Modeling CNS neurodegeneration by overexpression of disease-causing proteins using viral vectors

Viral-based animal models in polyglutamine disorders

Polyglutamine disorders are a complex group of incurable neurodegenerative disorders caused by an abnormal expansion in the trinucleotide cytosine-adenine-guanine tract of the affected gene. To better understand these disorders, our dependence on animal models persists, primarily relying on transgenic models. In an effort to complement and deepen our knowledge, researchers have also developed animal models of polyglutamine disorders employing viral vectors. Viral vectors have been extensively used to deliver genes to the brain, not only for therapeutic purposes but also for the development of animal models, given their remarkable flexibility. In a time- and cost-effective manner, it is possible to use different transgenes, at varying doses, in diverse targeted tissues, at different ages, and in different species, to recreate polyglutamine pathology. This paper aims to showcase the utility of viral vectors in disease modelling, share essential considerations for developing animal models with viral vectors, and provide a comprehensive review of existing viral-based animal models for polyglutamine disorders.

Advances in AAV technology for delivering genetically encoded cargo to the nonhuman primate nervous system

Modern neuroscience approaches including optogenetics, calcium imaging, and other genetic manipulations have facilitated our ability to dissect specific circuits in rodent models to study their role in neurological disease. These approaches regularly use viral vectors to deliver genetic cargo (e.g., opsins) to specific tissues and genetically-engineered rodents to achieve cell-type specificity. However, the translatability of these rodent models, cross-species validation of identified targets, and translational efficacy of potential therapeutics in larger animal models like nonhuman primates remains difficult due to the lack of efficient primate viral vectors. A refined understanding of the nonhuman primate nervous system promises to deliver insights that can guide the development of treatments for neurological and neurodegenerative diseases. Here, we outline recent advances in the development of adeno-associated viral vectors for optimized use in nonhuman primates. These tools promise to help open new avenues for study in translational neuroscience and further our understanding of the primate brain.

Selecting the Best Animal Model of Parkinson's Disease for Your Research Purpose: Insight from in vivo PET Imaging Studies

Parkinson's disease (PD) is a debilitating neurodegenerative multisystem disorder leading to motor and non-motor symptoms in millions of individuals. Despite intense research, there is still no cure, and early disease biomarkers are lacking. Animal models of PD have been inspired by basic elements of its pathogenesis, such as dopamine dysfunction, alpha-synuclein accumulation, neuroinflammation and disruption of protein degradation, and these have been crucial for a deeper understanding of the mechanisms of pathology, the identification of biomarkers, and evaluation of novel therapies. Imaging biomarkers are non-invasive tools to assess disease progression and response to therapies; their discovery and validation have been an active field of translational research. Here, we highlight different considerations of animal models of PD that can be applied to future research, in terms of their suitability to answer different research questions. We provide the reader with important considerations of the best choice of model to use based on the disease features of each model, including issues related to different species. In addition, positron emission tomography studies conducted in PD animal models in the last 5 years are presented. With a variety of different species, interventions and genetic information, the choice of the most appropriate model to answer research questions can be daunting, especially since no single model recapitulates all aspects of this complex disorder. Appropriate animal models in conjunction with in vivo molecular imaging tools, if selected properly, can be a powerful combination for the assessment of novel therapies and developing tools for early diagnosis.

A New Technical Approach for Cross-species Examination of Neuronal Wiring and Adult Neuron-glia Functions

Advances in single cell sequencing have enabled the identification of a large number of genes, expressed in many different cell types, and across a variety of model organisms. In particular, the nervous system harbors an immense number of interacting cell types, which are poorly characterized. Future loss- and gain-of-function experiments will be essential in determining how novel genes play critical roles in diverse cellular, as well as evolutionarily adapted, contexts. However, functional analysis across species is often hampered by technical limitations, in non-genetic animal systems. Here, we describe a new single plasmid system, misPiggy. The system is based around the hyperactive piggyBac transposon system, which combines stable genomic integration of transgenes (for long-term expression) with large cargo capacity. Taking full advantage of these characteristics, we engineered novel expression modules into misPiggy that allow for cell-type specific loss- and gain-of-gene function. These modules work widely across species from frog to ferret. As a proof of principle, we present a loss-of-function analysis of the neuronal receptor Deleted in Colorectal Cancer (DCC) in retinal ganglion cells (RGCs) of Xenopus tropicalis tadpoles. Single axon tracings of mosaic knock-out cells reveal a specific cell-intrinsic requirement of DCC, specifically in axonal arborization within the frog tectum, rather than retina-to-brain axon guidance. Furthermore, we report additional technical advances that enable temporal control of knock-down or gain-of-function analysis. We applied this to visualize and manipulate labeled neurons, astrocytes and other glial cells in the central nervous system (CNS) of mouse, rat and ferret. We propose that misPiggy will be a valuable tool for rapid, flexible and cost-effective screening of gene function across a variety of animal models.

Rat Models of Vocal Deficits in Parkinson's Disease

Parkinson's disease (PD) is a progressive, degenerative disorder that affects 10 million people worldwide. More than 90% of individuals with PD develop hypokinetic dysarthria, a motor speech disorder that impairs vocal communication and quality of life. Despite the prevalence of vocal deficits in this population, very little is known about the pathological mechanisms underlying this aspect of disease. As such, effective treatment options are limited. Rat models have provided unique insights into the disease-specific mechanisms of vocal deficits in PD. This review summarizes recent studies investigating vocal deficits in 6-hydroxydopamine (6-OHDA), alpha-synuclein overexpression, DJ1-/-, and Pink1-/- rat models of PD. Model-specific changes to rat ultrasonic vocalization (USV), and the effects of exercise and pharmacologic interventions on USV production in these models are discussed.

