Structural and functional aspects of liver sinusoidal endothelial cell fenestrae: a review

This review provides a detailed overview of the current state of knowledge about the ultrastructure and dynamics of liver sinusoidal endothelial fenestrae. Various aspects of liver sinusoidal endothelial fenestrae regarding their structure, origin, species specificity, dynamics and formation will be explored. In addition, the role of liver sinusoidal endothelial fenestrae in relation to lipoprotein metabolism, fibrosis and cancer will be approached.     

An endothelin A receptor antagonist induces dilatation of sinusoidal endothelial fenestrae: implications for endothelin-1 in hepatic microcirculation

Background Sinusoidal endothelial fenestrae (SEF) regulate the sinusoidal circulation by altering their diameter and number. This study documented the effects of endothelin (ET) receptor antagonists on SEF and hepatic microcirculation. Methods The portal pressure and hepatic tissue blood flow were measured with a hydromanometer and a laser Doppler blood flow meter, respectively. BQ-123 (ET_A receptor antagonist) or BQ-788 (ET_B receptor antagonist) was continuously infused into normal rats at the rate of 10 nmol/min for 10 min. The sinusoids were observed at 60 min after the infusion by scanning electron microscopy. The localization of ET-1 and ET_A and ET_B receptors was examined by the indirect immunoperoxidase method. Results When BQ-123 was infused, the portal pressure gradually decreased with time, and it showed a significant reduction compared with the control groups. On the other hand, a decrease in portal pressure was not evident in the BQ-788-infused groups. Hepatic tissue blood flow was maintained at the value prior to the infusion in both groups. BQ-123 also caused a marked dilatation of the SEF. The diameters of the SEF after BQ-123 infusion were almost three times those of normal SEF. ET-1 was evenly present along the sinusoidal walls, and the reaction products of the ET_A receptors were recognized along the portal vein and in the sinusoidal cells, that is, the hepatic stellate cells and endothelial cells. Conclusions Action of ET-1 via the ET_A receptors may regulate the size of SEF in addition to hepatic microcirculation.     

Three‐dimensional organization of fenestrae labyrinths in liver sinusoidal endothelial cells

Background/Aims: Liver sinusoidal endothelial cell (LSEC) fenestrae are membrane‐bound pores that are grouped in sieve plates and act as a bidirectional guardian in regulating transendothelial liver transport. The high permeability of the endothelial lining is explained by the presence of fenestrae and by various membrane‐bound transport vesicles. The question as to whether fenestrae relate to other transport compartments remains unclear and has been debated since their discovery almost 40 years ago. Methods: In this study, novel insights concerning the three‐dimensional (3D) organization of the fenestrated cytoplasm were built on transmission electron tomographical observations on isolated and cultured whole‐mount LSECs. Classical transmission electron microscopy and atomic force microscopy imaging was performed to accumulate cross‐correlative structural evidence. Results and Conclusions: The data presented here indicate that different arrangements of fenestrae have to be considered: i.e. open fenestrae that lack any structural obstruction mainly located in the thin peripheral cytoplasm and complexes of multifolded fenestrae organized as labyrinth‐like structures that are found in the proximity of the perinuclear area. Fenestrae in labyrinths constitute about one‐third of the total LSEC porosity. The 3D reconstructions also revealed that coated pits and small membrane‐bound vesicles are exclusively interspersed in the non‐fenestrated cytoplasmic arms.     

肝窦内皮细胞与肝窦毛细血管化研究进展

肝窦内皮细胞具有开放的、没有横膈膜的窗孔,内皮下没有基底膜。这种结构有利于调控肝细胞与肝窦血液的物质交换。肝窦内皮细胞能分泌内皮素-1和一氧化氮,并通过窗孔的变化,对肝脏微循环进行调节,同时可分泌大量的形成基底膜的成分,在肝窦毛细血管化的过程中起主导作用。肝窦内皮细胞表型标志发生变化,Ⅷ因子相关抗原、CD44等表达增强,也是肝窦毛细血管化的重要标志。笔者对肝窦内皮细胞的结构及其在肝窦毛细血管化中的功能和表型变化进行综述。     

Endothelial nitric oxide synthase and caveolin-1 are co-localized in sinusoidal endothelial fenestrae.

