American Heart Association

basic sciences

Upregulation of ACE2 Expression and its Relationship to Increased Shear Stress: A Protective Mechanism Lost in Cardiovascular Disease

Ying Gue, PhD

Kaneko N, Satta S, Komuro Y, Muthukrishnan SD, Kakarla V, Guo L, An J, Elahi F, Kornblum HI, Liebeskind DS, et al. Flow-Mediated Susceptibility and Molecular Response of Cerebral Endothelia to SARS-CoV-2 Infection. Stroke. 2020.

On March 11, 2020, the World Health Organisation (WHO) officially announced the Coronavirus Disease 2019 (COVID-19) as a pandemic. The outbreak originated in China, and. as of now, has over 3 million cases with over 200,000 deaths as a result. The disease is attributed to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

As the SARS-CoV-2 enters the host cell by binding the spike (S) protein to angiotensin converting enzyme 2 (ACE2), there has been an increased interest in studying the effect of ACE2 and the virus. Observational studies have shown that conditions which result in reduced ACE2 tissue expression will have a more severe disease course, notably males, advanced age, and patients with hypertension. Apart from the respiratory sequalae associated with COVID-19 infection, severe disease is further complicated by thromboembolic phenomenon including stroke. However, the mechanism underlying cerebral endothelial and response to COVID-19 infection remains unclear.

In this article by Kaneko et al., utilizing ex vivo models of human brain endothelial tissues, the authors were able to establish firstly, expression of ACE2 by human brain endothelial cells is low; secondly, ACE2 expression is increased in the presence of shear stress which facilitates binding of S protein of SARS-COV-2 to ACE2; and lastly, the binding triggers unique genes in human endothelial cells which are up-regulated to combat the infection.

Following Calcium Waves in Microglial Cells

Aurora Semerano, MD

Liu L, Kearns KN, Eli I, Sharifi KA, Soldozy S, Carlson EW, Scott KW, Sluzewski MF, Acton ST, Stauderman KA, et al. Microglial Calcium Waves During the Hyperacute Phase of Ischemic Stroke. Stroke. 2020.

Microglia are the main resident immune cell population of the central nervous system and play a key role in brain development, homeostasis, and repair. During ischemic stroke, microglia are rapidly activated and are characterized by morphological, proliferative, and functional alterations. The role of microglia activation in ischemic stroke remains highly controversial in the preclinical setting and depends on multiple factors, including the experimental conditions and the phase of the disease. More recently, an additional role for microglial cells has been proposed, since they have been found to be implied in the occurrence, the sensing, and the response to cortical spreading depolarization (CSD).1 CSD is defined as a slowly propagating (2–5 mm/min) wave of rapid, near-complete depolarization of neurons and astrocytes followed by a period of electrical suppression of a distinct population of cortical neurons. CSD is considered as the biological substrate of migraine aura, but it has been shown to occur in other neurological conditions, such as ischemic stroke, subarachnoid hemorrhage, and traumatic brain injury.2 In other words, CSD consists in a deep perturbation of the ionic environment in the brain, which has been associated with excitotoxicity damage and vaso-occlusive phenomenons after brain injury.

Pre-Clinical Evidence of the Neuro-Recovery Effects of Vascular Endothelial Growth Factor-Activating Glycosaminoglycan Sugar

Lin Kooi Ong, PhD

Chan SJ, Esposito E, Hayakawa K, Mandaville E, Smith RAA, Guo S, Niu W, Wong PT-H, Cool SM, Lo EH, Nurcombe V. Vascular Endothelial Growth Factor 165-Binding Heparan Sulfate Promotes Functional Recovery From Cerebral Ischemia. Stroke. 2020;51:2844–2853.

Angiogenesis and neurogenesis are crucial processes for brain recovery after stroke. While the brain has the capacity to form new cerebral blood vessels and to generate new neurons from neural stem cells after stroke, these self-repair mechanisms are limited. Therefore, strategies to promote brain restorative processes beyond the endogenous recovery are highly desirable. In this study, Chan and colleagues demonstrated that an exogenously applied heparan sulfate with increased affinity for vascular endothelial growth factor was able to enhance angiogenesis and neurogenesis within the peri-infarct regions, as well as to promote neurological recovery after experimental stroke.

