Perivascular Mast Cells Govern Shear Stress-Induced Arteriogenesis by Orchestrating Leukocyte Function

The Complement System Is Essential for Arteriogenesis by Enhancing Sterile Inflammation as a Relevant Step in Collateral Artery Growth

Arteriogenesis is an inflammatory driven mechanism, describing the growth of a natural bypass from pre-existing collateral arteries to compensate for an occluded artery. The complement system component C3 is a potent natural inflammatory activator. Here, we investigated its impact on the process of collateral artery growth using C3-deficient (C3 -/-) and wildtype control mice in a murine hindlimb model of arteriogenesis. Induction of arteriogenesis by unilateral femoral artery ligation resulted in decreased perfusion recovery in C3 -/- mice on day 7 as shown by Laser Doppler imaging. Immunofluorescence staining revealed a reduced vascular cell proliferation in C3 -/- mice. Gene expression analysis displayed a significant reduction in monocyte chemoattractant protein-1 (MCP-1) expression in C3 -/- mice. Interestingly, 3 days after induction of arteriogenesis, the number of macrophages (CD68+) recruited to growing collaterals was not affected by C3 deficiency. However, a significant reduction in inflammatory M1-like polarized macrophages (CD68+/MRC1-) was noted. Forced mast cell activation by Compound 48/80 as well as exogenous MCP-1 application rescued the number of M1-like polarized macrophages along with perfusion recovery in C3 -/- mice. In summary, this study demonstrates that complement C3 influences arteriogenesis by mediating MCP-1 expression, which is essential for the induction and enhancement of sterile inflammation.

Sulfur dioxide inhibits mast cell degranulation by sulphenylation of galectin-9 at cysteine 74

Objectives

Mast cell (MC) degranulation is a key process in allergic reactions and inflammatory responses. Aspartate aminotransferase 1 (AAT1)-derived endogenous sulfur dioxide (SO2) is an important regulator of MC function. However, the mechanism underlying its role in MC degranulation remains unclear. This study aimed to investigate the mechanism by which endogenous SO2 controlled MC degranulation.

Methods

HMC-1 and Rat basophilic leukemia cell MC line (RBL-2H3) were used in the cell experiments. SO2 content was detected by in situ fluorescent probe. MC degranulation represented by the release rate of MC β-hexosaminidase was determined using a colorimetric assay. Sulfenylation of galectin-9 (Gal-9) in MCs and purified protein was detected using a biotin switch assay. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used to determine the exact sulfenylation sites of Gal-9 by SO2. Animal models of passive cutaneous anaphylaxis (PCA) and hypoxia-driven pulmonary vascular remodeling were used to investigate the effect of SO2 on mast cell activation in vivo. Site-directed mutation of Gal-9 was conducted to confirm the exact site of SO2 and support the significance of SO2/Gal-9 signal axis in the regulation of MC degranulation.

Results

Degranulation was increased in AAT1-knockdowned MCs, and SO2 supplementation reversed the increase in MC degranulation. Furthermore, deficiency of endogenous SO2 contributed to IgE-mediated degranulation in vitro. Besides, SO2 inhibited IgE-mediated and hypoxia-driven MC degranulation in vivo. Mechanistically, LC-MS/MS analysis and site-directed mutation results showed that SO2 sulfenylated Gal-9 at cysteine 74. Sulfenylation of the 74th cysteine of Gal-9 protein was required in the SO2-inhibited MC degranulation under both physiological and pathophysiological conditions.

Conclusion

These findings elucidated that SO2 inhibited MC degranulation via sulfenylating Gal-9 under both physiological and pathophysiological conditions, which might provide a novel treatment approach for MC activation-related diseases.

The Intriguing World of Vascular Remodeling, Angiogenesis, and Arteriogenesis

Vascular remodeling is a very general feature related to angiogenesis and arteriogenesis, which are involved in neovascularization processes [...].

Cellular and Molecular Mechanisms of Mast Cells in Atherosclerotic Plaque Progression and Destabilization

Mast cells (MCs) are commonly recognized for their crucial involvement in the pathogenesis of allergic diseases, but over time, it has come to light that they also play a role in the pathophysiology of non-allergic disorders including atherosclerosis. The involvement of MCs in the pathology of atherosclerosis is supported by their accumulation in atherosclerotic plaques upon their progression and the association of intraplaque MC numbers with acute cardiovascular events. MCs that accumulate within the atherosclerotic plaque release a cocktail of mediators through which they contribute to neovascularization, plaque progression, instability, erosion, rupture, and thrombosis. At a molecular level, MC-released proteases, especially cathepsin G, degrade low-density lipoproteins (LDL) and mediate LDL fusion and binding of LDL to proteoglycans (PGs). Through a complicated network of chemokines including CXCL1, MCs promote the recruitment of among others CXCR2+ neutrophils, therefore, aggravating the inflammation of the plaque environment. Additionally, MCs produce extracellular traps which worsen inflammation and contribute to atherothrombosis. Altogether, evidence suggests that MCs actively, via several underlying mechanisms, contribute to atherosclerotic plaque destabilization and acute cardiovascular syndromes, thus, making the study of interventions to modulate MC activation an interesting target for cardiovascular medicine.

Treatment with Cobra Venom Factor Decreases Ischemic Tissue Damage in Mice

Tissue ischemia, caused by the blockage of blood vessels, can result in substantial damage and impaired tissue performance. Information regarding the functional contribution of the complement system in the context of ischemia and angiogenesis is lacking. To investigate the influence of complement activation and depletion upon femoral artery ligation (FAL), Cobra venom factor (CVF) (that functionally resembles C3b, the activated form of complement component C3) was applied in mice in comparison to control mice. Seven days after induction of muscle ischemia through FAL, gastrocnemius muscles of mice were excised and subjected to (immuno-)histological analyses. H&E and apoptotic cell staining (TUNEL) staining revealed a significant reduction in ischemic tissue damage in CVF-treated mice compared to controls. The control mice, however, exhibited a significantly higher capillary-to-muscle fiber ratio and a higher number of proliferating endothelial cells (CD31+/CD45-/BrdU+). The total number of leukocytes (CD45+) substantially decreased in CVF-treated mice versus control mice. Moreover, the CVF-treated group displayed a shift towards the M2-like anti-inflammatory and regenerative macrophage phenotype (CD68+/MRC1+). In conclusion, our findings suggest that treatment with CVF leads to reduced ischemic tissue damage along with decreased leukocyte recruitment but increased numbers of M2-like polarized macrophages, thereby enhancing tissue regeneration, repair, and healing.

Rag1 Deficiency Impairs Arteriogenesis in Mice

Increasing evidence suggests that lymphocytes play distinct roles in inflammation-induced tissue remodeling and tissue damage. Arteriogenesis describes the growth of natural bypasses from pre-existing collateral arteries. This process compensates for the loss of artery function in occlusive arterial diseases. The role of innate immune cells is widely understood in the process of arteriogenesis, whereas the role of lymphocytes remains unclear and is the subject of the present study. To analyze the role of lymphocytes, we induced arteriogenesis in recombination activating gene-1 (Rag1) knockout (KO) mice by unilateral ligation of the femoral artery. The lack of functional lymphocytes in Rag1 KO mice resulted in reduced perfusion recovery as shown by laser Doppler imaging. Additionally, immunofluorescence staining revealed a reduced vascular cell proliferation along with a smaller inner luminal diameter in Rag1 KO mice. The perivascular macrophage polarization around the growing collateral arteries was shifted to more pro-inflammatory M1-like polarized macrophages. Together, these data suggest that lymphocytes are crucial for arteriogenesis by modulating perivascular macrophage polarization.

