The plasticity of posterior communicating artery influences on the outcome of white matter injury induced by chronic cerebral hypoperfusion in rats

The dilemma of neuroprotection trials in times of successful endovascular recanalization

Background

The "translational roadblock" between successful animal stroke studies and neutral clinical trials is usually attributed to conceptual weaknesses. However, we hypothesized that rodent studies cannot inform the human disease due to intrinsic pathophysiological differences between rodents and humans., i.e., differences in infarct evolution.

Methods

To verify our hypothesis, we employed a mixed study design and compared findings from meta-analyses of animal studies and a retrospective clinical cohort study. For animal data, we systematically searched pubmed to identify all rodent studies, in which stroke was induced by MCAO and at least two sequential MRI scans were performed for infarct volume assessment within the first two days. For clinical data, we included 107 consecutive stroke patients with large artery occlusion, who received MRI scans upon admission and one or two days later.

Results

Our preclinical meta-analyses included 50 studies with 676 animals. Untreated animals had a median post-reperfusion infarct volume growth of 74%. Neuroprotective treatments reduced this infarct volume growth to 23%. A retrospective clinical cohort study showed that stroke patients had a median infarct volume growth of only 2% after successful recanalization. Stroke patients with unsuccessful recanalization, by contrast, experienced a meaningful median infarct growth of 148%.

Conclusion

Our study shows that rodents have a significant post-reperfusion infarct growth, and that this post-reperfusion infarct growth is the target of neuroprotective treatments. Stroke patients with successful recanalization do not have such infarct growth and thus have no target for neuroprotection.

Impairment of attention network function in posterior circulation ischemia-evidence from the Attention Network Test

Objective

This study aimed to investigate the effect of posterior circulation ischemia (PCI) on attention network function and to determine whether PCI is holistic or selective attention network deficit and which attention network is affected.

Methods

Thirty-six PCI patients aged 30 to 75 were assessed using the Attention Network Test and the Mini-Mental State Examination (MMSE). There were no significant differences in age, sex, and education between PCI group and the control group (n = 32). All data were statistically analyzed by SPSS 23.0 software.

Result

There were no significant difference in the MMSE scores between the two groups. Compared with the control group, the PCI group had significantly shorter response time for alerting and orienting network. The executive control network response time was significantly longer in PCI group than in the control group. The overall mean response time was also significantly longer in PCI group than in normal control group. There was no significant difference in mean accuracy between the two groups.

Conclusion

The alerting, orienting, and executive control networks were significantly less efficient in PCI group than in the control group (P < 0.01). This indicates impaired attention network in PCI patients. Since transient nerve seizures caused by vertebrobasilar ischemia may precede posterior circulation stroke, early assessment of cognitive function in patients with PCI is particularly important, and ANT is an excellent tool for this assessment.

Pitfalls of Commonly Used Ischemic and Dementia Models Due to Early Seizure by Carotid Ligation

While the bilateral common carotid artery (CCA) ligation model is widely used in cerebrovascular disease and dementia studies, it can frequently cause seizures. We examined the validity of seizure as an experimental model of ischemia. Eight-week-old male Wistar and Sprague-Dawley (SD) rats were implanted with electrocorticography (ECoG) electrodes and bilateral CCA ligation was performed and compared to the sham groups. ECoG monitoring was used to confirm the seizure discharge and count the number of spikes in the interictal phase 2 h after ligation, followed by power spectral analysis. Magnetic resonance imaging (MRI) was performed 6 h after bilateral CCA ligation to assess fractional anisotropy (FA), apparent diffusion coefficient (ADC), and cerebral blood flow (CBF) values. Magnetic resonance spectroscopy (MRS) was also performed and the ischemic parameters and electrophysiological changes were compared. The Wistar rat group had significantly higher mortality, frequency of seizures, incidence of non-convulsive seizures, and number of spikes in the interictal period compared to those in the SD rat group. Power spectral analysis showed increased power in the delta band in both Wistar and SD rat groups. MRI, after CCA ligation, showed significantly lower ADC values, lower glutamine and glutamate levels, and higher lactate values in Wistar rats, although there was no difference in FA values. Metabolic and electrophysiological changes after CCA ligation differed according to the rat strain. Wistar rats were prone to increased lactate and decreased glutamine and glutamate levels and the development of status epilepticus. Seizures can affect the results of ischemic experiments.

