Organic and inorganic lead inhibit neurite growth in vertebrate and invertebrate neurons in culture

Sensitivity of neural stem cell survival, differentiation and neurite outgrowth within 3D hydrogels to environmental heavy metals

We investigated the sensitivity of embryonic murine neural stem cells exposed to 10 pM-10 μM concentrations of three heavy metals (Cd, Hg, Pb), continuously for 14 days within 3D collagen hydrogels. Critical endpoints for neurogenesis such as survival, differentiation and neurite outgrowth were assessed. Results suggest significant compromise in cell viability within the first four days at concentrations ≥10 nM, while lower concentrations induced a more delayed effect. Mercury and lead suppressed neural differentiation at as low as 10 pM concentration within 7 days, while all three metals inhibited neural and glial differentiation by day 14. Neurite outgrowth remained unaffected at lower cadmium or mercury concentrations (≤100 pM), but was completely repressed beyond day 1 at higher concentrations. Higher metal concentrations (≥100 pM) suppressed NSC differentiation to motor or dopaminergic neurons. Cytokines and chemokines released by NSCs, and the sub-cellular mechanisms by which metals induce damage to NSCs have been quantified and correlated to phenotypic data. The observed degree of toxicity in NSC cultures is in the order: lead>mercury>cadmium. Results point to the use of biomimetic 3D culture models to screen the toxic effects of heavy metals during developmental stages, and investigate their underlying mechanistic pathways.

Lead-induced stress response in endoplasmic reticulum of astrocytes in CNS

ABSTRACT Lead is one of the most widespread toxicants in the environment, and its neurotoxicity contributes to a major medical issue. Numerous studies have shown that astrocytes are the main sites of Pb deposition in the central nervous system. A large amount of lead depositing in the astrocyte cells would result in the accumulation of unfolded protein in the endoplasmic reticulum (ER), which up-regulates the expression of molecular chaperones and meanwhile inhibits the cell-cycle progression and the transcription of certain proteins. The unfolded protein response (UPR) could down-regulate the expression of protein cyclinD1 and cause the stagnation of cell-cycle in primary-cultured astrocytes of rat. However, lead neither has obvious effects on the expression of C/EBP homologous protein (CHOP) nor achieves cell apoptosis in the progress of lead-induced UPR. When the stagnation of cell-cycle happens, glucose regulated protein of 78 kDa (GRP78) and other chaperones come to themselves to transport a body of unfolded-protein, consequently making cells survive.

Chelation of GRP78 with lead and its localization changes in the astroglia of rats exposed to lead

To observe the chelation of GRP78 with lead (Pb) and its localization changes, astroglial cells from Wistar rat brain were primarily cultured in medium with acetate Pb. The processes were terminated at different time points. The immunoprecipitation (IP) and Western blotting were used for GRP78 purification and expression and the Pb concentration was determined by employing atomic absorption spectrophotometry (AAS). The localization change of GRP78 was observed with colloid gold immunoelectron microscopy. The results showed that the expression of GRP78 was increased significantly in the cells treated with 1.0 micromol/L acetate Pb for 24 h and peaked at 96-192 h (P<0.01), and at the 12th day, the expression of GRP78 began to decrease but was still higher than normal (P<0.05). Pb content started to increase when cells were treated by acetate Pb for 24 h, and the peak appeared at 8 day (P<0.01), and then Pb content decreased gradually, but was still higher than normal (P<0.05). GRP78 protein expression began to remarkably increase when it transferred from ER to the cytosol around the nuclei 24 h after treatment with Pb. It is concluded that GRP78 in astroglia could strongly chelate with Pb ions and it might be a target protein of Pb.

Moderate lead exposure elicits neurotrophic effects in cerebral cortical precursor cells in culture

