Defective glutamate and K+ clearance by cortical astrocytes in familial hemiplegic migraine type 2.

Clizia Capuani,Angelita Tottene,Marcello Melone,Giovanna Crivellaro,Giorgio Casari,Fiorenzo Conti,Mirko Santello,Daniela Pietrobon,Luca Bragina

doi:10.15252/emmm.201505944

Abstract

Migraine is a common disabling brain disorder. A subtype of migraine with aura (familial hemiplegic migraine type 2: FHM2) is caused by loss‐of‐function mutations in α2 Na+,K+ ATPase (α2 NKA), an isoform almost exclusively expressed in astrocytes in adult brain. Cortical spreading depression (CSD), the phenomenon that underlies migraine aura and activates migraine headache mechanisms, is facilitated in heterozygous FHM2‐knockin mice with reduced expression of α2 NKA. The mechanisms underlying an increased susceptibility to CSD in FHM2 are unknown. Here, we show reduced rates of glutamate and K+ clearance by cortical astrocytes during neuronal activity and reduced density of GLT‐1a glutamate transporters in cortical perisynaptic astrocytic processes in heterozygous FHM2‐knockin mice, demonstrating key physiological roles of α2 NKA and supporting tight coupling with GLT‐1a. Using ceftriaxone treatment of FHM2 mutants and partial inhibition of glutamate transporters in wild‐type mice, we obtain evidence that defective glutamate clearance can account for most of the facilitation of CSD initiation in FHM2‐knockin mice, pointing to excessive glutamatergic transmission as a key mechanism underlying the vulnerability to CSD ignition in migraine.

Highlights

Migraine is a common disabling brain disease, which manifests itself as recurrent attacks of typically throbbing and unilateral headache with certain associated features such as nausea and hypersensitivity to sensory stimuli
To investigate whether the reduced membrane expression of the a2 NKA in heterozygous W887R/+ FHM type 2 (FHM2)-KI mice causes a reduced rate of Glu clearance by cortical astrocytes during neuronal activity, we took advantage of the fact that i) Glu uptake into astrocytes by Glu transporters (GluTs) is electrogenic, and can be monitored electrophysiologically, and ii) the time course of the Glu transporter current elicited in astrocytes upon extracellular neuronal stimulation in hippocampal slices reflects, to some extent, the time course of Glu clearance by astrocytes and provides a relative indication of how rapidly synaptically released Glu is taken up from extracellular space (Bergles & Jahr, 1997; Diamond, 2005)
We have studied the functional consequences of the reduced membrane expression of the a2 NKA pump in heterozygous W887R/+ FHM2-KI mice on the rate of Glu clearance by astrocytes during neuronal activity, as measured from the decay kinetics of the synaptically activated Glu transporter current recorded in cortical astrocytes

Summary

Introduction

Migraine is a common disabling brain disease, which manifests itself as recurrent attacks of typically throbbing and unilateral headache with certain associated features such as nausea and hypersensitivity to sensory stimuli. The mechanisms underlying an increased susceptibility to CSD in FHM2 are unknown, impaired Glu and/or K+ clearance by astrocytes have been suggested as hypothetical mechanisms (Moskowitz et al, 2004; Pietrobon, 2007) To test these hypotheses and to gain insights into the physiological role of the a2 NKA, we investigated the functional consequences of the W887R FHM2-causing mutation on Glu and K+ clearance by cortical astrocytes during neuronal activity in acute cortical slices. Using an in vitro model of CSD and ceftriaxone treatment in FHM2-KI mice as well as pharmacological inhibition of a fraction of GluTs in wild-type (WT) mice, we provide evidence that the defective Glu clearance can account for most of the facilitation of CSD initiation in the FHM2 mouse model

Results

A KCl puffer

Discussion

Materials and Methods