Harmful effects of cadmium on olfactory system in mice

Characterization of pathological changes in the olfactory system of mice exposed to methylmercury

Methylmercury (MeHg) is a well-known environmental neurotoxicant that causes severe brain disorders such as Minamata disease. Although some patients with Minamata disease develop olfactory dysfunction, the underlying pathomechanism is largely unknown. We examined the effects of MeHg on the olfactory system using a model of MeHg poisoning in which mice were administered 30 ppm MeHg in drinking water for 8 weeks. Mice exposed to MeHg displayed significant mercury accumulation in the olfactory pathway, including the nasal mucosa, olfactory bulb, and olfactory cortex. The olfactory epithelium was partially atrophied, and olfactory sensory neurons were diminished. The olfactory bulb exhibited an increase in apoptotic cells, hypertrophic astrocytes, and amoeboid microglia, mainly in the granular cell layer. Neuronal cell death was observed in the olfactory cortex, particularly in the ventral tenia tecta. Neuronal cell death was also remarkable in higher-order areas such as the orbitofrontal cortex. Correlation analysis showed that neuronal loss in the olfactory cortex was strongly correlated with the plasma mercury concentration. Our results indicate that MeHg is an olfactory toxicant that damages the central regions involved in odor perception. The model described herein is useful for analyzing the mechanisms and treatments of olfactory dysfunction in MeHg-intoxicated patients.

Cadmium neurotoxicity and therapeutic strategies

Cadmium (Cd) is a multitarget, carcinogenic, nonessential environmental pollutant. Due to its toxic effects at very low concentrations, lengthy biological half-life, and low excretion rate, exposure to Cd carries a concern. Prolonged exposure to Cd causes severe injury to the nervous system of both humans and animals. Nevertheless, the precise mechanisms responsible for the neurotoxic effects of Cd have yet to be fully elucidated. The accurate chemical mechanism potentially entails the destruction of metal-ion homeostasis, inducing oxidative stress, apoptosis, and autophagy. Here we review the evidence of the neurotoxic effects of Cd and corresponding strategies to protect against Cd-induced central nervous system injury.

Common and Trace Metals in Alzheimer's and Parkinson's Diseases

Trace elements and metals play critical roles in the normal functioning of the central nervous system (CNS), and their dysregulation has been implicated in neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD). In a healthy CNS, zinc, copper, iron, and manganese play vital roles as enzyme cofactors, supporting neurotransmission, cellular metabolism, and antioxidant defense. Imbalances in these trace elements can lead to oxidative stress, protein aggregation, and mitochondrial dysfunction, thereby contributing to neurodegeneration. In AD, copper and zinc imbalances are associated with amyloid-beta and tau pathology, impacting cognitive function. PD involves the disruption of iron and manganese levels, leading to oxidative damage and neuronal loss. Toxic metals, like lead and cadmium, impair synaptic transmission and exacerbate neuroinflammation, impacting CNS health. The role of aluminum in AD neurofibrillary tangle formation has also been noted. Understanding the roles of these elements in CNS health and disease might offer potential therapeutic targets for neurodegenerative disorders. The Codex Alimentarius standards concerning the mentioned metals in foods may be one of the key legal contributions to safeguarding public health. Further research is needed to fully comprehend these complex mechanisms and develop effective interventions.

Single-cell transcriptomic analysis reveals the adverse effects of cadmium on the trajectory of neuronal maturation

Cadmium (Cd) is an extensively existing environmental pollutant that has neurotoxic effects. However, the molecular mechanism of Cd on neuronal maturation is unveiled. Single-cell RNA sequencing (scRNA-seq) has been widely used to uncover cellular heterogeneity and is a powerful tool to reconstruct the developmental trajectory of neurons. In this study, neural stem cells (NSCs) from subventricular zone (SVZ) of newborn mice were treated with CdCl2 for 24 h and differentiated for 7 days to obtain neuronal lineage cells. Then scRNA-seq analysis identified five cell stages with different maturity in neuronal lineage cells. Our findings revealed that Cd altered the trajectory of maturation of neuronal lineage cells by decreasing the number of cells in different stages and hindering their maturation. Cd induced differential transcriptome expression in different cell subpopulations in a stage-specific manner. Specifically, Cd induced oxidative damage and changed the proportion of cell cycle phases in the early stage of neuronal development. Furthermore, the autocrine and paracrine signals of Wnt5a were downregulated in the low mature neurons in response to Cd. Importantly, activation of Wnt5a effectively rescued the number of neurons and promoted their maturation. Taken together, the findings of this study provide new and comprehensive insights into the adverse effect of Cd on neuronal maturation.

Immunomodulation by heavy metals as a contributing factor to inflammatory diseases and autoimmune reactions: Cadmium as an example

Cadmium (Cd) represents a unique hazard because of the long biological half-life in humans (20-30 years). This metal accumulates in organs causing a continuum of responses, with organ disease/failure as extreme outcome. Some of the cellular and molecular alterations in target tissues can be related to immune-modulating potential of Cd. This metal may cause adverse responses in which components of the immune system function as both mediators and effectors of Cd tissue toxicity, which, in combination with Cd-induced alterations in homeostatic reparative activities may contribute to tissue dysfunction. In this work, current knowledge concerning inflammatory/autoimmune disease manifestations found to be related with cadmium exposure are summarized. Along with epidemiological evidence, animal and in vitro data are presented, with focus on cellular and molecular immune mechanisms potentially relevant for the disease susceptibility, disease promotion, or facilitating development of pre-existing pathologies.

