Conditional deletions refine the embryonic requirement for Dlk1

Untangling the genetics of beta cell dysfunction and death in type 1 diabetes

Type 1 diabetes (T1D) is a complex multi-system disease which arises from both environmental and genetic factors, resulting in the destruction of insulin-producing pancreatic beta cells. Over the past two decades, human genetic studies have provided new insight into the etiology of T1D, including an appreciation for the role of beta cells in their own demise. Here, we outline models supported by human genetic data for the role of beta cell dysfunction and death in T1D. We highlight the importance of strong evidence linking T1D genetic associations to bona fide candidate genes for mechanistic and therapeutic consideration. To guide rigorous interpretation of genetic associations, we describe molecular profiling approaches, genomic resources, and disease models that may be used to construct variant-to-gene links and to investigate candidate genes and their role in T1D. We profile advances in understanding the genetic causes of beta cell dysfunction and death at individual T1D risk loci. We introduce genetic risk prediction models and discuss how they can be used to address disease heterogeneity. Finally, we present areas where investment will be critical for future use of genetics to address open questions and to develop new treatment and prevention strategies for T1D.

An integrative single-cell multi-omics profiling of human pancreatic islets identifies T1D associated genes and regulatory signals

Genome wide association studies (GWAS) have identified over 100 signals associated with type 1 diabetes (T1D). However, translating any given T1D GWAS signal into mechanistic insights, including putative causal variants and the context (cell type and cell state) in which they function, has been limited. Here, we present a comprehensive multi-omic integrative analysis of single-cell/nucleus resolution profiles of gene expression and chromatin accessibility in healthy and autoantibody+ (AAB+) human islets, as well as islets under multiple T1D stimulatory conditions. We broadly nominate effector cell types for all T1D GWAS signals. We further nominated higher-resolution contexts, including effector cell types, regulatory elements, and genes for three independent T1D risk variants acting through islet cells within the pancreas at the DLK1/MEG3, RASGRP1, and TOX loci. Subsequently, we created isogenic gene knockouts DLK1-/-, RASGRP1-/-, and TOX-/-, and the corresponding regulatory region knockout, RASGRP1Δ, and DLK1Δ hESCs. Loss of RASGRP1 or DLK1, as well as knockout of the regulatory region of RASGRP1 or DLK1, increased β cell apoptosis. Additionally, pancreatic β cells derived from isogenic hESCs carrying the risk allele of rs3783355A/A exhibited increased β cell death. Finally, RNA-seq and ATAC-seq identified five genes upregulated in both RASGRP1-/- and DLK1-/- β-like cells, four of which are associated with T1D. Together, this work reports an integrative approach for combining single cell multi-omics, GWAS, and isogenic hESC-derived β-like cells to prioritize the T1D associated signals and their underlying context-specific cell types, genes, SNPs, and regulatory elements, to illuminate biological functions and molecular mechanisms.

Imprinted Dlk1 dosage as a size determinant of the mammalian pituitary gland

Co-regulated genes of the Imprinted Gene Network are involved in the control of growth and body size, and imprinted gene dysfunction underlies human paediatric disorders involving the endocrine system. Imprinted genes are highly expressed in the pituitary gland, among them, Dlk1, a paternally expressed gene whose membrane-bound and secreted protein products can regulate proliferation and differentiation of multiple stem cell populations. Dosage of circulating DLK1 has been previously implicated in the control of growth through unknown molecular mechanisms. Here we generate a series of mouse genetic models to modify levels of Dlk1 expression in the pituitary gland and demonstrate that the dosage of DLK1 modulates the process of stem cell commitment with lifelong impact on pituitary gland size. We establish that stem cells are a critical source of DLK1, where embryonic disruption alters proliferation in the anterior pituitary, leading to long-lasting consequences on growth hormone secretion later in life.

Dlk1 maintains adult mice long-term HSCs by activating Notch signaling to restrict mitochondrial metabolism

BACKGROUND

Adult hematopoietic stem cells (HSCs) homeostasis is critically important in maintaining lifelong hematopoiesis. However, how adult HSCs orchestrate its homeostasis remains not fully understood. Imprinted gene Dlk1 has been shown to play critical role in mouse embryonic hematopoiesis and in regulation of stem cells, but its physiological roles in adult HSCs are unknown.

METHODS

We performed gene expression analysis of Dlk1, and constructed conditional Dlk1 knockout (KO) mice by crossing Mx1 cre mice with Dlk^flox/flox mice. Western blot and quantitative PCR were used to detect Dlk1 KO efficiency. Flow cytometry was performed to investigate the effects of Dlk1 KO on HSCs, progenitors and linage cells in primary mice. Competitive HSCs transplantation and secondary transplantation was used to examine the effects of Dlk1 KO on long-term hematopoietic repopulation potential of HSCs. RNA-Seq and cell metabolism assays was used to determine the underlying mechanisms.

RESULTS

Dlk1 was highly expressed in adult mice long-term HSCs (LT-HSCs) relative to progenitors and mature lineage cells. Dlk1 KO in adult mice HSCs drove HSCs enter active cell cycle, and expanded phenotypical LT-HSCs, but undermined its long-term hematopoietic repopulation potential. Dlk1 KO resulted in an increase in HSCs' metabolic activity, including glucose uptake, ribosomal translation, mitochondrial metabolism and ROS production, which impaired HSCs function. Further, Dlk1 KO in adult mice HSCs attenuated Notch signaling, and re-activation of Notch signaling under Dlk1 KO decreased the mitochondrial activity and ROS production, and rescued the changes in frequency and absolute number of HSCs. Scavenging ROS by antioxidant N-acetylcysteine could inhibit mitochondrial metabolic activity, and rescue the changes in HSCs caused by Dlk1 KO.

