A clinically relevant dose of cyclophosphamide chemotherapy impairs memory performance on the delayed spatial alternation task that is sustained over time as mice age

BACKGROUND

Cyclophosphamide chemotherapy is a mainstay of adjuvant breast cancer treatment. Unfortunately, this drug is associated with cognitive impairments in cancer patients that may accelerate cognitive aging. Memory is particularly affected in many patients. In order to better understand the precise cognitive impairments caused by this chemotherapy agent, we investigated a clinically relevant dose and administration paradigm on delayed spatial memory abilities in C57BL/6 mice. We utilized a delayed alternation paradigm similar to a delayed match to sample paradigm reported to be sensitive in human neurotoxicology research.

METHODS

A dose of 200mg/kg cyclophosphamide was administered intravenously (at weekly intervals) for 4 weeks to C57BL/6 mice starting at 6 ½ months of age. Memory was tested in mice using a reward-based delayed spatial alternation paradigm with delay values of 1.5, 3, 6.1, 12.4 and 25s presented randomly over 80 sessions (16 reinforcers per session), and testing began at the initiation of chemotherapy through 3 months.

RESULTS

At the longest delay, i.e., that requiring the greatest memory, mice treated with chemotherapy exhibited a significant decline over time in percent correct which leveled off compared to controls that continued to improve slightly.

CONCLUSIONS

Our clinically relevant model shows cyclophosphamide chemotherapy causes a slight decline in delayed spatial memories at the longest delay that is sustained over time as mice age.

Aryl hydrocarbon receptor deletion in cerebellar granule neuron precursors impairs neurogenesis

The aryl hydrocarbon receptor (AhR) is a ligand-activated member of the basic-helix-loop-helix/PER-ARNT-SIM(PAS) transcription factor superfamily that also mediates the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Increasing evidence suggests that AhR influences the development of many tissues, including the central nervous system. Our previous studies suggest that sustained AhR activation by TCDD and/or AhR deletion disrupts cerebellar granule neuron precursor (GNP) development. In the current study, to determine whether endogenous AhR controls GNP development in a cell-autonomous manner, we created a GNP-specific AhR deletion mouse, AhR(fx/fx) /Math1(CRE/+) (AhR CKO). Selective AhR deletion in GNPs produced abnormalities in proliferation and differentiation. Specifically, fewer GNPs were engaged in S-phase, as demonstrated by ∼25% reductions in thymidine (in vitro) and Bromodeoxyuridine (in vivo) incorporation. Furthermore, total granule neuron numbers in the internal granule layer at PND21 and PND60 were diminished in AhR conditional knockout (CKO) mice compared with controls. Conversely, differentiation was enhanced, including ∼40% increase in neurite outgrowth and 50% increase in GABARα6 receptor expression in deletion mutants. Our results suggest that AhR activity plays a role in regulating granule neuron number and differentiation, possibly by coordinating this GNP developmental transition. These studies provide novel insights for understanding the normal roles of AhR signaling during cerebellar granule cell neurogenesis and may have important implications for the effects of environmental factors in cerebellar dysgenesis.

The effects of postnatal exposure to low-dose bisphenol-A on activity-dependent plasticity in the mouse sensory cortex

Bisphenol-A (BPA) is a monomer used in the production of polycarbonate plastics, epoxies and resins and is present in many common household objects ranging from water bottles, can linings, baby bottles, and dental resins. BPA exposure has been linked to numerous negative health effects throughout the body, although the mechanisms of BPA action on the developing brain are still poorly understood. In this study, we sought to investigate whether low dose BPA exposure during a developmental phase when brain connectivity is being organized can cause long-term deleterious effects on brain function and plasticity that outlast the BPA exposure. Lactating dams were orally exposed to 25 μg/kg/day of BPA (one half the U.S. Environmental Protection Agency's 50 μg/kg/day rodent dose reference) or vehicle alone from postnatal day (P)5 to P21. Pups exposed to BPA in their mother's milk exhibited deficits in activity-dependent plasticity in the visual cortex during the visual critical period (P28). To determine the possible mechanisms underlying BPA action, we used immunohistochemistry to examine histological markers known to impact cortical maturity and developmental plasticity and quantified cortical dendritic spine density, morphology, and dynamics. While we saw no changes in parvalbumin neuron density, myelin basic protein expression or microglial density in BPA-exposed animals, we observed increases in spine density on apical dendrites in cortical layer five neurons but no significant alterations in other morphological parameters. Taken together our results suggest that exposure to very low levels of BPA during a critical period of brain development can have profound consequences for the normal wiring of sensory circuits and their plasticity later in life.

Deletion or activation of the aryl hydrocarbon receptor alters adult hippocampal neurogenesis and contextual fear memory

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates the toxicity of dioxin and serves multiple developmental roles. In the adult brain, while we now localize AhR mRNA to nestin-expressing neural progenitor cells in the dentate gyrus (DG) of the hippocampus, its function is unknown. This study tested the hypothesis that AhR participates in hippocampal neurogenesis and associated functions. AhR deletion and activation by the potent environmental toxicant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), adversely impacted neurogenesis and cognition. Adult AhR-deficient mice exhibited impaired hippocampal-dependent contextual fear memory while hippocampal-independent memory remained intact. AhR-deficient mice displayed reduced cell birth, decreased cell survival, and diminished neuronal differentiation in the DG. Following TCDD exposure, wild-type mice exhibited impaired hippocampal-dependent contextual memory, decreased cell birth, reduced neuronal differentiation, and fewer mature neurons in the DG. Glial differentiation and apoptosis were not altered in either TCDD-exposed or AhR-deficient mice. Finally, defects observed in TCDD-exposed mice were dependent on AhR, as TCDD had no negative effects in AhR-deficient mice. Our findings suggest that AhR should be further evaluated as a potential transcriptional regulator of hippocampal neurogenesis and function, although other sites of action may also warrant consideration. Moreover, TCDD exposure should be considered as an environmental risk factor that disrupts adult neurogenesis and potentially related memory processes.