Modification of the Peripheral Olfactory System by Electronic Cigarettes

Electronic cigarettes (e-cigs) are used by millions of adolescents and adults worldwide. Commercial e-liquids typically contain flavorants, propylene glycol, and vegetable glycerin with or without nicotine. These chemical constituents are detected and evaluated by chemosensory systems to guide and modulate vaping behavior and product choices of e-cig users. The flavorants in e-liquids are marketing tools. They evoke sensory percepts of appealing flavors through activation of chemical sensory systems to promote the initiation and sustained use of e-cigs. The vast majority of flavorants in e-liquids are volatile odorants, and as such, the olfactory system plays a dominant role in perceiving these molecules that enter the nasal cavity either orthonasally or retronasally during vaping. In addition to flavorants, e-cig aerosol contains a variety of by-products generated through heating the e-liquids, including odorous irritants, toxicants, and heavy metals. These harmful substances can directly and adversely impact the main olfactory epithelium (MOE). In this article, we first discuss the olfactory contribution to e-cig flavor perception. We then provide information on MOE cell types and their major functions in olfaction and epithelial maintenance. Olfactory detection of flavorants, nicotine, and odorous irritants and toxicants are also discussed. Finally, we discuss the cumulated data on modification of the MOE by flavorant exposure and toxicological impacts of formaldehyde, acrolein, and heavy metals. Together, the information presented in this overview may provide insight into how e-cig exposure may modify the olfactory system and adversely impact human health through the alteration of the chemosensory factor driving e-cig use behavior and product selections. © 2021 American Physiological Society. Compr Physiol 11:2621-2644, 2021.

Advances in intranasal application of stem cells in the treatment of central nervous system diseases

Stem cells are characterized by their self-renewal and multipotency and have great potential in the therapy of various disorders. However, the blood-brain barrier (BBB) limits the application of stem cells in the therapy of neurological disorders, especially in a noninvasive way. It has been shown that small molecular substances, macromolecular proteins, and even stem cells can bypass the BBB and reach the brain parenchyma following intranasal administration. Here, we review the possible brain-entry routes of transnasal treatment, the cell types, and diseases involved in intranasal stem cell therapy, and discuss its advantages and disadvantages in the treatment of central nervous system diseases, to provide a reference for the application of intranasal stem cell therapy.

Epigenetic influence of environmentally neurotoxic metals

Continuous globalization and industrialization have ensured metals are an increasing aspect of daily life. Their usefulness in manufacturing has made them vital to national commerce, security and global economy. However, excess exposure to metals, particularly as a result of environmental contamination or occupational exposures, has been detrimental to overall health. Excess exposure to several metals is considered environmental risk in the aetiology of several neurological and neurodegenerative diseases. Metal-induced neurotoxicity has been a major health concern globally with intensive research to unravel the mechanisms associated with it. Recently, greater focus has been directed at epigenetics to better characterize the underlying mechanisms of metal-induced neurotoxicity. Epigenetic changes are those modifications on the DNA that can turn genes on or off without altering the DNA sequence. This review discusses how epigenetic changes such as DNA methylation, post translational histone modification and noncoding RNA-mediated gene silencing mediate the neurotoxic effects of several metals, focusing on manganese, arsenic, nickel, cadmium, lead, and mercury.

Associations between Cadmium Exposure and Taste and Smell Dysfunction: Results from the National Health and Nutrition Examination Survey (NHANES), 2011-2014

Background: Cadmium is a ubiquitous environmental pollutant and has been associated with many adverse health outcomes. However, little is known about the effect of cadmium exposure on taste and smell dysfunction. Methods: We used the National Health and Nutrition Examination Survey (NHANES) 2011–2014 to investigate the associations between blood cadmium and taste and smell dysfunction among 5038 adults aged 40–80 years old. Taste and smell dysfunction were defined by questionnaires, examinations, or both criteria. Results: In survey weighted logistic regression models adjusting for age, gender, race/ethnicity, income-to-poverty ratio (IPR), and education, individuals with a blood cadmium level in the highest tertiles had significantly higher odds of having perceived smell dysfunction (odds ratio (OR) = 1.41, 95% confidence interval (CI): 1.08, 1.84), perceived taste dysfunction (OR = 1.48, 95% CI: 1.16, 1.89), and taste dysfunction defined by both self-reported and objectively measured data (OR = 1.46, 95% CI: 1.03, 2.07). After further adjusting for body mass index (BMI), cigarette smoking, and alcohol drinking, consistent results were observed for perceived taste dysfunction (OR = 1.49, 95% CI: 1.10, 2.00), and no significant associations were found between cadmium exposure and other outcomes. Conclusions: Our findings suggest that cadmium exposure is associated with perceived taste dysfunction.

