Identification of a molecular locus for normalizing dysregulated GABA release from interneurons in the Fragile X brain

Abstract

Principal neurons encode information by varying their firing rate and patterns precisely fine-tuned through GABAergic interneurons. Dysregulation of inhibition can lead to neuropsychiatric disorders, yet little is known about the molecular basis underlying inhibitory control. Here, we find that excessive GABA release from basket cells (BCs) attenuates the firing frequency of Purkinje neurons (PNs) in the cerebellum of Fragile X Mental Retardation 1 (Fmr1) knockout (KO) mice, a model of Fragile X Syndrome (FXS) with abrogated expression of the Fragile X Mental Retardation Protein (FMRP). This over-inhibition originates from increased excitability and Ca transients in the presynaptic terminals, where Kv1.2 potassium channels are downregulated. By paired patch-clamp recordings, we further demonstrate that acutely introducing an N-terminal fragment of FMRP into BCs normalizes GABA release in the Fmr1-KO synapses. Conversely, direct injection of an inhibitory FMRP antibody into BCs, or membrane depolarization of BCs, enhances GABA release in the wild type synapses, leading to abnormal inhibitory transmission comparable to the Fmr1-KO neurons. We discover that the N-terminus of FMRP directly binds to a phosphorylated serine motif on the C-terminus of Kv1.2; and that loss of this interaction in BCs exaggerates GABA release, compromising the firing activity of PNs and thus the output from the cerebellar circuitry. An allosteric Kv1.2 agonist, docosahexaenoic acid, rectifies the dysregulated inhibition in vitro as well as acoustic startle reflex and social interaction in vivo of the Fmr1-KO mice. Our results unravel a novel molecular locus for targeted intervention of FXS and perhaps autism.

Mechanisms underlying auditory processing deficits in Fragile X syndrome

“…68,75 In both cases, the direct binding of the channel subunit to FMRP promotes channel opening, increasing current amplitude. 67,82,87 Accordingly, the amplitude of the low-threshold potassium currents is reduced in MNTB neurons from Fmr1 knockout mice, promoting the abnormal firing patterns shown in Figure 3. 64 Novel drugs that alter the voltage-dependence of Kv3.1 channels have been shown to rectify these abnormal firing patterns by increasing low-threshold currents and decreasing high-threshold currents in Fmr1 knockout animals.…”

“…These are required for the high degree of temporal accuracy with which principal neurons of MNTB lock their action potentials to incoming stimuli . In both cases, the direct binding of the channel subunit to FMRP promotes channel opening, increasing current amplitude . Accordingly, the amplitude of the low‐threshold potassium currents is reduced in MNTB neurons from Fmr1 knockout mice, promoting the abnormal firing patterns shown in Figure .…”

“…67,[82][83][84][85][86][87] In each case, the interaction with FMRP alters the amplitude, kinetic behavior, or trafficking of the channel. Binding of FMRP to channels was first demonstrated for K Na 1.1 channels, also termed Slack channels, 67,82 and has also been documented for Kv1.2 87 and Cav2.2, 85 all of which are expressed in MNTB. The two potassium channels, Kv1.2 and K Na 1.1, that are regulated in this way play a very different function in the excitability of auditory brainstem neurons from that of the Kv3.1 channel.…”

Interneuron NMDA receptors change the gear of motor learning in the cerebellar machine

“…; Yang et al . ). The need for more detailed cellular and molecular analyses of the cerebellar circuitry and plasticity is pressing.…”

“…Since the 19th century the cerebellum has been known for its function in motor control including the maintenance of equilibrium (balance, posture, eye movement), coordination of the timing and force of muscle groups, adjustment of muscle tone, learning motor skills and speech. Supported by the rich connections between the cerebellum, the cerebral cortex and the limbic system, the cerebellum is increasingly recognized for its non-motor functions such as cognition (language and social interaction), motivation and emotions and consequently for its role in autism-spectrum, obsessive-compulsive, attention-deficit hyperactivity and bipolar disorders, and schizophrenia (Schmahmann et al 2007;Yang et al 2018). The need for more detailed cellular and molecular analyses of the cerebellar circuitry and plasticity is pressing.…”