September 17, 2024

Acutely blocking excessive mitochondrial fission prevents chronic neurodegeneration after traumatic brain injury

Cell Reports Medicine Publication

Summary

Progression of acute traumatic brain injury (TBI) into chronic neurodegeneration is a major health problem with no protective treatments. Here, we report that acutely elevated mitochondrial fission after TBI in mice triggers chronic neurodegeneration persisting 17 months later, equivalent to many human decades. We show that increased mitochondrial fission after mouse TBI is related to increased brain levels of mitochondrial fission 1 protein (Fis1) and that brain Fis1 is also elevated in human TBI. Pharmacologically preventing Fis1 from binding its mitochondrial partner, dynamin-related protein 1 (Drp1), for 2 weeks after TBI normalizes the balance of mitochondrial fission/fusion and prevents chronically impaired mitochondrial bioenergetics, oxidative damage, microglial activation and lipid droplet formation, blood-brain barrier deterioration, neurodegeneration, and cognitive impairment. Delaying treatment until 8 months after TBI offers no protection. Thus, time-sensitive inhibition of acutely elevated mitochondrial fission may represent a strategy to protect human TBI patients from chronic neurodegeneration.

Introduction

Traumatic brain injury (TBI) is a leading cause of chronic neurodegeneration, and upward of 5 million Americans are currently living with consequent symptoms, such as chronically impaired cognition and increased risk of developing neurodegenerative diseases of aging, including Alzheimer’s disease (AD) and Parkinson’s disease (PD).1,2,3,4,5,6 Strikingly, the current annual incidence of TBI is ∼3.5 million in the USA and 70 million worldwide,7,8 and even subconcussive TBI can impair cognitive function.9 Unfortunately, there are currently no therapies that prevent chronic neurodegeneration after TBI. Our study was designed to address this unmet need.

TBI encompasses concussion, contusion, diffuse axonal injury, and open or closed head injury and is commonly sustained from falls, motor vehicle accidents, explosive forces, military combat, sports injuries, violent assaults, and other accidents. Thus, most forms of TBI are multifactorial. We considered this when designing a laboratory mouse model of multimodal TBI10 to test our hypothesis that mitochondrial fission would be elevated by TBI, akin to what is observed in other chronic neurodegenerative conditions.11,12,13,14 Specifically, we used a model of TBI that provides a comprehensive perspective on TBI’s impact by combining readily calibratable and reproducible aspects of global concussive injury, acceleration/deceleration trauma, and early blast wave exposure, the physics of which we have rigorously characterized.10 Key outcome parameters in this model align with the anomalies observed in human TBI patients, including early axonal degeneration followed by chronic neuronal cell death, cognitive and motor deficits, blood-brain barrier (BBB) deterioration, chronic neuroinflammation, systemic metabolic alterations in the blood, and blood biomarkers.10,15,16,17,18,19,20,21,22,23

Although the brain composes only 2% of the body’s mass, it comprises approximately 20% of the body’s total energy consumption.24 As neurons generate only 10% of total ATP through glycolysis,25 proper mitochondrial functioning is particularly critical for maintaining neuronal energy demand through the tricarboxylic acid cycle. Under healthy conditions, mitochondria maintain an equilibrium between fission and fusion that optimally serves the cell’s needs. Mitochondrial fission is mediated by translocation of the cytosolic GTPase dynamin-related protein 1 (Drp1) to the outer mitochondrial membrane, where it binds to adaptor proteins such as mitochondrial fission 1 (Fis1). While some mitochondrial fission is required for mitochondrial biogenesis and transport, aberrantly excessive mitochondrial fission is associated with excessive synthesis of reactive oxygen species (ROS), mitochondrial membrane depolarization, bioenergetic defects, mitophagy, and neuronal cell death.11 Importantly, pathologically elevated mitochondrial fission is a feature of many chronic neurodegenerative diseases, including AD, PD, and Huntington’s disease (HD).11,12,13,14 Thus, we hypothesized that mitochondrial fission might also be aberrantly elevated by TBI, and, if so, then its inhibition would mitigate the resulting chronic neurodegeneration.

Previous attempts have been made to block mitochondrial fission using the pharmacologic agent mitochondrial division inhibitor 1 (Mdivi-1), but this is a non-specific agent with numerous off-target effects, including inhibition of mitochondrial respiratory complexes and increased generation of ROS.26,27 Therefore, we chose to pharmacologically inhibit mitochondrial fission with P110, a selective small peptide inhibitor of pathologic mitochondrial fission that readily enters the brain as a transactivator of transcription (TAT) peptide28,29,30,31,32 and functions by blocking the binding interaction of Fis1 and Drp1 under stressed conditions.32 Protective efficacy of P110 requires the presence of Drp1 and does not affect expression of any fusion- or fission-related proteins.28,32 Treatment with P110 reduces mitochondrial fragmentation, mitochondrial damage, and tissue injury without affecting normal physiological mitochondrial dynamics and function.13,28,33,34,35 Notably, subcutaneous administration of P110 is protective in several preclinical animal models of neurodegenerative disease.13,28,33,34,35,36 In addition, acute or chronic administration of P110 has had no reported toxic effects in mice.28,34,36

Here, we show an early and rapid rise in Fis1 expression that corresponds with pathologically elevated mitochondrial fission after TBI. We also show that early transient treatment with P110 after TBI permanently restores normal homeostatic mitochondrial fission and blocks progression of acute TBI into chronic neurodegeneration in mice, even 17 months post-injury. This is the equivalent of decades in people.37 Measures of chronic neurodegeneration that are prevented by acute P110 treatment include impaired mitochondrial bioenergetics, oxidative damage, microglial activation and lipid droplet formation, BBB deterioration, neurodegeneration, and cognitive impairment. Delaying P110 treatment until after chronic neurodegeneration has developed, however, is not protective in any of these measures. Thus, early restoration of normal homeostatic mitochondrial fission after TBI prevents transition to chronic neurodegeneration.