Atrial appendage ACE2, aging and cardiac surgical patients: A platform for understanding aging-related COVID-19 vulnerabilities

Abstract

Purpose of review

Hospitalizations for COVID-19 dramatically increase with age. This is likely due to increases in fragility across biological repair systems and a weakened immune system, including loss of the cardiorenal protective arm of the renin angiotensin system (RAS), composed of angiotensin converting enzyme-2 (ACE2)/angiotensin-(1–7) [Ang-(1–7)] and its actions through the Mas receptor. The purpose of this review is to explore how cardiac ACE2 changes with age, cardiac pathologies, comorbid conditions and pharmaceutical regimens in order to shed light on a potential hormonal unbalance facilitating SARs-CoV-2 vulnerabilities in older adults.

Recent findings

Increased ACE2 gene expression has been reported in human hearts with myocardial infarction, cardiac remodeling and heart failure. We also found ACE2 mRNA in atrial appendage tissue from cardiac surgical patients to be positively associated with age, elevated by certain comorbid conditions (e.g., COPD and previous stroke) and increased in conjunction with patients’ chronic use of anti-thrombotic agents and thiazides diuretics, but not drugs that block the renin angiotensin system.

Summary

Cardiac ACE2 may have bifunctional roles in COVID-19 since ACE2 mediates cellular susceptibility to SARS-CoV-2 infection, but also protects the heart via the ACE2/Ang-(1–7) pathway. Linking tissue ACE2 from cardiac surgery patients to their comorbid conditions and medical regimens provides a unique platform to address the influence that altered expression of the ACE2/Ang-(1–7)/Mas receptor axis might impact SARs-CoV-2 vulnerability in older adults.

Introduction

Hospitalizations for coronavirus disease 2019 (COVID-19) increase dramatically with age (1) and the oldest of the old (those 85 years and older) account for nearly one-third of COVID-19 deaths in the United States (https://www.cdc.gov/nchs/nvss/vsrr/covid_weekly/index.htm#AgeAndSex accessed October 30). Fragility and a weakened immune system are likely contributors to the increase in aging-related adverse sequela of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection. Aggravating factors might also include non-O (2–4), and particularly A blood types (5), and altered expression of renin angiotensin system factors, specifically reduced regulatory capacity of ACE2/Ang-(1–7)/Mas-receptor axis. Indeed, the role of ACE2 in the pathogenesis of heart disease extends beyond that of hydrolyzing angiotensin II into angiotensin-(1–7), since its functional activities may not always associate with favorable outcomes (6, 7), and because it is the receptor of SARS-CoV-2 (8). Thus, the purpose of this review is to briefly discuss why COVID-19 targets older adults, and to explore how cardiac ACE2 might change with age, cardiac pathologies, comorbid conditions and pharmaceutical regimens, in order to shed light on a potential biochemical link to SARs-CoV-2 infection and disease severity with advancing age.

Aging and Covid-19

It is well accepted that hospitalization for COVID-19 increases with age. Persons 85 years and older are 130 times more likely to be hospitalized for Covid-19 than those between ages 18 and 29 years (https://www.cdc.gov/coronavirus/2019-ncov/need-extra-precautions/older-adults.html). To better understand the impact of age on Covid-19, a look at how age impacts the risk of death is important. 16.5 % of the US population is 65 years and older, and this age group accounts for 85% of all-cause deaths, while 1 in 20 adults are in the 85 years and up category, accounting for an astonishing one-third of all Covid-19 related-deaths. Besides advanced age as one of the highest risk factors for COVID-19 related deaths, having comorbid diseases, being male, living in a nursing home, being Black, Hispanic or Latinx, sustaining a low socioeconomic status (e.g. annual income < $25,000 [U.S.]), having blood type A, and having two copies of the APOE e4 gene associate with severe SAR-Cov2 infection, including death (9–12).