Heavy Metals Exposure and Alzheimer's Disease and Related Dementias

Alzheimer's disease and related dementias lack effective treatment or cures and are major public health challenges. Risk for Alzheimer's disease and related dementias is partially attributable to environmental factors. The heavy metals lead, cadmium, and manganese are widespread and persistent in our environments. Once persons are exposed to these metals, they are adept at entering cells and reaching the brain. Lead and cadmium are associated with numerous health outcomes even at low levels of exposure. Although manganese is an essential metal, deficiency or environmental exposure or high levels of the metal can be toxic. In cell and animal model systems, lead, cadmium, and manganese are well documented neurotoxicants that contribute to canonical Alzheimer's disease pathologies. Adult human epidemiologic studies have consistently shown lead, cadmium, and manganese are associated with impaired cognitive function and cognitive decline. No longitudinal human epidemiology study has assessed lead or manganese exposure on Alzheimer's disease specifically though two studies have reported a link between cadmium and Alzheimer's disease mortality. More longitudinal epidemiologic studies with high-quality time course exposure data and incident cases of Alzheimer's disease and related dementias are warranted to confirm and estimate the proportion of risk attributable to these exposures. Given the widespread and global exposure to lead, cadmium, and manganese, even small increases in the risks of Alzheimer's disease and related dementias would have a major population impact on the burden on disease. This article reviews the experimental and epidemiologic literature of the associations between lead, cadmium, and manganese on Alzheimer's disease and related dementias and makes recommendations of critical areas of future investment.

Evaluation of the Nose-to-Brain Transport of Different Physicochemical Forms of Uranium after Exposure via Inhalation of a UO4 Aerosol in the Rat

BACKGROUND

Health-risk issues are raised concerning inhalation of particulate pollutants that are thought to have potential hazardous effects on the central nervous system. The brain is presented as a direct target of particulate matter (PM) exposure because of the nose-to-brain pathway involvement. The main cause of contamination in nuclear occupational activities is related to exposure to aerosols containing radionuclides, particularly uranium dust. It has been previously demonstrated that instilled solubilized uranium in the rat nasal cavity is conveyed to the brain via the olfactory nerve.

OBJECTIVE

The aim of this study was to analyze the anatomical localization of uranium compounds in the olfactory system after in vivo exposure to a polydisperse aerosol of uranium tetraoxide (UO4) particles.

METHODS

The olfactory neuroepithelium (OE) and selected brain structures-olfactory bulbs (OB), frontal cortex (FC), hippocampus (HIP), cerebellum (Cer), and brainstem (BS)-were microdissected 4 h after aerosol inhalation via a nose-only system in adult rats. Tissues were subjected to complementary analytical techniques.

RESULTS

Uranium concentrations measured by inductively coupled plasma mass spectrometry (ICP-MS) were significantly higher in all brain structures from exposed animals compared with their respective controls. We observed that cerebral uranium concentrations followed an anteroposterior gradient with typical accumulation in the OB, characteristic of a direct olfactory transfer of inhaled compounds. Secondary ion mass spectrometry (SIMS) microscopy and transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (TEM-EDX) were used in order to track elemental uranium in situ in the olfactory epithelium. Elemental uranium was detected in precise anatomical regions: olfactory neuron dendrites, paracellular junctions of neuroepithelial cells, and olfactory nerve tracts (around axons and endoneural spaces).

CONCLUSION

These neuroanatomical observations in a rat model are consistent with the transport of elemental uranium in different physicochemical forms (solubilized, nanoparticles) along olfactory nerve bundles after inhalation of UO4 microparticles. This work contributes to knowledge of the mechanistic actions of particulate pollutants on the brain. https://doi.org/10.1289/EHP4927.

Cadmium Exposure Impairs Cognition and Olfactory Memory in Male C57BL/6 Mice

Cadmium (Cd) is a heavy metal of high interest to the superfund initiative. Recent epidemiology studies have suggested a possible association between Cd exposure and cognitive as well as olfactory impairments in humans. However, studies in animal models are needed to establish a direct causal relationship between Cd exposure and impairments in cognition and olfaction. This study aims to investigate the toxic effect of Cd on cognition and olfactory function in mice. One group of 8-week-old C57BL/6 male mice was exposed to 3 mg/l Cd (in the form of CdCl2) through drinking water for 20 weeks for behavior tests and final blood Cd concentration analysis. The behavior tests were conducted before, during, and after Cd exposure to analyze the effects of Cd on cognition and olfactory function. Upon completion of behavior tests, blood was collected to measure final blood Cd concentration. Two additional groups of mice were similarly exposed to Cd for 5 or 13 weeks for peak blood Cd concentration measurement. The peak blood Cd concentration was 2.125-2.25 μg/l whereas the final blood Cd concentration was 0.18 μg/l. At this exposure level, Cd impaired hippocampus-dependent learning and memory in novel object location test, T-maze test, and contextual fear memory test. It also caused deficits in short-term olfactory memory and odor-cued olfactory learning and memory. Results in this study demonstrate a direct relationship between Cd exposure and cognitive as well as olfactory impairments in an animal model.

