Background: Chronic cerebral hypoperfusion (CCH) is a clinical syndrome, which is characterized by significantly decreased cerebral blood flow (CBF). CCH is a common consequence of cerebrovascular and cardiovascular diseases and the elderly. CCH results in a series of pathological damages, increasing cell death, autophagy dysfunction, amyloid β (Aβ) peptide accumulation, blood-brain barrier (BBB) disruption, and endothelial damage, which are found in CCH models. In addition, CCH is a prominent risk factor of cognitive impairment, such as vascular dementia, and CCH contributes to the occurrence and development of Alzheimer’s disease. Therefore, the treatment of patients with CCH is of great value. It has been confirmed that remote ischemic conditioning (RIC) is a safe, promising treatment for acute and chronic cerebrovascular diseases. RIC significantly increases CBF in both CCH models and patients, inhibits neuronal apoptosis, reduces Aβ deposition, protects BBB integrity and endothelial function, alleviates neuroinflammation, improves cognitive impairment, and exerts neuroprotection. Summary: With the development of animal models, the pathophysiological mechanisms of CCH and RIC are increasingly revealed. Key Messages: We discuss the mechanisms related to hypoperfusion in the brain and explore the potential treatment of RIC for CCH to promote its transformation and application in humans.
Chronic cerebral hypoperfusion (CCH) refers to a chronic process of pathological reduction in cerebral blood flow (CBF) due to a group of cerebral vascular and circulatory disorders. The etiology of CCH includes arteriosclerosis of large vessels (intracranial and extracranial large artery stenosis or occlusion), small vessel disease induced by hypertension or diabetes and vascular amyloidosis, hereditary vessel diseases such as cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy, and hemodynamic abnormalities such as heart failure and hypotension [1, 2]. The pathophysiological mechanisms of CCH include autophagy disorders, neuronal cell death/apoptosis, blood-brain barrier (BBB), accumulation of the amyloid β (Aβ) peptide, endothelial damages, impaired glucose metabolism, glutamate excitotoxicity, neuroinflammation, oxidative stress, and so on [2‒7].
CCH is considered as a major contributor of vascular cognitive impairment and neurodegenerative processes like Alzheimer’s disease (AD) [7‒10]; in addition, CCH may lead to anxiety, depression , and acute ischemic stroke if without treatment; therefore, early and effective treatment for CCH would be of great importance. In addition to the risk factors management, several drugs such as minocycline, cilostazol, and edaravone were used for CCH therapy in animal models , but there has been a major translational block in human . Increasing studies revealed that remote ischemic conditioning (RIC) might alleviate the pathological changes and cognitive deficits induced by CCH, being a promising treatment for CCH and vascular cognitive impairment .
RIC includes three methods: preconditioning (initiated before ischemia), per-conditioning (initiated during ischemia), and postconditioning (initiated after ischemia and during reperfusion) . RIC was first conducted by Murry et al.  in 1986. They described that repetitive occlusion (5 min) and reperfusion (5 min) of the left anterior descending artery by 4 cycles could reduce infarction area induced by following 45-min period of occlusion of the same artery in the canine heart. The result demonstrated that repetitive, transient, and sublethal ischemia-reperfusion may trigger endogenous protection against the following ischemic injury. After that, numerous studies were conducted to invest the protection of ischemic conditioning to multiple organs [15‒19]. Despite its advantages, the ischemic conditioning in situ is invasive and unpractical in human. Therefore, RIC, a noninvasive and well-tolerated therapy, was adopted. In RIC, repeated ischemic/reperfusion was performed on limbs and exerted protective effects against the subsequent ischemic injury in distant organs such as the brain, heart, kidneys, lungs, liver, skin flaps, ovaries, intestine, stomach, and pancreas [10, 20, 21].