Two key aspects related to the increase in aging-related SARs-CoV-2 susceptibility is that the aging body is more fragile, and the aging immune system is markedly different from that of a youthful immune system. With respect to the former, aging leads to a slight imbalance between the wear and tear or damage that normally occurs in the body and the ability of the body to repair itself (13). Both nuclear and mitochondrial DNA repair mechanisms, for example, are essential to maintain cell and whole-body health (14). Defects in repair processes of DNA, such as base excision, nucleotide excision, recombination, and mismatch repair, decline with age, leading to DNA damage accumulation, which play a role in many aging-associated diseases. Interestingly, long-term treatment with ACE-inhibitors or angiotensin receptor blockers increase the lifespan in rodents (15–17). Irrespective of its role in decreasing vasoconstriction and limiting cardiac hypertrophy and associated adverse consequences of systemic hyperfunction (e.g., stroke, myocardial infarction and renal insufficiency), RAS blockade’s benefit in aging involves downregulation of mammalian or mechanistic target of rapamycin (mTOR) (18), prevention of mitochondrial decrease and/or damage and reduced formation of reactive oxygen species (19–22).

Proteins, which are a product of DNA, are also key to the maintenance of cell structure, repair of cellular damage, and catalytic reactions. Unfolded and misfolded proteins need to be degraded and eliminated. With aging there is a loss of protein homeostasis, or impaired proteostasis, which leads to “sticky” aggregates and contributes to age-related diseases such Alzheimer’s and Parkinson’s (23). Drugs targeted at augmenting DNA and protein repair processes, such as Metformin are currently being studied by translational geroscience investigators to offset age-related disease vulnerabilities (24), including that of Covid-19 (ClinicalTrials.gov. NCT04625985; Metformin glycinate, treatment of patients with COVID-19 and severe respiratory syndrome secondary to SAR-CoV-2). Interestingly, in a large retrospective study of adults with type 2 diabetes or obesity, metformin was associated with significantly decreased mortality in women admitted to the hospital with COVID-19, with no significant mortality reduction in men (25). As reviewed by Brandi and Giustina (26), and Bienvenu et al. (27) distinct features of older men as compared to women of same age, including more cardiovascular comorbidities, heightened effects of hypogonadism (i.e., increased inflammatory response, reduced muscular strength) as well hyperandrogenism from exogenous testosterone replacement (i.e., activation of virus entrance in the cells, increased venous thromboembolic events), and absence of biallelic expression of immunologic proteins on the X chromosome, which might impede humoral and cell-mediated immune responses to antigenic stimulation, may account for more severe complications and lethality in males versus females infected with SARS-CoV-2. Whether therapeutic dimorphism with respect to treatment contributes to differences in COVID-19 disease severity between sexes, is not yet known. To briefly review, the drugs approved (https://www.covid19treatmentguidelines.nih.gov/therapeutic-management/) or considered investigational (https://www.fda.gov/vaccines-blood-biologics/investigational-new-drug-ind-or-device-exemption-ide-process-cber/recommendations-investigational-covid-19-convalescent-plasma) in the management of SARS-CoV-2 positive patients outside of the hospital, at high risk for disease progression, include monoclonal antibodies (e.g., bamlanivimab and casirivimab plus imdevimab) while those hospitalized with COVID-19 might receive the antiviral redesivir, antibiotics (e.g., azithromycin), steroid treatment with dexamethasone, and/or convalescent plasma.

What we do know is that immune response mechanisms decline with normal aging. When SARS Cov-2 infects cells and replicates, a robust immune system is more likely to fend off the infection than an unhealthy immune system. Older people have weakened immune systems (Table 1), which often become overwhelmed by infectious insults such as influenza or SARs-CoV-2, resulting in prolonged convalescence, long-standing physical disabilities and premature death (28). The only way to limit the adverse effects of COVID-19 disease sequelae is prevention. That is, prevention in terms of maintaining healthy aging, following primary care provider recommendations, regular exercise, a healthy diet, and by practicing good hygiene, including physical distancing, wearing masks, and frequent hand washing. At the time of this review, the European Medicines Agency and the U.S. Food and Drug Administration commenced the distribution of the Pfizer/BioNTech and Moderna RNA-based coronavirus vaccine for emergency use to help protect against SAR2-CoV-2 infection and prevent the spread of Covid-19 pandemic, in conjunction with all above-mentioned preventive measures.

