Calcium-Dependent Regulation of Neuronal Excitability Is Rescued in Fragile X Syndrome by a Tat-Conjugated N-Terminal Fragment of FMRP

Fragile X syndrome (FXS) arises from the loss of fragile X messenger ribonucleoprotein (FMRP) needed for normal neuronal excitability and circuit functions. Recent work revealed that FMRP contributes to mossy fi ber long-term potentiation by adjusting the Kv4 A -type current availability through interactions with a Cav3-Kv4 ion channel complex, yet the mechanism has not yet been de fi ned. In this study using wild -type and Fmr1 knock -out (KO) tsA-201 cells and cerebellar sections from male Fmr1 KO mice, we show that FMRP associates with all subunits of the Cav3.1-Kv4.3-KChIP3 complex and is critical to enabling calcium -dependent shifts in Kv4.3 inactivation to modulate the A -type current. Speci fi cally, upon depolarization Cav3 calcium in fl ux activates dualspeci fi c phosphatase 1/6 (DUSP1/6) to deactivate ERK1/2 (ERK) and lower phosphorylation of Kv4.3, a signaling pathway that does not function in Fmr1 KO cells. In Fmr1 KO mouse tissue slices, cerebellar granule cells exhibit a hyperexcitable response to membrane depolarizations. Either incubating Fmr1 KO cells or in vivo administration of a tat -conjugated FMRP N -terminus fragment (FMRP-N-tat) rescued Cav3-Kv4 function and granule cell excitability, with a decrease in the level of DUSP6. Together these data reveal a Cav3-activated DUSP signaling pathway critical to the function of a FMRP-Cav3-Kv4 complex that is misregulated in Fmr1 KO conditions. Moreover, FMRP-N-tat restores function of this complex to rescue calcium -dependent control of neuronal excitability as a potential therapeutic approach to alleviating the symptoms of FXS. Signi fi cance Statement Changes in neuronal excitability and ion channel functions have been a focus in studies of fragile X syndrome (FXS). Previous work identi fi ed fragile X messenger ribonucleoprotein (FMRP) regulates ion channel through either protein translation or direct protein - protein interactions. The current study reveals FMRP is required for the function of a Cav3-Kv4 complex by affecting a Cav3-DUSP-ERK signaling pathway to increase A -type current. In Fmr1 knock -out cells, calcium -dependent modulation of the A -type current is lost, leading to hyperexcitability of cerebellar granule cells. Pretreating with FMRP-N-tat restores the Cav3-Kv4 function and granule cell excitability, supporting FMRP-N-tat as a potential therapeutic for FXS.