FMRP regulates presynaptic localization of neuronal voltage gated calcium channels

Fragile X syndrome (FXS), the most common form of inherited intellectual disability and autism, results from the loss of fragile X mental retardation protein (FMRP). We have recently identified a direct interaction of FMRP with voltage-gated Ca²⁺ channels that modulates neurotransmitter release. In the present study we used a combination of optophysiological tools to investigate the impact of FMRP on the targeting of voltage-gated Ca²⁺ channels to the active zones in neuronal presynaptic terminals. We monitored Ca²⁺ transients at synaptic boutons of dorsal root ganglion (DRG) neurons using the genetically-encoded Ca²⁺ indicator GCaMP6f tagged to synaptophysin. We show that knock-down of FMRP induces an increase of the amplitude of the Ca²⁺ transient in functionally-releasing presynaptic terminals, and that this effect is due to an increase of N-type Ca²⁺ channel contribution to the total Ca²⁺ transient. Dynamic regulation of Ca_V2.2 channel trafficking is key to the function of these channels in neurons. Using a Ca_V2.2 construct with an α-bungarotoxin binding site tag, we further investigate the impact of FMRP on the trafficking of Ca_V2.2 channels. We show that forward trafficking of Ca_V2.2 channels from the endoplasmic reticulum to the plasma membrane is reduced when co-expressed with FMRP. Altogether our data reveal a critical role of FMRP on localization of Ca_V channels to the presynaptic terminals and how its defect in a context of FXS can profoundly affect synaptic transmission.