The aryl hydrocarbon receptor contributes to the proliferation of human medulloblastoma cells

The aryl hydrocarbon receptor (AhR), a ligand-activated member of the basic helix-loop-helix (bHLH)/PER-ARNT-SIM (PAS) transcription superfamily, is known to regulate the toxicity of polyaromatic halogenated hydrocarbon environmental chemicals, most notably dioxin. However, the AhR has also been implicated in multiple stages of tumorigenesis. Medulloblastoma (MB), a primary cerebellar brain tumor arising in infants and children, is thought to originate from abnormally proliferating cerebellar granule neuron precursors (GNPs). GNPs express high levels of the AhR in the external germinal layer of the developing cerebellum. Moreover, our laboratory has previously reported that either abnormal activation or deletion of the AhR leads to dysregulation of GNP cell cycle activity and maturation. These observations led to the hypothesis that the AhR promotes the growth of MB. Therefore, this study evaluated whether the AhR serves a pro-proliferative role in an immortalized MB tumor cell line (DAOY). We produced a stable AhR knockdown DAOY cell line [AhR short hairpin RNA (shRNA)], which exhibited a 70% reduction in AhR protein levels. Compared with wild-type DAOY cells, AhR shRNA DAOY cells displayed an impaired G(1)-to-S cell cycle transition, decreased DNA synthesis, and reduced proliferation. Furthermore, these cell cycle perturbations were correlated with decreased levels of the pro-proliferative gene Hes1 and increased levels of the cell cycle inhibitor p27(kip1). Supplementation experiments with human AhR restored the proliferative activity in AhR shRNA DAOY cells. Taken together, our data show that the AhR promotes proliferation of MB cells, suggesting that this pathway should be considered as a potential therapeutic target for MB treatment.

An azobenzene photoswitch sheds light on turn nucleation in amyloid-β self-assembly

Amyloid-β (Aβ) self-assembly into cross-β amyloid fibrils is implicated in a causative role in Alzheimer's disease pathology. Uncertainties persist regarding the mechanisms of amyloid self-assembly and the role of metastable prefibrillar aggregates. Aβ fibrils feature a sheet-turn-sheet motif in the constituent β-strands; as such, turn nucleation has been proposed as a rate-limiting step in the self-assembly pathway. Herein, we report the use of an azobenzene β-hairpin mimetic to study the role turn nucleation plays on Aβ self-assembly. [3-(3-Aminomethyl)phenylazo]phenylacetic acid (AMPP) was incorporated into the putative turn region of Aβ42 to elicit temporal control over Aβ42 turn nucleation; it was hypothesized that self-assembly would be favored in the cis-AMPP conformation if β-hairpin formation occurs during Aβ self-assembly and that the trans-AMPP conformer would display attenuated fibrillization propensity. It was unexpectedly observed that the trans-AMPP Aβ42 conformer forms fibrillar constructs that are similar in almost all characteristics, including cytotoxicity, to wild-type Aβ42. Conversely, the cis-AMPP Aβ42 congeners formed nonfibrillar, amorphous aggregates that exhibited no cytotoxicity. Additionally, cis-trans photoisomerization resulted in rapid formation of native-like amyloid fibrils and trans-cis conversion in the fibril state reduced the population of native-like fibrils. Thus, temporal photocontrol over Aβ turn conformation provides significant insight into Aβ self-assembly. Specifically, Aβ mutants that adopt stable β-turns form aggregate structures that are unable to enter folding pathways leading to cross-β fibrils and cytotoxic prefibrillar intermediates.

Turn nucleation perturbs amyloid β self-assembly and cytotoxicity

The accumulation of senile plaques composed of amyloid β (Aβ) fibrils is a hallmark of Alzheimer's disease, although prefibrillar oligomeric species are believed to be the primary neurotoxic congeners in the pathogenesis of Alzheimer's disease. Uncertainty regarding the mechanistic relationship between Aβ oligomer and fibril formation and the cytotoxicity of these aggregate species persists. β-Turn formation has been proposed to be a potential rate-limiting step during Aβ fibrillogenesis. The effect of turn nucleation on Aβ self-assembly was probed by systematically replacing amino acid pairs in the putative turn region of Aβ (residues 24-27) with d-ProGly ((D)PG), an effective turn-nucleating motif. The kinetic, thermodynamic, and cytotoxic effects of these mutations were characterized. It was found that turn formation dramatically accelerated Aβ fibril self-assembly dependent on the site of turn nucleation. The cytotoxicity of the three (D)PG-containing Aβ variants was significantly lower than that of wild-type Aβ40, presumably due to decreased oligomer populations as a function of a more rapid progression to mature fibrils; oligomer populations were not eliminated, however, suggesting that turn formation is also a feature of oligomer structures. These results indicate that turn nucleation is a critical step in Aβ40 fibril formation.