A randomized controlled trial which recruited 26 patients with acute ischemic stroke happened within 24 h and excluded patients receiving thrombolysis and severe disability (modified Rankin Scale score>3), and significant comorbidities showed that RIC was well tolerated and feasible and might improve National Institutes of Health Stroke Scale (NIHSS) score in 90 days . Meng et al.  demonstrated that RIC is also protective for people with intracranial atherosclerosis. In their study, 68 cases diagnosed symptomatic intracranial arterial stenosis were enrolled, and the results proved that RIC can remarkably reduce the incidence of recurrent stroke at 90 and 300 days, shorten the recovery time (modified Rankin Scale score 0–1), and improve cerebral perfusion. Another clinical trial confirmed that RIC significantly decreased not only the incidence of recurrent stroke in 180 days but also the inflammatory stress . RIC can also benefit people with small-vessel disease in visuospatial and executive ability . Other studies showed that RIC significantly lowered the standard biomarkers levels at 6 and 24 h after severe traumatic brain injury , and RIC is well tolerated in patients with aneurysm subarachnoid hemorrhage and associated with good clinical outcomes .
Many studies in CCH models and patients showed that RIC could significantly increase CBF, improve cognitive deficits, prevent neuronal cell death, protect endothelium function, alleviate neuroinflammation and oxidative stress, decrease infarct volume, and alleviate BBB leakage [27‒32]. These results suggest that RIC might be a promising treatment for CCH, but it needs more clinical studies to confirm that if RIC is suitable for CCH in humans.
RIC Can Inhibit Apoptosis
Apoptosis is a programmed cell death regulated by genes . The number of TUNEL-positive cells and the expression of apoptosis-related proteins changed in CCH models . Bax is an initiating apoptotic protein, caspase-3 is a proapoptotic protein, whereas Bcl-2 is an antiapoptotic protein which could prevent neuronal apoptosis. The expression levels of Bax and caspase-3 were significantly increased in CCH mice when compared with those in the sham group, whereas the Bcl-2 expression level was significantly reduced [35‒37]. Therefore, the Bcl-2/Bax ratio decreased in CCH models .
RIC therapy significantly suppressed apoptosis in CCH and ischemia/reperfusion injury. In myocardium ischemia/reperfusion injury models, remote ischemic postconditioning may increase the level of Bcl-2/Bax and reduce the level of cleaved caspase-3 [39, 40]. RIPC inhibited apoptosis and reduced apoptotic-cell proportion in ischemic stroke mice via the mitochondrial pathway [41‒43]. Two weeks of remote ischemic postconditioning therapy prevented cell death and vacuolization in the cortex, corpus callosum, and hippocampus CA1 region and decreased average neuropathological score due to bilateral common carotid artery stenosis (BCAS) [44, 45]. In ischemic stroke rats, RIC ameliorated neuronal death detected by the TUNEL technique and ameliorated neuronal apoptosis via both extrinsic and intrinsic apoptosis pathways . According to animal studies, inhibiting neuronal apoptosis might be an important aspect by which RIC exerted neuroprotection. However, if this happens in humans, it needs further more studies to expose.
The eNOS/NO system play a crucial role in the angiogenesis and regulation of CBF after stroke [47‒49]. The production of NO by vascular endothelium relied mainly derived from eNOS . eNOS catalyzed the conversion of L-arginine to L-citrulline generating NO. NO was important for cerebral autoregulation, cerebral blood vessel expansion, inhibiting platelet adhesion, and promoting smooth muscle proliferation [2, 45, 49]. Nitrite was the storage pool of NO generated by eNOS and was reduced to NO in hypoxic conditions which acted as a mediator of preconditioning . In eNOS/mice, angiogenesis was significantly impaired in the ischemic hind limb . Torre et al.  demonstrated that the eNOS levels were significantly decreased in the hippocampal endothelium in CCH rats, and the eNOS inhibitor L-NIO markedly worsened spatial cognitive impairment, which indicated that nitric oxide derived from eNOS played a critical role during CCH by regulating microvascular tone and CBF. However, another study showed that the phosphorylated endothelial nitric oxide synthase level was increased in the hippocampus of CCH models for 2 weeks. RIC treatment could sustain the increased level of phosphorylated endothelial nitric oxide synthase up to 4 weeks, elevate cerebral vessel number further, and improve spatial learning and memory compared to those without RIC treatment, and these effects could be reversed by intraperitoneal injection of NOS inhibitors . It was reported that RIC treatment could also upregulate the plasma nitrite level which was related to the improvement of CBF . Another experiment found that RIC upregulated mRNA expression of eNOS by about 10 folds in the blood vessels from the conditioning site and increased the level of NO . Moreover, in the eNOS knockout mice, the neuroprotection of rapid ischemic preconditioning was absent . These results suggested RIC exerted the neuroprotective effect partly through the eNOS/NO/nitrite system. Nitrite could be a promising biomarker in the peripheral blood because it was easily measured.
