Angiodiversity and organotypic functions of sinusoidal endothelial cells

Targeting the tumour vasculature: from vessel destruction to promotion

As angiogenesis was recognized as a core hallmark of cancer growth and survival, several strategies have been implemented to target the tumour vasculature. Yet to date, attempts have rarely been so diverse, ranging from vessel growth inhibition and destruction to vessel normalization, reprogramming and vessel growth promotion. Some of these strategies, combined with standard of care, have translated into improved cancer therapies, but their successes are constrained to certain cancer types. This Review provides an overview of these vascular targeting approaches and puts them into context based on our subsequent improved understanding of the tumour vasculature as an integral part of the tumour microenvironment with which it is functionally interlinked. This new knowledge has already led to dual targeting of the vascular and immune cell compartments and sets the scene for future investigations of possible alternative approaches that consider the vascular link with other tumour microenvironment components for improved cancer therapy.

The Molecular Mechanisms of Portal Vein Thrombosis in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) represents the sixth most diagnosed cancer worldwide and is the second leading cause of cancer-related death in the world. The association of HCC and portal vein thrombosis (PVT) represents an advanced stage of the tumor. PVT has a prevalence of about 25-50% in HCC, determining poor prognosis and a remarkable reduction in therapeutic perspectives in these patients, leading to severe complications such as ascites, metastasis, an increase in portal hypertension and potentially fatal gastrointestinal bleeding. The aim of this review is to evaluate the molecular mechanisms that are at the basis of PVT development, trying to evaluate possible strategies in the early detection of patients at high risk of PVT.

Impact of heterogeneity in liver matrix and intrahepatic cells on the progression of hepatic fibrosis

Liver fibrosis is a disease with a high prevalence worldwide. The development of hepatic fibrosis results from a combination of factors within the liver, such as extracellular matrix (ECM) deposition, hepatic stellate cells (HSCs) activation, collagen cross-linking, and inflammatory response. Heterogeneity in fibrotic liver is the result of a combination of heterogeneity in the intrahepatic microenvironment as well as heterogeneous expression of fibrosis-associated enzymes and cells, complicating the study of the mechanisms underlying the progression of liver fibrosis. The role of this heterogeneity on the crosstalk between cells and matrix and on the fibrotic process is worth exploring. In this paper, we will describe the phenomenon and mechanism of heterogeneity of liver matrix and intrahepatic cells in the process of hepatic fibrosis and discuss the crosstalk between heterogeneous factors on the development of fibrosis. The elucidation of heterogeneity is important for a deeper understanding of the pathological mechanisms of liver fibrosis as well as for clinical diagnosis and targeted therapies.

A systems view of the vascular endothelium in health and disease

The dysfunction of blood-vessel-lining endothelial cells is a major cause of mortality. Although endothelial cells, being present in all organs as a single-cell layer, are often conceived as a rather inert cell population, the vascular endothelium as a whole should be considered a highly dynamic and interactive systemically disseminated organ. We present here a holistic view of the field of vascular research and review the diverse functions of blood-vessel-lining endothelial cells during the life cycle of the vasculature, namely responsive and relaying functions of the vascular endothelium and the responsive roles as instructive gatekeepers of organ function. Emerging translational perspectives in regenerative medicine, preventive medicine, and aging research are developed. Collectively, this review is aimed at promoting disciplinary coherence in the field of angioscience for a broader appreciation of the importance of the vasculature for organ function, systemic health, and healthy aging.

Transcriptome Analyses of Liver Sinusoidal Endothelial Cells Reveal a Consistent List of Candidate Genes Associated with Endothelial Dysfunction and the Fibrosis Progression

Liver fibrosis is an important step in the transformation of chronic liver disease into cirrhosis and liver cancer, and structural changes and functional disorders of liver sinusoidal endothelial cells (LSECs) are early events in the occurrence of liver fibrosis. Therefore, it is necessary to identify the key regulatory genes of endothelial dysfunction in the process of liver fibrosis to provide a reference for the diagnosis and treatment of liver fibrosis. In this study, we identified 230 common differentially expressed genes (Co-DEGs) by analyzing transcriptomic data of primary LSECs from three different liver fibrosis mouse models (carbon tetrachloride; choline-deficient, l-amino acid-defined diet; and nonalcoholic steatohepatitis). Enrichment analysis revealed that the Co-DEGs were mainly involved in regulating the inflammatory response, immune response, angiogenesis, formation and degradation of the extracellular matrix, and mediating chemokine-related pathways. A Venn diagram analysis was used to identify 17 key genes related to the progression of liver cirrhosis. Regression analysis using the Lasso-Cox method identified genes related to prognosis among these key genes: SOX4, LGALS3, SERPINE2, CD52, and LPXN. In mouse models of liver fibrosis (bile duct ligation and carbon tetrachloride), all five key genes were upregulated in fibrotic livers. This study identified key regulatory genes for endothelial dysfunction in liver fibrosis, namely SOX4, LGALS3, SERPINE2, CD52, and LPXN, which will provide new targets for the development of therapeutic strategies targeting endothelial dysfunction in LSECs and liver fibrosis.

Self-Organization of Sinusoidal Vessels in Pluripotent Stem Cell-derived Human Liver Bud Organoids

The induction of tissue-specific vessels in in vitro living tissue systems remains challenging. Here, we directly differentiated human pluripotent stem cells into CD32b+ putative liver sinusoidal progenitors (iLSEP) by dictating developmental pathways. By devising an inverted multilayered air-liquid interface (IMALI) culture, hepatic endoderm, septum mesenchyme, arterial and sinusoidal quadruple progenitors self-organized to generate and sustain hepatocyte-like cells neighbored by divergent endothelial subsets composed of CD32blowCD31high, LYVE1+STAB1+CD32bhighCD31lowTHBD-vWF-, and LYVE1-THBD+vWF+ cells. Wnt2 mediated sinusoidal-to-hepatic intercellular crosstalk potentiates hepatocyte differentiation and branched endothelial network formation. Intravital imaging revealed iLSEP developed fully patent human vessels with functional sinusoid-like features. Organoid-derived hepatocyte- and sinusoid-derived coagulation factors enabled correction of in vitro clotting time with Factor V, VIII, IX, and XI deficient patients' plasma and rescued the severe bleeding phenotype in hemophilia A mice upon transplantation. Advanced organoid vascularization technology allows for interrogating key insights governing organ-specific vessel development, paving the way for coagulation disorder therapeutics.

Endothelial Slc35a1 Deficiency Causes Loss of LSEC Identity and Exacerbates Neonatal Lipid Deposition in the Liver in Mice

BACKGROUND & AIMS

The functional maturation of the liver largely occurs after birth. In the early stages of life, the liver of a newborn encounters enormous high-fat metabolic stress caused by the consumption of breast milk. It is unclear how the maturing liver adapts to high lipid metabolism. Liver sinusoidal endothelial cells (LSECs) play a fundamental role in establishing liver vasculature and are decorated with many glycoproteins on their surface. The Slc35a1 gene encodes a cytidine-5'-monophosphate (CMP)-sialic acid transporter responsible for transporting CMP-sialic acids between the cytoplasm and the Golgi apparatus for protein sialylation. This study aimed to determine whether endothelial sialylation plays a role in hepatic vasculogenesis and functional maturation.

