Kuz and TACE can activate Notch independent of ligand

A central mechanism in activation of the Notch signaling pathway is cleavage of the Notch receptor by ADAM metalloproteases. ADAMs also cleave Delta, the ligand for Notch, thereby downregulating Notch signals. Two ADAMs, Kuzbanian (Kuz) and TNF-alpha converting enzyme (TACE), are known to process both Delta and Notch, yet the role of these cleavages in signal propagation has remained controversial. Using an in vitro model, we show that Kuz regulates Notch signaling primarily by activating the receptor and has little overall effect on signaling via disabling Delta. We confirm that Kuz-dependent activation of Notch requires stimulation of Notch by Delta. However, over-expression of Kuz gives ligand-independent Notch activation. In contrast, TACE, which is elevated in expression in the developing Drosophila nervous system, can efficiently activate Notch in a ligand-independent manner. Altogether, these data demonstrate the potential for Kuz and TACE to participate in context- and mechanism-specific modes of Notch activation.     

Bile Acids: Chemistry, Pathochemistry, Biology, Pathobiology, and Therapeutics

Bile acids and bile alcohols in the form of their conjugates are amphipathic end products of cholesterol metabolism with multiple physiological functions. The great variety of bile acids and bile alcohols that are present in vertebrates are tabulated. Bile salts have an enterohepatic circulation resulting from efficient vectorial transport of bile salts through the hepatocyte and the ileal enterocyte; such transport leads to the accumulation of a pool of bile salts that cycles between the liver and intestine. Bile salt anions promote lipid absorption, enhance tryptic cleavage of dietary proteins, and have antimicrobial effects. Bile salts are signaling molecules, activating nuclear receptors in the hepatocyte and ileal enterocyte, as well as an increasing number of G-protein coupled receptors. Bile acids are used therapeutically to correct deficiency states, to decrease the cholesterol saturation of bile, or to decrease the cytotoxicity of retained bile acids in cholestatic liver disease.     

Myosin I: From yeast to human

Myosin I is a non-filamentous, single-headed, actin-binding motor protein and is present in a wide range of species from yeast to man. The role of these class I myosins have been studied extensively in simple eukaryotes, showing their role in diverse processes such as actin cytoskeleton organization, cell motility, and endocytosis. Recently, studies in metazoans have begun to reveal more specialized functions of myosin I. It will be a major challenge in the future to examine the physiological functions of each class I myosin in different cell types of metazoans.     

The Role of the Agouti-Related Protein in Energy Balance Regulation

The Agouti-Related Protein (AgRP) is a powerful orexigenic peptide that increases food intake when ubiquitously overexpressed or when administered centrally. AgRP-deficiency, on the other hand, leads to increased metabolic rate and a longer lifespan when mice consume a high fat diet. In humans, AgRP polymorphisms have been consistently associated with resistance to fatness in Blacks and Whites and resistance to the development of type-2 diabetes in African Blacks. Systemically administered AgRP accumulates in the liver, the adrenal gland and fat tissue while recent findings suggest that AgRP may also have inverse agonist effects, both centrally and peripherally. AgRP could thus modulate energy balance via different actions. Its absence or reduced functionality may offer a benefit both in terms of bringing about negative energy balance in obesigenic environments, as well as leading to an increased lifespan.     

Role of full-length osteoprotegerin in tumor cell biology

Osteoprotegerin (OPG) is a soluble tumor necrosis factor receptor family member, which potently inhibits RANKL-mediated osteoclastogenesis. Numerous constructs have been created for therapeutic purposes in which the heparin-binding and death homology domains of OPG were removed and the remaining peptide (amino acids 22–194) was fused to the Fc domain of human IgG1 (OPG-Fc). The administration of OPG-Fc efficiently counteracted bone loss in a variety of preclinical models of cancers. However, several in vitro studies have shown that native or recombinant full-length OPG not only neuralizes RANKL, but also the death-inducing ligand TRAIL, suggesting that OPG might potentially counteract the anti-tumor activity of TRAIL. Additional evidence suggests that full-length OPG possesses RANKL- and TRAIL-independent biological properties, mainly related to the promotion of endothelial cell survival and angiogenesis. Finally, breast tumor cells overexpressing OPG have shown increased bone metastatic potential in vivo. The relevance of these apparently conflicting findings in tumor cell biology is highlighted. Received 2 September 2008; received after revision 29 September 2008; accepted 13 October 2008     

