Angiotensin 1-7 alleviates aging-associated muscle weakness and bone loss, but is not associated with accelerated aging in ACE2-knockout mice

Muscle mass, muscle strength and the renin-angiotensin system

The renin-angiotensin system (RAS) is a classically known circulatory regulatory system. In addition to the previously known multi-organ circulatory form of the RAS, the existence of tissue RASs in individual organs has been well established. Skeletal muscle has also been identified as an organ with a distinct RAS. In recent years, the effects of RAS activation on skeletal muscle have been elucidated from several perspectives: differences in motor function due to genetic polymorphisms of RAS components, skeletal muscle dysfunction under conditions of excessive RAS activation such as heart failure, and the effects of the use of RAS inhibitors on muscle strength. In addition, the concept of the RAS itself has recently been expanded with the discovery of a 'protective arm' of the RAS formed by factors such as angiotensin-converting enzyme 2 and angiotensin 1-7. This has led to a new understanding of the physiological function of the RAS in skeletal muscle. This review summarizes the diverse physiological functions of the RAS in skeletal muscle and considers the potential of future therapeutic strategies targeting the RAS to overcome problems such as sarcopenia and muscle weakness associated with chronic disease.

Angiotensin receptor blockers, but not angiotensin-converting enzyme inhibitors, inhibit abnormal bone changes in spondyloarthritis

Spondyloarthritis (SpA) is a chronic inflammatory disease that results in bone ankylosis. The tissue renin-angiotensin system (RAS) is an emerging pathway potentially implicated in SpA-associated bone changes. The aim of the present study was to determine the mechanisms underlying this relationship. Sakaguchi (SKG) mice injected with curdlan (SKGc), animal models for SpA, were treated with RAS modulators, angiotensin II receptor blockers (ARBs) or angiotensin-converting enzyme inhibitors (ACEis). Disease activity was assessed using clinical scores and computed tomography scans. Mouse primary bone marrow monocytes (BMMs), osteoblast (OB) progenitor cells, peripheral blood monocytes (PBMCs), and bone-derived cells (BdCs) from patients with radiographic axial SpA (r-axSpA) were used to investigate the role of RAS in SpA pathogenesis. The expression of RAS components was significantly increased in SKGc mouse joints, and ARBs significantly reduced erosion and systemic bone loss, whereas ACEis did not. Osteoclast (OC) differentiation from primary BMMs, mediated by TRAF6, was inhibited by ARBs but promoted by ACEis; the modulators also exerted opposite effects on OB differentiation. Expression of RAS molecules was higher in PBMCs and BdCs of patients with r-axSpA than in control participants. ARBs inhibited OB differentiation in the BdCs of patients with r-axSpA, whereas ACEis did not. Neither ARBs nor ACEis affected OB differentiation in the control participants. In SpA, a condition characterized by RAS overexpression, ARBs, but not ACEis, inhibited OC and OB differentiation and bone progression. The findings should be taken into account when treating patients with SpA using RAS modulators.

Acute bone loss following SARS-CoV-2 infection in mice

The novel coronavirus disease 2019 (COVID-19) is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and has infected more than 650 million people worldwide. Approximately 23% of these patients developed lasting "long-haul" COVID symptoms, including fatigue, joint pain, and systemic hyperinflammation. However, the direct clinical impact of SARS-CoV-2 infection on the skeletal system including bone and joint health has not been determined. Utilizing a humanized mouse model of COVID-19, this study provides the first direct evidence that SARS-CoV-2 infection leads to acute bone loss, increased osteoclast number, and thinner growth plates. This bone loss could decrease whole-bone mechanical strength and increase the risk of fragility fractures, particularly in older patients, while thinner growth plates may create growth disturbances in younger patients. Evaluating skeletal health in patients that have recovered from COVID-19 will be crucial to identify at-risk populations and develop effective countermeasures.