BACKGROUND/AIMS: Nitric oxide is synthesized in diverse mammalian tissues by a family of calmodulin-dependent nitric oxide synthases (NOS). Caveolin, the principal structural protein in caveolae, interacts with endothelial NOS leading to enzyme inhibition in a reversible process modulated by Ca++-calmodulin. The aim of the present study was to clarify the ultrastructural localization of eNOS and caveolin-1 in hepatic sinusoidal endothelium by an electron immunogold method. METHODS: Male Wistar rats were used. Liver tissues and hepatic sinusoidal endothelial cells isolated from rat livers by collagenase infusion were studied. For immunohistochemistry, liver specimens were reacted with anti-eNOS or anti-caveolin-1 antibody. The ultrastructural localization of eNOS or caveolin-1 was identified by electron microscopy using an immunogold post-embedding method. RESULTS: Immunohistochemical studies using liver tissues localized endothelial NOS in hepatic sinusoidal lining cells, portal veins and hepatic arteries; and caveolin-1 in sinusoidal lining cells, bile canaliculi, portal vein and hepatic arteries. Immunogold particles indicating the presence of eNOS and caveolin-1 were demonstrated on the plasma membrane of sinusoidal endothelial fenestrae in liver tissue and also in isolated sinusoidal endothelial cells. CONCLUSION: Endothelial NOS and caveolin are co-localized on sinusoidal endothelial fenestrae, suggesting that interaction of the two may modulate cellular regulation of NO synthesis.     

缺血后处理对肝脏缺血再灌注中肝窦内皮细胞损伤的保护作用

目的　探讨缺血后处理对肝脏缺血再灌注中肝窦内皮细胞损伤的保护作用。方法　建立大鼠局部肝脏缺血再灌注模型 ,将 2 4只健康雄性Wistar大鼠随机分为假手术、缺血再灌注、缺血后处理 3组 ,以缺血再灌前、反复多次的短暂预再灌、停灌作后处理 ,观察各组血浆肝酶及透明质酸 (HA)水平变化和肝组织中丙二醛 (MDA)、超氧化物歧化酶(SOD)、内皮素 1(ET 1)含量 ,并行肝组织病理形态学检查。结果　与缺血再灌注组相比 ,缺血后处理组肝酶的漏出、血浆HA水平及肝组织中MDA、ET 1的含量明显降低 (P <0 .0 1) ,而SOD活性则显著升高 (P <0 .0 1) ,肝组织病理学损伤亦明显减轻。结论　缺血后处理可通过抑制再灌注后氧自由基的过量生成而保护肝窦内皮细胞 ,减轻肝脏缺血再灌注损伤。     

4,5,7-三羟基异黄酮对肝纤维化大鼠肝窦内皮细胞窗孔、增殖及合成一氧化氮的影响

目的 探讨酪氨酸蛋白激酶抑制剂4、5、7-三羟基异黄酮(genistein)对肝窦内皮细胞(sinusoidalendothelial cell, SEC)窗孔、增殖及合成一氧化氮(nitric oxide, NO)的影响。方法 用胶原酶原位灌注,Percoll不连续密度梯度离心法分离正常及CCl_4实验性肝纤维化大鼠的SEC并进行体外培养。采用扫描电镜技术,MTT法及硝酸还原酶法,分别观察genistein对SEC细胞窗孔、增殖及合成NO的影响。结果 Genistein对肝纤维化各级 SEC的窗孔数目及大小均无明显的影响。经不同浓度的genistein作用24h后,肝纤维化各级大鼠SEC的生长均受抑制,以100 μmol/L浓度的genistein对肝纤维化Ⅰ级大鼠SEC的作用最为明显[细胞增殖率为-(15.38±6.26)% vs(4.91±2.16)%,t=13.7. P＜0.05]。100 μmol/L浓度的genistein作用24h,可明显促进肝纤维化Ⅰ级大鼠SEC NO的合成[NO合成为(25.4±3.8)μmol/L vs(16.6±3.3)μmol/L,t=6.79,P＜0.05];但对肝纤维化Ⅱ级、Ⅲ级大鼠SEC的NO合成的影响却不明显。结论 体外实验中genistein可以抑制肝纤维化Ⅰ级SEC增殖,促进SEC细胞NO的合成;对肝纤维化SEC细胞的功能有一定的调节作用。     

