Angiotensin II Regulation of Proliferation, Differentiation, and Engraftment of Hematopoietic Stem Cells

Activation of limbal epithelial proliferation is partly controlled by the ACE2-LCN2 pathway

In response to corneal injury, an activation of corneal epithelial stem cells and their direct progeny the early transit amplifying (eTA) cells to rapidly proliferate is critical for proper re-epithelialization. Thus, it is important to understand how such stem/eTA cell activation is regulated. Angiotensin-converting enzyme 2 (ACE2) is predominantly expressed in the stem/eTA-enriched limbal epithelium but its role in the limbal epithelium was unclear. Single cell RNA sequencing (scRNA-seq) suggested that Ace2 involved the proliferation of the stem/eTA cells. Ace2 was reduced following corneal injury. Such reduction enhanced limbal epithelial proliferation and downregulated LCN2, a negative regulator of proliferation in a variety of tissues, via upregulating TGFA and consequently activating epidermal growth factor receptor (EGFR). Inhibition of EGFR or overexpression of LCN2 reversed the increased proliferation in limbal epithelial cells lacking ACE2. Our findings demonstrate that after corneal injury, ACE2 is downregulated, which activates limbal epithelial cell proliferation via a TGFA/EGFR/LCN2 pathway.

The potential role of renin angiotensin system in acute leukemia: a narrative review

Acute leukemias (ALs) are the most common cancers in pediatric population. There are two types of ALs: acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML). Some studies suggest that the Renin Angiotensin System (RAS) has a role in ALs. RAS signaling modulates, directly and indirectly, cellular activity in different cancers, affecting tumor cells and angiogenesis. Our review aimed to summarize the role of RAS in ALs and to explore future perspectives for the treatment of these hematological malignancies by modulating RAS molecules. The database including Pubmed, Scopus, Cochrane Library, and Scielo were searched to find articles about RAS molecules in ALL and in pediatric patients. The search terms were "RAS", "Acute Leukemia", "ALL", "Angiotensin-(1-7)", "Pediatric", "Cancer", "Angiotensin II", "AML". In the bone marrow, RAS has been found to play a key role in blood cell formation, affecting several processes including apoptosis, cell proliferation, mobilization, intracellular signaling, angiogenesis, fibrosis, and inflammation. Local tissue RAS modulates tumor growth and metastasis through autocrine and paracrine actions. RAS mainly acts via two molecules, Angiotensin II (Ang II) and Angiotensin (1-7) [Ang-(1-7)]. While Ang II promotes tumor cell growth and stimulates angiogenesis, Ang-(1-7) inhibits the proliferation of neoplastic cells and the angiogenesis, suggesting a potential therapeutic role of this molecule in ALL. The interaction between ALs and RAS reveals a complex network of molecules that can affect the hematopoiesis and the development of hematological cancers. Understanding these interactions could pave the way for innovative therapeutic approaches targeting RAS components.

The contribution of the AT1 receptor to erythropoiesis

The renin-angiotensin system (RAS) comprises a broad set of functional peptides and receptors that play a role in cardiovascular homeostasis and contribute to cardiovascular pathologies. Angiotensin II (Ang II) is the most potent peptide hormone produced by the RAS due to its high abundance and its strong and pleiotropic impact on the cardiovascular system. Formation of Ang II takes place in the bloodstream and additionally in tissues in the so-called local RAS. Of the two Ang II receptors (AT1 and AT2) that Ang II binds to, AT1 is the most expressed throughout the mammalian body. AT1 expression is not restricted to cells of the cardiovascular system but in fact AT1 protein is found in nearly all organs, hence, Ang II takes part in several modulatory physiological processes one of which is erythropoiesis. In this review, we present multiple evidence supporting that Ang II modulates physiological and pathological erythropoiesis processes trough the AT1 receptor. Cumulative evidence indicates that Ang II by three distinct mechanisms influences erythropoiesis: 1) stimulation of renal erythropoietin synthesis; 2) direct action on bone marrow precursor cells; and 3) modulation of sympathetic nerve activity to the bone marrow. The text highlights clinical and preclinical evidence focusing on mechanistic studies using rodent models.

Trained Immunity in Perivascular Adipose Tissue of Abdominal Aortic Aneurysm—A Novel Concept for a Still Elusive Disease

Abdominal aortic aneurysm (AAA) is a chronic, life-threatening vascular disease whose only therapeutic option is a surgical repair to prevent vessel rupture. The lack of medical therapy results from an inadequate understanding of the etiopathogenesis of AAA. Many studies in animal and human models indicate a ‘short-circuiting’ of the regulation of the inflammatory-immune response as a major player in the AAA chronic process. In this regard, perivascular adipose tissue (PVAT) has received increasing interest because its dysfunction affects large arteries primarily through immune cell infiltration. Consistently, we have recently produced evidence that innate and adaptive immune cells present in the PVAT of AAAs contribute to sustaining a damaging inflammatory loop. However, it is still unclear how the complex crosstalk between adaptive and innate immunity can be self-sustaining. From our perspective, trained immunity may play a role in this crosstalk. Trained immunity is defined as a form of innate immune memory resulting in enhanced responsiveness to repeated triggers. Specific innate stimuli and epigenetic and metabolic reprogramming events induce and shape trained immunity in myeloid progenitor cells improving host defense, but also contributing to the progression of immune-mediated and chronic inflammatory diseases. Here we present this hypothesis with data from the literature and our observations to support it.

Assessing the impact of blood pressure in the development of inflammatory bowel disease

The purpose of this study is to investigate the potential causal relationships between blood pressure and inflammatory bowel disease (IBD) by using the bidirectional Mendelian randomization (MR) approach. Summary‐level data for blood pressure was extracted from the hitherto largest genome‐wide study (GWAS) with 759 601 participants of European‐descent. We used 56 single nucleotide polymorphisms (SNPs) as instrumental variables (IVs) for blood pressure. Summary statistics for IBD were derived from a GWAS with an overall 59 957 participants of European ancestry, of which 109 IVs were selected. Several robust analytical methods, including inverse‐variance weighted (IVW) method, weighted‐median method, MR‐Egger regression, MR‐PRESSO test, maximum likelihood method, “leave‐one‐out” and multivariable MR analysis were used to evaluate the causal associations between blood pressure and IBD. Genetically predicted higher systolic blood pressure (SBP) was associated with an increased risk of IBD (odds ratio (OR) = 1.05, 95% confidence interval (CI):1.02–1.08, P = .001 by IVW). Subgroup analysis showed that higher SBP was positively associated with Crohn's disease (CD) (OR = 1.06, 95% CI:1.03–1.09, P = 9.18 × 10⁻⁵) and ulcerative colitis (UC) (OR = 1.05, 95% CI:1.01–1.09, P = .017) risk, respectively. In reverse‐direction MR analysis, the authors observed no evidence for the causal effect of IBD on blood pressure. Our findings suggested that high SBP was associated with an increased risk of IBD (for both UC and CD). Further studies are required to clarify the underlying mechanism of this causal association.