Subchronic polychlorinated biphenyl (Aroclor 1254) exposure produces oxidative damage and neuronal death of ventral midbrain dopaminergic systems

Recent epidemiologic studies have demonstrated a link between organochlorine and pesticide exposure to an enhanced risk for neurodegenerative disorders such as Parkinson's disease (PD). A common biological phenomenon underlying cell injury associated with both polychlorinated biphenyl (PCB) exposure and dopaminergic neurodegeneration during aging is oxidative stress (OS). In this study, we tested the hypothesis that oral PCB exposure, via food ingestion, impairs dopamine systems in the adult murine brain. We determined whether PCB exposure was associated with OS in dopaminergic neurons, a population of cells that selectively degenerate in PD. After 4 weeks of oral exposure to the PCB mixture Aroclor 1254, several congeners, mostly ortho substituted, accumulated throughout the brain. Significant increases in locomotor activity were observed within 2 weeks, which persisted after cessation of PCB exposure. Stereologic analyses revealed a significant loss of dopaminergic neurons within the substantia nigra and ventral tegmental area. However, striatal dopamine levels were elevated, suggesting that compensatory mechanisms exist to maintain dopamine homeostasis, which could contribute to the observed increases in locomotor activity following PCB exposure. Biochemical experiments revealed alterations in OS markers, including increases in SOD and HO-1 levels and the presence of oxidatively modified lipids and proteins. These findings were accompanied by elevated iron levels within the striatal and midbrain regions, perhaps due to the observed dysregulation of transferrin receptors and ferritin levels following PCB exposure. In this study, we suggest that both OS and the uncoupling of iron regulation contribute to dopamine neuron degeneration and hyperactivity following PCB exposure.

Neural precursor cell proliferation is disrupted through activation of the aryl hydrocarbon receptor by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Neurogenesis involves the proliferation of multipotent neuroepithelial stem cells followed by differentiation into lineage-restricted neural precursor cells (NPCs) during the embryonic period. Interestingly, these progenitor cells express robust levels of the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor that regulates expression of genes important for growth regulation, and xenobiotic metabolism. Upon binding 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a pervasive environmental contaminant and potent AhR ligand, AhR, is activated and disrupts gene expression patterns to produce cellular toxicity. Because of its widespread distribution in the brain during critical proliferative phases of neurogenesis, it is conceivable that AhR participates in NPC expansion. Therefore, this study tested the hypothesis that AhR activation by TCDD disrupts signaling events that regulate NPC proliferation. The C17.2 NPC line served as a model system to (1) assess whether NPCs are targets for TCDD-induced neurotoxicity and (2) characterize the effects of TCDD on NPC proliferation. We demonstrated that C17.2 NPCs express an intact AhR signaling pathway that becomes transcriptionally active after TCDD exposure. (3)H-thymidine and alamar blue reduction assays indicated that TCDD suppresses NPC proliferation in a concentration-dependent manner without the loss of cell viability. Cell cycle distribution analysis by flow cytometry revealed that TCDD-induced growth arrest results from an impaired G1 to S cell cycle transition. Moreover, TCDD exposure altered p27( kip1) and cyclin D1 cell cycle regulatory protein expression levels consistent with a G1 phase arrest. Initial studies in primary NPCs isolated from the ventral forebrain of embryonic mice demonstrated that TCDD reduced cell proliferation through a G1 phase arrest, corroborating our findings in the C17.2 cell line. Together, these observations suggest that the inappropriate or sustained activation of AhR by TCDD during neurogenesis can interfere with signaling pathways that regulate neuroepithelial stem cell/NPC proliferation, which could adversely impact final cell number in the brain and lead to functional impairments.

IGF-1 partially restores chemotherapy-induced reductions in neural cell proliferation in adult C57BL/6 mice

Chemotherapeutic agents produce persistent difficulties in memory through an unknown mechanism. We tested the hypothesis that chemotherapeutic agents readily able to cross the blood-brain barrier (cyclophosphamide and fluorouracil), as opposed to those not known to readily cross the barrier (paclitaxel and doxorubicin), reduce neural cell proliferation following chemotherapy. We found that 5-bromo-2-deoxyuridine labeling following chemotherapy given to C57BL/6 mice revealed a similar reduction in neural cell proliferation in the dentate gyrus for all four agents. Insulin-like growth factor 1, a molecule implicated in promoting neurogenesis, counteracted the effects of high doses of chemotherapy on neural cell proliferation.