Intra-striatal AAV2.retro administration leads to extensive retrograde transport in the rhesus macaque brain: implications for disease modeling and therapeutic development

Recently, AAV2.retro, a new capsid variant capable of efficient retrograde transport in brain, was generated in mice using a directed evolution approach. However, it remains unclear to what degree transport will be recapitulated in the substantially larger and more complex nonhuman primate (NHP) brain. Here, we compared the biodistribution of AAV2.retro with its parent serotype, AAV2, in adult macaques following delivery into the caudate and putamen, brain regions which comprise the striatum. While AAV2 transduction was primarily limited to the injected brain regions, AAV2.retro transduced cells in the striatum and in dozens of cortical and subcortical regions with known striatal afferents. We then evaluated the capability of AAV2.retro to deliver disease-related gene cargo to biologically-relevant NHP brain circuits by packaging a fragment of human mutant HTT, the causative gene mutation in Huntington’s disease. Following intra-striatal delivery, pathological mHTT-positive protein aggregates were distributed widely among cognitive, motor, and limbic cortico-basal ganglia circuits. Together, these studies demonstrate strong retrograde transport of AAV2.retro in NHP brain, highlight its utility in developing novel NHP models of brain disease and suggest its potential for querying circuit function and delivering therapeutic genes in the brain, particularly where treating dysfunctional circuits, versus single brain regions, is warranted.

Microbubble-facilitated ultrasound pulsation promotes direct α-synuclein gene delivery

Intra-neuronal α-synuclein (αSNCA) aggregation are the leading cause of dopaminergic neuron degeneration in Parkinson's disease (PD). Most PD patients is linked with αSNCA gene mutations. Gene therapy shows therapeutic potential by packing gene into viral vectors to improve gene expression through stereotactic brain injections. However, through intracranial injection, the gene expression is typically limited with tissue distribution tightly adjacent to the injection track, when expressing therapeutic genes for a wider CNS region is preferable. We use microbubble-facilitated ultrasound pulsations (MB-USP) as a new gene delivering tool to enhance the limit gene delivery of local injection in brain and evaluate the feasibility using αSNCA as model gene. We demonstrate that MB-USP can transfect naked constructs DNA of αSNCA gene into two types of neuron cells and enhance the gene expression. We confirm α-synuclein fusion protein functionality, showing that α-synuclein fusion protein significantly reduce the mitochondrial activity. We show MB-USP improves in vivo gene transfer in the brain with naked construct local injection, significantly enhances α-synuclein expression level to 1.68-fold, and broaden its distribution to 25-fold. In vivo fused α-synuclein protein aggregation is also found in gene-injected mice brains by MB-USP. MB-USP provides an alternative to α-synuclein over expression in vitro and in vivo model for investigation of α-synuclein related PD therapeutic strategies.

Serum Amyloid Alpha Is Downregulated in Peripheral Tissues of Parkinson's Disease Patients

We report the changed levels of serum amyloid alpha, an immunologically active protein, in Parkinson’s disease (PD) patients’ peripheral tissues. We have previously shown that Saa-1 and -2 (serum amyloid alpha-1,-2, genes) were among the top downregulated genes in PD patients’ skin, using whole-genome RNA sequencing. In the current study, we characterized the gene and protein expression profiles of skin and blood samples from patients with confirmed PD diagnosis and age/sex matched controls. qRT-PCR analysis of PD skin demonstrated downregulation of Saa-1 and -2 genes in PD patients. However, the lowered amount of protein could not be visualized using immunohistochemistry, due to low quantity of SAA (Serum Amyloid Alpha, protein) in skin. Saa-1 and -2 expression levels in whole blood were below detection threshold based on RNA sequencing, however significantly lowered protein levels of SAA1/2 in PD patients’ serum were shown with ELISA, implying that SAA is secreted into the blood. These results show that SAA is differentially expressed in the peripheral tissues of PD patients.

Mesenchymal Stem Cells Stabilize Axonal Transports for Autophagic Clearance of α-Synuclein in Parkinsonian Models

Genome-wide association studies have identified two loci, SNCA and the microtubule (MT)-associated protein tau, as common risk factors for Parkinson's disease (PD). Specifically, α-synuclein directly destabilizes MT via tau phosphorylation and induces axonal transport deficits that are the primary events leading to an abnormal accumulation of α-synuclein that causes nigral dopaminergic cell loss. In this study, we demonstrated that mesenchymal stem cells (MSCs) could modulate cytoskeletal networks and trafficking to exert neuroprotective properties in wild-type or A53T α-synuclein overexpressing cells and mice. Moreover, we found that eukaryotic elongation factor 1A-2, a soluble factor derived from MSCs, stabilized MT assembly by decreasing calcium/calmodulin-dependent tau phosphorylation and induced autophagolysosome fusion, which was accompanied by an increase in the axonal motor proteins and increased neuronal survival. Our data suggest that MSCs have beneficial effects on axonal transports via MT stability by controlling α-synuclein-induced tau phosphorylation, indicating that MSCs may exert a protective role in the early stages of axonal transport defects in α-synucleinopathies. Stem Cells 2017;35:1934-1947.

Inhibition of Prolyl Oligopeptidase Restores Spontaneous Motor Behavior in the α-Synuclein Virus Vector-Based Parkinson's Disease Mouse Model by Decreasing α-Synuclein Oligomeric Species in Mouse Brain

Decreased clearance of α-synuclein (aSyn) and aSyn protein misfolding and aggregation are seen as major factors in the pathogenesis of Parkinson's disease (PD) and other synucleinopathies that leads to disruption in neuronal function and eventually to cell death. Prolyl oligopeptidase (PREP) can accelerate the aSyn aggregation process, while inhibition of PREP by a small molecule inhibitor decreases aSyn oligomer formation and enhances its clearance via autophagy in different aSyn overexpressing cell types and in transgenic PD animal models. In this study, we investigated the impact of chronic PREP inhibition by a small molecule inhibitor, 4-phenylbutanoyl-l-prolyl-2(S)-cyanopyrrolidine (KYP-2047), on aSyn oligomerization, clearance, and underlying spontaneous motor behavior in a virus vector-based aSyn overexpression mouse model 4 weeks after aSyn microinjections and after the onset of symptomatic forepaw bias. Following 4 weeks of PREP inhibition, we saw an improved spontaneous forelimb use in mice that correlated with a decreased immunoreactivity against oligomer-specific forms of aSyn. Additionally, KYP-2047 had a trend to enhance dopaminergic systems activity. Our results suggest that PREP inhibition exhibits a beneficial effect on the aSyn clearance and aggregation in a virus mediated aSyn overexpression PD mouse model and that PREP inhibitors could be a novel therapeutic strategy for synucleinopathies.

