Author: Paul Humphreys

Stroke is the second leading cause of death worldwide and although survival rates are increasing year after year, the majority of stroke survivors live with a major disability as a result of severe brain damage. Ischemic stroke is the most common form of stroke where a blockage, or occlusion, in a blood vessel cuts off the blood supply to the brain. The initial ischemia, or lack of blood supply, not only causes damage but initiates neuro-inflammation which can result in the most severe tissue damage that leads to permanent disability.

Post-stroke neuro-inflammation

In normal conditions, the resident immune cells of the brain, called microglia, play an important role in brain homeostasis. However, onset of stroke results in microglia entering an active state in which they release pro-inflammatory cytokines and proteolytic enzymes that degrade the extra cellular matrix (Figure 1A). The release of cytokines leads to the recruitment and infiltration of other circulating immune cells which eventually leads to the breakdown of the blood-brain-barrier. Breakdown of which is akin to the breach of a dam, with invasive cells and molecules entering the brain that cause more damage. This disruption usually occurs within the acute (first few days after initial stroke) phase, but events that take place in the delayed sub-acute (weeks to months) phase can play a key role in the extent of recovery.

In the sub-acute phase, microglia cells switch from a pro-inflammatory to a neuro-protective phenotype in which they produce anti-inflammatory cytokines to limit inflammation (Figure 1B). The switch in the role of microglial leads to the growth of new blood vessels (angiogenesis) and the remodelling of brain tissue (neurogenesis) which is crucial for the recovery of the blood brain barrier and re-oxygenation of damaged tissue. However, repair at this stage is generally limited and a fluid-filled cavity can form in the brain that prevents further recovery.

Figure 1 – The general mechanisms of neuro-inflammation post-stroke

Treatments for ischemic stroke

Treatments for stroke are currently limited by unwanted side effects, they can only be administered to a restricted number of patients and generally have to be applied within a narrow treatment window of between 4-5 hours. Novel therapies targeting post-stroke inflammation have so far not progressed into a clinical treatment as of yet. Regenerative medicine, which incorporates interdisciplinary sciences, may help to overcome some of the challenges of previous therapeutic approaches. Stem cell therapies, gene therapies and hydrogels mixed with anti-inflammatory factors are all being explored as potential therapies. Final year PhD student Olivera Rajkovic has recently published a review that covers regenerative therapies for post-stroke neuro-inflammation which compliments her PhD project that utilises nano-particles to mop up reactive oxygen species (ROS) post-stroke. I asked her a little about her regenerative medicine approach to brain repair after stroke.

What are reactive oxygen species (ROS) and what do they have to do with stroke?

ROS are highly reactive molecules that have been implicated in brain injury after ischaemic stroke. There is evidence that a rapid increase in the production of ROS immediately after acute ischaemic stroke rapidly overwhelm antioxidant defences, causing further tissue damage. These ROS induce lipid, protein, and DNA damage. Eventually, this leads to cell death via either the apoptotic or the necrotic pathway.

What is the theory behind using nanoparticles?

Poor brain delivery is a major contributing factor to the continued failure of potential ischaemic stroke treatments. Nanoparticles (NPs) are promising candidates for overcoming the challenges of brain drug delivery, as it is well established that the blood brain barrier is highly disrupted after cerebral ischaemia. This can be passively targeted with nanoparticles through an enhanced permeability and retention (EPR) effect similar to that observed in tumours. In addition, nanoparticles are able to provide protection to therapeutic agents while efficiently delivering them into damaged areas. They can also be easily modified (with surfactants or ligands) to enhance blood brain barrier penetration.

What were some of your key findings?

We found that polypropylene nanoparticles, or PPS-NPs, exhibit antioxidant properties for multiple types of reactive oxygen species in vitro and have negligible cytotoxicity. We established that PPS-NPs have a prolonged blood circulation half-life of 9.5 h and our study demonstrated that in addition to acting as a sponge for reactive oxygen species, PPS-NPs are highly anti-inflammatory both in vitro and in a mouse model of stroke. Crucially, we demonstrated the ability of PPS-NPs to accumulate in the ischaemic brain and reduce infarct volume, blood brain barrier damage, neuronal loss, and recover neurological function. Collectively, our results suggest that PPS-NPs may be an effective post-stroke treatment option reducing reactive oxygen species-induced inflammation and damage.

You can read Olivera’s review here (https://doi.org/10.3389/fneur.2018.00734) and the twitter feed of her group at Manchester here.

By Paul Humphreys