TORC1‐dependent epilepsy caused by acute biallelic Tsc1 deletion in adult mice

Abstract

Commentary The dentate gyrus (DG) is thought to serve as a gate regulating the spread of excitatory input from the entorhinal cortex into the hippocampus (1). Breakdown of this gating function in the DG has been hypothesized to promote development of epileptogenesis in temporal lobe epilepsy (1, 2). A variety of pathological changes in DG granule cells in animal models and patients with temporal lobe epilepsy may contribute to disrupted DG function, including somatic hypertrophy, formation of basilar dendrites, ectopic granule cells within the hilus, and mossy fiber sprouting (3). These and other cellular and molecular abnormalities within the DG may lead to the formation of aberrant, excitatory circuits that result in temporal lobe epilepsy. However, as previous studies linking DG dysfunction and epileptogenesis have primarily been correlative in nature, direct proof that such DG abnormalities can definitively cause temporal lobe epilepsy has been lacking. In an elegant but conceptually straightforward study, Pun and colleagues provide compelling evidence that pathological disruption of the dentate gyrus is capable of causing temporal lobe epilepsy. To test this longstanding hypothesis, they took advantage of a specific genetic manipulation involving activation of the mammalian target of rapamycin complex 1 (mTORC1) pathway within DG granule cells. The mTORC1 pathway regulates a number of important cellular processes and has been implicated in promoting epileptogenesis in a variety of types of epilepsy (4). This is especially well-established in animal models of the genetic epilepsy, tuberous sclerosis complex; however, there is also some evidence for a role of mTORC1 in models of acquired epilepsy, such as due to brain injury following status epilepticus or trauma. Using targeted genetic techniques, Pun and colleagues inactivated the phos-phatase and tensin homolog (PTEN) gene primarily in DG granule cells in 2-week-old mice, as well as incidentally in a small population of inhibitory interneurons in the olfactory bulb. As PTEN acts as an upstream regulator of the mTORC1 pathway, loss of PTEN led to abnormal hyperactivation of the mTORC1 pathway in the targeted neurons of the knockout mice. Remarkably, epilepsy occurred in almost all of the PTEN knockout mice within 4-6 weeks of inducing the PTEN inac-tivation. As documented by intracranial EEG recordings, the seizures appeared to originate focally within the hippocam-pus, not neocortex. Quantitative assessment found that PTEN inactivation in as few as 9% of DG granule cells was enough to cause epilepsy. Furthermore, the DG granule cells in these mice developed a number of pathological abnormalities seen in human patients and other animal models of temporal lobe epilepsy, including neuronal hypertrophy, basal dendrite formation, increased dendritic spine density, ectopic neurons, and mossy fiber sprouting. Importantly, treatment with the mTORC1 inhibitor, rapamycin, significantly attenuated the development of epilepsy and DG pathological changes, indicating th...

The Neurology of mTOR

“…However, it is also likely that mTOR regulates neuronal excitability in already established neural circuits. Consistent with this model, Tsc1 inactivation in adult mice is rapidly followed by mTOR-dependent seizures(Abs et al, 2013). …”

Animal models of tumour-associated epilepsy

“…On the molecular level, homozygous Tsc1 deletion was associated with a significant reduction of the Tsc1 gene product (hamartin) as well as disinhibition of mTOR activity. Albeit this manipulation was lethal within weeks, the results demonstrated that acute loss of Tsc1 in the adult brain was sufficient to induce epileptogenesis, since structural abnormalities were absent in these mice and rapamycin treatment reduced seizure rates and increased survival (Abs et al, 2013). Moreover, these studies suggest that altered mTOR signalling may be a key mechanism of epileptogenesis in TSC.…”

“…But seizures did occur when Tsc1 was acutely inactivated in all (i.e. neuronal and non-neuronal) adult cells (Abs et al, 2013). On the molecular level, homozygous Tsc1 deletion was associated with a significant reduction of the Tsc1 gene product (hamartin) as well as disinhibition of mTOR activity.…”

The mTOR signalling cascade: paving new roads to cure neurological disease

“…Of course, a critical unanswered question remains about the extent to which seizures in these disorders reflect network disruption owing to the cortical malformation versus the effects of hyperactivated mTOR signalling. In one compelling study, inducible knockout of Tsc1 in adult rats led to rapid onset of severe seizures without having effects on brain structure, suggesting that mTOR activation is particularly relevant for seizure onset 54 .…”