Normally, the huntingtin protein resides mostly in the cytoplasm, but when the protein bears an N-terminal polyglutamine expansion, it forms nuclear aggregates. Many models of HD, in both fruit flies and mice, express small N-terminal fragments of the larger protein, which is difficult to express because of its size. The N-terminal fragments contain the polyglutamine repeat region and cause neurodegeneration. However, the truncated proteins most likely do not participate in the normal function of the full-length protein, which has been studied far less.

To try to understand this normal function, joint first authors Eliana Romero, Guang-Ho Cha, and Patrik Verstreken created the first fly model of HD that expresses the full-length human huntingtin protein with a 128-residue glutamine repeat (128Qhttfl). They found that the expanded protein accumulates in the cytosol of neurons, and forms no detectable nuclear or cytosolic aggregates during the lifetime of the flies. Nor did the researchers find evidence of axonal transport blockage, which has been observed with htt fragments and full-length protein (Trushina et al., 2004). The full-length protein caused a progressive neurodegeneration when expressed either in neurons of the eye or in CNS motor neurons, Romero and colleagues showed. The loss of CNS motor neurons severely impaired the flies’ ability to fly later in life.

Four possible toxic mechanisms have been suggested for expanded htt, namely, disruption of gene transcription, blockage of axonal transport, changes in neurotransmission, and alterations in calcium homeostasis. Because the Botas group observed neurotoxicity without nuclear aggregation or apparent changes in axonal transport, they focused on the last two possibilities. Assessing the distribution of huntingtin in synaptic boutons, the investigators noted that some boutons had it, some did not. Overall, synaptic morphology and localization of several synaptic proteins were not changed in flies expressing expanded htt. The investigators did find changes in synaptic function, however. Electrophysiological recording from neuromuscular junction synapses revealed enhanced neurotransmission in the htt flies, which was due to increased neurotransmitter release from presynaptic vesicles.

This hyperactivity of synapses suggested to the investigators that reducing synaptic activity might be a strategy to ameliorate HD. To test this idea, the researchers took a genetic approach to ask if partial loss-of-function mutations in several proteins involved in vesicular trafficking and neurotransmitter release would suppress the huntingtin phenotype. HD flies with one copy of the synaptic vesicle proteins Snap, syntaxin1A, or the syntaxin-associated protein Rop all showed normal synaptic activity. The same mutations also suppressed neurodegeneration and loss of motor functions in the flies.

Expanded huntingtin is known to disturb calcium homeostasis. In keeping with that, the investigators measured higher presynaptic calcium concentrations in neuromuscular junction synapses in their HD flies. Calcium levels reverted to normal in the syntaxin1A mutant, consistent with a role for calcium channels in aberrant neurotransmitter release. Voltage-gated calcium channels convey the calcium influx that triggers neurotransmitter release, and have been shown to associate with expanded htt. In agreement with this, the researchers found that partial loss-of-function mutants in a voltage-gated calcium channel also normalized synaptic calcium and neurotransmission, and suppressed neurodegeneration in the htt flies.

“All together, these data suggest that the presynaptic accumulation of 128Qhttfl impairs the function of factors involved in neurotransmitter release,” the authors write. Their results further support the idea that htt-stimulated neurodegeneration is caused by increased synaptic transmission, an idea fully consistent with many studies showing association of expanded htt with synaptic proteins, and evidence from mouse models of altered neurotransmitter release. The results raise the possibility of synaptic activity as a therapeutic target for HD, including specifically the genetic suppressors identified in this study. In particular, calcium channel antagonists, the authors write, “offer an attractive therapeutic option due to their specificity and wide usage.”

The synaptic actions of expanded htt likely represent an early effect of the full-length mutated protein, as the current results indicate they do not require nuclear accumulation. The authors speculate that cleavage of the protein later on could lead to nuclear accumulation, transcriptional deregulation, and axonal impairments that would compound the toxic effects of the full-length protein. In this regard, htt begins to look more like other neurotoxic proteins including amyloid-β, where early conformers of the protein increase synaptic activity (see ARF related news story), which could serve as an initial insult that gets the ball rolling toward the ultimate demise of neurons, and disease.—Pat McCaffrey.

Reference:
Romero E, Cha G, Verstreken P, Ly CV, Hughes RE, Bellen HJ, Botas J. Suppression of neurodegeneration and increased neurotransmission caused by expanded full-length huntingtin accumulating in the cytoplasm. Neuron. 2008 Jan 10; 57:27-20. Abstract