FMRP regulates neurotransmitter release and synaptic information transmission by modulating action potential duration via BK channels

Abstract

Loss of FMRP causes fragile X syndrome (FXS), but the physiological functions of FMRP remain highly debatable. Here we show that FMRP regulates neurotransmitter release in CA3 pyramidal neurons by modulating action potential (AP) duration. Loss of FMRP leads to excessive AP broadening during repetitive activity, enhanced presynaptic calcium influx, and elevated neurotransmitter release. The AP broadening defects caused by FMRP loss have a cell-autonomous presynaptic origin and can be acutely rescued in postnatal neurons. These presynaptic actions of FMRP are translation independent and are mediated selectively by BK channels via interaction of FMRP with BK channel's regulatory β4 subunits. Information-theoretical analysis demonstrates that loss of these FMRP functions causes marked dysregulation of synaptic information transmission. FMRP-dependent AP broadening is not limited to the hippocampus, but also occurs in cortical pyramidal neurons. Our results thus suggest major translation-independent presynaptic functions of FMRP that may have important implications for understanding FXS neuropathology.

Figure 1. FMRP Regulates Action Potential Duration in CA3 Pyramidal Neurons in a Translation-Independent and Cell-Autonomous Presynaptic Manner

(A) Sample traces of AP trains recorded in CA3 PCs of WT (in black, here and throughout) or Fmr1 KO mice (in red, here and throughout) in response to 25 stimuli at 60 Hz. 4 APs at 0.2 Hz were evoked prior to each train to determine baseline AP duration. Blue lines show the AP width measured at −10 mV level. (B) Analysis of AP duration in (A). AP duration was normalized to its own baseline for WT or Fmr1 KO mice. (C) Effect of intracellular perfusion of FMRP_1-298 in Fmr1 KO CA3 PCs on AP broadening. (D) Heat-inactivated FMRP_1-298 failed to affect AP broadening in Fmr1 KO mice. (E) Effect of intracellular perfusion of anti-FMRP Ab in WT CA3 on AP broadening. (F) Intracellular perfusion of Anti-FMRP Ab had no effect on AP duration in Fmr1 KO mice. (G, H) Anisomycin failed to block the effects of anti-FMRP on AP duration in WT mice (G), or the effects of FMRP_1-298 on AP duration in Fmr1 KO mice (H).

Figure 2. FMRP modulates TEA-Sensitive Voltage-Gated K⁺ Conductances in CA3 Pyramidal Neurons

(A, B, C) Three major VGKC components, I_D, I_A and I_K isolated in WT and Fmr1 KO CA3 PCs using AP-clamp. Currents during the AP burst were normalized to their own baseline. Representative traces show averaged currents evoked by the first and 25th APs of the burst, scaled for comparison to the normalized current evoked by the first AP of the burst in a corresponding neuron. Bar graphs show averaged currents evoked by the last two APs of the burst, expressed as percentage of baseline. ns, not significant; **p <0.01.

Figure 3. FMRP Regulation of Action Potential Duration is Mediated by BK Channels

(A–F) Normalized AP width in WT and Fmr1 KO CA3 PCs in the absence of K⁺ channel blockers (A), or in the presence of: 3 μM 4-AP (B); M channel blocker XE991 (C); Kv2 channel blocker sStromatoxin-1 (sSTX) (D); BK channel blocker paxilline (E); BK channel blocker iberiotoxin (F). (G,H) Summary of the data in (A–F) showing actual width of baseline AP (G) or of burst AP (averaged from the last 2 APs of burst) (H). ns, not significant; *p<0.05. (I, J) BK channel blockers (I), but not M channel blocker XE 991 (J) masked the effect of anti-FMRP Ab on AP broadening in WT mice.

Figure 4. FMRP Modulates Spontaneous Action Potential Duration and fAHP in CA3 Pyramidal Neurons via BK Channels

(A) Duration of spontaneous APs for WT and Fmr1 KO neurons. Insert (a): sample spontaneous APs; blue lines denote AP width and RMP. Insert (b): Enlargement of box in (a) indicating measurements of fAHP amplitude. Black and red lines indicate the lowest points of fAHP for WT and KO mice, respectively. (B) Data from (A) averaged within frequency bands shown on X-axis. **p<0.01. (C) Scatter plots of fAHP amplitude against firing frequencies for WT and Fmr1 KO neurons. The bar graph represents the mean fAHP amplitudes. **p<0.01. (D) Scatter plots of actual AP widths against firing frequencies in the presence of BK channel blockers. Insert: sample APs. (E) Data from (D) presented as frequency-banded mean AP width. ns, not significant.

