Spontaneous white matter lesion in brain of stroke-prone renovascular hypertensive rats: a study from MRI, pathology and behavior

Cerebral Perfusion Characteristics and Dynamic Brain Structural Changes in Stroke-Prone Renovascular Hypertensive Rats: A Preclinical Model for Cerebral Small Vessel Disease

Hypertension is a leading cause of cerebral small vessel disease (CSVD) and vascular dementia in elderly individuals. We aimed to assess cerebral perfusion and dynamic changes in brain structure in stroke-prone renovascular hypertensive rats (RHRSPs) with different durations of hypertension and to investigate whether they have pathophysiological features similar to those of humans with CSVD. The RHRSP model was established using the two-kidney, two-clip (2k2c) method, and the Morris water maze (MWM) test, MRI, immunohistochemistry, and biochemical analysis were performed at multiple time points for up to six months following the 2k2c operation. Systolic blood pressure was significantly greater in the RHRSP group than in the sham-operated group at week 4 post-surgery and continued to increase over time, leading to cognitive decline by week 20. Arterial spin labeling revealed cerebral hypoperfusion in the RHRSP group at 8 weeks, accompanied by vascular remodeling and decreased vessel density. Diffusion tensor imaging and Luxol fast blue staining indicated that white matter disintegration and demyelination gradually progressed in the corpus callosum and that myelin basic protein levels decreased. Eight weeks after surgery, blood-brain barrier (BBB) leakage into the corpus callosum was observed. The albumin leakage area was negatively correlated with the myelin sheath area (r=-0.88, p<0.001). RNA-seq analysis revealed downregulation of most angiogenic genes and upregulation of antiangiogenic genes in the corpus callosum of RHRSPs 24 weeks after surgery. RHRSPs developed cerebral hypoperfusion, BBB disruption, spontaneous white matter damage, and cognitive impairment as the duration of hypertension increased. RHRSPs share behavioral and neuropathological characteristics with CSVD patients, making them suitable animal models for preclinical trials related to CSVD.

Basic Mechanisms of Brain Injury and Cognitive Decline in Hypertension

Dementia affects almost 50 million adults worldwide, and remains a major cause of death and disability. Hypertension is a leading risk factor for dementia, including Alzheimer disease and Alzheimer disease-related dementias. Although this association is well-established, the mechanisms underlying hypertension-induced cognitive decline remain poorly understood. By exploring the mechanisms mediating the detrimental effects of hypertension on the brain, studies have aimed to provide therapeutic insights and strategies on how to protect the brain from the effects of blood pressure elevation. In this review, we focus on the basic mechanisms contributing to the cerebrovascular adaptions to elevated blood pressure and hypertension-induced microvascular injury. We also assess the cellular mechanisms of neurovascular unit dysfunction, focusing on the premise that cognitive impairment ensues when the dynamic metabolic demands of neurons are not met due to neurovascular uncoupling, and summarize cognitive deficits across various rodent models of hypertension as a resource for investigators. Despite significant advances in antihypertensive therapy, hypertension remains a critical risk factor for cognitive decline, and several questions remain about the development and progression of hypertension-induced cognitive impairment.

Human mini-brains for reconstituting central nervous system disorders

Neurological disorders in the central nervous system (CNS) are progressive and irreversible diseases leading to devastating impacts on patients' life as they cause cognitive impairment, dementia, and even loss of essential body functions. The development of effective medicines curing CNS disorders is, however, one of the most ambitious challenges due to the extremely complex functions and structures of the human brain. In this regard, there are unmet needs to develop simplified but physiopathologically-relevant brain models. Recent advances in the microfluidic techniques allow multicellular culture forming miniaturized 3D human brains by aligning parts of brain regions with specific cells serving suitable functions. In this review, we overview designs and strategies of microfluidics-based human mini-brains for reconstituting CNS disorders, particularly Alzheimer's disease (AD), Parkinson's disease (PD), traumatic brain injury (TBI), vascular dementia (VD), and environmental risk factor-driven dementia (ERFD). Afterward, the applications of the mini-brains in the area of medical science are introduced in terms of the clarification of pathogenic mechanisms and identification of promising biomarkers. We also present expanded model systems ranging from the CNS to CNS-connecting organ axes to study the entry pathways of pathological risk factors into the brain. Lastly, the advantages and potential challenges of current model systems are addressed with future perspectives.