The team first purified heparan sulfate variant 7, a glycosaminoglycan sugar which has increased affinity for vascular endothelial growth factor, and tagged the molecule with fluorescent dye. The team experimentally induced stroke in rats using transient middle cerebral artery occlusion, and then they delivered heparan sulfate (or placebo) into the right lateral ventricle of the brain at day 4 after experimental stroke. The rats were assessed for neurological deficits, and rats treated with heparan sulfate showed a modest improvement in the modified neurological score 7 days after treatment (heparan sulfate vs placebo; 7.3±0.4 vs 8.8±0.5). Furthermore, the team tracked the distribution of the fluorescent tagged heparan sulfate and found the signals co-localized with endothelial cells (Collagen IV) and in neural stem cells (Nestin) within the peri-infarct regions. Histology analysis showed that heparan sulfate treatment enhances angiogenesis and neurogenesis (by approximately 3 to 7 folds) within the peri-infarct regions, without compromising the blood brain barrier integrity. The team also performed a series of cell culture studies and demonstrated that the heparan sulfate most likely stimulates vascular endothelial growth factor signaling.

Caveolae, a Target for Stroke Therapeutics?

Melissa Trotman-Lucas, PhD

Blochet C, Buscemi L, Clément T, Gehri S, Badaut J, Hirt L. Involvement of caveolin-1 in neurovascular unit remodeling after stroke: Effects on neovascularization and astrogliosis. JCBFM. 2020;40:163-176.

Despite the collective history of failed neuroprotective therapies aimed at treating ischemic injury, the need to discover alternative stroke therapies is still present. However, despite improvements in the detection and treatment of ischemic strokes, a significant proportion of patients are ineligible for treatment and, therefore, unable to benefit. This impacts patient outcome, leaving many individuals with lifelong disabilities. Currently, the neurovascular unit (NVU) is being considered as a viable therapeutic target. This complex combination of capillaries, endothelial cells, pericytes, astrocytes and neurons closely controls connectivity between the brain and the blood. Events in the NVU contribute to cell death and neurological dysfunction during infarction, but also certain cell types within the unit have been shown to play a role in the preservation of post-stroke brain function. Endothelial cells, for instance, are key to the dynamic process of neovascularization, whereby these cells proliferate, migrate and differentiate following ischemic injury. Neovascularization is thought to be a key process in ischemia recovery, stimulating blood flow, vascular collateralization and neuroplasticity. In addition to the role of endothelial cells post-injury, astrocytes are also understood to be prominent in post-stroke recovery, transforming in the presence of molecules released during ischemic damage such as cytokines. These transformed astrocytes termed reactive astrocytes are known to be important in the formation of a glial scar that surrounds the damaged tissue. Interestingly, for some time it has been thought that the glial scar hindered axonal growth during brain recovery; however, recent evidence suggests the opposite and indeed promotion of axon development.

Orai2 Mediates Protection From Ischemia-Induced Capacitative Calcium Entry in Neurons

Melissa Trotman-Lucas, PhD

Stegner D, Hofmann S, Schuhmann MK, Kraft P, Herrmann AM, Popp S, et al. Loss of Orai2-Mediated Capacitative Ca2+ Entry Is Neuroprotective in Acute Ischemic Stroke. Stroke. 2019;50:3238–3245.

Neuroprotective treatments aim to prevent cellular death, particularly neuronal cell death, during ischemic/hypoxic conditions such as in acute ischemic stroke. Maintenance of neuroprotection long-term requires a return of blood and oxygen to the ischemic territory area, not regularly achieved in previous clinical trials. Yet, with recanalization rates of greater than 80%, afforded by the advent of mechanical thrombectomy following ischemic stroke, the return of flow to the ischemic territory is now more consistent. Nevertheless, following the return of blood flow to this area, cellular damage continues to spread and grow due to the phenomenon of “ischemic-reperfusion injury.” This progressive increase in damage indicates a continued need to investigate neuroprotective treatments that could be used alongside recanalization, both thrombolysis and thrombectomy, to recover/prevent long-term cellular damage.

It is well established that calcium (Ca2+) overload plays a key role in neuronal death during ischemic/hypoxic events. In addition to voltage-gated and receptor-mediated Ca2+ influx, there is a further route of entry into the cell, a route switched on in response to depleted intracellular Ca2+ stores. This additional entry route is called capacitative or store operated Ca2+ entry and is thought to contribute to the stabilization of both cytosolic Ca2+ concentration and intracellular Ca2+ stores.

