Prdm12 represses the expression of the visceral neuron determinants Phox2a/b in developing somatosensory ganglia

Prdm12 is a transcriptional regulator essential for the emergence of the somatic nociceptive lineage during sensory neurogenesis. The exact mechanisms by which Prdm12 promotes nociceptor development remain, however, poorly understood. Here, we report that the trigeminal and dorsal root ganglia hypoplasia induced by the loss of Prdm12 involves Bax-dependent apoptosis and that it is accompanied by the ectopic expression of the visceral sensory neuron determinants Phox2a and Phox2b, which is, however, not sufficient to impose a complete fate switch in surviving somatosensory neurons. Mechanistically, our data reveal that Prdm12 is required from somatosensory neural precursors to early post-mitotic differentiating nociceptive neurons to repress Phox2a/b and that its repressive function is context dependent. Together, these findings reveal that besides its essential role in nociceptor survival during development, Prdm12 also promotes nociceptor fate via an additional mechanism, by preventing precursors from engaging into an alternate Phox2 driven visceral neuronal type differentiation program.

Axon guidance genes control hepatic artery development

Earlier data on liver development demonstrated that morphogenesis of the bile duct, portal mesenchyme and hepatic artery is interdependent, yet how this interdependency is orchestrated remains unknown. Here, using 2D and 3D imaging, we first describe how portal mesenchymal cells become organised to form hepatic arteries. Next, we examined intercellular signalling active during portal area development and found that axon guidance genes are dynamically expressed in developing bile ducts and portal mesenchyme. Using tissue-specific gene inactivation in mice, we show that the repulsive guidance molecule BMP co-receptor A (RGMA)/neogenin (NEO1) receptor/ligand pair is dispensable for portal area development, but that deficient roundabout 2 (ROBO2)/SLIT2 signalling in the portal mesenchyme causes reduced maturation of the vascular smooth muscle cells that form the tunica media of the hepatic artery. This arterial anomaly does not impact liver function in homeostatic conditions, but is associated with significant tissular damage following partial hepatectomy. In conclusion, our work identifies new players in development of the liver vasculature in health and liver regeneration.

Variations in the poly-histidine repeat motif of HOXA1 contribute to bicuspid aortic valve in mouse and zebrafish

Bicuspid aortic valve (BAV), the most common cardiovascular malformation occurs in 0.5-1.2% of the population. Although highly heritable, few causal mutations have been identified in BAV patients. Here, we report the targeted sequencing of HOXA1 in a cohort of BAV patients and the identification of rare indel variants in the homopolymeric histidine tract of HOXA1. In vitro analysis shows that disruption of this motif leads to a significant reduction in protein half-life and defective transcriptional activity of HOXA1. In zebrafish, targeting hoxa1a ortholog results in aortic valve defects. In vivo assays indicates that these variants behave as dominant negatives leading abnormal valve development. In mice, deletion of Hoxa1 leads to BAV with a very small, rudimentary non-coronary leaflet. We also show that 17% of homozygous Hoxa1^-1His knock-in mice present similar phenotype. Genetic lineage tracing in Hoxa1^-/- mutant mice reveals an abnormal reduction of neural crest-derived cells in the valve leaflet, which is caused by a failure of early migration of these cells.

BRAFV600E Expression in Thyrocytes Causes Recruitment of Immunosuppressive STABILIN-1 Macrophages

Simple Summary

Incidence of thyroid cancer, including papillary thyroid cancer, is rapidly increasing. Oncogenes, such as the BRAF^V600E, have been identified, and their effect on thyroid cancer cells have been studied in vitro and in mouse models. What is less understood is the impact of these mutations on thyroid cancer microenvironment and, in turn, the effect of changes in the microenvironment on tumor progression. We investigated the modifications in the cellular composition of thyroid cancer microenvironment using an inducible mouse model. We focused on a subpopulation of macrophages, expressing the STABILIN-1 protein, recruited in the thyroid tumor microenvironment following BRAF^V600E expression. CRISPR/Cas9 genetic inactivation of Stablin-1 did not change macrophage recruitment but highlighted the immunosuppressive role of STABILIN-1-expressing macrophages. The identification of a similar subpopulation of STABILIN-1 macrophages in human thyroid diseases supports a conserved role for these macrophages and offers an opportunity for intervention.

Abstract

Papillary thyroid carcinoma (PTC) is the most frequent histological subtype of thyroid cancers (TC), and BRAF^V600E genetic alteration is found in 60% of this endocrine cancer. This oncogene is associated with poor prognosis, resistance to radioiodine therapy, and tumor progression. Histological follow-up by anatomo-pathologists revealed that two-thirds of surgically-removed thyroids do not present malignant lesions. Thus, continued fundamental research into the molecular mechanisms of TC downstream of BRAF^V600E remains central to better understanding the clinical behavior of these tumors. To study PTC, we used a mouse model in which expression of BRAF^V600E was specifically switched on in thyrocytes by doxycycline administration. Upon daily intraperitoneal doxycycline injection, thyroid tissue rapidly acquired histological features mimicking human PTC. Transcriptomic analysis revealed major changes in immune signaling pathways upon BRAF^V600E induction. Multiplex immunofluorescence confirmed the abundant recruitment of macrophages, among which a population of LYVE-1+/CD206+/STABILIN-1+ was dramatically increased. By genetically inactivating the gene coding for the scavenger receptor STABILIN-1, we showed an increase of CD8+ T cells in this in situ BRAF^V600E-dependent TC. Lastly, we demonstrated the presence of CD206+/STABILIN-1+ macrophages in human thyroid pathologies. Altogether, we revealed the recruitment of immunosuppressive STABILIN-1 macrophages in a PTC mouse model and the interest to further study this macrophage subpopulation in human thyroid tissues.

