Tracking Fenestrae Dynamics in Live Murine Liver Sinusoidal Endothelial Cells

Nucleic acid spheres for treating capillarisation of liver sinusoidal endothelial cells in liver fibrosis

Liver sinusoidal endothelial cells (LSECs) lose their characteristic fenestrations and become capillarized during the progression of liver fibrosis. Mesenchymal stem cell (MSC) transplantation can reverse this capillarization and reduce fibrosis, but MSC therapy has practical limitations that hinder its clinical use. Here, with the help of artificial intelligence (AI), we show that MSCs secrete a microRNA (miR-325-3p) that helps restore LSEC fenestrations (tiny pores) by modulating their cytoskeleton, effectively reversing capillarization. We further develop a spherical nucleic acid (SNA) nanoparticle carrying miR-325-3p as an alternative to MSC therapy. This SNA specifically enters fibrotic LSECs via the scavenger receptor A (Scara). In three mouse models of liver fibrosis, the SNA treatment restores LSEC fenestrations, reverses capillarization, and significantly reduces fibrosis without adverse effects. Our findings highlight the potential of SNA-based therapy for liver fibrosis, paving the way for targeted nucleic acid treatments directed at LSECs and offering hope for patients.

Hydrogen peroxide damage to rat liver sinusoidal endothelial cells is prevented by n-acetyl-cysteine but not GSH

BACKGROUND

Reactive oxygen species (ROS) are prevalent in the liver during intoxication, infection, inflammation, and aging. Changes in liver sinusoidal endothelial cells (LSEC) are associated with various liver diseases.

METHODS

Isolated rat LSEC were studied under oxidative stress induced by H2O2 at different concentrations (0.5-1000 µM) and exposure times (10-120 min). LSEC functions were tested in a dose-dependent and time-dependent manner.

RESULTS

(1) Cell viability, reducing potential, and scavenging function decreased as H2O2 concentration and exposure time increased; (2) intracellular ROS levels rose with higher H2O2 concentrations; (3) fenestrations exhibited a dynamic response, initially closing but partially reopening at H2O2 concentrations above 100 µM after about 1 hour; (4) scavenging function was affected after just 10 minutes of exposure, with the impact being irreversible and primarily affecting degradation rather than receptor-mediated uptake; (5) the tubulin network was disrupted in high H2O2 concentration while the actin cytoskeleton appears to remain largely intact. Finally, we found that reducing agents and thiol donors such as n-acetyl cysteine and glutathione (GSH) could protect cells from ROS-induced damage but could not reverse existing damage as pretreatment with n-acetyl cysteine, but not GSH, reduced the negative effects of ROS exposure.

CONCLUSIONS

The results suggest that LSEC does not store an excess amount of GSH but rather can readily produce it in the occurrence of oxidative stress conditions. Moreover, the observed thresholds in dose-dependent and time-dependent changes, as well as the treatments with n-acetyl cysteine/GSH, confirm the existence of a ROS-depleting system in LSEC.

Early and late phases of liver sinusoidal endothelial cell (LSEC) defenestration in mouse model of systemic inflammation

BACKGROUND

Liver sinusoidal endothelial cells (LSECs) have transcellular pores, called fenestrations, participating in the bidirectional transport between the vascular system and liver parenchyma. Fenestrated LSECs indicate a healthy phenotype of liver while loss of fenestrations (defenestration) in LSECs is associated with liver pathologies.

METHODS

We introduce a unique model of systemic inflammation triggered by the deletion of Mcpip1 in myeloid leukocytes (Mcpip1fl/flLysMCre) characterised by progressive alterations in LSEC phenotype. We implement multiparametric characterisation of LSECs by using novel real-time atomic force microscopy supported with scanning electron microscopy and quantitative fluorescence microscopy. In addition, we provide genetic profiling, searching for characteristic genes encoding proteins that might be connected with the structure of fenestrations.

RESULTS

We demonstrate that LSECs in Mcpip1fl/flLysMCre display two phases of defenestration: the early phase, with modest defenestration that was fully reversible using cytochalasin B and the late phase, with severe defenestration that is mostly irreversible. By thorough analysis of LSEC porosity, elastic modulus and actin abundance in Mcpip1fl/flLysMCre and in response to cytochalasin B, we demonstrate that proteins other than actin must be additionally responsible for inducing open fenestrations. We highlight several genes that were severely affected in the late but not in the early phase of LSEC defenestration shedding a light on complex structure of individual fenestrations.

CONCLUSIONS

The presented model of LSEC derived from Mcpip1fl/flLysMCre provides a valuable reference for developing novel strategies for LSEC refenestration in the early and late phases of liver pathology.

Crosstalk in extrahepatic and hepatic system in NAFLD/NASH

Non-alcoholic fatty liver disease (NAFLD) has emerged as the most prevalent chronic liver disease globally. Non-alcoholic steatohepatitis (NASH) represents an extremely progressive form of NAFLD, which, without timely intervention, may progress to cirrhosis or hepatocellular carcinoma. Presently, a definitive comprehension of the pathogenesis of NAFLD/NASH eludes us, and pharmacological interventions targeting NASH specifically remain constrained. The aetiology of NAFLD encompasses a myriad of external factors including environmental influences, dietary habits and gender disparities. More significantly, inter-organ and cellular interactions within the human body play a role in the development or regression of the disease. In this review, we categorize the influences affecting NAFLD both intra- and extrahepatically, elaborating meticulously on the mechanisms governing the onset and progression of NAFLD/NASH. This exploration delves into progress in aetiology and promising therapeutic targets. As a metabolic disorder, the development of NAFLD involves complexities related to nutrient metabolism, liver-gut axis interactions and insulin resistance, among other regulatory functions of extraneous organs. It further encompasses intra-hepatic interactions among hepatic cells, Kupffer cells (KCs) and hepatic stellate cells (HSCs). A comprehensive understanding of the pathogenesis of NAFLD/NASH from a macroscopic standpoint is instrumental in the formulation of future therapies for NASH.

Semaphorin-3A regulates liver sinusoidal endothelial cell porosity and promotes hepatic steatosis

Prevalence of metabolic dysfunction-associated steatotic liver disease (MASLD), formerly known as non-alcoholic fatty liver disease, increases worldwide and associates with type 2 diabetes and other cardiometabolic diseases. Here we demonstrate that Sema3a is elevated in liver sinusoidal endothelial cells of animal models for obesity, type 2 diabetes and MASLD. In primary human liver sinusoidal endothelial cells, saturated fatty acids induce expression of SEMA3A, and loss of a single allele is sufficient to reduce hepatic fat content in diet-induced obese mice. We show that semaphorin-3A regulates the number of fenestrae through a signaling cascade that involves neuropilin-1 and phosphorylation of cofilin-1 by LIM domain kinase 1. Finally, inducible vascular deletion of Sema3a in adult diet-induced obese mice reduces hepatic fat content and elevates very low-density lipoprotein secretion. Thus, we identified a molecular pathway linking hyperlipidemia to microvascular defenestration and early development of MASLD.

Chronic heart failure induces early defenestration of liver sinusoidal endothelial cells (LSECs) in mice

AIM

Chronic heart failure (CHF) is often linked to liver malfunction and systemic endothelial dysfunction. However, whether cardio-hepatic interactions in heart failure involve dysfunction of liver sinusoidal endothelial cells (LSECs) is not known. Here we characterize LSECs phenotype in early and end stages of chronic heart failure in a murine model.

