Parkinson's Disease Research Paper

Parkinson’s Disease and Effective Therapies

Carolyn Ardizzone

Molecular Neurobiology: Spring 2015

Introduction

Parkinson’s disease is a neurodegenerative disease affecting about 1 million people in the United States today (14). It is second in prevalence only to Alzheimer’s disease, with 1% of the population over 60 years old and 5% of the population over 85 years old with the disease (18). The average age of onset of the disease falls around 60 years old, but about 15% of people are diagnosed before age 50 (1). Although it is known to be a disease of the substantia nigra in the basal ganglia of the brain, the mechanisms producing the associated symptoms are yet to be discovered. Today, many of the therapies available to patients are developed

not based on information about what happens at a cellular level, but instead as trial and error of discovering what works. Therapies often target the major symptoms of the disease including tremors, bradykinesia, gait dysfunction, voice volume dampening, rigidity, and postural instability (10). This paper will focus on current diagnosis and etiology of the disease, along with what is currently known about effective treatments for these patients.
The basic mechanism of motor control dysfunction stems from the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc), with symptoms of idiopathic Parkinson’s manifesting after about 70-80% of this neuronal loss (23).

The SNpc is part of the larger basal ganglia that affects the motor cortex via the thalamus in the central nervous system. The thalamus intrinsically sends tonic inhibitory input to the motor cortex, suppressing motor function. Two pathways in the basal ganglia are capable of influencing this usual thalamic tonic inhibition – the indirect pathway and the direct pathway. The direct pathway flows from the cerebral cortex to the striatum to the internal segment of the globus pallidus, which synapses with the thalamus. The indirect pathway takes a similar route, but first detours from the striatum to the external segment of the globus pallidus, then the subthalamic nucleus, and then back to the internal segment of the globus pallidus. The net effect of the transient cortical activation of the direct pathway is an increase in motor movements, while transient cortical activation of the indirect pathway inhibits motor movements. The SNpc is relevant here because its neurons synapse with those of the striatum, an intermediate of both the direct and indirect pathways. However, the effect of its dopaminergic neurons on the cells of the striatum depends on the type of receptor present on the post-synaptic cells. The cells of the striatum with D1

receptors (the cells of the direct pathway) exhibit an excitatory response to dopamine while the cells with D2 receptors (the cells of the indirect pathway) exhibit an inhibitory response. This means that when dopamine is released from the SNpc, it normally activates the direct pathway and inhibits the indirect pathway, leading to an overall increase in motor cortex excitation. In Parkinson’s disease, when these dopaminergic neurons are degenerated, there is less input to excite the motor cortex, which results in a disease characterized by less motor movement and control (15).

Clinical Diagnoses
Accurately diagnosing patients with Parkinson’s disease has proven to be a difficult task due to lack of specific biomarkers or other certain determinants of the disease, and diagnoses between medical practitioners are currently often inconsistent. Many people have proposed their own criteria with some including two cardinal symptoms, some necessitating three, while others exclude those with co-morbidities or whom experience effects from drugs (17, 4, 8). Several studies have analyzed different sets of diagnostic criteria to determine how they affect prevalence of Parkinson’s disease. When considering broad, intermediate, and strict diagnostic criteria, researchers find that occurrence decreases in that order (2). Mortality rates between patients with Parkinson’s and those without the disease depends on what diagnostic criteria are used, but in most cases we see a two-fold increase in mortality in these individuals, and in one large study using data from insurance companies and drug prescriptions they found a two-and-a-half-fold increase (7).
According to one study by the World War II Veteran Twins registry, genetic factors seems to be highly influential in early-onset Parkinson’s patients, but less so for late-onset Parkinson’s patients for which the environment seems to have a larger role (20). As of 2010, there were eleven genes associated with Parkinson’s, mostly determined through linkage analysis, a technique in which a genetic marker is used to locate genes of interest via co-segregation. These genes include the autosomal dominant Parkinson’s-linked alpha-synuclein, PARK3, Ubiquitin-carboxy-terminal hydrolase L1 (UCH-L1), Leucine-rich repeat kinase 2, and autosomal recessive Parkinson’s-linked Parkin, PTEN-induced putative kinase 1, DJ-1, and ATPase type 13A2. These different genes are correlated with different aspects of the disease such as age of onset, presented symptoms, and, of course, different biological pathway dysfunction (22).
Besides possible genetic contributions to the disease, Parkinson’s has also been linked to exposure of some particular chemicals, such as pesticides. The idea that pesticides might have an effect on the disease picked up steam when 1-methyl-4-phenyl-1, 2, 3, 6,-tetrahydropyridine exposure was found to cause loss of dopaminergic neurons in the brain and persistent parkinsonism (11). Many studies have found that some pesticides increase incidence of Parkinson’s, but most of the studies identified were based off of hospital information, and were by no means causational studies (22). In order to determine real effects of pesticides, more in-depth studies must be conducted.