Toll-like receptor-2 gene knockout results in neurobehavioral dysfunctions and multiple brain structural and functional abnormalities in mice

OBJECTIVE

Toll-like receptor-2 (TLR2), a member of TLR family, plays an important role in the induction and regulation of immune/inflammation. TLR2 gene knockout (TLR2KO) mice have been widely used for animal models of neurological diseases. Since there is close relationship between immune system and neurobehavioral functions, it is important to clarify the exact role of TLR2 defect itself in neurobehavioral functions. The present study aimed to investigate the effect of TLR2KO on neurobehavioral functions in mice and the mechanisms underlying the observed changes.

METHODS

Male TLR2KO and wild type (WT) mice aged 3, 7, and 12 months were used for neurobehavioral testing and detection of protein expression by Western blot. Brain magnetic resonance imaging (MRI), electrophysiological recording, and Evans blue (EB) assay were applied to evaluate regional cerebral blood flow (rCBF), synaptic function, and blood-brain barrier (BBB) integrity in 12-month-old TLR2KO and age-matched WT mice.

RESULTS

Compared to WT mice, TLR2KO mice showed decreased cognitive function and locomotor activity, as well as increased anxiety, which developed from middle age (before 7-month-old) to old age. In addition, significantly reduced regional cerebral blood flow (rCBF), inhibited long-term potentiation (LTP), and increased blood-brain barrier (BBB) permeability were observed in 12-month-old TLR2KO mice. Furthermore, compared with age-matched WT mice, significant reduction in protein levels of tight junction proteins (ZO-1, Occludin, and Claudin-5) and increased neurofilament protein (SMI32) were observed in 7 and 12-month-old TLR2KO mice, and that myelin basic protein (MBP) decreased in 12-month-old TLR2KO mice.

CONCLUSION

Our data demonstrated that TLR2 defect resulted in significantly observable neurobehavioral dysfunctions in mice starting from middle age, as well as multiple abnormalities in brain structure, function, and molecular metabolism.

High-Mobility Group Box 1 Neutralization Prevents Chronic Cerebral Hypoperfusion-Induced Optic Tract Injuries in the White Matter Associated with Down-regulation of Inflammatory Responses

Chronic cerebral hypoperfusion (CCH)-induced white matter lesions (WMLs) are region-specific with the optic tract (OT) displaying the most severe damages and leading to visual-based behavioral impairment. Previously we have demonstrated that anti-high-mobility group box 1 (HMGB1) neutralizing antibody (Ab) prevents CCH-induced hippocampal damages via inhibition of neuroinflammation. Here we tested the protective role of the Ab on CCH-induced OT injuries. Rats were treated with permanent occlusion of common carotid arteries (2-VO) or a sham surgery, and then administered with PBS, anti-HMGB1 Ab, or paired control Ab. Pupillary light reflex examination, visual water maze, and tapered beam-walking were performed 28 days post-surgery to investigate the behavioral deficits. Meanwhile, WMLs were measured by Klüver-Barrera (KB) and H&E staining, and glial activation was further assessed to evaluate inflammatory responses in OT. Results revealed that anti-HMGB1 Ab ameliorated the morphological damages (grade scores, vacuoles, and thickness) in OT area and preserved visual abilities. Additionally, the increased levels of inflammatory responses and expressions of TLR4 and NF-κB p65 and phosphorylated NF-κB p65 (p-p65) in OT area were partly down-regulated after anti-HMGB1 treatment. Taken together, these findings suggested that HMGB1 neutralization could ease OT injuries and visual-guided behavioral deficits via suppressing inflammatory responses.