A Targetable, Noncanonical Signal Transducer and Activator of Transcription 3 Activation Induced by the Y-Less Region of IL-22 Receptor Orchestrates Imiquimod-Induced Psoriasis-Like Dermatitis in Mice

Exacerbated IL-22 activity induces tissue inflammation and immune disorders such as psoriasis. However, because IL-22 is also essential for tissue repair and defense at barrier interfaces, targeting IL-22 activity to treat psoriasis bears the risk of deleterious effects at mucosal sites such as the gut. We previously showed in vitro that IL-22 signaling relies on IL-22 receptor alpha (IL-22Rα) Y-dependent and -independent pathways. The second depends on the C-terminal Y-less region of IL-22Rα and leads to a massive signal transducer and activator of transcription 3 (STAT3) activation. Because STAT3 activation is associated with the development of psoriasis, we hypothesized that the specific inhibition of the noncanonical STAT3 activation by the Y-less region of IL-22Rα could reduce psoriasis-like disease while leaving intact its tissue defense functions in the gut. We show that mice expressing a C-terminally truncated version of IL-22Rα (ΔCtermut/mut mice) are protected from the development of psoriasis-like dermatitis lesions induced by imiquimod to a lesser extent than Il22ra-/- mice. In contrast, only Il22ra-/- mice lose weight after Citrobacter rodentium infection. Altogether, our data suggest that specific targeting of the noncanonical STAT3 activation by IL-22 could serve to treat psoriasis-like skin inflammation without affecting IL-22‒dependent tissue repair or barrier defense at other sites.

Spatio-temporal expression pattern and role of the tight junction protein MarvelD3 in pancreas development and function

Tight junction complexes are involved in the establishment and maintenance of cell polarity and the regulation of signalling pathways, controlling biological processes such as cell differentiation and cell proliferation. MarvelD3 is a tight junction protein expressed in adult epithelial and endothelial cells. In Xenopus laevis, MarvelD3 morphants present differentiation defects of several ectodermal derivatives. In vitro experiments further revealed that MarvelD3 couples tight junctions to the MEKK1-JNK pathway to regulate cell behaviour and survival. In this work, we found that MarvelD3 is expressed from early developmental stages in the exocrine and endocrine compartments of the pancreas, as well as in endothelial cells of this organ. We thoroughly characterized MarvelD3 expression pattern in developing pancreas and evaluated its function by genetic ablation. Surprisingly, inactivation of MarvelD3 in mice did not alter development and differentiation of the pancreatic tissue. Moreover, tight junction formation and organization, cell polarization, and activity of the JNK-pathway were not impacted by the deletion of MarvelD3.

Generation and characterization of a tamoxifen-inducible Vsx1-CreERT2 line to target V2 interneurons in the mouse developing spinal cord

In the spinal cord, ventral interneurons regulate the activity of motor neurons, thereby controlling motor activities including locomotion. Interneurons arise during embryonic development from distinct progenitor domains orderly distributed along the dorso-ventral axis of the neural tube. The p2 progenitor domain generates at least five V2 interneuron populations. However, identification and characterization of all V2 populations remain currently incomplete and the mechanisms that control their development remain only partly understood. In this study, we report the generation of a Vsx1-CreER^T2 BAC transgenic mouse line that drives CreER^T2 recombinase expression mimicking endogenous Vsx1 expression pattern in the developing spinal cord. We showed that the Vsx1-CreER^T2 transgene can mediate recombination in V2 precursors with a high efficacy and specificity. Lineage tracing demonstrated that all the V2 interneurons in the mouse developing spinal cord derive from cells expressing Vsx1. Finally, we confirmed that V2 precursors generate additional V2 populations that are not characterized yet. Thus, the Vsx1-CreER^T2 line described here is a useful genetic tool for lineage tracing and for functional studies of the mouse spinal V2 interneurons.

Dynamic Regulation of Expression of KRAS and Its Effectors Determines the Ability to Initiate Tumorigenesis in Pancreatic Acinar Cells

Pancreatic acinar cells are a cell type of origin for pancreatic cancer that become progressively less sensitive to tumorigenesis induced by oncogenic Kras mutations after birth. This sensitivity is increased when Kras mutations are combined with pancreatitis. Molecular mechanisms underlying these observations are still largely unknown. To identify these mechanisms, we generated the first CRISPR-edited mouse models that enable detection of wild-type and mutant KRAS proteins in vivo. Analysis of these mouse models revealed that more than 75% of adult acinar cells are devoid of detectable KRAS protein. In the 25% of acinar cells expressing KRAS protein, transcriptomic analysis highlighted a slight upregulation of the RAS and MAPK pathways. However, at the protein level, only marginal pancreatic expression of essential KRAS effectors, including C-RAF, was observed. The expression of KRAS and its effectors gradually decreased after birth. The low sensitivity of adult acinar cells to Kras mutations resulted from low expression of KRAS and its effectors and the subsequent lack of activation of RAS/MAPK pathways. Pancreatitis triggered expression of KRAS and its effectors as well as subsequent activation of downstream signaling; this induction required the activity of EGFR. Finally, expression of C-RAF in adult pancreas was required for pancreatic tumorigenesis. In conclusion, our study reveals that control of the expression of KRAS and its effectors regulates the sensitivity of acinar cells to transformation by oncogenic Kras mutations. SIGNIFICANCE: This study generates new mouse models to study regulation of KRAS during pancreatic tumorigenesis and highlights a novel mechanism through which pancreatitis sensitizes acinar cells to Kras mutations.

Knockout of hyaluronidase Spam1 reduces age-related bone and cartilage changes in mouse knee

OBJECTIVE

To investigate the role of Spam1 hyaluronidase in age-related bone and cartilage changes in the mouse knee.

DESIGN

Spam1^-/- and WT mice were euthanised at different ages from 10 to 52 weeks. The right hindlimbs were dissected, scanned with peripheral Quantitative Computed Tomography (pQCT) and then decalcified for histological analysis (modified Mankin score). In other mice, cartilages of both tibiae were sampled at 10, 30 and 52 weeks of age for RNA extraction and qPCR analysis. We assessed the expression of hyaluronidases Hyal1 and Hyal2, hyaluronan synthase HAS2, extracellular matrix proteases Mmp13 and Adamts-5, and type 2 collagen.

RESULTS

Spam1^-/- mice did not exhibit specific morphological characters up to 52 weeks of age. From 20 weeks, the proximal tibia of Spam1^-/- mice had a significantly lower bone mineral density than WT mice. At 52 weeks, the modified Mankin score was significantly lower in Spam1^-/- than WT mice. Spam1^-/- chondrocytes expressed significantly less Hyal2 than WT ones at all ages and less Mmp13 at 52 weeks. Through all the experiment, the Hyal1 expression of Spam1^-/- chondrocytes remained similar as that of WT chondrocytes.

