Brain Barrier Systems Play No Small Roles in Toxicant-induced Brain Disorders

Essential Trace Elements Prevent the Impairment in the Retention Memory, Cerebral Cortex, and Cerebellum Damage in Male Rats Exposed to Quaternary Metal Mixture by Up-regulation, of Heme Oxygynase-1 and Down-regulation of Nuclear Factor Erythroid 2-related Factor 2-NOs Signaling Pathways

In the present study, we examined adverse effects of metals and metalloids in the Cerebral cortex (CC) and Cerebellum (CE). Group 1 comprised from the controls while other four groups of male Wistar rats were treated with following pattern: Group II (Heavy Metal Mixture HMM only: PbCl₂, 20 mg·kg^-1; CdCl₂, 1.61 mg·kg^-1; HgCl₂, 0.40 mg·kg^-1, and NaAsO₃,10 mg·kg^-1), Groups III (HMM + ZnCl₂); Group IV (HMM + Na₂SeO₃) and Group V (HMM + ZnCl₂ + Na₂SeO₃) for 60 days per os. HMM promoted oxidative stress in the CC and CE of treated rats compared to controls; moreover, exposure to HMM led to increased activity of the AChE and pro-inflammatory cytokines; also, HMM promoted accumulation of caspase 3 and other transcriptional factors such as Nrf2 and decreased levels of Hmox-1. Essential metals reduced increased bioaccumulation of Pb, Cd, As and Hg in CC and CE caused by HMM exposure. Also, all mentioned adverse effects were diminished by essential metals treatment (Se and Zn). HMM exposed rats had considerably less escape dormancy than controls. Histopathological analysis revealed moderate cell loss at the intermediate (Purkinje cell) and granular layer. Zinc and selenium supplementations could reverse adverse effects of heavy metals at various cellular levels in neurons.

Toxic and carcinogenic effects of hexavalent chromium in mammalian cells in vivo and in vitro: a recent update

Chromium VI (Cr (VI)) can cross cell membranes readily and causes the formation of Cr-DNA adducts, genomic damages, elevation of reactive oxygen species (ROS) and alteration of survival signaling pathways, as evidenced by the modulation in p53 signaling pathway. Mammals, including humans are exposed to Cr, including Cr (VI), frequently through inhalation, drinking water, and food. Several studies demonstrated that Cr (VI) induces cellular death through apoptosis and autophagy, genotoxicity, functional alteration of mitochondria, endocrine and reproductive impairments. In the present review, studies on deleterious effects of Cr (VI) exposure to mammalian cells (in vivo and in vitro) have been documented. Special attention is paid to the underlying molecular mechanism of Cr (VI) toxicity.

The development and function of the brain barriers - an overlooked consideration for chemical toxicity

It is well known that the adult brain is protected from some infections and toxic molecules by the blood-brain and the blood-cerebrospinal fluid barriers. Contrary to the immense data collected in other fields, it is deeply entrenched in environmental toxicology that xenobiotics easily permeate the developing brain because these barriers are either absent or non-functional in the fetus and newborn. Here we review the cellular and physiological makeup of the brain barrier systems in multiple species, and discuss decades of experiments that show they possess functionality during embryogenesis. We next present case studies of two chemical classes, perfluoroalkyl substances (PFAS) and bisphenols, and discuss their potential to bypass the brain barriers. While there is evidence to suggest these pollutants may enter the developing and/or adult brain parenchyma, many studies suffer from confounding technical variables which complicates data interpretation. In the future, a more formal consideration of brain barrier biology could not only improve understanding of chemical toxicokinetics but could assist in prioritizing environmental xenobiotics for their neurotoxicity risk.

A Historical Review of Brain Drug Delivery

The history of brain drug delivery is reviewed beginning with the first demonstration, in 1914, that a drug for syphilis, salvarsan, did not enter the brain, due to the presence of a blood-brain barrier (BBB). Owing to restricted transport across the BBB, FDA-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Drugs that do not cross the BBB can be re-engineered for transport on endogenous BBB carrier-mediated transport and receptor-mediated transport systems, which were identified during the 1970s-1980s. By the 1990s, a multitude of brain drug delivery technologies emerged, including trans-cranial delivery, CSF delivery, BBB disruption, lipid carriers, prodrugs, stem cells, exosomes, nanoparticles, gene therapy, and biologics. The advantages and limitations of each of these brain drug delivery technologies are critically reviewed.

Age-dependent decline of copper clearance at the blood-cerebrospinal fluid barrier

The homeostasis of copper (Cu) in the central nervous system is regulated by the blood-brain barrier and blood-cerebrospinal (CSF) barrier (BCB) in the choroid plexus. While proteins responsible for Cu uptake, release, storage and intracellular trafficking exist in the choroid plexus, the influence of age on Cu clearance from the CSF via the choroid plexus and how Cu transporting proteins contribute to the process are unelucidated. This study was designed to test the hypothesis that the aging process diminishes Cu clearance from the CSF of rats by disrupting Cu transporting proteins in the choroid plexus. Data from ventriculo-cisternal perfusion experiments demonstrated greater ⁶⁴Cu radioactivity in the CSF effluents of older rats (18 months) compared to younger (1 month) and adult (2 months) rats, suggesting much slower removal of Cu by the choroid plexus in old animals. Studies utilizing qPCR and immunofluorescence revealed an age-specific expression pattern of Cu transporting proteins in the choroid plexus. Moreover, proteomic analyses unraveled age-specific proteomes in the choroid plexus with distinct pathway differences, particularly associated with extracellular matrix and neurodevelopment between young and old animals. Taken together, these findings support an age-dependent deterioration in CSF Cu clearance, which appears to be associated with altered subcellular distribution of Cu transporting proteins and proteomes in the choroid plexus.