Antimycin A-induced defenestration in rat hepatic sinusoidal endothelial cells

Liver sinusoidal endothelial cells (LSECs) possess fenestrae arranged in sieve plates. Hepatic endothelial fenestrae are open pores approximately 100 to 200 nm in diameter. Alterations in their number or diameter by hormones, xenobiotics, and diseases have important implications for hepatic microcirculation and function. Numerous reports of hepatotoxin-induced defenestration suggest that the cytoskeleton and the energy status of hepatic endothelial cells play a key role in the regulation of fenestrae. Therefore, we investigated the effect of antimycin A, an inhibitor of mitochondrial energy production, on the number of fenestrae in cultured LSECs using high-resolution microscopy and immunocytochemistry. Prolonged incubation (greater than 30 min) with antimycin A resulted in defenestrated cells and coincided with the appearance of F-actin dots, whereas the distribution of G-actin remained unchanged. Adenosine triphosphate (ATP) was depleted dramatically to less than 5% within 30 minutes within the LSECs. After treatment with antimycin A, unusual elevated fenestrated complexes were apparent, organized as a meshwork of anastomosing fenestrae at the center of and above the sieve plates. The position and appearance of these novel structures and their association with defenestration suggest that they are implicated in the process of defenestration. In conclusion, the results of experiments with antimycin A suggest that ATP is needed to maintain fenestrae and the underlying fenestrae-associated cytoskeleton rings that maintain fenestrae patency. Antimycin A-induced defenestration of LSECs is associated with the development of a structure in the sieve plate that appears to be intrinsically involved in defenestration.     

Vascular endothelial growth factor increases fenestral permeability in hepatic sinusoidal endothelial cells

Abstract: Vascular endothelial growth factor (VEGF) is an important regulator of vasculogenesis and vascular permeability. Hepatic sinusoidal endothelial cells (SECs) possess sieve‐like pores that form an anastomosing labyrinth structure by the deeply invaginated plasma membrane. Caveolin is the principal structural protein in caveolae. In this study, we examined the role of VEGF on the fenestration and permeability of SECs and the relation with caveolin‐1. SECs isolated from rat livers by collagenase infusion method were cultured for 24 h with (10 or 100 ng/ml) or without VEGF. The cells were then examined by transmission and scanning electron microscopy (EM). The expression of caveolin was investigated by confocal immunofluorescence, immunogold EM, and Western blot. Endocytosis and intracellular traffic was studied using horseradish peroxidase (HRP) reaction as a marker of fluid phase transport in SECs. Both transmission and scanning EM showed an increased number of sinusoidal endothelial fenestrae (SEF) in SECs cultured with VEGF. By confocal immunofluorescence, SECs cultured with VEGF displayed prominent caveolin‐1‐positive aggregates in the cytoplasm, especially surrounding the nucleus region. Immunogold EM depicted increased caveolin‐1 reactivity on vesicles and vacuoles of VEGF‐treated SECs compared with VEGF‐nontreated cells. However, there was no change in the level of caveolin‐1 protein expression on Western blot. After HRP injection, an increase of electron‐dense tracer filled the SEF in cells treated with VEGF. Our results suggested that VEGF induced fenestration in SECs, accompanied by an increased number of caveolae‐like vesicles. Increased caveolin‐1 might be associated with vesicle formation but not with fenestration. Increased fenestration may augment hepatic sinusoidal permeability and trans‐endothelial transport.     

Endothelial nitric oxide synthase and caveolin‐1 are co‐localized in sinusoidal endothelial fenestrae

Abstract: Background/Aims: Nitric oxide is synthesized in diverse mammalian tissues by a family of calmodulin‐dependent nitric oxide synthases (NOS). Caveolin, the principal structural protein in caveolae, interacts with endothelial NOS leading to enzyme inhibition in a reversible process modulated by Ca⁺⁺‐calmodulin. The aim of the present study was to clarify the ultrastructural localization of eNOS and caveolin‐1 in hepatic sinusoidal endothelium by an electron immunogold method. Methods: Male Wistar rats were used. Liver tissues and hepatic sinusoidal endothelial cells isolated from rat livers by collagenase infusion were studied. For immunohistochemistry, liver specimens were reacted with anti‐eNOS or anti‐caveolin‐1 antibody. The ultrastructural localization of eNOS or caveolin‐1 was identified by electron microscopy using an immunogold post‐embedding method. Results: Immunohistochemical studies using liver tissues localized endothelial NOS in hepatic sinusoidal lining cells, portal veins and hepatic arteries; and caveolin‐1 in sinusoidal lining cells, bile canaliculi, portal vein and hepatic arteries. Immunogold particles indicating the presence of eNOS and caveolin‐1 were demonstrated on the plasma membrane of sinusoidal endothelial fenestrae in liver tissue and also in isolated sinusoidal endothelial cells. Conclusion: Endothelial NOS and caveolin are co‐localized on sinusoidal endothelial fenestrae, suggesting that interaction of the two may modulate cellular regulation of NO synthesis.     

