Impaired retrograde transport by the Dynein/Dynactin complex contributes to Tau-induced toxicity.

The gene mapt codes for the microtubule-associated protein Tau. The R406W amino acid substitution in Tau is associated with frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) characterized by Tau-positive filamentous inclusions. These filamentous Tau inclusions are present in a group of neurodegenerative diseases known as tauopathies, including Alzheimer's disease (AD). To gain more insights into the pathomechanism of tauopathies, we performed an RNAi-based large-scale screen in Drosophila melanogaster to identify genetic modifiers of Tau[R406W]-induced toxicity. A collection of RNAi lines, putatively silencing more than 7000 genes, was screened for the ability to modify Tau[R406W]-induced toxicity in vivo. This collection covered more than 50% of all protein coding fly genes and more than 90% of all fly genes known to have a human ortholog. Hereby, we identified 62 genes that, when silenced by RNAi, modified Tau-induced toxicity specifically. Among these 62 modifiers were three subunits of the Dynein/Dynactin complex. Analysis on segmental nerves of fly larvae showed that pan neural Tau[R406W] expression and concomitant silencing of Dynein/Dynactin complex members synergistically caused strong pathological changes within the axonal compartment, but only minor changes at synapses. At the larval stage, these alterations did not cause locomotion deficits, but became evident in adult flies. Our data suggest that Tau-induced detrimental effects most likely originate from axonal rather than synaptic dysfunction and that impaired retrograde transport intensifies detrimental effects of Tau in axons. In conclusion, our findings contribute to the elucidation of disease mechanisms in tauopathies like FTDP-17 or AD.

CDK5 protects from caspase-induced Ataxin-3 cleavage and neurodegeneration.

Spinocerebellar ataxia type 3 (SCA3) is one of at least nine inherited neurodegenerative diseases caused by an expansion of a polyglutamine tract within corresponding disease-specific proteins. In case of SCA3, mutation of Ataxin-3 results in aggregation of misfolded protein, formation of intranuclear as well as cytosolic inclusion bodies and cell death in distinct neuronal populations. Since cyclin-dependent kinase-5 (CDK5) has been shown to exert beneficial effects on aggregate formation and cell death in various polyglutamine diseases, we tested its therapeutic potential for SCA3. Our data show increased caspase-dependent Ataxin-3 cleavage, aggregation, and neurodegeneration in the absence of sufficient CDK5 activity. This disease-propagating effect could be reversed by mutation of the caspase cleavage site in Ataxin-3. Moreover, reduction of CDK5 expression levels by RNAi in vivo enhances SCA3 toxicity as assayed in a Drosophila model for SCA3. In summary, we present CDK5 as a potent neuroprotectant, regulating cleavage and thereby toxicity of Ataxin-3 and other polyglutamine proteins. We propose that increased caspase-dependent cleavage of mutated Ataxin-3, because of missing CDK5 shielding, leads to aggregation and cell death. Moreover, reduction of CDK5 expression levels by RNAi in vivo enhances SCA3 toxicity as assayed in a Drosophila model for SCA3. We think that CDK5 functions as a shield against cleavage-induced toxification and thereby is an interesting target for therapeutic intervention in polyQ disease in general.

Quantitative interaction proteomics of neurodegenerative disease proteins.

Several proteins have been linked to neurodegenerative disorders (NDDs), but their molecular function is not completely understood. Here, we used quantitative interaction proteomics to identify binding partners of Amyloid beta precursor protein (APP) and Presenilin-1 (PSEN1) for Alzheimer's disease (AD), Huntingtin (HTT) for Huntington's disease, Parkin (PARK2) for Parkinson's disease, and Ataxin-1 (ATXN1) for spinocerebellar ataxia type 1. Our network reveals common signatures of protein degradation and misfolding and recapitulates known biology. Toxicity modifier screens and comparison to genome-wide association studies show that interaction partners are significantly linked to disease phenotypes in vivo. Direct comparison of wild-type proteins and disease-associated variants identified binders involved in pathogenesis, highlighting the value of differential interactome mapping. Finally, we show that the mitochondrial protein LRPPRC interacts preferentially with an early-onset AD variant of APP. This interaction appears to induce mitochondrial dysfunction, which is an early phenotype of AD.

Large-scale screen for modifiers of ataxin-3-derived polyglutamine-induced toxicity in Drosophila.

Polyglutamine (polyQ) diseases represent a neuropathologically heterogeneous group of disorders. The common theme of these disorders is an elongated polyQ tract in otherwise unrelated proteins. So far, only symptomatic treatment can be applied to patients suffering from polyQ diseases. Despite extensive research, the molecular mechanisms underlying polyQ-induced toxicity are largely unknown. To gain insight into polyQ pathology, we performed a large-scale RNAi screen in Drosophila to identify modifiers of toxicity induced by expression of truncated Ataxin-3 containing a disease-causing polyQ expansion. We identified various unknown modifiers of polyQ toxicity. Large-scale analysis indicated a dissociation of polyQ aggregation and toxicity.