Stabilin 1 and 2 are important regulators for cellular uptake of apolipoprotein B-containing lipoproteins in zebrafish

Zebrafish ApoB-Containing Lipoprotein Metabolism: A Closer Look

Zebrafish have become a powerful model of mammalian lipoprotein metabolism and lipid cell biology. Most key proteins involved in lipid metabolism, including cholesteryl ester transfer protein, are conserved in zebrafish. Consequently, zebrafish exhibit a human-like lipoprotein profile. Zebrafish with mutations in genes linked to human metabolic diseases often mimic the human phenotype. Zebrafish larvae develop rapidly and externally around the maternally deposited yolk. Recent work revealed that any disturbance of lipoprotein formation leads to the accumulation of cytoplasmic lipid droplets and an opaque yolk, providing a visible phenotype to investigate disturbances of the lipoprotein pathway, already leading to discoveries in MTTP (microsomal triglyceride transfer protein) and ApoB (apolipoprotein B). By 5 days of development, the digestive system is functional, making it possible to study fluorescently labeled lipid uptake in the transparent larvae. These and other approaches enabled the first in vivo description of the STAB (stabilin) receptors, showing lipoprotein uptake in endothelial cells. Various zebrafish models have been developed to mimic human diseases by mutating genes known to influence lipoproteins (eg, ldlra, apoC2). This review aims to discuss the most recent research in the zebrafish ApoB-containing lipoprotein and lipid metabolism field. We also summarize new insights into lipid processing within the yolk cell and how changes in lipid flux alter yolk opacity. This curious new finding, coupled with the development of several techniques, can be deployed to identify new players in lipoprotein research directly relevant to human disease.

Leveraging the transcriptome to further our understanding of GWAS findings: eQTLs associated with genes related to LDL and LDL subclasses, in a cohort of African Americans

Background: GWAS discoveries often pose a significant challenge in terms of understanding their underlying mechanisms. Further research, such as an integration with expression quantitative trait locus (eQTL) analyses, are required to decipher the mechanisms connecting GWAS variants to phenotypes. An eQTL analysis was conducted on genes associated with low-density lipoprotein (LDL) cholesterol and its subclasses, with the aim of pinpointing genetic variants previously implicated in GWAS studies focused on lipid-related traits. Notably, the study cohort consisted of African Americans, a population characterized by a heightened prevalence of hypercholesterolemia. Methods: A comprehensive differential expression (DE) analysis was undertaken, with a dataset of 17,948 protein-coding mRNA transcripts extracted from the whole-blood transcriptomes of 416 samples to identify mRNA transcripts associated with LDL, with further granularity delineated between small LDL and large LDL subclasses. Subsequently, eQTL analysis was conducted with a subset of 242 samples for which whole-genome sequencing data were available to identify single-nucleotide polymorphisms (SNPs) associated with the LDL-related mRNA transcripts. Lastly, plausible functional connections were established between the identified eQTLs and genetic variants reported in the GWAS catalogue. Results: DE analysis revealed 1,048, 284, and 94 mRNA transcripts that exhibited differential expression in response to LDL, small LDL, and large LDL, respectively. The eQTL analysis identified a total of 9,950 significant SNP-mRNA associations involving 6,955 SNPs including a subset 101 SNPs previously documented in GWAS of LDL and LDL-related traits. Conclusion: Through comprehensive differential expression analysis, we identified numerous mRNA transcripts responsive to LDL, small LDL, and large LDL. Subsequent eQTL analysis revealed a rich landscape of eQTL-mRNA associations, including a subset of eQTL reported in GWAS studies of LDL and related traits. The study serves as a testament to the important role of integrative genomics in unraveling the enigmatic GWAS relationships between genetic variants and the complex fabric of human traits and diseases.

Oit3, a promising hallmark gene for targeting liver sinusoidal endothelial cells

Liver sinusoidal endothelial cells (LSECs) play a pivotal role in maintaining liver homeostasis and influencing the pathological processes of various liver diseases. However, neither LSEC-specific hallmark genes nor a LSEC promoter-driven Cre mouse line has been introduced before, which largely restricts the study of liver diseases with vascular disorders. To explore LSEC-specific hallmark genes, we compared the top 50 marker genes between liver endothelial cells (ECs) and liver capillary ECs and identified 18 overlapping genes. After excluding globally expressed genes and those with low expression percentages, we narrowed our focus to two final candidates: Oit3 and Dnase1l3. Through single-cell RNA sequencing (scRNA-seq) and analysis of the NCBI database, we confirmed the extrahepatic expression of Dnase1l3. The paired-cell sequencing data further demonstrated that Oit3 was predominantly expressed in the midlobular liver ECs. Subsequently, we constructed inducible Oit3-CreERT2 transgenic mice, which were further crossed with ROSA26-tdTomato mice. Microscopy validated that the established Oit3-CreERT2-tdTomato mice exhibited significant fluorescence in the liver rather than in other organs. The staining analysis confirmed the colocalization of tdTomato and EC markers. Ex-vivo experiments further confirmed that isolated tdTomato+ cells exhibited well-differentiated fenestrae and highly expressed EC markers, confirming their identity as LSECs. Overall, Oit3 is a promising hallmark gene for tracing LSECs. The establishment of Oit3-CreERT2-tdTomato mice provides a valuable model for studying the complexities of LSECs in liver diseases.

High-density lipoproteins and non-alcoholic fatty liver disease

Background and aims

Non-alcoholic fatty liver disease (NAFLD), a high incidence liver pathology, is associated with a ∼1.5-fold higher cardiovascular disease risk. This phenomenon is generally attributed to the NAFLD-associated increase in circulating levels of pro-atherogenic apolipoprotein B100-containing small dense low-density lipoprotein and plasma hypertriglyceridemia. However, also a significant reduction in cholesterol transported by anti-atherogenic high-density lipoproteins (HDL) is frequently observed in subjects suffering from NAFLD as compared to unaffected people. In this review, we summarize data regarding the relationship between NAFLD and plasma HDL-cholesterol levels, with a special focus on highlighting potential causality between the NAFLD pathology and changes in HDL metabolism.