Mechanism of superoxide anion production by hepatic sinusoidal endothelial cells and Kupffer cells during short-term ethanol perfusion in the rat

BACKGROUND/AIMS: The aim of this study was to clarify the candidate cells for and the mechanism of superoxide anion (O2*-) release into the hepatic sinusoids during short-term exposure to ethanol. METHODS: The rat liver was perfused continuously with ethanol (a substrate for alcohol dehydrogenase) or tert-buthanol (not a substrate for alcohol dehydrogenase) for 20 min at a final concentration of 40 mM. In order to detect O2*- production, MCLA (2-methyl-6-[p-methoxyphenyl]-3,7-dihydroimidazo[1,2-a]pyrazin-3-one), a Cypridina luciferin analogue, was simultaneously infused and MCLA-enhanced chemiluminescence was measured. The effects of gadolinium chloride (GdCL3) (a suppressor of Kupffer cells (KCs)), staurosporine (ST) (an inhibitor of serine-threonine kinases, including protein kinase C), diphenyleneiodonium chloride (DPI) (an inhibitor of NADPH oxidase), ibuprofen (IB) (an inhibitor of cyclooxygenase) and 4-methylpyrazole (4MP) (an inhibitor of ethanol metabolism) on the ethanol-induced chemiluminescence were also evaluated. Sites where O2*- could be released were determined by histochemical detection of nitro blue tetrazolium reduction. RESULTS: Both ethanol and tert-buthanol rapidly caused O2*- release. GdCL3 suppressed the ethanol-induced O2*- release by 61%. Staurosporine and DPI, but neither IB nor 4-MP, also significantly inhibited the ethanol-induced O2*- release. In the histochemical examination, ethanol-stimulated liver showed blue formazan precipitate on both sinusoidal endothelial cells (SECs) and Kupffer cells (KCs), whereas the GdCl3-pretreated liver had the precipitate only on SECs. CONCLUSIONS: This study shows that ethanol itself stimulates both SECs and KCs to release O2*- via activation of NADPH oxidase probably involving protein kinase C (PKC).     

Effects of intravenous ethanol on hepatic and pancreatic blood flow in dogs

Changes in hepatic and pancreatic blood flow in response to ethanol infusion were determined simultaneously and continuously in anesthetized dogs, using a transit-time ultrasonic flowmeter and a laser-Doppler flowmeter. In addition, the effect of intravenous ethanol on exocrine pancreatic secretion was investigated. With a background infusion of secretin, ethanol (1.3 g/kg body wt) was infused intravenously over a 40-min period. Ethanol infusion significantly increased blood flow in the common hepatic artery (by 49%, at the time of the cessation of ethanol infusion), and this increased flow was maintained for 60 min after the cessation of ethanol infusion. In contrast, blood flow in the portal vein was not altered significantly by ethanol. Pancreatic blood flow and secretion showed no significant difference from those seen in the controls. Our data suggest that intravenous ethanol induces a redistribution of the splanchnic blood flow. The increased hepatic arterial flow seen in response to ethanol may play an important role in preventing ethanol-induced hypoxic liver damage.     

How molecular microscopy revealed new insights into the dynamics of hepatic endothelial fenestrae in the past decade.

This review discusses the current state of knowledge about the ultrastructure of hepatic endothelial fenestrae. The application of different high-resolution correlative microscopic methods during the past decade facilitated the accumulation of new insights in the morpho-functional and structural organization of the liver sieve. The data gathered unambiguously show the involvement of special domains in de novo formation and disappearance of fenestrae, and focuses future research into the (supra)molecular structure of the fenestrae-forming center, defenestration center and fenestrae-associated cytoskeleton ring by using cryo-electron microscopic tomography.     

Cardiovascular effect of intravenous lipid A in rabbits

The in vivo cardiovascular effect of intravenous administration of monophosphoryl lipid A (mp-lipid A) and diphosphoryl lipid A (dp-lipid A) in awake New Zealand white rabbits was investigated. Observed changes were evaluated in comparison to a control group and an endotoxin-treated group. Rabbits given lipid A showed a significant depression in cardiac index (p less than .025), mean arterial pressure (p less than .025, dp-lipid A only), arterial carbon dioxide tension (p less than .025), and total leukocyte count (p less than .05) compared to controls. Animals receiving lipid A tended to respond overall in a manner closely matching that of the endotoxin group. Dosages of lipid A given were approximately 3.5 times larger than the endotoxin dosages with respect to actual number of molecules administered (1.25-2.0 times larger by mass). These results indicate that lipid A is active in producing the cardiovascular and leukopenic effects characteristic of experimental septic shock.