Parkinson's disease: a genetic perspective

Parkinson's disease (PD) is a common neurodegenerative disorder in the aging population, affecting more than 1% over the age of 65 years. Certain rare forms of the disease are monogenic, representing 5-10% of PD patients, but there is increasing evidence that multiple genetic risk factors are important also for common forms of PD. To date, 13 genetic loci, PARK1-13, have been suggested for rare forms of PD such as autosomal dominant and autosomal recessive PD. At six of these loci, genes have been identified and reported by several groups to carry mutations that are linked to affected family members. Genes in which mutations have been linked to familial PD have also been shown to be candidate genes for idiopathic forms of PD, as those same genes may also carry other mutations that merely increase the risk. Four of the PARK genes, SNCA at PARK1, UCH-L1 at PARK5, PINK1 at PARK6 and LRRK2 at PARK8, have been implicated in sporadic PD. There are indeed multiple genetic risk factors that combine in different ways to increase or decrease risk, and several of these need to be identified in order to begin unwinding the causative pathways leading to the different forms of PD. In this review, we present the molecular genetics of PD that are understood today, to help explain the pathways leading to neurodegeneration.     

Parkinson's disease: mechanisms and models

Parkinson's disease (PD) results primarily from the death of dopaminergic neurons in the substantia nigra. Current PD medications treat symptoms; none halt or retard dopaminergic neuron degeneration. The main obstacle to developing neuroprotective therapies is a limited understanding of the key molecular events that provoke neurodegeneration. The discovery of PD genes has led to the hypothesis that misfolding of proteins and dysfunction of the ubiquitin-proteasome pathway are pivotal to PD pathogenesis. Previously implicated culprits in PD neurodegeneration, mitochondrial dysfunction and oxidative stress, may also act in part by causing the accumulation of misfolded proteins, in addition to producing other deleterious events in dopaminergic neurons. Neurotoxin-based models (particularly MPTP) have been important in elucidating the molecular cascade of cell death in dopaminergic neurons. PD models based on the manipulation of PD genes should prove valuable in elucidating important aspects of the disease, such as selective vulnerability of substantia nigra dopaminergic neurons to the degenerative process.     

Experimental models of Parkinson's disease

Research into the pathogenesis of Parkinson's disease has been rapidly advanced by the development of animal models. Initial models were developed by using toxins that specifically targeted dopamine neurons, the most successful of which used 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, a toxin that causes parkinsonism in man. More recently, the identification of alpha-synuclein mutations as a rare cause of Parkinson's disease has led to the development of alpha-synuclein transgenic mice and Drosophila. Here, I discuss the merits and limitations of these different animal models in our attempts to understand the physiology of Parkinson's disease and to develop new therapies.     

Animal models of Parkinson's disease

Animal models are important tools in experimental medical science to better understand pathogenesis of human diseases. Once developed, these models can be exploited to test therapeutic approaches for treating functional disturbances observed in the disease of interest. On the basis of experimental and clinical findings, Parkinson's disease (PD) was the first neurological disease to be modeled and, subsequently, to be treated by neurotransmitter replacement therapy. Agents that selectively disrupt or destroy catecholaminergic systems, such as reserpine, methamphetamine, 6-hydroxydopamine and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine have been used to develop PD models. Recently, it has been found that agricultural chemicals, such as rotenone and paraquat, when administered systemically, can reproduce specific features of PD in rodents, apparently via oxidative damage. Transgenic animals that over-express alpha-synuclein are used to study the role of this protein in dopaminergic degeneration. This review critically discusses animal models of PD and compares them with characteristics of the human disease.     

Animal models of Parkinson's disease

Animal models of Parkinson's disease (PD) have been widely used in the past four decades to investigate the pathogenesis and pathophysiology of this neurodegenerative disorder. These models have been classically based on the systemic or local (intracerebral) administration of neutoxins that are able to replicate most of the pathological and phenotypic features of PD in mammals (i.e. rodents or primates). In the last decade, the advent of the 'genetic era' of PD has provided a phenomenal enrichment of the research possibilities in this field, with the development of various mammalian (mice and, more recently, rats) and non-mammalian transgenic models that replicate most of the disease-causing mutations identified for monogenic forms of familial PD. Both toxic and transgenic classes of animal PD models have their own specificities and limitations, which must be carefully taken into consideration when choosing the model to be used. If a substantial and reproducible nigrostriatal lesion is required (e.g. for testing therapeutic interventions aimed at counteracting PD-related cell death), a classic toxic model such as one based on the administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine or 6-hydroxydopamine will adequately serve the purpose. On the other hand, if selected molecular mechanisms of PD pathogenesis must be investigated, transgenic models will offer invaluable insights. Therefore, until the 'perfect' model is developed, indications to use one model or another will depend on the specific objectives that are being pursued.     

