Continuing
with the theme of animal models, here is an essay I wrote as part of my MSc
neuroscience about the use of animals in Parkinson’s research. Many people,
myself included, feel uncomfortable about the use of animals in brain research,
an issue for which there are no easy answers. There is quite a bit of technical
terminology in here, but you can easily skim over the fancy words without
losing the overall meaning.
Describe and discuss the relevance of animal models in Parkinson’s
disease
Introduction
When James Parkinson published the first description of the medical condition that now bears his name, he speculated that a cure would “ere long be discovered” (Parkinson 1817). He was overly optimistic: over 200 years later there is no clinically proven way to halt or reverse the progression of Parkinson’s disease (PD) (Vijiaratnam et al. 2021). There was, however, a major breakthrough in 1957 when Swedish neuropharmacologist Arvid Carlsson performed a seminal experiment first on mice and then on rabbits. He administered reserpine to the animals which reduced their dopamine levels, effectively tranquilising them, then demonstrated that treatment with levodopa (L-dopa) could rapidly reverse the effect (Carlsson et al. 1957). Today, as a direct result of Carlsson’s animal experiments, levodopa remains the “gold standard” drug to treat symptoms of PD, and is used by many of the estimated 10 million people in the world with the condition (Schapira et al. 2009).
This essay first surveys the animal models used for PD research then explores their role across various therapeutic strategies, including pharmaceutical development, deep brain stimulation and neuronal regeneration. But, given that over 60 years since the discovery of levodopa, there are still no disease-modifying therapies generally available for PD, this essay also addresses the question of whether too much reliance has been placed on animal models in recent decades and what their role should be in the future.
Animal models used in PD research
In the context of a neurodegenerative condition such as PD, the purpose of an animal model is to replicate some aspect of the condition in a simplified way that can be used to elucidate its pathophysiology and/or develop potential therapies. Given that the aetiopathogenesis of PD is not yet fully understood, a number of different mechanisms have been exploited for the development of animal models. These fall into four categories (pharmacological depletion of dopamine, neurotoxins, oxidative stress and genetic factors) and are summarised in Table 1. The corresponding roles these play in selected postulated mechanisms of PD are illustrated in Figure 1.
Table 1. Summary of animal models commonly used in
PD research (adapted from Betarbet et al. 2002, and Konnova & Swanberg 2018).
|
Primary target mechanism |
Model |
Main symptoms induced |
Comments |
|
Pharmacological
dopamine depletion |
Reserpine |
Akinesia,
catalepsy |
|
|
Environmental
toxin |
MPTP |
Akinesia,
rigidity |
Commonly
used model |
|
Rotenone |
Akinesia,
rigidity, tremor |
Pesticides
and herbicides; human toxicity debated |
|
|
Paraquat
/ Maneb |
Reduced
motor activity |
||
|
Oxidative
stress |
6-OHDA |
Akinesia |
Commonly
used model |
|
Methamphetamine |
No
clear PD symptoms |
|
|
|
Genetic
risk factors |
PINK1,
Parkin etc. |
Various |
|
Figure 1. Selected postulated pathogenic mechanisms in PD and associated models
(adapted from Przedborski 2017 and Dawson et al. 2010)
In terms of species used, 85% of PD experiments using animal
models are performed on mice and rats (Figure 2). These are the
laboratory animals of choice because they are easy to breed and manipulate, are
relatively inexpensive, and because degeneration of nigrostriatal dopaminergic
neurons leads to impaired motor function mirroring the effect PD has on motor
function in humans (Konnova & Swanberg 2018).
One of the most commonly used PD models in recent decades has been the administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). This was originally synthesised as a type of heroin which caused severe Parkinsonism in a number of narcotics users in California in 1982. A researcher described the effect as like a textbook case of advanced PD, saying:
“Patients exhibited virtually all of the motor features of typical Parkinson’s disease, including tremor and asymmetry of findings. They even exhibited non-motor aspects of the disease such as facial seborrhea and mild deficits in higher cognitive function. Furthermore, these patients all had a dramatic and near immediate response to L-dopa.” (Langston 2017).
Further investigation revealed that MPTP was converted to MPP+ (1-methyl-4-phenylpyridinium) in the brain which is toxic to mitochondria (Langston et al. 1984). Around the same time, it was demonstrated that MPTP had the same effect on monkeys (Burns et al. 1984), causing a rapid depletion of dopaminergic neurons in the substantia nigra pars compacta whilst leaving the ventral tegmental area largely unaffected. Before the end of 1984, the effect had also been demonstrated in mice (Heikkila et al 1984), and thus MPTP was established as an apparently realistic model for PD and has been widely used since.
