Dll1 haploinsufficiency in adult mice leads to a complex phenotype affecting metabolic and immunological processes

Dll1 haploinsufficiency causes brain abnormalities with functional relevance

Introduction

The Notch pathway is fundamental for the generation of neurons during development. We previously reported that adult mice heterozygous for the null allele of the gene encoding the Delta-like ligand 1 for Notch (Dll1^lacZ ) have a reduced neuronal density in the substantia nigra pars compacta. The aim of the present work was to evaluate whether this alteration extends to other brain structures and the behavioral consequences of affected subjects.

Methods

Brains of Dll1 ^+/lacZ embryos and mice at different ages were phenotypically compared against their wild type (WT) counterpart. Afterwards, brain histological analyses were performed followed by determinations of neural cell markers in tissue slices. Neurological deficits were diagnosed by applying different behavioral tests to Dll1 ^+/lacZ and WT mice.

Results

Brain weight and size of Dll1 ^+/lacZ mice was significantly decreased compared with WT littermates (i.e., microcephaly), a phenotype detected early after birth. Interestingly, enlarged ventricles (i.e., hydrocephalus) was a common characteristic of brains of Dll1 haploinsufficient mice since early ages. At the cell level, general cell density and number of neurons in several brain regions, including the cortex and hippocampus, of Dll1 ^+/lacZ mice were reduced as compared with those regions of WT mice. Also, fewer neural stem cells were particularly found in the adult dentate gyrus of Dll1 ^+/lacZ mice but not in the subventricular zone. High myelination levels detected at early postnatal ages (P7-P24) were an additional penetrant phenotype in Dll1 ^+/lacZ mice, observation that was consistent with premature oligodendrocyte differentiation. After applying a set of behavioral tests, mild neurological alterations were detected that caused changes in motor behaviors and a deficit in object categorization.

Discussion

Our observations suggest that Dll1 haploinsufficiency limits Notch signaling during brain development which, on one hand, leads to reduced brain cell density and causes microcephaly and hydrocephalus phenotypes and, on the other, alters the myelination process after birth. The severity of these defects could reach levels that affect normal brain function. Therefore, Dll1 haploinsufficiency is a risk factor that predisposes the brain to develop abnormalities with functional consequences.

Micro ribonucleic acid-363 regulates the phosphatidylinositol 3-kinase/threonine protein kinase axis by targeting NOTCH1 and forkhead box C2, leading to hepatic glucose and lipids metabolism disorder in type 2 diabetes mellitus

AIMS/INTRODUCTION

Glucose metabolic disorder is the main cause for type 2 diabetes progression. Exploring the molecular mechanisms of metabolic disorder are crucial for type 2 diabetes treatment.

MATERIALS AND METHODS

Micro ribonucleic acid (miR)-363, NOTCH1 and forkhead box C2 (FOXC2) expressions in high glucose (HG)-treated HepG2 cells and the livers of type 2 diabetes mellitus rats were assessed using quantitative polymerase chain reaction. Protein levels of NOTCH1, FOXC2 and phosphatidylinositol 3-kinase (PI3K)/serine/threonine protein kinase (Akt)-related proteins were evaluated using western blot. Lipid accumulation was determined using Oil Red O staining. Then glucose consumption, blood glucose level and glycogen content were detected using kits. Finally, dual luciferase reporter assay was used to verify the binding relationship between miR-363 and NOTCH1, and the binding relationship between miR-363 and FOXC2.

RESULTS

MiR-363 was significantly upregulated in the livers of diabetic rats and HG-induced HepG2 cells, whereas NOTCH1 and FOXC2 were downregulated. In HG-induced HepG2 cells, miR-363 inhibitor markedly increased glucose consumption and uptake, and reduced accumulation of lipid droplets. Then NOTCH1 and FOXC2 were identified as downstream targets of miR-363. NOTCH1 overexpression or FOXC2 overexpression could ameliorate glucose and lipids metabolism disorder in type 2 diabetes model cells. In addition, we found that FOXC2 inhibition abolished the effect of NOTCH1 overexpression on HG-induced HepG2 cells. Finally, we proved that the PI3K/Akt pathway was the downstream pathway of FOXC2.

CONCLUSION

MiR-363 was considered as a key regulator of glucose and lipids metabolism in type 2 diabetes mellitus, which regulated PI3K/Akt axis by targeting NOTCH1 and FOXC2, thus leading to hepatic glucose and lipids metabolism disorder in type 2 diabetes.