Atherosclerosis and Inflammation: Insights from the Theory of General Pathological Processes

Recent advances have greatly improved our understanding of the molecular mechanisms behind atherosclerosis pathogenesis. However, there is still a need to systematize this data from a general pathology perspective, particularly with regard to atherogenesis patterns in the context of both canonical and non-classical inflammation types. In this review, we analyze various typical phenomena and outcomes of cellular pro-inflammatory stress in atherosclerosis, as well as the role of endothelial dysfunction in local and systemic manifestations of low-grade inflammation. We also present the features of immune mechanisms in the development of productive inflammation in stable and unstable plaques, along with their similarities and differences compared to canonical inflammation. There are numerous factors that act as inducers of the inflammatory process in atherosclerosis, including vascular endothelium aging, metabolic dysfunctions, autoimmune, and in some cases, infectious damage factors. Life-critical complications of atherosclerosis, such as cardiogenic shock and severe strokes, are associated with the development of acute systemic hyperinflammation. Additionally, critical atherosclerotic ischemia of the lower extremities induces paracoagulation and the development of chronic systemic inflammation. Conversely, sepsis, other critical conditions, and severe systemic chronic diseases contribute to atherogenesis. In summary, atherosclerosis can be characterized as an independent form of inflammation, sharing similarities but also having fundamental differences from low-grade inflammation and various variants of canonical inflammation (classic vasculitis).

Inflammation balance in skeletal muscle damage and repair

Responding to tissue injury, skeletal muscles undergo the tissue destruction and reconstruction accompanied with inflammation. The immune system recognizes the molecules released from or exposed on the damaged tissue. In the local minor tissue damage, tissue-resident macrophages sequester pro-inflammatory debris to prevent initiation of inflammation. In most cases of the skeletal muscle injury, however, a cascade of inflammation will be initiated through activation of local macrophages and mast cells and recruitment of immune cells from blood circulation to the injured site by recongnization of damage-associated molecular patterns (DAMPs) and activated complement system. During the inflammation, macrophages and neutrophils scavenge the tissue debris to release inflammatory cytokines and the latter stimulates myoblast fusion and vascularization to promote injured muscle repair. On the other hand, an abundance of released inflammatory cytokines and chemokines causes the profound hyper-inflammation and mobilization of immune cells to trigger a vicious cycle and lead to the cytokine storm. The cytokine storm results in the elevation of cytolytic and cytotoxic molecules and reactive oxygen species (ROS) in the damaged muscle to aggravates the tissue injury, including the healthy bystander tissue. Severe inflammation in the skeletal muscle can lead to rhabdomyolysis and cause sepsis-like systemic inflammation response syndrome (SIRS) and remote organ damage. Therefore, understanding more details on the involvement of inflammatory factors and immune cells in the skeletal muscle damage and repair can provide the new precise therapeutic strategies, including attenuation of the muscle damage and promotion of the muscle repair.

Cytokine storm: behind the scenes of the collateral circulation after acute myocardial infarction

At least 17 million people die from acute myocardial infarction (AMI) every year, ranking it first among causes of death of human beings, and its incidence is gradually increasing. Typical characteristics of AMI include acute onset and poor prognosis. At present, there is no satisfactory treatment, but development of coronary collateral circulation (CCC) can be key to improving prognosis. Recent research indicates that the levels of cytokines, including those related to promoting inflammatory responses and angiogenesis, increase after the onset of AMI. In the early phase of AMI, cytokines play a vital role in inducing development of collateral circulation. However, when myocardial infarction is decompensated, cytokine secretion increases greatly, which may induce a cytokine storm and worsen prognosis. Cytokines can regulate the activation of a variety of signal pathways and form a complex network, which may promote or inhibit the establishment of collateral circulation. We searched for published articles in PubMed and Google Scholar, employing the keyword "acute myocardial infarction", "coronary collateral circulation" and "cytokine storm", to clarify the relationship between AMI and a cytokine storm, and how a cytokine storm affects the growth of collateral circulation after AMI, so as to explore treatment methods based on cytokine agents or inhibitors used to improve prognosis of AMI.

Reactive Epidermal Hyperplasia and Angiogenesis of the Rear (REAR): A Proposed Unifying Name for Senile Gluteal Dermatosis and Prurigiform Angiomatosis

Senile gluteal dermatosis (SGD) is a common but seldom recognized condition. It is characterized clinically by unilateral or bilateral hyperkeratotic, lichenified plaques on the gluteal area, being attributed to prolonged sitting, particularly in the elderly. SGD also encompasses the recently proposed entity of prurigiform angiomatosis. Histologically, there are features of lichenification, such as epidermal hyperplasia and a preserved granular layer, with prominent dermal angioproliferation. We report 4 cases of this condition as well as novel findings of variably increased mast cells and superficial lymphatic vessels in addition to the proliferation of dermal blood vessels. We propose a unifying name for Reactive Epidermal hyperplasia and Angiogenesis of the Rear (REAR) to encapsulate the characteristic clinical and histological features of this distinct entity.

Cobra Venom Factor Boosts Arteriogenesis in Mice

Arteriogenesis, the growth of natural bypass blood vessels, can compensate for the loss of arteries caused by vascular occlusive diseases. Accordingly, it is a major goal to identify the drugs promoting this innate immune system-driven process in patients aiming to save their tissues and life. Here, we studied the impact of the Cobra venom factor (CVF), which is a C3-like complement-activating protein that induces depletion of the complement in the circulation in a murine hind limb model of arteriogenesis. Arteriogenesis was induced in C57BL/6J mice by femoral artery ligation (FAL). The administration of a single dose of CVF (12.5 µg) 24 h prior to FAL significantly enhanced the perfusion recovery 7 days after FAL, as shown by Laser Doppler imaging. Immunofluorescence analyses demonstrated an elevated number of proliferating (BrdU+) vascular cells, along with an increased luminal diameter of the grown collateral vessels. Flow cytometric analyses of the blood samples isolated 3 h after FAL revealed an elevated number of neutrophils and platelet-neutrophil aggregates. Giemsa stains displayed augmented mast cell recruitment and activation in the perivascular space of the growing collaterals 8 h after FAL. Seven days after FAL, we found more CD68+/MRC-1+ M2-like polarized pro-arteriogenic macrophages around growing collaterals. These data indicate that a single dose of CVF boosts arteriogenesis by catalyzing the innate immune reactions, relevant for collateral vessel growth.

Depletion of γδ T Cells Leads to Reduced Angiogenesis and Increased Infiltration of Inflammatory M1-like Macrophages in Ischemic Muscle Tissue

γδ T cells, a small subset of T cells in blood, play a substantial role in influencing immunoregulatory and inflammatory processes. The functional impact of γδ T cells on angiogenesis in ischemic muscle tissue has never been reported and is the topic of the present work. Femoral artery ligation (FAL) was used to induce angiogenesis in the lower leg of γδ T cell depleted mice and wildtype and isotype antibody-treated control groups. Gastrocnemius muscle tissue was harvested 3 and 7 days after FAL and assessed using (immuno-)histological analyses. Hematoxylin and Eosin staining showed an increased area of tissue damage in γδ T cell depleted mice 7 days after FAL. Impaired angiogenesis was demonstrated by lower capillary to muscle fiber ratio and decreased number of proliferating endothelial cells (CD31+/BrdU+). γδ T cell depleted mice showed an increased number of total leukocytes (CD45+), neutrophils (MPO+) and neutrophil extracellular traps (NETs) (MPO+/CitH3+), without changes in the neutrophils to NETs ratio. Moreover, the depletion resulted in a higher macrophage count (DAPI/CD68+) caused by an increase in inflammatory M1-like macrophages (CD68+/MRC1−). Altogether, we show that depletion of γδ T cells leads to increased accumulation of leukocytes and M1-like macrophages, along with impaired angiogenesis.

Integrated Analysis of Competitive Endogenous RNA Networks in Acute Ischemic Stroke

Background: Acute ischemic stroke (AIS) is a severe neurological disease with complex pathophysiology, resulting in the disability and death. The goal of this study is to explore the underlying molecular mechanisms of AIS and search for new potential biomarkers and therapeutic targets.

Methods: Integrative analysis of mRNA and miRNA profiles downloaded from Gene Expression Omnibus (GEO) was performed. We explored differentially expressed genes (DEGs) and differentially expressed miRNAs (DEMirs) after AIS. Target mRNAs of DEMirs and target miRNAs of DEGs were predicted with target prediction tools, and the intersections between DEGs and target genes were determined. Subsequently, Gene Ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) pathway enrichment analyses, Gene set enrichment analysis (GSEA), Gene set variation analysis (GSVA), competitive endogenous RNA (ceRNA) (lncRNA-miRNA-mRNA) network, protein–protein interaction (PPI) network, and gene transcription factors (TFs) network analyses were performed to identify hub genes and associated pathways. Furthermore, we obtained AIS samples with evaluation of immune cell infiltration and used CIBERSORT to determine the relationship between the expression of hub genes and infiltrating immune cells. Finally, we used the Genomics of Drug Sensitivity in Cancer (GDSC) database to predict the effect of the identified targets on drug sensitivity.