Lead (Pb) persists as an environmental toxicant despite aggressive environmental and occupational regulation. Neurotoxicological effects of acute Pb poisoning range from subtle cognitive deficits, to clumsiness and ataxia, to coma and seizures. In adult neurotoxicity, reductions of blood Pb levels are often associated with reversal of clinical signs. In children, however, the effects are more likely to endure, with even low levels of chronic Pb exposure correlating with decreasing IQ. These persistent effects likely result from neurodevelopmental insults, such as altered cell survival or maturation, although the mechanisms have not been fully defined. In the present study we define the effects of moderate-level Pb exposure on mammalian neurogenesis using a well-characterized cortical precursor model. Gestational day 14.5 rat cerebral cortical precursor cells were cultured in defined media and cell number, precursor proliferation, apoptosis, and neuritic process outgrowth were assessed following exposure to a range of Pb acetate concentrations. Surprisingly, whereas a concentration of 30 microg/ml Pb acetate was acutely toxic to neurons, concentrations between 1 and 10 microg/ml Pb acetate (approximately 3 microM and 30 microM Pb, respectively) increased cell number: 10 times as many cells exposed to 10 microg/ml Pb were present on day 4 as compared to control. The increase in cell number was not a result of increased proliferation, however, as DNA synthesis did not increase. Rather, Pb exposure maintained the survival of cortical precursors, as the progressive apoptosis occurring under control conditions was markedly reduced by the metal. Additionally, neuritic process initiation and outgrowth increased in a concentration-dependent manner, with processes four times as abundant on day 1 and twice as long on day 2. These results suggest that brief exposure to lead during neurogenesis directly affects cell survival and process development, potentially altering cortical arrangement. Consequently, alterations in neural circuitry may underlie some of the neurological effects of Pb exposure during brain development.

Astroglia as metal depots: molecular mechanisms for metal accumulation, storage and release

The brain is an organ that concentrates metals, and these metals are often localized to astroglia. An examination of metal physiology of brain cells, particularly astroglia, offers insights into the developmental neurotoxicity of certain metals, including lead (Pb), mercury (Hg), manganese (Mn), and copper (Cu). Xenobiotic metals probably accumulate in cells by exploiting the normal functions of proteins that transport and handle essential metals. In addition, essential metals may become toxic by accumulating at levels that exceed the normal metal buffering capacity of the cell. This review considers the uptake, accumulation, storage, and release of two xenobiotic metals, Pb and Hg, as well as two essential nutrient metals that are neurotoxic in high amounts, Mn and Cu. Evidence that each metal accumulates in astroglia is evaluated, together with the mechanisms the host cell may invoke to protect itself from cytoxicity.

Induction of vascular endothelial growth factor in human astrocytes by lead. Involvement of a protein kinase C/activator protein-1 complex-dependent and hypoxia-inducible factor 1-independent signaling pathway

The mechanism(s) underlying lead neurotoxicity are not fully elucidated. cDNA expression microarray analysis identified lead-sensitive genes in immortalized human fetal astrocytes (SV-FHA). Of the represented genes expressed, vascular endothelial growth factor (VEGF) was one of the most sensitive. Lead induced VEGF mRNA 3-fold and VEGF protein approximately 2-fold with maximum mRNA induction following incubation with 10 micrometer lead acetate for 24 h. Phorbol 12-myristate 13-acetate (PMA), a potent protein kinase C (PKC) activator, increased VEGF mRNA 2-fold and PKC inhibition by GF-109203 completely blocked VEGF induction by lead. Expression of dominant-negative PKC-epsilon, but not PKC-alpha, completely inhibited VEGF mRNA induction by lead. Lead activated the transcription factor AP-1 and increased AP-1-dependent luciferase expression >2-fold. Transfection of cells with a c-jun dominant-negative effectively inhibited both AP-1 activation and VEGF mRNA induction by lead. Hypoxia-inducible factor 1 (HIF-1) activity in SV-FHAs was moderately increased by lead (86%) and PMA (96%). Pretreatment with GF-109203 completely inhibited these effects of lead and PMA. However, lead did not alter HIF-1-dependent luciferase expression and a HIF-1alpha dominant-negative had no effects on the induction of VEGF mRNA by lead. These findings indicate that lead induces VEGF expression in SV-FHAs via a PKC/AP-1-dependent and HIF-1-independent signaling pathway.

