Single-walled carbon nanotube-induced mitotic disruption.

Carbon nanotubes were among the earliest products of nanotechnology and have many potential applications in medicine, electronics, and manufacturing. The low density, small size, and biological persistence of carbon nanotubes create challenges for exposure control and monitoring and make respiratory exposures to workers likely. We have previously shown mitotic spindle aberrations in cultured primary and immortalized human airway epithelial cells exposed to 24, 48 and 96 µg/cm(2) single-walled carbon nanotubes (SWCNT). To investigate mitotic spindle aberrations at concentrations anticipated in exposed workers, primary and immortalized human airway epithelial cells were exposed to SWCNT for 24-72 h at doses equivalent to 20 weeks of exposure at the Permissible Exposure Limit for particulates not otherwise regulated. We have now demonstrated fragmented centrosomes, disrupted mitotic spindles and aneuploid chromosome number at those doses. The data further demonstrated multipolar mitotic spindles comprised 95% of the disrupted mitoses. The increased multipolar mitotic spindles were associated with an increased number of cells in the G2 phase of mitosis, indicating a mitotic checkpoint response. Nanotubes were observed in association with mitotic spindle microtubules, the centrosomes and condensed chromatin in cells exposed to 0.024, 0.24, 2.4 and 24 µg/cm(2) SWCNT. Three-dimensional reconstructions showed carbon nanotubes within the centrosome structure. The lower doses did not cause cytotoxicity or reduction in colony formation after 24h; however, after three days, significant cytotoxicity was observed in the SWCNT-exposed cells. Colony formation assays showed an increased proliferation seven days after exposure. Our results show significant disruption of the mitotic spindle by SWCNT at occupationally relevant doses. The increased proliferation that was observed in carbon nanotube-exposed cells indicates a greater potential to pass the genetic damage to daughter cells. Disruption of the centrosome is common in many solid tumors including lung cancer. The resulting aneuploidy is an early event in the progression of many cancers, suggesting that it may play a role in both tumorigenesis and tumor progression. These results suggest caution should be used in the handling and processing of carbon nanotubes.

Ferritin, transferrin, and iron in selected regions of the adult and aged rat brain.

Iron is necessary for normal neural function but it must be stringently regulated to avoid iron-induced oxidative injury. The regulation of systemic iron is through the proteins transferrin (iron mobilization) and ferritin (iron sequestration). This study examines the cellular and regional distribution of iron and the iron-related proteins ferritin and transferrin in selected regions of the adult and aged rat brain. This information is a necessary prerequisite to understanding the mechanism by which iron homeostasis is maintained in the brain. The predominant cell type containing ferritin, transferrin, and iron throughout the brain at all ages is the oligodendrocyte. Neurons in most brain regions contain granular iron deposits which become more apparent with age. Ferritin and iron are also present in microglial cells in all brain regions, but are particularly abundant in the hippocampus. These latter cells visibly increase in number in all brain regions as the animal approaches senescence. Another area in which immunostaining is notable is surrounding the III ventricle, where transferrin is found in the choroid plexus and ependyma and ferritin and iron are present in tanycytes. The results of this study indicate an important role for neuroglia in the regulation of iron in the brain and also implies that a transport system may exist for the transfer of iron between the brain and cerebrospinal fluid. In the normal rodent brain, the principal cell of iron regulation is the oligodendrocyte; however, the role of microglial cells in the sequestration and detoxification of iron may be significant, particularly as the animal ages. With age there is an increase in stainable iron in neurons without a concomitant increase in neuronal ferritin immunostaining, suggesting a ferritin independent accumulation of neuronal iron with age.

A beta and perlecan in rat brain: glial activation, gradual clearance and limited neurotoxicity.