Result: We identified 293 DEGs and 26 DEMirs associated with AIS. DEGs were found to be mainly enriched in inflammation and immune-related signaling pathways through enrichment analysis. The ceRNA network included nine lncRNAs, 13 miRNAs, and 21 mRNAs. We used the criterion AUC >0.8, to screen a 3-gene signature (FBL, RPS3, and RPS15) and the aberrantly expressed miRNAs (hsa-miR-125a-5p, hsa-miR-125b-5p, hsa-miR-148b-3p, and hsa-miR-143-3p) in AIS, which were verified by a method of quantitative PCR (qPCR) in HT22 cells. T cells CD8, B cells naïve, and activated NK cells had statistical increased in number compared with the acute cerebral infarction group. By predicting the IC50 of the patient to the drug, AZD0530, Z.LLNle.CHO and NSC-87877 with significant differences between the groups were screened out. AIS demonstrated heterogeneity in immune infiltrates that correlated with the occurrence and development of diseases.

Conclusion: These findings may contribute to a better understanding of the molecular mechanisms of AIS and provide the basis for the development of novel treatment targets in AIS.

Structural Remodeling of the Extracellular Matrix in Arteriogenesis: A Review

Lower extremity arterial occlusive disease (AOD) results in significant morbidity and mortality for the population, with up to 10% of patients ultimately requiring amputation. An alternative method for non-surgical revascularization which is yet to be fully understood is the optimization of the body's own natural collateral arterial network in a process known as arteriogenesis. Under conditions of conductance vessel stenosis or occlusion resulting in increased flow, shear forces, and pressure gradients within collaterals, positive remodeling occurs to increase the diameter and capacity of these vessels. The creation of a distal arteriovenous fistula (AVF) will drive increased arteriogenesis as compared to collateral formation with the occlusion of a conductance vessel alone by further increasing flow through these arterioles, demonstrating the capacity for arteriogenesis to form larger, more efficient collaterals beyond what is spontaneously achieved after arterial occlusion. Arteries rely on an extracellular matrix (ECM) composed of elastic fibers and collagens that provide stability under hemodynamic stress, and ECM remodeling is necessary to allow for increased diameter and flow conductance in mature arterial structures. When positive remodeling occurs, digestion of lamella and the internal elastic lamina (IEL) by matrix metalloproteinases (MMPs) and other elastases results in the rearrangement and thinning of elastic structures and may be replaced with disordered elastin synthesis without recovery of elastic function. This results in transmission of wall strain to collagen and potential for aneurysmal degeneration along collateral networks, as is seen in the pancreaticoduodenal artery (PDA) after celiac occlusion and inferior mesenteric artery (IMA) with concurrent celiac and superior mesenteric artery (SMA) occlusions. Further understanding into the development of collaterals is required to both better understand aneurysmal degeneration and optimize collateral formation in AOD.

Impact of C57BL/6J and SV-129 Mouse Strain Differences on Ischemia-Induced Postnatal Angiogenesis and the Associated Leukocyte Infiltration in a Murine Hindlimb Model of Ischemia

Strain-related differences in arteriogenesis in inbred mouse strains have already been studied excessively. However, these analyses missed evaluating the mouse strain-related differences in ischemia-induced angiogenic capacities. With the present study, we wanted to shed light on the different angiogenic potentials and the associated leukocyte infiltration of C57BL/6J and SV-129 mice to facilitate the comparison of angiogenesis-related analyses between these strains. For the induction of angiogenesis, we ligated the femoral artery in 8-12-week-old male C57BL/6J and SV-129 mice and performed (immuno-) histological analyses on the ischemic gastrocnemius muscles collected 24 h or 7 days after ligation. As evidenced by hematoxylin and eosin staining, C57BL/6J mice showed reduced tissue damage but displayed an increased capillary-to-muscle fiber ratio and an elevated number of proliferating capillaries (CD31⁺/BrdU⁺ cells) compared to SV-129 mice, thus showing improved angiogenesis. Regarding the associated leukocyte infiltration, we found increased numbers of neutrophils (MPO⁺ cells), NETs (MPO⁺/CitH3⁺/DAPI⁺), and macrophages (CD68⁺ cells) in SV-129 mice, whereas macrophage polarization (MRC1^- vs. MRC1⁺) and total leukocyte infiltration (CD45⁺ cells) did not differ between the mouse strains. In summary, we show increased ischemia-induced angiogenic capacities in C57BL/6J mice compared to SV-129 mice, with the latter showing aggravated tissue damage, inflammation, and impaired angiogenesis.

Role of Vascular Smooth Muscle Cell Phenotype Switching in Arteriogenesis

Arteriogenesis is one of the primary physiological means by which the circulatory collateral system restores blood flow after significant arterial occlusion in peripheral arterial disease patients. Vascular smooth muscle cells (VSMCs) are the predominant cell type in collateral arteries and respond to altered blood flow and inflammatory conditions after an arterial occlusion by switching their phenotype between quiescent contractile and proliferative synthetic states. Maintaining the contractile state of VSMC is required for collateral vascular function to regulate blood vessel tone and blood flow during arteriogenesis, whereas synthetic SMCs are crucial in the growth and remodeling of the collateral media layer to establish more stable conduit arteries. Timely VSMC phenotype switching requires a set of coordinated actions of molecular and cellular mediators to result in an expansive remodeling of collaterals that restores the blood flow effectively into downstream ischemic tissues. This review overviews the role of VSMC phenotypic switching in the physiological arteriogenesis process and how the VSMC phenotype is affected by the primary triggers of arteriogenesis such as blood flow hemodynamic forces and inflammation. Better understanding the role of VSMC phenotype switching during arteriogenesis can identify novel therapeutic strategies to enhance revascularization in peripheral arterial disease.

C3 Deficiency Leads to Increased Angiogenesis and Elevated Pro-Angiogenic Leukocyte Recruitment in Ischemic Muscle Tissue

The complement system is a potent inflammatory trigger, activator, and chemoattractant for leukocytes, which play a crucial role in promoting angiogenesis. However, little information is available about the influence of the complement system on angiogenesis in ischemic muscle tissue. To address this topic and analyze the impact of the complement system on angiogenesis, we induced muscle ischemia in complement factor C3 deficient (C3-/-) and wildtype control mice by femoral artery ligation (FAL). At 24 h and 7 days after FAL, we isolated the ischemic gastrocnemius muscles and investigated them by means of (immuno-)histological analyses. C3-/- mice showed elevated ischemic damage 7 days after FAL, as evidenced by H&E staining. In addition, angiogenesis was increased in C3-/- mice, as demonstrated by increased capillary/muscle fiber ratio and increased proliferating endothelial cells (CD31+/BrdU+). Moreover, our results showed that the total number of leukocytes (CD45+) was increased in C3-/- mice, which was based on an increased number of neutrophils (MPO+), neutrophil extracellular trap formation (MPO+/CitH3+), and macrophages (CD68+) displaying a shift toward an anti-inflammatory and pro-angiogenic M2-like polarized phenotype (CD68+/MRC1+). In summary, we show that the deficiency of complement factor C3 increased neutrophil and M2-like polarized macrophage accumulation in ischemic muscle tissue, contributing to angiogenesis.