METHODS

Right ventricle (RV) function, features of congestive hepatopathy, and the phenotype of primary LSECs were characterized in Tgαq*44 mice, with cardiomyocyte-specific overexpression of the Gαq protein, at the age of 4- and 12-month representative for early and end-stage phases of CHF, respectively.

RESULTS

4- and 12-month-old Tgαq*44 mice displayed progressive impairment of RV function and alterations in hepatic blood flow velocity resulting in hepatic congestion with elevated GGT and bilirubin plasma levels and decreased albumin concentration without gross liver pathology. LSECs isolated from 4- and 12-month-old Tgαq*44 mice displayed significant loss of fenestrae with impaired functional response to cytochalasin B, significant changes in proteome related to cytoskeleton remodeling, and altered vasoprotective function. However, LSECs barrier function and bioenergetics were largely preserved. In 4- and 12-month-old Tgαq*44 mice, LSECs defenestration was associated with prolonged postprandial hypertriglyceridemia and in 12-month-old Tgαq*44 mice with proteomic changes of hepatocytes indicative of altered lipid metabolism.

CONCLUSION

Tgαq*44 mice displayed right-sided HF and altered hepatic blood flow leading to LSECs dysfunction involving defenestration, shift in eicosanoid profile, and proteomic changes. LSECs dysfunction appears as an early and persistent event in CHF, preceding congestive hepatopathy and contributing to alterations in lipoprotein transport and CHF pathophysiology.

Cirrhosis-downregulated LSECtin can be retrieved by cytokines, shifts the TLR-induced LSECs secretome and correlates with the hepatic Th response

BACKGROUND AND AIMS

We evaluated tolerogenic C-type lectin LSECtin loss in cirrhosis and its potential regulation by cytokines.

METHODS

Liver tissue from patients with cirrhosis and healthy controls, immortalised and generated LSECtin-CRISPR immortalised LSECs, and murine primary LSECs from the CCl4 model were handled.

RESULTS

LSECtin expression was reduced in liver tissue from cirrhotic patients, and it decreased from compensated to decompensated disease. Increased phosphorylation of MAPK, Akt and NFkB was observed upon LSECtin stimulation in LSEC murine cell line, showing a pattern of inflammatory and chemotactic cytokines either restrained (IL-10, CCL4) or unrestrained (TNF-α, IL-1β, IL-6, CCL2). CD44 attenuated whereas LAG-3 increased all substrates phosphorylation in combination with TLR4 and TLR2 ligands except for NFkB. TNF-α, IL-1 β, IL-6 and CCL2 were restrained by LSECtin crosslinking on TLRs studied. Conversely, IL-10 and CCL4 were upregulated, suggesting a LSECtin-TLRs synergistic effect. Also, LSECtin was significantly induced after IL-13 stimulation or combined with anti-inflammatory cytokines in cirrhotic and immortalised LSECs. Th17 and regulatory T cells were progressively increased in the hepatic tissue from compensated to decompensated patients. A significant inverse correlation was present between gene expression levels of CLEC4G/LSECtin and RORγT and FOXP3 in liver tissues.

CONCLUSION

LSECtin restrains TLR proinflammatory secretome induced on LSECs by interfering immune response control, survival and MAPKs signalling pathways. The cytokine-dependent induction of LSECtin and the association between LSECtin loss and Th17 cell subset expansion in the liver, provides a solid background for exploring LSECtin retrieval as a mechanism to reprogram LSEC homeostatic function hampered during cirrhosis.

Extra-corporeal non-liver transplant therapies for acute liver failure: Focus on plasma exchange and continuous renal replacement therapy

The acute inflammatory milieu in patients with acute liver failure (ALF) results in 'toxic' blood in these patients. In vitro experiments have shown that the plasma obtained from ALF patients is toxic to rabbit hepatocytes and inhibits regeneration of rat hepatocytes. Treatments such as plasma exchange and continuous renal replacement therapy to cleanse the blood have improved survival in ALF patients. In the liver microcirculation, the exchange of fluid across fenestrae in liver sinusoidal endothelial cells (LSECs) is vital for proper functioning of hepatocytes. Clogging of the liver filter bed by inflammatory debris and cells ('traffic jam hypothesis') impeding blood flow in sinusoids may in turn reduce the exchange of fluid across LSEC fenestrae and cause dysfunction and necrosis of hepatocytes in ALF patients. In mouse model of paracetamol overdose, disturbances in microcirculation in the liver preceded the development of injury and necrosis of hepatocytes. This may represent a reversible pathophysiological mechanism in ALF which may be improved by the anti-inflammatory effect of plasma exchange. Wider access to urgent plasma exchange is a major advantage compared to urgent liver transplantation to treat ALF patients worldwide, especially so in resource constrained settings. Continuous hemo-filtration or dialysis is used to reduce ammonia levels and treat cerebral edema in ALF patients. In this review, we discuss the different modalities to cleanse the blood in ALF patients, with an emphasis on plasma exchange, from a hepatology perspective.

FAK-p38 signaling serves as a potential target for reverting matrix stiffness-modulated liver sinusoidal endothelial cell defenestration

Liver sinusoidal endothelial cells (LSECs) are highly specific endothelial cells which play an essential role in the maintenance of liver homeostasis. During the progression of liver fibrosis, matrix stiffening promotes LSEC defenestration, however, the underlying mechanotransduction mechanism remains poorly understood. Here, we applied stiffness-tunable hydrogels to assess the matrix stiffening-induced phenotypic changes in primary mouse LSECs. Results indicated that increased stiffness promoted LSEC defenestration through cytoskeletal reorganization. LSECs sensed the increased matrix stiffness via focal adhesion kinase (FAK), leading to the activation of p38-mitogen activated protein kinase activated protein kinase 2 (MK2) pathway, thereby inducing actin remodeling via LIM Kinase 1 (LIMK1) and Cofilin. Interestingly, inhibition of FAK or p38-MK2 pathway was able to effectively restore the fenestrae to a certain degree in LSECs isolated from early to late stages of liver fibrosis mice. Thus, this study highlights the impact of mechanotransduction in LSEC defenestration, and provides novel insights for potential therapeutic interventions for liver fibrosis.

The Interplay between Liver Sinusoidal Endothelial Cells, Platelets, and Neutrophil Extracellular Traps in the Development and Progression of Metabolic Dysfunction-Associated Steatotic Liver Disease

Metabolic dysfunction-associated steatotic liver disease (MASLD) represents a societal burden due to the lack of effective treatment and incomplete pathophysiology understanding. This review explores the intricate connections among liver sinusoidal endothelial cells (LSECs), platelets, neutrophil extracellular traps (NETs), and coagulation disruptions in MASLD pathogenesis. In MASLD's early stages, LSECs undergo capillarization and dysfunction due to excessive dietary macronutrients and gut-derived products. Capillarization leads to ischemic changes in hepatocytes, triggering pro-inflammatory responses in Kupffer cells (KCs) and activating hepatic stellate cells (HSCs). Capillarized LSECs show a pro-inflammatory phenotype through adhesion molecule overexpression, autophagy loss, and increased cytokines production. Platelet interaction favors leucocyte recruitment, NETs formation, and liver inflammatory foci. Liver fibrosis is facilitated by reduced nitric oxide, HSC activation, profibrogenic mediators, and increased angiogenesis. Moreover, platelet attachment, activation, α-granule cargo release, and NETs formation contribute to MASLD progression. Platelets foster fibrosis and microthrombosis, leading to parenchymal extinction and fibrotic healing. Additionally, platelets promote tumor growth, epithelial-mesenchymal transition, and tumor cell metastasis. MASLD's prothrombotic features are exacerbated by insulin resistance, diabetes, and obesity, manifesting as increased von Willebrand factor, platelet hyperaggregability, hypo-fibrinolysis, and a prothrombotic fibrin clot structure. Improving LSEC health and using antiplatelet treatment appear promising for preventing MASLD development and progression.

