Dr Teresa Sanchez | The Enigma of the Vascular Endothelium: New Hope for Stroke Therapies and Beyond

Strokes cause brain injury that can be fatal, and those who survive are often left with long-term problems. Understanding what exactly happens to the blood vessels in the brain as a result of stroke is critical to the development of novel, life-saving treatments. Dr Teresa Sanchez carries out her vital research at Cornell University in New York, where she is providing new insights into the molecular and cellular mechanisms at play in various cerebrovascular diseases, including stroke.

Fighting a Killer

Cardiovascular diseases are the leading cause of death, according to the World Health Organization. In 2019 alone, they claimed around 17.9 million lives, representing a staggering 32% of deaths globally. They encompass a group of disorders which impact the heart and blood vessels, such as coronary heart disease and cerebrovascular disease. Over 85% of these deaths are specifically attributed to strokes and heart attacks. During a stroke, the blood supply to brain cells is restricted due to a blood clot blocking a cerebral artery or the rupture of a cerebral blood vessel, resulting in brain damage, disability, and, in some cases, death.

Thanks to recent advances in blood clot removal and neurosurgical repair of cerebral artery ruptures, mortality has significantly decreased. However, due to the lack of therapies to repair the extensive neurovascular damage at the level of the microcirculation (i.e., the smaller blood vessels downstream of the bigger cerebral arteries), survivors often require extended periods of rehabilitation to regain their independence, with some never making a complete recovery and needing ongoing care and support in their daily lives.

Dr Teresa Sanchez is the principal investigator at the Laboratory of Molecular and Translational Vascular Research at Weill Cornell Medical College, Cornell University. She investigates various conditions involving the vasculature (blood vessels), working to understand the role of certain molecules and receptors in the cells lining the blood vessels. Along with her team, she aims to develop new treatment strategies for cerebrovascular diseases and other vascular conditions.

Exploring Endothelial Dysfunction

Endothelial cells line the inside of blood vessels, and this thin layer of cells (the endothelium) helps to regulate immunity, inflammation, and the flow of blood. Dr Sanchez explains that dysfunction of the endothelium can play a critical role in organ failure and death, particularly in cerebrovascular diseases, where it can exacerbate brain injuries. It is also involved in sepsis and systemic inflammatory response syndrome, where the body has an extreme reaction in situations such as infections, trauma, surgery, cancers or cancer treatments, leading to leaky blood vessels, blood clots, and eventually organ dysfunction and tissue death.

The potential of targeting endothelial cells to treat these deadly conditions remains limited due to a lack of understanding of the specific molecular mechanisms involved in endothelial dysfunction. While many currently available treatments work by broadly suppressing inflammation, which, in turn, can lead to deleterious immune suppression, being able to specifically target the endothelium would help improve clinical outcomes, avoiding these adverse effects.

The Critical Role of Sphingosine-1-Phosphate

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid. These are a group of fat molecules with vital roles within the body. S1P is found in high quantities in the blood plasma and regulates the responses of the vascular and immune systems by interacting with its receptors, the S1PRs, on the endothelial and immune/blood cells. Dr Sanchez confirmed that the endothelium plays a critical role in inflammation, and she found that S1PRs regulate the permeability (leakiness) of the blood vessels.

In more recent work, Dr Sanchez and her team investigated the role of two S1P receptors known as S1PR1 and S1PR2 in the regulation of the cardiovascular and immune systems during inflammatory conditions like sepsis. S1PR1 regulates both endothelial and immune cell function; indeed, pharmacological modulators of S1PR1 are clinically used for the treatment of autoimmune disorders such as multiple sclerosis. Using genetically altered mice, human endothelial cell samples, and drugs that activate or inhibit S1PR receptors, the Sanchez laboratory discovered that S1PR2 plays a key role in the permeability and inflammatory responses of the vascular endothelium during endotoxaemia, which occurs when infectious bacteria disseminate in the body and release toxins into the blood resulting in a wide-spread inflammatory response affecting several organs.

Dr Sanchez’s studies revealed that inhibition of S1PR2 activity can prevent endothelial dysfunction and improve outcomes without interfering with the immune response. In contrast, while modulation of S1PR1 can improve vascular function, it also leads to undesirable effects like suppression of the immune system, impairing the body’s defence against infections. They also provided detailed mechanisms on the downstream signalling (the cascade of reactions after the receptor has been activated) of S1PR2 in vascular inflammation. Clearly, S1PR2 is a key regulator of permeability and inflammation of the vascular endothelium and a potential novel therapeutic target to help strengthen vascular integrity (control of the leakiness of blood vessels) in inflammatory vascular disorders.

Paradox of Ischaemic Reperfusion Injury

Ischaemic strokes occur when the blood supply to parts of the brain is restricted by a blood clot or narrowing of blood vessels due to a build-up of fatty deposits. Current therapy options involve clot-busting drugs and long-term medicines to prevent further clots, with surgical options to clear blockages in the bigger blood vessels available in some circumstances. Dr Sanchez explains that the use and effectiveness of these current reperfusion therapies to re-establish blood flow are limited due to the complications of ischaemic reperfusion injury.

