The asymmetric Pitx2 gene regulates gut muscular-lacteal development and protects against fatty liver disease

An atypical basement membrane forms a midline barrier during left-right asymmetric gut development in the chicken embryo

Correct intestinal morphogenesis depends on the early embryonic process of gut rotation, an evolutionarily conserved program in which a straight gut tube elongates and forms into its first loops. However, the gut tube requires guidance to loop in a reproducible manner. The dorsal mesentery (DM) connects the gut tube to the body and directs the lengthening gut into stereotypical loops via left-right (LR) asymmetric cellular and extracellular behavior. The LR asymmetry of the DM also governs blood and lymphatic vessel formation for the digestive tract, which is essential for prenatal organ development and postnatal vital functions including nutrient absorption. Although the genetic LR asymmetry of the DM has been extensively studied, a divider between the left and right DM has yet to be identified. Setting up LR asymmetry for the entire body requires a Lefty1+ midline barrier to separate the two sides of the embryo, without it, embryos have lethal or congenital LR patterning defects. Individual organs including the brain, heart, and gut also have LR asymmetry, and while the consequences of left and right signals mixing are severe or even lethal, organ-specific mechanisms for separating these signals remain poorly understood. Here, we uncover a midline structure composed of a transient double basement membrane, which separates the left and right halves of the embryonic chick DM during the establishment of intestinal and vascular asymmetries. Unlike other basement membranes of the DM, the midline is resistant to disruption by intercalation of Netrin4 (Ntn4). We propose that this atypical midline forms the boundary between left and right sides and functions as a barrier necessary to establish and protect organ asymmetry.

TRIB3 Is a Hub Gene in Steatohepatitis and Aggravates Lipid Deposition and Inflammation in Hepatocytes

Background

Non-alcoholic fatty liver disease (NAFLD), also known as Metabolic dysfunction-associated fatty liver disease (MASLD), has become one of the most common chronic liver diseases worldwide, approximately 30% of adults and 70%~80% of patients with obesity and diabetes suffer from NAFLD.

Objective

We attempted to find a potential hub gene in NAFLD hepatocyte cell model induced by palmitic acid and oil acid (PAOA), and investigated the function of the hub-gene.

Methods

We searched and downloaded the GSE122660 dataset from GEO-DataSets, and differentially expressed genes (DEGs) were analyzed using R software. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses were performed to identify the significantly activated signaling pathways in steatohepatitis. A protein-protein interaction (PPI) network was constructed to identify hub genes among the DEGs. qRT-PCR, Western blotting, and Oil Red O staining were used to explore the function of hub genes in PAOA-induced hepatocytes in vitro.

Results

A total of 255 DEGs were identified in the GSE122660 dataset and were primarily associated with inflammation-and lipid metabolism-related pathways. The tribbles pseudokinase 3 (TRIB3) was highlighted as a hub gene. We found that TRIB3 was upregulated in CDHFDinduced NAFLD mouse liver tissue and hepatocyte cell models. Furthermore, TRIB3 aggravated PAOA-induced lipid accumulation and inflammation in hepatocytes in vitro.

Conclusion

The present study identified TRIB3 as a hub gene for steatohepatitis and aggravated lipid accumulation and inflammation in vitro. Therefore, targeting TRIB3 could be a potential pharmacological strategy for NAFLD treatment.

Transport functions of intestinal lymphatic vessels

Lymphatic vessels are crucial for fluid absorption and the transport of peripheral immune cells to lymph nodes. However, in the small intestine, the lymphatic fluid is rich in diet-derived lipids incorporated into chylomicrons and gut-specific immune cells. Thus, intestinal lymphatic vessels have evolved to handle these unique cargoes and are critical for systemic dietary lipid delivery and metabolism. This Review covers mechanisms of lipid absorption from epithelial cells to the lymphatics as well as unique features of the gut microenvironment that affect these functions. Moreover, we discuss details of the intestinal lymphatics in gut immune cell trafficking and insights into the role of inter-organ communication. Lastly, we highlight the particularities of fat absorption that can be harnessed for efficient lipid-soluble drug distribution for novel therapies, including the ability of chylomicron-associated drugs to bypass first-pass liver metabolism for systemic delivery. In all, this Review will help to promote an understanding of intestinal lymphatic-systemic interactions to guide future research directions.