CONCLUSION

Spam1 knockout reduced significantly cartilage degradation in mouse knee whereas the chondrocyte expression of Hyal 1, Hyal 2 and Mmp13 was modified, suggesting a role of this hyaluronidase in cartilage metabolism.

Abstract A07: Inflammation enables pancreatic acinar cells to overcome resistance to oncogenic Kras by increasing its expression and plasma membrane localization

Kras mutations are not sufficient to induce precancerous lesions in the pancreas, unlike in other organs such as lungs. Pancreatic intraepithelial neoplasia (PanIN) appear when mutated Kras is associated with pancreatitis. Since acinar cells are thought to be the cell origin of PanIN and pancreatic ductal adenocarcinoma (PDAC), our aim is to understand how acinar cells initially resist to Kras mutations and how pancreatitis enables mutated Kras to promote pancreatic neoplasia. Pancreatitis and associated inflammation were induced by injections of cerulein in tamoxifen-treated Elastase-CreER/LSLKrasG12D mice, in which mutated Kras expression is induced exclusively in acinar cells. RNAseq analysis on FACS-sorted acinar cells revealed that Kras expression and several cancer-associated pathways were rapidly induced in the presence of inflammation. In situ hybridization, RTqPCR, and Western blot (WB) experiments confirmed that Kras and/or KrasG12D mRNA and protein were significantly increased in acinar cells after cerulein treatment. To further explore the control of Kras expression, we generated a novel mouse model in which a citrine gene (a variant of GFP) is fused in frame with the endogenous Kras gene, allowing to detect Kras on tissue sections using a GFP antibody. We demonstrated by WB that the protein expression of Kras, KrasG12D and citrine-Kras fusion was similarly regulated in response to cerulein treatment, suggesting the involvement of the same regulatory elements on all three alleles. The ability of citrine-Kras fusion to bind GTP and translocate to the plasma membrane in acini was not altered compared to wild-type Kras protein. Little or no citrine-Kras was visualized in acinar cells of tamoxifen-treated Elastase-CreER/LSLKrasG12D/citrine-Kras mice. However, a 30-fold increase of citrine-Kras at the cell membrane of neoplastic lesions was detected after cerulein treatment. This indicates that inflammation is required for expression and plasma membrane targeting of Kras and KrasG12D in metaplastic acini. Importantly, protein expression of FT-α/β, Rce1 and ICMT, three enzymes necessary for Kras targeting to the membrane was greatly increased in murine acinar cells during pancreatitis. Basal FT-α/β, Rce1 and ICMT levels were found to be very low in normal mouse and human pancreata, compared to other organs, and their expression increased in primary mouse and human acini grown under conditions inducing the formation of preneoplastic lesions. In addition, computational analysis of a PDAC cohort (n=178) from The Cancer Genome Atlas (TCGA) revealed a positive correlation between expression of KrasG12D and its regulating enzymes. We conclude that Kras and KrasG12D are normally not present in acinar cells in vivo and that inflammation induces a coordinated program driving their expression and translocation to plasma membrane. Our findings provide a first clue to the long-standing observation that in several organs, Kras mutations, alone, are not sufficient to affect tissue homeostasis.

Citation Format: Mohamad Nabil Assi, Jean-Nicolas Lodewyckx, Younes Achouri, Claude Gérard, Nicolas Dauguet, Isabelle Houbracken, Yves Heremans, Donatienne Tyteca, Ilse Rooman, Luc Bouwens, Frédéric Lemaigre, Patrick Jacquemin. Inflammation enables pancreatic acinar cells to overcome resistance to oncogenic Kras by increasing its expression and plasma membrane localization [abstract]. In: Proceedings of the AACR Special Conference on Targeting RAS-Driven Cancers; 2018 Dec 9-12; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(5_Suppl):Abstract nr A07.

Monocytes undergo multi-step differentiation in mice during oral infection by Toxoplasma gondii

Monocytes play a major role in the defense against pathogens. They are rapidly mobilized to inflamed sites where they exert both proinflammatory and regulatory effector functions. It is still poorly understood how this dynamic and exceptionally plastic system is controlled at the molecular level. Herein, we evaluated the differentiation process that occurs in Ly6Chi monocytes during oral infection by Toxoplasma gondii. Flow cytometry and single-cell analysis revealed distinct activation status and gene expression profiles in the bone marrow, the spleen and the lamina propria of infected mice. We provide further evidence that acquisition of effector functions, such as the capacity to produce interleukin-27, is accompanied by distinct waves of epigenetic programming, highlighting a role for STAT1/IRF1 in the bone marrow and AP-1/NF-κB in the periphery. This work broadens our understanding of the molecular events that occur in vivo during monocyte differentiation in response to inflammatory cues.

EOMES interacts with RUNX3 and BRG1 to promote innate memory cell formation through epigenetic reprogramming

Memory CD8⁺ T cells have the ability to provide lifelong immunity against pathogens. Although memory features generally arise after challenge with a foreign antigen, naïve CD8 single positive (SP) thymocytes may acquire phenotypic and functional characteristics of memory cells in response to cytokines such as interleukin-4. This process is associated with the induction of the T-box transcription factor Eomesodermin (EOMES). However, the underlying molecular mechanisms remain ill-defined. Using epigenomic profiling, we show that these innate memory CD8SP cells acquire only a portion of the active enhancer repertoire of conventional memory cells. This reprograming is secondary to EOMES recruitment, mostly to RUNX3-bound enhancers. Furthermore, EOMES is found within chromatin-associated complexes containing BRG1 and promotes the recruitment of this chromatin remodelling factor. Also, the in vivo acquisition of EOMES-dependent program is BRG1-dependent. In conclusion, our results support a strong epigenetic basis for the EOMES-driven establishment of CD8⁺ T cell innate memory program.

IL-24 contributes to skin inflammation in Para-Phenylenediamine-induced contact hypersensitivity

Para-Phenylenediamine (PPD) is an aromatic amine used in hair dyes and in temporary black henna tattoos, which is a frequent cause of allergic contact dermatitis (ACD). ACD is a skin inflammatory reaction characterized by modifications such as spongiosis, exocytosis and acanthosis. The aim of this study is to characterize the expression and the role of IL-20-related cytokines, including IL-19, IL-20, IL-22 and IL-24, in ACD. The expression of IL19, IL20, IL22 and IL24 is increased in affected skin from PPD allergic patients compared with uninvolved skin. In addition, the expression of these cytokines positively correlates with clinical symptoms. To assess their role in ACD, we set up a mouse model of PPD-induced allergic contact dermatitis and we showed that, in contrast to Il22-deficient mice, Il22ra1-, Il20rb- and Il24-deficient mice are partially protected against development of PPD-induced contact hypersensitivity. These mice have decreased ear thickening and less acanthosis compared with WT mice after PPD treatment. In addition, the absence of IL-22R, IL-20R2 or IL-24 affects the recruitment of neutrophils into the skin but not the total IgE production. Taken together, these results demonstrate the implication of IL-24 via the IL-20R type II receptor in the inflammatory process of ACD.

