Huntingtin is a cell-autonomous regulator of neuropeptide trafficking and clock output

ABSTRACT Huntington’s disease is a severe neurodegenerative condition arising from an abnormal CAG repeat expansion in the HTT gene, which leads to the production of a mutant Huntingtin protein carrying an extended polyglutamine stretch. Although the field has largely centred on the toxic effects gained by this mutant protein, growing evidence points to the loss of normal wild-type Huntingtin function as an additional contributor to disease progression. Despite this, the cell-intrinsic roles of wild-type Huntingtin in neuronal biology remain poorly defined, in part because disentangling its specific contributions from broader network-level effects has proven technically challenging. To address this knowledge gap, we took advantage of the Drosophila huntingtin homolog ( htt ) and selectively manipulated its expression in the small lateral ventral neurons (sLNvs), a discrete cluster of just eight circadian pacemaker neurons that govern behavioral rhythmicity and sleep. Through targeted genetic knock-down, we show that reducing htt levels in sLNvs weakens the robustness of free-running circadian rhythms and substantially increases sleep in female flies. These behavioral changes are not rooted in developmental abnormalities, as restricting htt knock-down to adult flies reproduces the sleep phenotype across both beam-crossing and video-based locomotion assays. At the cellular level, htt loss disrupts dense core vesicle trafficking along sLNv axons, altering the fraction of motile vesicles and their velocity, and abolishing the time-of-day-dependent fluctuations in vesicle dynamics observed in these neurons. Complementary electrophysiological recordings using whole-cell patch-clamp further reveal that htt knock-down lowers action potential firing rates without perturbing resting membrane potential. Together, these results identify huntingtin as a cell-autonomous regulator of neuropeptide trafficking, neuronal excitability and circadian output. Beyond advancing our understanding of wild-type huntingtin physiology, this work carries direct relevance for HD therapeutic strategies, particularly those involving huntingtin-lowering approaches, by highlighting functions that may be unintentionally compromised.