Differential nickel-induced responses of olfactory sensory neuron populations in zebrafish

The olfactory epithelium of fish includes three main types of olfactory sensory neurons (OSNs). Whereas ciliated (cOSNs) and microvillous olfactory sensory neurons (mOSNs) are common to all vertebrates, a third, smaller group, the crypt cells, is exclusive for fish. Dissolved pollutants reach OSNs, thus resulting in impairment of the olfactory function with possible neurobehavioral damages, and nickel represents a diffuse olfactory toxicant. We studied the effects of three sublethal Ni²⁺ concentrations on the different OSN populations of zebrafish that is a widely used biological model. We applied image analysis with cell count and quantification of histochemically-detected markers of the different types of OSNs. The present study shows clear evidence of a differential responses of OSN populations to treatments. Densitometric values for G_{α olf}, a marker of cOSNs, decreased compared to control and showed a concentration-dependent effect in the ventral half of the olfactory rosette. The densitometric analysis of TRPC2, a marker of mOSNs, revealed a statistically significant reduction compared to control, smaller than the decrease for G_{α olf} and without concentration-dependent effects. After exposure, olfactory epithelium stained with anti-calretinin, a marker of c- and mOSNs, revealed a decrease in thickness while the sensory area appeared unchanged. The thickness reduction together with increased densitometric values for HuC/D, a marker of mature and immature neurons, suggests that the decrements in G_{α olf} and TRPC2 immunostaining may depend on cell death. However, reductions in the number of apical processes and of antigen expression could be a further explanation. We hypothesize that cOSNs are more sensitive than mOSNs to Ni²⁺ exposure. Difference between subpopulations of OSNs or differences in water flux throughout the olfactory cavity could account for the greater susceptibility of the OSNs located in the ventral half of the olfactory rosette. Cell count of anti-TrkA immunopositive cells reveals that Ni²⁺ exposure does not affect crypt cells. The results of this immunohistochemical study are not in line with those obtained by electro-olfactogram.

Cadmium-induced neurodegeneration and activation of noncanonical sonic hedgehog pathway in rat cerebellum

BACKGROUND

Cadmium is a nonessential toxic heavy metal, which enters the body easily and damages the cellular system. The sonic hedgehog (Shh) signaling pathway is one of the key regulatory pathways, which define neural growth and development.

OBJECTIVES

This study aimed to explore how cadmium exposure affects neural activities, Shh signaling cascade, and its downstream target genes.

METHODS

Total 18 male Wistar rats were randomly divided into two groups, control and test groups. Test rats were administered with 3 mg cadmium/kg body weight, while the control rats were treated with vehicle continuously for 28 days. Thereafter, rats were killed and the isolated brain samples were examined using oxidative stress assessment, histological and immunohistological behavioral assessment, polymerase chain reaction (PCR), and the comet assay.

RESULTS

A disturbed oxidative balance, DNA damage, and an upregulated Shh signaling pathway were observed in cadmium-treated samples. Loss of structural integrity in cerebellum and loss of motor activity were observed in cadmium-treated rats.

Olfactory dysfunction revisited: a reappraisal of work-related olfactory dysfunction caused by chemicals

Occupational exposure to numerous individual chemicals has been associated with olfactory dysfunction, mainly in individual case descriptions. Comprehensive epidemiological investigations into the olfactotoxic effect of working substances show that the human sense of smell may be impaired by exposure to metal compounds involving cadmium, chromium and nickel, and to formaldehyde. This conclusion is supported by the results of animal experiments. The level of evidence for a relationship between olfactory dysfunction and workplace exposure to other substances is relatively weak.

Olfactory dysfunction revisited: a reappraisal of work-related olfactory dysfunction caused by chemicals

Occupational exposure to numerous individual chemicals has been associated with olfactory dysfunction, mainly in individual case descriptions. Comprehensive epidemiological investigations into the olfactotoxic effect of working substances show that the human sense of smell may be impaired by exposure to metal compounds involving cadmium, chromium and nickel, and to formaldehyde. This conclusion is supported by the results of animal experiments. The level of evidence for a relationship between olfactory dysfunction and workplace exposure to other substances is relatively weak.

Associations between blood cadmium levels and cognitive function in a cross-sectional study of US adults aged 60 years or older

OBJECTIVES

The relationship between cadmium exposure and cognition has been well studied in children. However, the association between environmental cadmium exposure and cognitive function has not been researched extensively in older adults. Our goal was to evaluate the association between cognitive function and blood cadmium levels in US adults aged 60 years or older.

DESIGN

A cross-sectional study.

SETTING

The US National Health and Nutrition Examination Survey (NHANES).

PARTICIPANTS

A total of 2068 adults aged 60 years or older who completed four cognitive assessment tests and blood cadmium detection in two waves of NHANES (2011-2014).

MAIN OUTCOME MEASURES

Cognitive assessment was conducted by household interview or at a Mobile Examination Center (MEC) using the Consortium to Establish a Registry for Alzheimer's Disease (CERAD) Word List Learning Test, the CERAD Word List Recall Test, the Animal Fluency Test and the Digit Symbol Substitution Test (DSST). We created a composite cognitive z-score to represent global cognitive function.

RESULTS

The median blood cadmium concentration in the study participants was 0.35 µg/L, and the IQR was 0.24-0.56 µg/L. In linear regression analyses, adjusting for demographics, behaviour and medical history, blood cadmium as a continuous variable was inversely associated with the composite z-score (μg/L, β=-0.11, 95% CI -0.20 to -0.03). Similarly, there was a significant association between quartiles of blood cadmium and composite z-score, with somewhat lower scores in the upper quartile of exposure (blood cadmium ≥0.63 µg/L) compared with those in the lower quartile of exposure (blood cadmium <0.25 µg/L) (μg/L, β=-0.14, 95% CI -0.25 to -0.03), and there was a trend by quartiles of blood cadmium (P<0.0001).

CONCLUSIONS

Our findings suggest that increased blood cadmium is associated with worse cognitive function in adults aged 60 years or older in the USA.

