Longitudinal optical imaging technique to visualize progressive axonal damage after brain injury in mice reveals responses to different minocycline treatments

Abstract

A high-resolution, three-dimensional, optical imaging technique for the murine brain was developed to identify the effects of different therapeutic windows for preclinical brain research. This technique tracks the same cells over several weeks. We conducted a pilot study of a promising drug to treat diffuse axonal injury (DAI) caused by traumatic brain injury, using two different therapeutic windows, as a means to demonstrate the utility of this novel longitudinal imaging technique. DAI causes immediate, sporadic axon damage followed by progressive secondary axon damage. We administered minocycline for three days commencing one hour after injury in one treatment group and beginning 72 hours after injury in another group to demonstrate the method’s ability to show how and when the therapeutic drug exerts protective and/or healing effects. Fewer varicosities developed in acutely treated mice while more varicosities resolved in mice with delayed treatment. For both treatments, the drug arrested development of new axonal damage by 30 days. In addition to evaluation of therapeutics for traumatic brain injury, this hybrid microlens imaging method should be useful to study other types of brain injury and neurodegeneration and cellular responses to treatment.

Acute minocycline administration reduces brain injury and improves long-term functional outcomes after delayed hypoxemia following traumatic brain injury

“…Previous preclinical studies have reported efficacy of minocycline for brain injury with dosing ranging from 22.5 to 90 mg/kg/dose [ 19 , 24 , 32 , 49 ]. We designed a two-step funnel design for four different dosing regimens of minocycline with an initial evaluation of efficacy using short-term neuropathology followed by selecting the most efficacious dosing for long-term behavioral outcome studies.…”

“…Minocycline is a broad-spectrum tetracycline antibiotic, which has been studied as a neuroprotective agent in TBI [ 32 , 43 , 46 ]. Although minocycline is proposed to have pleiotropic effects on the brain, the main mechanism of action for neuroprotection is thought to be suppression of microglial activation after TBI [ 43 ].…”

Rapid and Reversible Development of Axonal Varicosities: A New Form of Neural Plasticity

“…By combining the FPI model and in vivo imaging using highresolution gradient index lens technology, the Murray group was able to examine axonal changes in the same mouse and visualized axonal morphology from external capsule in both preinjury and post-injury stages for up to 60 days after injury. They found axonal varicosities and undulated axons 1 h post injury (Pernici et al, 2019(Pernici et al, , 2020. Administration of minocycline to reduce microglia activation for three days starting 1 or 72 h after injury inhibited axonal varicosity formation or promoted axon recovery, respectively (Pernici et al, 2020).…”

“…They found axonal varicosities and undulated axons 1 h post injury (Pernici et al, 2019(Pernici et al, , 2020. Administration of minocycline to reduce microglia activation for three days starting 1 or 72 h after injury inhibited axonal varicosity formation or promoted axon recovery, respectively (Pernici et al, 2020). Despite clear advantages, the in vivo imaging often involves extensive experimental procedure in order to examine a limited number of axons in a highly focused brain regions.…”

Acute axon damage and demyelination are mitigated by 4-aminopyridine (4-AP) therapy after experimental traumatic brain injury

“…At an early stage of damage, axons have the potential to recover but once mechanical or molecular processes fragment the axon, then irreversible axon degeneration proceeds [ 35 , 48 , 51 , 52 , 57 , 79 , 80 ]. Use of transgenic mice with fluorescent labeling of axons has shown that axonal swellings can appear within minutes of an insult and recover within hours, or persist for weeks or more, and can degenerate to form terminal end bulbs at sites of disconnection [ 35 , 57 , 59 , 78 ]. Axons can also continue to initiate distal, or Wallerian, degeneration in the days to weeks after an initial mechanical injury [ 52 ].…”