In Vivo models of alpha-synuclein

Animal models involving manipulation of a-syn expression have unquestionably led to a better understanding of the correlation between a-syn, neurotoxicity, aggregation, and neurodegeneration. In a transgenic nematode model,overexpression of a-syn results in increased lifespan but impairs survival and function of the eight dopamine containing cells intrinsic to the animals. MPPþ exposure induces
dopamine-neuron death and worm lethality in a-syn transgenic worms. In this model, the major cause of MPPþ toxicity links with ATP depletion. The power of Caenorhabditis elegans as a model system lies with rapid and powerful genetic interaction studies to identify protein interactors capable of modifying a-syn action. An RNA interference screen to identify critical proteins involved in a-syn
protection identified a number of proteins involved in the endocytic pathway in addition to chaperones and other proteins. The usual caveat with a-syn–overexpression
model systems, but particularly for models involving organisms that do not natively express an a-syn–like protein, is that overexpressed a-syn might not adopt physiologically relevant cell functionality and that the cell impairment or deficiency has no overlap with the dysfunction occurring in PD. Thus, lower organisms seem an ideal tool for hypothesis generation but ultimately require translation to mammalian systems. Successful examples of translation from yeast to mouse models have highlighted new pathways with potential therapeutic targets.
Similar to nematodes, Drosophila does not possess clear homologues to a-syn. In models that involve overexpression of human a-syn, flies demonstrate loss of dopamine neurons associated with progressive loss of motor dysfunctions and
the presence of filamentous intraneuronal inclusions.
These flies also exhibited age-dependent retinal degeneration and premature loss of climbing activity. Induction of chaperone pathways rescues cells from the apparent effects of a-synuclein expression. The reproducibility of a-syn– induced dopaminergic cell death in flies has been a matter of contention among different laboratories, with some groups reporting cell shrinkage due to a-syn overexpression in particular dopamine neuron clusters that may be masked as cell loss when counted by using particular methods. The utility of Drosophila models of a-syn overexpression remains in question until the technical issues that prevent an understanding of phenotype are clearly defined.
As opposed to reports of dopaminergic cell death in the worm and fly, mouse transgenic models overexpressing human a-syn have not yet demonstrated overt degeneration in substantia nigra neurons. Transgenic mouse models driving
a-syn with various promoters such as PDGFb, mouse thymus cell antigen 1:Thy1, TH promoter, and prion (PrP) promoter have been described. These transgenic animals demonstrate markedly different phenotypes, making broad-based conclusions difficult
to draw. In mice overexpressing human a-syn and a-syn with PD-associated mutations driven by the PrP promoter, phenotype is related to dose, and the PD-mutation A53T
demonstrates greater in vivo neurotoxicity as compared with other variants; moreover, these mice develop adult-onset neurodegenerative disease with a progressive motoric dysfunction leading to death. Transgenic animals further
demonstrate an early phenotype before pathologic lesions form. Other important observations from transgenic mouse experiments include loss of straital dopaminergic terminals in case of PDGFb promoter-WT–a-synuclein expression, decreased rotarod performance and the presence of detergent-soluble and -insoluble a-syn species in Thy1-promoter-WT and A53T-a-syn expression. Without neurodegeneration in the substantia nigra, the challenge lies with picking a phenotype among the plethora of observations robust enough to screen potential therapies for efficacy and
yet possess reasonable homology to mechanisms thought to underlie pathogenesis in human PD. The lack of Lewy body formation in transgenic mice and selective degeneration of substantia nigra neurons might disqualify existing transgenic
mice as an appropriate model system for therapeutic testing. The next generation of transgenic might include conditional and regionally specific expression, or crosses of existing transgenics to mice that modify expression of a critical a-syn
modifier. As opposed to that of traditional transgenic mice, neurodegeneration in the substantia nigra due to a-syn expression via viral-vector–based delivery has been described in both rats and mice. Viral-based gene transduction in living animals results in acute and targeted gene expression, so-called somatic transgenics. Adenoassociated viral (AAV) vectors and HIV-1–derived lentiviral
vectors successfully direct high-levels of a-syn expression and loss of nigral and dopaminergic neurons in rodents. Co-delivery of the early-onset PD–associated protein
parkin prevents dopaminergic degeneration, but in the same model, delivery of GDNF does not prevent neurodegeneration. The authors speculate that GDNF treatment cannot modulate the cellular toxicity related to mutant a-syn accumulation. Virus-based models suggest a-syn as a viable target for therapeutic intervention. Whether a-syn viral transduction in rodents represents a viable in vivo model in which therapeutic
approaches prove efficacy remains speculative. Issues including a high technical proficiency requirement for model implementation, high variation between experiments, interlaboratory variation in reproducing the critical cell death
phenotype, and a high degree of labor in counting cells by stereology all prevent widespread use of the model system. Further, the lack of cell death in traditional transgenics might translate to a more cautious approach in interpreting viral transduction experiments, in which inflammation or viral transduction pathways may provide a necessary ‘‘second-hit’’ in causing cell death that may or may not have relevance to PD. Alternatively, acute somatic transgenics may not have
compensatory pathways that block cell death in the traditional transgenics. Transgenics that conditionally and acutely upregulate a-syn to the levels obtained through viral transduction will help resolve the issues and may provide the most
powerful model system.