6-Hydroxydopamine (6-OHDA) causes oxidative stress but does not cross the blood-brain barrier so has to be injected directly. When administered to the substantia nigra, dopaminergic neurons rapidly degenerate; and when administered to the striatum it produces a much slower degeneration of the nigrostriatal system (Betarbet et al. 2002). The 6-OHDA model does not mimic the clinical features of PD as closely as MPTP but is nevertheless also widely used.
A number of pesticides, herbicides and fungicides are now generally believed to be risk factors for PD (Pezzoli & Cereda 2013) and in particular the herbicides paraquat and maneb, and the pesticide rotenone, have been used in rodent models of PD. However, whilst they have been shown to induce clear PD symptoms in rodent models, the level of toxicity in humans is debated (Blandini & Armentero 2012).
Reserpine and methamphetamine both cause dopamine depletion with little impact on the cell bodies (Betarbet et al. 2002) though the mechanisms are different, with methamphetamine believed to cause oxidative stress whereas reserpine interferes with action of dopamine transporters (Yaffe et al. 2018).
Finally, there are genetic models. As of 2019, Approximately 90 mutations, many of them single nucleotide polymorphisms, had been identified as PD risk factors in humans (Nalls et al. 2019). Of these a number have been replicated in animal models (Table 2), though their ability to replicate the dopaminergic neurodegeneration or clinical features of PD is limited (Blandini & Armentero 2012).
Table 2. PD mutations that have been used in animal
models (reproduced from Konnova & Swanberg 2018).
|
Locus |
Gene |
Mutation |
Inheritance |
|
PARK1 |
SNCA |
A30P, A53T, E46K |
Dominant |
|
PARK2 |
Parkin |
Various |
Recessive |
|
PARK4 |
SNCA |
Duplication and triplication |
Dominant |
|
PARK5 |
UCH-L1 |
I93M, S18Y |
Dominant |
|
PARK6 |
PINK1 |
G309D, exonic deletions |
Recessive |
|
PARK7 |
DJ-1 |
L166P, homozygous exon deletion |
Recessive |
|
PARK8 |
LRRK2 |
G2109S, R1441C/G |
Dominant |
Applications of PD animal models
The models described above have been used for the pre-clinical testing of a large number of candidate drug treatments for PD. This includes dopamine agonists, of which pramipexole is a recent example having been licensed by the US Food and Drug Administration[1] and European Medicines Agency[2] in 1997. This had been tested on a variety of models, including the MPTP model on monkeys, the 6-ODHA model on rats (Dooley & Markham 1998), the MPTP model on mice (Kitamura et al. 1997) and the methamphetamine model on mice (Hall et al. 1996).
Animal models are used to test theories about PD pathogenesis. For instance, experiments transplanting gut microbiota from humans with PD to mice resulting in parkinsonian motor impairment, lend support to the recent idea that PD may, in some cases, start in the gut (Sampson et al. 2016, Liddle 2018).
In addition to direct models of PD, animals have also been key to the understanding of brain structures compromised in PD, notably the basal ganglia, and this led serendipitously to the discovery and clinical development of deep brain stimulation (DBS), an increasingly common surgical procedure to alleviate motor symptoms in the more advanced stages of PD. In the 1970s and 1980s, a series of experiments on monkeys revealed the physiology of the basal ganglia-thalamocortical (BGTC) circuitry (Vitek & Johnson 2019), in particular the role of the so-called direct and indirect pathways in controlling motor function. This led to the use for a few years from the mid-1980s of pallidotomy as a treatment for PD (Laitinen et al. 1992). Although successful in some patients in relieving the motor symptoms of PD, there were frequent complications including cognitive changes, and in other cases, PD symptoms were either not improved or worsened (Vitek & Johnson 2019). However, the use of electric stimulation during these procedures as a testing mechanism led to the idea that the application of small electric currents in certain areas of the BGTC circuit via surgically implanted electrodes might directly alleviate PD motor symptoms. This resulted in DBS, a procedure that gives “significant improvement in motor function in patients with Parkinson’s disease where the condition cannot further be improved with medical therapy” (Obeso et al. 2001). Exactly how DBS works is not fully known, and monkey models continue to be used to improve understanding (e.g., Hashimoto et al. 2003).