Lineage-Restricted Mammary Stem Cells Sustain the Development, Homeostasis, and Regeneration of the Estrogen Receptor Positive Lineage

The mammary gland (MG) is composed of different cell lineages, including the basal and the luminal cells (LCs) that are maintained by distinct stem cell (SC) populations. LCs can be subdivided into estrogen receptor (ER)⁺ and ER⁻ cells. LCs act as the cancer cell of origin in different types of mammary tumors. It remains unclear whether the heterogeneity found in luminal-derived mammary tumors arises from a pre-existing heterogeneity within LCs. To investigate LC heterogeneity, we used lineage tracing to assess whether the ER⁺ lineage is maintained by multipotent SCs or by lineage-restricted SCs. To this end, we generated doxycycline-inducible ER-rtTA mice that allowed us to perform genetic lineage tracing of ER⁺ LCs and study their fate and long-term maintenance. Our results show that ER⁺ cells are maintained by lineage-restricted SCs that exclusively contribute to the expansion of the ER⁺ lineage during puberty and their maintenance during adult life.

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ER⁺ stem cells mediate expansion and maintenance of the ER⁺ lineage

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ER⁺ stem cells expand and differentiate into ER⁺ cells following transplantation

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ER⁺ stem cells survive involution and repopulate the ER⁺ lineage

MicroRNA-337-3p controls hepatobiliary gene expression and transcriptional dynamics during hepatic cell differentiation

Transcriptional networks control the differentiation of the hepatocyte and cholangiocyte lineages from embryonic liver progenitor cells and their subsequent maturation to the adult phenotype. However, how relative levels of hepatocyte and cholangiocyte gene expression are determined during differentiation remains poorly understood. Here, we identify microRNA (miR)-337-3p as a regulator of liver development. miR-337-3p stimulates expression of cholangiocyte genes and represses hepatocyte genes in undifferentiated progenitor cells in vitro and in embryonic mouse livers. Beyond the stage of lineage segregation, miR-337-3p controls the transcriptional network dynamics of developing hepatocytes and balances both cholangiocyte populations that constitute the ductal plate. miR-337-3p requires Notch and transforming growth factor-β signaling and exerts a biphasic control on the hepatocyte transcription factor hepatocyte nuclear factor 4α by modulating its activation and repression. With the help of an experimentally validated mathematical model, we show that this biphasic control results from an incoherent feedforward loop between miR-337-3p and hepatocyte nuclear factor 4α.

CONCLUSION

Our results identify miR-337-3p as a regulator of liver development and highlight how tight quantitative control of hepatic cell differentiation is exerted through specific gene regulatory network motifs. (Hepatology 2018;67:313-327).

Overexpression of Interleukin-4 in the Thyroid of Transgenic Mice Upregulates the Expression of Duox1 and the Anion Transporter Pendrin

BACKGROUND

The dual oxidases (Duox) are involved in hydrogen peroxide generation, which is essential for thyroid hormone synthesis, and therefore they are markers of thyroid function. During inflammation, cytokines upregulate DUOX gene expression in the airway and the intestine, suggesting a role for these proteins in innate immunity. It was previously demonstrated that interleukin-4 (IL-4) upregulates DUOX gene expression in thyrocytes. Although the role of IL-4 in autoimmune thyroid diseases has been studied extensively, the effects of IL-4 on thyroid physiology remain largely unknown. Therefore, a new animal model was generated to study the impact of IL-4 on thyroid function.

METHODS

Transgenic (Thyr-IL-4) mice with thyroid-targeted expression of murine IL-4 were generated. Transgene expression was verified at the mRNA and protein level in thyroid tissues and primary cultures. The phenotype of the Thyr-IL-4 animals was characterized by measuring serum thyroxine (T4) and thyrotropin levels and performing thyroid morphometric analysis, immunohistochemistry, whole transcriptome sequencing, quantitative reverse transcription polymerase chain reaction, and ex vivo thyroid function assays.

RESULTS

Thyrocytes from two Thyr-IL-4 mouse lines (#30 and #52) expressed IL-4, which was secreted into the extracellular space. Although 10-month-old transgenic animals had T4 and thyrotropin serum levels in the normal range, they had altered thyroid follicular structure with enlarged follicles composed of elongated thyrocytes containing numerous endocytic vesicles. These follicles were positive for T4 staining the colloid, indicating their capacity to produce thyroid hormones. RNA profiling of Thyr-IL-4 thyroid samples revealed modulation of multiple genes involved in inflammation, while no major leukocyte infiltration could be detected. Upregulated expression of Duox1, Duoxa1, and the pendrin anion exchanger gene (Slc26a4) was detected. In contrast, the iodide symporter gene Slc5a5 was markedly downregulated resulting in impaired iodide uptake and reduced thyroid hormone levels in transgenic thyroid tissue. Hydrogen peroxide production was increased in Thyr-IL-4 thyroid tissue compared with wild-type animals, but no significant oxidative stress could be detected.

CONCLUSIONS

This is the first study to show that ectopic expression of IL-4 in thyroid tissue upregulates Duox1/Duoxa1 and Slc26a4 expression in the thyroid. The present data demonstrate that IL-4 could affect thyroid morphology and function, mainly by downregulating Slc5a5 expression, while maintaining a normal euthyroid phenotype.