A beta1-40 and perlecan (A beta + perlecan) were infused into rat hippocampus for 1 week via osmotic pumps. At the end of the infusion a deposit of A beta immunoreactive material was found surrounding the infusion site. No neurons could be identified within this A beta deposit. The neuron-free area resulting from A beta + perlecan was significantly larger than that found after infusions of A beta40-1 and perlecan (reverse A beta + perlecan), perlecan alone or phosphate-buffered saline vehicle. Following infusion of A beta + perlecan, the glial cells segregated in a manner similar to that associated with compacted amyloid plaques in Alzheimer's disease (AD). Activated microglia/macrophages were prevalent within the A beta deposit while the perimeter of the deposit was delimited by reactive astrocytes. Thioflavin S and Congo red staining indicated a beta-pleated sheet conformation of the A beta deposits, implying formation of fibrils. Intact, apparently healthy neurons were found immediately adjacent to the A beta + perlecan deposit. In contrast, reverse A beta peptide did not form congophilic deposits despite the presence of perlecan. Apoptotic profiles visualized with bisbenzamide or TUNEL staining of fragmented DNA were not seen at any of the infusion sites, yet were readily seen in hippocampal sections from animals treated with kainic acid. At 8 weeks, A beta immunoreactivity, Thioflavin S and Congo red staining was reduced, indicating that A beta was being cleared. There also was no evidence of neuron loss by Nissl or TUNEL staining. The zone of apparent necrosis did not expand between 1 and 8 weeks, and in some instances appeared to contract. The consistency of the A beta + perlecan infusion method in producing reliable A beta amyloid deposits permits estimates of the rate at which fibrillar A beta amyloid can be removed from the brain, and may provide a useful model to study this process in vivo. However, the absence of clearly identifiable degenerating/dying neurons at the 1 or 8 week survival times suggests that either fibrillar A beta + perlecan slowly displaced the brain parenchyma during infusion, or neurons were killed very gradually during the process of clearing the A beta.

Chemically induced neuronal damage and gliosis: enhanced expression of the proinflammatory chemokine, monocyte chemoattractant protein (MCP)-1, without a corresponding increase in proinflammatory cytokines(1).

Enhanced expression of proinflammatory cytokines and chemokines has long been linked to neuronal and glial responses to brain injury. Indeed, inflammation in the brain has been associated with damage that stems from conditions as diverse as infection, multiple sclerosis, trauma, and excitotoxicity. In many of these brain injuries, disruption of the blood-brain barrier (BBB) may allow entry of blood-borne factors that contribute to, or serve as the basis of, brain inflammatory responses. Administration of trimethyltin (TMT) to the rat results in loss of hippocampal neurons and an ensuing gliosis without BBB compromise. We used the TMT damage model to discover the proinflammatory cytokines and chemokines that are expressed in response to neuronal injury. TMT caused pyramidal cell damage within 3 days and a substantial loss of these neurons by 21 days post dosing. Marked microglial activation and astrogliosis were evident over the same time period. The BBB remained intact despite the presence of multiple indicators of TMT-induced neuropathology. TMT caused large increases in whole hippocampal-derived monocyte chemoattractant protein (MCP)-1 mRNA (1,000%) by day 3 and in MCP-1 (300%) by day 7. The mRNA levels for tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta and IL-6, cytokines normally expressed during the earliest stage of inflammation, were not increased up to 21 days post dosing. Lipopolysaccharide, used as a positive control, caused large inductions of cytokine mRNA in liver, as well as an increase in IL-1beta in hippocampus, but it did not result in the induction of astrogliosis. The data suggest that enhanced expression of the proinflammatory cytokines, TNF-alpha, IL-1beta and IL-6, is not required for neuronal and glial responses to injury and that MCP-1 may serve a signaling function in the damaged CNS that is distinct from its role in proinflammatory events.

Obesity exacerbates chemically induced neurodegeneration.

