ATP1A2 Mutations in Migraine: Seeing through the Facets of an Ion Pump onto the Neurobiology of Disease

Abstract

Mutations in four genes have been identified in familial hemiplegic migraine (FHM), from which CACNA1A (FHM type 1) and SCN1A (FHM type 3) code for neuronal voltage-gated calcium or sodium channels, respectively, while ATP1A2 (FHM type 2) encodes the α2 isoform of the Na(+),K(+)-ATPase's catalytic subunit, thus classifying FHM primarily as an ion channel/ion transporter pathology. FHM type 4 is attributed to mutations in the PRRT2 gene, which encodes a proline-rich transmembrane protein of as yet unknown function. The Na(+),K(+)-ATPase maintains the physiological gradients for Na(+) and K(+) ions and is, therefore, critical for the activity of ion channels and transporters involved neuronal excitability, neurotransmitter uptake or Ca(2+) signaling. Strikingly diverse functional abnormalities have been identified for disease-linked ATP1A2 mutations which frequently lead to changes in the enzyme's voltage-dependent properties, kinetics, or apparent cation affinities, but some mutations are truly deleterious for enzyme function and thus cause full haploinsufficiency. Here, we summarize structural and functional data about the Na(+),K(+)-ATPase available to date and an overview is provided about the particular properties of the α2 isoform that explain its physiological relevance in electrically excitable tissues. In addition, current concepts about the neurobiology of migraine, the correlations between primary brain dysfunction and mechanisms of headache pain generation are described, together with insights gained recently from modeling approaches in computational neuroscience. Then, a survey is given about ATP1A2 mutations implicated in migraine cases as documented in the literature with focus on mutations that were described to completely destroy enzyme function, or lead to misfolded or mistargeted protein in particular model cell lines. We also discuss whether or not there are correlations between these most severe mutational effects and clinical phenotypes. Finally, perspectives for future research on the implications of Na(+),K(+)-ATPase mutations in human pathologies are presented.

Keywords: Na+/K+-ATPase; familial hemiplegic migraine; human ATP1A2; neuronal hyperexcitability; protein expression; protein stability; protein targeting; structure-function studies.

Figure 1

The Post-Albers reaction mechanism of the Na⁺,K⁺-ATPase. See text for details.

Figure 2

Structure of the Na⁺,K⁺-ATPase (PDB code 3B8E; Morth et al., 2007) and location of migraine-associated ATP1A2 mutations. The domains of the cytoplasmic part are A (actuator; light orange), N (nucleotide binding; orange) and P (phosphorylation; pink) domain, transmembrane helices are depicted in gray, except for the about 70 Å-long central TM5 helix (orange). The β- and γ-subunits are shown in magenta and blue, respectively, two Rb⁺ ions in the cation binding pocket are shown as purple spheres. Amino acids mutated in migraine cases as listed in Supplementary Table 1 are shown in stick representation. More than 80% of mutations fall into four clusters, one around the catalytic P domain, one in a central region between P and TM domain, one within the extracellularly-facing part of the TM domain, and one around the enzyme's C-terminus.

Figure 3

ATP1A2 and ATP1A3 in the tripartite synapse (Perea et al., 2009). Most glutamatergic synapses are in contact with astrocytic processes, which express a high density of excitatory amino acid transporters (EAATs, e.g., GLT1, GLAST), which are crucial for the synaptic glutamate (Glu) clearance. In astrocytes, glutamate is converted to glutamine by glutamine synthetase as part of glutamate recycling. Ion channels involved in electrical excitation are indicated by the respective cations, Na⁺, K⁺, and Ca²⁺. Abbreviations: GLAST (EAAT1), glutamate/aspartate transporter; GLT1 (EAAT2), glutamate transporter; GLNT, glutamine transporter; GluR, (metabotropic) glutamate receptor; NKA2/3, Na⁺, K⁺-ATPase α₂-α₃-subunit; NMDA, ionotropic glutamate receptor; NCX, Na⁺,Ca²⁺-exchanger; Gln, glutamine.

Figure 4

Stationary Na⁺/K⁺ pump currents of the Na⁺,K⁺-ATPase (human ATP1A2) from two-electrode voltage clamp experiments on X. laevis oocytes. (A) Pump currents in response to different extracellular [K⁺] at −30 mV holding potential, (B) I-V curves for different [K⁺]_ext at high [Na⁺]_ext (100 mM). (C) K_0.5(K⁺ext) values from fits of a Hill function to the [K⁺] dependent pump current amplitudes at different potentials from (B).

Figure 5

Properties of ouabain-sensitive transient currents of the Na⁺,K⁺-ATPase. (A) Transient currents evoked by pulses from −30 mV to voltages between +60 and −140 mV in −40 mV decrements measured on human ATP1A2 in two-electrode voltage clamp experiments on X. laevis oocytes (“ON” currents), and from pulses back to −30 mV (“OFF” pulses). (B) Reciprocal time constants from fits of a single exponential function to the current traces in (A). (C) Q(V) distribution from the improper integrals of the transient current signals (“OFF” pulses) from (A) with the parameters obtained from fits of a Boltzmann-type function to the data. The Q(V) distribution from “ON” transient currents would be obtained by multiplying the above curve with (−1).

Figure 6

Leak currents of the Na⁺,K⁺-ATPase upon mutation of the α-subunit's C-terminus. (A) Ouabain-sensitive transient currents measured at [K⁺]_o = 0, [Na⁺]_o = 100 mM (pH 7.4) upon pulses from −30 mV to voltages between +60 and −140 mV in −40 mV decrements for mutant ATP1A2-ΔYY in two-electrode voltage clamp experiments on X. laevis oocytes (“ON” currents), and from pulses back to −30 mV (“OFF” pulses). (B) [Na⁺]_o dependence of the current-voltage curves of the steady-state inward leak currents (closed symbols), the leak currents of the WT enzyme at zero [Na⁺] are shown as open squares.

Figure 7

Location of ATP1A2 mutations with most severe consequences on function. Mutated amino acids, for which functional consequences are described in the text, are shown in stick representation. Mutant P979L was shown to be fully functional in the Xenopus oocyte system, whereas temperature-dependent effects on protein stability and plasma membrane targeting were observed in mammalian cells (Tavraz et al., 2008, 2009).