Molecular Mechanisms Associated with ROS-Dependent Angiogenesis in Lower Extremity Artery Disease

Currently, atherosclerosis, which affects the vascular bed of all vital organs and tissues, is considered as a leading cause of death. Most commonly, atherosclerosis involves coronary and peripheral arteries, which results in acute (e.g., myocardial infarction, lower extremities ischemia) or chronic (persistent ischemia leading to severe heart failure) consequences. All of them have a marked unfavorable impact on the quality of life and are associated with increased mortality and morbidity in human populations. Lower extremity artery disease (LEAD, also defined as peripheral artery disease, PAD) refers to atherosclerotic occlusive disease of the lower extremities, where partial or complete obstruction of peripheral arteries is observed. Decreased perfusion can result in ischemic pain, non-healing wounds, and ischemic ulcers, and significantly reduce the quality of life. However, the progressive atherosclerotic changes cause stimulation of tissue response processes, like vessel wall remodeling and neovascularization. These mechanisms of adapting the vascular network to pathological conditions seem to play a key role in reducing the impact of the changes limiting the flow of blood. Neovascularization as a response to ischemia induces sprouting and expansion of the endothelium to repair and grow the vessels of the circulatory system. Neovascularization consists of three different biological processes: vasculogenesis, angiogenesis, and arteriogenesis. Both molecular and environmental factors that may affect the process of development and growth of blood vessels were analyzed. Particular attention was paid to the changes taking place during LEAD. It is important to consider the molecular mechanisms underpinning vessel growth. These mechanisms will also be examined in the context of diseases commonly affecting blood vessel function, or those treatable in part by manipulation of angiogenesis. Furthermore, it may be possible to induce the process of blood vessel development and growth to treat peripheral vascular disease and wound healing. Reactive oxygen species (ROS) play an important role in regulation of essential cellular signaling pathways such as cell differentiation, proliferation, migration and apoptosis. With regard to the repair processes taking place during diseases such as LEAD, prospective therapeutic methods have been described that could significantly improve the treatment of vessel diseases in the future. Summarizing, regenerative medicine holds the potential to transform the therapeutic methods in heart and vessel diseases treatment.

Absence of Cold-Inducible RNA-Binding Protein (CIRP) Promotes Angiogenesis and Regeneration of Ischemic Tissue by Inducing M2-Like Macrophage Polarization

Cold-inducible RNA-binding protein (CIRP) is an intracellular RNA-chaperone and extracellular promoter of inflammation, which is increasingly expressed and released under conditions of hypoxia and cold stress. The functional relevance of CIRP for angiogenesis and regeneration of ischemic muscle tissue has never been investigated and is the topic of the present study. We investigated the role of CIRP employing CIRP deficient mice along with a hindlimb model of ischemia-induced angiogenesis. 1 and 7 days after femoral artery ligation or sham operation, gastrocnemius muscles of CIRP-deficient and wildtype mice were isolated and processed for (immuno-) histological analyses. CIRP deficient mice showed decreased ischemic tissue damage as evidenced by Hematoxylin and Eosin staining, whereas angiogenesis was enhanced as demonstrated by increased capillary/muscle fiber ratio and number of proliferating endothelial (CD31⁺/BrdU⁺) cells on day 7 after surgery. Moreover, CIRP deficiency resulted in a reduction of total leukocyte count (CD45⁺), neutrophils (myeloperoxidase, MPO⁺), neutrophil extracellular traps (NETs) (MPO⁺/CitH3⁺), and inflammatory M1-like polarized macrophages (CD68⁺/MRC1^-), whereas the number of tissue regenerating M2-like polarized macrophages (CD68⁺/MRC1^-) was increased in ischemic tissue samples. In summary, we show that the absence of CIRP ameliorates angiogenesis and regeneration of ischemic muscle tissue, most likely by influencing macrophage polarization in direction to regenerative M2-like macrophages.

Extracellular RNA as a Versatile DAMP and Alarm Signal That Influences Leukocyte Recruitment in Inflammation and Infection

Upon vascular injury, tissue damage, ischemia, or microbial infection, intracellular material such as nucleic acids and histones is liberated and comes into contact with the vessel wall and circulating blood cells. Such "Danger-associated molecular patterns" (DAMPs) may thus have an enduring influence on the inflammatory defense process that involves leukocyte recruitment and wound healing reactions. While different species of extracellular RNA (exRNA), including microRNAs and long non-coding RNAs, have been implicated to influence inflammatory processes at different levels, recent in vitro and in vivo work has demonstrated a major impact of ribosomal exRNA as a prominent DAMP on various steps of leukocyte recruitment within the innate immune response. This includes the induction of vascular hyper-permeability and vasogenic edema by exRNA via the activation of the "vascular endothelial growth factor" (VEGF) receptor-2 system, as well as the recruitment of leukocytes to the inflamed endothelium, the M1-type polarization of inflammatory macrophages, or the role of exRNA as a pro-thrombotic cofactor to promote thrombosis. Beyond sterile inflammation, exRNA also augments the docking of bacteria to host cells and the subsequent microbial invasion. Moreover, upon vessel occlusion and ischemia, the shear stress-induced release of exRNA initiates arteriogenesis (i.e., formation of natural vessel bypasses) in a multistep process that resembles leukocyte recruitment. Although exRNA can be counteracted for by natural circulating RNase1, under the conditions mentioned, only the administration of exogenous, thermostable, non-toxic RNase1 provides an effective and safe therapeutic regimen for treating the damaging activities of exRNA. It remains to be investigated whether exRNA may also influence viral infections (including COVID-19), e.g., by supporting the interaction of host cells with viral particles and their subsequent invasion. In fact, as a consequence of the viral infection cycle, massive amounts of exRNA are liberated, which can provoke further tissue damage and enhance virus dissemination. Whether the application of RNase1 in this scenario may help to limit the extent of viral infections like COVID-19 and impact on leukocyte recruitment and emigration steps in immune defense in order to limit the extent of associated cardiovascular diseases remains to be studied.

Mast Cell Functions Linking Innate Sensing to Adaptive Immunity

Although mast cells (MCs) are known as key drivers of type I allergic reactions, there is increasing evidence for their critical role in host defense. MCs not only play an important role in initiating innate immune responses, but also influence the onset, kinetics, and amplitude of the adaptive arm of immunity or fine-tune the mode of the adaptive reaction. Intriguingly, MCs have been shown to affect T-cell activation by direct interaction or indirectly, by modifying the properties of antigen-presenting cells, and can even modulate lymph node-borne adaptive responses remotely from the periphery. In this review, we provide a summary of recent findings that explain how MCs act as a link between the innate and adaptive immunity, all the way from sensing inflammatory insult to orchestrating the final outcome of the immune response.

RNase A Treatment Interferes With Leukocyte Recruitment, Neutrophil Extracellular Trap Formation, and Angiogenesis in Ischemic Muscle Tissue

Background: RNase A (the bovine equivalent to human RNase 1) and RNase 5 (angiogenin) are two closely related ribonucleases. RNase 5 is described as a powerful angiogenic factor. Whether RNase A shares the same angiogenic characteristic, or interferes with vessel growth as demonstrated for arteriogenesis, has never been investigated and is the topic of this present study.

Methods and Results: To investigate whether RNase A shows a pro‐ or anti-angiogenic effect, we employed a murine hindlimb model, in which femoral artery ligation (FAL) results in arteriogenesis in the upper leg, and, due to provoked ischemia, in angiogenesis in the lower leg. C57BL/6J male mice underwent unilateral FAL, whereas the contralateral leg was sham operated. Two and seven days after the surgery and intravenous injection of RNase A (50 μg/kg dissolved in saline) or saline (control), the gastrocnemius muscles of mice were isolated from the lower legs for (immuno-) histological analyses. Hematoxylin and Eosin staining evidenced that RNase A treatment resulted in a higher degree of ischemic tissue damage. This was, however, associated with reduced angiogenesis, as evidenced by a reduced capillary/muscle fiber ratio. Moreover, RNase A treatment was associated with a significant reduction in leukocyte infiltration as shown by CD45⁺ (pan-leukocyte marker), Ly6G⁺ or MPO⁺ (neutrophils), MPO⁺/CitH₃⁺ [neutrophil extracellular traps (NETs)], and CD68⁺ (macrophages) staining. CD68/MRC1 double staining revealed that RNase A treated mice showed a reduced percentage of M1-like polarized (CD68⁺/MRC1⁻) macrophages whereas the percentage of M2-like polarized (CD68⁺/MRC1⁺) macrophages was increased.

Conclusion: In contrast to RNase 5, RNase A interferes with angiogenesis, which is linked to reduced leukocyte infiltration and NET formation.