CONCLUSION

Our study showed that Dlk1 played an essential role in maintaining HSC homeostasis, which is realized by governing cell cycle and restricting mitochondrial metabolic activity.

Balanced gene dosage control rather than parental origin underpins genomic imprinting

Mammalian parental imprinting represents an exquisite form of epigenetic control regulating the parent-specific monoallelic expression of genes in clusters. While imprinting perturbations are widely associated with developmental abnormalities, the intricate regional interplay between imprinted genes makes interpreting the contribution of gene dosage effects to phenotypes a challenging task. Using mouse models with distinct deletions in an intergenic region controlling imprinting across the Dlk1-Dio3 domain, we link changes in genetic and epigenetic states to allelic-expression and phenotypic outcome in vivo. This determined how hierarchical interactions between regulatory elements orchestrate robust parent-specific expression, with implications for non-imprinted gene regulation. Strikingly, flipping imprinting on the parental chromosomes by crossing genotypes of complete and partial intergenic element deletions rescues the lethality of each deletion on its own. Our work indicates that parental origin of an epigenetic state is irrelevant as long as appropriate balanced gene expression is established and maintained at imprinted loci.

Here the authors investigate whether for imprinted genes the parent-of-origin of the expressed allele or rather appropriate gene dosage is more important for normal development. Using the differentially methylated region of Dlk1-Dio3 gene involved in imprinting, they show that correct parent-of-origin imprinting pattern is secondary to balanced gene dosage.

Case Report: Hypoglycemia Due to a Novel Activating Glucokinase Variant in an Adult - a Molecular Approach

We present a case of an obese 22-year-old man with activating GCK variant who had neonatal hypoglycemia, re-emerging with hypoglycemia later in life. We investigated him for asymptomatic hypoglycemia with a family history of hypoglycemia. Genetic testing yielded a novel GCK missense class 3 variant that was subsequently found in his mother, sister and nephew and reclassified as a class 4 likely pathogenic variant. Glucokinase enables phosphorylation of glucose, the rate-limiting step of glycolysis in the liver and pancreatic β cells. It plays a crucial role in the regulation of insulin secretion. Inactivating variants in GCK cause hyperglycemia and activating variants cause hypoglycemia. Spleen-preserving distal pancreatectomy revealed diffuse hyperplastic islets, nuclear pleomorphism and periductular islets. Glucose stimulated insulin secretion revealed increased insulin secretion in response to glucose. Cytoplasmic calcium, which triggers exocytosis of insulin-containing granules, revealed normal basal but increased glucose-stimulated level. Unbiased gene expression analysis using 10X single cell sequencing revealed upregulated INS and CKB genes and downregulated DLK1 and NPY genes in β-cells. Further studies are required to see if alteration in expression of these genes plays a role in the metabolic and histological phenotype associated with glucokinase pathogenic variant. There were more large islets in the patient's pancreas than in control subjects but there was no difference in the proportion of β cells in the islets. His hypoglycemia was persistent after pancreatectomy, was refractory to diazoxide and improved with pasireotide. This case highlights the variable phenotype of GCK mutations. In-depth molecular analyses in the islets have revealed possible mechanisms for hyperplastic islets and insulin hypersecretion.

Partners in Crime: Beta-Cells and Autoimmune Responses Complicit in Type 1 Diabetes Pathogenesis

Type 1 diabetes (T1D) is an autoimmune disease characterized by autoreactive T cell-mediated destruction of insulin-producing pancreatic beta-cells. Loss of beta-cells leads to insulin insufficiency and hyperglycemia, with patients eventually requiring lifelong insulin therapy to maintain normal glycemic control. Since T1D has been historically defined as a disease of immune system dysregulation, there has been little focus on the state and response of beta-cells and how they may also contribute to their own demise. Major hurdles to identifying a cure for T1D include a limited understanding of disease etiology and how functional and transcriptional beta-cell heterogeneity may be involved in disease progression. Recent studies indicate that the beta-cell response is not simply a passive aspect of T1D pathogenesis, but rather an interplay between the beta-cell and the immune system actively contributing to disease. Here, we comprehensively review the current literature describing beta-cell vulnerability, heterogeneity, and contributions to pathophysiology of T1D, how these responses are influenced by autoimmunity, and describe pathways that can potentially be exploited to delay T1D.

Screening of patients born small for gestational age with the Silver-Russell syndrome phenotype for DLK1 variants

Silver-Russell syndrome (SRS) is a rare imprinting disorder associated with prenatal and postnatal growth retardation. Loss of methylation (LOM) on chromosome 11p15 is observed in 40 to 60% of patients and maternal uniparental disomy (mUPD) for chromosome 7 (upd(7)mat) in ~5 to 10%. Patients with LOM or mUPD 14q32 can present clinically as SRS. Delta like non-canonical Notch ligand 1 (DLK1) is one of the imprinted genes expressed from chromosome 14q32. Dlk1-null mice display fetal growth restriction (FGR) but no genetic defects of DLK1 have been described in human patients born small for gestational age (SGA). We screened a cohort of SGA patients with a SRS phenotype for DLK1 variants using a next-generation sequencing (NGS) approach to search for new molecular defects responsible for SRS. Patients born SGA with a clinical suspicion of SRS and normal methylation by molecular testing at the 11p15 or 14q32 loci and upd(7)mat were screened for DLK1 variants using targeted NGS. Among 132 patients, only two rare variants of DLK1 were identified (NM_003836.6:c.103 G > C (p.(Gly35Arg) and NM_003836.6: c.194 A > G p.(His65Arg)). Both variants were inherited from the mother of the patients, which does not favor a role in pathogenicity, as the mono-allelic expression of DLK1 is from the paternal-inherited allele. We did not identify any pathogenic variants in DLK1 in a large cohort of SGA patients with a SRS phenotype. DLK1 variants are not a common cause of SGA.

