Using Drosophila models of neurodegenerative diseases for drug discovery

Introduction: Neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease and Huntington's disease are increasing in prevalence as our aging population increases in size. Despite this, currently there are no disease-modifying drugs available for the treatment of these conditions. Drosophila melanogaster is a highly tractable model organism that has been successfully used to emulate various aspects of these diseases in vivo. These Drosophila models have not been fully exploited in drug discovery and design strategies.

Areas covered: This review explores how Drosophila models can be used to facilitate drug discovery. Specifically, we review their uses as a physiologically-relevant medium to high-throughput screening tool for the identification of therapeutic compounds and discuss how they can aid drug discovery by highlighting disease mechanisms that may serve as druggable targets in the future. The reader will appreciate how the various attributes of Drosophila make it an unsurpassed model organism and how Drosophila models of neurodegeneration can contribute to drug discovery in a variety of ways.

Expert opinion: Drosophila models of human neurodegenerative diseases can make a significant contribution to the unmet need of disease-modifying therapeutic intervention for the treatment of these increasingly common neurodegenerative conditions.     

Non-mammalian animal models of Parkinson's disease for drug discovery

Importance of the field: Parkinson's disease (PD) is a prevalent neurodegenerative disease affecting millions of predominantly elderly individuals worldwide. Despite intensive efforts devoted to drug discovery, the disease remains incurable. Compounding this problem is the current lack of a truly representative mammalian model of PD. However, a number of non-mammalian models of PD have been created in recent years that hold tremendous promise to accelerate our understanding of the disease as well as to transform the drug discovery process. Areas covered in this review: This review provides an overview of the various Caenorhabditis elegans and Drosophila genetic models of PD that have been generated to date and discusses the utility of these model systems in the identification of molecules of potential therapeutic value for the PD patient. What the reader will gain: Readers will appreciate the strengths (and limitations) of C. elegans and Drosophila in modeling salient features of the disease as well as their usefulness in uncovering novel gene-gene interaction and pathways relevant to PD pathogenesis. Readers will also appreciate how technological advancements have allowed the direct evaluation of novel compounds in these living models of PD in a virtually high-throughput manner. Take home message: Non-mammalian models of PD provide a valuable in vivo platform for drug screening. Unlike cell-based systems, these living models with an intact nervous system allow for a more meaningful evaluation of the neuroprotective properties of genetic and chemical modifiers to be conducted.     

Fruitful research: drug target discovery for neurodegenerative diseases in Drosophila

Introduction: Although vertebrate model systems have obvious advantages in the study of human disease, invertebrate organisms have contributed enormously to this field as well. The conservation of genome structure and physiology among organisms poses unexpected peculiarities, and the redundancy in certain gene families or the presence of polymorphisms that can slightly alter gene expression can, in certain instances, bring invertebrate systems, such as Drosophila, closer to humans than mice and vice versa. This necessitates the analysis of disease pathways in multiple model organisms.

Areas covered: The author highlights findings from Drosophila models of neurodegenerative diseases that have occurred in the past few years. She also highlights and discusses various molecular, genetic and genomic tools used in flies, as well as methods for generating disease models. Finally, the author describes Drosophila models of Alzheimer's, Parkinson's tri-nucleotide repeat diseases, and Fragile X syndrome and summarizes insights in disease mechanisms that have been discovered directly in fly models.

Expert opinion: Full genome genetic screens in Drosophila can lead to the rapid identification of drug target candidates that can be subsequently validated in a vertebrate system. In addition, the Drosophila models of neurodegeneration may often show disease phenotypes that are absent in equivalent mouse models. The author believes that the extensive contribution of Drosophila to both new disease drug target discovery, in addition to target validation, makes them indispensible to drug discovery and development.     

Biomarkers of neurodegeneration for diagnosis and monitoring therapeutics

Rapid progress towards understanding the molecular underpinnings of neurodegenerative disorders such as Alzheimer's disease is revolutionizing drug discovery for these conditions. Furthermore, the development of models for these disorders is accelerating efforts to translate insights related to neurodegenerative mechanisms into disease-modifying therapies. However, there is an urgent need for biomarkers to diagnose neurodegenerative disorders early in their course, when therapy is likely to be most effective, and to monitor responses of patients to new therapies. As research related to this need is currently most advanced for Alzheimer's disease, this Review focuses on progress in the development and validation of biomarkers to improve the diagnosis and treatment of Alzheimer's disease and related disorders.     

