A putative Drosophila homolog of the Huntington's disease gene

Huntington's Disease: Complex Pathogenesis and Therapeutic Strategies

Huntington's disease (HD) arises from the abnormal expansion of CAG repeats in the huntingtin gene (HTT), resulting in the production of the mutant huntingtin protein (mHTT) with a polyglutamine stretch in its N-terminus. The pathogenic mechanisms underlying HD are complex and not yet fully elucidated. However, mHTT forms aggregates and accumulates abnormally in neuronal nuclei and processes, leading to disruptions in multiple cellular functions. Although there is currently no effective curative treatment for HD, significant progress has been made in developing various therapeutic strategies to treat HD. In addition to drugs targeting the neuronal toxicity of mHTT, gene therapy approaches that aim to reduce the expression of the mutant HTT gene hold great promise for effective HD therapy. This review provides an overview of current HD treatments, discusses different therapeutic strategies, and aims to facilitate future therapeutic advancements in the field.

Drosophila melanogaster as a model to study autophagy in neurodegenerative diseases induced by proteinopathies

Proteinopathies are a large group of neurodegenerative diseases caused by both genetic and sporadic mutations in particular genes which can lead to alterations of the protein structure and to the formation of aggregates, especially toxic for neurons. Autophagy is a key mechanism for clearing those aggregates and its function has been strongly associated with the ubiquitin-proteasome system (UPS), hence mutations in both pathways have been associated with the onset of neurodegenerative diseases, particularly those induced by protein misfolding and accumulation of aggregates. Many crucial discoveries regarding the molecular and cellular events underlying the role of autophagy in these diseases have come from studies using Drosophila models. Indeed, despite the physiological and morphological differences between the fly and the human brain, most of the biochemical and molecular aspects regulating protein homeostasis, including autophagy, are conserved between the two species.In this review, we will provide an overview of the most common neurodegenerative proteinopathies, which include PolyQ diseases (Huntington's disease, Spinocerebellar ataxia 1, 2, and 3), Amyotrophic Lateral Sclerosis (C9orf72, SOD1, TDP-43, FUS), Alzheimer's disease (APP, Tau) Parkinson's disease (a-syn, parkin and PINK1, LRRK2) and prion diseases, highlighting the studies using Drosophila that have contributed to understanding the conserved mechanisms and elucidating the role of autophagy in these diseases.

Huntingtin Plays a Role in the Physiological Response to Ethanol in Drosophila

BACKGROUND

Huntingtin (htt) protein is an essential regulator of nervous system function through its various neuroprotective and pro-survival functions, and loss of wild-type htt function is implicated in the etiology of Huntington's disease. While its pathological role is typically understood as a toxic gain-of-function, some neuronal phenotypes also result from htt loss. Therefore, it is important to understand possible roles for htt in other physiological circumstances.

OBJECTIVE

To elucidate the role of htt in the context of ethanol exposure, we investigated how loss of htt impacts behavioral and physiological responses to ethanol in Drosophila.

METHODS

We tested flies lacking htt for ethanol sensitivity and tolerance, preference for ethanol using capillary feeder assays, and recovery of mobility after intoxication. Levels of dopamine neurotransmitter and numbers of dopaminergic cells in brains lacking dhtt were also measured.

RESULTS

We found that dhtt-null flies are both less sensitive and more tolerant to ethanol exposure in adulthood. Moreover, flies lacking dhtt are more averse to alcohol than controls, and they recover mobility faster following acute ethanol intoxication. We showed that dhtt mediates these effects at least in part through the dopaminergic system, as dhtt is required to maintain normal levels of dopamine in the brain and normal numbers of dopaminergic cells in the adult protocerebrum.

CONCLUSIONS

Our results demonstrate that htt regulates the physiological response to ethanol and indicate a novel neuroprotective role for htt in the dopaminergic system, raising the possibility that it may be involved more generally in the response to toxic stimuli.

HAP40 is a conserved central regulator of Huntingtin and a potential modulator of Huntington's disease pathogenesis

Perturbation of huntingtin (HTT)'s physiological function is one postulated pathogenic factor in Huntington's disease (HD). However, little is known how HTT is regulated in vivo. In a proteomic study, we isolated a novel ~40kDa protein as a strong binding partner of Drosophila HTT and demonstrated it was the functional ortholog of HAP40, an HTT associated protein shown recently to modulate HTT's conformation but with unclear physiological and pathologic roles. We showed that in both flies and human cells, HAP40 maintained conserved physical and functional interactions with HTT. Additionally, loss of HAP40 resulted in similar phenotypes as HTT knockout. More strikingly, HAP40 strongly affected HTT's stability, as depletion of HAP40 significantly reduced the levels of endogenous HTT protein while HAP40 overexpression markedly extended its half-life. Conversely, in the absence of HTT, the majority of HAP40 protein were degraded, likely through the proteasome. Further, the affinity between HTT and HAP40 was not significantly affected by polyglutamine expansion in HTT, and contrary to an early report, there were no abnormal accumulations of endogenous HAP40 protein in HD cells from mouse HD models or human patients. Lastly, when tested in Drosophila models of HD, HAP40 partially modulated the neurodegeneration induced by full-length mutant HTT while showed no apparent effect on the toxicity of mutant HTT exon 1 fragment. Together, our study uncovers a conserved mechanism governing the stability and in vivo functions of HTT and demonstrates that HAP40 is a central and positive regulator of endogenous HTT. Further, our results support that mutant HTT is toxic regardless of the presence of its partner HAP40, and implicate HAP40 as a potential modulator of HD pathogenesis through its multiplex effect on HTT's function, stability and the potency of mutant HTT's toxicity.

Diffusion Tensor Imaging in Preclinical and Human Studies of Huntington's Disease: What Have we Learned so Far?

BACKGROUND

Huntington's Disease is an irreversible neurodegenerative disease characterized by the progressive deterioration of specific brain nerve cells. The current evaluation of cellular and physiological events in patients with HD relies on the development of transgenic animal models. To explore such events in vivo, diffusion tensor imaging has been developed to examine the early macro and microstructural changes in brain tissue. However, the gap in diffusion tensor imaging findings between animal models and clinical studies and the lack of microstructural confirmation by histological methods has questioned the validity of this method.

OBJECTIVE

This review explores white and grey matter ultrastructural changes associated to diffusion tensor imaging, as well as similarities and differences between preclinical and clinical Huntington's Disease studies.

METHODS

A comprehensive review of the literature using online-resources was performed (Pub- Med search).

RESULTS

Similar changes in fractional anisotropy as well as axial, radial and mean diffusivities were observed in white matter tracts across clinical and animal studies. However, comparative diffusion alterations in different grey matter structures were inconsistent between clinical and animal studies.