Is the anti-aging effect of ACE2 due to its role in the renin-angiotensin system?-Findings from a comparison of the aging phenotypes of ACE2-deficient, Tsukuba hypertensive, and Mas-deficient mice

Angiotensin converting enzyme 2 (ACE2) functions as an enzyme that produces angiotensin 1-7 (A1-7) from angiotensin II (AII) in the renin-angiotensin system (RAS). We evaluated aging phenotypes, especially skeletal muscle aging, in ACE2 systemically deficient (ACE2 KO) mice and found that ACE2 has an antiaging function. The characteristic aging phenotype observed in ACE2 KO mice was not reproduced in mice deficient in the A1-7 receptor Mas or in Tsukuba hypertensive mice, a model of chronic AII overproduction, suggesting that ACE2 has a RAS-independent antiaging function. In this review, the results we have obtained and related studies on the aging regulatory mechanism mediated by RAS components will be presented and summarized. We evaluated the aging phenotype of ACE2 systemically deficient (ACE2 KO) mice, particularly skeletal muscle aging, and found that ACE2 has an antiaging function. The characteristic aging phenotype observed in ACE2 KO mice was not reproduced in Mas KO mice, angiotensin 1-7 receptor-deficient mice or in Tsukuba hypertensive mice, a model of chronic angiotensin II overproduction, suggesting that the antiaging functions of ACE2 are independent of the renin-angiotensin system (RAS).

The fibrotic niche impairs satellite cell function and muscle regeneration in mouse models of Marfan syndrome

AIM

It has been suggested that the proliferation and early differentiation of myoblasts are impaired in Marfan syndrome (MFS) mice during muscle regeneration. However, the underlying cellular and molecular mechanisms remain poorly understood. Here, we investigated muscle regeneration in MFS mouse models by analyzing the influence of the fibrotic niche on satellite cell function.

METHODS

In vivo, ex vivo, and in vitro experiments were performed. In addition, we evaluated the effect of the pharmacological inhibition of fibrosis using Ang-(1-7) on regenerating skeletal muscles of MFS mice.

RESULTS

The skeletal muscle of MFS mice shows an increased accumulation of collagen fibers (81.2%), number of fibroblasts (157.1%), and Smad2/3 signaling (110.5%), as well as an aberrant number of fibro-adipogenic progenitor cells in response to injury compared with wild-type mice. There was an increased number of proinflammatory and anti-inflammatory macrophages (3.6- and 3.1-fold, respectively) in regenerating muscles of wild-type mice, but not in the regenerating muscles of MFS mice. Our data show that proliferation and differentiation of satellite cells are altered (p ≤ 0.05) in MFS mice. Myoblast transplantation assay revealed that the regenerating muscles from MFS mice have reduced satellite cell self-renewal capacity (74.7%). In addition, we found that treatment with Ang-(1-7) reduces fibrosis (71.6%) and ameliorates satellite cell dysfunction (p ≤ 0.05) and muscle contractile function (p ≤ 0.05) in MFS mice.

CONCLUSION

The fibrotic niche, caused by Fbn1 mutations, reduces the myogenic potential of satellite cells, affecting structural and functional muscle regeneration. In addition, the fibrosis inhibitor Ang-(1-7) partially counteracts satellite cell abnormalities and restores myofiber size and contractile force in regenerating muscles.

Control of aging by the renin–angiotensin system: a review of C. elegans, Drosophila, and mammals

The free-living, non-parasitic nematode Caenorhabditis elegans is a premier model organism for the study of aging and longevity due to its short lifespan, powerful genetic tools, and conservation of fundamental mechanisms with mammals. Approximately 70 percent of human genes have homologs in C. elegans, including many that encode proteins in pathways that influence aging. Numerous genetic pathways have been identified in C. elegans that affect lifespan, including the dietary restriction pathway, the insulin/insulin-like growth factor (IGF) signaling pathway, and the disruption of components of the mitochondrial electron transport chain. C. elegans is also a powerful system for performing drug screens, and many lifespan-extending compounds have been reported; notably, several FDA-approved medications extend the lifespan in C. elegans, raising the possibility that they can also extend the lifespan in humans. The renin–angiotensin system (RAS) in mammals is an endocrine system that regulates blood pressure and a paracrine system that acts in a wide range of tissues to control physiological processes; it is a popular target for drugs that reduce blood pressure, including angiotensin-converting enzyme (ACE) inhibitors and angiotensin II receptor blockers (ARBs). Emerging evidence indicates that this system influences aging. In C. elegans, decreasing the activity of the ACE homolog acn-1 or treatment with the ACE-inhibitor Captopril significantly extends the lifespan. In Drosophila, treatment with ACE inhibitors extends the lifespan. In rodents, manipulating the RAS with genetic or pharmacological interventions can extend the lifespan. In humans, polymorphisms in the ACE gene are associated with extreme longevity. These results suggest the RAS plays a conserved role in controlling longevity. Here, we review studies of the RAS and aging, emphasizing the potential of C. elegans as a model for understanding the mechanism of lifespan control.

