Calcium/calmodulin-dependent protein kinase II activity and expression are altered in the hippocampus of Pb2+-exposed rats

Potential of Quercetin to Protect Cadmium Induced Cognitive Deficits in Rats by Modulating NMDA-R Mediated Downstream Signaling and PI3K/AKT-Nrf2/ARE Signaling Pathways in Hippocampus

Exposure to cadmium, a heavy metal distributed in the environment is a cause of concern due to associated health effects in population around the world. Continuing with the leads demonstrating alterations in brain cholinergic signalling in cadmium induced cognitive deficits by us; the study is focussed to understand involvement of N-Methyl-D-aspartate receptor (NMDA-R) and its postsynaptic signalling and Nrf2-ARE pathways in hippocampus. Also, the protective potential of quercetin, a polyphenolic bioflavonoid, was assessed in cadmium induced alterations. Cadmium treatment (5 mg/kg, body weight, p.o., 28 days) decreased mRNA expression and protein levels of NMDA receptor subunits (NR1, NR2A) in rat hippocampus, compared to controls. Cadmium treated rats also exhibited decrease in levels of NMDA-R associated downstream signalling proteins (CaMKIIα, PSD-95, TrkB, BDNF, PI3K, AKT, Erk1/2, GSK3β, and CREB) and increase in levels of SynGap in hippocampus. Further, decrease in protein levels of Nrf2 and HO1 associated with increase in levels of Keap1 exhibits alterations in Nrf2/ARE signalling in hippocampus of cadmium treated rats. Degeneration of pyramidal neurons in hippocampus was also evident on cadmium treatment. Simultaneous treatment with quercetin (25 mg/kg body weight p.o., 28 days) was found to attenuate cadmium induced changes in hippocampus. The results provide novel evidence that cadmium exposure may disrupt integrity of NMDA receptors and its downstream signaling targets by affecting the Nrf2/ARE signaling pathway in hippocampus and these could contribute in cognitive deficits. It is further interesting that quercetin has the potential to protect cadmium induced changes by modulating Nrf2/ARE signaling which was effective to control NMDA-R and PI3K/AKT cell signaling pathways.

Developmental Pb exposure increases AD risk via altered intracellular Ca2+ homeostasis in hiPSC-derived cortical neurons

Exposure to environmental chemicals such as lead (Pb) during vulnerable developmental periods can result in adverse health outcomes later in life. Human cohort studies have demonstrated associations between developmental Pb exposure and Alzheimer's disease (AD) onset in later life which were further corroborated by findings from animal studies. The molecular pathway linking developmental Pb exposure and increased AD risk, however, remains elusive. In this work, we used human iPSC-derived cortical neurons as a model system to study the effects of Pb exposure on AD-like pathogenesis in human cortical neurons. We exposed neural progenitor cells derived from human iPSC to 0, 15, and 50 ppb Pb for 48 h, removed Pb-containing medium, and further differentiated them into cortical neurons. Immunofluorescence, Western blotting, RNA-sequencing, ELISA, and FRET reporter cell lines were used to determine changes in AD-like pathogenesis in differentiated cortical neurons. Exposing neural progenitor cells to low-dose Pb, mimicking a developmental exposure, can result in altered neurite morphology. Differentiated neurons exhibit altered calcium homeostasis, synaptic plasticity, and epigenetic landscape along with elevated AD-like pathogenesis markers, including phosphorylated tau, tau aggregates, and Aβ42/40. Collectively, our findings provide an evidence base for Ca dysregulation caused by developmental Pb exposure as a plausible molecular mechanism accounting for increased AD risk in populations with developmental Pb exposure.