Experimental models of Parkinson's disease: insights from many models.

Toxin-induced and genetic experimental models have been invaluable in investigating idiopathic Parkinson's disease (PD). The neurotoxins--reserpine, 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and methamphetamine--have been used to develop parkinsonian models in a wide variety of species. Both 6-OHDA and MPTP can replicate the neurochemical, morphologic, and behavioral changes seen in human disease. The unilateral 6-OHDA rat model is an excellent model for testing and determining modes of action of new pharmacologic compounds. The nonhuman primate MPTP-induced parkinsonian model has behavioral features that best approximate idiopathic PD. These induced and genetic models have been used to study the pathophysiology of the degenerating nigrostriatal system and to evaluate novel therapeutic strategies. Important differences within these models provide insights into various aspects of the dopaminergic phenotype and its role as a target in disease. These models provide an avenue to evaluate many anti-parkinsonian compounds, such as levodopa, which was first evaluated in an animal model and is the gold standard of parkinsonian treatment today.     

帕金森病动物模型:揭开人类帕金森病奥秘的钥匙

帕金森病（Parkinson‘s disease,PD)是一种以黑质致密部多巴胺能神经元的特异性,进行性坏死为特征的神经系统退行性疾病,尽管PD的发病原因还不十分清楚,目前的证据表明环境因素和遗传因素是PD可能的主要致病因素,针对这两大因素,神经科学家们发展了很多制造PD动物模型的方法,MPTP（1-甲基-4-氨基-1,2,3,6-四氢吡啶）模型是目前比较经典的模型。而新近兴起的基因工程模型亦具有广阔的应用前景咯种PD动物模型推动着人们对PD认识的深入和治疗手段的发展,恰当的动物模型必将成为最终揭开人类帕金森病奥秘的钥匙。     

Nonhuman primate models of Parkinson's disease

Nonhuman primate (NHP) models of Parkinson's disease (PD) play an essential role in the understanding of PD pathophysiology and the assessment of PD therapies. NHP research enabled the identification of environmental risk factors for the development of PD. Electrophysiological studies in NHP models of PD identified the neural circuit responsible for PD motor symptoms, and this knowledge led to the development of subthalamic surgical ablation and deep brain stimulation. Similar to human PD patients, parkinsonian monkeys are responsive to dopamine replacement therapies and present complications associated with their long-term use, a similarity that facilitated the assessment of new symptomatic treatments, such as dopaminergic agonists. New generations of compounds and novel therapies that use directed intracerebral delivery of drugs, cells, and viral vectors benefit from preclinical evaluation in NHP models of PD. There are several NHP models of PD, each with characteristics that make it suitable for the study of different aspects of the disease or potential new therapies. Investigators who use the models and peer scientists who evaluate their use need information about the strengths and limitations of the different PD models and their methods of evaluation. This article provides a critical review of available PD monkey models, their utilization, and how they compare to emerging views of PD as a multietiologic, multisystemic disease. The various models are particularly useful for representing different aspects of PD at selected time points. This conceptualization provides clues for the development of new NHP models and facilitates the clinical translation of findings. As ever, successful application of any model depends on matching the model to the scientific question to be answered. Adequate experimental designs, with multiple outcome measures of clinical relevance and an appropriate number of animals, are essential to minimize the limitations of models and increase their predictive clinical validity.     

