Defining Endothelial Cell-Derived Factors That Promote Pericyte Recruitment and Capillary Network Assembly

Surgical Bioengineering of the Microvasculature and Challenges in Clinical Translation

Tissue and organ dysfunction are major causes of worldwide morbidity and mortality with all medical specialties being impacted. Tissue engineering is an interdisciplinary field relying on the combination of scaffolds, cells, and biologically active molecules to restore form and function. However, clinical translation is still largely hampered by limitations in vascularization. Consequently, a thorough understanding of the microvasculature is warranted. This review provides an overview of (1) angiogenesis, including sprouting angiogenesis, intussusceptive angiogenesis, vascular remodeling, vascular co-option, and inosculation; (2) strategies for vascularized engineered tissue fabrication such as scaffold modulation, prevascularization, growth factor utilization, and cell-based approaches; (3) guided microvascular development via scaffold modulation with electromechanical cues, 3D bioprinting, and electrospinning; (4) surgical approaches to bridge the micro- and macrovasculatures in order to hasten perfusion; and (5) building specific vasculature in the context of tissue repair and organ transplantation, including skin, adipose, bone, liver, kidney, and lung. Our goal is to provide the reader with a translational overview that spans developmental biology, tissue engineering, and clinical surgery.

Endothelial-pericyte interactions regulate angiogenesis via VEGFR2 signaling during retinal development and disease

Pericytes stabilize the microvasculature by enhancing endothelial barrier integrity, resulting in functional networks. During retinal development, pericyte recruitment is crucial for stabilizing nascent angiogenic vasculature. However, in adulthood, disrupted endothelial-pericyte interactions lead to vascular dropout and pathological angiogenesis in ocular microvascular diseases, and strategies to stabilize the retinal vasculature are lacking. We demonstrate that direct endothelial-pericyte contact downregulates pVEGFR2 in endothelial cells, which enhances pericyte migration and promotes endothelial cell barrier function. Intravitreal injection of a VEGFR2 inhibitor in mouse models of the developing retina and oxygen-induced retinopathy increased pericyte recruitment and aided vascular stability. The VEGFR2 inhibitor further rescued ischemic retinopathy by enhancing vascularization and tissue growth while reducing vascular permeability. Our findings offer a druggable target to support the growth of functional and mature microvasculature in ocular microvascular diseases and tissue regeneration overall.

The Triad of Blood-Brain Barrier Integrity: Endothelial Cells, Astrocytes, and Pericytes in Perinatal Stroke Pathophysiology

Pediatric stroke, a significant cause of long-term neurological deficits in children, often arises from disruptions within neurovascular unit (NVU) components. The NVU, a dynamic ensemble of astrocytes, endothelial cells, pericytes, and microglia, is vital for maintaining cerebral homeostasis and regulating vascular brain development. Its structural integrity, particularly at the blood-brain barrier (BBB), depends on intercellular junctions and the basement membrane, which together restrict paracellular transport and shield the brain from systemic insults. Dysfunction in this intricate system is increasingly linked to pediatric stroke and related cerebrovascular conditions. Mutations disrupting endothelial cell adhesion or pericyte-endothelial interactions can compromise BBB stability, leading to pathological outcomes such as intraventricular hemorrhage in the germinal matrix, a hallmark of vascular brain immaturity. Additionally, inflammation, ferroptosis, necroptosis, and autophagy are key cellular processes influencing brain damage and repair. Excessive activation of these mechanisms can exacerbate NVU injury, whereas targeted therapeutic modulation offers potential pathways to mitigate damage and support recovery. This review explores the cellular and molecular mechanisms underlying NVU dysfunction, BBB disruption, and subsequent brain injury in pediatric stroke. Understanding the interplay between genetic mutations, environmental stressors, and NVU dynamics provides new insights into stroke pathogenesis. The susceptibility of the germinal matrix to vascular rupture further emphasizes the critical role of NVU integrity in early brain development. Targeting inflammatory pathways and cell death mechanisms presents promising strategies to preserve NVU function and improve outcomes for affected neonates.

Glycoprotein 130 Antagonism Counteracts Metabolic and Inflammatory Alterations to Enhance Right Ventricle Function in Pulmonary Artery Banded Pigs

Background

Right ventricular dysfunction (RVD) is a risk factor for death in multiple cardiovascular diseases, but RV-enhancing therapies are lacking. Inhibition of glycoprotein-130 (GP130) signaling with the small molecule SC144 improves RV function in rodent RVD via anti-inflammatory and metabolic mechanisms. However, SC144's efficacy and molecular effects in a translational large animal model of RVD are unknown.

Methods

4-week-old castrated male pigs underwent pulmonary artery banding (PAB). After 3 weeks, PAB pigs were randomized into 2 groups (daily injections of SC144 [2.2 mg/kg, PAB-SC144, n=5] or vehicle [PAB-Veh, n=5] for 3 weeks). Five age-matched pigs served as controls. Cardiac MRI quantified RV size/function. Right heart catheterization evaluated hemodynamics. Single-nucleus RNA sequencing delineated cell-type specific changes between experimental groups. Electron microscopy evaluated RV mitochondrial morphology. Phosphoproteomics identified dysregulated RV kinases. Lipidomics and metabolomics quantified lipid species and metabolites in RV tissue. Quantitative proteomics examined RV mitochondrial protein regulation.

Results

SC144 significantly improved RV ejection fraction (Control: 60±4%, PAB-Veh: 22±10%, PAB-SC144: 37±6%) despite similar RV afterload. Single-nucleus RNA sequencing demonstrated PAB-Veh pigs had lower cardiomyocyte and higher macrophage/lymphocyte/pericyte/endothelial cell abundances as compared to control, and many of these changes were blunted by SC144. SC144 combatted the downregulation of cardiomyocyte metabolic genes induced by PAB. Kinome enrichment analysis suggested SC144 counteracted RV mTORC1 activation. Correspondingly, SC144 rebalanced RV autophagy pathway proteins and improved mitochondrial morphology. Integrated lipidomics, metabolomics, and proteomics analyses revealed SC144 restored fatty acid metabolism. Finally, CellChat analysis revealed SC144 restored pericyte-endothelial cell cross-talk.

Conclusion

GP130 antagonism blunts elevated immune cell abundance, reduces pro-inflammatory gene transcription in macrophages and lymphocytes, rebalances autophagy and preserves fatty acid metabolism in cardiomyocytes, and restores endothelial cell and pericyte communication to improve RV function.

Repeated treatment with VEGF receptor inhibitors induces phenotypic changes in endothelial cells and pericytes in the rat retina

Abnormal ocular angiogenesis is a major cause of visual impairment and vision loss in neovascularization-related diseases. Currently, anti-vascular endothelial growth factor (VEGF) drugs are used to treat ocular neovascularization, but repeated injections are needed to maintain their therapeutic effects. However, repeated injection of anti-VEGF drugs may affect the retinal blood vessel phenotype and diminish therapeutic effects. In this study, we aimed to investigate the phenotypic changes in endothelial cells and pericytes caused by the repeated interruption of the VEGF receptor signaling pathway in neonatal rats. KRN633 (10 mg/kg), a VEGF receptor tyrosine kinase inhibitor, was subcutaneously administered on postnatal day (P)-7 and P8 (first round), P14 and P15 (second round), and P21 and P22 (third round). The rat eyes were collected on P7, P9, P14, P16, P21, P23, P28, and P35. Using retinal flat-mount specimens stained with specific markers for vascular endothelial cells, basement membranes, and pericytes, the arteriolar tortuosity, capillary area density, and distribution of pericytes were evaluated. Significant loss of capillaries was observed the day after the first round of KRN633 treatment, after which aggressive angiogenesis occurred, leading to the formation of tortuous arterioles. Rats that completed second and third rounds of KRN633 treatment showed more severe abnormalities in the retinal vasculature than those that only completed first round treatment. Repeated treatment with KRN633 decreased the anti-angiogenic effects but increased the immunoreactivity of α-smooth muscle actin in the pericytes on veins and capillaries. α-Smooth muscle actin expression was inversely correlated to anti-angiogenic effects. Overall, these results revealed that repeated interruption of VEGF receptor signaling pathway altered the phenotypes of endothelial cells and pericytes and induced anti-VEGF drug resistance. Therefore, careful follow-up is necessary when using anti-VEGF drugs to treat abnormal angiogenesis-associated ocular diseases.