Skeletal Muscle and COVID-19: The Potential Involvement of Bioactive Sphingolipids

SARS-CoV-2 virus infection is the cause of the coronavirus disease 2019 (COVID-19), which is still spreading over the world. The manifestation of this disease can range from mild to severe and can be limited in time (weeks) or persist for months in about 30–50% of patients. COVID-19 is considered a multiple organ dysfunction syndrome and the musculoskeletal system manifestations are beginning to be considered of absolute importance in both COVID-19 patients and in patients recovering from the SARS-CoV-2 infection. Musculoskeletal manifestations of COVID-19 and other coronavirus infections include loss of muscle mass, muscle weakness, fatigue or myalgia, and muscle injury. The molecular mechanisms by which SARS-CoV-2 can cause damage to skeletal muscle (SkM) cells are not yet well understood. Sphingolipids (SLs) represent an important class of eukaryotic lipids with structural functions as well as bioactive molecules able to modulate crucial processes, including inflammation and viral infection. In the last two decades, several reports have highlighted the role of SLs in modulating SkM cell differentiation, regeneration, aging, response to insulin, and contraction. This review summarizes the consequences of SARS-CoV-2 infection on SkM and the potential involvement of SLs in the tissue responses to virus infection. In particular, we highlight the role of sphingosine 1-phosphate signaling in order to aid the prediction of novel targets for preventing and/or treating acute and long-term musculoskeletal manifestations of virus infection in COVID-19.

Tryptophan Metabolism and COVID-19-Induced Skeletal Muscle Damage: Is ACE2 a Key Regulator?

The severity of coronavirus disease 2019 (COVID-19) is characterized by systemic damage to organs, including skeletal muscle, due to excessive secretion of inflammatory cytokines. Clinical studies have suggested that the kynurenine pathway of tryptophan metabolism is selectively enhanced in patients with severe COVID-19. In addition to acting as a receptor for severe acute respiratory syndrome coronavirus 2, the causative virus of COVID-19, angiotensin converting enzyme 2 (ACE2) contributes to tryptophan absorption and inhibition of the renin-angiotensin system. In this article, we review previous studies to assess the potential for a link between tryptophan metabolism, ACE2, and skeletal muscle damage in patients with COVID-19.

Combined Administration of Andrographolide and Angiotensin- (1-7) Synergically Increases the Muscle Function and Strength in Aged Mice

BACKGROUND

Sarcopenia is a progressive and generalized skeletal muscle disorder characterized by muscle weakness, loss of muscle mass, and decline in the capacity of force generation. Aging can cause sarcopenia. Several therapeutic strategies have been evaluated to prevent or alleviate this disorder. One of them is angiotensin 1-7 [Ang-(1-7)], an anti-atrophic peptide for skeletal muscles that regulates decreased muscle mass for several causes, including aging. Another regulator of muscle mass and function is andrographolide, a bicyclic diterpenoid lactone that decreases the nuclear factor kappa B (NF-κB) signaling and attenuates the severity of some muscle diseases.

OBJECTIVE

Evaluate the effect of combined administration of Ang-(1-7) with andrographolide on the physical performance, muscle strength, and fiber´s diameter in a murine model of sarcopenia by aging.

METHODS

Aged male mice of the C57BL/6J strain were treated with Andrographolide, Ang-(1-7), or combined for three months. The physical performance, muscle strength, and fiber´s diameter were measured.

RESULTS

The results showed that aged mice (24 months old) treated with Ang-(1-7) or Andrographolide improved their performance on a treadmill test, muscle strength, and their fiber´s diameter compared to aged mice without treatment. The combined administration of Ang-(1-7) with andrographolide to aged mice has an enhanced synergically effect on physical performance, muscle strength, and fiber´s diameter.

CONCLUSION

Our results indicated that in aged mice, the effects of andrographolide and Ang-(1-7) on muscle function, strength, and fiber´s diameter are potentiated.