Lead targets GRP78, a molecular chaperone, in C6 rat glioma cells

Exposure to potentially neurotoxic levels of lead (Pb) occurs in about 9% of American children under 6 years of age. Astroglia in the brain serve as a Pb depot, sequestering Pb and preventing its contact with the more sensitive neurons. Astroglia have the capacity to adapt to Pb exposure, and as such are able to tolerate relatively high intracellular Pb accumulation. This tolerance mechanism has yet to be defined in biochemical terms. In the present study, we present evidence that glucose-regulated protein (GRP78), a molecular chaperone in the ER, participates directly or indirectly in the tolerance mechanism. Exposure of cultured C6 rat glioma cells, an astroglia-like cell line, to 1 microM Pb acetate for 1 week raised the intracellular levels of two proteins, one of which was identified by sequence analysis as GRP78. GRP78 accumulation started within 1 day and progressed with time of exposure. Studies in vitro showed that GRP78 bound tightly to affinity columns with Pb(2+) as the affinity ligand and bound weakly when either Zn(2+) or Ni(2+) replaced the Pb(2+). The reduced form of GSH and BSA did not compete with GRP78 to chelate Pb(2+). However, the heavy metal binding domain (HMB) of Menkes protein competed with GRP78 for chelating Pb(2+). The data provide evidence that GRP78 may be a component of the Pb tolerance mechanism through its direct interaction with Pb(2+). Its increased synthesis could be part of the adaptive response to Pb exposure.

In vitro neurotoxic effects of 1 GeV/n iron particles assessed in retinal explants

The heavy ion component of the cosmic radiation remains problematic to the assessment of risk in manned space flight. The biological effectiveness of HZE particles has yet to be established, particularly with regard to nervous tissue. Using heavy ions accelerated at the AGS of Brookhaven National Laboratory, we study the neurotoxic effects of iron particles. We exposed retinal explants, taken from chick embryos, to determine the dose response relationships for neurite outgrowth. Morphometric techniques were used to evaluate the in vitro effects of 1 GeV/a iron particles (LET 148 keV/micrometer). Iron particles produced a dose-dependent reduction of neurite outgrowth with a maximal effect achieved with a dose of 100 cGy. Doses as low as 10-50 cGy were able to induce reductions of the neurite outgrowth as compared to the control group. Neurite generation is a more sensitive parameter than neurite elongation, suggesting different mechanism of radiation damage in our model. These results showed that low doses/fluences of iron particles could impair the retinal ganglion cells' capacity to generate neurites indicating the highly neurotoxic capability of this heavy charged particle.

Developmental effects of chronic low-level lead exposure on voltage-gated calcium channels in brain synaptosomes obtained from the neonatal and the adult rats

Effects of chronic low level (1 mg/kg/day) lead exposure were studied on (1) the density and the binding properties of L, N, and P type voltage-gated Ca2+ influx channels (VGCCs), and (2) the depolarization-induced rise in [Ca2+]i in synaptosomes obtained from the brains of the neonatal (postnatal-day-5) and the adult (postnatal-week-20) rats. Lead exposure started prenatally and continued for either up to postnatal-day-5 or up to postnatal-week-20. The KD and the Bmax values for the binding of nifedipine (antagonist of L type channels), omega-CgTx (a specific antagonist of N type channels) and omega-AgaTx (antagonist of P type channels) to VGCCs in the neonatal samples were less then those in the adult samples. Depolarization increased (1) the density and the antagonist binding-affinity of VGCCs and (2) increased [Ca2+]i in both the neonatal and the adult samples. The depolarization-induced increase in [Ca2+]i in the neonatal samples was lower than that in the adult samples. Chronic low-level lead exposure decreased the densities of L, N, and P type VGCCs and attenuated the depolarization-induced increase in [Ca2+]i in synaptosomes. Chronic low-level lead exposure, however, did not affect the relative ratio of L, N, and P channels, the affinity of VGCCs for antagonists, and the depolarization-induced increase in antagonist binding to VGCCs in synaptosomes. Thus chronic low-level lead exposure during early development and adulthood may decrease the synthesis of VGCCs but not their antagonist binding-affinity in both the neonatal and the adult rats.

The influence of Pb2+ on expression of acetylcholinesterase and the acetylcholine receptor

This paper examines the influence of inorganic lead (Pb2+) on the presence of acetylcholinesterase (AchE) molecular forms and the acetylcholine receptor (AchR) in two types of excitable tissue, primary cultures of skeletal muscle and neural retina from embryonic chick. Treatment of skeletal muscle with Pb2+ is observed to cause reductions in the 5/7S and 19S but not the 11.4S molecular forms of AchE. The reductions are dose-dependent, requiring submicromolar concentrations, slow in onset, requiring incubation times greater than 24 hr, and tissue specific, being pronounced in skeletal muscle but absent from neural retina. Significantly, the reductions in AchE occur without corresponding reductions in amounts of AchR and without reduction in activity of protein kinase C (PKC). These studies illustrate a tissue-specific action of inorganic lead that is not mediated through PKC.

