New directions in the chemistry of natural products: The organic chemist as a pathfinder for biochemistry and medicine

Zusammenfassung Im Rückblick und Ausblick wird anhand einiger ausgewählter Arbeiten aus dem Laboratorium des Verfassers gezeigt, wie die Ziele und Methodologie der modernen Naturstoff-Forschung sich im Laufe der letzten 20 Jahre geändert haben. Noch immer kommen die unerwartetsten Anregungen für ungewöhnliche Strukturen aus der Natur, so zum Beispiel die neuen Tier-AlkaloideHistrionicotoxin und seine Begleiter, die an einem Spiro-cyclohexyl-piperidin-Skelett, Azetylen- und Allen-Seitenketten enthalten. Mehr noch als die Struktur interessiert beimBatrachotoxin die selektive Wirkung auf den passiven Transport von Natrium-Ionen durch Membranen elektrogener Gewebe, ein Effekt, der durchTetrodotoxin reversibel aufgehoben werden kann. Heute zählt man auch die Eiweiss-Stoffe zu den wichtigen Naturprodukten. Ihre Erforschung wurde durch die Einführung selektiver Spaltungsmethoden ermöglicht und erleichtert, was am Beispiel desImmunoglobulins und desKollagens gezeigt wird. Diese Spaltungsmethoden beruhen auf der Ausnutzung benachbarter Gruppen-Effekte, die weiterhin auch als Basis zum Ausbau von Enzym-Modellen dienen. Die Beschäftigung mit der Chemie der Überträgersubstanzen der Nervenfortleitung, wie Dopamin und Noradrenalin, führte zur Auffindung von6-Hydroxydopamin, das zum ersten Mal selektiv «chemische Sympathektomie» erlaubt. In der physiologischen Inaktivierung dieser Nervenhormone spielt dieBrenzkatechin O-Methyltransferase eine grosse Rolle im Stoffwechsel, der man durch Reindarstellung und Beschreibung des Enzyms näherzukommen versucht. Im Stoffwechsel von aromatischen Substraten bedeutet die ersteIsolierung eines Arenoxyds einen wichtigen Fortschritt. Ein derart labiles Stoffwechselprodukt spielt in der langfristigen Toxikologie aromatischer Arzneimittel und in der Ätiologie von Krebs durch cancerogene Kohlenwasserstoffe eine Rolle. Zum Studium von Oxydationsmechanismen mikrosomaler oder Modell-Hydroxylierungen wird als Kriterium mehr und mehr die sogenannteNIH-Verschiebung herangezogen.

---

---

13. Paul Karrer Lecture, presented 30 June 1971, in the Aula of the University of Zurich. German title: Neue Ziele in der Naturstoffchemie: Der organische Chemiker als Wegbereiter der Biochemie und Medizin.     

Turning points in the evolution of peroxidase–catalase superfamily: molecular phylogeny of hybrid heme peroxidases

Heme peroxidases and catalases are key enzymes of hydrogen peroxide metabolism and signaling. Here, the reconstruction of the molecular evolution of the peroxidase–catalase superfamily (annotated in pfam as PF00141) based on experimentally verified as well as numerous newly available genomic sequences is presented. The robust phylogenetic tree of this large enzyme superfamily was obtained from 490 full-length protein sequences. Besides already well-known families of heme b peroxidases arranged in three main structural classes, completely new (hybrid type) peroxidase families are described being located at the border of these classes as well as forming (so far missing) links between them. Hybrid-type A peroxidases represent a minor eukaryotic subfamily from Excavates, Stramenopiles and Rhizaria sharing enzymatic and structural features of ascorbate and cytochrome c peroxidases. Hybrid-type B peroxidases are shown to be spread exclusively among various fungi and evolved in parallel with peroxidases in land plants. In some ascomycetous hybrid-type B peroxidases, the peroxidase domain is fused to a carbohydrate binding (WSC) domain. Both here described hybrid-type peroxidase families represent important turning points in the complex evolution of the whole peroxidase–catalase superfamily. We present and discuss their phylogeny, sequence signatures and putative biological function.     

Role of AGC kinases in plant growth and stress responses

AGC kinases are important regulators of cell growth, metabolism, division, and survival in mammalian systems. Mutation or deregulation of members of this family of protein kinases contribute to the pathogenesis of many human diseases, including cancer and diabetes. Although AGC kinases are conserved in the plant kingdom, little is known about their molecular functions and targets. Some of the best-studied plant AGC kinases mediate auxin signaling and are thereby involved in the regulation of growth and morphogenesis. Furthermore, certain members are regulated by lipid-derived signals via the 3-phosphoinositide-dependent kinase 1 (PDK1) and the kinase target of rapamycin (TOR), similar to its animal counterparts. In this review, we discuss recent findings on plant AGC kinases that unravel important roles in the regulation of plant growth, immunity and cell death, and connections to stress-induced mitogen-activated protein kinase signaling cascades.     

RGM is a repulsive guidance molecule for retinal axons

Axons rely on guidance cues to reach remote targets during nervous system development. A well-studied model system for axon guidance is the retinotectal projection. The retina can be divided into halves; the nasal half, next to the nose, and the temporal half. A subset of retinal axons, those from the temporal half, is guided by repulsive cues expressed in a graded fashion in the optic tectum, part of the midbrain. Here we report the cloning and functional characterization of a membrane-associated glycoprotein, which we call RGM (repulsive guidance molecule). This molecule shares no sequence homology with known guidance cues, and its messenger RNA is distributed in a gradient with increasing concentration from the anterior to posterior pole of the embryonic tectum. Recombinant RGM at low nanomolar concentration induces collapse of temporal but not of nasal growth cones and guides temporal retinal axons in vitro, demonstrating its repulsive and axon-specific guiding activity.     