Mechanism of autophagy mediated by IGF-1 signaling pathway in the neurotoxicity of lead in pubertal rats

Lead can damage neuron synapses in the hippocampus and cause synaptic plasticity losses, and learning, memory, and intelligence impairments. Previous studies have focused on the functional and structural plasticity of hippocampal synapses; however, the specific molecular mechanisms behind such impairments are not fully understood. This study aimed to elucidate the molecular mechanisms of cognitive impairment in rats following chronic lead exposure and mitigate or prevent lead toxicity in the central nervous system. We found that lead exposure caused significant damage to rat nervous systems, that is, compared with the control group, the lead treatment group had more autophagosomes in their hippocampal neurons; lower serum and hippocampal IGF-1 levels; lower hippocampal IGF-1, IGF-1R, PI3K, Akt, and mTOR gene expression; and upregulated hippocampal autophagy-associated proteins levels. Brain stereotactic technology was used to conduct autophagy inhibitor in vivo intervention experiments, and the results of these experiments suggest that the autophagy inhibitor DC661 inhibited lead-exposure-induced autophagy and autophagy-related gene expression in the rat hippocampus, possibly through activation of the IGF-1 pathway. Overall, our findings suggest that lead might activate hippocampal autophagy through the IGF-1/PI3K/Akt/mTOR signaling pathway. Therefore, this study provides a novel molecular mechanism underlying developmental toxicity in pubertal rats induced by lead exposure and provides a new target for anticipation and reversal of such neurotoxicity.

The Calcium/Calmodulin-Dependent Kinases II and IV as Therapeutic Targets in Neurodegenerative and Neuropsychiatric Disorders

CaMKII and CaMKIV are calcium/calmodulin-dependent kinases playing a rudimentary role in many regulatory processes in the organism. These kinases attract increasing interest due to their involvement primarily in memory and plasticity and various cellular functions. Although CaMKII and CaMKIV are mostly recognized as the important cogs in a memory machine, little is known about their effect on mood and role in neuropsychiatric diseases etiology. Here, we aimed to review the structure and functions of CaMKII and CaMKIV, as well as how these kinases modulate the animals’ behavior to promote antidepressant-like, anxiolytic-like, and procognitive effects. The review will help in the understanding of the roles of the above kinases in the selected neurodegenerative and neuropsychiatric disorders, and this knowledge can be used in future drug design.

Cognitive Impairment Induced by Lead Exposure during Lifespan: Mechanisms of Lead Neurotoxicity

Lead (Pb) is considered a strong environmental toxin with human health repercussions. Due to its widespread use and the number of people potentially exposed to different sources of this heavy metal, Pb intoxication is recognized as a public health problem in many countries. Exposure to Pb can occur through ingestion, inhalation, dermal, and transplacental routes. The magnitude of its effects depends on several toxicity conditions: lead speciation, doses, time, and age of exposure, among others. It has been demonstrated that Pb exposure induces stronger effects during early life. The central nervous system is especially vulnerable to Pb toxicity; Pb exposure is linked to cognitive impairment, executive function alterations, abnormal social behavior, and fine motor control perturbations. This review aims to provide a general view of the cognitive consequences associated with Pb exposure during early life as well as during adulthood. Additionally, it describes the neurotoxic mechanisms associated with cognitive impairment induced by Pb, which include neurochemical, molecular, and morphological changes that jointly could have a synergic effect on the cognitive performance.

Effects and Molecular Mechanism of L-Type Calcium Channel on Fluoride-Induced Kidney Injury

This study aimed to investigate the role and molecular mechanism of L-type calcium channel (LTCC) on fluoride exposure-induced kidney injury. Subchronic and chronic fluoride exposures were included in the experiment. Each part contained 140 ICR male mice. They were randomly divided into 7 groups: control group, high-fluoride group (NaF 30 mg/L), low-fluoride group (NaF 5 mg/L), high/low-fluoride + agonist (FPL64176) group, high/low-fluoride + inhibitor (nifedipine) group. One week before the end of fluoride exposure, each mouse in the fluoride exposure group was injected intraperitoneally with LTCC agonist (FPL64176) or inhibitor (nifedipine) (5 mg/kg day). The apoptosis of kidney cell was observed by TUNEL, and the protein expression levels of Cav1.2 and CaM, CaMKII, Bcl-2, and Bax were detected by Western blot. Compared with the control group, the protein expression levels of Cav1.2, CaM, and Bax significantly increased, and those of CaMKII and Bcl-2 significantly decreased, the ratio of Bax/Bcl-2 also significantly increased, and the number of apoptotic kidney cells significantly increased in the high/low-fluoride group and in the high/low-fluoride + agonist group. The above indicators and fluoride exposure concentrations showed in time- and dose-dependent changes. Compared with the high/low-fluoride + agonist group, the protein expression level of the molecular in the kidney cells above mentioned was significantly opposite and the number of apoptotic kidney cells significantly decreased in the high/low-fluoride + inhibitor group. In conclusion, LTCC mediates the kidney injury induced by fluoride exposure in mice. Fluoride exposure induced abnormal expression of the Cav1.2 protein, Ca²⁺ signal transduction pathway, and apoptosis-regulated proteins, which is one of the molecular mechanisms. Nifedipine may be a new and effective anti-fluoride drug.