Hypertension, Neurovascular Dysfunction, and Cognitive Impairment

Hypertension affects a significant proportion of the adult and aging population and represents an important risk factor for vascular cognitive impairment and late-life dementia. Chronic high blood pressure continuously challenges the structural and functional integrity of the cerebral vasculature, leading to microvascular rarefaction and dysfunction, and neurovascular uncoupling that typically impairs cerebral blood supply. Hypertension disrupts blood-brain barrier integrity, promotes neuroinflammation, and may contribute to amyloid deposition and Alzheimer pathology. The mechanisms underlying these harmful effects are still a focus of investigation, but studies in animal models have provided significant molecular and cellular mechanistic insights. Remaining questions relate to whether adequate treatment of hypertension may prevent deterioration of cognitive function, the threshold for blood pressure treatment, and the most effective antihypertensive drugs. Recent advances in neurovascular biology, advanced brain imaging, and detection of subtle behavioral phenotypes have begun to provide insights into these critical issues. Importantly, a parallel analysis of these parameters in animal models and humans is feasible, making it possible to foster translational advancements. In this review, we provide a critical evaluation of the evidence available in experimental models and humans to examine the progress made and identify remaining gaps in knowledge.

Mesenchymal Stem Cells Improve Cognitive Impairment and Reduce Aβ Deposition via Promoting AQP4 Polarity and Relieving Neuroinflammation in Rats With Chronic Hypertension-Induced Cerebral Small-Vessel Disease

Cerebral small-vessel disease (CSVD) is the main cause of vascular cognitive impairment (VCI), and the accumulation of amyloid β-protein (Aβ) may be significantly involved in CSVD-induced VCI. The imbalance between Aβ production and clearance is believed to be an important pathological mechanism of Aβ deposition in Alzheimer disease. In this study, we aimed to disclose the roles of aquaporin 4 (AQP4) and neuroinflammation in CSVD, which were the key factors for Aβ clearance and production, respectively, and the effect of mesenchymal stem cells (MSCs) on Aβ deposition and these two factors. The stroke-prone renovascular hypertensive (RHRSP) rats were grouped and received MSC and MSC + AS1517499 (an inhibitor of pSTAT6). The latter was used to explore the underlying mechanism. The cognitive function, white matter lesions, Aβ expression, expression, and polarity of AQP4, neuroinflammation and the STAT6 pathway were investigated. Compared with sham-operated rats, RHRSP rats showed spatial cognitive impairment, white matter lesions and Aβ deposition. Moreover, AQP4 polarity disorder and neuroinflammatory activation were found, which were linked to Aβ deposition. Treatment with MSCs markedly improved cognitive tasks and reduced Aβ deposition but failed to reduce white-matter lesions. Furthermore, MSCs not only promoted AQP4 polarity but also alleviated neuroinflammation probably through the STAT6 pathway. The present study demonstrated that Aβ deposition, AQP4 polarity disorder and neuroinflammation might be involved in CSVD and the regulatory effects of MSCs on them suggested potential therapeutic value for CSVD.

EP3 Receptor Deficiency Improves Vascular Remodeling and Cognitive Impairment in Cerebral Small Vessel Disease

Aging and hypertension are major risk factors for cerebral small vessel disease (CSVD). Anti-hypertensive therapy has achieved effective; however, incomplete results in treating CSVD, suggesting the need for additional treatments. Targeting abnormal inflammatory responses has become a topic of research interest. Small artery remodeling is the main pathological feature of CSVD. Inhibition of the E-prostanoid 3 (EP3) receptor has been shown to attenuate vascular remodeling in peripheral organs; however, little is known about its role in CSVD. Therefore, we investigated whether the deletion of EP3 attenuates the development of CSVD in an animal model-- stroke-prone renovascular hypertensive rat (RHRsp). We found that the cerebral small arteries of RHRsp exhibited increased EP3 expression. Despite no alleviation of hypertension, the deletion of EP3 still attenuated the cerebral small artery remodeling of RHRsp, as evidenced by reduced overexpression of extracellular matrix (ECM) in the vessel. In vitro experiments indicated that EP3 deletion regulated the expression of ECM by downregulating TGF-β1/Smad signaling. Furthermore, the Morris water maze test and magnetic resonance test demonstrated that EP3 knockout attenuated cognitive impairment of the RHRsp, possibly through increased cerebral blood flow. Together, our results indicate that the deletion of EP3 attenuates vascular remodeling and vascular cognitive impairment induced by hypertension, and blockade of the EP3 receptor may be a promising strategy for the treatment of CSVD.