Table 1.

Effects of aging on innate and adaptive immunity and clinical consequences

		Innate Immunity
Components	Skin and Mucous	Dendritic Cells	Natural Killer	Neutrophils	Macrophages
	↓ Replacement	↓ Ability to stimulate lymphocytes	↓ Cytotoxic capacity	↓ Chemotaxis and phagocytosis	↓ Expression and function of TLR
	↓ Barrier function	↓ Chemotaxis	↓ Secretion of INF-𝛄	↓ Superoxide anion production	↓ Production of MHC
	↓ IgA after age 60	↓ Presentation of antigens	↓ Response to IL-2	↓ Activation signal transduction	↓ Production of TNF-𝛼 and IL-6
				↓ Production of INF
Clinical consequence	↓ mucociliary function → reduced cough, less viral clearance and mucous clearance	Loss of self-tolerance and increased reactivity to self; increase in autoantibodies	Slower resolution of inflammatory responses; increased incidence of bacterial and fungal infection	Delayed pathogen clearance; increase tissue damage through excess protease activity. Consistent with this, older adults experience greater end-organ damage after severe infections	Reduced fever; less malaise; less efficient viral clearance; reduced wound healing
	Adaptive immunity
Components	B lymphocytes	T lymphocytes
	↓ Antibody production	↓ CD4+/CD8+ lymphocytes
	↓ Lymphocyte number	↓ Membrane receptors important in lymphocyte activation
	↓ Production of IL-2
Clinical consequence	Reduced ability to effectively respond against viruses and bacteria; decreased antibody response to vaccination.	Reduced protection from vaccine; reduced longevity of protection from vaccine

Table created using data from Fuentes E, Fuentes M, Alarcon M, Palomo I. Immune system dysfunction in the elderly. An Acad Bras Cienc 2017;89(1):285–299.

NOTE: IgA, Immunoglobulin A; INF- 𝛄, interferon gamma; IL-2, interleukin-2; INF, interferon; TLR, toll-like receptor; MHC, major histocompatibility complex; TNF-𝛼, tumor necrosis factor alpha; IL-6, interleukin-6

ACE2/RAS, aging and COVID-19

As ACE2 is the primary vector for cellular entry of SARS-CoV-2 infection and key component of the counterregulatory renin angiotensin system axis, it is valuable to review ACE2’s biochemical functions and potential role as a “player” in aging-related COVID-19 disease vulnerability and severity. The characterization of an angiotensin converting enzyme (ACE) homologue, named ACE2, functioning as a mono-carboxy peptidase and insensitive to pharmacological inhibition, expanded our knowledge of the biotransformation mechanisms of generating the biological active peptide hormones from the substrate angiotensinogen (29) (Figure 1). ACE2 is now recognized to modulate the net tissue activity of angiotensin II (Ang II) by virtue of its high hydrolytic activity in converting the peptide into angiotensin-(1–7) [Ang-(1–7)]. In turn, Ang-(1–7) by coupling into the Mas receptor initiates a signaling cascade which opposes profibrotic and oxidative stress actions of transforming growth factor beta/TGF-β₁/Smad pathway and hypertrophic and proliferative changes in cardiac and arterial vasculature via inhibition of mitogen-activated protein kinase stimulation (16, 30). The counterbalancing axis of the renin angiotensin system (RAS), as originally proposed by Ferrario’s laboratory (31), constitutes a servomechanism limiting Ang II circulatory and tissue mechanisms from triggering imbalance. Acting as a mono-carboxypeptidase, ACE2 also forms Ang-1–9 through the hydrolysis of Ang I (7). This latter reaction is several hundred times slower than Ang II hydrolysis by ACE2 to form Ang(1–7) (32), yet may also have a role in ACE2 -mediated cardioprotection (33). ACE2 also cleaves several other non-RAS peptides including bradykinin into (des-Arg9)-bradykinin, a member of the kininogen-kinin system (34) which has potent inflammatory actions via activation of types 1 and 2 bradykinin receptors (35), apelin-13, a peptide proposed to cause vasoconstriction and known to regulate fluid homeostasis (36), and dynorphin A, one of several potential mediators of cardiovascular-related inflammation (37).

