ABSTRACT
Synaptic vesicle (SV) recycling defects are linked to neurodevelopmental disorders, including fragile X syndrome (FXS), which results from loss of fragile X mental retardation protein (FMRP) encoded by the FMR1 gene. Hyperexcitability of neuronal circuits is a key feature of FXS, therefore we investigated whether SV recycling was affected by the absence of FMRP during increased neuronal activity. We revealed that primary neuronal cultures from a Fmr1 knockout rat model display a specific defect in activity-dependent bulk endocytosis (ADBE). This defect resulted in an inability of Fmr1 knockout neurons to sustain SV recycling during high frequency stimulation. Using a molecular replacement strategy, we also revealed that a human FMRP mutant that cannot bind BK channels failed to correct ADBE dysfunction in knockout neurons, however this dysfunction was corrected by BK channel agonists. Therefore, FMRP performs a key role in sustaining neurotransmitter release via selective control of ADBE, suggesting intervention via this endocytosis mode may correct hyperexcitabiltiy observed in FXS.
SUMMARY Fragile X syndrome (FXS) is caused by loss of fragile X mental retardation protein (FMRP). Bonnycastle et al show that FMRP is specifically required for activity-dependent bulk endocytosis (ADBE), revealing 1) FMRP sustains neurotransmitter release and 2) intervention via ADBE may correct circuit hyperexcitabilty in FXS.
Competing Interest Statement
The authors have declared no competing interest.
ABBREVIATIONS
- SV
- synaptic vesicle
- FMRP
- fragile X mental retardation protein
- FXS
- fragile X syndrome
- AP
- action potential
- ADBE
- activity-dependent bulk endocytosis
- BK
- big potassium
- RRP
- readily releasable pool
- ID
- intellectual disability
- HRP
- horseradish peroxidase
- KO
- knockout
- WT
- wild-type
- DIV
- day in vitro
- TMR
- tetramethylrhodamine
- BSA
- bovine serum albumin
- ROI
- region of interest
- mCer
- mCerulean
- sypHy
- synaptophysin-pHluorin