Potential Benefits and Harms of Novel Antidiabetic Drugs During COVID-19 Crisis

Patients with diabetes have been reported to have enhanced susceptibility to severe or fatal COVID-19 infections, including a high risk of being admitted to intensive care units with respiratory failure and septic complications. Given the global prevalence of diabetes, affecting over 450 million people worldwide and still on the rise, the emerging COVID-19 crisis poses a serious threat to an extremely large vulnerable population. However, the broad heterogeneity and complexity of this dysmetabolic condition, with reference to etiologic mechanisms, degree of glycemic derangement and comorbid associations, along with the extensive sexual dimorphism in immune responses, can hamper any patient generalization. Even more relevant, and irrespective of glucose-lowering activities, DPP4 inhibitors and GLP1 receptor agonists may have a favorable impact on the modulation of viral entry and overproduction of inflammatory cytokines during COVID-19 infection, although current evidence is limited and not univocal. Conversely, SGLT2 inhibitors may increase the likelihood of COVID-19-related ketoacidosis decompensation among patients with severe insulin deficiency. Mindful of their widespread popularity in the management of diabetes, addressing potential benefits and harms of novel antidiabetic drugs to clinical prognosis at the time of a COVID-19 pandemic deserves careful consideration.

A Simple and Effective Flow Cytometry-Based Method for Identification and Quantification of Tissue Infiltrated Leukocyte Subpopulations in a Mouse Model of Peripheral Arterial Disease

Arteriogenesis, the growth of a natural bypass from pre-existing arteriolar collaterals, is an endogenous mechanism to compensate for the loss of an artery. Mechanistically, this process relies on a locally and temporally restricted perivascular infiltration of leukocyte subpopulations, which mediate arteriogenesis by supplying growth factors and cytokines. Currently, the state-of-the-art method to identify and quantify these leukocyte subpopulations in mouse models is immunohistology. However, this is a time consuming procedure. Here, we aimed to develop an optimized protocol to identify and quantify leukocyte subpopulations by means of flow cytometry in adductor muscles containing growing collateral arteries. For that purpose, adductor muscles of murine hindlimbs were isolated at day one and three after induction of arteriogenesis, enzymatically digested, and infiltrated leukocyte subpopulations were identified and quantified by flow cytometry, as exemplary shown for neutrophils and macrophages (defined as CD45⁺/CD11b⁺/Ly6G⁺ and CD45⁺/CD11b⁺/F4/80⁺ cells, respectively). In summary, we show that flow cytometry is a suitable method to identify and quantify leukocyte subpopulations in muscle tissue, and provide a detailed protocol. Flow cytometry constitutes a timesaving tool compared to histology, which might be used in addition for precise localization of leukocytes in tissue samples.

Shear Stress-Induced miR-143-3p in Collateral Arteries Contributes to Outward Vessel Growth by Targeting Collagen V-α2

OBJECTIVE

Arteriogenesis, describing the process of collateral artery growth, is activated by fluid shear stress (FSS). Since this vascular mechanotransduction may involve microRNAs (miRNAs), we investigated the FSS-induced expression of vascular cell miRNAs and their functional impact on collateral artery growth during arteriogenesis. Approach and Results: To this end, rats underwent femoral artery ligation and arteriovenous anastomosis to increase collateral blood flow to maximize FSS and trigger collateral vessel remodeling. Five days after surgery, a miRNA expression profile was obtained from collateral tissue, and upregulation of 4 miRNAs (miR-24-3p, miR-143-3p, miR-146a-5p, and miR-195-5p) was verified by quantitative polymerase chain reaction. Knockdown of miRNAs at the same time of the surgery in an in vivo mouse ligation and recovery model demonstrated that inhibition of miR-143-3p only severely impaired blood flow recovery due to decreased arteriogenesis. In situ hybridization revealed distinct localization of miR-143-3p in the vessel wall of growing collateral arteries predominantly in smooth muscle cells. To investigate the mechanotransduction of FSS leading to the increased miR-143-3p expression, cultured endothelial cells were exposed to FSS. This provoked the expression and release of TGF-β (transforming growth factor-β), which increased the expression of miR-143-3p in smooth muscle cells in the presence of SRF (serum response factor) and myocardin. COL5A2 (collagen type V-α2)-a target gene of miR-143-3p predicted by in silico analysis-was found to be downregulated in growing collaterals.

CONCLUSIONS

These results indicate that the increased miR-143-3p expression in response to FSS might contribute to the reorganization of the extracellular matrix, which is important for vascular remodeling processes, by inhibiting collagen V-α2 biosynthesis.

Why Should Growth Hormone (GH) Be Considered a Promising Therapeutic Agent for Arteriogenesis? Insights from the GHAS Trial

Despite the important role that the growth hormone (GH)/IGF-I axis plays in vascular homeostasis, these kind of growth factors barely appear in articles addressing the neovascularization process. Currently, the vascular endothelium is considered as an authentic gland of internal secretion due to the wide variety of released factors and functions with local effects, including the paracrine/autocrine production of GH or IGF-I, for which the endothelium has specific receptors. In this comprehensive review, the evidence involving these proangiogenic hormones in arteriogenesis dealing with the arterial occlusion and making of them a potential therapy is described. All the elements that trigger the local and systemic production of GH/IGF-I, as well as their possible roles both in physiological and pathological conditions are analyzed. All of the evidence is combined with important data from the GHAS trial, in which GH or a placebo were administrated to patients suffering from critical limb ischemia with no option for revascularization. We postulate that GH, alone or in combination, should be considered as a promising therapeutic agent for helping in the approach of ischemic disease.

Local Mast Cell Activation Promotes Neovascularization

Mast cells have been associated with arteriogenesis and collateral formation. In advanced human atherosclerotic plaques, mast cells have been shown to colocalize with plaque neovessels, and mast cells have also been associated with tumor vascularization. Based on these associations, we hypothesize that mast cells promote angiogenesis during ischemia. In human ischemic muscle tissue from patients with end-stage peripheral artery disease, we observed activated mast cells, predominantly located around capillaries. Also, in mouse ischemic muscles, mast cells were detected during the revascularization process and interestingly, mast cell activation status was enhanced up to 10 days after ischemia induction. To determine whether mast cells contribute to both arteriogenesis and angiogenesis, mast cells were locally activated immediately upon hind limb ischemia in C57Bl/6 mice. At day 9, we observed a 3-fold increase in activated mast cell numbers in the inguinal lymph nodes. This was accompanied by an increase in the amount of Ly6C^high inflammatory monocytes. Interestingly, local mast cell activation increased blood flow through the hind limb (46% at day 9) compared to that in non-activated control mice. Histological analysis of the muscle tissue revealed that mast cell activation did not affect the number of collaterals, but increased the collateral diameter, as well as the number of CD31⁺ capillaries. Together, these data illustrate that locally activated mast cell contribute to arteriogenesis and angiogenesis.

Arteriogenesis of the Spinal Cord-The Network Challenge

Spinal cord ischemia (SCI) is a clinical complication following aortic repair that significantly impairs the quality and expectancy of life. Despite some strategies, like cerebrospinal fluid drainage, the occurrence of neurological symptoms, such as paraplegia and paraparesis, remains unpredictable. Beside the major blood supply through conduit arteries, a huge collateral network protects the central nervous system from ischemia—the paraspinous and the intraspinal compartment. The intraspinal arcades maintain perfusion pressure following a sudden inflow interruption, whereas the paraspinal system first needs to undergo arteriogenesis to ensure sufficient blood supply after an acute ischemic insult. The so-called steal phenomenon can even worsen the postoperative situation by causing the hypoperfusion of the spine when, shortly after thoracoabdominal aortic aneurysm (TAAA) surgery, muscles connected with the network divert blood and cause additional stress. Vessels are a conglomeration of different cell types involved in adapting to stress, like endothelial cells, smooth muscle cells, and pericytes. This adaption to stress is subdivided in three phases—initiation, growth, and the maturation phase. In fields of endovascular aortic aneurysm repair, pre-operative selective segmental artery occlusion may enable the development of a sufficient collateral network by stimulating collateral vessel growth, which, again, may prevent spinal cord ischemia. Among others, the major signaling pathways include the phosphoinositide 3 kinase (PI3K) pathway/the antiapoptotic kinase (AKT) pathway/the endothelial nitric oxide synthase (eNOS) pathway, the Erk1, the delta-like ligand (DII), the jagged (Jag)/NOTCH pathway, and the midkine regulatory cytokine signaling pathways.

