Seminar: Iryna Ethell PhD (UC Riverside)

Iryna Ethell PhD Professor of Biomedical Sciences
Associate Dean for Academic Affairs, School of Medicine
University of California – Riverside

Seminar Title: “Role of Matrix Metalloproteinase 9, Perineuronal Nets and Cortical Circuits in Auditory Hyperexcitability in Fragile X Syndrome Mouse Model”

Host: Patricia Maness PhD

Fragile X Syndrome (FXS) is the most prevalent cause of inherited intellectual disability and autism with symptoms including cognitive and communication deficits. The Fmr1 knockout (KO) mouse is an established model of FXS as it displays similar behavioral impairments, including hypersensitivity to sound that is seen in humans with FXS.  Our study revealed abnormal responses to sound in auditory cortex of Fmr1 KO mice, which were normalized following genetic deletion of Mmp-9.  However, the mechanisms underlying the auditory deficits are not known. In the normal auditory cortex Mmp-9 levels are higher during onset of hearing in mice and are gradually decreased as auditory circuits develop and mature. Our data show that MMP-9 levels are abnormally high in the auditory cortex of Fmr1 KO mice during development and remain high in the adult, suggesting that elevated MMP-9 levels may be responsible for the abnormal auditory deficits in Fmr1 KO mice. MMP-9 can impact the inhibition by disrupting perineuronal nets (PNN) that are critical in supporting fast spiking Parvalbumin (PV) interneurons. Our results support this assumption and show that PV cell development and PNN formation around GABAergic interneurons were impaired in P21 auditory cortex of Fmr1 KO mice, which may impact the development of inhibitory circuits. Indeed, in vivo single unit recordings from auditory cortex neurons showed enhanced spontaneous and sound-driven responses in developing Fmr1 KO mice, which were normalized following genetic reduction of MMP-9. Genetic reduction of MMP-9 also restored PNN formation to WT levels in L4 auditory cortex of Fmr1 KO mice. We also found that enhanced PNN formation following acute pharmacological treatment of Fmr1 KO mice with MMP-9 inhibitor SB-3CT reverses auditory processing deficits by measuring resting state and evoked neural oscillations in freely moving mice implanted with electroencephalography (EEG) electrodes. These findings indicate that elevated MMP-9 levels contribute to the development of sensory hypersensitivity by influencing formation of PNNs around PV interneurons suggesting MMP9 as a new therapeutic target to reduce sensory deficits in FXS and potentially other autism spectrum disorders.

MMP-9 and extracellular matrix (ECM) are well-positioned to regulate cortical circuits by modulating interactions between excitatory neurons and PV cells. In vitro slice recordings showed increased cortical excitability in mouse somatosensory cortex following deletion of Fmr1 only from excitatory neurons by affecting excitability of inhibitory PV interneurons (Hays et al., 2011). In our studies, we found that cell specific deletion of Fmr1 from excitatory neurons during early postnatal P14-P21 window also affects the development of PV cells and PNNs in mouse auditory cortex, which remains impaired at P60. Embryonic and early postnatal (P14-P21) cre-mediated deletion of floxed Fmr1 gene was achieved in forebrain excitatory neurons using Nex1 (Cre^Nex1) and CaMK2a (Cre^CaMk2a) promoters, respectively. We found that MMP-9 gelatinase activity, mTOR/Akt phosphorylation and resting EEG gamma power were enhanced in the adult auditory cortex of both conditional KO mice, whereas the density of PV cells, PNNs and PV cells with PNN was reduced in these mice compared to control mice. Together, these findings suggest that the communications between excitatory neurons and PV interneurons may play an important role in the proper development of auditory circuits and can be influence by the changes in the extracellular matrix. This work was supported by grants from FRAXA Research Foundation, NIH 1U54HD082008 and the US Army Medical Research W81XWH-15-1-0436