A fundamental regulator of neuronal network development and plasticity is the extracellular matrix (ECM) of the brain. The ECM provides a scaffold stabilizing synaptic circuits, while the proteolytic cleavage of its components and cell surface proteins are thought to have permissive roles in the regulation of plasticity. The enzymatic proteolysis is thought to be crucial for homeostasis between stability and reorganizational plasticity and facilitated largely by a family of proteinases named matrix metalloproteinases (MMPs). Here, we investigated whether MMP2 and MMP9 play a role in mediating adult primary visual cortex (V1) plasticity as well as stroke-induced impairments of visual cortex plasticity in mice. In healthy adult mice, selective inhibition of MMP2/9 for 7 d suppressed ocular dominance plasticity. In contrast, brief inhibition of MMP2/9 after a cortical stroke rescued compromised plasticity. Our data indicate that the proteolytic activity of MMP2 and MMP9 is critical and required to be within a narrow range to allow adult visual plasticity.SIGNIFICANCE STATEMENTLearning and recovery from injuries depend on the plasticity of neuronal connections. The brain's extracellular matrix (ECM) provides a scaffold for stabilizing synaptic circuits, while its enzymatic proteolysis is hypothesized to regulate homeostasis between stability and reorganizational plasticity. ECM digestion is facilitated by a family of matrix metalloproteinases (MMPs). Here, we show that treatments that inhibit MMP2/9 can either inhibit or rescue cortical plasticity depending on cortical state: in the visual cortex of healthy adult mice, inhibition of MMP2/9 suppressed cortical plasticity. In contrast, brief inhibition of MMP2/9 after a stroke rescued compromised plasticity. Our data provide strong evidence that an optimal level of MMP2/9 proteolytic activity is crucial for adult visual plasticity.

Development of the central nervous system (CNS) depends on accurate spatiotemporal control of signaling pathways and transcriptional programs. Forkhead Box G1 (FOXG1) is one of the master regulators that play fundamental roles in forebrain development; from the timing of neurogenesis, to the patterning of the cerebral cortex. Mutations in the FOXG1 gene cause a rare neurodevelopmental disorder called FOXG1 syndrome, also known as congenital form of Rett syndrome. Patients presenting with FOXG1 syndrome manifest a spectrum of phenotypes, ranging from severe cognitive dysfunction and microcephaly to social withdrawal and communication deficits, with varying severities. To develop and improve therapeutic interventions, there has been considerable progress towards unravelling the multi-faceted functions of FOXG1 in the neurodevelopment and pathogenesis of FOXG1 syndrome. Moreover, recent advances in genome editing and stem cell technologies, as well as the increased yield of information from high throughput omics, have opened promising and important new avenues in FOXG1 research. In this review, we provide a summary of the clinical features and emerging molecular mechanisms underlying FOXG1 syndrome, and explore disease-modelling approaches in animals and human-based systems, to highlight the prospects of research and possible clinical interventions.

Forkhead box G1 (FOXG1) has important functions in neuronal differentiation and balances excitatory/inhibitory network activity. Thus far, molecular processes underlying FOXG1 function are largely unexplored. Here, we present a multiomics data set exploring how FOXG1 impacts neuronal maturation at the chromatin level in the mouse hippocampus. At a genome-wide level, FOXG1 i) both represses and activates transcription, ii) binds mainly to enhancer regions, iii) reconfigures the epigenetic landscape through bidirectional alteration of H3K27ac, H3K4me3, and chromatin accessibility, and iv) operates synergistically with NEUROD1. Interestingly, we could not detect a clear hierarchy of FOXG1 and NEUROD1, but instead, provide the evidence that they act in a highly cooperative manner to control neuronal maturation. Genes affected by the chromatin alterations impact synaptogenesis and axonogenesis. Inhibition of histone deacetylases partially rescues transcriptional alterations upon FOXG1 reduction. This integrated multiomics view of changes upon FOXG1 reduction reveals an unprecedented multimodality of FOXG1 functions converging on neuronal maturation. It fuels therapeutic options based on epigenetic drugs to alleviate, at least in part, neuronal dysfunction.