Effects of cadmium on Bcl-2/Bax expression ratio in rat cortex brain and hippocampus

To investigate the underlying mechanism of neurotoxicity of cadmium, we examined the effects of intraperitoneal injection of cadmium on messenger RNA (mRNA) expression of Bcl-2 (B-cell lymphoma 2) and Bax (Bcl2-associated x) genes and caspase-3/7 activation in rat hippocampus and frontal cortex. Twenty-eight male Wistar rats weighing 200–250 g were randomly divided into four groups. Control group received saline and three other groups received cadmium at doses of 1, 2 and 4 mg/kg (body weight) for 15 successive days. One day after the last injection, the hippocampus and frontal cortex were dissected and removed and then the expression of Bcl-2 and Bax genes was evaluated using real-time polymerase chain reaction and apoptotic studies was done using caspase-3/7 activation assay. Cadmium reduced the mRNA level of Bcl-2 in the control group at doses of 1 ( p < 0.01), 2 and 4 mg/kg ( p < 0.001) in rat hippocampus and cortex cells. The mRNA level of Bax increased significantly compared to the control group at the doses of 1 ( p < 0.05), 2 and 4 mg/kg ( p < 0.001) in rat hippocampus. The mRNA level of Bax was increased significantly compared to the control group at the doses of 2 and 4 mg/kg ( p < 0.001) in rat cortex cells. Cadmium increased caspase-3/7 activity at doses of 1, 2 and 4 mg/kg in rat hippocampus. Caspase-3/7 activity was increased significantly at dose of 4 mg/kg in rat cortex. This decreased Bcl-2/Bax mRNA ratio induces cell apoptosis. Apoptotic effect of cadmium may be through the mitochondrial pathway by the activation of caspase-3/7.

Differential response of olfactory sensory neuron populations to copper ion exposure in zebrafish

The peripheral olfactory system of fish is in direct contact with the external aqueous environment, so dissolved contaminants can easily impair sensory functions and cause neurobehavioral injuries. The olfactory epithelium of fish is arranged in lamellae forming a rosette in the olfactory cavity and contains three main types of olfactory sensory neurons (OSNs): ciliated (cOSNs) and microvillous olfactory sensory neurons (mOSNs), common to all vertebrates, and a third minor group of olfactory neurons, crypt cells, absent in tetrapods. Since copper is a ubiquitously diffusing olfactory toxicant and a spreading contaminant in urban runoff, we investigated the effect of low copper concentration on the three different OSNs in the olfactory epithelium of zebrafish, a model system widely used in biological research. Image analysis was applied for morphometry and quantification of immunohistochemically detected OSNs. Copper exposure resulted in an evident decrease in olfactory epithelium thickness. Moreover, after exposure, the lamellae of the dorsal and ventral halves of the olfactory rosettes showed a different increase in their sensory areas, suggesting a lateral migration of new cells into non-sensory regions. The results of the present study provide clear evidence of a differential response of the three neural cell populations of zebrafish olfactory mucosa after 96h of exposure to copper ions at the sublethal concentration of 30μgL^-1. Densitometric values of cONS, immunostained with anti-G _αolf, decreased of about 60% compared to the control. When the fish were transferred to water without copper addition and examined after 3, 10 and 30days, we observed a partial restoration of anti-G _αolf staining intensity to normal condition. The recovery of cOSNs appeared sustained by neuronal proliferation, quantified with anti-PCNA immunostaining, in particular in the early days after exposure. The densitometric analysis applied to mOSNs, immunostained with anti-TRPC2, revealed a statistically significant decrease of about 30% compared to the control. For cOSNs and mOSNs, the decrement in staining intensity may be indicative of cell death, but reduction in antigen expression may not be excluded. In the post-exposure period of 1 month we did not find recovery of mOSNs. We hypothesize that cOSNs are more sensitive than mOSNs to copper treatment, but also more prompted to tissue repair. Anti-TrkA-immunopositive crypt cells appeared not to be affected by copper exposure since statistical analysis excluded any significant difference between the control and treated fish. Comparative studies on OSNs would greatly enhance our understanding of the mechanisms of olfaction.

Mechanisms of divalent metal toxicity in affective disorders

Metals are required for proper brain development and play an important role in a number of neurobiological functions. The divalent metal transporter 1 (DMT1) is a major metal transporter involved in the absorption and metabolism of several essential metals like iron and manganese. However, non-essential divalent metals are also transported through this transporter. Therefore, altered expression of DMT1 can modify the absorption of toxic metals and metal-induced toxicity. An accumulating body of evidence has suggested that increased metal stores in the brain are associated with elevated oxidative stress promoted by the ability of metals to catalyze redox reactions, resulting in abnormal neurobehavioral function and the progression of neurodegenerative diseases. Metal overload has also been implicated in impaired emotional behavior, although the underlying mechanisms are not well understood with limited information. The current review focuses on psychiatric dysfunction associated with imbalanced metabolism of metals that are transported by DMT1. The investigations with respect to the toxic effects of metal overload on behavior and their underlying mechanisms of toxicity could provide several new therapeutic targets to treat metal-associated affective disorders.

Intranasal exposure to uranium results in direct transfer to the brain along olfactory nerve bundles

AIMS

Uranium olfactory uptake after intranasal exposure raises some concerns for people potentially exposed to airborne radionuclide contamination as the brain could be a direct target for these contaminants. A model of nasal instillation was used to elucidate the transport mechanisms of uranium to the brain and to map its localization.