Figure 1.

Schematic of biochemical pathways to biologically active angiotensins. In the classical or canonical RAS pathway, the protease renin cleaves the substrate angiotensinogen to form angiotensin I (Ang I), and then, ACE removes two amino acids from Ang I to yield Ang II. Ang II can bind to the angiotensin type 1 receptor (AT1R) to exert actions, such as cell growth and proliferation, oxidative stress, inflammation, fibrosis and vasoconstriction. To offset these adverse effects, Ang II can be further degraded to Ang -(1–7), and Ang I can be metabolized by neprilysn to form Ang-(1–7). Ang-(1–7) binds to its receptor, Mas, to exert actions that counteract AT1R activation. In addition, Ang I can be hydrolyzed by ACE2 to form Ang-(1–9), and Ang-(1–9) is then metabolized by ACE to form Ang-(1–7). Recently, a noncanonical pathway leading to Ang II formation, via the conversion of Ang I or angiotensin-(1–12) [Ang-(1–12)] by chymase, was deemed to be the primary contributor in humans to tissue Ang II-induced CVD sequela (93).

Evidence for a role of the ACE2/Ang-(1–7)/Mas-receptor in cardiovascular disease pathogenesis is documented by the presence of low Ang-(1–7) expression in untreated essential hypertensive subjects (38, 39), association of blood pressure control with elevated Ang-(1–7) values in patients treated with ACE inhibitors (ACE-I) (40) or Ang II receptor blockers (ARBs) (41, 42), and increased expression of ACE2 in diseases of the heart and the blood vessels (34, 43–48).

Indeed, complexities in RAS biochemistry resurfaced when the harmful effects of ACE2 were linked to COVID-19 virus-mediated death. Individuals with severe SARs-CoV-2 infection have been shown to have high ACE2 and Ang II levels, suggesting a RAS-related role in COVID-19 pathogenesis. As ACE2 is the central molecular target that interacts with spike-like glycoproteins that are present on SARs-CoV-2 viral surface, high viral loads may lead to ACE2 tissue depletion. Whether this, in turn, triggers an increase in ACE2 transcription, with subsequent increases in nitric oxide and radical oxygen species from an exuberant formation of Ang-(1–7), bradykinin and des-Arg9-bradykinin, is not clear. However, critically ill COVID-19 patients in the intensive care unit are often in a state of refractory shock. Whether the cytokine storm underlying this shock state might be triggered by a feed-forward activation of the RAS and kallikrein system, remains speculative. However, given that kallikrein converts prorenin into renin (49), the interaction between the RAS and the kallikrein system becomes highly suspected to explain the severe vasodilatory, pro-inflammatory and procoagulant syndrome associated with COVID-19 disease severity (35).

