CNS & Neurological Disorders - Drug Targets (Formerly Current Drug Targets - CNS & Neurological Disorders) - Volume 9, Issue 4, 2010

Age-related neurodegenerative diseases represent a growing socio-economic burden to developed societies due to an increased life-expectancy and the lack of effective treatments. The pathological hallmarks of the majority of these diseases, including Alzheimer's (AD) and Parkinson's disease (PD), are proteinaceous inclusions and the progressive loss of neurons. These inclusions are characterised by β-sheet-rich fibrillar aggregates commonly termed amyloids that accumulate in the extracellular milieu or intracellular compartments of affected neurons. Protein aggregates are commonly derived from misfolded proteins that are prone to amyloidogenesis due to structural modifications, such as mutant forms of α-synuclein (in PD) or aberrantly cleaved forms of amyloid precursor protein (Aβ in AD). Amyloidogenesis occurs when the normally reversible balance between monomers and oligomers destabilises towards the predominant formation of oligomers, which subsequently undergo a conformational conversion into amyloid fibrils. Although still debated, the prevalent hypothesis is that amyloid fibrils represent a cellular protection mechanism, whereas soluble intermediate oligomeric and pre-fibrillar protein species are the major cause of cellular toxicity. However, the precise mechanism(s) leading to cell death are not understood. Despite this, there is an urgent need for new therapeutic strategies aimed at slowing or halting the progression of neurodegenerative diseases. A significant contribution in this direction has been the discovery that catechins derived from green tea are potent inhibitors of protein fibrillarization. Namely, epi-gallocatechin-3-gallate (EGCG) has been shown to antagonise both the formation of aggregates and the cellular toxicity of several aggregation-prone proteins including α-synuclein and Aβ. These studies already showed that EGCG can prevent amyloidogenesis, at least in cell-based model systems. The potential clinical application of such a drug could be to halt, or at least slow disease progression. However, because the majority of proteinopathies are sporadic, they are usually diagnosed once clinical symptoms have become apparent due to the loss of large populations of nerve cells. The evident question then arises whether EGCG might also be effective in disassembling toxic aggregation intermediates. To address this question, Erich Wanker and colleagues used in vitro aggregation assays and cell-based models of disease to demonstrate that EGCG remodels mature α-synuclein and Aβ fibrils and reduces cellular toxicity. Following on their previous work on EGCG, Bieschke et al. now show that EGCG affects the ordered fibrillar structure of α-synuclein and Aβ aggregates, leading to amyloid remodelling. They show that EGCG binding to aggregates precedes, and is necessary and sufficient for structural change. Most importantly, this study provides evidence that EGCG treatment remodels toxic β-sheet-rich amyloid protofibrils into smaller amorphous protein aggregates, thereby reducing cellular toxicity. The authors then tested several other EGCG-related polyphenols and found that the gallate moiety present in a variety of green tea catechins harbours the critical structural module required for efficient amyloid conversion from toxic proto-fibrils into amorphous, benign protein aggregates. The work by Bieschke et al. increases our understanding of the causes underlying amyloid-driven neurodegeneration, and demonstrates a potential therapeutic benefit of gallate in neurodegenerative diseases characterised by the formation of amyloid lesions. The fact that gallate can change the properties of amyloids from being neurotoxic to benign should make it a vital tool that allows further dissection of the structural and molecular basis of amyloid toxicity. In addition, this report represents a major step towards the identification of drugs with the potential to target proteinopathies. So far, the efficacy of EGCG has been shown in vitro and in cell-based model systems, but also in fly models of Huntington disease. The next step, of course, will be to show whether this holds true also for mammalian in vivo models, with the ultimate goal (and hope) that EGCG will be successful in clinical trials.

