key: cord-1031080-hgamtz7y
authors: Lin, Han-Bin; Liu, Peter P.
title: COVID-19 and the Heart: ACE2 Level is Not Destiny, But the Company It Keeps May Hold the Fate
date: 2020-07-25
journal: JACC Basic Transl Sci
DOI: 10.1016/j.jacbts.2020.07.005
sha: 113ed47912e0ae79fb453f8fb1534b1ad9489776
doc_id: 1031080
cord_uid: hgamtz7y

[Figure: see text]

Since the first report of COVID-19 from Wuhan, China, the pandemic's infectivity and diverse outcomes have stunned the world. A consistent feature of COVID-19 is its predilection of inflicting adverse outcomes in patients with cardiovascular disease, or risk factors (1) . While many factors contribute to this association, a major mechanistic underpinning is the fact that the novel SARS-Cov-2 virus uses the transmembrane enzyme angiotensin converting enzyme 2 (ACE2) as its key internalizing receptor (2) .

ACE2 has a key salutary function in the renin-angiotensin system (RAS) by converting the pro-inflammatory vasoconstrictive octapeptide angiotensin II (1) (2) (3) (4) (5) (6) (7) (8) , to anti-inflammatory vasodilatory angiotensin (1) (2) (3) (4) (5) (6) (7) . But it also raised several key questions in COVID-19, along with controversy. First, is ACE2 expression increased in cardiovascular diseases, hence contributes to the worse outcomes of COVID-19 in cardiovascular patients? Furthermore, is the ACE2 expression affected by cardiovascular medications (with a raging controversy involving the use of RAS inhibitors). Thirdly, does the SARS-CoV-2 virus preferentially target the myocyte in the heart, if they express the ACE2 receptor?

The study by Bristow and colleagues in this issue of JACC Basic Translational Science helps to shed light on some of these questions (3) . This relatively carefully performed study in a small cohort of patients with non-ischemic dilated cardiomyopathy (DCM) helps to answer the question of ACE2 expression in disease and treatment, and which of the co-receptors for COVID-19 infection is present with ACE2 in the heart.

Bristow's team studied 46 DCM patients with serial septal myocardial biopsies subjected to extensive gene expression analysis after treatment with β-blockade. During follow-up, 30 of the 46 patients were classified as responders to β-blockade, with ejection fraction improving ≥ 8% at 12 months. The remainder were classified as non-responders. The analysis showed that ACE2 gene expression was up-regulated by an average of 1.97 fold in all of the DCM patients, compared to normal controls. Surprisingly, the patients who were responders to β-blockade with reverse LV remodeling, the initially elevated ACE2 expression actually returned to normal at 12 months follow-up. This was not observed with RAS inhibitor use/dosage.

From an infection permissiveness point of view, the 5 proteases that can function as the SARS-CoV-2 co-receptor, including TMPRSS2, that allow membrane fusion with the virus to facilitate transfer of the viral RNA to the host, all were present, but showed no change with DCM. However, integrins have been found more recently as an alternate co-receptor binding the RGD domain on the exposed loops of the viral capsid protein(4). Interestingly, integrin ITGA5 tracked in line with ACE2, and was up regulated with DCM, and down regulated with βblockade particularly in those who reverse remodeled. Thus integrins like ITA5 may function as an alternative co-receptor to ACE2 in facilitating SARS-CoV-2 infection intracellularly.

Bristow's study thus helps to illuminate the following questions.

While it is known that RAS is highly activated in patients with cardiovascular diseases, the direct demonstration of increased tissue ACE2 expression in the diseased human heart is actually rare, and often sampled at end stage. There have been reports from Chinese and German investigators indicating that ACE2 expression is particularly up-regulated in the pericytes of the heart, in contrast to low levels in the myocytes (5) . Interestingly, ACE2 can be cleaved by the disintegrin ADAM17 to be shed into the circulation and function as a biomarker. Indeed, circulation ACE2 levels have been found to be increased in patients with heart failure, with a suggestion that this is accompanied by reduced myocardial expression.

The Bristow study challenges the latter scenario, and demonstrates clearly in patients with DCM, there is a very significant up-regulation of ACE2 in the myocardium. The Bristow cohort is particularly advantageous because these patients are relatively young, and are at earlier stages of their disease. This avoids the typical confounding effects when tissue samples are taken from end stage heart failure patients with multiple co-morbidities and complex drug regimen.

The increased ACE2 expression most likely represents a compensatory mechanism to counterbalance the increase in RAS activity in progressive heart failure. This would also support, although not demonstrate directly, that the shedding of ACE2 in the circulation is a consequence of the increased receptor turnover, and not the exhaustion of ACE2 in the myocardium. Thus increased circulating ACE2 levels likely reflect increased tissue ACE2 levels.