METHODS

Endothelial-specific Slc35a1 knockout mice were generated. Liver tissues were collected for histologic analysis, lipidomic profiling, RNA sequencing, confocal immunofluorescence, and immunoblot analyses.

RESULTS

Endothelial Slc35a1-deficient mice exhibited excessive neonatal hepatic lipid deposition, severe liver damage, and high mortality. Endothelial deletion of Slc35a1 led to sinusoidal capillarization and disrupted hepatic zonation. Mechanistically, vascular endothelial growth factor receptor 2 (VEGFR2) in LSECs was desialylated and VEGFR2 signaling was enhanced in Slc35a1-deficient mice. Inhibition of VEGFR2 signaling by SU5416 alleviated lipid deposition and restored hepatic vasculature in Slc35a1-deficient mice.

CONCLUSIONS

Our findings suggest that sialylation of LSECs is critical for maintaining hepatic vascular development and lipid homeostasis. Targeting VEGFR2 signaling may be a new strategy to prevent liver disorders associated with abnormal vasculature and lipid deposition.

Single-cell transcriptome reveals a novel mechanism of C-Kit+-liver sinusoidal endothelial cells in NASH

AIM

To understand how liver sinusoidal endothelial cells (LSECs) respond to nonalcoholic steatohepatitis (NASH).

METHODS

We profiled single-LSEC from livers of control and MCD-fed mice. The functions of C-Kit+-LSECs were determined using coculture and bone marrow transplantation (BMT) methods.

RESULTS

Three special clusters of single-LSEC were differentiated. C-Kit+-LSECs of cluster 0, Msr1+-LSECs of cluster 1 and Bmp4+Selp+-VECs of cluster 2 were revealed, and these cells with diverse ectopic expressions of genes participated in regulation of endothelial, fibrosis and lipid metabolism in NASH. The number of C-Kit+-primary LSECs isolated from MCD mice was lower than control mice. Immunofluorescence co-staining of CD31 and C-KIT showed C-Kit+-LSECs located in hepatic sinusoid were also reduced in NASH patients and MCD mice, compared to AIH patients and control mice respectively. Interestingly, lipotoxic hepatocytes/HSCs cocultured with C-Kit+-LSECs or the livers of MCD mice receipting of C-Kit+-BMCs (bone marrow cells) showed less steatosis, inflammation and fibrosis, higher expression of prolipolytic FXR and PPAR-α, lower expression of TNF-α and α-SMA. Furthermore, coculturing or BMT of C-Kit+-endothelial derived cells could increase the levels of hepatic mitochondrial LC3B, decrease the degree of mitochondrial damage and ROS production through activating Pink1-mediated mitophagy pathway in NASH.

CONCLUSIONS

Hence, a novel transcriptomic view of LSECs was revealed to have heterogeneity and complexity in NASH. Importantly, a cluster of C-Kit+-LSECs was confirmed to recovery Pink1-related mitophagy and NASH progression.

Single-Cell Transcriptomics Reveals the Ameliorative Effect of Oridonin on Septic Liver Injury

Sepsis is a life-threatening syndrome leading to hemodynamic instability and potential organ dysfunction. Oridonin, commonly used in Traditional Chinese Medicine (TCM), exhibits significant anti-inflammation activity. To explore the protective mechanisms of oridonin against the pathophysiological changes, the authors conducted single-cell transcriptome (scRNA-seq) analysis on septic liver models induced by cecal ligation and puncture (CLP). They obtained a total of 63,486 cells, distributed across 11 major cell clusters, and concentrated their analysis on four specific clusters (hepatocytes/Heps, macrophages, endothelial/Endos and T/NK) based on their changes in proportion during sepsis and under oridonin treatment. Firstly, biological changes in Hep, which are related to metabolic dysregulation and pro-inflammatory signaling, are observed during sepsis. Secondly, they uncovered the dynamic profiles of macrophage's phenotype, indicating that a substantial number of macrophages exhibited a M1-skewed phenotype associated with pro-inflammatory characteristics in septic model. Thirdly, they detected an upregulation of both inflammatory cytokines and transcriptomic factor Nfkb1 expression within Endo, along with slight capillarization during sepsis. Moreover, excessive accumulation of cytotoxic NK led to an immune imbalance. Though, oridonin ameliorated inflammatory-related responses and improved the liver dysfunction in septic mice. This study provides fundamental evidence of the protective effects of oridonin against sepsis-induced cytokine storm.

Identification of myeloid-derived growth factor as a mechanically-induced, growth-promoting angiocrine signal for human hepatocytes

Recently, we have shown that after partial hepatectomy (PHx), an increased hepatic blood flow initiates liver growth in mice by vasodilation and mechanically-triggered release of angiocrine signals. Here, we use mass spectrometry to identify a mechanically-induced angiocrine signal in human hepatic endothelial cells, that is, myeloid-derived growth factor (MYDGF). We show that it induces proliferation and promotes survival of primary human hepatocytes derived from different donors in two-dimensional cell culture, via activation of mitogen-activated protein kinase (MAPK) and signal transducer and activator of transcription 3 (STAT3). MYDGF also enhances proliferation of human hepatocytes in three-dimensional organoids. In vivo, genetic deletion of MYDGF decreases hepatocyte proliferation in the regenerating mouse liver after PHx; conversely, adeno-associated viral delivery of MYDGF increases hepatocyte proliferation and MAPK signaling after PHx. We conclude that MYDGF represents a mechanically-induced angiocrine signal and that it triggers growth of, and provides protection to, primary mouse and human hepatocytes.

Endothelial and tumor-intrinsic mechanisms of hepatic melanoma metastasis

The treatment of metastatic cutaneous melanoma was fundamentally improved by the discovery and introduction of immune checkpoint inhibitors, such as anti-PD-1 and anti-CTLA-4 antibodies, and targeted therapy with BRAF and MEK inhibition. Unfortunately, many patients suffer a relapse due to resistance mechanisms that in part are mediated by organ-specific metastatic sites. Especially, brain and liver metastases are negative predictive factors for both treatment modalities. There is still high unmet clinical need to prevent and treat spread to these organs. Therefore, experimental research should focus on mechanisms of hepatic melanoma metastasis to better understand this process and to identify therapeutic targets.

Endotheliale und tumor-intrinsische Mechanismen der hepatischen Melanommetastasierung: Endothelial and tumor-intrinsic mechanisms of hepatic melanoma metastasis

Die Behandlung des metastasierten kutanen Melanoms wurde durch die Entdeckung und Zulassung von Immuncheckpoint‐Inhibitoren (ICI) wie Anti‐PD‐1‐ und Anti‐CTLA‐4‐Antikörpern sowie der zielgerichteten Therapie mit BRAF‐ und MEK‐Inhibition grundlegend verbessert. Allerdings entwickeln viele Patienten Rezidive aufgrund von Resistenzmechanismen, die zum Teil durch Metastasierung in bestimmte Organe verursacht werden. Insbesondere Hirn‐ und Lebermetastasen sind negative prädiktive Faktoren für beide Behandlungsmodalitäten. Bislang gibt es nur begrenzte Möglichkeiten, organspezifische Metastasierung aufzuhalten oder zu behandeln. Daher muss sich die experimentelle Forschung unter anderem Mechanismen der hepatischen Metastasierung des Melanoms widmen, um diesen Prozess besser zu verstehen und therapeutische Ziele ausfindig zu machen.

