The fly as a model for neurodegenerative diseases: is it worth the jump?

Role of non-coding RNAs in neurodegeneration and stress response in Drosophila

The inherent limitations of genetic analysis in humans and other mammals as well as striking conservation of most genes controlling nervous system functioning in flies and mammals made Drosophila an attractive model to investigate various aspects of brain diseases. Since RNA research has made great progress in recent years here we present an overview of studies demonstrating the role of various non-coding RNAs in neurodegeneration and stress response in Drosophila as a model organism. We put special emphasis on the role of non-coding micro RNAs, hsr-omega transcripts, and artificial small highly structured RNAs as triggers of neuropathology including aggregates formation, cognitive abnormalities and other symptoms. Cellular stress is a conspicuous feature of many neurodegenerative diseases and the production of specialized proteins protects the nerve cells against aggregates formation. Therefore, herein we describe some data implicating various classes of non-coding RNAs in stress response in Drosophila. All these findings highlight Drosophila as an important model system to investigate various brain diseases potentially mediated by some non-coding RNAs including polyglutamine diseases, Alzheimer's disease, Huntigton's disease, and many others.

Genome-wide RNAi screen and in vivo protein aggregation reporters identify degradation of damaged proteins as an essential hypertonic stress response

The damaging effects of hypertonic stress on cellular proteins are poorly defined, and almost nothing is known about the pathways that detect and repair hypertonicity-induced protein damage. To begin addressing these problems, we screened approximately 19,000 Caenorhabditis elegans genes by RNA interference (RNAi) feeding and identified 40 that are essential for survival during acute hypertonic stress. Half (20 of 40) of these genes encode proteins that function to detect, transport, and degrade damaged proteins, including components of the ubiquitin-proteasome system, endosomal sorting complexes, and lysosomes. High-molecular-weight ubiquitin conjugates increase during hypertonic stress, suggesting a global change in the ubiquitinylation state of endogenous proteins. Using a polyglutamine-containing fluorescent reporter, we demonstrate that cell shrinkage induces rapid protein aggregation in vivo and that many of the genes that are essential for survival during hypertonic stress function to prevent accumulation of aggregated proteins. High levels of urea, a strong protein denaturant, do not cause aggregation, suggesting that factors such as macromolecular crowding also contribute to protein aggregate formation during cell shrinkage. Acclimation of C. elegans to mild hypertonicity dramatically increases the osmotic threshold for protein aggregation, demonstrating that protein aggregation-inhibiting pathways are activated by osmotic stress. Our studies demonstrate that hypertonic stress induces protein damage in vivo and that detection and degradation of damaged proteins are essential mechanisms for survival under hypertonic conditions.

Suppression of neurodegeneration and increased neurotransmission caused by expanded full-length huntingtin accumulating in the cytoplasm

Huntington's disease (HD) is a dominantly inherited neurodegenerative disorder caused by expansion of a translated CAG repeat in the N terminus of the huntingtin (htt) protein. Here we describe the generation and characterization of a full-length HD Drosophila model to reveal a previously unknown disease mechanism that occurs early in the course of pathogenesis, before expanded htt is imported into the nucleus in detectable amounts. We find that expanded full-length htt (128Qhtt(FL)) leads to behavioral, neurodegenerative, and electrophysiological phenotypes. These phenotypes are caused by a Ca2+-dependent increase in neurotransmitter release efficiency in 128Qhtt(FL) animals. Partial loss of function in synaptic transmission (syntaxin, Snap, Rop) and voltage-gated Ca2+ channel genes suppresses both the electrophysiological and the neurodegenerative phenotypes. Thus, our data indicate that increased neurotransmission is at the root of neuronal degeneration caused by expanded full-length htt during early stages of pathogenesis.

A Drosophila model for LRRK2-linked parkinsonism

Mutations in the leucine-rich repeat kinase (LRRK2) gene cause late-onset autosomal dominant Parkinson's disease (PD) with pleiomorphic pathology. Previously, we and others found that expression of mutant LRRK2 causes neuronal degeneration in cell culture. Here we used the GAL4/UAS system to generate transgenic Drosophila expressing either wild-type human LRRK2 or LRRK2-G2019S, the most common mutation associated with PD. Expression of either wild-type human LRRK2 or LRRK2-G2019S in the photoreceptor cells caused retinal degeneration. Expression of LRRK2 or LRRK2-G2019S in neurons produced adult-onset selective loss of dopaminergic neurons, locomotor dysfunction, and early mortality. Expression of mutant G2019S-LRRK2 caused a more severe parkinsonism-like phenotype than expression of equivalent levels of wild-type LRRK2. Treatment with l-DOPA improved mutant LRRK2-induced locomotor impairment but did not prevent the loss of tyrosine hydroxylase-positive neurons. To our knowledge, this is the first in vivo"gain-of-function" model which recapitulates several key features of LRRK2-linked human parkinsonism. These flies may provide a useful model for studying LRRK2-linked pathogenesis and for future therapeutic screens for PD intervention.

Identification of novel genes that modify phenotypes induced by Alzheimer's beta-amyloid overexpression in Drosophila

Sustained increases in life expectancy have underscored the importance of managing diseases with a high incidence in late life, such as various neurodegenerative conditions. Alzheimer's disease (AD) is the most common among these, and consequently significant research effort is spent on studying it. Although a lot is known about the pathology of AD and the role of beta-amyloid (Abeta) peptides, the complete network of interactions regulating Abeta metabolism and toxicity still eludes us. To address this, we have conducted genetic interaction screens using transgenic Drosophila expressing Abeta and we have identified mutations that affect Abeta metabolism and toxicity. These analyses highlight the involvement of various biochemical processes such as secretion, cholesterol homeostasis, and regulation of chromatin structure and function, among others, in mediating toxic Abeta effects. Several of the mutations that we identified have not been linked to Abeta toxicity before and thus constitute novel potential targets for AD intervention. We additionally tested these mutations for interactions with tau and expanded-polyglutamine overexpression and found a few candidate mutations that may mediate common mechanisms of neurodegeneration. Our data offer insight into the toxicity of Abeta and open new areas for further study into AD pathogenesis.

Separating cause from effect: how does insulin/IGF signalling control lifespan in worms, flies and mice?

Ageing research has been revolutionized by the use of model organisms to discover genetic alterations that can extend lifespan. In the last 5 years alone, it has become apparent that single gene mutations in the insulin and insulin-like growth-factor signalling pathways can lengthen lifespan in worms, flies and mice, implying evolutionary conservation of mechanisms. Importantly, this research has also shown that these mutations can keep the animals healthy and disease-free for longer and can alleviate specific ageing-related pathologies. These findings are striking in view of the negative effects that disruption of these signalling pathways can also produce. Here, we summarize the body of work that has lead to these discoveries and point out areas of interest for future work in characterizing the genetic, molecular and biochemical details of the mechanisms to achieving a longer and healthier life.