Prolonged mild hypoxic exposure increases brain capillary density as an integral part of the acclimatization process. The major pathways responsible for brain angiogenesis are a hypoxia inducible factor-1 (HIF-1α) dependent upregulation of vascular endothelial growth factor (VEGF), and a HIF-1α independent upregulation of cycloxygenase-2 (COX-2) and angiopoietin-2 (Ang-2). Upon return to normoxia, capillary density is restored to baseline through an angiolytic mechanism. The dynamic changes in the capillary density in response to changes in oxygen availability have led to the concept of "angioplasticity" which describes a balance between angiogenesis and angiolysis driven by the availability of oxygen/glucose with respect to energy demand. The goals of these investigations are to understand and to be able to manipulate the molecular mechanisms responsible for brain microvascular remodeling. These mechanisms that allow the neurovascular unit to adapt to environmental challenges are also likely involved in the vascular remodeling that occurs with learning, and which may be diminished with age. These processes can have an important contribution to the pathophysiology of ischemia and other metabolic and oxidative stresses. We seek to identify the relative roles of the components of the neurovascular unit in the regulation of the major modulatory molecules: HIF-1, HIF-2 and COX-2 that govern angiogenesis by using transgenic mice to isolate the cell-type specific modulators.

Metabolic rate for glucose (CMRglu), cerebral blood flow (CBF) and oxidative stress defense are known to decline with age and result in a decline in glucose oxidative metabolism or “hypometabolism”. As a result, the coupling between CMRglu and CBF deteriorates with age suggesting dysfunction of the neurovascular unit. Cell death in brain is associated with energy imbalances related to glucose metabolism that are linked to insufficient replenishing or restoring of the citric acid cycle intermediates. Neurodegeneration associated with oxidative stress in the aged makes recovery from stroke, cardiac arrest and other pathophysiological conditions challenging, resulting in increased morbidity/mortality. Neuroprotection in ketotic rat brain following transient focal ischemia is related to succinate-mediated stabilization of hypoxic-inducible factor-1 alpha (HIF-1α). Ketone bodies (beta-hydroxybutyrate and acetoacetate; BHB, AcAc) are alternate energy substrates to glucose that can be utilized by brain, especially during long term fasting or a ketogenic diet. We are investigating the neuroprotective effects of ketosis following transient global and focal ischemia, with direct implication for clinical treatments of patients at risk for stroke, especially in the aged. PET imaging and metabolic flux analyses (via stable isotopomer analysis) will be used to determine if cerebral metabolic rate for glucose (CMRglu) and brain energetics (energy balance / substrate utilization) are stabilized with diet-induced ketosis in the aged rat.

In the CNS there is a continuous balance between angiogenic and angiolytic signals, responding to the tissue metabolic energy demands and the availability of oxygen and substrate (glucose); and this balance is assessed at the local level of the neurovascular unit. A more recent refinement of this hypothesis specifically assigns the VEGF upregulation to the astrocyte and HIF-2α. The cerebral vasculature continues during postnatal development with the peak of angiogenetic activity between P10 and P23, stabilizing by about P30. Perinatal hypoxic (or hypoxic/hypercapnic) exposure influences this process. For example, 2 hours of severe hypoxic exposure at P2 results in an increased cerebral capillary density when measured at P60. We study the mechanisms and consequences of this perinatal hypoxia +/-hypercapnia induced alteration in the brain microvasculature; deduce the role of VEGF in altered adult capillary density in mice exposed to brief, severe perinatal hypoxia +/-hypercapnia;and, evaluate the effect of perinatal exposure to hypoxia +/-hypercapnia on hypoxia induced angiogenesis. In this project we will confirm that capillary density differences in the adult correlate with VEGF levels and determine the mechanism regulating VEGF. We will show that perinatal hypercapnia with hypoxia impairs angioplasticity. We will begin the exploration of the mechanism of the CO2 impairment of angioplasticity using relevant transgenic mouse strains that exhibit mimicking phenotypes.