Egalitarianism in young children

Human social interaction is strongly shaped by other-regarding preferences, that is, a concern for the welfare of others. These preferences are important for a unique aspect of human sociality-large scale cooperation with genetic strangers-but little is known about their developmental roots. Here we show that young children's other-regarding preferences assume a particular form, inequality aversion that develops strongly between the ages of 3 and 8. At age 3-4, the overwhelming majority of children behave selfishly, whereas most children at age 7-8 prefer resource allocations that remove advantageous or disadvantageous inequality. Moreover, inequality aversion is strongly shaped by parochialism, a preference for favouring the members of one's own social group. These results indicate that human egalitarianism and parochialism have deep developmental roots, and the simultaneous emergence of altruistic sharing and parochialism during childhood is intriguing in view of recent evolutionary theories which predict that the same evolutionary process jointly drives both human altruism and parochialism.     

Neuronal circuitry mechanism regulating adult quiescent neural stem-cell fate decision

Adult neurogenesis arises from neural stem cells within specialized niches. Neuronal activity and experience, presumably acting on this local niche, regulate multiple stages of adult neurogenesis, from neural progenitor proliferation to new neuron maturation, synaptic integration and survival. It is unknown whether local neuronal circuitry has a direct impact on adult neural stem cells. Here we show that, in the adult mouse hippocampus, nestin-expressing radial glia-like quiescent neural stem cells (RGLs) respond tonically to the neurotransmitter γ-aminobutyric acid (GABA) by means of γ2-subunit-containing GABAA receptors. Clonal analysis of individual RGLs revealed a rapid exit from quiescence and enhanced symmetrical self-renewal after conditional deletion of γ2. RGLs are in close proximity to terminals expressing 67-kDa glutamic acid decarboxylase (GAD67) of parvalbumin-expressing (PV+) interneurons and respond tonically to GABA released from these neurons. Functionally, optogenetic control of the activity of dentate PV+ interneurons, but not that of somatostatin-expressing or vasoactive intestinal polypeptide (VIP)-expressing interneurons, can dictate the RGL choice between quiescence and activation. Furthermore, PV+ interneuron activation restores RGL quiescence after social isolation, an experience that induces RGL activation and symmetrical division. Our study identifies a niche cell–signal–receptor trio and a local circuitry mechanism that control the activation and self-renewal mode of quiescent adult neural stem cells in response to neuronal activity and experience.     

Localization and functionality of microsporidian iron-sulphur cluster assembly proteins

Microsporidia are highly specialized obligate intracellular parasites of other eukaryotes (including humans) that show extreme reduction at the molecular, cellular and biochemical level. Although microsporidia have long been considered as early branching eukaryotes that lack mitochondria, they have recently been shown to contain a tiny mitochondrial remnant called a mitosome. The function of the mitosome is unknown, because microsporidians lack the genes for canonical mitochondrial functions, such as aerobic respiration and haem biosynthesis. However, microsporidial genomes encode several components of the mitochondrial iron-sulphur (Fe-S) cluster assembly machinery. Here we provide experimental insights into the metabolic function and localization of these proteins. We cloned, functionally characterized and localized homologues of several central mitochondrial Fe-S cluster assembly components for the microsporidians Encephalitozoon cuniculi and Trachipleistophora hominis. Several microsporidial proteins can functionally replace their yeast counterparts in Fe-S protein biogenesis. In E. cuniculi, the iron (frataxin) and sulphur (cysteine desulphurase, Nfs1) donors and the scaffold protein (Isu1) co-localize with mitochondrial Hsp70 to the mitosome, consistent with it being the functional site for Fe-S cluster biosynthesis. In T. hominis, mitochondrial Hsp70 and the essential sulphur donor (Nfs1) are still in the mitosome, but surprisingly the main pools of Isu1 and frataxin are cytosolic, creating a conundrum of how these key components of Fe-S cluster biosynthesis coordinate their function. Together, our studies identify the essential biosynthetic process of Fe-S protein assembly as a key function of microsporidian mitosomes.     

A mitochondrial remnant in the microsporidian Trachipleistophora hominis

Microsporidia are obligate intracellular parasites of several eukaryotes. They have a highly complex and unique infection apparatus but otherwise appear structurally simple. Microsporidia are thought to lack typical eukaryotic organelles, such as mitochondria and peroxisomes. This has been interpreted as support for the hypothesis that these peculiar eukaryotes diverged before the mitochondrial endosymbiosis, which would make them one of the earliest offshoots in eukaryotic evolution. But microsporidial nuclear genes that encode orthologues of typical mitochondrial heatshock Hsp70 proteins have been detected, which provides evidence for secondary loss of the organelle or endosymbiont. In addition, gene trees and more sophisticated phylogenetic analyses have recovered microsporidia as the relatives of fungi, rather than as basal eukaryotes. Here we show that a highly specific antibody raised against a Trachipleistophora hominis Hsp70 protein detects the presence, under light and electron microscopy, of numerous tiny ( approximately 50 x 90 nm) organelles with double membranes in this human microsporidial parasite. The finding of relictual mitochondria in microsporidia provides further evidence of the reluctance of eukaryotes to lose the mitochondrial organelle, even when its canonical function of aerobic respiration has been apparently lost.