Connecting cell-cycle activation to neurodegeneration in Drosophila

Studies in cell-culture systems and in postmortem tissue from human disease have suggested a connection between cell-cycle activation and neurodegeneration. The fruit fly Drosophila melanogaster has recently emerged as a powerful model system in which to model neurodegenerative diseases. Here we review work in the fly that has begun to address some of the important questions regarding the relationship between cell-cycle activation and neurodegeneration in vivo, including recent data implicating cell-cycle activation as a downstream effector of tau-induced neurodegeneration. We suggest how powerful research tools in Drosophila might be utilized to approach fundamental questions that remain.     

Drosophila as a model system for studying lifespan and neuroprotective activities of plant-derived compounds

The fruit fly, Drosophila melanogaster, has been intensively used as a genetic model system for basic and applied research on human neurological diseases because of advantages over mammalian model systems such as ease of laboratory maintenance and genetic manipulations. Disease-associated gene mutations, whether endogenous or transgenically-inserted, often cause phenotypes in vivo that are similar to the clinical features of the human disorder. The Drosophila genome is simpler than that of mammals, in terms of gene and chromosome number, but nonetheless demonstrates extraordinary phylogenetic conservation of gene structure and function, especially notable among the genes whose mutations cause neurodevelopmental, neuropsychiatric, or neurodegenerative disorders. In addition, its well-established neuroanatomical, developmental, and molecular genetic research techniques allow many laboratories worldwide to study complex biological and genetic processes. Based on these merits of the Drosophila model system, it has been used for screening lifespan expansion and neuroprotective activities of plant extracts or their secondary metabolites to counteract pathological events such as mitochondrial damage by oxidative stress, which may cause sporadic neurodegenerative diseases. In this review, we have summarized that the fruit fly can be used for early-stage drug discovery and development to identify novel plant-derived compounds to protect against neurodegeneration in Alzheimer's disease and Parkinson's disease, and other neurological disorders caused by oxidative stress. Thus, the Drosophila system can directly or indirectly contribute to translational research for new therapeutic strategies to prevent or ameliorate neurodegenerative diseases.     

Drosophila models of human neurodegenerative disease

Drosophila has provided a powerful genetic system in which to elucidate fundamental cellular pathways in the context of a developing and functioning nervous system. Recently, Drosophila has been applied toward elucidating mechanisms of human neurodegenerative disease, including Alzheimer's, Parkinson's and Huntington's diseases. Drosophila allows study of the normal function of disease proteins, as well as study of effects of familial mutations upon targeted expression of human mutant forms in the fly. These studies have revealed new insight into the normal functions of such disease proteins, as well as provided models in Drosophila that will allow genetic approaches to be applied toward elucidating ways to prevent or delay toxic effects of such disease proteins. These, and studies to come that follow from the recently completed sequence of the Drosophila genome, underscore the contributions that Drosophila as a model genetic system stands to contribute toward the understanding of human neurodegenerative disease.     

Characterization of a Drosophila Alzheimer's disease model: pharmacological rescue of cognitive defects

Transgenic models of Alzheimer's disease (AD) have made significant contributions to our understanding of AD pathogenesis, and are useful tools in the development of potential therapeutics. The fruit fly, Drosophila melanogaster, provides a genetically tractable, powerful system to study the biochemical, genetic, environmental, and behavioral aspects of complex human diseases, including AD. In an effort to model AD, we over-expressed human APP and BACE genes in the Drosophila central nervous system. Biochemical, neuroanatomical, and behavioral analyses indicate that these flies exhibit aspects of clinical AD neuropathology and symptomology. These include the generation of Aβ(40) and Aβ(42), the presence of amyloid aggregates, dramatic neuroanatomical changes, defects in motor reflex behavior, and defects in memory. In addition, these flies exhibit external morphological abnormalities. Treatment with a γ-secretase inhibitor suppressed these phenotypes. Further, all of these phenotypes are present within the first few days of adult fly life. Taken together these data demonstrate that this transgenic AD model can serve as a powerful tool for the identification of AD therapeutic interventions.     