Human neurodegenerative diseases are devastating illnesses that predominantly affect elderly people, and represent a tremendous unmet medical need. Consider, for example, Alzheimer's disease. Memory progressively fails, complex tasks become even more difficult, and oncefamiliar situations and people suddenly appear strange, even threatening. Over years, afflicted patients lose virtually all abilities and succumb to the disease. The majority of chronic neurodegenerative diseases are associated with the accumulation of misfolded proteins into aggregates that contain fibrillar structures, eventually causing the progressive loss of neurons in the brain and nervous system. Most of these proteinopathies are sporadic and the cause of pathogenesis remains elusive. Heritable forms are associated with genetic defects, suggesting that the affected protein is causally related to disease formation and/or progression. The limitations of human genetics, however, make it necessary to use model systems to analyse affected genes and pathways in more detail. Animal models have contributed considerably to advancing our understanding of the pathophysiological mechanisms underlying neurodegenerative disorders and have pointed to novel strategies for drug development. The successful use of animal models in drug discovery relies on both the development of valid disease models and the availability of adequate testing paradigms for evaluating the effects of different therapeutic approaches. In the opening article, Wilcock provides an overview of AD, and mouse models used for its study. AD is a progressive, neurodegenerative disorder characterized pathologically by amyloid plaques composed of aggregated amyloid β-peptide (Aβ), neurofibrillary tangles composed of aggregated, hyperphosphorylated tau protein, and neuron loss. While the disease was first described in 1906, transgenic mouse models for the study of AD pathologies have only been available for fifteen years. Despite the generation of many different mouse models that develop amyloid plaques or neurofibrillary tangles, mouse models demonstrating the two pathologies together have only recently been made. Also, neuron loss has been difficult to achieve in many models. Most recently, several transgenic mice have been generated that do demonstrate all three pathological characteristics of AD; amyloid plaques, neurofibrillary tangles and neuron loss. This review discusses the advances made in our understanding of AD pathology using transgenic mouse models, and some of the limitations associated with studying these mice and how transgenic mouse models have contributed to the development of therapeutics for the treatment of AD. A critical requirement in the development of AD therapeutics is a demonstration of the in vivo efficacy of compounds in pre-clinical disease relevant models. One of the most frequently used models in AD research are transgenic mice over-expressing mutant forms of human amyloid precursor protein (APP) that are associated with early-onset familial AD. Hussain carries on this theme by highlighting how APP transgenic mouse models have successfully been used in drug discovery to support the progression of Aβ lowering therapeutics to clinical trials to ultimately test the 'amyloid hypothesis' of AD. These mice exhibit an age-dependent accumulation and deposition of Aβ as extracellular plaques in the brain, and thereby depict one of the key pathologies observed in the brains of AD patients. Although these mouse models do not recapitulate all the pathological features of AD, they have been invaluable in the development of therapeutic agents aimed at lowering Aβ production, inhibiting Aβ deposition or facilitating Aβ clearance. Further development of these APP transgenic models led to the incorporation of transgenes for human mutant presenilins, resulting in an accelerated Aβ deposition rate and human mutant tau protein leading to neurofibrillary tangle-like pathology. The latter was a major advance in the development of AD models, as it allowed researchers to investigate the interplay between the two key pathologies of AD. Tauopathies, including AD, are neurodegenerative diseases characterized by the deposition of hyperphosphorylated tau protein in the CNS, and are the major cause of dementia in later life. In their review, Noble, Hanger, and Gallo discuss the advances made in developing mouse models that recapitulate, to varying extents, the development of human tau pathology, and the learning and memory deficits characteristic of some tauopathies. Such models have been used to shown promising disease-modifying effects in pre-clinical testing of new therapeutics. Some of the most enlightening models developed to date either constitutively or inducibly express pathogenic tau mutations. These animals have been instrumental in defining critical disease-related mechanisms, including the observation that tangles are not the toxic form of tau in disease. The authors appraise the strengths and weaknesses of well characterised transgenic models that emulate human tauopathy, and then summarise the use of tau mice for the development and evaluation of new therapeutic approaches, and their utility in identifying novel drug targets. In addition, they review the parameters to be considered in the development of the next generation of mouse models of tauopathy, aimed at further increasing our understanding of disease aetiology and in evaluating novel treatments....

