Reduced Gamma Oscillations in a Mouse Model of Intellectual Disability: A Role for Impaired Repetitive Neurotransmission?

Intellectual disability affects 2-3% of the population; mutations of the X-chromosome are a major cause of moderate to severe cases. The link between the molecular consequences of the mutation and impaired cognitive function remains unclear. Loss of function mutations of oligophrenin-1 (OPHN1) disrupt Rho-GTPase signalling. Here we demonstrate abnormal neurotransmission at CA3 synapses in hippocampal slices from Ophn1-/y mice, resulting from a substantial decrease in the readily releasable pool of vesicles. As a result, synaptic transmission fails at high frequencies required for oscillations associated with cognitive functions. Both spontaneous and KA-induced gamma oscillations were reduced in Ophn1-/y hippocampal slices. Spontaneous oscillations were rapidly rescued by inhibition of the downstream signalling pathway of oligophrenin-1. These findings suggest that the intellectual disability due to mutations of oligophrenin-1 results from a synaptopathy and consequent network malfunction, providing a plausible mechanism for the learning disabilities. Furthermore, they raise the prospect of drug treatments for affected individuals

Electrophysiological characterization of a mouse deficient for oligophrenin1: a mouse model of X-linked mental retardation

Mental retardation is the most common brain disease. One of the first genes identified in X-linked mental retardation (XLMR) was the OPHN-1 gene. Mutation of this gene has been described in patients with moderate to severe cognitive impairments. MR is characterized by reduced cognitive function with or without other clinical features, thus providing a direct approach to study the neurobiology of cognition and pathogenesis of MR. I propose in this thesis to clarify the underlying mechanisms responsible for the learning impairments. My first approach was to investigate the functioning of a neuronal population using extracellular recording of fast oscillations which are thought to underlie higher cognitive performance. I showed that $Ophn-1$ null mice displayed weaker gamma oscillations. Thereafter, Investigation of the synaptic properties of CA3 pyramidal neurons using the patch-clamp technique has been undertaken. I have shown reduced inputs of excitatory and inhibitory neurotransmission to CA3 pyramidal neurons accompanied with reduced frequency dependent facilitation of the inhibitory neurotransmission at 33Hz. Finally, a reduction in readily releasable pool size in inhibitory synapses of CA3 area was unravelled. This defect explained the reduction of frequency of sIPSCs and consequently the reduction in gamma oscillations power in Ophn-1$^{-/y}$ slices

Locking the dimeric GABA(B) G-protein-coupled receptor in its active state.

International audienceG-protein-coupled receptors (GPCRs) play a major role in cell-cell communication in the CNS. These proteins oscillate between various inactive and active conformations, the latter being stabilized by agonists. Although mutations can lead to constitutive activity, most of these destabilize inactive conformations, and none lock the receptor in an active state. Moreover, GPCRs are known to form dimers, but the role of each protomer in the activation process remains unclear. Here, we show that the heterodimeric GPCR for the main inhibitory neurotransmitter, the GABA(B) receptor, can be locked in its active state by introducing two cysteines expected to form a disulphide bridge to maintain the binding domain of the GABA(B1) subunit in a closed form. This constitutively active receptor cannot be inhibited by antagonists, but its normal functioning, activation by agonists, and inhibition by antagonists can be restored after reduction with dithiothreitol. These data show that the closed state of the binding domain of GABA(B1) is sufficient to turn ON this heterodimeric receptor and illustrate for the first time that a GPCR can be locked in an active conformation

Coordinated Regulation of Synaptic Plasticity at Striatopallidal and Striatonigral Neurons Orchestrates Motor Control

The basal ganglia play a critical role in shaping motor behavior. For this function, the activity of medium spiny neurons (MSNs) of the striatonigral and striatopallidal pathways must be integrated. It remains unclear whether the activity of the two pathways is primarily coordinated by synaptic plasticity mechanisms. Using a model of Parkinson’s disease, we determined the circuit and behavioral effects of concurrently regulating cell-type-specific forms of corticostriatal long-term synaptic depression (LTD) by inhibiting small-conductance Ca2+-activated K+ channels (SKs) of the dorsolateral striatum. At striatopallidal synapses, SK channel inhibition rescued the disease-linked deficits in endocannabinoid (eCB)-dependent LTD. At striatonigral cells, inhibition of these channels counteracted a form of adenosine-mediated LTD by activating the ERK cascade. Interfering with eCB-, adenosine-, and ERK signaling in vivo alleviated motor abnormalities, which supports that synaptic modulation of striatal pathways affects behavior. Thus, our results establish a central role of coordinated synaptic plasticity at MSN subpopulations in motor control

Reduced facilitation in response to high frequency stimulation in <i>Ophn1</i>^−/y neurons.

