Acute Axon Damage and Demyelination are Mitigated by 4-Aminopyridine (4-AP) Therapy after Experimental Traumatic Brain Injury
Damage to long axons in white matter tracts is a major pathology in closed head traumatic brain injury (TBI). Acute TBI treatments are needed that protect against axon damage and promote recovery of axon function to prevent long term symptoms and neurodegeneration. Our prior characterization of axon damage and demyelination after TBI led us to examine repurposing of 4-aminopyridine (4-AP), an FDA-approved inhibitor of voltage-gated potassium (Kv) channels. 4-AP is currently indicated to provide symptomatic relief for patients with chronic stage multiple sclerosis, which involves axon damage and demyelination. We tested clinically relevant dosage of 4-AP as an acute treatment for experimental TBI and found multiple benefits in corpus callosum axons. This randomized, controlled pre-clinical study focused on the first week after TBI, when axons are particularly vulnerable. 4-AP treatment initiated one day post-injury dramatically reduced axon damage detected by intra-axonal fluorescence accumulations in Thy1-YFP mice of both sexes. Detailed electron microscopy in C57BL/6 mice showed that 4-AP reduced pathological features of mitochondrial swelling, cytoskeletal disruption, and demyelination. Furthermore, 4-AP improved the molecular organization of axon nodal regions by restoring disrupted paranode domains and reducing Kv1.2 channel dispersion. 4-AP treatment did not resolve deficits in action potential conduction across the corpus callosum, based on ex vivo electrophysiological recordings at 7 days post-TBI. Thus, this first study of repurposing 4-AP as an acute treatment for TBI demonstrates pre-clinical efficacy in decreasing pathological hallmarks of axon damage. Studies beyond this acute phase are now warranted to assess functional utility and outcome trajectory. SIGNIFICANCE STATEMENTTraumatic brain injury (TBI) is an acute injury that, if unresolved, can progress to cause persistent, debilitating symptoms. Currently, no treatments effectively prevent damage to long myelinated axons in white matter tracts, which is a hallmark pathology of TBI. 4-aminopyridine (4-AP) is FDA-approved to treat chronic symptoms in patients with multiple sclerosis, which involves autoimmune damage to myelinated axons. As the first assessment of repurposing 4-AP as an acute treatment for TBI, our randomized, controlled studies tested the hypothesis that low-dose 4-AP initiated one day after experimental TBI will reduce acute axon damage and demyelination. We found that 4-AP treatment significantly reduced the progression of axon pathology and demyelination during the first week after TBI using clinically relevant experimental conditions.