![PD animal models](https://upload.wikimedia.org/wikipedia/commons/0/06/PD_animal_models.png)
Animal models of Parkinson's disease are essential in the research field and widely used to study
Parkinson's disease
Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...
. Parkinson's disease is a
neurodegenerative disorder
A neurodegenerative disease is caused by the progressive loss of structure or function of neurons, in the process known as neurodegeneration. Such neuronal damage may ultimately involve cell death. Neurodegenerative diseases include amyotrophic ...
, characterized by the loss of dopaminergic neurons in the
substantia nigra
The substantia nigra (SN) is a basal ganglia structure located in the midbrain that plays an important role in reward and movement. ''Substantia nigra'' is Latin for "black substance", reflecting the fact that parts of the substantia nigra app ...
pars compacta
The pars compacta (SNpc) is a portion of the ''substantia nigra'', located in the midbrain. It is formed by dopaminergic neurons and located medial to the pars reticulata. Parkinson's disease is characterized by the death of dopaminergic neurons ...
(SNpc). The loss of the
dopamine
Dopamine (DA, a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic compound, organic chemical of the catecholamine and phenethylamine families. Dopamine const ...
neurons in the brain, results in motor dysfunction, ultimately causing the four cardinal symptoms of PD: tremor, rigidity, postural instability, and
bradykinesia
Hypokinesia is one of the classifications of movement disorders, and refers to decreased bodily movement. Hypokinesia is characterized by a partial or complete loss of muscle movement due to a disruption in the basal ganglia. Hypokinesia is a sym ...
. It is the second most prevalent neurodegenerative disease, following
Alzheimer's disease
Alzheimer's disease (AD) is a neurodegeneration, neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in short-term me ...
. It is estimated that nearly one million people could be living with PD in the United States.
There are a variety of models that can be utilized to be able to address important aspects of Parkinson's disease. Researchers can consider disease progression, cell death, behavioral characteristics, and more PD phenotypes. Parkinson's disease animal models are divided into two categories:
neurotoxin
Neurotoxins are toxins that are destructive to nerve tissue (causing neurotoxicity). Neurotoxins are an extensive class of exogenous chemical neurological insultsSpencer 2000 that can adversely affect function in both developing and mature ner ...
models and genetic models. Neurotoxin models include chemically induced toxicity in the brain; whereas, genetic models include genes that are mutated and induce PD phenotypes.
Neurotoxin models
6-OHDA
![Dopamine](https://upload.wikimedia.org/wikipedia/commons/2/2f/Dopamine.svg)
6-Hydroxydopamine, better known as
6-OHDA
Oxidopamine, also known as 6-hydroxydopamine (6-OHDA) or 2,4,5-trihydroxyphenethylamine, is a neurotoxic synthetic organic compound used by researchers to selectively destroy dopaminergic and noradrenergic neurons in the brain.
The main use for ...
, is a widely used neurotoxin in PD models. It is structurally similar to dopamine, only differentiating by an additional
hydroxyl group
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy g ...
in the 6-OHDA structure (Figure 1 & Figure 2).
Through scientific studies, this neurotoxin has been used in rodents (rats and mice), guinea pigs, cats, dogs, and monkeys. 6-OHDA does not cross the
blood-brain-barrier (BBB) making the chemical more selective for dopaminergic neurons. This model requires injecting the 6-OHDA directly into the nigrostriatal pathway, targeting the
dopamine transporter
The dopamine transporter (also dopamine active transporter, DAT, SLC6A3) is a membrane-spanning protein that pumps the neurotransmitter dopamine out of the synaptic cleft back into cytosol. In the cytosol, other transporters sequester the dopam ...
(DAT).This can be performed through stereotaxic injections (both unilateral and bilateral are experimentally permissible) and will eventually cause loss of dopamine neurons in the SNpc and loss of dopamine terminals in the
striatum
The striatum, or corpus striatum (also called the striate nucleus), is a nucleus (a cluster of neurons) in the subcortical basal ganglia of the forebrain. The striatum is a critical component of the motor and reward systems; receives glutamate ...
since the
nigrostriatal pathway The nigrostriatal pathway is a bilateral dopaminergic pathway in the brain that connects the substantia nigra pars compacta (SNc) in the midbrain with the dorsal striatum (i.e., the caudate nucleus and putamen) in the forebrain. It is one of the fou ...
is being affected.
