Small N-terminal mutant huntingtin fragments, but not wild type, are mainly present in monomeric form: Implications for pathogenesis

Formation and toxicity of soluble polyglutamine oligomers in living cells

BACKGROUND

Aggregation and cytotoxicity of mutant proteins containing an expanded number of polyglutamine (polyQ) repeats is a hallmark of several diseases, including Huntington's disease (HD). Within cells, mutant Huntingtin (mHtt) and other polyglutamine expansion mutant proteins exist as monomers, soluble oligomers, and insoluble inclusion bodies (IBs). Determining which of these forms constitute a toxic species has proven difficult. Recent studies support a role for IBs as a cellular coping mechanism to sequester levels of potentially toxic soluble monomeric and oligomeric species of mHtt.

METHODOLOGY/PRINCIPAL FINDINGS

When fused to a fluorescent reporter (GFP) and expressed in cells, the soluble monomeric and oligomeric polyglutamine species are visually indistinguishable. Here, we describe two complementary biophysical fluorescence microscopy techniques to directly detect soluble polyglutamine oligomers (using Htt exon 1 or Htt(ex1)) and monitor their fates in live cells. Photobleaching analyses revealed a significant reduction in the mobilities of mHtt(ex1) variants consistent with their incorporation into soluble microcomplexes. Similarly, when fused to split-GFP constructs, both wildtype and mHtt(ex1) formed oligomers, as evidenced by the formation of a fluorescent reporter. Only the mHtt(ex1) split-GFP oligomers assembled into IBs. Both FRAP and split-GFP approaches confirmed the ability of mHtt(ex1) to bind and incorporate wildtype Htt into soluble oligomers. We exploited the irreversible binding of split-GFP fragments to forcibly increase levels of soluble oligomeric mHtt(ex1). A corresponding increase in the rate of IBs formation and the number formed was observed. Importantly, higher levels of soluble mHtt(ex1) oligomers significantly correlated with increased mutant cytotoxicity, independent of the presence of IBs.

CONCLUSIONS/SIGNIFICANCE

Our study describes powerful and sensitive tools for investigating soluble oligomeric forms of expanded polyglutamine proteins, and their impact on cell viability. Moreover, these methods should be applicable for the detection of soluble oligomers of a wide variety of aggregation prone proteins.

Preventing Ataxin-3 protein cleavage mitigates degeneration in a Drosophila model of SCA3

Protein cleavage is a common feature in human neurodegenerative disease. Ataxin-3 protein with an expanded polyglutamine (polyQ) repeat causes spinocerebellar ataxia type-3 (SCA3), also called Machado–Joseph disease, and is cleaved in mammalian cells, transgenic mice and SCA3 patient brain tissue. However, the pathological significance of Ataxin-3 cleavage has not been carefully examined. To gain insight into the significance of Ataxin-3 cleavage, we developed a Drosophila SL2 cell-based model as well as transgenic fly models. Our data indicate that Ataxin-3 protein cleavage is conserved in the fly and may be caspase-dependent as reported previously. Importantly, comparison of flies expressing either wild-type or caspase-site mutant proteins indicates that Ataxin-3 cleavage enhances neuronal loss in vivo. This genetic in vivo confirmation of the pathological role of Ataxin-3 cleavage indicates that therapies targeting Ataxin-3 cleavage might slow disease progression in SCA3 patients.

Conformational targeting of fibrillar polyglutamine proteins in live cells escalates aggregation and cytotoxicity

BACKGROUND

Misfolding- and aggregation-prone proteins underlying Parkinson's, Huntington's and Machado-Joseph diseases, namely alpha-synuclein, huntingtin, and ataxin-3 respectively, adopt numerous intracellular conformations during pathogenesis, including globular intermediates and insoluble amyloid-like fibrils. Such conformational diversity has complicated research into amyloid-associated intracellular dysfunction and neurodegeneration. To this end, recombinant single-chain Fv antibodies (scFvs) are compelling molecular tools that can be selected against specific protein conformations, and expressed inside cells as intrabodies, for investigative and therapeutic purposes.

METHODOLOGY/PRINCIPAL FINDINGS

Using atomic force microscopy (AFM) and live-cell fluorescence microscopy, we report that a human scFv selected against the fibrillar form of alpha-synuclein targets isomorphic conformations of misfolded polyglutamine proteins. When expressed in the cytoplasm of striatal cells, this conformation-specific intrabody co-localizes with intracellular aggregates of misfolded ataxin-3 and a pathological fragment of huntingtin, and enhances the aggregation propensity of both disease-linked polyglutamine proteins. Using this intrabody as a tool for modulating the kinetics of amyloidogenesis, we show that escalating aggregate formation of a pathologic huntingtin fragment is not cytoprotective in striatal cells, but rather heightens oxidative stress and cell death as detected by flow cytometry. Instead, cellular protection is achieved by suppressing aggregation using a previously described intrabody that binds to the amyloidogenic N-terminus of huntingtin. Analogous cytotoxic results are observed following conformational targeting of normal or polyglutamine-expanded human ataxin-3, which partially aggregate through non-polyglutamine domains.

CONCLUSIONS/SIGNIFICANCE

These findings validate that the rate of aggregation modulates polyglutamine-mediated intracellular dysfunction, and caution that molecules designed to specifically hasten aggregation may be detrimental as therapies for polyglutamine disorders. Moreover, our findings introduce a novel antibody-based tool that, as a consequence of its general specificity for fibrillar conformations and its ability to function intracellularly, offers broad research potential for a variety of human amyloid diseases.

The potential role of ribosomal frameshifting in generating aberrant proteins implicated in neurodegenerative diseases

Aberrant forms of proteins ubiquitin B and beta-amyloid precusor protein, UBB+1 and APP+1, are implicated in human neurodegenerative diseases. They have their carboxyl-terminal regions derived from an alternative reading frame. Transcription slippage has been invoked to explain the production of these proteins from abnormal mRNA. However, ribosomal frameshifting on wild-type mRNA may account for the great majority of the aberrant protein. Ribosomal frameshifting may also be involved in the progression of triplet expansion diseases such as Huntington's and spinocerebellar ataxias. In a particular spinocerebellar ataxia, SCA3, Toulouse and colleagues recently discovered -1 frameshifting in a transcript containing an expanded CAG-repeat. Antibiotics that affect mammalian ribosomes may have complex effects on frameshifting and disease progression.