Serotonin 5-HT7 receptors require cyclin-dependent kinase 5 to rescue hippocampal synaptic plasticity in a mouse model of Fragile X Syndrome

Abstract

Fragile X Syndrome is a genetic form of intellectual disability associated with autism, epilepsy and mood disorders. Electrophysiology studies in Fmr1 knockout (KO) mice, a murine model of Fragile X Syndrome, have demonstrated alterations of synaptic plasticity, with exaggerated long-term depression induced by activation of metabotropic glutamate receptors (mGluR-LTD) in Fmr1 KO hippocampus. We have previously demonstrated that activation of serotonin 5-HT7 receptors reverses mGluR-LTD in the hippocampus of wild-type and Fmr1 KO mice, thus correcting a synaptic dysfunction typically observed in this disease model. Here we show that pharmacological inhibition of cyclin-dependent kinase 5 (Cdk5, a signaling molecule recently shown to be a modulator of brain synaptic plasticity) enhanced mGluR-LTD in wild-type hippocampal neurons, which became comparable to exaggerated mGluR-LTD observed in Fmr1 KO neurons. Furthermore, Cdk5 inhibition prevented 5-HT7 receptor-mediated reversal of mGluR-LTD both in wild-type and in Fmr1 KO neurons. Our results show that Cdk5 modulates hippocampal synaptic plasticity. 5-HT7 receptors require Cdk5 to modulate synaptic plasticity in wild-type and rescue abnormal plasticity in Fmr1 KO neurons, pointing out Cdk5 as a possible novel target in Fragile X Syndrome.

Keywords: 5-HT7 receptors; Cdk5; Fragile X Syndrome; Serotonin; hippocampus; mGluR-LTD.

FIGURE 1

Inhibition of Cdk5 enhanced mGluR‐LTD in CA1 neurons from wild‐type mice and prevented 5‐HT7 receptor‐mediated effect on mGluR‐LTD. AMPA receptor‐mediated excitatory post‐synaptic currents (EPSCs) were recorded in the presence of D‐AP5 (50 µM) and bicuculline (5 µM) under whole‐cell patch clamp in the CA3‐CA1 synapse in hippocampal slices from wild‐type mice. (a) Bath application of the group I mGluR agonist DHPG (100 µM, 5 min) induced a long‐term depression (mGluR‐LTD) of EPSC amplitude (white dots, n = 11). When the Cdk5 inhibitor roscovitine (1.6 µM) was added to intracellular medium, DHPG‐induced mGluR‐LTD was enhanced (light grey dots, n = 7) with respect to control. (b) When DHPG application was followed by application of the 5‐HT7 receptor agonist LP‐211 (10 nM, 5 min), mGluR‐LTD was completely reversed (dark grey dots, n = 6). In the presence of intracellular roscovitine (1.6 µM), application of LP‐211 did not modify the amount of mGluR‐LTD (black dots, n = 6). (c) The bar graph shows that the amount of mGluR‐LTD measured 40 min after DHPG application (mean EPSC amplitude in all tested neurons, expressed as % of baseline EPSC amplitude; EPSC values of single neurons are displayed for each bar) in the four different experimental conditions (control; roscovitine; LP‐211; LP‐211 + roscovitine) was significantly different (p = 0.0006 by one‐way ANOVA followed by Tukey's multiple comparisons test). *p < 0.05; ***p < 0.001

FIGURE 2

Inhibition of Cdk5 did not modify mGluR‐LTD in CA1 neurons from Fmr1 KO mice and prevented 5‐HT7 receptor‐mediated effect on mGluR‐LTD. AMPA receptor‐mediated excitatory post‐synaptic currents (EPSCs) were recorded from CA1 neurons in the presence of D‐AP5 (50 µM) and bicuculline (5 µM) in hippocampal slices from Fmr1 KO mice. (a) Bath application of DHPG (100 µM, 5 min) induced mGluR‐LTD (white dots; n = 8). In the presence of intracellular roscovitine (1.6 µM) the amount of mGluR‐LTD was not modified (grey dots, n = 6) with respect to control conditions. (b) Application of LP‐211 (10 nM, 5 min) completely reversed mGluR‐LTD in control conditions (dark grey dots, n = 8) but had no effect on mGluR‐LTD in the presence of intracellular roscovitine (black dots, n = 7). (c) The bar graph shows the amount of mGluR‐LTD measured 40 min after DHPG application (mean EPSC amplitude in all tested neurons, expressed as % of baseline EPSC amplitude; EPSC values of single neurons are displayed for each bar). The amount of mGluR‐LTD in the four experimental conditions (control; roscovitine; LP‐211; LP‐211 + roscovitine) was significantly different (*p = 0.0331 by one‐way ordinary ANOVA followed by Tukey's multiple comparisons test)