Angiotensin II receptor blocker and statin combination therapy associated with higher skeletal muscle index in patients with cardiovascular disease: A retrospective study

WHAT IS KNOWN AND OBJECTIVE

Reduction in skeletal muscle mass is the most important component in diagnosing sarcopenia. Ageing and chronic heart failure due to cardiovascular diseases (CVDs) accelerate the reduction of skeletal muscles. However, there are no currently available drugs that are effective for sarcopenia. The purpose of this study was to explore the association between prescribed medications and skeletal muscle mass in patients with CVD.

METHODS

This was a single-centre, retrospective, cross-sectional study. The subjects were 636 inpatients with CVD who took prescribed medicines for at least 4 weeks at the time of admission. Skeletal muscle volume was assessed using a bioelectrical impedance assay.

RESULTS AND DISCUSSION

Single regression analysis showed that 10 and 3 medications were positively and negatively associated with skeletal muscle index (SMI), respectively. Stepwise multivariate regression analysis revealed that angiotensin II receptor blocker (ARB)/statin combination, dipeptidyl peptidase-4 inhibitor, and antihyperuricemic agents were positively associated with SMI while diuretics and antiarrhythmic agents were negatively associated with SMI. After adjustment using propensity score matching, the SMI was found to be significantly higher in ARB/statin combination users than in non-users.

WHAT IS NEW AND CONCLUSION

Combination use of ARB/statin was associated with a higher SMI in patients with CVD. A future randomised, controlled trial is warranted to determine whether the ARB/statin combination will increase the SMI and prevent sarcopenia in patients with CVD.

Ang-(1-7) protects skeletal muscle function in aged mice

Background

The angiotensin-converting enzyme 2 (ACE2)/angiotensin 1–7 (Ang-(1–7)) axis has been shown to protect against the age-associated decline in skeletal muscle function. Here, we investigated the protective effects of ACE2 in mitigating the age-associated decline of skeletal muscle function and to identify the potential underlying molecular mechanisms.

Methods

We measured the expression levels of Ang-(1–7) in C57BL/6J mice of different ages and correlated these levels with measures of skeletal muscle function. We also investigated the expression of myocyte enhancer factor 2 A (MEF2A) in ACE2 knockout (ACE2KO) mice and its relationship with muscle function. We then treated aged ACE2KO mice for four weeks with Ang-(1–7) and characterized the levels of MEF2A and skeletal muscle function before and after treatment. We assessed the impact of Ang-(1–7) on the growth and differentiation of C2C12 cells in vitro and assessed changes in expression of the glucose transporter type 4 (Glut4).

Results

Aged mice showed reduced skeletal muscle function and levels of Ang-(1–7) expression in comparison to young and middle-aged mice. In ACE2KO mice, skeletal muscle function and MEF2A protein expression were significantly lower than in age-matched wild-type (WT) mice. After one month of Ang-(1–7) treatment, skeletal muscle function in the aged ACE2KO mice improved, while MEF2A protein expression was similar to that in the untreated group. In C2C12 cells, Ang-(1–7) was shown to promote along with the upregulated expression of Glut4.

Conclusions

The ACE2/ Ang-(1–7) axis has a protective function in skeletal muscle and administration of exogenous Ang-(1–7) can delay the age-related decline in the function of skeletal muscle.

Double Deletion of Angiotensin II Type 2 and Mas Receptors Accelerates Aging-Related Muscle Weakness in Male Mice

Background The activation of AT2 (angiotensin II type 2 receptor ) and Mas receptor by angiotensin II and angiotensin-(1-7), respectively, is the primary process that counteracts activation of the canonical renin-angiotensin system (RAS). Although inhibition of canonical RAS could delay the progression of physiological aging, we recently reported that deletion of Mas had no impact on the aging process in mice. Here, we used male mice with a deletion of only AT2 or a double deletion of AT2 and Mas to clarify whether these receptors contribute to the aging process in a complementary manner, primarily by focusing on aging-related muscle weakness. Methods and Results Serial changes in grip strength of these mice up to 24 months of age showed that AT2/Mas knockout mice, but not AT2 knockout mice, had significantly weaker grip strength than wild-type mice from the age of 18 months. AT2/Mas knockout mice exhibited larger sizes, but smaller numbers and increased frequency of central nucleation (a marker of aged muscle) of single skeletal muscle fibers than AT2 knockout mice. Canonical RAS-associated genes, inflammation-associated genes, and senescence-associated genes were highly expressed in skeletal muscles of AT2/Mas knockout mice. Muscle angiotensin II content increased in AT2/Mas knockout mice. Conclusions Double deletion of AT2 and Mas in mice exaggerated aging-associated muscle weakness, accompanied by signatures of activated RAS, inflammation, and aging in skeletal muscles. Because aging-associated phenotypes were absent in single deletions of the receptors, AT2 and Mas could complement each other in preventing local activation of RAS during aging.