<p>(<b>a</b>) Representative traces illustrating IPSC summation in response to 10 stimuli delivered at 33 Hz. The responses to first stimuli were normalised. In contrast to <i>Ophn1</i>^+/y neurons, <i>Ophn1</i>^−/y IPSCs (<i>grey trace</i>) showed no summation. (<b>b</b>) IPSC amplitude plotted against stimulus number for 33 Hz trains in <i>Ophn1</i>^+/y (•, n = 20) and <i>Ophn1</i>^−/y neurons (○, n = 14). (<b>c</b>) Maximal IPSC amplitude (mean of the last 5 stimuli) plotted against stimulus frequency. (<i>p</i>: *<0.05, **<0.01, ***<0.001).</p

KA-induced and spontaneous gamma oscillations are differentially affected by ROCK inhibition.

<p>Y–27632 (10 µM) was applied for 20 minutes prior to application of KA (50 nM). (<b>a</b>) Oscillations recorded at 60 minutes post KA application were reduced by Y-27632 application. In contrast, spontaneous oscillations recorded prior to KA application were enhanced in <i>Ophn1</i>^−/y slices (<b>b</b>) (<i>p</i>: *<0.05, ***<0.005).</p

<i>Ophn1</i>^−/y slices show reduced postsynaptic potentials.

<p>(<b>a</b>) Representative traces of postsynaptic potentials from <i>Ophn1</i>^+/y (black traces) and <i>Ophn1</i>^−/y (grey traces) slices. (<b>b</b>) Stimulus response curve of postsynaptic potentials recorded from the s. radiatum of CA3. PSP slopes were significantly smaller in <i>Ophn1</i>^−/y (n = 5) than in <i>Ophn1</i>^+/y slices (n = 12; p = 0.011, ANOVA). Representative traces of spontaneous EPSCs in <i>Ophn1</i>^+/y (<b>c</b>) and <i>Ophn1</i>^−/y (<b>d</b>). Representative individual spontaneous EPSCs are shown in the right panel. Cumulative frequency plots show that the interevent intervals (<b>e</b>), but not amplitude (<b>f</b>) of EPSCs are altered in <i>Ophn1⁻</i>^/y neurons compared to <i>Ophn1</i>^+/y neurons. (<i>inset</i>) Mean frequency and amplitude of spontaneous EPSCs.</p

Smaller gamma oscillations in <i>Ophn1</i>^−/y slices.

<p>(<b>a</b>) Application of 50 nM KA induced neuronal synchrony in the gamma frequency range; the power of these oscillations increased over time (<b>e</b>). (<b>b</b>) Spectrogram illustrating the development of the dominant frequency of gamma oscillations in <i>Ophn1</i>^+/y (<i>left panel</i>) and <i>Ophn1</i>^−/y (<i>right panel</i>) slices. (<b>c</b>) Power spectra for <i>Ophn1</i>^+/y (<i>left panel</i>) and <i>Ophn1</i>^−/y (<i>right panel</i>) slices at t = 60 minutes. (<b>d</b>) The peak frequency did not differ significantly between <i>Ophn1</i>^+/y (•) and <i>Ophn1</i>^−/y (○) slices. (<b>e</b>) Summated power of gamma oscillations was reduced in <i>Ophn1</i>^−/y slices.</p

Gamma oscillation synchrony and coherence unchanged in <i>Ophn1</i>^−/y slices.

<p>(<b>a</b>) Average gamma waveforms for <i>Ophn1</i>^+/y (<i>black trace</i>) and <i>Ophn1</i>^−/y (<i>grey trace</i>) slices revealed a reduced amplitude without alteration in gamma waveform kinetics (<b>b</b>; <i>grey trace</i>, normalised <i>Ophn1</i>^−/y). The reduced gamma power in <i>Ophn1</i>^−/y slices was not associated with an altered spatial distribution; (<b>c</b>) waveform averages phase-zeroed at the peak of the oscillation recorded from CA3c (<i>black trace</i>), CA3b (<i>dotted trace</i>), CA3a (<i>short dashed trace</i>) and CA1 (<i>long dashed trace</i>) in an <i>Ophn1</i>^+/y slice. (<b>d</b>) Cross-correlation (<i>left panel</i>) and phase lead (<i>right panel</i>) for CA regions with CA3c as the reference, data are expressed as mean±s.e.m. No differences were observed between <i>Ophn1</i>^+/y (<i>filled symbols</i>) and <i>Ophn1</i>^−/y (<i>open symbols</i>) slices.</p

RRP is reduced in CA3 synapses.

<p>Evoked IPSCs recorded during a 20<i>Ophn1</i>^+/y (<b>a</b>, <i>black trace</i>) and <i>Ophn1</i>^−/y neuron (<i>grey trace</i>). The traces are averages of 5 sweeps. (<b>b</b>) The corresponding cumulative evoked IPSC amplitude plot (<i>Ophn1^+/y</i>,•; <i>Ophn1</i>^−/y,○). Data between 1–2 s were fitted by linear regression and back-extrapolated to time 0 to estimate the RRP size (<b>b, c</b>). (<b>c</b>) The mean amplitude of IPSC₁ was unaltered in <i>Ophn1</i>^−/y neurons. (<b>d</b>) Mean P_ves was increased in <i>Ophn1</i>^−/y neurons, whilst the mean number of vesicles (N_syn) forming the RRP was reduced (<b>e</b>) (<i>p</i>: **<0.01, ***<0.005).</p