A mouse model of L-2-hydroxyglutaric aciduria, a disorder of metabolite repair

The purpose of the present work was to progress in our understanding of the pathophysiology of L-2-hydroxyglutaric aciduria, due to a defect in L-2-hydroxyglutarate dehydrogenase, by creating and studying a mouse model of this disease. L-2-hydroxyglutarate dehydrogenase-deficient mice (l2hgdh-/-) accumulated L-2-hydroxyglutarate in tissues, most particularly in brain and testis, where the concentration reached ≈ 3.5 μmol/g. Male mice showed a 30% higher excretion of L-2-hydroxyglutarate compared to female mice, supporting that this dicarboxylic acid is partially made in males by lactate dehydrogenase C, a poorly specific form of this enzyme exclusively expressed in testes. Involvement of mitochondrial malate dehydrogenase in the formation of L-2-hydroxyglutarate was supported by the commensurate decrease in the formation of this dicarboxylic acid when down-regulating this enzyme in mouse l2hgdh-/- embryonic fibroblasts. The concentration of lysine and arginine was markedly increased in the brain of l2hgdh-/- adult mice. Saccharopine was depleted and glutamine was decreased by ≈ 40%. Lysine-α-ketoglutarate reductase, which converts lysine to saccharopine, was inhibited by L-2-hydroxyglutarate with a Ki of ≈ 0.8 mM. As low but significant activities of the bifunctional enzyme lysine-α-ketoglutarate reductase/saccharopine dehydrogenase were found in brain, these findings suggest that the classical lysine degradation pathway also operates in brain and is inhibited by the high concentrations of L-2-hydroxyglutarate found in l2hgdh-/- mice. Pathological analysis of the brain showed significant spongiosis. The vacuolar lesions mostly affected oligodendrocytes and myelin sheats, as in other dicarboxylic acidurias, suggesting that the pathophysiology of this model of leukodystrophy may involve irreversible pumping of a dicarboxylate in oligodendrocytes. Neurobehavioral testing indicated that the mice mostly suffered from a deficit in learning capacity. In conclusion, the findings support the concept that L-2-hydroxyglutaric aciduria is a disorder of metabolite repair. The accumulation of L-2-hydroxyglutarate exerts toxic effects through various means including enzyme inhibition and glial cell swelling.

Liver progenitor cells yield functional hepatocytes in response to chronic liver injury in mice

BACKGROUND & AIMS

Self-renewal of mature hepatocytes promotes homeostasis and regeneration of adult liver. However, recent studies have indicated that liver progenitor cells (LPC) could give rise to hepatic epithelial cells during normal turnover of the liver and after acute injury. We investigated the capacity of LPC to differentiate into hepatocytes in vivo and contribute to liver regeneration.

METHODS

We performed lineage tracing experiments, using mice that express tamoxifen-inducible Cre recombinase under control of osteopontin regulatory region crossed with yelow fluorescent protein reporter mice, to follow the fate of LPC and biliary cells. Adult mice received partial (two-thirds) hepatectomy, acute or chronic administration of carbon tetrachloride (CCl(4)), choline-deficient diet supplemented with ethionine, or 3,5-diethoxycarbonyl-1,4-dihydrocollidine diet.

RESULTS

LPC and/or biliary cells generated 0.78% and 2.45% of hepatocytes during and upon recovery of mice from liver injury, respectively. Repopulation efficiency by LPC and/or biliary cells increased when extracellular matrix and laminin deposition were reduced. The newly formed hepatocytes integrated into hepatic cords, formed biliary canaliculi, expressed hepato-specific enzymes, accumulated glycogen, and proliferated in response to partial hepatectomy, as neighboring native hepatocytes. By contrast, LPC did not contribute to hepatocyte regeneration during normal liver homeostasis, in response to surgical or toxic loss of liver mass, during chronic liver injury (CCl(4)-induced), or during ductular reactions.

CONCLUSIONS

LPC or biliary cells terminally differentiate into functional hepatocytes in mice with liver injury.

A feedback loop between the liver-enriched transcription factor network and miR-122 controls hepatocyte differentiation

BACKGROUND & AIMS

Hepatocyte differentiation is controlled by liver-enriched transcription factors (LETFs). We investigated whether LETFs control microRNA expression during development and whether this control is required for hepatocyte differentiation.

METHODS

Using in vivo DNA binding assays, we identified miR-122 as a direct target of the LETF hepatocyte nuclear factor (HNF) 6. The role and mechanisms of the HNF6-miR-122 gene cascade in hepatocyte differentiation were studied in vivo and in vitro by gain-of-function and loss-of-function experiments, using developing mice and zebrafish as model organisms.

RESULTS

HNF6 and its paralog Onecut2 are strong transcriptional stimulators of miR-122 expression. Specific levels of miR-122 were required for proper progression of hepatocyte differentiation; miR-122 stimulated the expression of hepatocyte-specific genes and most LETFs, including HNF6. This indicates that HNF6 and miR-122 form a positive feedback loop. Stimulation of hepatocyte differentiation by miR-122 was lost in HNF6-null mice, revealing that a transcription factor can mediate microRNA function. All hepatocyte-specific genes whose expression was stimulated by miR-122 bound HNF6 in vivo, confirming their direct regulation by this factor.

CONCLUSIONS

Hepatocyte differentiation is directed by a positive feedback loop that includes a transcription factor (HNF6) and a microRNA (miR-122) that are specifically expressed in liver. These findings could lead to methods to induce differentiation of hepatocytes in vitro and improve our understanding of liver cell dedifferentiation in pathologic conditions.

Transcriptional mechanisms of EphA7 gene expression in the developing cerebral cortex

The patterning of cortical areas is controlled by a combination of intrinsic factors that are expressed in the cortex and external signals such as inputs from the thalamus. EphA7 is a guidance receptor that is involved in key aspects of cortical development and is expressed in gradients within developing cortical areas. Here, we identified a regulatory element of the EphA7 promoter, named pA7, that can recapitulate salient features of the pattern of expression of EphA7, including cortical gradients. Using a pA7-Green fluorescent Protein (GFP) mouse reporter line, we isolated cortical neuron populations displaying different levels of EphA7/GFP expression. Transcriptome analysis of these populations enabled to identify many differentially expressed genes, including 26 transcription factors with putative binding sites in the pA7 element. Among these, Pbx1 was found to bind directly to the EphA7 promoter in the developing cortex. All genes validated further were confirmed to be expressed differentially in the developing cortex, similarly to EphA7. Their expression was unchanged in mutant mice defective for thalamocortical projections, indicating a transcriptional control largely intrinsic to the cortex. Our study identifies a novel repertoire of cortical neuron genes that may act upstream of, or together with EphA7, to control the patterning of cortical areas.

