed to chlorpyrifos, a widely used organophosphate pesticide. Electrical stimulation of the vagus nerves caused frequency-dependent bronchoconstriction that was significantly potentiated in animals 24 h or 7 days after a single subcutaneous injection of either 390 mg/kg or 70 mg/kg of chlorpyrifos, respectively. Mechanisms by which chlorpyrifos may cause airway hyperreactivity include inhibition of acetylcholinesterase (AChE) or dysfunction of M3 muscarinic receptors on airway smooth muscle or of autoinhibitory M2 muscarinic receptors on parasympathetic nerves in the lung. AChE activity in the lung was significantly inhibited 24 h after treatment with 390 mg/kg of chlorpyrifos, but not 7 days after injection of 70 mg/kg of chlorpyrifos. Acute exposure to eserine (250 microg/ml) also significantly inhibited lung AChE but did not potentiate vagally induced bronchoconstriction. Neuronal M2 receptor function was tested using the M2 agonist pilocarpine, which inhibits vagally induced bronchoconstriction in control animals. In chlorpyrifos-treated animals, pilocarpine dose-response curves were shifted significantly to the right, demonstrating decreased responsiveness of neuronal M2 receptors. In contrast, chlorpyrifos treatment did not alter methacholine-induced bronchoconstriction, suggesting that chlorpyrifos does not alter M3 muscarinic receptor function on airway smooth muscle. These data demonstrate that organophosphate insecticides can cause airway hyperreactivity in the absence of AChE inhibition by decreasing neuronal M2 receptor function.
    AJP Lung Cellular and Molecular Physiology 06/2004; 286(5):L963-9. DOI:10.1152/ajplung.00343.2003 · 4.08 Impact Factor
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    ABSTRACT: Perinatal exposure to polychlorinated biphenyls (PCBs) is linked to cognitive deficits in humans and experimental animals; however, the mechanism(s) underlying this effect remain speculative. Apoptosis is essential to normal brain development, and perturbation of normal spatiotemporal patterns of apoptosis can cause persistent neural deficits. We tested the hypothesis that PCBs alter apoptosis in neuronal cell types critical to cognitive function. Primary cultures of rat cortical and hippocampal neurons were treated for 48 h with Aroclor 1254 or the congeners PCB 77 and 47, which represent coplanar and noncoplanar PCBs that bind the arylhydrocarbon receptor (AhR) with high and low affinity, respectively. Using Hoechst dye and an ELISA for DNA oligonucleosomes, we observed that Aroclor 1254 (10 microM) and PCB 47 (1 microM) significantly increased DNA fragmentation in hippocampal but not cortical neurons, and this effect was blocked by the caspase inhibitors, z-VAD-fmk and DEVD-CHO. In contrast, PCB 77 had no effect on apoptosis in either neuronal cell type, suggesting that PCB-induced apoptosis occurs independent of the AhR. The proapoptotic activity of PCBs was inhibited by the ryanodine receptor (RyR) antagonist FLA 365 and by the antioxidant alpha-tocopherol but not by antagonists of the IP(3) receptor (xestospongin C), L-type calcium channel (verapamil), or NMDA receptor (APV). These data indicate that noncoplanar PCBs induce apoptosis in hippocampal neurons subsequent to RyR activation and increased reactive oxygen species and suggest that altered regional profiles of apoptosis may be an important mechanism underlying the developmental neurotoxicity of PCBs.
    Toxicology and Applied Pharmacology 08/2003; 190(1):72-86. DOI:10.1016/S0041-008X(03)00156-X · 3.71 Impact Factor

Publication Stats

217 Citations
15.20 Total Impact Points


  • 2008
    • Johns Hopkins University
      Baltimore, Maryland, United States
  • 2003–2005
    • Johns Hopkins Bloomberg School of Public Health
      • Department of Environmental Health Sciences
      Baltimore, Maryland, United States