Defining potential roles of Pb(2+) in neurotoxicity from a calciomics approach

Metal ions play crucial roles in numerous biological processes, facilitating biochemical reactions by binding to various proteins. An increasing body of evidence suggests that neurotoxicity associated with exposure to nonessential metals (e.g., Pb(2+)) involves disruption of synaptic activity, and these observed effects are associated with the ability of Pb(2+) to interfere with Zn(2+) and Ca(2+)-dependent functions. However, the molecular mechanism behind Pb(2+) toxicity remains a topic of debate. In this review, we first discuss potential neuronal Ca(2+) binding protein (CaBP) targets for Pb(2+) such as calmodulin (CaM), synaptotagmin, neuronal calcium sensor-1 (NCS-1), N-methyl-d-aspartate receptor (NMDAR) and family C of G-protein coupled receptors (cGPCRs), and their involvement in Ca(2+)-signalling pathways. We then compare metal binding properties between Ca(2+) and Pb(2+) to understand the structural implications of Pb(2+) binding to CaBPs. Statistical and biophysical studies (e.g., NMR and fluorescence spectroscopy) of Pb(2+) binding are discussed to investigate the molecular mechanism behind Pb(2+) toxicity. These studies identify an opportunistic, allosteric binding of Pb(2+) to CaM, which is distinct from ionic displacement. Together, these data suggest three potential modes of Pb(2+) activity related to molecular and/or neural toxicity: (i) Pb(2+) can occupy Ca(2+)-binding sites, inhibiting the activity of the protein by structural modulation, (ii) Pb(2+) can mimic Ca(2+) in the binding sites, falsely activating the protein and perturbing downstream activities, or (iii) Pb(2+) can bind outside of the Ca(2+)-binding sites, resulting in the allosteric modulation of the protein activity. Moreover, the data further suggest that even low concentrations of Pb(2+) can interfere at multiple points within the neuronal Ca(2+) signalling pathways to cause neurotoxicity.

Expression of calmodulin-related genes in lead-exposed mice

The toxic metal lead is a widespread environmental polutant that can adversely affect human health. However, the underlying mechanisms of lead-induced toxicity are still largely unknown. The mechanism of lead toxicity was presumed to involve cross reaction between Pb²⁺ and Ca²⁺ with calmodulin dependent systems. The aim of the present study was thus to identify differential expression of calmodulin-related genes in the spleen of lead-exposed mice. We performed microarray analysis to identify differentially expressed genes. RNAs from spleen tissue of lead exposed animals (n=6) and controls (n=6) were converted to labeled cRNA and hybridized to Illumina mouse WG-6_v2_Bead Chip. Expression profiles were analyzed using Illumina BeadStudio Application. Real-time RT-PCR was conducted to validate the microarray data. By microarray analysis 5 calmodulin-related genes (MAP2K6, CAMKK2, CXCR4, PHKA2, MYLK) were found to be differently expressed in lead exposed compared with control mice (p<0.05). The results of Real-time RT-PCR showed that MAP2K6 and CAMKK2 were up-regulated and CXCR4 was down-regulated in lead exposure, but there were no significant differences in PHKA2 and MYLK expression between the lead exposed and control group. These results show that lead exposure produced significant changes in expression of a variety of genes in the spleen and can affect calmodulin-related gene expression.