Gene expression profiling and therapeutic interventions in neurodegenerative diseases: a comprehensive study on potentiality and limits

INTRODUCTION: Neurodegenerative diseases are incurable debilitating disorders of the nervous system that affect approximately 30 million people worldwide. Despite profuse efforts attempting to define the molecular mechanisms underlying neurodegeneration, many aspects of these pathologies remain elusive. The novelty of their mechanisms represents a challenge to biology, to their related biomarkers identification and drug discovery. Because of their multifactorial aspects and complexity, gene expression analysis platforms have been extensively used to investigate altered pathways during degeneration and to identify potential biomarkers and drug targets. AREAS COVERED: This work offers an overview of the gene expression profiling studies carried out on Alzheimer's disease, Huntington's disease, Parkinson's disease and prion disease specimens. Therapeutic approaches are also discussed. EXPERT OPINION: Although many therapeutic approaches have been tested, some of them acting on several altered cellular pathways, no effective cures for these neurodegenerative diseases have been identified. Microarray technology must be associated with functional proteomics and physiology in an effort to identify specific and selective biomarkers and druggable targets, thus allowing the successful discovery of disease-modifying therapeutic treatments.     

Invertebrate disease models in neurotherapeutic discovery

Models that reproduce many of the cellular and molecular aspects of various human neurodegenerative disorders have been developed in the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans. An understanding of the underlying molecular and genetic mechanisms of disease pathogenesis is being gained from studies utilizing the wealth of genetic and molecular tools available for these invertebrate model organisms. This review focuses on recent studies that lay a foundation for utilizing these disease models in drug discovery and for continued genetic dissection of disease mechanisms.     

Human disease models in Drosophila melanogaster and the role of the fly in therapeutic drug discovery

The common fruit fly, Drosophila melanogaster, is a well studied and highly tractable genetic model organism for understanding molecular mechanisms of human diseases. Many basic biological, physiological, and neurological properties are conserved between mammals and D. melanogaster, and nearly 75% of human disease-causing genes are believed to have a functional homolog in the fly. In the discovery process for therapeutics, traditional approaches employ high-throughput screening for small molecules that is based primarily on in vitro cell culture, enzymatic assays, or receptor binding assays. The majority of positive hits identified through these types of in vitro screens, unfortunately, are found to be ineffective and/or toxic in subsequent validation experiments in whole-animal models. New tools and platforms are needed in the discovery arena to overcome these limitations. The incorporation of D. melanogaster into the therapeutic discovery process holds tremendous promise for an enhanced rate of discovery of higher quality leads. D. melanogaster models of human diseases provide several unique features such as powerful genetics, highly conserved disease pathways, and very low comparative costs. The fly can effectively be used for low- to high-throughput drug screens as well as in target discovery. Here, we review the basic biology of the fly and discuss models of human diseases and opportunities for therapeutic discovery for central nervous system disorders, inflammatory disorders, cardiovascular disease, cancer, and diabetes. We also provide information and resources for those interested in pursuing fly models of human disease, as well as those interested in using D. melanogaster in the drug discovery process.     

Zebrafish: from disease modeling to drug discovery

The study of zebrafish, a leading model organism for developmental biology, is rapidly expanding to include human disease. Zebrafish models based on known disease mechanisms have been developed in several therapeutic areas, including blood diseases, diabetes, muscular dystrophy, neurodegenerative disease, angiogenesis and lipid metabolism. This review summarizes recent progress in disease model development, and outlines the potential of zebrafish to contribute to drug discovery through the identification of novel drug targets, validation of those targets and screening for new therapeutic compounds.     

Approaches for discovering anti-prion compounds: lessons learned and challenges ahead

Introduction: Recent years have witnessed major advances in our understanding of the molecular bases of prion diseases. These studies not only highlight the protein misfolding as a potential initiator of a neurodegenerative process, they also provide a foundation for considering whether such a process can be common to many neurodegenerative diseases, including Alzheimer’s disease. This makes prion diseases a sort of prototype of neurodegenerative disease, endowed with some intrinsic positive features in terms of drug development. Thanks to the fact that disappearance of the scrapie protein can serve as a clear readout of drug efficiency, phenotypic approaches have high potential for prion disease drug discovery.

Areas covered: In this review, the authors discuss phenotypic screening and how it lends itself to drug repositioning. Furthermore, they discuss the advantages of working with a molecule with proven safety, tolerability and drug-like properties in combination with a reliable phenotypic screening and how it could improve the success rate for prion drug development. They also provide examples of several interesting candidates that have been identified using this approach, including quinacrine, astemizole, guanabenz and doxycycline.

