Roles of NAMPT and NAD+ in hypoxic conditioning-induced neurovascular protection in subarachnoid hemorrhage

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Project Summary/Abstract Aneurysmal subarachnoid hemorrhage (SAH) is a highly morbid condition, in large part due to secondary brain injury from Early Brain Injury (EBI) and Delayed Cerebral Ischemia (DCI). EBI occurs 1-3 days after ictus and is characterized by blood brain barrier breakdown, neuroinflammation, and neuronal cell death. DCI occurs 4-12 days after ictus and results from a combination of large artery vasospasm and microcirculatory deficits. Given that EBI and DCI are caused by wide-ranging neurovascular deficits, we believe that effective SAH therapy will require a multiplicity of protective effects to maximize the chance of efficacy. We therefore applied a powerful protection strategy with known pleiotropic effects – Conditioning-based therapy – to experimental models of SAH. Conditioning is a concept whereby the brain's inherent resistance to injury can be enhanced by exposure to non-harmful stress stimuli. Previously, we showed that hypoxic conditioning initiated before SAH (Hypoxic Preconditioning) provides robust protection against DCI in an eNOS-dependent manner. Recently, we extended upon these results in three important ways: 1) We showed that hypoxic conditioning initiated 3h after SAH (Hypoxic Post-Conditioning; HPostC) also produces robust neurovascular protection; 2) We showed that the NAD+-dependent deacetylase, Sirtuin 1 (SIRT1), is a key mediator of this protection; and 3) We showed preliminarily that Nicotinamide phosphoribosyltransferase (NAMPT) is likely a key upstream molecule driving the neurovascular protection afforded by HPostC. NAMPT is the rate-limiting enzyme in the NAD+ salvage pathway that converts nicotinamide (NAM) to nicotinamide mononucleotide (NMN) enabling biosynthesis of NAD+, which is an essential co-factor of SIRT1 leading to its activation. In the present grant, we will test our central hypothesis is that NAMPT-driven NAD+ production plays a causal role in the neurovascular protection afforded by HPostC in SAH, and that this protection is either partially or completely SIRT1-mediated. The Specific Aims are (1) Test the hypothesis that NAMPT is necessary for the EBI and DCI protection afforded by HPostC in SAH; (2) Test the hypothesis that therapeutic strategies designed to augment NAMPT activity or increase NAD+ levels mimic the EBI and DCI protection afforded by HPostC in SAH; and if so, determine if this protection is partially or completely SIRT1-mediated; and (3) Determine the translational potential of therapeutic strategies targeting NAMPT and NAD+ by assessing their impact on long- term cognitive deficits after SAH. Methods used include: (a) Two complementary mouse models of SAH; (b) Assessment of NAMPT, NAD+, and SIRT1 levels; (c) Assessment of neuroinflammation, neuronal cell death, vasospasm, microcirculatory deficits, and short- and long-term neurobehavioral deficits; (d) Pharmacologic and genetic inhibition of NAMPT and SIRT1; and (e) Pharmacologic and genetic augmentation of NAMPT or NAD+. Overall, the work proposed in the present grant has the potential to identify an entirely new therapies for the treatment of patients with ruptured brain aneurysms – NAMPT activation or NAD+ augmentation. If successful, these studies will result in an improved understanding of the breadth, mechanism, and sustainability of HPostC- induced neurovascular protection in SAH and determine the translatability of NAMPT- and NAD+-directed therapeutics.