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For original transcript: http://www.abc.net.au/lateline/content/2012/s3545855.htm
Daniel Lai (z3393749)
Geoffrey Hung (z3380054)
Laurene Lebelt (z3432578)
Matthew Wong (z3375126)

Abbreviations

BBB, blood brain barrier; DA, dopamine; DAT, dopamine transporter; IP, intraperitoneal; MAO-B, monoamine oxidase-B; MPTP, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine; PD, Parkinson's disease; PQ, paraquat; ROS, reactive oxygen species; SN, substantia nigra.

1. Introduction


Parkinson’s disease (PD) is a disorder in which dopamine generating cells have depleted causing the sufferer to have motion impairments such as resting tremors and slowed movement. It is a major disorder still under intense investigation as the causes of this disease are not completely understood. This media item was produced by ABC Lateline and recently aired on July, 2012.

Specifically, it addresses current concerns in Australia over the commonly used pesticide, paraquat (PQ), which has been implicated in the eitology of PD. PQ has been under review since 1997 in Australia and the pesticide has already been banned in over 30 countries. The main allegations of PQ are still being assessed as the company producing the product denies the link between PD and PQ. PQ, as the news item mentions, has developed a ‘bad reputation’ and has been grouped with other pesticides known definitively to induce symptoms of PD such as rotenone.

This news item is of particular interest because PQ has significant use in improving agricultural yields by inhibiting weed growth. Consequently, banning PQ can have detrimental impacts on the agricultural industry in Australia. It is evident that banning PQ purely on a ‘bad reputation’ is unjustified given its usefulness whereas continuing its use could potentially increase the incidence of PD of those employed in the agricultural industry. Currently, the definitive link between PD and PQ is based upon epidemiological studies which are purely correlative.

To gain a deeper insight into this issue, a clearer understanding of how PQ is directly involved in the etiology of PD beyond epidemiological evidence is essential. This allows for a more informed choice on whether the ban on PQ is warranted. The rest of this article specifically addresses how environmental agents might be directly involved in the development of PD, although the influence of genetic factors must not be forgotten.

2. Neuroscientific Context


2.1 Parkinson's Disease

2.1.1 What is Parkinson’s Disease?
Parkinson’s disease is one the most common neurodegenerative disorders, yet it remains one of the most mysterious to date. In all Parkinson’s disease patients, there is the death of dopaminergic cells, and hence depletion of dopamine (see Figure 1), which is an essential neurotransmitter responsible for regulating processes such as hunger, sleep but most importantly, motor function. This lack of dopamine will lead to an imbalance of neurons directly involved in motility, affecting normal motor function. People diagnosed with Parkinson’s disease exhibit symptoms such as tremors, rigidity, akinesia and muscle stiffness. The true cause of Parkinson’s disease is still unclear, due to a culmination of different factors at work – both genetic and environmental. Due to constraints, the environmental basis of PD will be discussed.

CNS102.jpg
Figure 1. Parkinson's disease patients have depleted dopamine, shown here by reduced substantia nigra (black lining) on left

2.2 Environmental Causes

2.2.1 Oxidative stress
A very common theory underlying Parkinson’s disease pathology is causation by oxidative stress. All reactive oxidants and free radicals can contribute to oxidative stress. This implies anti-oxidants can help to alleviate symptoms, and reduce the risk of developing Parkinson’s disease. For instance, epidemiological studies suggest that there is an inverse correlation between smoking and Parkinson’s disease in both men and women (Hernan et al., 2001). This may be due to increased levels of reducing carbon monoxide in circulation, which may shield against oxidative stress.

2.2.2 MPTP
Common environmental toxins may exert their effects by oxidative stress, such as MPTP. The Parkinsonian effects of MPTP were discovered in the 1980s, when a group of drug users in California took cocaine contaminated with MPTP. They were stricken with symptoms characteristic of Parkinson’s disease. The effects were shown to be irreversible, and quickly brought attention to the existence of environmental causes of Parkinson’s disease. It is now known that MPTP is metabolised to MPP+ via MAO-B, which can then inhibit complex I of mitochondria and prevent oxidation of NADH (Nicklas et al., 1985; Jenner, 1989).

2.2.3 Rotenone
Rotenone, another possible culprit implicating the disease is widely used in chemical pesticides. Because it is lipid-soluble, it has easy access into all organs, including the brain. Studies suggest that rotenone causes mitochondria dysfunction, by impairing oxidative phosphorylation, through inhibition of activity at complex I (Betarbet et al., 2000). This inhibition, although affecting the whole brain produces selective degeneration of dopaminergic cells. Sherer et al. (2003) hypothesised that the toxicity of rotenone is associated with increased oxidative stress, often arising from the production of reactive oxygen species in complex I. This was supported by results that showed increased resistance to cell death when an antioxidant was added. On a demographic scale, it appears that people who are more exposed to it, for instance in agricultural areas, have a greater risk of being affected by the disease (Tanner et al., 2011).