Obesity is a major risk factor associated with a variety of human disorders. While its involvement in disorders such as diabetes, coronary heart disease and cancer have been well characterized, it remains to be determined if obesity has a detrimental effect on the nervous system. To address this issue we determined whether obesity serves as a risk factor for neurotoxicity. Model neurotoxicants, methamphetamine (METH) and kainic acid (KA), which are known to cause selective neurodegeneration of anatomically distinct areas of the brain, were evaluated using an animal model of obesity, the ob/ob mouse. Administration of METH and KA resulted in mortality among ob/ob mice but not among their lean littermates. While METH caused dopaminergic nerve terminal degeneration as indicated by decreased striatal dopamine (49%) and tyrosine hydroxylase protein (68%), as well as an increase in glial fibrillary acidic protein by 313% in the lean mice, these effects were exacerbated under the obese condition (96%, 86% and 602%, respectively). Similarly, a dosage of KA that did not increase glial fibrillary acidic protein in lean mice increased the hippocampal content of this protein (93%) in ob/ob mice. KA treatment resulted in extensive neuronal degeneration as determined by Fluoro-Jade B staining, decreased hippocampal microtubule-associated protein-2 immunoreactivity and increased reactive gliosis in ob/ob mice. The neurotoxic outcome in ob/ob mice remained exacerbated even when lean and ob/ob mice were dosed with METH or KA based only on a lean body mass. Administration of METH or KA resulted in up-regulation of the mitochondrial uncoupling protein-2 to a greater extent in the ob/ob mice, an effect known to reduce ATP yield and facilitate oxidative stress and mitochondrial dysfunction. These events may underlie the enhanced neurotoxicity seen in the obese mice. In summary, our results implicate obesity as a risk factor associated with chemical- and possibly disease-induced neurodegeneration.

Age exaggerates proinflammatory cytokine signaling and truncates signal transducers and activators of transcription 3 signaling following ischemic stroke in the rat.

Neuroinflammation is associated with glial activation following a variety of brain injuries, including stroke. While activation of perilesional astrocytes and microglia following ischemic brain injury is well documented, the influence of age on these cellular responses after stroke is unclear. This study investigated the influence of advanced age on neuronal degeneration, neuroinflammation, and glial activation in female Sprague-Dawley rats after reversible embolic occlusion of the middle cerebral artery (MCAO). Results indicate that in comparison to young adult rats (3 months), aged rats (18 months) showed enhanced neuronal degeneration, altered microglial response, and a markedly increased expression of proinflammatory cytokines/chemokines following MCAO. In addition, the time-course for activation of signal transducers and activators of transcription 3 (STAT3), the signaling mechanism that regulates astrocyte reactivity, was truncated in the aged rats after MCAO. Moreover, the expression of suppressor of cytokine signaling 3 (SOCS3), which is associated with termination of astrogliosis, was enhanced as a function of age after MCAO. These findings are suggestive of an enhanced proinflammatory response and a truncated astroglial response as a function of advanced age following MCAO. These data provide further evidence of the prominent role played by age in the molecular and cellular responses to ischemic stroke and suggest that astrocytes may represent targets for future therapies aimed at improving stroke outcome.

Induction of aneuploidy by single-walled carbon nanotubes.

Engineered carbon nanotubes are newly emerging manufactured particles with potential applications in electronics, computers, aerospace, and medicine. The low density and small size of these biologically persistent particles makes respiratory exposures to workers likely during the production or use of commercial products. The narrow diameter and great length of single-walled carbon nanotubes (SWCNT) suggest the potential to interact with critical biological structures. To examine the potential of nanotubes to induce genetic damage in normal lung cells, cultured primary and immortalized human airway epithelial cells were exposed to SWCNT or a positive control, vanadium pentoxide. After 24 hr of exposure to either SWCNT or vanadium pentoxide, fragmented centrosomes, multiple mitotic spindle poles, anaphase bridges, and aneuploid chromosome number were observed. Confocal microscopy demonstrated nanotubes within the nucleus that were in association with cellular and mitotic tubulin as well as the chromatin. Our results are the first to report disruption of the mitotic spindle by SWCNT. The nanotube bundles are similar to the size of microtubules that form the mitotic spindle and may be incorporated into the mitotic spindle apparatus.

Iron regulation in the brain: histochemical, biochemical, and molecular considerations.

