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

Michal Vascak,John T Povlishock,Kimberle M Jacobs,Matthew Baer,Jianli Sun

doi:10.3389/fncel.2017.00157

Michal Vascak, John T Povlishock + Show 3 more

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https://doi.org/10.3389/fncel.2017.00157

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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.

Highlights

Traumatic brain injury (TBI) constitutes a major global healthcare problem that extracts a devastating personal and societal toll (Langlois et al, 2006)
We evaluated the adjusted coefficient of determination (R2), which describes how much of the variability in the data is accounted for in the multilevel model
The neocortical pyramidal neuron axon initial segment (AIS) is a major determinant of network function and homeostatic plasticity (Grubb et al, 2011; Bender and Trussell, 2012; Kole and Stuart, 2012)

Summary

Introduction

Traumatic brain injury (TBI) constitutes a major global healthcare problem that extracts a devastating personal and societal toll (Langlois et al, 2006). TBI research focused on severe head injuries associated with high-speed motor vehicle incidents. The incidence of severe TBI decreased dramatically (Thurman et al, 1999; Coronado et al, 2011), with milder forms of TBI elicited by contact sports and blast waves during combat predominating (Borg et al, 2004; Cassidy et al, 2004; Styrke et al, 2007; Hoge et al, 2008). Mild TBI (mTBI) is frequently termed the ‘‘silent epidemic’’ because of its relatively subtle nature typically occurring without any evidence of macroscopic brain damage such as contusion or hematoma formation (Alexander, 1995; Povlishock and Katz, 2005). Despite the focus of research shifting to mTBI (CDC, 2003), its cellular and physiological substrates remain controversial (Shaw, 2002; Büki and Povlishock, 2006; Cohen et al, 2007; Andriessen et al, 2010; Johnson et al, 2013)

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