PT  - JOURNAL ARTICLE
AU  - Bonnycastle, Katherine
AU  - Kind, Peter C.
AU  - Cousin, Michael A.
TI  - FMRP sustains presynaptic function via control of activity-dependent bulk endocytosis
AID  - 10.1101/2020.09.10.291062
DP  - 2021 Jan 01
TA  - bioRxiv
PG  - 2020.09.10.291062
4099  - http://biorxiv.org/content/early/2021/02/17/2020.09.10.291062.short
4100  - http://biorxiv.org/content/early/2021/02/17/2020.09.10.291062.full
AB  - 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 StatementThe authors have declared no competing interest.SVsynaptic vesicleFMRPfragile X mental retardation proteinFXSfragile X syndromeAPaction potentialADBEactivity-dependent bulk endocytosisBKbig potassiumRRPreadily releasable poolIDintellectual disabilityHRPhorseradish peroxidaseKOknockoutWTwild-typeDIVday in vitroTMRtetramethylrhodamineBSAbovine serum albuminROIregion of interestmCermCeruleansypHysynaptophysin-pHluorin