Mural Cells Initiate Endothelial-to-Mesenchymal Transition in Adjacent Endothelial Cells in Extracranial AVMs

Extracranial arteriovenous malformations (eAVMs) are complex vascular lesions characterized by anomalous arteriovenous connections, vascular instability, and disruptions in endothelial cell (EC)-to-mural cell (MC) interactions. This study sought to determine whether eAVM-MCs could induce endothelial-to-mesenchymal transition (EndMT), a process known to disrupt vascular integrity, in the eAVM microenvironment. eAVM and paired control tissues were analyzed using RT-PCR for EC (CD31, CD34, and CDH5) and EndMT-specific markers (SNAI1, SNAI2, ACTA2/α-SMA, N-cadherin/CDH2, VIM). Immunohistochemistry (IHC) was also performed to analyze MC- (PDGFR-β and α-SMA), EC (CD31, CD34, and CDH5), and EndMT-specific markers (CDH2 and SNAI1) in sequential paraffin-embedded sections of eAVM patient biopsies and in adjacent normal tissue biopsies from the same patients. Furthermore, eAVM-MCs and MCs from normal paired tissues (NMCs) were then isolated from fresh human surgical samples using flow cytometry and co-cultured with normal human umbilical vein vascular endothelial cells (HUVECs), followed by analysis of CD31 by immunofluorescence. RT-PCR analysis did not show a significant difference in the expression of EC markers between eAVM tissues and controls, whereas expression of EndMT-specific markers was upregulated in eAVM tissues compared to controls. IHC of eAVMs and paired control tissues demonstrated regions of significant perivascular eAVM-MC expansion (PDGFR-β+, and α-SMA+) surrounding dilated, morphologically abnormal vessels. These regions contained endothelium undergoing EndMT as evidenced by loss of CD31, CD34, and CDH5 expression and upregulation of the EndMT-associated genes CDH2 and SNAI1. Isolated eAVM-MCs induced loss of CD31 in HUVECs when grown in co-culture, while NMCs did not. This study suggests that the eAVM endothelium surrounded by expanded eAVM-MCs undergoes EndMT, potentially leading to the formation of dilated and fragile vessels, and implicates the eAVM-MCs in EndMT initiation and eAVM pathology.

Enhancing human capillary tube network assembly and maturation through upregulated expression of pericyte-derived TIMP-3

In this study, we identify and characterize new molecular determinants that optimize human capillary tube network assembly. Our lab has previously reported a novel, serum free-defined 3D co-culture model using human endothelial cells (ECs) and human pericytes whereby EC-lined tubes form and co-assemble with pericytes, but when these cultures are maintained at or beyond 5 days, tubes become progressively wider and unstable. To address this issue, we generated novel human pericytes that carry a tissue inhibitor of metalloproteinase (TIMP)-3 transgene which can be upregulated following doxycycline addition. EC-pericyte co-cultures established in the presence of doxycycline demonstrated marked enhancement of capillary network assembly including dramatic narrowing of capillary tube widths to an average of 8 µm (physiologic capillary tube width), increased tube lengths, increased tube branching, and robust stimulation of basement membrane matrix assembly, particularly with collagen type IV and fibronectin deposition compared to controls. These substantial changes depend not only on induction of pericyte TIMP-3, but also on recruitment of pericytes to EC tubes. Blockade of pericyte recruitment prevents these dramatic capillary network alterations suggesting that EC-pericyte interactions and induction of pericyte TIMP-3 are necessary together to coordinate and facilitate capillary assembly and maturation. Overall, this work is critical for our basic understanding of capillary formation, but also for the ability to reproducibly generate stabilized networks of capillary tubes.

17β-Estradiol Abrogates TNF-α-Induced Human Brain Vascular Pericyte Migration by Downregulating miR-638 via ER-β

Pericytes (PCs) contribute to brain capillary/BBB integrity and PC migration is a hallmark for brain capillary leakage following pro-inflammatory insults. Estradiol promotes endothelial barrier integrity by inhibiting tumor necrosis factor-alpha (TNF-α)-induced PC migration. However, the underlying mechanisms remain unclear. Since micro-RNAs (miRs) regulate BBB integrity and increases in miR638 and TNF-α occur in pathological events associated with capillary leakage, we hypothesize that TNF-α mediates its capillary disruptive actions via miR638 and that estradiol blocks these actions. Using quantitative reverse transcription PCR, we first assessed the modulatory effects of TNF-α on miR638. The treatment of PCs with TNF-α significantly induced miR638. Moreover, transfection with miR638 mimic induced PC migration, whereas inhibitory miR638 (anti-miR) abrogated the pro-migratory actions of TNF-α, suggesting that TNF-α stimulates PC migration via miR638. At a molecular level, the pro-migratory effects of miR638 involved the phosphorylation of ERK1/2 but not Akt. Interestingly, estradiol downregulated the constitutive and TNF-α-stimulated expression of miR638 and inhibited the TNF-α-induced migration of PCs. In PCs treated with estrogen receptor (ER) ER-α, ER-β, and GPR30 agonists, a significant downregulation in miR638 expression was solely observed in response to DPN, an ER-β agonist. DPN inhibited the pro-migratory effects of TNF-α but not miR638. Additionally, the ectopic expression of miR638 prevented the inhibitory effects of DPN on TNF-α-induced PC migration, suggesting that interference in miR638 formation plays a key role in mediating the inhibitory actions of estradiol/DPN. In conclusion, these findings provide the first evidence that estradiol inhibits TNF-α-induced PC migration by specifically downregulating miR638 via ER-β and may protect the neurovascular unit during injury/stroke via this mechanism.

The generation of stable microvessels in ischemia is mediated by endothelial cell derived TRAIL

Reversal of ischemia is mediated by neo-angiogenesis requiring endothelial cell (EC) and pericyte interactions to form stable microvascular networks. We describe an unrecognized role for tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) in potentiating neo-angiogenesis and vessel stabilization. We show that the endothelium is a major source of TRAIL in the healthy circulation compromised in peripheral artery disease (PAD). EC deletion of TRAIL in vivo or in vitro inhibited neo-angiogenesis, pericyte recruitment, and vessel stabilization, resulting in reduced lower-limb blood perfusion with ischemia. Activation of the TRAIL receptor (TRAIL-R) restored blood perfusion and stable blood vessel networks in mice. Proof-of-concept studies showed that Conatumumab, an agonistic TRAIL-R2 antibody, promoted vascular sprouts from explanted patient arteries. Single-cell RNA sequencing revealed heparin-binding EGF-like growth factor in mediating EC-pericyte communications dependent on TRAIL. These studies highlight unique TRAIL-dependent mechanisms mediating neo-angiogenesis and vessel stabilization and the potential of repurposing TRAIL-R2 agonists to stimulate stable and functional microvessel networks to treat ischemia in PAD.

Rictor, an mTORC2 Protein, Regulates Murine Lymphatic Valve Formation Through the AKT-FOXO1 Signaling

BACKGROUND

Lymphatic valves are specialized structures in collecting lymphatic vessels and are crucial for preventing retrograde lymph flow. Mutations in valve-forming genes have been clinically implicated in the pathology of congenital lymphedema. Lymphatic valves form when oscillatory shear stress from lymph flow signals through the PI3K/AKT pathway to promote the transcription of valve-forming genes that trigger the growth and maintenance of lymphatic valves. Conventionally, in many cell types, AKT is phosphorylated at Ser473 by the mTORC2 (mammalian target of rapamycin complex 2). However, mTORC2 has not yet been implicated in lymphatic valve formation.

METHODS

In vivo and in vitro techniques were used to investigate the role of Rictor, a critical component of mTORC2, in lymphatic endothelium.

RESULTS

Here, we showed that embryonic and postnatal lymphatic deletion of Rictor, a critical component of mTORC2, led to a significant decrease in lymphatic valves and prevented the maturation of collecting lymphatic vessels. RICTOR knockdown in human dermal lymphatic endothelial cells not only reduced the level of activated AKT and the expression of valve-forming genes under no-flow conditions but also abolished the upregulation of AKT activity and valve-forming genes in response to oscillatory shear stress. We further showed that the AKT target, FOXO1 (forkhead box protein O1), a repressor of lymphatic valve formation, had increased nuclear activity in Rictor knockout mesenteric lymphatic endothelial cells in vivo. Deletion of Foxo1 in Rictor knockout mice restored the number of valves to control levels in lymphatic vessels of the ear and mesentery.

CONCLUSIONS

Our work identifies a novel role for RICTOR in the mechanotransduction signaling pathway, wherein it activates AKT and prevents the nuclear accumulation of the valve repressor, FOXO1, which ultimately enables the formation and maintenance of lymphatic valves.

Selective mural cell recruitment of pericytes to networks of assembling endothelial cell-lined tubes

Mural cells are critically important for the development, maturation, and maintenance of the blood vasculature. Pericytes are predominantly observed in capillaries and venules, while vascular smooth muscle cells (VSMCs) are found in arterioles, arteries, and veins. In this study, we have investigated functional differences between human pericytes and human coronary artery smooth muscle cells (CASMCs) as a model VSMC type. We compared the ability of these two mural cells to invade three-dimensional (3D) collagen matrices, recruit to developing human endothelial cell (EC)-lined tubes in 3D matrices and induce vascular basement membrane matrix assembly around these tubes. Here, we show that pericytes selectively invade, recruit, and induce basement membrane deposition on EC tubes under defined conditions, while CASMCs fail to respond equivalently. Pericytes dramatically invade 3D collagen matrices in response to the EC-derived factors, platelet-derived growth factor (PDGF)-BB, PDGF-DD, and endothelin-1, while minimal invasion occurs with CASMCs. Furthermore, pericytes recruit to EC tube networks, and induce basement membrane deposition around assembling EC tubes (narrow and elongated tubes) when these cells are co-cultured. In contrast, CASMCs are markedly less able to perform these functions showing minimal recruitment, little to no basement membrane deposition, with wider and shorter tubes. Our new findings suggest that pericytes demonstrate much greater functional ability to invade 3D matrix environments, recruit to EC-lined tubes and induce vascular basement membrane matrix deposition in response to and in conjunction with ECs.