Modeling subcortical ischemic white matter injury in rodents: unmet need for a breakthrough in translational research

Subcortical ischemic white matter injury (SIWMI), pathological correlate of white matter hyperintensities or leukoaraiosis on magnetic resonance imaging, is a common cause of cognitive decline in elderly. Despite its high prevalence, it remains unknown how various components of the white matter degenerate in response to chronic ischemia.This incomplete knowledge is in part due to a lack of adequate animal model. The current review introduces various SIWMI animal models and aims to scrutinize their advantages and disadvantages primarily in regard to the pathological manifestations of white matter components. The SIWMI animal models are categorized into 1) chemically induced SIWMI models, 2) vascular occlusive SIWMI models, and 3) SIWMI models with comorbid vascular risk factors. Chemically induced models display consistent lesions in predetermined areas of the white matter, but the abrupt evolution of lesions does not appropriately reflect the progressive pathological processes in human white matter hyperintensities. Vascular occlusive SIWMI models often do not exhibit white matter lesions that are sufficiently unequivocal to be quantified. When combined with comorbid vascular risk factors (specifically hypertension), however, they can produce progressive and definitive white matter lesions including diffuse rarefaction, demyelination, loss of oligodendrocytes, and glial activation, which are by far the closest to those found in human white matter hyperintensities lesions. However, considerable surgical mortality and unpredictable natural deaths during a follow-up period would necessitate further refinements in these models. In the meantime, in vitro SIWMI models that recapitulate myelinated white matter track may be utilized to study molecular mechanisms of the ischemic white matter injury. Appropriate in vivo and in vitro SIWMI models will contribute in a complementary manner to making a breakthrough in developing effective treatment to prevent progression of white matter hyperintensities.

Genetic risk of dementia modifies obesity effects on white matter myelin in cognitively healthy adults

APOE-ε4 is a major genetic risk factor for late-onset Alzheimer's disease that interacts with other risk factors, but the nature of such combined effects remains poorly understood. We quantified the impact of APOE-ε4, family history (FH) of dementia, and obesity on white matter (WM) microstructure in 165 asymptomatic adults (38-71 years old) using quantitative magnetization transfer and neurite orientation dispersion and density imaging. Microstructural properties of the fornix, parahippocampal cingulum, and uncinate fasciculus were compared with those in motor and whole-brain WM regions. Widespread interaction effects between APOE, FH, and waist-hip ratio were found in the myelin-sensitive macromolecular proton fraction from quantitative magnetization transfer. Among individuals with the highest genetic risk (FH+ and APOE-ε4), obesity was associated with reduced macromolecular proton fraction in the right parahippocampal cingulum, whereas no effects were present for those without FH. Risk effects on apparent myelin were moderated by hypertension and inflammation-related markers. These findings suggest that genetic risk modifies the impact of obesity on WM myelin consistent with neuroglia models of aging and late-onset Alzheimer's disease.

Concomitant enlargement of perivascular spaces and decrease in glymphatic transport in an animal model of cerebral small vessel disease

OBJECTIVES

To observe glymphatic transport and evaluate enlarged perivascular spaces (PVSs) in spontaneously hypertensive rats (SHRs).

METHODS

SHRs were used as an animal model of cerebral small vessel disease, and Wistar Kyoto (WKY) rats were used as the control group. Histopathology was used to evaluate the enlargement of PVSs. A fluorescent tracer was infused into the cisterna magna of rats, and the proportion of the brain parenchyma area exposed to the fluorescent tracer was later quantified to evaluate the influx and efflux function of the glymphatic system. The global and polarized expression of aquaporin protein 4 (AQP4) was analyzed by immunofluorescence.