Liver sinusoidal endothelial cells show reduced scavenger function and downregulation of Fc gamma receptor IIb, yet maintain a preserved fenestration in the Glmpgt/gt mouse model of slowly progressing liver fibrosis

Liver sinusoidal endothelial cells (LSECs) are fenestrated endothelial cells with a unique, high endocytic clearance capacity for blood-borne waste macromolecules and colloids. This LSEC scavenger function has been insufficiently characterized in liver disease. The Glmpgt/gt mouse lacks expression of a subunit of the MFSD1/GLMP lysosomal membrane protein transporter complex, is born normal, but soon develops chronic, mild hepatocyte injury, leading to slowly progressing periportal liver fibrosis, and splenomegaly. This study examined how LSEC scavenger function and morphology are affected in the Glmpgt/gt model. FITC-labelled formaldehyde-treated serum albumin (FITC-FSA), a model ligand for LSEC scavenger receptors was administered intravenously into Glmpgt/gt mice, aged 4 months (peak of liver inflammation), 9-10 month, and age-matched Glmpwt/wt mice. Organs were harvested for light and electron microscopy, quantitative image analysis of ligand uptake, collagen accumulation, LSEC ultrastructure, and endocytosis receptor expression (also examined by qPCR and western blot). In both age groups, the Glmpgt/gt mice showed multifocal liver injury and fibrosis. The uptake of FITC-FSA in LSECs was significantly reduced in Glmpgt/gt compared to wild-type mice. Expression of LSEC receptors stabilin-1 (Stab1), and mannose receptor (Mcr1) was almost similar in liver of Glmpgt/gt mice and age-matched controls. At the same time, immunostaining revealed differences in the stabilin-1 expression pattern in sinusoids and accumulation of stabilin-1-positive macrophages in Glmpgt/gt liver. FcγRIIb (Fcgr2b), which mediates LSEC endocytosis of soluble immune complexes was widely and significantly downregulated in Glmpgt/gt liver. Despite increased collagen in space of Disse, LSECs of Glmpgt/gt mice showed well-preserved fenestrae organized in sieve plates but the frequency of holes >400 nm in diameter was increased, especially in areas with hepatocyte damage. In both genotypes, FITC-FSA also distributed to endothelial cells of spleen and bone marrow sinusoids, suggesting that these locations may function as possible compensatory sites of clearance of blood-borne scavenger receptor ligands in liver fibrosis.

Effect of caffeine and other xanthines on liver sinusoidal endothelial cell ultrastructure

Xanthines such as caffeine and theobromine are among the most consumed psychoactive stimulants in the world, either as natural components of coffee, tea and chocolate, or as added ingredients. The present study assessed if xanthines affect liver sinusoidal endothelial cells (LSEC). Cultured primary rat LSEC were challenged with xanthines at concentrations typically obtained from normal consumption of xanthine-containing beverages, food or medicines; and at higher concentrations below the in vitro toxic limit. The fenestrated morphology of LSEC were examined with scanning electron and structured illumination microscopy. All xanthine challenges had no toxic effects on LSEC ultrastructure as judged by LSEC fenestration morphology, or function as determined by endocytosis studies. All xanthines in high concentrations (150 μg/mL) increased fenestration frequency but at physiologically relevant concentrations, only theobromine (8 μg/mL) showed an effect. LSEC porosity was influenced only by high caffeine doses which also shifted the fenestration distribution towards smaller pores. Moreover, a dose-dependent increase in fenestration number was observed after caffeine treatment. If these compounds induce similar changes in vivo, age-related reduction of LSEC porosity can be reversed by oral treatment with theobromine or with other xanthines using targeted delivery.

Lipid oxidation dysregulation: an emerging player in the pathophysiology of sepsis

Sepsis is a life-threatening organ dysfunction caused by abnormal host response to infection. Millions of people are affected annually worldwide. Derangement of the inflammatory response is crucial in sepsis pathogenesis. However, metabolic, coagulation, and thermoregulatory alterations also occur in patients with sepsis. Fatty acid mobilization and oxidation changes may assume the role of a protagonist in sepsis pathogenesis. Lipid oxidation and free fatty acids (FFAs) are potentially valuable markers for sepsis diagnosis and prognosis. Herein, we discuss inflammatory and metabolic dysfunction during sepsis, focusing on fatty acid oxidation (FAO) alterations in the liver and muscle (skeletal and cardiac) and their implications in sepsis development.

Determining Spatial Variability of Elastic Properties for Biological Samples Using AFM

Measuring the mechanical properties (i.e., elasticity in terms of Young's modulus) of biological samples using Atomic Force Microscopy (AFM) indentation at the nanoscale has opened new horizons in studying and detecting various pathological conditions at early stages, including cancer and osteoarthritis. It is expected that AFM techniques will play a key role in the future in disease diagnosis and modeling using rigorous mathematical criteria (i.e., automated user-independent diagnosis). In this review, AFM techniques and mathematical models for determining the spatial variability of elastic properties of biological materials at the nanoscale are presented and discussed. Significant issues concerning the rationality of the elastic half-space assumption, the possibility of monitoring the depth-dependent mechanical properties, and the construction of 3D Young's modulus maps are also presented.

The gut–liver axis in sepsis: interaction mechanisms and therapeutic potential

Sepsis is a potentially fatal condition caused by dysregulation of the body's immune response to an infection. Sepsis-induced liver injury is considered a strong independent prognosticator of death in the critical care unit, and there is anatomic and accumulating epidemiologic evidence that demonstrates intimate cross talk between the gut and the liver. Intestinal barrier disruption and gut microbiota dysbiosis during sepsis result in translocation of intestinal pathogen-associated molecular patterns and damage-associated molecular patterns into the liver and systemic circulation. The liver is essential for regulating immune defense during systemic infections via mechanisms such as bacterial clearance, lipopolysaccharide detoxification, cytokine and acute-phase protein release, and inflammation metabolic regulation. When an inappropriate immune response or overwhelming inflammation occurs in the liver, the impaired capacity for pathogen clearance and hepatic metabolic disturbance can result in further impairment of the intestinal barrier and increased disruption of the composition and diversity of the gut microbiota. Therefore, interaction between the gut and liver is a potential therapeutic target. This review outlines the intimate gut–liver cross talk (gut–liver axis) in sepsis.

