Stroke: how could gene and cell therapy help?

Stroke is the second leading cause of death worldwide, and the major cause of disability in Europe. A stroke happens when the blood supply to part of the brain is severely reduced, often with severe effects on the body. The long-term effects depend on the extent of stroke and where it occurs. About one-third of stroke sufferers recover quite well, but most still experience some permanent effects, and some strokes cause severe disability. Could stem cells and gene editing techniques help?

What do we know?

The severe reductions in blood flow that occur in strokes can seriously damage parts of the brain, or even be fatal.

Anyone of any age can have a stroke, but age, comorbidities (e.g. diabetes, hypertension, or certain other conditions), family health history and lifestyle can affect the risk of having a stroke.

The best treatment for someone suffering a stroke is to get treatment as quickly as possible to restore blood flow.

Brain (neural) and vascular stem and progenitor cells promote neurovascular remodeling and rewiring of the brain and spinal cord. Scienctists are investigating whether they may improve neurological function following a stroke.

What are researchers working on?

Scientists seek to understand the signals controlling neural stem cells, and the mechanism by which stem cells facilitate recovery, in order to design better treatments.

Researchers are working to develop medications and interventions that promote the generation, proliferation and migration of neural stem and progenitor cells towards damaged areas to amplify and facilitate the repair and neurovascular restorative processes.

There are limited numbers of neural stem cells in our brains. It is possible to make large numbers of neural stem cells in laboratories using stem cells called Induced Pluripotent Stem Cells (iPSCs). However, these cells may not be effective in promoting neurological recovery and repair post-stroke.. Further studies are required to demonstrate that lab-made cells are both safe and effective.

Many studies now indicate that the therapeutic mechanisms driving neural stem cell treatment for stroke and neural injury are mediated by lipid (fat)-encased nanoparticles produced by these stem cells. These nanoparticles are also referred to as extracellular vesicles (EVs). EVs transfer genetic instructions, proteins, and lipids to recipient cells. A subpopulation of these EVs, called exosomes (30-150nm) have been shown to play a key, role in mediating stem cell therapeutic benefit. Major research is ongoing to investigate the potential of these exosomes, as well as to engineer the molecular content of these EVs, to understand and improve their therapeutic benefits for stroke, neural injury and neurodegenerative diseases.

What are the challenges?

Neural stem cell therapies stimulate neurovascular repair and remodeling, and amplify the body’s own repair mechanisms following a stroke or neural injury. Understanding the molecular signals and processes by which neural stem cells promote neurological recovery would allow us to maximise their therapeutic potential for stroke and neural injury.

Stroke does not only cause not only provokes motor and sensory issues. It also leads to cognitive and emotional issues, including depression and social withdrawal. In addition, a stroke may cause secondary organ dysfunction (e.g. cardiac, renal, pulmonary, liver, and intestinal problems) which further exacerbates the outcomes. It is therefore important to treat stroke as a systemic injury, and treat not only the brain, but the person. How stem and neural cell-based therapeutics impact cognitive, emotional and systemic response to stroke requires further investigation.

What is a stroke?

A stroke happens when the blood supply to one or more parts of the brain is reduced or completely blocked. The blockage may be temporary or permanent and it can be caused in two different ways:

In ischaemic stroke a blood clot obstructs the supply of blood to a region in the brain
In hemorrhagic stroke a blood vessel bursts and bleeds into the brain.

All parts of the brain need a good blood supply to work properly. When the flow of blood is restricted or stopped, vital nutrients and oxygen cannot reach the cells in the brain. The affected cells are damaged and may die. The effects on the body depend on which part of the brain is damaged and how long the blockage remains. A stroke can affect movement, speech, thought processes, behaviour, and memory. It can cause paralysis and motor deficits in one or more parts of the body, or loss of control of bodily functions. Approximately 40% of people affected by stroke will have permanent symptoms that result in them needing special care. Post-stroke, neurological deficits tend to decrease with time. However, only around 10% of patients recover fully.

Anyone of any age can have a stroke, but there are some important risk factors. The chance of having a stroke increases with age, and with comorbidities, such as diabetes. Certain ethnic groups are more at risk, and a family history of stroke increases the chance that you may be affected. There are also risk factors that we may reduce through lifestyle changes, such as making sure any high blood pressure is treated, eating a healthy diet low in fat and salt, stopping smoking and staying physically active.

How is stroke treated now?

Getting treatment from experienced medical professionals quickly is the most important factor for treating stroke. The sooner the blood flow can be restored, the less damage the brain will suffer, which increases the chances of a good recovery.