CONCLUSION

Diffusion tensor imaging can be related to specific structural anomalies in specific cellular populations. However, some differences between animal and clinical studies could derive from the contrasting neuroanatomy or connectivity across species. Such differences should be considered before generalizing preclinical results into the clinical practice. Moreover, current limitations of this technique to accurately represent complex multicellular events at the single micro scale are real. Future work applying complex diffusion models should be considered.

Huntington's Disease-An Outlook on the Interplay of the HTT Protein, Microtubules and Actin Cytoskeletal Components

Huntington's disease is a severe and currently incurable neurodegenerative disease. An autosomal dominant mutation in the Huntingtin gene (HTT) causes an increase in the polyglutamine fragment length at the protein N-terminus. The consequence of the mutation is the death of neurons, mostly striatal neurons, leading to the occurrence of a complex of motor, cognitive and emotional-volitional personality sphere disorders in carriers. Despite intensive studies, the functions of both mutant and wild-type huntingtin remain poorly understood. Surprisingly, there is the selective effect of the mutant form of HTT even on nervous tissue, whereas the protein is expressed ubiquitously. Huntingtin plays a role in cell physiology and affects cell transport, endocytosis, protein degradation and other cellular and molecular processes. Our experimental data mining let us conclude that a significant part of the Huntingtin-involved cellular processes is mediated by microtubules and other cytoskeletal cell structures. The review attempts to look at unresolved issues in the study of the huntingtin and its mutant form, including their functions affecting microtubules and other components of the cell cytoskeleton.

Peripheral Expression of Mutant Huntingtin is a Critical Determinant of Weight Loss and Metabolic Disturbances in Huntington's Disease

Deteriorating weight loss in patients with Huntington's disease (HD) is a complicated peripheral manifestation and the cause remains poorly understood. Studies suggest that body weight strongly influences the clinical progression rate of HD and thereby offers a valuable target for therapeutic interventions. Mutant huntingtin (mHTT) is ubiquitously expressed and could induce toxicity by directly acting in the peripheral tissues. We investigated the effects of selective expression of mHTT exon1 in fat body (FB; functionally equivalent to human adipose tissue and liver) using transgenic Drosophila. We find that FB-autonomous expression of mHTT exon1 is intrinsically toxic and causes chronic weight loss in the flies despite progressive hyperphagia, and early adult death. Moreover, flies exhibit loss of intracellular lipid stores, and decline in the systemic levels of lipids and carbohydrates which aggravates over time, representing metabolic defects. At the cellular level, besides impairment, cell death also occurs with the formation of mHTT aggregates in the FB. These findings indicate that FB-autonomous expression of mHTT alone is sufficient to cause metabolic abnormalities and emaciation in vivo without any neurodegenerative cues.

Modern approaches for modelling dystonia and Huntington's disease in vitro and in vivo

Dystonia associated with Huntington's disease, Parkinson's disease or other neurodegenerative diseases substantially affects patients' quality of life and is a major health problem worldwide. The above-mentioned diseases are characterized by neurodegeneration accompanied by motor and cognitive impairment and often have complex aetiology. A frequent feature of these conditions is the abnormal accumulation of protein aggregates within specific neuronal populations in the affected brain regions. Familial neurodegenerative diseases are associated with a number of genetic mutations. Identification of these mutations allowed creation of modern model systems for studying neurodegeneration, either in cultured cells or in model animals. Animal models, especially mouse models, have contributed considerably to improving our understanding of the pathophysiology of neurodegenerative diseases. These models have allowed study of the pathogenic mechanisms and development of new disease-modifying strategies and therapeutic approaches. However, due to the complex nature of these pathologies and the irreversible damage that they cause to the neural tissue, effective therapies against neurodegeneration remain to be elaborated. In this review, we provide an overview of cellular and animal models developed for studying neurodegenerative diseases, including Huntington's disease and dystonia of different origins.

Deletion of Htt cause alterations in cAMP signaling and spatial patterning in Dictyostelium discoideum

In the present study, we have analyzed in detail the functions of Htt during growth and development of the protist, Dictyostelium discoideum by creating mutants (both overexpressor and knockout). The mRNA was present at all stages of growth and development. Overexpression of htt did not show any major anomaly, while deletion resulted in delayed aggregation territory formation and showed asynchronous development especially after slug stage. The slugs formed by htt ^- cells showed aberration in anterior-posterior boundary, showing increased prestalk region. DdHtt regulates STAT transcription factors in the tip organizer region that help maintain patterning and culmination. In chimeras with the wild-type, htt ^- cells preferentially localized to the tip of the slug and basal disc regions of the fruiting body showing prestalk/stalk bias, while the overexpressing cells majorly populated the prespore/spore region showing spore bias. These differences could be attributed to protein kinase A (PKA)-regulated cyclic adenosine monophosphate (cAMP) signaling.

Evolving Notch polyQ tracts reveal possible solenoid interference elements

Polyglutamine (polyQ) tracts in regulatory proteins are extremely polymorphic. As functional elements under selection for length, triplet repeats are prone to DNA replication slippage and indel mutations. Many polyQ tracts are also embedded within intrinsically disordered domains, which are less constrained, fast evolving, and difficult to characterize. To identify structural principles underlying polyQ tracts in disordered regulatory domains, here I analyze deep evolution of metazoan Notch polyQ tracts, which can generate alleles causing developmental and neurogenic defects. I show that Notch features polyQ tract turnover that is restricted to a discrete number of conserved “polyQ insertion slots”. Notch polyQ insertion slots are: (i) identifiable by an amphipathic “slot leader” motif; (ii) conserved as an intact C-terminal array in a 1-to-1 relationship with the N-terminal solenoid-forming ankyrin repeats (ARs); and (iii) enriched in carboxamide residues (Q/N), whose sidechains feature dual hydrogen bond donor and acceptor atoms. Correspondingly, the terminal loop and β-strand of each AR feature conserved carboxamide residues, which would be susceptible to folding interference by hydrogen bonding with residues outside the ARs. I thus suggest that Notch polyQ insertion slots constitute an array of AR interference elements (ARIEs). Notch ARIEs would dynamically compete with the delicate serial folding induced by adjacent ARs. Huntingtin, which harbors solenoid-forming HEAT repeats, also possesses a similar number of polyQ insertion slots. These results suggest that intrinsically disordered interference arrays featuring carboxamide and polyQ enrichment may constitute coupled proteodynamic modulators of solenoids.

