Apelin signaling drives vascular endothelial cells toward a pro-angiogenic state

Apelin Alleviates Meniscus Endothelial Cell Apoptosis in Osteoarthritis

Osteoarthritis (OA) is a degenerative disease characterized by articular cartilage and/or chondrocyte destruction, and although it has long been considered as a primary disease, the importance of meniscus endothelial cell modulation in the subchondral microenvironment has recently drawn attention. Previous studies have shown that apelin could potentially inhibit cellular apoptosis; however, it remains unclear whether apelin could play a protective role in protecting the endothelium in the OA meniscus. In this study, with the advantages of single-cell RNA sequencing (scRNA-seq) data, in combination with flow cytometry, we identified two endothelial subclusters in the meniscus, featured by high expression of Homeobox A13 (HOXA13) and Ras Protein-Specific Guanine Nucleotide Releasing Factor 2 (RASGRF2), respectively. Compared with control patients, both subclusters decreased in absolute cell numbers and exhibited downregulated APJ endogenous ligand (APLN, coding for apelin) and upregulated apelin receptor (APLNR, coding apelin receptor). Furthermore, we confirmed that in OA, decreased endothelial cell numbers, including both subclusters, were related to intrinsic apoptosis factors: one more relevant to caspase 3 (CASP3) and the other to BH3-Interacting Domain Death agonist (BID). In vitro culturing of meniscal endothelial cells purified from patients proved that apelin could significantly inhibit apoptosis by downregulating these two factors in endothelial cell subclusters, suggesting that apelin could potentially serve as a therapeutic target for patients with OA.

Size-Reduced Macrocyclic Analogues of [Pyr1]-apelin-13 Showing Negative Gα12 Bias Still Produce Prolonged Cardiac Effects

We previously reported a series of macrocyclic analogues of [Pyr¹]-apelin-13 (Ape13) with increased plasma stability and potent APJ agonist properties. Based on the most promising compound in this series, we synthesized and then evaluated novel macrocyclic compounds of Ape13 to identify agonists with specific pharmacological profiles. These efforts led to the development of analogues 39 and 40, which possess reduced molecular weight (MW 1020 Da vs Ape13, 1534 Da). Interestingly, compound 39 (K_i 0.6 nM), which does not activate the Gα₁₂ signaling pathway while maintaining potency and efficacy similar to Ape13 to activate Gα_i1 (EC₅₀ 0.8 nM) and β-arrestin2 recruitment (EC₅₀ 31 nM), still exerts cardiac actions. In addition, analogue 40 (K_i 5.6 nM), exhibiting a favorable Gα₁₂-biased signaling and an increased in vivo half-life (t_1/2 3.7 h vs <1 min of Ape13), produces a sustained cardiac response up to 6 h after a single subcutaneous bolus injection.

Apelin pathway in cardiovascular, kidney, and metabolic diseases: Therapeutic role of apelin analogs and apelin receptor agonists

The apelin/apelin receptor (ApelinR) signal transduction pathway exerts essential biological roles, particularly in the cardiovascular system. Disturbances in the apelin/ApelinR axis are linked to vascular, heart, kidney, and metabolic disorders. Therefore, the apelinergic system has surfaced as a critical therapeutic strategy for cardiovascular diseases (including pulmonary arterial hypertension), kidney disease, insulin resistance, hyponatremia, preeclampsia, and erectile dysfunction. However, apelin peptides are susceptible to rapid degradation through endogenous peptidases, limiting their use as therapeutic tools and translational potential. These proteases include angiotensin converting enzyme 2, neutral endopeptidase, and kallikrein thereby linking the apelin pathway with other peptide systems. In this context, apelin analogs with enhanced proteolytic stability and synthetic ApelinR agonists emerged as promising pharmacological alternatives. In this review, we focus on discussing the putative roles of the apelin pathway in various physiological systems from function to dysfunction, and emphasizing the therapeutic potential of newly generated metabolically stable apelin analogs and non-peptide ApelinR agonists.

Andrographolide, a novel inducer of apelin gene expression

ETHNOPHARMACOLOGICAL RELEVANCE

Andrographis paniculata (A. paniculata) has been used as a traditional medicine in Asia and Scandinavia for centuries to remedy several illnesses. It has since been shown to possess antibacterial, antifungal, antiviral, anti-neoplasm, hepatoprotective, hypoglycemic, hypocholesterolemic, and energetic effects.