METHODS

Increasing concentrations of depleted uranium containing solutions were instilled in the nasal cavity of adult male rats. Uranium concentrations were measured using inductively coupled plasma-mass spectrometry (ICP-MS) 4 h after instillation. Olfactory neuroepithelium cytoarchitecture was studied using immunohistochemistry experiments. Secondary ion mass spectrometry (SIMS) microscopy was performed to localize uranium in the olfactory system.

RESULTS

ICP-MS analyses showed a frontal accumulation of uranium in the olfactory bulbs associated with a smaller increase in more caudal brain regions (frontal cortex, hippocampus and cerebellum). Uranium concentrations in the olfactory bulbs do not reach a saturation point. Olfactory nerve bundle integrity is not affected by uranium as revealed by immunohistochemistry. SIMS microscopy allowed us to show that uranium localization is mainly restricted to the olfactory neuroepithelium and around olfactory nerve bundles. It is subsequently detected in the olfactory nerve layer of the olfactory bulb.

DISCUSSION

These results suggest the existence of a transcellular passage from the mucosa to the perineural space around axon bundles. Uranium bypasses the blood brain barrier and is conveyed to the brain via the cerebrospinal fluid along the olfactory nerve. Future studies might need to integrate this new contamination route to assess uranium neurotoxicity after nasal exposure.

Clinical trials of amyloid-based immunotherapy for Alzheimer's disease: end of beginning or beginning of end?

INTRODUCTION

Amyloid deposit and hyperphosphorylated Tau protein contribute to pathological changes seen in Alzheimer's disease (AD) and imply that removal may reverse the cognitive decline. Immunotherapy is a potential way of reducing the load of amyloid or Tau in the brain.

AREAS COVERED

This review summarizes recent clinical trials that have investigated immunotherapy to treat AD and its potential mechanisms. In addition, the potential opportunities as well as challenges of immunotherapy for AD in clinical trials are also discussed.

EXPERT OPINION

Amyloid-based immunotherapy for AD is a novel method with potential; however, some clinical trials were terminated because of the adverse effects. Further studies need to determine the following questions: (i) which is better, passive, or active immunotherapy; (ii) which could be used for the vaccine, amyloid or Tau; (iii) which is better, short- or long-antigen vaccine; and (iv) the route of delivery for antigen or antibody.

Cadmium and its neurotoxic effects

Cadmium (Cd) is a heavy metal that has received considerable concern environmentally and occupationally. Cd has a long biological half-life mainly due to its low rate of excretion from the body. Thus, prolonged exposure to Cd will cause toxic effect due to its accumulation over time in a variety of tissues, including kidneys, liver, central nervous system (CNS), and peripheral neuronal systems. Cd can be uptaken from the nasal mucosa or olfactory pathways into the peripheral and central neurons; for the latter, Cd can increase the blood brain barrier (BBB) permeability. However, mechanisms underlying Cd neurotoxicity remain not completely understood. Effect of Cd neurotransmitter, oxidative damage, interaction with other metals such as cobalt and zinc, estrogen-like, effect and epigenetic modification may all be the underlying mechanisms. Here, we review the in vitro and in vivo evidence of neurotoxic effects of Cd. The available finding indicates the neurotoxic effects of Cd that was associated with both biochemical changes of the cell and functional changes of central nervous system, suggesting that neurotoxic effects may play a role in the systemic toxic effects of the exposure to Cd, particularly the long-term exposure.

Copper-induced deregulation of microRNA expression in the zebrafish olfactory system

Although environmental trace metals, such as copper (Cu), can disrupt normal olfactory function in fish, the underlying molecular mechanisms of metal-induced olfactory injury have not been elucidated. Current research has suggested the involvement of epigenetic modifications. To address this hypothesis, we analyzed microRNA (miRNA) profiles in the olfactory system of Cu-exposed zebrafish. Our data revealed 2, 10, and 28 differentially expressed miRNAs in a dose-response manner corresponding to three increasing Cu concentrations. Numerous deregulated miRNAs were involved in neurogenesis (e.g., let-7, miR-7a, miR-128, and miR-138), indicating a role for Cu-mediated toxicity via interference with neurogenesis processes. Putative gene targets of deregulated miRNAs were identified when interrogating our previously published microarray database, including those involved in cell growth and proliferation, cell death, and cell morphology. Moreover, several miRNAs (e.g., miR-203a, miR-199*, miR-16a, miR-16c, and miR-25) may contribute to decreased mRNA levels of their host genes involved in olfactory signal transduction pathways and other critical neurological processes via a post-transcriptional mechanism. Our findings provide novel insight into the epigenetic regulatory mechanisms of metal-induced neurotoxicity of the fish olfactory system and identify novel miRNA biomarkers of metal exposures.