Epithelial: Endothelial cross-talk regulates exocrine differentiation in developing pancreas

Endothelial cells are required to initiate pancreas development from the endoderm. They also control the function of endocrine islets after birth. Here we investigate in developing pancreas how the endothelial cells become organized during branching morphogenesis and how their development affects pancreatic cell differentiation. We show that endothelial cells closely surround the epithelial bud at the onset of pancreas morphogenesis. During branching morphogenesis, the endothelial cells become preferentially located near the central (trunk) epithelial cells and remain at a distance from the branch tips where acinar cells differentiate. This correlates with predominant expression of the angiogenic factor vascular endothelial growth factor-A (VEGF-A) in trunk cells. In vivo ablation of VEGF-A expression by pancreas-specific inactivation of floxed Vegfa alleles results in reduced endothelial development and in excessive acinar differentiation. On the contrary, acinar differentiation is repressed when endothelial cells are recruited around tip cells that overexpress VEGF-A. Treatment of embryonic day 12.5 explants with VEGF-A or with VEGF receptor antagonists confirms that acinar development is tightly controlled by endothelial cells. We also provide evidence that endothelial cells repress the expression of Ptf1a, a transcription factor essential for acinar differentiation, and stimulate the expression of Hey-1 and Hey-2, two repressors of Ptf1a activity. In explants, we provide evidence that VEGF-A signaling is required, but not sufficient, to induce endocrine differentiation. In conclusion, our data suggest that, in developing pancreas, epithelial production of VEGF-A determines the spatial organization of endothelial cells which, in turn, limit acinar differentiation of the epithelium.

Maged1, a new regulator of skeletal myogenic differentiation and muscle regeneration

BACKGROUND

In normal adult skeletal muscle, cell turnover is very slow. However, after an acute lesion or in chronic pathological conditions, such as primary myopathies, muscle stem cells, called satellite cells, are induced to proliferate, then withdraw definitively from the cell cycle and fuse to reconstitute functional myofibers.

RESULTS

We show that Maged1 is expressed at very low levels in normal adult muscle but is strongly induced after injury, during the early phase of myoblast differentiation. By comparing in vitro differentiation of myoblasts derived from wild-type or Maged1 knockout mice, we observed that Maged1 deficiency results in reduced levels of p21CIP1/WAF1, defective cell cycle exit and impaired myotube maturation. In vivo, this defect results in delayed regeneration of injured muscle.

CONCLUSIONS

These data demonstrate for the first time that Maged1 is an important factor required for proper skeletal myoblast differentiation and muscle healing.

Identification of the cell lineage at the origin of basal cell carcinoma

For most types of cancers, the cell at the origin of tumour initiation is still unknown. Here, we used mouse genetics to identify cells at the origin of basal cell carcinoma (BCC), which is one of the most frequently occurring types of cancer in humans, and can result from the activation of the Hedgehog signalling pathway. Using mice conditionally expressing constitutively active Smoothened mutant (SmoM2), we activated Hedgehog signalling in different cellular compartments of the skin epidermis and determined in which compartments Hedgehog activation induces BCC formation. Activation of SmoM2 in hair follicle bulge stem cells and their transient amplifying progenies did not induce cancer formation, demonstrating that BCC does not originate from bulge stem cells, as previously thought. Using clonal analysis, we found that BCC arises from long-term resident progenitor cells of the interfollicular epidermis and the upper infundibulum. Our studies uncover the cells at the origin of BCC in mice and demonstrate that expression of differentiation markers in tumour cells is not necessarily predictive of the cancer initiating cells.

Epidermal progenitors give rise to Merkel cells during embryonic development and adult homeostasis

Lineage-tracing experiments show that the origin of specialized mechanosensory Merkel cells in the skin is epidermal progenitors, not the neural crest.

Merkel cells (MCs) are located in the touch-sensitive area of the epidermis and mediate mechanotransduction in the skin. Whether MCs originate from embryonic epidermal or neural crest progenitors has been a matter of intense controversy since their discovery >130 yr ago. In addition, how MCs are maintained during adulthood is currently unknown. In this study, using lineage-tracing experiments, we show that MCs arise through the differentiation of epidermal progenitors during embryonic development. In adults, MCs undergo slow turnover and are replaced by cells originating from epidermal stem cells, not through the proliferation of differentiated MCs. Conditional deletion of the Atoh1/Math1 transcription factor in epidermal progenitors results in the absence of MCs in all body locations, including the whisker region. Our study demonstrates that MCs arise from the epidermis by an Atoh1-dependent mechanism and opens new avenues for study of MC functions in sensory perception, neuroendocrine signaling, and MC carcinoma.

Phosphoserine aminotransferase deficiency: a novel disorder of the serine biosynthesis pathway

We present the first two identified cases of phosphoserine aminotransferase deficiency. This disorder of serine biosynthesis has been identified in two siblings who showed low concentrations of serine and glycine in plasma and cerebrospinal fluid. Clinically, the index patient presented with intractable seizures, acquired microcephaly, hypertonia, and psychomotor retardation and died at age 7 mo despite supplementation with serine (500 mg/kg/d) and glycine (200 mg/kg/d) from age 11 wk. The younger sibling received treatment from birth, which led to a normal outcome at age 3 years. Measurement of phosphoserine aminotransferase activity in cultured fibroblasts in the index patient was inconclusive, but mutational analysis revealed compound heterozygosity for two mutations in the PSAT1 gene--one frameshift mutation (c.delG107) and one missense mutation (c.299A-->C [p.Asp100Ala])--in both siblings. Expression studies of the p.Asp100Ala mutant protein revealed a V(max) of only 15% of that of the wild-type protein.

2-Keto-4-methylthiobutyrate, an intermediate in the methionine salvage pathway, is a good substrate for CtBP1

In the present work, we have studied the kinetic properties of the catalytic domain of CtBP1, a co-repressor belonging to the d-2-hydroxyacid dehydrogenase family and known to reduce pyruvate in the presence of NADH. CtBP1 acted on a variety of alpha-keto acids, for which it displayed biphasic curves with inhibition at elevated concentrations, as observed with other dehydrogenases of the same family. Based on catalytic efficiencies, the best substrate was 2-keto-4-methylthiobutyrate, an intermediate of the methionine salvage pathway. It was about 20-fold better than 2-ketoisocaproate and glyoxylate, and 80-fold better than pyruvate. From these data we conclude that 2-keto-4-methylthiobutyrate may be an important regulator of CtBP activity, possibly linking gene repression to the activity of the methionine salvage and spermine synthesis pathways.