This is a paradoxical situation whereby tissue damage is caused by returning the blood supply after a period of restricted oxygen. The restoration of the blood supply in the bigger blood vessels results in inflammation and increases in permeability of the smaller blood vessels, the cerebral microvasculature, leading to a detrimental chain of events resulting in haemorrhagic transformation, where the brain tissue begins to bleed uncontrollably. Despite decades of intensive research, currently there are no effective therapies to prevent these pathological processes, and for this reason, stroke patients suffer chronic motor and cognitive impairments, which is not only devastating for the patients but also a significant burden to society and a major driver of health care costs. 

S1P has been shown to be highly potent in controlling vascular integrity by interacting with its receptors on the endothelial cells. Dr Sanchez continued to investigate the fascinating role of S1PR, this time with a focus on its role during stroke. Using mice with genetic deletion of S1PR2 and an S1PR2 antagonist, JTE013, they showed that S1PR2 plays a critical role in cerebrovascular permeability and the development of neurovascular injuries and intracerebral haemorrhage (bleeding in the brain) in mouse models of ischaemic stroke. A change in the activity of certain enzymes when S1PR2 was blocked was also found; matrix metalloproteinase (MMP)-9 activity decreased in vivo, and lower gelatinase activity was detected in the cerebral microvessels.

The researchers also found that, in contrast to S1PR2, S1PR1 is an endogenous protective mechanism limiting endothelial injury and mice that had a specific deletion of S1PR1 in endothelial cells had significantly worsened outcomes in stroke. Thus, the balance of expression and activity of these two S1P receptors in the endothelium determines stroke outcomes, being S1PR1 protective and S1PR2 deleterious. Dr Sanchez further investigated the potential relevance of these findings in humans, and they discovered that both S1PR1 and S1PR2 are expressed in the cerebrovascular endothelium in the human brain, which further underscores the therapeutic potential of the pathway in cerebrovascular diseases.

This work establishes the novel and important role of S1PR2 in the development of cerebrovascular complications of ischaemic reperfusion injury in experimental stroke models. Dr Sanchez highlights the potential of S1PR2 as a novel therapeutic target for cerebrovascular protection in stroke and other conditions of hypoxic (low oxygen) or inflammatory injury in cerebral blood vessels.

Characterisation of Cerebral Microvessels

When Dr Sanchez and her team delved further into the molecular mechanisms underlying the dysfunction of the cerebral microvessels, they found that many experimental approaches used to characterise cerebral microvessels lacked standardisation. They saw that being able to isolate intact microvessels with consistent cellular compositions in a reproducible manner was needed to ensure a fair comparison during studies.

To address this, the team created an optimised protocol describing the isolation of microvessels from parts of the mouse brain that yields samples with consistent amounts of suitable components required for these types of studies, such as blood-brain barrier components (the protective lining of the brain that separates it from the blood). This method does not require the use of enzymatic digestion, which is widely used in research and often causes unwanted molecular and metabolic changes that can impact experimental results and lead to artefacts. Importantly, this protocol allows the accurate quantification of the changes in gene expression in stroke models and the activation of the signalling pathways in various experimental conditions. The team also describes a number of ways to streamline some of the technical processes, including the isolation of the genomic DNA and bisulfite treatment for assessing DNA methylation, which are molecular changes that occur in the DNA and control the expression of genes.

This optimised protocol should help improve the understanding of the molecular mechanisms that govern cerebral microvascular dysfunction, which, in turn, could help with the development of novel therapies for stroke and other neurological conditions.

Mysteries of Stroke Microvascular Dysfunction – Solved!

Dr Sanchez conducted a remarkable study to profile the molecular changes in gene activity in the cerebral microvasculature caused by strokes. Using their optimised protocols, the team identified the changes in the expression of certain genes and, thus, the production of proteins and other molecules involved with the inner workings of the vascular endothelial cells caused by stroke. They compared the changes that they discovered in mouse cerebral microvasculature to the alterations seen in human non-fatal brain stroke lesions.

This work revealed that there were hundreds of similar alterations in the mouse stroke microvessels and human stroke lesions. They identified shared molecular features associated with vascular diseases, such as Serpine1/Plasminogen Activator Inhibitor-1 and Hemoxygenase-1, endothelial activation like Angiopoietin-2, and changes in sphingolipid metabolism and signalling including S1PR2. Knowing which genes, molecules, and receptors are linked to inflammation, vascular dysfunction, and permeability in the brain microvessels opens up many novel avenues of research. Dr Sanchez explains that finding these shared molecular features between mouse and human strokes, which are all potent modulators of endothelial function, has allowed them to identify multiple new druggable targets.

Novel Treatments on the Horizon

Stroke is one of the leading causes of long-term disability and death globally, and despite decades upon decades of intensive research, the treatment options for ischaemic stroke are surprisingly limited. Cerebral microvascular dysfunction contributes to brain injuries and compromises the effectiveness of currently used reperfusion treatments for ischaemic stroke. Understanding the molecular mechanisms controlling this cerebral microvascular dysfunction has been a major obstacle to the development of new therapies.

The groundbreaking research carried out by Dr Sanchez and her team is pathing the way for novel therapies. Critically, their research has highlighted the relevance of cerebral microvascular dysfunction and created a knowledge platform for future investigations of new endothelial-targeted treatments for stroke. These findings could also advance the development of therapies for other conditions involving vascular dysfunction. Dr Sanchez’s laboratory continues this vital research, looking further into stroke and cerebrovascular diseases, sepsis and systemic inflammatory response syndrome, Alzheimer’s disease, vascular dementia, and other brain and vascular disorders.