SIGNIFICANCE STATEMENT

Alpha-synuclein (aSyn) has been implicated in Parkinson's disease, with aSyn aggregates believed to exert toxic effects on neurons, while prolyl oligopeptidase (PREP) has been shown to interact with aSyn both in cells and cell free conditions, thus enhancing its aggregation. We demonstrate the possibility to abolish motor imbalance caused by aSyn viral vector injection with chronic 4 week PREP inhibition by a potent small-molecule PREP inhibitor, 4-phenylbutanoyl-l-prolyl-2(S)-cyanopyrrolidine (KYP-2047). Treatment was initiated postsymptomatically, 4 weeks after aSyn injection. KYP-2047-treated animals had a significantly decreased amount of oligomeric aSyn particles and improved dopamine system activity compared to control animals. To our knowledge, this is the first time viral overexpression of aSyn has been countered and movement impairments abolished after their onset.

Viral Vector-Based Modeling of Neurodegenerative Disorders: Parkinson's Disease

Gene therapy methods are increasingly used to model Parkinson's disease (PD) in animals in an effort to test experimental therapeutics within a more relevant context to disease pathophysiology and neuropathology. We have detailed several criteria that are critical or advantageous to accurately modeling PD in a murine model or in a nonhuman primate. Using these criteria, we then evaluate approaches made to model PD using viral vectors to date, including both adeno-associated viruses and lentiviruses. Lastly, we comment on the consideration of aging as a critical factor for modeling PD.

Gene Transfer of Brain-derived Neurotrophic Factor (BDNF) Prevents Neurodegeneration Triggered by FXN Deficiency

Friedreich's ataxia is a predominantly neurodegenerative disease caused by recessive mutations that produce a deficiency of frataxin (FXN). Here, we have used a herpesviral amplicon vector carrying a gene encoding for brain-derived neurotrophic factor (BDNF) to drive its overexpression in neuronal cells and test for its effect on FXN-deficient neurons both in culture and in the mouse cerebellum in vivo. Gene transfer of BDNF to primary cultures of mouse neurons prevents the apoptosis which is triggered by the knockdown of FXN gene expression. This neuroprotective effect of BDNF is also observed in vivo in a viral vector-based knockdown mouse cerebellar model. The injection of a lentiviral vector carrying a minigene encoding for a FXN-specific short hairpin ribonucleic acid (shRNA) into the mouse cerebellar cortex triggers a FXN deficit which is accompanied by significant apoptosis of granule neurons as well as loss of calbindin in Purkinje cells. These pathological changes are accompanied by a loss of motor coordination of mice as assayed by the rota-rod test. Coinjection of a herpesviral vector encoding for BDNF efficiently prevents both the development of cerebellar neuropathology and the ataxic phenotype. These data demonstrate the potential therapeutic usefulness of neurotrophins like BDNF to protect FXN-deficient neurons from degeneration.

Assessment of brain metabolite correlates of adeno-associated virus-mediated over-expression of human alpha-synuclein in cortical neurons by in vivo (1) H-MR spectroscopy at 9.4 T

In this study, we used proton-localized spectroscopy ((1) H-MRS) for the acquisition of the neurochemical profile longitudinally in a novel rat model of human wild-type alpha-synuclein (α-syn) over-expression. Our goal was to find out if the increased α-syn load in this model could be linked to changes in metabolites in the frontal cortex. Animals injected with AAV vectors encoding for human α-syn formed the experimental group, whereas green fluorescent protein expressing animals were used as the vector-treated control group and a third group of uninjected animals were used as naïve controls. Data were acquired at 2, 4, and 8 month time points. Nineteen metabolites were quantified in the MR spectra using LCModel software. On the basis of 92 spectra, we evaluated any potential gender effect and found that lactate (Lac) levels were lower in males compared to females, while the opposite was observed for ascorbate (Asc). Next, we assessed the effect of age and found increased levels of GABA, Tau, and GPC+PCho. Finally, we analyzed the effect of treatment and found that Lac levels (p = 0.005) were specifically lower in the α-syn group compared to the green fluorescent protein and control groups. In addition, Asc levels (p = 0.05) were increased in the vector-injected groups, whereas glucose levels remained unchanged. This study indicates that the metabolic switch between glucose-lactate could be detectable in vivo and might be modulated by Asc. No concomitant changes were found in markers of neuronal integrity (e.g., N-acetylaspartate) consistent with the fact that α-syn over-expression in cortical neurons did not result in neurodegeneration in this model. We acquired the neurochemical profile longitudinally in a rat model of human wild-type alpha-synuclein (α-syn) over-expression in cortical neurons. We found that Lactate levels were reduced in the α-syn group compared to the control groups and Ascorbate levels were increased in the vector-injected groups. No changes were found in markers of neuronal integrity consistent with the fact that α-syn over-expression did not result in frank neurodegeneration.