The aryl hydrocarbon receptor has a normal function in the regulation of hematopoietic and other stem/progenitor cell populations

The aryl hydrocarbon receptor (AhR) is known mainly as the mediator for the toxicity of certain xenobiotics. However, there is also much information to indicate that this transcription factor has important biological functions. Here we review the evidence that the AhR has a significant role in the regulation of hematopoietic stem cells (HSCs). Data to support this come from studies with xenobiotic AhR ligands, phenotypic analyses of mice lacking AhR, examining the presence and regulation of the AhR within HSCs, knowledge of genes and signaling pathways regulated by the AhR, and investigations of hematopoietic disorders. Based on this information, we hypothesize that AhR expression is necessary for the proper maintenance of quiescence in HSCs, and that AhR down-regulation is essential for "escape" from quiescence and subsequent proliferation of these cells. This implicates the AhR as a negative regulator of hematopoiesis with a function of curbing excessive or unnecessary proliferation. This provides an important advantage by preventing the premature exhaustion of HSCs and sensitivity to genetic alterations, thus preserving HSC function and long-term multi-lineage generation over the lifespan of the organism. This also implicates a role of the AhR in aging processes. AhR dysregulation may result in the altered ability of HSCs to sense appropriate signals in the bone marrow microenvironment leading to hematopoietic disease. It is also reasonable to hypothesize that this protein has an important function in the regulation of other tissue stem cell populations. Suggestive evidence is consistent with a role in skin and neural stem cells.

2,3,7,8-Tetracholorodibenzo-p-dioxin exposure disrupts granule neuron precursor maturation in the developing mouse cerebellum

The widespread environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been linked to developmental neurotoxicity associated with abnormal cerebellar maturation in both humans and rodents. TCDD mediates toxicity via binding to the aryl hydrocarbon receptor (AhR), a transcription factor that regulates the expression of xenobiotic metabolizing enzymes and growth regulatory molecules. Our previous studies demonstrated that cerebellar granule neuron precursor cells (GNPs) express transcriptionally active AhR during critical developmental periods. TCDD exposure also impaired GNP proliferation and survival in vitro. Therefore, this study tested the hypothesis that TCDD exposure disrupts cerebellar development by interfering with GNP differentiation. In vivo experiments indicated that TCDD exposure on postnatal day (PND) 6 resulted in increased expression of a mitotic marker and increased thickness of the external granule layer (EGL) on PND10. Expression of the early differentiation marker TAG-1 was also more pronounced in postmitotic, premigratory granule neurons of the EGL, and increased apoptosis of GNPs was observed. On PND21, expression of the late GNP differentiation marker GABA(A alpha 6) receptor (GABAR(A alpha 6)) and total estimated cell numbers were both reduced following exposure on PND6. Studies in unexposed adult AhR(-/-) mice revealed lower GABAR(A alpha 6) levels and DNA content. In vitro studies showed elevated expression of the early differentiation marker p27/Kip1 and the GABAR(A alpha 6) in GNPs following TCDD exposure, and the expression patterns of proteins related to granule cell neurite outgrowth, beta III-tubulin and polysialic acid neural cell adhesion molecule, were consistent with enhanced neuroblast differentiation. Together, our data suggest that TCDD disrupts a normal physiological role of AhR, resulting in compromised GNP maturation and neuroblast survival, which impacts final cell number in the cerebellum.

Heme-oxygenase-1 promotes polychlorinated biphenyl mixture aroclor 1254-induced oxidative stress and dopaminergic cell injury

Dopaminergic (DAergic) systems have been identified as putative targets for polycholorinated biphenyl (PCB) actions. However, the precise mechanisms leading to neurotoxicity are unresolved. Reactive oxygen species (ROS) were recently shown to mediate injury in DAergic MN9D cells following exposure to Aroclor 1254 (A1254), a commercial PCB mixture. The oxidative stress response in DAergic cells included a persistent expression of heme oxygenase-1 (HO-1). This study tested the hypothesis that a sustained PCB-induced HO-1 response leads to abnormally high Fe levels, which generates ROS production and mediates death in the MN9D DAergic cell model. Accordingly, results indicated that A1254 augmented intracellular Fe levels in MN9D cells after 24 h. Fe chelation by desferoxamine or pharmacologic inhibition of HO activity with tin-protoporphyrin reduced Fe accumulation, ROS production, and cytotoxicity following A1254 exposure. HO-1 over-expression predisposed MN9D DAergic cells to enhanced ROS production and cell death in response to PCBs. Conversely, antisense inhibition of HO-1 expression prevented PCB-induced ROS production and cell death. These observations suggest that enhanced HO-1 catalytic activity and subsequent liberation of Fe participate in neurotoxic DAergic cell injury caused by A1254 exposure in vitro.

Polychlorinated biphenyl mixture aroclor 1254-induced oxidative stress plays a role in dopaminergic cell injury

Oxidative stress (OS) is thought to participate in the pathogenesis of neurodegenerative disorders, including Parkinson's disease (PD). Excessive reactive oxygen species (ROS) production can occur during the normal aging process or following exposure to environmental toxicants. Dopamine neurons, which degenerate during PD, are particularly sensitive to oxidative stress. Polychlorinated biphenyls (PCBs), persistent and widespread pollutants, have been shown to adversely impact dopaminergic (DAergic) pathways, but the role ROS play in neurotoxicity remains unclear. To test the hypothesis that PCB exposure compromises dopamine neurons by stimulating ROS production, the direct toxicity and oxidative stress response following PCB exposure was examined both in MN9D dopamine cells and primary mesencephalic cultures. PCBs induced a time- and concentration-dependent increase in ROS production, which preceded cytotoxicity. Whereas intracellular GSH depletion exacerbated PCB effects, antioxidant pretreatment attenuated ROS production and cell death. Coincident alterations in antioxidant defense enzymes also accompanied ROS production, including decreased MnSOD and increased CuZnSOD protein levels. The robust elevation in heme oxygenase-1 levels further support the activation of oxidative stress mechanisms following PCB exposure. Furthermore, PCBs produced concentration-dependent reductions in intracellular dopamine levels and elevated dopamine turnover. Although the intracellular source of ROS remains unknown, these results suggest that sublethal PCB concentrations activate an oxidative stress-related pathway, which potentially disrupts dopamine neuron function.

