Identification of the bacterial metabolite aerugine as potential trigger of human dopaminergic neurodegeneration.

The causes of nigrostriatal cell death in idiopathic Parkinson's disease are unknown, but exposure to toxic chemicals may play some role. We followed up here on suggestions that bacterial secondary metabolites might be selectively cytotoxic to dopaminergic neurons. Extracts from Streptomyces venezuelae were found to kill human dopaminergic neurons (LUHMES cells). Utilizing this model system as a bioassay, we identified a bacterial metabolite known as aerugine (C10H11NO2S; 2-[4-(hydroxymethyl)-4,5-dihydro-1,3-thiazol-2-yl]phenol) and confirmed this finding by chemical re-synthesis. This 2-hydroxyphenyl-thiazoline compound was previously shown to be a product of a wide-spread biosynthetic cluster also found in the human microbiome and in several pathogens. Aerugine triggered half-maximal dopaminergic neurotoxicity at 3-4 µM. It was less toxic for other neurons (10-20 µM), and non-toxic (at <100 µM) for common human cell lines. Neurotoxicity was completely prevented by several iron chelators, by distinct anti-oxidants and by a caspase inhibitor. In the Caenorhabditis elegans model organism, general survival was not affected by aerugine concentrations up to 100 µM. When transgenic worms, expressing green fluorescent protein only in their dopamine neurons, were exposed to aerugine, specific neurodegeneration was observed. The toxicant also exerted functional dopaminergic toxicity in nematodes as determined by the "basal slowing response" assay. Thus, our research has unveiled a bacterial metabolite with a remarkably selective toxicity toward human dopaminergic neurons in vitro and for the dopaminergic nervous system of Caenorhabditis elegans in vivo. These findings suggest that microbe-derived environmental chemicals should be further investigated for their role in the pathogenesis of Parkinson's disease.

Nickel-Induced Developmental Neurotoxicity in C. elegans Includes Cholinergic, Dopaminergic and GABAergic Degeneration, Altered Behaviour, and Increased SKN-1 Activity.

Nickel (Ni) is a ubiquitous metal in the environment with increasing industrial application. While environmental and occupational exposure to Ni compounds has been known to result in toxicities to several organs, including the liver, kidney, lungs, skin and gonads, neurotoxic effects have not been extensively investigated. In this present study, we investigated specific neuronal susceptibility in a C. elegans model of acute Ni neurotoxicity. Wild-type worms and worms expressing green fluorescent protein (GFP) in either cholinergic, dopaminergic or GABAergic neurons were treated with NiCl2 for 1 h at the first larval (L1) stage. The median lethal dose (LD50) was calculated to be 5.88 mM in this paradigm. Morphology studies of GFP-expressing worms showed significantly increasing degeneration of cholinergic, dopaminergic and GABAergic neurons with increasing Ni concentration. Significant functional changes in locomotion and basal slowing response assays reflected that cholinergic and dopaminergic neuronal function, respectively, were impaired due to Ni treatment. Interestingly, a small but significant number of worms exhibited shrinker phenotype upon Ni exposure but no loopy head foraging behaviour was observed suggesting that function of D-type GABAergic neurons of C elegans may be specifically attenuated while the RME subset of GABAergic neurons are not. GFP expression due to induction of glutathione S-transferase 4 (gst-4), a target of Nrf2 homolog skn-1, was increased in a Pgst-4::GFP worm highlighting Ni-induced oxidative stress. RT-qPCR verified upregulation of this expression of gst-4 immediately after exposure. These data suggest that oxidative stress is associated with neuronal damage and altered behaviour due to developmental Ni exposure.

The effects of manganese overexposure on brain health.

Manganese (Mn) is the twelfth most abundant element on the earth and an essential metal to human health. Mn is present at low concentrations in a variety of dietary sources, which provides adequate Mn content to sustain support various physiological processes in the human body. However, with the rise of Mn utility in a variety of industries, there is an increased risk of overexposure to this transition metal, which can have neurotoxic consequences. This risk includes occupational exposure of Mn to workers as well as overall increased Mn pollution affecting the general public. Here, we review exposure due to air pollution and inhalation in industrial settings; we also delve into the toxic effects of manganese on the brain such as oxidative stress, inflammatory response and transporter dysregulation. Additionally, we summarize current understandings underlying the mechanisms of Mn toxicity.