Cold to Hot: Tumor Immunotherapy by Promoting Vascular Normalization Based on PDGFB Nanocomposites

Immunotherapy is a promising cancer therapeutic strategy. However, the "cold" tumor immune microenvironment (TIME), characterized by insufficient immune cell infiltration and immunosuppressive status, limits the efficacy of immunotherapy. Tumor vascular abnormalities due to defective pericyte coverage are gradually recognized as a profound determinant in "cold" TIME establishment by hindering immune cell trafficking. Recently, several vascular normalization strategies by improving pericyte coverage have been reported, whereas have unsatisfactory efficacy and high rates of resistance. Herein, a combinatorial strategy to induce tumor vasculature-targeted pericyte recruitment and zinc ion-mediated immune activation with a platelet-derived growth factor B (PDGFB)-loaded, cyclo (Arg-Gly-Asp-D-Phe-Lys)-modified zeolitic imidazolate framework 8 (PDGFB@ZIF8-RGD) nanoplatform is proposed. PDGFB@ZIF8-RGD effectively induced tumor vascular normalization, which facilitated trafficking and infiltration of immune effector cells, including natural killer (NK) cells, M1-like macrophages and CD8+ T cells, into tumor microenvironment. Simultaneously, vascular normalization promoted the accumulation of zinc ions inside tumors to trigger effector cell immune activation and effector molecule production. The synergy between these two effects endowed PDGFB@ZIF8-RGD with superior capabilities in reprogramming the "cold" TIME to a "hot" TIME, thereby initiating robust antitumor immunity and suppressing tumor growth. This combinatorial strategy for improving immune effector cell infiltration and activation is a promising paradigm for solid tumor immunotherapy.

Investigation of the differentiation potential of pericyte cells as an alternative source of mesenchymal stem cells

BACKGROUND

The mesenchymal stem cells (MSCs) with characterized by their multipotency and capacity to differentiate into various tissue cell types, have led to their incorporation in regenerative medicine research. However, the limited numbers of MSCs in the human body and their diverse differentiation capabilities in tissues highlight the need for exploring alternative regenerative cell sources. In this study, therefore, we conducted molecular level examinations to determine whether pericytes, specialized cell communities situated near blood vessels, could serve as a substitute for human bone marrow-derived mesenchymal stem cells (hBM-MSCs). In this context, the potential application of pericytes surrounds the vessels when MSCs are insufficient for functional purposes.

METHODS

The pericytes utilized in this investigation were derived from the placenta and characterized at the third passage. Similarly, the hBM-MSCs were also characterized at the third passage. The pluripotent properties of the two cell types were assessed at the gene expression level. Thereafter, both pericytes and hBM-MSCs were directed towards adipogenic, osteogenic and chondrogenic differentiation. The cells in both groups were examined on days 7, 14, and, 21 and their differentiation status was compared both immunohistochemically and through gene expression analysis.

RESULTS

Upon comparing the pluripotency characteristics of placental pericytes and hBM-MSCs, it was discovered that there was a substantial upregulation of the pluripotency genes FoxD3, Sox2, ZPF42, UTF1, and, Lin28 in both cell types. However, no significant expression of the genes Msx1, Nr6a1, Pdx1, and, GATA6 was observed in either cell type. It was also noted that pericytes differentiate into adipogenic, osteogenic and, chondrogenic lineages similar to hBM-MSCs.

DISCUSSION

As a result, it has been determined that pericytes exhibit high differentiation and proliferation properties similar to those of MSCs, and therefore can be considered a suitable alternative cell source for regenerative medicine and tissue engineering research, in cases where MSCs are not available or insufficient. It is notable that pericytes have been suggested as a potential substitute in studies where MSCs are lacking.

Defining the Functional Influence of Endothelial Cell-Expressed Oncogenic Activating Mutations on Vascular Morphogenesis and Capillary Assembly

This study sought to define key molecules and signals controlling major steps in vascular morphogenesis, and how these signals regulate pericyte recruitment and pericyte-induced basement membrane deposition. The morphogenic impact of endothelial cell (EC) expression of activating mutants of Kirsten rat sarcoma virus (kRas), mitogen-activated protein kinase 1 (Mek1), phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA), Akt serine/threonine kinase 1 (Akt1), Ras homolog enriched in brain (Rheb) Janus kinase 2 (Jak2), or signal transducer and activator of transcription 3 (Stat3) expression versus controls was evaluated, along with EC signaling events, pharmacologic inhibitor assays, and siRNA suppression experiments. Primary stimulators of EC lumen formation included kRas, Akt1, and Mek1, whereas PIK3CA and Akt1 stimulated a specialized type of cystic lumen formation. In contrast, the key drivers of EC sprouting behavior were Jak2, Stat3, Mek1, PIK3CA, and mammalian target of rapamycin (mTor). These conclusions are further supported by pharmacologic inhibitor and siRNA suppression experiments. EC expression of active Akt1, kRas, and PIK3CA led to markedly dysregulated lumen formation coupled to strongly inhibited pericyte recruitment and basement membrane deposition. For example, activated Akt1 expression in ECs excessively stimulated lumen formation, decreased EC sprouting behavior, and showed minimal pericyte recruitment with reduced mRNA expression of platelet-derived growth factor-BB, platelet-derived growth factor-DD, and endothelin-1, critical EC-derived factors known to stimulate pericyte invasion. The study identified key signals controlling fundamental steps in capillary morphogenesis and maturation and provided mechanistic details on why EC activating mutations induced a capillary deficiency state with abnormal lumens, impaired pericyte recruitment, and basement deposition: predisposing stimuli for the development of vascular malformations.

Mechanisms of angiogenesis in tumour

Angiogenesis is essential for tumour growth and metastasis. Antiangiogenic factor-targeting drugs have been approved as first line agents in a variety of oncology treatments. Clinical drugs frequently target the VEGF signalling pathway during sprouting angiogenesis. Accumulating evidence suggests that tumours can evade antiangiogenic therapy through other angiogenesis mechanisms in addition to the vascular sprouting mechanism involving endothelial cells. These mechanisms include (1) sprouting angiogenesis, (2) vasculogenic mimicry, (3) vessel intussusception, (4) vascular co-option, (5) cancer stem cell-derived angiogenesis, and (6) bone marrow-derived angiogenesis. Other non-sprouting angiogenic mechanisms are not entirely dependent on the VEGF signalling pathway. In clinical practice, the conversion of vascular mechanisms is closely related to the enhancement of tumour drug resistance, which often leads to clinical treatment failure. This article summarizes recent studies on six processes of tumour angiogenesis and provides suggestions for developing more effective techniques to improve the efficacy of antiangiogenic treatment.

Overexpression of Wnt5a promoted the protective effect of mesenchymal stem cells on Lipopolysaccharide-induced endothelial cell injury via activating PI3K/AKT signaling pathway

BACKGROUND

Lung endothelial barrier injury plays an important role in the pathophysiology of acute lung injury/acute respiratory distress syndrome (ALI/ARDS). Mesenchymal stem cells (MSCs) therapy has shown promise in ARDS treatment and restoration of the impaired barrier function. It has been reported that Wnt5a shows protective effects on endothelial cells. Therefore, the study aimed to investigate whether overexpression of Wnt5a could promote the protective effects of MSCs on Lipopolysaccharide (LPS)-induced endothelial cell injury.

METHODS

To evaluate the protective effects of MSCs overexpressing Wnt5a, we assessed the migration, proliferation, apoptosis, and angiogenic ability of endothelial cells. We assessed the transcription of protective cellular factors using qPCR and determined the molecular mechanism using Western blot analysis.

RESULTS

Overexpression of Wnt5a upregulated the transcription of protective cellular factors in MSCs. Co-culture of MSCWnt5a promoted endothelial migration, proliferation and angiogenesis, and inhibited endothelial cell apoptosis through the PI3K/AKT pathway.

CONCLUSIONS

Overexpression of Wnt5a promoted the therapeutic effect of MSCs on endothelial cell injury through the PI3K/AKT signaling. Our study provides a novel approach for utilizing genetically modified MSCs in the transplantation therapy for ARDS.

The TGFβ1/SMADs/Snail1 signaling axis mediates pericyte-derived fibrous scar formation after spinal cord injury

AIMS

The deposition of fibrous scars after spinal cord injury (SCI) affects axon regeneration and the recovery of sensorimotor function. It has been reported that microvascular pericytes in the neurovascular unit are the main source of myofibroblasts after SCI, but the specific molecular targets that regulate pericyte participation in the formation of fibrous scars remain to be clarified.