Single-cell dynamics of liver development in postnatal pigs

The postnatal development of the liver, an essential organ for metabolism and immunity, remains poorly characterized at the single-cell resolution. Here, we generated single-nucleus and single-cell transcriptomes of 84,824 pig liver cells at four postnatal time points: day 30, 42, 150, and 730. We uncovered 23 cell types, including three rare cell types: plasmacytoid dendritic cells, CAVIN3+IGF2+ endothelial cells, and EBF1+ fibroblasts. The latter two were verified by multiplex immunohistochemistry. Trajectory and gene regulatory analyses revealed 33 genes that encode transcription factors associated with hepatocyte development and function, including NFIL3 involved in regulating hepatic metabolism. We characterized the spatiotemporal heterogeneity of liver endothelial cells, identified and validated leucine zipper protein 2 (LUZP2) as a novel adult liver sinusoidal endothelial cell-specific transcription factor. Lymphoid cells (NK and T cells) governed the immune system of the pig liver since day 30. Furthermore, we identified a cluster of tissue-resident NK cells, which displayed virus defense functions, maintained proliferative features at day 730, and manifested a higher conservative transcription factor expression pattern in humans than in mouse liver. Our study presents the most comprehensive postnatal liver development single-cell atlas and demonstrates the metabolic and immune changes across the four age stages.

Organotypic heterogeneity in microvascular endothelial cell responses in sepsis-a molecular treasure trove and pharmacological Gordian knot

In the last decades, it has become evident that endothelial cells (ECs) in the microvasculature play an important role in the pathophysiology of sepsis-associated multiple organ dysfunction syndrome (MODS). Studies on how ECs orchestrate leukocyte recruitment, control microvascular integrity and permeability, and regulate the haemostatic balance have provided a wealth of knowledge and potential molecular targets that could be considered for pharmacological intervention in sepsis. Yet, this information has not been translated into effective treatments. As MODS affects specific vascular beds, (organotypic) endothelial heterogeneity may be an important contributing factor to this lack of success. On the other hand, given the involvement of ECs in sepsis, this heterogeneity could also be leveraged for therapeutic gain to target specific sites of the vasculature given its full accessibility to drugs. In this review, we describe current knowledge that defines heterogeneity of organ-specific microvascular ECs at the molecular level and elaborate on studies that have reported EC responses across organ systems in sepsis patients and animal models of sepsis. We discuss hypothesis-driven, single-molecule studies that have formed the basis of our understanding of endothelial cell engagement in sepsis pathophysiology, and include recent studies employing high-throughput technologies. The latter deliver comprehensive data sets to describe molecular signatures for organotypic ECs that could lead to new hypotheses and form the foundation for rational pharmacological intervention and biomarker panel development. Particularly results from single cell RNA sequencing and spatial transcriptomics studies are eagerly awaited as they are expected to unveil the full spatiotemporal signature of EC responses to sepsis. With increasing awareness of the existence of distinct sepsis subphenotypes, and the need to develop new drug regimen and companion diagnostics, a better understanding of the molecular pathways exploited by ECs in sepsis pathophysiology will be a cornerstone to halt the detrimental processes that lead to MODS.

The modes of angiogenesis: an updated perspective

Following the process of vasculogenesis during development, angiogenesis generates new vascular structures through a variety of different mechanisms or modes. These different modes of angiogenesis involve, for example, increasing microvasculature density by sprouting of endothelial cells, splitting of vessels to increase vascular surface area by intussusceptive angiogenesis, fusion of capillaries to increase blood flow by coalescent angiogenesis, and the recruitment of non-endothelial cells by vasculogenic mimicry. The recent reporting on coalescent angiogenesis as a new mode of vessel formation warrants a brief overview of angiogenesis mechanisms to provide a more complete picture. The journal Angiogenesis is devoted to the delineation of the different modes and mechanisms that collectively dictate blood vessel formation, inhibition, and function in health and disease.

Endothelial cell SMAD6 balances Alk1 function to regulate adherens junctions and hepatic vascular development

BMP signaling is crucial to blood vessel formation and function, but how pathway components regulate vascular development is not well-understood. Here, we find that inhibitory SMAD6 functions in endothelial cells to negatively regulate ALK1-mediated responses, and it is required to prevent vessel dysmorphogenesis and hemorrhage in the embryonic liver vasculature. Reduced Alk1 gene dosage rescued embryonic hepatic hemorrhage and microvascular capillarization induced by Smad6 deletion in endothelial cells in vivo. At the cellular level, co-depletion of Smad6 and Alk1 rescued the destabilized junctions and impaired barrier function of endothelial cells depleted for SMAD6 alone. Mechanistically, blockade of actomyosin contractility or increased PI3K signaling rescued endothelial junction defects induced by SMAD6 loss. Thus, SMAD6 normally modulates ALK1 function in endothelial cells to regulate PI3K signaling and contractility, and SMAD6 loss increases signaling through ALK1 that disrupts endothelial cell junctions. ALK1 loss-of-function also disrupts vascular development and function, indicating that balanced ALK1 signaling is crucial for proper vascular development and identifying ALK1 as a 'Goldilocks' pathway in vascular biology that requires a certain signaling amplitude, regulated by SMAD6, to function properly.

Progress on the pathological tissue microenvironment barrier-modulated nanomedicine

Imbalance in the tissue microenvironment is the main obstacle to drug delivery and distribution in the human body. Before penetrating the pathological tissue microenvironment to the target site, therapeutic agents are usually accompanied by three consumption steps: the first step is tissue physical barriers for prevention of their penetration, the second step is inactivation of them by biological molecules, and the third step is a cytoprotective mechanism for preventing them from functioning on specific subcellular organelles. However, recent studies in drug-hindering mainly focus on normal physiological rather than pathological microenvironment, and the repair of damaged physiological barriers is also rarely discussed. Actually, both the modulation of pathological barriers and the repair of damaged physiological barriers are essential in the disease treatment and the homeostasis maintenance. In this review, we present an overview describing the latest advances in the generality of these pathological barriers and barrier-modulated nanomedicine. Overall, this review holds considerable significance for guiding the design of nanomedicine to increase drug efficacy in the future.

Single-cell sequencing reveals the existence of fetal vascular endothelial stem cell-like cells in mouse liver

BACKGROUND

A resident vascular endothelial stem cell (VESC) population expressing CD157 and CD200 has been identified recently in the adult mouse. However, the origin of this population and how it develops has not been characterized, nor has it been determined whether VESC-like cells are present during the perinatal period. Here, we investigated the presence of perinatal VESC-like cells and their relationship with the adult VESC-like cell population.

