Stimulation of posttraumatic neurogenesis by IGF-1 overexpression

Traumatic brain injury (TBI) is associated with cognitive deficits that depend in part on hippocampal neuron death and dysfunction. Administration of insulin-like growth factor-1 (IGF-1) has been shown to improve learning and memory after TBI in rodents. However, little is known about the cellular substrates underlying these functional effects. Using a transgenic mouse model in which IGF-1 is conditionally overexpressed in the brain by astrocytes, we have shown that elevating brain levels of IGF-1 protects against trauma-induced death of mature hippocampal neurons. However, following contusion TBI, one of the most vulnerable populations of cells in the hippocampus are immature neurons, the vast majority of which die within three days. A recovery of immature neuron numbers over a period of weeks after injury is driven by an increase in neural progenitor cell proliferation. Nevertheless, only a small fraction of these newborn neurons appear to survive to maturity. We have provocative data demonstrating that IGF-1 overexpression significantly enhances the recovery of immature neurons in the subgranular zone by promoting neuronal differentiation. While interest in therapies aimed at enhancing hippocampal neurogenesis after CNS injury is high, it is critical to establish that newborn neurons develop and integrate in a functionally appropriate or beneficial manner. Our preliminary data suggests that IGF-1 not only increases the number of newborn neurons, it also promotes the development of a more normal dendritic arbor. We hypothesize that IGF-1 acts through the mTOR pathway to stimulate the differentiation and dendritic development of newborn neurons, resulting in improved integration into the hippocampal circuitry and increased survival and maturation. To test this hypothesis, in Aim 1 we will determine the timing and cellular localization of phosphorylation of key signaling molecules in the PI3K pathway downstream of the IGF-1 receptor, including Akt, GSK3â and S6 (as a marker of mTOR activity). After determining an optimal dose of the selective mTOR inhibitor rapamycin via phospho-S6 immunoblotting, this dose will be tested in vivo to establish whether neurogenic effects of IGF-1 are mediated through the mTOR pathway. In Aim 2, we will use a viral vector approach to label dividing cells to allow visualization of, and electrophysiological recording from, newborn neurons in hippocampal slices taken from IGF-1 transgenic (Tg) and wildtype mice after CCI brain injury. Electrophysiological properties and anatomical features will be evaluated to assess appropriate development and connectivity of immature neurons. Long-term survival of newborn neurons will be quantified in a separate cohort receiving bromodeoxyuridine injections to label proliferating cells. These studies will provide new insights into intracellular signaling cascades underlying IGF-1 mediated neurogenesis and plasticity in the injured hippocampus. They will also, for the first time, address the functional effectiveness of IGF-1 stimulated hippocampal neurogenesis in TBI, in order to inform the design of future therapeutic strategies targeting neurogenesis in TBI and other CNS disorders. 4