Epileptogenesis and Reduced Inward Rectifier Potassium Current in Tuberous Sclerosis Complex‐1–Deficient Astrocytes

Abstract

Summary:Purpose: Individuals with tuberous sclerosis complex (TSC) frequently have intractable epilepsy. To gain insights into mechanisms of epileptogenesis in TSC, we previously developed a mouse model of TSC with conditional inactivation of the Tsc1 gene in glia (Tsc1 GFAP CKO mice). These mice develop progressive seizures, suggesting that glial dysfunction may be involved in epileptogenesis in TSC. Here, we investigated the hypothesis that impairment of potassium uptake through astrocyte inward rectifier potassium (Kir) channels may contribute to epileptogenesis in Tsc1 GFAP CKO mice. Methods: Kir channel function and expression were examined in cultured Tsc1-deficient astrocytes. Kir mRNA expression was analyzed in astrocytes microdissected from neocortical sections of Tsc1 GFAP CKO mice. Physiological assays of astrocyte Kir currents and susceptibility to epileptiform activity induced by increased extracellular potassium were further studied in situ in hippocampal slices.Results: Cultured Tsc1-deficient astrocytes exhibited reduced Kir currents and decreased expression of specific Kir channel protein subunits, Kir2.1 and Kir6.1. mRNA expression of the same Kir subunits also was reduced in astrocytes from neocortex of Tsc1 GFAP CKO mice. By using pharmacologic modulators of signalling pathways implicated in TSC, we showed that the impairment in Kir channel function was not affected by rapamycin inhibition of the mTOR/S6K pathway, but was reversed by decreasing CDK2 activity with roscovitine or retinoic acid. Last, hippocampal slices from Tsc1 GFAP CKO mice exhibited decreased astrocytic Kir currents, as well as increased susceptibility to potassium-induced epileptiform activity.Conclusions: Impaired extracellular potassium uptake by astrocytes through Kir channels may contribute to neuronal hyperexcitability and epileptogenesis in a mouse model of TSC.

Silencing the Kir4.1 Potassium Channel Subunit in Satellite Glial Cells of the Rat Trigeminal Ganglion Results in Pain-Like Behavior in the Absence of Nerve Injury

“…An increase in cortical neurons excitability has been described in mice with reduced Kir currents and lower expression of Kir2.1 and Kir6.1 channel subunits (Jansen et al, 2005). It has also been shown that extracellular K ϩ increases in Kir4.1 Ϫ/Ϫ mice (Neusch et al, 2006;Djukic et al, 2007), and, although an increase in neuronal excitability was not reported, in one case there was an enhancement of synaptic potentiation.…”

Single-cell Tsc1 knockout during corticogenesis generates tuber-like lesions and reduces seizure threshold in mice

“…This was unexpected, considering that astrocytes in human tubers show increased GFAP immunoreactivity and cell size and loss of GS (28,34). We did not explore other features such as K + channel and glutamate transporter expression that are altered in epileptic astrocytes or Tsc1 null astrocytes (36,(50)(51)(52). However, human tubers, which have variable degrees of gliosis, are removed from patients who undergo surgical treatment for focal epilepsy mapped to the resected tubers (53).…”

Downregulation of Kir4.1 inward rectifying potassium channel subunits by RNAi impairs potassium transfer and glutamate uptake by cultured cortical astrocytes

“…The K 1 siphoning process in the cortex may involve different mediators than in the retina where it has been suggested that Kir2.1 channels, in retinal M€ uller (glial) cells, may be responsible for K 1 influx within the synaptic layers, whereas Kir4.1 channels may be mainly responsible for K 1 release into the vitreous and the blood vessels (Kofuji and Newman, 2004;Kofuji et al, 2002). Expression of functional Kir2-type channel subunits in astrocytes remains controversial (Jansen et al, 2005;Pruss et al, 2005;Schr€ oder et al, 2002;Stonehouse et al, 1999). However, in the present study, we demonstrate an almost Fig.…”

“…The most obvious is that elevations in potassium due to decreased potassium buffering will lead to hyperexcitability of neurons and, ultimately, to seizures. Indeed, Jansen et al (2005) have recently demonstrated alterations of Kir channels in astrocytes associated with epileptogenesis in a mouse model of the tuberous sclerosis complex. Furthermore, mislocalization of aquaporin-4 which is normally colocalized with Kir4.1 protein (Nagelhus et al, 1999(Nagelhus et al, , 2004 results in delayed K 1 clearance and increased seizure activity in mice (Amiry-Moghaddam et al, 2003).…”