RIC May Improve CBF and Cognitive Decline
Cerebral hypoperfusion is an important contributor to cognitive decline in both humans and animal models [56‒60]. The cognitive decline was partly attributed to the decline of CBF. In CCH models, CBF was reduced as much as 35%∼45% or even more than 60% of its pre-occlusion level [61, 62]. Therefore, increase in CBF may be an effective way to ameliorate cognitive decline. Many studies demonstrated that RIC could improve CBF and cognitive impairment induced by cerebral hypoperfusion. CBF decreased after BCAS in the CCH model and showed slightly but insignificantly improvement over days . The CBF reduction induced by hypoperfusion could be reversed by RIC application . CBF was significantly improved in 2-week, 1-month, and 4-month RIC therapy as compared to BCAS-sham RIC, and the improvement effect of CBF could sustain to 1 week after the cessation of RIC therapy [45, 63]. However, as for the effect of improving CBF, 1-month RIC therapy was as effective as 4-month RIC therapy at 6 months after BCAS . RIC therapy could improve spatial learning and memory impairment tested by the Morris water maze test at 6 weeks after BCAS but not at 4 weeks . RIC could also improve nonspatial working memory of BCAS mice tested by the NOR test . Recent clinical research indicated that RIC could improve small vessel disease-related mild cognitive impairment after 1 year of therapy . Therefore, RIC could be a promising treatment for neurodegenerative and neurovascular diseases.
White matter damage was a critical pathological change in vascular dementia, and the degree of leukoaraiosis was associated with cognitive impairment in the elderly [64‒66]. White matter lesions were the common pathological changes in CCH models, and hypoperfusion was early found in the leukoaraiosis area [67, 68]. Anyhow, RIC might robustly alleviate white matter damages. In an animal study, the application of RIC therapy for 2 weeks after BCAS operation prevented the degeneration of the white matter . The result was consistent with another study. Zhou et al.  demonstrated that RIC therapy for 1 or 4 months significantly reversed the white matter lesion induced by BCAS mice . Studies about patients with intracranial atherosclerotic stenosis or cerebral small-vessel disease also suggested that long-term RIC therapy ameliorated white matter damage and improved cognitive impairment [24, 61].
RIC May Regulate Autophagy
Autophagy is an intracellular degradation pathway, through which some unnecessary cellular components such as damaged proteins and organelles were degraded into metabolic elements and recycled for restoring cellular homeostasis and normal cellular functions [60, 69, 70]. Autophagy can be activated in response to diverse physiological and pathological process, such as starvation, oxidative stress, inflammatory, and immune responses, and autophagy dysregulation was observed in a wide range of disorders . Accumulating evidences indicated that autophagy played essential roles in neuron physiopathological processes, and autophagy might be a promising therapeutic strategy for vascular disease  and neurodegenerative diseases . Cerebral hypoperfusion was associated with abnormal autophagy and neurological dysfunctions , and autophagy modulation might play important roles in improving these disorders.