Dissection of the molecular mechanisms that control the nuclear accumulation of transport factors importin-α and CAS in stressed cells

The physiological state of eukaryotic cells controls nuclear trafficking of numerous cargos. For example, stress results in the inhibition of classical protein import, which is characterized by the redistribution of several transport factors. As such, importin-alpha and cellular apoptosis susceptibility protein (CAS) accumulate in nuclei of heat-shocked cells; however, the mechanisms underlying this relocation are not fully understood. We now show that heat upregulates the initial docking of importin-alpha at the nuclear envelope and stimulates the translocation of CAS into the nuclear interior. Moreover, heat exposure compromises the exit of importin-alpha from nuclei and drastically increases its retention in the nucleoplasm, whereas CAS nuclear exit and retention are less affected. Taken together, our results support the idea that heat shock regulates importin-alpha and CAS nuclear accumulation at several levels. The combination of different stress-induced changes leads to the nuclear concentration of both transport factors in heat-stressed cells.     

The chemical versatility of the β–α–β fold: Catalytic promiscuity and divergent evolution in the tautomerase superfamily

Tautomerase superfamily members have an amino-terminal proline and a β–α–β fold, and include 4-oxalocrotonate tautomerase (4-OT), 5-(carboxymethyl)-2-hydroxymuconate isomerase (CHMI), trans- and cis-3-chloroacrylic acid dehalogenase (CaaD and cis-CaaD, respectively), malonate semialdehyde decarboxylase (MSAD), and macrophage migration inhibitory factor (MIF), which exhibits a phenylpyruvate tautomerase (PPT) activity. Pro-1 is a base (4-OT, CHMI, the PPT activity of MIF) or an acid (CaaD, cis-CaaD, MSAD). Components of the catalytic machinery have been identified and mechanistic hypotheses formulated. Characterization of new homologues shows that these mechanisms are incomplete. 4-OT, CaaD, cis-CaaD, and MSAD also have promiscuous activities with a hydratase activity in CaaD, cis-CaaD, and MSAD, PPT activity in CaaD and cis-CaaD, and CaaD and cis-CaaD activities in 4-OT. The shared promiscuous activities provide evidence for divergent evolution from a common ancestor, give hints about mechanistic relationships, and implicate catalytic promiscuity in the emergence of new enzymes. Received 22 May 2008; received after revision 20 June 2008; accepted 02 July 2008     

Hydroxylation of demethoxy-Q6 constitutes a control point in yeast coenzyme Q6 biosynthesis

Coenzyme Q is a lipid molecule required for respiration and antioxidant protection. Q biosynthesis in Saccharomyces cerevisiae requires nine proteins (Coq1p–Coq9p). We demonstrate in this study that Q levels are modulated during growth by its conversion from demethoxy-Q (DMQ), a late intermediate. Similar conversion was produced when cells were subjected to oxidative stress conditions. Changes in Q₆/DMQ₆ ratio were accompanied by changes in COQ7 gene mRNA levels encoding the protein responsible for the DMQ hydroxylation, the penultimate step in Q biosynthesis pathway. Yeast coq null mutant failed to accumulate any Q late biosynthetic intermediate. However, in coq7 mutants the addition of exogenous Q produces the DMQ synthesis. Similar effect was produced by over-expressing ABC1/COQ8. These results support the existence of a biosynthetic complex that allows the DMQ₆ accumulation and suggest that Coq7p is a control point for the Q biosynthesis regulation in yeast. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Received 04 September 2008; received after revision 22 October 2008; accepted 23 October 2008     