Opportunities, Challenges and Pitfalls of Using Cannabidiol as an Adjuvant Drug in COVID-19

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection may lead to coronavirus disease 2019 (COVID-19) which, in turn, may be associated with multiple organ dysfunction. In this review, we present advantages and disadvantages of cannabidiol (CBD), a non-intoxicating phytocannabinoid from the cannabis plant, as a potential agent for the treatment of COVID-19. CBD has been shown to downregulate proteins responsible for viral entry and to inhibit SARS-CoV-2 replication. Preclinical studies have demonstrated its effectiveness against diseases of the respiratory system as well as its cardioprotective, nephroprotective, hepatoprotective, neuroprotective and anti-convulsant properties, that is, effects that may be beneficial for COVID-19. Only the latter two properties have been demonstrated in clinical studies, which also revealed anxiolytic and antinociceptive effects of CBD (given alone or together with Δ9-tetrahydrocannabinol), which may be important for an adjuvant treatment to improve the quality of life in patients with COVID-19 and to limit post-traumatic stress symptoms. However, one should be aware of side effects of CBD (which are rarely serious), drug interactions (also extending to drugs acting against COVID-19) and the proper route of its administration (vaping may be dangerous). Clearly, further clinical studies are necessary to prove the suitability of CBD for the treatment of COVID-19.

Aging, Immunity, and COVID-19: How Age Influences the Host Immune Response to Coronavirus Infections?

The novel coronavirus severe acute respiratory syndrome coronavirus 2 causing the Coronavirus disease (COVID-19) pandemic has ravaged the world with over 72 million total cases and over 1.6 million deaths worldwide as of early December 2020. An overwhelming preponderance of cases and deaths is observed within the elderly population, and especially in those with pre-existing conditions and comorbidities. Aging causes numerous biological changes in the immune system, which are linked to age-related illnesses and susceptibility to infectious diseases. Age-related changes influence the host immune response and therefore not only weaken the ability to fight respiratory infections but also to mount effective responses to vaccines. Immunosenescence and inflamm-aging are considered key features of the aging immune system wherein accumulation of senescent immune cells contribute to its decline and simultaneously increased inflammatory phenotypes cause immune dysfunction. Age-related quantitative and qualitative changes in the immune system affect cells and soluble mediators of both the innate and adaptive immune responses within lymphoid and non-lymphoid peripheral tissues. These changes determine not only the susceptibility to infections, but also disease progression and clinical outcomes thereafter. Furthermore, the response to therapeutics and the immune response to vaccines are influenced by age-related changes within the immune system. Therefore, better understanding of the pathophysiology of aging and the immune response will not only help understand age-related diseases but also guide targeted management strategies for deadly infectious diseases like COVID-19.

ACE2, angiotensin 1-7 and skeletal muscle: review in the era of COVID-19

Angiotensin converting enzyme-2 (ACE2) is a multifunctional transmembrane protein recently recognised as the entry receptor of the virus causing COVID-19. In the renin–angiotensin system (RAS), ACE2 cleaves angiotensin II (Ang II) into angiotensin 1-7 (Ang 1-7), which is considered to exert cellular responses to counteract the activation of the RAS primarily through a receptor, Mas, in multiple organs including skeletal muscle. Previous studies have provided abundant evidence suggesting that Ang 1-7 modulates multiple signalling pathways leading to protection from pathological muscle remodelling and muscle insulin resistance. In contrast, there is relatively little evidence to support the protective role of ACE2 in skeletal muscle. The potential contribution of endogenous ACE2 to the regulation of Ang 1-7-mediated protection of these muscle pathologies is discussed in this review. Recent studies have suggested that ACE2 protects against ageing-associated muscle wasting (sarcopenia) through its function to modulate molecules outside of the RAS. Thus, the potential association of sarcopenia with ACE2 and the associated molecules outside of RAS is also presented herein. Further, we introduce the transcriptional regulation of muscle ACE2 by drugs or exercise, and briefly discuss the potential role of ACE2 in the development of COVID-19.