The prion hypothesis of Parkinson's disease

The discovery of alpha-synuclein's prion-like behaviors in mammals, as well as a non-Mendelian type of inheritance, has led to a new concept in biology, the "prion hypothesis" of Parkinson's disease. The misfolding and aggregation of alpha-synuclein (α-syn) within the nervous system occur in many neurodegenerative diseases including Parkinson's disease (PD), Lewy body dementia (LBD), and multiple system atrophy (MSA). The molecular basis of synucleinopathies appears to be tightly coupled to α-syn's conformational conversion and fibril formation. The pathological form of α-syn consists of oligomers and fibrils with rich in β-sheets. The conversion of its α-helical structure to the β-sheet rich fibril is a defining pathologic feature of α-syn. These kinds of disorders have been classified as protein misfolding diseases or proteopathies which share key biophysical and biochemical characteristics with prion diseases. In this review, we highlight α-syn's prion-like activities in PD and PD models, describe the idea of a prion-like mechanism contributing to PD pathology, and discuss several key molecules that can modulate the α-syn accumulation and propagation.

Gene transfer engineering for astrocyte-specific silencing in the CNS

Cell-type-specific gene silencing is critical to understand cell functions in normal and pathological conditions, in particular in the brain where strong cellular heterogeneity exists. Molecular engineering of lentiviral vectors has been widely used to express genes of interest specifically in neurons or astrocytes. However, we show that these strategies are not suitable for astrocyte-specific gene silencing due to the processing of small hairpin RNA (shRNA) in a cell. Here we develop an indirect method based on a tetracycline-regulated system to fully restrict shRNA expression to astrocytes. The combination of Mokola-G envelope pseudotyping, glutamine synthetase promoter and two distinct microRNA target sequences provides a powerful tool for efficient and cell-type-specific gene silencing in the central nervous system. We anticipate our vector will be a potent and versatile system to improve the targeting of cell populations for fundamental as well as therapeutic applications.

AAV-Mediated Gene Therapy for Research and Therapeutic Purposes

Adeno-associated virus (AAV) is a small, nonenveloped virus that was adapted 30 years ago for use as a gene transfer vehicle. It is capable of transducing a wide range of species and tissues in vivo with no evidence of toxicity, and it generates relatively mild innate and adaptive immune responses. We review the basic biology of AAV, the history of progress in AAV vector technology, and some of the clinical and research applications where AAV has shown success.

Spinal cord injury and the neuron-intrinsic regeneration-associated gene program

Spinal cord injury (SCI) affects millions of people worldwide and causes a significant physical, emotional, social and economic burden. The main clinical hallmark of SCI is the permanent loss of motor, sensory and autonomic function below the level of injury. In general, neurons of the central nervous system (CNS) are incapable of regeneration, whereas injury to the peripheral nervous system is followed by axonal regeneration and usually results in some degree of functional recovery. The weak neuron-intrinsic regeneration-associated gene (RAG) response upon injury is an important reason for the failure of neurons in the CNS to regenerate an axon. This response consists of the expression of many RAGs, including regeneration-associated transcription factors (TFs). Regeneration-associated TFs are potential key regulators of the RAG program. The function of some regeneration-associated TFs has been studied in transgenic and knock-out mice and by adeno-associated viral vector-mediated overexpression in injured neurons. Here, we review these studies and propose that AAV-mediated gene delivery of combinations of regeneration-associated TFs is a potential strategy to activate the RAG program in injured CNS neurons and achieve long-distance axon regeneration.

Neural stem cell transplantation in an animal model of traumatic brain injury

The central nervous system (CNS) can be damaged by a wide range of conditions resulting in loss of specific populations of neurons and/or glial cells and in the development of defined psychiatric or neurological symptoms of varying severity. As the CNS has limited inherent capacity to regenerate lost tissue and self-repair, the development of therapeutic strategies for the treatment of CNS insults remains a serious scientific challenge with potential important clinical applications. In this context, strategies involving transplantation of specific cell populations, such as stem cells and neural stem cells (NSCs), to replace damaged cells offers an opportunity for the development of cell-based therapies. Along these lines, in this review we describe a protocol which involves transplantation of NPCs, genetically engineered to overexpress the neurogenic molecule Cend1 and have thus the potency to differentiate with higher frequency towards the neuronal lineage in a rodent model of stab wound cortical injury.

Understanding cognitive deficits in Parkinson's disease: lessons from preclinical animal models

Parkinson's disease (PD) has been, until recently, mainly defined by the presence of characteristic motor symptoms, such as rigidity, tremor, bradykinesia/akinesia, and postural instability. Accordingly, pharmacological and surgical treatments have so far addressed these motor disturbances, leaving nonmotor, cognitive deficits an unmet clinical condition. At the preclinical level, the large majority of studies aiming at defining mechanisms and testing novel therapies have similarly focused on the motor aspects of PD. Unfortunately, deterioration of the executive functions, such as attention, recognition, working memory, and problem solving, often appear in an early, premotor phase of the disease and progressively increase in intensity, negatively affecting the quality of life of ∼50%-60% of PD patients. At present, the cellular mechanisms underlying cognitive impairments in PD patients are largely unknown and an adequate treatment is still missing. The preclinical research has recently developed new animal models that may open new perspectives for a more integrated approach to the treatment of both motor and cognitive symptoms of the disease. This review will provide an overview on the cognitive symptoms occurring in early PD patients and then focus on the rodent and nonhuman primate models so far available for the study of discriminative and spatial memory attention and learning abilities related to this pathological condition.

Parkinson's disease and alpha synuclein: is Parkinson's disease a prion-like disorder?