The neurotoxin can be injected has been shown to be injected into the striatum and the substantia nigra. However, injections into the SNpc is estimated to degrade about 60% of
tyrosine hydroxylase
Tyrosine hydroxylase or tyrosine 3-monooxygenase is the enzyme responsible for catalyzing the conversion of the amino acid L-tyrosine to L-3,4-dihydroxyphenylalanine (L-DOPA). It does so using molecular oxygen (O2), as well as iron (Fe2+) and t ...
(TH+) neurons as well as loss of TH positive terminals in the striatum. A limitation to using 6-OHDA is that the potency of the neurotoxin causes rapid
apoptosis
Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...
, which makes it difficult to study Parkinson's disease progression.
The mechanism of action of 6-OHDA occurs through the aggregation of toxins and the conversion into
catecholaminergic
Catecholaminergic means "related to catecholamines". The catecholamine neurotransmitters include dopamine, epinephrine (adrenaline), and norepinephrine (noradrenaline).
A catecholaminergic agent (or drug) is a chemical which functions to direc ...
neurons. Since the structures of both dopamine and 6-OHDA are similar, the dopamine transporter takes up the 6-OHDA and induces
toxicity
Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...
.
This toxicity emerges from the production of
free radicals
In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron.
With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spont ...
from the additional hydroxyl group in the neurotoxin's structure. There is also oxidative stress occurring mediated through the inhibition of the cell's mitochondrial complex I, producing ROS (reactive oxygen species), which causes a decrease or loss in respiratory activity. In addition, there is also the proposed mechanism of
oxidative stress
Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily Detoxification, detoxify the reactive intermediates or to repair the resulting damage. Disturbances ...
inducing
neuroinflammation
Neuroinflammation is inflammation of the nervous tissue. It may be initiated in response to a variety of cues, including infection, traumatic brain injury,Ebert SE, Jensen P, Ozenne B, Armand S, Svarer C, Stenbaek DS ''et al.'' Molecular imaging of ...
(Figure 3).
![MPTP Mechanism](https://upload.wikimedia.org/wikipedia/commons/4/44/MPTP_Mechanism.jpg)
MPTP
1-Methyl-4-phenyl-1,2,3,6-tetraydropyridine (MPTP) is a widely used neurotoxin in Parkinson's disease research (Figure 4). In contrast to 6-OHDA, MPTP crosses the BBB which making the neurotoxin even more selective for dopaminergic neurons. Due to the ability to cross the blood-brain-barrier, MPTP is administered peripherally, subcutaneously. This neurotoxin is known to replicate oxidative stress, ROS, energy failure, and inflammation; which are all hallmarks in Parkinson's disease. However, it does not produce Lewy body pathology. The mechanism of action of MPTP is due to its conversion to
1-methyl-4-phenylpyridinium (MPP
+) caused by the interaction of MPTP with
monoamine oxidase B
Monoamine oxidase B, also known as MAOB, is an enzyme that in humans is encoded by the ''MAOB'' gene.
The protein encoded by this gene belongs to the flavin monoamine oxidase family. It is an enzyme located in the outer mitochondrial membrane. ...
(MAO-B). (Figure5). MPTP enters astrocytes and is metabolized to MPP
+ before being released. Once released into the extracellular space, MPP
+ is taken up into the neuron by DAT and is stored in vesicles by the up take of vesicular monoamine transporter (VMAT2). In the neuron, MPP
+ inhibits the function of complex 1 of electron transport chain, which decreases ATP production and releases ROS.
Herbicides (rotenone and paraquat)
Rotenone
Rotenone is an odorless, colorless, crystalline isoflavone used as a broad-spectrum insecticide, piscicide, and pesticide. It occurs naturally in the seeds and stems of several plants, such as the jicama vine plant, and the roots of several member ...
is a chemical compound (Figure 6) that can be derived from the plants: ''
Derris elliptica
''Derris eliptica'' is a species of leguminous plant from Southeast Asia and the southwest Pacific islands, including New Guinea. The roots of ''D. elliptica'' contain rotenone, a strong insecticide and fish poison.
Also known as ''derris powd ...
'', ''D mallaccensis'', ''Lonchocarpus utilis'', and ''L urucu''.
It is a known neurotoxin that is selective to dopaminergic neurons when administered to rodents via stereotaxic injections. However, it targets the striatum and not the substantia nigra. Moreover, rotenone can cross the BBB and spread through the
central nervous system
The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...