The Ever-Changing Morphology of Hippocampal Granule Neurons in Physiology and Pathology

Newborn neurons are continuously added to the hippocampal dentate gyrus throughout adulthood. In this review, we analyze the maturational stages that newborn granule neurons go through, with a focus on their unique morphological features during each stage under both physiological and pathological circumstances. In addition, the influence of deleterious (such as schizophrenia, stress, Alzheimer's disease, seizures, stroke, inflammation, dietary deficiencies, or the consumption of drugs of abuse or toxic substances) and neuroprotective (physical exercise and environmental enrichment) stimuli on the maturation of these cells will be examined. Finally, the regulation of this process by proteins involved in neurodegenerative and neurological disorders such as Glycogen synthase kinase 3β, Disrupted in Schizophrenia 1 (DISC-1), Glucocorticoid receptor, pro-inflammatory mediators, Presenilin-1, Amyloid precursor protein, Cyclin-dependent kinase 5 (CDK5), among others, will be evaluated. Given the recently acquired relevance of the dendritic branch as a functional synaptic unit required for memory storage, a full understanding of the morphological alterations observed in newborn neurons may have important consequences for the prevention and treatment of the cognitive and affective alterations that evolve in conjunction with impaired adult hippocampal neurogenesis.

The effects of preweaning manganese exposure on spatial learning ability and p-CaMKIIα level in the hippocampus

BACKGROUND

The effects and mechanisms of preweaning Manganese (Mn) exposure on cognitive dysfunction remain unclear.

OBJECTIVE

This study evaluated the effects of preweaning Mn exposure on spatial learning and memory as well as the protein expression of CaMKIIα and p-CaMKIIα.

METHODS

We treated neonate rats with Mn(2+) doses of 0 (control group), 10, 20 and 30mg of Mn(2+) per kg body weight (Mn-exposed groups) over postnatal day (PND) 1-21 by intraperitoneal injection. The ability of spatial learning and memory was tested on PND 22 using the Morris water maze (MWM), while the protein expressions of CaMKIIα and p-CaMKIIα in the hippocampus were evaluated by Western blotting. The levels of Mn in the blood and hippocampus were measured by inductively coupled plasma-mass spectrometry (ICP-MS).

RESULTS

The rats in Mn-exposed groups showed a significant delay in spatial learning ability on the third day of the MWM without dose-dependent differences, but there was no effect on the spatial memory ability. p-CaMKIIα, but not CaMKIIα protein expression significantly reduced in the Mn-exposed group.

CONCLUSION

These findings suggested that the inhibition of p-CaMKIIα could be one of the mechanisms involved in the occurrence of Mn-induced cognitive impairments.

Protective effects of lithium against lead-induced toxicities in multiple systems of adult mouse

Occupational and environmental exposures to lead (Pb), one of the toxic metal pollutants, is of global concern.

Mechanisms of lead and manganese neurotoxicity

Human exposure to neurotoxic metals is a global public health problem. Metals which cause neurological toxicity, such as lead (Pb) and manganese (Mn), are of particular concern due to the long-lasting and possibly irreversible nature of their effects. Pb exposure in childhood can result in cognitive and behavioural deficits in children. These effects are long-lasting and persist into adulthood even after Pb exposure has been reduced or eliminated. While Mn is an essential element of the human diet and serves many cellular functions in the human body, elevated Mn levels can result in a Parkinson's disease (PD)-like syndrome and developmental Mn exposure can adversely affect childhood neurological development. Due to the ubiquitous presence of both metals, reducing human exposure to toxic levels of Mn and Pb remains a world-wide public health challenge. In this review we summarize the toxicokinetics of Pb and Mn, describe their neurotoxic mechanisms, and discuss common themes in their neurotoxicity.

