Chronic low-level Pb exposure during development decreases the expression of the voltage-dependent anion channel in auditory neurons of the brainstem

doi:10.1016/j.neuro.2010.08.004

NeuroToxicology

Volume 31, Issue 6, December 2010, Pages 662-673

https://doi.org/10.1016/j.neuro.2010.08.004 Get rights and content

Abstract

Lead (Pb) exposure is a risk factor for neurological dysfunction. How Pb produces these behavioral deficits is unknown, but Pb exposure during development is associated with auditory temporal processing deficits in both humans and animals. Pb disrupts cellular energy metabolism and efficient energy production is crucial for auditory neurons to maintain high rates of synaptic activity. The voltage-dependent anion channel (VDAC) is involved in the regulation of mitochondrial physiology and is a critical component in controlling mitochondrial energy production. We have previously demonstrated that VDAC is an in vitro target for Pb, therefore, VDAC may represent a potential target for Pb in the auditory system. In order to determine whether Pb alters VDAC expression in central auditory neurons, CBA/CaJ mice (n = 3–5/group) were exposed to 0.01 mM, or 0.1 mM Pb acetate during development via drinking water. At P21, immunohistochemistry reveals a significant decrease for VDAC in neurons of the Medial Nucleus of the Trapezoid Body. Western blot analysis confirms that Pb results in a significant decrease for VDAC. Decreases in VDAC expression could lead to an upregulation of other cellular energy producing systems as a compensatory mechanism, and a Pb-induced increase in brain type creatine kinase is observed in auditory regions of the brainstem. In addition, comparative proteomic analysis shows that several proteins of the glycolytic pathway, the phosphocreatine circuit, and oxidative phosphorylation are also upregulated in response to developmental Pb exposure. Thus, Pb-induced decreases in VDAC could have a significant effect on the function of auditory neurons.

Introduction

Lead (Pb) is a naturally occurring toxic heavy metal that has been widely distributed throughout the environment due to its extensive use in a variety of industrial procedures and products. Environmental Pb enters biological systems through ingestion and respiration (Toscano and Guilarte, 2005) and continues to be a serious problem in many parts of the U.S. The U.S. Centers for Disease Control and Prevention have determined that blood Pb levels of 10 μg/dl should prompt public health actions, however recent studies in humans and animals have shown that the neurotoxic effects of Pb occurs at even lower blood Pb levels (Gilbert and Weiss, 2006). Low-level Pb exposure is a risk factor for learning disabilities and attention deficit hyperactivity disorder (ADHD) (Lidsky and Schneider, 2003, Braun et al., 2006). Many children with these behavioral syndromes also demonstrate deficits in auditory temporal processing, suggesting a disturbing link between developmental Pb exposure, behavioral dysfunction and auditory temporal processing (Gray, 1999, Otto and Fox, 1993, Lurie et al., 2006, Breier et al., 2003, Montgomery et al., 2005).

Auditory temporal processing involves the processing of central auditory neuronal signals in time and space, allowing the listener to resolve complex sounds and to recognize specific signals within a noise background. Children exposed to Pb show decreased performance in tests requiring appropriately timed reactions and demonstrate increased latencies in brainstem auditory evoked potentials (Finkelstein et al., 1998, Holdstein et al., 1986). In animals, chickens exposed to low levels of Pb show deficits in backward masking, a test of central auditory temporal processing (Gray, 1999). We have found that mice exposed to low levels of Pb demonstrate alterations of two measures of central auditory brainstem function, the brainstem conduction time and gap encoding in the inferior colliculus (Jones et al., 2008). Taken together, these studies suggest that the auditory system is a target for Pb.