Modulation of Antioxidant Defense Systems by the Environmental Pesticide Maneb in Dopaminergic Cells

A lack of evidence supporting a role of heritability in the development of idiopathic Parkinson's disease (PD) has implicated exposures to environmental contaminants in the disease etiology. Epidemiological and clinical studies, as well as animal models of the PD phenotype, have consistently linked agrichemical exposure with dopaminergic (DAergic) damage, particularly through oxidative stress mechanisms. Maneb (MB) is a dithiocarbamate (DTC) fungicide that has specifically been implicated to have adverse effects on dopamine (DA) systems, but the role MB plays in modulating the oxidative state of DAergic cells has not previously been described. Since glutathione (GSH) is a major cellular antioxidant, it was hypothesized that exposure to MB would disrupt this system. The current study primarily utilized the PC12 cell line, which displays a catecholaminergic phenotype. Low concentrations of MB (50-1000 ng/ml) had little effect on cell viability, as measured by LDH release. These same concentrations, however, led to increases in GSH and its oxidized form, GSSG. Effects on viability and GSH were correlated to a primary mesencephalic culture system. Furthermore, these effects were markedly different from those observed with the classical oxidative stressor and pesticide, paraquat (PQ). To determine how MB would affect cells in which antioxidant systems were compromised, PC12 cells were treated with L-buthionine-(S,R)-sulfoximine (BSO) to deplete cellular GSH, followed by treatment with MB. Results suggest that following an insult to the GSH antioxidant system, MB can act as an additional insult to the system and prevent the normal recovery of those defenses. Altered protein levels of heme oxygenase-1 (HO-1) further indicated an oxidative stress response elicited by MB in PC12 cells. DAergic neurons, as a population, are inherently vulnerable to oxidative stress, and the disruption of antioxidant systems by the fungicide MB may contribute to the neurodegeneration of these cells, especially with concurrent exposures to other environmentally relevant oxidative stressors, such as PQ.

Aryl hydrocarbon receptor expression and activity in cerebellar granule neuroblasts: implications for development and dioxin neurotoxicity

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent teratogen that produces neurobehavioral abnormalities associated with both cognitive and locomotor systems, yet the precise regional and cellular targets of developmental neurotoxicity remain largely unknown. Most, if not all, TCDD-induced pathology is mediated via binding to the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor that belongs to the basic helix-loop-helix/Per-Arnt-Sim (bHLH/PAS) superfamily. Upon ligand binding, AhR translocates to the nucleus, dimerizes with the AhR nuclear translocator protein (Arnt), and regulates transcription by interaction with dioxin-response elements (DREs) in target genes, most notably specific cytochrome P450 (CYP) family members. To assess whether developing cerebellar granule neuroblasts are potential direct targets for TCDD toxicity, AhR expression and transcriptional activity were examined. AhR and Arnt proteins were present in mouse cerebellum from birth throughout postnatal development. AhR protein levels peaked between postnatal day (PND) 3-10, a critical period for granule neuroblast growth and maturation. Transcriptionally active AhR was detected in immature cerebellar granule cells in a transgenic dioxin-responsive lacZ mouse model after acute TCDD exposure. AhR and Arnt were also expressed in cerebellar granule neuroblast cultures. AhR localized to the nucleus in granule cells 15 min after TCDD treatment. TCCD elicited time-dependent and concentration-dependent increases in CYP1A1 and 1B1 mRNA and protein levels. Moreover, TCDD treatment reduced both thymidine incorporation and granule neuroblast survival in a concentration-dependent manner. These data suggest that (1) granule neuroblasts are direct targets for developmental AhR-mediated TCDD neurotoxicity and (2) TCDD exposure may disrupt granule cell neurogenesis.

2,5-Hexanedione-induced changes in the monomeric neurofilament protein content of rat spinal cord fractions

Quantitative morphometric analyses have demonstrated that axon atrophy is the primary neuropathic feature in the CNS and PNS of rats intoxicated with 2,5-hexanedione (HD). Axon caliber is maintained by the exchange of mobile neurofilament (NF) subunits with the stationary polymer and, therefore, HD might produce atrophy by disrupting cytoskeletal turnover. To evaluate this possibility, groups of rats were exposed to HD at dosing schedules (175 mg/kg x 101 days or 400 mg/kg x 26 days) that produced moderate levels of neurological deficits and prevalent axon atrophy in spinal cord white matter tracts. Lumbar spinal cord regions from HD-intoxicated rats and their age-matched controls were Triton-extracted and separated by differential fractionation into a low-speed, insoluble pellet (P1) of NF polymer and a high-speed supernatant fraction (S2), which presumably contained mobile monomer. Cytoskeletal protein contents (NF-L, -M, -H, and beta-tubulin) in each fraction were determined by immunoblot analysis. Results show that regardless of HD dose-rate, the NF polymer in P1 remained unaffected, although soluble monomer in the S2 fraction was depleted significantly (60-80% reduction). Fractional beta-tubulin contents were inconsistently affected and abnormal higher-molecular-weight NF proteins were detected in the P1 fraction only. Studies with antibodies directed against phosphorylated (RT97) and nonphosphorylated (SMI32) epitopes on NF-H and measurements of corresponding isoelectric range suggested that alterations in phosphorylation were not involved. The selective depletion of Triton-soluble protein suggested that HD adduction of NFs interfered with the dynamic interactions of the polymeric and mobile monomeric pools.