Characterization of phospholipid hydroperoxide glutathione metabolizing peroxidase (gpx4) isoforms in Coho salmon olfactory and liver tissues and their modulation by cadmium

Exposure to environmental contaminants, including various pesticides and trace metals, can disrupt critical olfactory-driven behaviors of fish such as homing to natal streams, mate selection, and an ability to detect predators and prey. These neurobehavioral injuries have been linked to reduced survival and population declines. Despite the importance of maintaining proper olfactory signaling processes in the presence of chemical exposures, little is known regarding chemical detoxification in the salmon olfactory system, and in particular, the antioxidant defenses that maintain olfactory function. An understudied, yet critical component of cellular antioxidant defense is phospholipid hydroperoxide glutathione peroxidase (PHGPx/GPx4), an isoform within the family of selenium-dependent glutathione peroxidase (GPx) enzymes that can directly reduce lipid peroxides and other membrane-bound complex hydroperoxides. In this study, we cloned two gpx4 isoforms (gpx4a and gpx4b) from Coho salmon olfactory tissues and compared their modulation in olfactory and liver tissues by cadmium, an environmental pollutant and olfactory toxicant that cause oxidative damage as a mechanism of toxicity. Amino acid sequence comparisons of the two gpx4 isoforms shared 71% identity, and also relatively high sequence identities when compared with other fish GPx4 isoforms. Sequence comparisons with human GPx4 indicated conservation of three important active sites at selenocysteine (U46), glutamine (Q81), and tryptophan (W136), suggesting similar catalytic activity between fish and mammalian GPx4 isoforms. Tissue profiling confirmed the expression of gpx4a and gpx4b in all ten Coho tissues examined. The expression of gpx4 mRNAs in the Coho olfactory system was accompanied by comparably high initial rates of GPx4 enzymatic activity in mitochondrial and cytosolic fractions. Exposure to low (3.7 ppb) and high (347 ppb) environmental Cd concentrations for 24-48 h significantly decreased gpx4a expression in Coho olfactory rosettes, whereas olfactory gpx4b mRNA expression was not modulated by exposures at these concentrations. In summary, Coho salmon express two paralogs of gpx4, a key enzyme in the maintenance of signal transduction processes that protect against cellular oxidative damage. The Cd-associated downregulation of salmon olfactory gpx4a expression in particular, may be associated with the loss of olfactory signal transduction that accompanies metal-associated loss of olfaction in salmonids.

Clinical trials of intranasal delivery for treating neurological disorders--a critical review

INTRODUCTION

The intranasal delivery of therapeutics to the brain has achieved great success in preclinical studies. These findings are important because there are many neurological disorders without feasible treatments, due to a lack of effective drug delivery methods to the brain. Translating such intranasal delivery strategies from bench to bedside is an important step for curing these neurological diseases.

AREAS COVERED

This review summarizes recent clinical trials that have investigated the intranasal delivery of drugs to the brain to treat neurological disorders and their potential mechanisms of action. In addition, the potential opportunities as well as challenges of intranasal delivery in clinical trials are discussed.

EXPERT OPINION

The intranasal delivery of drugs to the brain is a novel method with great potential, and it may provide an extraordinary approach to overcome the existing barriers of drug delivery for treating some neurological disorders. Intranasal delivery of central nervous system therapeutics has shown promise in several clinical trials, which demonstrates both the need and importance of further research.

Functional rehabilitation of cadmium-induced neurotoxicity despite persistent peripheral pathophysiology in the olfactory system

Intranasal exposure to the heavy metal cadmium has been linked to olfactory dysfunction and neurotoxicity. Here, we combine optical imaging of in vivo neurophysiology, genetically defined anatomical tract tracing, mass spectrometry, and behavioral psychophysical methods to evaluate the persistent harmful effects of acute intranasal exposure to cadmium in a mouse model and to investigate the functional consequences of sensory rehabilitation training. We find that an acute intranasal instillation of cadmium chloride leads to an accumulation of cadmium in the brain's olfactory bulb that persists for at least 4 weeks. This is accompanied by persistent severe pathophysiology of the olfactory nerve, a gradual reduction in axonal projections from the olfactory epithelium, and complete impairment on an olfactory detection task. Remarkably, 2 weeks of odorant-guided operant conditioning training proved sufficient to restore olfactory detection performance to control levels in cadmium-exposed mice. Optical imaging from rehabilitated mice showed that this training did not cause any detectable restoration of olfactory nerve function, suggesting that the recovery of function was mediated by central neuroplasticity in which the brain learned to interpret the degraded sensory input. These data demonstrate that sensory learning can mask even severe damage from neurotoxicants and suggest that explicit sensory training may be useful in rehabilitation of olfactory dysfunction.

In vivo visualization of olfactory pathophysiology induced by intranasal cadmium instillation in mice

Intranasal exposure to cadmium has been related to olfactory dysfunction in humans and to nasal epithelial damage and altered odorant-guided behavior in rodent models. The pathophysiology underlying these deficits has not been fully elucidated. Here we use optical imaging techniques to visualize odorant-evoked neurotransmitter release from the olfactory nerve into the brain's olfactory bulbs in vivo in mice. Intranasal cadmium chloride instillations reduced this sensory activity by up to 91% in a dose-dependent manner. In the olfactory bulbs, afferents from the olfactory epithelium could be quantified by their expression of a genetically encoded fluorescent marker for olfactory marker protein. At the highest dose tested, cadmium exposure reduced the density of these projections by 20%. In a behavioral psychophysical task, mice were trained to sample from an odor port and make a response when they detected an odorant against a background of room air. After intranasal cadmium exposure, mice were unable to detect the target odor. These experiments serve as proof of concept for a new approach to the study of the neural effects of inhaled toxicants. The use of in vivo functional imaging of the neuronal populations exposed to the toxicant permits the direct observation of primary pathophysiology. In this study optical imaging revealed significant reductions in odorant-evoked release from the olfactory nerve at a cadmium chloride dose two orders of magnitude less than that required to induce morphological changes in the nerve in the same animals, demonstrating that it is a more sensitive technique for assessing the consequences of intranasal neurotoxicant exposure. This approach is potentially useful in exploring the effects of any putative neurotoxicant that can be delivered intranasally.