Expert opinion: The availability of persistently scrapie-infected murine neuroblastoma cells has greatly helped to identify compounds that inhibit prion formation. However, a human neuronal model infected with the human isoform would ultimately serve as the ideal disease model toward the discovery of effective drugs.     

Gene expression profiling and therapeutic interventions in neurodegenerative diseases: a comprehensive study on potentiality and limits

Introduction: Neurodegenerative diseases are incurable debilitating disorders of the nervous system that affect approximately 30 million people worldwide. Despite profuse efforts attempting to define the molecular mechanisms underlying neurodegeneration, many aspects of these pathologies remain elusive. The novelty of their mechanisms represents a challenge to biology, to their related biomarkers identification and drug discovery. Because of their multifactorial aspects and complexity, gene expression analysis platforms have been extensively used to investigate altered pathways during degeneration and to identify potential biomarkers and drug targets.

Areas covered: This work offers an overview of the gene expression profiling studies carried out on Alzheimer's disease, Huntington's disease, Parkinson's disease and prion disease specimens. Therapeutic approaches are also discussed.

Expert opinion: Although many therapeutic approaches have been tested, some of them acting on several altered cellular pathways, no effective cures for these neurodegenerative diseases have been identified. Microarray technology must be associated with functional proteomics and physiology in an effort to identify specific and selective biomarkers and druggable targets, thus allowing the successful discovery of disease-modifying therapeutic treatments.     

Animal models in the drug discovery pipeline for Alzheimer's disease

With increasing feasibility of predicting conversion of mild cognitive impairment to dementia based on biomarker profiling, the urgent need for efficacious disease-modifying compounds has become even more critical. Despite intensive research, underlying pathophysiological mechanisms remain insufficiently documented for purposeful target discovery. Translational research based on valid animal models may aid in alleviating some of the unmet needs in the current Alzheimer's disease pharmaceutical market, which includes disease-modification, increased efficacy and safety, reduction of the number of treatment unresponsive patients and patient compliance. The development and phenotyping of animal models is indeed essential in Alzheimer's disease-related research as valid models enable the appraisal of early pathological processes - which are often not accessible in patients, and subsequent target discovery and evaluation. This review paper summarizes and critically evaluates currently available animal models, and discusses their value to the Alzheimer drug discovery pipeline. Models dealt with include spontaneous models in various species, including senescence-accelerated mice, chemical and lesion-induced rodent models, and genetically modified models developed in Drosophila melanogaster, Caenorhabditis elegans, Danio rerio and rodents. Although highly valid animal models exist, none of the currently available models recapitulates all aspects of human Alzheimer's disease, and one should always be aware of the potential dangers of uncritical extrapolating from model organisms to a human condition that takes decades to develop and mainly involves higher cognitive functions.     

Asparagine endopeptidase is an innovative therapeutic target for neurodegenerative diseases

Introduction: Asparagine endopeptidase (AEP) is a pH-dependent endolysosomal cysteine protease that cleaves its substrates after asparagine residues. Our most recent study identifies that it possesses the delta-secretase activity, and that it is implicated in numerous neurological diseases such as Alzheimer’s disease (AD) and stroke. Accumulating evidence supports that the inhibition of AEP exhibits beneficial effects for treating these devastating diseases.

Areas covered: Based on recent evidence, it is clear that AEP cleaves its substrate, such as amyloid precursor protein (APP), tau and SET, and plays a critical role in neuronal cell death in various neurodegenerative diseases and stroke. In this article, the basic biology of AEP, its knockout phenotypes in mouse models, its substrates in neurodegenerative diseases, and its small peptidyl inhibitors and prodrugs are discussed. In addition, we discuss the potential of AEP as a novel therapeutic target for neurodegenerative diseases.

Expert opinion: AEP plays a unique role in numerous biological processes, depending on both pH and context. Most striking is our most recent finding; that AEP is activated in an age-dependent manner and simultaneously cleaves both APP and tau, thereby unifying both major pathological events in AD. Thus, AEP acts as an innovative trigger for neurodegenerative diseases. Inhibition of AEP will provide a disease-modifying treatment for neurodegenerative diseases including AD.     