.
Since rotenone can cross the blood-brain-barrier, it can be administered peripherally as well. Although, peripheral injections can lead to system toxicity.
The exact mechanism of action of rotenone is still unclear, but one aspect that is known is that the herbicide accumulates and clusters in the neuron in organelles like the mitochondria, which disrupts the
oxidative phosphorylation
Oxidative phosphorylation (UK , US ) or electron transport-linked phosphorylation or terminal oxidation is the metabolic pathway in which cells use enzymes to oxidize nutrients, thereby releasing chemical energy in order to produce adenosine tri ...
mechanism in the cell and inhibits the respiratory chain complex I. Limitations of using rotenone is the lack of reproducibility of results throughout experiments and the quantity of aggregates and lesion. In addition, there is an elevated mortality rate in the animals induced with rotenone.
![Paraquat 200](https://upload.wikimedia.org/wikipedia/commons/f/f9/Paraquat_200.svg)
Paraquat
1,1'-dimethyl-4-4'-bipyridinium dichloride (Paraquat) is a nonselective
herbicide
Herbicides (, ), also commonly known as weedkillers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page fo ...
(Figure 7). Human exposure to this chemical is highly toxic. Its chemical structure is very similar to MPP
+, therefore, it was thought to act as a neurotoxin as well.
Paraquat has the capability to cross the BBB and is selective to dopamine neurons when injected via stereotaxic injections in the brain. Similar to rotenone, paraquat can also be administered peripherally, however, this can lead to systemic toxicity.
It is found to decrease dopamine concentration and produce parkinsonian phenotypes (both physically and behaviorally). Mechanistically, paraquat targets the dopamine transporter to be transported into dopaminergic neurons and ends up in the striatum. It lingers in the midbrain for approximately four weeks. However, since it is capable to cross the BBB, the toxin can be found in other regions such as the
pineal gland
The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep, sleep patterns in both circadian rhythm, circ ...
, cerebral ventricles,
olfactory bulb
The olfactory bulb (Latin: ''bulbus olfactorius'') is a grey matter, neural structure of the vertebrate forebrain involved in olfaction, the sense of odor, smell. It sends olfactory information to be further processed in the amygdala, the orbitof ...
,
hypothalamus
The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
, and the
area postrema
The area postrema, a paired structure in the medulla oblongata of the brainstem, is a circumventricular organ having permeable capillaries and sensory neurons that enable its dual role to detect circulating chemical messengers in the blood and t ...
. Several studies have demonstrated the relationship between paraquat and oxidative stress indicating that this may be another mechanism of paraquat induced neurodegeneration. In addition, the herbicide is accumulated in the lungs and kidney, resulting high toxicity; as well, as death.
Genetic models
alpha-synuclein
Alpha-synuclein
Alpha-synuclein is a protein that, in humans, is encoded by the ''SNCA'' gene. Alpha-synuclein is a neuronal protein that regulates synaptic vesicle trafficking and subsequent neurotransmitter release.
It is abundant in the brain, while smaller a ...
(α-synuclein) is an
endogenous
Endogenous substances and processes are those that originate from within a living system such as an organism, tissue, or cell.
In contrast, exogenous substances and processes are those that originate from outside of an organism.
For example, es ...
protein that is encoded by the ''SNCA'' gene and known as the pathological hallmark of Parkinson's disease.
It is found in distinct regions of the body, but in PD, alpha-synuclein accumulation in the brain is of main importance. This protein misfolds and accumulates creating insoluble aggregates in the brain known as
Lewy bodies
Lewy bodies are the inclusion bodies – abnormal aggregations of protein – that develop inside nerve cells affected by Parkinson's disease (PD), the Lewy body dementias ( Parkinson's disease dementia and dementia with Lewy bodies (DLB)), and ...
(found in the soma) and Lewy neurites (found in the neuropil)(Figure 8). This pathology is well known as synucleinopathies.
The inclusions/aggregates lead to dopamine neuronal depletion in the SNpc as well as dopamine terminal loss in the striatum from the projection of SNpc neurons through the nigrostriatal pathway. In addition, studies have shown that there is progressive formation of α-synuclein inclusions in distinct brain areas like the hippocampus, the cortex, and amygdala.
