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Cell Death Differ 9:1207–1211 Animal Models of Neurodegenerative Diseases Imad Ghorayeb, Guylène Page, Afsaneh Gaillard, and Mohamed Jaber Abstract The use of animals as models of neurodegenerative disorders has allowed the determination of biological targets and biomarkers of several diseases, has yielded new therapeutical perspectives, and is essential before performing novel clinical assays. This review discusses the nature, use, and limits of animal models and how to obtain them for several neurodegenerative disorders such as multiple sys- tem atrophy, amyotrophic lateral sclerosis, and Huntington’s disease, with a special emphasis on Parkinson’s and Alzheimer’s diseases. When possible, rodent, inverte- brate and primate models are presented and discussed in relation to human disease. Finally, we highlight discrepancies between animal models and human neuropathol- ogy leading to question the pertinence of some of these findings to human disorders probably because of the wide spectrum of parameters defining a disease. Another point raised by these studies is the growing necessity to standardize the experimen- tal procedures used to obtain an animal model, housing and breeding conditions, assessments of phenotypes investigated and, ultimately the interpretation of results obtained and their relevance to the pathology. Keywords Parkinson’s disease · Huntington disease · Alzheimer’s disease · Amyotrophic lateral sclerosis · Multiple system atrophy · Tauopathies · Nucleotide repeats Contents 1 Introduction 50 2 Alzheimer’s Disease (AD) 52 2.1 The Human Disease 52 2.2 Rodent Models 53 2.3 Invertebrate Models 62 M. Jaber (B) Institut de Physiologie e t Biologie Cellulaires, CNRS 6187, Université de Poitiers, 86022 Poitiers Cedex, France e-mail: mohamed.jaber@unv-poitiers.fr 49 J.P. Blass (ed.), Neurochemical Mechanisms in Disease, Advances in Neurobiology 1, DOI 10.1007/978-1-4419-7104-3_3, C Springer Science+Business Media, LLC 2011 50 I. Ghorayeb et al. 2.4 Primate Models 63 2.5 Perspectives 64 3 Parkinson’s Disease (PD) 65 3.1 The Human Disease 65 3.2 Rodent Animal Models 66 3.3 Nonhuman Primate Models 72 4 Multiple System Atrophy (MSA) 73 4.1 The Human Disease 73 4.2 Rodent Animal Models 74 4.3 Primate Animal Models 76 5 Amyotrophic Lateral Sclerosis (ALS) 76 5.1 The Human Disease 76 5.2 Animal Models 77 6 Huntington’s Disease (HD) 79 6.1 The Human Disease 79 6.2 Rodent Animal Models 80 6.3 Invertebrate Animal Models 82 6.4 Primate Animal Models 83 7 Conclusion 84 References 85 1 Introduction Given the inherent complexity of neuronal systems and the disease process, ani- mal models have become mandatory in neuroscience research in general and for understanding the pathogenesis of neurodegenerative diseases in particular. Indeed, investigation of human pathologies relies mostly on postmortem human brains and on clinical criteria that neither allow the identification of the causal chains that have led to a disease nor the biological basis of a given pathology. Thus, animal models of neurodegenerative disorders have become a widespread laboratory “tool”. Use and housing of these models require animal facilities dedicated to research purposes and that are controlled by specific policies, guidelines, and procedures at local, national, and international levels. Whatever the country and the regulations, the accredita- tion process is long and difficult to obtain and projects involving animal models are reviewed on a regular basis to ensure the welfare of the animals, the appropriate- ness of the species used for a given investigation, the adequacy of the experimental procedures with the postulated hypothesis, and that a minimal number of animals is used for a given study. The use of primates is often dependent on the s olidity of previous research performed in lower species such as rodents (often mice and rats) but also worms (Caenorhabditis elegans), flies (Drosophila Melanogaster), and zebrafish. Primate animal models are still an essential step before reaching human clinical research for obvious and frequently confirmed similarities between the two species, be it behavioral (large clinical repertoire), anatomical, physiological, or genetic. Major Animal Models of Neurodegenerative Diseases 51 constraints of an ethical, practical, and cost nature have limited the use of primates to a few specialized centers. Thus, mainstream