Dlk1 dosage regulates hippocampal neurogenesis and cognition

Neurogenesis in the adult brain gives rise to functional neurons, which integrate into neuronal circuits and modulate neural plasticity. Sustained neurogenesis throughout life occurs in the subgranular zone (SGZ) of the dentate gyrus in the hippocampus and is hypothesized to be involved in behavioral/cognitive processes such as memory and in diseases. Genomic imprinting is of critical importance to brain development and normal behavior, and exemplifies how epigenetic states regulate genome function and gene dosage. While most genes are expressed from both alleles, imprinted genes are usually expressed from either the maternally or the paternally inherited chromosome. Here, we show that in contrast to its canonical imprinting in nonneurogenic regions, Delta-like homolog 1 (Dlk1) is expressed biallelically in the SGZ, and both parental alleles are required for stem cell behavior and normal adult neurogenesis in the hippocampus. To evaluate the effects of maternally, paternally, and biallelically inherited mutations within the Dlk1 gene in specific behavioral domains, we subjected Dlk1-mutant mice to a battery of tests that dissociate and evaluate the effects of Dlk1 dosage on spatial learning ability and on anxiety traits. Importantly, reduction in Dlk1 levels triggers specific cognitive abnormalities that affect aspects of discriminating differences in environmental stimuli, emphasizing the importance of selective absence of imprinting in this neurogenic niche.

Dlk1 regulates quiescence in calcitonin receptor-mutant muscle stem cells

Muscle stem cells, also called muscle satellite cells (MuSCs), are responsible for skeletal muscle regeneration and are sustained in an undifferentiated and quiescent state under steady conditions. The calcitonin receptor (CalcR)-protein kinase A (PKA)-Yes-associated protein 1 (Yap1) axis is one pathway that maintains quiescence in MuSCs. Although CalcR signaling in MuSCs has been identified, the critical CalcR signaling targets are incompletely understood. Here, we show the relevance between the ectopic expression of delta-like non-canonical Notch ligand 1 (Dlk1) and the impaired quiescent state in CalcR-conditional knockout (cKO) MuSCs. Dlk1 expression was rarely detected in both quiescent and proliferating MuSCs in control mice, whereas Dlk1 expression was remarkably increased in CalcR-cKO MuSCs at both the mRNA and protein levels. It is noteworthy that all Ki67+ non-quiescent CalcR-cKO MuSCs express Dlk1, and non-quiescent CalcR-cKO MuSCs are enriched in the Dlk1+ fraction by cell sorting. Using mutant mice, we demonstrated that PKA-activation or Yap1-depletion suppressed Dlk1 expression in CalcR-cKO MuSCs, which suggests that the CalcR-PKA-Yap1 axis inhibits the expression of Dlk1 in quiescent MuSCs. Moreover, the loss of Dlk1 rescued the quiescent state in CalcR-cKO MuSCs, which indicates that the ectopic expression of Dlk1 disturbs quiescence in CalcR-cKO. Collectively, our results suggest that ectopically expressed Dlk1 is responsible for the impaired quiescence in CalcR-cKO MuSCs.

DLK1 Expressed in Mouse Orexin Neurons Modulates Anxio-Depressive Behavior but Not Energy Balance

Lateral hypothalamic area (LHA) neurons expressing the neuropeptide orexin (OX) are implicated in obesity and anxio-depression. However, these neurons release OX as well as a host of other proteins that might contribute to normal physiology and disease states. We hypothesized that delta-like homolog 1 (DLK1), a protein reported to be co-expressed by all OX neurons, contributes to the regulation of energy balance and/or anxio-depression. Consistent with previous reports, we found that all rat OX neurons co-express DLK1. Yet, in mice and humans only a subset of OX neurons co-expressed DLK1. Since human OX-DLK1 distribution is more similar to mice than rats, mice are a comparable model to assess the human physiologic role of DLK1. We therefore used a viral lesion strategy to selectively delete DLK1 within the LHA of adult mice (DLK1^Null) to reveal its role in body weight and behavior. Adult-onset DLK1 deletion had no impact on body weight or ingestive behavior. However, DLK1^Null mice engaged in more locomotor activity than control mice and had decreased anxiety and depression measured via the elevated plus maze and forced swim tests. These data suggest that DLK1 expression via DLK1-expressing OX neurons primarily contributes to anxio-depression behaviors without impacting body weight.

Roles of Epigenetic Modifications in the Differentiation and Function of Pancreatic β-Cells

Diabetes, a metabolic disease with multiple causes characterized by high blood sugar, has become a public health problem. Hyperglycaemia is caused by deficiencies in insulin secretion, impairment of insulin function, or both. The insulin secreted by pancreatic β cells is the only hormone in the body that lowers blood glucose levels and plays vital roles in maintaining glucose homeostasis. Therefore, investigation of the molecular mechanisms of pancreatic β cell differentiation and function is necessary to elucidate the processes involved in the onset of diabetes. Although numerous studies have shown that transcriptional regulation is essential for the differentiation and function of pancreatic β cells, increasing evidence indicates that epigenetic mechanisms participate in controlling the fate and regulation of these cells. Epigenetics involves heritable alterations in gene expression caused by DNA methylation, histone modification and non-coding RNA activity that does not result in DNA nucleotide sequence alterations. Recent research has revealed that a variety of epigenetic modifications play an important role in the development of diabetes. Here, we review the mechanisms by which epigenetic regulation affects β cell differentiation and function.