METHODS

We applied single-cell RNA sequencing of endothelial cells (ECs) from embryonic day (E) 14, E18, postnatal day (P) 7, P14, and week (W) 8 liver and investigated transcriptomic changes during liver EC development. We performed flow cytometry, immunofluorescence, colony formation assays, and transplantation assays to validate the presence of and to assess the function of CD157+ and CD200+ ECs in the perinatal period.

RESULTS

We identified CD200- expressing VESC-like cells in the perinatal period. These cells formed colonies in vitro and had high proliferative ability. The RNA velocity tool and transplantation assay results indicated that the projected fate of this population was toward adult VESC-like cells expressing CD157 and CD200 1 week after birth.

CONCLUSION

Our study provides a comprehensive atlas of liver EC development and documents VESC-like cell lineage commitment at single-cell resolution.

Pathological angiogenesis: mechanisms and therapeutic strategies

In multicellular organisms, angiogenesis, the formation of new blood vessels from pre-existing ones, is an essential process for growth and development. Different mechanisms such as vasculogenesis, sprouting, intussusceptive, and coalescent angiogenesis, as well as vessel co-option, vasculogenic mimicry and lymphangiogenesis, underlie the formation of new vasculature. In many pathological conditions, such as cancer, atherosclerosis, arthritis, psoriasis, endometriosis, obesity and SARS-CoV-2(COVID-19), developmental angiogenic processes are recapitulated, but are often done so without the normal feedback mechanisms that regulate the ordinary spatial and temporal patterns of blood vessel formation. Thus, pathological angiogenesis presents new challenges yet new opportunities for the design of vascular-directed therapies. Here, we provide an overview of recent insights into blood vessel development and highlight novel therapeutic strategies that promote or inhibit the process of angiogenesis to stabilize, reverse, or even halt disease progression. In our review, we will also explore several additional aspects (the angiogenic switch, hypoxia, angiocrine signals, endothelial plasticity, vessel normalization, and endothelial cell anergy) that operate in parallel to canonical angiogenesis mechanisms and speculate how these processes may also be targeted with anti-angiogenic or vascular-directed therapies.

Angiocrine Signaling in Sinusoidal Health and Disease

Liver sinusoidal endothelial cells (LSECs) are key players in maintaining hepatic homeostasis. They also play crucial roles during liver injury by communicating with liver cell types as well as immune cells and promoting portal hypertension, fibrosis, and inflammation. Cutting-edge technology, such as single cell and spatial transcriptomics, have revealed the existence of distinct LSEC subpopulations with a clear zonation in the liver. The signals released by LSECs are commonly called "angiocrine signaling." In this review, we summarize the role of angiocrine signaling in health and disease, including zonation in healthy liver, regeneration, fibrosis, portal hypertension, nonalcoholic fatty liver disease, alcohol-associated liver disease, aging, drug-induced liver injury, and ischemia/reperfusion, as well as potential therapeutic advances. In conclusion, sinusoidal endotheliopathy is recognized in liver disease and promising preclinical studies are paving the path toward LSEC-specific pharmacotherapies.

New therapeutic concepts against ischemia-reperfusion injury in organ transplantation

INTRODUCTION

Ischemia-reperfusion injury (IRI) involves a positive amplification feedback loop that stimulates innate immune-driven tissue damage associated with organ procurement from deceased donors and during transplantation surgery. As our appreciation of its basic immune mechanisms has improved in recent years, translating putative biomarkers into therapeutic interventions in clinical transplantation remains challenging.

AREAS COVERED

This review presents advances in translational/clinical studies targeting immune responses to reactive oxygen species in IRI-stressed solid organ transplants, especially livers. Here we focus on novel concepts to rejuvenate suboptimal donor organs and improve transplant function using pharmacologic and machine perfusion (MP) strategies. Cellular damage induced by cold ischemia/warm reperfusion and the latest mechanistic insights into the microenvironment's role that leads to reperfusion-induced sterile inflammation is critically discussed.

EXPERT OPINION

Efforts to improve clinical outcomes and increase the donor organ pool will depend on improving donor management and our better appreciation of the complex mechanisms encompassing organ IRI that govern the innate-adaptive immune interface triggered in the peritransplant period and subsequent allo-Ag challenge. Computational techniques and deep machine learning incorporating the vast cellular and molecular mechanisms will predict which peri-transplant signals and immune interactions are essential for improving access to the long-term function of life-saving transplants.

Association of Differentially Altered Liver Fibrosis with Deposition of TGFBi in Stabilin-Deficient Mice

Liver sinusoidal endothelial cells (LSECs) control clearance of Transforming growth factor, beta-induced, 68kDa (TGFBi) and Periostin (POSTN) through scavenger receptors Stabilin-1 (Stab1) and Stabilin-2 (Stab2). Stabilin inhibition can ameliorate atherosclerosis in mouse models, while Stabilin-double-knockout leads to glomerulofibrosis. Fibrotic organ damage may pose a limiting factor in future anti-Stabilin therapies. While Stab1-deficient (Stab1-/-) mice were shown to exhibit higher liver fibrosis levels upon challenges, fibrosis susceptibility has not been studied in Stab2-deficient (Stab2-/-) mice. Wildtype (WT), Stab1-/- and Stab2-/- mice were fed experimental diets, and local ligand abundance, hepatic fibrosis, and ligand plasma levels were measured. Hepatic fibrosis was increased in both Stab1-/- and Stab2-/- at baseline. A pro-fibrotic short Methionine-Choline-deficient (MCD) diet induced slightly increased liver fibrosis in Stab1-/- and Stab2-/- mice. A Choline-deficient L-amino acid-defined (CDAA) diet induced liver fibrosis of similar distribution and extent in all genotypes (WT, Stab1-/- and Stab2-/-). A hepatic abundance of Stabilin ligand TGFBi correlated very highly with liver fibrosis levels. In contrast, plasma levels of TGFBi were increased only in Stab2-/- mice after the CDAA diet but not the MCD diet, indicating the differential effects of these diets. Here we show that a single Stabilin deficiency of either Stab1 or Stab2 induces mildly increased collagen depositions under homeostatic conditions. Upon experimental dietary challenge, the local abundance of Stabilin ligand TGFBi was differentially altered in Stabilin-deficient mice, indicating differentially affected LSEC scavenger functions. Since anti-Stabilin-directed therapies are in clinical evaluation for the treatment of diseases, these findings bear relevance to treatment with novel anti-Stabilin agents.

The tissue-specific transcriptional landscape underlines the involvement of endothelial cells in health and disease

Endothelial cells (ECs) that line vascular and lymphatic vessels are being increasingly recognized as important to organ function in health and disease. ECs participate not only in the trafficking of gases, metabolites, and cells between the bloodstream and tissues but also in the angiocrine-based induction of heterogeneous parenchymal cells, which are unique to their specific tissue functions. The molecular mechanisms regulating EC heterogeneity between and within different tissues are modeled during embryogenesis and become fully established in adults. Any changes in adult tissue homeostasis induced by aging, stress conditions, and various noxae may reshape EC heterogeneity and induce specific transcriptional features that condition a functional phenotype. Heterogeneity is sustained via specific genetic programs organized through the combinatory effects of a discrete number of transcription factors (TFs) that, at the single tissue-level, constitute dynamic networks that are post-transcriptionally and epigenetically regulated. This review is focused on outlining the TF-based networks involved in EC specialization and physiological and pathological stressors thought to modify their architecture.