Characterization of axonal transport defects in Drosophila Huntingtin mutants

Polyglutamine (polyQ) expansion within Huntingtin (Htt) causes the fatal neurodegenerative disorder Huntington’s Disease (HD). Although Htt is ubiquitously expressed and conserved from Drosophila to humans, its normal biological function is still being elucidated. Here we characterize a role for the Drosophila Htt homolog (dHtt) in fast axonal transport (FAT). Generation and expression of transgenic dHtt-mRFP and human Htt-mRFP fusion proteins in Drosophila revealed co-localization with mitochondria and synaptic vesicles undergoing FAT. However, Htt was not ubiquitously associated with the transport machinery, as it was excluded from dense-core vesicles and APLIP1 containing vesicles. Quantification of cargo movement in dHtt deficient axons revealed that mitochondria and synaptic vesicles show a decrease in the distance and duration of transport, and an increase in the number of pauses. In addition, the ratio of retrograde to anterograde flux was increased in mutant animals. Densecore vesicles did not display similar defects in processivity, but did show altered retrograde to anterograde flux along axons. Given the co-localization with mitochondria and synaptic vesicles, but not dense-core vesicles, the data suggest dHtt likely acts locally at cargo interaction sites to regulate processivity. An increase in dynein heavy chain expression was also observed in dHtt mutants, suggesting that the altered flux observed for all cargo may represent secondary transport changes occurring independent of dHtt’s primary function. Expression of dHtt in a milton (HAP1) mutant background revealed that the protein does not require mitochondria or HAP1 to localize along axons, suggesting Htt has an independent mechanism for coupling with motors to regulate their processivity during axonal transport.

Deletion of the huntingtin proline-rich region does not significantly affect normal huntingtin function in mice

The N-terminus of Huntingtin, the protein encoded by the Huntington's disease gene, contains a stretch of polyglutamine residues that is expanded in Huntington's disease. The polyglutamine stretch is flanked by two conserved protein domains in vertebrates: an N1-17 domain, and a proline-rich region (PRR). The PRR can modulate the structure of the adjacent polyglutamine stretch, and is a binding site for several interacting proteins. To determine the role of the PRR in Huntingtin function, we have generated a knock-in allele of the mouse Huntington's disease gene homolog that expresses full-length normal huntingtin lacking the PRR. Mice that are homozygous for the huntingtin PRR deletion are born at the normal Mendelian frequency, suggesting that the PRR is not required for essential huntingtin functions during embryonic development. Moreover, adult homozygous mutants did not exhibit any significant differences from wild-type controls in general motor function and motor learning. However, 18 month-old male, but not female, homozygous PRR deletion mutants exhibited deficits in the Morris water task, suggesting that age-dependent spatial learning and memory may be affected in a sex-specific fashion by the huntingtin PRR deletion.

Studying polyglutamine diseases in Drosophila

Polyglutamine (polyQ) diseases are a family of dominantly transmitted neurodegenerative disorders caused by an abnormal expansion of CAG trinucleotide repeats in the protein-coding regions of the respective disease-causing genes. Despite their simple genetic basis, the etiology of these diseases is far from clear. Over the past two decades, Drosophila has proven to be successful in modeling this family of neurodegenerative disorders, including the faithful recapitulation of pathological features such as polyQ length-dependent formation of protein aggregates and progressive neuronal degeneration. Additionally, it has been valuable in probing the pathogenic mechanisms, in identifying and evaluating disease modifiers, and in helping elucidate the normal functions of disease-causing genes. Knowledge learned from this simple invertebrate organism has had a large impact on our understanding of these devastating brain diseases.

Using Drosophila models of Huntington's disease as a translatable tool

The Huntingtin (Htt) protein is essential for a wealth of intracellular signaling cascades and when mutated, causes multifactorial dysregulation of basic cellular processes. Understanding the contribution to each of these intracellular pathways is essential for the elucidation of mechanisms that drive pathophysiology. Using appropriate models of Huntington's disease (HD) is key to finding the molecular mechanisms that contribute to neurodegeneration. While mouse models and cell lines expressing mutant Htt have been instrumental to HD research, there has been a significant contribution to our understating of the disease from studies utilizing Drosophila melanogaster. Flies have an Htt protein, so the endogenous pathways with which it interacts are likely conserved. Transgenic flies engineered to overexpress the human mutant HTT gene display protein aggregation, neurodegeneration, behavioral deficits and a reduced lifespan. The short life span of flies, low cost of maintaining stocks and genetic tools available for in vivo manipulation make them ideal for the discovery of new genes that are involved in HD pathology. It is possible to do rapid genome wide screens for enhancers or suppressors of the mutant Htt-mediated phenotype, expressed in specific tissues or neuronal subtypes. However, there likely remain many yet unknown genes that modify disease progression, which could be found through additional screening approaches using the fly. Importantly, there have been instances where genes discovered in Drosophila have been translated to HD mouse models.

Huntingtin functions as a scaffold for selective macroautophagy

Selective macroautophagy is an important protective mechanism against diverse cellular stresses. In contrast to the well-characterized starvation-induced autophagy, the regulation of selective autophagy is largely unknown. Here, we demonstrate that Huntingtin, the Huntington’s disease gene product, functions as a scaffold protein for selective macroautophagy but it is dispensable for nonselective macroautophagy. In Drosophila, Huntingtin genetically interacts with autophagy pathway components. In mammalian cells, Huntingtin physically interacts with the autophagy cargo receptor p62 to facilitate its association with the integral autophagosome component LC3 and with lysine-63-linked ubiquitin-modified substrates. Maximal activation of selective autophagy during stress is attained by the ability of Huntingtin to bind ULK1, a kinase that initiates autophagy, which releases ULK1 from negative regulation via mTOR. Our data uncover an important physiological function of Huntingtin and provide a missing link in the activation of selective macroautophagy in metazoans.

The Drosophila Huntington's disease gene ortholog dhtt influences chromatin regulation during development

Huntington's disease is an autosomal dominant neurodegenerative disorder caused by a CAG expansion mutation in HTT, the gene encoding huntingtin. Evidence from both human genotype-phenotype relationships and mouse model systems suggests that the mutation acts by dysregulating some normal activity of huntingtin. Recent work in the mouse has revealed a role for huntingtin in epigenetic regulation during development. Here, we examine the role of the Drosophila huntingtin ortholog (dhtt) in chromatin regulation in the development of the fly. Although null dhtt mutants display no overt phenotype, we found that dhtt acts as a suppressor of position-effect variegation (PEV), suggesting that it influences chromatin organization. We demonstrate that dhtt affects heterochromatin spreading in a PEV model by modulating histone H3K9 methylation levels at the heterochromatin-euchromatin boundary. To gain mechanistic insights into how dhtt influences chromatin function, we conducted a candidate genetic screen using RNAi lines targeting known PEV modifier genes. We found that dhtt modifies phenotypes caused by knockdown of a number of key epigenetic regulators, including chromatin-associated proteins, histone demethylases (HDMs) and methyltransferases. Notably, dhtt strongly modifies phenotypes resulting from loss of the HDM dLsd1, in both the ovary and wing, and we demonstrate that dhtt appears to act as a facilitator of dLsd1 function in regulating global histone H3K4 methylation levels. These findings suggest that a fundamental aspect of huntingtin function in heterochromatin/euchromatin organization is evolutionarily conserved across phyla.