Major controversy has arisen regarding ACE2 regulation by drug treatment, because of preclinical studies that suggested exposure to ACE inhibitors may up-regulate the expression of ACE2. This has resulted in confusion with respect to RAS inhibitor continuation or termination in COVID-19. The Bristow study did not find any changes in ACE2 gene expression levels with RAS inhibitor exposure or dosages, given a relatively small sample size. This question ultimately can only be addressed by large global prospective randomized trials, for which we are coordinating one -"The COVID-RASi Trial", along with other investigator initiated trials ongoing around the world.

However, Bristow and colleagues have found an intriguing dynamic pattern of ACE expression with β-blockade. Of the 30 responders, there was actual normalization of the ACE2 expression compared to baseline, in contrast to the lack of significant change in non-responders, This underscores two very important concepts. The first is that ACE2 expression levels, even in established conditions such as dilated cardiomyopathy, is dynamic and can be potentially modified. Secondly, therapeutic agents that can modify the natural history of the underlying disease, and reverse remodel the cardiomyopathy, is also most effective in normalizing ACE2 expression. This also indicated that ACE2 up-regulation is not inherent to DCM, but a response to its secondary RAS activation.

Patients with severe COVID-19 infection often are accompanied by the release of cardiac enzymes such as troponins, suggesting ongoing cardiac injury. This injury signal is also highly prognostic. The questions remain of whether SARS-CoV-2 can directly infect the myocardium and cause myocyte damage. To date, there are only a few case reports of cardiac pathology in COVID-19, and most don't show evidence of viral genetic material in the myocyte. This is in contrast to evidence of virus in the lungs, in the immune cell such as macrophages, or the vascular system. The question is how readily SARS-CoV-2 virus can infect the myocytes, or could the myocardial injury be indirect through perturbations in the immune or vascular system? While the Bristow study does not directly address this question, it does provide insight into the permissiveness of viral entry. The focus is particularly on the dynamics of potential co-receptors in the heart, such as the membrane associated proteases, including TMPRSS2. The presence of membrane proteases such as TMPRSS2 helps to induce membrane fusion following virus-ACE2 engagement, and significantly increases the efficiency of infection.

According to human tissue atlas database, proteases such as TMPRSS2 are highly expressed in the lung, gastrointestinal and kidney tissues. The levels in the myocardium are relatively low, and according to the Bristow data, there is no dynamic up-regulation of the proteases either with DCM, or down-regulation with β-blockade, despite significant changes with ACE2. This may suggest that while ACE2 is expressed in significant quantities in the myocardium, but without the significant dynamic presence of the proteases as co-receptors, such as TMPRSS2, the in vivo access of the SARS-CoV-2 virus to the myocardium may be limited.

Interesting gene expression association studies done in this paper showed that alternate co-receptors for SARS-CoV-2, such as integrins binding to the RGD motif in the spike protein, may be particularly relevant for the cardiovascular system(4). Indeed, integrin α-5, encoded by the gene ITGA5, was increased in the heart of DCM patients. ITGA5 also showed co-regulation with β-blockade. Integrin α-5 is an important component of fibronectin, an essential matrix protein especially in the vasculature, and the maturation of adipocytes. This may in part contribute to the COVID-19's propensity for hypertension and obesity, and its involvement of the vasculature. However, these are hypotheses generated by the intriguing new data, and will require validation with properly designed experiments. Therefore, the overall implication of the study by Bristow and colleagues is important.

The first is that ACE2 expression levels are indeed increased in the heart early in conditions as dilated cardiomyopathy leading to heart failure. Furthermore, the increased ACE2 levels are not coupled to the underlying condition, but appear to be dynamic, and can be normalized by effective treatment to reverse the disease process. This would imply that in the COVID-19 era, more aggressive treatment for patients with cardiovascular disease or risk factors are even more importantly, to mitigate their COVID-19 risk and decrease activation of the RAS system. Finally, the elucidation of potential additional SARS-CoV-2 co-receptors, may help to identify additional tools for the prevention or treatment of COVID-19.

COVID-19 has exposed many of the weaknesses of our cardiovascular patients, and also issues within our health system. Careful rethinking of our strategies of prevention and mitigation of these cardiovascular conditions will help not only with our patients, but our health system and society at large. 

The Science Underlying COVID-19: Implications for the Cardiovascular System

SARS-CoV-2 Cell Entry Depends on ACE2 and TMPRSS2 and Is Blocked by a Clinically Proven Protease Inhibitor

Dynamic Regulation of SARS-CoV-2 Binding and Cell Entry Mechanisms in Remodeled Human Ventricular Myocardium

A potential role for integrins in host cell entry by SARS-CoV-2

Cell type-specific expression of the putative SARS-CoV-2 receptor ACE2 in human hearts