Drosophila as an In Vivo Model for Human Neurodegenerative Disease

With the increase in the ageing population, neurodegenerative disease is devastating to families and poses a huge burden on society. The brain and spinal cord are extraordinarily complex: they consist of a highly organized network of neuronal and support cells that communicate in a highly specialized manner. One approach to tackling problems of such complexity is to address the scientific questions in simpler, yet analogous, systems. The fruit fly, Drosophila melanogaster, has been proven tremendously valuable as a model organism, enabling many major discoveries in neuroscientific disease research. The plethora of genetic tools available in Drosophila allows for exquisite targeted manipulation of the genome. Due to its relatively short lifespan, complex questions of brain function can be addressed more rapidly than in other model organisms, such as the mouse. Here we discuss features of the fly as a model for human neurodegenerative disease. There are many distinct fly models for a range of neurodegenerative diseases; we focus on select studies from models of polyglutamine disease and amyotrophic lateral sclerosis that illustrate the type and range of insights that can be gleaned. In discussion of these models, we underscore strengths of the fly in providing understanding into mechanisms and pathways, as a foundation for translational and therapeutic research.     

Using Drosophila as a model insect

The fruit fly Drosophila melanogaster has become such a popular model organism for studying human disease that it is often described as a little person with wings. This view has been strengthened with the sequencing of the Drosophila genome and the discovery that 60% of human disease genes have homologues in the fruit fly. In this review, I discuss the approach of using Drosophila not only as a model for metazoans in general but as a model insect in particular. Specifically, I discuss recent work on the use of Drosophila to study the transmission of disease by insect vectors and to investigate insecticide function and development.     

Ovine PrP transgenic Drosophila show reduced locomotor activity and decreased survival

Drosophila have emerged as a model system to study mammalian neurodegenerative diseases. In the present study we have generated Drosophila transgenic for ovine PrP (prion protein) to begin to establish an invertebrate model of ovine prion disease. We generated Drosophila transgenic for polymorphic variants of ovine PrP by PhiC31 site-specific germ-line transformation under expression control by the bi-partite GAL4/UAS (upstream activating sequence) system. Site-specific transgene insertion in the fly genome allowed us to test the hypothesis that single amino acid codon changes in ovine PrP modulate prion protein levels and the phenotype of the fly when expressed in the Drosophila nervous system. The Arg(154) ovine PrP variants showed higher levels of PrP expression in neuronal cell bodies and insoluble PrP conformer than did His(154) variants. High levels of ovine PrP expression in Drosophila were associated with phenotypic effects, including reduced locomotor activity and decreased survival. Significantly, the present study highlights a critical role for helix-1 in the formation of distinct conformers of ovine PrP, since expression of His(154) variants were associated with decreased survival in the absence of high levels of PrP accumulation. Collectively, the present study shows that variants of the ovine PrP are associated with different spontaneous detrimental effects in ovine PrP transgenic Drosophila.     

Drosophila in the study of neurodegenerative disease

As populations benefit from increasing lifespans, neurodegenerative diseases have emerged as a critical health concern. How can the fruit fly, Drosophila melanogaster, contribute to curing human diseases of the nervous system? A growing number of neurodegenerative diseases, as well as other human diseases, are being modeled in Drosophila and used as a platform to identify and validate cellular pathways that contribute to neurodegeneration and to identify promising therapeutic targets by using a variety of approaches from screens to target validation. The unique properties and tools available in the Drosophila system, coupled with the fact that testing in vivo has proven highly productive, have accelerated the progress of testing therapeutic strategies in mice and, ultimately, humans. This review highlights selected recent applications to illustrate the use of Drosophila in studying neurodegenerative diseases.     

Neurodegenerative mutants in Drosophila: a means to identify genes and mechanisms involved in human diseases?