How ACE2 might change during aging is likely dependent on multiple factors including, sex, tissue and cell type, and pathological conditions. In healthy humans, ACE2 expression decreases with age in colon, blood and adrenal gland, brain, nerve, adipose, and salivary gland in males, but only does so in the first three tissues in female (50). In human dermal fibroblasts, ACE2 expression is increased with age (51). In pathological conditions, no differences in activity of ACE2 in bronchoalveolar lavage fluid were noted between young and old patients with acute respiratory distress syndrome (ARDS) (52), while in ventilated patients, ACE2 strongly upregulated with increasing age (53). Increased ACE2 expression and activity has also been reported in diseases of the human heart and blood vessels (34, 43–48). Taken together with the fact that the heart is among the top ten organs that express ACE2 (54) (55), and that COVID-19 infection can result in myocardial complications (56–58), we retrospectively analyzed right atrial appendage tissue samples for ACE2 gene expression levels from 34 consented cardiac surgical patients [IRB #22619] and compared levels with respect to comorbid conditions and chronic medication usage (59). Our goal was to establish more definitive knowledge of how this tissue enzyme is altered in various pathological conditions and under different chronic medical regimens, particularly since high atrial expression of Ang II-forming substrates and chymase gene expression were found to be augmented in the left atria of patients with resistant atrial fibrillation (60). We hypothesized that an increase in ACE2 gene expression in the atria of patients with pre-existing chronic obstructive lung disease (COPD), atrial fibrillation (AF), hypertension and treated with ACEIs or ARBs could explain, in part, why the myocardium might be particularly vulnerable to the virus.

We summarize our findings herein to enlighten anesthesiologists and perioperative care physicians on possible biochemical reasons for increases in COVID-19 disease vulnerability among older adults. Indeed, age related cardiac syndromes such arrhythmias, myocarditis, and acute coronary syndrome are among the manifestations of the SARs-CoV-2 infection (61–64). Moreover, patients with preexisting cardiovascular disease, a common medical problem among older adults, have heightened vulnerabilities to SARs-CoV-2 and more dire consequences to COVID-19 (65–69).

In brief, our study cohort consisted of patients undergoing heart surgery for CABG (n=23), aortic valve replacement [AVR) (n=6)], CABG + AVR (n=4), and CABG + mitral valve replacement [MVR (n=1)]. The majority of patients had a history of hypertension (82%), and over a third had diabetes (38%). Other comorbidities included past or current history of AF (18%), COPD (15%) and stroke (15%). With regard to baseline medical therapy, 88% were on anti-thrombolytic agents, 76% were on statins, 68% were on beta-blockers, 49% were on ACE inhibitors (ACEIs) or angiotensin receptor blockers (ARBs), and 21% were on thiazide diuretics. Preoperative echocardiographic indices reveal that the baseline ejection fraction of the patients was within normal limits (median LVEF = 55 (IQR, 45– 55%)), while the right atrial diameter was on the upper end of normal (4.2 (IQR, 3.7 – 4.6 cm)). Atrial appendage ACE2 mRNA normalized to respective glyceraldehyde-3-phosphate dehydrogenase (GAPDH) values ranged from 0.49 – 37.01, and levels were positively related to age (Figure 2).

Figure 2.

Relationship between age and atrial appendage ACE2 gene transcripts obtained from 34 cardiac surgical patients.

Cardiac ACE2 and comorbid conditions

To determine if having a pre-existing condition increased atrial appendage ACE2 expression, analysis of variance was performed. Our results showed no effect of ACE2 for hypertension, diabetes, and atrial fibrillation. Patients with COPD and a stroke history had significantly higher cardiac ACE2 gene expression levels than those without COPD (P<0.003) or a previous stroke (P = 0.014). Figure 3 illustrates expression levels of ACE2 mRNA (normalized to respective GAPDH values) across comorbid conditions. As gene transcripts for renin, ACE, and chymase in the right atrial appendage tissue were also higher in COPD patients versus non-COPD patients (data not shown), the increase ACE2 might represent a compensatory mechanism. COPD and related pulmonary hypertension can increase right ventricular afterload and cause stretching of the right atria, leading to an activation of the local RAS (52). Also, since most COPD patients are/were smokers, it is worth noting that smoking can increase ACE2 expression in the human lung (56, 57).

Figure 3.

Histogram of atrial tissue ACE2 mRNA for patients with or without the following comorbid conditions: chronic obstructive pulmonary disease (COPD), stroke history, hypertension, diabetes, and kidney disease. Values represent mean and standard error of the mean. Astereisk (*) denotes a significant main effect for COPD vs. non-COPD and for previous stroke vs. no stroke history with respect to ACE2 gene levels.

