Cell‐specific alterations of glutamate receptor expression in tuberous sclerosis complex cortical tubers

Abstract

Objective-Genetic loss of TSC1/TSC2 function in tuberous sclerosis complex (TSC) results in overactivation of the mammalian target of rapamycin complex 1 pathway, leading to cellular dysplasia. We hypothesized that the dysplastic cells in TSC tubers are heterogeneous, including separable classes on a neuronal-glial spectrum, and that these dysplastic cells express glutamate receptor (GluR) patterns consistent with increased cortical network excitability.Methods-Surgically resected human cortical tubers and nondysplastic epileptic cortical samples were analyzed by double-label immunocytochemistry for coexpression of neuronal and glial markers, the TSC1/TSC2 pathway downstream molecule phospho-S6 (pS6) and GluR subunits, and compared with control cortical tissue. Western blotting was used to quantify changes in GluR subunit expression in tubers versus controls.Results-We demonstrate that cortical tubers contain a broad spectrum of cell types including disoriented pyramidal cells, dysplastic neurons, giant neuroglial cells, dysplastic astroglia, and reactive astrocytes. Dysplastic neurons, giant cells, and dysplastic astroglia express high levels of pS6 and demonstrate altered GluR subunit composition, resembling those of normal immature neurons and glia. In contrast, nondysplastic neurons in TSC and non-TSC epileptic lesions express lower pS6 levels and display changes in GluR subunit expression that are distinct from the patterns seen in tuber dysplastic cells.Interpretation-This work significantly expands the spectrum of abnormal cells recognized in tubers beyond the classic tuber giant cell and demonstrates cell-specific abnormalities in GluR expression that may contribute to seizure pathogenesis in TSC. Furthermore, these results suggest that subunit-specific antagonists may be of potential use in the treatment of epilepsy in TSC.Tuberous sclerosis complex (TSC) is an autosomal dominant disorder caused by inactivating mutations in either the TSC1 or TSC2 gene. 1,2 TSC brain lesions are malformations of cortical development that include cortical tubers, white matter abnormalities, subependymal nodules, Although previous reports have recognized the occurrence of various dysplastic cell types within tubers beyond the classical giant cell, 4,20,21 we set out to classify the cellular pathology of these lesions in detail, using combined morphological and immunocytochemical criteria, including the use of neuronal and glial markers and phospho-S6 (pS6) to assess the mammalian target of rapamycin (mTOR) pathway dysfunction. In addition, given that TSC is a malformation of development, we explored the hypothesis that the abnormal cell types present in tubers express immature iGluRs patterns known to promote cortical network hyperexcitability, thereby possibly contributing to the epileptogenicity of these lesions. White and coworkers 22 previously reported that dysplastic neurons and giant cells express abnormal patterns of glutamate and GABA receptor subunit messenger RNA. To further determine whether neurotrans...

Epilepsy Related to Developmental Tumors and Malformations of Cortical Development

“…The giant cells in TSC are histologically similar to balloon cells detected in FCD type IIb in that they exhibit an enlarged cell body and opalescent, glassy-appearing, eosinophilic cytoplasm [6]. Both giant cells and balloon cells express cell proteins characteristic of neuroglial progenitor cells such as SOX2, nestin, vimentin, and c-myc, suggesting a failure to differentiate prior to migration into the cortex [137,[191][192][193]; for reviews see [117] and [185]). With the recent advances in both fetal ultrasonography and magnetic resonance imaging, tubers can be detected during the prenatal period in TSC patients [194][195][196].…”

“…For example, large-scale gene expression studies indicate alterations in glutamatergic and GABAergic synaptic transmission in cortical tubers, involving reduced expression of the glial glutamate transporter (GLT-1 or EAAT2), reduced expression levels of GABA A receptor subunits, and reduced expression of potassium channels [135]. Selective alterations of iGlu subunits, as well as of mGluR subtypes (with prominent group I mGluR expression, mGluR1, and mGluR5) have been reported in tubers [193,223,224]. In addition (as discussed above) an excitatory action of GABA has been reported in TSC [162].…”

mTOR Signaling in Epilepsy: Insights from Malformations of Cortical Development

“…Numerous studies have shown activated mTORC1 substrates phospho-p70S6 kinase, phospho-S6, and phospho-4E-BP1 in resected and postmortem TSC tuber samples (Baybis et al 2004;Miyata et al 2004;Talos et al 2008). Indeed, two recent studies have shown mTORC1 activation in fetal tubers (Tsai et al 2012;Prabowo et al 2013).…”

Regulable neural progenitor-specific Tsc1 loss yields giant cells with organellar dysfunction in a model of tuberous sclerosis complex

“…TSC cortical tubers exhibit disrupted neuronal laminar architecture, hypomyelination, and the hallmark enlarged, undifferentiated, "balloon-like" giant cells (7,8), as well as dysmorphic and enlarged neurons, hypertrophic astrocytes, and a variety of reactive cells (9)(10)(11). Severe epilepsy is seen in most patients with TSC, and the number and size of tubers correlate with an increased risk for intractable seizures (3,5), which are associated with poor cognitive outcome (6).…”