Cholinergic denervation-like changes in rat hippocampus following developmental lead exposure

We investigated the effects of developmental lead exposure from embryonic day 16 (E16) through postnatal day 28 (PN28), on cholinergic and catecholaminergic markers in the septohippocampal pathway in rats through fourth month of age. Lead exposure resulted in a persistent 30-40% reduction of [3H]hemicholinium-3 ([3H]HC-3) binding in the hippocampus through PN120, and 20-30% reduction of septal and hippocampal choline acetyltransferase (ChAT) activity which persisted through PN84 but returned to control levels in both septum and hippocampus at PN112. The muscarinic ligand [3H]quinuclidinyl benzylate ([3H]QNB) binding was reduced in the septum at PN28 but did not differ significantly from controls at PN56-PN112. Neither short- nor long-term effects of Pb exposure on [3H]QNB binding were seen in the hippocampus. Similar to the effects of fimbria-fornix transection, Pb exposure resulted in a long-term 50-90% increase of tyrosine hydroxylase(TH) activity in the hippocampus, although neither treatment affected TH activity in the septum. The lead-induced increase in hippocampal TH was significantly attenuated by superior cervical ganglionectomy. It is concluded that the effects of perinatal lead exposure resemble in several respects those seen following surgical disruption of the septohippocampal pathway in adult animals. The denervation-like effects in the hippocampus may be an important factor in long-term learning and cognitive impairments following developmental exposure to low-levels of lead.

The effects of triethyl lead on the development of hippocampal neurons in culture

Triethyl lead is the major metabolite of tetraethyl lead, which is used in industrial processes and as an antiknock additive to gasoline. We tested the hypothesis that low levels of triethyl lead (0.1 nmol/L to 5 mumol/L) interfere with the normal development of cultured E18 rat hippocampal neurons, possibly through increases in intracellular free calcium ion concentration, [Ca2+]in. The study assessed survival and differentiation using morphometric analysis of individual neurons. We also looked at short-term (up to 3.75-h) changes in intracellular calcium using the calcium-sensitive dye fura-2. Survival of neurons was significantly reduced at 5 mumol/L, and overall production of neurites was reduced at > or = 2 mumol/L. The length of axons and the number of axons and dendrites were reduced at > or = 1 mumol/L. Neurite branching was inhibited at 10 nmol/L for dendrites and 100 nmol/L for axons. Increases in intracellular calcium were observed during a 3.75-h exposure of newly plated neurons to 5 mumol/L triethyl lead. These increases were prevented by BAPTA-AM; which clamps [Ca2+]in at about 100 nmol/L. Culturing neurons with BAPTA-AM and 5 mumol/L triethyl lead did not reverse the effects of triethyl lead, suggesting that elevation of [Ca2+]in is not responsible for decreases in survival and neurite production. Triethyl lead has been shown to disrupt cytoskeletal elements, particularly neurofilaments, at very low levels, suggesting a possible mechanism for its inhibition of neurite branching at nanomolar concentrations.

Inhibition of astroglia-induced endothelial differentiation by inorganic lead: a role for protein kinase C

Microvascular endothelial function in developing brain is particularly sensitive to lead toxicity, and it has been hypothesized that this results from the modulation of protein kinase C (PKC) by lead. We examined the effects of inorganic lead on an in vitro model of central nervous system endothelial differentiation in which astroglial cells induce central nervous system endothelial cells to form capillary-like structures. Capillary-like structure formation within C6 astroglial-endothelial cocultures was inhibited by lead acetate with 50% maximal inhibition at 0.5 microM total lead. Inhibition was independent of effects on cell viability or growth. Under conditions that inhibited capillary-like structure formation, we found that lead increased membrane-associated PKC in both C6 astroglial and endothelial cells. Prolonged exposure of C6 cells to 5 microM lead for up to 16 h resulted in a time-dependent increase in membranous PKC as determined by immunoblot analysis. Membranous PKC increased after 5-h exposures to as little as 50 nM lead and was maximal at approximately 1 microM. Phorbol esters were used to determine whether PKC modulation was causally related to the inhibition of endothelial differentiation by lead. Phorbol 12-myristate 13-acetate (10 nM) inhibited capillary-like structure formation by 65 +/- 5%, whereas 4 alpha-phorbol 12,13-didecanoate was without effect. These findings suggest that inorganic lead induces cerebral microvessel dysfunction by interfering with PKC modulation in microvascular endothelial or perivascular astroglial cells.