Hypoxia Inducible Factor 1? Mediates Endogenous Neuroprotective Role In Rat Model Of Chronic Cerebral Hypoperfusion

Background:Chronic cerebral hypoperfusion (CCH) has been commonly associated with Alzheimer's disease and vascular dementia, which is one of the initial conditions that are critical to the development of cognitive dysfunction. Chronic Cerebral hypoperfusion can cause disorder in neuronal electrical activity, inhibition of protein synthesis, apoptosis, inflammation and oxidative stress, which may be pathophysiological mechanisms that contribute to cognitive impairment. Despite the detrimental effects of hypoperfusion, the decline in cellular oxygen levels does, however, induce compensatory or endogenous adaptive and regenerative mechanisms to defend against hypoperfusion injury and to allow recovery of brain function. Thus, exploring the mechanisms of endogenous neuroprotection during CCH is valuable for the therapy of AD and VaD. Regulation of the endogenous adaptive processes involves the concerted activation of various transcription factors, including HIF-la. The hypoxia-inducible factor la (HIFla)-mediated signaling pathway is an adaptive and protective mechanism that is triggered by hypoxia, ischemia, and other pathophysiological conditions. HIF-la target genes include pro-survival genes involved in angiogenesis, energy metabolism, erythropoiesis, vasomotor control, and cell proliferation, such as vascular endothelial growth factor (VEGF), glucose transporter-1 (GLUT-1), and erythropoietin (EPO). Paradoxically, HIF-la is also involved in hypoxia-dependent inflammation, apoptosis, and cellular stress. Some of the pro-death genes that are involved include Bcl-2/adenovirus EIB 19 kDa-interacting protein 3 (BNIP-3), NADPH oxidase activator 1 (Noxa), NIP3-like protein X (NIX), and others. Consequently, despite the undisputed role of HIF-la in regulating oxygen homeostasis, both neuroprotective and detrimental effects of HIF-la have been observed in various acute ischemia models. Some research has indicated a role for HIF-la in mediating both pro-death and pro-survival responses that may depend on the duration of the ischemia and the types of pathological stimuli. CCH is a less severe form of ischemia that develops more slowly than an acute ischemic stroke. Therefore, HIF1?-induced long-term effects may be important factors in the development of CCH, which may provide a potential target for dementia therapy.Objects:The aim of our study was to ascertain the role of endogenous HIF-la stabilization during chronic cerebral hypoperfusion in rats.Methods:1. Permanent bilateral common carotid artery occlusion (2VO) was used to induce chronic global cerebral hypoperfusion in rats. The expression of HIF-la protein and the transcription of downstream genes in hippocampus were measured at different time points, including 0 h,12 h,24 h,3 days,7 days,14 days,28 days,42 days, and 56 days after 2VO. 2. Lentivirus expressing HIF-la small hairpin RNA was injected into bilateral hippocampus and bilateral ventricles to knock down HIF-1? gene express in hippocampus and some other brain areas. One week following the injection, permanent bilateral common carotid artery occlusions (2-vessel occlusions (2VOs)) were performed to induce CCH in male adult Wistar rats. Four weeks later, we assessed the effect of HIF-la knockdown on cerebral capillary density, oxidative stress, inflammatory-related glial cell activation of the brain, and cognitive function.Results:1. HIF-la increased as early as 12 h after the occlusion and remained high for at least 56 days. Interestingly, mRNA levels of both pro-apoptotic (Bcl-2/adenovirus EIB 19 kDa-interacting protein 3, NADPH oxidase activator 1, and NIP3-like protein X) and pro-survival (vascular endothelial growth factor, glucose transporter-1) genes were up-regulated at the early stage after 2VO, followed by a gradual decline to baseline/control levels. 2. After 2VO, rats with lentivirus-mediated HIF-la knockdown exhibited reduced cerebral capillary density, increased oxidative damage, and increased density of glial fibrillary acidic protein-positive astrocytes and ionized calcium binding adapter molecule 1-positive microglia in cortex and some subregion of hippocampus. Furthermore, HIF-la knockdown also aggravated consequential spatial learning and memory impairment induced by 2VO.Conclusion:Our findings suggest that HIF-mediated pro-survival responses are dominant in rats with CCH. The accumulated HIF-la after CCH mediates endogenous adaptive process to defend against more severe hypoperfusion injury of brain.