Intestinal Lymphatic Biology, Drug Delivery, and Therapeutics: Current Status and Future Directions

Historically, the intestinal lymphatics were considered passive conduits for fluids, immune cells, dietary lipids, lipid soluble vitamins, and lipophilic drugs. Studies of intestinal lymphatic drug delivery in the late 20th century focused primarily on the drugs' physicochemical properties, especially high lipophilicity, that resulted in intestinal lymphatic transport. More recent discoveries have changed our traditional view by demonstrating that the lymphatics are active, plastic, and tissue-specific players in a range of biological and pathological processes, including within the intestine. These findings have, in turn, inspired exploration of lymph-specific therapies for a range of diseases, as well as the development of more sophisticated strategies to actively deliver drugs or vaccines to the intestinal lymph, including a range of nanotechnologies, lipid prodrugs, and lipid-conjugated materials that "hitchhike" onto lymphatic transport pathways. With the increasing development of novel therapeutics such as biologics, there has been interest in whether these therapeutics are absorbed and transported through intestinal lymph after oral administration. Here we review the current state of understanding of the anatomy and physiology of the gastrointestinal lymphatic system in health and disease, with a focus on aspects relevant to drug delivery. We summarize the current state-of-the-art approaches to deliver drugs and quantify their uptake into the intestinal lymphatic system. Finally, and excitingly, we discuss recent examples of significant pharmacokinetic and therapeutic benefits achieved via intestinal lymphatic drug delivery. We also propose approaches to advance the development and clinical application of intestinal lymphatic delivery strategies in the future. SIGNIFICANCE STATEMENT: This comprehensive review details the understanding of the anatomy and physiology of the intestinal lymphatic system in health and disease, with a focus on aspects relevant to drug delivery. It highlights current state-of-the-art approaches to deliver drugs to the intestinal lymphatics and the shift toward the use of these strategies to achieve pharmacokinetic and therapeutic benefits for patients.

An atypical basement membrane forms a midline barrier during left-right asymmetric gut development in the chicken embryo

Correct intestinal morphogenesis depends on the early embryonic process of gut rotation, an evolutionarily conserved program in which a straight gut tube elongates and forms into its first loops. However, the gut tube requires guidance to loop in a reproducible manner. The dorsal mesentery (DM) connects the gut tube to the body and directs the lengthening gut into stereotypical loops via left-right (LR) asymmetric cellular and extracellular behavior. The LR asymmetry of the DM also governs blood and lymphatic vessel formation for the digestive tract, which is essential for prenatal organ development and postnatal vital functions including nutrient absorption. Although the genetic LR asymmetry of the DM has been extensively studied, a divider between the left and right DM has yet to be identified. Setting up LR asymmetry for the entire body requires a Lefty1+ midline barrier to separate the two sides of the embryo, without it, embryos have lethal or congenital LR patterning defects. Individual organs including the brain, heart, and gut also have LR asymmetry, and while the consequences of left and right signals mixing are severe or even lethal, organ-specific mechanisms for separating these signals are poorly understood. Here, we uncover a midline structure composed of a transient double basement membrane, which separates the left and right halves of the embryonic chick DM during the establishment of intestinal and vascular asymmetries. Unlike other basement membranes of the DM, the midline is resistant to disruption by intercalation of Netrin4 (Ntn4). We propose that this atypical midline forms the boundary between left and right sides and functions as a barrier necessary to establish and protect organ asymmetry.

Villus myofibroblasts are developmental and adult progenitors of mammalian gut lymphatic musculature

Inside the finger-like intestinal projections called villi, strands of smooth muscle cells contract to propel absorbed dietary fats through the adjacent lymphatic capillary, the lacteal, sending fats into the systemic blood circulation for energy production. Despite this vital function, mechanisms of formation, assembly alongside lacteals, and maintenance of villus smooth muscle are unknown. By combining single-cell RNA sequencing and quantitative lineage tracing of the mouse intestine, we identified a local hierarchy of subepithelial fibroblast progenitors that differentiate into mature smooth muscle fibers via intermediate contractile myofibroblasts. This continuum persists as the major mechanism for villus musculature renewal throughout adult life. The NOTCH3-DLL4 signaling axis governs the assembly of smooth muscle fibers alongside their adjacent lacteals and is required for fat absorption. Our studies identify the ontogeny and maintenance of a poorly defined class of intestinal smooth muscle, with implications for accelerated repair and recovery of digestive function following injury.