There are 50 ways to leave your lover (Simon 1987) but many more to kill your brain cells. Several neurodegenerative diseases in humans, like Alzheimer’s disease, have been intensely studied but the underlying cellular and molecular mechanisms are still unknown for most of them. For those syndromes where associated gene products have been identified their biochemistry and physiological as well as pathogenic function is often still under debate. This is in part due to the inherent limitations of genetic analyses in humans and other mammals and therefore experimentally accessible invertebrate in vivo models, such as Caenorhabditis elegans and Drosophila melanogaster, have recently been introduced to investigate neurodegenerative syndromes. Several laboratories have used transgenic approaches in Drosophila to study the human genes associated with neurodegenerative diseases. This has added substantially to our understanding of the mechanisms leading to neurodegenerative diseases in humans. The isolation and characterization of Drosophila mutants, which display a variety of neurodegenerative phenotypes, also provide valuable insights into genes, pathways, and mechanisms causing neurodegeneration. So far only about two dozen such mutants have been described but already their characterization reveals an involvement of various cellular functions in neurodegeneration, ranging from preventing oxidative stress to RNA editing. Some of the isolated genes can already be associated with human neurodegenerative diseases and hopefully the isolation and characterization of more of these mutants, together with an analysis of homologous genes in vertebrate models, will provide insights into the genetic and molecular basis of human neurodegenerative diseases.     

HDAC6 at the intersection of autophagy, the ubiquitin-proteasome system and neurodegeneration

The two major intracellular catabolic pathways, the ubiquitin-proteasome system (UPS) and macroautophagy (autophagy), have each been implicated as playing roles in neurodegenerative proteinopathies. We have investigated the relationship between the UPS and autophagy using Drosophila models of neurodegenerative diseases. We identified histone deacetylase 6 (HDAC6) as a genetic modifier of polyglutamine-induced neurodegeneration and determined that its mechanism of action is autophagy-dependent. The ability of HDAC6 to suppress degeneration has been extended to additional neurodegenerative disease models, including a fly model expressing pathological Abeta fragments, presented here, but is not a universal modifier of degenerative phenotypes. Importantly, HDAC6 was also found to suppress degeneration associated with proteasome mutations in an autophagy-dependent manner, revealing a compensatory relationship between these two degradation pathways. Our findings indicate that HDAC6 facilitates degradation of potentially noxious protein substrates, contributing vitally to the neuroprotective role of autophagy.     

Drosophila as a model host for Pseudomonas aeruginosa infection

Using the fruit fly Drosophila melanogaster as model host, we have identified mutants of the bacterium Pseudomonas aeruginosa with reduced virulence. Strikingly, all strains strongly impaired in fly killing also lacked twitching motility; most such strains had a mutation in pilGHIJKL chpABCDE, a gene cluster known to be required for twitching motility and potentially encoding a signal transduction system. The pil chp genes appear to control the expression of additional virulence factors, however, since the wild-type fly-killing phenotype of a subset of mutants isolated on the basis of their compact colony morphology indicated that twitching motility itself was not required for full virulence in the fly.     

Understanding the importance of autophagy in human diseases using Drosophila

Autophagy is a lysosome-dependent intracellular degradation pathway that has been implicated in the pathogenesis of various human diseases, either positively or negatively impacting disease outcomes depending on the specific context. The majority of medical conditions including cancer, neurodegenerative diseases, infections and immune system disorders and inflammatory bowel disease could probably benefit from therapeutic modulation of the autophagy machinery. Drosophila represents an excellent model animal to study disease mechanisms thanks to its sophisticated genetic toolkit, and the conservation of human disease genes and autophagic processes. Here, we provide an overview of the various autophagy pathways observed both in flies and human cells(macroautophagy, microautophagy and chaperone-mediated autophagy), and discuss Drosophila models of the above-mentioned diseases where fly research has already helped to understand how defects in autophagy genes and pathways contribute to the relevant pathomechanisms.     

The ubiquitin proteasome system in synaptic and axonal degeneration: a new twist to an old cycle

The ubiquitin proteasome system (UPS) contributes to the pathophysiology of neurodegenerative diseases, and it is also a major determinant of synaptic protein degradation and activity. Recent studies in rodents and in the fruit fly Drosophila have shown that the activity of the UPS is involved in axonal degeneration. Increased knowledge of the UPS in synaptic and axonal reactions may provide novel drug targets for treatments of neuronal injuries and neurodegenerative disorders.     

