Quantitative microglia analyses reveal diverse morphologic responses in the rat cortex after diffuse brain injury

Helena Morrison,Kimberly Young,Mahir Qureshi,Rachel K Rowe,Jonathan Lifshitz

doi:10.1038/s41598-017-13581-z

Abstract

Determining regions of altered brain physiology after diffuse brain injury is challenging. Microglia, brain immune cells with ramified and dynamically moving processes, constantly surveil the parenchyma for dysfunction which, when present, results in a changed morphology. Our purpose was to define the spatiotemporal changes in microglia morphology over 28 days following rat midline fluid percussion injury (mFPI) as a first step in exploiting microglia morphology to reflect altered brain physiology. Microglia morphology was quantified from histological sections using Image J skeleton and fractal analysis procedures at three time points and in three regions post-mFPI: impact site, primary somatosensory cortex barrel field (S1BF), and a remote region. Microglia ramification (process length/cell and endpoints/cell) decreased in the impact and S1BF but not the remote region (p < 0.05). Microglia complexity was decreased in the S1BF (p = 0.003) and increased in the remote region (p < 0.02). Rod-shaped microglia were present in the S1BF and had a 1.8:1.0 length:width ratio. An in-depth quantitative morphologic analysis revealed diverse and widespread changes to microglia morphology in the cortex post-mFPI. Due to their close link to neuronal function, changes in microglia morphology, summarized in this study, likely reflect altered physiology with diverse and widespread impact on neuronal and circuit function.

Highlights

A hyper-ramified/hyper-complex morphology has not been correlated to neuropathological indicators of injury, such as IL1-β, CD68, or caspase, but instead increased expression of β1-integrin[44]
The hyper-ramified response has been described as an indicator or early response to injury[22,46]. It remains unclear if the hyper-ramified/hyper-complex morphology should be considered as part of the injury response, a change in the extracellular milieu, or in response to intense neuronal activity
We propose that hyper-complex microglia in the remote region represent a combination of protection against a spreading wave of neuropathology or downstream second-order neuropathology from remodeled circuits which may result in increased neuronal excitatory signaling

Summary

Introduction

In addition to de-ramification, our analysis revealed that microglia become progressively more complex and cell density increases over 28 days post-injury in the remote brain region. A hyper-ramified/hyper-complex morphology has not been correlated to neuropathological indicators of injury, such as IL1-β, CD68, or caspase, but instead increased expression of β1-integrin[44].

Results

Conclusion