Activity-dependent relocation of the axon initial segment fine-tunes neuronal excitability

Abstract

In neurons, the axon initial segment (AIS) is a specialised region near the start of the axon that is the site of action potential initiation [1][2][3][4][5][6] . The precise location of the AIS varies across and within different neuronal types 7,8 , and has been linked to cells' information-processing capabilities 8 ; however, the factors determining AIS position in individual neurons remain unknown. Here we show that changes in electrical activity can alter the location of the AIS. In dissociated hippocampal cultures, chronic depolarization with high potassium moves multiple components of the AIS, including voltage-gated sodium channels, up to 17 μm away from the soma of excitatory neurons. This movement reverses when neurons are returned to non-depolarized conditions, and depends upon the activation of T-and/or L-type voltage-gated calcium channels. The AIS also moved distally when we combined long-term LED (light-emitting diode) photostimulation with sparse neuronal expression of the light-activated cation channel channelrhodopsin-2; here, burst patterning of activity was successful where regular stimulation at the same frequency failed. Furthermore, changes in AIS position correlate with alterations in current thresholds for action potential spiking. Our results show that neurons can regulate the position of an entire subcellular structure according to their ongoing levels and patterns of electrical activity. This novel form of activity-dependent plasticity may fine-tune neuronal excitability during development.In dissociated hippocampal neurons, global depolarization with 15 mM extracellular potassium from 12 to 14 days in vitro (DIV) produced a significant distal shift in AIS location ( Fig. 1a-d and Supplementary Figs 1-3). Labelling for the AIS scaffolding protein ankyrin G 9 , showed that start, maximum, and end AIS positions were all significantly relocated away from the soma (Fig. 1a,b; start: Mann-Whitney U-test, P < 0.0001; maximum: P < 0.0001; end: P < 0.0001; n = 885 cells, 36 coverslips), leaving the length of the AIS unchanged ( Fig. 1a,b; P = 0.11). We also observed significant activity-dependent distal shifts in other AIS-specific proteins 10,11 , including the scaffolding protein βIV spectrin ( Fig. 1c,d; start: Mann-Whitney U-test, P < 0.0001; maximum: P < 0.0001; end: P < 0.0001; n = 1065 cells, 44 coverslips), the extracellular matrix binding protein neurofascin ( Fig. 1d; start: Mann-Whitney U-test, P < 0.0001; maximum: P = 0.002; end: P = 0.01; n = 96 cells, 4 coverslips), the ion channel-associated protein FGF14 ( Fig. 1d; start: MannWhitney U-test, P < 0.0001; maximum: P < 0.0001; end: P < 0.0001; n = 89 cells, 4 coverslips), and, vitally, the voltage-gated sodium channels (VGSCs) essential for actionCorrespondence and requests for materials should be addressed to M.G. (matthew.grubb@kcl.ac.uk) or J.B. (juan.burrone@kcl.ac.uk).. Author contributions M.G. planned and performed all experiments and analysis, and wrote the paper. J.B. produced simulation data, planned experiments,...

Loss of Cyclin-Dependent Kinase 5 from Parvalbumin Interneurons Leads to Hyperinhibition, Decreased Anxiety, and Memory Impairment

“…Recent findings demonstrate that the size of the AIS can be regulated by presynaptic activity and that this variability serves as a component of synaptic plasticity that fine-tunes neuronal excitability (Grubb and Burrone, 2010;Kuba et al, 2010). To determine the potential effect of an increased inhibitory synaptic activity on AIS parameters, we calculated the average length of the AIS in excitatory neurons in the CA1 area of the hippocampus of PVCre; fCdk5 and control mice.…”

Giant ankyrin-G: A critical innovation in vertebrate evolution of fast and integrated neuronal signaling

“…AISs, in addition to generating APs, also are innervated by GABAergic axo-axonic interneurons, which are key elements in neural circuits (2). AISs are capable of plasticity in response to neural activity and may have a role in adaptive responses of the nervous system, including some forms of learning and memory (3,4). AISs also are involved in epilepsy as well as major psychiatric diseases (5).…”

Projections, Partaken Circuits and Axon Initial Segments of Cortical Principal Neurons

“…Activity-dependent AIS plasticity has been observed in cultured hippocampal neurons. Increasing chronically neuronal activity over 48 hours resulted in a significant shift in AIS position with the entire structure (ankyrin G, ßIV spectrin, and Na + channels) moving distally away from the soma [75]. Importantly, hippocampal AIS relocation is a bidirectional phenomenon because the AIS can shift proximally after neurons are returned to baseline activity conditions.…”

“…The distinct length of the AIS of hippocampal pyramidal neurons (≈30 microns) and the neurons of the nucleus magnocellularis (≈10 microns) has been suggested to be the cause of these different plastic changes in the AIS. Indeed, cell type-based variability in AIS plasticity is seen within hippocampal cultures, where GAD 65-expressing interneurons display little or no shift in AIS location upon high potassium stimulation [75]. Dopaminergic neurons in dissociated cultures of rat olfactory bulb show inverse AIS plasticity: their initial segments move proximally after 48 hours depolarization.…”

“…This complexity underlies what is now being recognized as a highly dynamic structure [73]. AIS structure and composition vary considerably across, and even within, neuronal classes [74,75], seemingly tuned to the computational demands of the cell. A recent activity of the cell can affect AIS ion channel kinetics and availability, thus altering action potential waveform, timing, and probability.…”