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.

Liver angiocrine factors

Endothelial cells secrete growth factors, chemokines, and extracellular matrix components, including angiocrine factors or angiokines, involved in the regulation of organ morphogenesis, homeostasis, and regeneration. The concepts of angiocrine signaling have been demonstrated in the liver, pancreas, brain, lung, heart, kidney, skin, bone marrow, as well as in pathological conditions, including cancer. The aim of this review article is to analyze the role of angiocrine factors in the liver in physiological as well as in pathological conditions.

ALK1 controls hepatic vessel formation, angiodiversity, and angiocrine functions in hereditary hemorrhagic telangiectasia of the liver

BACKGROUND AND AIMS

In hereditary hemorrhagic telangiectasia (HHT), severe liver vascular malformations are associated with mutations in the Activin A Receptor-Like Type 1 ( ACVRL1 ) gene encoding ALK1, the receptor for bone morphogenetic protein (BMP) 9/BMP10, which regulates blood vessel development. Here, we established an HHT mouse model with exclusive liver involvement and adequate life expectancy to investigate ALK1 signaling in liver vessel formation and metabolic function.

APPROACH AND RESULTS

Liver sinusoidal endothelial cell (LSEC)-selective Cre deleter line, Stab2-iCreF3 , was crossed with Acvrl1 -floxed mice to generate LSEC-specific Acvrl1 -deficient mice ( Alk1HEC-KO ). Alk1HEC-KO mice revealed hepatic vascular malformations and increased posthepatic flow, causing right ventricular volume overload. Transcriptomic analyses demonstrated induction of proangiogenic/tip cell gene sets and arterialization of hepatic vessels at the expense of LSEC and central venous identities. Loss of LSEC angiokines Wnt2 , Wnt9b , and R-spondin-3 ( Rspo3 ) led to disruption of metabolic liver zonation in Alk1HEC-KO mice and in liver specimens of patients with HHT. Furthermore, prion-like protein doppel ( Prnd ) and placental growth factor ( Pgf ) were upregulated in Alk1HEC-KO hepatic endothelial cells, representing candidates driving the organ-specific pathogenesis of HHT. In LSEC in vitro , stimulation or inhibition of ALK1 signaling counter-regulated Inhibitors of DNA binding (ID)1-3, known Alk1 transcriptional targets. Stimulation of ALK1 signaling and inhibition of ID1-3 function confirmed regulation of Wnt2 and Rspo3 by the BMP9/ALK1/ID axis.

CONCLUSIONS

Hepatic endothelial ALK1 signaling protects from development of vascular malformations preserving organ-specific endothelial differentiation and angiocrine signaling. The long-term surviving Alk1HEC-KO HHT model offers opportunities to develop targeted therapies for this severe disease.

Temporally and regionally distinct morphogenetic processes govern zebrafish caudal fin blood vessel network expansion

Blood vessels form elaborate networks that depend on tissue-specific signalling pathways and anatomical structures to guide their growth. However, it is not clear which morphogenetic principles organize the stepwise assembly of the vasculature. We therefore performed a longitudinal analysis of zebrafish caudal fin vascular assembly, revealing the existence of temporally and spatially distinct morphogenetic processes. Initially, vein-derived endothelial cells (ECs) generated arteries in a reiterative process requiring vascular endothelial growth factor (Vegf), Notch and cxcr4a signalling. Subsequently, veins produced veins in more proximal fin regions, transforming pre-existing artery-vein loops into a three-vessel pattern consisting of an artery and two veins. A distinct set of vascular plexuses formed at the base of the fin. They differed in their diameter, flow magnitude and marker gene expression. At later stages, intussusceptive angiogenesis occurred from veins in distal fin regions. In proximal fin regions, we observed new vein sprouts crossing the inter-ray tissue through sprouting angiogenesis. Together, our results reveal a surprising diversity among the mechanisms generating the mature fin vasculature and suggest that these might be driven by separate local cues.

Endothelial Cell SMAD6 Balances ACVRL1/Alk1 Function to Regulate Adherens Junctions and Hepatic Vascular Development

BMP signaling is critical to blood vessel formation and function, but how pathway components regulate vascular development is not well-understood. Here we find that inhibitory SMAD6 functions in endothelial cells to negatively regulate ALK1/ACVRL1-mediated responses, and it is required to prevent vessel dysmorphogenesis and hemorrhage in the embryonic liver vasculature. Reduced Alk1 gene dosage rescued embryonic hepatic hemorrhage and microvascular capillarization induced by Smad6 deletion in endothelial cells in vivo . At the cellular level, co-depletion of Smad6 and Alk1 rescued the destabilized junctions and impaired barrier function of endothelial cells depleted for SMAD6 alone. At the mechanistic level, blockade of actomyosin contractility or increased PI3K signaling rescued endothelial junction defects induced by SMAD6 loss. Thus, SMAD6 normally modulates ALK1 function in endothelial cells to regulate PI3K signaling and contractility, and SMAD6 loss increases signaling through ALK1 that disrupts endothelial junctions. ALK1 loss-of-function also disrupts vascular development and function, indicating that balanced ALK1 signaling is crucial for proper vascular development and identifying ALK1 as a "Goldilocks" pathway in vascular biology regulated by SMAD6.

Assessing the effects of aging on the liver endothelial cell landscape using single-cell RNA sequencing

Endothelial cell (EC) function declines with age and contributes to the development of many vascular-related disease processes. Currently, the effects of aging on the molecular regulatory mechanisms of liver ECs have not been fully elucidated. Here, we employed single-cell RNA sequencing to map the transcriptome of ECs and analyzed their relationship with aging. We identified 8 different EC subtypes, interestingly, 2 of which were specially expressed in aged mice ECs namely aged capillary ECs (Aged ECs) and pro-inflammation capillary ECs (Proinfla.ECs). Double immunostaining for an EC marker (Cd31) and a marker of these specialized EC phenotypes confirmed the single-cell RNA sequencing data. Gene ontology analysis revealed that Aged ECs and Proinfla.ECs were associated with inflammatory response. Then we found that liver proliferating capillary ECs (Prolife.ECs) were most affected by senescence. Single-cell transcript analysis suggests that Prolife.ECs and angiogenic capillary ECs may form a poor microenvironment that promotes angiogenesis and tumorigenesis. Pseudo-temporal trajectories revealed that Prolife.ECs have different differentiation pathways in young and aged mice. In aged mice, Prolife.ECs could specifically differentiate into an unstable state, which was mainly composed of angiogenic capillary ECs. Intercellular communication revealed inflammatory activation in old group. Overall, this work compared the single-cell RNA profiles of liver ECs in young and aged mice. These findings provide a new insight into liver aging and its molecular mechanisms, and further exploration of Aged ECs and Proinfla.ECs may help to elucidate the molecular mechanisms associated with senescence.