Evidence for dynamic and multiple roles for huntingtin in Ciona intestinalis

Although mutations in the huntingtin gene (HTT) due to poly-Q expansion cause neuropathology in humans (Huntington’s disease; HD), the normal function(s) of the gene and its protein (HTT) remain obscure. With new information from recently sequenced invertebrate genomes, the study of new animal models opens the possibility of a better understanding of HTT function and its evolution. To these ends, we studied huntingtin expression pattern and dynamics in the invertebrate chordate Ciona intestinalis. Ciona huntingtin (Ci-HTT) shows a biphasic expression pattern during larval development and prior to metamorphosis. A single form of huntingtin protein is present until the early larval stages, at which time two different mass proteins become evident in the metamorphically competent larva. An antibody against Ci-HTT labeled 50 cells in the trunk mesenchyme regions in pre-hatching and hatched larvae and probably represents the distribution of the light form of the protein. Dual labeling with anti-Ci-HTT and anti-aldoketoreductase confirmed the presence of Ci-HTT in mesenchyme cells. Suppression of Ci-HTT RNA by a morpholino oligonucleotide reduced the number and apparent mobility of Ci-HTT positive cells. In Ciona, HTT expression has a dynamic temporal and spatial expression pattern that in ontogeny precedes metamorphosis. Although our results may reflect a derived function for the protein in pre- and post-metamorphic events in Ciona, we also note that as in vertebrates, there is evidence for multiple differential temporal expression, indicating that this protein probably has multiple roles in ontogeny and cell migration.

Huntingtin is critical both pre- and postsynaptically for long-term learning-related synaptic plasticity in Aplysia

Patients with Huntington’s disease exhibit memory and cognitive deficits many years before manifesting motor disturbances. Similarly, several studies have shown that deficits in long-term synaptic plasticity, a cellular basis of memory formation and storage, occur well before motor disturbances in the hippocampus of the transgenic mouse models of Huntington’s disease. The autosomal dominant inheritance pattern of Huntington’s disease suggests the importance of the mutant protein, huntingtin, in pathogenesis of Huntington’s disease, but wild type huntingtin also has been shown to be important for neuronal functions such as axonal transport. Yet, the role of wild type huntingtin in long-term synaptic plasticity has not been investigated in detail. We identified a huntingtin homolog in the marine snail Aplysia, and find that similar to the expression pattern in mammalian brain, huntingtin is widely expressed in neurons and glial cells. Importantly the expression of mRNAs of huntingtin is upregulated by repeated applications of serotonin, a modulatory transmitter released during learning in Aplysia. Furthermore, we find that huntingtin expression levels are critical, not only in presynaptic sensory neurons, but also in the postsynaptic motor neurons for serotonin-induced long-term facilitation at the sensory-to-motor neuron synapse of the Aplysia gill-withdrawal reflex. These results suggest a key role for huntingtin in long-term memory storage.

Animal models of neurodegenerative diseases

The prevalence of neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), increases with age, and the number of affected patients is expected to increase worldwide in the next decades. Accurately understanding the etiopathogenic mechanisms of these diseases is a crucial step for developing disease-modifying drugs able to preclude their emergence or at least slow their progression. Animal models contribute to increase the knowledge on the pathophysiology of neurodegenerative diseases. These models reproduce different aspects of a given disease, as well as the histopathological lesions and its main symptoms. The purpose of this review is to present the main animal models for AD, PD, and Huntington's disease.

Modeling Huntington disease in Drosophila: Insights into axonal transport defects and modifiers of toxicity

Huntington disease (HD) is an inherited neurodegenerative disorder caused by a polyglutamine (polyQ) expansion in the huntingtin (Htt) gene. Despite years of research, there is no treatment that extends life for patients with the disorder. Similarly, little is known about which cellular pathways that are altered by pathogenic Huntingtin (Htt) protein expression are correlated with neuronal loss. As part of a longstanding effort to gain insights into HD pathology, we have been studying the protein in the context of the fruitfly Drosophila melanogaster. We generated transgenic HD models in Drosophila by engineering flies that carry a 12-exon fragment of the human Htt gene with or without the toxic trinucleotide repeat expansion. We also created variants with a monomeric red fluorescent protein (mRFP) tag fused to Htt that allows in vivo imaging of Htt protein localization and aggregation. While wild-type Htt remains diffuse throughout the cytoplasm of cells, pathogenic Htt forms insoluble aggregates that accumulate in neuronal soma and axons. Aggregates can physically block transport of numerous organelles along the axon. We have also observed that aggregates are formed quickly, within just a few hours of mutant Htt expression. To explore mechanisms of neurodegeneration in our HD model, we performed in vivo and in vitro screens to search for modifiers of viability and pathogenic Htt aggregation. Our results identified several novel candidates for HD therapeutics that can now be tested in mammalian models of HD. Furthermore, these experiments have highlighted the complex relationship between aggregates and toxicity that exists in HD.

A huntingtin peptide inhibits polyQ-huntingtin associated defects

BACKGROUND

Huntington's disease (HD) is caused by the abnormal expansion of the polyglutamine tract in the human Huntingtin protein (polyQ-hHtt). Although this mutation behaves dominantly, huntingtin loss of function also contributes to HD pathogenesis. Indeed, wild-type Huntingtin plays a protective role with respect to polyQ-hHtt induced defects.

METHODOLOGY/PRINCIPAL FINDINGS

The question that we addressed here is what part of the wild-type Huntingtin is responsible for these protective properties. We first screened peptides from the Huntingtin protein in HeLa cells and identified a 23 aa peptide (P42) that inhibits polyQ-hHtt aggregation. P42 is part of the endogenous Huntingtin protein and lies within a region rich in proteolytic sites that plays a critical role in the pathogenesis process. Using a Drosophila model of HD, we tested the protective properties of this peptide on aggregation, as well as on different polyQ-hHtt induced neuronal phenotypes: eye degeneration (an indicator of cell death), impairment of vesicular axonal trafficking, and physiological behaviors such as larval locomotion and adult survival. Together, our results demonstrate high protective properties for P42 in vivo, in whole animals. These data also demonstrate a specific role of P42 on Huntington's disease model, since it has no effect on other models of polyQ-induced diseases, such as spinocerebellar ataxias.

