The mTOR kinase inhibitor Everolimus decreases S6 kinase phosphorylation but fails to reduce mutant huntingtin levels in brain and is not neuroprotective in the R6/2 mouse model of Huntington's disease

Abstract

BackgroundHuntington's disease (HD) is a progressive neurodegenerative disorder caused by a CAG repeat expansion within the huntingtin gene. Mutant huntingtin protein misfolds and accumulates within neurons where it mediates its toxic effects. Promoting mutant huntingtin clearance by activating macroautophagy is one approach for treating Huntington's disease (HD). In this study, we evaluated the mTOR kinase inhibitor and macroautophagy promoting drug everolimus in the R6/2 mouse model of HD.ResultsEverolimus decreased phosphorylation of the mTOR target protein S6 kinase indicating brain penetration. However, everolimus did not activate brain macroautophagy as measured by LC3B Western blot analysis. Everolimus protected against early declines in motor performance; however, we found no evidence for neuroprotection as determined by brain pathology. In muscle but not brain, everolimus significantly decreased soluble mutant huntingtin levels.ConclusionsOur data suggests that beneficial behavioral effects of everolimus in R6/2 mice result primarily from effects on muscle. Even though everolimus significantly modulated its target brain S6 kinase, this did not decrease mutant huntingtin levels or provide neuroprotection.

A small-molecule scaffold induces autophagy in primary neurons and protects against toxicity in a Huntington disease model

“…S1). Our results are consistent with those observed by others: If rapamycin induces autophagy in neurons, it does so weakly (8,17). Indeed, it was suggested that rapamycin inhibits polyglutamine protein aggregation independent of autophagy (19).…”

“…Indeed, inhibition of mTOR with rapamycin and its analogue, everolimus, strongly induced autophagy in a nonneuronal cell line but very weakly, if at all, in neurons. A similar result was observed in a mouse model of HD (8). Because autophagy induction and the resulting turnover of misfolded toxic proteins have been proposed as a neuroprotective strategy for many neurodegenerative diseases, our quest for strong autophagy inducers in neurons has therapeutical implications.…”

“…In cell line and fly models, rapamycin slows accumulation of misfolded proteins implicated in several neurodegenerative diseases (1). In a mouse model of HD, everolimus, a rapamycin analogue, induced autophagy in muscle but not in brain (8). Compounds that decrease inositol levels also induce autophagy (5,9).…”

Rapamycin induces autophagic flux in neurons

“…As two independent laboratories, we have both shown increases in autophagic flux in primary neurons treated with rapamycin using a range of assays for autophagosome formation (2) as well as assays for autophagosome maturation (2, 3). The second line of evidence presented by Tsvetkov et al (1) asserted that a previous paper found that rapamycin did not induce autophagy in the mouse brain (4). This study, which showed modest protective effects of a rapamycin analog in a mouse model of Huntington's disease, analyzed brain LC3-II levels without lysosomal inhibitors and found nonsignificant increases.…”

It may be possible to delay the onset of neurodegenerative diseases with an immunosuppressive drug (rapamycin)

“…However, the effects of rapamycin on autophagy are less prominent than those on peripheral tissues and non-neuronal cells. Everolimus, a rapamycin analogue, stimulates autophagy in muscle but not in brain and does not reduce mutant huntingtin levels in the R6/2 mouse model of HD [11]. Another paper reported that mTOR-dependent but everolimus-independent mechanisms regulate autophagy and huntingtin-aggregate degradation in cells of neuronal origin [12].…”