Traumatic brain injury (TBI) in children is the leading cause of death and disability. Although clinical studies have shown that the developing brain is particularly vulnerable to injury, the basis for this vulnerability remains unclear. TBI to the developing murine brain results in prolonged trafficking of neutrophils into cortical and subcortical structures. We hypothesize that neutrophil elastase (NE) is a determinant of cell injury and a self-propagating inflammatory loop that is driven by an imbalance between NE and its inhibitor, A1- protease inhibitor (A1-PI). With broad substrate specificity and in the absence f adequate local inhibition, excessive NE activity may lead to degradation of the extracellular matrix and exert direct toxic effects on resident cells, events that signal further recruitment of leukocytes into the damaged tissue. We will examine the mechanism underlying proteolytic imbalance, focusing on matrix metalloproteinase-9 (MMP-9). Like NE, MMP-9 is released from activated neutrophils, thus positioning it in proximity to NE. Here we will determine if excessive NE activity arises from inactivation of A1-PI by neutrophil-derived MMP-9. Our general hypothesis is that unopposed NE activity is a key mediator of cell injury after TBI and that strategies to reduce or eliminate its activity will confer neuroprotection and establish an environment that is favorable to brain development and cognitive recovery.
Specific Aim 1 will test the hypothesis that unopposed proteolytic activity of NE contributes to a self-propagating inflammatory response.
Specific Aim 2 will test the hypothesis that NE, released from activated neutrophils, contributes to cell injury and oxidative stress.
Specific Aim 3 will determine if MMP-9 inactivates a1-PI thus allowing NE to produce neural injury and further neutrophil recruitment.
Specific Aim 4 will test the hypothesis that pharmacologic blockade of NE will result in long-term structural and behavioral recovery. To address these aims we will use a murine model of TBI at postnatal day 21 and complimentary pharmacologic and genetic strategies to modulate NE activity. We will compare indices of tissue damage in brain injured wildtype (WT) and NE knockout (KO) mice and WT mice treated with a specific inhibitor of NE. To determine the dependency of NE-directed pathogenesis on the over-all activational state of neutrophils, we will study neutrophil-specific SYK-conditional KOs that show a reduced activational state while still retaining NE activity. With state-of-the-art magnetic resonance imaging and a comprehensive battery of behavioral assays, we will further determine if pharmacologic blockage of NE activity supports structural recovery and improves long-term cognitive outcomes. Together, these studies provide an important foundation for understanding the unique vulnerability of the young brain to TBI and for developing the therapies that are specifically tailored to the brain-injured child.

Public Health Relevance

Although traumatic brain injury (TBI) is the most frequent cause of acquired brain injury and morbidity in children and adversely impacts cognitive development, there has been little progress toward understanding how best to support recovery processes. Using genetic and pharmacologic approaches in a murine model of TBI, we will determine how unchecked, neutrophil elastase (NE)-directed proteolysis establishes an environment that is unfavorable to recovery and if early blockade of this activity supports cognitive recovery. These studies establish the basis for developing a NE-targeted therapeutic for the brain-injured child.

National Institute of Health (NIH)
National Institute of Neurological Disorders and Stroke (NINDS)
Research Project (R01)
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Special Emphasis Panel (ZRG1-BDCN-W (04))
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Hicks, Ramona R
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University of California San Francisco
Schools of Medicine
San Francisco
United States
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Semple, Bridgette D; O'Brien, Terence J; Gimlin, Kayleen et al. (2017) Interleukin-1 Receptor in Seizure Susceptibility after Traumatic Injury to the Pediatric Brain. J Neurosci 37:7864-7877
Semple, Bridgette D; Carlson, Jaclyn; Noble-Haeusslein, Linda J (2016) Pediatric Rodent Models of Traumatic Brain Injury. Methods Mol Biol 1462:325-43
Ryan, Nicholas P; Catroppa, Cathy; Godfrey, Celia et al. (2016) Social dysfunction after pediatric traumatic brain injury: A translational perspective. Neurosci Biobehav Rev 64:196-214
Semple, Bridgette D; Sadjadi, Raha; Carlson, Jaclyn et al. (2016) Long-Term Anesthetic-Dependent Hypoactivity after Repetitive Mild Traumatic Brain Injuries in Adolescent Mice. Dev Neurosci 38:220-238
Bajwa, Nikita M; Halavi, Shina; Hamer, Mary et al. (2016) Mild Concussion, but Not Moderate Traumatic Brain Injury, Is Associated with Long-Term Depression-Like Phenotype in Mice. PLoS One 11:e0146886
Smith, Douglas H; Hicks, Ramona R; Johnson, Victoria E et al. (2015) Pre-Clinical Traumatic Brain Injury Common Data Elements: Toward a Common Language Across Laboratories. J Neurotrauma 32:1725-35
Semple, Bridgette D; Trivedi, Alpa; Gimlin, Kayleen et al. (2015) Neutrophil elastase mediates acute pathogenesis and is a determinant of long-term behavioral recovery after traumatic injury to the immature brain. Neurobiol Dis 74:263-80
Semple, Bridgette D; Lee, Sangmi; Sadjadi, Raha et al. (2015) Repetitive concussions in adolescent athletes - translating clinical and experimental research into perspectives on rehabilitation strategies. Front Neurol 6:69
Semple, Bridgette D; Noble-Haeusslein, Linda J; Gooyit, Major et al. (2015) Early Gelatinase Activity Is Not a Determinant of Long-Term Recovery after Traumatic Brain Injury in the Immature Mouse. PLoS One 10:e0143386
Bondi, Corina O; Semple, Bridgette D; Noble-Haeusslein, Linda J et al. (2015) Found in translation: Understanding the biology and behavior of experimental traumatic brain injury. Neurosci Biobehav Rev 58:123-46

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