Resistance of immature hippocampus to morphologic and physiologic alterations following status epilepticus or kindling

Seizures in adult rats result in long-term deficits in learning and memory, as well as an enhanced susceptibility to further seizures. In contrast, fewer lasting changes have been found following seizures in rats younger than 20 days old. This age-dependency could be due to differing amounts of hippocampal neuronal damage produced by seizures at different ages. To determine if there is an early developmental resistance to seizure-induced hippocampal damage, we compared the effects of kainic acid (KA)-induced status epilepticus and amygdala kindling on hippocampal dentate gyrus anatomy and electrophysiology, in immature (16 day old) and adult rats. In adult rats, KA status epilepticus resulted in numerous silver-stained degenerating dentate hilar neurons, pyramidal cells in fields CA1 and CA3, and marked numerical reductions in CA3c pyramidal neuron counts (-57%) in separate rats. Two weeks following the last kindled seizure, some, but significantly less, CA3c pyramidal cell loss was observed (-26%). Both KA status epilepticus and kindling in duced mossy-fiber sprouting, as evidenced by ectopic Timm staining in supragranular layers of the dentate gyrus. In hippocampal slices from adult rats, paired-pulse stimulation of perforant path axons revealed a persistent enhancement of dentate granule-cell inhibition following KA status epilepticus or kindling. While seizures induced by KA or kindling in 16-day-old rats were typically more severe than in adults, the immature hippocampus exhibited markedly less KA-induced cell loss (-22%), no kindling-induced loss, no detectable synaptic rearrangement, and no change in dentate inhibition. These results demonstrate that, in immature rats, neither severe KA-induced seizures nor repeated kindled seizures produce the kind of hippocampal damage and changes associated with even less severe seizures in adults. The lesser magnitude of seizure-induced hippocampal alterations in immature rats may explain their greater resistance to long-term effects of seizures on neuronal function, as well as future seizure susceptibility. Conversely, hippocampal neuron loss and altered synaptic physiology in adults may contribute to increased sensitivity to epileptogenic stimuli, spontaneous seizures, and behavioral deficits.

Gamma-diketone peripheral neuropathy III. Neurofilament gene expression

Evidence suggests the morphologic hallmark of gamma-diketone neuropathy is axon atrophy and that this effect is associated with reduced neurofilament (NF) subunit protein content (Toxicol Appl Pharmacol 2000;165:141-7). To investigate the mechanism of diminished NF content, subunit (NF-L, -M and -H) gene expression was quantified in dorsal root ganglion (DRG) of slightly affected and moderately intoxicated groups of rats exposed to 2,5-hexanedione (HD) at one of three daily dosing rates (175, 250 and 400 mg/kg per day). Results show that sensory ganglia from slightly affected rats exhibited no changes in gene expression, whereas at a moderate level of neurotoxicity, each dosing protocol was associated with small but significant reductions (approximately 20%) in mean NF subunit mRNA. This was not a generalized effect on expression of cytoskeletal components in sensory ganglia since tubulin message levels were not affected. Although the observed reduction in NF gene expression might be related to diminished levels of subunit proteins in peripheral nerve, the actual contribution is likely to be minimal. The magnitude of effect was small and did not correspond to the dose-rate dependent effect of HD on respective isotype proteins. The mechanism of gamma-diketone-induced axon atrophy is unknown but might involve local changes in axonal NF phosphorylation and degradation.

Effect of nicotine on cerebellar granule neuron development

To assess the role of nicotinic cholinergic receptors (nAChR) on neuronal maturation, nAChR expression and the direct effects of nAChR activation were examined in cerebellar external granular layer (EGL) precursors isolated in vitro. Treatment of EGL neuroblasts with nicotine elicited a concentration-dependent increase in DNA content and synthesis, implying an increase in cell numbers. Pretreatment of cultures with the nAChR antagonist dihydro-beta-erythroidine (DHBE) attenuated nicotine-induced changes in DNA abundance and synthesis. Furthermore, chronic nicotine treatment for 4-7 days promoted EGL cell survival. Epibatidine but not cytisine stimulated granule neuroblast DNA synthesis and survival. Survival effects mediated by nicotine and epibatidine were attenuated by pretreating cultures with DHBE. Immunocytochemical analysis revealed that EGL neurons possessed alpha3, but not alpha4, nAChR immunoreactivity. Quantitative autoradiography was used to determine which nAChRs are present during the period of granule cell neurogenesis in vivo. On postnatal day 5, the EGL was intensely labelled by [3H]-epibatidine but virtually devoid of [3H]-A85380 binding, suggesting that a high concentration of alpha3 AChRs is present in granule neuroblasts. The pharmacology of [3H]-epibatidine displacement from EGL neurons also suggested an interaction with the alpha3-nAChR subunits. Together these data provide novel evidence that the activation of nAChRs directly affect the development of primary cerebellar neuroblasts and further suggest that the effects are mediated through the alpha3-nAChR subtype.