Methods and results

Publications in PUBMED describing the relationship between HDL levels and NAFLD susceptibility and/or disease severity, either in human clinical settings or genetically-modified mouse models, were critically reviewed for subsequent inclusion in this manuscript. Furthermore, relevant literature describing effects on lipid loading in cultured hepatocytes of models with genetic alterations related to HDL metabolism have been summarized.

Conclusions

Although in vitro observations suggest causality between HDL formation by hepatocytes and protection against NAFLD-like lipid accumulation, current literature remains inconclusive on whether relative HDL deficiency is actually driving the development of fatty liver disease in humans. In light of the current obesity pandemic and the associated marked rise in NAFLD incidence, it is of clear scientific and societal interest to gain further insight into the relationship between HDL-cholesterol levels and fatty liver development to potentially uncover the therapeutic potential of pharmacological HDL level and/or function modulation.

Remodeling the hepatic fibrotic microenvironment with emerging nanotherapeutics: a comprehensive review

Liver fibrosis could be the last hope for treating liver cancer and remodeling of the hepatic microenvironment has emerged as a strategy to promote the ablation of liver fibrosis. In recent years, especially with the rapid development of nanomedicine, hepatic microenvironment therapy has been widely researched in studies concerning liver cancer and fibrosis. In this comprehensive review, we summarized recent advances in nano therapy-based remodeling of the hepatic microenvironment. Firstly, we discussed novel strategies for regulatory immune suppression caused by capillarization of liver sinusoidal endothelial cells (LSECs) and macrophage polarization. Furthermore, metabolic reprogramming and extracellular matrix (ECM) deposition are caused by the activation of hepatic stellate cells (HSCs). In addition, recent advances in ROS, hypoxia, and impaired vascular remodeling in the hepatic fibrotic microenvironment due to ECM deposition have also been summarized. Finally, emerging nanotherapeutic approaches based on correlated signals were discussed in this review. We have proposed novel strategies such as engineered nanotherapeutics targeting antigen-presenting cells (APCs) or direct targeting T cells in liver fibrotic immunotherapy to be used in preventing liver fibrosis. In summary, this comprehensive review illustrated the opportunities in drug targeting and nanomedicine, and the current challenges to be addressed.

Zebrafish as outgroup model to study evolution of scavenger receptor class B type I functions

BACKGROUND AND AIMS

Scavenger receptor class B1 (SCARB1) - also known as the high-density lipoprotein (HDL) receptor - is a multi-ligand scavenger receptor that is primarily expressed in liver and steroidogenic organs. This receptor is known for its function in reverse cholesterol transport (RCT) in mammals and hence disruption leads to a massive increase in HDL cholesterol in these species. The extracellular domain of SCARB1 - which is important for cholesterol handling - is highly conserved across multiple vertebrates, except in zebrafish.

METHODS

To examine the functional conservation of SCARB1 among vertebrates, two stable scarb1 knockout zebrafish lines, scarb1 715delA (scarb1 -1 nt) and scarb1 715_716insGG (scarb1 +2 nt), were created using CRISPR-Cas9 technology.

RESULTS

We demonstrate that, in zebrafish, SCARB1 deficiency leads to disruption of carotenoid-based pigmentation, reduced fertility, and a decreased larvae survival rate, whereas steroidogenesis was unaltered. The observed reduced fertility is driven by defects in female fertility (-50 %, p < 0.001). Importantly, these alterations were independent of changes in free (wild-type 2.4 ± 0.2 μg/μl versus scarb1^-/- 2.0 ± 0.1 μg/μl) as well as total (wild-type 4.2 ± 0.4 μg/μl versus scarb1^-/- 4.0 ± 0.3 μg/μl) plasma cholesterol levels. Uptake of HDL in the liver of scarb1^-/- zebrafish larvae was reduced (-86.7 %, p < 0.001), but this coincided with reduced perfusion of the liver. No effect was observed on lipoprotein uptake in the caudal vein. SCARB1 deficient canaries, which also lack carotenoids in their plumage, similarly as scarb1^-/- zebrafish, failed to show an increase in plasma free- and total cholesterol levels.

CONCLUSION

Our findings suggest that the specific function of SCARB1 in maintaining plasma cholesterol could be an evolutionary novelty that became prominent in mammals, while other known functions were already present earlier during vertebrate evolution.

The Crosstalk Between Liver Sinusoidal Endothelial Cells and Hepatic Microenvironment in NASH Related Liver Fibrosis

Chronic liver injury can be caused by many factors, including virus infection, alcohol intake, cholestasis and abnormal fat accumulation. Nonalcoholic steatohepatitis (NASH) has become the main cause of liver fibrosis worldwide. Recently, more and more evidences show that hepatic microenvironment is involved in the pathophysiological process of liver fibrosis induced by NASH. Hepatic microenvironment consists of various types of cells and intercellular crosstalk among different cells in the liver sinusoids. Liver sinusoidal endothelial cells (LSECs), as the gatekeeper of liver microenvironment, play an irreplaceable role in the homeostasis and alterations of liver microenvironment. Many recent studies have reported that during the progression of NASH to liver fibrosis, LSECs are involved in various stages mediated by a series of mechanisms. Therefore, here we review the key role of crosstalk between LSECs and hepatic microenvironment in the progression of NASH to liver fibrosis (steatosis, inflammation, and fibrosis), as well as promising therapeutic strategies targeting LSECs.