Investigating Stem Cell Treatment in Stroke-Associated Retinal Ischemia

Melissa Trotman-Lucas, PhD

Nguyen H, Lee JY, Sanberg PR, Napoli E, Borlongan CV. Eye Opener in Stroke: Mitochondrial Dysfunction and Stem Cell Repair in Retinal Ischemia. Stroke. 2019;50:2197–2206.

Retinal damage is a significant stroke-associated outcome, with 92% of patients suffering visual impairments during the initial recovery phase. Partial, and in some cases complete, recovery can occur; however, 20% of patients will suffer persistent or permanent visual impairment. The anatomic link between the ophthalmic artery and the middle cerebral artery is a potential factor in the reduction of retinal blood flow during middle cerebral artery occlusion (MCAO). The reduced blood flow leads to ischemic retinal damage and subsequent visual impairment. Visual disability is much less evident than other motor or speech deficits following stroke, yet can significantly affect a patient’s rehabilitation, including functional recovery and quality of life. Reduction in rapid eye movements, visual acuity and visual field defects can occur, affecting daily activities and capabilities such as reading, mobility, postural stability, spatial recognition and more. These directly affect a patient’s independence, increasing fear and disorientation, reducing confidence and leading to social withdrawal. Current therapy for stroke-associated visual impairment is neurovisual rehabilitation, involving guiding a patient to learn coping strategies to improve quality of life. Retinal ischemia also shares parallels in pathology to other ocular vascular diseases, such as diabetic retinopathy, glaucoma, retinal vein occlusion and central retinal artery occlusion. There is a need to better understand the similarities and relationship between cerebral ischemia and retinal ischemia, particularly ischemic stroke that incorporates retinal ischemic damage. The lack of current understanding may contribute to the absence of effective treatments for this prevalent post-stroke outcome.

Burst of Stress Hormone is Necessary for Poststroke Survival

Lin Kooi Ong, PhD

Yang J, Kim E, Beltran C, Cho S. Corticosterone-Mediated Body Weight Loss Is an Important Catabolic Process for Poststroke Immunity and Survival. Stroke. 2019;50:2539–2546.

“Stress is bad for you, right? Well, not necessarily. The devil is in the dose, and how you perceive the stress.”1
—Dr. Lila Landowski, University of Tasmania

In response to stroke, the hypothalamic-pituitary-adrenal axis is activated and releases glucocorticoids such as cortisol. Cortisol is involved in stress responses, regulation of energy and immune reactions. Yang et al. aimed to elucidate the acute actions of corticosterone in body weight loss, immunity responses and survival after experimental stroke in mice. Corticosterone is a major catabolic steroid hormone produced from the adrenal gland in rodents (cortisol in humans). The team observed a small burst of corticosterone levels around 1 to 3 hours and decreased back to baseline around 6 hours after stroke. Further, they noted that the degree of weight loss at day 3 estimates the severity of the stroke as well as the infiltration of peripheral immune cells into the brain. As the role of corticosterone during the acute stroke phase is controversial, the team tested the concept by surgically removing the adrenal glands prior to induction of experimental stroke. All mice with intact adrenal glands survived after stroke, and only 1 out of 8 mice following adrenalectomy survived. Interestingly, treating adrenalectomy mice with corticosterone partially rescued the survival rate to 4 out of 10. Based on the findings, the authors suggested that the adrenal corticosterone-mediated catabolic process is necessary for poststroke immunity and survival.

Microstimulation Guidance Results in Accurate and Reproducible Preclinical Lacunar Infarcts, Modeled in the Rat

Melissa Trotman-Lucas, PhD

Wen TC, Sindhurakar A, Contreras Ramirez V, Park H, Gupta D, Carmel JB. Targeted Infarction of the Internal Capsule in the Rat Using Microstimulation Guidance. Stroke. 2019;50:2531-2538.