Alzheimer's disease is a progressive, neurodegenerative disorder characterized by a devastating cognitive decline. The disease is identified pathologically by amyloid plaques composed of aggregated amyloid-β peptide, neurofibrillary tangles composed of aggregated, hyperphosphorylated tau protein and neuron loss. While the disease was first described in 1906, transgenic mouse models for the study of Alzheimer's disease pathologies have only been available to scientists for fifteen years. Despite the generation of many different mouse models that develop amyloid plaques or neurofibrillary tangles, it has only been in recent years that mouse models demonstrating the two pathologies together have been made. Also, neuron loss has been difficult to achieve in many models. Most recently, several transgenic mouse lines have been generated that do demonstrate all three pathological characteristics of Alzheimer's disease: amyloid plaques, neurofibrillary tangles and neuron loss. This review will focus on the advances made in our understanding of Alzheimer's disease pathology using the transgenic mouse models. It will also discuss the limitations associated with studying some of these mice and how transgenic mouse models have contributed to the development of therapeutics for the treatment of Alzheimer's disease.

A critical requirement in the development of Alzheimer's disease (AD) therapeutics is a demonstration of the in vivo efficacy of compounds in pre-clinical disease relevant models. One of the most frequently used models in AD research are transgenic mice overexpressing mutant forms of human amyloid precursor protein (APP) that are associated with early-onset familial AD. These mice exhibit an age-dependent accumulation and deposition of amyloid β-peptide (Aβ) as extracellular plaques in the brain, and thereby depict one of the key pathologies observed in the brains of AD patients. Although these mouse models do not recapitulate all the pathological features of AD, they have been invaluable in the development of therapeutic agents aimed at lowering Aβ production, inhibiting Aβ deposition or facilitating Aβ clearance. Further development of these APP transgenic models led to the incorporation of transgenes for human mutant presenilins, resulting in an accelerated Aβ deposition rate and human mutant tau protein leading to neurofibrillary tangle-like pathology. The latter was a major advance in the development of AD models, as it allowed researchers to investigate the interplay between the two key pathologies of AD. This review highlights how APP transgenic mouse models have successfully been used in drug discovery to support the progression of Aβ lowering therapeutics to clinical trials to ultimately test the 'amyloid hypothesis' of AD.

Tauopathies, including Alzheimer's disease, are neurodegenerative diseases characterized by the deposition of hyperphosphorylated tau protein in the central nervous system, and are the major cause of dementia in later life. Considerable advances have been made in developing mouse models that recapitulate, to varying extents, the development of human tau pathology, and the learning and memory deficits characteristic of some tauopathies. Furthermore, such models have been used to show promising disease-modifying effects in pre-clinical testing of new therapeutics. Various strategies have been utilised to generate mouse models of tauopathies. Some of the most enlightening models developed to date either constitutively or inducibly express pathogenic tau mutations. These animals have been instrumental in defining critical disease-related mechanisms, including the observation that tangles are not the toxic form of tau in disease. Here, we discuss the strengths and weaknesses of well characterised transgenic models that emulate human tauopathy, and include a comprehensive listing of the main phenotypic characteristics of all reported tau transgenic rodents. We summarise the use of tau mice for the development and evaluation of new therapeutic approaches, and their utility in identifying novel drug targets. In addition, we review the parameters to be considered in the development of the next generation of rodent models of tauopathy, aimed at further increasing our understanding of disease aetiology and in evaluating novel treatments.

Individuals with trisomy 21, also known as Down syndrome (DS), develop a clinical syndrome including almost identical neuropathological characteristics of Alzheimer's disease (AD) observed in non-DS individuals. The main difference is the early age of onset of AD pathology in individuals with DS, with high incidence of clinical symptoms in the late 40- early 50 years of age. The neuropathology of AD in persons with DS is superimposed with the developmental abnormalities causing alterations of neuronal morphology and function. Despite the ubiquitous occurrence of AD neuropathology, clinical signs of dementia do not occur in all adults with DS even at older ages. Phenotype analysis of DS mouse models has revealed a differential age-related neurodegenerative pattern that correlates with specific biochemical and molecular alterations at the cellular level. In fact, several individual genes found in trisomy in DS have been functionally related to neuronal degeneration. Thus, mouse models overexpressing HSA21 gene(s) are fundamental to understand the neurodegenerative process in DS, as described in the present review. In addition, these models might allow to define and evaluate potential drug targets and to develop therapeutic strategies that may interfere or delay the onset of AD.