AIMS OF THE STUDY

This study sought to investigate the effect of Andrographolide on apelin gene expression and serum levels of glucose.

MATERIALS AND METHODS

In this study, 18 male rats were used. They were divided into three groups of six, including i) negative control group, ii) 3.5 mg/kg Andrographolide group, and iii) 7 mg/kg Andrographolide group. Apelin gene expression was investigated by real-time PCR method. Serum levels of glucose were measured by the photometric method.

RESULTS

The results of this study revealed that 3.5 and 7 mg doses per kg of body weight of andrographolide, for six days, significantly increased hepatic expression of apelin gene in male Wistar rats, as compared with the control group (p < 0.05). Serum levels of glucose at doses of 3.5 and 7 mg/kg of andrographolide, and in the control group, were 71.5 ± 8.96, 51.5 ± 2.64, and 93.87 ± 14.27 mg/dl, respectively. Andrographolide induced a decrease in serum levels of HDL-c and an increase in LDL-c/HDL-c ratio.

CONCLUSIONS

Our results suggest that Andrographolide can elicit an increase of hepatic apelin gene expression and a decrease in serum levels of blood glucose.

Imbalanced Activation of Wnt-/β-Catenin-Signaling in Liver Endothelium Alters Normal Sinusoidal Differentiation

Endothelial wingless-related integration site (Wnt)-/β-catenin signaling is a key regulator of the tightly sealed blood–brain barrier. In the hepatic vascular niche angiokine-mediated Wnt signaling was recently identified as an important regulator of hepatocyte function, including the determination of final adult liver size, liver regeneration, and metabolic liver zonation. Within the hepatic vasculature, the liver sinusoidal endothelial cells (LSECs) are morphologically unique and functionally specialized microvascular endothelial cells (ECs). Pathological changes of LSECs are involved in chronic liver diseases, hepatocarcinogenesis, and liver metastasis. To comprehensively analyze the effects of endothelial Wnt-/β-catenin signaling in the liver, we used endothelial subtype-specific Clec4g-iCre mice to generate hepatic ECs with overexpression of Ctnnb1. In the resultant Clec4g-iCre^tg/wt;Ctnnb1(Ex3)^fl/wt (Ctnnb1^OE−EC) mice, activation of endothelial Wnt-/β-catenin signaling resulted in sinusoidal transdifferentiation with disturbed endothelial zonation, that is, loss of midzonal LSEC marker lymphatic vessel endothelial hyaluronic acid receptor 1 (Lyve1) and enrichment of continuous EC genes, such as cluster of differentiation (CD)34 and Apln. Notably, gene set enrichment analysis revealed overrepresentation of brain endothelial transcripts. Activation of endothelial Wnt-/β-catenin signaling did not induce liver fibrosis or alter metabolic liver zonation, but Ctnnb1^OE−EC mice exhibited significantly increased plasma triglyceride concentrations, while liver lipid content was slightly reduced. Ctnnb1 overexpression in arterial ECs of the heart has been reported previously to cause cardiomyopathy. As Clec4g-iCre is active in a subset of cardiac ECs, it was not unexpected that Ctnnb1^OE−EC mice showed reduced overall survival and cardiac dysfunction. Altogether, balanced endothelial Wnt-/β-catenin signaling in the liver is required for normal LSEC differentiation and for maintenance of normal plasma triglyceride levels.

Molecular and Cellular Mechanisms of Vascular Development in Zebrafish

The establishment of a functional cardiovascular system is crucial for the development of all vertebrates. Defects in the development of the cardiovascular system lead to cardiovascular diseases, which are among the top 10 causes of death worldwide. However, we are just beginning to understand which signaling pathways guide blood vessel growth in different tissues and organs. The advantages of the model organism zebrafish (Danio rerio) helped to identify novel cellular and molecular mechanisms of vascular growth. In this review we will discuss the current knowledge of vasculogenesis and angiogenesis in the zebrafish embryo. In particular, we describe the molecular mechanisms that contribute to the formation of blood vessels in different vascular beds within the embryo.