Intergenic sequences harboring potential enhancer elements contribute to Axenfeld-Rieger syndrome by regulating PITX2

Recent studies have uncovered that noncoding sequence variants may relate to Axenfeld-Rieger syndrome (ARS), a rare developmental anomaly with genetic heterogeneity. However, how these genomic regions are functionally and structurally associated with ARS is still unclear. In this study, we performed genome-wide linkage analysis and whole-genome sequencing in a Chinese family with ARS and identified a heterozygous deletion of about 570 kb (termed LOH-1) in the intergenic sequence between paired-like homeodomain transcription factor 2 (PITX2) and family with sequence similarity 241 member A. Knockout of LOH-1 homologous sequences caused ARS phenotypes in mice. RNA-Seq and real-time quantitative PCR revealed a significant reduction in Pitx2 gene expression in LOH-1-/- mice, while forkhead box C1 expression remained unchanged. ChIP-Seq and bioinformatics analysis identified a potential enhancer region (LOH-E1) within LOH-1. Deletion of LOH-E1 led to a substantial downregulation of the PITX2 gene. Mechanistically, we found a sequence (hg38 chr4:111,399,594-111,399,691) that is on LOH-E1 could regulate PITX2 by binding to RAD21, a critical component of the cohesin complex. Knockdown of RAD21 resulted in reduced PITX2 expression. Collectively, our findings indicate that a potential enhancer sequence that is within LOH-1 may regulate PITX2 expression remotely through cohesin-mediated loop domains, leading to ARS when absent.

Origin and adult renewal of the gut lacteal musculature from villus myofibroblasts

Intestinal smooth muscles are the workhorse of the digestive system. Inside the millions of finger-like intestinal projections called villi, strands of smooth muscle cells contract to propel absorbed dietary fats through the adjacent lymphatic vessel, called the lacteal, sending fats into the blood circulation for energy production. Despite this vital function, how villus smooth muscles form, how they assemble alongside lacteals, and how they repair throughout life remain unknown. Here we combine single-cell RNA sequencing of the mouse intestine with quantitative lineage tracing to reveal the mechanisms of formation and differentiation of villus smooth muscle cells. Within the highly regenerative villus, we uncover a local hierarchy of subepithelial fibroblast progenitors that progress to become mature smooth muscle fibers, via an intermediate contractile myofibroblast-like phenotype. This continuum persists in the adult intestine as the major source of renewal of villus smooth muscle cells during adult life. We further found that the NOTCH3-DLL4 signaling axis governs the assembly of villus smooth muscles alongside their adjacent lacteal, and we show that this is necessary for gut absorptive function. Overall, our data shed light on the genesis of a poorly defined class of intestinal smooth muscle and pave the way for new opportunities to accelerate recovery of digestive function by stimulating muscle repair.

Mechanisms and functions of intestinal vascular specialization

The intestinal vasculature has been studied for the last 100 years, and its essential role in absorbing and distributing ingested nutrients is well known. Recently, fascinating new insights into the organization, molecular mechanisms, and functions of intestinal vessels have emerged. These include maintenance of intestinal epithelial cell function, coping with microbiota-induced inflammatory pressure, recruiting gut-specific immune cells, and crosstalk with other organs. Intestinal function is also regulated at the systemic and cellular levels, such that the postprandial hyperemic response can direct up to 30% of systemic blood to gut vessels, while micron-sized endothelial cell fenestrations are necessary for nutrient uptake. In this review, we will highlight past discoveries made about intestinal vasculature in the context of new findings of molecular mechanisms underpinning gut function. Such comprehensive understanding of the system will pave the way to breakthroughs in nutrient uptake optimization, drug delivery efficiency, and treatment of human diseases.

Chylomicrons Regulate Lacteal Permeability and Intestinal Lipid Absorption

BACKGROUND

Lymphatic vessels are responsible for tissue drainage, and their malfunction is associated with chronic diseases. Lymph uptake occurs via specialized open cell-cell junctions between capillary lymphatic endothelial cells (LECs), whereas closed junctions in collecting LECs prevent lymph leakage. LEC junctions are known to dynamically remodel in development and disease, but how lymphatic permeability is regulated remains poorly understood.

METHODS

We used various genetically engineered mouse models in combination with cellular, biochemical, and molecular biology approaches to elucidate the signaling pathways regulating junction morphology and function in lymphatic capillaries.

RESULTS

By studying the permeability of intestinal lacteal capillaries to lipoprotein particles known as chylomicrons, we show that ROCK (Rho-associated kinase)-dependent cytoskeletal contractility is a fundamental mechanism of LEC permeability regulation. We show that chylomicron-derived lipids trigger neonatal lacteal junction opening via ROCK-dependent contraction of junction-anchored stress fibers. LEC-specific ROCK deletion abolished junction opening and plasma lipid uptake. Chylomicrons additionally inhibited VEGF (vascular endothelial growth factor)-A signaling. We show that VEGF-A antagonizes LEC junction opening via VEGFR (VEGF receptor) 2 and VEGFR3-dependent PI3K (phosphatidylinositol 3-kinase)/AKT (protein kinase B) activation of the small GTPase RAC1 (Rac family small GTPase 1), thereby restricting RhoA (Ras homolog family member A)/ROCK-mediated cytoskeleton contraction.