ACE2/ACE imbalance and impaired vasoreparative functions of stem/progenitor cells in aging

Aging increases risk for ischemic vascular diseases. Bone marrow–derived hematopoietic stem/progenitor cells (HSPCs) are known to stimulate vascular regeneration. Activation of either the Mas receptor (MasR) by angiotensin-(1-7) (Ang-(1-7)) or angiotensin-converting enzyme-2 (ACE2) stimulates vasoreparative functions in HSPCs. This study tested if aging is associated with decreased ACE2 expression in HSPCs and if Ang-(1-7) restores vasoreparative functions. Flow cytometric enumeration of Lin⁻CD45^lowCD34⁺ cells was carried out in peripheral blood of male or female individuals (22–83 years of age). Activity of ACE2 or the classical angiotensin-converting enzyme (ACE) was determined in lysates of HSPCs. Lin⁻Sca-1⁺cKit⁺ (LSK) cells were isolated from young (3–5 months) or old (20–22 months) mice, and migration and proliferation were evaluated. Old mice were treated with Ang-(1-7), and mobilization of HSPCs was determined following ischemia induced by femoral ligation. A laser Doppler blood flow meter was used to determine blood flow. Aging was associated with decreased number (Spearman r = − 0.598, P < 0.0001, n = 56), decreased ACE2 (r = − 0.677, P < 0.0004), and increased ACE activity (r = 0.872, P < 0.0001) (n = 23) in HSPCs. Migration or proliferation of LSK cells in basal or in response to stromal-derived factor-1α in old cells is attenuated compared to young, and these dysfunctions were reversed by Ang-(1-7). Ischemia increased the number of circulating LSK cells in young mice, and blood flow to ischemic areas was recovered. These responses were impaired in old mice but were restored by treatment with Ang-(1-7). These results suggest that activation of ACE2 or MasR would be a promising approach for enhancing ischemic vascular repair in aging.

Bone biology and COVID-19 infection: Is ACE2 a potential influence factor?

The outbreak of coronavirus disease 2019 (COVID-19) has posed a severe threat to global health management system since it has been detected in the human body. This pandemic was prompted by severe acute respiratory syndrome coronaviruses 2 (SARS-CoV-2) and rapidly developed into a public emergency with an alarming increase in cases and deaths. The increasing explorations to SARS-CoV-2 infection guide us to consider whether bone lesion is followed by this pathologic process. We especially focus on the underlying pathobiology that SARS-CoV-2 possibly mediated in bone remodeling and analyze the association of bone destruction with ACE2 in COVID-19 incidence, for preferable understanding the pathogenesis and providing necessary clinical management in orthopedics.

Angiotensin-(1-7) Improves Integrated Cardiometabolic Function in Aged Mice

Angiotensin (Ang)-(1-7) is a beneficial renin–angiotensin system (RAS) hormone that elicits protective cardiometabolic effects in young animal models of hypertension, obesity, and metabolic syndrome. The impact of Ang-(1-7) on cardiovascular and metabolic outcomes during aging, however, remains unexplored. This study tested the hypothesis that Ang-(1-7) attenuates age-related elevations in blood pressure and insulin resistance in mice. Young adult (two-month-old) and aged (16-month-old) male C57BL/6J mice received Ang-(1-7) (400 ng/kg/min) or saline for six-weeks via a subcutaneous osmotic mini-pump. Arterial blood pressure and metabolic function indices (body composition, insulin sensitivity, and glucose tolerance) were measured at the end of treatment. Adipose and cardiac tissue masses and cardiac RAS, sympathetic and inflammatory marker gene expression were also measured. We found that chronic Ang-(1-7) treatment decreased systolic and mean blood pressure, with a similar trend for diastolic blood pressure. Ang-(1-7) also improved insulin sensitivity in aged mice to levels in young mice, without effects on glucose tolerance or body composition. The blood pressure–lowering effects of Ang-(1-7) in aged mice were associated with reduced sympathetic outflow to the heart. These findings suggest Ang-(1-7) may provide a novel pharmacological target to improve age-related cardiometabolic risk.