Oxidative stress has been consistently linked to ageing-related neurodegenerative diseases leading to the generation of lipid peroxides, carbonyl proteins and oxidative DNA damage in tissue samples from affected brains. Studies from mouse models that express disease-specific mutant proteins associated to the major neurodegenerative processes have underscored a critical role of mitochondria in the pathogenesis of these diseases. There is strong evidence that mitochondrial dysfunction is an early event in neurodegeneration. Mitochondria are the main cellular source of reactive oxygen species and key regulators of cell death. Moreover, mitochondria are highly dynamic organelles that divide, fuse and move along axons and dendrites to supply cellular energetic demands; therefore, impairment of any of these processes would directly impact on neuronal viability. Most of the disease-specific pathogenic mutant proteins have been shown to target mitochondria, promoting oxidative stress and the mitochondrial apoptotic pathway. In addition, disease-specific mutant proteins may also impair mitochondrial dynamics and recycling of damaged mitochondria via autophagy. Collectively, these data suggest that ROS-mediated defective mitochondria may accumulate during and contribute to disease progression. Strategies aimed to improve mitochondrial function or ROS scavenging may thus be of potential clinical relevance.

Parkinson's disease (PD) is a chronic progressive neurodegenerative movement disorder characterized by a profound and selective loss of nigrostriatal dopaminergic neurons. Another neurodegenerative disorder, Huntington's disease (HD), is characterized by striking movement abnormalities and the loss of medium-sized spiny neurons in the striatum. Current medications only provide symptomatic relief and fail to halt the death of neurons in these disorders. A major hurdle in the development of neuroprotective therapies is due to limited understanding of disease processes leading to the death of neurons. The etiology of dopaminergic neuronal demise in PD is elusive, but a combination of genetic and environmental factors seems to play a critical role. The majority of PD cases are sporadic; however, the discovery of genes linked to rare familial forms of disease and studies from experimental animal models has provided crucial insights into molecular mechanisms of disease pathogenesis. HD, on the other hand, is one of the few neurodegenerative diseases with a known genetic cause, namely an expanded CAG repeat mutation, extending a polyglutamine tract in the huntingtin protein. One of the most important advances in HD research has been the generation of various mouse models that enable the exploration of early pathological, molecular, and cellular abnormalities produced by the mutation. In addition, these models for both HD and PD have made possible the testing of different pharmacological approaches to delay the onset or slow the progression of disease. This article will provide an overview of the genetics underlying PD and HD, the animal models developed, and their potential utility to the study of disease pathophysiology.

α-Synuclein is a soluble, natively unfolded protein that is highly enriched in the presynaptic terminals of neurons in the central nervous system. Interest in α-synuclein has increased markedly following the discovery of a relationship between its dysfunction and several neurodegenerative diseases, including Parkinson's disease. The physiological functions of α-synuclein remain to be fully defined, although recent data suggest a role in regulating membrane stability and neuronal plasticity. In addition, there is increasing evidence pointing to phosphorylation as playing an important role in the oligomerization, fibrillogenesis, Lewy body formation, and neurotoxicity of α-synuclein in Parkinson's disease. Immunohistochemical and biochemical studies reveal that the majority of α-synuclein within inclusions from patients with Parkinson's disease and other synucleinopathies is phosphorylated at Ser129. α-Synuclein can be phosphorylated in vitro also at Ser87, and three C-terminal tyrosine residues (Tyr125, Tyr 133, and Tyr136). Tyrosine 125 phosphorylation diminishes during the normal aging process in both humans and flies. Notably, cortical tissue from patients with Parkinson's disease-related synucleinopathy dementia with Lewy bodies showed less phosphorylation at Tyr125. While phosphorylation at Ser87 is enhanced in synucleinopathies, it inhibits α-synuclein oligomerization, and influences synuclein-membrane interactions. The possibility that α-synuclein neurotoxicity in Parkinson's disease and related synucleinopathies may result from an imbalance between the detrimental, oligomer-promoting effect of Ser129 phosphorylation and a neuroprotective action of Ser87/Tyr125 phosphorylation that inhibits toxic oligomer formation merits consideration, as will be discussed in this article.