A vHNF1/TCF2-HNF6 cascade regulates the transcription factor network that controls generation of pancreatic precursor cells

Generation of pancreatic precursor cells in the endoderm is controlled by a network of transcription factors. Hepatocyte nuclear factor-6 (HNF6) is a key player in this network, because it controls the initiation of the expression of pancreatic and duodenal homeobox 1 (Pdx1), the earliest marker of pancreatic precursor cells. To further characterize this network, we have investigated how the expression of HNF6 is controlled in mouse endoderm, by using in vitro and in vivo protein-DNA interaction techniques combined with endoderm electroporation, transgenesis, and gene inactivation in embryos. We delineated Hnf6 regulatory regions that confer expression of a reporter gene in the embryonic endoderm but not in extraembryonic visceral endoderm. HNF6 expression in the embryonic endoderm was found to depend on an intronic enhancer. This enhancer contains functional binding sites for the tissue-specific factors of the forkhead box A and HNF1 families. Among the latter, variant HNF1 (vHNF1)/TCF2, which is expressed before HNF6 in the endoderm, was found to be critical for HNF6 expression. Therefore, the sequential activation of vHNF1, HNF6, and Pdx1 in the endoderm appears to control the generation of pancreatic precursors. This cascade may be used to benchmark in vitro differentiation of pancreatic precursor cells from embryonic stem cells, for cell therapy of diabetes.

Mutations in phenotypically mild D-2-hydroxyglutaric aciduria

D-2-hydroxyglutaric aciduria is a neurometabolic disorder with mild and severe phenotypes. Recently, we reported pathogenic mutations in the D-2-hydroxyglutarate dehydrogenase gene as the cause of the severe phenotype of D-2-hydroxyglutaric aciduria in two patients. Here, we report two novel pathogenic mutations in this gene in one patient with a mild presentation and two asymptomatic siblings with D-2-hydroxyglutaric aciduria from two unrelated consanguineous Palestinian families: a splice error (IVS4-2A-->G) and a missense mutation (c.1315A-->G;p.Asn439Asp). Overexpression of this mutant protein showed marked reduction of the enzyme activity.

Long chain fatty acyl-CoA synthetase 5 expression is induced by insulin and glucose: involvement of sterol regulatory element-binding protein-1c

In a screen for sterol regulatory element-binding protein (SREBP)-1c target genes in the liver, we identified long chain fatty acyl-CoA synthetase 5 (ACS-5). Hepatic ACS-5 mRNA is poorly expressed during fasting and diabetes and strongly induced by carbohydrate refeeding and insulin treatment. In cultured hepatocytes, insulin and a high glucose concentration induce ACS-5 mRNA. Adenoviral overexpression of a nuclear form of SREBP-1c in liver of diabetic mice or in cultured hepatocytes mimics the effect of insulin to induce ACS-5. By contrast, a dominant negative form of SREBP-1c abolishes the effect of insulin on ACS-5 expression. The dietary and SREBP-1c-mediated insulin regulation of ACS-5 expression indicate that ACS-5 is involved in the anabolic fate of fatty acids.

Mutations in the D-2-hydroxyglutarate dehydrogenase gene cause D-2-hydroxyglutaric aciduria

d-2-hydroxyglutaric aciduria is a neurometabolic disorder with both a mild and a severe phenotype and with unknown etiology. Recently, a novel enzyme, d-2-hydroxyglutarate dehydrogenase, which converts d-2-hydroxyglutarate into 2-ketoglutarate, and its gene were identified. In the genes of two unrelated patients affected with d-2-hydroxyglutaric aciduria, we identified disease-causing mutations. One patient was homozygous for a missense mutation (c.1331T-->C; p.Val444Ala). The other patient was compound heterozygous for a missense mutation (c.440T-->G; p.Ile147Ser) and a splice-site mutation (IVS1-23A-->G) that resulted in a null allele. Overexpression studies in HEK-293 cells of proteins containing the missense mutations showed a marked reduction of d-2-hydroxyglutarate dehydrogenase activity, proving that mutations in the d-2-hydroxyglutarate dehydrogenase gene cause d-2-hydroxyglutaric aciduria.

Identification of a dehydrogenase acting on D-2-hydroxyglutarate

Extracts of frozen rat liver were found to catalyse the formation of 3H2O from DL-2-hydroxy[2-3H]glutarate. Three peaks of enzyme activities were observed on separation by chromatography on DEAE-Sepharose. The first and second peaks corresponded to an enzyme acting on L-2-hydroxyglutarate and the third peak corresponded to an enzyme acting on D-2-hydroxyglutarate, as indicated by competitive inhibition of the detritiation of the racemic radioactive compound by the unlabelled L- and D-isomers respectively. The enzyme acting on the D-form was further characterized. It was independent of NAD or NADP and it converted D-2-hydroxyglutarate into a-ketoglutarate, transferring electrons to artificial electron acceptors. It also oxidized D-lactate, D-malate and meso-tartrate and was stimulated by Zn2+, Co2+ and Mn2+, but not by Mg2+ or Ca2+. Subcellular fractionation indicated that it was present in the mitochondrial fraction. The enzyme was further purified by chromatography on Blue Trisacryl and phenyl-Sepharose, up to a stage where only a few bands were still visible by SDS/PAGE. Among the four candidate polypeptides that were identified by MS, one corresponded to a predicted mitochondrial protein homologous with FAD-dependent D-lactate dehydrogenase. The corresponding human protein was expressed in HEK-293 cells and it was shown to catalyse the detritiation of DL-2-hydroxy[2-3H]glutarate with similar properties as the purified rat enzyme.

Sterol regulatory element binding protein-1c expression and action in rat muscles: insulin-like effects on the control of glycolytic and lipogenic enzymes and UCP3 gene expression

Sterol regulatory element binding protein-1c (SREBP-1c) is a transcription factor that mediates insulin effects on hepatic gene expression. It is itself transcriptionally stimulated by insulin in hepatocytes. Here we show that SREBP-1c mRNA is expressed in adult rat skeletal muscles and that this expression is decreased by diabetes. The regulation of SREBP-1c expression was then assessed in cultures of adult muscle satellite cells. These cells form spontaneously contracting multinucleated myotubes within 7 days of culture. SREBP-1c mRNA is expressed in contracting myotubes. A 4-h treatment with 100 nmol/l insulin increases SREBP-1c expression and nuclear abundance by two- to threefold in myotubes. In cultured myotubes, insulin increases the expression of glycolytic and lipogenic enzyme genes and inhibits the 9-cis retinoic acid-induced UCP3 expression. These effects of insulin are mimicked by adenovirus-mediated expression of a transcriptionally active form of SREBP-1c. We conclude that in skeletal muscles, SREBP-1c expression is sensitive to insulin and can transduce the positive and negative actions of the hormone on specific genes and thus has a pivotal role in long-term muscle insulin sensitivity.