Chronic cerebral hypoperfusion and plasticity of the posterior cerebral artery following permanent bilateral common carotid artery occlusion

Vascular dementia (VaD) is a group of heterogeneous diseases with the common feature of cerebral hypoperfusion. To identify key factors contributing to VaD pathophysiology, we performed a detailed comparison of Wistar and Sprague-Dawley (SD) rats subjected to permanent bilateral common carotid artery occlusion (BCCAo). Eight-week old male Wistar and SD rats underwent BCCAo, followed by a reference memory test using a five-radial arm maze with tactile cues. Continuous monitoring of cerebral blood flow (CBF) was performed with a laser Doppler perfusion imaging (LDPI) system. A separate cohort of animals was sacrificed for evaluation of the brain vasculature and white matter damage after BCCAo. We found reference memory impairment in Wistar rats, but not in SD rats. Moreover, our LDPI system revealed that Wistar rats had significant hypoperfusion in the brain region supplied by the posterior cerebral artery (PCA). Furthermore, Wistar rats showed more profound CBF reduction in the forebrain region than did SD rats. Post-mortem analysis of brain vasculature demonstrated greater PCA plasticity at all time points after BCCAo in Wistar rats. Finally, we confirmed white matter rarefaction that was only observed in Wistar rats. Our studies show a comprehensive and dynamic CBF status after BCCAo in Wistar rats in addition to severe PCA dolichoectasia, which correlated well with white matter lesion and memory decline.

Carotid artery stenosis in hypertensive rats impairs dilatory pathways in parenchymal arterioles

Hypertension is a leading risk factor for vascular cognitive impairment and is strongly associated with carotid artery stenosis. In normotensive rats, chronic cerebral hypoperfusion induced by bilateral common carotid artery stenosis (BCAS) leads to cognitive impairment that is associated with impaired endothelium-dependent dilation in parenchymal arterioles (PAs). The aim of this study was to assess the effects of BCAS on PA function and structure in stroke-prone spontaneously hypertensive rats, a model of human essential hypertension. Understanding the effects of hypoperfusion on PAs in a hypertensive model could lead to the identification of therapeutic targets for cognitive decline in a model that reflects the at-risk population. We hypothesized that BCAS would impair endothelium-dependent dilation in PAs and induce artery remodeling compared with sham rats. PAs from BCAS rats had endothelial dysfunction, as assessed using pressure myography. Inhibition of nitric oxide and prostaglandin production had no effect on PA dilation in sham or BCAS rats. Surprisingly, inhibition of epoxyeicosatrienoic acid production increased dilation in PAs from BCAS rats but not from sham rats. Similar results were observed in the presence of inhibitors for all three dilatory pathways, suggesting that epoxygenase inhibition may have restored a nitric oxide/prostaglandin-independent dilatory pathway in PAs from BCAS rats. PAs from BCAS rats underwent remodeling with a reduced wall thickness. These data suggest that marked endothelial dysfunction in PAs from stroke-prone spontaneously hypertensive rats with BCAS may be associated with the development of vascular cognitive impairment. NEW & NOTEWORTHY The present study assessed the structure and function of parenchymal arterioles in a model of chronic cerebral hypoperfusion and hypertension, both of which are risk factors for cognitive impairment. We observed that impaired dilation and artery remodeling in parenchymal arterioles and abolished cerebrovascular reserve capacity may mediate cognitive deficits.

