The development of transgenic animal models is dependent on identifying the potential role of genes of interest to the etiology of PD. A transgenic mouse is an animal in which a specific gene of interest has been altered through one of several techniques, including (i) the excision of the host gene (knock-out), (ii) the introduction of a mutant gene (knock-in), and (iii) the alteration of gene expression (knock-down, null, or over-expression). In PD, one source of transgenic targeting is derived from genes identified through epidemiological and linkage analysis studies. Alpha-synuclein and parkin are examples of genes that have been identified through linkage analysis in familial forms of parkinsonism. Once the transgene has been constructed, the degree of its expression and its impact on the phenotype of the animal depends on many factors, including the selection of sequence (mutant vs. wild-type), site of genomic integration, number of copies recombined, selection of transcription promoter, and upstream controlling elements (enhancers). Other important factors may include the background strain and age of the animal. These different features may account for some of the biochemical and pathological variations observed among transgenic mouse lines.
Rare cases of autosomal dominant familial forms of PD (the Contursi, German, and Iowa kindreds) have been linked to A30P and A53T substitution mutations in the gene encoding alpha-synuclein or triplicate of the gene (141-143). The normal function of alpha-synuclein is unknown, but its localization and developmental expression suggests a role in neuroplasticity, neurotransmission, and vesicular function (144-146). The disruption of normal neuronal function may lead to the loss of synaptic maintenance and subsequent degeneration. It is interesting that mice with knockout of alpha-synuclein are viable suggesting that a "gain-in-function" phenotype or other protein-protein interactions may contribute to neurodegeneration. Although no mutant forms of alpha-synuclein have been identified in idiopathic PD, its localization to Lewy bodies (including PD and related disorders) has suggested a patho-physiological link between alpha-synuclein aggregation and neurodegenerative disease (147,148). An interesting caveat is that the mutant allele of alpha-synuclein in the Contursi kindred is identical to the wild-type mouse suggesting that protein expression and/or protein-protein interactions, leading to a yet unidentified gain of function may be more important than loss of function due to missense mutation. Since the identification of alpha-synuclein in familial PD, many groups have developed transgenic mouse models (149-163).
A review of the transgene construction parameters (species and/or mutant forms), promoter selection (neuron or glia specific), and gene and protein expression patterns or levels demonstrates a high degree of variability in the resulting trans-genic strains. Some transgenic mouse lines show neurochemical or pathological changes in dopaminergic neurons (including inclusions, decreased striatal dopamine, and loss of striatal tyrosine hydroxylase immunoreactivity) and behavioral deficits (rotarod and attenuation of dopamine-dependent locomotor response to amphetamine), whereas other lines show no deficits. No group has reported the specific loss of substantia nigra dopaminergic neurons despite inclusion pathology or cell death in other areas of the brain. This range of results with different alpha-synuclein constructs from different laboratories underscores the important link between protein expression (mutant vs. wild-type alleles) and pathological and behavioral outcome. Important applications of alpha-synuclein transgenic mice are occurring at the level of understanding the role of this protein in basal ganglia function. For example, the response of alpha-synuclein expression to neurotoxic injury as well as interactions with other proteins, including parkin, will provide valuable insights into mechanisms important to neurodegeneration (164). Some groups report evidence of neuronal dysfunction (either physiological or motor behavioral changes) without cell death. This suggests that cell death may in fact be a component of the late phase in the progression of basal ganglia degeneration while neuronal dysfunction may occur at the level of the synapse and connectivity.
An autosomal recessive form of juvenile parkinsonism (AR-JP) led to the identification of a gene on chromosome 6q27 called parkin (165,166). Mutations in parkin may account for the majority of autosomal recessive familial cases of PD. Parkin protein has a large N-terminal ubiquitin-like domain and C-terminal cysteine ring structure and is expressed in the brain (167-169). Recent biochemical studies indicate that parkin protein may play a critical role in mediating interactions with a number of different proteins involved in the proteasome-mediated degradation pathway, including alpha-synuclein (164,170). Null mutations in mice appear normal with respect to motor behavior with no evidence of cell loss; however, striatal dopamine levels are elevated with enhanced synaptic excitability in striatal neurons (171). Mutations of the parkin gene have been introduced into transgenic mice. At present, there is very little known about pathological or behavioral alterations due to mutations in parkin protein. However, parkin transgenic models enable investigation of the ubiquitin-mediated protein degradation pathways and its relationship to neurodegenerative disease.
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