Astroglial neural stem cells (NSC) expressing GFAP and Sox2 proliferate in germinal areas such as the subventricular zone (SVZ) in the recovery period following hypoxic injury. While we know a great deal about how GFAP+ NSC generate neurons for the olfactory bulb in the normal developing brain, we are still far from understanding whether and how these precursors contribute to brain repair after injury. In the first specific aim, we address this clinically relevant issue by asking whether reactive GFAP+ NSC give rise to new cortical neurons and oligodendrocytes in the cerebral cortex of juvenile mice that survive chronic sublethal hypoxia, a model for premature birth. We will investigate what types of neurons arise from these cells in normal and hypoxic conditions and whether these neurons are electrophysiologically active and normally integrated in the cortical circuitry. This will be accomplished by genetically marking astroglial lineage cells with EGFP and gal reporter genes, using a transgenic mouse carrying a drug-inducible Cre recombinase gene expressed under the GFAP promoter. In the second specific aim, we will investigate whether the GFAP+ cells that give rise to cortical neurons are located in the SVZ and give rise to precursors that subsequently migrate to the cerebral cortex, or whether they are located within the cortex. For this, we will microinject a lentiviral vector transducing Cre under the GFAP promoter to permanently mark cortical or SVZ GFAP+ cells with heritable gal expression. To understand whether hypoxic rearing changes the intrinsic potential of GFAP+ NSC or affects their environment, we will FACS-purify GFAP+cells from the SVZ of normoxic or hypoxic mice and follow their fate after transplantation in the brain of normoxic or hypoxic recipients. The Fibroblast Growth Factor Receptor 1 (Fgfr1) gene product is upregulated by hypoxia in GFAP+ NSC of the SVZ and appears to be important for recovery.
In specific aim 3, we will delete the Fgfr1 gene within GFAP+ cells using inducible Cre recombination in vivo. This will test whether Fgfr1 is required before or after the hypoxic insult within GFAP+ cells to promote their proliferation and differentiation in the immature hypoxic brain. Our results may lead to novel therapeutic approaches involving exogenous or endogenous astroglial precursors aimed at improving recovery in pediatric patients suffering from hypoxic encephalopathy.
Hypoxia low oxygen is a frequent cause of brain injury in premature birth and other birth-related complications. Hypoxia often results in smaller brain volume and mental disability;however a portion of the affected children undergoes a remarkable improvement in both cerebral cortical gray matter volume and cognitive scores. Using a mouse model of chronic sublethal hypoxia that reproduces both the initial brain atrophy and the subsequent recovery, we will investigate the cellular and molecular mechanisms underlying recovery. We will focus on specific growth factor that affect the degree to which neural stem cells naturally present in the brain generate neurons and glia that survive long-term in the injured brain.