Ischemic stroke: experimental models and reality

The vast majority of cerebral stroke cases are caused by transient or permanent occlusion of a cerebral blood vessel (“ischemic stroke”) eventually leading to brain infarction. The final infarct size and the neurological outcome depend on a multitude of factors such as the duration and severity of ischemia, the existence of collateral systems and an adequate systemic blood pressure, etiology and localization of the infarct, but also on age, sex, comorbidities with the respective multimedication and genetic background. Thus, ischemic stroke is a highly complex and heterogeneous disorder. It is immediately obvious that experimental models of stroke can cover only individual specific aspects of this multifaceted disease. A basic understanding of the principal molecular pathways induced by ischemia-like conditions comes already from in vitro studies. One of the most frequently used in vivo models in stroke research is the endovascular suture or filament model in rodents with occlusion of the middle cerebral artery (MCA), which causes reproducible infarcts in the MCA territory. It does not require craniectomy and allows reperfusion by withdrawal of the occluding filament. Although promptly restored blood flow is far from the pathophysiology of spontaneous human stroke, it more closely mimics the therapeutic situation of mechanical thrombectomy which is expected to be increasingly applied to stroke patients. Direct transient or permanent occlusion of cerebral arteries represents an alternative approach but requires craniectomy. Application of endothelin-1, a potent vasoconstrictor, allows induction of transient focal ischemia in nearly any brain region and is frequently used to model lacunar stroke. Circumscribed and highly reproducible cortical lesions are characteristic of photothrombotic stroke where infarcts are induced by photoactivation of a systemically given dye through the intact skull. The major shortcoming of this model is near complete lack of a penumbra. The two models mimicking human stroke most closely are various embolic stroke models and spontaneous stroke models. Closeness to reality has its price and goes along with higher variability of infarct size and location as well as unpredictable stroke onset in spontaneous models versus unpredictable reperfusion in embolic clot models.

Salvia miltiorrhiza extract protects white matter and the hippocampus from damage induced by chronic cerebral hypoperfusion in rats

BACKGROUND

Salvia miltiorrhiza (SM), an herbal plant, is traditionally used in the treatment of cardiovascular and cerebrovascular diseases in Asian countries. SM has multiple biological effects including anti-inflammatory activity. The present study is aimed at investigating the effects of SM extract in rats with chronic cerebral hypoperfusion.

METHODS

Chronic cerebral hypoperfusion was induced in male Wistar rats by permanent bilateral common carotid artery occlusion (BCCAo). The rats were divided into 3 groups: sham-control, BCCAo treated with vehicle, and BCCAo treated with SM extract. Vehicle or SM extract (200 mg/kg) were administered daily by oral gavage beginning on day 21 after BCCAo and continuing to day 42. Immunohistochemical analyses were used to measure Iba-1-positive microglia and myelin basic protein (MBP) in white matter and hippocampal tissue. In addition, the expression levels of proinflammatory cytokines, including TNF-α, IL-1β, and IL-6, and the toll-like receptor (TLR) pathway in the hippocampus, were analyzed by western blot.

RESULTS

Administration of SM extract attenuated the activation of microglial cells in the white matter and hippocampus after BCCAo. SM extract also prevented neuroinflammation after BCCAo by reducing hippocampal levels of TNF-α, IL-1β, and IL-6, and increasing the reduced levels of MBP in the white matter and hippocampus. Further, the administration of SM extract alleviated the up-regulation of hippocampal TLR4 and myeloid differentiation primary response gene 88 (MyD88) in rats with chronic BCCAo.

CONCLUSIONS

Our findings suggest that SM may be a promising therapeutic candidate in vascular dementia because of its protective effects against damage to the white matter and hippocampus after BCCAo.

CX3CR1 RNAi inhibits hypoxia-induced microglia activation via p38MAPK/PKC pathway

There is accumulating evidence which demonstrates that chronic cerebral ischaemia can induce white matter lesions (WMLs), and microglia-activation-mediated cytokines and proteases releasing during the ischaemia might play a vital role in pathogenesis. In addition, hypoxia-induced upregulated expression of fractalkine promotes the activation of microglia and their migration to the lesions through interaction with its receptor CX3CR1. However, the specific mechanisms involved in fractalkine/CX3CR1-mediated microglial activation have not been fully identified. In the present study, we constructed lentivirus encoding shRNA against CX3CR1 and transduced into microglial cells in under hypoxic conditions. Moreover, we analysed the proliferation, cytokine secretion and signal-pathway activation of the microglia. We found that CX3CR1 RNAi-mediated gene downregulation could attenuate hypoxic-induced microglial proliferation, cytokine secretion [including tumuor necrosis factor-α (TNF-α), interleukin-1β (IL-1β)] and matrix metalloproteinase-2 (MMP-2) synthesis. These effects were shown to be nediated through p38MAPK/PKC activation. Therefore, our results reveal a novel mechanism of fractalkine/CX3CR1 involvement in activation of microglia. Thus CX3CR1 RNAi might provide a therapeutic strategy which could be useful in chronic cerebral ischaemia.