Zhang et al.  discussed the neuroprotection of autophagy inhibition on white matter lesions and spatial working memory in cerebral hypoperfusion mice. They revealed that autophagy activity in white matter changed over time, which was initiated at 3 days, suppressed at 10 days, and activated again at 30 days after BCAS operation, and the application of the autophagy inhibitor wortmannin could alleviate BCAS-induced white matter lesions and improve the spatial working memory. Another study demonstrated that CCH could improve the excessive autophagy activity and exacerbate the synaptic damage in the rat hippocampus, and URB, an autophagy inhibitor, could inhibit abnormal excessive autophagy and alleviate synaptic degradation to exert neuroprotective effects .
Autophagy was divided into three distinct classes, namely, microautophagy, chaperone-mediated autophagy, and macroautophagy. Macroautophagy was the major type. Autophagy was initiated by the formation of a double-membranous structure, the autophagosome. The autophagosome was then fused with a lysosome to generate an autolysosome, where the degradation occurred by specific acidic hydrolases . Beclin1 was essential for the activation and regulation of autophagy, LC3 contributed to the elongation and maturation of autophagosomes, and p62 accumulation was indicative of impaired autophagic degradation . RIC may regulate autophagy by multiple pathways, although the exact mechanisms remain unknown. RIC promoted Bcl-2 phosphorylation via the activation of the AKT pathway and promoted dissociation of the Bcl-2 and Beclin1 complex, thereby leading to the induction of autophagy to exert neuroprotection in ischemic stroke rats . In the ischemia/reperfusion rats, RIC might reverse the increase of the LC3-II/LC3-I ratio, Beclin1, and autophagosome numbers to relieve the induction of autophagy through the activation of the mTOR/p70S6K signaling pathway to protect against cerebral I/R injury . Wang et al.  showed that the combined application of remote ischemic perconditioning and remote ischemic postconditioning increased the immunoreactivity of LC3 and Beclin1 and decreased the immunoreactivity of P62, which suggested that the increase of autophagosome resulted from enhanced autophagy activation.
RIC Can Reduce the Deposit of Aβ
Aβ is a 36–43-amino acid peptide, which is derived from an amyloid precursor protein after β- and γ-secretase-mediated sequential cleavage. Aβ40 and Aβ42 are the main forms of existing Aβ in the brain [38, 62]. Increased Aβ deposition is one of the characteristics of AD pathology . Several studies have reported that CCH increased the Aβ levels and aggravated the pathology of AD [33, 80‒83]. As the increased deposition of Aβ in the brain was resulted from the imbalance between the production and clearance, the overproduction and impaired clearance led to the deposition of Aβ in CCH models. In BCAS models, the Aβ generation and accumulation were increased as much as 4-fold increase in brain areas including the frontal cortex, hippocampus CA1, and hippocampus DG by enhancing the β- and γ-secretase levels and the increasing trends sustained until 6 months after BCAS operation [38, 44], although the expression of Aβ in the temporal cortex was not elevated . The disruption of BBB influenced the clearance of Aβ . It was conducted that the CCH disrupted the structural integrity of the neuronal-glial-vascular units and disrupted the BBB microarchitecture to reduce the clearance of Aβ . Impaired clearance through the glymphatic pathway from ischemic or infarcts also might contribute to the deposition of Aβ .
RIC therapy might reduce the accumulation of Aβ induced by CCH and prevent the progression of AD pathology. In BCAS mice, 2 weeks after remote ischemic postconditioning therapy, the Aβ42 content was reduced by 2-fold . Studies showed that RIC treatment decreased leakage of BBB [27, 86]. The decreased leakage of BBB after RIC treatment was supposed to be a way through which RIC decreased the content of Aβ in CCH models.
CCH is a common condition in people, and CCH may lead to critical pathology changes and cognitive deficits, threatening people’s health and life quality. RIC may restore CBF and cognitive functions and alleviate the pathological changes induced by CCH, becoming a potential treatment for CCH. The mechanism of neuroprotection against CCH provided by RIC needs to be investigated deeply in the future.
Conflict of Interest Statement
The authors have no conflicts of interest to declare.
The authors did not receive any funding.
Xiao Ma contributed to the literature search and the manuscript writing. Chenhua Ji contributed to the conception of study and manuscript modification.