Biological Significance of NOTCH Signaling Strength

The evolutionarily conserved NOTCH signaling displays pleotropic functions in almost every organ system with a simple signaling axis. Different from many other signaling pathways that can be amplified via kinase cascades, NOTCH signaling does not contain any intermediate to amplify signal. Thus, NOTCH signaling can be activated at distinct signaling strength levels, disruption of which leads to various developmental disorders. Here, we reviewed mechanisms establishing different NOTCH signaling strengths, developmental processes sensitive to NOTCH signaling strength perturbation, and transcriptional regulations influenced by NOTCH signaling strength changes. We hope this could add a new layer of diversity to explain the pleotropic functions of NOTCH signaling pathway.

The Roles of Notch Signaling in Liver Development and Disease

The Notch signaling pathway plays major roles in organ development across animal species. In the mammalian liver, Notch has been found critical in development, regeneration and disease. In this review, we highlight the major advances in our understanding of the role of Notch activity in proper liver development and function. Specifically, we discuss the latest discoveries on how Notch, in conjunction with other signaling pathways, aids in proper liver development, regeneration and repair. In addition, we review the latest in the role of Notch signaling in the pathogenesis of liver fibrosis and chronic liver disease. Finally, recent evidence has shed light on the emerging connection between Notch signaling and glucose and lipid metabolism. We hope that highlighting the major advances in the roles of Notch signaling in the liver will stimulate further research in this exciting field and generate additional ideas for therapeutic manipulation of the Notch pathway in liver diseases.

Haploinsufficiency of the Notch Ligand DLL1 Causes Variable Neurodevelopmental Disorders

Notch signaling is an established developmental pathway for brain morphogenesis. Given that Delta-like 1 (DLL1) is a ligand for the Notch receptor and that a few individuals with developmental delay, intellectual disability, and brain malformations have microdeletions encompassing DLL1, we hypothesized that insufficiency of DLL1 causes a human neurodevelopmental disorder. We performed exome sequencing in individuals with neurodevelopmental disorders. The cohort was identified using known Matchmaker Exchange nodes such as GeneMatcher. This method identified 15 individuals from 12 unrelated families with heterozygous pathogenic DLL1 variants (nonsense, missense, splice site, and one whole gene deletion). The most common features in our cohort were intellectual disability, autism spectrum disorder, seizures, variable brain malformations, muscular hypotonia, and scoliosis. We did not identify an obvious genotype-phenotype correlation. Analysis of one splice site variant showed an in-frame insertion of 12 bp. In conclusion, heterozygous DLL1 pathogenic variants cause a variable neurodevelopmental phenotype and multi-systemic features. The clinical and molecular data support haploinsufficiency as a mechanism for the pathogenesis of this DLL1-related disorder and affirm the importance of DLL1 in human brain development.

The developmental biology of genetic Notch disorders

Notch signaling regulates a vast array of crucial developmental processes. It is therefore not surprising that mutations in genes encoding Notch receptors or ligands lead to a variety of congenital disorders in humans. For example, loss of function of Notch results in Adams-Oliver syndrome, Alagille syndrome, spondylocostal dysostosis and congenital heart disorders, while Notch gain of function results in Hajdu-Cheney syndrome, serpentine fibula polycystic kidney syndrome, infantile myofibromatosis and lateral meningocele syndrome. Furthermore, structure-abrogating mutations in NOTCH3 result in CADASIL. Here, we discuss these human congenital disorders in the context of known roles for Notch signaling during development. Drawing on recent analyses by the exome aggregation consortium (EXAC) and on recent studies of Notch signaling in model organisms, we further highlight additional Notch receptors or ligands that are likely to be involved in human genetic diseases.

Hepatic Notch1 deletion predisposes to diabetes and steatosis via glucose-6-phosphatase and perilipin-5 upregulation

Notch signaling pathways have recently been implicated in the pathogenesis of metabolic diseases. However, the role of hepatic Notch signaling in glucose and lipid metabolism remains unclear and needs further investigation as it might be a candidate therapeutic target in metabolic diseases such as nonalcoholic steatohepatitis (NASH) and nonalcoholic fatty liver disease (NAFLD). We used hepatocyte-specific Notch1 knockout (KO) mice and liver biopsies from NASH and NAFLD patients to analyze the role of Notch1 in glucose and lipid metabolism. Hepatocyte-specific Notch1 KO mice were fed with a high fat diet (HFD) or a regular diet (RD). We assessed the metabolic phenotype, glucose and insulin tolerance tests, and liver histology. Hepatic mRNA expression was profiled by Affymetrix Mouse Gene arrays and validated by quantitative reverse transcription PCR (qPCR). Akt phosphorylation was visualized by immunoblotting. Gene expression was analyzed in liver biopsies from NASH, NAFLD, and control patients by qPCR. We found that Notch1 KO mice had elevated fasting glucose. Gene expression analysis showed an upregulation of glucose-6-phosphatase, involved in the final step of gluconeogenesis and glucose release from glycogenolysis, and perilipin-5, a regulator of hepatic lipid accumulation. When fed with an HFD KO mice developed overt diabetes and hepatic steatosis. Akt was highly phosphorylated in KO animals and the Foxo1 target gene expression was altered. Accordingly, a reduction in Notch1 and increase in glucose-6-phosphatase and perilipin-5 expression was observed in liver biopsies from NAFLD/NASH compared with controls. Notch1 is a regulator of hepatic glucose and lipid homeostasis. Hepatic impairment of Notch1 expression may be involved in the pathogenesis of human NAFLD/NASH.