research in neurodegenerative dis- ease has focused on rodent animal models that were used to better understand the pathology and the underlying biological mechanisms, develop standardized diag- noses (biological tests, identification of biomarkers, etc.), and search for potential new treatments for these diseases. The use of rodent models was also strengthened given the possibility of performing genetic manipulations to mimic some of the genetic features of the diseases. Here, we detail several animal models of neurodegenerative disorders with a spe- cial focus on the two major ones in humans, namely Parkinson’s and Alzheimer’s diseases. As detailed in this chapter, some animal models can present sponta- neous syndromes, often due to analogous mutations with the human disease but are more generally obtained following toxin injections, physical (mechanical lesions), or genetic manipulations. Some animal models can mimic the behavioral conse- quences of a given neurodegenerative disease with drawbacks related to specific animal behavior that is only remotely related to humans. A disease gene-based model (also referred to as an “etiological model”) can indeed reproduce the eti- ology of a genetically determined form of a given disease, although adaptations that can occur following a genetic manipulation throughout development can be quite different between the animal and the human. Because manipulation of the mouse genome has become standardized and available at relatively moderate costs, the use of genetically altered mice strains to model neurodegenerative disorders has become increasingly widespread. This use will tend to be generalized following the publica- tion of the assembled mouse genome sequence (Botcherby, 2002). Transgenic mice can be generated to overexpress a gene to reproduce a gain of function mutation, to knock out a gene for a nonexpression mutation, and to mutate a gene to express an altered protein. Many other variances are available where a gene is silenced during development only or expressed/knocked-out in a specific brain area, for instance. These transgenic mice are used to map disease features, determine genetic and environmental factors that can precipitate disease progression, detail behavioral and cellular consequences of altering the expression of a disease-related gene, and test potential therapeutics. Meticulous gene manipulations have generated a wealth of information regarding the etiology of pathology, the identification of its biological basis and the behavioral consequences of such a manipulation. However, increasing concern i s raised as to the adequacy of these animal models to human diseases. Indeed, it is safe to state early in this book chapter that animal models generated so far fail to reproduce faithfully the myriad biochemical, cellular, and behavioral changes reported in a given neurodegenerative disease i n humans. The ideal animal model reproducing all hallmarks of a given neurodegenerative disorder is an unattainable aim, as it is expected to develop specific and reproducible behavioral symptoms and biological features related to the disease along with slow onset and selective cell loss. Instead, an animal model is considered acceptable when it demonstrates its usefulness in understanding the pathogenesis of a disease, its behavioral, cellular, and molecular consequences and in exploring potential treatment avenues. This can sometimes 52 I. Ghorayeb et al. be achieved even when animal models show striking differences with the human pathology. In this line, the general message that can be drawn throughout this review is that we have reached a point in research using animal models where it has become a pressing necessity to standardize not only the experimental procedures used to obtain an animal model, but also the housing and breeding conditions, age and sex of the animals, qualitative and quantitative assessments of phenotypes investigated and, ultimately the interpretation of results obtained and their relevance to the pathology. 