We have recently demonstrated that the voltage-dependent anion channel (VDAC) is a novel target for Pb in CNS neurons in vitro (Prins et al., 2010). VDAC is an ion channel located in the mitochondrial outer membrane that plays a central role in regulating energy metabolism in neurons by maintaining cellular ATP levels and regulating calcium buffering (Shoshan-Barmatz et al., 2006, Shoshan-Barmatz and Gincel, 2003). In vitro exposure to lower levels of Pb results in decreased VDAC transcription and expression in two different neuronal cell lines (Prins et al., 2010). Further, the decrease in VDAC is correlated with a decrease in cellular ATP levels, suggesting a connection between decreased VDAC expression and decreased cellular ATP levels. It is not known whether developmental Pb exposure results in loss of VDAC protein in brainstem auditory neurons in vivo. Because auditory neurons have high and fluctuating energy requirements (Trussell, 1999, Hiel et al., 1996), a decrease in VDAC expression could alter the function of auditory neurons by disrupting energy buffering systems within these neurons.

In order to determine if VDAC represent a potential target for Pb in central auditory neurons, the current study examines the expression of VDAC in the brainstem following developmental Pb exposure. We find that chronic low-level Pb exposure during development results in the decreased expression of VDAC in the murine auditory brainstem. Immunohistochemical analysis of the auditory brainstem nucleus, the Medial Nucleus of the Trapezoid Body (MNTB), demonstrates that neurons in the MNTB show a significant decrease in VDAC staining following Pb exposure. In addition, western blot analysis of a ventral brainstem region (VBS) containing several auditory nuclei, including the MNTB, and the medial and lateral superior olivary nuclei, reveal a significant decrease in VDAC expression.

A comparative proteomic analysis of the VBS was then conducted to determine if other energy pathways, such as the glycolytic pathway, were upregulated in response to the Pb-induced decrease in VDAC. If decreased VDAC levels result in the decreased production of ATP, then one would expect to see an increase in other cellular energy producing systems to compensate. We found that several proteins of the glycolytic pathway, the phosphocreatine circuit, and oxidative phosphorylation were upregulated in response to developmental Pb exposure.

Section snippets

Chronic Pb exposure

Breeding pairs of CBA/CaJ mice were obtained from The Jackson Laboratory (Bar Harbor, Maine) and maintained in microisolator units in the University of Montana specific pathogen free animal facility. Cages, bedding, and food were sterilized by autoclaving and mice were handled with aseptic gloves. Mice were allowed food and water ad libitum. All animal use was in accordance with NIH and University of Montana IACUC (Institutional Animal Care and Use Committee) guidelines. Thirteen breeding pairs

Mouse blood lead levels

The present study used three different doses of Pb in the drinking water; the no Pb control, low Pb (0.01 mM), and high Pb (0.1 mM). Blood lead levels (mean ± SEM) of the mice were as follows: no Pb controls (<1.38 ± 0.14 μg/dl), low Pb (8.0 ± 0.4 μg/dl), and high Pb (42.3 ± 1.97 μg/dl). Neither the low nor the high Pb dose resulted in any change in body size and weight, and thus were considered sub-toxic doses. In addition, the blood lead levels of our high Pb animals have been commonly used to demonstrate

Discussion

The current study demonstrates that VDAC1 is a novel target of Pb in central auditory neurons and shows a decrease (2.1-fold by proteomic analysis) with Pb exposure. In addition, Pb appears to primarily affect energy buffering systems within the auditory region of the brainstem. Our immunohistochemical analysis reveals a significant decrease in VDAC expression in medial nucleus of the trapezoid body (MNTB) neurons following Pb exposure. Our western blot analysis demonstrates that decreased VDAC

Conclusions

The current study identifies VDAC as a novel target of Pb in auditory brainstem neurons. The decrease in VDAC expression is particularly notable since the protein plays a central role in regulating cellular energy metabolism and VDAC is thought to play an important role in neurons with high-energy requirements. Further, these results support the hypothesis that Pb preferentially impacts energy production systems in the auditory brainstem. The physiological consequences resulting from disrupted

Funding

This work was supported by the National Institutes of Health [P20 RR17670, P20 R015583 D.I.L].