Dlk1-Mediated Temporal Regulation of Notch Signaling Is Required for Differentiation of Alveolar Type II to Type I Cells during Repair

Lung alveolar type I cells (AT1) and alveolar type II cells (AT2) regulate the structural integrity and function of alveoli. AT1, covering ∼95% of the surface area, are responsible for gas exchange, whereas AT2 serve multiple functions, including alveolar repair through proliferation and differentiation into AT1. However, the signaling mechanisms for alveolar repair remain unclear. Here, we demonstrate, in Pseudomonas aeruginosa-induced acute lung injury in mice, that non-canonical Notch ligand Dlk1 (delta-like 1 homolog) is essential for AT2-to-AT1 differentiation. Notch signaling was activated in AT2 at the onset of repair but later suppressed by Dlk1. Deletion of Dlk1 in AT2 induced persistent Notch activation, resulting in stalled transition to AT1 and accumulation of an intermediate cell population that expressed low levels of both AT1 and AT2 markers. Thus, Dlk1 expression leads to precisely timed inhibition of Notch signaling and activates AT2-to-AT1 differentiation, leading to alveolar repair.

The imprinted gene Delta like non-canonical notch ligand 1 (Dlk1) associates with obesity and triggers insulin resistance through inhibition of skeletal muscle glucose uptake

BACKGROUND

The imprinted gene Delta like non-canonical Notch ligand 1 (Dlk1) is considered an inhibitor of adipogenesis, but its in vivo impact on fat mass indeed remains elusive and controversial.

METHODS

Fat deposits were assessed by MRI and DXA scanning in two cohorts of non-diabetic men, whereas glucose disposal rate (GDR) was determined during euglycemic hyperinsulinemic clamp. Blood analyte measurements were used for correlation and mediation analysis to investigate how age, BMI, and fat percentage affect the relation between DLK1 and GDR. Confirmatory animal studies performed in normal (NC) and high fat diet (HFD) fed Dlk1^+/+ and Dlk1^-/- mice included DXA scanning, glucose tolerance tests (GTTs), blood measurements, and skeletal muscle glucose uptake studies by positron emission tomography (PET), histology, qRT-PCR, and in vitro cell studies.

FINDINGS

Overall, DLK1 is positively correlated with fat amounts, which is consistent with a negative linear relationship between DLK1 and GDR. This relationship is not mediated by age, BMI, or fat percentage. In support, DLK1 also correlates positively with HOMA-IR and ADIPO-IR in these humans, but has no linear relationship with the early diabetic inflammation marker MCP-1. In Dlk1^-/- mice, the increase in fat percentage and adipocyte size induced by HFD is attenuated, and these animals are protected against insulin resistance. These Dlk1 effects seem independent of gluconeogenesis, but at least partly relies on increased in vivo glucose uptake in skeletal muscles by Dlk1 regulating the major glucose transporter Glut4 in vivo as well as in two independent cell lines.

INTERPRETATION

Thus, instead of an adipogenic inhibitor, Dlk1 should be regarded as a factor causally linked to obesity and insulin resistance, and may be used to predict development of type 2 diabetes. FUND: The Danish Diabetes Academy supported by the Novo Nordisk Foundation, The Danish National Research Council (#09-073648), The Lundbeck Foundation, University of Southern Denmark, and Dep. Of Clinical Biochemistry and Pharmacology/Odense University Hospital, the Swedish Research Council, the Swedish Diabetes Foundation, the Strategic Research Program in Diabetes at Karolinska Institute and an EFSD/Lilly grant.

Expression and Functional Analyses of Dlk1 in Muscle Stem Cells and Mesenchymal Progenitors during Muscle Regeneration

Delta like non-canonical Notch ligand 1 (Dlk1) is a paternally expressed gene which is also known as preadipocyte factor 1 (Pref-1). The accumulation of adipocytes and expression of Dlk1 in regenerating muscle suggests a correlation between fat accumulation and Dlk1 expression in the muscle. Additionally, mice overexpressing Dlk1 show increased muscle weight, while Dlk1-null mice exhibit decreased body weight and muscle mass, indicating that Dlk1 is a critical factor in regulating skeletal muscle mass during development. The muscle regeneration process shares some features with muscle development. However, the role of Dlk1 in regeneration processes remains controversial. Here, we show that mesenchymal progenitors also known as adipocyte progenitors exclusively express Dlk1 during muscle regeneration. Eliminating developmental effects, we used conditional depletion models to examine the specific roles of Dlk1 in muscle stem cells or mesenchymal progenitors. Unexpectedly, deletion of Dlk1 in neither the muscle stem cells nor the mesenchymal progenitors affected the regenerative ability of skeletal muscle. In addition, fat accumulation was not increased by the loss of Dlk1. Collectively, Dlk1 plays essential roles in muscle development, but does not greatly impact regeneration processes and adipogenic differentiation in adult skeletal muscle regeneration.