An assessment of the translational relevance of Drosophila in drug discovery

Introduction: Drosophila melanogaster offers a powerful expedient and economical system with facile genetics. Because of the high sequence and functional conservation with human disease-associated genes, it has been cardinal in deciphering disease mechanisms at the genetic and molecular level. Drosophila are amenable to and respond well to pharmaceutical treatment which coupled to their genetic tractability has led to discovery, repositioning, and validation of a number of compounds.

Areas covered: This review summarizes the generation of fly models of human diseases, their advantages and use in elucidation of human disease mechanisms. Representative studies provide examples of the utility of this system in modeling diseases and the discovery, repositioning and testing on pharmaceuticals to ameliorate them.

Expert opinion: Drosophila offers a facile and economical whole animal system with many homologous organs to humans, high functional conservation and established methods of generating and validating human disease models. Nevertheless, it remains relatively underused as a drug discovery tool probably because its relevance to mammalian systems remains under question. However, recent exciting success stories using Drosophila disease models for drug screening, repositioning and validation strongly suggest that fly models should figure prominently in the drug discovery pipeline from bench to bedside.     

The therapeutic potentials of neurotrophic factors for diseases of the nervous system

Neurotrophic factors are secreted proteins that regulate the long-term survival and differentiation of neurones in the peripheral and central nervous systems (PNS and CNS). Their profound effects on the structural integrity of the nervous system have led to intense interest in the potential use of the factors as therapeutic agents for neurodegenerative diseases. In the present review, we summarise the current data on the physiological role of these proteins based on in vitro investigations and animal models, and discuss their clinical potential as therapeutic agents in the treatment of neurological disorders. A rationale for neurotrophic factor therapy can be built for a specific neurological disease once a particular neurotrophic factor is identified to regulate vulnerable neurones in the affected areas. Progress has been made in the use of neurotrophic factors as an alternative therapy to conventional drug treatment, particularly for Alzheimer’s disease, Parkinson’s disease and peripheral neuropathy. Production of chimeric neurotrophins may combine the beneficial aspects of a number of neurotrophic factors. Delivery of exogenous protein factors to the CNS remains one of the major obstacles for trophic factor therapy. Second generation approaches (including transplantation of genetically engineered cells, direct gene transfer into the brain and pharmacological manipulation of the levels of endogenous neurotrophic factors) may prove to be effective in overcoming the technical problems surrounding direct CNS delivery. The discovery of new trophic molecules and development of novel delivery methods represent two important aspects of neurotrophic therapy for neurodegenerative diseases.     

Cell death assays for neurodegenerative disease drug discovery

ABSTRACT

Introduction: Neurodegenerative diseases affect millions of people worldwide. Neurodegeneration is gradual over time, characterized by neuronal death that causes deterioration of cognitive or motor functions, ultimately leading to the patient’s death. Currently, there are no treatments that effectively slow the progression of any neurodegenerative disease, but improved microscopy assays and models for neurodegeneration could lead the way to the discovery of disease-modifying therapeutics.

Areas covered: Herein, the authors describe cell-based assays used to discover drugs with the potential to slow neurodegeneration, and their associated disease models. They focus on microscopy technologies that can be adapted to a high-throughput screening format that both detect cell death and monitor early signs of neurodegeneration and functional changes to identify drugs that the block early stages of neurodegeneration.

Expert opinion: Many different phenotypes have been used in screens for the development of therapeutics towards neurodegenerative disease. The context of each phenotype in relation to neurodegeneration must be established to identify therapeutics likely to successfully target and treat disease. The use of improved models of neurodegeneration, statistical analyses, computational models, and improved markers of neuronal death will help in this pursuit and lead to better screening methods to identify therapeutic compounds against neurodegenerative disease.     

Disease-related determinants of susceptibility to drug-induced idiosyncratic hepatotoxicity

Idiosyncratic adverse drug reactions, including unpredictable hepatotoxicity, remain a serious challenge in drug development. Besides patient-specific susceptibility factors (genetic and/or acquired), determinants of the underlying disease may also predispose the patient to a drug's potential toxicity. Examples include viral infections, inflammatory conditions, neurodegenerative diseases and type II diabetes. This review focuses on diseases (therapeutic indications) often associated with mitochondrial abnormalities, and which are treated with drugs mechanistically linked to potential mitochondrial toxicity, thus superimposing these mitochondrial events. The need for an increased use of animal models of human disease in mechanistic investigations and drug candidate selection will also be emphasized.     