Pharmacological Intervention
It is thought that neurodegeneration likely proceeds through a multi-step pathway. Cells are intimately connected with their molecular environments - their molecular prodromes - and changes in these environments can cause cellular disruptions. Initially in response to this disruption, a cell becomes somewhat damaged, but still able to receive input normally and produce normal output due to compensatory mechanisms. Eventually, though, the cell can no longer overcome the disruption, and outward dysfunction can be recognized. Finally, the cell breaks down, and this is the stage at which autopsies or brain scans can detect neurodegeneration. Schapira et al. suggests that any of these stages could be potential targets for drug therapies (16). Our current drug development lacks this specificity, but there are some medications available that help relieve symptoms.
Knowing that the dopamine pathway is affected in the basal ganglia, but not the exact mechanisms of dysfunction, scientists have developed several drugs targeting dopamine release in the brain. Although no drugs so far have been successful in preventing neuronal degradation (23), a levodopa, a chemical precursor of dopamine, and carbidopa mixture is the most widely utilized combination of drugs that help with symptom management (14). When administered, levodopa enters the brain and gets taken up by neurons, which then transform it into dopamine that is released and interacts with dopamine receptors in the striatum. Carbidopa is a molecule that inhibits the enzyme amino acid decarboxylase that is found in the blood and normally breaks down levodopa before it can reach the brain (13). One randomized, placebo-controlled, double-blind study by Fahn et al. evaluated the effect of different dosages of levodopa versus placebo in 361 subjects with Parkinson’s for 40 weeks with a two week washout period. The dependent variables observed were the average score on the Unified Parkinson’s Disease Rating Scale (UPDRS) and brain imaging data scores to compare the groups (6). The UPDRS is the assessment often used in clinical settings to diagnose patients with Parkinson’s, specifically to monitor disability and impairment (12). The study found that levodopa actually reduced the worsening of symptoms instead of hastening them as compared to the normal, placebo-mediated rate of decline as determined by the subjects’ UPDRS scores after the washout period. It should be noted that this could have been the result of an insufficient washout period, and that some effects due to levodopa were still present two weeks after medication stopped. During the 40 weeks of medication, there was a clear, significant dose-dependent decrease in symptoms. The group with the highest dosage of levodopa saw the greatest benefits, although all treatment groups saw benefits as compared to the placebo group. The downside of the high levodopa dosage was an increased incidence of adverse events – specifically, hypertonia, dyskinesia (which will be discussed shortly), headache, nausea, and infection (6).
As for the other dependent variable - since dopamine and levodopa have been shown to degenerate cultured dopamine neurons via reactive oxygen species, many have been concerned that the ingestion of levodopa might increase the rate of neurodegeneration in Parkinson’s patients. Brain imaging techniques in the study used odine-123–labeled 2-b-carboxymethoxy- 3-b-(4-iodophenyl)tropane ([123I]b-CIT) to assess striatal dopamine transporter density in 142 of the 316 subjects. The results showed that the subjects in the levodopa groups experienced significantly greater decreases in [123I]b-CIT uptake than did the placebo group, but the results are not strong enough to be conclusive.
Levodopa-induced dyskinesia, as shown as an adverse effect in the above study, is an area of concern for many.

At first it was hypothesized that dyskinesias might be caused by hypersensitive dopamine receptors in the striatum. If this were the case, however, dyskinesia would appear shortly after an initial dose of levodopa, but we instead see a gradual digression over multiple years. More recently there has been evidence that the knockout of D1 receptors in mice protects against levodopa-induced dyskinesia (LID) while knockout of D2 receptors produces no such protection. Furthermore, D1 receptors are not up-regulated after levodopa administration, no matter the length of treatment, in either humans or animal models with Parkinson’s, although there is not a solid consensus on these results. Regardless of the impact on the expression of these D1 receptors, there seems to be a link between the receptors’ sensitivity and the severity of LID in individuals. The patients that do not have dyskinesia seem to have a decrease in D1 receptor sensitivity following levodopa treatment while those who do have dyskinesia see an increase in receptor sensitivity. This sensitivity could be due to the location of D1 receptors on the cell membrane, as those with dyskinesia tend to have the same number of overall receptors, but have more receptors displaced from the synaptic membrane as compared to non-dyskinetic animals. There are many other theorized mechanisms for the dyskinesia that levodopa