However, according to the
Braak staging
Braak staging refers to two methods used to classify the degree of pathology in Parkinson's disease and Alzheimer's disease. These methods are used both in research and for the clinical diagnosis of these diseases and are obtained by performing ...
, α-synuclein aggregates initially develop in the olfactory bulb and the lower
brainstem
The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...
; propagating towards the higher brainstem and the substantia nigra; reaching the
mesocortex Mesocortex (also called juxtallocortex) is the transitional areas of the cerebral cortex, formed at borders between true isocortex and true allocortex (either paleocortex or archicortex). Parts of mesocortex that lie closer to the true isocortex an ...
and the
thalamus
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
; and, ultimately covering the neocortex.
Braak staging is a widely used method to measure the stage of pathology (stage 1 being the lowest level of pathology and stage 4 being the highest) of Parkinson's disease; used both in basic research and clinically.
There are proposed mechanisms by which α-synuclein acts, in terms of pathology, one being the inhibition of the autophagy-lysosome pathway. This pathway is highly important as it is responsible for intracellular degradation.
Therefore, as α-synuclein fibrils inhibits the function of
autophagy
Autophagy (or autophagocytosis; from the Ancient Greek , , meaning "self-devouring" and , , meaning "hollow") is the natural, conserved degradation of the cell that removes unnecessary or dysfunctional components through a lysosome-dependent re ...
impairing the removal of aggregated protein, there is the production of more α-synuclein inclusions since it cannot be degraded. Other pathological mechanisms include the oxidative stress, dysfunction of the mitochondria, and neuroinflammation.
Alpha-synuclein preformed fibrils
![Alpha-synuclein Genetic Model](https://upload.wikimedia.org/wikipedia/commons/7/78/Alpha-synuclein_Genetic_Model.png)
The pre-formed fibril model was developed as a way to study the propagation of α-synuclein. This model consists of injecting extracellular α-synuclein fibrils via stereotaxic injections to induce intracellular α-synuclein aggregation. Consequently, this will induce parkinsonian
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
s.
The α-synuclein pre-formed fibrils (PFFs) are made ''in vitro'' utilizing recombinant α-synuclein
monomer
In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization.
Classification
Mo ...
s which will aggregate and form fibrils.
The fibrils can then be manipulated to form different conformations like being
sonicated to form short fibrils or form heterogenous mixes of fibrils with
oligomer
In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relativ ...
s and monomers.
Once the fibrils are generated, they can be injected into the brain, where
hyperphosphorylation
Hyperphosphorylation occurs when a biochemical with multiple phosphorylation sites is fully saturated. Hyperphosphorylation is one of the signaling mechanisms used by the cell to regulate mitosis. When these mechanisms fail, developmental problems ...
of endogenous α- synuclein (pα-syn) will occur and induce aggregation, forming cytoplasmic Lewy body and Lewy neurite inclusions. This method can be injected in brain regions like the SNpc and the cortex, however, the most common region to inject PFFs is into the striatum. Moreover, the spread of α-synuclein PFFs to brain regions occur through the uptake of the fibrils by dopamine neuron terminals that make their way up to the soma in the SNpc (Figure 9).
A limitation to the pre-formed fibril model is that although it is a widely used model, it lacks overt neurodegeneration.
Alpha-synuclein viral vector mediated overexpression
The different synuclein models that have been widely used have also faced challenges of targeting the fibrils to the SNpc, thus, lacking abundant neurodegeneration. Through the viral vector-mediated delivery of alpha-synuclein, the vectors can target the dopaminergic neurons directly. Vectors like
lentivirus
''Lentivirus'' is a genus of retroviruses that cause chronic and deadly diseases characterized by long incubation periods, in humans and other mammalian species. The genus includes the human immunodeficiency virus (HIV), which causes AIDS. Lent ...
and
adeno-associated virus
Adeno-associated viruses (AAV) are small viruses that infect humans and some other primate species. They belong to the genus ''Dependoparvovirus'', which in turn belongs to the family ''Parvoviridae''. They are small (approximately 26 nm in di ...
have been used in this method.
This method allows for targeting of nigrostriatal neurons, where α-synuclein protein can be overexpressed and there can be a production of alpha-synuclein- leading to accumulated phosphorylated α-synuclein in the SNpc, and overt dopaminergic neurodegeneration, including loss of dopamine terminals in the striatum.