Rate of neurotoxicant exposure determines morphologic manifestations of distal axonopathy

Exposure to a variety of agricultural, industrial, and pharmaceutical chemicals produces nerve damage classified as a central-peripheral distal axonopathy. Morphologically, this axonopathy is characterized by distal axon swellings and secondary degeneration. Over the past 25 years substantial research efforts have been devoted toward deciphering the molecular mechanisms of these presumed hallmark neuropathic features. However, recent studies suggest that axon swelling and degeneration are related to subchronic low-dose neurotoxicant exposure rates (i.e., mg toxicant/kg/day) and not to the development of neurophysiological deficits or behavioral toxicity. This suggests these phenomena are nonspecific and of uncertain pathophysiologic relevance. This possibility has significant implications for research investigating mechanisms of neurotoxicity, development of exposure biomarkers, design of risk assessment models, neurotoxicant classification schemes, and clinical diagnosis and treatment of toxic neuropathies. In this commentary we will review the evidence for the dose-related dependency of distal axonopathies and discuss how this concept might influence our current understanding of chemical-induced neurotoxicities.

gamma-diketone peripheral neuropathy. II. Neurofilament subunit content

Quantitative morphometric analyses have demonstrated that axon atrophy is the primary neuropathic alteration in peripheral nerve of 2,5-hexanedione (HD)-intoxicated rats (Lehning et al., Toxicol. Appl. Pharmacol. 165, 127-140, 2000). Research suggests that axon caliber is regulated by neurofilament (NF) content and density. Therefore, as a possible mechanism of atrophy, NF subunit (NF-L, -M, and -H) proteins were quantitated in moderately affected rats intoxicated with HD at three daily dosing rates (175, 250, and 400 mg/kg/day). Analyses of subunit protein contents in proximal sciatic nerves indicated uniformly small decreases, which corresponded to minimal changes in axon area occurring in this region. In distal tibial nerve, subunit proteins were decreased substantially (40-70%) when rats were exposed to the 175 and 250 mg/kg/day doses. These reductions in NFs corresponded to significant decreases (approximately 50%) in tibial axon area induced by lower dosing rates. In contrast, 400 mg/kg/day produced similar changes in caliber but smaller reductions (18-25%) in NF-L, -M, and -H levels. This suggests that a decrement in axonal NF content is unlikely to be solely responsible for gamma-diketone-induced axon atrophy and that the corresponding mechanism probably involves additional changes in factors regulating NF density. Analysis of NF content in peripheral nerve also identified the presence of anomolous higher molecular weight NF-H proteins. However, the neurotoxicological significance of these abnormal subunits is uncertain based on their limited occurrence and inconsistent spatiotemporal expression.

Heparin-binding epidermal growth factor-like growth factor in hippocampus: modulation of expression by seizures and anti-excitotoxic action

The expression of heparin-binding epidermal growth factor-like growth factor (HB-EGF), an EGF receptor ligand, was investigated in rat forebrain under basal conditions and after kainate-induced excitotoxic seizures. In addition, a potential neuroprotective role for HB-EGF was assessed in hippocampal cultures. In situ hybridization analysis of HB-EGF mRNA in developing rat hippocampus revealed its expression in all principle cell layers of hippocampus from birth to postnatal day (P) 7, whereas from P14 through adulthood, expression decreased in the pyramidal cell layer versus the dentate gyrus granule cells. After kainate-induced excitotoxic seizures, levels of HB-EGF mRNA increased markedly in the hippocampus, as well as in several other cortical and limbic forebrain regions. In the hippocampus, HB-EGF mRNA expression increased within 3 hr after kainate treatment, continued to increase until 24 hr, and then decreased; increases occurred in the dentate gyrus granule cells, in the molecular layer of the dentate gyrus, and in and around hippocampal pyramidal CA3 and CA1 neurons. At 48 hr after kainate treatment, HB-EGF mRNA remained elevated in vulnerable brain regions of the hippocampus and amygdaloid complex. Western blot analysis revealed increased levels of HB-EGF protein in the hippocampus after kainate administration, with a peak at 24 hr. Pretreatment of embryonic hippocampal cell cultures with HB-EGF protected neurons against kainate toxicity. The kainate-induced elevation of [Ca2+]i in hippocampal neurons was not altered in cultures pretreated with HB-EGF, suggesting an excitoprotective mechanism different from that of previously characterized excitoprotective growth factors. Taken together, these results suggest that HB-EGF may function as an endogenous neuroprotective agent after seizure-induced neural activity/injury.

Opposing actions of the EGF family and opioids: heparin binding-epidermal growth factor (HB-EGF) protects mouse cerebellar neuroblasts against the antiproliferative effect of morphine

Endogenous opioids and opiate drugs of abuse inhibit the proliferation of cerebellar external granular layer (EGL) neuroblasts by mechanisms that are incompletely understood. Opioids do not act alone, rather multiple extracellular factors regulate granule cell neurogenesis and these undoubtedly act in concert with opioids to shape developmental outcome. We examined whether, heparin binding-epidermal growth factor-like growth factor (HB-EGF), a recently described member of the epidermal growth factor (EGF) family, might compete with an inhibitory opioid signal. The results confirmed our ongoing studies that morphine inhibited neuroblast proliferation, while HB-EGF enhanced cell replication. HB-EGF not only counteracted the antiproliferative morphine signal, but invariably enhanced DNA synthesis irrespective of morphine treatment. Our findings suggest that regional and temporal differences in the availability of endogenous HB-EGF may serve to limit the response of EGL neuroblasts to opioids, and HB-EGF may be neuroprotective in opiate drug abuse. If similar responses occur in vivo, then the EGF family and the opioid system may represent distinct and contrasting components of an extracellular signaling system serving to coordinate EGL neurogenesis.

