Cytoplasmic aggregates trap polyglutamine-containing proteins and block axonal transport in a Drosophila model of Huntington's disease

Abstract

Huntington's disease is an autosomal dominant neurodegenerative disorder caused by expansion of a polyglutamine tract in the huntingtin protein that results in intracellular aggregate formation and neurodegeneration. Pathways leading from polyglutamine tract expansion to disease pathogenesis remain obscure. To elucidate how polyglutamine expansion causes neuronal dysfunction, we generated Drosophila transgenic strains expressing human huntingtin cDNAs encoding pathogenic (Htt-Q128) or nonpathogenic proteins (Htt-Q0). Whereas expression of Htt-Q0 has no discernible effect on behavior, lifespan, or neuronal morphology, pan-neuronal expression of Htt-Q128 leads to progressive loss of motor coordination, decreased lifespan, and time-dependent formation of huntingtin aggregates specifically in the cytoplasm and neurites. Huntingtin aggregates sequester other expanded polyglutamine proteins in the cytoplasm and lead to disruption of axonal transport and accumulation of aggregates at synapses. In contrast, Drosophila expressing an expanded polyglutamine tract alone, or an expanded polyglutamine tract in the context of the spinocerebellar ataxia type 3 protein, display only nuclear aggregates and do not disrupt axonal trafficking. Our findings indicate that nonnuclear events induced by cytoplasmic huntingtin aggregation play a central role in the progressive neurodegeneration observed in Huntington's disease.

Modeling Huntington disease inDrosophila

“…The polyQ proteins also localize to distinct areas of the neuron. 10 These differences are likely accounted for by protein context or differential protein interactions. Additional evidence for the importance of protein context comes from studies showing that expression of a polyQ stretch alone can be more toxic than a polyQ stretch embedded in a diseasecausing protein.…”

“…A previous HD model in the lab suggested axonal transport defects were likely to be a contributing factor to pathology. 10 This model contained the first 548 amino acids of Htt, with a polyQ stretch of 128 repeats, representing the pathogenic form of the protein, or Q0, a non-pathogenic form (Fig. 1A).…”

Transcellular spreading of huntingtin aggregates in the Drosophila brain

“…To gain a better understanding of how this protein spreading contributes to disease pathology, we sought to study this phenomenon in Drosophila. Drosophila has been used to create useful models of many neurodegenerative diseases, including Parkinson's disease (12) and the polyglutamine (polyQ) expansion diseases spinocerebellar ataxia type 1 (13) and type 3 (14) as well as Huntington's disease (15)(16)(17)(18). These models have proven to reproduce many of the key structural and functional deficits associated with disease pathology and provide insight into the underlying mechanisms.…”

Morphometric Analysis of Huntington’s Disease Neurodegeneration in Drosophila

“…There are several elav -GAL4 drivers. The X chromosome elav driver is the strongest and is an enhancer trap insert into the endogenous elav locus while the other elav drivers (e.g., on chr II) are fusions of the elav promoter and GAL4, and are weaker [ 39 ] . The GMR driver is often used to express Htt and other degenerative genes in the eyes [ 9,19,40 ] but some considerations should be noted.…”

“…To understand degenerative mechanisms and discover methods of suppression, this dominant gain-of-function disease has been modeled by expressing mutant human huntingtin ( mHtt ) in Drosophila melanogaster . Multiple transgenic Drosophila models of HD disease have been generated [6][7][8][9][10][11][12][13][14][15] . Fly models recapitulate the core phenotypes observed in HD patients, including late onset, progressive cellular pathology as a function of polyQ repeat length, motor dysfunction, protein inclusions, transcriptional dysregulation, mitochondrial dysfunction, and shortened adult lifespan; hence, they provide powerful genetic systems for dissecting the neuronal degeneration induced by glutamine repeat-containing proteins.…”