Clinical data show the outcomes of SARS-CoV-2 infection are more severe in COPD patients (28, 58). A recent meta-analysis with a total of 2,002 cases reported that COPD presence associated with nearly a 4-fold higher risk of developing severe COVID-19, while death was reported in 60% of COPD patients compared to 34.3% of non-COPD patients (59). Whether COVID-19 severity in COPD patients is due to an indirect effect of elevated cardiac ACE2 on lung function is not known. It also remains to be determined whether there is a similar elevation in lung ACE2 expression in COPD vs. non-COPD individuals.

Having a previous stroke was also linked to higher ACE2 gene expression levels in our study. Atrial appendage tissue renin and ACE mRNA levels were also increased in stroke vs. non-stroke patients. Ang II is a key mediator by which hypertension exerts its deleterious vascular effects and hypertension is a major risk factor for stroke, with detrimental actions on the cerebral circulation which underlies most cerebrovascular diseases (60). In our small cohort, all patients with a history of stroke were also hypertensive, which might indicate an activated RAS leading to increased cardiac renin and ACE gene expression. In this case, the increase in ACE2 mRNA expression might signify a compensatory mechanism in response to an activated RAS. Although it is not clear from our findings what role an ACE2-to-stroke relationship might have in disease sequelae after SARS-CoV-2 infection, a pooled analysis of four clinical studies found that those patients with a stroke history had a 2.5-fold increase risk of having more severe-COVID-19 symptoms (61) than those without a stroke history. Understanding the full effect of the cardiac RAS, and specifically elevated ACE2, in predicting extracardiac sequelae from SARS-CoV-2 infection will require postmortem molecular and biochemical analyses of infected tissues from those who succumb to COVID-19.

Cardiac ACE2 and chronic RAS blockade

Because cardiac ACE2 levels have been shown to be influenced by chronic RAS blockade in animal studies (70, 71), we also sought to determine the impact of various drug regimens on ACE2 mRNA expression. Interestingly the chronic use of thrombolytic agents and thiazide diuretics led to significant elevations in ACE2 gene expression, whereas neither angiotensin receptor blockers (ARBs) nor angiotensin converting enzyme inhibitors (ACEIs) affected mRNA levels of this enzyme. This held true even after ACE2 mRNA levels vs. RAS inhibitors were compared in a stepwise manner to use of beta blockers, calcium channel blockers, nitrates, and thiazide diuretics. ACE2 gene expression levels also failed to associate with statin use.

It is interesting that ACEi and ARBs were not among the medications linked to elevated atrial tissue ACE2 gene expression. In contrast, Hu et al (72) found that ACE2 mRNA in atrial tissue AF not on ACEi. From a pharmacologic point of view, Ferrario et al (70, 71, 73) and others (74) have shown in preclinical models that blocking the ACE/Ang II/AT1R axis, through limiting the formation and actions of Ang II, potentiates the expression and, in some instances, the activity of ACE2 in the heart and other cardiovascular tissues. Whether differences in disease traits could account for discrepancies between these clinical studies remains uncertain. Even so, the finding that cardiac ACE2 gene expression failed to relate to chronic use of RAS blockade provides insight into the ongoing discussion regarding RAS inhibitors and COVID-19 vulnerabilities. That is, if RAS inhibition leads to upregulation of ACE2, acute respiratory syndrome and myocarditis in SARS-CoV-2‒infected patients might be attenuated by reducing Ang II formation and blocking its actions at the level of the angiotensin type-1 receptor, and conceivably through the local formation of Ang-(1–7). Paradoxically, an increase in ACE2 expression may facilitate virus access into host tissues, thus aggravating the clinical picture. To date, a small observational study (42 patients) and one comprehensive monitoring study involving 576 COVID-19 patients report favorable effects with respect to COVID-19 outcomes in patients on RAS blockers (75, 76), while findings from several large retrospective studies and two meta-analyses of the literature show ACEI/ARB use is not associated with an increase or decrease in adverse events from COVID-19 (77–81). Moreover, in the BRACE-CORONA trial, the first randomized data assessing the role of continuing versus stopping ACE inhibitors and ARBs in patients hospitalized with COVID-19, suspending ACEIs and ARBs for 30 days did not impact the number of days alive and out of hospital (82).