The classical animal models of Parkinson's disease (PD) rely on the use of neurotoxins, including 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP), 6-hydroxydopamine and, more recently, the agricultural chemicals paraquat and rotenone, to deplete dopamine (DA). These neurotoxins elicit motor deficits in different animal species although MPTP fails to induce a significant dopaminergic neurodegeneration in rats. In the attempt to better reproduce the key features of PD, in particular the progressive nature of neurodegeneration, alternative PD models have been developed, based on the genetic and neuropathological links between α-synuclein (α-syn) and PD. In vivo microdialysis was used to investigate extracellular striatal DA dynamics in MPTP- and α-syn-generated rodent models of PD. Acute and sub-acute MPTP intoxication of mice both induce prolonged release of striatal DA. Such DA release may be considered the first step in MPTP-induced striatal DA depletion and nigral neuron death, mainly through reactive oxygen species generation. Although MPTP induces DA reduction, neurochemical and motor recovery starts immediately after the end of treatment, suggesting that compensatory mechanisms are activated. Thus, the MPTP mouse model of PD may be unsuitable for closely reproducing the features of the human disease and predicting potential long-term therapeutic effects, in terms of both striatal extracellular DA and behavioral outcome. In contrast, the α-syn-generated rat model of PD does not suffer from a massive release of striatal DA during induction of the nigral lesion, but rather is characterized by a prolonged reduction in baseline DA and nicotine-induced increases in dialysate DA levels. These results are suggestive of a stable nigrostriatal lesion with a lack of dopaminergic neurochemical recovery. The α-syn rat model thus reproduces the initial stage and slow development of PD, with a time-dependent impairment in motor function. This article will describe the above experimental PD models and demonstrate the utility of microdialysis for their characterization.

Amyotrophic Lateral Sclerosis (ALS), which accounts for the majority of motor neuron disorders, is a progressive and fatal neurodegenerative disease leading to complete paralysis of skeletal muscles and premature death usually by respiratory failure. About 10% of all ALS cases are inherited, with the responsible genes having been identified in approximately 30% of these individuals. Mutations in the copper-zinc superoxide dismutase (SOD1) gene were the first to be recognized nearly twenty years ago, and since then different animal models, in particular transgenic rodents, have been developed. They replicate many of the clinical, neuropathological and molecular features of ALS patients and have contributed significantly to our understanding of the pathogenic mechanisms of this disease. Although results obtained so far with mutant SOD1 mice have not translated into effective therapies in ALS patients, these models still represent the only experimentally accessible system to study multiple aspects of disease pathogenesis and to provide proof-of-principle for the development of new therapeutic strategies. This review will examine the most recent discoveries obtained from these animal models in an attempt to elucidate the complex mechanisms of the disease. In particular it will focus on the contribution of multiple cell types in governing the disease development and progression.

Human neurodegenerative diseases are devastating illnesses that predominantly affect elderly people. The majority of the diseases are associated with pathogenic oligomers from misfolded proteins, eventually causing the formation of aggregates and the progressive loss of neurons in the brain and nervous system. Several of these proteinopathies are sporadic and the cause of pathogenesis remains elusive. Heritable forms are associated with genetic defects, suggesting that the affected protein is causally related to disease formation and/or progression. The limitations of human genetics, however, make it necessary to use model systems to analyse affected genes and pathways in more detail. During the last two decades, research using the genetically amenable fruitfly has established Drosophila melanogaster as a valuable model system in the study of human neurodegeneration. These studies offer reliable models for Alzheimer's, Parkinson's, and motor neuron diseases, as well as models for trinucleotide repeat expansion diseases, including ataxias and Huntington's disease. As a result of these studies, several signalling pathways including phosphatidylinositol 3-kinase (PI3K)/Akt and target of rapamycin (TOR), c-Jun N-terminal kinase (JNK) and bone morphogenetic protein (BMP) signalling, have been shown to be deregulated in models of proteinopathies, suggesting that two or more initiating events may trigger disease formation in an age-related manner. Moreover, these studies also demonstrate that the fruitfly can be used to screen chemical compounds for their potential to prevent or ameliorate the disease, which in turn can directly guide clinical research and the development of novel therapeutic strategies for the treatment of human neurodegenerative diseases.