METHODS

In this study, a rat model of spinal cord dorsal hemisection injury was used. After SCI, epigallocatechin gallate (EGCG) was intraperitoneally injected to block the TGFβ1 signaling pathway or LV-Snail1-shRNA was immediately injected near the core of the injury using a microsyringe to silence Snail1 expression. Western blotting and RT-qPCR were used to analyze protein expression and transcription levels in tissues. Nissl staining and immunofluorescence analysis were used to analyze neuronal cell viability, scar tissue, and axon regeneration after SCI. Finally, the recovery of hind limb function after SCI was evaluated.

RESULTS

The results showed that targeted inhibition of Snail1 could block TGFβ1-induced pericyte-myofibroblast differentiation in vitro. In vivo experiments showed that timely blockade of Snail1 could reduce fibrous scar deposition after SCI, promote axon regeneration, improve neuronal survival, and facilitate the recovery of lower limb motor function.

CONCLUSION

In summary, Snail1 promotes the deposition of fibrous scars and inhibits axonal regeneration after SCI by inducing the differentiation of pericytes into myofibroblasts. Snail1 may be a promising therapeutic target for SCI.

Platelet-derived Growth Factor Activates Pericytes in the Microvessels of Chronic Subdural Hematoma Outer Membranes

Angiogenesis is one of the growth mechanisms of chronic subdural hematoma (CSDH). Pericytes have been implicated in the capillary sprouting during angiogenesis and are involved in brain ischemia and diabetic retinopathy. This study examined the pericyte expressions in CSDH outer membranes obtained during trepanation surgery. Eight samples of CSDH outer membranes and 35 samples of CSDH fluid were included. NG2, N-cadherin, VE-cadherin, Tie-2, endothelial nitric oxide synthase (eNOS), platelet-derived growth factor (PDGF) receptor-β (PDGFR-β), a well-known marker of pericytes, phosphorylated PDGFR-β at Tyr751, and β-actin expressions, were examined using western blot analysis. PDGFR-β, N-cadherin, and Tie-2 expression levels were also examined using immunohistochemistry. The concentrations of PDGF-BB in CSDH fluid samples were measured using enzyme-linked immunosorbent assay kits. NG2, N-cadherin, VE-cadherin, Tie-2, eNOS, PDGFR-β, and eNOS expressions in CSDH outer membranes were confirmed in all cases. Furthermore, phosphorylated PDGFR-β at Tyr751 was also detected. In addition, PDGFR-β, N-cadherin, and Tie-2 expressions were localized to the endothelial cells of the vessels within CSDH outer membranes by immunohistochemistry. The concentration of PDGF-BB in CSDH fluids was significantly higher than that in cerebrospinal fluid. These findings indicate that PDGF activates pericytes in the microvessels of CSDH outer membranes and suggest that pericytes are crucial in CSDH angiogenesis through the PDGF/PDGFR-β signaling pathway.

Spatiotemporal signaling underlies progressive vascular rarefaction in myocardial infarction

Therapeutic angiogenesis represents a promising avenue to revascularize the ischemic heart. Its limited success is partly due to our poor understanding of the cardiac stroma, specifically mural cells, and their response to ischemic injury. Here, we combine single-cell and positional transcriptomics to assess the behavior of mural cells within the healing heart. In response to myocardial infarction, mural cells adopt an altered state closely associated with the infarct and retain a distinct lineage from fibroblasts. This response is concurrent with vascular rarefaction and reduced vascular coverage by mural cells. Positional transcriptomics reveals that the infarcted heart is governed by regional-dependent and temporally regulated programs. While the remote zone acts as an important source of pro-angiogenic signals, the infarct zone is accentuated by chronic activation of anti-angiogenic, pro-fibrotic, and inflammatory cues. Together, our work unveils the spatiotemporal programs underlying cardiac repair and establishes an association between vascular deterioration and mural cell dysfunction.

Pericyte dysfunction is a key mediator of the risk of cerebral ischemia

Pericytes are critical yet understudied cells that are a central component of the neurovascular unit. They are connected to the cerebrovascular endothelium and help control vascular contractility and maintain the blood-brain barrier. Pericyte dysfunction has the potential to mediate many of the deleterious vascular consequences of ischemic stroke. Current therapeutics are designed to be administered after stroke onset and limit damage, but there are few options to target vascular risk factors pre-ischemia which likely contribute to stroke outcomes. Here, we focus on the role of pericytes in health and disease, and discuss how pericyte dysfunction can increase the risk of ischemic injury. Additionally, we note that despite the importance of pericytes in cerebrovascular disease, there are relatively few current therapeutic options that target pericyte function.

Elucidating the Morphogenic and Signaling Roles of Defined Growth Factors Controlling Human Endothelial Cell Lumen Formation Versus Sprouting Behavior

Five growth factors [ie, insulin, fibroblast growth factor-2 (FGF-2), stem cell factor, IL-3, and stromal-derived factor 1α] in combination are necessary for human endothelial cells (ECs) to undergo tube morphogenesis, a process requiring both lumen formation and sprouting behavior. This study investigated why these factors are required by subdividing the factors into 4 separate groups: insulin-only, insulin and FGF-2, no FGF-2 (all factors but without FGF-2), and all factors. The study found that the insulin-only condition failed to support EC morphogenesis or survival, the insulin and FGF-2 condition supported primarily EC lumen formation, and the no FGF-2 condition supported EC sprouting behavior. By comparison, the all-factors condition more strongly stimulated both EC lumen formation and sprouting behavior, and signaling analysis revealed prolonged stimulation of multiple promorphogenic signals coupled with inhibition of proregressive signals. Pharmacologic inhibition of Jak kinases more selectively blocked EC sprouting behavior, whereas inhibition of Raf, phosphatidylinositol 3-kinase, and Akt kinases showed selective blockade of lumen formation. Inhibition of Src family kinases and Notch led to increased sprouting coupled to decreased lumen formation, whereas inhibition of Pak, Mek, and mammalian target of rapamycin kinases blocked both sprouting and lumen formation. These findings reveal novel downstream biological and signaling activities of defined factors that are required for the assembly of human EC-lined capillary tube networks.

Extracellular Matrix Stiffness Regulates Microvascular Stability by Controlling Endothelial Paracrine Signaling to Determine Pericyte Fate

BACKGROUND

The differentiation of pericytes into myofibroblasts causes microvascular degeneration, ECM (extracellular matrix) accumulation, and tissue stiffening, characteristics of fibrotic diseases. It is unclear how pericyte-myofibroblast differentiation is regulated in the microvascular environment. Our previous study established a novel 2-dimensional platform for coculturing microvascular endothelial cells (ECs) and pericytes derived from the same tissue. This study investigated how ECM stiffness regulated microvascular ECs, pericytes, and their interactions.

METHODS

Primary microvessels were cultured in the TGM2D medium (tubular microvascular growth medium on 2-dimensional substrates). Stiff ECM was prepared by incubating ECM solution in regular culture dishes for 1 hour followed by PBS wash. Soft ECM with Young modulus of ≈6 kPa was used unless otherwise noted. Bone grafts were prepared from the rat skull. Immunostaining, RNA sequencing, RT-qPCR (real-time quantitative polymerase chain reaction), Western blotting, and knockdown experiments were performed on the cells.

RESULTS

Primary microvascular pericytes differentiated into myofibroblasts (NG2+αSMA+) on stiff ECM, even with the TGFβ (transforming growth factor beta) signaling inhibitor A83-01. Soft ECM and A83-01 cooperatively maintained microvascular stability while inhibiting pericyte-myofibroblast differentiation (NG2+αSMA-/low). We thus defined 2 pericyte subpopulations: primary (NG2+αSMA-/low) and activated (NG2+αSMA+) pericytes. Soft ECM promoted microvascular regeneration and inhibited fibrosis in bone graft transplantation in vivo. As integrins are the major mechanosensor, we performed RT-qPCR screening of integrin family members and found Itgb1 (integrin β1) was the major subunit downregulated by soft ECM and A83-01 treatment. Knocking down Itgb1 suppressed myofibroblast differentiation on stiff ECM. Interestingly, ITGB1 phosphorylation (Y783) was mainly located on microvascular ECs on stiff ECM, which promoted EC secretion of paracrine factors, including CTGF (connective tissue growth factor), to induce pericyte-myofibroblast differentiation. CTGF knockdown or monoclonal antibody treatment partially reduced myofibroblast differentiation, implying the participation of multiple pathways in fibrosis formation.

CONCLUSIONS

ECM stiffness and TGFβ signaling cooperatively regulate microvascular stability and pericyte-myofibroblast differentiation. Stiff ECM promotes EC ITGB1 phosphorylation (Y783) and CTGF secretion, which induces pericyte-myofibroblast differentiation.