Early postnatal lead exposure induces tau phosphorylation in the brain of young rats

Cognitive impairment is a common feature of both lead exposure and hyperphosphorylation of tau. We, therefore, investigated whether lead exposure would induce tau hyperphosphorylation. Wistar rat pups were exposed to 0.2% lead acetate via their dams' drinking water from postnatal day 1 to 21. Lead in blood and brain were measured by atomic absorption spectrophotometry and the expression of tau, phosphorylated tau and various serine/threonine protein phosphatases (PP1, PP2A, PP2B and PP5) in the brain was analyzed by Western blot. Lead exposure significantly impaired learning and resulted in a significant reduction in the expression of tau but increased the phosphorylation of tau at Ser199/202, Thr212/Ser214 and Thr231. PP2A expression decreased, whereas, PP1 and PP5 expression increased in lead-exposed rats. These results demonstrate that early postnatal exposure to lead decrease PP2A expression and induce tau hyperphosphorylation at several serine and threonine residues. Hyperphosphorylation of tau may be a mechanism of Pb-induced deficits in learning and memory.

Arsenite exposure altered the expression of NMDA receptor and postsynaptic signaling proteins in rat hippocampus

Chronic arsenic exposure has an adverse effect on neurobehavioral function. Our previous study demonstrated an elevated arsenic level, ultra-structure changes and reduced NR2A gene expression in hippocampus, and impaired spatial learning in arsenite-exposed rats. The NMDA receptor and the postsynaptic signaling proteins CaMKII, postsynaptic density protein 95 (PSD-95), synaptic Ras GTPase-activating protein (SynGAP) and nuclear activated extracellular-signal regulated kinase (ERK1/2) play important roles in synaptic plasticity, learning and memory. We hypothesized that the above molecular expression changes may contribute to arsenic neurotoxicity. In present study, the expression of NMDA receptor and postsynaptic signaling proteins in hippocampus were evaluated in rats exposed to 0, 2.72, 13.6 and 68 mg/L sodium arsenite for 3 months. Decreased protein expression of NR2A, PSD-95 and p-CaMKII α in the hippocampus of arsenite-exposed rats was observed, while the expression of SynGAP, a negative regulator of Ras-MAPK activity, was increased when compared with the controls. Additionally, decreased p-ERK1/2 activity was found in the hippocampus of arsenite-exposed rats. These data suggest that altered expression of NMDA receptor complex and postsynaptic signaling proteins may explain arsenic-induced neurotoxicity.

Dysregulation of BDNF-TrkB signaling in developing hippocampal neurons by Pb(2+): implications for an environmental basis of neurodevelopmental disorders

Dysregulation of synaptic development and function has been implicated in the pathophysiology of neurodegenerative disorders and mental disease. A neurotrophin that has an important function in neuronal and synaptic development is brain-derived neurotrophic factor (BDNF). In this communication, we examined the effects of lead (Pb(2+)) exposure on BDNF-tropomyosin-related kinase B (TrkB) signaling during the period of synaptogenesis in cultured neurons derived from embryonic rat hippocampi. We show that Pb(2+) exposure decreases BDNF gene and protein expression, and it may also alter the transport of BDNF vesicles to sites of release by altering Huntingtin phosphorylation and protein levels. Combined, these effects of Pb(2+) resulted in decreased concentrations of extracellular mature BDNF. The effect of Pb(2+) on BDNF gene expression was associated with a specific decrease in calcium-sensitive exon IV transcript levels and reduced phosphorylation and protein expression of the transcriptional repressor methyl-CpG-binding protein (MeCP2). TrkB protein levels and autophosphorylation at tyrosine 816 were significantly decreased by Pb(2+) exposure with a concomitant increase in p75 neurotrophin receptor (p75(NTR)) levels and altered TrkB-p75(NTR) colocalization. Finally, phosphorylation of Synapsin I, a presynaptic target of BDNF-TrkB signaling, was significantly decreased by Pb(2+) exposure with no effect on total Synapsin I protein levels. This effect of Pb(2+) exposure on Synapsin I phosphorylation may help explain the impairment in vesicular release documented by us previously (Neal, A. P., Stansfield, K. H., Worley, P. F., Thompson, R. E., and Guilarte, T. R. (2010). Lead exposure during synaptogenesis alters vesicular proteins and impairs vesicular release: Potential role of N-Methyl-D-aspartate receptor (NMDAR) dependent BDNF signaling. Toxicol. Sci. 116, 249-263) because it controls vesicle movement from the reserve pool to the readily releasable pool. In summary, the present study demonstrates that Pb(2+) exposure during the period of synaptogenesis of hippocampal neurons in culture disrupts multiple synaptic processes regulated by BDNF-TrkB signaling with long-term consequences for synaptic function and neuronal development.