Opioids disrupt Ca2+ homeostasis and induce carbonyl oxyradical production in mouse astrocytes in vitro: transient increases and adaptation to sustained exposure

Pharmacologically distinct subpopulations of astroglia express mu, delta, and/or kappa opioid receptors. Activation of mu, delta, or kappa opioid receptors can destabilize intracellular calcium ([Ca2+]i) in astrocytes leading to cellular hypertrophy and reactive injury. To assess whether acute or sustained opioid exposure might adversely affect astroglial function by disrupting Ca2+ homeostasis or by producing reactive oxygen species, fura-2 and a novel fluorescent-tagged biotin-4-amidobenzoic hydrazide reagent, respectively, were used to detect [Ca2+]i and carbonyl oxidation products within individual murine astrocytes. Acute (3 h) exposure to mu; (H-Tyr-Pro-Phe (N-Me) -D-Pro-NH2; PLO17), delta ([D-Pen2, D-Pen5]-enkephalin), and kappa (trans-(+/-)-3, 4-dichloro-N-methyl-N-[2-(1-pyrr olidinyl) cyclohexyl] benzeneacetamide methanesulfonate; U50,488H) opioid agonists caused significant mean increases in [Ca2+]i and in the levels of oxidative products in astrocytes. In contrast, following 72 h of continuous opioid exposure, [Ca2+]i and carbonyl levels returned to normal, irrespective of opioid treatment. These preliminary findings indicate that opioids initially destabilize [Ca2+]i and increase reactive oxygen species in astrocytes; however, astrocytes later recover and adapt to sustained opioid exposure.

Relationship of lead-induced proteins to stress response proteins in astroglial cells

Astroglial cells are resistant to cell death and morphologic damage following lead (Pb) exposure at concentrations which elicit detrimental effects in neurons. A possible explanation may be that astroglial cells respond to Pb by increasing the expression of specific proteins, such as heat-shock proteins (HSPs), which confer resistance to low levels of Pb. However, there has been relatively limited information regarding the ability of Pb to evoke the synthesis of HSPs. In the current study, pulse-labeling of cultured astroglial proteins with [3H]-leucine was used to evaluate the nature of Pb-induced changes in protein expression. The effect of Pb on newly synthesized proteins was compared to the response elicited by heat-shock and oxidative injury. Immunoblot analysis was utilized to examine alterations in levels of various stress proteins including HSP27, HSP70, HSP90, and heme oxygenase-1 (HO-1). Even though Pb induced the synthesis of proteins with estimated molecular weights of 23 kDa, 32 kDa, 70 kDa, and 90 kDa, the accumulation of HSPs other than HO-1 was not observed. Hyperthermia and treatment with Na arsenite both resulted in enhanced expression of HSP70 and HO-1. In addition, exposure to hydrogen peroxide (H2O2), cadmium (Cd), and lipopolysaccharide (LPS) stimulated a rise in HO-1 levels. Although cellular insult failed to elicit an increase in either HSP27 or HSP90, cultured astroglia expressed readily detectable levels of both these proteins. Furthermore, Pb exposure resulted in the development of crosstolerance to subsequent injury by treatment with either Cd or H2O2. The results of this study indicate that Pb triggers a less conventional stress response in astroglial cells, which may provide enhanced resistance to the toxic effects of Pb.

Induction of newly synthesized proteins in astroglial cells exposed to lead

Although astroglial cells accumulate lead (Pb), they appear to be less sensitive to its overt deleterious effects than are neurons. One possible mechanism in which as rocytes adapt to Pb may be by upregulating the expression of particular genes. This study evaluated the effect of exogenous Pb on proteins synthesized in primary rat astroglial cultures. Following incubations with 0-50 microM Pb acetate cells were radiolabeled with leucine for various time intervals. Proteins were subsequently analyzed using SDS-PAGE followed by fluorography. Changes observed within the first 24 hr resembled a classical stress response and included increased synthesis of proteins with apparent molecular weights of 90, 70, 32-35, and 23 kDa. The most notable change was the enhanced synthesis and abundance of the 23-kDa protein (p23) which persisted throughout a 14-day treatment period, whereas the synthesis of the other proteins declined. This protein did not appear to be induced in rat lung fibroblasts treated with similar concentrations of Pb. Subcellular fractionation indicated that p23 was localized to the cytosol. Treatment with actinomycin D or cycloheximide prior to the addition of Pb precluded induction of p23. Pulse labeling of cells following a 24-hr Pb exposure revealed that p23 continued to be synthesized for 12 but not 24 hr following removal of Pb. Pulse-chase experiments indicated that the protein was stable for at least 18 hr following the removal of Pb. Two-dimensional gel electrophoresis revealed that the 23-kDa Pb-induced protein consisted of multiple-charged species with pl values ranging from 4.3 to 7.8.