CONCLUSIONS/SIGNIFICANCE

Altogether our data show that P42, a 23 aa-long hHtt peptide, plays a protective role with respect to polyQ-hHtt aggregation as well as cellular and behavioral dysfunctions induced by polyQ-hHtt in vivo. Our study also confirms the correlation between polyQ-hHtt aggregation and neuronal defects. Finally, these results strongly suggest a therapeutic potential for P42, specific of Huntington's disease.

Choosing and using Drosophila models to characterize modifiers of Huntington's disease

HD (Huntington's disease) is a fatal inherited gain-of-function disorder caused by a polyQ (polyglutamine) expansion in the htt (huntingtin protein). Expression of mutant htt in model organisms is sufficient to recapitulate many of the cellular defects found in HD patients. Many groups have independently developed Drosophila models of HD, taking advantage of its rapid life cycle, carefully annotated genome and well-established molecular toolkits. Furthermore, unlike simpler models, Drosophila have a complex nervous system, displaying a range of carefully co-ordinated behaviours which offer an exquisitely sensitive readout of neuronal disruption. Measuring HD-associated changes in behaviour in Drosophila therefore offers a window into the earliest stages of HD, when therapeutic interventions might be particularly effective. The present review describes a number of recently developed Drosophila models of HD and offers practical guidance on the advantages and disadvantages of various experimental approaches that can be used to screen these models for modifiers of mutant htt-mediated toxicity.

An evolutionary recent neuroepithelial cell adhesion function of huntingtin implicates ADAM10-Ncadherin

The Huntington's disease gene product, huntingtin, is indispensable for neural tube formation, but its role is obscure. We studied neurulation in htt-null embryonic stem cells and htt-morpholino zebrafish embryos and found a previously unknown, evolutionarily recent function for this ancient protein. We found that htt was essential for homotypic interactions between neuroepithelial cells; it permitted neurulation and rosette formation by regulating metalloprotease ADAM10 activity and Ncadherin cleavage. This function was embedded in the N terminus of htt and was phenocopied by treatment of htt knockdown zebrafish with an ADAM10 inhibitor. Notably, in htt-null cells, reversion of the rosetteless phenotype occurred only with expression of evolutionarily recent htt heterologues from deuterostome organisms. Conversely, all of the heterologues that we tested, including htt from Drosophila melanogaster and Dictyostelium discoideum, exhibited anti-apoptotic activity. Thus, anti-apoptosis may have been one of htt’s ancestral function(s), but, in deuterostomes, htt evolved to acquire a unique regulatory activity for controlling neural adhesion via ADAM10-Ncadherin, with implications for brain evolution and development.

High-content chemical and RNAi screens for suppressors of neurotoxicity in a Huntington's disease model

To identify Huntington's Disease therapeutics, we conducted high-content small molecule and RNAi suppressor screens using a Drosophila primary neural culture Huntingtin model. Drosophila primary neurons offer a sensitive readout for neurotoxicty, as their neurites develop dysmorphic features in the presence of mutant polyglutamine-expanded Huntingtin compared to nonpathogenic Huntingtin. By tracking the subcellular distribution of mRFP-tagged pathogenic Huntingtin and assaying neurite branch morphology via live-imaging, we identified suppressors that could reduce Huntingtin aggregation and/or prevent the formation of dystrophic neurites. The custom algorithms we used to quantify neurite morphologies in complex cultures provide a useful tool for future high-content screening approaches focused on neurodegenerative disease models. Compounds previously found to be effective aggregation inhibitors in mammalian systems were also effective in Drosophila primary cultures, suggesting translational capacity between these models. However, we did not observe a direct correlation between the ability of a compound or gene knockdown to suppress aggregate formation and its ability to rescue dysmorphic neurites. Only a subset of aggregation inhibitors could revert dysmorphic cellular profiles. We identified lkb1, an upstream kinase in the mTOR/Insulin pathway, and four novel drugs, Camptothecin, OH-Camptothecin, 18β-Glycyrrhetinic acid, and Carbenoxolone, that were strong suppressors of mutant Huntingtin-induced neurotoxicity. Huntingtin neurotoxicity suppressors identified through our screen also restored viability in an in vivo Drosophila Huntington's Disease model, making them attractive candidates for further therapeutic evaluation.

Molecular biology of Huntington's disease

It has been more than 17 years since the causative mutation for Huntington's disease was discovered as the expansion of the triplet repeat in the N-terminal portion of the Huntingtin (HTT) gene. In the intervening time, researchers have discovered a great deal about Huntingtin's involvement in a number of cellular processes. However, the role of Huntingtin in the key pathogenic mechanism leading to neurodegeneration in the disease process has yet to be discovered. Here, we review the body of knowledge that has been uncovered since gene discovery and include discussions of the HTT gene, CAG triplet repeat expansion, HTT expression, protein features, posttranslational modifications, and many of its known protein functions and interactions. We also highlight potential pathogenic mechanisms that have come to light in recent years.

Genetics and neuropathology of Huntington's disease

Huntington's disease (HD) is an autosomal dominant progressive neurodegenerative disorder that prominently affects the basal ganglia, leading to affective, cognitive, behavioral and motor decline. The basis of HD is a CAG repeat expansion to >35 CAG in a gene that codes for a ubiquitous protein known as huntingtin, resulting in an expanded N-terminal polyglutamine tract. The size of the expansion is correlated with disease severity, with increasing CAG accelerating the age of onset. A variety of possibilities have been proposed as to the mechanism by which the mutation causes preferential injury to the basal ganglia. The present chapter provides a basic overview of the genetics and pathology of HD.

Molecular Mechanisms and Potential Therapeutical Targets in Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a CAG repeat expansion in the gene encoding for huntingtin protein. A lot has been learned about this disease since its first description in 1872 and the identification of its causative gene and mutation in 1993. We now know that the disease is characterized by several molecular and cellular abnormalities whose precise timing and relative roles in pathogenesis have yet to be understood. HD is triggered by the mutant protein, and both gain-of-function (of the mutant protein) and loss-of-function (of the normal protein) mechanisms are involved. Here we review the data that describe the emergence of the ancient huntingtin gene and of the polyglutamine trait during the last 800 million years of evolution. We focus on the known functions of wild-type huntingtin that are fundamental for the survival and functioning of the brain neurons that predominantly degenerate in HD. We summarize data indicating how the loss of these beneficial activities reduces the ability of these neurons to survive. We also review the different mechanisms by which the mutation in huntingtin causes toxicity. This may arise both from cell-autonomous processes and dysfunction of neuronal circuitries. We then focus on novel therapeutical targets and pathways and on the attractive option to counteract HD at its primary source, i.e., by blocking the production of the mutant protein. Strategies and technologies used to screen for candidate HD biomarkers and their potential application are presented. Furthermore, we discuss the opportunities offered by intracerebral cell transplantation and the likely need for these multiple routes into therapies to converge at some point as, ideally, one would wish to stop the disease process and, at the same time, possibly replace the damaged neurons.