Deletion of Dlk1 increases the vulnerability to developing anxiety-like behaviors and ethanol consumption in mice

Anxiety and alcohol use disorders (AUD) often present together, constituting a significant public health problem worldwide. In this study, we investigated the role of DLK1, a ligand of the Delta/NOTCH epidermal growth factor (EGF)-like protein family, reported to play a role in DA neurons differentiation in the striatum, as a neurobiological factor involved in the mechanisms regulating this psychiatric comorbidity. We exposed Dlk1 knockout mice (Dlk1-/- mice) to the open-field (OF), the light-dark box (LBD) and the elevated plus maze (EPM) tests, evaluating motivation to drink and ethanol consumption using the oral ethanol self-administration (OEA) paradigm. Quantitative real time polymerase chain reaction (qPCR) studies were carried out to evaluate alterations in targets closely related to DA neurotransmission in the reward system, tyrosine hydroxylase (Th) in the ventral tegmental area (VTA), and μ-opioid receptor (Oprm1) in the nucleus accumbens (NAc). No differences were observed in the total or peripheral distances travelled by Dlk1-/- compared to wild-type (WT) mice in OF. However, central distance travelled significantly decreased in Dlk1-/- mice. Deletion of Dlk1 increased anxiety-like behaviors in the LDB and EPM, and, Dlk1-/- mice also presented higher ethanol intake and motivation to drink (number of effective responses) in the OEA. In addition, Th and Oprm1 gene expression was reduced in the VTA and NAc of Dlk1-/- mice. We conclude that deletion of Dlk1 increases anxiety-related behaviors and vulnerability to ethanol consumption and modifies the gene expression of key targets closely related with DA neurotransmission involved in the reinforcing actions of ethanol.

Expression pattern of delta-like 1 homolog in developing sympathetic neurons and chromaffin cells

Delta-like 1 homolog (DLK1) is a member of the epidermal growth factor (EGF)-like family and an atypical notch ligand that is widely expressed during early mammalian development with putative functions in the regulation of cell differentiation and proliferation. During later stages of development, DLK1 is downregulated and becomes increasingly restricted to specific cell types, including several types of endocrine cells. DLK1 has been linked to various tumors and associated with tumor stem cell features. Sympathoadrenal precursors are neural crest derived cells that give rise to either sympathetic neurons of the autonomic nervous system or the endocrine chromaffin cells located in the adrenal medulla or extraadrenal positions. As these cells are the putative cellular origin of neuroblastoma, one of the most common malignant tumors in early childhood, their molecular characterization is of high clinical importance. In this study we have examined the precise spatiotemporal expression of DLK1 in developing sympathoadrenal cells. We show that DLK1 mRNA is highly expressed in early sympathetic neuron progenitors and that its expression depends on the presence of Phox2B. DLK1 expression becomes quickly restricted to a small subpopulation of cells in sympathetic ganglia, while virtually all chromaffin cells in the adrenal medulla and the Organ of Zuckerkandl still express high levels of DLK1 at late gestational stages.

Intrauterine hyperglycemia induces intergenerational Dlk1-Gtl2 methylation changes in mouse placenta

An intrauterine hyperglycemic environment has long-lasting effects on the offspring. Recent studies focused on fetal tissues, whereas we studied the development and molecular alteration of the placenta. By intercrossing male and female adult control (C) and first-generation offspring mice with gestational diabetes mellitus (F1-GDM), we obtained four groups of second generation (F2) offspring: 1) C♂-C♀, 2) C♂-GDM♀, 3) GDM♂-C♀, 4) GDM♂- GDM♀. Placental weights in F1-GDM offspring were lower than in the control group. Placental weights in F2-offspring decreased through the paternal line. Placental RNA was extracted and analyzed using microarrays on day18.5 of pregnancy. This revealed 35 upregulated imprinted genes and 10 down-regulated imprinted genes. Dlk1and Gtl2 were especially down-regulated and up-regulated, respectively, due to their abnormal methylation status. These findings suggest that intrauterine hyperglycemia decreased placental weight in the first generation, and this was transmitted paternally to the second generation in mice. They also suggest intrauterine hyperglycemia leads to abnormal placental Dlk1-Gtl2 expression due to DNA methylation in first and second generation mice.

Gene expression and epigenetic aberrations in F1-placentas fathered by obese males

Gene expression and/or epigenetic deregulation may have consequences for sperm and blastocysts, as well as for the placenta, together potentially contributing to problems observed in offspring. We previously demonstrated specific perturbations of fertilization, blastocyst formation, implantation, as well as aberrant glucose metabolism and adiposity in offspring using a mouse model of paternal obesity. The current investigation analyzed gene expression and methylation of specific CpG residues in F1 placentas of pregnancies fathered by obese and normal-weight male mice, using real-time PCR and bisulfite pyrosequencing. Our aim was to determine if paternal obesity deregulated placental gene expression and DNA methylation when compared to normal-weight males. Gene methylation of sperm DNA was analyzed and compared to placentas to address epigenetic transmission. Of the 10 paternally expressed genes (Pegs), 11 genes important for development and transport of nutrients, and the long-terminal repeat Intracisternal A particle (IAP) elements, derived from a member of the class II endogenous retroviral gene family, we observed a significant effect of paternal diet-induced obesity on deregulated expression of Peg3, Peg9, Peg10, and the nutrient transporter gene Slc38a2, and aberrant DNA methylation of the Peg9 promoter in F1 placental tissue. Epigenetic changes in Peg9 were also found in sperm from obese fathers. We therefore propose that paternal obesity renders changes in gene expression and/or methylation throughout the placental genome, which could contribute to the reproductive problems related to fertility and to the metabolic, long-term health impact on offspring.