Lead (Pb) exposure induces dopaminergic neurotoxicity in Caenorhabditis elegans: Involvement of the dopamine transporter.

Lead (Pb) is an environmental neurotoxicant, and has been implicated in several neurological disorders of dopaminergic dysfunction; however, the molecular mechanism of its toxicity has yet to be fully understood. This study investigated the effect of Pb exposure on dopaminergic neurodegeneration and function, as well as expression level of several dopaminergic signaling genes in wild type (N2) and protein kinase C (pkc) mutant Caenorhabditis elegans. Both N2 and pkc mutant worms were exposed to Pb2+ for 1 h. Thereafter, dopaminergic (DAergic) neurodegeneration, behavior and gene expression levels were assessed. The results revealed that Pb2+ treatment affects dopaminergic cell morphology and structure in worms expressing green fluorescent protein (GFP) under a DAergic cell specific promoter. Also, there was a significant impairment in dopaminergic neuronal function as tested by basal slowing response (BSR) in wild-type, N2 worms, but no effect was observed in pkc mutant worms. Furthermore, Pb2+ exposure increased dat-1 gene expression level when compared with N2 worms, but no alteration was observed in the pkc mutant strains. LC-MS analysis revealed a significant decrease in dopamine content in worms treated with Pb2+ when compared with controls. In summary, our results revealed that Pb2+ exposure induced dopaminergic dysfunction in C. elegans by altering dat-1 gene levels, but pkc mutants showed significant resistance to Pb2+ toxicity. We conclude that PKC activation is directly involved in the neurotoxicity of Pb.

Triclosan induces PC12 cells injury is accompanied by inhibition of AKT/mTOR and activation of p38 pathway.

Triclosan (TCS) has been widely used as a disinfectant and antiseptic in multiple consumer and healthcare products due to its clinical effectiveness against various bacteria, fungi and protozoa. Recently, several studies have reported the adverse effects of TCS on various nerve cells, arousing concerns about its potential neurotoxicity. The present study aimed to investigate the neurotoxicity of TCS in rat pheochromocytoma PC12 cells. After differentiation, the stabilized PC12 cells were treated with 1, 10, 50 µM TCS for 12 h. At the end of the treatment, the generation of reactive oxygen species (ROS), protein expression of apoptotic-related genes, AMPK-AKT/mTOR, as well as p38 in PC12 cells were determined. The concentrations were chosen based on the results of cell viability and lactic dehydrogenase (LDH) assays in response to TCS treatment (ranging from 0.001 to 100 µM) for varied time periods. The results showed that TCS is cytotoxic to PC12 cells, causing decreased cell viability accompanied by increased LDH release. TCS treatment at 10 and 50 µM for 12 h increased the mRNA and protein expression of the pro-apoptotic gene Bax, while Bcl-2 levels remained unchanged. Moreover, an increase in the generation of reactive oxygen species (ROS) was found in TCS-treated PC12 cells at the concentrations of 1 and 10 µM. Pretreatment with 100 µM N-acetyl cysteine (NAC- ROS scavenger) for 1 h normalized the ROS generations in TCS-treated PC12 cells. Additionally, the suppression of the phosphorylation of Akt and mTOR was observed in TCS-treated PC12 cells at 10 and 50 µM for 12 h, concomitant with the activation of p38 MAPK pathway at 50 µM TCS. However, there were no effects of TCS on the phosphorylation of AMPK in these cells. Taken together, these results suggest that TCS may cause adverse effects and oxidative stress in PC12 cells accompanied by inhibition of Akt/mTOR and activation of p38.

Role of Caenorhabditis elegans AKT-1/2 and SGK-1 in Manganese Toxicity.