Trends in the molecular pathogenesis and clinical therapeutics of common neurodegenerative disorders

The term neurodegenerative disorders, encompasses a variety of underlying conditions, sporadic and/or familial and are characterized by the persistent loss of neuronal subtypes. These disorders can disrupt molecular pathways, synapses, neuronal subpopulations and local circuits in specific brain regions, as well as higher-order neural networks. Abnormal network activities may result in a vicious cycle, further impairing the integrity and functions of neurons and synapses, for example, through aberrant excitation or inhibition. The most common neurodegenerative disorders are Alzheimer’s disease, Parkinson’s disease, Amyotrophic Lateral Sclerosis and Huntington’s disease. The molecular features of these disorders have been extensively researched and various unique neurotherapeutic interventions have been developed. However, there is an enormous coercion to integrate the existing knowledge in order to intensify the reliability with which neurodegenerative disorders can be diagnosed and treated. The objective of this review article is therefore to assimilate these disorders’ in terms of their neuropathology, neurogenetics, etiology, trends in pharmacological treatment, clinical management, and the use of innovative neurotherapeutic interventions.

Inactivation of Drosophila Huntingtin affects long-term adult functioning and the pathogenesis of a Huntington's disease model

A polyglutamine expansion in the huntingtin (HTT) gene causes neurodegeneration in Huntington's disease (HD), but the in vivo function of the native protein (Htt) is largely unknown. Numerous biochemical and in vitro studies have suggested a role for Htt in neuronal development, synaptic function and axonal trafficking. To test these models, we generated a null mutant in the putative Drosophila HTT homolog (htt, hereafter referred to asdhtt) and, surprisingly, found that dhtt mutant animals are viable with no obvious developmental defects. Instead, dhtt is required for maintaining the mobility and long-term survival of adult animals, and for modulating axonal terminal complexity in the adult brain. Furthermore, removing endogenous dhtt significantly accelerates the neurodegenerative phenotype associated with a Drosophila model of polyglutamine Htt toxicity (HD-Q93), providing in vivo evidence that disrupting the normal function of Htt might contribute to HD pathogenesis.

Protective role of Engrailed in a Drosophila model of Huntington's disease

Huntington's disease (HD) is caused by the expansion of the polyglutamine (polyQ) tract in the human Huntingtin (hHtt) protein (polyQ-hHtt). Although this mutation behaves dominantly, htt loss of function may also contribute to HD pathogenesis. Using a Drosophila model of HD, we found that Engrailed (EN), a transcriptional activator of endogenous Drosophila htt (dhtt), is able to prevent aggregation of polyQ-hHtt. To interpret these findings, we tested and identified a protective role of N-terminal fragments of both Drosophila and Human wild-type Htt onto polyQ-hHtt-induced cellular defects. In addition, N-terminal parts of normal hHtt were also able to rescue eye degeneration due to the loss of Drosophila endogenous dhtt function. Thus, our data indicate that Drosophila and Human Htt share biological properties, and confirm a model whereby EN activates endogenous dhtt, which in turn prevents polyQ-hHtt-induced phenotypes. The protective role of wild-type hHtt N-terminal parts, specifically onto polyQ-hHtt-induced cellular toxicity suggests that the HD may be considered as a dominant negative disease rather than solely dominant.

Of flies and man: Drosophila as a model for human complex traits

Understanding the genetic and environmental factors affecting human complex genetic traits and diseases is a major challenge because of many interacting genes with individually small effects, whose expression is sensitive to the environment. Dissection of complex traits using the powerful genetic approaches available with Drosophila melanogaster has provided important lessons that should be considered when studying human complex traits. In Drosophila, large numbers of pleiotropic genes affect complex traits; quantitative trait locus alleles often have sex-, environment-, and genetic background-specific effects, and variants associated with different phenotypic are in noncoding as well as coding regions of candidate genes. Such insights, in conjunction with the strong evolutionary conservation of key genes and pathways between flies and humans, make Drosophila an excellent model system for elucidating the genetic mechanisms that affect clinically relevant human complex traits, such as alcohol dependence, sleep, and neurodegenerative diseases.

Characterization, developmental expression and evolutionary features of the huntingtin gene in the amphioxus Branchiostoma floridae

BACKGROUND

Huntington's disease is an inherited neurodegenerative disorder that is caused by the expansion of an N-terminal polyQ stretch in the huntingtin protein. In order to investigate the hypothesis that huntingtin was already involved in development of the nervous system in the last common ancestor of chordates, we isolated and characterised the huntingtin homologue from the amphioxus Branchiostoma floridae. In the present paper the amphioxus general term must be referred to Branchiostoma floridae.

RESULTS

In this report, we show that the exon-intron organization of the amphioxus huntingtin gene is highly conserved with that of other vertebrates species. The AmphiHtt protein has two glutamine residues in the position of the typical vertebrate polyQ tract. Sequence conservation is greater along the entire length of the protein than in a previously identified Ciona huntingtin. The first three N-terminal HEAT repeats are highly conserved in vertebrates and amphioxus, although exon rearrangement has occurred in this region. AmphiHtt expression is detectable by in situ hybridization starting from the early neurula stage, where it is found in cells of the neural plate. At later stages, it is retained in the neural compartment but also it appears in limited and well-defined groups of non-neural cells. At subsequent larval stages, AmphiHtt expression is detected in the neural tube, with the strongest signal being present in the most anterior part.

CONCLUSION

The cloning of amphioxus huntingtin allows to infer that the polyQ in huntingtin was already present 540 million years ago and provides a further element for the study of huntingtin function and its evolution along the deuterostome branch.

Normal huntingtin function: an alternative approach to Huntington's disease

Several neurological diseases are characterized by the altered activity of one or a few ubiquitously expressed cell proteins, but it is not known how these normal proteins turn into harmful executors of selective neuronal cell death. We selected huntingtin in Huntington's disease to explore this question because the dominant inheritance pattern of the disease seems to exclude the possibility that the wild-type protein has a role in the natural history of this condition. However, even in this extreme case, there is considerable evidence that normal huntingtin is important for neuronal function and that the activity of some of its downstream effectors, such as brain-derived neurotrophic factor, is reduced in Huntington's disease.