RESULTS

Compared with WKY rats, SHRs exhibited obviously enlarged PVSs and significantly decreased influx and efflux function of the glymphatic system. The results showed a significant decrease in AQP4 polarity in SHRs, but a difference in global AQP4 expression was not observed between SHRs and WKY rats.

CONCLUSIONS

Impaired glymphatic transport may be involved in the pathogenesis of arteriolosclerotic cerebral small vessel disease, and enlarged PVSs may be a manifestation of the impaired glymphatic system.

The effect and mechanism of combination of total paeony glycosides and total ligustici phenolic acids against focal cerebral ischemia

The root of Paeonia lactiflora Pall. (Chishao, CS) and Ligusticum chuanxiong Hort. (Chuanxiong, CX) were widely used as a drug pair in Chinese Medicine, and the combination of CS and CX showed a more significant inhibition on neuronal apoptosis in our previous study. In the present study, total paeony glycosides (TPGs) from CS and total ligustici phenolic acids (TLPAs) from CX were combined to evaluate the synergistic effects against focal cerebral ischemia both in vitro and in vivo. The combination of TPGs and TLPAs at 7:3 had the best anti-oxidative stress and anti-inflammatory effect on OGD-induced HUVEC. Additionally, the infarction area proportion and neuron apoptosis of rats by TPGs:TLPAs (7:3) was significantly lower than their alone in MCAO rats. Moreover, TPGs: TLPAs of 7:3 showed a more significant effect on decreasing the expression of MMP-2 and MMP-9, and increasing the protein expression or mRNA level of TIMP-1 than other combinations. The optimal ratio of TPGs and TLPAs at 7:3 could bring more remarkable protective effects against focal cerebral ischemia in MCAO rats by alleviating oxidative stress, inflammatory and neuronal apoptosis to protect the blood-brain barrier. Overall, the present study provided benefical evidence for clinical application of CS and CX as a "drug pair".

A Novel Model of Mixed Vascular Dementia Incorporating Hypertension in a Rat Model of Alzheimer's Disease

Alzheimer's disease (AD) and mixed dementia (MxD) comprise the majority of dementia cases in the growing global aging population. MxD describes the coexistence of AD pathology with vascular pathology, including cerebral small vessel disease (SVD). Cardiovascular disease increases risk for AD and MxD, but mechanistic synergisms between the coexisting pathologies affecting dementia risk, progression and the ultimate clinical manifestations remain elusive. To explore the additive or synergistic interactions between AD and chronic hypertension, we developed a rat model of MxD, produced by breeding APPswe/PS1ΔE9 transgenes into the stroke-prone spontaneously hypertensive rat (SHRSP) background, resulting in the SHRSP/FAD model and three control groups (FAD, SHRSP and non-hypertensive WKY rats, n = 8-11, both sexes, 16-18 months of age). After behavioral testing, rats were euthanized, and tissue assessed for vascular, neuroinflammatory and AD pathology. Hypertension was preserved in the SHRSP/FAD cross. Results showed that SHRSP increased FAD-dependent neuroinflammation (microglia and astrocytes) and tau pathology, but plaque pathology changes were subtle, including fewer plaques with compact cores and slightly reduced plaque burden. Evidence for vascular pathology included a change in the distribution of astrocytic end-foot protein aquaporin-4, normally distributed in microvessels, but in SHRSP/FAD rats largely dissociated from vessels, appearing disorganized or redistributed into neuropil. Other evidence of SVD-like pathology included increased collagen IV staining in cerebral vessels and PECAM1 levels. We identified a plasma biomarker in SHRSP/FAD rats that was the only group to show increased Aqp-4 in plasma exosomes. Evidence of neuron damage in SHRSP/FAD rats included increased caspase-cleaved actin, loss of myelin and reduced calbindin staining in neurons. Further, there were mitochondrial deficits specific to SHRSP/FAD, notably the loss of complex II, accompanying FAD-dependent loss of mitochondrial complex I. Cognitive deficits exhibited by FAD rats were not exacerbated by the introduction of the SHRSP phenotype, nor was the hyperactivity phenotype associated with SHRSP altered by the FAD transgene. This novel rat model of MxD, encompassing an amyloidogenic transgene with a hypertensive phenotype, exhibits several features associated with human vascular or "mixed" dementia and may be a useful tool in delineating the pathophysiology of MxD and development of therapeutics.