2.3 Paraquat

Despite epidemiological data suggesting a role for Paraquat (PQ) in the etiology of Parkinson’s disease (PD), the underlying mechanisms of paraquat induced PD remains a highly disputed area of research. Specifically, ambiguities arise from conflicting results concerning the ability for paraquat to cross the blood brain barrier (BBB) and how paraquat selectively targets and damages dopamine (DA) neurons in the substantia nigra (SN) in order to produce the symptoms PD. Furthermore, issues regarding the manner in which PQ exposure occurs and the dosage also add to the uncertainties of PQ’s neurotoxic effects.

2.3.1 Mechanism
The mechanism for Paraquat’s toxicity is generally attributed to a redox cycling process where reactive oxygen species (ROS), such as superoxide (O2-), are generated (see Figure 2).

paraquatmechanism.jpg
Figure 2. The dication of paraquat, PQ2+ (on the left), is reduced to PQ+ (on the right) and PQ+ then acts to reduce molecular oxygen, O2 to O2- regenerating the parent PQ2+ in the process (Inchem, 2009).

Accumulation of ROS within cells is generally cytotoxic as it can have deleterious effects on DNA, RNA and proteins (Cabiscol et al., 2000).

2.3.2 Selectivity of Paraquat
Elucidating the means by which the above, non-specific mechanism is propagated selectively in the DA neurons of the SN is important in substantiating the link between PD and paraquat. MPP+, the metabolite of MPTP responsible for the symptoms of PD, shares a chemical structure similar to that of paraquat (see Figure 3). This had led to the suggestion that paraquat may operate under the same mechanisms as MPP+ to induce neuronal damage. To validate this claim, it must be shown that paraquat, like MPP+, can be transported by the dopamine transporter (DAT) into DA neurons and that it must inhibit mitochondrial complex I.


similarityPQMPP.jpg
Figure 3. Structural comparison of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), its metabolite 1-methyl-4-phenylpyridinium (MPP+) and paraquat (PQ). (Dinis-Oliveira, 2006)


Richardson, Quan, Sherer, Greenamyre and Miller (2005) have shown that unlike MPP+, PQ does not seem to impair the functioning of complex I in mitochondria and is not a substrate of the DAT. This suggests that the underlying mechanisms for the neurotoxic effects of PQ are different to that of MPP
and may be non-specific.

However, Rappold et al. (2011) have shown that whilst PQ2+ does not act as a substrate for the DAT, PQ+does. They also showed that in the presence of the reducing agent, NADPH oxidase expressed in microglia, PQ2+ was converted to PQ+ and subsequently taken into DA neurons via the DAT. This finding provides evidence for a possible mechanism in which paraquat can selectively target and damage DA neurons via intracellular redox cycling (see Figure 4).
PQmechanism.jpg
Figure 4. The main pathway for PQ mediated toxicity is highlighted. PQ2+ is reduced to PQ+ by NADPH oxidase extracellularly. PQ+ can then enter dopamine neurons through the dopamine transporter. An intracellular redox cycling process is then established in dopamine neurons. (Rappold et al., 2011)

Rappold et al. (2011) have suggested that DA neurons are specifically vulnerable to the neurotoxic effects of PQ because of the pathways involved in DA metabolism. Yamato et al. (2010), showed how inducing excessive DA metabolism via the monamine oxidase pathway can lead to the formation of ROS. This demonstrates how alterations in DA metabolism can have deleterious effects on DA neurons. Since, Rappoldet al. (2011) showed that ROS formation in DA cells was enhanced in the presence of both PQ+ and a nontoxic concentration of DA compared to a control, it is possible that PQ may affect DA metabolism in a way that enhances ROS formation in DA neurons.

Whilst it seems that PQ does not inhibit mitochondrial complex I, it has been demonstrated that PQ can nonetheless selectively target and damage DA neurons. This may be due to the notion that PQ+ is a substrate for DAT and that PQ may alter DA metabolism in a manner that induces damage to DA neurons via increased ROS formation.