These findings strengthen the recommendations of the American Heart Association and international regulatory agencies about not withdrawing or switching ACEI/ARB treatments to modify COVID-19 prognosis. Until prospective, randomized-controlled studies are completed that investigate the extent of risk reduction for in-hospital death from COVID-19, a de novo trial of RAS inhibitor initiation in patients hospitalized with COVID-19 is not advocated.

Cardiac ACE2 and non-RAS related medication

Anti-thrombotic agents are another class of medications commonly used by older patients with ischemic and valvular heart disease; use of these medications were associated with elevated ACE2 gene expression in our study. To our knowledge, no clinical or basic science reports have described the potential modulation of tissue ACE2 expression or activity by antithrombotic therapy. Whether our finding could be confounded by underlying pathological conditions common to cardiovascular disease patients, such as atherosclerosis, should be considered. Previous studies have reported that ACE2 is capable of modulating thrombus formation. Fraga-Silva et al (83) demonstrated in spontaneously hypertensive rats that a decrease in ACE2 activity in thrombi was associated with an increase in thrombus formation, as well as pharmacologic activation of ACE2-attenuated platelet attachment of vessels and thrombus formation. ACE2 overexpression has also been shown in vitro and in vivo to inhibit the development of early atherosclerotic lesions in rabbit aortas (84, 85), while ACE2 deletion, either whole-body deletion or in bone marrow-derived cells, increased the development of atherosclerosis in LDL receptor-null mice (86). Increases in ACE2 expression or activity within a thrombus might also represent a counterregulatory response within atherosclerotic plaques to macrophage-related increases in proinflammatory cytokines (87). In the context of COVID-19, coagulopathic complications are common (88), and anti-coagulant and anti-thrombotic therapies have proven beneficial, as evidenced by the Antithrombotic Therapy to Ameliorate Complications of COVID-19 trial (NCT04372589). Given this, and the positive link between cardiac ACE2 gene expression and anti-thrombotic use observed in our cardiac surgical patient cohort, postmortem studies focused on COVID-19 pathogenicity are needed to determine whether genes encoding proteins involved in viral entry (e.g., ACE2, transmembrane protease serine 2 (TMPRSS2), and ADAM metallopeptidase domain 17 (ADAM17)), and/or the ACE/Ang II/AT1R and/or the interferon/interleukin-related pathways (e.g., interleukin-6) are up or down-regulated in tissues post-thrombotic syndrome and if the expression of these genes correlate with anti-thrombotic treatment. This information could tell us whether direct viral entry is involved in the thrombus formation or if the procoagulant state of the tissue is a reflection of RAS activation and cytokine-mediated inflammation. Some drugs are already available, or are being repurposed, to dissect these pathways that complicate COVID-19 infection (89), with the aim of preventing viral entry, halting viral replication, and limiting tissue damage.

Thiazide diuretics were also positively associated with atrial appendage ACE2 gene expression. Although only 7 of 34 patients were taking thiazides in our small cohort, including hydrochlorothiazide (HCTZ) and chlorthalidone, association of thiazide use with local ACE2 mRNA expression levels is consistent with previous work. In studies by the Ferrario laboratory (90), chronic low-dose HCTZ treatment increased cardiac ACE2 mRNA and activity in normotensive rats, an effect that was not related to its anti-hypertensive actions. The inhibitory effect of HCTZ on carbonic anhydrase activity (91) may account for its interaction with tissue ACE2. Using a systems biology approach, Emameh and colleagues (92) recently reported co-expression of ACE2 with carbonic anhydrase and neprilysin, another RAS-related enzyme that can produce Ang-(1–7), in various human tissues and its potential pathogenesis of SARS-CoV-2. Although we do not know if an ACE2-neprilysin-carbonic anhydrase protein network is affected by thiazide use in our cardiac surgical patients, this possibility is important to consider in the quest to improve our understanding of the relationship between ACE2 and COVID-19 susceptibility, disease progression, and treatment.