Conflicts of interest statement

The author's declare that there are no conflicts of interest.

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Using auditory brainstem evoked responses (ABR) in human subjects, some investigators have localized Pb-induced injury to the auditory brainstem pathways [14–19]. Similarly, experimental studies in Pb-exposed animals have reported neural damage at the level of the auditory brainstem [20–22]. For example, recent experimental studies in animals that were systematically exposed to Pb have shown specific areas of neuronal damage in the superior olivary complex, an integral component of the bilateral ascending auditory tracts and nuclei in the brainstem [20,23].
The purpose of this study was to investigate the neurological integrity and physiological status of the auditory brainstem tracts and nuclei in children with chronic lead (Pb) exposure using non-invasive acoustic stapedius reflex (ASR) measurements of afferent and efferent-neuromuscular auditory function. Following audiological examinations, uncrossed (ipsilateral) and crossed (contralateral) brainstem ASR responses were evoked by pure tone (500, 1000, and 2000 Hz), and broadband noise (bandwidth: 125–4000 Hz) stimulus activators. The ASR threshold (ASRT), amplitude growth, and decay/fatigue were measured by conventional clinical middle ear immittance methods in a group of Andean children (age range: 2–18 years) with a history of chronic environmental Pb exposure from occupational Pb glazing. Blood lead (PbB) levels of the study group (n = 117) ranged from 4.0 to 83.7 μg/dL with a mean PbB level of 33.5 μg/dL (SD: 23.6; median: 33.0: CDC III Classification). The PbB distribution data indicated that 77.8% (n = 91) of the children had PbB levels greater than the CDC action line of 10 μg/dL. Repeatable, normal ASRTs were elicited for ipsilateral (mean: ≤ 90 dB HL) and contralateral (mean: ≤ 97 dB HL) stimulation for each acoustic activator. Spearman Rho correlation analysis indicated no significant association between PbB level and ipsilateral or contralateral ASRT for any of the stimulus activators. The ASR amplitude growth results showed typical growth functions with no Pb-associated aberrations. No statistical association was found between ASR decay/adaptation (ASRD) and PbB level for any of the stimulus activators. The results of stapedius muscle reflex testing using several stimulus activators showed no significant relationship between PbB level and the physiological integrity of the auditory brainstem mediated ASR responses in children with chronic Pb exposure and elevated PbB levels.

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Chronic low-level Pb exposure during development decreases the expression of the voltage-dependent anion channel in auditory neurons of the brainstem

Abstract

Introduction

Section snippets

Chronic Pb exposure

Mouse blood lead levels

Discussion

Conclusions

Funding

Conflicts of interest statement

Brain Res Brain Res Rev

Brain Res Bull

Brain Res Brain Res Rev

Neurotoxicology

Hear Res

Biochim Biophys Acta

Neurotoxicology

Brain Lang

Biochim Biophys Acta

Brain Res Brain Res Rev

Toxicol Appl Pharmacol

Biochim Biophys Acta

Altered expression and localization of creatine kinase B, heterogeneous nuclear ribonucleoprotein F, and high mobility group box 1 protein in the nuclear matrix associated with colon cancer

Cancer Res

The creatine–creatine phosphate energy shuttle

Annu Rev Biochem

Exposures to environmental toxicants and attention deficit hyperactivity disorder in U.S. children

Environ Health Perspect

Auditory temporal processing in children with specific reading disability with and without attention deficit/hyperactivity disorder

J Speech Lang Hear Res

Phosphotransfer networks and cellular energetics

J Exp Biol

Chronic low-level lead exposure affects the monoaminergic system in the mouse superior olivary complex

J Comp Neurol

Chronic developmental lead exposure and hippocampal long-term potentiation: biphasic dose-response relationship

Neurotoxicology

Effect of inorganic lead on rat brain mitochondrial respiration and energy production

J Neurochem

Early lead exposure affects auditory temporal processing in chicks

J Environ Med