2.3.3 The blood brain barrier
Before paraquat can selectively target and damage nigrostriatal neurons via oxidative stress, it needs to cross the BBB in a sufficient quantity. In terms of passive diffusion, the BBB is only permeable to small and lipid-soluble molecules. This presents a problem for a relatively large and divalent cation such as Paraquat (PQ2+) and raises the question as to how brain structures protected by the BBB can even sequester this toxic herbicide.

Since some molecules, such as glucose and some amino acids, required by the brain are large and/or polar, mechanisms must exist in which those molecules cross the BBB. It may be possible that PQ2+ enters the brain via these same mechanisms. This seems plausible given that administration of the amino acid to L-valine served to prevent damage to DA neurons in the SN to some extent (Chanyachukul, 2004). This suggests that L-valine may act to competitively inhibit PQ uptake through a neutral amino acid transporter, but this remains to be verified. Nonetheless, it is possible that this transporter may provide one of the routes for PQ to bypass the BBB.

However, it was found that PQ failed to cross the BBB in the adult rhesus macaque with PQ mainly accumulating in brain structures that lacked protection from the BBB (Bartlett et al., 2009). This discrepancy has yet to be resolved satisfactorily and has been mainly attributed to the use of specific strains of mice (C57Bl/6) which may be more susceptible to PQ toxicity (Berry et al., 2010). Since no adequate mechanism has been found that can account for how PQ crosses the BBB, further studies are required to substantiate the in vivo effects of PQ on DA neurons in the SN.

2.3.4 Dosage and Administration
McCormack et al. (2005) have determined that 8-9 week old male C57BL/6 mice exposed to PQ experienced a reduction in DA neurons in the SN in dose-dependent manner, with increasing dosage leading to increasing loss of SN neurons. Specifically, PQ was found to be associated with a loss of 25-30% loss of DA neurons in the SN after three exposures of 10mg/kg over a course of 3 weeks (one injection of PQ per week). The effect of the third injection was found to be negligible in contributing to the overall loss of DA neurons. This suggests that DA neurons may have some capacity to protect themselves against PQ, limiting the neurotoxic effects of chronic exposures to PQ.

Criticisms of this study have been raised in that the reduction in DA neurons associated with PQ is much less pronounced compared to MPP+ (Miller, 2005). Symptoms of PD typically do not manifest until a loss of about 70% of DA neurons in the SN (Parkinson’s Australia, 2012). Whilst increasing the dosage beyond 10mg/kg might serve to produce the neurotoxic that characterises PD, it is not without deleterious consequences to the respiratory system. Ishidia et al. (2006) provided evidence for the role of PQ exposure in the development of pulmonary fibrosis in eight-week-old male C57BL/6 mice injected with 20mg/kg PQ twice a week. As a result, it is difficult to implement higher dosages of PQ to in vivo studies without subsequently triggering the more immediate and possibly fatal effects on the respiratory system, such that any neurodegenerative effects induced by PQ become trivial in comparison.

Furthermore, the administration of PQ in the study by McCormack et al. (2005) was via intraperitoneal (IP) injection. This raises the question whether the same neurotoxic effects can be observed when exposure to PQ reflects more realistic circumstances in the occupational environment such as through inhalation, ingestion or skin contact. Rojo, Cavada, de Sagarra & Cuadrado (2007) found in their rodent model (eight-week-old male C57BL/6 mice) that chronic inhalation of paraquat, failed to produce a noticeable loss of DA neurons in the SN. Due to the polarity of PQ2+, only a small percentage (5-10%) of a single dose of PQ is absorbed via the gastrointestinal tract with the majority of the dose (45%) being excreted from the body after two days (Berry, La Vecchia & Nicotera, 2010). These findings suggest that exposure to PQ via ingestion or inhalation may serve to inhibit or reduce the neurotoxic effects of PQ observed from IP injection. Whether PQ can assert the same neurotoxic effects via skin contact is still unknown and is a subject of further study.

3. Critical Analysis


ABC’s Lateline is funded by the Australian government and as such commercial interest is not its primary concern. The media program is known for its provocative and insightful news stories which aim to stimulate debate and move beyond one-sided perspectives. Lateline tends to interview the individuals at the forefront of the subject of interest to keep up to date with the latest developments. CEO of Parkinson’s Australia, Daryl Smeaton, and Associate Professor John Powers were interviewed in this media piece specifically.

The media item is presented with an objective tone and does not sensationalise this topic through the superfluous use of cinematic techniques. However, the way the information was presented in this news item appeared to be one-sided in perspective to some extent with the impression that PQ research is progressing towards a single definitive conclusion; the link between PQ and PD is absolutely certain. This gives a false sense of homogeneity in the literature on PQ neurotoxicity and this is far from reality as our research has revealed.