CONCLUSIONS

Our results reveal that antagonistic inputs into ROCK-dependent cytoskeleton contractions regulate the interconversion of lymphatic junctions in the intestine and in other tissues, providing a tunable mechanism to control the lymphatic barrier.

Endothelial FOXC1 and FOXC2 promote intestinal regeneration after ischemia-reperfusion injury

Intestinal ischemia underlies several clinical conditions and can result in the loss of the intestinal mucosal barrier. Ischemia-induced damage to the intestinal epithelium is repaired by stimulation of intestinal stem cells (ISCs), and paracrine signaling from the vascular niche regulates intestinal regeneration. Here, we identify FOXC1 and FOXC2 as essential regulators of paracrine signaling in intestinal regeneration after ischemia-reperfusion (I/R) injury. Vascular endothelial cell (EC)- and lymphatic EC (LEC)-specific deletions of Foxc1, Foxc2, or both in mice worsen I/R-induced intestinal damage by causing defects in vascular regrowth, expression of chemokine CXCL12 and Wnt activator R-spondin 3 (RSPO3) in blood ECs (BECs) and LECs, respectively, and activation of Wnt signaling in ISCs. Both FOXC1 and FOXC2 directly bind to regulatory elements of the CXCL12 and RSPO3 loci in BECs and LECs, respectively. Treatment with CXCL12 and RSPO3 rescues the I/R-induced intestinal damage in EC- and LEC-Foxc mutant mice, respectively. This study provides evidence that FOXC1 and FOXC2 are required for intestinal regeneration by stimulating paracrine CXCL12 and Wnt signaling.

Brain-gut-liver axis: Chronic psychological stress promotes liver injury and fibrosis via gut in rats

Background

The effect of chronic psychological stress on hepatitis and liver fibrosis is concerned. However, its mechanism remains unclear. We investigated the effect and mechanism of chronic psychological stress in promoting liver injury and fibrosis through gut.

Methods

Sixty male SD rats were randomly assigned to 6 groups. Rat models of chronic psychological stress (4 weeks) and liver fibrosis (8 weeks) were established. The diversity of gut microbiota in intestinal feces, permeability of intestinal mucosa, pathologies of intestinal and liver tissues, collagen fibers, protein expressions of toll-like receptor 4 (TLR4), myeloid differentiation factor 88 (MyD88), nuclear factor kappa β (NF-κβ), tumor necrosis factor α (TNF-α) and interleukin 1 (IL-1) in liver tissue, liver function and coagulation function in blood and lipopolysaccharide (LPS) in portal vein blood were detected and analyzed.

Results

The diversities and abundances of gut microbiota were significant differences in rats among each group. The pathological lesions of intestinal and liver tissues, decreased expression of occludin protein in intestinal mucosa, deposition of collagen fibers and increased protein expression of TLR4, MyD88, NF-κβ, TNF-α and IL-1 in liver tissue, increased LPS level in portal vein blood, and abnormalities of liver function and coagulation function, were observed in rats exposed to chronic psychological stress or liver fibrosis. There were significant differences with normal rats. When the dual intervention factors of chronic psychological stress and liver fibrosis were superimposed, the above indicators were further aggravated.

Conclusion

Chronic psychological stress promotes liver injury and fibrosis, depending on changes in the diversity of gut microbiota and increased intestinal permeability caused by psychological stress, LPS that enters liver and acts on TLR4, and active LPS-TLR4 pathway depend on MyD88. It demonstrates the possibility of existence of brain-gut-liver axis.

Protocol to detect smooth muscle actin-alpha and measure oxidative damage in neonatal mouse intestine

This protocol describes how to characterize α-Smooth muscle actin (αSMA) spatiotemporal expression during mouse small intestinal development. Specific tissue fixation preserves αSMA arrangement in low αSMA expressing cells that are conventionally undetectable under αSMA immunofluorescent stain due to inappropriate fixative-caused artificial actin depolymerization. Parallel analysis of αSMA carbonylation allows estimation of oxidative damage in gut muscular lineage. This approach improves the molecular specificity offered by commercialized kits that estimate total protein carbonyl level in cell lysates without protein specificity.

For complete details on the use and execution of this protocol, please refer to Hu et al. (2021).

•

Optimal fixative improves αSMA immunofluorescent staining in the mouse intestine

•

Improved cellular resolution of αSMA protein in whole mount immunofluorescent staining

•

Semiquantitative measurement of oxidative damage on specific tissue proteins in vivo

•

This protocol identifies the size distribution of total protein carbonyl in animal tissue

Publisher’s note: Undertaking any experimental protocol requires adherence to local institutional guidelines for laboratory safety and ethics.