Rodent models of ischemic stroke lack translational relevance... are baboon models the answer?

Rodent models of ischemic stroke are associated with many issues and limitations, which greatly diminish the translational potential of these studies. Recent studies demonstrate that significant differences exist between rodent and human ischemic stroke. These differences include the physical characteristics of the stroke, as well as changes in the subsequent inflammatory and molecular pathways following the acute ischemic insult. Non-human primate (NHP) models of ischemic stroke, however, are much more similar to humans. In addition to evident anatomical similarities, the physiological responses that NHPs experience during ischemic stroke are much more applicable to the human condition and thus make it an attractive model for future research. The baboon ischemic stroke model, in particular, has been studied extensively in comparison to other NHP models. Here we discuss the major shortcomings associated with rodent ischemic stroke models and provide a comparative overview of baboon ischemic stroke models. Studies have shown that baboons, although more difficult to obtain and handle, are more representative of ischemic events in humans and may have greater translational potential that can offset these deficiencies. There remain critical issues within these baboon stroke studies that need to be addressed in future investigations. The most critical issue revolves around the size and the variability of baboon ischemic stroke. Compared to rodent models, however, issues such as these can be addressed in future studies. Importantly, baboon models avoid many drawbacks associated with rodent models including vascular variability and inconsistent inflammatory responses - issues that are inherent to the species and cannot be avoided.

Radiation induced brain injury: assessment of white matter tracts in a pre-clinical animal model using diffusion tensor MR imaging

We aim to study radiation induced white matter injury in a pre-clinical model using Diffusion tensor MR imaging (DTI). Nineteen 12-week old Sprague-Dawley rats were irradiated to the right hemisphere using a linear accelerator. The dose distribution map was coregistered to the DTI map to generate the actual radiation dose to each white matter tract. Rats underwent longitudinal DTI scans at five time points from 4 to 48 weeks post-radiation with histological evaluations. Fractional anisotropy (FA) of the external capsule, fornix, cerebral peduncle, anterior commissure, optic tract and optic nerve was evaluated. Radiation dose was highest at the ipsilateral external capsule and fornix (29.4 ± 1.3 and 29.8 ± 1.1 Gy, respectively). Optic nerve received 50 % dose to the external capsule and other white matter tracts received 80 % dose. Significantly lower FA was firstly found in the ipsilateral external capsule at 4 weeks post-radiation and in the ipsilateral fornix at 40 weeks post-radiation compared to the contralateral side. Significantly lower FA was found in contralateral optic nerve compared to ipsilateral optic nerve at 48 weeks post-radiation despite ipsilateral optic nerves receiving higher radiation dose than contralateral optic nerve (p = 0.021). No differences were found in other white matter regions until 48 weeks. Histology indicated demyelination, axonal degeneration and coagulative necrosis in all injured white matter. DTI can serve as a promising tool for assessment of radiation induced white matter injury and regional radiosensitivity of white matter tracts.

Different strokes for different folks: the rich diversity of animal models of focal cerebral ischemia

No single animal model is able to encompass all of the variables known to affect human ischemic stroke. This review highlights the major strengths and weaknesses of the most commonly used animal models of acute ischemic stroke in the context of matching model and experimental aim. Particular emphasis is placed on the relationships between outcome and underlying vascular variability, physiologic control, and use of models of comorbidity. The aim is to provide, for novice and expert alike, an overview of the key controllable determinants of experimental stroke outcome to help ensure the most effective application of animal models to translational research.