Huntingtin gene evolution in Chordata and its peculiar features in the ascidian Ciona genus

Background

To gain insight into the evolutionary features of the huntingtin (htt) gene in Chordata, we have sequenced and characterized the full-length htt mRNA in the ascidian Ciona intestinalis, a basal chordate emerging as new invertebrate model organism. Moreover, taking advantage of the availability of genomic and EST sequences, the htt gene structure of a number of chordate species, including the cogeneric ascidian Ciona savignyi, and the vertebrates Xenopus and Gallus was reconstructed.

Results

The C. intestinalis htt transcript exhibits some peculiar features, such as spliced leader trans-splicing in the 98 nt-long 5' untranslated region (UTR), an alternative splicing in the coding region, eight alternative polyadenylation sites, and no similarities of both 5' and 3'UTRs compared to homologs of the cogeneric C. savignyi. The predicted protein is 2946 amino acids long, shorter than its vertebrate homologs, and lacks the polyQ and the polyP stretches found in the the N-terminal regions of mammalian homologs. The exon-intron organization of the htt gene is almost identical among vertebrates, and significantly conserved between Ciona and vertebrates, allowing us to hypothesize an ancestral chordate gene consisting of at least 40 coding exons.

Conclusion

During chordate diversification, events of gain/loss, sliding, phase changes, and expansion of introns occurred in both vertebrate and ascidian lineages predominantly in the 5'-half of the htt gene, where there is also evidence of lineage-specific evolutionary dynamics in vertebrates. On the contrary, the 3'-half of the gene is highly conserved in all chordates at the level of both gene structure and protein sequence. Between the two Ciona species, a fast evolutionary rate and/or an early divergence time is suggested by the absence of significant similarity between UTRs, protein divergence comparable to that observed between mammals and fishes, and different distribution of repetitive elements.

Animal models of Huntington's disease: implications in uncovering pathogenic mechanisms and developing therapies

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder, which is caused by an abnormal expansion of Cytosine Adenine Guanine (CAG) trinucleotide repeat in the gene making huntingtin (Htt). Despite intensive research efforts devoted to investigate molecular mechanisms of pathogenesis, effective therapy for this devastating disease is still not available at present. The development of various animal models of HD has offered alternative approaches in the study of HD molecular pathology. Many HD models, including chemical-induced models and genetic models, mimic some aspects of HD symptoms and pathology. To date, however, there is no ideal model which replicates all of the essential features of neuropathology and progressive motor and cognitive impairments of human HD. As a result, our understanding of molecular mechanisms of pathogenesis in HD is still limited. A new model is needed in order to uncover the pathogenesis and to develop novel therapies for HD. In this review we discussed usefulness and limitations of various animal and cellular models of HD in uncovering molecular mechanisms of pathogenesis and developing novel therapies for HD.

Is Huntington's a glutamine storage disease?

Huntington's disease is a neurological disorder caused by the expansion of a polyglutamine tract in the protein huntingtin. Several other neurological diseases also result from the expansion of polyglutamine regions in different proteins. Despite intense efforts, no definitive biochemical or physiological role for huntingtin has been described, nor has a function been assigned to the polyglutamine region in unaffected individuals. This article presents the hypothesis that polyglutamine expansions within huntingtin and other polyglutamine proteins provide a function in and of themselves. Incorporating multiple glutamine residues into a protein during synthesis, and releasing them during protein turnover, may represent a means of minimizing interruptions in brain levels of glutamine and glutamate during periods of malnutrition. The number and variety of different proteins containing polyglutamine expansions can be interpreted as a series of evolutionary "experiments" toward a nontoxic form for glutamine storage.

Expression and Characterization of Full-length Human Huntingtin, an Elongated HEAT Repeat Protein

Huntington disease is an inherited neurodegenerative disorder that is caused by expanded CAG trinucleotide repeats, resulting in a polyglutamine stretch of >37 on the N terminus of the protein huntingtin (htt). htt is a large (347 kDa), ubiquitously expressed protein. The precise functions of htt are not clear, but its importance is underscored by the embryonic lethal phenotype in htt knock-out mice. Despite the fact that the htt gene was cloned 13 years ago, little is known about the properties of the full-length protein. Here we report the expression and preliminary characterization of recombinant full-length wild-type human htt. Our results support a model of htt composed entirely of HEAT repeats that stack to form an elongated superhelix.

Deletion of the triplet repeat encoding polyglutamine within the mouse Huntington's disease gene results in subtle behavioral/motor phenotypes in vivo and elevated levels of ATP with cellular senescence in vitro

Huntingtin (htt), the protein encoded by the Huntington's disease (HD) gene, contains a polymorphic stretch of glutamines (polyQ) near its N-terminus. When the polyQ stretch is expanded beyond 37Q, HD results. However, the role of the normal polyQ stretch in the function of htt is still unknown. To determine the contribution of the polyQ stretch to normal htt function, we have generated mice with a precise deletion of the short CAG triplet repeat encoding 7Q in the mouse HD gene (Hdh(DeltaQ)). Hdh(DeltaQ/DeltaQ) mice are born with normal Mendelian frequency and exhibit no gross phenotypic differences in comparison to control littermates, suggesting that the polyQ stretch is not essential for htt's functions during embryonic development. Adult mice, however, commit more errors initially in the Barnes circular maze learning and memory test and perform slightly better than wild-type controls in the accelerating rotarod test for motor coordination. To determine whether these phenotypes may reflect an altered cellular physiology in the Hdh(DeltaQ) mice, we characterized the growth and energy status of primary embryonic and adult Hdh(DeltaQ/DeltaQ) fibroblasts in culture. The Hdh(DeltaQ) fibroblasts exhibited elevated levels of ATP, but senesced prematurely in comparison with wild-type fibroblasts. Taken altogether, these results suggest that htt's polyQ stretch is required for modulating longevity in culture and support the hypothesis that the polyQ stretch may also modulate a htt function involved in regulating energy homeostasis.

Neural expression of the Huntington's disease gene as a chordate evolutionary novelty

Huntington's disease is a progressive neuro-degenerative disorder in humans, which is scharacterized by onset of dementia, muscular ataxia, and death. Huntington's disease is caused by the expansion of the polyglutamine (polyQ) tract in the N-terminus of the HD protein (Huntingtin). CAG expansion is a dominant gain of function mutation that affects striated neurons in the brain (Cattaneo, 2003, News Physiol Sci 18:34). The evolutionary origins of the vertebrate Hd gene are not well understood. In order to address the evolutionary history of the Hd gene, we have cloned and characterized the expression of the Hd gene in two invertebrate deuterostomes, an echinoderm and an ascidian, and have examined the expression patterns in a phylogenetic context. Echinoderms are basal deuterostomes and ascidians are basal chordates; both are useful for understanding the origins of and evolutionary trends in genes important in vertebrates such as the Huntigton's disease gene. Expression of Hd RNA is detected at all stages of development in both the echinoderm and ascidian studied. In the echinoderm Heliocidaris erythrogramma, Hd is expressed in coelomic mesodermal tissue derivatives, but not in the central nervous system. In the ascidian Halocynthia roretzi expression is located in both mesoderm and nervous tissue. We suggest that the primitive deuterostome expression pattern is not neural. Thus, neural expression of the Hd gene in deuterostomes may be a novel feature of the chordate lineage, and the original role(s) of HD in deuterostomes may have been non-neural.