Despite recognition that iron is important for normal neurological function, the proteins involved in maintaining iron homeostasis within the brain have until recently received little attention. In the past few years, studies aimed at determining both general and cellular control of iron in the brain have increased. Histological studies indicate that maintenance of iron homeostasis in the brain is the responsibility of neuroglia and possibly the choroid plexus. Transferrin, the iron transport protein, has been found predominantly in oligodendrocytes in the brain and in myelinating Schwann cells in the peripheral nervous system. The messenger RNA for transferrin is located in the brain in oligodendrocytes and the choroid plexus. Most of the transferrin protein and transcript expression in the brain is dependent on the presence of a mature population of oligodendrocytes. Transferrin is also involved in the transport of iron across the blood-brain barrier via transferrin receptors on brain capillary endothelial cells. The transferrin receptor is also present on cells within the brain. Ferritin, the iron storage protein, and iron are found in the brain in oligodendrocytes and microglia. Additional cells in which iron and ferritin are found are tanycytes, which are associated with the third ventricle. This latter observation raises interesting possibilities regarding the transport of iron from cerebrospinal fluid into the brain. The high iron requirement of the brain coupled with the high susceptibility of the brain to iron-generated peroxidative damage requires stringent regulation of the availability of iron. Consequently, the iron regulatory proteins are central to understanding mechanisms controlling iron-dependent activity at the cellular level, as well as protection of the brain from oxidative damage. The behavior of brain iron regulatory proteins will be a significant factor in future studies of the neurological diseases resulting from brain iron imbalance. We review the contributions of our laboratory to this field over the past 6 years, discuss current projects, and suggest future directions for study.

Isoforms of ferritin have a specific cellular distribution in the brain.

Ferritin is the major iron storage protein and accounts for the majority of the iron in the brain. Thus, ferritin is a key component in protecting the brain from iron induced oxidative damage. The high lipid content, high rate of oxidative metabolism, and high iron content combine to make the brain the organ most susceptible to oxidative stress. The role of oxidative damage and disruption of brain iron homeostasis is considered clinically important to normal aging and a potential pathogenic component of a number of neurologic disorders including Alzheimer's disease and Parkinson's disease. Little is known, however, of the mechanism by which the brain maintains iron homeostasis at either the whole organ or cellular level. In this study we report the cellular distribution of the two isoforms of ferritin in the brain of adult subhuman primates. A subset of neurons immunolabel specifically for the H-chain ferritin protein, whereas cells resembling microglia are immunolabeled only after exposure to the L-chain ferritin antibody. Only one cell type immunostains for both H- and L-chain ferritin; these cells are morphologically similar and have the same distribution pattern as oligodendrocytes. Neither ferritin isoform is usually detected in astrocytes. These data indicate considerable differences in iron sequestration and use between neurons and glia and among neuronal and glial subtypes. This information will be essential in determining the role of each of these cells in maintaining general brain iron homeostasis and the relative abilities of these cells to withstand oxidative stress.

Regional and cellular localization of presenilin-2 RNA in rat and human brain.

In situ hybridization probes selective for presenilin-2 (PS-2) were used to determine the regional and cellular expression pattern of PS-2 mRNA in rat and human brain. In rat brain, the greatest expression of PS-2 mRNA is in the granule cell layers of the dentate gyrus and cerebellum. Molecular layers within these structures are virtually devoid of signal. Cortical expression of PS-2 message is restricted to neuronal layers, while the hybridization signal is weak or absent in molecular layers and white matter. Kidney, liver, and spleen display moderate levels of PS-2 message. A PS-2 sense strand probe produced no specific signals in any tissue. In human brain, the greatest hybridization signal for PS-2 is present in the granule cells of the cerebellum. Within hippocampus, the granule cell layer of dentate is strongly labeled, with CA3 pyramidal neurons also clearly visible. A laminar expression pattern is seen in the neuronal layers of human frontal and temporal cortex, with the deeper laminae having the strongest signals. These data are consistent with a primarily neuronal localization of PS-2 mRNA within the brains of both rat and human. Within the limitations of the analysis, it appears that virtually every neuron is labeled, and differences in the intensity of labeling are associated with both neuron size/density and brain region. The distribution of PS-2 RNA is not restricted to those regions having the greatest pathology in Alzheimer's disease. However, one unusual pathological feature of PS-2 mutations causing AD is the presence of cerebellar amyloid plaques in some cases. It is intriguing, in this context, that PS-2 RNA is enriched in the cerebellum, especially in human specimens.