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FEATURED ARTICLES With friends like these... Neuroscience Gateway (May 2007) | doi:10.1038/aba1747

Using a proteomics approach, researchers identify 234 proteins that bind to the protein associated with Huntington's disease and may modify disease progression.

Powerful people involved in scandals often take their associates down with them when they fall from grace. Similarly, some mutant proteins involved in neurodegeneration bind and sequester proteins important in cellular function in inclusion bodies that clog cellular functions. Huntington's disease is a fatal neurodegenerative disorder caused by the polyglutamine expansion of the huntingtin protein, which forms potentially toxic cellular aggregates. Kaltenbach et al. identify over 200 proteins, including several potential therapeutic targets, that bind to normal and polyglutamine-expanded huntingtin in a high-throughput proteomics screen reported in PLoS Genetics.

Normally, the amino terminus of huntingtin contains several repeats of the amino acid glutamine. In people with Huntington's disease, the huntingtin polyglutamine tract contains more than 35 glutamines. Symptoms of Huntington's disease, including involuntary movements (chorea), muscle rigidity and cognitive dysfuntion, begin during middle age. There is no cure, and most patients die within 15 years of symptom onset.

The authors screened for proteins in mouse and human brain that interact with normal and polyglutamine-expanded huntingtin by affinity purification followed by mass spectrometry and yeast two- hybrid. They used 'baits' from various regions of huntingtin protein. Regardless of the number of glutamine repeats, only baits from the amino terminus of huntingtin (amino acids 1 to 90) formed complexes with other proteins. After stringent filtering, affinity purification identified 130 and yeast two-hybrid identified 104 mouse and human proteins that bound to normal or polyglutamine-expanded huntingtin.

The two proteomics techniques identified different sets of huntingtin-interacting proteins. Affinity purification pulled out more proteins involved in metabolism, whereas yeast two-hybrid screening pulled out more proteins involved in transcription, cell signaling and protein turnover. Only 4 proteins were common to both lists: glyceraldehyde-3-phosphate dehydrogenase (GAPDH), clathrin, pyruvate kinase and tyrosine 3- monooxygenase/tryptophan 5-monooxygenase activation protein, beta (YWHAB).

Do the identified proteins affect neurodegeneration? Polyglutamine-expanded huntingtin induces degeneration of Drosophila photoreceptors. The authors mutated or overexpressed genes encoding 60 of the 234 huntingtin- interacting proteins in Drosophila expressing polyglutamine- expanded huntingtin. Altered expression of 27 of the proteins (45%) modified photoreceptor degeneration. Loss of function of 17 proteins reduced neurodegeneration, suggesting that drugs that inhibit these proteins might prolong survival for people with Huntington's disease.

SNARE proteins are important in the fusion of neurotransmitter- fillled synaptic vesicles to neuron membranes. Wild-type and mutant huntingtin associated with several SNARE proteins, including syntaxin 1a (STX1a), N-ethylmaleimide sensitive fusion protein attachment protein alpha (NAPA), the delta subunit of the voltage-gated calcium channel (CACNA2D1) and synaptosomal- associated protein 25 (SNAP25). For STX1a, NAPA and CACNA2D1, partial knockdown decreased and overexpression increased neurodegeneration induced by polyglutamine-expanded huntingtin. Reduced expression of STX1a increased lifespan and improved neuromuscular function for Drosophila expressing polyglutamine-expanded huntingtin.

The SNARE proteins did not show a preference for wild-type or polyglutamine-expanded huntingtin, suggesting that huntingtin normally interacts with SNARE proteins, and the polyglutamine expansion of huntingtin either disturbs the interaction or disturbs SNARE protein function. Perhaps other polyglutamine expansion disorders, including spinocerebellar ataxia and Kennedy's disease, also prevent the normal function of synaptic proteins.

Debra Speert,

1. L. S.Kaltenbachet al.Huntingtin interacting proteins are genetic modifiers of neurodegeneration.PLoS Genetics.3,e82–(2007)