Deletion of Fmr1 from Forebrain Excitatory Neurons Triggers Abnormal Cellular, EEG, and Behavioral Phenotypes in the Auditory Cortex of a Mouse Model of Fragile X Syndrome

Abstract

Fragile X syndrome (FXS) is a leading genetic cause of autism with symptoms that include sensory processing deficits. In both humans with FXS and a mouse model [Fmr1 knockout (KO) mouse], electroencephalographic (EEG) recordings show enhanced resting state gamma power and reduced sound-evoked gamma synchrony. We previously showed that elevated levels of matrix metalloproteinase-9 (MMP-9) may contribute to these phenotypes by affecting perineuronal nets (PNNs) around parvalbumin (PV) interneurons in the auditory cortex of Fmr1 KO mice. However, how different cell types within local cortical circuits contribute to these deficits is not known. Here, we examined whether Fmr1 deletion in forebrain excitatory neurons affects neural oscillations, MMP-9 activity, and PV/PNN expression in the auditory cortex. We found that cortical MMP-9 gelatinase activity, mTOR/Akt phosphorylation, and resting EEG gamma power were enhanced in CreNex1/Fmr1Flox/y conditional KO (cKO) mice, whereas the density of PV/PNN cells was reduced. The CreNex1/Fmr1Flox/y cKO mice also show increased locomotor activity, but not the anxiety-like behaviors. These results indicate that fragile X mental retardation protein changes in excitatory neurons in the cortex are sufficient to elicit cellular, electrophysiological, and behavioral phenotypes in Fmr1 KO mice. More broadly, these results indicate that local cortical circuit abnormalities contribute to sensory processing deficits in autism spectrum disorders.

Mechanisms underlying auditory processing deficits in Fragile X syndrome

“…To begin to evaluate the relative contribution of subcortical inputs and local cortical circuits to observed changes in cortical recordings, a conditional knockout in which FMRP was deleted specifically from forebrain excitatory neurons using the Nex1 promoter was generated. 129 Similar to global knockout animals, this conditional knockout strain exhibits elevated gamma frequency band power in EEGs as well as delayed development of PNN and PV cells, indicating that these deficits arise locally in the cortex. However, the deficit in temporal synchronization of responses to time varying stimuli was absent when FMRP expression is preserved at subcortical levels, demonstrating that the change following global Fmr1 knockout is a secondary consequence of altered subcortical inputs.…”

“…This is a complex issue because of the nature of ascending and descending connections in the auditory system. To begin to evaluate the relative contribution of subcortical inputs and local cortical circuits to observed changes in cortical recordings, a conditional knockout in which FMRP was deleted specifically from forebrain excitatory neurons using the Nex1 promoter was generated . Similar to global knockout animals, this conditional knockout strain exhibits elevated gamma frequency band power in EEGs as well as delayed development of PNN and PV cells, indicating that these deficits arise locally in the cortex.…”

Detection of neurophysiological features in female R255X MeCP2 mutation mice

“…Critically, ERP methods can be developed and used across species, providing the ability to translate findings readily from preclinical studies to clinical trials. Within neurodevelopmental disorders, great progress has been made in Fragile X syndrome: parallel studies on ERP in people with Fragile X syndrome and preclinical animal models suggested that they could be employed as biomarkers to evaluate disease severity or predict treatment response ( Ethridge et al, 2016 ; Ethridge et al, 2017 ; Ethridge et al, 2019 ; Lovelace, 2020 ; Sinclair et al, 2017b ; Van der Molen et al, 2012 ; Wen et al, 2019 ). Previous work in people with RTT has found changes in both visual ( LeBlanc et al, 2015 ) and auditory ERP ( Stauder et al, 2006 ), and similar changes in both visual and auditory ERPs have been observed in mouse models of RTT ( Durand et al, 2012 ; Goffin et al, 2012 ; Goffin et al, 2014 ).…”

Astroglial FMRP modulates synaptic signaling and behavior phenotypes in FXS mouse model

“…Fig. S1F) but not AMPAR-(Cre -Fmr1 loxP-neo/y , 2.47 ± 0.03 pA/pF, n=32; Cre + Fmr1 loxP-neo/y , 2.41 ± 0.03 pA/pF, n=33, p=0 19). responses in Cre + Fmr1 loxP-neo/y cortical neurons are significantly lower than that in control Cre -Fmr1 loxP-neo/y cortical neurons(Fig.…”

“…Selective ablation of FMRP in adult neural stem cells leads to reduced hippocampal neurogenesis and disrupts hippocampusdependent learning (18). Forebrain excitatory neuron-selective deletion of the fmr1 gene is sufficient to enhance cortical MMP-9 gelatinase activity, mTOR/Akt phosphorylation, and resting EEG gamma power (19). By generating astroglial Fmr1 cKO and cON mice in which FMRP expression is selectively diminished or restored (20), we previously showed that selective loss of astroglial FMRP dysregulates astroglial glutamate transporter GLT1 expression which contributes to neuronal excitability as well as increased spine density (20).…”