2 Alzheimer’s Disease (AD) 2.1 The Human Disease Late-onset Alzheimer’s disease (AD) is the most prevalent subtype of age-related dementia accounting for 60% of cases of dementia and with a mean prevalence estimate of 3.4% (Kalaria et al., 2008). If growth in the older population continues, it is projected that the prevalence of AD will nearly quadruple in the next 50 years, by which time approximately 1 in 45 individuals will be afflicted with the disease (Brookmeyer et al., 1998). In AD, neurodegeneration targets specific brain regions early in its course, especially cholinergic basal forebrain and medial temporal lobe structures. The sequential involvement of the posterior cingulated, temporal, and parietal cortical regions completes the progression of the disease. The neuropathological hall- marks of AD include massive neuronal cell and synapse loss at specific sites and the presence of senile plaques and neurofibrillary tangles (NFTs). The senile plaques are formed from deposits of amyloid-β peptide (Aβ) that is derived from the amyloid precursor protein (APP) whereas the NFTs contain hyperphosphory- lated microtubule-associated protein (MAP) tau. Phosphorylation of both APP and tau represents a biochemical link between the two characteristic lesions of AD (Duyckaerts et al., 2008). Most AD cases occur sporadically (SAD), although inheritance of certain suscep- tibility genes enhances the risk. In early-onset familial AD (FAD), which accounts for less than 5% of the total number of AD cases, autosomal dominant mutations have been identified in three genes: APP, presenilin 1 (PS-1), and presenilin 2 (PS-2), each of which leads to an overabundance of Aβ (Gotz and Ittner, 2008). The presenilins are components of the proteolytic γ-secretase complex that, together with β-secretase, generates Aβ fragments from the cleavage of APP. Most FAD cases are caused by mutations in PSEN1 and PSEN2, of which over 130 have been identified. In SAD, various susceptibility genes have been identified, includ- ing apolipoprotein E (ApoE). It is actually considered that the genetic risk factor that accounts for more cases of AD than any other is the ApoE4 allele located on chromosome 19 (Bertram and Tanzi, 2008). Because neuropathological confirmation is required for the diagnosis of definite AD, only diagnosis of probable and possible AD can be made in living patients Animal Models of Neurodegenerative Diseases 53 according to the commonly used criteria for AD diagnosis. These include pro- gressive memory loss with cognitive deficits in at least two cognitive domains (McKhann et al., 1984). As the disease progresses, the characteristic clinical fea- tures of aphasia, apraxia, and agnosia emerge along with consequent amnesia and personality changes. At present, there are no known curative or preventive measures for AD and current symptomatic treatments of AD are of limited benefit, as they are not directed at the underlying biological basis of the disease. 2.2 Rodent Models The identification of the genetic defects and mutations that cause FAD has led to the generation of transgenic rodent AD models. Nowadays, mice are the most popular animal models for AD, although rat models are developed as well. Furthermore, invertebrate models of AD have been developed and are presented at the end of this section. 2.2.1 Pharmacological Models of Alzheimer’s Disease Aβ neurotoxicity is studied in rodents (mouse and rat) after i ntracerebral injections of Aβ peptides (Aβ 1–40/42 ,Aβ 22/25–35 ) previously fibrillary aggregated by incuba- tion at 37 ◦ C for 4 days minimum. Usually, rodents are intracerebroventricularily injected with a dose range between 3 and 9 nmol for a mouse and a dose of 15 nmol for a rat (Maurice et al., 1996; Stepanichev et al., 2003). Some authors injected the aggregated Aβ peptide directly into the hippocampus or into frontal and cin- gulated cortices uni- or bilaterally (Cetin and Dincer, 2007; Gonzalo-Ruiz et al., 2006). Examination of Congo red-stained tissue sections demonstrated the presence of numerous amyloid deposits throughout the brain areas and a decrease in cre- syl violet-stained cells indicating a significant cell loss. Furthermore, Aβ-injected mice showed learning and memory deficits after 1 week postinjection (Fu et al., 2006; Gonzalo-Ruiz et al., 2006; Maurice et al., 1996; Stepanichev et al., 2003). Although these Aβ-injected rodent models did not encompass all of the neuropatho- logical effects observed in AD, they are useful to understand the toxicity of amyloid deposits, in particular in the cholinergic system, and to screen for neuroprotective molecules active on the amyloid process (Fu et al., 2006; Gonzalo-Ruiz et al., 2006). 