Loss of Huntingtin stimulates capture of retrograde dense-core vesicles to increase synaptic neuropeptide stores

Abstract

The Huntington’s disease protein Huntingtin (Htt) regulates axonal transport of dense-core vesicles (DCVs) containing neurotrophins and neuropeptides. DCVs travel down axons to reach nerve terminals where they are either captured in synaptic boutons to support later release or reverse direction to reenter the axon as part of vesicle circulation. Currently, the impact of Htt on DCV dynamics in the terminal is unknown. Here we report that knockout of Drosophila Htt selectively reduces retrograde DCV flux at proximal boutons of motoneuron terminals. However, initiation of retrograde transport at the most distal bouton and transport velocity are unaffected suggesting that synaptic capture rate of these retrograde DCVs could be altered. In fact, tracking DCVs shows that retrograde synaptic capture efficiency is significantly elevated by Htt knockout or knockdown. Furthermore, synaptic boutons contain more neuropeptide in Htt knockout larvae even though bouton size, single DCV fluorescence intensity, neuropeptide release in response to electrical stimulation and subsequent activity-dependent capture are unaffected. Thus, loss of Htt increases synaptic capture as DCVs travel by retrograde transport through boutons resulting in reduced transport toward the axon and increased neuropeptide in the terminal. These results therefore identify native Htt as a regulator of synaptic capture and neuropeptide storage.

Excess Rab4 rescues synaptic and behavioral dysfunction caused by defective HTT-Rab4 axonal transport in Huntington’s disease

“…Since Drosophila NMJs undergo rapid structural and functional changes during larval growth [83], type-1 synaptic boutons between muscles 6/7 at larval abdominal segments A4-A5 of third instar larval were examined. Similar to previous studies [10], htt reduction did not dramatically alter the coordinated growth of NMJs (Fig. S2a).…”

Limited distal organelles and synaptic function in extensive monoaminergic innervation

“…This process has been termed capture and has been measured as the decrease in DCV flux entering and leaving a bouton (Shakiryanova et al, 2006;Wong et al, 2012;Bulgari et al, 2014Bulgari et al, , 2017Cavolo et al, 2016). Notably, this parameter encompasses capture initiation, which when measured over minutes does not quantify the much longer duration of capture (Shakiryanova et al, 2006).…”

“…Furthermore, synaptic capture of DCVs increases following activity and is constitutively elevated by a transcription factor in specialized neuroendocrine neurons (Shakiryanova et al, 2006;Bulgari et al, 2014;Cavolo et al, 2016). Disease-related proteins such as the fragile-X syndrome protein and huntingtin also influence synaptic neuropeptide stores by regulating direction-specific DCV capture (Cavolo et al, 2016;Bulgari et al, 2017). Thus, synaptic capture is a major determinant of presynaptic accumulation of DCVs and hence affects the capacity for release of DCV contents (e.g.…”

Simulating Reversibility of Dense Core Vesicles Capture in En Passant Boutons: Using Mathematical Modeling to Understand the Fate of Dense Core Vesicles in En Passant Boutons

“…Another possible explanation is that kinesin motors more easily release DCVs than dynein motors. The process of DCV release by motors may be differentially controlled (Shakiryanova et al [19], Bulgari et al [3], Cavolo et al [16]).…”

“…Malfunctions in DCV transport can lead to devastating neurodegenerative diseases. Bulgari et al [3] reported that the loss of Huntingtin protein results in more capture of retrogradely transported DCVs and thus more DCV accumulation in the nerve terminal. They also suggested a possible link between DCV transport and Huntington's disease, caused by accumulation of mutant Huntingtin protein and loss of wild type Huntingtin, through modification of synaptic capture of DCVs.…”