Functions and transport of silicon in plants

Silicon exerts beneficial effects on plant growth and production by alleviating both biotic and abiotic stresses including diseases, pests, lodging, drought, and nutrient imbalance. Recently, two genes (Lsi1 and Lsi2) encoding Si transporters have been identified from rice. Lsi1 (low silicon 1) belongs to a Nod26-like major intrinsic protein subfamily in aquaporin, while Lsi2 encodes a putative anion transporter. Lsi1 is localized on the distal side of both exodermis and endodermis in rice roots, while Lsi2 is localized on the proximal side of the same cells. Lsi1 shows influx transport activity for Si, while Lsi2 shows efflux transport activity. Therefore, Lsi1 is responsible for transport of Si from the external solution to the root cells, whereas Lsi2 is an efflux transporter responsible for the transport of Si from the root cells to the apoplast. Coupling of Lsi1 with Lsi2 is required for efficient uptake of Si in rice. Received 21 December 2007; received after revision 29 April 2008; accepted 15 May 2008     

Purinergic systems in microglia

Accumulating findings indicate that nucleotides play an important role in microglia through P2 purinoceptors. P2 purinoceptors are divided into two families, ionotropic receptors (P2X) and metabotropic receptors (P2Y). P2X receptors (7 types; P2X₁ – P2X₇) contain intrinsic pores that open by binding with ATP. P2Y receptors (8 types; P2Y_{1, 2, 4, 6, 11, 12, 13 and 14}) are activated by nucleotides and couple to intracellular second-messenger systems through heteromeric G-proteins. Nucleotides are released or leaked from non-excitable cells as well as neurons in physiological and pathophysiological conditions. Microglia express many types of P2 purinoceptors and are known as resident macrophages in the CNS. ATP and other nucleotides work as ‘warning molecules’ especially through activating microglia in pathophysiological conditions. Microglia play a key role in neuropathic pain, chemotaxis and phagocytosis through nucleotide-evoked activation of P2X₄, P2Y₁₂ and P2Y₆ receptors, respectively. These findings indicate that extracellular nucleotides are important players in the central stage of microglial function. Received 19 April 2008; received after revision 20 May 2008; accepted 23 May 2008     

Stem cell therapy in stroke

Recent work has focused on cell transplantation as a therapeutic option following ischemic stroke, based on animal studies showing that cells transplanted to the brain not only survive, but also lead to functional improvement. Neural degeneration after ischemia is not selective but involves different neuronal populations, as well as glial and endothelial cell types. In models of stroke, the principal mechanism by which any improvement has been observed, has been attributed to the release of trophic factors, possibly promoting endogenous repair mechanisms, reducing cell death and stimulating neurogenesis and angiogenesis. Initial human studies indicate that stem cell therapy may be technically feasible in stroke patients, however, issues still need to be addressed for use in human subjects. Received 23 June 2008; received after revision 24 September 2008; accepted 30 September 2008     

Physiological arousal: a role for hypothalamic systems

The lateral hypothalamus (LH) has long been known as a homeostasis center of the brain that modulates feeding behavior, arousal and reward. The hypocretins (Hcrts, also called orexins) and melanin-concentrating hormone (MCH) are neuropeptides produced in two intermingled populations of a few thousand neurons in the LH. The Hcrts have a prominent role in regulating the stability of arousal, since Hcrt system deficiency leads to narcolepsy. MCH is an important modulator of energy balance, as MCH system deficiency in mice leads to leanness and increased metabolism. Recently, MCH has been proposed to modulate rapid eye movement sleep in rodents. In this review, we propose a working model of the cross-talk between Hcrt and MCH circuits that may provide an arousal balance system to regulate complex goal-oriented behaviors.     

The cold case: Are rhinoviruses perfectly adapted pathogens?

Rhinoviruses, which cause common cold, belong to the Picornaviridae family, small non-enveloped viruses (diameter 15-30 nm) containing a single-stranded RNA genome (about 7 kb). Over 100 different rhinoviral serotypes have been identified thus far, establishing rhinoviruses as the most diverse group of Picornaviridae. Based on receptor binding properties, rhinoviruses are divided into two classes: the major group binding to intracellular adhesion molecule-1 and the minor group binding to the very low density lipoprotein receptors. Interactions between virus and the receptor molecules cause a conformational change in the capsid, which is a prerequisite for viral uptake. Rhinoviruses trigger a chemokine response upon infection that may lead to exacerbation of the symptoms of common cold, i.e. asthma and inflammation. The following review aims to summarize the knowledge about rhinoviral infections and discusses therapeutical approaches against this almost perfectly adapted pathogen.     