Role of cysteine in the dietary control of the expression of 3-phosphoglycerate dehydrogenase in rat liver

Shifting rats to a protein-free, carbohydrate-rich diet, although not starvation, resulted in the appearance of mRNA for, and activity of, 3-phosphoglycerate dehydrogenase (3-PGDH) in liver as well as in a marked decrease in plasma cystine concentration. Refeeding with protein caused a 50% decrease in the mRNA in 8 h and its complete disappearance within 24 h, followed by a slower disappearance of the enzymic activity. Intraperitoneal administration of cysteine or methionine to protein-starved rats decreased the mRNA by 50-60% after 8 h. However, the repeated administration of cysteine failed to cause the complete disappearance of this mRNA in 24 h. In hepatocytes in primary culture, cysteine plus methionine and glucagon had, independently, an approx. 4-fold inhibitory effect on the abundance of the 3-PGDH mRNA and caused its almost complete disappearance when tested together. Insulin had an approx. 2-fold stimulatory effect, which was antagonized by cysteine plus methionine but was still apparent in the presence of glucagon. Nuclear run-on experiments and analysis of the stability of the mRNA with 5,6-dichlorobenzimidazole riboside, an inhibitor of RNA polymerase II, suggested that the effect of cysteine plus methionine was due to destabilization of the mRNA, whereas the effect of glucagon was exerted on transcription. Cysteine, but not methionine, inhibited the accumulation of 3-PGDH mRNA in FTO2B hepatoma cells. In conclusion, the dietary control of the expression of the 3-PGDH gene in liver seems to involve the negative effects of cysteine and glucagon and the positive effect of insulin.

Kinetic properties and tissular distribution of mammalian phosphomannomutase isozymes

Human tissues contain two types of phosphomannomutase, PMM1 and PMM2. Mutations in the PMM2 gene are responsible for the most common form of carbohydrate-deficient glycoprotein syndrome [Matthijs, Schollen, Pardon, Veiga-da-Cunha, Jaeken, Cassiman and Van Schaftingen (1997) Nat. Genet. 19, 88-92]. The protein encoded by this gene has now been produced in Escherichia coli and purified to homogeneity, and its properties have been compared with those of recombinant human PMM1. PMM2 converts mannose 1-phosphate into mannose 6-phosphate about 20 times more rapidly than glucose 1-phosphate to glucose 6-phosphate, whereas PMM1 displays identical Vmax values with both substrates. The Ka values for both mannose 1,6-bisphosphate and glucose 1,6-bisphosphate are significantly lower in the case of PMM2 than in the case of PMM1. Like PMM1, PMM2 forms a phosphoenzyme with the chemical characteristics of an acyl-phosphate. PMM1 and PMM2 hydrolyse different hexose bisphosphates (glucose 1,6-bisphosphate, mannose 1,6-bisphosphate, fructose 1,6-bisphosphate) at maximal rates of approximately 3.5 and 0.3% of their PMM activity, respectively. Fructose 1,6-bisphosphate does not activate PMM2 but causes a time-dependent stimulation of PMM1 due to the progressive formation of mannose 1,6-bisphosphate from fructose 1,6-bisphosphate and mannose 1-phosphate. Experiments with specific antibodies, kinetic studies and Northern blots indicated that PMM2 is the only detectable isozyme in most rat tissues except brain and lung, where PMM1 accounts for about 66 and 13% of the total activities, respectively.

Sequence of a putative glucose 6-phosphate translocase, mutated in glycogen storage disease type Ib

We report the sequence of a human cDNA that encodes a 46 kDa transmembrane protein homologous to bacterial transporters for phosphate esters. This protein presents at its carboxy terminus the consensus motif for retention in the endoplasmic reticulum. Northern blots of rat tissues indicate that the corresponding mRNA is mostly expressed in liver and kidney. In two patients with glycogen storage disease type Ib, mutations were observed that either replaced a conserved Gly to Cys or introduced a premature stop codon. The encoded protein is therefore most likely the glucose 6-phosphate translocase that is functionally associated with glucose-6-phosphatase.

Cloning, sequencing and expression of rat liver 3-phosphoglycerate dehydrogenase

Rat liver d-3-phosphoglycerate dehydrogenase was purified to homogeneity and digested with trypsin, and the sequences of two peptides were determined. This sequence information was used to screen a rat hepatoma cDNA library. Among 11 positive clones, two covered the whole coding sequence. The deduced amino acid sequence (533 residues; Mr 56493) shared closer similarity with Bacillus subtilis 3-phosphoglycerate dehydrogenase than with the enzymes from Escherichia coli, Haemophilus influenzae and Saccharomyces cerevisiae. In all cases the similarity was most apparent in the substrate- and NAD+-binding domains, and low or insignificant in the C-terminal domain. A corresponding 2.1 kb mRNA was present in rat tissues including kidney, brain and testis, whatever the dietary status, and also in livers of animals fed a protein-free, carbohydrate-rich diet, but not in livers of control rats, suggesting transcriptional regulation. The full-length rat 3-phosphoglycerate dehydrogenase was expressed in E. coli and purified. The recombinant enzyme and the protein purified from liver displayed hyperbolic kinetics with respect to 3-phosphoglycerate, NAD+ and NADH, but substrate inhibition by 3-phosphohydroxypyruvate was observed; this inhibition was antagonized by salts. Similar properties were observed with a truncated form of 3-phosphoglycerate dehydrogenase lacking the C-terminal domain, indicating that the latter is not implicated in substrate inhibition or in salt effects. By contrast with the bacterial enzyme, rat 3-phosphoglycerate dehydrogenase did not catalyse the reduction of 2-oxoglutarate, indicating that this enzyme is not involved in human D- or L-hydroxyglutaric aciduria.