Finally, animals can also play a role in neuronal regeneration therapies, for example in grafting dopaminergic cells into the striatum of a patient with PD (cells are typically grafted into the striatum where the dopamine is actually used rather than the substantia nigra pars compacta). In this case, animal models can be used to test ideas such as implantation of induced pluripotent stem cells (Schweitzer et al. 2020) or in vivo cell reprogramming (Heinrich et al. 2015) prior to human experimentation. Animals can also be sources of cells for xenotransplantation given the ethical and practical challenges with using human embryonic stem cells. For example, this has been tried with some success with transplantation of neural cells from pigs into humans (Deacon et al. 1997).
Discussion: are animal models still relevant?
Over recent years there have been numerous candidate pharmaceuticals for PD that have shown promise in animal models only to fail to demonstrate efficacy in clinical trials in humans. Examples of recent clinical trial failures following success in mouse models include nilotinib (Simuni et al. 2021, Tanebe et al. 2014) and venglustat (Veil et al. 2021). As of mid-2021, there were at least 44 drugs in Phase 1 or Phase 2 trials (Prasad & Hung 2021), many with pre-clinical testing on rodent models of PD – will all of these also fail to meet their primary outcome targets?
Taking the MPTP model as an example, in hindsight it is clear that a drug that rapidly causes the death of dopaminergic neurons in the substantia nigra is different from a disease that results in gradual degeneration over decades and over many regions of the brain (Epsay & Stecher 2020, Vijiaratnam et al. 2021). The reality is that animal models of PD, particularly in rodents, do not fully mirror the condition in humans, where neurodegeneration can be found not just in the dopaminergic system but also in the noradrenergic, serotonergic and cholinergic systems in addition to the cerebral cortex, olfactory system and autonomic nervous system (Dauer & Przedborski 2003). Moreover, PD has many non-motor symptoms (Chaudhuri et al. 2006), is heterogeneous in its clinical presentation and likely has several underlying pathogenic mechanisms (Elkouzi et al. 2019), meaning that different animal models are potentially needed for different subtypes of PD. Taking all these factors into account, there is a significant risk of wasted effort in experimentation with inadequate models, and a risk of ethical challenges regarding the unnecessary use of animals.
Nevertheless, when focussed on symptomatic relief, animal models have directly resulted in the development of life-changing therapies for many PD patients, namely treatment with levodopa, dopamine agonists and deep brain stimulation, as well as significantly furthering understanding of related neurophysiology.
Conclusion: the future of animal models in PD
The sensible approach for the future appears to be: continue to use animal models, but selectively. Existing animal models have proven invaluable in the development of therapeutic strategies to provide relief from the symptoms of PD. However, the same models are ineffective at replicating the exact biochemical mechanisms underlying PD in humans and should be used in the development of disease-modifying agents only after careful consideration. Preferably, better models need to be found, most likely using transgenic approaches, or experiments need to be performed directly on humans. Using models like MPTP to try and develop cures for PD will probably continue to lead down costly and time-consuming blind alleys.
References
Betarbet, R., Sherer, T. & Greenamyre, J. (2002). “Animal models of Parkinson’s Disease.” BioEssays 24:308-318. https://doi.org/10.1002/bies.10067
Bladini, F. & Armentero, M. (2012). “Animal models of Parkinson’s disease.” FEBS Journal 279 (7): 1156-1166. https://doi.org/10.1111/j.1742-4658.2012.08491.x
Burns R., Markey S., Phillips J, & Chiueh C. (1984). “The neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in the monkey and man.” Canadian Journal of Neurological Sciences 11: 166-168. https://doi.org/10.1017/S0317167100046345
Carlsson, A., Lindqvist, M. & Magnusson, T. (1957). “3,4-Dihydroxyphenylalanine and 5-Hydroxytryptophan as Reserpine Antagonists.” Nature 180: 1200. https://doi.org/10.1038/1801200a0
Chaudhuri, K., Healy, D. & Schapira, A. (2006). “Non-motor symptoms of Parkinson's disease: diagnosis and management.” The Lancet Neurology 5 (3): 235-245. https://doi.org/10.1016/S1474-4422(06)70373-8.