Figure 5. FMRP Modulates Calcium Sensitivity of BK Channels

(A,B) Sample traces (A) and summary data (B) for BK currents isolated using AP clamp in WT and Fmr1 KO neurons. Right panels in (A) are BK currents evoked by the first 5 APs of the train. (C) Sample BK currents evoked using the paired-ramp protocol shown. Two identical ramps were from −110mV to +40mV with a rate of 0.1mV/ms and 50ms hold at +40mV. Time between ramps (5ms) is expanded for presentation. Blue lines indicate maximal BK currents of the 1st ramp. (D) Summary of the data from (C) **, p < 0.01; ^##, p < 0.01. (E) Same as (C, D) in the presence of BAPTA. ns, not significant. (F–H) Sample AP traces (F), normalized AP widths (G) and actual AP widths (H) during 25 AP trains at 60Hz in Fmr1 KO and WT mice in the presence of BAPTA. (I) Scatter plots of actual AP width against firing frequencies for spontaneous APs in the presence of BAPTA. Insert shows sample APs. (J) Frequency-banded mean spontaneous AP duration (data from (I)). ns, not significant. (K) Mean fAHP amplitude in the presence of BAPTA. Data are from APs at instantaneous firing frequencies <20 Hz, since no measurable fAHP was evident above ~20 Hz in the presence of BAPTA. ns, not significant.

Figure 6. FMRP Interacts with the β4 Subunit of BK Channels to Regulate Action Potential Duration in CA3 Pyramidal Neurons

(A). FMRP was immunoprecipitated from protein extract prepared from WT or Fmr1 KO mice hippocampi and analyzed by western blot with the indicated antibodies. GAPDH is used as a loading control for the lysate and as a negative control for the IP. A duplicate experiment is shown. (B). β4 was immunoprecipitated from protein extract prepared from WT or Fmr1 KO mice hippocampi and analyzed by western blot with the indicated antibodies. (C–E) Sample AP traces (C), normalized AP widths (D) and actual AP widths (E) during 25 AP trains at 60Hz in sloβ4 KO mice before and after intracellular perfusion of anti-FMRP Ab.

Figure 7. FMRP Regulates Presynaptic Calcium Influx and Synaptic Transmission in CA3 Pyramidal Neurons

(A, B) Representative image of a CA3 PC filled with fluorescent dyes Fluo 5F and Alexa 594 (A). A high magnification image from the same neuron shows a presynaptic bouton on a CA3 axon (B). Square indicates the location of the presynaptic bouton in (A). Line indicates a direction of the line scan. (C) Representative calcium transients measured in presynaptic terminals of CA3 PCs using two-photon microscopy in response to a 15-AP train (60-Hz) 10 min or 45 min following break-in in the absence (black) or presence (purple) of anti-FMRP Ab. Bar graph shows mean relative amplitudes of calcium transients measured 45 minutes after break-in for neurons without (black) or with (purple) anti-FMRP Ab. Data are normalized to respective values measured 10 min after break-in. (D) VGCC currents isolated via AP clamp during 25-AP 60Hz WT trains. Sample averaged VGCC current traces are shown for the first and 25^th AP in the train, scaled to normalized currents on the first AP of the train in a corresponding neuron. (E) VGCC currents isolated via a step protocol (from −65 to 0 mV for 200 ms). Non-L-type Ca²⁺ currents were isolated in the presence of nimodipine (10 μM). “Steady” denotes the currents averaged from the last 25 ms of the step. (F,G) Normalized EPSCs recorded in CA1 PC in response to SC stimulation with a train of 25-stimuli at 60 Hz in WT and Fmr1 KO mice under basal conditions (F) or in the presence of BK channel blockers. Upper panel shows EPSC traces during 60-Hz burst with the 1st EPSC of the burst scaled to the same value (100%) in WT and Fmr1 KO for visual comparison. Stimulus artifacts in traces were removed for clarity. Lower panel shows average synaptic responses normalized to their own low-frequency (0.2 Hz) baseline.

Figure 8. Axonal/Presynaptic Actions of FMRP on BK Channel Activity Modulate Action Potential Waveform and Synaptic Information Transmission in CA3-CA1 Synapses

(A) Sample traces of extracellular recordings of cAPs and fEPSPs in CA1 stratum radiatum in response to SC stimulation with a train of 25 stimuli at 60Hz in WT or Fmr1 KO mice. Traces are plotted with the first fEPSP scaled to the same value (100%) in WT and Fmr1 KO mice for visual comparison. Right panel shows the 1st and last (25th) responses on an extended time scale. Stimulus artifacts in traces were removed for clarity. (B, C) Normalized cAP amplitude (B) and normalized fEPSP initial slope (C) during 25 stimuli at 60Hz in Fmr1 KO and WT mice. Bar graph compares the average of the last two measurements of the train. **p<0.01. (D,E,F) Same as (A,B,C), respectively, in the presence of BK channel blockers. (G) Plot of normalized fEPSP slope against normalized cAPs amplitude in WT mice without BK blockers (data from (B,C). The plot was fitted by Equation Y = X^A, where Y is fEPSP and X is cAP. (H, I) Prediction of fEPSP changes from corresponding cAP amplitude in the absence (H) or presence (I) of BK channel blockers. (J–L). Information-theoretic analysis of synaptic information transmission in WT and Fmr1 KO mice. Mutual information is plotted as a function of input spike number for a range of input spike frequencies (J). Parameters of optimal information transmission are plotted for the peak location (K) and peak width (L). Input spike frequencies tested are shown in (J) above each trace.