2.2.2 Transgenic Mouse Models of Alzheimer’s Disease APP Mice After the first discovery of the mutation in the APP gene by Hardy and Allsop (1991), authors described the first NSEAPP mouse model of AD (Quon et al., 1991). Then, other human APP transgenics were developed: PDAPP, Tg2576, APP23, TgCRND8, and J20. The APP transgene carried one or two mutations at the β-secretase site (Swedish mutation) and/or at the γ-secretase site (London mutation) and was driven by various mouse promoters for gene coding for neuron-specific enolase (NSE), platelet-derived growth factor (PDGF), the prion protein (Prp), or 54 I. Ghorayeb et al. thymus antigen (Thy-1.2). For Thy-1.2, the thymus-specific intronic regulatory element has been removed to target expression specifically to the mouse brain (Andra et al., 1996). Most app transgenes utilize a cDNA encoding the APP 695 isoform, which is the predominant species expressed in the brain, or the longer APP 751 species. Except for NSEAPP mice that show diffuse (preamyloid) plaques, all others displayed amyloid plaques which resembled the mature (neuritic) plaques characteristic of AD with positive thioflavin-S staining. These amyloid deposits were observed at 6–12 months according to the model from the hippocampus to cortical and limbic areas in a progressive manner showing regional specificity like that seen in AD pathology. In TgCRND8 mice expressing both the Swedish and London mutations under the Prp promoter, thioflavine-S-positive amyloid deposits became evident by 3 months of age (Chishti et al., 2001). The amyloid plaques were associated with dystrophic neurites, gliosis, and synaptic loss only in PDAPP mice. Despite the extent of amyloid burden, clear neurodegeneration has not been demonstrated except in the hippocampal CA1 region (14% of neuronal loss) of 14–18-month-old APP23 mice with an apparent correlation with senile plaques load (Calhoun et al., 1999). A positive immunoreactivity of phosphorylated tau protein was detected, however, no paired-helicoidal filament (PHFs) was noted in these transgenic APP mice. To date, it seems that the APP23 mice are the only strain to show a cerebral amyloid angiopathy (CAA). Clinically, the Aβ form of CAA is a significant contributor to haemorrhagic s troke, and up to 90% of AD patients may develop CAA over the disease course. Modest cholinergic deficits have also been reported in aged APP23 mice (Boncristiano et al., 2002). Behavioural studies described age-dependent cognitive deficits assessed by using a Morris water maze. This behavioural test measures spatial reference memory. In these transgenic APP mice, both their acquisition of hidden platform locations and their retention of spatial reference information are affected (Table 1). APP/PS-1 Mice Most FAD cases are caused by the mutations in PS-1 and PS-2. Presenilins are polytopic transmembrane proteins which are, in combination with three or other pro- teins (aph-1, pen-2, and nicastrin), required for an efficient γ-secretase complex and activity to generate amyloid peptides (Edbauer et al., 2003). Although pathogenic mutations in APP and presenilins do not coexist in human AD, it was tempting to cross APP and PS-1 mutant mice and to assess whether mutant PS-1 would cause elevated Aβ levels. Overexpression alone of PS-1 M146L, M146V FAD-associated mutations induced a selective increase of Aβ 42 production. Crossing APP trans- genic mice with PS-1 mutant mice causes an elevation of Aβ 42 /Aβ 40 levels and an acceleration of amyloid deposits by 4 months of age in APP SWE /PS-1 dE9 mice (Garcia-Alloza et al., 2006), by 6 months of age in PSAPP (Tg2576 mouse × PS- 1 M146L mouse), compared to 9 months in Tg2576 mice and by 1 month of age in TgCRND8/PS-1 mice compared to 3 months TgCRND8 mice (Chishti et al., 2001; Holcomb et al., 1998). In various double APP/PS-1 transgenic mice, no clear evidence for neurodegeneration in either frontal cortex or CA1 hippocampus was . mohamed.jaber@unv-poitiers.fr 49 J.P. Blass (ed.), Neurochemical Mechanisms in Disease, Advances in Neurobiology 1, DOI 10.1007/978-1-4419-7104-3_3, C Springer Science+Business Media, LLC 2011 50 I. Ghorayeb. neurodegenerative diseases in particular. Indeed, investigation of human pathologies relies mostly on postmortem human brains and on clinical criteria that neither allow the identification of the causal chains. authors injected the aggregated Aβ peptide directly into the hippocampus or into frontal and cin- gulated cortices uni- or bilaterally (Cetin and Dincer, 2007; Gonzalo-Ruiz et al., 2006). Examination