Patients who reach hospital within the first 3-4 hours of the onset of a stroke can be given “clot-busting” medication to break down the blockage. Receiving these medications significantly reduces disability post-stroke, and improves long-term quality of life.

For certain types of stroke (e.g., large artery occlusions), doctors may insert a catheter a tube into the blocked blood vessel and mechanically withdraw the clot (mechanical thrombectomy), thereby opening the occluded vessel. Under the right conditions, mechanical thrombectomy may be performed up to 24 hours after the onset of the stroke.

A stroke is a medical emergency, and it is essential that medical emergency assistance be immediately contacted even with the slightest indication that you may be having a stroke. The Stroke Association provides clear advice on how to recognize the symptoms of a stroke. If these symptoms appear, act quickly .

After a stroke, therapies are focused on helping the brain’s undamaged areas to help in recovery of lost or compromised neurological function, such as walking or talking. This is termed neurorehabilitation. Neurorehabilitation involves a wide range of professionals, including neurologists, speech therapists, nurses and physiotherapists. In some cases, healthy areas of the brain can learn to take over from those areas that were damaged by the stroke. Unfortunately, severely damaged parts of the brain cannot recover because the body cannot replace the lost brain cells. This is where scientists hope that stem cells may play a role, helping us to find ways to boost the body’s recovery and restorative systems.

How might gene and cell therapies help?

One reason that helping people recover from a stroke can be difficult is that stroke damages many different types of cells in the brain. There are no studies suggesting that stem cell therapies could be used to regrow or replace damaged tissue, and there are many challenges to this approach. These include:

Understanding how to enable these cells to organise themselves in same way as inside the healthy brain
Recreating complex connections across different areas of the brain
Joining the cells up with the brain’s blood supply
These are essential to allow ‘repaired’ central nervous system (brain and spinal cord) to resume neurological functions lost or damaged by the stroke.

Scientists are researching approaches to encourage the production of neural stem cells and progenitor cells in the nervous system. Researchers are also investigating how to enable the ‘re-wiring’ and remodelling of the neurons, glia, and vascular cells (which together form the ‘neurovascular unit’). Scientists hope that these approaches, if successful, could be used to promote neurological recovery after a stroke.

Current research

Early stem cell treatments for stroke

The first studies aimed at developing cell replacement treatments for stroke were done using brain cells derived from a type of tumour called a teratocarcinoma. Researchers found that they could use stem cells from the teratocarcinoma to produce neurons (nerve cells of the brain) in the lab. They then transplanted these lab-grown neurons into the brains of rats after a stroke, and showed that the transplanted cells were able to integrate into the rats’ brains. This research led to a clinical trial in 2000 to assess the safety of human-teratocarcinoma-derived neurons transplanted into the brain of stroke patients. However, although this initial clinical study suggested that the transplanted cells survived and might even have had some benefits in a very small number of patients, a further study in 2005 failed to find any improvement in patients. The origin of these cells in a tumour, combined with the lack of improvement shown in patients has led researchers to focus on other possible stem cell sources.

Neural stem cells and stroke

Brain stem cells, known as neural stem cells, are one of the main types of stem cell being studied in relation to treating stroke. These stem cells are able to divide and multiply, and to form all the different types of cells in the brain. They can be obtained from foetal tissue and from certain parts of the adult brain. However, both these sources provide a very limited number of cells. These cells also have the disadvantage that they are not identical to the patient’s own cells. This means they might be rejected upon transplantation unless drugs are given to suppress the patient’s immune system (immunosuppressants). In addition, the use of foetal tissue is the subject of ongoing ethical debate, while obtaining neural stem cells from an adult brain requires a major operation and carries significant risks for the donor.

Despite the challenges associated with obtaining neural stem cells, some promising results have been achieved in studies on rodents. This research suggests that, when injected into the brain, neural stem cells can move selectively towards damaged areas. Once there, the cells can help replace damaged tissue and encourage the brain’s own repair mechanisms into action by releasing substances that reduce inflammation and improve survival of existing neurons. Transplanting neural stem cells into the brain nevertheless remains a very difficult and long-term challenge.

Other research suggests that an alternative approach may also be useful. There is some evidence that certain chemicals can be used to encourage the neural stem cells that are already in the brain to divide, multiply, and move towards damaged areas. This may open up new ways to treat stroke by using medication.

Embryonic stem cells and stroke

Embryonic stem cells and induced pluripotent stem cells (iPS cells) have been used to grow neural stem cells in the lab in large numbers. Both embryonic stem cells and iPS cells are pluripotent – they can make all the different types of cells in the body. Learning how to control this process to produce neural stem cells addresses some of the problems faced by researchers looking for a source of cells for treatments. However, the powerful properties of embryonic stem cells and iPS cells also mean they have the ability to form tumours. This risk must be understood and managed before we can consider clinical trials of potential new treatments in people with stroke.