Pathophysiology, Management, and Therapeutics in Subarachnoid Hemorrhage and Delayed Cerebral Ischemia: An Overview

Subarachnoid hemorrhage (SAH) is a type of hemorrhagic stroke resulting from the rupture of an arterial vessel within the brain. Unlike other stroke types, SAH affects both young adults (mid-40s) and the geriatric population. Patients with SAH often experience significant neurological deficits, leading to a substantial societal burden in terms of lost potential years of life. This review provides a comprehensive overview of SAH, examining its development across different stages (early, intermediate, and late) and highlighting the pathophysiological and pathohistological processes specific to each phase. The clinical management of SAH is also explored, focusing on tailored treatments and interventions to address the unique pathological changes that occur during each stage. Additionally, the paper reviews current treatment modalities and pharmacological interventions based on the evolving guidelines provided by the American Heart Association (AHA). Recent advances in our understanding of SAH will facilitate clinicians' improved management of SAH to reduce the incidence of delayed cerebral ischemia in patients.

EDNRA-Expressing Mesenchymal Cells Are Expanded in Myeloma Interstitial Bone Marrow and Associated with Disease Progression

Multiple myeloma (MM) induces dysfunctional bone marrow (BM) mesenchymal cells and neoangiogenesis. Pericytes and smooth muscle cells (SMCs) could detach from vessels and become cancer-associated fibroblasts. We found that the pericyte and SMC marker endothelin receptor type A (EDNRA) is overexpressed in whole MM bone biopsies; we sought to characterize its expression. EDNRA expression gradually increased with disease progression. High-risk MM patients had higher EDNRA expression than low-risk MM patients and EDNRA expression was highest in focal lesions. High EDNRA expression was associated with high expression of pericyte markers (e.g., RGS5, POSTN, and CD146) and the angiogenic marker FLT1. A single-cell analysis of unexpanded BM mesenchymal cells detected EDNRA expression in a subset of cells that coexpressed mesenchymal cell markers and had higher expression of proliferation genes. Immunohistochemistry revealed that the number of EDNRA+ cells in the interstitial BM increased as MM progressed; EDNRA+ cells were prevalent in areas near the MM focal growth. EDNRA+ cells were detached from CD34+ angiogenic cells and coexpressed RGS5 and periostin. Therefore, they likely originated from pericytes or SMCs. These findings identify a novel microenvironmental biomarker in MM and suggest that the presence of detached EDNRA+ cells indicates disrupted vasculature and increased angiogenesis.

Human blood vessel organoids reveal a critical role for CTGF in maintaining microvascular integrity

The microvasculature plays a key role in tissue perfusion and exchange of gases and metabolites. In this study we use human blood vessel organoids (BVOs) as a model of the microvasculature. BVOs fully recapitulate key features of the human microvasculature, including the reliance of mature endothelial cells on glycolytic metabolism, as concluded from metabolic flux assays and mass spectrometry-based metabolomics using stable tracing of 13C-glucose. Pharmacological targeting of PFKFB3, an activator of glycolysis, using two chemical inhibitors results in rapid BVO restructuring, vessel regression with reduced pericyte coverage. PFKFB3 mutant BVOs also display similar structural remodelling. Proteomic analysis of the BVO secretome reveal remodelling of the extracellular matrix and differential expression of paracrine mediators such as CTGF. Treatment with recombinant CTGF recovers microvessel structure. In this work we demonstrate that BVOs rapidly undergo restructuring in response to metabolic changes and identify CTGF as a critical paracrine regulator of microvascular integrity.

No-Reflow Post-Recanalization in Acute Ischemic Stroke: Mechanisms, Measurements, and Molecular Markers

Acute ischemic stroke remains the primary cause of disability worldwide. For patients with large vessel occlusions, intravenous thrombolysis followed by mechanical thrombectomy remains the standard of care. Revascularization of the large vessel is typically successful. However, despite reopening of the occluded vessel, many patients fail to return to independence. Functional failure, despite macrovascular recanalization, is often referred to as the no-reflow phenomenon. Even with an extensive characterization of reperfusion in animal models, numerous mechanisms may explain no-reflow. Further, uniform measurements of this microvascular dysfunction and prognostic markers associated with no-reflow are lacking. In this review, we highlight a number of mechanisms that may explain no-reflow, characterize current multimodal measurements, and assess its molecular markers.

Extracellular Matrix Remodeling in Vascular Disease: Defining Its Regulators and Pathological Influence

Because of structural and cellular differences (ie, degrees of matrix abundance and cross-linking, mural cell density, and adventitia), large and medium-sized vessels, in comparison to capillaries, react in a unique manner to stimuli that induce vascular disease. A stereotypical vascular injury response is ECM (extracellular matrix) remodeling that occurs particularly in larger vessels in response to injurious stimuli, such as elevated angiotensin II, hyperlipidemia, hyperglycemia, genetic deficiencies, inflammatory cell infiltration, or exposure to proinflammatory mediators. Even with substantial and prolonged vascular damage, large- and medium-sized arteries, persist, but become modified by (1) changes in vascular wall cellularity; (2) modifications in the differentiation status of endothelial cells, vascular smooth muscle cells, or adventitial stem cells (each can become activated); (3) infiltration of the vascular wall by various leukocyte types; (4) increased exposure to critical growth factors and proinflammatory mediators; and (5) marked changes in the vascular ECM, that remodels from a homeostatic, prodifferentiation ECM environment to matrices that instead promote tissue reparative responses. This latter ECM presents previously hidden matricryptic sites that bind integrins to signal vascular cells and infiltrating leukocytes (in coordination with other mediators) to proliferate, invade, secrete ECM-degrading proteinases, and deposit injury-induced matrices (predisposing to vessel wall fibrosis). In contrast, in response to similar stimuli, capillaries can undergo regression responses (rarefaction). In summary, we have described the molecular events controlling ECM remodeling in major vascular diseases as well as the differential responses of arteries versus capillaries to key mediators inducing vascular injury.

The role of coagulome in the tumor immune microenvironment

The rising incidence and persistent thrombosis in multiple cancers including those that are immunosuppressive highlight the need for understanding the tumor coagulome system and its role beyond hemostatic complications. Immunotherapy has shown significant benefits in solid organ tumors but has been disappointing in the treatment of hypercoagulable cancers, such as glioblastoma and pancreatic ductal adenocarcinomas. Thus, targeting thrombosis to prevent immunosuppression seems a clinically viable approach in cancer treatment. Hypercoagulable tumors often develop fibrin clots within the tumor microenvironment (TME) that dictates the biophysical characteristics of the tumor tissue. The application of systems biology and single-cell approaches highlight the potential role of coagulome or thrombocytosis in shaping the tumor immune microenvironment (TIME). In-depth knowledge of the tumor coagulome would provide unprecedented opportunities to better predict the hemostatic complications, explore how thrombotic stroma modulates tumor immunity, reexamine the significance of clinical biomarkers, and enable steering the stromal versus systemic immune response for boosting the effectiveness of immune checkpoint inhibitors in cancer treatment. We focus on the role of coagulation factors in priming a suppressive TIME and the huge potential of existing anticoagulant drugs in the clinical settings of cancer immunotherapy.

Optimization of Culture Media and Cell Ratios for 3D In Vitro Skeletal Muscle Tissues with Endothelial Cells

A major challenge of engineering larger macroscale tissues in vitro is the limited diffusion of nutrients and oxygen to the interior. For skeletal muscle, this limitation results in millimeter scale outcomes to avoid necrosis. One method to address this constraint may be to vascularize in vitro-grown muscle tissue, to support nutrient (culture media) flow into the interior of the structure. In this exploratory study, we examine culture conditions that enable myogenic development and endothelial cell survival within tissue engineered 3D muscles. Myoblasts (C2C12s), endothelial cells (HUVECs), and endothelial support cells (C3H 10T1/2s) were seeded into Matrigel-fibrin hydrogels and cast into 3D printed frames to form 3D in vitro skeletal muscle tissues. Our preliminary results suggest that the simultaneous optimization of culture media formulation and cell concentrations is necessary for 3D cultured muscles to exhibit robust myosin heavy chain expression and GFP expression from GFP-transfected endothelial cells. The ability to form differentiated 3D muscles containing endothelial cells is a key step toward achieving vascularized 3D muscle tissues, which have potential use as tissue for implantation in a medical setting, as well as for future foods such as cultivated meats.

Pericytes and vascular smooth muscle cells in central nervous system arteriovenous malformations

Previously considered passive support cells, mural cells-pericytes and vascular smooth muscle cells-have started to garner more attention in disease research, as more subclassifications, based on morphology, gene expression, and function, have been discovered. Central nervous system (CNS) arteriovenous malformations (AVMs) represent a neurovascular disorder in which mural cells have been shown to be affected, both in animal models and in human patients. To study consequences to mural cells in the context of AVMs, various animal models have been developed to mimic and predict human AVM pathologies. A key takeaway from recently published work is that AVMs and mural cells are heterogeneous in their molecular, cellular, and functional characteristics. In this review, we summarize the observed perturbations to mural cells in human CNS AVM samples and CNS AVM animal models, and we discuss various potential mechanisms relating mural cell pathologies to AVMs.