Population genetic approaches to neurological disease: Parkinson's disease as an example

Parkinson's disease (PD) is a common, progressive, incurable disabling condition. The cause is unknown but over the past few years tremendous progress in our understanding of the genetic bases of this condition has been made. To date, this has almost exclusively come from the study of relatively rare Mendelian forms of the disease and there are no currently, widely accepted common variants known to increase susceptibility.The role that the "Mendelian" genes play in common sporadic forms of PD is unknown. Moreover, most studies in PD can really be described as candidate polymorphism studies rather than true and complete assessments of the genes themselves. We provide a model of how one might tackle some of these issues using Parkinson's disease as an illustration. One of the emerging hypotheses of gene environment interaction in Parkinson's disease is based on drug metabolizing (or xenobiotic) enzymes and their interaction with putative environmental toxins. This motivated us to describe a tagging approach for an extensive but not exhaustive list of 55 drug metabolizing enzyme genes. We use these data to illustrate the power, and some of the limitations of a haplotype tagging approach. We show that haplotype tagging is extremely efficient and works well with only a modest increase in effort through different populations. The tagging approach works much less well if the minor allele frequency is below 5%. However, it will now be possible using these tags to evaluate these genes comprehensively in PD and other neurodegenerative conditions.     

PEP-1-PEA-15 protects against toxin-induced neuronal damage in a mouse model of Parkinson's disease

Background

PEA-15 is abundantly expressed in both neurons and astrocytes throughout the brain. It is a multifunctional protein with the ability to increase cell survival via anti-apoptotic and anti-proliferative properties. However, the function of PEA-15 in neuronal diseases such as Parkinson's disease (PD) remains unclear. In this study, we investigated the protective effects of PEA-15 on neuronal damage induced by MPP⁺ in neuroblastoma SH-SY5Y and BV2 microglia cells and in a MPTP-induced PD mouse model using cell-permeable PEP-1-PEA-15.

Methods

PEP-1-PEA-15 was purified using affinity chromatography. Cell viability and DNA fragmentation were examined by MTT assay and TUNEL staining. Dopaminergic neuronal cell death in the animal model was examined by immunohistochemistry.

Results

PEP-1-PEA-15 transduced into the SH-SY5Y and BV2 cells in a time- and dose-dependent manner. Transduced PEP-1-PEA-15 protected against MPP⁺-induced toxicity by inhibiting intracellular ROS levels and DNA fragmentation. Further, it enhanced the expression levels of Bcl-2 and caspase-3 while reducing the expression levels of Bax and cleaved caspase-3. We found that PEP-1-PEA-15 transduced into the substantia nigra and prevented dopaminergic neuronal cell death in a MPTP-induced PD mouse. Also, we showed the neuroprotective effects in the model by demonstrating that treatment with PEP-1-PEA-15 ameliorated MPTP-induced behavioral dysfunctions and increased dopamine levels in the striatum.

Conclusions

PEP-1-PEA-15 can efficiently transduce into cells and protects against neurotoxin-induced neuronal cell death in vitro and in vivo.

General significance

These results demonstrate the potential for PEP-1-PEA-15 to provide a new strategy for protein therapy treatment of a variety of neurodegenerative diseases including PD.     

Rodent models of genetic disease

The genetic study of rats and mice using natural variants, natural mutations, chemical or radiation induced mutations, engineered mutations and conditional engineered mutations has provided the tools for investigating the genetics of disease. The completion of the mouse genomic sequence and progress towards sequencing the rat genome in the past year will enable the molecular identification of quantitative trait loci and induced mutations. Sequence-based single nucleotide polymorphism discovery and a greater understanding of the haplotype structure of inbred strains is revitalising quantitative trait locus mapping and there are now plans for an ambitious eight-way recombinant inbred cross and renewed interest in existing resources such as heterogeneous stocks. In the past year there have been refinements to ENU mutagenesis approaches including balancer chromosomes and a new gene-driven approach.     

Analysis of dopaminergic neuronal dysfunction in genetic and toxin‐induced models of Parkinson's disease in Drosophila

Drosophila melanogaster has contributed significantly to the understanding of disease mechanisms in Parkinson's disease (PD) as it is one of the very few PD model organisms that allow the study of age‐dependent behavioral defects, physiology and histology, and genetic interactions among different PD‐related genes. However, there have been contradictory results from a number of recent reports regarding the loss of dopaminergic neurons in different PD fly models. In an attempt to re‐evaluate and clarify this issue, we have examined three different genetic (α‐synuclein, Pink1, parkin) and two toxin‐based (rotenone and paraquat) models of the disease for neuronal cell loss. Our results showed no dopaminergic neuronal loss in all models tested. Despite this surprising result, we found additional phenotypes showing the dysfunctional status of the dopaminergic neurons in most of the models analyzed. A common feature found in most models is a quantifiable decrease in the fluorescence of a green‐fluorescent protein reporter gene in dopaminergic neurons that correlates well with other phenotypes found for these models and can be reliably used as a hallmark of the neurodegenerative process when modeling diseases affecting the dopaminergic system in Drosophila.

