Brain pathophysiology in SARS-CoV-2 disease

SARS-CoV-2, the virus underlying the current COVID-19 pandemic, not only affects peripheral tissues, it also targets the brain causing microvascular lesions, microhemorrhages and neurological manifestations. The internalization of SARS-CoV-2 is initiated by the binding of the virus spike protein to angiotensin converting enzyme 2 (ACE2) on the membrane of host cells including endothelial cells throughout cerebral capillaries. ACE2 is internalized along with the virus thereby leading to a state of ACE2 deficiency. ACE2 is a critical member of the renin-angiotensin system (RAS). This enzyme catabolizes the octapeptide hormone angiotensin-[1-8] thereby protecting cells and tissues from the vasoconstrictor, pro-inflammatory and pro-thrombotic effects of overactive angiotensin type 1 receptors (AT1Rs). Blocking AT1Rs with an AT1R antagonist protects mice from behavioral impairments due to ACE2 deficiency. Lipopolysaccharide (LPS) causes microglia activation and neuronal cell loss and is widely used as an experimental model of neuroinflammation. The brain pathophysiology induced by LPS shares many similarities with SARS-CoV-2 infection. We hypothesize that under conditions of reduced ACE2 (i.e., ACE2 knockout mice or SARS-CoV-2-infected hamsters), AT1R activity is upregulated in the microvasculature. In the presence of an inflammatory insult (i.e., LPS or SARS-CoV-2), AT1Rs promote endothelial dysfunction in the microvasculature through pro-inflammatory and pro-thrombotic signaling pathways leading to blood brain barrier injury. Deficits in cognition and increased anxiety ensue. We will test this overall hypothesis through the following specific aims: Determine the mechanisms of the pro-injury (Aim 1) and protective (Aim 2) arms of the RAS that regulate the pathophysiology of the brain in animal models of neuroinflammation and COVID-19. (Aim 3) Determine the mechanisms underlying the effects of biological sex and age in the brain pathophysiology induced by LPS and SARS-CoV-2. Studying RAS mechanisms in the brain will provide insight into on-going and future clinical trials of therapeutics for treating brain injury associated with COVID-19 and other diseases of neuroinflammation. In addition, focusing on mechanisms underlying the effects of biological sex and age on microvasculature pathophysiology in models of neuroinflammation and COVID-19 will shed light into why male sex and age are major risk factors for COVID-19 severity.