Fibroblasts constitute the major mesenchymal cell type in the connective tissue and their functions are remarkably diverse: here, by characterising lineages of mouse skin fibroblasts, it is shown that distinct subpopulations contribute to skin development and repair during injury. Fibroblasts are unremarkable looking cells found in most tissues in the body, where they are mainly concerned with making the collagen that supports other cell types. The cells all look much the same yet are functionally diverse, prompting the question, is there just one cell type responding differently to different stimuli, or do individual cells specialize? A transplantation and lineage tracing study in mice now shows that skin connective tissue arises from two distinct fibroblast lineages that also contribute differentially to skin development and repair after injury. One cell type forms the lower dermis and the other the upper dermis. The latter lineage is required for hair follicle production. In wounded adult skin, the initial wave of dermal repair is mediated by the 'lower' lineage, which may explain the absence of hair follicles in newly closed wounds. The authors develop a comprehensive lineage tree for all fibroblast-derived cell types in mouse dermis, including smooth muscle cells and adipocytes. Fibroblasts are the major mesenchymal cell type in connective tissue and deposit the collagen and elastic fibres of the extracellular matrix (ECM)1. Even within a single tissue, fibroblasts exhibit considerable functional diversity, but it is not known whether this reflects the existence of a differentiation hierarchy or is a response to different environmental factors. Here we show, using transplantation assays and lineage tracing in mice, that the fibroblasts of skin connective tissue arise from two distinct lineages. One forms the upper dermis, including the dermal papilla that regulates hair growth and the arrector pili muscle, which controls piloerection. The other forms the lower dermis, including the reticular fibroblasts that synthesize the bulk of the fibrillar ECM, and the preadipocytes and adipocytes of the hypodermis. The upper lineage is required for hair follicle formation. In wounded adult skin, the initial wave of dermal repair is mediated by the lower lineage and upper dermal fibroblasts are recruited only during re-epithelialization. Epidermal β-catenin activation stimulates the expansion of the upper dermal lineage, rendering wounds permissive for hair follicle formation. Our findings explain why wounding is linked to formation of ECM-rich scar tissue that lacks hair follicles2,3,4. They also form a platform for discovering fibroblast lineages in other tissues and for examining fibroblast changes in ageing and disease.

Life-long neurogenesis is known to occur in some areas of the mammalian adult brain, including in the subventricular zone (SVZ), where interaction between neural precursor cells and resident astrocytes can produced migratory neuroblasts. Here, Anne Ferguson-Smith and colleagues reveal a differential role for the same developmental gene, Dlk1, in neural precursor cells and astrocytes. The gene product, the Notch ligand DLK1, has two isoforms. One is an inductive niche factor secreted by astrocytes, and the other is a membrane-bound isoform required by the neural stem cells themselves to respond to secreted DLK1. Selective changes of imprinting with age modulate the role of Dlk1 in each cell type in mice, regulating neurogenesis when establishing the adult neurogenic niche. The modulation of genomic imprinting in a stem-cell environment adds a previously unrecognized element to epigenetic regulation. The gene for the atypical NOTCH ligand delta-like homologue 1 (Dlk1) encodes membrane-bound and secreted isoforms that function in several developmental processes in vitro and in vivo. Dlk1, a member of a cluster of imprinted genes, is expressed from the paternally inherited chromosome1,2. Here we show that mice that are deficient in Dlk1 have defects in postnatal neurogenesis in the subventricular zone: a developmental continuum that results in depletion of mature neurons in the olfactory bulb. We show that DLK1 is secreted by niche astrocytes, whereas its membrane-bound isoform is present in neural stem cells (NSCs) and is required for the inductive effect of secreted DLK1 on self-renewal. Notably, we find that there is a requirement for Dlk1 to be expressed from both maternally and paternally inherited chromosomes. Selective absence of Dlk1 imprinting in both NSCs and niche astrocytes is associated with postnatal acquisition of DNA methylation at the germ-line-derived imprinting control region. The results emphasize molecular relationships between NSCs and the niche astrocyte cells of the microenvironment, identifying a signalling system encoded by a single gene that functions coordinately in both cell types. The modulation of genomic imprinting in a stem-cell environment adds a new level of epigenetic regulation to the establishment and maintenance of the niche, raising wider questions about the adaptability, function and evolution of imprinting in specific developmental contexts.

