Low expression of Kv7/M channels facilitates intrinsic and network bursting in the developing rat hippocampus

KV7 channelopathies

KV7 voltage-gated potassium channels, encoded by the KCNQ gene family, have caught increasing interest of the scientific community for their important physiological roles, which are emphasized by the fact that four of the five so far identified members are related to different hereditary diseases. Furthermore, these channels prove to be attractive pharmacological targets for treating diseases characterized by membrane hyperexcitability. KV7 channels are expressed in brain, heart, thyroid gland, pancreas, inner ear, muscle, stomach, and intestines. They give rise to functionally important potassium currents, reduction of which results in pathologies such as long QT syndrome, diabetes, neonatal epilepsy, neuromyotonia, or progressive deafness. Here, we summarize some key traits of KV7 channels and review how their molecular deficiencies could explain diverse disease phenotypes. We also assess the therapeutic potential of KV7 channels; in particular, how the activation of KV7 channels by the compounds retigabine and R-L3 may be useful for treatment of epilepsy or cardiac arrhythmia.

Spontaneous bursting activity in the developing entorhinal cortex

Periodic spontaneous activity represents an important attribute of the developing nervous system. The entorhinal cortex (EC) is a crucial component of the medial temporal lobe memory system. Yet, little is known about spontaneous activity in the immature EC. Here, we investigated spontaneous field potential (fp) activity and intrinsic firing patterns of medial EC layer III principal neurons in brain slices obtained from rats at the first two postnatal weeks. A fraction of immature layer III neurons spontaneously generated prolonged (2-20 s) voltage-dependent intrinsic bursting activity. Prolonged bursts were dependent on the extracellular concentration of Ca(2+) ([Ca(2+)](o)). Thus, reduction of [Ca(2+)](o) increased the fraction of neurons with prolonged bursting by inducing intrinsic bursts in regularly firing neurons. In 1 mm [Ca(2+)](o), the percentages of neurons showing prolonged bursts were 53%, 81%, and 29% at postnatal day 5 (P5)-P7, P8-P10, and P11-P13, respectively. Prolonged intrinsic bursting activity was blocked by buffering intracellular Ca(2+) with BAPTA, and by Cd(2+), flufenamic acid (FFA), or TTX, and was suppressed by nifedipine and riluzole, suggesting that the Ca(2+)-sensitive nonspecific cationic current (I(CAN)) and the persistent Na(+) current (I(Nap)) underlie this effect. Indeed, a 0.2-1 s suprathreshold current step stimulus elicited a terminated plateau potential in these neurons. fp recordings at P5-P7 showed periodic spontaneous glutamate receptor-mediated events (sharp fp events or prolonged fp bursts) which were blocked by FFA. Slow-wave network oscillations become a dominant pattern at P11-P13. We conclude that prolonged intrinsic bursting activity is a characteristic feature of developing medial EC layer III neurons that might be involved in neuronal and network maturation.

Functional analysis of novel KCNQ2 mutations found in patients with Benign Familial Neonatal Convulsions

Benign Familial Neonatal Convulsions (BFNC) are a rare epilepsy disorder with an autosomal-dominant inheritance. It is linked to mutations in the potassium channel genes KCNQ2 and KCNQ3. These encode for Kv7.2 and Kv7.3 potassium ion channels, which produce an M-current that regulates the potential firing action in neurons through modulation of the membrane potential. We report on the biophysical and biochemical properties of V589X, T359K and P410fs12X mutant-KCNQ2 ion channels that were detected in three BFNC families. Mutant KCNQ2 cDNAs were co-expressed with WT-KCNQ2 and KCNQ3 cDNAs in HEK293 cells to mimic heterozygous expression of the KCNQ2 mutations in BFNC patients. The resulting potassium currents were measured using patch-clamp techniques and showed an approximately 75% reduction in current and a depolarized shift in the voltage dependence of activation. Furthermore, the time-constant of activation of M-currents in cells expressing T359K and P410fs12X was slower compared to cells expressing only wild-type proteins. Immunofluorescent labeling of HEK293 cells stably expressing GFP-tagged KCNQ2-WT or mutant alpha-subunits indicated cell surface expression of WT, V589X and T359K mutants, suggesting a loss-of-function, while P410fs12X was predominantly retained in the ER and sub-cellular compartments outside the ER suggesting an effectively haplo-insufficient effect.

Compensatory enhancement of intrinsic spiking upon NKCC1 disruption in neonatal hippocampus

Depolarizing and excitatory GABA actions are thought to be important in cortical development. We show here that GABA has no excitatory action on CA3 pyramidal neurons in hippocampal slices from neonatal NKCC1(-/-) mice that lack the Na-K-2Cl cotransporter isoform 1. Strikingly, NKCC1(-/-) slices generated endogenous network events similar to giant depolarizing potentials (GDPs), but, unlike in wild-type slices, the GDPs were not facilitated by the GABA(A) agonist isoguvacine or blocked by the NKCC1 inhibitor bumetanide. The developmental upregulation of the K-Cl cotransporter 2 (KCC2) was unperturbed, whereas the pharmacologically isolated glutamatergic network activity and the intrinsic excitability of CA3 pyramidal neurons were enhanced in the NKCC1(-/-) hippocampus. Hence, developmental expression of KCC2, unsilencing of AMPA-type synapses, and early network events can take place in the absence of excitatory GABAergic signaling in the neonatal hippocampus. Furthermore, we show that genetic as well as pharmacologically induced loss of NKCC1-dependent excitatory actions of GABA results in a dramatic compensatory increase in the intrinsic excitability of glutamatergic neurons, pointing to powerful homeostatic regulation of neuronal activity in the developing hippocampal circuitry.

At immature mossy fibers-CA3 connections, activation of presynaptic GABA(B) receptors by endogenously released GABA contributes to synapses silencing

Early in postnatal life correlated GABAergic activity in the hippocampus is thought to play a crucial role in synaptogenesis and in the development of adult neuronal networks. Unlike adulthood, at this developmental stage, mossy fibers (MF) which are the axons of granule cells, release GABA into CA3 principal cells and interneurons. Here, we tested the hypothesis that at MF-CA3 connections, tonic activation of GABA(B) autoreceptors by GABA is responsible for the low probability of release and synapse silencing. Blocking GABA(B) receptors with CGP55845 enhanced the probability of GABA release and switched on silent synapses while the opposite was observed with baclofen. Both these effects were presynaptic and were associated with changes in paired-pulse ratio and coefficient of variation. In addition, enhancing the extracellular GABA concentration by repetitive stimulation of MF or by blocking the GABA transporter GAT-1, switched off active synapses, an effect that was prevented by CGP55845. In the presence of CGP55845, stimulation of MF-induced synaptic potentiation. The shift of E(GABA) from the depolarizing to the hyperpolarizing direction with bumetanide, a blocker of the cation-chloride co-transporter NKCC1, prevented synaptic potentiation and caused synaptic depression, suggesting that the depolarizing action of GABA observed in the presence of CGP55845 is responsible for the potentiating effect. It is proposed that, activation of GABA(B) receptors by spillover of GABA from MF terminals reduces the probability of release and contributes to synapses silencing. This would act as a filter to prevent excessive activation of the auto-associative CA3 network and the emergence of seizures.