Genomic copy number variation in Mus musculus

BACKGROUND

Copy number variation is an important dimension of genetic diversity and has implications in development and disease. As an important model organism, the mouse is a prime candidate for copy number variant (CNV) characterization, but this has yet to be completed for a large sample size. Here we report CNV analysis of publicly available, high-density microarray data files for 351 mouse tail samples, including 290 mice that had not been characterized for CNVs previously.

RESULTS

We found 9634 putative autosomal CNVs across the samples affecting 6.87% of the mouse reference genome. We find significant differences in the degree of CNV uniqueness (single sample occurrence) and the nature of CNV-gene overlap between wild-caught mice and classical laboratory strains. CNV-gene overlap was associated with lipid metabolism, pheromone response and olfaction compared to immunity, carbohydrate metabolism and amino-acid metabolism for wild-caught mice and classical laboratory strains, respectively. Using two subspecies of wild-caught Mus musculus, we identified putative CNVs unique to those subspecies and show this diversity is better captured by wild-derived laboratory strains than by the classical laboratory strains. A total of 9 genic copy number variable regions (CNVRs) were selected for experimental confirmation by droplet digital PCR (ddPCR).

CONCLUSION

The analysis we present is a comprehensive, genome-wide analysis of CNVs in Mus musculus, which increases the number of known variants in the species and will accelerate the identification of novel variants in future studies.

The intracellular domains of Notch1 and Notch2 are functionally equivalent during development and carcinogenesis

Although Notch1 and Notch2 are closely related paralogs and function through the same canonical signaling pathway, they contribute to different outcomes in some cell and disease contexts. To understand the basis for these differences, we examined in detail mice in which the Notch intracellular domains (N1ICD and N2ICD) were swapped. Our data indicate that strength (defined here as the ultimate number of intracellular domain molecules reaching the nucleus, integrating ligand-mediated release and nuclear translocation) and duration (half-life of NICD-RBPjk-MAML-DNA complexes, integrating cooperativity and stability dependent on shared sequence elements) are the factors that underlie many of the differences between Notch1 and Notch2 in all the contexts we examined, including T-cell development, skin differentiation and carcinogenesis, the inner ear, the lung and the retina. We were able to show that phenotypes in the heart, endothelium, and marginal zone B cells are attributed to haploinsufficiency but not to intracellular domain composition. Tissue-specific differences in NICD stability were most likely caused by alternative scissile bond choices by tissue-specific γ-secretase complexes following the intracellular domain swap. Reinterpretation of clinical findings based on our analyses suggests that differences in outcome segregating with Notch1 or Notch2 are likely to reflect outcomes dependent on the overall strength of Notch signals.

Delineation of candidate genes responsible for structural brain abnormalities in patients with terminal deletions of chromosome 6q27

Patients with terminal deletions of chromosome 6q present with structural brain abnormalities including agenesis of corpus callosum, hydrocephalus, periventricular nodular heterotopia, and cerebellar malformations. The 6q27 region harbors genes that are important for the normal development of brain and delineation of a critical deletion region for structural brain abnormalities may lead to a better genotype-phenotype correlation. We conducted a detailed clinical and molecular characterization of seven unrelated patients with deletions involving chromosome 6q27. All patients had structural brain abnormalities. Using array comparative genomic hybridization, we mapped the size, extent, and genomic content of these deletions. The smallest region of overlap spans 1.7 Mb and contains DLL1, THBS2, PHF10, and C6orf70 (ERMARD) that are plausible candidates for the causation of structural brain abnormalities. Our study reiterates the importance of 6q27 region in normal development of brain and helps identify putative genes in causation of structural brain anomalies.