Metal toxicity at the synapse: presynaptic, postsynaptic, and long-term effects

Metal neurotoxicity is a global health concern. This paper summarizes the evidence for metal interactions with synaptic transmission and synaptic plasticity. Presynaptically metal ions modulate neurotransmitter release through their interaction with synaptic vesicles, ion channels, and the metabolism of neurotransmitters (NT). Many metals (e.g., Pb(2+), Cd(2+), and Hg(+)) also interact with intracellular signaling pathways. Postsynaptically, processes associated with the binding of NT to their receptors, activation of channels, and degradation of NT are altered by metals. Zn(2+), Pb(2+), Cu(2+), Cd(2+), Ni(2+), Co(2+), Li(3+), Hg(+), and methylmercury modulate NMDA, AMPA/kainate, and/or GABA receptors activity. Al(3+), Pb(2+), Cd(2+), and As(2)O(3) also impair synaptic plasticity by targeting molecules such as CaM, PKC, and NOS as well as the transcription machinery involved in the maintenance of synaptic plasticity. The multiple effects of metals might occur simultaneously and are based on the specific metal species, metal concentrations, and the types of neurons involved.

Enhanced nitric oxide production during lead (Pb²⁺) exposure recovers protein expression but not presynaptic localization of synaptic proteins in developing hippocampal neurons

We have previously reported that lead (Pb(2+)) exposure results in both presynaptic and postsynaptic changes in developing neurons as a result of inhibition of the N-methyl-d-aspartate receptor (NMDAR). NMDAR inhibition by Pb(2+) during synaptogenesis disrupts downstream trans-synaptic signaling of brain-derived neurotrophic factor (BDNF) and exogenous addition of BDNF can recover the effects of Pb(2+) on both presynaptic protein expression and presynaptic vesicular release. NMDAR activity can modulate other trans-synaptic signaling pathways, such as nitric oxide (NO) signaling. Thus, it is possible that other trans-synaptic pathways in addition to BDNF signaling may be disrupted by Pb(2+) exposure. The current study investigated whether exogenous addition of NO could recover the presynaptic vesicular proteins lost as a result of Pb(2+) exposure during synaptogenesis, namely Synaptophysin (Syn) and Synaptobrevin (Syb). We observed that exogenous addition of NO during Pb(2+) exposure results in complete recovery of whole-cell Syn levels and partial recovery of Syn and Syb synaptic targeting in Pb(2+)-exposed neurons.

Expression profiling of Ca(2+)/calmodulin-dependent signaling molecules in the rat dorsal and ventral hippocampus after acute lead exposure

The septal and temporal poles of the hippocampus differ markedly in their anatomical organization, but whether these distinct regions exhibit differential neurochemical profiles underlying lead (Pb(2+)) neurotoxicity remains to be determined. In the present study, we examined changes in the expression of Ca(2+)/calmodulin-dependent enzymes, including calpain, calcineurin, phospho-CaMKII (Thr286) and neuronal nitric oxide synthase (nNOS), in the rat dorsal and ventral hippocampus (DH and VH) after acute Pb(2+) exposure. Five days after Pb(2+) exposure, we observed constitutively active forms of calcineurin (45 kDa and 48 kDa) in ventral portions of the hippocampus, a result consistent with the observed calpain activation that is indicated by the breakdown of spectrin in this region. Our data demonstrate that nNOS expression is significantly higher in the ventral region of the hippocampus when compared to the dorsal region, whereas phosphorylation of CaMKII (Thr286) is less pronounced in the ventral portion of the hippocampus and more pronounced in dorsal regions after acute Pb(2+) exposure. Thus, it appears likely that the ventral region of hippocampus is more vulnerable to the neurotoxic effects of Pb(2+) than the dorsal region. Taken together, the present data suggest that acute lead exposure leads to differential expression patterns of Ca(2+)/calmodulin-dependent enzymes along the dorsoventral axis of the hippocampus.