Altered protein handling is thought to play a key role in the etiopathogenesis of Parkinson's disease (PD), as the disorder is characterized neuropathologically by the accumulation of intraneuronal protein aggregates (Lewy bodies and Lewy neurites). Attention has particularly focused on the α-synuclein protein, as it is the principal component of Lewy pathology. Moreover, point mutations in the α-synuclein gene cause rare familial forms of PD. Importantly, duplication/triplication of the wild type α-synuclein gene also cause a form of PD, indicating that increased levels of the normal α-synuclein protein is sufficient to cause the disease. Further, single nucleotide polymorphisms in the α-synuclein gene are associated with an increased risk of developing sporadic PD. Recent evidence now suggests the possibility that α-synuclein is a prion-like protein and that PD is a prion-like disease. Within cells, α-synuclein normally adopts an α-helical conformation. However, under certain circumstances, the protein can undergo a profound conformational transition to a β-sheet-rich structure that polymerizes to form toxic oligomers and amyloid plaques. Recent autopsy studies of patients with advanced PD who received transplantation of fetal nigral mesencephalic cells more than a decade earlier demonstrated that typical Lewy pathology had developed within grafted neurons. This suggests that α-synuclein in an aberrantly folded, β-sheet-rich form had migrated from affected to unaffected neurons. Laboratory studies confirm that α-synuclein can transfer from affected to unaffected nerve cells, where it appears that the misfolded protein can act as a template to promote misfolding of host α-synuclein. This leads to the formation of larger aggregates, neuronal dysfunction, and neurodegeneration. Indeed, recent reports demonstrate that a single intracerebral inoculation of misfolded α-synuclein can induce Lewy-like pathology in cells that can spread from affected to unaffected regions and can induce neurodegeneration with motor disturbances in both transgenic and normal mice. Further, inoculates derived from the brains of elderly α-synuclein-overexpressing transgenic mice have now been shown to accelerate the disease process when injected into the brains of young transgenic animals. Collectively, these findings support the hypothesis that α-synuclein is a prion-like protein that can adopt a self-propagating conformation that causes neurodegeneration. We propose that this mechanism plays an important role in the development of PD and provides novel targets for candidate neuroprotective therapies.

Selective and brain-permeable polo-like kinase-2 (Plk-2) inhibitors that reduce α-synuclein phosphorylation in rat brain

Polo-like kinase-2 (Plk-2) has been implicated as the dominant kinase involved in the phosphorylation of α-synuclein in Lewy bodies, which are one of the hallmarks of Parkinson's disease neuropathology. Potent, selective, brain-penetrant inhibitors of Plk-2 were obtained from a structure-guided drug discovery approach driven by the first reported Plk-2-inhibitor complexes. The best of these compounds showed excellent isoform and kinome-wide selectivity, with physicochemical properties sufficient to interrogate the role of Plk-2 inhibition in vivo. One such compound significantly decreased phosphorylation of α-synuclein in rat brain upon oral administration and represents a useful probe for future studies of this therapeutic avenue toward the potential treatment of Parkinson's disease.

Role of Glyoxalase 1 (Glo1) and methylglyoxal (MG) in behavior: recent advances and mechanistic insights

Glyoxalase 1 (GLO1) is a ubiquitous cellular enzyme that participates in the detoxification of methylglyoxal (MG), a cytotoxic byproduct of glycolysis that induces protein modification (advanced glycation end-products, AGEs), oxidative stress, and apoptosis. The concentration of MG is elevated under high-glucose conditions, such as diabetes. As such, GLO1 and MG have been implicated in the pathogenesis of diabetic complications. Recently, findings have linked GLO1 to numerous behavioral phenotypes, including psychiatric diseases (anxiety, depression, schizophrenia, and autism) and pain. This review highlights GLO1's association with behavioral phenotypes, describes recent discoveries that have elucidated the underlying mechanisms, and identifies opportunities for future research.

Characterization of a rat model of Huntington's disease based on targeted expression of mutant huntingtin in the forebrain using adeno-associated viral vectors

Huntington's disease (HD) is a fatal neurodegenerative disorder caused by an expanded CAG repeat in the huntingtin (htt) gene. Neuropathology is most severe in the striatum and cerebral cortex. As mutant htt is ubiquitously expressed, it has not been possible to establish clear structure-to-function relationships for the clinical aspects. In the present study, we have injected recombinant adeno-associated viral vectors of serotype 5 (rAAV5) expressing an 853-amino-acid fragment of htt with either 79 (mutant) or 18 (wild-type) glutamines (Q) in the dorsal striatum of neonatal rats to achieve expression of htt in the forebrain. Rats were followed for 6 months and compared with control rats. Neuropathological assessment showed long-term expression of the green fluorescent protein (GFP) transgene (used as a marker protein) and accumulation of htt inclusions in the cerebral cortex with the rAAV5-htt-79Q vectors. We estimated that around 10% of NeuN-positive cells in the cerebral cortex and 2% of DARPP-32 neurons in the striatum were targeted with the GFP-expressing vector. Formation of intracellular htt inclusions was not associated with neuronal loss, gliosis or microglia activation and did not lead to altered motor activity or changes in body weight. However, the same mutant htt vector caused orexin loss in the hypothalamus - another area known to be affected in HD. In conclusion, our results demonstrate that widespread forebrain expression of mutant htt can be achieved using rAAV5-vectors and suggest that this technique can be further explored to study region-specific effects of mutant htt or other disease-causing genes in the brain.

Cognitive deficits in animal models of basal ganglia disorders

The two most common neurological disorders of the basal ganglia are Parkinson's disease (PD) and Huntington's disease (HD). The most overt symptoms of these diseases are motoric, reflecting the loss of the striatal medium spiny neurons in HD and ascending substantia nigra dopaminergic cells in PD. However, both disease processes induce insidious psychiatric and cognitive syndromes that can manifest well in advance of the onset of motor deficits. These early deficits provide an opportunity for prophylactic therapeutic intervention in order to retard disease progression from the earliest possible point. In order to exploit this opportunity, animal models of HD and PD are being probed for the specific cognitive deficits represented in the disease states. At the neuronal level, these deficits are typically, but not exclusively, mediated by disruption of parallel corticostriatal loops that integrate motor information with sensory and higher order, "executive" cognitive functions. Dysfunction in these systems can be probed with sensitive behavioural tests that selectively probe these cognitive functions in mouse models with focal lesions of striatal or cortical regions, or of specific neurotransmitter systems. Typically these tests were designed and validated in rats. With the advent of genetically modified mouse models of disease, validated tests provide an opportunity to screen mouse models of disease for early onset cognitive deficits. This review seeks to draw together the literature on cognitive deficits in HD and PD, to determine the extent to which these deficits are represented in the current animal models of disease, and to evaluate the viability of selecting cognitive deficits as potential therapeutic targets. This article is part of a Special Issue entitled 'Animal Models'.