Fetus-derived DLK1 is required for maternal metabolic adaptations to pregnancy and is associated with fetal growth restriction

Pregnancy is a state of high metabolic demand. Fasting diverts metabolism to fatty acid oxidation, and the fasted response occurs much more rapidly in pregnant women than in non-pregnant women. The product of the imprinted DLK1 gene (delta-like homolog 1) is an endocrine signaling molecule that reaches a high concentration in the maternal circulation during late pregnancy. By using mouse models with deleted Dlk1, we show that the fetus is the source of maternal circulating DLK1. In the absence of fetally derived DLK1, the maternal fasting response is impaired. Furthermore, we found that maternal circulating DLK1 levels predict embryonic mass in mice and can differentiate healthy small-for-gestational-age (SGA) infants from pathologically small infants in a human cohort. Therefore, measurement of DLK1 concentration in maternal blood may be a valuable method for diagnosing human disorders associated with impaired DLK1 expression and to predict poor intrauterine growth and complications of pregnancy.

Preadipocyte factor 1 induces pancreatic ductal cell differentiation into insulin-producing cells

The preadipocyte factor 1 (Pref-1) is involved in the proliferation and differentiation of various precursor cells. However, the intracellular signaling pathways that control these processes and the role of Pref-1 in the pancreas remain poorly understood. Here, we showed that Pref-1 induces insulin synthesis and secretion via two independent pathways. The overexpression of Pref-1 activated MAPK signaling, which induced nucleocytoplasmic translocation of FOXO1 and PDX1 and led to the differentiation of human pancreatic ductal cells into β-like cells and an increase in insulin synthesis. Concurrently, Pref-1 activated Akt signaling and facilitated insulin secretion. A proteomics analysis identified the Rab43 GTPase-activating protein as a downstream target of Akt. A serial activation of both proteins induced various granular protein syntheses which led to enhanced glucose-stimulated insulin secretion. In a pancreatectomised diabetic animal model, exogenous Pref-1 improved glucose homeostasis by accelerating pancreatic ductal and β-cell regeneration after injury. These data establish a novel role for Pref-1, opening the possibility of applying this molecule to the treatment of diabetes.

Molecular basis of imprinting disorders affecting chromosome 14: lessons from murine models

Uniparental inheritance of chromosome 14q32 causes developmental failure during gestation and early postnatal development due to mis-expression of a cluster of imprinted genes under common epigenetic control. Two syndromes associated with chromosome 14q32 abnormalities have been described, Kagami–Ogata and Temple syndromes. Both of these syndromes are characterised by specific impairments of intrauterine development, placentation and early postnatal survival. Such abnormalities arise because the processes of intrauterine growth and postnatal adaptation are critically modulated by the dosage of imprinted genes in the chromosome 14q32 cluster. Much of our understanding of how the imprinted genes in this cluster are regulated, as well as their individual functions in the molecular pathways controlling growth and postnatal adaptation, has come from murine models. Mouse chromosome 12qF1 contains an imprinted region syntenic to human chromosome 14q32, collectively referred to as the Dlk1–Dio3 cluster. In this review, we will summarise the wealth of information derived from animal models of chromosome 12 imprinted gene mis-regulation, and explore the relationship between the functions of individual genes and the phenotypic result of their mis-expression. As there is often a considerable overlap between the functions of genes in the Dlk1–Dio3 cluster, we propose that the expression dosage of these genes is controlled by common regulatory mechanisms to co-ordinate the timing of growth and postnatal adaptation. While the diseases associated with mis-regulated chromosome 14 imprinting are rare, studies carried out in mice on the functions of the affected genes as well as their normal regulatory mechanisms have revealed new mechanistic pathways for the control of growth and survival in early life.

Maternal and zygotic Zfp57 modulate NOTCH signaling in cardiac development

Zfp57 is a maternal-zygotic effect gene that maintains genomic imprinting. Here we report that Zfp57 mutants exhibited a variety of cardiac defects including atrial septal defect (ASD), ventricular septal defect (VSD), thin myocardium, and reduced trabeculation. Zfp57 maternal-zygotic mutant embryos displayed more severe phenotypes with higher penetrance than the zygotic ones. Cardiac progenitor cells exhibited proliferation and differentiation defects in Zfp57 mutants. ZFP57 is a master regulator of genomic imprinting, so the DNA methylation imprint was lost in embryonic heart without ZFP57. Interestingly, the presence of imprinted DLK1, a target of ZFP57, correlated with NOTCH1 activation in cardiac cells. These results suggest that ZFP57 may modulate NOTCH signaling during cardiac development. Indeed, loss of ZFP57 caused loss of NOTCH1 activation in embryonic heart with more severe loss observed in the maternal-zygotic mutant. Maternal and zygotic functions of Zfp57 appear to play redundant roles in NOTCH1 activation and cardiomyocyte differentiation. This serves as an example of a maternal effect that can influence mammalian organ development. It also links genomic imprinting to NOTCH signaling and particular developmental functions.

What does genetics tell us about imprinting and the placenta connection?

Genomic imprinting is an epigenetic gene silencing phenomenon that is specific to eutherians in the vertebrate lineage. The acquisition of both placentation and genomic imprinting has spurred interest in the possible evolutionary link for many years. In this review we examine the genetic evidence and find that while many imprinted domains are anchored by genes required for proper placenta development in a parent of origin fashion, an equal number of imprinted genes have no apparent function that depends on imprinting. Examination of recent data from studies of molecular and genetic mechanisms points to a maternal control of the selection and maintenance of imprint marks, reinforcing the importance of the oocyte in the healthy development of the placenta and fetus.