Platelet-Derived Growth Factor-BB Priming Enhances Vasculogenic Capacity of Bone Marrow-Derived Endothelial Precursor Like Cells

BACKGROUND

Human endothelial progenitor cells (EPCs) were first identified in the peripheral blood and later in the cord blood and bone marrow (BM) with different vascularization capacity and different surface marker profiles. However, their identity and functional roles in neovascularization have not been clearly demonstrated in vivo and in vitro.

METHODS

Characterization of BM-EPC like cells were performed by fluorescence-activated cell sorting, immunofluorescence staining, enzyme-linked immunosorbent assay, Matrigel tube formation assay, and western blot analysis.

RESULTS

BM-EPC like cells were identified by selective adhesion to fibronectin and collagen from BM mononuclear cells, which generate fast-growing colonies with spindle morphology, express surface markers of CD105, vWF, UEA-I lectin binding, secrete HGF, VEGF, TGF-beta1 but can be distinguished from circulating EPC and endothelial cells by no expression of surface markers such as CD31, CD309, CD45, and CD34. These BM-EPC like cells shared many cell surface markers of BM-mesenchymal stem cells (MSC) but also can be distinguished by their vasculogenic property and other unique surface markers. Furthermore, the vasculogenic capacity of BM-EPC like cells were enhanced by co-culture of BM-MSC or PDGF-BB priming. PDGF-BB stimulated cell migration, proliferation, and secretion of laminin β-1, which was proposed as one of the mechanisms involved in the better vascularization of BM-EPC like cells.

CONCLUSION

PDGF-BB priming may be applied to improve the potency and function of BM-EPC like cells as vasculogenic cell therapy for the ischemic vascular repair.

Endothelial-derived angiocrine factors as instructors of embryonic development

Blood vessels are well-known to play roles in organ development and repair, primarily owing to their fundamental function in delivering oxygen and nutrients to tissues to promote their growth and homeostasis. Endothelial cells however are not merely passive conduits for carrying blood. There is now evidence that endothelial cells of the vasculature actively regulate tissue-specific development, morphogenesis and organ function, as well as playing roles in disease and cancer. Angiocrine factors are growth factors, cytokines, signaling molecules or other regulators produced directly from endothelial cells to instruct a diverse range of signaling outcomes in the cellular microenvironment, and are critical mediators of the vascular control of organ function. The roles of angiocrine signaling are only beginning to be uncovered in diverse fields such as homeostasis, regeneration, organogenesis, stem-cell maintenance, cell differentiation and tumour growth. While in some cases the specific angiocrine factor involved in these processes has been identified, in many cases the molecular identity of the angiocrine factor(s) remain to be discovered, even though the importance of angiocrine signaling has been implicated. In this review, we will specifically focus on roles for endothelial-derived angiocrine signaling in instructing tissue morphogenesis and organogenesis during embryonic and perinatal development.

A novel retinoic acid drug, EYE-502, inhibits choroidal neovascularization by targeting endothelial cells and pericytes

Choroidal neovascularization (CNV) occurs in neovascular age-related macular degeneration (AMD) and often leads to permanent visual impairment. Intravitreal injection of anti-vascular endothelial growth factor (VEGF) agents is the gold standard for the treatment of CNV. However, anti-VEGF treatment did not always cause vision improvement and sometimes had detrimental effects on normal retinal tissues. Herein, we identified a novel retinoic acid drug, EYE-502, which had great therapeutic effects on CNV. Administration of EYE-502 could inhibit VEGF-induced dysfunction of endothelial cells (ECs) and reduce platelet-derived growth factor (PDGF)-induced recruitment of pericytes to ECs in vitro. Administration of EYE-502 could reduce the area of choroidal sprouting and laser-induced CNV, exhibiting similar anti-angiogenic effects as aflibercept. Moreover, administration of EYE-502 could reduce pericyte coverage in the sprouting vessels and choroidal neovascularization. Mechanistically, EYE-502 primarily bound to retinoic acid receptors (RARs) and exerted the anti-angiogenic effects by targeting ECs and pericytes via affecting the activation of Wnt/β-catenin and PDGF/PDGFR/PI3K/Akt signaling. Taken together, this study reports a novel retinoic acid drug, EYE-502, which can exert the anti-angiogenic effects by simultaneous targeting of ECs and pericytes.

Insights into Endothelin Receptors in Pulmonary Hypertension

Pulmonary hypertension (PH) is a disease which affects the cardiopulmonary system; it is defined as a mean pulmonary artery pressure (mPAP) > 20 mmHg as measured by right heart catheterization at rest, and is caused by complex and diverse mechanisms. In response to stimuli such as hypoxia and ischemia, the expression and synthesis of endothelin (ET) increase, leading to the activation of various signaling pathways downstream of it and producing effects such as the induction of abnormal vascular proliferation during the development of the disease. This paper reviews the regulation of endothelin receptors and their pathways in normal physiological processes and disease processes, and describes the mechanistic roles of ET receptor antagonists that are currently approved and used in clinical studies. Current clinical researches on ET are focused on the development of multi-target combinations and novel delivery methods to improve efficacy and patient compliance while reducing side effects. In this review, future research directions and trends of ET targets are described, including monotherapy and precision medicine.

Rictor induces AKT signaling to regulate lymphatic valve formation

Lymphatic valves are specialized structures of the collecting lymphatic vessels and are crucial for preventing retrograde lymph flow. Mutations in valve-forming genes have been clinically implicated in the pathology of congenital lymphedema. Lymphatic valves form when oscillatory shear stress (OSS) from lymph flow signals through the PI3K/AKT pathway to promote the transcription of valve-forming genes that trigger the growth and maintenance of lymphatic valves throughout life. Conventionally, in other tissue types, AKT activation requires dual kinase activity and the mammalian target of rapamycin complex 2 (mTORC2) commands this process by phosphorylating AKT at Ser473. Here we showed that embryonic and postnatal lymphatic deletion of Rictor , a critical component of mTORC2, led to a significant decrease in lymphatic valves and prevented the maturation of collecting lymphatic vessels. RICTOR knockdown in human lymphatic endothelial cells (hdLECs) not only significantly reduced the level of activated AKT and the expression of valve-forming genes under no-flow conditions, but also abolished the upregulation of AKT activity and valve-forming genes in response to flow. We further showed that the AKT target, FOXO1, a repressor of lymphatic valve formation, had increased nuclear activity in Rictor knockout mesenteric LECs, in vivo . Deletion of Foxo1 in Rictor knockout mice restored the number of valves to control levels in both mesenteric and ear lymphatics. Our work revealed a novel role of RICTOR signaling in the mechanotransduction signaling pathway, wherein it activates AKT and prevents the nuclear accumulation of the valve repressor, FOXO1, which ultimately allows the formation and maintenance of a normal lymphatic valve.

Extracellular Matrix Regulation of Vascular Morphogenesis, Maturation, and Stabilization

The extracellular matrix represents a critical regulator of tissue vascularization during embryonic development and postnatal life. In this perspective, we present key information and concepts that focus on how the extracellular matrix controls capillary assembly, maturation, and stabilization, and, in addition, contributes to tissue stability and health. In particular, we present and discuss mechanistic details underlying (1) the role of the extracellular matrix in controlling different steps of vascular morphogenesis, (2) the ability of endothelial cells (ECs) and pericytes to coassemble into elongated and narrow capillary EC-lined tubes with associated pericytes and basement membrane matrices, and (3) the identification of specific growth factor combinations ("factors") and peptides as well as coordinated "factor" and extracellular matrix receptor signaling pathways that are required to form stabilized capillary networks.

Pericytes in the tumor microenvironment

Pericytes are a type of mural cell located between the endothelial cells of capillaries and the basement membrane, which function to regulate the capillary vasomotor and maintain normal microcirculation of local tissues and organs and have been identified as a significant component in the tumor microenvironment (TME). Pericytes have various interactions with different components of the TME, such as constituting the pre-metastatic niche, promoting the growth of cancer cells and drug resistance through paracrine activity, and inducing M2 macrophage polarization. While changes in the TME can affect the number, phenotype, and molecular markers of pericytes. For example, pericyte detachment from endothelial cells in the TME facilitates tumor cells in situ to invade the circulating blood and is beneficial to local capillary basement membrane enzymatic hydrolysis and endothelial cell proliferation and budding, which contribute to tumor angiogenesis and metastasis. In this review, we discuss the emerging role of pericytes in the TME, and tumor treatment related to pericytes. This review aimed to provide a more comprehensive understanding of the function of pericytes and the relationship between pericytes and tumors and to provide ideas for the treatment and prevention of malignant tumors.

