By Dr Neena Kalia, University of Birmingham, UK
One useful consequence of the pandemic is that it has put a spotlight on the critical role of the microcirculation (blood circulation in the smallest vessels) in disease and as an important therapeutic target.
COVID-19 was initially thought to be a disease of the lungs but, as the pandemic progressed, it was clear that microcirculatory dysfunction was a leading factor in the disease. Terms such as ‘sticky blood’, clots, leaky blood vessels and cytokine storm made for regular news headlines.
Whilst the global focus is currently on the devastating impact of COVID-19 infection on the microcirculation, one should not forget that microvascular perturbations also play a role in many common non-infectious diseases.
The microcirculation is the most important compartment of the cardiovascular system providing essential oxygen and nutrients to the surrounding tissue and removing the build-up of toxic metabolites. However, many things can go wrong with it and when it fails to function properly, the surrounding tissue can be left significantly underperfused (deprived of sufficient blood).
Cardiovascular disease (CVD) is the number one cause of death globally with an estimated 17.9 million people dying each year. Four out of five of these deaths is due to heart attacks and strokes.
Even during the pandemic, CVD remains the leading cause of death worldwide across all age groups. The timely use of stents, also called primary percutaneous coronary interventions (PCI), to open occluded coronary arteries and initiate myocardial reperfusion, has revolutionised the treatment for patients suffering a heart attack.
However, despite such interventions, a significant proportion of patients still incur extensive muscle damage and develop heart failure. The number of patients this affects is not trivial when you take into account the global prevalence of the disease.
So why, despite (macro) perfusion being restored in the culprit artery, does this poor prognosis still exist?
As a microcirculation researcher I would be biased, but not wrong, to suggest that one should go and look for the answer within the coronary microcirculation. Of course, I would not be the first to make this suggestion.
With remarkable foresight, Likoff and colleagues first suggested that abnormalities in the coronary microcirculation could be responsible for the signs and symptoms of ischaemic heart diseases as early as 1967.
However, the focus of clinical attention until lately has been on the angiographically visible larger arteries. This is due to the fact that it is not possible to see coronary blood vessels under 200mm in diameter with current clinical tools, let alone image cellular events taking place inside them.
Even experimentally, real-time application of in vivo imaging techniques such as intravital microscopy to the beating heart has been notoriously challenging, not least due to inherent contractile motion, additional movement due to the proximity of the inflating/deflating lungs and the low transparency of the tissue.
Having spent a career spanning over 25 years intravitally imaging various organs, I was up for the challenge of trying to image the anaesthetised mouse beating heart and was not going to let a little bit of motion get in the way.
With BHF support, my group established a novel method, recently published in Cardiovascular Research, which allowed us to directly visualise at a cellular level even the tiniest coronary capillaries in real-time. To model the clinical heart attack and subsequent PCI scenario, we reversibly tied one of the coronary arteries to mimic the clinical scenario of a blocked artery and, after a short ischaemic duration, initiated reperfusion.
Fluorescently labelled albumin identified immediate resumption of blood flow within the larger vessels of the heart. This, and the fact that myocardial infarction still developed in the heart, was not different to what is observed clinically.
However, what intravital imaging detected was multiple patchy areas in which the dense network of coronary capillaries were devoid of perfusion – a clear mismatch between macro- and microcirculation perfusion.
Within minutes of reperfusion, fluorescently labelled neutrophils, monocytes and platelets began lining the capillaries, with platelets aggregating and forming numerous occlusive microthrombi. This is a critical situation for the surrounding heart muscle. Capillaries blocked with aggregates of platelets can no longer adequately supply blood to the heart muscle. Recruited neutrophils do not sit idly – misguided into thinking they have been recruited to fight invading pathogens, they release a host of ‘nasties’ that destroys healthy muscle and damages the all-important microvessels instead.
Like most experimental studies, our research was conducted in the default adult male mouse. Yet, heart attacks is a disease of the elderly, with advancing age associated with worse prognosis after a stent is put in.
To test whether this could be linked to a greater susceptibility of the aged coronary microcirculation to reperfusion injury, we recently conducted studies in adult and aged female mice. (We used female mice as aged female mice are more readily available because they do not fight or bite each other, something which is common in the more aggressive male mice.)
Compared to adult hearts, functional capillary density was further reduced and neutrophil recruitment was, remarkably, higher. Even in the absence of injury, the aged coronary microcirculation had a raised inflammatory cell presence.
This was an important observation as it directly supported the concept of ‘inflammaging’ (a chronic low-grade inflammation that develops with age) and the paradigm that comorbidities could drive poor outcomes post-PCI through existing basal microcirculatory dysfunction.
Interestingly, occlusive platelet microthrombi were a key pathological feature in male hearts but a heightened inflammatory response dominated in females. Whether these specific gender-related responses explain the worse clinical outcomes of females post-PCI than males is under investigation.
We have identified new mechanisms that explain the age and gender-related differences in the microcirculatory response to reperfusion injury, including a role for novel members of the interleukin-1 superfamily.
Furthermore, we continue to use this model to provide original contributions on the architecture and cellular perturbations of the coronary microcirculation in the setting of other comorbidities for CVD including diabetes and renal insufficiency.
Join us at Physiology 2021 for the symposium ‘The Many Facets of the Microcirculation in Health and Disease’ to hear more about this work and listen to other microcirculation experts discussing its importance in experimental and clinical studies of acute renal failure, Alzheimer’s disease and diabetes.
Dr Neena Kalia heads the Microcirculation Research Group at the University of Birmingham. She is the Director of a world-leading Intravital Imaging Facility, which is open for business for any researcher interested in real-time imaging of the microcirculation in health and disease.
References:
- Kavanagh et al. (2019). Imaging the injured beating heart intravitally and the vasculoprotection afforded by haematopoietic stem cells. Cardiovascular Research 115(13), 1918-1932.