Isoprenoid biosynthesis in hereditary periodic fever syndromes and inflammation

Mevalonate kinase (MK) is an essential enzyme in the isoprenoid biosynthesis pathway which produces numerous biomolecules (isoprenoids) involved in a variety of cellular processes. The indispensability of MK and isoprenoid biosynthesis for human health is demonstrated by the identification of its deficiency as the biochemical and molecular cause of the inherited autoinflammatory disorders mevalonic aciduria and hyperimmunoglobulinemia D and periodic fever syndrome. Since the discovery of the genetic defect, considerable progress has been made in understanding the molecular, biochemical and immunological basis of MK deficiency. Important questions such as which specific protein(s) and/or signaling pathway(s) are affected, however, remain unanswered. Resolving the complete pathophysiology of this disorder is a major challenge, but eventually will give insight into the in vivo role of MK and isoprenoid biosynthesis in inflammation and fever. This may open novel options for antiinflammatory therapies in general. Here, we give a general introduction on isoprenoid biosynthesis, the regulation thereof and deficiencies therein. We review the molecular, biochemical and immunological aspects of MK deficiency and discuss the relations between isoprenoid biosynthesis and inflammation. Finally, we compare MK deficiency with other autoinflammatory syndromes.     

Bile acids induce energy expenditure by promoting intracellular thyroid hormone activation

While bile acids (BAs) have long been known to be essential in dietary lipid absorption and cholesterol catabolism, in recent years an important role for BAs as signalling molecules has emerged. BAs activate mitogen-activated protein kinase pathways, are ligands for the G-protein-coupled receptor (GPCR) TGR5 and activate nuclear hormone receptors such as farnesoid X receptor alpha (FXR-alpha; NR1H4). FXR-alpha regulates the enterohepatic recycling and biosynthesis of BAs by controlling the expression of genes such as the short heterodimer partner (SHP; NR0B2) that inhibits the activity of other nuclear receptors. The FXR-alpha-mediated SHP induction also underlies the downregulation of the hepatic fatty acid and triglyceride biosynthesis and very-low-density lipoprotein production mediated by sterol-regulatory-element-binding protein 1c. This indicates that BAs might be able to function beyond the control of BA homeostasis as general metabolic integrators. Here we show that the administration of BAs to mice increases energy expenditure in brown adipose tissue, preventing obesity and resistance to insulin. This novel metabolic effect of BAs is critically dependent on induction of the cyclic-AMP-dependent thyroid hormone activating enzyme type 2 iodothyronine deiodinase (D2) because it is lost in D2-/- mice. Treatment of brown adipocytes and human skeletal myocytes with BA increases D2 activity and oxygen consumption. These effects are independent of FXR-alpha, and instead are mediated by increased cAMP production that stems from the binding of BAs with the G-protein-coupled receptor TGR5. In both rodents and humans, the most thermogenically important tissues are specifically targeted by this mechanism because they coexpress D2 and TGR5. The BA-TGR5-cAMP-D2 signalling pathway is therefore a crucial mechanism for fine-tuning energy homeostasis that can be targeted to improve metabolic control.     

A simple inkwriter for class experiments

Zusammenfassung Unter Verwendung eines Polythänschlauches wurde ein Tintenschreiber (Gewicht 90–110 mg) zur Registrierung von Kontraktionen isolierter Organe konstruiert. Dieser Schreiber lässt sich mit Erfolg im physiologischen und pharmakologischen Pratikum verwenden.     

Thermoperiodically induced diapause in a mite in constant darkness is vitamin A dependent

Summary By rearing the predacious miteAmblyseius potentillae in a daily temperature cycle in constant darkness it could be shown that diapause may be thermoperiodically induced. When the same experiments were performed using diets without vitamin A it appeared that vitamin A is necessary to achieve a state of reproductive diapause in this mite.2 February 1987     

Domainal evolution of a prokaryotic DNA repair protein and its relationship to active-transport proteins

The ABC excision nuclease of Escherichia coli is an ATP-dependent DNA repair enzyme composed of three protein subunits, UvrA, UvrB and UvrC. The DNA sequences of all three genes have been reported. UvrA, the component that binds directly to the DNA, and UvrB, which attaches itself to the UvrA-DNA complex, both contain consensus sequences though to be diagnostic of ATP-binding sites, although the UvrC sequence does not. We now report that a computer analysis of the UvrA sequence has revealed an unusual series of internal duplications centering around putative metal-binding sites which may be involved in the interaction with DNA. We also find a strong evolutionary relationship to a family of prokaryotic membrane-associated active-transport proteins.     

Pyruvate dehydrogenase kinase 4 expression is synergistically induced by AMP-activated protein kinase and fatty acids

Organs are flexible as to which substrates they will use to maintain energy homeostasis. Under well-fed conditions, glucose is a preferred substrate for oxidation. During fasting, fatty acid oxidation will become a more important energy source. Glucose oxidation is decreased by fatty acids, a process in which the pyruvate dehydrogenase complex (PDH) and its regulator pyruvate dehydrogenase kinase 4 (PDK4) play important roles. It is currently unknown how energy status influences PDH activity. We show that AMP-activated protein kinase (AMPK) activation by hypoxia and AICAR treatment combined with fatty acid administration synergistically induce PDK4 expression. We provide evidence that AMPK activation modulates ligand-dependent activation of peroxisome proliferator-activated receptor. Finally, we show that this synergistic induction of PDK4 decreases cellular glucose oxidation. In conclusion, AMPK and fatty acids play a direct role in fuel selection in response to cellular energy status in order to spare glucose. S. M. Houten, M. Chegary: These two authors contributed equally to this work. Received 11 July 2008; received after revision 26 January 2009; accepted 02 February 2009     

Insulin binds to brain blood vessels in vivo.

The brain has generally been considered an insulin-independent organ, because insulin does not apparently exert a direct effect on brain glucose consumption. Recently, however, insulin receptors have been detected throughout the central nervous system (CNS) of several species. Since important insights into the functional significance of brain insulin receptors might be provided by identification of the cell type(s) possessing these receptors, we have attempted to localise them morphologically using light and electron microscope autoradiography. We report here results indicating that blood vessels throughout the CNS of the rat bind plasma insulin rapidly and with considerable specificity.     

Mutations in MVK, encoding mevalonate kinase, cause hyperimmunoglobulinaemia D and periodic fever syndrome.

Hyperimmunoglobulinaemia D and periodic fever syndrome (HIDS; MIM 260920) is an autosomal recessive disorder characterized by recurrent episodes of fever associated with lymphadenopathy, arthralgia, gastrointestinal dismay and skin rash. Diagnostic hallmark of HIDS is a constitutively elevated level of serum immunoglobulin D (IgD), although patients have been reported with normal IgD levels. To determine the underlying defect in HIDS, we analysed urine of several patients and discovered increased concentrations of mevalonic acid during severe episodes of fever, but not between crises. Subsequent analysis of cells from four unrelated HIDS patients revealed reduced activities of mevalonate kinase (MK; encoded by the gene MVK), a key enzyme of isoprenoid biosynthesis. Sequence analysis of MVK cDNA from the patients identified three different mutations, one of which was common to all patients. Expression of the mutant cDNAs in Escherichia coli showed that all three mutations affect the activity of the encoded proteins. Moreover, immunoblot analysis demonstrated a deficiency of MK protein in patient fibroblasts, indicating a protein-destabilizing effect of the mutations.     

Crucial role for DNA ligase III in mitochondria but not in Xrcc1-dependent repair

Mammalian cells have three ATP-dependent DNA ligases, which are required for DNA replication and repair. Homologues of ligase I (Lig1) and ligase IV (Lig4) are ubiquitous in Eukarya, whereas ligase III (Lig3), which has nuclear and mitochondrial forms, appears to be restricted to vertebrates. Lig3 is implicated in various DNA repair pathways with its partner protein Xrcc1 (ref. 1). Deletion of Lig3 results in early embryonic lethality in mice, as well as apparent cellular lethality, which has precluded definitive characterization of Lig3 function. Here we used pre-emptive complementation to determine the viability requirement for Lig3 in mammalian cells and its requirement in DNA repair. Various forms of Lig3 were introduced stably into mouse embryonic stem (mES) cells containing a conditional allele of Lig3 that could be deleted with Cre recombinase. With this approach, we find that the mitochondrial, but not nuclear, Lig3 is required for cellular viability. Although the catalytic function of Lig3 is required, the zinc finger (ZnF) and BRCA1 carboxy (C)-terminal-related (BRCT) domains of Lig3 are not. Remarkably, the viability requirement for Lig3 can be circumvented by targeting Lig1 to the mitochondria or expressing Chlorella virus DNA ligase, the minimal eukaryal nick-sealing enzyme, or Escherichia coli LigA, an NAD(+)-dependent ligase. Lig3-null cells are not sensitive to several DNA-damaging agents that sensitize Xrcc1-deficient cells. Our results establish a role for Lig3 in mitochondria, but distinguish it from its interacting protein Xrcc1.