BEYOND THE ENDOTHELIUM: THE ROLE OF MURAL CELLS IN VASCULAR BIOLOGY: In vitro systems to study endothelial/pericyte cell interactions

The vasculature is crucial for tissue development and survival, and the stability of blood vessels to perform these functions relies on the interplay between endothelial cells (ECs) and mural cells. Pericytes are a subtype of mural cells found in the microvasculature that extend their processes to wrap around the endothelial monolayer. Pericytes are recruited during vessel growth through the excretion of soluble factors from ECs where they stabilize angiogenic sprouts and induce maturation of the resident cells. Alterations in these interactions between ECs and pericytes are associated with aberrant vessel growth and disrupted vasculature function characteristic of numerous diseases. Therefore, deeper understanding of the cross-talk between these cell types has numerous implications for understanding morphogenesis and elucidating disease mechanisms. In this review, we highlight recent advances and current trends studying the interactions between ECs and pericytes in vitro. We begin by analyzing three-dimensional hydrogel platforms that mimic the tissue extracellular matrix to investigate signaling pathways and altered vascular function in disease-specific cells. We next examine how microfluidic vasculature-on-a-chip platforms have elucidated the interplay of these vascular cells during angiogenesis and vascular network formation under controlled physiochemical cues and interstitial flow. Additionally, studies have utilized microvessels to measure the effect of shear stress on barrier function through the control of luminal flow and the impact of inflammation on these vascular cell interactions. Finally, we briefly highlight self-assembling human blood vessel organoids, an emerging high-throughput platform to study ECs and pericyte interactions.

The heterogeneity of fibrosis and angiogenesis in endometriosis revealed by single-cell RNA-sequencing

Fibrosis, angiogenesis and chronic inflammation are the inherent characteristics of endometriosis (EMS). The cellular heterogeneity of ectopic and non-ectopic endometrium by single-cell RNA-sequencing (scRNA-seq)at secretory phase without the disturbance of hormone drugs hasn't been explored so far. In this study, scRNA-seq was adopted to explore the properties of ectopic endometrium (ECE), eutopic endometrium (EUE) and normal endometrium (NOE) at secretory phase. We found that (i) The proportion of myofibroblasts, pericytes, endothelial cells and macrophages in ECE overwhelms that of non-ectopic tissues (EUE and NOE), and Myofibro.C2 was the predominant myofibroblast sub-cluster in ECE. (ii) Myofibroblasts were mainly fibroblast-to-myofibroblast transdifferentiation (FMT) and pericytes were endothelial cell-dependent differentiation in ECE. (iii) Both myofibroblasts and pericytes had a low differentiation potential. (iv) The increased inflammation score, deceased NK cells, T cell exhaustion score and antigen-presenting capacity in ECE confirmed the inflammatory properties and immunodeficiency of ECE. These findings suggested that myofibroblasts, pericytes and macrophages may be the potential targets for anti-fibrotic, anti-angiogenic and anti-inflammatory therapy of EMS.

Type H vessel/platelet-derived growth factor receptor β+ perivascular cell disintegration is involved in vascular injury and bone loss in radiation-induced bone damage

Collapse of the microvascular system is a prerequisite for radiation-induced bone loss. Since type H vessels, a specific bone vessel subtype surrounded by platelet-derived growth factor receptor β⁺ (PDGFRβ⁺ ) perivascular cells (PVCs), has been recently identified to couple angiogenesis and osteogenesis, we hypothesize that type H vessel injury initiates PDGFRβ⁺ PVC dysfunction, which contributes to the abnormal angiogenesis and osteogenesis after irradiation. In this study, we found that radiation led to the decrease of both type H endothelial cell (EC) and PDGFRβ⁺ PVC numbers. Remarkably, results from lineage tracing showed that PDGFRβ⁺ PVCs detached from microvessels and converted the lineage commitment from osteoblasts to adipocytes, leading to vascular injury and bone loss after irradiation. These phenotype transitions above were further verified to be associated with the decrease in hypoxia-inducible factor-1α (HIF-1α)/PDGF-BB/PDGFRβ signalling between type H ECs and PDGFRβ⁺ PVCs. Pharmacological blockade of HIF-1α/PDGF-BB/PDGFRβ signalling induced a phenotype similar to radiation-induced bone damage, while the rescue of this signalling significantly alleviated radiation-induced bone injury. Our findings show that the decrease in HIF-1α/PDGF-BB/PDGFRβ signalling between type H ECs and PDGFRβ⁺ PVCs after irradiation affects the homeostasis of EC-PVC coupling and plays a part in vascular damage and bone loss, which has broad implications for effective translational therapies.

Sema4D-plexin-B1 signaling in recruiting dental stem cells for vascular stabilization on a microfluidic platform

The recruitment of mural cells is critical for stabilization of nascent vessels. Stem cells from human exfoliated deciduous teeth (SHED) are considered to have mural cell-like properties. However, the signaling mechanisms that regulate the cross-talk between endothelial cells and SHED in recruiting them as mural cells is much less well understood. Herein, using a 3D biomimetic microfluidic device, for the first time, we unraveled the role of semaphorin 4D (Sema4D)-plexin-B1 signaling in the recruitment of SHED as mural cells during angiogenic sprouting and vasculature formation by endothelial cells (ECs) in a 3D fibrin matrix. The specific compartmentalized design of the microfluidic chip facilitated recreation of the multi-step dynamic process of angiogenesis in a time and space dependent manner. Enabled by the chip design, different morphogenic steps of angiogenesis including endothelial proliferation, migration & invasion, vascular sprout formation and recruitment of mural cells as well as functional aspects including perfusion and permeability were examined under various pharmacological and genetic manipulations. The results showed that Sema4D facilitates the interaction between endothelial cells and SHED and promotes the recruitment of SHED as mural cells in vascular stabilization. Our results further demonstrated that Sema4D exerts these effects by acting on endothelial-plexin-B1 by inducing expression of platelet-derived growth factor (PDGF)-BB, which is a major mural cell recruitment factor.

The GPI-Anchored Protein Thy-1/CD90 Promotes Wound Healing upon Injury to the Skin by Enhancing Skin Perfusion

Wound healing is a highly regulated multi-step process that involves a plethora of signals. Blood perfusion is crucial in wound healing and abnormalities in the formation of new blood vessels define the outcome of the wound healing process. Thy-1 has been implicated in angiogenesis and silencing of the Thy-1 gene retards the wound healing process. However, the role of Thy-1 in blood perfusion during wound closure remains unclear. We proposed that Thy-1 regulates vascular perfusion, affecting the healing rate in mouse skin. We analyzed the time of recovery, blood perfusion using Laser Speckle Contrast Imaging, and tissue morphology from images acquired with a Nanozoomer tissue scanner. The latter was assessed in a tissue sample taken with a biopsy punch on several days during the wound healing process. Results obtained with the Thy-1 knockout (Thy-1^−/−) mice were compared with control mice. Thy-1^−/− mice showed at day seven, a delayed re-epithelialization, increased micro- to macro-circulation ratio, and lower blood perfusion in the wound area. In addition, skin morphology displayed a flatter epidermis, fewer ridges, and almost no stratum granulosum or corneum, while the dermis was thicker, showing more fibroblasts and fewer lymphocytes. Our results suggest a critical role for Thy-1 in wound healing, particularly in vascular dynamics.

Identification of a new way to induce differentiation of dermal fibroblasts into vascular endothelial cells

Background

Human dermal fibroblasts (HDFs) have the potential to differentiate into vascular endothelial cells (VECs), but their differentiation rate is low and the mechanism involved is not clear. The small molecule pathway controls the phenotype of fibroblasts by activating cellular signaling pathways, which is a more convenient method in the differentiation strategy of HDFs into VECs.

Methods

In this study, HDFs were treated with the different doses of CPP ((E)-4-(4-(4-(7-(diethylamino)-2-oxo-2H-chromene-3-carbonyl) piperazin-1-yl) styryl)-1-methylpyridin-1-ium iodide), and the mRNA and protein levels of HDFs were detected by qPCR, Western blot, flow cytometry and immunofluorescent staining. The matrigel assays, acetylated-LDL uptake and angiogenesis assays of chick embryo chorioallantoic membrane (CAM) and hindlimb ischemia model of nude mice were performed to evaluate the functions of VECs derived from HDFs.

Results

Here, we report that the small chemical molecule, CPP, can effectively induce HDFs to differentiate into VECs. First, we observed the morphological changes of HDFS treated with CPP. Flow cytometry, Western blot and qRT-PCR analyses showed that CPP effectively decreased the level of the HDFs-marker Vimentin and increased levels of the VEC-markers CD31, CD133, TEK, ERG, vWF, KDR and CDH5. Detection of the percentage of CD31-positive cells by immunofluorescent staining confirmed that CPP can effectively induce HDFs to differentiate into VECs. The results of Matrigel assays, DiI-ac-LDL uptake, angiogenesis assays on CAM and hindlimb ischemia model of nude mice showed that CPP-induced HDFs have the functions of VECs in vitro and in vivo. Western blot and qRT-PCR analysis showed that CPP induces HDFs to differentiate into VECs by promoting the expression of pro-angiogenic factors (VEGF, FGF-2 and PDGF-BB).