Current understanding of hexavalent chromium [Cr(VI)] neurotoxicity and new perspectives

Hexavalent chromium [Cr(VI)] is a global environmental pollutant that increases risk for several types of cancers and is increasingly being recognized as a neurotoxicant. Traditionally, the brain has been viewed as a largely post-mitotic organ due to its specialized composition of neurons, and consequently, clastogenic effects were not considered in neurotoxicology. Today, we understand the brain is composed of at least eight distinct cell types - most of which continue mitotic activity throughout lifespan. We have learned these dividing cells play essential roles in brain and body health. This review focuses on Cr(VI), a potent clastogen and known human carcinogen, as a potentially neurotoxic agent targeting mitotic cells of the brain. Despite its well-established role as a human carcinogen, Cr(VI) neurotoxicity studies have failed to find a significant link to brain cancers. In the few studies that did find a link, Cr(VI) was identified as a risk for gliomas. Instead, in the human brain, Cr(VI) appears to have more subtle deleterious effects that can impair childhood learning and attention development, olfactory function, social memory, and may contribute to motor neuron diseases. Studies of Cr(VI) neurotoxicity with animal and cell culture models have demonstrated elevated markers of oxidative damage and redox stress, with widespread neurodegeneration. One study showed mice exposed to Cr(VI)-laden tannery effluent exhibited longer periods of aggressive behavior toward an "intruder" mouse and took longer to recognize mice previously encountered, recapitulating the social memory deficits observed in humans. Here we conducted a critical review of the available literature on Cr(VI) neurotoxicity and synthesize the collective observations to thoroughly evaluate Cr(VI) neurotoxicity - much remains to be understood and recognized.

Exposure to Traffic-Generated Pollutants Exacerbates the Expression of Factors Associated with the Pathophysiology of Alzheimer’s Disease in Aged C57BL/6 Wild-Type Mice

Background: Multiple studies report a strong correlation between traffic-generated air pollution-exposure and detrimental outcomes in the central nervous system (CNS), including Alzheimer’s disease (AD). Incidence of AD is rapidly increasing and, worldwide, many live in regions where pollutants exceed regulatory standards. Thus, it is imperative to identify environmental pollutants that contribute to AD, and the mechanisms involved. Objective: We investigated the effects of mixed gasoline and diesel engine emissions (MVE) on the expression of factors involved in progression of AD in the hippocampus and cerebrum in a young versus aged mouse model. Methods: Young (2 months old) and aged (18 months old) male C57BL/6 mice were exposed to either MVE (300μg/m3 PM) or filtered air (FA) for 6 h/d, 7 d/wk, for 50 d. Immunofluorescence and RT-qPCR were used to quantify oxidative stress (8-OHdG) and expression of amyloid-β protein precursor (AβPP), β secretase (BACE1), amyloid-β (Aβ), aryl hydrocarbon receptor (AhR), cytochrome P450 (CYP) 1B1, angiotensin-converting enzyme (ACE1), and angiotensin II type 1 (AT1) receptor in the cerebrum and hippocampus, in addition to cerebral microvascular tight junction (TJ) protein expression. Results: We observed age-related increases in oxidative stress, AhR, CYP1B1, Aβ, BACE1, and AT1 receptor in the CA1 region of the hippocampus, and elevation of cerebral AβPP, AhR, and CYP1B1 mRNA, associated with decreased cerebral microvascular TJ protein claudin-5. MVE-exposure resulted in further promotion of oxidative stress, and significant increases in AhR, CYP1B1, BACE1, ACE1, and Aβ, compared to the young and aged FA-exposed mice. Conclusion: Such findings suggest that MVE-exposure exacerbates the expression of factors in the CNS associated with AD pathogenesis in aged populations.

Localization of ZIP14 and ZIP8 in HIBCPP Cells

The blood-cerebrospinal fluid barrier (BCB) is important in maintaining brain manganese (Mn) homeostasis. This barrier consists of a single layer of epithelial cells, connected by tight junctions, that restrict the passage of nutrients to only allow molecules to be carried through the membrane by a transporter. These epithelial cells are polarized with asymmetrical blood-facing and cerebrospinal fluid-facing sides. Here, we have established a polarized model of a human choroid plexus papilloma cell line, HIBCPP. For the first time, Mn importers ZIP14 and ZIP8 were identified in HIBCPP cells and were found to be enriched at the basolateral and apical sides of the cell monolayer, respectively. The localization of each ZIP protein adds to the understanding of Mn transport across the HIBCPP BCB model to help understand the mechanism of Mn homeostasis within the brain.