This one-sidedness might stem from the need to simplify the topic so that it has greater coherence and impact on its general audience. Due to a general audience, the explanation for PD was simple, relating to the destruction of brain cells related to movement. Scientific information presented did not appear to be compromised, since although mechanisms were not outlined nor dopamine, it was still a valid account of the neurodegenerative disorder at the basic level. By improving the accessibility of the content to a general audience they can be more effectively oriented and informed of the issue. Consequently, the amount of information that can be explored in detail becomes limited.

Despite these shortcomings, the news item is much more balanced in its discussion of the potential gains and losses to be had in either banning or continuing PQ use. For example, Daryl Smeaton makes the point that “the use of chemicals is important in agriculture” emphasising that the value of PQ to the agricultural industry has been considered. Furthermore the point that PQ should be banned based on its “bad reputation” was made clearly emphasising that the primary motive for decisions to ban PQ are aimed at minimising risks and potential harms. From this perspective, the media item makes a good argument in that chemicals with potential risks should wait approval rather than disapproval and orients debate around the issue of risks vs. rewards.

In conclusion, there is a sense of one-sidedness in the news item in terms of the specific ‘science’ that was presented. However, it can be argued that this one-sidedness is inevitable when a simplified approach is taken and that it is necessary to reach a wider, general audience. The news item does stimulate debate over the idea of whether the risks outweigh the gains in terms of continuing the use of PQ. Overall, the quality of the item was good with interviewees from the forefront of the issue and a largely non-sensationalised approach.

4. Appendix


Search Strategy
After a few days of forming our group, many media items and articles had been discussed. We decided on either a news report on a pesticide with links to Parkinson's disease, or on an article on omega 3 and its effects on learning in children. However, choosing the news report on the pesticides seemed to be in favour due to the fact that some group members had previous knowledge on Parkinson's disease. The media item made us question why a pesticide is still in use after links to Parkinson’s disease and having been banned in over 30 countries. Dr. Richard Vickery approved our media item and our research begun.

The media item is a news report from ABC Lateline and had aired recently on the 13th July 2012. We initially thought that there would not be much information specifically about the link between paraquat and Parkinson’s disease. However, a quick Google search of “Paraquat and Parkinson’s Disease” revealed a wealth of studies investigating this topic. At first, we looked at journal article reviews concerning the epidemiological data and experimental data to help orient ourselves to the topic. From there we were able to search for specific studies that answered the question of how paraquat and other pesticides were specifically involved in the development of PD. The articles were found through the use of UNSW Library databases for scientific journals. Many sources were compared and only relevant and relatively recent articles were used as sources to our wikipage with a few exceptions.

Reviewer Comments
Based on review comments, we have added captions on all diagrams to help explain and make them easier to understand. We also introduced an abbreviation list at the start of the page to clear up confusion over the acronyms. We added some more detail into scientific integrity for analysis as reviewers suggested our critical analysis was lacking. Specifically though we did not include critical analysis of the studies used in our neuroscientific context as one reviewer suggested as the critical analysis section of the assignment only requires a critical analysis of the media item. We simplified some of the neuroscientific context and verified terms that were not so well understood. We fixed up some of the in-text references for consistency and proof read and edited some grammatical errors. General consistency regarding font and size of text and spacing has also been fixed.

One reviewer commented, “However, I sometimes felt as though this section [neuroscientific context] was a bit too in-depth for the general purpose of the group project and was a bit dry/hard to get through.” We also felt that there was an overwhelming amount of information on our wiki page. Despite the reviewers having commented positively on some of this information, we nonetheless had to remove some of it in order to keep within the word limit and stay relevant to the topic. The information we removed was done so that only the most relevant information was included onto the page.

Overall, the comments were very helpful and informative to our group in terms of what areas needed to be amended.

5. References


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Berry, C., La Vecchia, C., Nicotera, P. (2010). Paraquat and Parkinson's disease. Nature, 17(7), 1115-1125.

Betarbet, R., Sherer, T. B., MacKenzie, G., Garcia-Osuna, M., Panov, A. V., & Greenamyre, J. T. (2000). Chronic systemic pesticide exposure reproduces features of Parkinson's disease. Nature Neuroscience, 3(12), 1301-1306.

Cabiscol, E., Tamarit, J.,Ros, J. (2000) Oxidative stress in bacteria and protein damage by reactive oxygen species. International Microbiology, 3(1), 3-8.