Bone Marrow Origin of Coronary Artery Diseases: The Impact of the Local Renin-Angiotensin System

The renin-angiotensin system (RAS) has both important systemic circulatory and local effects. The effects of local cardiac RAS on the cardiovascular system have been increasingly researched. In this study, we review the relationship between local bone marrow and local cardiac RAS and their impacts on atherosclerosis.

Immune-based therapies in cardiovascular and metabolic diseases: past, present and future

Cardiometabolic disorders were originally thought to be driven primarily by changes in lipid metabolism that cause the accumulation of lipids in organs, thereby impairing their function. Thus, in the setting of cardiovascular disease, statins - a class of lipid-lowering drugs - have remained the frontline therapy. In the past 20 years, seminal discoveries have revealed a central role of both the innate and adaptive immune system in driving cardiometabolic disorders. As such, it is now appreciated that immune-based interventions may have an important role in reducing death and disability from cardiometabolic disorders. However, to date, there have been a limited number of clinical trials exploring this interventional strategy. Nonetheless, elegant preclinical research suggests that immune-targeted therapies can have a major impact in treating cardiometabolic disease. Here, we discuss the history and recent advancements in the use of immunotherapies to treat cardiometabolic disorders.

Neutrophils in cardiovascular disease: warmongers, peacemakers, or both?

Neutrophils, the most abundant of all leucocytes and the first cells to arrive at the sites of sterile inflammation/injury act as a double-edged sword. On one hand, they inflict a significant collateral damage to the tissues and on the other hand, they help facilitate wound healing by a number of mechanisms. Recent studies have drastically changed the perception of neutrophils from being simple one-dimensional cells with an unrestrained mode of action to a cell type that display maturity and complex behaviour. It is now recognized that neutrophils are transcriptionally active and respond to plethora of signals by deploying a wide variety of cargo to influence the activity of other cells in the vicinity. Neutrophils can regulate macrophage behaviour, display innate immune memory, and play a major role in the resolution of inflammation in a context-dependent manner. In this review, we provide an update on the factors that regulate neutrophil production and the emerging dichotomous role of neutrophils in the context of cardiovascular diseases, particularly in atherosclerosis and the ensuing complications, myocardial infarction, and heart failure. Deciphering the complex behaviour of neutrophils during inflammation and resolution may provide novel insights and in turn facilitate the development of potential therapeutic strategies to manage cardiovascular disease.

An Insight on Multicentric Signaling of Angiotensin II in Cardiovascular system: A Recent Update

The multifaceted nature of the renin-angiotensin system (RAS) makes it versatile due to its involvement in pathogenesis of the cardiovascular disease. Angiotensin II (Ang II), a multifaceted member of RAS family is known to have various potential effects. The knowledge of this peptide has immensely ameliorated after meticulous research for decades. Several studies have evidenced angiotensin I receptor (AT1 R) to mediate the majority Ang II-regulated functions in the system. Functional crosstalk between AT1 R mediated signal transduction cascades and other signaling pathways has been recognized. The review will provide an up-to-date information and recent discoveries involved in Ang II receptor signal transduction and their functional significance in the cardiovascular system for potential translation in therapeutics. Moreover, the review also focuses on the role of stem cell-based therapies in the cardiovascular system.

RAAS: A Convergent Player in Ischemic Heart Failure and Cancer

The current global prevalence of heart failure is estimated at 64.34 million cases, and it is expected to increase in the coming years, especially in countries with a medium-low sociodemographic index where the prevalence of risk factors is increasing alarmingly. Heart failure is associated with many comorbidities and among them, cancer has stood out as a contributor of death in these patients. This connection points out new challenges both in the context of the pathophysiological mechanisms involved, as well as in the quality of life of affected individuals. A hallmark of heart failure is chronic activation of the renin-angiotensin-aldosterone system, especially marked by a systemic increase in levels of angiotensin-II, a peptide with pleiotropic activities. Drugs that target the renin-angiotensin-aldosterone system have shown promising results both in the prevention of secondary cardiovascular events in myocardial infarction and heart failure, including a lower risk of certain cancers in these patients, as well as in current cancer therapies; therefore, understanding the mechanisms involved in this complex relationship will provide tools for a better diagnosis and treatment and to improve the prognosis and quality of life of people suffering from these two deadly diseases.

Angiotensin II modulates the murine hematopoietic stem cell and progenitors cocultured with stromal S17 cells

Although the existence of the renin-angiotensin system (RAS) in the bone marrow is clear, the exact role of this system in hematopoiesis has not yet been fully characterized. Here the direct role of angiotensin II (AngII) in hematopoietic stem cells (HSCs), common myeloid progenitors (CMPs), granulocyte/monocyte progenitors (GMPs), and megakaryocytes/erythroid progenitors (MEPs), using a system of coculture with stromal S17 cells. Flow cytometry analysis showed that AngII increases the percentage of HSC and GMP, while reducing CMP with no effect on MEP. According to these data, AngII increased the total number of mature Gr-1⁺ /Mac-1⁺ cells without changes in Terr119⁺ cells. AngII does not induce cell death in the population of LSK cells. In these populations, treatment with AngII decreases the expression of Ki67⁺ protein with no changes in the Notch1 expression, suggesting a role for AngII on the quiescence of immature cells. In addition, exposure to AngII from murine bone marrow cells increased the number of CFU-GM and BFU-E in a clonogenic assay. In conclusion, our data showed that AngII is involved in the regulation of hematopoiesis with a special role in HSC, suggesting that AngII should be evaluated in coculture systems, especially in cases that require the expansion of these cells in vitro, still a significant challenge for therapeutic applications in humans.

Downregulation of Membrane-bound Angiotensin Converting Enzyme 2 (ACE2) Receptor has a Pivotal Role in COVID-19 Immunopathology

BACKGROUND

The Coronavirus Disease 2019 (COVID-19) is becoming the major health issue in recent human history with thousands of deaths and millions of cases worldwide. Newer research and old experience with other coronaviruses highlighted a probable underlying mechanism of disturbance of the renin-angiotensin system (RAS) that is associated with the intrinsic effects of SARS-CoV-2 infection.

OBJECTIVE

In this review, we aimed to describe the intimate connections between the RAS components, the immune system and COVID-19 pathophysiology.

METHODS

This non-systematic review article summarizes recent evidence on the relationship between COVID-19 and the RAS.

RESULTS

Several studies have indicated that the downregulation of membrane-bound ACE2 may exert a key role for the impairment of immune functions and for COVID-19 patients' outcomes. The downregulation may occur by distinct mechanisms, particularly: (1) the shedding process induced by the SARS-CoV-2 fusion pathway, which reduces the amount of membrane-bound ACE2, stimulating more shedding by the high levels of Angiotensin II; (2) the endocytosis of ACE2 receptor with the virus itself and (3) by the interferon inhibition caused by SARS-CoV-2 effects on the immune system, which leads to a reduction of ACE2 receptor expression.