The STING in Non-Alcoholic Fatty Liver Diseases: Potential Therapeutic Targets in Inflammation-Carcinogenesis Pathway

Non-alcoholic fatty liver disease (NAFLD), an important chronic disease, is one of the major causes of high mortality and creates a substantial financial burden worldwide. The various immune cells in the liver, including macrophages, NK cells, dendritic cells, and the neutrophils involved in the innate immune response, trigger inflammation after recognizing the damage signaled from infection or injured cells and tissues. The stimulator of interferon genes (STING) is a critical molecule that binds to the cyclic dinucleotides (CDNs) generated by the cyclic GMP-AMP synthase (cGAS) to initiate the innate immune response against infection. Previous studies have demonstrated that the cGAS-STING pathway plays a critical role in inflammatory, auto-immune, and anti-viral immune responses. Recently, studies have focused on the role of STING in liver diseases, the results implying that alterations in its activity may be involved in the pathogenesis of liver disorders. Here, we summarize the function of STING in the development of NAFLD and present the current inhibitors and agonists targeting STING.

WITHDRAWN: Foreword to the special issue on different approaches to force spectroscopy in the research of cell pathologies

The Publisher regrets that this article is an accidental duplication of an article that has already been published in Micron, Volume 161, October 2022, 103325, https://doi.org/10.1016/j.micron.2022.103325. The duplicate article has therefore been withdrawn. The full Elsevier Policy on Article Withdrawal can be found at https://www.elsevier.com/about/our-business/policies/article-withdrawal.

Comprehensive Review of the Vascular Niche in Regulating Organ Regeneration and Fibrosis

The vasculature occupies a large area of the body, and none of the physiological activities can be carried out without blood vessels. Blood vessels are not just passive conduits and barriers for delivering blood and nutrients. Meanwhile, endothelial cells covering the vascular lumen establish vascular niches by deploying some growth factors, known as angiocrine factors, and actively participate in the regulation of a variety of physiological processes, such as organ regeneration and fibrosis and the occurrence and development of cancer. After organ injury, vascular endothelial cells regulate the repair process by secreting various angiocrine factors, triggering the proliferation and differentiation process of stem cells. Therefore, analyzing the vascular niche and exploring the factors that maintain vascular homeostasis can provide strong theoretical support for clinical treatment targeting blood vessels. Here we mainly discuss the regulatory mechanisms of the vascular niche in organ regeneration and fibrosis.

Tuning of Liver Sieve: The Interplay between Actin and Myosin Regulatory Light Chain Regulates Fenestration Size and Number in Murine Liver Sinusoidal Endothelial Cells

Liver sinusoidal endothelial cells (LSECs) facilitate the efficient transport of macromolecules and solutes between the blood and hepatocytes. The efficiency of this transport is realized via transcellular nanopores, called fenestrations. The mean fenestration size is 140 ± 20 nm, with the range from 50 nm to 350 nm being mostly below the limits of diffraction of visible light. The cellular mechanisms controlling fenestrations are still poorly understood. In this study, we tested a hypothesis that both Rho kinase (ROCK) and myosin light chain (MLC) kinase (MLCK)-dependent phosphorylation of MLC regulates fenestrations. We verified the hypothesis using a combination of several molecular inhibitors and by applying two high-resolution microscopy modalities: structured illumination microscopy (SIM) and scanning electron microscopy (SEM). We demonstrated precise, dose-dependent, and reversible regulation of the mean fenestration diameter within a wide range from 120 nm to 220 nm and the fine-tuning of the porosity in a range from ~0% up to 12% using the ROCK pathway. Moreover, our findings indicate that MLCK is involved in the formation of new fenestrations—after inhibiting MLCK, closed fenestrations cannot be reopened with other agents. We, therefore, conclude that the Rho-ROCK pathway is responsible for the control of the fenestration diameter, while the inhibition of MLCK prevents the formation of new fenestrations.

Defenestrated endothelium delays liver-directed gene transfer in hemophilia A mice

Hemophilia A is an inherited bleeding disorder caused by defective or deficient coagulation factor VIII (FVIII) activity. Until recently, the only treatment for prevention of bleeding involved IV administration of FVIII. Gene therapy with adeno-associated vectors (AAVs) has shown some efficacy in patients with hemophilia A. However, limitations persist due to AAV-induced cellular stress, immunogenicity, and reduced durability of gene expression. Herein, we examined the efficacy of liver-directed gene transfer in FVIII knock-out mice by AAV8-GFP. Surprisingly, compared with control mice, FVIII knockout (F8TKO) mice showed significant delay in AAV8-GFP transfer in the liver. We found that the delay in liver-directed gene transfer in F8TKO mice was associated with absence of liver sinusoidal endothelial cell (LSEC) fenestration, which led to aberrant expression of several sinusoidal endothelial proteins, causing increased capillarization and decreased permeability of LSECs. This is the first study to link impaired liver-directed gene transfer to liver-endothelium maladaptive structural changes associated with FVIII deficiency in mice.

From fixed-dried to wet-fixed to live - comparative super-resolution microscopy of liver sinusoidal endothelial cell fenestrations

Fenestrations in liver sinusoidal endothelial cells (LSEC) are transcellular nanopores of 50-350 nm diameter that facilitate bidirectional transport of solutes and macromolecules between the bloodstream and the parenchyma of the liver. Liver diseases, ageing, and various substances such as nicotine or ethanol can negatively influence LSECs fenestrations and lead to defenestration. Over the years, the diameter of fenestrations remained the main challenge for imaging of LSEC in vitro. Several microscopy, or rather nanoscopy, approaches have been used to quantify fenestrations in LSEC to assess the effect of drugs and, and toxins in different biological models. All techniques have their limitations, and measurements of the "true" size of fenestrations are hampered because of this. In this study, we approach the comparison of different types of microscopy in a correlative manner. We combine scanning electron microscopy (SEM) with optical nanoscopy methods such as structured illumination microscopy (SIM) or stimulated emission depletion (STED) microscopy. In addition, we combined atomic force microscopy (AFM) with SEM and STED, all to better understand the previously reported differences between the reports of fenestration dimensions. We conclude that sample dehydration alters fenestration diameters. Finally, we propose the combination of AFM with conventional microscopy that allows for easy super-resolution observation of the cell dynamics with additional chemical information that can be traced back for the whole experiment. Overall, by pairing the various types of imaging techniques that provide topological 2D/3D/label-free/chemical information we get a deeper insight into both limitations and strengths of each type microscopy when applied to fenestration analysis.

Evaluation of Liposome-Loaded Microbubbles as a Theranostic Tool in a Murine Collagen-Induced Arthritis Model

Rheumatoid arthritis (RA) is an autoimmune disease characterized by severe inflammation of the synovial tissue. Here, we assess the feasibility of liposome-loaded microbubbles as theranostic agents in a murine arthritis model. First, contrast-enhanced ultrasound (CEUS) was used to quantify neovascularization in this model since CEUS is well-established for RA diagnosis in humans. Next, the potential of liposome-loaded microbubbles and ultrasound (US) to selectively enhance liposome delivery to the synovium was evaluated with in vivo fluorescence imaging. This procedure is made very challenging by the presence of hard joints and by the limited lifetime of the microbubbles. The inflamed knee joints were exposed to therapeutic US after intravenous injection of liposome-loaded microbubbles. Loaded microbubbles were found to be quickly captured by the liver. This resulted in fast clearance of attached liposomes while free and long-circulating liposomes were able to accumulate over time in the inflamed joints. Our observations show that murine arthritis models are not well-suited for evaluating the potential of microbubble-mediated drug delivery in joints given: (i) restricted microbubble passage in murine synovial vasculature and (ii) limited control over the exact ultrasound conditions in situ given the much shorter length scale of the murine joints as compared to the therapeutic wavelength.