Dauer, W. & Przedborski, S. (2003). “Parkinson’s Disease: Mechanisms and Models.” Neuron 39: 889-909. https://doi.org/10.1016/S0896-6273(03)00568-3
Dawson, T., Ko, H. & Dawsom, V. (2010). “Genetic Animal Models of Parkinson’s Disease.” Neuron 66 (5): 646-661. https://doi.org/10.1016/j.neuron.2010.04.034
Deacon, T., Schumacher, J., Dinsmore, J., Thomas, C., Palmer, P., Koif, S., Edge, A., Penney, D. Assissieh, S., Dempsey, P. & Isacson, O. (1997). “Histological evidence of fetal pig neural cell survival after transplantation into a patient with Parkinson's disease.” Nature Medicine 3 (3): 350-353. https://doi.org/10.1038/nm0397-350
Dooley, M. & Markham, A. (1998). “Pramipexole: A Review of its use in the Management of Early and Advanced Parkinson’s Disease.” Drugs & Aging 12: 495-514. https://doi.org/10.2165/00002512-199812060-00007
Elkouzi, A., Vedam-Mai, V., Eisinger, R. & Okun, M. (2019). “Emerging therapies in Parkinson disease — repurposed drugs and new approaches.” Nature reviews neurology 15: 204-223. https://doi.org/10.1038/s41582-019-0155-7
Epsay, A. & Stecher, B. (2020). “Brain fables: the hidden history of neurodegenerative diseases and a blueprint to conquer them,” chapter 2. Cambridge: Cambridge University Press.
Hall, E. Andrus, P., Oostveen, J., Althaus, J., & Von Voigtlander, P. (1996). “Neuroprotective effects of the dopamine D2/D3 agonist pramipexole against postischemic or methamphetamine-induced degeneration of nigrostriatal neurons.” Brain Research 742, Issues 1-2: 80-88. https://doi.org/10.1016/S0006-8993(96)00968-7
Hashimoto, T., Elder, C., Okun, M., Patrick, S. & Vitek, J. (2003). “Stimulation of the Subthalamic Nucleus Changes the Firing Pattern of Pallidal Neurons.” The Journal of Neuroscience 23 (5): 1916-1923. https://doi.org/10.1523/JNEUROSCI.23-05-01916.2003
Heikkila R., Cabbat F., Manzino L, & Duvoisin, R. (1984). “Effects of 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine on neostriatal dopamine in mice.” Neuropharmacology 23, 711-713. https://doi.org/10.1016/0028-3908(84)90170-9
Heinrich, C., Spagnoli, F. & Berninger, B. (2015). “In vivo reprogramming for tissue repair.” Nature Cell Biology 17 (3): 204-211. https://doi.org/10.1038/ncb3108
Kitamura, Y., Kohno, Y. Nakazawa, M., & Nomura, Y. (1997). “Inhibitory effects of talipexole and pramipexole on MPTP-induced dopamine reduction in the striatum of C57BL/6N mice.” Japanese Journal of Pharmacology 74 (1): 51-7. https://doi.org/10.1254/jjp.74.51
Konnova, E. & Swanberg, M. (2018). “Animal models of Parkinson’s Disease.” In Parkinson’s Disease, Pathogenesis and Clinical Aspects. Brisbane: Codon Publications. https://doi.org/10.15586/codonpublications.parkinsonsdisease.2018.ch5
Langston, J. (2017). “The MPTP story.” Journal of Parkinson’s Disease 7: S11-S19. https://doi.org/10.3233/JPD-179006
Langston J., Irwin I., Langston E., & Forno L. (1984). “1-Methyl-4-phenylpyridinium ion (MPP+): Identification of a metabolite of MPTP, a toxin selective to the substantia nigra.” Neuroscience Letters, 48, 87-92. https://doi.org/10.1016/0304-3940(84)90293-3
Latinen, L., Bergenheim, A., & Hariz, M. (1992). “Leksell's posteroventral pallidotomy in the treatment of Parkinson's disease.” Journal of Neurosurgery 76 (1): 53-61. https://doi.org/10.3171/jns.1992.76.1.0053
Liddle, R. (2018). “Parkinson’s Disease from the Gut.” Brain Research 1693 (B) 201-206. https://doi.org/10.1016/j.brainres.2018.01.010
Nalls M., Blauwendraat C., Vallerga C., Heilbron K., Bandres-Ciga S., Chang D., Tan M., Kia D., Noyce A., Xue A., Bras J., Young E., von Coelln R., Simón-Sánchez J., Schulte C., Sharma M., Krohn L., Pihlstrøm L., Siitonen A., Iwaki H., Leonard H., Faghri F., Gibbs J., Hernandez D., Scholz S., Botia J., Martinez M., Corvol J., Lesage S., Jankovic J., Shulman L., Sutherland M., Tienari P., Majamaa K., Toft M., Andreassen O., Bangale T., Brice A., Yang J., Gan-Or Z., Gasser T., Heutink P., Shulman J., Wood N., Hinds D., Hardy J., Morris H., Gratten J., Visscher P., Graham R., & Singleton A.; 23andMe Research Team; System Genomics of Parkinson's Disease Consortium; International Parkinson's Disease Genomics Consortium. (2019). “Identification of novel risk loci, causal insights, and heritable risk for Parkinson's disease: a meta-analysis of genome-wide association studies.” Lancet Neurology 18 (12):1091-1102. https://doi.org/10.1016/S1474-4422(19)30320-5
Obeso, J. et al. (2001). “Deep-Brain Stimulation of the Subthalamic Nucleus or the Pars Interna of the Globus Pallidus in Parkinson's Disease.” New England Journal of Medicine 345: 956-963. https://doi.org/10.1056/NEJMoa000827
Parkinson, J. (1817). “An essay on the shaking palsy.” London: Sherwood, Neely & Jones. Available from Project Gutenberg at https://www.gutenberg.org/ebooks/23777.