Intranasal delivery of stem cells to the brain

INTRODUCTION

Stem cell-based therapy has proved to be a promising treatment option for neurological disorders. However, there are difficulties in successfully administrating these stem cells. For example, the brain-blood barrier impedes the entrance of stem cells into the CNS after systemic administration. Direct transplantation or injection may result in brain injury, and these strategies are clinically less feasible. Intranasal administration is a non-invasive and effective alternative for the delivery of drugs, vector-encoded viruses or even phages to the CNS. Recent studies have in fact demonstrated that stem cells may enter the CNS after intranasal administration. These results suggest that intranasal delivery may provide an alternative strategy for stem cell-based therapy.

AREAS COVERED

This review summarizes current studies that have applied the intranasal delivery of stem cells into the brain. In addition, the distribution and fate of stem cells in the brain and the potential opportunities as well as challenges of intranasal stem cell delivery are also discussed.

EXPERT OPINION

Intranasal delivery of stem cells is a new method with great potential for the transplantation of stem cells into the brain, and it may provide an extraordinary approach to overcoming the existing barriers of stem cell delivery for the treatment of many neurological disorders. This potential benefit emphasizes the importance of future research into intranasal delivery of stem cells.

Lipopolysaccharide-QD micelles induce marked induction of TLR2 and lipid droplet accumulation in olfactory bulb microglia

The intranasal entry of biological and artificial nanoparticles can induce inflammatory responses both locally and more widely in surrounding tissues. The aim of this study was to assess the microglia activation induced by nanoparticles with different surfaces in (i) a transgenic mouse (Toll-like receptor (TLR)-2-luciferase (Luc) reporter) which allowed the biophotonic imaging of microglial activation/innate immune response after intranasal delivery of nanoparticles and (ii) in microglial dispersed cells in vitro. Cadmium selenide nanoparticles (quantum dots, QD), surface-exchanged with lipopolysaccharide (LPS) to form micelles, were tested to assess microglia activation and lipid droplet formation in both model systems. In vivo imaging revealed a robust increase in the extent of microglial activation/TLR2 response, initially in the olfactory bulb, but also in other more caudal brain regions. The increased TLR2 expression was complemented with enhanced CD68 expression in activated microglia in the same regions. Intense in vitro microglial activation by LPS-QD micelles was accompanied by a significant enhancement of nitric oxide production and formation of large lipid droplets, suggesting the possibility of this organelle acting as an inflammatory biomarker in response to nanoparticles, and not simply as a storage site in fat tissues.

[Biological roles of trace elements in the brain with special focus on Zn and Fe]

INTRODUCTION

Many metals like iron (Fe), copper (Cu) or zinc (Zn) fulfil various essential biological functions and are thus vital for all living organisms. For instance, they play important roles in nervous tissue, participating in a wide range of processes such as neurotransmitter synthesis, myelination or synaptic transmission.

STATE OF THE ART

As in other tissues, brain cells tightly control the concentration of metals but any excess or deficit can lead to deleterious responses and alter cognitive functions. Of note, certain metals such as Zn, Fe or Cu accumulate in specific brain structures over lifespan and several neurodegenerative diseases are associated with a dysregulation of the homeostatic mechanisms controlling the concentration of these cations.

CONCLUSION AND PERSPECTIVES

This review will address some of the cellular and molecular processes controlling the entry and distribution of selected metals (mainly Zn and Fe) in the brain, as well as their roles in synaptic transmission, in the pathogenesis of some neurologic diseases such as Parkinson's disease and Alzheimer's disease, and their impact on cognitive functions.

Role of the olfactory receptor neurons in the direct transport of inhaled uranium to the rat brain

Uranium presents numerous industrial and military uses and one of the most important risks of contamination is dust inhalation. In contrast to the other modes of contamination, the inhaled uranium has been proposed to enter the brain not only by the common route of all modes of exposure, the blood pathway, but also by a specific inhalation exposure route, the olfactory pathway. To test whether the inhaled uranium enter the brain directly from the nasal cavity, male Sprague-Dawley rats were exposed to both inhaled and intraperitoneally injected uranium using the (236)U and (233)U, respectively, as tracers. The results showed a specific frontal brain accumulation of the inhaled uranium which is not observed with the injected uranium. Furthermore, the inhaled uranium is higher than the injected uranium in the olfactory bulbs (OB) and tubercles, in the frontal cortex and in the hypothalamus. In contrast, the other cerebral areas (cortex, hippocampus, cerebellum and brain residue) did not show any preferential accumulation of inhaled or injected uranium. These results mean that inhaled uranium enters the brain via a direct transfer from the nasal turbinates to the OB in addition to the systemic pathway. The uranium transfer from the nasal turbinates to the OB is lower in animals showing a reduced level of olfactory receptor neurons (ORN) induced by an olfactory epithelium lesion prior to the uranium inhalation exposure. These results give prominence to a role of the ORN in the direct transfer of the uranium from the nasal cavity to the brain.