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.

Multimodal on-chip nanoscopy and quantitative phase imaging reveals the nanoscale morphology of liver sinusoidal endothelial cells

Visualization of three-dimensional (3D) morphological changes in the subcellular structures of a biological specimen is a major challenge in life science. Here, we present an integrated chip-based optical nanoscopy combined with quantitative phase microscopy (QPM) to obtain 3D morphology of liver sinusoidal endothelial cells (LSEC). LSEC have unique morphology with small nanopores (50-300 nm in diameter) in the plasma membrane, called fenestrations. The fenestrations are grouped in discrete clusters, which are around 100 to 200 nm thick. Thus, imaging and quantification of fenestrations and sieve plate thickness require resolution and sensitivity of sub-100 nm along both the lateral and the axial directions, respectively. In chip-based nanoscopy, the optical waveguides are used both for hosting and illuminating the sample. The fluorescence signal is captured by an upright microscope, which is converted into a Linnik-type interferometer to sequentially acquire both superresolved images and phase information of the sample. The multimodal microscope provided an estimate of the fenestration diameter of 119 ± 53 nm and average thickness of the sieve plates of 136.6 ± 42.4 nm, assuming the constant refractive index of cell membrane to be 1.38. Further, LSEC were treated with cytochalasin B to demonstrate the possibility of precise detection in the cell height. The mean phase value of the fenestrated area in normal and treated cells was found to be 161 ± 50 mrad and 109 ± 49 mrad, respectively. The proposed multimodal technique offers nanoscale visualization of both the lateral size and the thickness map, which would be of broader interest in the fields of cell biology and bioimaging.

Alcohol-associated capillarization of sinusoids: A critique since the discovery by Schaffner and Popper in 1963

This article reviews the literature on capillarization of hepatic sinusoids since its discovery in 1963. Liver sinusoidal endothelial cells are uniquely fenestrated and lack an underlying basement membrane. In chronic liver disease, the sinusoids capillarize and transform into systemic capillaries, a process termed capillarization of sinusoids. The histopathology is marked by defenestration, basement membrane formation, and space of Disse fibrogenesis. Capillarized sinusoids compromise the bidirectional exchange of materials between sinusoids and hepatocytes, leading to hepatocellular dysfunction. Sinusoidal capillarization was first described in active cirrhosis of alcoholics in 1963. Since then, it has been found in early and progressive stages of alcoholic hepatic fibrosis before the onset of cirrhosis. The sinusoidal structure is not altered in alcoholic steatosis without fibrosis. Defenestration impairs the ability of the endothelium to filter chylomicron remnants from sinusoids into the Disse's space, contributing to alcohol-induced postprandial hyperlipidemia and possibly atherosclerosis. Ethanol also modulates the fenestration dynamics in animals. In baboons, chronic alcohol consumption diminishes endothelial porosity in concomitance with hepatic fibrogenesis and in rats defenestrates the endothelium in the absence of fibrosis, and sometimes capillarizes the sinusoids. Acute ethanol ingestion enlarges fenestrations in rats and contracts fenestrations in rabbits. In sinusoidal endothelial cell culture, ethanol elicits fenestration dilation, which is likely related to its interaction with fenestration-associated cytoskeleton. Ethanol potentiates sinusoidal injury caused by cocaine, acetaminophen or lipopolysaccharide in mice and rats. Understanding ethanol's mechanisms on pathogenesis of sinusoidal capillarization and fenestration dynamics will lead to development of methods to prevent risks for atherosclerosis in alcoholism.

Research progress of sinusoidal endothelial cells in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is a growing health problem associated with metabolic syndrome. Liver sinusoidal endothelial cells (LSECs) are highly specialized endothelial cells located between the blood and other liver cell types. They are composed of window pores, have high endocytosis, and play an important role in maintaining the overall liver homeostasis. Under pathological conditions, LSECs may be the key event of a variety of chronic liver diseases. In this review, we introduce the unique physiological structure and function of LSECs, summarize the main changes of LSECs in NAFLD (including sinohepatic capillarization, angiogenesis, vasoconstriction, proinflammatory effect, and fibrosis) and their pathogenesis, and discuss the influence of LSECs on the progression of NAFLD, with an aim to demonstrate the potential efficacy of LSECS targeted therapy for NAFLD.

The wHole Story About Fenestrations in LSEC

The porosity of liver sinusoidal endothelial cells (LSEC) ensures bidirectional passive transport of lipoproteins, drugs and solutes between the liver capillaries and the liver parenchyma. This porosity is realized via fenestrations - transcellular pores with diameters in the range of 50-300 nm - typically grouped together in sieve plates. Aging and several liver disorders severely reduce LSEC porosity, decreasing their filtration properties. Over the years, a variety of drugs, stimulants, and toxins have been investigated in the context of altered diameter or frequency of fenestrations. In fact, any change in the porosity, connected with the change in number and/or size of fenestrations is reflected in the overall liver-vascular system crosstalk. Recently, several commonly used medicines have been proposed to have a beneficial effect on LSEC re-fenestration in aging. These findings may be important for the aging populations of the world. In this review we collate the literature on medicines, recreational drugs, hormones and laboratory tools (including toxins) where the effect LSEC morphology was quantitatively analyzed. Moreover, different experimental models of liver pathology are discussed in the context of fenestrations. The second part of this review covers the cellular mechanisms of action to enable physicians and researchers to predict the effect of newly developed drugs on LSEC porosity. To achieve this, we discuss four existing hypotheses of regulation of fenestrations. Finally, we provide a summary of the cellular mechanisms which are demonstrated to tune the porosity of LSEC.

Quantitative analysis methods for studying fenestrations in liver sinusoidal endothelial cells. A comparative study

Liver Sinusoidal Endothelial Cells (LSEC) line the hepatic vasculature providing blood filtration via transmembrane nanopores called fenestrations. These structures are 50-300 nm in diameter, which is below the resolution limit of a conventional light microscopy. To date, there is no standardized method of fenestration image analysis. With this study, we provide and compare three different approaches: manual measurements, a semi-automatic (threshold-based) method, and an automatic method based on user-friendly open source machine learning software. Images were obtained using three super resolution techniques - atomic force microscopy (AFM), scanning electron microscopy (SEM), and structured illumination microscopy (SIM). Parameters describing fenestrations such as diameter, area, roundness, frequency, and porosity were measured. Finally, we studied the user bias by comparison of the data obtained by five different users applying provided analysis methods.