Cellular heterogeneity and stem cells of vascular endothelial cells in blood vessel formation and homeostasis: Insights from single-cell RNA sequencing

Vascular endothelial cells (ECs) that constitute the inner surface of blood vessels are essential for new vessel formation and organ homeostasis. ECs display remarkable phenotypic heterogeneity across different organs and the vascular tree during angiogenesis and homeostasis. Recent advances in single cell RNA sequencing (scRNA-seq) technologies have allowed a new understanding of EC heterogeneity in both mice and humans. In particular, scRNA-seq has identified new molecular signatures for arterial, venous and capillary ECs in different organs, as well as previously unrecognized specialized EC subtypes, such as the aerocytes localized in the alveolar capillaries of the lung. scRNA-seq has also revealed the gene expression profiles of specialized tissue-resident EC subtypes that are capable of clonal expansion and contribute to adult angiogenesis, a process of new vessel formation from the pre-existing vasculature. These specialized tissue-resident ECs have been identified in various different mouse tissues, including aortic endothelium, liver, heart, lung, skin, skeletal muscle, retina, choroid, and brain. Transcription factors and signaling pathways have also been identified in the specialized tissue-resident ECs that control angiogenesis. Furthermore, scRNA-seq has also documented responses of ECs in diseases such as cancer, age-related macular degeneration, Alzheimer's disease, atherosclerosis, and myocardial infarction. These new findings revealed by scRNA-seq have the potential to provide new therapeutic targets for different diseases associated with blood vessels. In this article, we summarize recent advances in the understanding of the vascular endothelial cell heterogeneity and endothelial stem cells associated with angiogenesis and homeostasis in mice and humans, and we discuss future prospects for the application of scRNA-seq technology.

Lyve-1 deficiency enhances the hepatic immune microenvironment entailing altered susceptibility to melanoma liver metastasis

BACKGROUND

Hyaluronan receptor LYVE-1 is expressed by liver sinusoidal endothelial cells (LSEC), lymphatic endothelial cells and specialized macrophages. Besides binding to hyaluronan, LYVE-1 can mediate adhesion of leukocytes and cancer cells to endothelial cells. Here, we assessed the impact of LYVE-1 on physiological liver functions and metastasis.

METHODS

Mice with deficiency of Lyve-1 (Lyve-1-KO) were analyzed using histology, immunofluorescence, microarray analysis, plasma proteomics and flow cytometry. Liver metastasis was studied by intrasplenic/intravenous injection of melanoma (B16F10 luc2, WT31) or colorectal carcinoma (MC38).

RESULTS

Hepatic architecture, liver size, endothelial differentiation and angiocrine functions were unaltered in Lyve-1-KO. Hyaluronan plasma levels were significantly increased in Lyve-1-KO. Besides, plasma proteomics revealed increased carbonic anhydrase-2 and decreased FXIIIA. Furthermore, gene expression analysis of LSEC indicated regulation of immunological pathways. Therefore, liver metastasis of highly and weakly immunogenic tumors, i.e. melanoma and colorectal carcinoma (CRC), was analyzed. Hepatic metastasis of B16F10 luc2 and WT31 melanoma cells, but not MC38 CRC cells, was significantly reduced in Lyve-1-KO mice. In vivo retention assays with B16F10 luc2 cells were unaltered between Lyve-1-KO and control mice. However, in tumor-free Lyve-1-KO livers numbers of hepatic CD4⁺, CD8⁺ and regulatory T cells were increased. In addition, iron deposition was found in F4/80⁺ liver macrophages known to exert pro-inflammatory effects.

CONCLUSION

Lyve-1 deficiency controlled hepatic metastasis in a tumor cell-specific manner leading to reduced growth of hepatic metastases of melanoma, but not CRC. Anti-tumorigenic effects are likely due to enhancement of the premetastatic hepatic immune microenvironment influencing early liver metastasis formation.

Endothelial Cell Fate Determination: A Top Notch Job in Vascular Decision-Making

As vascular networks form, endothelial cells (ECs) undergo cell fate decisions that determine whether they become tip or stalk cells of the developing vascular plexus or mature into arterial, venous, or lymphatic endothelium. EC fate decisions are coordinated with neighboring cells to initiate sprouting, maintain endothelial barrier, or ensure appropriate specialization of vessels. We describe mechanisms that control EC fate at specific steps in these processes, with an emphasis on the role of the Notch signaling pathway. Specific EC fate determination steps that are highlighted are tip/stalk selection during sprouting angiogenesis, venous-arterial specification, arteriogenesis, lymphatic vessel specification, and lymphatic valve formation.

LncRNA Airn maintains LSEC differentiation to alleviate liver fibrosis via the KLF2-eNOS-sGC pathway

BACKGROUND

Long noncoding RNAs (lncRNAs) have emerged as important regulators in a variety of human diseases. The dysregulation of liver sinusoidal endothelial cell (LSEC) phenotype is a critical early event in the fibrotic process. However, the biological function of lncRNAs in LSEC still remains unclear.

METHODS

The expression level of lncRNA Airn was evaluated in both human fibrotic livers and serums, as well as mouse fibrotic livers. Gain- and loss-of-function experiments were performed to detect the effect of Airn on LSEC differentiation and hepatic stellate cell (HSC) activation in liver fibrosis. Furthermore, RIP, RNA pull-down-immunoblotting, and ChIP experiments were performed to explore the underlying mechanisms of Airn.

RESULTS

We have identified Airn was significantly upregulated in liver tissues and LSEC of carbon tetrachloride (CCl₄)-induced liver fibrosis mouse model. Moreover, the expression of AIRN in fibrotic human liver tissues and serums was remarkably increased compared with healthy controls. In vivo studies showed that Airn deficiency aggravated CCl₄- and bile duct ligation (BDL)-induced liver fibrosis, while Airn over-expression by AAV8 alleviated CCl₄-induced liver fibrosis. Furthermore, we revealed that Airn maintained LSEC differentiation in vivo and in vitro. Additionally, Airn inhibited HSC activation indirectly by regulating LSEC differentiation and promoted hepatocyte (HC) proliferation by increasing paracrine secretion of Wnt2a and HGF from LSEC. Mechanistically, Airn interacted with EZH2 to maintain LSEC differentiation through KLF2-eNOS-sGC pathway, thereby maintaining HSC quiescence and promoting HC proliferation.

CONCLUSIONS

Our work identified that Airn is beneficial to liver fibrosis by maintaining LSEC differentiation and might be a serum biomarker for liver fibrogenesis.

Therapeutic Applications of Nanomedicine in Metabolic Diseases by Targeting the Endothelium

The endothelial cells not only constitute the barrier between the blood and interstitial space, but also actively regulate vascular tone, blood flow, and the function of adjacent parenchymal cells. The close anatomical relationship between endothelial cells and highly vascularized metabolic organs suggests that the crosstalk between these units is vital for systemic metabolic homeostasis. Here, we review recent studies about the pivotal role of endothelial cells in metabolic diseases. Specifically, we discuss how the dysfunction of endothelial cells directly contributes to the development of insulin resistance (IR), type 2 diabetes mellitus (T2DM), atherosclerosis, and non-alcoholic fatty liver disease (NAFLD) via communication with parenchymal cells. Furthermore, although many biological macromolecules have been shown to ameliorate the progression of metabolic diseases by improving endothelial function, the low solubility, poor bioavailability, or lack of specificity of these molecules limit their clinical application. Given the advantages in drug delivery of nanomedicine, we focus on summarizing the reports that improving endothelial dysfunction through nanomedicine-based therapies provides an opportunity for preventing metabolic diseases.