Antagonistic roles in fetal development and adult physiology for the oppositely imprinted Grb10 and Dlk1 genes

Background

Despite being a fundamental biological problem the control of body size and proportions during development remains poorly understood, although it is accepted that the insulin-like growth factor (IGF) pathway has a central role in growth regulation, probably in all animals. The involvement of imprinted genes has also attracted much attention, not least because two of the earliest discovered were shown to be oppositely imprinted and antagonistic in their regulation of growth. The Igf2 gene encodes a paternally expressed ligand that promotes growth, while maternally expressed Igf2r encodes a cell surface receptor that restricts growth by sequestering Igf2 and targeting it for lysosomal degradation. There are now over 150 imprinted genes known in mammals, but no other clear examples of antagonistic gene pairs have been identified. The delta-like 1 gene (Dlk1) encodes a putative ligand that promotes fetal growth and in adults restricts adipose deposition. Conversely, Grb10 encodes an intracellular signalling adaptor protein that, when expressed from the maternal allele, acts to restrict fetal growth and is permissive for adipose deposition in adulthood.

Results

Here, using knockout mice, we present genetic and physiological evidence that these two factors exert their opposite effects on growth and physiology through a common signalling pathway. The major effects are on body size (particularly growth during early life), lean:adipose proportions, glucose regulated metabolism and lipid storage in the liver. A biochemical pathway linking the two cell signalling factors remains to be defined.

Conclusions

We propose that Dlk1 and Grb10 define a mammalian growth axis that is separate from the IGF pathway, yet also features an antagonistic imprinted gene pair.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-014-0099-8) contains supplementary material, which is available to authorized users.

DLK1/PREF1 regulates nutrient metabolism and protects from steatosis

Nonalcoholic fatty liver disease (NAFLD) is associated with insulin resistance and obesity, as well as progressive liver dysfunction. Recent animal studies have underscored the importance of hepatic growth hormone (GH) signaling in the development of NAFLD. The imprinted Delta-like homolog 1 (Dlk1)/preadipocyte factor 1 (Pref1) gene encodes a complex protein producing both circulating and membrane-tethered isoforms whose expression dosage is functionally important because even modest elevation during embryogenesis causes lethality. DLK1 is up-regulated during embryogenesis, during suckling, and in the mother during pregnancy. We investigated the normal role for elevated DLK1 dosage by overexpressing Dlk1 from endogenous control elements. This increased DLK1 dosage caused improved glucose tolerance with no primary defect in adipose tissue expansion even under extreme metabolic stress. Rather, Dlk1 overexpression caused reduced fat stores, pituitary insulin-like growth factor 1 (IGF1) resistance, and a defect in feedback regulation of GH. Increased circulatory GH culminated in a switch in whole body fuel metabolism and a reduction in hepatic steatosis. We propose that the function of DLK1 is to shift the metabolic mode of the organism toward peripheral lipid oxidation and away from lipid storage, thus mediating important physiological adaptations associated with early life and with implications for metabolic disease resistance.

The Missing lnc(RNA) between the pancreatic β-cell and diabetes

Diabetes mellitus represents a group of complex metabolic diseases that result in impaired glucose homeostasis, which includes destruction of β-cells or the failure of these insulin-secreting cells to compensate for increased metabolic demand. Despite a strong interest in characterizing the transcriptome of the different human islet cell types to understand the molecular basis of diabetes, very little attention has been paid to the role of long non-coding RNAs (lncRNAs) and their contribution to this disease. Here we summarize the growing evidence for the potential role of these lncRNAs in β-cell function and dysregulation in diabetes, with a focus on imprinted genomic loci.

Dlk1 promotes a fast motor neuron biophysical signature required for peak force execution

Motor neurons, which relay neural commands to drive skeletal muscle movements, encompass types ranging from "slow" to "fast," whose biophysical properties govern the timing, gradation, and amplitude of muscle force. Here we identify the noncanonical Notch ligand Delta-like homolog 1 (Dlk1) as a determinant of motor neuron functional diversification. Dlk1, expressed by ~30% of motor neurons, is necessary and sufficient to promote a fast biophysical signature in the mouse and chick. Dlk1 suppresses Notch signaling and activates expression of the K(+) channel subunit Kcng4 to modulate delayed-rectifier currents. Dlk1 inactivation comprehensively shifts motor neurons toward slow biophysical and transcriptome signatures, while abolishing peak force outputs. Our findings provide insights into the development of motor neuron functional diversity and its contribution to the execution of movements.

Identifying placental epigenetic alterations in an intrauterine growth restriction (IUGR) rat model induced by gestational protein deficiency

Poor maternal nutrition during gestation can lead to intrauterine growth retardation (IUGR), a main cause of low birth weight associated with high neonatal morbidity and mortality. Such early uterine environmental exposures can impact the neonatal epigenome to render later-in-life disease susceptibility. We established in Wistar Han rats a mild IUGR model induced by gestational protein deficiency (i.e. 9% crude protein in low protein diet vs. 21% in control, from GD 0 to 21) to identify alterations in gene expression and methylation patterns in certain genes implicated in human IUGR or in placental development. We found differential gene expression of Wnt2 and Dlk1 between IUGR and control. Notably, Wnt2 exhibited significant decrease while Dlk1 increase in IUGR placentas, correlating to decrease in fetal and placental weight. Methylation patterns encompassing 30 CpGs in the Wnt2 promoter region revealed variability in both IUGR and control placentas, but a site-specific hypomethylation was evident in IUGR placentas. Our present findings further support a key role of maternal gestational nutrition in defining the neonatal epigenome.