Expanding role of delta-like 4 mediated notch signaling in cardiovascular and metabolic diseases

Cardiometabolic disease, a global health threat, has been linked to chronic inflammation, in which activated macrophages play a key role. Macrophages are highly heterogeneous hematopoietic cells found in nearly every tissue in the body. Various stimuli recruit monocytes into the cardiovascular system and metabolic organs, where they differentiate to macrophages, and activate these pro-inflammatory phagocytes, leading to the initiation and development of inflammation in these organs. Key regulators of macrophage activation therefore may serve as therapeutic targets for cardiometabolic disease. The Notch signaling pathway, involving 5 ligands and 4 receptors, regulates the differentiation of various cell types during development, and also contributes to the disease processes in adults. We found that the Notch ligand delta-like 4 (Dll4) activates macrophages in vitro as determined by the induction of genes and pathways associated with cardiovascular and metabolic disorders. Our recent study demonstrated in vivo that blockade of Dll4 by a neutralizing antibody attenuates key features typical of cardiovascular and metabolic diseases, such as accumulation of activated macrophages in arteries and fat; chronic atherosclerosis; arterial and valvular calcification; insulin resistance; and fatty liver. These results suggest that Dll4-mediated Notch signaling participates in the shared disease mechanisms for cardiovascular and metabolic disorders. This review summarizes the role of macrophages and Dll4/Notch signaling in the development of inflammation in both the cardiovascular system and metabolic organs. 

Therapeutic effects of anti-Delta1 mAb on Theiler's murine encephalomyelitis virus-induced demyelinating disease

We examined the role of Notch ligand Delta-like 1 (Delta1) in the development of Theiler's murine encephalomyelitis virus (TMEV)-induced demyelinating disease (TMEV-IDD). Blocking of Delta1 by anti-Delta1 monoclonal antibody (mAb) in the effector phase significantly suppressed the disease development of TMEV-IDD both clinically and histologically. The number of infiltrating inflammatory mononuclear cells in the spinal cords was also decreased in mice treated with anti-Delta1 mAb at the effector phase. Flow cytometric analysis of cytokine staining revealed that IFN-γ- or IL-4-producing CD4(+) splenocytes were significantly decreased in mice treated with anti-Delta1 mAb in the spleens, whereas IL-10-producing CD4(+) splenocytes were increased. Furthermore, IFN-γ-, TNF-α-, IL-4-, or IL-10-producing CD4(+) cells were decreased in spinal cords, and IL-17-producing CD4(+) cells were increased. These data suggest that Delta1 may play important roles in the development of TMEV-IDD and that antibodies to Delta1 could be used as a novel therapeutic treatment of demyelinating diseases such as human multiple sclerosis.

Notch ligand delta-like 4 blockade attenuates atherosclerosis and metabolic disorders

Atherosclerosis and insulin resistance are major components of the cardiometabolic syndrome, a global health threat associated with a systemic inflammatory state. Notch signaling regulates tissue development and participates in innate and adaptive immunity in adults. The role of Notch signaling in cardiometabolic inflammation, however, remains obscure. We noted that a high-fat, high-cholesterol diet increased expression of the Notch ligand Delta-like 4 (Dll4) in atheromata and fat tissue in LDL-receptor-deficient mice. Blockade of Dll4-Notch signaling using neutralizing anti-Dll4 antibody attenuated the development of atherosclerosis, diminished plaque calcification, improved insulin resistance, and decreased fat accumulation. These changes were accompanied by decreased macrophage accumulation, diminished expression of monocyte chemoattractant protein-1 (MCP-1), and lower levels of nuclear factor-κB (NF-κB) activation. In vitro cell culture experiments revealed that Dll4-mediated Notch signaling increases MCP-1 expression via NF-κB, providing a possible mechanism for in vivo effects. Furthermore, Dll4 skewed macrophages toward a proinflammatory phenotype ("M1"). These results suggest that Dll4-Notch signaling plays a central role in the shared mechanism for the pathogenesis of cardiometabolic disorders.

NOTCH, a new signaling pathway implicated in holoprosencephaly

Genetics of Holoprosencephaly (HPE), a congenital malformation of the developing human forebrain, is due to multiple genetic defects. Most genes that have been implicated in HPE belong to the sonic hedgehog signaling pathway. Here we describe a new candidate gene isolated from array comparative genomic hybridization redundant 6qter deletions, DELTA Like 1 (DLL1), which is a ligand of NOTCH. We show that DLL1 is co-expressed in the developing chick forebrain with Fgf8. By treating chick embryos with a pharmacological inhibitor, we demonstrate that DLL1 interacts with FGF signaling pathway. Moreover, a mutation analysis of DLL1 in HPE patients revealed a three-nucleotide deletion. These various findings implicate DLL1 in early patterning of the forebrain and identify NOTCH as a new signaling pathway involved in HPE.