Approximately 1.5 million traumatic brain injuries (TBIs) occur in the United States annually. Although much research has focused on neuronal death following TBI, TBI also initiates a proliferative response in some cell types, including neural progenitor cells (NPCs) and astrocytes. Proliferating NPCs are thought to replace dying neurons through neurogenesis, but the function of astrocyte proliferation is more controversial. Both normally rely on ?-catenin for proliferation, but the importance of the ?-catenin pathway in posttraumatic proliferation has not yet been established. Because quiescent NPCs and astrocytes express glial fibrillary acidic protein (GFAP), this study utilizes a novel transgenic mouse in which ?-catenin signaling is conditionally knocked-out using the Cre-lox system under the GFAP promoter (GFAP-tta/Tet-O-Cre/?-cateninflox/flox;cat-KO), allowing for the specific manipulation of NPCs and astrocytes after trauma. The central hypothesis is that ?-catenin- mediated NPC and astrocyte proliferation promotes recovery after contusion brain injury. The main goals of this proposal are to understand the extent to which ?-catenin signaling affects NPC and astrocyte proliferation after controlled cortical impact (CCI) brain injury (Aim 1), and to assess the role of dividing NPCs and astrocytes in the injured brain (Aim 2).
Aim 1 tests the hypothesis that ?-catenin signaling is critical for NPC and astrocyte proliferation following CCI. cat-KO and wild-type (WT) mice will receive moderate CCI injury and injection of 5-bromo-2-deoxyuridine (BrdU, a thymidine analog) to label dividing cells. Immunostaining will be used to compare numbers of BrdU-positive NPCs and astrocytes in cat-KO and WT mice.
Aim 2 tests the hypothesis that eliminating ?-catenin-dependent proliferation in GFAP-positive NPCs and astrocytes will impair spontaneous recovery after CCI. Astrocytes are normally key regulators of blood-brain barrier (BBB) integrity and dendritic/synaptic outgrowth. Because ?-catenin affects astrocyte proliferation, it is expected that cat-KO mice will show greater deficits than WT mice in BBB integrity (assessed by IgG or Evan's blue extravasation) and dendritic/synaptic recovery (assessed through immunostaining and Golgi staining). Because ?-catenin affects NPC proliferation, recovery of immature neuronal populations in the hippocampus (evaluated with doublecortin immunolabeling) is expected to be impaired in cat-KO mice compared to WT mice. The combined loss of astrocyte and NPC proliferation are predicted to result in greater cognitive impairment, which will be evaluated using a Morris Water Maze learning paradigm and Novel Object Recognition (memory) Task. These studies will provide novel insights on NPC and astrocyte proliferation in posttraumatic recovery, thereby identifying new therapeutic targets for posttraumatic intervention. The proposed studies will greatly enhance my training in the pathobiology of TBI and my technical expertise for studying neurodegenerative conditions.
Traumatic brain injury is the leading cause of death and disability under age 45. This research study will establish how proliferation of neural stem cells and astrocytes in the brain after trauma contributes to repair of the injured brain, ultimately leading to improved behavioral outcome.