Extracellular RNA released due to shear stress controls natural bypass growth by mediating mechanotransduction in mice

Fluid shear stress in the vasculature is the driving force for natural bypass growth, a fundamental endogenous mechanism to counteract the detrimental consequences of vascular occlusive disease, such as stroke or myocardial infarction. This process, referred to as "arteriogenesis," relies on local recruitment of leukocytes, which supply growth factors to preexisting collateral arterioles enabling them to grow. Although several mechanosensing proteins have been identified, the series of mechanotransduction events resulting in local leukocyte recruitment is not understood. In a mouse model of arteriogenesis (femoral artery ligation), we found that endothelial cells release RNA in response to increased fluid shear stress and that administration of RNase inhibitor blocking plasma RNases improved perfusion recovery. In contrast, treatment with bovine pancreatic RNase A or human recombinant RNase1 interfered with leukocyte recruitment and collateral artery growth. Our results indicated that extracellular RNA (eRNA) regulated leukocyte recruitment by engaging vascular endothelial growth factor receptor 2 (VEGFR2), which was confirmed by intravital microscopic studies in a murine cremaster model of inflammation. Moreover, we found that release of von Willebrand factor (VWF) as a result of shear stress is dependent on VEGFR2. Blocking VEGFR2, RNase application, or VWF deficiency interfered with platelet-neutrophil aggregate formation, which is essential for initiating the inflammatory process in arteriogenesis. Taken together, the results show that eRNA is released from endothelial cells in response to shear stress. We demonstrate this extracellular nucleic acid as a critical mediator of mechanotransduction by inducing the liberation of VWF, thereby initiating the multistep inflammatory process responsible for arteriogenesis.

The Extraordinary Role of Extracellular RNA in Arteriogenesis, the Growth of Collateral Arteries

Arteriogenesis is an intricate process in which increased shear stress in pre-existing arteriolar collaterals induces blood vessel expansion, mediated via endothelial cell activation, leukocyte recruitment and subsequent endothelial and smooth muscle cell proliferation. Extracellular RNA (eRNA), released from stressed cells or damaged tissue under pathological conditions, has recently been discovered to be liberated from endothelial cells in response to increased shear stress and to promote collateral growth. Until now, eRNA has been shown to enhance coagulation and inflammation by inducing cytokine release, leukocyte recruitment, and endothelial permeability, the latter being mediated by vascular endothelial growth factor (VEGF) signaling. In the context of arteriogenesis, however, eRNA has emerged as a transmitter of shear stress into endothelial activation, mediating the sterile inflammatory process essential for collateral remodeling, whereby the stimulatory effects of eRNA on the VEGF signaling axis seem to be pivotal. In addition, eRNA might influence subsequent steps of the arteriogenesis cascade as well. This article provides a comprehensive overview of the beneficial effects of eRNA during arteriogenesis, laying the foundation for further exploration of the connection between the damaging and non-damaging effects of eRNA in the context of cardiovascular occlusive diseases and of sterile inflammation.

Microphysiological Engineering of Self-Assembled and Perfusable Microvascular Beds for the Production of Vascularized Three-Dimensional Human Microtissues

The vasculature is an essential component of the circulatory system that plays a vital role in the development, homeostasis, and disease of various organs in the human body. The ability to emulate the architecture and transport function of blood vessels in the integrated context of their associated organs represents an important requirement for studying a wide range of physiological processes. Traditional in vitro models of the vasculature, however, largely fail to offer such capabilities. Here we combine microfluidic three-dimensional (3D) cell culture with the principle of vasculogenic self-assembly to engineer perfusable 3D microvascular beds in vitro. Our system is created in a micropatterned hydrogel construct housed in an elastomeric microdevice that enables coculture of primary human vascular endothelial cells and fibroblasts to achieve de novo formation, anastomosis, and controlled perfusion of 3D vascular networks. An open-top chamber design adopted in this hybrid platform also makes it possible to integrate the microengineered 3D vasculature with other cell types to recapitulate organ-specific cellular heterogeneity and structural organization of vascularized human tissues. Using these capabilities, we developed stem cell-derived microphysiological models of vascularized human adipose tissue and the blood-retinal barrier. Our approach was also leveraged to construct a 3D organotypic model of vascularized human lung adenocarcinoma as a high-content drug screening platform to simulate intravascular delivery, tumor-killing effects, and vascular toxicity of a clinical chemotherapeutic agent. Furthermore, we demonstrated the potential of our platform for applications in nanomedicine by creating microengineered models of vascular inflammation to evaluate a nanoengineered drug delivery system based on active targeting liposomal nanocarriers. These results represent a significant improvement in our ability to model the complexity of native human tissues and may provide a basis for developing predictive preclinical models for biopharmaceutical applications.

Insulin Treatment Forces Arteriogenesis in Diabetes Mellitus by Upregulation of the Early Growth Response-1 (Egr-1) Pathway in Mice

The process of arteriogenesis is severely compromised in patients with diabetes mellitus (DM). Earlier studies have reported the importance of Egr-1 in promoting collateral outward remodeling. However, the role of Egr-1 in the presence of DM in outward vessel remodeling was not studied. We hypothesized that Egr-1 expression may be compromised in DM which may lead to impaired collateral vessel growth. Here, we investigated the relevance of the transcription factor Egr-1 for the process of collateral artery growth in diabetic mice. Induction of arteriogenesis by femoral artery ligation resulted in an increased expression of Egr-1 on mRNA and protein level but was severely compromised in streptozotocin-induced diabetic mice. Diabetes mellitus mice showed a significantly reduced expression of Egr-1 endothelial downstream genes Intercellular Adhesion Molecule-1 (ICAM-1) and urokinase Plasminogen Activator (uPA), relevant for extravasation of leukocytes which promote arteriogenesis. Fluorescent-activated cell sorting analyses confirmed reduced leukocyte recruitment. Diabetes mellitus mice showed a reduced expression of the proliferation marker Ki-67 in growing collaterals whose luminal diameters were also reduced. The Splicing Factor-1 (SF-1), which is critical for smooth muscle cell proliferation and phenotype switch, was found to be elevated in collaterals of DM mice. Treatment of DM mice with insulin normalized the expression of Egr-1 and its downstream targets and restored leukocyte recruitment. SF-1 expression and the diameter of growing collaterals were normalized by insulin treatment as well. In summary, our results showed that Egr-1 signaling was impaired in DM mice; however, it can be rescued by insulin treatment.

Polymer scaffold architecture is a key determinant in mast cell inflammatory and angiogenic responses

Implanted polymer scaffolds can induce inflammation leading to the foreign body response (FBR), fibrosis, and implant failure. Thus, it is important to understand how immune cells interact with scaffolds to mitigate inflammation and promote a regenerative response. We previously demonstrated that macrophage phenotype is modulated by fiber and pore diameters of an electrospun scaffold. However, it is unclear if this effect is consistent among other innate immune cells. Mast cells are inflammatory sentinels that play a vital role in the FBR of implanted biomaterials, as well as angiogenesis. We determined if altering electrospun scaffold architecture modulates mast cell responses, with the goal of promoting regenerative cell-scaffold interactions. Polydioxanone (PDO) scaffolds were made from 60 mg/mL or 140 mg/mL PDO solutions, yielding structures with divergent fiber and pore diameters. Mouse mast cells plated on these scaffolds were activated with IL-33 or lipopolysaccharide (LPS). Relative to the 60 mg/mL scaffold, 140 mg/mL scaffolds yielded less IL-6 and TNF, and greater VEGF secretion. Pores >4-6 μm elicited less IL-6 and TNF secretion. IL-33-induced VEGF regulation was more complex, showing effects of both pore size and fiber diameter. These data indicate parameters that can predict mast cell responses to scaffolds, informing biomaterial design to increase wound healing and diminish implant rejection. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 884-892, 2019.

Mast cell tryptase - Marker and maker of cardiovascular diseases

Mast cells are tissue-resident cells, which have been proposed to participate in various inflammatory diseases, among them the cardiovascular diseases (CVDs). For mast cells to be able to contribute to an inflammatory process, they need to be activated to exocytose their cytoplasmic secretory granules. The granules contain a vast array of highly bioactive effector molecules, the neutral protease tryptase being the most abundant protein among them. The released tryptase may act locally in the inflamed cardiac or vascular tissue, so contributing directly to the pathogenesis of CVDs. Moreover, a fraction of the released tryptase reaches the systemic circulation, thereby serving as a biomarker of mast cell activation. Actually, increased levels of circulating tryptase have been found to associate with CVDs. Here we review the biological relevance of the circulating tryptase as a biomarker of mast cell activity in CVDs, with special emphasis on the relationship between activation of mast cells in their tissue microenvironments and the pathophysiological pathways of CVDs. Based on the available in vitro and in vivo studies, we highlight the potential molecular mechanisms by which tryptase may contribute to the pathogenesis of CVDs. Finally, the synthetic and natural inhibitors of tryptase are reviewed for their potential utility as therapeutic agents in CVDs.