CONCLUSION

Recent research provides evidence of a reduction of the components of the alternative RAS axis, including ACE2 and Angiotensin-(1-7). In contrast, increased levels of Angiotensin II can activate the AT1 receptor in several organs. Consequently, increased inflammation, thrombosis and angiogenesis occur in patients infected with SARS-COV-2. Attention should be paid to the interactions of the RAS and COVID-19, mainly in the context of novel vaccines and proposed medications.

ACE Inhibition Modulates Myeloid Hematopoiesis after Acute Myocardial Infarction and Reduces Cardiac and Vascular Inflammation in Ischemic Heart Failure

Aims: Angiotensin-converting-enzyme inhibitors (ACE inhibitors) are a cornerstone of drug therapy after myocardial infarction (MI) and improve left ventricular function and survival. We aimed to elucidate the impact of early treatment with the ACE inhibitor ramipril on the hematopoietic response after MI, as well as on the chronic systemic and vascular inflammation. Methods and Results: In a mouse model of MI, induced by permanent ligation of the left anterior descending artery, immediate initiation of treatment with ramipril (10 mg/k/d via drinking water) reduced cardiac inflammation and the number of circulating inflammatory monocytes, whereas left ventricular function was not altered significantly, respectively. This effect was accompanied by enhanced retention of hematopoietic stem cells, Lin⁻Sca1⁻c-Kit⁺CD34⁺CD16/32⁺ granulocyte–macrophage progenitors (GMP) and Lin⁻Sca1⁻c-Kit⁺CD150⁻CD48⁻ multipotent progenitors (MPP) in the bone marrow, with an upregulation of the niche factors Angiopoetin 1 and Kitl at 7 d post MI. Long-term ACE inhibition for 28 d limited vascular inflammation, particularly the infiltration of Ly6C^high monocytes/macrophages, and reduced superoxide formation, resulting in improved endothelial function in mice with ischemic heart failure. Conclusion: ACE inhibition modulates the myeloid inflammatory response after MI due to the retention of myeloid precursor cells in their bone marrow reservoir. This results in a reduction in cardiac and vascular inflammation with improvement in survival after MI.

Renin-angiotensin system is involved in embryonic emergence of hematopoietic stem/progenitor cells

Angiotensin-converting enzyme (ACE), a key element of the renin-angiotensin system (RAS), has recently been identified as a new marker of both adult and embryonic human hematopoietic stem/progenitor cells (HSPCs). However, whether a full renin-angiotensin pathway is locally present during the hematopoietic emergence is still an open question. In the present study, we show that this enzyme is expressed by hematopoietic progenitors in the developing mouse embryo. Furthermore, ACE and the other elements of RAS-namely angiotensinogen, renin, and angiotensin II type 1 (AT1) and type 2 (AT2) receptors-are expressed in the paraaortic splanchnopleura (P-Sp) and in its derivative, the aorta-gonad-mesonephros region, both in human and mouse embryos. Their localization is compatible with the existence of a local autocrine and/or paracrine RAS in these hemogenic sites. in vitro perturbation of the RAS by administration of a specific AT1 receptor antagonist inhibits almost totally the generation of blood CD45-positive cells from dissected P-Sp, implying that angiotensin II signaling is necessary for the emergence of hematopoietic cells. Conversely, addition of exogenous angiotensin II peptide stimulates hematopoiesis in culture, with an increase in the number of immature c-Kit⁺ CD41⁺ CD31⁺ CD45⁺ hematopoietic progenitors, compared to the control. These results highlight a novel role of local-RAS during embryogenesis, suggesting that angiotensin II, via activation of AT1 receptor, promotes the emergence of undifferentiated hematopoietic progenitors.

The varying roles of macrophages in kidney injury and repair

PURPOSE OF REVIEW

Macrophages play an important role in regulating homeostasis, kidney injury, repair, and tissue fibrogenesis. The present review will discuss recent advances that explore the novel subsets and functions of macrophage in the pathogenesis of kidney damage and hypertension.

RECENT FINDINGS

Macrophages differentiate into a variety of subsets in microenvironment-dependent manner. Although the M1/M2 nomenclature is still applied in considering the pro-inflammatory versus anti-inflammatory effects of macrophages in kidney injury, novel, and accurate macrophage phenotypes are defined by flow cytometric markers and single-cell RNA signatures. Studies exploring the crosstalk between macrophages and other cells are rapidly advancing with the additional recognition of exosome trafficking between cells. Using murine conditional mutants, actions of macrophage can be defined more precisely than in bone marrow transfer models. Some studies revealed the opposing effects of the same protein in renal parenchymal cells and macrophages, highlighting a need for the development of cell-specific immune therapies for translation.

SUMMARY

Macrophage-targeted therapies hold potential for limiting kidney injury and hypertension. To realize this potential, future studies will be required to understand precise mechanisms in macrophage polarization, crosstalk, proliferation, and maturation in the setting of renal disease.

Angiotensin II Induces Differentiation of Human Neuroblastoma Cells by Increasing MAP2 and ROS Levels

INTRODUCTION

The roles of angiotensin II (Ang II) in the brain are still under investigation. In this study, we investigated if Ang II influences differentiation of human neuroblastoma cells with simultaneous activation of NADPH oxidase and reactive oxygen species (ROS). Moreover, we investigated the Ang II receptor type involved during differentiation.

METHODS

Human neuroblastoma cells (SH-SY5Y; 5 × 10⁵ cells) were exposed to Ang II (600 nM) for 24 h. Differentiation was monitored by measuring MAP2 and NF-H levels. Cell size and ROS were analyzed by flow cytometry, and NADPH oxidase activation was assayed using apocynin (500 μM). Ang II receptors (ATR) activation was assayed using ATR blockers or Ang II metabolism inhibitors (10^-7 M).

RESULTS

(1) Cell size decreased significantly in Ang II-treated cells; (2) MAP2 and ROS increased significantly in Ang II-treated cells with no changes in viability; (3) MAP2 and ROS decreased significantly in cells incubated with Ang II plus apocynin. (4) A significant decrease in MAP2 was observed in cells exposed to Ang II plus PD123.319 (AT2R blocker).

CONCLUSION

Our findings suggest that Ang II influences differentiation of SH-SY5Y by increasing MAP2 through the AT2R. The increase in MAP2 and ROS were also mediated through NADPH oxidase with no cell death.

Immunity and Hypertension

Hypertension is the primary cause of cardiovascular mortality. Despite multiple existing treatments, only half of those with the disease achieve adequate control. Therefore, understanding the mechanisms causing hypertension is essential for the development of novel therapies. Many studies demonstrate that immune cell infiltration of the vessel wall, kidney and central nervous system, as well as their counterparts of oxidative stress, the renal renin-angiotensin system (RAS) and sympathetic tone play a critical role in the development of hypertension. Genetically modified mice lacking components of innate and/or adaptive immunity confirm the importance of chronic inflammation in hypertension and its complications. Depletion of immune cells improves endothelial function, decreases oxidative stress, reduces vascular tone and prevents renal interstitial infiltrates, sodium retention and kidney damage. Moreover, the ablation of microglia or central nervous system perivascular macrophages reduces RAS-induced inflammation and prevents sympathetic nervous system activation and hypertension. Therefore, understanding immune cell functioning and their interactions with tissues that regulate hypertensive responses may be the future of novel antihypertensive therapies.