The eye of Drosophila as a model system for studying intracellular signaling in ontogenesis and pathogenesis

Many human diseases are caused by malfunction of basic types of cellular activity such as proliferation, differentiation, apoptosis, cell polarization, and migration. In turn, these processes are associated with different routes of intracellular signal transduction. A number of model systems have been designed to study normal and abnormal cellular and molecular processes associated with pathogenesis. The developing eye of the fruit fly Drosophila melanogaster is one of these systems. The sequential development of compound eyes of this insect makes it possible to model human neurodegenerative diseases and mechanisms of carcinogenesis. In this paper we overview the program of the eye development in Drosophila, with emphasis on intracellular signaling pathways that regulate this complex process. We discuss in detail the roles of the Notch, Hedgehog, TGFβ, Wnt, and receptor tyrosine kinase signaling pathways in Drosophila eye development and human pathology. We also briefly describe the modern methods of experimentation with this model organism to analyze the function of human pathogenic proteins.     

HDAC6 at the Intersection of Autophagy,the Ubiquitin-proteasome System,and Neurodegeneration

The two major intracellular catabolic pathways, the ubiquitin-proteasome system (UPS) and macroautophagy (autophagy), have each been implicated as playing roles in neurodegenerative proteinopathies1, 2. We have investigated the relationship between the UPS and autophagy using Drosophila models of neurodegenerative diseases. We identified histone deacetylase 6 (HDAC6) as a genetic modifier of polyglutamine-induced neurodegeneration and determined that its mechanism of action is autophagy-dependent3. The ability of HDAC6 to suppress degeneration has been extended to additional neurodegenerative disease models, including a fly model expressing pathologic Aβ fragments presented here, but is not a universal modifier of degenerative phenotypes. Importantly, HDAC6 was also found to suppress degeneration associated with proteasome mutations in an autophagy-dependent manner, revealing a compensatory relationship between these two degradation pathways. Our findings indicate that HDAC6 facilitates degradation of potentially noxious protein substrates, contributing vitally to the neuroprotective role of autophagy.     

Pathogenic VCP/TER94 alleles are dominant actives and contribute to neurodegeneration by altering cellular ATP level in a Drosophila IBMPFD model

Inclusion body myopathy with Paget's disease of bone and frontotemporal dementia (IBMPFD) is caused by mutations in Valosin-containing protein (VCP), a hexameric AAA ATPase that participates in a variety of cellular processes such as protein degradation, organelle biogenesis, and cell-cycle regulation. To understand how VCP mutations cause IBMPFD, we have established a Drosophila model by overexpressing TER94 (the sole Drosophila VCP ortholog) carrying mutations analogous to those implicated in IBMPFD. Expression of these TER94 mutants in muscle and nervous systems causes tissue degeneration, recapitulating the pathogenic phenotypes in IBMPFD patients. TER94-induced neurodegenerative defects are enhanced by elevated expression of wild-type TER94, suggesting that the pathogenic alleles are dominant active mutations. This conclusion is further supported by the observation that TER94-induced neurodegenerative defects require the formation of hexamer complex, a prerequisite for a functional AAA ATPase. Surprisingly, while disruptions of the ubiquitin-proteasome system (UPS) and the ER-associated degradation (ERAD) have been implicated as causes for VCP-induced tissue degeneration, these processes are not significantly affected in our fly model. Instead, the neurodegenerative defect of TER94 mutants seems sensitive to the level of cellular ATP. We show that increasing cellular ATP by independent mechanisms could suppress the phenotypes of TER94 mutants. Conversely, decreasing cellular ATP would enhance the TER94 mutant phenotypes. Taken together, our analyses have defined the nature of IBMPFD-causing VCP mutations and made an unexpected link between cellular ATP level and IBMPFD pathogenesis.     