Physiological and pathological regulation of ACE2, the SARS-CoV-2 receptor

The renin-angiotensin system (RAS) is crucial for the physiology and pathology of all the organs. Angiotensin-converting enzyme 2 (ACE2) maintains the homeostasis of RAS as a negative regulator. Recently, ACE2 was identified as the receptor of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the coronavirus that is causing the pandemic of Coronavirus disease 2019 (COVID-19). Since SARS-CoV-2 must bind with ACE2 before entering the host cells in humans, the distribution and expression of ACE2 may be critical for the target organ of the SARS-CoV-2 infection. Moreover, accumulating evidence has demonstrated the implication of ACE2 in the pathological progression in tissue injury and several chronic diseases, ACE2 may also be essential in the progression and clinical outcomes of COVID-19. Therefore, we summarized the expression and activity of ACE2 in various physiological and pathological conditions, and discussed its potential implication in the susceptibility of SARS-CoV-2 infection and the progression and prognosis of COVID-19 patients in the current review.

Protective Effect of Angiotensin 1-7 on Sarcopenia Induced by Chronic Liver Disease in Mice

Sarcopenia associated with chronic liver disease (CLD) is one of the more common extrahepatic features in patients with these pathologies. Among the cellular alterations observed in the muscle tissue under CLD is the decline in the muscle strength and function, as well as the increased fatigue. Morphological changes, such as a decrease in the fiber diameter and transition in the fiber type, are also reported. At the molecular level, sarcopenia for CLD is characterized by: (i) a decrease in the sarcomeric protein, such as myosin heavy chain (MHC); (ii) an increase in the ubiquitin–proteasome system markers, such as atrogin-1/MAFbx1 and MuRF-1/TRIM63; (iii) an increase in autophagy markers, such as LC3II/LC3I ratio. Among the regulators of muscle mass is the renin-angiotensin system (RAS). The non-classical axis of RAS includes the Angiotensin 1–7 [Ang-(1-7)] peptide and its receptor Mas, which in skeletal muscle has anti-atrophic effect in models of muscle wasting induced by immobilization, lipopolysaccharide, myostatin or angiotensin II. In this paper, we evaluated the effect of Ang-(1-7) on the sarcopenia by CLD in a murine model induced by the 5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) hepatotoxin administered through diet. Our results show that Ang-(1-7) administration prevented the decline of the function and strength of muscle and increased the fatigue detected in the DDC-fed mice. Besides, we observed that the decreased fiber diameter and MHC levels, as well as the transition of fiber types, were all abolished by Ang-(1-7) in mice fed with DDC. Finally, Ang-(1-7) can decrease the atrogin-1 and MuRF-1 expression as well as the autophagy marker in mice treated with DDC. Together, our data support the protective role of Ang-(1-7) on the sarcopenia by CLD in mice.

Effect of renin-angiotensin system on senescence

The renin-angiotensin system (RAS) plays crucial roles in the control of blood pressure and sodium homeostasis. Moreover, RAS also acts as a key player in cell and organ senescence, mainly by activation of the classical axis of angiotensin (Ang) converting enzyme (ACE)/Ang II/Ang II type 1 receptor via overproduction of reactive oxygen species. Overactivation of the classical RAS axis induces organ dysfunction in the vasculature, brain, kidney and skeletal muscle, resulting in atherosclerosis, stroke, chronic kidney disease and sarcopenia. Moreover, RAS has been shown to regulate lifespan, using gene-modification models. Recently, mice lacking the Ang II type 1 receptor were shown to exhibit an increase in lifespan compared with control mice. Here, the effect of RAS on age-related tissue dysfunction in several organs is reviewed, including not only the classical axis but also protective functions of RAS such as the ACE2/Ang (1-7)/Mas axis. Geriatr Gerontol Int 2020; ••: ••-••.

Blood flow restriction exercise stimulates mobilization of hematopoietic stem/progenitor cells and increases the circulating ACE2 levels in healthy adults

