Reframing the axon initial segment: Giant ankyrin-G as a modulator of excitability and plasticity in neurodevelopment

ANK3 is a large, alternatively spliced gene that produces multiple ankyrin-G (AnkG) isoforms with distinct functions in neuronal organization and excitability.Among these are a group termed "giant AnkGs" (gAnkGs), which include a single 7.8 kilobase exon (giant exon) between the sequences encoding the UPA and death domains (Ensembl ID: ENSE00000997921) (1, 2).There has been growing interest in gAnkGs, due to their critical roles within neurons and their implications in neurodevelopmental disorders (NDDs).gAnkG isoforms are known for their roles at the axon initial segment (AIS), nodes of Ranvier, and at GABAergic synapses (3) and have repeatedly been implicated in autism spectrum disorder (ASD), bipolar disorder (BD), epilepsy, and a spectrum of NDDs through genome-wide association studies and rare genetic variant studies (Figure 1A-B) (4-8).In this issue of PNAS, Li et al. investigated a human missense variant, ANK3 p.T1861M, identified in children with NDDs presenting with ataxia, seizures, and ASD features.This variant lies within the giant exon, a domain central to AnkG's role in scaffolding proteins to the plasma membrane at the AIS (9).To mechanistically evaluate the pathogenicity of this variant, Li and colleagues employ a mouse knock-in model carrying the homologous variant, T1935M, and investigate its effects at the molecular, cellular, circuit, and behavioral levels.Their major discovery is that this mutation disrupts activity-dependent AIS plasticity, preventing normal developmental AIS shortening, and causing pyramidal neuron hyperexcitability due to abnormally elongated AISs.This defect emerges during a critical developmental window between postnatal days 28 and 60, around the time when AIS maturation fine-tunes neuronal excitability in the cortex (10).These data suggest a novel role for gAnkG in AIS remodeling and plasticity.AIS elongation was unexpected and contrasts with previously studied Ank3 models including Ank3 exon1b (Ank3-1b), Ank3 exon37 (exon37), and Ank3 W1989R mouse models.In forebrain, Ank3-1b -/-and exon37 -/-mice have neurons lacking AISs in parvalbumin (PV) interneurons or all neurons, respectively (9, 11).Ank3-1b +/-mice show reduced AnkG and voltage-gated sodium channel (Na V ) densities at the AIS of PV interneurons and shortened AIS length with no change to pyramidal neuron AISs (12); Ank3 W1989R/+ knock-in mice also exhibit shortened pyramidal cell AISs (7).