Molecular target discovery for neural repair in the functional genomics era

A comprehensive understanding of the molecular pathways activated by traumatic neural injury is of major importance for the development of treatments for spinal cord injury (SCI). High-throughput gene expression profiling is a powerful approach to reveal genome-wide changes in gene expression during a specific biological process. Microarray analysis of injured nerves or neurons would ideally generate new hypotheses concerning the progression or deregulation of injury- and repair-related biological processes, such as neural scar formation and axon regeneration. These hypotheses should subsequently be tested experimentally and would eventually provide the molecular substrates for the development of novel therapeutics. Over the last decade, this approach has elucidated numerous extrinsic (mostly neural scar-associated) as well as neuron-intrinsic genes that are regulated following an injury. To date, the main challenge is to translate the observed injury-induced gene expression changes into a mechanistic framework to understand their functional implications. To achieve this, research on neural repair will have to adopt the conceptual advances and analytical tools provided by the functional genomics and systems biology revolution. Based on progress made in bioinformatics, high-throughput and high-content functional cellular screening, and in vivo gene transfer technology, we propose a multistep "roadmap" that provides an integrated strategy for molecular target discovery for repair of the injured spinal cord.

Viral-mediated overexpression of mutant huntingtin to model HD in various species

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by an expansion of CAG repeats in the huntingtin (Htt) gene. Despite intensive efforts devoted to investigating the mechanisms of its pathogenesis, effective treatments for this devastating disease remain unavailable. The lack of suitable models recapitulating the entire spectrum of the degenerative process has severely hindered the identification and validation of therapeutic strategies. The discovery that the degeneration in HD is caused by a mutation in a single gene has offered new opportunities to develop experimental models of HD, ranging from in vitro models to transgenic primates. However, recent advances in viral-vector technology provide promising alternatives based on the direct transfer of genes to selected sub-regions of the brain. Rodent studies have shown that overexpression of mutant human Htt in the striatum using adeno-associated virus or lentivirus vectors induces progressive neurodegeneration, which resembles that seen in HD. This article highlights progress made in modeling HD using viral vector gene transfer. We describe data obtained with of this highly flexible approach for the targeted overexpression of a disease-causing gene. The ability to deliver mutant Htt to specific tissues has opened pathological processes to experimental analysis and allowed targeted therapeutic development in rodent and primate pre-clinical models.

Transplantation of embryonic neural stem/precursor cells overexpressing BM88/Cend1 enhances the generation of neuronal cells in the injured mouse cortex

The intrinsic inability of the central nervous system to efficiently repair traumatic injuries renders transplantation of neural stem/precursor cells (NPCs) a promising approach towards repair of brain lesions. In this study, NPCs derived from embryonic day 14.5 mouse cortex were genetically modified via transduction with a lentiviral vector to overexpress the neuronal lineage-specific regulator BM88/Cend1 that coordinates cell cycle exit and differentiation of neuronal precursors. BM88/Cend1-overexpressing NPCs exhibiting enhanced differentiation into neurons in vitro were transplanted in a mouse model of acute cortical injury and analyzed in comparison with control NPCs. Immunohistochemical analysis revealed that a smaller proportion of BM88/Cend1-overexpressing NPCs, as compared with control NPCs, expressed the neural stem cell marker nestin 1 day after transplantation, while the percentage of nestin-positive cells was significantly reduced thereafter in both types of cells, being almost extinct 1 week post-grafting. Both types of cells did not proliferate up to 4 weeks in vivo, thus minimizing the risk of tumorigenesis. In comparison with control NPCs, Cend1-overexpressing NPCs generated more neurons and less glial cells 1 month after transplantation in the lesioned cortex whereas the majority of graft-derived neurons were identified as GABAergic interneurons. Furthermore, transplantation of Cend1-overexpressing NPCs resulted in a marked reduction of astrogliosis around the lesioned area as compared to grafts of control NPCs. Our results suggest that transplantation of Cend1-overexpressing NPCs exerts beneficial effects on tissue regeneration by enhancing the number of generated neurons and restricting the formation of astroglial scar, in a mouse model of cortical brain injury.

Lentiviral vector-mediated gene transfer and RNA silencing technology in neuronal dysfunctions

Lentiviral-mediated gene transfer in vivo or in cultured mammalian neurons can be used to address a wide variety of biological questions, to design animal models for specific neurodegenerative pathologies, or to test potential therapeutic approaches in a variety of brain disorders. Lentiviruses can infect nondividing cells, thereby allowing stable gene transfer in postmitotic cells such as mature neurons. An important contribution has been the use of inducible vectors: the same animal can thus be used repeatedly in the doxycycline-on or -off state, providing a powerful mean for assessing the function of a gene candidate in a disorder within a specific neuronal circuit. Furthermore, lentivirus vectors provide a unique tool to integrate siRNA expression constructs with the aim to locally knockdown expression of a specific gene, enabling to assess the function of a gene in a very specific neuronal pathway. Lentiviral vector-mediated delivery of short hairpin RNA results in persistent knockdown of gene expression in the brain. Therefore, the use of lentiviruses for stable expression of siRNA in brain is a powerful aid to probe gene functions in vivo and for gene therapy of diseases of the central nervous system. In this chapter, I review the applications of lentivirus-mediated gene transfer in the investigation of specific gene candidates involved in major brain disorders and neurodegenerative processes. Major applications have been in polyglutamine disorders, such as synucleinopathies and Parkinson's disease, or in investigating gene function in Huntington's disease, dystonia, or muscular dystrophy. Recently, lentivirus gene transfer has been an invaluable tool for evaluation of gene function in behavioral disorders such as drug addiction and attention-deficit hyperactivity disorder or in learning and cognition.

