In the July 10 Cell, Frederic Saudou, Sandrine Humbert, and colleagues from the CNRS in Orsay, France, report that huntingtin (htt) enhances transport of brain-derived neurotrophic factor (BDNF) along microtubules. The results offer a plausible explanation as to why striatal neurons, which depend on cortex-derived BDNF for survival, are so specifically and effectively destroyed by Huntington’s disease.
PolyQ expansions in huntingtin are responsible for this lethal neurodegenerative disorder, which slowly kills striatal neurons. Exactly how polyQ-huntingtin wreaks havoc has been debated since the mutation was discovered. Some theories hinge on the fact that the mutant protein forms aggregates. These may be damaging in and of themselves, or they may sequester other essential proteins and thus prevent them from doing their job (see ARF related news story). Other theories have linked polyQ-huntingtin to regulation of BDNF activity (see ARF related news story). More recently, however, it has been suggested that polyQ-huntingtin blocks axonal transport (see ARF related news story). Now, first author Laurent Gauthier and colleagues seem to have linked the last two observations.
Gauthier et al. made the connection thanks to extremely fast 3D videomicroscopy. By labeling BDNF with a variant of green fluorescent protein (eGFP), they were able to measure its movement in vesicles. When Gauthier examined neural cell lines isolated from knock-in mice that expressed either wild-type or polyQ-huntingtin, they found no obvious difference in the localization of BDNF. The speed of BDNF transport, however, was another matter. The number of stationary vesicles was reduced by almost half when wild-type htt was used, mutant htt, however, had no such effect. In addition, the videomicroscopy showed that the vesicles were moving faster in the presence of wild-type htt. To prove that this vesicular fast track was indeed due to huntingtin, Gauthier knocked down its expression with siRNA. This put the brakes on the vesicles, restoring them to their original speed. Web videos documenting this effect are part of the supplemental material on Cell’s journal website.
The slowing, but not stopping, of the vesicles by mutant huntingtin fits with how the disease progresses, as it suggests that neurons succumb gradually, over time. The finding that cells heterozygous for polyQ-htt are as badly affected as homozygous cells indicates a dominant-negative effect, which also fits with Huntington's genetics.
But how exactly does htt affect BDNF transport? To address this question, Gauthier incubated cells with nocodozole, which depolymerizes microtubules. This resulted in a scattering of the BDNF vesicles and inhibition of their movement, suggesting that the factor is transported along microtubules. To find out what other components may keep the BDNF vesicle train on its tracks, Gauthier co-expressed eGFP-htt with huntingtin-associated protein 1, or HAP1. The latter was found to mediate BDNF transport. In fact, htt engineered to lack the HAP 1 binding site failed to stimulated movement of BDNF. HAP1 is well-known for its ability to bridge microtubules to huntingtin (see ARF related news story).
Where does this leave the aggregation hypothesis? In a curious twist, the authors also found evidence that the lethality of polyQ-huntingtin may be due, at least partly, to sequestration of other proteins—the culprits being HAP1, p150glued, dynein intermediate chain (IC), and kinesin heavy chain (HC). When the authors partitioned these components, they found that polyQ-htt has a higher affinity for HAP1 than wild-type htt. Not only that, but they found that this complex pulls in p150glued, and prevents the dynein and kinesin motor proteins from binding to microtubules. These findings appear to have physiological relevance, because by using sucrose gradients Gauthier found that distribution of dynein IC and p150glued is altered in brain samples from Huntington's patients.
As for treatment, the dominant-negative nature of the mutations suggests that inhibiting gene expression might be an approach worth testing. Toward this end, Beverly Davidson and colleagues from the University of Iowa reported in an independent study last week in Nature Medicine that they managed to relieve symptoms of spinocerebellar ataxia, another polyglutamine-expansion disease, in rodents treated with small interfering RNAs directed against the mutated gene. (For more on RNAi therapeutic approaches, see ARF Live Discussion.)—Tom Fagan