Circulating stem cells and cardiovascular outcomes: from basic science to the clinic

The cardiovascular and haematopoietic systems have fundamental inter-relationships during development, as well as in health and disease of the adult organism. Although haematopoietic stem cells (HSCs) emerge from a specialized haemogenic endothelium in the embryo, persistence of haemangioblasts in adulthood is debated. Rather, the vast majority of circulating stem cells (CSCs) is composed of bone marrow-derived HSCs and the downstream haematopoietic stem/progenitors (HSPCs). A fraction of these cells, known as endothelial progenitor cells (EPCs), has endothelial specification and vascular tropism. In general, the levels of HSCs, HSPCs, and EPCs are considered indicative of the endogenous regenerative capacity of the organism as a whole and, particularly, of the cardiovascular system. In the last two decades, the research on CSCs has focused on their physiologic role in tissue/organ homoeostasis, their potential application in cell therapies, and their use as clinical biomarkers. In this review, we provide background information on the biology of CSCs and discuss in detail the clinical implications of changing CSC levels in patients with cardiovascular risk factors or established cardiovascular disease. Of particular interest is the mounting evidence available in the literature on the close relationships between reduced levels of CSCs and adverse cardiovascular outcomes in different cohorts of patients. We also discuss potential mechanisms that explain this association. Beyond CSCs' ability to participate in cardiovascular repair, levels of CSCs need to be interpreted in the context of the broader connections between haematopoiesis and cardiovascular function, including the role of clonal haematopoiesis and inflammatory myelopoiesis.

Trained Immunity and Cardiometabolic Disease: The Role of Bone Marrow

Until recently, immunologic memory was considered an exclusive characteristic of adaptive immunity. However, recent advances suggest that the innate arm of the immune system can also mount a type of nonspecific memory responses. Innate immune cells can elicit a robust response to subsequent inflammatory challenges after initial activation by certain stimuli, such as fungal-derived agents or vaccines. This type of memory, termed trained innate immunity (also named innate immune memory), is associated with epigenetic and metabolic alterations. Hematopoietic progenitor cells, which are the cells responsible for the generation of mature myeloid cells at steady-state and during inflammation, have a critical contribution to the induction of innate immune memory. Inflammation-triggered alterations in cellular metabolism, the epigenome and transcriptome of hematopoietic progenitor cells in the bone marrow promote long-lasting functional changes, resulting in increased myelopoiesis and consequent generation of trained innate immune cells. In the present brief review, we focus on the involvement of hematopoietic progenitors in the process of trained innate immunity and its possible role in cardiometabolic disease.

Protective role of ACE2 and its downregulation in SARS-CoV-2 infection leading to Macrophage Activation Syndrome: Therapeutic implications

In light of the outbreak of the 2019 novel coronavirus disease (COVID-19), the international scientific community has joined forces to develop effective treatment strategies. The Angiotensin-Converting Enzyme (ACE) 2, is an essential receptor for cell fusion and engulfs the SARS coronavirus infections. ACE2 plays an important physiological role, practically in all the organs and systems. Also, ACE2 exerts protective functions in various models of pathologies with acute and chronic inflammation. While ACE2 downregulation by SARS-CoV-2 spike protein leads to an overactivation of Angiotensin (Ang) II/AT1R axis and the deleterious effects of Ang II may explain the multiorgan dysfunction seen in patients. Specifically, the role of Ang II leading to the appearance of Macrophage Activation Syndrome (MAS) and the cytokine storm in COVID-19 is discussed below. In this review, we summarized the latest research progress in the strategies of treatments that mainly focus on reducing the Ang II-induced deleterious effects rather than attenuating the virus replication.

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Protective role of ACE2 in the organs and system

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Downregulation of ACE2 expression by SARS-CoV-2 leads to Ang II-induced organ damage.

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The appearance of MAS in COVID-19 patient

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Suggested treatment to diminish the deleterious effect of Ang II or appearance of MAS

Targeting Angiotensin-Converting Enzyme-2/Angiotensin-(1-7)/Mas Receptor Axis in the Vascular Progenitor Cells for Cardiovascular Diseases

Bone marrow-derived hematopoietic stem/progenitor cells are vasculogenic and play an important role in endothelial health and vascular homeostasis by participating in postnatal vasculogenesis. Progenitor cells are mobilized from bone marrow niches in response to remote ischemic injury and migrate to the areas of damage and stimulate revascularization largely by paracrine activation of angiogenic functions in the peri-ischemic vasculature. This innate vasoprotective mechanism is impaired in certain chronic clinical conditions, which leads to the development of cardiovascular complications. Members of the renin-angiotensin system-angiotensin-converting enzymes (ACEs) ACE and ACE2, angiotensin II (Ang II), Ang-(1-7), and receptors AT1 and Mas-are expressed in vasculogenic progenitor cells derived from humans and rodents. Ang-(1-7), generated by ACE2, is known to produce cardiovascular protective effects by acting on Mas receptor and is considered as a counter-regulatory mechanism to the detrimental effects of Ang II. Evidence has now been accumulating in support of the activation of the ACE2/Ang-(1-7)/Mas receptor pathway by pharmacologic or molecular maneuvers, which stimulates mobilization of progenitor cells from bone marrow, migration to areas of vascular damage, and revascularization of ischemic areas in pathologic conditions. This minireview summarizes recent studies that have enhanced our understanding of the physiology and pharmacology of vasoprotective axis in bone marrow-derived progenitor cells in health and disease. SIGNIFICANCE STATEMENT: Hematopoietic stem progenitor cells (HSPCs) stimulate revascularization of ischemic areas. However, the reparative potential is diminished in certain chronic clinical conditions, leading to the development of cardiovascular diseases. ACE2 and Mas receptor are key members of the alternative axis of the renin-angiotensin system and are expressed in HSPCs. Accumulating evidence points to activation of ACE2 or Mas receptor as a promising approach for restoring the reparative potential, thereby preventing the development of ischemic vascular diseases.

The haematopoietic stem cell niche: a new player in cardiovascular disease?

Haematopoiesis, the process of blood production, can be altered during the initiation or progression of many diseases. Cardiovascular disease (CVD) has been shown to be heavily influenced by changes to the haematopoietic system, including the types and abundance of immune cells produced. It is now well established that innate immune cells are increased in people with CVD, and the mechanisms contributing to this can be vastly different depending on the risk factors or comorbidities present. Many of these changes begin at the level of the haematopoietic stem and progenitor cells (HSPCs) that reside in the bone marrow (BM). In general, the HSPCs and downstream myeloid progenitors are expanded via increased proliferation in the setting of atherosclerotic CVD. However, HSPCs can also be encouraged to leave the BM and colonise extramedullary sites (i.e. the spleen). Within the BM, HSPCs reside in specialized microenvironments, often referred to as a niche. To date in depth studies assessing the damage or dysregulation that occurs in the BM niche in varying CVDs are scarce. In this review, we provide a general overview of the complex components and interactions within the BM niche and how they influence the function of HSPCs. Additionally, we discuss the main findings regarding changes in the HSPC niche that influence the progression of CVD. We hypothesize that understanding the influence of the BM niche in CVD will aid in delineating new pathways for therapeutic interventions.

