Balloon cells (BCs) in focal cortical dysplasia (FCD) and giant cells (GCs) in tubers of the tuberous sclerosis complex (TSC) share phenotypic similarities. TSC1 or TSC2 gene mutations in TSC lead to mTOR pathway activation and p70S6kinase (phospho-S6K) and ribosomal S6 (phospho-S6) protein phosphorylation. Phospho-S6K, phospho-S6, and phospho-S6K-activated proteins phospho-STAT3 and phospho-4EBP1 were detected immunohistochemically in GCs, whereas only phospho-S6 was observed in BCs. Expression of four candidate gene families (cell signaling, cell adhesion, growth factor/receptor, and transcription factor mRNAs) was assayed in single, microdissected phospho-S6-immunolabeled BCs and GCs as a strategy to define whether BCs and GCs exhibit differential transcriptional profiles. Among 60 genes, differential expression of 24 mRNAs distinguished BCs from GCs and only 4 genes showed similar expression profiles between BCs and GCs. Tuberin mRNA levels were reduced in GCs from TSC patients with TSC2 gene mutations but were unchanged in BCs. Phospho-S6K, -S6, -STAT3, and -4EBP1 expression in GCs reflects loss of hamartin-tuberin-mediated mTOR pathway inhibition. Phospho-S6 expression alone in BCs does not support mTOR cascade activation in FCD. Differential gene expression profiles in BCs and GCs supports the hypothesis that these cell types derive by distinct pathogenic mechanisms.

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Polyhydramnios, megalencephaly, and symptomatic epilepsy syndrome (PMSE) is a rare human autosomalrecessive disorder characterized by abnormal brain development, cognitive disability, and intractable epilepsy. It is caused by homozygous deletions of STE20-related kinase adaptor α (STRADA). The underlying pathogenic mechanisms of PMSE and the role of STRADA in cortical development remain unknown. Here, we found that a human PMSE brain exhibits cytomegaly, neuronal heterotopia, and aberrant activation of mammalian target of rapamycin complex 1 (mTORC1) signaling. STRADα normally binds and exports the protein kinase LKB1 out of the nucleus, leading to suppression of the mTORC1 pathway. We found that neurons in human PMSE cortex exhibited abnormal nuclear localization of LKB1. To investigate this further, we modeled PMSE in mouse neural progenitor cells (mNPCs) in vitro and in developing mouse cortex in vivo by knocking down STRADα expression. STRADα-deficient mNPCs were cytomegalic and showed aberrant rapamycin-dependent activation of mTORC1 in association with abnormal nuclear localization of LKB1. Consistent with the observations in human PMSE brain, knockdown of STRADα in vivo resulted in cortical malformation, enhanced mTORC1 activation, and abnormal nuclear localization of LKB1. Thus, we suggest that the aberrant nuclear accumulation of LKB1 caused by STRADα deficiency contributes to hyperactivation of mTORC1 signaling and disruption of neuronal lamination during corticogenesis, and thereby the neurological features associated with PMSE.

A rare neurodevelopmental disorder in the Old Order Mennonite population called PMSE (polyhydramnios, megalencephaly, and symptomatic epilepsy syndrome; also called Pretzel syndrome) is characterized by infantile-onset epilepsy, neurocognitive delay, craniofacial dysmorphism, and histopathological evidence of heterotopic neurons in subcortical white matter and subependymal regions. PMSE is caused by a homozygous deletion of exons 9 to 13 of the LYK5/STRADA gene, which encodes the pseudokinase STRADA, an upstream inhibitor of mammalian target of rapamycin complex 1 (mTORC1). We show that disrupted pathfinding in migrating mouse neural progenitor cells in vitro caused by STRADA depletion is prevented by mTORC1 inhibition with rapamycin or inhibition of its downstream effector p70 S6 kinase (p70S6K) with the drug PF-4708671 (p70S6Ki). We demonstrate that rapamycin can rescue aberrant cortical lamination and heterotopia associated with STRADA depletion in the mouse cerebral cortex. Constitutive mTORC1 signaling and a migration defect observed in fibroblasts from patients with PMSE were also prevented by mTORC1 inhibition. On the basis of these preclinical findings, we treated five PMSE patients with sirolimus (rapamycin) without complication and observed a reduction in seizure frequency and an improvement in receptive language. Our findings demonstrate a mechanistic link between STRADA loss and mTORC1 hyperactivity in PMSE, and suggest that mTORC1 inhibition may be a potential treatment for PMSE as well as other mTOR-associated neurodevelopmental disorders.

Focal cortical dysplasia type IIB with Ballon cells is a developmental malformation of the cerebral cortex highly associated with epilepsy. As a strategy to define the embryonic origin and neurochemical phenotype of cells in this disease, we probed specimens (n = 10) resected during epilepsy surgery with a panel of 13 antibodies recognizing proteins associated with (i) specific progenitor cell types including brain lipid binding protein (BLBP), collapsin response mediator protein 4 (CRMP4), Dlx1, Dlx2, GFAPdelta, MASH1, Otx1, Pax6, vimentin and phosphorylated vimentin and (ii) excitatory or inhibitory neurochemical phenotypes such as the vesicular glutamate transporters-1 and 2 (VGLUT-1, VGLUT-2), or the vesicular GABA transporter (VGAT). Balloon cells and large dysplastic neurons in all specimens expressed Otx1, phospho-vimentin, Pax6 and BLBP, proteins normally expressed by cells in the embryonic ventricular zone. A subpopulation of balloon cells expressed MASH-1 also expressed in the ventricular zone. Most balloon cells and dysplastic neurons were VGLUT2 immunoreactive, whereas none expressed Dlx1 or Dlx2, markers for inhibitory cells derived from the medial ganglionic eminence and few expressed VGAT, found in GABAergic interneurons. Otx1 mRNA expression and Dlx1 mRNA absence was confirmed by single cell RT-PCR. A subpopulation of balloon cells was labelled with CRMP4 and GFAPdelta, markers specific for newly generated cells derived from the adult subventricular zone. Detection of Otx1, phospho-vimentin, Pax6 and BLBP expression but absence of Dlx1/Dlx2 expression suggests that balloon cells and dysplastic neurons derive from radial glial cells in the telencephalic ventricular zone and not the medial ganglionic eminence. VGLUT expression argues that dysplastic neurons may be glutamatergic. CRMP-4 and GFAPdelta expression suggests that new cells may arrive in focal cortical dysplasia, perhaps deriving in part from the subventricular zone. These findings provide a developmental lineage model in which balloon cells and dysplastic neurons are derived from radial glial progenitor cells.