Fat causes necrosis and inflammation in parenchymal cells in human steatotic liver

Adapted fixation methods for electron microscopy allowed us to study liver cell fine structure in 217 biopsies of intact human livers over the course of 10 years. The following novel observations and concepts arose: single fat droplets in parenchymal cells can grow to a volume four times larger than the original cell, thereby extremely marginalizing the cytoplasm with all organelles. Necrosis of single parenchymal cells, still containing one huge fat droplet, suggests death by fat in a process of single-cell steatonecrosis. In a later stage of single-cell steatonecrosis, neutrophils and erythrocytes surround the single fat droplet, forming an inflammatory fat follicle indicating the apparent onset of inflammation. Also, fat droplets frequently incorporate masses of filamentous fragments and other material, most probably representing Mallory substance. No other structure or material was found that could possibly represent Mallory bodies. We regularly observe the extrusion of huge fat droplets, traversing the peripheral cytoplasm of parenchymal cells, the Disse space and the endothelium. These fat droplets fill the sinusoid as a sinusoidal lipid embolus. In conclusion, adapted methods of fixation applied to human liver tissue revealed that single, huge fat droplets cause necrosis and inflammation in single parenchymal cells. Fat droplets also collect Mallory substance and give rise to sinusoidal fat emboli. Therefore, degreasing of the liver seems to be an essential therapeutic first step in the self-repairing of non-alcoholic fatty liver disease. This might directly reduce single-cell steatotic necrosis and inflammation as elements in non-alcoholic steatohepatitis progression.

Role of liver sinusoidal endothelial cells in liver diseases

Liver sinusoidal endothelial cells (LSECs) form the wall of the hepatic sinusoids. Unlike other capillaries, they lack an organized basement membrane and have cytoplasm that is penetrated by open fenestrae, making the hepatic microvascular endothelium discontinuous. LSECs have essential roles in the maintenance of hepatic homeostasis, including regulation of the vascular tone, inflammation and thrombosis, and they are essential for control of the hepatic immune response. On a background of acute or chronic liver injury, LSECs modify their phenotype and negatively affect neighbouring cells and liver disease pathophysiology. This Review describes the main functions and phenotypic dysregulations of LSECs in liver diseases, specifically in the context of acute injury (ischaemia-reperfusion injury, drug-induced liver injury and bacterial and viral infection), chronic liver disease (metabolism-associated liver disease, alcoholic steatohepatitis and chronic hepatotoxic injury) and hepatocellular carcinoma, and provides a comprehensive update of the role of LSECs as therapeutic targets for liver disease. Finally, we discuss the open questions in the field of LSEC pathobiology and future avenues of research.

Targeting Liver Sinusoidal Endothelial Cells: An Attractive Therapeutic Strategy to Control Inflammation in Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD), especially its advanced stage nonalcoholic steatohepatitis (NASH), has become a threatened public health problem worldwide. However, no specific drug has been approved for clinical use to treat patients with NASH, though there are many promising candidates against NAFLD in the drug development pipeline. Recently, accumulated evidence showed that liver sinusoidal endothelial cells (LSECs) play an essential role in the occurrence and development of liver inflammation in patients with NAFLD. LSECs, as highly specialized endothelial cells with unique structure and anatomical location, contribute to the maintenance of liver homeostasis and could be a promising therapeutic target to control liver inflammation of NAFLD. In this review, we outline the pathophysiological roles of LSECs related to inflammation of NAFLD, highlight the pro-inflammatory and anti-inflammatory effects of LSECs, and discuss the potential drug development strategies against NAFLD based on targeting to LSECs.

Angiodiversity and organotypic functions of sinusoidal endothelial cells

‘Angiodiversity’ refers to the structural and functional heterogeneity of endothelial cells (EC) along the segments of the vascular tree and especially within the microvascular beds of different organs. Organotypically differentiated EC ranging from continuous, barrier-forming endothelium to discontinuous, fenestrated endothelium perform organ-specific functions such as the maintenance of the tightly sealed blood–brain barrier or the clearance of macromolecular waste products from the peripheral blood by liver EC-expressed scavenger receptors. The microvascular bed of the liver, composed of discontinuous, fenestrated liver sinusoidal endothelial cells (LSEC), is a prime example of organ-specific angiodiversity. Anatomy and development of LSEC have been extensively studied by electron microscopy as well as linage-tracing experiments. Recent advances in cell isolation and bulk transcriptomics or single-cell RNA sequencing techniques allowed the identification of distinct LSEC molecular programs and have led to the identification of LSEC subpopulations. LSEC execute homeostatic functions such as fine tuning the vascular tone, clearing noxious substances from the circulation, and modulating immunoregulatory mechanisms. In recent years, the identification and functional analysis of LSEC-derived angiocrine signals, which control liver homeostasis and disease pathogenesis in an instructive manner, marks a major change of paradigm in the understanding of liver function in health and disease. This review summarizes recent advances in the understanding of liver vascular angiodiversity and the functional consequences resulting thereof.

KRAS or BRAF mutations cause hepatic vascular cavernomas treatable with MAP2K-MAPK1 inhibition

Human hepatic vascular cavernomas, the most common benign tumor of the liver, were described in the mid-1800s, yet the mechanisms for their formation and effective treatments remain unknown. Here, we demonstrate gain-of-function mutations in KRAS or BRAF genes within liver endothelial cells as a causal mechanism for hepatic vascular cavernomas. We identified gain-of-function mutations in KRAS or BRAF genes in pathological liver tissue samples from patients with hepatic vascular cavernomas. Mice expressing these human KRAS^G12D or BRAF^V600E mutations in hepatic endothelial cells recapitulated the human hepatic vascular cavernoma phenotype of dilated sinusoidal capillaries with defective branching patterns. KRAS^G12D or BRAF^V600E induced “zipper-like” contiguous expression of junctional proteins at sinusoidal endothelial cell–cell contacts, switching capillaries from branching to cavernous expansion. Pharmacological or genetic inhibition of the endothelial RAS–MAPK1 signaling pathway rescued hepatic vascular cavernoma formation in endothelial KRAS^G12D- or BRAF^V600E-expressing mice. These results uncover a major cause of hepatic vascular cavernomas and provide a road map for their personalized treatment.

Gut microbiota mediated molecular events and therapy in liver diseases

Gut microbiota is a community of microorganisms that reside in the gastrointestinal tract. An increasing number of studies has demonstrated that the gut-liver axis plays a critical role in liver homeostasis. Dysbiosis of gut microbiota can cause liver diseases, including nonalcoholic fatty liver disease and alcoholic liver disease. Preclinical and clinical investigations have substantiated that the metabolites and other molecules derived from gut microbiota and diet interaction function as mediators to cause liver fibrosis, cirrhosis, and final cancer. This effect has been demonstrated to be associated with dysregulation of intrahepatic immunity and liver metabolism. Targeting these findings have led to the development of novel preventive and therapeutic strategies. Here, we review the cellular and molecular mechanisms underlying gut microbiota-mediated impact on liver disease. We also summarize the advancement of gut microbiota-based therapeutic strategies in the control of liver diseases.

Characterizing liver sinusoidal endothelial cell fenestrae on soft substrates upon AFM imaging and deep learning

BACKGROUND

Liver sinusoidal endothelial cells (LSECs) display unique fenestrated morphology. Alterations in the size and number of fenestrae play a crucial role in the progression of various liver diseases. While their features have been visualized using atomic force microscopy (AFM), the in situ imaging methods and off-line analyses are further required for fenestra quantification.

METHODS

Primary mouse LSECs were cultured on a collagen-I-coated culture dish, or a polydimethylsiloxane (PDMS) or polyacrylamide (PA) hydrogel substrate. An AFM contact mode was applied to visualize fenestrae on individual fixed LSECs. Collected images were analyzed using an in-house developed image recognition program based on fully convolutional networks (FCN).