Noncanonical Mechanisms for Direct Bone Marrow Generating Ang II (Angiotensin II) Predominate in CD68 Positive Myeloid Lineage Cells

Bone marrow (BM) Ang II (angiotensin II) is a major participant in the regulation of hematopoiesis and immunity. The novel tissue substrate Ang-(1-12) [angiotensin-(1-12)] and its cleaving enzyme chymase are an essential source of Ang II production in cardiac tissue. We hypothesized this noncanonical chymase-mediated Ang II-producing mechanism exists in the BM tissue. Immunohistostaining and flow cytometry confirmed the presence of Ang-(1-12) immunoreaction in the BM of SD (Sprague Dawley) rats. Chymase-mediated Ang II-producing activity in BM was ≈1000-fold higher than ACE (angiotensin-converting enzyme)-mediated Ang II-producing activity (4531±137 and 4.2±0.3 fmol/min per mg, respectively; n=6; P<0.001) and 280-fold higher than chymase activity in the left ventricle of 16.3±1.7 fmol/min per mg (P<0.001). Adding a selective chymase inhibitor, TEI-F00806, eliminated almost all 125I-Ang II production. Flow cytometry demonstrated that delta median fluorescence intensity of chymase in cluster of differentiation 68 positive cells was significantly higher than that in cluster of differentiation 68 negative cells (1546±157 and 222±48 arbitrary units, respectively; P=0.0021). Cluster of differentiation 68 positive and side scatter low subsets, considered to be myeloid progenitors, express the highest chymase fluorescence intensity in rat BM. Chymase activity and cellular expression was similar in both male and female rats. In conclusion, myeloid lineage cells, especially myeloid progenitors, have an extraordinary Ang II-producing activity by chymase in the BM.

Gap Junctions in the Bone Marrow Lympho-Hematopoietic Stem Cell Niche, Leukemia Progression, and Chemoresistance

Abstract: The crosstalk between hematopoietic stem cells (HSC) and bone marrow (BM) microenvironment is critical for homeostasis and hematopoietic regeneration in response to blood formation emergencies after injury, and has been associated with leukemia transformation and progression. Intercellular signals by the BM stromal cells in the form of cell-bound or secreted factors, or by physical interaction, regulate HSC localization, maintenance, and differentiation within increasingly defined BM HSC niches. Gap junctions (GJ) are comprised of arrays of membrane embedded channels formed by connexin proteins, and control crucial signaling functions, including the transfer of ions, small metabolites, and organelles to adjacent cells which affect intracellular mechanisms of signaling and autophagy. This review will discuss the role of GJ in both normal and leukemic hematopoiesis, and highlight some of the most novel approaches that may improve the efficacy of cytotoxic drugs. Connexin GJ channels exert both cell-intrinsic and cell-extrinsic effects on HSC and BM stromal cells, involved in regenerative hematopoiesis after myelosuppression, and represent an alternative system of cell communication through a combination of electrical and metabolic coupling as well as organelle transfer in the HSC niche. GJ intercellular communication (GJIC) in the HSC niche improves cellular bioenergetics, and rejuvenates damaged recipient cells. Unfortunately, they can also support leukemia proliferation and survival by creating leukemic niches that provide GJIC dependent energy sources and facilitate chemoresistance and relapse. The emergence of new strategies to disrupt self-reinforcing malignant niches and intercellular organelle exchange in leukemic niches, while at the same time conserving normal hematopoietic GJIC function, could synergize the effect of chemotherapy drugs in eradicating minimal residual disease. An improved understanding of the molecular basis of connexin regulation in normal and leukemic hematopoiesis is warranted for the re-establishment of normal hematopoiesis after chemotherapy.

Microglial Cells Impact Gut Microbiota and Gut Pathology in Angiotensin II-Induced Hypertension

RATIONALE

Increased microglial activation and neuroinflammation within autonomic brain regions have been implicated in sustained hypertension, and their inhibition by minocycline-an anti-inflammatory antibiotic-produces beneficial effects. These observations led us to propose a dysfunctional brain-gut communication hypothesis for hypertension. However, it has been difficult to reconcile whether an anti-inflammatory or antimicrobial action is the primary beneficial effect of minocycline in hypertension. Accordingly, we utilized chemically modified tetracycline-3 (CMT-3)-a derivative of tetracycline that has potent anti-inflammatory activity-to address this question.

OBJECTIVE

Test the hypothesis that central administration of CMT-3 would inhibit microglial activation, attenuate neuroinflammation, alter selective gut microbial communities, protect the gut wall from developing hypertension-associated pathology, and attenuate hypertension.

METHODS AND RESULTS

Rats were implanted with radiotelemetry devices for recording mean arterial pressure. Ang II (angiotensin II) was infused subcutaneously using osmotic mini-pumps to induce hypertension. Another osmotic mini-pump was surgically implanted to infuse CMT-3 intracerebroventricularly. Intracerebroventricular CMT- 3 infusion was also investigated in SHR (spontaneously hypertensive rats). Physiological, pathological, immunohistological parameters, and fecal microbiota were analyzed. Intracerebroventricular CMT-3 significantly inhibited Ang II-induced increases in number of microglia, their activation, and proinflammatory cytokines in the paraventricular nucleus of hypothalamus. Further, intracerebroventricular CMT-3 attenuated increased mean arterial pressure, normalized sympathetic activity, and left ventricular hypertrophy in Ang II rats, as well as in the SHR. Finally, CMT-3 beneficially restored certain gut microbial communities altered by Ang II and attenuated pathological alterations in gut wall.

CONCLUSIONS

These observations demonstrate that inhibition of microglial activation alone was sufficient to induce significant antihypertensive effects. This was associated with unique changes in gut microbial communities and profound attenuation of gut pathology. They suggest, for the first time, a link between microglia and certain microbial communities that may have implications for treatment of hypertension.

IL-27 receptor-regulated stress myelopoiesis drives abdominal aortic aneurysm development

Abdominal aortic aneurysm (AAA) is a prevalent life-threatening disease, where aortic wall degradation is mediated by accumulated immune cells. Although cytokines regulate inflammation within the aorta, their contribution to AAA via distant alterations, particularly in the control of hematopoietic stem cell (HSC) differentiation, remains poorly defined. Here we report a pathogenic role for the interleukin-27 receptor (IL-27R) in AAA, as genetic ablation of IL-27R protects mice from the disease development. Mitigation of AAA is associated with a blunted accumulation of myeloid cells in the aorta due to the attenuation of Angiotensin II (Ang II)-induced HSC expansion. IL-27R signaling is required to induce transcriptional programming to overcome HSC quiescence and increase differentiation and output of mature myeloid cells in response to stress stimuli to promote their accumulation in the diseased aorta. Overall, our studies illuminate how a prominent vascular disease can be distantly driven by a cytokine-dependent regulation of bone marrow precursors.