Redox Control of Angiogenesis

SIGNIFICANCE

Angiogenesis is the formation of new vessels that sprout from existing vessels. This process is highly complex and requires a coordinated shift of the endothelial phenotype from a quiescent cell in the vessel wall into a migrating or proliferating cell. Such change in the life of the endothelial cell is induced by a variety of factors such as hypoxia, metabolic changes, or cytokines. Recent Advances: Within the last years, it became clear that the cellular redox state and oxidation of signaling molecules or phosphatases are critical modulators in angiogenesis.

CRITICAL ISSUES

According to the wide variety of stimuli that induce angiogenesis, a complex signaling network is needed to support a coordinated response of the endothelial cell. Reactive oxygen species (ROS) now are second messengers that either directly oxidize a target molecule or initiate a cascade of redox sensitive steps that transmit the signal. Further Directions: For the understanding of redox signaling, it is essential to recognize and accept that ROS do not represent master regulators of angiogenetic processes. They rather modulate existing signal cascades. This review summarizes some current findings on redox signaling in angiogenesis.

Characterization of mast cell-derived rRNA-containing microvesicles and their inflammatory impact on endothelial cells

Tissue-resident mast cells (MCs) are well known for their role in inflammatory responses and allergic and anaphylactic reactions, but they also contribute to processes of arterial remodeling. Although ribosomes and cytosolic RNAs are located around secretory granules in mature MCs, their functional role in MC responses remains unexplored. Previous studies by our group characterized extracellular RNA (eRNA) as an inflammatory and pathogenetic factor in vitro and in vivo. In the present study, RNA-containing MCs and eRNA were located in close proximity to growing collateral arteries in vivo. In vitro, various agonists were found to induce the degranulation of MCs and the concomitant release of eRNA in association with microvesicles (MVs). The liberation of eRNA from MCs was abolished by MC stabilizers or by preventing the increase of intracellular Ca²⁺ in MCs. eRNA was found to be mainly contained inside MVs, as demonstrated by electron microscopy and immunocytochemistry. The exposure to and the uptake of MC-released MVs by cultured endothelial cells increased their expression of cytokines, such as monocyte chemoattractant protein or IL-6, in a dose- and time-dependent manner. These results indicate that RNA-containing MC-derived MVs are likely to be involved in inflammatory responses, relevant, for example, to processes of vascular remodeling.-Elsemüller, A.-K., Tomalla, V., Gärtner, U., Troidl, K., Jeratsch, S., Graumann, J., Baal, N., Hackstein, H., Lasch, M., Deindl, E., Preissner, K. T., Fischer, S. Characterization of mast cell-derived rRNA-containing microvesicles and their inflammatory impact on endothelial cells.

Is there a Chance to Promote Arteriogenesis by DPP4 Inhibitors Even in Type 2 Diabetes? A Critical Review

Cardiovascular diseases (CVD) are still the prevailing cause of death not only in industrialized countries, but even worldwide. Type 2 diabetes mellitus (type 2 DM) and hyperlipidemia, a metabolic disorder that is often associated with diabetes, are major risk factors for developing CVD. Recently, clinical trials proved the safety of gliptins in treating patients with type 2 DM. Gliptins are dipeptidyl-peptidase 4 (DPP4/CD26) inhibitors, which stabilize glucagon-like peptide-1 (GLP-1), thereby increasing the bioavailability of insulin. Moreover, blocking DPP4 results in increased levels of stromal cell derived factor 1 (SDF-1). SDF-1 has been shown in pre-clinical animal studies to improve heart function and survival after myocardial infarction, and to promote arteriogenesis, the growth of natural bypasses, compensating for the function of an occluded artery. Clinical trials, however, failed to demonstrate a superiority of gliptins compared to placebo treated type 2 DM patients in terms of cardiovascular (CV) outcomes. This review highlights the function of DPP4 inhibitors in type 2 DM, and in treating cardiovascular diseases, with special emphasis on arteriogenesis. It critically addresses the potency of currently available gliptins and gives rise to hope by pointing out the most relevant questions that need to be resolved.

The Role of Midkine in Arteriogenesis, Involving Mechanosensing, Endothelial Cell Proliferation, and Vasodilation

Mechanical forces in blood circulation such as shear stress play a predominant role in many physiological and pathophysiological processes related to vascular responses or vessel remodeling. Arteriogenesis, defined as the growth of pre-existing arterioles into functional collateral arteries compensating for stenosed or occluded arteries, is such a process. Midkine, a pleiotropic protein and growth factor, has originally been identified to orchestrate embryonic development. In the adult organism its expression is restricted to distinct tissues (including tumors), whereby midkine is strongly expressed in inflamed tissue and has been shown to promote inflammation. Recent investigations conferred midkine an important function in vascular remodeling and growth. In this review, we introduce the midkine gene and protein along with its cognate receptors, and highlight its role in inflammation and the vascular system with special emphasis on arteriogenesis, particularly focusing on shear stress-mediated vascular cell proliferation and vasodilatation.

Estimating hemodynamic shear stress in murine peripheral collateral arteries by two-photon line scanning

Changes in wall shear stress of blood vessels are assumed to be an important component of many physiological and pathophysiological processes. However, due to technical limitations experimental in vivo data are rarely available. Here, we investigated two-photon excitation fluorescence microscopy as an option to measure vessel diameter as well as blood flow velocities in a murine hindlimb model of arteriogenesis (collateral artery growth). Using line scanning at high frequencies, we measured the movement of blood cells along the vessel axis. We found that peak systolic blood flow velocity averaged 9 mm/s and vessel diameter 42 µm in resting collaterals. Induction of arteriogenesis by femoral artery ligation resulted in a significant increase in centerline peak systolic velocity after 1 day with an average of 51 mm/s, whereas the averaged luminal diameter of collaterals (52 µm) changed much less. Thereof calculations revealed a significant fourfold increase in hemodynamic wall shear rate. Our results indicate that two-photon line scanning is a suitable tool to estimate wall shear stress e.g., in experimental animal models, such as of arteriogenesis, which may not only help to understand the relevance of mechanical forces in vivo, but also to adjust wall shear stress in ex vivo investigations on isolated vessels as well as cell culture experiments.

A new role of mast cells in arteriogenesis

Arteriogenesis is defined as the growth of functional collateral arteries from pre-existing arterio-arteriolar anastomoses. The role of mast cells in arteriogenesis is largely unexplored. Recent evidences suggest that mast cells together with other inflammatory cells, including monocytes-macrophages, lymphocytes, NK cells and endothelial precursor cells (EPCs) may be involved in this process. This review article analyzes the literature concerning this new aspect of biological activity of mast cells.

Self-extracellular RNA acts in synergy with exogenous danger signals to promote inflammation

Self-extracellular RNA (eRNA), released from stressed or injured cells upon various pathological situations such as ischemia-reperfusion-injury, has been shown to act as an alarmin by inducing procoagulatory and proinflammatory responses. In particular, M1-polarization of macrophages by eRNA resulted in the expression and release of a variety of cytokines, including tumor necrosis factor (TNF)-α or interleukin-6 (IL-6). The present study now investigates in which way self-eRNA may influence the response of macrophages towards various Toll-like receptor (TLR)-agonists. Isolated agonists of TLR2 (Pam2CSK4), TLR3 (PolyIC), TLR4 (LPS), or TLR7 (R848) induced the release of TNF-α in a concentration-dependent manner in murine macrophages, differentiated from bone marrow-derived stem cells by mouse colony stimulating factor. Here, the presence of eRNA shifted the dose-response curve for Pam2CSK4 (Pam) considerably to the left, indicating that eRNA synergistically enhanced the cytokine liberation from macrophages even at very low Pam-levels. The synergistic activation of TLR2 by eRNA/Pam was duplicated by other TLR2-agonists such as FSL-1 or Pam3CSK4. In contrast, for TLR4-agonists such as LPS a synergistic effect of eRNA was much weaker, and was not existent for TLR3-, or TLR7-agonists. The synergistic eRNA/Pam action was dependent on the NFκB-signaling pathway as well as on p38MAP- and MEK1/ERK-kinases and was prevented by predigestion of eRNA with RNase1 or by antibodies against TLR2. Thus, the presence of self-eRNA as alarming molecule sensitizes innate immune responses towards pathogen-associated molecular patterns (PAMPs) in a synergistic way and may thereby contribute to the differentiated outcome of inflammatory responses.