causes that have yet to be explored, including some involving D4 receptors instead of D1 receptors (19). Clearly, there is much to be discovered before we can make any drugs that specifically target the problem of interest while minimizing the impact of adverse side effects or avoiding them altogether.
In terms of targeting the products of some of the genes mentioned earlier that are implicated in genetic forms of the disease, alpha-synuclein seems to be one of the most studied. It is the protein most often found in patients with Parkinson’s, and its toxicity is thought to derive from the mis-folded form (24). The failure to clear these proteins could be due to an overproduction of the protein or insufficient means of breaking it down. In either case, a drug that targets the breakdown of alpha-synuclein could prove to be very helpful in halting progression of the disease. There are several factors that have deemed efforts so far ineffective, including toxic, unexpected side effects on other bodily functions (16).
More recently, researchers have become dissatisfied with single target therapeutic drugs, and have realized the importance of developing a multi-molecule drug cocktail which can act on multiple targets in a complex pathway (23). The drug entacapon has recently been added to the levodopa/carbidopa combination, but the downside is that side effects increase in number and possibly severity with this addition. With many of these multipotent drugs still in clinical trials, we have yet to see the long-term impacts or even the short-term impacts in humans. As more and more data are gathered on the efficacy and safety of these drugs, we will be better able to prescribe effectively for each patient.
Non-Invasive Rhythmic Stimulation
In addition to pharmacological therapies to help with gait dysfunction and other parkinsonisms, those with a temporal component have proven to be especially effective. According to a paper by Dr. Ivry at University of California, Berkeley, Parkinson’s patients have been reported to have impaired ability estimating time intervals and there is evidence for dopaminergic pathways being involved in speed regulation of internal clock mechanisms. In particular, dopaminergic neurons terminating on D2 receptors in the striatum have been shown to be potential pacemaker units, providing convincing evidence that temporal aspects of therapy might be effective (9). Rhythmic auditory stimulation (RAS) is one of those therapies that takes advantage of this idea. Initial studies of RAS looked at the effect of an external auditory cue on multiple aspects of gait patterns in both medicated and non-medicated Parkinson’s patients and healthy, normal controls. The Parkinson’s patients were less well able to synchronize their gait cycles to the cue than were the control subjects, but most were still able to entrain to the rhythmic cue despite their basal ganglia dysfunction. The faster cues resulted in significant changes in stride length and velocity (21).
To further study these results, the same research group began a three-week RAS training trial with three randomized groups. The first group received no training at all during the time, the second group received gait training with no RAS, and the third group received gait training of the same intensity as the second group, but with the addition of RAS. After three weeks of training, the groups were tested with no rhythmic cues on both a flat surface and a sloped surface. Significant differences in velocity (25%), stride length (12%), and rhythm (10%) were only found between baseline and post-test results in the group that received three weeks of RAS gait training. Patients’ recall of the gait tempo they had heard on their last day of training was 95% accurate during post-testing, a result that countered previous findings that accurate time recall and time estimation are weakened in Parkinson’s patients. These improvements lasted three weeks without re-training with a 10% decline at week 4 and a much steeper decline after that. This is important information to consider when developing a schedule for therapy, as improvements may be lost during long breaks.
This same group also conducted a study looking at electromyography, a measure of muscle activation, in control versus Parkinson’s patients when exposed to RAS training for three weeks. They collected data on variability and bilateral symmetry in three leg muscles and found that after three weeks, RAS training significantly reduced differences between healthy individuals’ EMG parameters and initial Parkinson’s patients EMG parameter recordings (21).
Music-based Movement therapy (MbMt) is a subtype of RAS that incorporates music into the therapeutic approach to enhance enjoyment and distract from fatigue. There are MbMts that use dancing as a form of rehabilitation and there are MbMts that incorporate specific gait-related exercises. It has been shown that MbMts incorporating specific gait-related exercises are more beneficial in terms of increasing stride length and gait velocity. Engaging in this type of therapy with others who also have Parkinson’s might be helpful in reducing stress and eliciting pleasure sensations related to reward in these patients (5).

Discussion
What we currently know about Parkinson’s disease, it’s etiology and effective treatments, is limited, but knowledge is progressing quickly as more research on the subject continues. Although pharmacological treatments such as levodopa are currently available to temporarily alleviate symptoms, these medications come with serious side effects and are not preventative or even specifically targeted. In order to develop more effective, more targeted drugs to fight the progression of the disease, research needs to focus on identifying biological markers that can help identify more precisely those who have Parkinson’s disease. However, in order for this research to occur, there needs to be a consensus within the medical community on what diagnostic criteria will be used to definitively diagnose patients. It is this interconnected relationship that makes progress all the more difficult. Until these two things occur, therapists can continue to implement physical interventions in addition to patients’ prescribed pharmacological interventions to help alleviate symptoms further without harmful side effects.