Microglia, autonomic nervous system, immunity and hypertension: Is there a link?

Hypertension ranks the most common risk factor for cardiovascular diseases, and it affects almost one third of adult population globally. Emerging evidence indicates that immune activation is highly involved in the entire progress of hypertension and end organ damage. In addition to immunity, autonomic nervous system, particularly sympathetic nervous system, is one of the most conserved systems to maintain body homeostasis. Immune and sympathetic activities are found simultaneously increased in hypertension, suggesting a synergistic action of these two systems in the progression of this disease. Microglia, the primary immune cells in the central nervous system, have been suggested in the regulation of sympathetic outflow; depletion of microglia alters neuroinflammation and pressor responses in hypertensive models. In this review, we firstly updated the current understanding on microglial ontogeny and functions in both steady state and diseases. Then we reviewed on the interaction between autonomic nervous system and peripheral immunity in hypertension. Microglia bridge the central and peripheral inflammation via regulating the sympathetic nerve activity in hypertension. Future exploration of the molecular linkage of this pathway may provide novel therapeutic angel for hypertension and related cardiovascular diseases.

The gut microbiota and the brain-gut-kidney axis in hypertension and chronic kidney disease

Crosstalk between the gut microbiota and the host has attracted considerable attention owing to its involvement in diverse diseases. Chronic kidney disease (CKD) is commonly associated with hypertension and is characterized by immune dysregulation, metabolic disorder and sympathetic activation, which are all linked to gut dysbiosis and altered host-microbiota crosstalk. In this Review, we discuss the complex interplay between the brain, the gut, the microbiota and the kidney in CKD and hypertension and explain our brain-gut-kidney axis hypothesis for the pathogenesis of these diseases. Consideration of the role of the brain-gut-kidney axis in the maintenance of normal homeostasis and of dysregulation of this axis in CKD and hypertension could lead to the identification of novel therapeutic targets. In addition, the discovery of unique microbial communities and their associated metabolites and the elucidation of brain-gut-kidney signalling are likely to fill fundamental knowledge gaps leading to innovative research, clinical trials and treatments for CKD and hypertension.

Hypertension as a Metabolic Disorder and the Novel Role of the Gut

Purpose of Review

Hypertension is related to impaired metabolic homeostasis and can be regarded as a metabolic disorder. This review presents possible mechanisms by which metabolic disorders increase blood pressure (BP) and discusses the importance of the gut as a novel modulator of BP.

Recent Findings

Obesity and high salt intake are major risk factors for hypertension. There is a hypothesis of “salt-induced obesity”; i.e., high salt intake may tie to obesity. Heightened sympathetic nervous system (SNS) activity, especially in the kidney and brain, increases BP in obese patients. Adipokines, including adiponectin and leptin, and renin-angiotensin-aldosterone system (RAAS) contribute to hypertension. Adiponectin induced by a high-salt diet may decrease sodium/glucose cotransporter (SGLT) 2 expression in the kidney, which results in reducing BP. High salt can change secretions of adipokines and RAAS-related components. Evidence has been accumulating linking the gastrointestinal tract to BP. Glucagon-like peptide-1 (GLP-1) and ghrelin decrease BP in both rodents and humans. The sweet taste receptor in enteroendocrine cells increases SGLT1 expression and stimulates sodium/glucose absorption. Roux-en-Y gastric bypass improves glycemic and BP control due to reducing the activity of SGLT1. Na/H exchanger isoform 3 (NHE3) increases BP by stimulating the intestinal absorption of sodium. Gastrin functions as an intestinal sodium taste sensor and inhibits NHE3 activity. Intestinal mineralocorticoid receptors also regulate sodium absorption and BP due to changing ENaC activity. Gastric sensing of sodium induces natriuresis, and gastric distension increases BP. Changes in the composition and function of gut microbiota contribute to hypertension. A high-salt/fat diet may disrupt the gut barrier, which results in systemic inflammation, insulin resistance, and increased BP. Gut microbiota regulates BP by secreting vasoactive hormones and short-chain fatty acids. BP-lowering effects of probiotics and antibiotics have been reported. Bariatric surgery improves metabolic disorders and hypertension due to increasing GLP-1 secretion, decreasing leptin secretion and SNS activity, and changing gut microbiome composition. Strategies targeting the gastrointestinal system may be therapeutic options for improving metabolic abnormalities and reducing BP in humans.

Summary

SNS, brain, adipocytes, RAAS, the kidney, the gastrointestinal tract, and microbiota play important roles in regulating BP. Most notably, the gut could be a novel target for treatment of hypertension as a metabolic disorder.

The bidirectional interaction between the sympathetic nervous system and immune mechanisms in the pathogenesis of hypertension

Over the last few years, evidence has accumulated to suggest that hypertension is, at least in part, an immune-mediated inflammatory disorder. Many links between immunity and hypertension have been established and provide a complex framework of mechanistic interactions contributing to the rise in BP. These include immune-mediated inflammatory processes affecting regulatory brain nuclei and interactions with other mediators of cardiovascular regulation such as the sympathetic nervous system. Sympathoexcitation differentially regulates T-cells based upon activation status of the immune cell as well as the resident organ. Exogenous and endogenous triggers activate signalling pathways in innate and adaptive immune cells resulting in pro-inflammatory cytokine production and activation of T-lymphocytes in the cardiovascular and renal regions, now considered major factors in the development of essential hypertension. The inflammatory cascade is sustained and exacerbated by the immune flow via the brain-bone marrow-spleen-gastrointestinal axis and thereby further aggravating immune-mediated pathways resulting in a vicious cycle of established hypertension and target organ damage. This review summarizes the evidence and recent advances in linking immune-mediated inflammation, sympathetic activation and their bidirectional interactions with the development of hypertension. LINKED ARTICLES: This article is part of a themed section on Immune Targets in Hypertension. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.12/issuetoc.

Delayed Captopril Administration Mitigates Hematopoietic Injury in a Murine Model of Total Body Irradiation

The increasing potential for accidental radiation exposure from either nuclear accidents or terrorist activities has escalated the need for radiation countermeasure development. We previously showed that a 30-day course of high-dose captopril, an ACE inhibitor, initiated 1–4 h after total body irradiation (TBI), improved Hematopoietic Acute Radiation Syndrome (H-ARS) and increased survival in mice. However, because of the time likely required for the deployment of a stockpiled radiation countermeasure to a radiation mass casualty site, there is a need for therapies that can be administered 24–48 hours after initial exposure. Using C57BL/6 mice exposed to an LD_50-80/30 of ⁶⁰Co TBI (7.75–7.9 Gy, 0.615 Gy/min), we show that low-dose captopril administration, initiated as late as 48 h post-TBI and continued for 14 days, significantly enhanced overall survival similarly to high-dose, rapid administration. Captopril treatment did not affect radiation-induced cell cycle arrest genes or the immediate loss of hematopoietic precursors. Reduced mortality was associated with the recovery of bone marrow cellularity and mature blood cell recovery at 21–30 days post-irradiation. Captopril reduced radiation-induced cytokines EPO, G-CSF, and SAA in the plasma. Finally, delayed captopril administration mitigated brain micro-hemorrhage at 21 days post-irradiation. These data indicate that low dose captopril administered as late as 48 h post-TBI for only two weeks improves survival that is associated with hematopoietic recovery and reduced inflammatory response. These data suggest that captopril may be an ideal countermeasure to mitigate H-ARS following accidental radiation exposure.