Atomic model of human cystic fibrosis transmembrane conductance regulator: Membrane-spanning domains and coupling interfaces

We describe herein an atomic model of the outward-facing three-dimensional structure of the membrane-spanning domains (MSDs) and nucleotide-binding domains (NBDs) of human cystic fibrosis transmembrane conductance regulator (CFTR), based on the experimental structure of the bacterial transporter Sav1866. This model, which is in agreement with previous experimental data, highlights the role of some residues located in the transmembrane passages and directly involved in substrate translocation and of some residues within the intracellular loops (ICL1-ICL4) making MSD/NBD contacts. In particular, our model reveals that D173 ICL1 and N965 ICL3 likely interact with the bound nucleotide and that an intricate H-bond network (involving especially the ICL4 R1070 and the main chain of NBD1 F508) may stabilize the interface between MSD2 and the NBD1F508 region. These observations allow new insights into the ATP-binding sites asymmetry and into the molecular consequences of the F508 deletion, which is the most common cystic fibrosis mutation.     

Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a highly aggressive neuroendocrine carcinoma of the skin. More than one-third of MCC patients will die from this cancer, making it twice as lethal as malignant melanoma. Despite the fact that MCC is still a very rare tumor, its incidence is rapidly increasing; the American Cancer Society estimates for 2008 almost 1 500 new cases in the USA. These clinical observations are especially disturbing as the pathogenesis of MCC is not yet fully understood; however, a number of recent reports contribute to a better understanding of its pathogenesis. Here we describe findings regarding the role of Wnt, MAPK and Akt signaling as well as possible aberrations in the p14ARF/p53/RB tumor suppressor network in MCC. Most important, and possibly with high impact on future therapeutic approaches is the demonstration that a polyomavirus has frequently integrated in the genome of the MCC cells prior to tumor development. Received 12 August 2008; received after revision 06 October 2008; accepted 22 October 2008     

hShroom1 links a membrane bound protein to the actin cytoskeleton

hShroom1 (hShrm1) is a member of the Apx/Shroom (Shrm) protein family and was identified from a yeast two-hybrid screen as a protein that interacts with the cytoplasmic domain of melanoma cell adhesion molecule (MCAM). The characteristic signature of the Shrm family is the presence of a unique domain, ASD2 (Apx/Shroom domain 2). mRNA analysis suggests that hShrm1 is expressed in brain, heart, skeletal muscle, colon, small intestine, kidney, placenta and lung tissue, as well a variety of melanoma and other cell lines. Co-immunoprecipitation and bioluminescence resonance energy transfer (BRET) experiments indicate that hShrm1 and MCAM interact in vivo and by immunofluorescence microscopy some co-localization of these proteins is observed. hShrm1 partly co-localises with β-actin and is found in the Triton X-100 insoluble fraction of melanoma cell extracts. We propose that hShrm1 is involved in linking MCAM to the cytoskeleton. D. E. Dye, S. Karlen: These authors contributed equally to this work. Received 09 October 2008; received after revision 23 November 2008; accepted 09 December 2008     

A novel member of the Rhomboid family, RHBDD1, regulates BIK-mediated apoptosis

Rhomboid family members are widely conserved and found in all three kingdoms of life. They are serine proteases and serve important regulatory functions. In the present study, a novel gene highly expressed in the testis, RHBDD1, is shown to be a new member of the Rhomboid family, participating in the cleavage of BIK, a proapoptotic member of the Bcl-2 family. The RHBDD1-involved proteolytic modification is upstream of the BIK protein degradation pathway. Mutagenesis studies show that the amino acid residues glycine142 and serine144 of RHBDD1 are crucial for its activity in cleaving BIK at a site located in the transmembrane region. Overexpression or knock-down of RHBDD1 in HEK 293T cells can reduce or enhance BIK-mediated apoptosis, respectively. The present findings suggest that, by acting as a serine protease, RHBDD1 modulates BIK-mediated apoptotic activity. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Received 31 July 2008; received after revision 16 September 2008; accepted 19 September 2008