Mild Traumatic Brain Injury Evokes Pyramidal Neuron Axon Initial Segment Plasticity and Diffuse Presynaptic Inhibitory Terminal Loss

Abstract

The axon initial segment (AIS) is the site of action potential (AP) initiation, thus a crucial regulator of neuronal activity. In excitatory pyramidal neurons, the high density of voltage-gated sodium channels (NaV1.6) at the distal AIS regulates AP initiation. A surrogate AIS marker, ankyrin-G (ankG) is a structural protein regulating neuronal functional via clustering voltage-gated ion channels. In neuronal circuits, changes in presynaptic input can alter postsynaptic output via AIS structural-functional plasticity. Recently, we showed experimental mild traumatic brain injury (mTBI) evokes neocortical circuit disruption via diffuse axonal injury (DAI) of excitatory and inhibitory neuronal systems. A key finding was that mTBI-induced neocortical electrophysiological changes involved non-DAI/ intact excitatory pyramidal neurons consistent with AIS-specific alterations. In the current study we employed Thy1-yellow fluorescent protein (YFP)-H mice to test if mTBI induces AIS structural and/or functional plasticity within intact pyramidal neurons 2 days after mTBI. We used confocal microscopy to assess intact YFP+ pyramidal neurons in layer 5 of primary somatosensory barrel field (S1BF), whose axons were continuous from the soma of origin to the subcortical white matter (SCWM). YFP+ axonal traces were superimposed on ankG and NaV1.6 immunofluorescent profiles to determine AIS position and length. We found that while mTBI had no effect on ankG start position, the length significantly decreased from the distal end, consistent with the site of AP initiation at the AIS. However, NaV1.6 structure did not change after mTBI, suggesting uncoupling from ankG. Parallel quantitative analysis of presynaptic inhibitory terminals along the postsynaptic perisomatic domain of these same intact YFP+ excitatory pyramidal neurons revealed a significant decrease in GABAergic bouton density. Also within this non-DAI population, patch-clamp recordings of intact YFP+ pyramidal neurons showed AP acceleration decreased 2 days post-mTBI, consistent with AIS functional plasticity. Simulations of realistic pyramidal neuron computational models using experimentally determined AIS lengths showed a subtle decrease is NaV1.6 density is sufficient to attenuate AP acceleration. Collectively, these findings highlight the complexity of mTBI-induced neocortical circuit disruption, involving changes in extrinsic/presynaptic inhibitory perisomatic input interfaced with intrinsic/postsynaptic intact excitatory neuron AIS output.

Sensory input drives rapid homeostatic scaling of the axon initial segment in mouse barrel cortex

“…1B). AIS in layer V were generally shorter than in layer II/III (average adult S1BF layer II/III: ~22 µm, layer V: ~19 µm) in accordance with previously published data from visual and somatosensory cortex (Gutzmann et al, 2014;Vascak et al, 2017).…”

“…These observations are in good support of AIS remodeling observed in disease or injury models (reviewed in (Buffington and Rasband, 2011). In addition to the up-or downregulation of excitability by AIS changes, cell type-specific remodeling of the AIS has also been found to occur either via long-term (days to weeks) (Gutzmann et al, 2014;Kim et al, 2019;Kuba et al, 2010;Schlüter et al, 2017;Vascak et al, 2017)) or short-term manipulations of a few hours (Bender et al, 2010;Evans et al, 2015;Lezmy et al, 2017;Martinello et al, 2015)).…”

Lasting deficit in inhibitory control with mild traumatic brain injury

“…However, substantial evidence suggests that concussion (i.e., a brief loss of consciousness) induced by a closed-head impact is likely caused by a sudden abnormal discharge of neuronal activity both in the cortical and subcortical systems (e.g., the reticular activating system or RAS) although the exact mechanism remains unclear 23 . The extent of structural injuries at the cellular, cortical, and subcortical level, and their white matter connections is also difficult to assess after mTBI even with the most up-to-date technologies 24 – 28 . The majority of the patients with mTBI often appear asymptomatic post-injury after a recovery period of a few days or weeks and able to carry on cognitive tasks normally 29 – 31 .…”

Diffuse Axonal Injury: A Devastating Pathology

“…It is the biochemical response to the impact stimuli which causes that largest impact on axons. However, impact does in fact cause some lesions but not comparable to the damage done by biochemical cascades which follow the impact [12,13]. Biochemical pathways within our bodies are greatly responsible for the axonal disconnect which occurs secondary to the traumatic impact.…”