Angiotensin II Signal Transduction: An Update on Mechanisms of Physiology and Pathophysiology

The renin-angiotensin-aldosterone system plays crucial roles in cardiovascular physiology and pathophysiology. However, many of the signaling mechanisms have been unclear. The angiotensin II (ANG II) type 1 receptor (AT₁R) is believed to mediate most functions of ANG II in the system. AT₁R utilizes various signal transduction cascades causing hypertension, cardiovascular remodeling, and end organ damage. Moreover, functional cross-talk between AT₁R signaling pathways and other signaling pathways have been recognized. Accumulating evidence reveals the complexity of ANG II signal transduction in pathophysiology of the vasculature, heart, kidney, and brain, as well as several pathophysiological features, including inflammation, metabolic dysfunction, and aging. In this review, we provide a comprehensive update of the ANG II receptor signaling events and their functional significances for potential translation into therapeutic strategies. AT₁R remains central to the system in mediating physiological and pathophysiological functions of ANG II, and participation of specific signaling pathways becomes much clearer. There are still certain limitations and many controversies, and several noteworthy new concepts require further support. However, it is expected that rigorous translational research of the ANG II signaling pathways including those in large animals and humans will contribute to establishing effective new therapies against various diseases.

Effect of irradiation and bone marrow transplantation on angiotensin II-induced aortic inflammation in ApoE knockout mice

BACKGROUND AND AIMS

Angiotensin II (Ang II) infusion promotes the development of aortic aneurysms and accelerates atherosclerosis in ApoE^-/- mice. In order to elucidate the role of hematopoietic cells in these pathologies, irradiation and bone marrow transplantation (BMT) are commonly utilized. The aim of this study was to investigate the effects of irradiation and BMT on abdominal and thoracic aortic aneurysm formation and acute leukocyte recruitment in the aortic root and descending aorta, in an experimental mouse model of aortic aneurysm formation.

METHODS

ApoE^-/- mice were either lethally irradiated and reconstituted with ApoE^-/- bone marrow or non-irradiated. Following engraftment, mice were treated with Ang II to induce aortic inflammation and accelerate atherosclerosis.

RESULTS

Ang II infusion (0.8 mg/kg/day) in BMT mice resulted in reduced aortic aneurysms and atherosclerosis with decreased leukocyte infiltration in the aorta compared to non-BMT mice, when receiving the same dose of Ang II. Furthermore, the reduced aortic infiltration in BMT mice was accompanied by increased levels of monocytes in the spleen and bone marrow. A dose of 3 mg/kg/day Ang II was required to achieve a similar incidence of aneurysm formation as achieved with 0.8 mg/kg/day in non-BMT mice.

CONCLUSIONS

This study provides evidence that BMT can alter inflammatory cell recruitment in experimental mouse models of aortic aneurysm formation and atherosclerosis and suggests that irradiation and BMT have a considerably more complex effect on vascular inflammation, which should be evaluated.

Visceral Adipose Tissue Accumulation and Residual Cardiovascular Risk

PURPOSE OF THE REVIEW

Low-grade systemic inflammation increases residual cardiovascular risk. The pathogenesis of low-grade systemic inflammation is not well understood.

RECENT FINDINGS

Visceral adipose tissue accumulates when the subcutaneous adipose tissue can no longer store excess nutrients. Visceral adipose tissue inflammation initially facilitates storage of nutrients but with time become maladaptive and responsible for low-grade systemic inflammation. Control of low-grade systemic inflammation requires reversal of visceral adipose tissue accumulation with intense and sustained aerobic exercise or bariatric surgery. Alternatively, pharmacologic inhibition of the inflammatory signaling pathway may be considered. Reversal visceral adipose tissue accumulation lowers residual cardiovascular risk.

Salt, Hypertension, and Immunity

The link between inappropriate salt retention in the kidney and hypertension is well recognized. However, growing evidence suggests that the immune system can play surprising roles in sodium homeostasis, such that the study of inflammatory cells and their secreted effectors has provided important insights into salt sensitivity. As part of the innate immune system, myeloid cells have diverse roles in blood pressure regulation, ranging from prohypertensive actions in the kidney, vasculature, and brain, to effects in the skin that attenuate blood pressure elevation. In parallel, T lymphocyte subsets, as key constituents of the adaptive immune compartment, have variable effects on renal sodium handling and the hypertensive response, accruing from the functions of the cytokines that they produce. Conversely, salt can directly modulate the phenotypes of myeloid and T cells, illustrating bidirectional regulatory mechanisms through which sodium and the immune system coordinately impact blood pressure. This review details the complex interplay between myeloid cells, T cells, and salt in the pathogenesis of essential hypertension.

A Single Angiotensin II Hypertensive Stimulus Is Associated with Prolonged Neuronal and Immune System Activation in Wistar-Kyoto Rats

Activation of autonomic neural pathways by chronic hypertensive stimuli plays a significant role in pathogenesis of hypertension. Here, we proposed that even a single acute hypertensive stimulus will activate neural and immune pathways that may be important in initiation of memory imprinting seen in chronic hypertension. We investigated the effects of acute angiotensin II (Ang II) administration on blood pressure, neural activation in cardioregulatory brain regions, and central and systemic immune responses, at 1 and 24 h post-injection. Administration of a single bolus intra-peritoneal (I.P.) injection of Ang II (36 μg/kg) resulted in a transient increase in the mean arterial pressure (MAP) (by 22 ± 4 mmHg vs saline), which returned to baseline within 1 h. However, in contrast to MAP, neuronal activity, as measured by manganese-enhanced magnetic resonance (MEMRI), remained elevated in several cardioregulatory brain regions over 24 h. The increase was predominant in autonomic regions, such as the subfornical organ (SFO; ~20%), paraventricular nucleus of the hypothalamus (PVN; ~20%) and rostral ventrolateral medulla (RVLM; ~900%), among others. Similarly, systemic and central immune responses, as evidenced by circulating levels of CD4⁺/IL17⁺ T cells, and increased IL17 levels and activation of microglia in the PVN, respectively, remained elevated at 24 h following Ang II challenge. Elevated Fos expression in the PVN was also present at 24 h (by 73 ± 11%) following Ang II compared to control saline injections, confirming persistent activation of PVN. Thus, even a single Ang II hypertensive stimulus will initiate changes in neuronal and immune cells that play a role in the developing hypertensive phenotype.