The first uses of embryonic stem cells in stroke research date back to 2005, when neural stem cells produced from embryonic stem cells were injected into rat brains. The transplanted neural stem cells were seen to produce different types of specialized neurons (nerve cells) inside the brain. In 2006 a research group from Germany demonstrated that neural stem cells made in this way not only survived and made new nerve cells inside the brain, but the neurons they produced could also make connections to existing neurons of the brain. During 2008 and 2009 different research groups demonstrated that transplanted neurons produced from human embryonic stem cells were able to integrate into rat brains after they had undergone an ischemic stroke. The scientists observed an improvement in the movement of the animals after the transplant. A study led by groups from Sweden and Germany reported similar results in mice and rats using neural stem cells made from human iPS cells.

Despite these promising laboratory results, much more research is needed before it will be possible to consider using embryonic stem cells or iPS cells for stroke treatment in patients. In order to produce methods for transplantation that will be safe and effective, Scientists need to understand precisely how to guide the pluripotent stem cells to produce only the type of neural cell required, and to subsequently study the long-term impact of transplants.

Mesenchymal stem cells and stroke

Mesenchymal stem cells (MSCs) are one of the most commonly used types of stem cell in clinical trials on stroke to date. They can be easily obtained and grown from a patient’s bone marrow, and can produce fat, cartilage, and bone cells. Mesenchymal –stem cell-like cells can also be derived from other sources such umbilical cord and adipose (fatty) tissue. However, the exact identity and nature of these cells is still the subject of some scientific debate.

MSCs from bone marrow and cells obtained from adipose tissue have been injected into the brain or into a leg vein of rats with stroke-like brain damage. In these studies, animals that received injected cells show improved neurological recovery. compared to animals that were not given an injection. The injected cells appear to move to the damaged area of the brain, but this is not thought to be essential for a beneficial effect since MSCs cannot make new brain cells. Instead, the researchers carrying out such studies think the injected MSCs produce and release substances that reduce inflammation and stimulate self-repair within the brain. More research is needed to understand fully how this might work before effective therapies can be developed.

In 2005 a group of researchers in South Korea reported a clinical trial with five patients who received an injection of MSCs into their brains. After one year, the results suggested that the injection of MSCs was safe but there was no clear evidence that the cells had improved the patients’ condition. The same scientists reported a similar study in 2010 in which a larger number of patients had been given MSCs and studied for the following five years. The results were very similar to the first study. The question therefore remains open as to whether MSCs are able to provide a benefit to stroke patients.

Genetic engineering and stroke

Scientists are currently investigating how to use genetic engineering techniques to change the molecular content of extracellular vesicles (EVs) produced by stem and progenitor cells. Researchers hope that this could be a means to promote neurological recovery post-stroke. Scientists are particularly interested in small EVs (exosomes). These are very efficient at transferring their molecular contents to the recipient cells. These exosomes also contain non-coding RNA,
which acts as a molecular ‘switch’. These switches influence multiple molecular pathways, including those related to neurological recovery. This means that both stem cells and their exosomes can affect multiple restorative pathways within living tissue.

Preclinical studies have shown that genetically engineered stem cells, and their EVs, can amplify recovery via these restorative pathways.

A discussion of where current research and emerging technology could lead in the coming decades. It is important in this section that we offer a balanced and accurate account based on the consensus within the research community, and do not make therapy options which are currently speculative sound as thought they are more established or closer to the clinic than they are.

Clinical trials and the current status of gene and cell therapies for stroke

No stem cell therapies are yet available for stroke. Researchers around the world are using different types of stem cells to study how the brain works and to investigate how the damage caused by stroke may one day be repaired.

Several clinical trials assessing the therapeutic effects of stem cells for stroke have been completed or are ongoing within Europe and internationally.

There are still many open questions about the possible use of stem cells to treat stroke. For example: What is the ideal source of stem cells? How should they be transplanted or can the stem cells already inside the patient’s brain be used? How many cells are needed and what would be the right time to carry out a treatment after stroke? What are the specific conditions needed to grow the right type of cells to high purity and safety standards? Ongoing research aims to answer these and other important questions.

More recent research has capitalised on the ability of biological nanoparticles (e.g. exosomes or EVs), released from stem cells which foster neurological recovery post stroke. There are multiple potential benefits using these exosomes vs. using the actual stem cells. Among other benefits: