Daniel Jahn

A truncated lamin A in the Lmna−/− mouse line: Implications for the understanding of laminopathies

During recent years a number of severe clinical syndromes, collectively termed laminopathies, turned out to be caused by various, distinct mutations in the human LMNA gene. Arising from this, remarkable progress has been made to unravel the molecular pathophysiology underlying these disorders. A great benefit in this context was the generation of an A-type lamin deficient mouse line (Lmna−/−) by Sullivan and others,1 which has become one of the most frequently used models in the field and provided profound insights to many different aspects of A-type lamin function. Here, we report the unexpected finding that these mice express a truncated Lmna gene product on both transcriptional and protein level. Combining different approaches including mass spectrometry, we precisely define this product as a C-terminally truncated lamin A mutant that lacks domains important for protein interactions and post-translational processing. Based on our findings we discuss implications for the interpretation of previous studies using Lmna−/− mice and the concept of human laminopathies.

The Meiotic Nuclear Lamina Regulates Chromosome Dynamics and Promotes Efficient Homologous Recombination in the Mouse

The nuclear lamina is the structural scaffold of the nuclear envelope and is well known for its central role in nuclear organization and maintaining nuclear stability and shape. In the past, a number of severe human disorders have been identified to be associated with mutations in lamins. Extensive research on this topic has provided novel important clues about nuclear lamina function. These studies have contributed to the knowledge that the lamina constitutes a complex multifunctional platform combining both structural and regulatory functions. Here, we report that, in addition to the previously demonstrated significance for somatic cell differentiation and maintenance, the nuclear lamina is also an essential determinant for germ cell development. Both male and female mice lacking the short meiosis-specific A-type lamin C2 have a severely defective meiosis, which at least in the male results in infertility. Detailed analysis revealed that lamin C2 is required for telomere-driven dynamic repositioning of meiotic chromosomes. Loss of lamin C2 affects precise synapsis of the homologs and interferes with meiotic double-strand break repair. Taken together, our data explain how the nuclear lamina contributes to meiotic chromosome behaviour and accurate genome haploidization on a mechanistic level.

Mechanisms of enterohepatic fibroblast growth factor 15/19 signaling in health and disease

The gut-derived hormone fibroblast growth factor 15/19 (FGF15/19) is an emerging versatile regulator of various metabolic pathways. As such, FGF15/19 has been implicated in homeostatic control of bile acid, carbohydrate and lipid metabolism in multiple target organs including the liver, adipose tissue and brain. In line with this, growing evidence suggests that dysregulation of FGF15/19 contributes to a number of metabolic and bile acid-associated disorders such as fatty liver disease, Type 2 diabetes and different gastrointestinal dysfunctions. In this review we summarize the current knowledge on the organ-specific functions of FGF15/19 and address their underlying molecular mechanisms. Moreover, recent advances in the characterization of factors that control the release of the hormone in the gut will be discussed and linked to the current view of how alterations of FGF15/19 signaling may contribute to disease development. Finally, the suitability of FGF15/19 as a potential therapeutic target will be critically reviewed.

Aldosterone induces fibrosis, oxidative stress and DNA damage in livers of male rats independent of blood pressure changes

Mineralocorticoid receptor blockers show antifibrotic potential in hepatic fibrosis. The mechanism of this protective effect is not known yet, although reactive oxygen species seem to play an important role. Here, we investigated the effects of elevated levels of aldosterone (Ald), the primary ligand of the mineralocorticoid receptor, on livers of rats in a hyperaldosteronism model: aldosterone-induced hypertension. Male Sprague-Dawley rats were treated for 4 weeks with aldosterone. To distinguish if damage caused in the liver depended on increased blood pressure or on increased Ald levels, the mineralocorticoid receptor antagonist spironolactone was given in a subtherapeutic dose, not normalizing blood pressure. To investigate the impact of oxidative stress, the antioxidant tempol was administered. Aldosterone induced fibrosis, detected histopathologically, and by expression analysis of the fibrosis marker, α-smooth muscle actin. Further, the mRNA amount of the profibrotic cytokine TGF-β was increased significantly. Fibrosis could be reduced by scavenging reactive oxygen species, and also by blocking the mineralocorticoid receptor. Furthermore, aldosterone treatment caused oxidative stress and DNA double strand breaks in livers, as well as the elevation of DNA repair activity. An increase of the transcription factor Nrf2, the main regulator of the antioxidative response could be observed, and of its target genes heme oxygenase-1 and γ-glutamylcysteine synthetase. All these effects of aldosterone were prevented by spironolactone and tempol. Already after 4 weeks of treatment, aldosteroneinfusion induced fibrosis in the liver. This effect was independent of elevated blood pressure. DNA damage caused by aldosterone might contribute to fibrosis progression when aldosterone is chronically increased.

Non-Alcoholic Steatohepatitis: From Pathophysiology to Novel Therapies.

Non-alcoholic fatty liver (NAFL) disease is defined by an accumulation of liver fat exceeding 5% of its weight in the absence of significant alcoholic intake. In 5-20%, there is a progression from NAFL to non-alcoholic steatohepatitis (NASH). Until now, it is not well understood why only some patients develop NASH, and currently, no drugs are licensed for this indication. Different T-cell populations such as T-regulatory, Th1 and Th17 cells play a central role in the immunopathogenesis of fatty liver disease and open the option of future interleukin (IL)-17-based therapeutics. The inflammatory process underlying NASH is furthermore characterized by elevated expression of pro-inflammatory cytokines such as TNFα and IL-1β. Anakinra, a recombinant version of IL-1Ra shows promising metabolic effects with improved hyperglycemia and beta-cell secretory function in a double-blind placebo controlled randomized trial in type 2 diabetic patients but such studies are still in their preliminary stages for NASH. Several studies point out that bile acid farnesoid X receptor (FXR)-mediated signals (such as the enterohepatic hormone fibroblast growth factor 15/19) are involved in the regulation of triglyceride and glucose metabolism. Recent clinical trials have revealed a beneficial impact of the FXR agonist obeticholic acid on body weight, insulin sensitivity and liver histology in patients with NASH. Further potential novel therapeutic targets in NASH are currently in phase II clinical development.

Structural and functional adaptations of the mammalian nuclear envelope to meet the meiotic requirements

Numerous studies in the past years provided definite evidence that the nuclear envelope is much more than just a simple barrier. It rather constitutes a multifunctional platform combining structural and dynamic features to fulfill many fundamental functions such as chromatin organization, regulation of transcription, signaling, but also structural duties like maintaining general nuclear architecture and shape. One additional and, without doubt, highly impressive aspect is the recently identified key function of selected nuclear envelope components in driving meiotic chromosome dynamics, which in turn is essential for accurate recombination and segregation of the homologous chromosomes. Here, we summarize the recent work identifying new key players in meiotic telomere attachment and movement and discuss the latest advances in our understanding of the actual function of the meiotic nuclear envelope.

Role of PTEN in Oxidative Stress and DNA Damage in the Liver of Whole-Body Pten Haplodeficient Mice

Type 2 diabetes (T2DM) and obesity are frequently associated with non-alcoholic fatty liver disease (NAFLD) and with an elevated cancer incidence. The molecular mechanisms of carcinogenesis in this context are only partially understood. High blood insulin levels are typical in early T2DM and excessive insulin can cause elevated reactive oxygen species (ROS) production and genomic instability. ROS are important for various cellular functions in signaling and host defense. However, elevated ROS formation is thought to be involved in cancer induction. In the molecular events from insulin receptor binding to genomic damage, some signaling steps have been identified, pointing at the PI3K/AKT pathway. For further elucidation Phosphatase and Tensin homolog (Pten), a tumour suppressor phosphatase that plays a role in insulin signaling by negative regulation of PI3K/AKT and its downstream targets, was investigated here. Dihydroethidium (DHE) staining was used to detect ROS formation in immortalized human hepatocytes. Comet assay and micronucleus test were performed to investigate genomic damage in vitro. In liver samples, DHE staining and western blot detection of HSP70 and HO-1 were performed to evaluate oxidative stress response. DNA double strand breaks (DSBs) were detected by immunohistostaining. Inhibition of PTEN with the pharmacologic inhibitor VO-OHpic resulted in increased ROS production and genomic damage in a liver cell line. Knockdown of Pten in a mouse model yielded increased oxidative stress levels, detected by ROS levels and expression of the two stress-proteins HSP70 and HO-1 and elevated genomic damage in the liver, which was significant in mice fed with a high fat diet. We conclude that PTEN is involved in oxidative stress and genomic damage induction in vitro and that this may also explain the in vivo observations. This further supports the hypothesis that the PI3K/AKT pathway is responsible for damaging effects of high levels of insulin.

Alterations in Enterohepatic Fgf15 Signaling and Changes in Bile Acid Composition Depend on Localization of Murine Intestinal Inflammation.

Background:Fibroblast growth factor (FGF) 15/19 is part of the gut-liver crosstalk accounting for bile acid (BA) metabolism regulation. Dysregulation of fibroblast growth factor 15/19 signaling is observed in different pathological conditions, for example, in gastrointestinal diseases such as inflammatory bowel disease (IBD). To understand the molecular bases, we analyzed the enterohepatic regulation of Fgf15-mediated pathway in 2 different inflammatory bowel disease mouse models. Methods:Target genes of the BA-farnesoid-X-receptor (Fxr)-Ffg15 axis were quantified by RT-PCR or western blotting in gut and liver of dextran sulfate sodium (DSS)-treated and IL10(−/−) mice. Serum Fgf15 levels were analyzed by ELISA. Biliary and fecal BA composition was differentiated by HPLC-MS/MS. Results:Dextran sulfate sodium-treated mice with ileum-sparing colitis showed higher Fgf15 serum levels. In contrast, IL10(−/−) mice with ileitis had a trend toward decreased Fgf15 serum levels compared with controls and increased expression of Asbt as a negative Fxr-target gene. In hepatic tissue of both models, no histological changes, but higher interleukin 6 (IL-6) mRNA expression and down-regulation of Fxr and Cytochrom P450 7a1 mRNA expression were observed. Fibroblast growth factor receptor 4 up-regulation was in line with higher Fgf15 serum levels in dextran sulfate sodium-treated mice. A distinct fecal BA profile was observed in both models with significantly higher levels of taurine-conjugated BA in particular tauro-&bgr;-muricholic acid in IL10(−/−) mice. Conclusions:Ileum-sparing colitis is characterized by activation of Fxr-Fgf15 signaling with higher expression of Fxr-target gene Fgf15, whereas ileal inflammation showed no signs of Fxr-Fgf15 activation. Abundance of BA such as T-&bgr;-MCA may be important for intestinal Fxr activation in mice.

Farnesoid X receptor-dependent and -independent pathways mediate the transcriptional control of human fibroblast growth factor 19 by vitamin A.

Fibroblast growth factor 19 (FGF19) is a gut-derived hormone that controls bile acid (BA), carbohydrate and lipid metabolism. Whereas strong evidence supports a key role of BAs and farnesoid X receptor (FXR) for the control of FGF19 expression, information on other regulators is limited. In mice, FGF15 expression (ortholog of human FGF19) is induced by vitamin A (VitA) in an FXR-dependent manner. However, the significance of this finding for human FGF19 is currently unclear. Here, we demonstrate that VitA derivatives induce FGF19 in human intestinal cell lines by a direct transcriptional mechanism. In contrast to mouse FGF15, however, this direct regulation is not dependent on FXR but mediated by retinoic acid receptors (RARs) and their interaction with a novel DR-5 element in the human FGF19 gene. In addition to this direct effect, VitA derivatives impacted on the BA-mediated control of FGF19 by regulation of FXR protein levels. In conclusion, VitA regulates human FGF19 expression through FXR-dependent and -independent pathways. Moreover, we suggest that considerable mechanistic differences exist between humans and mice with regard to the nuclear receptors controlling the VitA-FGF15/19 axis. These findings may implicate a clinical relevance of RAR-activating VitA derivatives for the regulation of FGF19 levels in humans.

Nuclear Lamins in Mammalian Spermatogenesis

Nuclear lamins are important structural protein components of the nuclear envelope. The composition and properties of the nuclear lamina of spermatogenic cells differ significantly from those of somatic cells of the same species. Mammalian spermatogenic cells selectively express short lamin isoforms showing several peculiarities. In this chapter, we summarize what is known about germ line-specific lamins and discuss their possible relevance for germ cell differentiation and fertility.