Pezzoli, G. & Cereda, E. (2013). “Exposure to pesticides or solvents and risk of Parkinson disease”. Neurology 80 (22): 2035-2041. https://doi.org/10.1212/WNL.0b013e318294b3c8
Prasad, E. & Hung, S. (2021). “Current therapies in clinical trials of Parkinson’s Disease: A 2021 update.” Pharmaceuticals 24:717. https://doi.org/10.3390/ph14080717
Przedborski, S. (2017). “The two-century journey of Parkinson disease research.” Nature Reviews Neuroscience 18: 251-259. https://doi.org/10.1038/nrn.2017.25
Sampson,
T. et al. (2016). “Gut Microbiota Regulate Motor Deficits and Neuroinflammation
in a Model of Parkinson’s Disease.” Cell 167: 1469-1480. https://doi.org/10.1016/j.cell.2016.11.018
Schapira, A., Emre, M., Jenner, P. & Poewe, W. (2009). “Levodopa in the treatment of Parkinson’s disease.” European Journal of Neurology 16: 982-989. https://doi.org/10.1111/j.1468-1331.2009.02697.x
Schweitzer,J. et al. (2020). “Personalized iPSC-Derived Dopamine Progenitor Cells for Parkinson’s Disease.” N Engl J Med 2020; 382:1926-1932. https://doi.org/10.1056/NEJMoa1915872
Simuni, T., Fiske, B, Merchant, K. et al. (2021). “Efficacy of Nilotiib in Patients with Moderately Advanced Parkinson’s Disease.” JAMA Neurol. 2021;78(3):312–320. https://doi.org/10.1001/jamaneurol.2020.4725
Tanabe, A, Yamamura,Y. Kasahara, J., Morigaki, R, Kaji, R & Goto, S. (2014). “A novel tyrosine kinase inhibitor AMN107 (nilotinib) normalizes striatal motor behaviors in a mouse model of Parkinson’s disease.” Frontiers in Cellular Neurosciene 8: 50. https://doi.org/10.3389/fncel.2014.00050
Viel, C., Clarke, J., Kayatekin, C. et al. (2021). “Preclinical pharmacology of glucosylceramide synthase inhibitor venglustat in a GBA-related synucleinopathy model.” Sci Rep 11, 20945. https://doi.org/10.1038/s41598-021-00404-5
Vijiaratnam, N., Simuni, T., Bandmann, O., Morris, H. & Foltynie, T. (2021). “Progress towards therapies for disease modification in Parkinson’s disease.” The Lancet Neurology 20 (7): 559-572. https://doi.org/10.1016/S1474-4422(21)00061-2
Vitek, J. & Johnson, L. (2019). “Understanding Parkinson’s disease and deep brain stimulation: Role of monkey models.” Proceedings of the National Academy of Sciences of the USA vol. 116 no. 52: 26259-26265. https://doi.org/10.1073/pnas.1902300116
Yaffe, D., Forrest, L. & Schuldiner, S. (2018). "The ins and outs of vesicular monoamine transporters". The Journal of General Physiology. 150 (5): 671–682. https://doi.org/10.1085/jgp.201711980