Multimerin-2 orchestrates the cross-talk between endothelial cells and pericytes: A mechanism to maintain vascular stability

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The ECM Multimerin-2 is a substrate for pericyte adhesion.

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The recruitment of pericytes leads to enhanced Multimerin-2 expression by endothelial cells.

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Multimerin-2 induces the expression of important cytokines both in endothelial cells and pericytes.

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The deposition of Multimerin-2 is key for the endothelial cell/pericyte crosstalk required for the establishment of vascular stability.

Tumor angiogenesis is vital for the growth and development of various solid cancers and as such is a valid and promising therapeutic target. Unfortunately, the use of the currently available anti-angiogenic drugs increases the progression-free survival by only a few months. Conversely, targeting angiogenesis to prompt both vessel reduction and normalization, has been recently viewed as a promising approach to improve therapeutic efficacy. As a double-edged sword, this line of attack may on one side halt tumor growth as a consequence of the reduction of nutrients and oxygen supplied to the tumor cells, and on the other side improve drug delivery and, hence, efficacy. Thus, it is of upmost importance to better characterize the mechanisms regulating vascular stability. In this context, recruitment of pericytes along the blood vessels is crucial to their maturation and stabilization. As the extracellular matrix molecule Multimerin-2 is secreted by endothelial cells and deposited also in juxtaposition between endothelial cells and pericytes, we explored Multimerin-2 role in the cross-talk between the two cell types. We discovered that Multimerin-2 is an adhesion substrate for pericytes. Interestingly, and consistent with the notion that Multimerin-2 is a homeostatic molecule deposited in the later stages of vessel formation, we found that the interaction between endothelial cells and pericytes promoted the expression of Multimerin-2. Furthermore, we found that Multimerin-2 modulated the expression of key cytokines both in endothelial cells and pericytes. Collectively, our findings posit Multimerin-2 as a key molecule in the cross-talk between endothelial cells and pericytes and suggest that the expression of this glycoprotein is required to maintain vascular stability.

Progress in understanding of the stalk and tip cells formation involvement in angiogenesis mechanisms

Vascular sprouting is a key process of angiogenesis and mainly related to the formation of stalk and tip cells. Many studies have found that angiogenesis has a great clinical significance in promoting the functional repair of impaired tissues and anti-angiogenesis is a key to treatment of many tumors. Therefore, how the pathways regulate angiogenesis by regulating the formation of stalk and tip cells is an urgent problem for researchers. This review mainly summarizes the research progress of pathways affecting the formation of stalk and tip cells during angiogenesis in recent years, including the main signaling pathways (such as VEGF-VEGFR-Dll4-Notch signaling pathway, ALK-Smad signaling pathway,CCN1-YAP/YAZ signaling pathway and other signaling pathways) and cellular actions (such as cellular metabolisms, intercellular tension and other actions), aiming to further give the readers an insight into the mechanism of regulating the formation of stalk and tip cells during angiogenesis and provide more targets for anti-angiogenic drugs.

The Role of the Skeletal Muscle Secretome in Mediating Endurance and Resistance Training Adaptations

Exercise, in the form of endurance or resistance training, leads to specific molecular and cellular adaptions not only in skeletal muscles, but also in many other organs such as the brain, liver, fat or bone. In addition to direct effects of exercise on these organs, the production and release of a plethora of different signaling molecules from skeletal muscle are a centerpiece of systemic plasticity. Most studies have so far focused on the regulation and function of such myokines in acute exercise bouts. In contrast, the secretome of long-term training adaptation remains less well understood, and the contribution of non-myokine factors, including metabolites, enzymes, microRNAs or mitochondrial DNA transported in extracellular vesicles or by other means, is underappreciated. In this review, we therefore provide an overview on the current knowledge of endurance and resistance exercise-induced factors of the skeletal muscle secretome that mediate muscular and systemic adaptations to long-term training. Targeting these factors and leveraging their functions could not only have broad implications for athletic performance, but also for the prevention and therapy in diseased and elderly populations.

APLN/APLNR Signaling Controls Key Pathological Parameters of Glioblastoma

Simple Summary

The neurovascular peptide Apelin and its receptor APLNR are upregulated during glioblastoma pathology. Here we summarize their role in the brain tumor microenvironment composed of neurons, astrocytes, and the vascular and immune systems. Targeting APLN/APLNR signaling promises to unfold multimodal actions in future GBM therapy, acting as an anti-angiogenic and an anti-invasive treatment, and offering the possibility to reduce neurological symptoms and increase overall survival simultaneously.