Non-mammalian animal models of Parkinson's disease for drug discovery

Importance of the field: Parkinson's disease (PD) is a prevalent neurodegenerative disease affecting millions of predominantly elderly individuals worldwide. Despite intensive efforts devoted to drug discovery, the disease remains incurable. Compounding this problem is the current lack of a truly representative mammalian model of PD. However, a number of non-mammalian models of PD have been created in recent years that hold tremendous promise to accelerate our understanding of the disease as well as to transform the drug discovery process.

Areas covered in this review: This review provides an overview of the various Caenorhabditis elegans and Drosophila genetic models of PD that have been generated to date and discusses the utility of these model systems in the identification of molecules of potential therapeutic value for the PD patient.

What the reader will gain: Readers will appreciate the strengths (and limitations) of C. elegans and Drosophila in modeling salient features of the disease as well as their usefulness in uncovering novel gene–gene interaction and pathways relevant to PD pathogenesis. Readers will also appreciate how technological advancements have allowed the direct evaluation of novel compounds in these living models of PD in a virtually high-throughput manner.

Take home message: Non-mammalian models of PD provide a valuable in vivo platform for drug screening. Unlike cell-based systems, these living models with an intact nervous system allow for a more meaningful evaluation of the neuroprotective properties of genetic and chemical modifiers to be conducted.     

Advances in tau-based drug discovery

INTRODUCTION: Tauopathies, including Alzheimer's disease (AD) and some frontotemporal dementias, are neurodegenerative diseases characterised by pathological lesions comprised of tau protein. There is currently a significant and urgent unmet need for disease-modifying therapies for these conditions and recently attention has turned to tau as a potential target for intervention. AREAS COVERED: Increasing evidence has highlighted pathways associated with tau-mediated neurodegeneration as important targets for drug development. Here, the authors review recently published papers in this area and summarise the genetic and pharmacological approaches that have shown efficacy in reducing tau-associated neurodegeneration. These include the use of agents to prevent abnormal tau processing and increase tau clearance, therapies targeting the immune system, and the manipulation of tau pre-mRNA to modify tau isoform expression. EXPERT OPINION: Several small molecule tau-based treatments are currently being assessed in clinical trials, the outcomes of which are eagerly awaited. Current evidence suggests that therapies targeting tau are likely, at least in part, to form the basis of an effective and safe treatment for Alzheimer's disease and related neurodegenerative disorders in which tau deposition is evident.     

Review on the protective effects of PACAP in models of neurodegenerative diseases in vitro and in vivo

Pituitary adenylate cyclase activating polypeptide (PACAP) is a pleiotropic and multifunctional peptide exerting its effects via 3 main receptors (PAC1, VPAC1 and VPAC2). PACAP is now considered to be a potent neurotrophic and neuroprotective peptide. It plays an important role during the embryonic development of the nervous system. PACAP also protects neurons against various toxic insults in neuronal cultures of diverse origins. In vivo, PACAP shows neuroprotection in models of ischemic and traumatic brain injuries, and those of neurodegenerative diseases. The present review summarizes the findings on the neuroprotective potential of PACAP in models of neurodegenerative diseases, with special focus on in vitro and in vivo models of Parkinson`s disease, Huntington chorea and Alzheimer`s disease. Based on these observations, both endogenous and exogenously administered PACAP or its novel analogs, fragments offer a novel therapeutic approach in treatment of neurodegenerative diseases.     

Structure-based drug discovery and protein targets in the CNS

Structure-based methods are having an increasing role and impact in drug discovery. The crystal structures of an increasing number of therapeutic targets are becoming available. These structures can transform our understanding of how these proteins perform their biological function and often provide insights into the molecular basis of disease. In addition, the structures can help the discovery process. Methods such as virtual screening and experimental fragment screening can provide starting hit compounds for a discovery project. Crystal structures of compounds bound to the protein can direct or guide the medicinal chemistry optimisation to improve drug-like properties - not only providing ideas on how to improve binding affinity or selectivity, but also showing where the compound can be modified in attempting to modulate physico-chemical properties and biological efficacy. The majority of drug discovery projects against globular protein targets now use these methods at some stage.This review provides a summary of the range of structure-based drug discovery methods that are in use and surveys the suitability of the methods for targets currently identified for CNS drugs. Until recently, structure-based discovery was difficult or unknown for these targets. The recent determination of the structures of a number of GPCR proteins, together with the steady increase in structures for other membrane proteins, is opening up the possibility for these structure-based methods to find increased use in drug discovery for CNS diseases and conditions.