Excessive levels of the essential metal manganese (Mn) may cause a syndrome similar to Parkinson's disease. The model organism Caenorhabditis elegans mimics some of Mn effects in mammals, including dopaminergic neurodegeneration, oxidative stress, and increased levels of AKT. The evolutionarily conserved insulin/insulin-like growth factor-1 signaling pathway (IIS) modulates worm longevity, metabolism, and antioxidant responses by antagonizing the transcription factors DAF-16/FOXO and SKN-1/Nrf-2. AKT-1, AKT-2, and SGK-1 act upstream of these transcription factors. To study the role of these proteins in C. elegans response to Mn intoxication, wild-type N2 and loss-of-function mutants were exposed to Mn (2.5 to 100 mM) for 1 h at the L1 larval stage. Strains with loss-of-function in akt-1, akt-2, and sgk-1 had higher resistance to Mn compared to N2 in the survival test. All strains tested accumulated Mn similarly, as shown by ICP-MS. DAF-16 nuclear translocation was observed by fluorescence microscopy in WT and loss-of-function strains exposed to Mn. qRT-PCR data indicate increased expression of γ-glutamyl cysteine synthetase (GCS-1) antioxidant enzyme in akt-1 mutants. The expression of sod-3 (superoxide dismutase homologue) was increased in the akt-1 mutant worms, independent of Mn treatment. However, dopaminergic neurons degenerated even in the more resistant strains. Dopaminergic function was evaluated with the basal slowing response behavioral test and dopaminergic neuron integrity was evaluated using worms expressing green fluorescent protein (GFP) under the dopamine transporter (DAT-1) promoter. These results suggest that AKT-1/2 and SGK-1 play a role in C. elegans response to Mn intoxication. However, tissue-specific responses may occur in dopaminergic neurons, contributing to degeneration.

C. elegans as a model in developmental neurotoxicology.

Due to many advantages Caenorhabditis elegans (C. elegans) has become a preferred model of choice in many fields, including neurodevelopmental toxicity studies. This review discusses the benefits of using C. elegans as an alternative to mammalian systems and gives examples of the uses of the nematode in evaluating the effects of major known neurodevelopmental toxins, including manganese, mercury, lead, fluoride, arsenic and organophosphorus pesticides. Reviewed data indicates numerous similarities with mammals in response to these toxins. Thus, C. elegans studies have the potential to predict possible effects of developmental neurotoxicants in higher animals, and may be used to identify new molecular pathways behind neurodevelopmental disruptions, as well as new toxicants.

Null allele mutants of trt-1, the catalytic subunit of telomerase in Caenorhabditis elegans, are less sensitive to Mn-induced toxicity and DAergic degeneration.

Exposure to manganese (Mn) represents an environmental risk factor for Parkinson's disease (PD). Recent evidence suggests that telomerase reverse transcriptase (TERT), the catalytic subunit of mammalian telomerase participates in non-telomeric functions and may play a role in cellular protection from oxidative stress and DNA damage. trt-1 is the catalytic subunit of telomerase in Caenorhabditis elegans (C. elegans). The present study investigated the relationship between trt-1 mutation and Mn-induced neurotoxicity. Wild-type (wt) and trt-1 worms were subjected to an acute Mn treatment of 1h at the first larval (L1) stage. Survival assay and behavior (Basal slowing response, chemotaxis) were assessed. Dopaminergic (DAergic) neurodegeneration was evaluated in successful crosses of trt-1 worms expressing green fluorescent protein (GFP) (dat-1:GFP worms). trt-1 worms were less sensitive to Mn-induced lethality compared to wt worms. Mn induced DAergic degeneration in wt worms, but not in trt-1 worms. Basal slowing was altered in both wt and trt-1 worms; however trt-1 worms were significantly less affected in their basal slowing behavior compared to wt worms. Mn treatment did not affect chemotaxis by NaCl in either wt or trt-1 mutants worms. Combined, the results establish that null mutation in trt-1 improves survival and attenuates damage to the DAergic system.