A double agent in Huntington's disease: Rad23b increases the degradation tag on toxic huntingtin while impairing the proteasome meant to destroy it
A double agent in Huntington's disease: Rad23b increases the degradation tag on toxic huntingtin while impairing the proteasome meant to destroy it
In a paper published on September 24, Taiwanese biologists showed that a short regulatory RNA, miR-196a, lowers Rad23b levels. In cell experiments this reduced aggregates of mutant huntingtin, the protein that clumps inside neurons in this disease; in mice, extra Rad23b had the opposite effect and increased them.
Huntington's disease arises when a DNA repeat in the HTT gene expands. The resulting huntingtin protein carries an abnormally long stretch of a single amino acid, glutamine, and forms aggregates that disrupt neuronal function. Normally the cell tags an unwanted protein with a small label, ubiquitin, and delivers it to the proteasome, a complex that cuts proteins into fragments.
miR-196a had already been linked to a reduction in such aggregates, but the gene it acts on remained unclear: HTT itself has no binding site for this microRNA. The authors compared proteins in nerve cells with and without miR-196a: out of 2681 proteins detected, they narrowed the list to six candidates after filtering and then confirmed two. For Rad23b the direct link was verified separately: mutating the binding site in its mRNA abolished the suppressive effect of miR-196a.
Rad23b is a shuttle protein that ferries ubiquitin-tagged proteins to the proteasome. In the experiments it bound mutant huntingtin and increased the amount of ubiquitin on it. Yet in the same cell experiments Rad23b reduced the chymotrypsin-like activity of the proteasome, one of its proteolytic reactions. As a result, huntingtin accumulated ever more ubiquitin tags while its aggregates continued to grow. When Rad23b was knocked out or suppressed, aggregates decreased; a proteasome inhibitor restored them, but an autophagy inhibitor (autophagy being the cell's alternative degradation pathway) did not.
This chain of events was also tested in mice carrying a Huntington's disease model. Additional Rad23b worsened their performance in motor tests; their brains contained more mutant huntingtin aggregates and more cells showing signs of death. In an independent dataset, Rad23b mRNA levels were likewise higher in laboratory neurons derived from people with manifest symptoms than in carriers of the same mutation whose symptoms had not yet appeared.
The authors suggest that the protein's role depends on the type of aggregate. They cite earlier work on diseases linked to the C9ORF72 gene, where Rad23b becomes trapped in different protein aggregates and restoring its levels reduced them. For huntingtin aggregates the authors propose a different model: Rad23b remains bound to its cargo while simultaneously weakening proteasome function. In this way, the ubiquitin tag on a protein no longer guarantees that the cell will actually dispose of it.
The disease was first described by the American physician George Huntington in 1872, based on his observations of three generations of a single family. More than 150 years later, there is still no drug that slows the disease itself.
In July, Roche terminated its trial of tominersen: the drug lowered mutant huntingtin levels in the cerebrospinal fluid, but after 16 months the patients' ability to move, think, and live independently was no different from the placebo group. The new finding offers a clue as to why reducing the amount of protein alone may not suffice: even huntingtin that is heavily tagged with ubiquitin will remain in the cell if the proteasome itself is being impaired by the very protein that attaches those tags.