Lead exposure during synaptogenesis alters NMDA receptor targeting via NMDA receptor inhibition

N-methyl-D-aspartate receptor (NMDAR) ontogeny and subunit expression are altered during developmental lead (Pb²+) exposure. However, it is unknown whether these changes occur at the synaptic or cellular level. Synaptic and extra-synaptic NMDARs have distinct cellular roles, thus, the effects of Pb²+ on NMDAR synaptic targeting may affect neuronal function. In this communication, we show that Pb²+ exposure during synaptogenesis in hippocampal neurons altered synaptic NMDAR composition, resulting in a decrease in NR2A-containing NMDARs at established synapses. Conversely, we observed increased targeting of the obligatory NR1 subunit of the NMDAR to the postsynaptic density (PSD) based on the increased colocalization with the postsynaptic protein PSD-95. This finding together with increased binding of the NR2B-subunit specific ligand [³H]-ifenprodil, suggests increased targeting of NR2B-NMDARs to dendritic spines as a result of Pb²+ exposure. During brain development, there is a shift of NR2B- to NR2A-containing NMDARs. Our findings suggest that Pb²+ exposure impairs or delays this developmental switch at the level of the synapse. Finally, we show that alter expression of NMDAR complexes in the dendritic spine is most likely due to NMDAR inhibition, as exposure to the NMDAR antagonist aminophosphonovaleric acid (APV) had similar effects as Pb²+ exposure. These data suggest that NMDAR inhibition by Pb²+ during synaptogensis alters NMDAR synapse development, which may have lasting consequences on downstream signaling.

Molecular neurobiology of lead (Pb(2+)): effects on synaptic function

Lead (Pb(2+)) is a ubiquitous environmental neurotoxicant that continues to threaten public health on a global scale. Epidemiological studies have demonstrated detrimental effects of Pb(2+) on childhood IQ at very low levels of exposure. Recently, a mechanistic understanding of how Pb(2+) affects brain development has begun to emerge. The cognitive effects of Pb(2+) exposure are believed to be mediated through its selective inhibition of the N-methyl-D: -aspartate receptor (NMDAR). Studies in animal models of developmental Pb(2+) exposure exhibit altered NMDAR subunit ontogeny and disruption of NMDAR-dependent intracellular signaling. Additional studies have reported that Pb(2+) exposure inhibits presynaptic calcium (Ca(2+)) channels and affects presynaptic neurotransmission, but a mechanistic link between presynaptic and postsynaptic effects has been missing. Recent work has suggested that the presynaptic and postsynaptic effects of Pb(2+) exposure are both due to inhibition of the NMDAR by Pb(2+), and that the presynaptic effects of Pb(2+) may be mediated by disruption of NMDAR activity-dependent signaling of brain-derived neurotrophic factor (BDNF). These findings provide the basis for the first working model to describe the effects of Pb(2+) exposure on synaptic function. Here, we review the neurotoxic effects of Pb(2+) exposure and discuss the known effects of Pb(2+) exposure in light of these recent findings.

Ca2+/calmodulin-dependent protein kinase II-dependent remodeling of Ca2+ current in pressure overload heart failure

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activity is increased in heart failure (HF), a syndrome characterized by markedly increased risk of arrhythmia. Activation of CaMKII increases peak L-type Ca(2+) current (I(Ca)) and slows I(Ca) inactivation. Whether these events are linked mechanistically is unknown. I(Ca) was recorded in acutely dissociated subepicardial and subendocardial murine left ventricular (LV) myocytes using the whole cell patch clamp method. Pressure overload heart failure was induced by surgical constriction of the thoracic aorta. I(Ca) density was significantly larger in subepicardial myocytes than in subendocardial/myocytes. Similar patterns were observed in the cell surface expression of alpha1c, the channel pore-forming subunit. In failing LV, I(Ca) density was increased proportionately in both cell types, and the time course of I(Ca) inactivation was slowed. This typical pattern of changes suggested a role of CaMKII. Consistent with this, measurements of CaMKII activity revealed a 2-3-fold increase (p < 0.05) in failing LV. To test for a causal link, we measured frequency-dependent I(Ca) facilitation. In HF myocytes, this CaMKII-dependent process could not be induced, suggesting already maximal activation. Internal application of active CaMKII in failing myocytes did not elicit changes in I(Ca). Finally, CaMKII inhibition by internal diffusion of a specific peptide inhibitor reduced I(Ca) density and inactivation time course to similar levels in control and HF myocytes. I(Ca) density manifests a significant transmural gradient, and this gradient is preserved in heart failure. Activation of CaMKII, a known pro-arrhythmic molecule, is a major contributor to I(Ca) remodeling in load-induced heart failure.