Adeno-associated viral vector serotypes 1 and 5 targeted to the neonatal rat and pig striatum induce widespread transgene expression in the forebrain

Viral vector-mediated gene transfer has emerged as a powerful means to target transgene expression in the central nervous system. Here we characterized the efficacy of serotypes 1 and 5 recombinant adeno-associated virus (rAAV) vectors encoding green fluorescent protein (GFP) after stereotaxic delivery to the neonatal rat and minipig striatum. The efficiency of GFP expression and the phenotype of GFP-positive cells were assessed within the forebrain at different time points up to 12 months after surgery. Both rAAV1-GFP and rAAV5-GFP delivery resulted in transduction of the striatum as well as striatal input and output areas, including large parts of the cortex. In both species, rAAV5 resulted in a more widespread transgene expression compared to rAAV1. In neonatal rats, rAAV5 also transduced several other areas such as the olfactory bulbs, hippocampus, and septum. Phenotypic analysis of the GFP-positive cells, performed using immunohistochemistry and confocal microscopy, showed that most of the GFP-positive cells by either serotype were NeuN-positive neuronal profiles. The rAAV5 vector further displayed the ability to transduce non-neuronal cell types in both rats and pigs, albeit at a low frequency. Our results show that striatal delivery of rAAV5 vectors in the neonatal brain represents a useful tool to express genes of interest both in the basal ganglia and the neocortex. Furthermore, we apply, for the first time, viral vector-mediated gene transfer to the pig brain providing the opportunity to study effects of genetic manipulation in this non-primate large animal species. Finally, we generated an atlas of the Göttingen minipig brain for guiding future studies in this large animal species.

GFAP reactivity, apolipoprotein E redistribution and cholesterol reduction in human astrocytes treated with alpha-synuclein

Alpha-synuclein (alpha-syn) is an abundant neuronal protein expressed at the synapse. In neurodegenerative disease alpha-syn accumulates in the extracellular space. Astrocytes present at neural synapses are thought to contribute to synaptogenesis through cholesterol release and normally exhibit increased glial fibrillary acid protein (GFAP) reactivity and apolipoprotein E (apoE) expression in neurodegenerative disease states. We proposed that extracellular alpha-syn treatment of human astrocytes would impact cholesterol levels and expression of GFAP and apolipoprotein E (apoE). Human astrocytes were treated with alpha-syn at different concentrations and time points to determine the effective membrane permeability of the peptide. After alpha-syn treatment, we analyzed apoE and cholesterol levels in the astrocyte membrane. Lastly, we performed immunocytochemistry for GFAP in control and alpha-syn treated cells. Our results indicate membrane apoE was reduced and redistributed from a nuclear and membranous dominated expression to the cytosol. Cholesterol levels were also reduced in the astrocyte cell membrane. GFAP expression was sharply increased in alpha-syn treated cells indicating that alpha-syn may contribute to reactive gliosis. Our results support the conclusion that astrocytes play a role in pathological mechanisms in synucleinopathies.

Differential transduction following basal ganglia administration of distinct pseudotyped AAV capsid serotypes in nonhuman primates

We examined the transduction efficiency of different adeno-associated virus (AAV) capsid serotypes encoding for green fluorescent protein (GFP) flanked by AAV2 inverted terminal repeats in the nonhuman primate basal ganglia as a prelude to translational studies, as well as clinical trials in patients with Parkinson's disease (PD). Six intact young adult cynomolgus monkeys received a single 10 microl injection of AAV2/1-GFP, AAV2/5-GFP, or AAV2/8-GFP pseudotyped vectors into the caudate nucleus and putamen bilaterally in a pattern that resulted in each capsid serotype being injected into at least four striatal sites. GFP immunohistochemistry revealed excellent transduction rates for each AAV pseudotype. Stereological estimates of GFP+ cells within the striatum revealed that AAV2/5-GFP transduces significantly higher number of cells than AAV2/8-GFP (P < 0.05) and there was no significant difference between AAV2/5-GFP and AAV2/1-GFP (P = 0.348). Consistent with this result, Cavalieri estimates revealed that AAV2/5-GFP resulted in a significantly larger transduction volume than AAV2/8-GFP (P < 0.05). Each pseudotype transduced striatal neurons effectively [>95% GFP+ cells colocalized neuron-specific nuclear protein (NeuN)]. The current data suggest that AAV2/5 and AAV2/1 are superior to AAV2/8 for gene delivery to the nonhuman primate striatum and therefore better candidates for therapeutic applications targeting this structure.

Versatile somatic gene transfer for modeling neurodegenerative diseases

A growing variety of technical approaches allow control over the expression of selected genes in living organisms. The ability to deliver functional exogenous genes involved in neurodegenerative diseases has opened pathological processes to experimental analysis and targeted therapeutic development in rodent and primate preclinical models. Biological adaptability, economic animal use, and reduced model development costs complement improved control over spatial and temporal gene expression compared with conventional transgenic models. A review of viral vector studies, typically adeno-associated virus or lentivirus, for expression of three proteins that are central to major neurodegenerative diseases, will illustrate how this approach has powered new advances and opportunities in CNS disease research.