The Gut, Its Microbiome, and Hypertension

PURPOSE OF THE REVIEW

Evidence is rapidly accumulating implicating gut dysbiosis in hypertension (HTN). However, we are far from understanding whether this is a cause or consequence of HTN, and how to best translate this fundamental knowledge to advance the management of HTN. This review aims to summarize recent advances in the field, illustrate the connections between the gut and hypertension, and establish that the gut microbiota (GM)-gut interaction is centrally positioned for consideration as an innovative approach for HTN therapeutics.

RECENT FINDINGS

Animal models of HTN have shown that gut pathology occurs in HTN, and provides some clues to mechanisms linking the dysbiosis, gut pathology, and HTN. Circumstantial evidence links gut dysbiosis and HTN. Gut pathology, apparent in animal HTN models, has not been fully investigated in hypertensive patients. Objective evidence and an understanding of mechanisms could have a major impact for new antihypertensive therapies and/or improved applications of current ones.

The role of macrophages in hypertension and its complications

Circulating monocytes and tissue macrophages play complex roles in the pathogenesis of hypertension, a highly prevalent disease associated with catastrophic cardiovascular morbidity. In the vasculature and kidney, macrophage-derived reactive oxygen species (ROS) and inflammatory cytokines induce endothelial and epithelial dysfunction, respectively, resulting in vascular oxidative stress and impairment of sodium excretion. By contrast, VEGF-C-expressing macrophages in the skin can facilitate the removal of excess interstitial stores of sodium by stimulating lymphangiogenesis. Inappropriate activation of the renin-angiotensin system (RAS) contributes to essential hypertension in a majority of patients, and macrophages express the type 1 (AT₁) receptor for angiotensin II (Ang II). While proinflammatory macrophages clearly contribute to RAS-dependent hypertension, activation of the AT₁ receptor directly on macrophages suppresses their M1 polarization and limits tubular and interstitial damage to the kidney during hypertension. Thus, stimulating the macrophage AT₁ receptor ameliorates the target organ damage and immune stimulation provoked by AT₁ receptor activation in intrinsic renal and vascular cells. The proinflammatory cytokines TNF-α and IL-1β produced by M1 macrophages drive blood pressure elevation and consequent target organ damage. However, additional studies are needed to identify the tissues in which these cytokines act and the signaling pathways they stimulate during hypertension. Moreover, identifying the precise myeloid cell subsets that contribute to hypertension should guide the development of more precise immunomodulatory therapies for patients with persistent blood pressure elevation and progressive end-organ injury.

Extended time-lapse in vivo imaging of tibia bone marrow to visualize dynamic hematopoietic stem cell engraftment

Homing, engraftment and proliferation of hematopoietic stem/progenitor cell (HSC/HPCs) are crucial steps required for success of a bone marrow transplant. Observation of these critical events is limited by the opaque nature of bone. Here we demonstrate how individual HSCs engraft in long bones by thinning one side of the tibia for direct and unbiased observation. Intravital imaging enabled detailed visualization of single Sca-1⁺, c-Kit⁺, Lineage⁻ (SKL) cell migration to bone marrow niches and subsequent proliferation to reconstitute hematopoiesis. This longitudinal study allowed direct observation of dynamic HSC/HPC activities during engraftment in full color for up to six days in live recipients. Individual SKL cells, but not mature or committed progenitor cells, preferentially homed to a limited number of niches near highly vascularized endosteal regions, and clonally expanded. Engraftment of SKL cells in P-selectin and osteopontin knockout mice showed abnormal homing and expansion of SKL cells. CD150⁺, CD48⁻ SKL populations initially engrafted in the central marrow region, utilizing only a subset of niches occupied by the parent SKL cells. Our study demonstrates that time-lapse imaging of tibia can be a valuable tool to understand the dynamic characteristics of functional HSC and niche components in various mouse models.

Hypertension-Linked Pathophysiological Alterations in the Gut

RATIONALE

Sympathetic nervous system control of inflammation plays a central role in hypertension. The gut receives significant sympathetic innervation, is densely populated with a diverse microbial ecosystem, and contains immune cells that greatly impact overall inflammatory homeostasis. Despite this uniqueness, little is known about the involvement of the gut in hypertension.

OBJECTIVE

Test the hypothesis that increased sympathetic drive to the gut is associated with increased gut wall permeability, increased inflammatory status, and microbial dysbiosis and that these gut pathological changes are linked to hypertension.

METHODS AND RESULTS

Gut epithelial integrity and wall pathology were examined in spontaneously hypertensive rat and chronic angiotensin II infusion rat models. The increase in blood pressure in spontaneously hypertensive rat was associated with gut pathology that included increased intestinal permeability and decreased tight junction proteins. These changes in gut pathology in hypertension were associated with alterations in microbial communities relevant in blood pressure control. We also observed enhanced gut-neuronal communication in hypertension originating from paraventricular nucleus of the hypothalamus and presenting as increased sympathetic drive to the gut. Finally, angiotensin-converting enzyme inhibition (captopril) normalized blood pressure and was associated with reversal of gut pathology.

CONCLUSIONS

A dysfunctional sympathetic-gut communication is associated with gut pathology, dysbiosis, and inflammation and plays a key role in hypertension. Thus, targeting of gut microbiota by innovative probiotics, antibiotics, and fecal transplant, in combination with the current pharmacotherapy, may be a novel strategy for hypertension treatment.

Brain-Gut-Bone Marrow Axis: Implications for Hypertension and Related Therapeutics

Hypertension is the most prevalent modifiable risk factor for cardiovascular disease and disorders directly influencing cardiovascular disease morbidity and mortality, such as diabetes mellitus, chronic kidney disease, obstructive sleep apnea, etc. Despite aggressive attempts to influence lifestyle modifications and advances in pharmacotherapeutics, a large percentage of patients still do not achieve recommended blood pressure control worldwide. Thus, we think that mechanism-based novel strategies should be considered to significantly improve control and management of hypertension. The overall objective of this review is to summarize implications of peripheral- and neuroinflammation as well as the autonomic nervous system-bone marrow communication in hematopoietic cell homeostasis and their impact on hypertension pathophysiology. In addition, we discuss the novel and emerging field of intestinal microbiota and roles of gut permeability and dysbiosis in cardiovascular disease and hypertension. Finally, we propose a brain-gut-bone marrow triangular interaction hypothesis and discuss its potential in the development of novel therapies for hypertension.

Novel Pathophysiological Mechanisms in Hypertension

Hypertension is the most common disease affecting humans and imparts a significant cardiovascular and renal risk to patients. Extensive research over the past few decades has enhanced our understanding of the underlying mechanisms in hypertension. However, in most instances, the cause of hypertension in a given patient continues to remain elusive. Nevertheless, achieving aggressive blood pressure goals significantly reduces cardiovascular morbidity and mortality, as demonstrated in the recently concluded SPRINT trial. Since a large proportion of patients still fail to achieve blood pressure goals, knowledge of novel pathophysiologic mechanisms and mechanism based treatment strategies is crucial. The following chapter will review the novel pathophysiological mechanisms in hypertension, with a focus on role of immunity, inflammation and vascular endothelial homeostasis. The therapeutic implications of these mechanisms will be discussed where applicable.