Exposure of C6 glioma cells to Pb(II) increases the phosphorylation of p38MAPK and JNK1/2 but not of ERK1/2

Pb(II) is a neurotoxic pollutant that produces permanent cognitive deficits in children. Pb(II) can modulate cell signaling pathways and cell viability in a variety of cell types. However, these actions are not well demonstrated on glial cells, which represent an important target for metals into the central nervous system. The present work was undertaken to determine the ability of Pb(II) in modulating the activity of mitogen activated protein kinases (MAPKs) in cultures of C6 rat glioma cells, a useful functional model for the study of astrocytes. Additionally, cell viability was analyzed by measurement of MTT reduction. Cells were exposed to lead acetate 0.1, 1, 10 microM for 24 and 48 h. MAPKs activation - in particular ERK1/2, p38(MAPK) and JNK1/2 - were analyzed by western blotting. Results showed that 10 microM Pb(II) treatment for 24 h caused a discrete stimulation of p38(MAPK) phosphorylation. However, 1 and 10 microM Pb(II) treatment for 48 h provoked a significant stimulation in the phosphorylation state of p38(MAPK) and JNK1/2. The phosphorylation state of ERK1/2 was not modified by any Pb(II) treatment. Moreover, data indicate that at 48 h treatment even 1 microM Pb(II) can be cytotoxic, causing impairment on cell viability. Therefore, depending on a long incubation period, a significant concomitant activation of p38(MAPK) and JNK1/2 by Pb(II) took place in parallel with the impairment of C6 glioma cells viability.

Transient-outward K+ channel inhibition facilitates L-type Ca2+ current in heart

BACKGROUND

Transient outward current (I(to)) and L-type calcium current (I(Ca)) are important repolarization currents in cardiac myocytes. These two currents often undergo disease-related remodeling while other currents are spared, suggesting a functional coupling between them. Here, we investigated the effects of I(to) channel blockers, 4-aminopyridine (4-AP) and heteropodatoxin-2 (HpTx2), on I(Ca) in cardiac ventricular myocytes.

METHODS AND RESULTS

I(Ca) was recorded in enzymatically dissociated mouse and guinea pig ventricular myocytes using the whole-cell voltage clamp method. In mouse ventricular myocytes, 4-AP (2 mM) significantly facilitated I(Ca) by increasing current amplitude and slowing inactivation. These effects were not voltage-dependent. Similar facilitating effects were seen when equimolar Ba2+ was substituted for external Ca2+, indicating that Ca2+ influx is not required. Measurements of Ca2+/calmodulin-dependent protein kinase (CaMKII) activity revealed significant increases in cells treated with 4-AP. Pretreatment of cells with 10 microM KN93, a specific inhibitor of CaMKII, abolished the effects of 4-AP on I(Ca.) To test the requirement of I(to), we studied guinea pig ventricular myocytes, which do not express I(to) channels. In these cells, 2 mM 4-AP had no effect on I(Ca) amplitude or kinetics. In both cell types, Ca2+-induced I(Ca) facilitation, a CaMKII-dependent process, was observed. However, 4-AP abolished Ca2+-induced I(Ca) facilitation exclusively in mouse ventricular myocytes.

CONCLUSION

4-AP, an I(to) blocker, facilitates L-type Ca2+ current through a mechanism involving the I(to) channel and CaMKII activation. These data indicate a functional association of I(Ca) and I(to) in cardiac myocytes.