Axon initial segment dysfunction in epilepsy

Abstract

The axon initial segment (AIS) contains the site of action potential initiation and plays a major role in neuronal excitability. AIS function relies on high concentrations of different ion channels and complex regulatory mechanisms that orchestrate molecular microarchitecture. We review recent evidence that a large number of ion channels associated with epilepsy are enriched at the AIS, making it a 'hotspot' for epileptogenesis. Furthermore, we present novel data on the clustering of GABRgamma2 receptors in the AIS of cortical and hippocampal neurons in a knock in mouse model of a human genetic epilepsy. This article highlights the molecular coincidence of epilepsy mutations at the AIS and reviews pathogenic mechanisms converging at the AIS.

Figure 1. Neuronal anatomy, axonal compartments and the AIS

The AIS (highlighted in red) includes the first 30–70 μm – depending on the cell type – of axon adjacent to the axon hillock.

Figure 2. AIS function in acute brain slices can be analysed using somatic patch clamp recordings

A, averaged AP waveform. B, time-aligned fist derivative of AP waveform (dV/dt). C, time-aligned second derivative (d²V/dt²). Characteristic signature of sequential AP initiation in the AIS (open arrowhead) and soma (filled arrowhead). Time bar 1 ms, V_m bar 20 mV (A), dV/dt bar 100 mV s⁻¹ (B), and d²V/dt² bar 1000 mV s⁻² (C).

Figure 3. Three major Na⁺ channel α-subunits localise to the AIS

Our own immunohistochemical stainings reproduced the interneuron-specificity of Na_V 1.1 expression suggested by Lorincz & Nusser (2008) (but also see Duflocq et al. 2008) as well as gradient distribution of Na_V1.2 and Na_V1.6. Green: staining against Na⁺ channel α-subunit, red: staining against AnkG. A, inhibitory interneuron-specific expression of Na_V1.1 in proximal half of the AIS (inhibitory neuron in stratum radiatum in hippocampus). B, in adult mice Na_V1.2 is concentrated in the proximal half of pyramidal cell AIS (CA3). C, Na_V1.6 gradient with peak concentration in distal AIS (CA3). Scale bar 5 μm.

Figure 4. Brain region specific distribution of γ2

A–E, antibody staining against γ2 (green) and AnkG (red) in wild-type tissue. A, CA3 AISs with high γ2 cluster densities. B, fewer γ2 clusters in CA1 compared to CA3. C, sparse γ2 clusters in S1 AIS. D, difference in the number of γ2 clusters per 10 μm length of AIS for CA3, CA1 and S1. E, no difference in numbers of γ2 positive postsynapses in wild-type (RR) and heterozygous mutant (RQ) CA1 neurons. F, virus-mediated expression of EGFP-γ2(wild-type) in CA3 neurons. G, virus-mediated expression of EGFP-γ2(Q43) in CA3 neurons. No γ2 containing clusters were detected. Note accumulation of EGFP-γ2(Q43) in intracellular compartments (asterisk). Scale bars 10 μm.