Cerebral Venous Collagen Remodeling in a Modified White Matter Lesions Animal Model

To mimic the expected pathological changes of white matter lesions (WMLs) and increase the stability, we applied modified two-vessel occlusion (modified 2VO) (1-week interval bilateral carotid artery occlusion) in stroke-prone renovascular hypertensive rats (RHRSP) and established a modified WMLs model (RHRSP/modified 2VO) that compared their phenotypes with RHRSP and sham-operated rats. In addition, we tried to differentiate small veins from small arteries through the presence of smooth muscle to study the pathological changes of small veins detailed in the model. RHRSP/modified 2VO rats showed higher stability and more extensive white matter damage without an obvious increase in mortality rate at 12 weeks after the modified 2VO operation compared to RHRSP rats. RHRSP/modified 2VO rats showed more severe small venous collagen deposition than RHRSP rats, and the majority of the deposition was collagen I and IV rather than collagen III. In addition, RHRSP/modified 2VO rats possessed cognitive impairment, mild wall thickness and blood-brain barrier disruption. Our findings suggest that the modified WMLs model (RHRSP/modified 2VO) mimics cognitive impairment and small vessel pathological changes similar to WMLs in humans. Differentiating small veins from small arteries through smooth muscle is feasible, and marked small venous deposition may play an important role in the hypertensive white matter lesions.

Blood-brain barrier dysfunction and recovery after ischemic stroke

The blood-brain barrier (BBB) plays a vital role in regulating the trafficking of fluid, solutes and cells at the blood-brain interface and maintaining the homeostatic microenvironment of the CNS. Under pathological conditions, such as ischemic stroke, the BBB can be disrupted, followed by the extravasation of blood components into the brain and compromise of normal neuronal function. This article reviews recent advances in our knowledge of the mechanisms underlying BBB dysfunction and recovery after ischemic stroke. CNS cells in the neurovascular unit, as well as blood-borne peripheral cells constantly modulate the BBB and influence its breakdown and repair after ischemic stroke. The involvement of stroke risk factors and comorbid conditions further complicate the pathogenesis of neurovascular injury by predisposing the BBB to anatomical and functional changes that can exacerbate BBB dysfunction. Emphasis is also given to the process of long-term structural and functional restoration of the BBB after ischemic injury. With the development of novel research tools, future research on the BBB is likely to reveal promising potential therapeutic targets for protecting the BBB and improving patient outcome after ischemic stroke.

Different effects of running wheel exercise and skilled reaching training on corticofugal tract plasticity in hypertensive rats with cortical infarctions

The approaches that facilitate white matter plasticity may prompt functional recovery after a stroke. The effects of different exercise methods on motor recovery in stroke rats have been investigated. However, it is not clear whether their effects on axonal plasticity different. The aim of this study was to compare the effects of the forced running wheel exercise (RWE) and skilled reaching training (SRT) on axonal plasticity and motor recovery. Cortical infarctions were generated in stroke-prone renovascular hypertensive rats. The rats were randomly divided into the following three groups: the control (Con) group, the RWE group, and the SRT group. A sham group was also included. The mNSS and forelimb grip strength tests were performed on days 3, 7, 14, 21, 28, 35, and 42 after ischemia. The anterograde tract tracer biotinylated dextran amine (BDA) was injected into the rats to trace the axonal plasticity of the contralesional corticofugal tracts. Compared with the Con group, the mNSS scores in the SRT and RWE groups decreased on day 28 (P<0.05) and on days 35 and 42 (P<0.01). The grip strength in the SRT group increased relative to that in the RWE group at 42day post-ischemia (P<0.01). Both the RWE and SRT groups exhibited enhanced plasticity of the contralesional corticofugal tract axons at the level of the red nucleus (P<0.01) and the cervical enlargement (P<0.01). More contralateral corticorubral tract remodeling was observed at the red nucleus level in the SRT group than in the RWE group (P<0.001). Taken together, these results suggest that SRT may enhance axon plasticity in the corticorubral tract more effectively than the forced RWE and is associated with better motor recovery after cerebral ischemia.