Moreover, the use of the viral vectors, allows for a longer lasting expression of α-synuclein. Delivery of the α-synuclein through the viral vector is conducted through stereotaxic injections into the brain similar to the injections of α-synuclein pre-formed fibrils.
In addition, the optimal pα-synuclein expression in this method is around week 4 post-injection.
In contrast to the PFF model, the α-synuclein inclusions are nuclear and demonstrates an anterograde transport in which the pα-syn travels from the soma of the neuron to the terminals, where expression are maintained within medium spiny neurons.
LRRK-2
Leucine-rich repeat kinase (LRRK2) is a protein, that when
mutated
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitos ...
, is implicated in PD pathology. It is associated with both familial (most prevalent causes of familial PD) and sporadic PD. There are key mutations of the LRRK2 protein, like G2019s which is the most common
missense mutation
In genetics, a missense mutation is a point mutation in which a single nucleotide change results in a codon that codes for a different amino acid. It is a type of nonsynonymous substitution.
Substitution of protein from DNA mutations
Missense m ...
and R1441C/G.
Most studies have been conducted on ''C.elegans'', ''Drosophila'', and rodents (mice and rats). It is still unclear as to the mechanism of action of LRRK2, however, the kinase activity is of importance and its ability to function as a GTPase is also a factor in its neurotoxicity.
Unlike the other Parkinson's disease genetic models, LRRK2 can exhibit both Lewy body pathology and tau pathology, but it is also unclear as to its relationship.
On the other hand, results from LRRK2 mutation studies have demonstrated deficits in dopamine transmission, as well as axonal degeneration. Similar to other genetic animal models, LRRK2 mutations also produce dopaminergic neuron loss in the substantia nigra and Lewy body pathology. LRRK2 knockout models have also been studied and show the increase of protein aggregation and accumulation which also includes α-synuclein; but, it does not decrease degeneration of nigrostriatal neurons.
A limitation to the LRRK2 animal model is that although there is a loss in dopaminergic neurons, neurodegeneration is very low.
PINK1
Pten-Induced Kinase 1 (PINK1) mutations are associated with autosomal recessive parkinsonism.
It is a neuroprotective kinase predominantly found in the mitochondria and cytoplasmic areas of the cell. PINK1 is also a serine/threonine protein kinase and is associated with the mitochondria. PINK1, in research studies, is generally used as a knockout (KO) model. The mechanism of action of this gene involves the recruitment of the Parkin gene from the cytoplasm to the mitochondria. Once recruited, this leads to augmented ubiquitin activity and therefore induces
mitophagy Mitophagy is the selective degradation of mitochondria by autophagy. It often occurs to defective mitochondria following damage or stress. The process of mitophagy was first described over a hundred years ago by Margaret Reed Lewis and Warren Harmo ...
. Mitophagy is a pathway in which the mitochondria is degraded. Both PINK1 and Parkin share functions in the same pathway, therefore, their activities are similar.
Some studies have demonstrated expression of the PINK1 mutation in rodents, inducing dopaminergic neuron loss and motor defects.
Other studies are more associated with PINK1 KO. PINK1 knockouts show reduction in dopamine levels in the striatum.
They express very low levels of dopaminergic neuron loss and do not present the formation of Lewy bodies. However, the KO models demonstrate mitochondria dysfunction and oxidative stress. On the other hand, studies are demonstrating loss of dopaminergic neurons and showing motor deficits in rats.
DJ-1
Protein Deglycase (DJ-1) mutations are associated with recessive forms of familial parkinsonism.
It is a molecular chaperone that undergoes
reduction-oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
(redox) reaction and plays a major role in the inhibition of alpha synuclein aggregate formation.
It is believed that this is possible due to DJ-1
antioxidant
Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricant ...
properties, therefore, inhibiting oxidative stress in the cell which is what induces pathological phenotypes. To demonstrate the proposed neuroprotective properties of DJ-1, knockout studies of this gene have shown motor deficits in mice, less dopamine levels in the striatum, and no evidence of Lewy body aggregation.
In addition to the knockout model, DJ-1 is very sensitive to neurotoxins (MPTP, 6-OHDA, etc.). Studies have demonstrated that under those conditions, DJ-1 expresses dopaminergic neuron loss in the SNpc and motor defects.
Summary
Table 1
represents a summary of the PD animal models and details regarding their mechanisms of action, pathogenesis, and limitations.
References
{{reflist
Parkinson's disease
Animal models