     

Cellular models to investigate biochemical pathways in Parkinson's disease

Cellular models are instrumental in dissecting a complex pathological process into simpler molecular events. Parkinson's disease is multifactorial and clinically heterogeneous; the aetiology of the sporadic (and most common) form is still unclear and only a few molecular mechanisms have been clarified so far in the neurodegenerative cascade. In such a multifaceted picture, it is particularly important to identify experimental models that simplify the study of the different networks of proteins/genes involved. Cellular models that reproduce some of the features of the neurons that degenerate in Parkinson's disease have contributed to many advances in our comprehension of the pathogenic flow of the disease. In particular, the pivotal biochemical pathways (i.e. apoptosis and oxidative stress, mitochondrial impairment and dysfunctional mitophagy, unfolded protein stress and improper removal of misfolded proteins) have been widely explored in cell lines, challenged with toxic insults or genetically modified. The central role of α-synuclein has generated many models aiming to elucidate its contribution to the dysregulation of various cellular processes. In conclusion, classical cellular models appear to be the correct choice for preliminary studies on the molecular action of new drugs or potential toxins and for understanding the role of single genetic factors. Moreover, the availability of novel cellular systems, such as cybrids or induced pluripotent stem cells, offers the chance to exploit the advantages of an in vitro investigation, although mirroring more closely the cell population being affected.     

Immune changes of Schistosoma japonicum infections in various rodent disease models

Rodent models for Schistosoma japonicum infections have demonstrated that these animals possess a degree of resistance to schistosome infections that may be both T and B lymphocyte-mediated. However, their exact role is not well-defined and other immune mechanisms are likely to also play a role in protecting against infection. Immunosuppressed and immunocompetent reed voles (Microtus fortis, Mf), rats and mice (n=24/group) were infected with S. japonicum, and animals were sacrificed 42 days later under anesthesia. Neither worms nor eggs were observed in infected immunosuppressed Mf or rats, with the exception of one rat that presented with few eggs. In immunosuppressed mice, changes in the number and size of the worms were not significantly different compared to immunocompetent mice, but worm fecundity was affected. The size and number of granulomas in immunosuppressed animals was also reduced. Analysis of serum antibodies specific to schistosome adult worm antigen at 3 weeks post-infection demonstrated that the levels of antibodies in the sera of rats were significantly higher than in Mf and mice. In addition, Mf serum levels of IL-4 and IL-12 were significantly higher than levels observed in rats and mice. Antibodies and cytokines in the sera of Mf peaked 3 weeks post-infection and then began to decrease, while antibody responses in rats and mice increased gradually between weeks 3-7 post-infection. It is possible that T and B cells have a dual role in both mediating protection and exacerbating disease outcomes.     

A neuroprotective role for angiogenin in models of Parkinson's disease

We previously observed marked down-regulation of the mRNA for angiogenin, a potent inducer of neovascularization, in a mouse model of Parkinson's disease (PD) based on over-expression of alpha-synuclein. Angiogenin has also been recently implicated in the pathogenesis of amyotrophic lateral sclerosis. In this study, we confirmed that mouse angiogenin-1 protein is dramatically reduced in this transgenic alpha-synuclein mouse model of PD, and examined the effect of angiogenin in cellular models of PD. We found that endogenous angiogenin is present in two dopamine-producing neuroblastoma cell lines, SH-SY5Y and M17, and that exogenous angiogenin is taken up by these cells and leads to phosphorylation of Akt. Applied angiogenin protects against the cell death induced by the neurotoxins 1-methyl-4-phenylpyridinium and rotenone and reduces the activation of caspase 3. Together our data supports the importance of angiogenin in protecting against dopaminergic neuronal cell death and suggests its potential as a therapy for PD.