A Network Model to Explore the Effect of the Micro-environment on Endothelial Cell Behavior during Angiogenesis

Angiogenesis is an important adaptation mechanism of the blood vessels to the changing requirements of the body during development, aging, and wound healing. Angiogenesis allows existing blood vessels to form new connections or to reabsorb existing ones. Blood vessels are composed of a layer of endothelial cells (ECs) covered by one or more layers of mural cells (smooth muscle cells or pericytes). We constructed a computational Boolean model of the molecular regulatory network involved in the control of angiogenesis. Our model includes the ANG/TIE, HIF, AMPK/mTOR, VEGF, IGF, FGF, PLCγ/Calcium, PI3K/AKT, NO, NOTCH, and WNT signaling pathways, as well as the mechanosensory components of the cytoskeleton. The dynamical behavior of our model recovers the patterns of molecular activation observed in Phalanx, Tip, and Stalk ECs. Furthermore, our model is able to describe the modulation of EC behavior due to extracellular micro-environments, as well as the effect due to loss- and gain-of-function mutations. These properties make our model a suitable platform for the understanding of the molecular mechanisms underlying some pathologies. For example, it is possible to follow the changes in the activation patterns caused by mutations that promote Tip EC behavior and inhibit Phalanx EC behavior, that lead to the conditions associated with retinal vascular disorders and tumor vascularization. Moreover, the model describes how mutations that promote Phalanx EC behavior are associated with the development of arteriovenous and venous malformations. These results suggest that the network model that we propose has the potential to be used in the study of how the modulation of the EC extracellular micro-environment may improve the outcome of vascular disease treatments.

Midkine Controls Arteriogenesis by Regulating the Bioavailability of Vascular Endothelial Growth Factor A and the Expression of Nitric Oxide Synthase 1 and 3

Midkine is a pleiotropic factor, which is involved in angiogenesis. However, its mode of action in this process is still ill defined. The function of midkine in arteriogenesis, the growth of natural bypasses from pre-existing collateral arteries, compensating for the loss of an occluded artery has never been investigated. Arteriogenesis is an inflammatory process, which relies on the proliferation of endothelial cells and smooth muscle cells. We show that midkine deficiency strikingly interferes with the proliferation of endothelial cells in arteriogenesis, thereby interfering with the process of collateral artery growth. We identified midkine to be responsible for increased plasma levels of vascular endothelial growth factor A (VEGFA), necessary and sufficient to promote endothelial cell proliferation in growing collaterals. Mechanistically, we demonstrate that leukocyte domiciled midkine mediates increased plasma levels of VEGFA relevant for upregulation of endothelial nitric oxide synthase 1 and 3, necessary for proper endothelial cell proliferation, and that non-leukocyte domiciled midkine additionally improves vasodilation.

The data provided on the role of midkine in endothelial proliferation are likely to be relevant for both, the process of arteriogenesis and angiogenesis. Moreover, our data might help to estimate the therapeutic effect of clinically applied VEGFA in patients with vascular occlusive diseases.

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Leukocyte domiciled midkine is decisive for collateral endothelial cell proliferation in arteriogenesis.

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Midkine controls the bioavailability of VEGFA mediating endothelial Nos1 and Nos3 expression.

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Nos1 and Nos3, relevant for endothelial cell proliferation, can substitute for each other.

Arteriogenesis is a life and tissue saving process as it compensates for the loss of an occluded artery. Decoding the underlying molecular mechanisms is a prerequisite for the development of novel therapeutic options to treat patients with vascular occlusive diseases. Lautz et al. identified midkine to be responsible for the increased bioavailability of VEGFA during arteriogenesis, necessary and sufficient to promote endothelial cell proliferation. These data might help to estimate the therapeutic effect of clinically applied VEGFA. As the identified mechanisms might also apply for angiogenesis, they are likely to be of broader relevance, e.g. in terms of tumor treatment.

Teriparatide Treatment Improves Bone Defect Healing Via Anabolic Effects on New Bone Formation and Non-Anabolic Effects on Inhibition of Mast Cells in a Murine Cranial Window Model

Investigations of teriparatide (recombinant parathyroid hormone [rPTH]) as a potential treatment for critical defects have demonstrated the predicted anabolic effects on bone formation, and significant non-anabolic effects on healing via undefined mechanisms. Specifically, studies in murine models of structural allograft healing demonstrated that rPTH treatment increased angiogenesis (vessels <30 μm), and decreased arteriogenesis (>30 μm) and mast cell numbers, which lead to decreased fibrosis and accelerated healing. To better understand these non-anabolic effects, we interrogated osteogenesis, vasculogenesis, and mast cell accumulation in mice randomized to placebo (saline), rPTH (20 μg/kg/2 days), or the mast cell inhibitor sodium cromolyn (SC) (24 μg/kg/ 2days), via longitudinal micro-computed tomography (μCT) and multiphoton laser scanning microscopy (MPLSM), in a critical calvaria defect model. μCT demonstrated that SC significantly increased defect window closure and new bone volume versus placebo (p < 0.05), although these effects were not as great as rPTH. Interestingly, both rPTH and SC have similar inhibitory effects on arteriogenesis versus placebo (p < 0.05) without affecting total vascular volume. MPLSM time-course studies in untreated mice revealed that large numbers of mast cells were detected 1 day postoperation (43 ± 17), peaked at 6 days (76 ± 6), and were still present in the critical defect at the end of the experiment on day 30 (20 ± 12). In contrast, angiogenesis was not observed until day 4, and functional vessels were first observed on 6 days, demonstrating that mast cell accumulation precedes vasculogenesis. To confirm a direct role of mast cells on osteogenesis and vasculogenesis, we demonstrated that specific diphtheria toxin-α deletion in Mcpt5-Cre-iDTR mice results in similar affects as SC treatment in WT mice. Collectively, these findings demonstrate that mast cells inhibit bone defect healing by stimulating arteriogenesis associated with fibrotic scaring, and that an efficacious non-anabolic effect of rPTH therapy on bone repair is suppression of arteriogenesis and fibrosis secondary to mast cell inhibition. © 2017 American Society for Bone and Mineral Research.

How neutrophils resist shear stress at blood vessel walls: molecular mechanisms, subcellular structures, and cell-cell interactions

Neutrophils are the first cells arriving at sites of tissue injury or infection to combat invading pathogens. Successful neutrophil recruitment to sites of inflammation highly depends on specific molecular mechanisms, fine-tuning the received information into signaling pathways and converting them into well-described recruitment steps. This review highlights the impact of vascular flow conditions on neutrophil recruitment and the multitude of mechanisms developed to enable this sophisticated process under wall shear stress conditions. The recruitment process underlies a complex interplay between adhesion and signaling molecules, as well as chemokines, in which neutrophils developed specific mechanisms to travel to sites of lesion in low and high shear stress conditions. Rolling, as the first step in the recruitment process, highly depends on endothelial selectins and their ligands on neutrophils, inducting of intracellular signaling and subsequently activating β₂ integrins, enabling adhesion and postadhesion events. In addition, subcellular structures, such as microvilli, tethers, and slings allow the cell to arrest, even under high wall shear stress. Thereby, microvilli that are pulled out from the cell body form tethers that develop into slings upon their detachment from the substrate. In addition to the above-described primary capture, secondary capture of neutrophils via neutrophil-neutrophil or neutrophil-platelet interaction promotes the process of neutrophil recruitment to sites of lesion. Thus, precise mechanisms based on a complex molecular interplay, subcellular structures, and cell-cell interactions turn the delicate process of neutrophil trafficking during flow into a robust response allowing effective neutrophil accumulation at sites of injury.