Abstract

Glioblastoma (GBM) is the most common and aggressive primary brain tumor in adults. GBM-expansion depends on a dense vascular network and, coherently, GBMs are highly angiogenic. However, new intratumoral blood vessels are often aberrant with consequences for blood-flow and vascular barrier function. Hence, the delivery of chemotherapeutics into GBM can be compromised. Furthermore, leaky vessels support edema-formation, which can result in severe neurological deficits. The secreted signaling peptide Apelin (APLN) plays an important role in the formation of GBM blood vessels. Both APLN and the Apelin receptor (APLNR) are upregulated in GBM cells and control tumor cell invasiveness. Here we summarize the current evidence on the role of APLN/APLNR signaling during brain tumor pathology. We show that targeting APLN/APLNR can induce anti-angiogenic effects in GBM and simultaneously blunt GBM cell infiltration. In addition, we discuss how manipulation of APLN/APLNR signaling in GBM leads to the normalization of tumor vessels and thereby supports chemotherapy, reduces edema, and improves anti-tumorigenic immune reactions. Hence, therapeutic targeting of APLN/APLNR signaling offers an interesting option to address different pathological hallmarks of GBM.

Zebrafish Vascular Development: General and Tissue-Specific Regulation

Circulation is required for the delivery of oxygen and nutrition to tissues and organs, as well as waste collection. Therefore, the heart and vessels develop first during embryogenesis. The circulatory system consists of the heart, blood vessels, and blood cells, which originate from the mesoderm. The gene expression pattern required for blood vessel development is predetermined by the hierarchical and sequential regulation of genes for the differentiation of mesodermal cells. Herein, we review how blood vessels form distinctly in different tissues or organs of zebrafish and how vessel formation is universally or tissue-specifically regulated by signal transduction pathways and blood flow. In addition, the unsolved issues of mutual contacts and interplay of circulatory organs during embryogenesis are discussed.

ACE2 in the second act of COVID-19 syndrome: Peptide dysregulation and possible correction with oestrogen

Coronavirus disease 2019 (COVID-19) has become the most critical pandemic of the 21st Century and the most severe since the 1918 influenza pandemic. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infects the host by binding to angiotensin-converting enzyme 2 (ACE2). The role of ACE2 in the pathophysiology of coronavirus disease 2019 (COVID-19) is a topic of debate, with clinical and experimental evidence indicating a multifaceted relationship between ACE2 activity and disease severity. Here, we review the mechanisms by which the peptidergic substrates and products of ACE and ACE2 contribute to physiological and pathophysiological processes and hypothesise how down-regulation of ACE2 by SARS-CoV-2 cellular entry disrupts homeostasis. A better understanding of the endocrinology of the disease, in particular the neuroendocrinology of ACE2 during COVID-19, may contribute to the timely design of new therapeutic strategies, including the regulation of ACE2 itself by steroid hormones, to ameliorate the severity of COVID-19.

Wnt-Dependent Oligodendroglial-Endothelial Interactions Regulate White Matter Vascularization and Attenuate Injury

Recent studies have indicated oligodendroglial-vascular crosstalk during brain development, but the underlying mechanisms are incompletely understood. We report that oligodendrocyte precursor cells (OPCs) contact sprouting endothelial tip cells in mouse, ferret, and human neonatal white matter. Using transgenic mice, we show that increased or decreased OPC density results in cognate changes in white matter vascular investment. Hypoxia induced increases in OPC numbers, vessel density and endothelial cell expression of the Wnt pathway targets Apcdd1 and Axin2 in white matter, suggesting paracrine OPC-endothelial signaling. Conditional knockout of OPC Wntless resulted in diminished white matter vascular growth in normoxia, whereas loss of Wnt7a/b function blunted the angiogenic response to hypoxia, resulting in severe white matter damage. These findings indicate that OPC-endothelial cell interactions regulate neonatal white matter vascular development in a Wnt-dependent manner and further suggest this mechanism is important in attenuating hypoxic injury.