Single-cell transcriptomic analysis of vascular endothelial cells in zebrafish embryos

Vascular endothelial cells exhibit substantial phenotypic and transcriptional heterogeneity which is established during early embryogenesis. However, the molecular mechanisms involved in establishing endothelial cell diversity are still not well understood. Zebrafish has emerged as an advantageous model to study vascular development. Despite its importance, the single-cell transcriptomic profile of vascular endothelial cells during zebrafish development is still missing. To address this, we applied single-cell RNA-sequencing (scRNA-seq) of vascular endothelial cells isolated from zebrafish embryos at the 24 hpf stage. Six distinct clusters or subclusters related to vascular endothelial cells were identified which include arterial, two venous, cranial, endocardial and endothelial progenitor cell subtypes. Furthermore, we validated our findings by characterizing novel markers for arterial, venous, and endocardial cells. We experimentally confirmed the presence of two transcriptionally different venous cell subtypes, demonstrating heterogeneity among venous endothelial cells at this early developmental stage. This dataset will be a valuable resource for future functional characterization of vascular endothelial cells and interrogation of molecular mechanisms involved in the establishment of their heterogeneity and cell-fate decisions.

The rising impact of angiogenesis research

While inhibiting pathological angiogenesis has been long associated with the field of oncology, recent advances in angiogenesis research have impacted the progress of disease treatment for additional non-malignant diseases or chronic conditions in the fields of ophthalmology, cardiology, and gynecology. Moreover, stimulators of angiogenesis find application in ischemic diseases, while inhibitors of angiogenesis are being used to limit blood vessel formation, but in judicious ways that modify or "reprogram" the vasculature as a reinforcement for immunotherapy. We have noticed an increasing impact, as evidenced by increases in the total number of citations, in the literature surrounding the angiogenesis field suggesting that targeting angiogenesis per se is well established as a tractable approach for therapy in diverse conditions.

Vascular Remodeling Is a Crucial Event in the Early Phase of Hepatocarcinogenesis in Rodent Models for Liver Tumorigenesis

Simple Summary

Hepatocellular carcinoma (HCC) is a highly vascularized tumor and remodeling of the tumor vasculature is one of the hallmarks of tumor progression. Mouse models are elegant tools to study the onset and progression of liver tumors. However, only few data exist on the vasculature and vascular remodeling processes especially in the early phase of hepatocarcinogenesis. The aim of this study was therefore to perform a comprehensive characterization and comparison of the vasculature in mouse models used for hepatocarcinogenesis studies. For this purpose, we characterized the preneoplastic foci of cellular alteration (FCA) and hepatocellular carcinoma (HCC) by using tissue-based techniques and computer-assisted analysis to better understand if and how vascular remodeling appears in rodent models for liver tumorigenesis. Our findings demonstrated crucial differences in the number and size of the vessels, degree of maturation and intratumoral localization of the vasculature in FCA and HCC, clearly indicating that vascular remodeling is an important step in the early phase of liver tumorigenesis of rodent models.

Abstract

The investigation of hepatocarcinogenesis is a major field of interest in oncology research and rodent models are commonly used to unravel the pathophysiology of onset and progression of hepatocellular carcinoma. HCC is a highly vascularized tumor and vascular remodeling is one of the hallmarks of tumor progression. To date, only a few detailed data exist about the vasculature and vascular remodeling in rodent models used for hepatocarcinogenesis. In this study, the vasculature of HCC and the preneoplastic foci of alteration (FCA) of different mouse models with varying genetic backgrounds were comprehensively characterized by using immunohistochemistry (CD31, Collagen IV, αSMA, Desmin and LYVE1) and RNA in situ hybridization (VEGF-A). Computational image analysis was performed to evaluate selected parameters including microvessel density, pericyte coverage, vessel size, intratumoral vessel distribution and architecture using the Aperio ImageScope and Definiens software programs. HCC presented with a significantly lower number of vessels, but larger vessel size and increased coverage, leading to a higher degree of maturation, whereas FCA lesions presented with a higher microvessel density and a higher amount of smaller but more immature vessels. Our results clearly demonstrate that vascular remodeling is present and crucial in early stages of experimental hepatocarcinogenesis. In addition, our detailed characterization provides a strong basis for further angiogenesis studies in these experimental models.

Organ-specific endothelial cell heterogenicity and its impact on regenerative medicine and biomedical engineering applications

Endothelial cells (ECs) are a key cellular component of the vascular system as they form the inner lining of the blood vessels. Recent findings highlight that ECs express extensive phenotypic heterogenicity when following the vascular tree from the major vasculature down to the organ capillaries. However, in vitro models, used for drug development and testing, or to study the role of ECs in health and disease, rarely acknowledge this EC heterogenicity. In this review, we highlight the main differences between different EC types, briefly summarize their different characteristics and focus on the use of ECs in in vitro models. We introduce different approaches on how ECs can be utilized in co-culture test systems in the field of brain, pancreas, and liver research to study the role of the endothelium in health and disease. Finally, we discuss potential improvements to current state-of-the-art in vitro models and future directions.

Organ-Specific Endothelial Cell Differentiation and Impact of Microenvironmental Cues on Endothelial Heterogeneity

Endothelial cells throughout the body are heterogeneous, and this is tightly linked to the specific functions of organs and tissues. Heterogeneity is already determined from development onwards and ranges from arterial/venous specification to microvascular fate determination in organ-specific differentiation. Acknowledging the different phenotypes of endothelial cells and the implications of this diversity is key for the development of more specialized tissue engineering and vascular repair approaches. However, although novel technologies in transcriptomics and proteomics are facilitating the unraveling of vascular bed-specific endothelial cell signatures, still much research is based on the use of insufficiently specialized endothelial cells. Endothelial cells are not only heterogeneous, but their specialized phenotypes are also dynamic and adapt to changes in their microenvironment. During the last decades, strong collaborations between molecular biology, mechanobiology, and computational disciplines have led to a better understanding of how endothelial cells are modulated by their mechanical and biochemical contexts. Yet, because of the use of insufficiently specialized endothelial cells, there is still a huge lack of knowledge in how tissue-specific biomechanical factors determine organ-specific phenotypes. With this review, we want to put the focus on how organ-specific endothelial cell signatures are determined from development onwards and conditioned by their microenvironments during adulthood. We discuss the latest research performed on endothelial cells, pointing out the important implications of mimicking tissue-specific biomechanical cues in culture.