Synphilin suppresses α-synuclein neurotoxicity in a Parkinson's disease Drosophila model

Parkinson's disease (PD) is the second most common neurodegenerative disorder in humans. It affects 1% of the population over 65-years old. Its causes are environmental and genetic. As the world population ages, there is an urgent need for better and more detailed animal models for this kind of disease. In this work we show that the use of transgenic Drosophila is comparable to more complicated and costly animal models such as mice. The Drosophila model behaves very similar to the equivalent transgenic mice model. We show that both Synphilin-1 and α-synuclein are toxic by themselves, but when co-expressed, they suppress their toxicity reciprocally. Importantly, the symptoms induced in the fly can be treated and partially reverted using standard PD pharmacological treatments. This work showcases Drosophila as a detailed and multifaceted model for Parkinson's disease, providing a convenient platform in which to study and find new genetic modifiers of PD. genesis 49:392-402, 2011.     

Drosophila melanogaster as a model organism for Alzheimer’s disease

Drosophila melanogaster provides an important resource for in vivo modifier screens of neurodegenerative diseases. To study the underlying pathogenesis of Alzheimer’s disease, fly models that address Tau or amyloid toxicity have been developed. Overexpression of human wild-type or mutant Tau causes age-dependent neurodegeneration, axonal transport defects and early death. Large-scale screens utilizing a neurodegenerative phenotype induced by eye-specific overexpression of human Tau have identified several kinases and phosphatases, apoptotic regulators and cytoskeleton proteins as determinants of Tau toxicity in vivo. The APP ortholog of Drosophila (dAPPl) shares the characteristic domains with vertebrate APP family members, but does not contain the human Aβ42 domain. To circumvent this drawback, researches have developed strategies by either direct secretion of human Aβ42 or triple transgenic flies expressing human APP, β-secretase and Drosophila γ-secretase presenilin (dPsn). Here, we provide a brief overview of how fly models of AD have contributed to our knowledge of the pathomechanisms of disease.     

Systematic Analysis of Fly Models with Multiple Drivers Reveals Different Effects of Ataxin-1 and Huntingtin in Neuron Subtype-Specific Expression

The fruit fly, Drosophila melanogaster, is a commonly used model organism for neurodegenerative diseases. Its major advantages include a short lifespan and its susceptibility to manipulation using sophisticated genetic techniques. Here, we report the systematic comparison of fly models of two polyglutamine (polyQ) diseases. We induced expression of the normal and mutant forms of full-length Ataxin-1 and Huntingtin exon 1 in cholinergic, dopaminergic, and motor neurons, and glial cells using cell type-specific drivers. We systematically analyzed their effects based on multiple phenotypes: eclosion rate, lifespan, motor performance, and circadian rhythms of spontaneous activity. This systematic assay system enabled us to quantitatively evaluate and compare the functional disabilities of different genotypes. The results suggest different effects of Ataxin-1 and Huntingtin on specific types of neural cells during development and in adulthood. In addition, we confirmed the therapeutic effects of LiCl and butyrate using representative models. These results support the usefulness of this assay system for screening candidate chemical compounds that modify the pathologies of polyQ diseases.     

Synaptic abnormalities in a Drosophila model of Alzheimer’s disease

Alzheimer’s disease (AD) is an age-related neurodegenerative disease characterized by memory loss and decreased synaptic function. Advances in transgenic animal models of AD have facilitated our understanding of this disorder, and have aided in the development, speed and efficiency of testing potential therapeutics. Recently, we have described the characterization of a novel model of AD in the fruit fly, Drosophila melanogaster, where we expressed the human AD-associated proteins APP and BACE in the central nervous system of the fly. Here we describe synaptic defects in the larval neuromuscular junction (NMJ) in this model. Our results indicate that expression of human APP and BACE at the larval NMJ leads to defective larval locomotion behavior, decreased presynaptic connections, altered mitochondrial localization in presynaptic motor neurons and decreased postsynaptic protein levels. Treating larvae expressing APP and BACE with the γ-secretase inhibitor L-685,458 suppresses the behavioral defects as well as the pre- and postsynaptic defects. We suggest that this model will be useful to assess and model the synaptic dysfunction normally associated with AD, and will also serve as a powerful in vivo tool for rapid testing of potential therapeutics for AD.KEY WORDS: APP, Alzheimer’s disease, Drosophila, BACE, Synapse, NMJ