MicroRNA-126-5p promotes endothelial proliferation and limits atherosclerosis by suppressing Dlk1

Atherosclerosis, a hyperlipidemia-induced chronic inflammatory process of the arterial wall, develops preferentially at sites where disturbed laminar flow compromises endothelial cell (EC) function. Here we show that endothelial miR-126-5p maintains a proliferative reserve in ECs through suppression of the Notch1 inhibitor delta-like 1 homolog (Dlk1) and thereby prevents atherosclerotic lesion formation. Endothelial recovery after denudation was impaired in Mir126(-/-) mice because lack of miR-126-5p, but not miR-126-3p, reduced EC proliferation by derepressing Dlk1. At nonpredilection sites, high miR-126-5p levels in endothelial cells confer a proliferative reserve that compensates for the antiproliferative effects of hyperlipidemia, such that atherosclerosis was exacerbated in Mir126(-/-) mice. In contrast, downregulation of miR-126-5p by disturbed flow abrogated EC proliferation at predilection sites in response to hyperlipidemic stress through upregulation of Dlk1 expression. Administration of miR-126-5p rescued EC proliferation at predilection sites and limited atherosclerosis, introducing a potential therapeutic approach.

The pancreatic β cell and type 1 diabetes: innocent bystander or active participant?

Type 1 diabetes mellitus (T1DM) is a chronic disease resulting from destruction of insulin-producing pancreatic β cells. Genetic and environmental factors contribute to T1DM onset. Use of high-throughput DNA sequencing has allowed geneticists to perform genome-wide association studies (GWAS) to identify novel gene loci associated with T1DM. Interestingly, >50% of these genes encode products that are expressed in β cells. These studies, coupled with emerging molecular evidence that β cells are impaired by gain-of-function or loss-of-function of these loci, suggest an active role for the β cell in eliciting its own demise. Although immune dysregulation plays a vital role in T1DM pathogenesis, understanding the mechanisms contributing to β cell failure may lead to new strategies to preserve or improve β cell function in patients with T1DM.

An obesogenic diet during mouse pregnancy modifies maternal nutrient partitioning and the fetal growth trajectory

In developed societies, high-sugar and high-fat (HSHF) diets are now the norm and are increasing the rates of maternal obesity during pregnancy. In pregnant rodents, these diets lead to cardiovascular and metabolic dysfunction in their adult offspring, but the intrauterine mechanisms involved remain unknown. This study shows that, relative to standard chow, HSHF feeding throughout mouse pregnancy increases maternal adiposity (+30%, P<0.05) and reduces fetoplacental growth at d 16 (-10%, P<0.001). At d 19, however, HSHF diet group pup weight had normalized, despite the HSHF diet group placenta remaining small and morphologically compromised. This altered fetal growth trajectory was associated with enhanced placental glucose and amino acid transfer (+35%, P<0.001) and expression of their transporters (+40%, P<0.024). HSHF feeding also up-regulated placental expression of fatty acid transporter protein, metabolic signaling pathways (phosphoinositol 3-kinase and mitogen-activated protein kinase), and several growth regulatory imprinted genes (Igf2, Dlk1, Snrpn, Grb10, and H19) independently of changes in DNA methylation. Obesogenic diets during pregnancy, therefore, alter maternal nutrient partitioning, partly through changes in the placental phenotype, which helps to meet fetal nutrient demands for growth near term. However, by altering provision of specific nutrients, dietary-induced placental adaptations have important roles in programming development with health implications for the offspring in later life.

Pituitary phenotypes of mice lacking the notch signalling ligand delta-like 1 homologue

The Notch signalling pathway ligand delta-like 1 homologue (Dlk1, also named Pref1) is expressed throughout the developing pituitary and becomes restricted to mostly growth hormone (GH) cells within the adult gland. We have investigated the role of Dlk1 in pituitary development and function from late embryogenesis to adulthood using a mouse model completely lacking the expression of Dlk1. We confirm that Dlk1-null mice are shorter and weigh less than wild-type littermates from late gestation, at parturition and in adulthood. A loss of Dlk1 leads to significant reduction in GH content throughout life, whereas other pituitary hormones are reduced to varying degrees depending on sex and age. Both the size of the pituitary and the proportion of hormone-producing cell populations are unchanged, suggesting that there is a reduction in hormone content per cell. In vivo challenge of mutant and wild-type littermates with growth hormone-releasing hormone and growth hormone-releasing hexapeptide shows that reduced GH secretion is unlikely to account for the reduced growth of Dlk1 knockout animals. These data suggest that loss of Dlk1 gives rise to minor pituitary defects manifesting as an age- and sex-dependent reduction in pituitary hormone contents. However, Dlk1 expression in other tissue is most likely responsible for the weight and length differences observed in mutant animals.

Pref-1, a gatekeeper of adipogenesis

Preadipocyte factor 1 (Pref-1, also called Dlk1/FA1) is a molecular gatekeeper of adipogenesis which acts by maintaining the preadipocyte state and preventing adipocyte differentiation. Pref-1 is made as an epidermal growth factor-like repeat containing transmembrane protein, and is cleaved by TNFα-converting enzyme (TACE) to generate a soluble form, which acts as an autocrine/paracrine factor. Pref-1 upregulates Sox9 expression by activating the ERK/MAPK pathway and the Pref-1 interaction with fibronectin is required for inhibition of adipogenesis. Pref-1 also prevents brown adipocyte differentiation and its thermogenic function. Here, we highlight the recent evidence for the role of Pref-1 in adipogenesis.