Lacunar stroke occurs when a penetrating arteriole becomes blocked, leading to ischemic damage to deep brain structures such as subcortical white matter or deep grey matter. Lacunes are small areas of cell damage that can cause significant disability to the sufferer. This type of cerebral ischemic occurrence accounts for a fifth of all strokes, resulting in varied clinical presentations. Lacunar stroke of the internal capsule, or IC, is associated with lasting motor deficits and poor recovery. It is critical to develop a reliable preclinical model, focusing on corticospinal tract (CST) damage located within the IC, as this damage is a reliable predictor of stroke severity and clinical outcome.

In this article commentary, the recent publication by Tong-Chun Wen and colleagues reporting lacunar infarction induction using microstimulation guidance is discussed. Microstimulation was used by the group as a guide as the IC is a small subcortical structure, within already limited rat white matter, with an elongated irregular shape that can vary with age, sex and strain. Wen et al. set out to develop a reliable preclinical IC stroke model that could produce tightly focused lesions to improve upon the various preclinical methods currently used, including chemical, physical and photo induced models. Chemical induction can result in off target injuries due to unpredictable diffusion of the inducting solution and physical induction, although closely imitating the mechanism of human lacunar stroke induction, requires significant surgical expertise. The group utilized photothrombolysis, where a photosensitive dye is first injected into the blood stream and then target vessels are illuminated by a laser light source inducing thrombus formation. The group enhanced this method with the addition of micro-electrical stimulation to guide placement of the illuminating optical fibre. This was achieved through the use of an optrode, a combined electrode and optical-fibre probe.

Article Commentary: “Cannabis and Cannabinoid Biology in Stroke”

Pamela Cheng, DO

Choi S-H, Mou Y, Silva AC. Cannabis and Cannabinoid Biology in Stroke: Controversies, Risks, and Promises. Stroke. 2019;50:2640–2645.

Stroke remains one of the leading causes of death and disability in the United States. Currently, the main therapeutic approach includes thrombolysis and mechanical thrombectomy. However, immediate reperfusion therapy does not tell the whole story. Following a stroke, there are complex biochemical events that occur that lead to excitotoxicity and oxidative stress, which may contribute to long-term functional outcomes. As such, there has been great interest and research in the field of neuroprotection. While there are currently no approved neuroprotective treatment options for stroke, there have been some promising yet conflicting results on the endocannabinoid system (ECS).

The ECS is composed of endogenous, lipid-based neurotransmitters that bind to the cannabinoid receptors. The ECS has shown promise in a wide range of pathological conditions and neurological disorders. In stroke, there is evidence that the ECS is altered in both animals and humans, and may contribute to the consequences of ischemic stroke. While studies have been conflicting in either supporting or refuting the use of cannabinoids, they remain a prominent research focus.

Poststroke Hypertension: Beneficial or Harmful?

Melissa Trotman-Lucas, PhD, BSc

Thakkar P, McGregor A, Barber PA, Paton JFR, Barrett C, McBryde F. Hypertensive Response to Ischemic Stroke in the Normotensive Wistar Rat: Mechanisms and Therapeutic Relevance. Stroke. 2019;50:2522-2530.

Sudden increased blood pressure (BP), known as hypertension, following an ischaemic insult is the scenario for ~80% of acute ischaemic stroke (AIS) patients. This poststroke hypertension is the subject of continued scientific debate, with both the mechanism and its therapeutic relevance still poorly understood. There are two potential sides to the role of poststroke hypertension in AIS tissue damage: exacerbation and protection. Exacerbation of damage to vulnerable ischemic tissue may occur alongside promotion of edema formation; moreover, this abrupt increase in BP can increase the risk of cardiovascular events, including further strokes and heart attacks. On the flip side, this increased BP may be a reaction by the body to increase blood supply to the brain tissue, increasing oxygenation of the penumbral tissue. Therefore, creating the conundrum that treatment to reduce BP levels in AIS may be protective but, on the other hand, it may also escalate tissue damage and increase the risk of a poorer patient outcome.

A recent study by Thakkar et al., published in Stroke, sought to answer whether a neurally mediated increase in systemic BP protects cerebral perfusion by opposing the increase in intracranial pressure (ICP) through increasing supply pressure to the tissues. Undertaking this by characterising the cerebrovascular, ICP and cerebral oxygenation responses in a rat AIS model. Testing also the physiological impact of hypertension prevention on the maintenance of oxygenation in the penumbra and on functional recovery poststroke.