mTOR-dependent abnormalities in autophagy characterize human malformations of cortical development: evidence from focal cortical dysplasia and tuberous sclerosis

Abstract

Focal cortical dysplasia (FCD) is a localized malformation of cortical development and is the commonest cause of severe childhood epilepsy in surgical practice. Children with FCD are severely disabled by their epilepsy, presenting with frequent seizures early in life. The commonest form of FCD in children is characterized by the presence of an abnormal population of cells, known as balloon cells. Similar pathological changes are seen in the cortical malformations that characterize patients with tuberous sclerosis complex (TSC). However, the cellular and molecular mechanisms that underlie the malformations of FCD and TSC are not well understood. We provide evidence for a defect in autophagy in FCD and TSC. We have found that balloon cells contain vacuoles that include components of the autophagy pathway. Specifically, we show that balloon cells contain prominent lysosomes by electron microscopy, immunohistochemistry for LAMP1 and LAMP2, LysoTracker labelling and enzyme histochemistry for acid phosphatase. Furthermore, we found that balloon cells contain components of the ATG pathway and that there is cytoplasmic accumulation of the regulator of autophagy, DOR. Most importantly we found that there is abnormal accumulation of the autophagy cargo protein, p62. We show that this defect in autophagy can be, in part, reversed in vitro by inhibition of the mammalian target of rapamycin (mTOR) suggesting that abnormal activation of mTOR may contribute directly to a defect in autophagy in FCD and TSC.

Epilepsy Related to Developmental Tumors and Malformations of Cortical Development

“…Moreover, similarly to FCD II, induction of apoptosis signaling pathways and premature activation of mechanisms of neurodegeneration, as well as a defect in autophagy [132], has also been observed in the tubers.…”

“…Moreover, dysmorphic neurons and ballon/giant cells show nuclear and cytoplasmic accumulation of p62 [121], a stress-inducible intracellular protein, which is known to regulate different signal transduction pathways, and has been recently identified as a key target of autophagy (for reviews see [130,131]. A recent study confirms the mTOR-dependent abnormalities in autophagy, indicating a defect in autophagy as a common feature of FCD II and TSC [132]. The mTOR signaling (overactivated in both TSC and FCD II) is, indeed, a major negative regulator of autophagy ( [133]; for a review see [134]), and gene expression analysis of TSC cortical tubers revealed a deregulation of genes associated with ubiquitination [135].…”

PI3K-AKT pathway polymerase chain reaction (PCR) array analysis of epilepsy induced by type II focal cortical dysplasia

“…The mammalian target of rapamycin (mTOR) is a highly conserved serine/threonine protein kinase, and much evidence has accumulated indicating that the mTOR signaling pathway plays an important role in the pathogenesis of epilepsy (Meng et al, 2013;Yasin et al, 2013;Lee et al, 2014;Scheffer et al, 2014). TSC is an autosomal dominant hereditary disease denoted by epilepsy relating to aberrant activation of mTOR caused by TSC1/TSC2 gene mutation.…”

Early lipofuscin accumulation in frontal lobe epilepsy

“…A few balloon cells in FCDIIB cases showed some fine granular PAS-positive staining in the cytoplasm (Fig 2C) or sometimes at the margins of the cell (Fig 2C, inset), reminiscent of the patterns reported in the cell culture of balloon cells, with accumulation of dense core lysosomes. 22 Balloon cells in FCDIIB cases also showed p62 immunopositivity, displaying both Four of these proteins (clathrin, dynamin, syntaxin binding protein, and vesicle-associated membrane protein 2) had been described in a previous study investigating the proteome of lipofuscin in aged and Alzheimer'sdiseased human brains (Ottis et al, 2012). At the presynaptic terminal of neurones, synaptic vesicles are actively loaded with neurotransmitters (NT) and then they are docked and "primed" at the "active zone" of the plasma membrane.…”