The cortical plate (CP) is composed of excitatory and inhibitory neurons, the latter of which originate in the ganglionic eminences. From their origin in the ventral telencephalon, maturing postmitotic interneurons migrate during embryonic development over some distance to reach their final destination in the CP. The histone methyltransferase Disruptor of Telomeric Silencing 1-like (DOT1L) is necessary for proper CP development and layer distribution of glutamatergic neurons. However, its specific role on cortical interneuron development has not yet been explored. Here, we demonstrate that DOT1L affects interneuron development in a cell autonomous manner. Deletion of Dot1l in Nkx2.1-expressing interneuron precursor cells results in an overall reduction and altered distribution of GABAergic interneurons in the CP from postnatal day 0 onwards. We observed an altered proportion of GABAergic interneurons in the cortex, with a significant decrease in parvalbumin-expressing interneurons. Moreover, a decreased number of mitotic cells at the embryonic day E14.5 was observed upon Dot1l deletion. Altogether, our results indicate that reduced numbers of cortical interneurons upon DOT1L deletion result from premature cell cycle exit, but effects on postmitotic differentiation, maturation, and migration are likely at play as well.

Development of the central nervous system (CNS) depends on accurate spatiotemporal control of signalling pathways and transcription programs. Forkhead Box G1 (FOXG1) is one of the master regulators that plays fundamental roles in forebrain development, from the timing of neurogenesis to the patterning of the cerebral cortex. Mutations in the FOXG1 gene cause a rare neurodevelopmental disorder called FOXG1 syndrome, also known as congenital form of Rett syndrome. Patients presenting with FOXG1 syndrome manifest a spectrum of phenotypes ranging from severe cognitive dysfunction and microcephaly to social withdrawal and communication deficits with varying severities. To develop and improve therapeutic interventions, there has been considerable progress towards unravelling the multi-faceted functions of FOXG1 in neurodevelopment and pathogenesis of FOXG1 syndrome. Moreover, recent advances in genome editing and stem cell technologies, as well as increased yield of information from high throughput omics opened promising and important new avenues in FOXG1 research. In this review, we provide a summary of clinical features and emerging molecular mechanisms underlying FOXG1 syndrome, and explore disease-modelling approaches in animals and human-based systems to highlight prospects of research and possible clinical interventions.

The choroid plexus is a secretory brain epithelium that produces cerebrospinal fluid and sources developmental signals. Choroid plexus cysts, reported as feature in FOXG1 syndrome patients, suggest that pathogenic FOXG1 variants may redirect early forebrain tissue to adapt a choroid plexus-like fate. FOXG1 syndrome is caused by diverse FOXG1 mutations, including gene deletions and nonsense mutations, yet it remains unclear whether these variants affect early developmental fate decisions including choroid plexus in the same way. We address this question in patient and CRISPR-edited human iPSC-derived cerebral organoids. Recurrent FOXG1 syndrome nonsense mutations, but not deletion of the entire coding gene, redirect developing neural tissue away from telencephalic identity and towards an ectopic choroid plexus-like fate. This fate switch occurs in mutation- and stage-dependent manners, but converges on reduced nuclear FOXG1 availability due to decreased FOXG1 dosage and cytoplasmic mislocalisation. Mechanistically, we identify WLS, an essential chaperone for WNT ligand secretion, as a FOXG1-repressed target. Loss of FOXG1 occupancy at the WLS and BMP4 locus derepresses both genes, increases WNT and BMP signalling, and promotes choroid plexus-associated gene expression. In mosaic organoids, this response extends beyond mutant cells: neighbouring control cells also exhibit increased WLS expression and choroid plexus-like features, consistent with coupling cell-autonomous fate induction to non-cell-autonomous propagation of choroid plexus-like identity. Together, our findings define a FOXG1-dependent mechanism that relies on its expression dynamic and critical thresholds, and in which nonsense mutation-specific loss of functional FOXG1 activates a WLS-driven WNT signalling axis. This provides a framework for how specific FOXG1 variants can converge on choroid plexus cyst-like phenotypes and reveals FOXG1 as a safeguard of early forebrain fate.