Cerebral Vascular Disease and Neurovascular Injury in Ischemic Stroke

The consequences of cerebrovascular disease are among the leading health issues worldwide. Large and small cerebral vessel disease can trigger stroke and contribute to the vascular component of other forms of neurological dysfunction and degeneration. Both forms of vascular disease are driven by diverse risk factors, with hypertension as the leading contributor. Despite the importance of neurovascular disease and subsequent injury after ischemic events, fundamental knowledge in these areas lag behind our current understanding of neuroprotection and vascular biology in general. The goal of this review is to address select key structural and functional changes in the vasculature that promote hypoperfusion and ischemia, while also affecting the extent of injury and effectiveness of therapy. In addition, as damage to the blood-brain barrier is one of the major consequences of ischemia, we discuss cellular and molecular mechanisms underlying ischemia-induced changes in blood-brain barrier integrity and function, including alterations in endothelial cells and the contribution of pericytes, immune cells, and matrix metalloproteinases. Identification of cell types, pathways, and molecules that control vascular changes before and after ischemia may result in novel approaches to slow the progression of cerebrovascular disease and lessen both the frequency and impact of ischemic events.

Cerebral small vessel disease and post-stroke cognitive impairment

Cerebral small vessel disease (CSVD) refers to a group of pathological processes with multifarious etiologies that affect the small arteries, arterioles, venules, and capillaries of the brain. Features seen on neuroimaging include white matter hyperintensities, lacunar infarction, cerebral microbleeds, brain atrophy, microinfarcts and enlarged perivascular spaces (EPVS). CSVD gives rise to one in five strokes worldwide and is a leading cause of cognitive impairment and dementia, especially in the elderly. Post-stroke cognitive impairment (PSCI) is one of the most common subtypes of cognitive impairment. The underlying mechanisms of PSCI are not known in detail. A growing body of evidence has been suggesting that CSVD plays an important role in the pathogenesis of PSCI. This article reviews the advances in research on the relationship between CSVD and PSCI.

Hypotensive effect of S-adenosyl-L-methionine in hypertensive rats is reduced by autonomic ganglia and KATP channel blockers

S-adenosyl-L-methionine (SAM) is an amino acid involved in a number of physiological processes in the nervous system. Some evidence suggests a therapeutic potential of SAM in hypertension. In this study we investigated the effect of intracerebroventricular (ICV) infusions of SAM on arterial blood pressure in rats. Mean arterial blood pressure (MABP) and heart rate (HR) were measured at baseline and during ICV infusion of either SAM or vehicle (aCSF; controls) in conscious, male normotensive Wistar Kyoto rats (WKY) and Spontaneously Hypertensive Rats (SHR). MABP and HR were not affected by the vehicle. WKY rats infused with SAM (10 μM, 100 μM and 1 mM) showed a biphasic hemodynamic response i.e., mild hypotension and bradycardia followed by a significant increase in MABP and HR. On the contrary, SHR infused with SAM showed a dose-dependent hypotensive response. In separate series of experiments, pretreatment with hexamethonium, a ganglionic blocker as well as pretreatment with glibenclamide, a KATP channel blocker reduced the hemodynamic effects of SAM. SAM may affect the nervous control of arterial blood pressure via the autonomic nervous system and KATP channel-dependent mechanisms.

Microvascular Dysfunction and Cognitive Impairment

The impact of vascular risk factors on cognitive function has garnered much interest in recent years. The appropriate distribution of oxygen, glucose, and other nutrients by the cerebral vasculature is critical for proper cognitive performance. The cerebral microvasculature is a key site of vascular resistance and a preferential target for small vessel disease. While deleterious effects of vascular risk factors on microvascular function are known, the contribution of this dysfunction to cognitive deficits is less clear. In this review, we summarize current evidence for microvascular dysfunction in brain. We highlight effects of select vascular risk factors (hypertension, diabetes, and hyperhomocysteinemia) on the pial and parenchymal circulation. Lastly, we discuss potential links between microvascular disease and cognitive function, highlighting current gaps in our understanding.