Conclusions

While it is not yet established if older individuals are more vulnerable to SARs-CoV-2 infection, they do have worse outcomes once infected when compared to younger adults and children. Normal declines in the biological aging process, such as increases in fragility in biological repair systems and a weakened immune system are deemed to be partially responsible for symptomatic COVID-19 spikes among older adults. Increased SARs-CoV-2 infection prevalence has also been reported to occur in non-O blood types, and disease severity (e.g., increased risk of mechanical ventilation, continuous renal replacement, and prolonged intensive care stay (56) has been linked to blood-type A. Our finding that ACE2 atrial tissue gene transcript increases with age, COPD, stroke history, and chronic use of anti-thrombotic agents and thiazide diuretics, but not ACEI or ARBs, of older cardiac surgery patients becomes provocative. Although there is no direct evidence of the connection of atrial tissue ACE2 mRNA and cardiac complications from SARS-CoV-2 infection, we believe our findings offer some insight into what conditions and drugs may lead to increased ACE2 expression, which in turn might influence the degree of SARS-CoV-2 infection, disease progression and possibly associated cardiac complications of older adults (Figure 4). The information we provide is not meant to change clinical practice of physician anesthesiologists and intensivists who are on the front lines of the COVID-19 pandemic, but rather to enhance the clinician’s understanding of how the functional existence of a disturbance in neurohormonal regulation of tissue RAS might influence the link of an augmented risk for SARs-CoV-2 infection in older adults with pre-existing cardiovascular conditions.

Figure 4.

Visual summary of clinical data showing patient factors that may modulate cardiac ACE2 gene expression levels. Higher ACE2 expression occurred with preexisting COPD (P<0.003) and stroke history (P=0.014), but not atrial fibrillation, hypertension, or diabetes. ACE2 expression also increased by chronic intake of thrombolytic agents (P=0.005) and thiazide diuretics (P=0.03), but not RAS inhibitors or statins. As a reminder, this was a small observational study involving patients who underwent cardiac surgery and, thus, was not designed to establish a causal relationship between ACE2 expression and COVID-19 disease severity. Even so, our findings offer insight into conditions and drugs that may lead to ↑ACE2 expression, which in turn might influence susceptibility to SARS-CoV-2 infection, COVID-19 progression, and associated cardiac complications in older adults.

Key points.

1. The presence of age-related comorbidities, such as heart disease, COPD, and stroke, associate with COVID-19 disease severity.

2. Biological changes that occur with advancing age, including impaired DNA repair systems, loss of protein homeostasis and a weakened immune system, may underlie the increases in aging-related vulnerability to SARS-CoV-2 infection.

3. SARs-CoV-2 infection severity has been linked to blood-type A.

4. Cardiac ACE2 may have bifunctional roles in COVID-19 since ACE2 mediates cellular susceptibility to SARS-CoV-2 infection, but also protects the heart via the ACE2/Ang-(1–7) pathway.

5. ACE2 atrial tissue gene transcripts from older cardiac surgery patients increase with age, COPD, stroke history, and chronic use of anti-thrombotic agents and thiazide diuretics, but not ACEI or ARBs.

Abbreviations

ACE2

Angiotensin converting enzyme-2

ACE

angiotensin converting enzyme

RAS

renin angiotensin system

COPD

chronic obstructive pulmonary disease

COVID-19

coronavirus disease 2019

SARS-CoV-2

Severe Acute Respiratory Syndrome Coronavirus 2

Ang II

angiotensin II

ACE-I

ACE inhibitors

ARBs

Ang II receptor blockers