Chanyachukul, T., Yoovathaworn, K., Thongsaard, W., Chongthammakun, S., Navasumrit, P., Satayavivad, J. (2004) Attenuation of paraquat-induced motor behavior and neurochemical disturbances by L-valine in vivo. Toxicology Letters, 150(3), 259-269.

Dinis-Oliveira, R. J., Remiao, F., Carmo, H., Duarte, J. A., Sanchez Navarro A., Bastos, M. L., Carvalho, F. (2006) Paraquat exposure as an etiological factor of Parkinson's disease. Neurotoxicology, 27(6), 1110-1122.

Hernan, M. A., Zhang, S. M., Rueda-DeCastro, A. M., Colditz, G. A., Speizer, F. E., & Ascherio, A. (2001). Cigarette smoking and the incidence of Parkinson's disease in two prospective studies. Annals of Neurology, 50(6), 780-786.

Inchem 2009, Paraquat, Canadian Centre for Occupational Health and Safety, Hamilton, Ontario, accessed 6 September 2012, <http://www.inchem.org/documents/jmpr/jmpmono/v86pr14.htm>

Ishida, Y., Takayasu, T., Kimura, A., Hayashi, T., Kakimoto, N., Miyashita, T., Kondo, T. (2006). Gene expression of cytokines and growth factors in the lungs after paraquat administration in mice. Legal Medicine, 8(2), 102-109.

Jenner, P. (1989). Clues to the mechanism underlying dopamine cell death in Parkinson's disease. Journal of Neurology, Neurosurgery, and Psychiatry., 52, 22-28.

McCormack A. L., Atienza, J. G., Johnston, L. C., Andersen, J. K., Vu S., Di Monte, D. A. (2005). Role of oxidative stress in paraquat-induced dopaminergic cell degeneration. Journal of Neurochemistry, 93(4): 1030-1037.

Miller, G. W. (2007). Paraquat: the red herring of Parkinson’s disease research. Toxicological Sciences, 100(1), 1–2.

Movement Disorder Virtual University. (2008). Parkinson's Disease: Epidemiology. Retrieved September 2, 2012, from http://www.mdvu.org/library/disease/pd/par_epi.asp

Nicklas, W. J., Vyas, I., & Heikkila, R. E. (1985). Inhibition of NADH-linked oxidation in brain mitochondria by 1-methyl-4-phenyl-pyridine, a metabolite of the neurotoxin, 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine. Life Sciences, 36(26), 2503-2508.

Parkinson’s Australia 2012. What is Parkinson's, Parkinson's Australia Inc., Mawson, Australian Capital Territory. Retrieved 8 September 2012, from http://www.parkinsons.org.au/about-ps/whatps.html

Rappold P, M., Cui, M., Chesser, A. S., Tibbett, J., Grima, J. C., Duan, L., Sen, N., Javitch, J. A., Tieu, K. (2011). Paraquat neurotoxicity is mediated by the dopamine transporter and organic cation transporter-3. Proceedings of the national academy of sciences, 108(51), 20766-20771.

Richardson, J. R., Quan, Y., Sherer, T. B., Greenamyre, J. T., Miller, G. W. (2005). Paraquat neurotoxicity is distinct from that of MPTP and rotenone.Toxicological Sciences, 88(1), 193-201.

Rojo, A. I., Cavada, C., de Sagarra, M. R., Cuadrado, A. (2007). Chronic inhalation of rotenone or paraquat does not induce Parkinson's disease symptoms in mice or rat. Experimental Neurology, 208(1), 120-126.

Sherer, T. B., Betarbet, R., Testa, C. M., Seo, B. B., Richardson, J. R., Kim, J. H., . . . Greenamyre, J. T. (2003). Mechanism of toxicity in rotenone models of Parkinson's disease. Journal of Neuroscience, 23(34), 10756-10764 begin_of_the_skype_highlighting 10756-10764 end_of_the_skype_highlighting.

Tanner, C. M., Kamel, F., Ross, G. W., Hoppin, J. A., Goldman, S. M., Korell, M., Marras, C., Bhudhikanok, G. S., Kasten, M., Chade, A. R., Comyns, K., Richards, M. B., Meng, C., Priestley, B., Fernandez, H. H., Cambi, F., Umbach, D. M., Blair, A., Sandler, D. P., Langston, J. W. (2011). Rotenone, paraquat, and Parkinson's disease. Environmental Health Perspective, 119(6), 866-872.

Yamato, M., Kudo, W., Shiba, T., Yamada, K-I., Wantanabe, T., Utsumi, H. (2010). Determination of reactive oxygen species associated with the degeneration of dopaminergic neurons during dopamine metabolism. Free Radical Research. 44(3), 249–257.