The predominantly HEAT-like motif structure of huntingtin and its association and coincident nuclear entry with dorsal, an NF-kB/Rel/dorsal family transcription factor

BACKGROUND

Huntington's disease (HD) pathogenesis is due to an expanded polyglutamine tract in huntingtin, but the specificity of neuronal loss compared with other polyglutamine disorders also implies a role for the protein's unknown inherent function. Huntingtin is moderately conserved, with 10 HEAT repeats reported in its amino-terminal half. HD orthologues are evident in vertebrates and Drosophila, but not in Saccharomyces cerevisiae, Caenorhabditis elegans or Arabidopsis thaliana, a phylogenetic profile similar to the NF-kB/Rel/dorsal family transcription factors, suggesting a potential functional relationship.

RESULTS

We initially tested the potential for a relationship between huntingtin and dorsal by overexpression experiments in Drosophila S2 cells. Drosophila huntingtin complexes via its carboxyl-terminal region with dorsal, and the two enter the nucleus concomitantly, partly in a lipopolysaccharide (LPS)- and Nup88-dependent manner. Similarly, in HeLa cell extracts, human huntingtin co-immunoprecipitates with NF-kB p50 but not with p105. By cross-species comparative analysis, we find that the carboxyl-terminal segment of huntingtin that mediates the association with dorsal possesses numerous HEAT-like sequences related to those in the amino-terminal segment. Thus, Drosophila and vertebrate huntingtins are composed predominantly of 28 to 36 degenerate HEAT-like repeats that span the entire protein.

CONCLUSION

Like other HEAT-repeat filled proteins, huntingtin is made up largely of degenerate HEAT-like sequences, suggesting that it may play a scaffolding role in the formation of particular protein-protein complexes. While many proteins have been implicated in complexes with the amino-terminal region of huntingtin, the NF-kB/Rel/dorsal family transcription factors merit further examination as direct or indirect interactors with huntingtin's carboxyl-terminal segment.

Trinucleotide repeats: mechanisms and pathophysiology

Within the closing decade of the twentieth century, 14 neurological disorders were shown to result from the expansion of unstable trinucleotide repeats, establishing this once unique mutational mechanism as the basis of an expanding class of diseases. Trinucleotide repeat diseases can be categorized into two subclasses based on the location of the trinucleotide repeats: diseases involving noncoding repeats (untranslated sequences) and diseases involving repeats within coding sequences (exonic). The large body of knowledge accumulating in this fast moving field has provided exciting clues and inspired many unresolved questions about the pathogenesis of diseases caused by expanded trinucleotide repeats. This review summarizes the current understanding of the molecular pathology of each of these diseases, starting with a clinical picture followed by a focused description of the disease genes, the proteins involved, and the studies that have lent insight into their pathophysiology.

Huntington's disease: from gene to potential therapy

Huntington's disease (HD) is a progressive, late-onset neurodegenerative illness with autosomal dominant inheritance that affects one in 10 000 individuals in Western Europe. The disease is caused by a polyglutamine repeat expansion located in the N-terminal region of the huntingtin protein. The mutation is likely to act by a gain of function, but the molecular mechanisms by which it leads to neuronal dysfunction and cell death are not yet known. The normal function of huntingtin in cell metabolism is also unclear. There is no therapy for HD. Research on HD should help elucidate the pathogenetic mechanism of this illness in order to develop successful treatments to prevent or slow down symptoms. This article presents new results in HD research focusing on in vivo and in vitro model systems, potential molecular mechanisms of HD, and the development of therapeutic strategies.

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 dFMR1, a Drosophila melanogaster homolog of the fragile X mental retardation protein

Fragile X syndrome is the most common inherited form of mental retardation. It is caused by loss of FMR1 gene activity due to either lack of expression or expression of a mutant form of the protein. In mammals, FMR1 is a member of a small protein family that consists of FMR1, FXR1, and FXR2. All three members bind RNA and contain sequence motifs that are commonly found in RNA-binding proteins, including two KH domains and an RGG box. The FMR1/FXR proteins also contain a 60S ribosomal subunit interaction domain and a protein-protein interaction domain which mediates homomer and heteromer formation with each family member. Nevertheless, the specific molecular functions of FMR1/FXR proteins are unknown. Here we report the cloning and characterization of a Drosophila melanogaster homolog of the mammalian FMR1/FXR gene family. This first invertebrate homolog, termed dfmr1, has a high degree of amino acid sequence identity/similarity with the defined functional domains of the FMR1/FXR proteins. The dfmr1 product binds RNA and is similar in subcellular localization and embryonic expression pattern to the mammalian FMR1/FXR proteins. Overexpression of dfmr1 driven by the UAS-GAL4 system leads to apoptotic cell loss in all adult Drosophila tissues examined. This phenotype is dependent on the activity of the KH domains. The ability to induce a dominant phenotype by overexpressing dfmr1 opens the possibility of using genetic approaches in Drosophila to identify the pathways in which the FMR1/FXR proteins function.

Identification and characterization of the miniature pig Huntington's disease gene homolog: evidence for conservation and polymorphism in the CAG triplet repeat

Huntington's disease (HD) is associated with a significant expansion of a CAG trinucleotide repeat, which results in a lengthened polyglutamine tract in the single gene product, huntingtin, on human 4p16.3. We isolated cDNA clones that encompassed the entire coding sequence of the miniature pig HD gene (Sus HD) from two porcine testis cDNA libraries. The cDNA contig revealed a 12,749-nucleotide transcript coding for a 345-kDa protein (3139 amino acid residues), which exhibited 96% peptide sequence homology to human huntingtin. Northern blot analysis revealed that the Sus HD gene was ubiquitously expressed as two large transcripts of approximately 11 and 13 kb in size in all the tested tissues, much like the human HD gene. The CAG trinucleotide repeat was found to be interrupted by CAA triplets and to encode 17 or 18 consecutive glutamine residues. In our laboratory stock of miniature pig, three allotypes in the triplet repeat sequence were found. Thus, the Sus HD gene closely resembles its human counterpart in terms of sequence and expression pattern. In particular, human-miniature pig similarities in the normal length of the CAG triplet repeat as well as its repeat-number polymorphism may indicate that miniature pig would provide a good animal model for Huntington's disease.