Adult CD34⁺ hematopoietic stem/progenitor cells (HSPC) in the systemic circulation are bone marrow-derived and have the propensity of maintaining cardiovascular health. Activation of angiotensin-converting enzyme-2 (ACE2)-angiotensin-(1-7)-Mas receptor pathway, the vascular protective axis of the renin-angiotensin system (RAS), stimulates vasculogenic functions of HSPCs. In a previous study, exposure to hypoxia increased the expressions of ACE2 and Mas, and stimulated ACE2 shedding. The current study tested if blood flow restriction exercise (BFR)-induced regional hypoxia recapitulates the in vitro observations in healthy adults. Hypoxia was induced by 80% limb occlusion pressure (LOP) via inflation cuff. Muscle oxygen saturation was determined using near-infrared spectroscopy. Peripheral blood was collected 30 min after quiet sitting (control) or after BFR. Lin^-CD45^lowCD34⁺ HSPCs were enumerated by flow cytometry, and ACE and ACE2 activities were determined in plasma and cell lysates and supernatants. Regional hypoxia resulted in muscle oxygen saturation of 17.5% compared with 49.7% in the control condition (P < 0.0001, n = 9). Circulating HSPCs were increased following BFR (834.8 ± 62.1/mL) compared with control (365 ± 59, P < 0.001, n = 7), which was associated with increased stromal-derived factor 1α and vascular endothelial growth factor receptor levels by four- and threefold, respectively (P < 0.001). ACE2 activity was increased in the whole cell lysates of HSPCs, resulting in an ACE2-to-ACE ratio of 11.7 ± 0.5 in BFR vs 9.1 ± 0.9 in control (P < 0.05). Cell supernatants have threefold increase in the ACE2-to-ACE ratio following BFR compared with control (P < 0.001). Collectively, these findings provide strong evidence for the upregulation of ACE2 by acute regional hypoxia in vivo. Hypoxic exercise regimens appear to be promising means of enhancing vascular regenerative capacity.NEW & NOTEWORTHY Although many studies have explored the mechanisms of skeletal muscle growth and adaptation with hypoxia exercise interventions, less attention has been given to the potential for vascular adaptation and regenerative capacity. This study shows for the first time an acute upregulation of the angiotensin-converting enzyme 2 and increase in CD34⁺ vasculogenic cells following an acute bout of blood flow restriction with low-intensity exercise. These rapid changes collectively promote skeletal muscle angiogenesis. Therefore, this study supports the potential of hypoxic exercise interventions with low intensity for vascular and muscle health.

Mechanism of increased risk of insulin resistance in aging skeletal muscle

As age increases, the risk of developing type 2 diabetes increases, which is associated with senile skeletal muscle dysfunction. During skeletal muscle aging, mitochondrial dysfunction, intramyocellular lipid accumulation, increased inflammation, oxidative stress, modified activity of insulin sensitivity regulatory enzymes, endoplasmic reticulum stress, decreased autophagy, sarcopenia and over-activated renin-angiotensin system may occur. These changes can impair skeletal muscle insulin sensitivity and increase the risk of insulin resistance and type 2 diabetes during skeletal muscle aging. This review of the mechanism of the increased risk of insulin resistance during skeletal muscle aging will provide a more comprehensive explanation for the increased incidence of type 2 diabetes in elderly individuals, and will also provide a more comprehensive perspective for the prevention and treatment of type 2 diabetes in elderly populations.

Different effects of the deletion of angiotensin converting enzyme 2 and chronic activation of the renin-angiotensin system on muscle weakness in middle-aged mice

Inhibition of the renin-angiotensin system (RAS) has been shown to alleviate muscle atrophy both under pathological conditions and during physiological aging. We recently reported that the deletion of angiotensin converting enzyme 2 (ACE2), which converts Angiotensin II to Angiotensin-(1-7) in mice, leads to the early manifestation of aging-associated muscle weakness along with the increased expression of p16^INK4a, a senescence-associated gene, and increased central nuclei in the tibialis anterior (TA) muscle in middle age. As ACE2 is multifunctional and functions beyond its role in the RAS, we investigated whether activation of the RAS primarily contributes to muscle weakness in ACE2 knockout (KO) mice by comparing these mice to Tsukuba hypertensive (TH) mice that overproduce human angiotensin II. The grip strength of young (6 months) and middle-aged (15 months) TH mice was consistently lower than that of wild-type mice at the same ages. Middle-aged TH mice were continuously lean with extremely reduced adiposity. Central nuclei in the gastrocnemius (GM) muscle were increased in ACE2KO mice, while no apparent morphological change was observed in the GM muscles of TH mice. Increased expression of p16^INK4a along with alterations in the expression of several sarcopenia-associated genes were observed in the GM muscles of ACE2KO mice but not TH mice. These findings suggest that chronic overactivation of the RAS does not primarily contribute to the early aging phenotypes of skeletal muscle in ACE2KO mice.