Harnessing reactive oxygen/nitrogen species and inflammation: Nanodrugs for liver injury

Overall, 12% of the global population (800 million) suffers from liver disease, which causes 2 million deaths every year. Liver injury involving characteristic reactive oxygen/nitrogen species (RONS) and inflammation plays a key role in progression of liver disease. As a key metabolic organ of the human body, the liver is susceptible to injury from various sources, including COVID-19 infection. Owing to unique structural features and functions of the liver, most current antioxidants and anti-inflammatory drugs are limited against liver injury. However, the characteristics of the liver could be utilized in the development of nanodrugs to achieve specific enrichment in the liver and consequently targeted treatment. Nanodrugs have shown significant potential in eliminating RONS and regulating inflammation, presenting an attractive therapeutic tool for liver disease through controlling liver injury. Therefore, the main aim of the current review is to provide a comprehensive summary of the latest developments contributing to our understanding of the mechanisms underlying nanodrugs in the treatment of liver injury via harnessing RONS and inflammation. Meanwhile, the prospects of nanodrugs for liver injury therapy are systematically discussed, which provides a sound platform for novel therapeutic insights and inspiration for design of nanodrugs to treat liver disease.

Developmental angiocrine diversification of endothelial cells for organotypic regeneration

Adult organs are vascularized by specialized blood vessels. In addition to inter-organ vascular heterogeneity, each organ is arborized by structurally and functionally diversified populations of endothelial cells (ECs). The molecular pathways that are induced to orchestrate inter- and intra- organ vascular heterogeneity and zonation are shaped during development and fully specified postnatally. Notably, intra-organ specialization of ECs is associated with induction of angiocrine factors that guide cross-talk between ECs and parenchymal cells, establishing co-zonated vascular regions within each organ. In this review, we describe how microenvironmental tissue-specific biophysical, biochemical, immune, and inflammatory cues dictate the specialization of ECs with zonated functions. We delineate how physiological and biophysical stressors in the developing liver, lung, and kidney vasculature induce specialization of capillary beds. Deciphering mechanisms by which vascular microvasculature diversity is attained could set the stage for treating regenerative disorders and promote healing of organs without provoking fibrosis.

In Vitro 3D Staphylococcus aureus Abscess Communities Induce Bone Marrow Cells to Expand into Myeloid-Derived Suppressor Cells

Staphylococcus aureus is the main causative pathogen of subcutaneous, bone, and implant-related infections, forming structures known as staphylococcal abscess communities (SACs) within tissues that also contain immunosuppressive myeloid-derived suppressor cells (MDSCs). Although both SACs and MDSCs are present in chronic S. aureus infections, it remains unknown whether SACs directly trigger MDSC expansion. To investigate this, a previously developed 3D in vitro SAC model was co-cultured with murine and human bone marrow cells. Subsequently, it was shown that SAC-exposed human CD11b^low/− myeloid cells or SAC-exposed murine CD11b⁺ Gr-1⁺ cells were immunosuppressive mainly by reducing absolute CD4⁺ and CD8α⁺ T cell numbers, as shown in T cell proliferation assays and with flow cytometry. Monocytic MDSCs from mice with an S. aureus bone infection also strongly reduced CD4⁺ and CD8α⁺ T cell numbers. Using protein biomarker analysis and an immunoassay, we detected in SAC–bone marrow co-cultures high levels of GM-CSF, IL-6, VEGF, IL-1β, TNFα, IL-10, and TGF-β. Furthermore, SAC-exposed neutrophils expressed Arg-1 and SAC-exposed monocytes expressed Arg-1 and iNOS, as shown via immunofluorescent stains. Overall, this study showed that SACs cause MDSC expansion from bone marrow cells and identified possible mediators to target as an additional strategy for treating chronic S. aureus infections.

The Scavenger Function of Liver Sinusoidal Endothelial Cells in Health and Disease

The aim of this review is to give an outline of the blood clearance function of the liver sinusoidal endothelial cells (LSECs) in health and disease. Lining the hundreds of millions of hepatic sinusoids in the human liver the LSECs are perfectly located to survey the constituents of the blood. These cells are equipped with high-affinity receptors and an intracellular vesicle transport apparatus, enabling a remarkably efficient machinery for removal of large molecules and nanoparticles from the blood, thus contributing importantly to maintain blood and tissue homeostasis. We describe here central aspects of LSEC signature receptors that enable the cells to recognize and internalize blood-borne waste macromolecules at great speed and high capacity. Notably, this blood clearance system is a silent process, in the sense that it usually neither requires or elicits cell activation or immune responses. Most of our knowledge about LSECs arises from studies in animals, of which mouse and rat make up the great majority, and some species differences relevant for extrapolating from animal models to human are discussed. In the last part of the review, we discuss comparative aspects of the LSEC scavenger functions and specialized scavenger endothelial cells (SECs) in other vascular beds and in different vertebrate classes. In conclusion, the activity of LSECs and other SECs prevent exposure of a great number of waste products to the immune system, and molecules with noxious biological activities are effectively “silenced” by the rapid clearance in LSECs. An undesired consequence of this avid scavenging system is unwanted uptake of nanomedicines and biologics in the cells. As the development of this new generation of therapeutics evolves, there will be a sharp increase in the need to understand the clearance function of LSECs in health and disease. There is still a significant knowledge gap in how the LSEC clearance function is affected in liver disease.

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

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

Endothelial Heterogeneity in Development and Wound Healing

The vasculature is comprised of endothelial cells that are heterogeneous in nature. From tissue resident progenitors to mature differentiated endothelial cells, the diversity of these populations allows for the formation, maintenance, and regeneration of the vascular system in development and disease, particularly during situations of wound healing. Additionally, the de-differentiation and plasticity of different endothelial cells, especially their capacity to undergo endothelial to mesenchymal transition, has also garnered significant interest due to its implication in disease progression, with emphasis on scarring and fibrosis. In this review, we will pinpoint the seminal discoveries defining the phenotype and mechanisms of endothelial heterogeneity in development and disease, with a specific focus only on wound healing.

Engineering the Vasculature of Stem-Cell-Derived Liver Organoids

The vasculature of stem-cell-derived liver organoids can be engineered using methods that recapitulate embryonic liver development. Hepatic organoids with a vascular network offer great application prospects for drug screening, disease modeling, and therapeutics. However, the application of stem cell-derived organoids is hindered by insufficient vascularization and maturation. Here, we review different theories about the origin of hepatic cells and the morphogenesis of hepatic vessels to provide potential approaches for organoid generation. We also review the main protocols for generating vascularized liver organoids from stem cells and consider their potential and limitations in the generation of vascularized liver organoids.

Endothelial cell plasticity at the single-cell level

The vascular endothelium is characterized by a remarkable level of plasticity, which is the driving force not only of physiological repair/remodeling of adult tissues but also of pathological angiogenesis. The resulting heterogeneity of endothelial cells (ECs) makes targeting the endothelium challenging, no less because many EC phenotypes are yet to be identified and functionally inventorized. Efforts to map the vasculature at the single-cell level have been instrumental to capture the diversity of EC types and states at a remarkable depth in both normal and pathological states. Here, we discuss new EC subtypes and functions emerging from recent single-cell studies in health and disease. Interestingly, such studies revealed distinct metabolic gene signatures in different EC phenotypes, which deserve further consideration for therapy. We highlight how this metabolic targeting strategy could potentially be used to promote (for tissue repair) or block (in tumor) angiogenesis in a tissue or even vascular bed-specific manner.