Rapamycin was isolated from bacteria found on Easter Island (Rapa Nui, in local parlance), and its potent immunosuppressive qualities have proven successful in preventing rejection of organ transplants. There are also hopes that rapamycin's inhibitory effects on cell growth could make it a useful cancer drug.

In previous in-vitro studies, authors David Rubinsztein, Brinda Ravikumar, and colleagues at Cambridge University in England have found evidence that cells could be protected from the effects of mutant htt (as well as polyalanine peptides) by upregulating autophagy, the process of sequestering defective proteins inside autophagic vacuoles, or autophagosomes ( Ravikumar et al., 2002; 2003). These researchers have also shown that α-synuclein is partly degraded by autophagosomes, raising the possibility that boosting this function could help treat Parkinson's disease (Webb et al., 2003).

Autophagy is partly controlled by the mammalian target of rapamycin (mTOR), a member of the phosphoinositide kinase-related kinase family. Also going by the names RAFT (rapamycin and FKBP target) or FRAP (FKBP-12-rapamycin associated protein), mTOR plays a critical role during embryogenesis, and has been implicated in insulin pathways. Some researchers have found that it is phosphorylated and activated by Akt/PKB, though this is not universally accepted. In turn, mTOR phosphorylates the transcriptional regulators 4E-BP1 (aka PHAS-1) and S6K1 (aka p70S6K).

It is probably through this latter pathway that mTOR helps to regulate autophagy, according to Rubinsztein's group. Inhibiting mTOR leads to increased autophagy, and in the present study, led by joint first authors Ravikumar and Coralie Vacher, the group proposes an explanation for the previously noted upregulation of autophagosomes in HD brain.

The researchers found that mTOR is sequestered in huntingtin aggregates from human HD tissue, as well as from mice and cells transfected with mutant huntingtin. In the latter, the mTOR sequestration was accompanied by lower soluble mTOR levels, reduced mTOR phosphorylation of S6K1 and BP1, and, in turn, reduced S6K1-driven translation. To complete the story, the presence of mutant htt also generated a greater number of autophagosomes. Thus, the sequestration of mTOR by htt aggregates could boost autophagosome numbers by limiting the available mTOR, suggest the authors. Of potential interest for other polyQ disorders, mTOR was also found sequestered with aggregates from several of the less common protein aggregating disorders, and S6K1 phosphorylation was reduced in several of these.

In previous in-vitro experiments, the researchers had found that they could reduce both the formation of htt aggregates and cell death with rapamycin. This was only possible early on, before aggregation. Ravikumar and colleagues now report that when given before aggregation begins, rapamycin retards neurodegeneration in a fly model of HD and reduces aggregate formation and behavioral deficits in a mouse model of HD.

The mouse experiment was done using an ester of rapamycin that is being evaluated in clinical trials for cancer, and which may have favorable pharmacologic and side effect profiles. Mice received continuous treatment (three times weekly, 20 mg/kg) from the age of four weeks. The rapamycin-treated mice had less tremor and better scores on grip strength, rotorod, and wire maneuver tests than placebo-treated littermates when tested at four time points between 16 and 22 weeks. The researchers also report that striatal htt aggregation was reduced in the rapamycin-treated mice.

One caveat noted by the authors is that the rapamycin-treated mice did not live longer. They gained less weight than control mice, which might affect survival; this side effect is not seen in humans. On balance, should rapamycin or analogues be used in Huntington's disease? The drug’s very potency could be a problem, leading to unacceptable side effects. In addition to immunosuppression and cell toxicity, rapamycin elevates both serum cholesterol and triglyceride levels. The authors also note that the drug may not be effective once htt aggregates have formed. Symptoms typically appear around age 40, and the disease is progressive. If the drug effectively prevents mutant htt-related aggregation and neurodegeneration in young adulthood, the authors argue, "… the benefits of delaying onset of Huntington's disease would outweigh the side effects of these drugs."—Hakon Heimer.

Reference:
Ravikumar B, Vacher C, Berger Z, Davies JE, Luo S, Oroz LG, Scaravilli F, Easton DF, Duden R, O'Kane CJ, Rubinsztein DC. Inhibition of mTOR induces autophagy and reduces toxicity of polyglutamine expansions in fly and mouse models of Huntington disease. Nat Genet. 2004 May 16 [Epub ahead of print] Abstract