Potential Targets for Acute Ischemic Stroke Treatment: The Role of S1PR1 and LOX-1 in Cerebrovascular Integrity and Inflammation

Abstract

Vascular pathology such as acute ischemic stroke (AIS) is a highly prevalent disease reported to affect 1 in 4 people over the age of 25 across their lifetime resulting in severe morbidity and mortality. AIS sequalae within the brain is multi-faceted and encompasses a variety of cell types including neurons, glia, local and peripheral immune cells, and cerebrovascular smooth muscle and endothelium. Cerebrovascular integrity loss and increased inflammation during an AIS is of significant concern as these factors profoundly impact the surrounding microenvironment leading to worsened neurological outcomes. Effective treatment for AIS revolves around prompt revascularization in which only ~3% of stroke patients qualify. Thusly, sphingosine-1-phosphate receptor (S1PR) modulators have emerged as a promising therapeutic candidate, due to protection of blood brain barrier (BBB) integrity and attenuation of inflammation. Pilot studies have demonstrated their effectiveness in improving clinical outcomes of AIS patients. Interestingly, elevation of lipid levels as seen in hyperlipidemia has become increasingly common globally and has been found to be an independent as well as a synergistic risk factor with hypertension on stroke incidence. Increased levels of oxidized low-density lipoprotein (oxLDL) as found in a significant portion of AIS patients, leads to a higher risk of death, poor functional outcome, and cerebrovascular dysfunction which warrants further investigation as a co-morbidity for AIS. Our preliminary data demonstrate that selective S1PR1 activation elicits neuroprotection during experimental AIS. However, to advance S1PR1 activation as a potential therapeutic target for AIS patients with high oxLDL concentrations, several critical questions remain unanswered. First, the protective mechanisms of S1PR1 activation on the cerebrovascular smooth muscle and endothelium following ischemic injury remains to be elucidated. Second, the role of lectin like oxLDL receptor 1 (LOX-1) following clinically relevant oxLDL exposure to alter vascular reactivity and inflammation remains unknown. Lastly, it is undetermined if oxLDL acts independently or in concert with ischemia to modify cerebrovascular smooth muscle and endothelial integrity, inflammation, and phenotype. Therefore, I hypothesized that cerebrovascular integrity dysfunction and inflammation following ischemic injury will be amplified by oxLDL via LOX-1 and mitigated by S1PR1 activation. The results of this dissertation show that the cerebrovascular smooth muscle and endothelium differentially express S1PRs, of which S1PR1, is upregulated following ischemic injury in a time- dependent manner. We also showed that activation of S1PR1 attenuated human cerebrovascular smooth muscle cell death, autophagic flux, and synthetic phenotypic switching as well as endothelial barrier dysfunction and inflammation following ischemic injury. In the follow-up study we demonstrated that oxLDL altered isolated murine vessel function, stiffness, remodeling, and inflammation in an endothelial, sex, age, and LOX-1 receptor dependent manner. Moreover, we observed differential effects of oxLDL and ischemic injury on human brain and aortic vascular smooth muscle cells, suggesting distinct responses between conduit and resistance vessels. Additionally, we observed that oxLDL exacerbated ischemic-mediated increase in endothelial inflammation and barrier integrity in a dose, time, and LOX-1 receptor dependent manner. The primary findings of this dissertation suggest that S1PR1 activation and LOX-1 inhibition may be novel and viable therapeutic targets within the cerebrovasculature following acute ischemic stroke injury.