RESULTS

Key scanning parameters were first optimized for visualizing the fenestrae on LSECs on culture dish, which was also applicable for the LSECs cultured on various hydrogels. The intermediate-magnification morphology images of LSECs were used for developing the FCN-based, fenestra recognition program. This program enabled us to recognize the vast majority of fenestrae from AFM images after twice trainings at a typical accuracy of 81.6% on soft substrate and also quantify the statistics of porosity, number of fenestrae and distribution of fenestra diameter.

CONCLUSIONS

Combining AFM imaging with FCN training is able to quantify the morphological distributions of LSEC fenestrae on various substrates.

SIGNIFICANCE

AFM images acquired and analyzed here provided the global information of surface ultramicroscopic structures over an entire cell, which is fundamental in understanding their regulatory mechanisms and pathophysiological relevance in fenestra-like evolution of individual cells on stiffness-varied substrates.

Prominent Receptors of Liver Sinusoidal Endothelial Cells in Liver Homeostasis and Disease

Liver sinusoidal endothelial cells (LSECs) are the most abundant non-parenchymal cells lining the sinusoidal capillaries of the hepatic system. LSECs are characterized with numerous fenestrae and lack basement membrane as well as a diaphragm. These unique morphological characteristics of LSECs makes them the most permeable endothelial cells of the mammalian vasculature and aid in regulating flow of macromolecules and small lipid-based structures between sinusoidal blood and parenchymal cells. LSECs have a very high endocytic capacity aided by scavenger receptors (SR), such as SR-A, SR-B (SR-B1 and CD-36), SR-E (Lox-1 and mannose receptors), and SR-H (Stabilins). Other high-affinity receptors for mediating endocytosis include the FcγRIIb, which assist in the antibody-mediated removal of immune complexes. Complemented with intense lysosomal activity, LSECs play a vital role in the uptake and degradation of many blood borne waste macromolecules and small (<280 nm) colloids. Currently, seven Toll-like receptors have been investigated in LSECs, which are involved in the recognition and clearance of pathogen-associated molecular pattern (PAMPs) as well as damage associated molecular pattern (DAMP). Along with other SRs, LSECs play an essential role in maintaining lipid homeostasis with the low-density lipoprotein receptor-related protein-1 (LRP-1), in juxtaposition with hepatocytes. LSECs co-express two surface lectins called L-Specific Intercellular adhesion molecule-3 Grabbing Non-integrin Receptor (L-SIGN) and liver sinusoidal endothelial cell lectin (LSECtin). LSECs also express several adhesion molecules which are involved in the recruitment of leukocytes at the site of inflammation. Here, we review these cell surface receptors as well as other components expressed by LSECs and their functions in the maintenance of liver homeostasis. We further discuss receptor expression and activity and dysregulation associated with the initiation and progression of many liver diseases, such as hepatocellular carcinoma, liver fibrosis, and cirrhosis, alcoholic and non-alcoholic fatty liver diseases and pseudocapillarization with aging.

Keap1 governs ageing-induced protein aggregation in endothelial cells

The breach of proteostasis, leading to the accumulation of protein aggregates, is a hallmark of ageing and age-associated disorders, up to now well-established in neurodegeneration. Few studies have addressed the issue of dysfunctional cell response to protein deposition also for the cardiovascular system. However, the molecular basis of proteostasis decline in vascular cells, as well as its relation to ageing, are not understood. Recent studies have indicated the associations of Nrf2 transcription factor, the critical modulator of cellular stress-response, with ageing and premature senescence. In this report, we outline the significance of protein aggregation in physiological and premature ageing of murine and human endothelial cells (ECs). Our study shows that aged donor-derived and prematurely senescent Nrf2-deficient primary human ECs, but not those overexpressing dominant-negative Nrf2, exhibit increased accumulation of protein aggregates. Such phenotype is also found in the aortas of aged mice and young Nrf2 tKO mice. Ageing-related loss of proteostasis in ECs depends on Keap1, well-known repressor of Nrf2, recently perceived as a key independent regulator of EC function and protein S-nitrosation (SNO). Deposition of protein aggregates in ECs is associated with impaired autophagy. It can be counteracted by Keap1 depletion, S-nitrosothiol reductant or rapamycin treatment. Our results show that Keap1:Nrf2 protein balance and Keap1-dependent SNO predominate Nrf2 transcriptional activity-driven mechanisms in governing proteostasis in ageing ECs.

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Physiological and premature ageing facilitates aggregation of proteins in ECs.

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Loss of proteostasis depends on Keap1-driven S-nitrosation in ageing ECs.

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Keap1:Nrf2 ratio predominates Nrf2 transcriptional activity in proteostasis control.

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Keap1 or SNO depletion, or rapamycin treatment restore proteostasis in ageing ECs.

Biophysical nanocharacterization of liver sinusoidal endothelial cells through atomic force microscopy

The structural-functional hallmark of the liver sinusoidal endothelium is the presence of fenestrae grouped in sieve plates. Fenestrae are open membrane bound pores supported by a (sub)membranous cytoskeletal lattice. Changes in number and diameter of fenestrae alter bidirectional transport between the sinusoidal blood and the hepatocytes. Their physiological relevance has been shown in different liver disease models. Although the structural organization of fenestrae has been well documented using different electron microscopy approaches, the dynamic nature of those pores remained an enigma until the recent developments in the research field of four dimensional (4-D) AFM. In this contribution we highlight how AFM as a biophysical nanocharacterization tool enhanced our understanding in the dynamic behaviour of liver sinusoidal endothelial fenestrae. Different AFM probing approaches, including spectroscopy, enabled mapping of topography and nanomechanical properties at unprecedented resolution under live cell imaging conditions. This dynamic biophysical characterization approach provided us with novel information on the 'short' life-span, formation, disappearance and closure of hepatic fenestrae. These observations are briefly reviewed against the existing literature.

Liver Sinusoidal Endothelial Cells Contribute to Hepatic Antigen-Presenting Cell Function and Th17 Expansion in Cirrhosis

Hepatic immune function is compromised during cirrhosis. This study investigated the immune features of liver sinusoidal endothelial cells (LSECs) in two experimental models of cirrhosis. Dendritic cells, hepatic macrophages, and LSECs were isolated from carbon tetrachloride and bile duct-ligated rats. Gene expression of innate receptors, bacterial internalization, co-stimulatory molecules induction, and CD4+ T cell activation and differentiation were evaluated. Induced bacterial peritonitis and norfloxacin protocols on cirrhotic rats were also carried out. LSECs demonstrated an active immunosurveillance profile, as shown by transcriptional modulation of different scavenger and cell-adhesion genes, and their contribution to bacterial internalization. LSECs significantly increased their expression of CD40 and CD80 and stimulated CD4+ T cell activation marker CD71 in both models. The pro-inflammatory Th17 subset was expanded in CCl₄-derived LSECs co-cultures. In the bile duct ligation (BDL) model, CD4+ T cell differentiation only occurred under induced bacterial peritonitis conditions. Differentiated pro-inflammatory Th cells by LSECs in both experimental models were significantly reduced with norfloxacin treatment, whereas Foxp3 tolerogenic Th CD4+ cells were expanded. Conclusion: LSECs’ participation in the innate-adaptive immune progression, their ability to stimulate pro-inflammatory CD4+ T cells expansion during liver damage, and their target role in norfloxacin-induced immunomodulation granted a specific competence to this cell population in cirrhosis.