Conclusions

Our data suggest that the small chemical molecule CPP efficiently induces the differentiation of HDFs into VECs. Simultaneously, this new inducer provides a potential to develop new approaches to restore vascular function for the treatment of ischemic vascular diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03185-4.

Comparison of the Behavior of Perivascular Cells (Pericytes and CD34+ Stromal Cell/Telocytes) in Sprouting and Intussusceptive Angiogenesis

Perivascular cells in the pericytic microvasculature, pericytes and CD34+ stromal cells/telocytes (CD34+SCs/TCs), have an important role in angiogenesis. We compare the behavior of these cells depending on whether the growth of endothelial cells (ECs) from the pre-existing microvasculature is toward the interstitium with vascular bud and neovessel formation (sprouting angiogenesis) or toward the vascular lumen with intravascular pillar development and vessel division (intussusceptive angiogenesis). Detachment from the vascular wall, mobilization, proliferation, recruitment, and differentiation of pericytes and CD34+SCs/TCs, as well as associated changes in vessel permeability and functionality, and modifications of the extracellular matrix are more intense, longer lasting over time, and with a greater energy cost in sprouting angiogenesis than in intussusceptive angiogenesis, in which some of the aforementioned events do not occur or are compensated for by others (e.g., sparse EC and pericyte proliferation by cell elongation and thinning). The governing mechanisms involve cell-cell contacts (e.g., peg-and-socket junctions between pericytes and ECs), multiple autocrine and paracrine signaling molecules and pathways (e.g., vascular endothelial growth factor, platelet-derived growth factor, angiopoietins, transforming growth factor B, ephrins, semaphorins, and metalloproteinases), and other factors (e.g., hypoxia, vascular patency, and blood flow). Pericytes participate in vessel development, stabilization, maturation and regression in sprouting angiogenesis, and in interstitial tissue structure formation of the pillar core in intussusceptive angiogenesis. In sprouting angiogenesis, proliferating perivascular CD34+SCs/TCs are an important source of stromal cells during repair through granulation tissue formation and of cancer-associated fibroblasts (CAFs) in tumors. Conversely, CD34+SCs/TCs have less participation as precursor cells in intussusceptive angiogenesis. The dysfunction of these mechanisms is involved in several diseases, including neoplasms, with therapeutic implications.

Ciliogenesis mechanisms mediated by PAK2-ARL13B signaling in brain endothelial cells is responsible for vascular stability

In the developing vasculature, cilia, microtubule-based organelles that project from the apical surface of endothelial cells (ECs), have been identified to function cell autonomously to promote vascular integrity and prevent hemorrhage. To date, the underlying mechanisms of endothelial cilia formation (ciliogenesis) are not fully understood. Understanding these mechanisms is likely to open new avenues for targeting EC-cilia to promote vascular stability. Here, we hypothesized that brain ECs ciliogenesis and the underlying mechanisms that control this process are critical for brain vascular stability. To investigate this hypothesis, we utilized multiple approaches including developmental zebrafish model system and primary cell culture systems. In the p21 activated kinase 2 (pak2a) zebrafish vascular stability mutant [redhead (rhd)] that shows cerebral hemorrhage, we observed significant decrease in cilia-inducing protein ADP Ribosylation Factor Like GTPase 13B (Arl13b), and a 4-fold decrease in cilia numbers. Overexpressing ARL13B-GFP fusion mRNA rescues the cilia numbers (1-2-fold) in brain vessels, and the cerebral hemorrhage phenotype. Further, this phenotypic rescue occurs at a critical time in development (24 h post fertilization), prior to initiation of blood flow to the brain vessels. Extensive biochemical mechanistic studies in primary human brain microvascular ECs implicate ligands platelet-derived growth factor-BB (PDGF-BB), and vascular endothelial growth factor-A (VEGF-A) trigger PAK2-ARL13B ciliogenesis and signal through cell surface VEGFR-2 receptor. Thus, collectively, we have implicated a critical brain ECs ciliogenesis signal that converges on PAK2-ARL13B proteins to promote vascular stability.

The Platelet-Derived Growth Factor Pathway in Pulmonary Arterial Hypertension: Still an Interesting Target?

The lack of curative options for pulmonary arterial hypertension drives important research to understand the mechanisms underlying this devastating disease. Among the main identified pathways, the platelet-derived growth factor (PDGF) pathway was established to control vascular remodeling and anti-PDGF receptor (PDGFR) drugs were shown to reverse the disease in experimental models. Four different isoforms of PDGF are produced by various cell types in the lung. PDGFs control vascular cells migration, proliferation and survival through binding to their receptors PDGFRα and β. They elicit multiple intracellular signaling pathways which have been particularly studied in pulmonary smooth muscle cells. Activation of the PDGF pathway has been demonstrated both in patients and in pulmonary hypertension (PH) experimental models. Tyrosine kinase inhibitors (TKI) are numerous but without real specificity and Imatinib, one of the most specific, resulted in beneficial effects. However, adverse events and treatment discontinuation discouraged to pursue this therapy. Novel therapeutic strategies are currently under experimental evaluation. For TKI, they include intratracheal drug administration, low dosage or nanoparticles delivery. Specific anti-PDGF and anti-PDGFR molecules can also be designed such as new TKI, soluble receptors, aptamers or oligonucleotides.

Silencing of YAP attenuates pericyte-myofibroblast transition and subretinal fibrosis in experimental model of choroidal neovascularization

Age-related macular degeneration (AMD) is the main reason of irreversible vision loss in the elderly. The subretinal fibrosis subsequent to choroidal neovascularization (CNV) is an important feature in the late stage of wet AMD and is considered to be one reason for incomplete response to anti-VEGF drugs. Recent studies have shown that pericyte-myofibroblast transition (PMT) is an important pathological process involving fibrotic diseases of various organs. However, the specific role and mechanism of PMT in the subretinal fibrosis of CNV have not been clarified. It has been clear that the Hippo pathway along with its downstream effector Yes-associated protein (YAP) plays an important role in both epithelial and endothelial myofibroblast development. Therefore, we speculate whether YAP participates in PMT of pericytes and promotes fibrosis of CNV. In this study, experimental CNV was induced by laser photocoagulation in C57BL/6J (B6) mice, and aberrant YAP overexpression was detected in the retinal pigment epithelial/choroid/sclera tissues of the laser-injured eyes. YAP knockdown reduced the proliferation, migration, and differentiation of human retinal microvascular pericytes in vitro. It also reduced subretinal fibrosis of laser-induced CNV in vivo. Moreover, by proteomics-based analysis of pericyte conditioned medium (PC-CM) and bioinformatic analyses, we identified that the crosstalk between Hippo/YAP and MAPK/Erk was involved in expression of filamin A in hypoxic pericytes. These findings suggest that Hippo/YAP and MAPK/Erk are linked together to mediate pericyte proliferation, migration as well as differentiation, which may embody potential implications for treatment in diseases related to CNV.

Platelet-derived growth factor signalling in neurovascular function and disease

Platelet-derived growth factors are critical for cerebrovascular development and homeostasis. Abnormalities in this signalling pathway are implicated in neurological diseases, especially those where neurovascular dysfunction and neuroinflammation plays a prominent role in disease pathologies, such as stroke and Alzheimer's disease; the angiogenic nature of this pathway also draws its significance in brain malignancies such as glioblastoma where tumour angiogenesis is profuse. In this review, we provide an updated overview of the actions of the platelet-derived growth factors on neurovascular function, their role in the regulation of perivascular cell types expressing the cognate receptors, neurological diseases associated with aberrance in signalling, and highlight the clinical relevance and therapeutic potentials of this pathway for central nervous system diseases.

Isolation and characterization of peritoneal microvascular pericytes

As a potential source of myofibroblasts, pericytes may play a role in human peritoneal fibrosis. The culture of primary vascular pericytes in animals has previously been reported, most of which are derived from cerebral and retinal microvasculature. Here, in the field of peritoneal dialysis, we describe a method to isolate and characterize mouse peritoneal microvascular pericytes. The mesenteric tissues of five mice were collected and digested by type II collagenase and type I DNase. After cell attachment, the culture fluid was replaced with pericyte‐conditioned medium. Pericytes with high purity (99.0%) could be isolated by enzymatic disaggregation combined with conditional culture and magnetic activated cell sorting. The primary cells were triangular or polygonal with protrusions, and confluent cell culture could be established in 3 days. The primary pericytes were positive for platelet‐derived growth factor receptor‐β, α‐smooth muscle actin, neuron‐glial antigen 2, and CD13. Moreover, they promoted formation of endothelial tubes, and pericyte–myofibroblast transition occurred after treatment with transforming growth factor‐β1. In summary, we describe here a reproducible isolation protocol for primary peritoneal pericytes, which may be a powerful tool for in vitro peritoneal fibrosis studies.