OBJECTIVE—Low birth weight (LBW) is associated with increased risk of obesity, diabetes, and cardiovascular disease during adult life. Moreover, this programmed disease risk can progress to subsequent generations. We previously described a mouse model of LBW, produced by maternal caloric undernutrition (UN) during late gestation. LBW offspring (F1-UN generation) develop progressive obesity and impaired glucose tolerance (IGT) with aging. We aimed to determine whether such metabolic phenotypes can be transmitted to subsequent generations in an experimental model, even in the absence of altered nutrition during the second pregnancy. RESEARCH DESIGN AND METHODS—We intercrossed female and male F1 adult control (C) and UN mice and characterized metabolic phenotypes in F2 offspring. RESULTS—We demonstrate that 1) reduced birth weight progresses to F2 offspring through the paternal line (C♀-C♂ = 1.64 g; C♀-UN♂ = 1.57 g, P < 0.05; UN♀-C♂ = 1.64 g; UN♀-UN♂ = 1.60 g, P < 0.05), 2) obesity progresses through the maternal line (percent body fat: C♀-C♂ = 22.4%; C♀-UN♂ = 22.9%; UN♀-C♂ = 25.9%, P < 0.05; UN♀-UN♂ = 27.5%, P < 0.05), and 3) IGT progresses through both parental lineages (glucose tolerance test area under curve C♀-C♂ = 100; C♀-UN♂ = 122, P < 0.05; UN♀-C♂ = 131, P < 0.05; UN♀-UN♂ = 151, P < 0.05). Mechanistically, IGT in both F1 and F2 generations is linked to impaired β-cell function, explained, in part, by dysregulation of Sur1 expression. CONCLUSIONS—Maternal undernutrition during pregnancy (F0) programs reduced birth weight, IGT, and obesity in both first- and second-generation offspring. Sex-specific transmission of phenotypes implicates complex mechanisms including alterations in the maternal metabolic environment (transmaternal inheritance of obesity), gene expression mediated by developmental and epigenetic pathways (transpaternal inheritance of LBW), or both (IGT).

To investigate the function of the Grb10 adapter protein, we have generated mice in which the Grb10 gene was disrupted by a gene-trap insertion. Our experiments confirm that Grb10 is subject to genomic imprinting with the majority of Grb10 expression arising from the maternally inherited allele. Consistent with this, disruption of the maternal allele results in overgrowth of both the embryo and placenta such that mutant mice are at birth ≈30% larger than normal. This observation establishes that Grb10 is a potent growth inhibitor. In humans, GRB10 is located at chromosome 7p11.2–p12 and has been associated with Silver–Russell syndrome, in which ≈10% of those affected inherit both copies of chromosome 7 from their mother. Our results indicate that changes in GRB10 dosage could, in at least some cases, account for the severe growth retardation that is characteristic of Silver–Russell syndrome. Because Grb10 is a signaling protein capable of interacting with tyrosine kinase receptors, we tested genetically whether Grb10 might act downstream of insulin-like growth factor 2, a paternally expressed growth-promoting gene. The result indicates that Grb10 action is essentially independent of insulin-like growth factor 2, providing evidence that imprinting acts on at least two major fetal growth axes in a manner consistent with parent–offspring conflict theory.

The Grb10 adapter protein is capable of interacting with a variety of receptor tyrosine kinases, including, notably, the insulin receptor. Biochemical and cell culture experiments have indicated that Grb10 might act as an inhibitor of insulin signaling. We have used mice with a disruption of the Grb10 gene (Grb10Delta2-4 mice) to assess whether Grb10 might influence insulin signaling and glucose homeostasis in vivo. Adult Grb10Delta2-4 mice were found to have improved whole-body glucose tolerance and insulin sensitivity, as well as increased muscle mass and reduced adiposity. Tissue-specific changes in insulin receptor tyrosine phosphorylation were consistent with a model in which Grb10, like the closely related Grb14 adapter protein, prevents specific protein tyrosine phosphatases from accessing phosphorylated tyrosines within the kinase activation loop. Furthermore, insulin-induced IRS-1 tyrosine phosphorylation was enhanced in Grb10Delta2-4 mutant animals, supporting a role for Grb10 in attenuation of signal transmission from the insulin receptor to IRS-1. We have previously shown that Grb10 strongly influences growth of the fetus and placenta. Thus, Grb10 forms a link between fetal growth and glucose-regulated metabolism in postnatal life and is a candidate for involvement in the process of fetal programming of adult metabolic health.

Visceral adipose tissue (VAT) has multiple roles in orchestrating whole-body energy homeostasis. In addition, VAT is now considered an immune site harboring an array of innate and adaptive immune cells with a direct role in immune surveillance and host defense. We report that conventional dendritic cells (cDCs) in VAT acquire a tolerogenic phenotype through upregulation of pathways involved in adipocyte differentiation. While activation of the Wnt/β-catenin pathway in cDC1 DCs induces IL-10 production, upregulation of the PPARγ pathway in cDC2 DCs directly suppresses their activation. Combined, they promote an anti-inflammatory milieu in vivo delaying the onset of obesity-induced chronic inflammation and insulin resistance. Under long-term over-nutrition, changes in adipocyte biology curtail β-catenin and PPARγ activation, contributing to VAT inflammation.