New aspects of hepatic endothelial cells in physiology and nonalcoholic fatty liver disease

The liver is the central metabolic hub for carbohydrate, lipid, and protein metabolism. It is composed of four major types of cells, including hepatocytes, endothelial cells (ECs), Kupffer cells, and stellate cells. Hepatic ECs are highly heterogeneous in both mice and humans, representing the second largest population of cells in liver. The majority of them line hepatic sinusoids known as liver sinusoidal ECs (LSECs). The structure and biology of LSECs and their roles in physiology and liver disease were reviewed recently. Here, we do not give a comprehensive review of LSEC structure, function, or pathophysiology. Instead, we focus on the recent progress in LSEC research and other hepatic ECs in physiology and nonalcoholic fatty liver disease and other hepatic fibrosis-related conditions. We discuss several current areas of interest, including capillarization, scavenger function, autophagy, cellular senescence, paracrine effects, and mechanotransduction. In addition, we summarize the strengths and weaknesses of evidence for the potential role of endothelial-to-mesenchymal transition in liver fibrosis.

Fenestral diaphragms and PLVAP associations in liver sinusoidal endothelial cells are developmentally regulated

Endothelial cells contain several nanoscale domains such as caveolae, fenestrations and transendothelial channels, which regulate signaling and transendothelial permeability. These structures can be covered by filter-like diaphragms. A transmembrane PLVAP (plasmalemma vesicle associated protein) protein has been shown to be necessary for the formation of diaphragms. The expression, subcellular localization and fenestra-forming role of PLVAP in liver sinusoidal endothelial cells (LSEC) have remained controversial. Here we show that fenestrations in LSEC contain PLVAP-diaphragms during the fetal angiogenesis, but they lose the diaphragms at birth. Although it is thought that PLVAP only localizes to diaphragms, we found luminal localization of PLVAP in adult LSEC using several imaging techniques. Plvap-deficient mice revealed that the absence of PLVAP and diaphragms did not affect the morphology, the number of fenestrations or the overall vascular architecture in the liver sinusoids. Nevertheless, PLVAP in fetal LSEC (fenestrations with diaphragms) associated with LYVE-1 (lymphatic vessel endothelial hyaluronan receptor 1), neuropilin-1 and VEGFR2 (vascular endothelial growth factor receptor 2), whereas in the adult LSEC (fenestrations without diaphragms) these complexes disappeared. Collectively, our data show that PLVAP can be expressed on endothelial cells without diaphragms, contradict the prevailing concept that biogenesis of fenestrae would be PLVAP-dependent, and reveal previously unknown PLVAP-dependent molecular complexes in LSEC during angiogenesis.

Actin-spectrin scaffold supports open fenestrae in liver sinusoidal endothelial cells

Fenestrae are open transmembrane pores that are a structural hallmark of healthy liver sinusoidal endothelial cells (LSECs). Their key role is the transport of solutes and macromolecular complexes between the sinusoidal lumen and the space of Disse. To date, the biochemical nature of the cytoskeleton elements that surround the fenestrae and sieve plates in LSECs remain largely elusive. Herein, we took advantage of the latest developments in atomic force imaging and super‐resolution fluorescence nanoscopy to define the organization of the supramolecular complex(es) that surround the fenestrae. Our data revealed that spectrin, together with actin, lines the inner cell membrane and provided direct structural support to the membrane‐bound pores. We conclusively demonstrated that diamide and iodoacetic acid (IAA) affect fenestrae number by destabilizing the LSEC actin‐spectrin scaffold. Furthermore, IAA induces rapid and repeatable switching between the open vs closed state of the fenestrae, indicating that the spectrin‐actin complex could play an important role in controlling the pore number. Our results suggest that spectrin functions as a key regulator in the structural preservation of the fenestrae, and as such, it might serve as a molecular target for altering transendothelial permeability.

LSEC Fenestrae Are Preserved Despite Pro-inflammatory Phenotype of Liver Sinusoidal Endothelial Cells in Mice on High Fat Diet

Healthy liver sinusoidal endothelial cells (LSECs) maintain liver homeostasis, while LSEC dysfunction was suggested to coincide with defenestration. Here, we have revisited the relationship between LSEC pro-inflammatory response, defenestration, and impairment of LSEC bioenergetics in non-alcoholic fatty liver disease (NAFLD) in mice. We characterized inflammatory response, morphology as well as bioenergetics of LSECs in early and late phases of high fat diet (HFD)-induced NAFLD. LSEC phenotype was evaluated at early (2-8 week) and late (15-20 week) stages of NAFLD progression induced by HFD in male C57Bl/6 mice. NAFLD progression was monitored by insulin resistance, liver steatosis and obesity. LSEC phenotype was determined in isolated, primary LSECs by immunocytochemistry, mRNA gene expression (qRT-PCR), secreted prostanoids (LC/MS/MS) and bioenergetics (Seahorse FX Analyzer). LSEC morphology was examined using SEM and AFM techniques. Early phase of NAFLD, characterized by significant liver steatosis and prominent insulin resistance, was related with LSEC pro-inflammatory phenotype as evidenced by elevated ICAM-1, E-selectin and PECAM-1 expression. Transiently impaired mitochondrial phosphorylation in LSECs was compensated by increased glycolysis. Late stage of NAFLD was featured by prominent activation of pro-inflammatory LSEC phenotype (ICAM-1, E-selectin, PECAM-1 expression, increased COX-2, IL-6, and NOX-2 mRNA expression), activation of pro-inflammatory prostaglandins release (PGE2 and PGF2α) and preserved LSEC bioenergetics. Neither in the early nor in the late phase of NAFLD, were LSEC fenestrae compromised. In the early and late phases of NAFLD, despite metabolic and pro-inflammatory burden linked to HFD, LSEC fenestrae and bioenergetics are functionally preserved. These results suggest prominent adaptive capacity of LSECs that might mitigate NAFLD progression.

Actin Depolymerization in Dedifferentiated Liver Sinusoidal Endothelial Cells Promotes Fenestrae Re-Formation

Liver sinusoidal endothelial cells (LSECs) possess fenestrae, which are key for the exchange between blood and hepatocytes. Alterations in their number or diameter have important implications for hepatic function in liver diseases. They are lost early in the development of hepatic fibrosis through a process called capillarization. In this study, we aimed to demonstrate whether in vitro dedifferentiated LSECs that have lost fenestrae are able to re-form these structures. Using stimulated emission depletion super-resolution microscopy in combination with transmission electron microscopy, we analyzed fenestrae formation in a model mimicking the capillarization process in vitro. Actin is known to be involved in fenestrae regulation in differentiated LSECs. Using cytochalasin D, an actin-depolymerizing agent, we demonstrated that dedifferentiated LSECs remain capable of forming fenestrae. Conclusion: We provide a new insight into the complex role of actin in fenestrae formation and in the control of their size and show that LSEC fenestrae re-formation is possible, suggesting that this process could be used during fibrosis regression to try to restore exchanges and hepatocyte functions.