Comparison of transduction efficiency of recombinant AAV serotypes 1, 2, 5, and 8 in the rat nigrostriatal system

Enhanced delivery and expression of genes in specific neuronal systems is critical for the development of genetic models of neurodegenerative disease and potential gene therapy. Recent discovery of new recombinant adeno-associated viral (rAAV) capsid serotypes has resulted in improved transduction efficiency, but expression levels, spread of transgene, and potential toxicity can differ depending on brain region and among species. We compared the transduction efficiency of titer-matched rAAV 2/1, 2/5, and 2/8 to the commonly used rAAV2/2 in the rat nigrostriatal system via expression of the reporter transgene, enhanced green fluorescent protein. Newer rAAV serotypes 2/1, 2/5, and 2/8 demonstrated marked increase in transduction and spread of enhanced green fluorescent protein expression in dopaminergic nigrostriatal neurons and projections to the striatum and globus pallidus compared to rAAV2/2 at 2 weeks post-injection. The number of nigral cells transduced was greatest for rAAV2/1, but for serotypes 2/5 and 2/8 was still two- to threefold higher than that for 2/2. Enhanced transduction did not cause an increase in glial cell response or toxicity. New rAAV serotypes thus promise improved gene delivery to nigrostriatal system with the potential for better models and therapeutics for Parkinson disease and other neurodegenerative disorders.

Animal models of neurodegenerative diseases

Animal models of neurodegenerative disease are excellent tools for studying pathogenesis and therapies including cellular transplantation. In this chapter, we describe different models of Huntington's disease and Parkinson's disease, stereotactic surgery (used in creation of lesion models and transplantation) and finally transplantation studies in these models.

AAV vector-mediated RNAi of mutant huntingtin expression is neuroprotective in a novel genetic rat model of Huntington's disease

We report the characterization of a new rapid-onset model of Huntington's disease (HD) generated by adeno-associated virus (AAV) vector-mediated gene transfer of N-terminal huntingtin (htt) constructs into the rat striatum. Expression of exon 1 of mutant htt containing 70 CAG repeats rapidly led to neuropathological features associated with HD. In addition, we report novel data relating to neuronal transduction of AAV vectors that modulated the phenotype observed in this model. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) revealed that AAV vector-mediated expression in the striatum increased by >100-fold as compared to the endogenous htt level. Moreover, AAV vectors exhibited nonuniform transduction patterns in striatal neuronal populations, as well as axonal transport leading to transduction and neuronal cell death in the globus pallidus and substantia nigra (SN). These findings may inform future studies that utilize AAV vectors for neurodegenerative disease modeling. Further, RNA interference (RNAi) of mutant htt expression mediated by virus vector delivery of short hairpin RNAs (shRNAs) ameliorates early-stage disease phenotypes in transgenic mouse models of HD. However, it has not been reported whether shRNA-mediated knockdown of mutant htt expression is neuroprotective. AAV-shRNA was shown to mediate a dramatic knockdown of HD70 expression, preventing striatal neurodegeneration and concomitant motor behavioral impairment. These results provide further support for the use of AAV vector-mediated RNAi as a therapeutic strategy for HD.

In vivo alpha-synuclein overexpression in rodents: a useful model of Parkinson's disease?

Mutations in alpha-synuclein were the first genetic defect linked to Parkinson's disease (PD). The relevance of alpha-synuclein to sporadic PD is strongly supported by the presence of alpha-synuclein aggregates in neurons of patients. This has prompted the development of numerous animal models based on alpha-synuclein overexpression, primarily through genetic methods in mice and viral transduction in rats. In mice, different promoters and transgenes lead to a wide variety of phenotypes accompanied by non-existent, late onset, or non-specific neurodegeneration. Rapid neurodegeneration, in contrast, is observed after viral transduction but is limited to the targeted region and does not mimic the broad pathology observed in the disease. Overall, each model reproduces a subset of features of PD and can be used to identify therapeutic targets and test disease-modifying therapies. The predictive value of all models of the disease, however, remains speculative in the absence of effective neuroprotective treatments for PD in humans.

Molecular pathogenesis, experimental models and new therapeutic strategies for Parkinson’s disease

Parkinson’s disease (PD) is a progressive neurodegenerative disorder characterized by rigidity, bradykinesia, postural instability and resting tremor. The major symptoms are related to the progressive loss of dopaminergic neurons in the substantia nigra pars compacta. The recent discovery of PARK genes causing familial forms of PD has led to a new approach in the study of the disease. The cause and pathogenesis of PD remains unknown; mitochondrial dysfunction, oxidative damage, endoplasmic reticulum stress, failure of the ubiquitin–proteasome system, environmental factors and genetic predisposition might all be involved. Toxin-induced PD animal models and genetic mouse models that mimic familial PD have contributed to investigating the molecular pathogenesis and treatment of the disease. Recently, neurogenesis in the striatum and subventricular zones in PD animal models have been reported. This review discusses molecular pathogenesis, experimental disease models and recent cell-based therapeutic approaches for PD.

Concise review: therapeutic strategies for Parkinson disease based on the modulation of adult neurogenesis

Parkinson disease (PD) is a progressive neurodegenerative disorder affecting millions of people worldwide. To date, treatment strategies are mainly symptomatic and aimed at increasing dopamine levels in the degenerating nigrostriatal system. Hope rests upon the development of effective neurorestorative or neuroregenerative therapies based on gene and stem cell therapy or a combination of both. The results of experimental therapies based on transplanting exogenous dopamine-rich fetal cells or glial cell line-derived neurotrophic factor overexpression into the brain of Parkinson disease patients encourage future cell- and gene-based strategies. The endogenous neural stem cells of the adult brain provide an alternative and attractive cell source for neuroregeneration. Prior to designing endogenous stem cell therapies, the possible impact of PD on adult neuronal stem cell pools and their neurogenic potential must be investigated. We review the experimental data obtained in animal models or based on analysis of patients' brains prior to describing different treatment strategies. Strategies aimed at enhancing neuronal stem cell proliferation and/or differentiation in the striatum or the substantia nigra will have to be compared in animal models and selected prior to clinical studies.