chapter 1 - Bentham Science

More documents

Recommendations

Info

Hemolymph Lipoproteins: Role in Insect Reproduction HPFP: Recent Advances in Insects and Other Arthropods Vol. 1 5 Until now, Vg primary proteins have been reported from several insect species (see for references a recent review by Tufail and Takeda [15]. Vgs are members of a larger superfamily of molecules known as large lipid transfer proteins [50]. Despite their overall low conservation across major taxonomic groups, their common function and disproportionate conservation of certain amino acids suggest a common phylogenetic ancestor [15, 13, 51]. 2.3. Structural Characteristics The Vg sequences are now available from 31 insect species (see 2.4, below). The sequence alignment shows that they can be aligned confidently along their entire lengths, with five particular subdomains (data not shown) where the sequences are well conserved [2, 15, 51, 52]. The most striking feature of all insect Vgs is the existence of polyserine stretches, which are at least conserved at the N-terminus of almost all insect Vgs [except Anthonomus grandis [53], Lymantria dispar [54], Pteromalus puparum [55] and Spodoptera litura [56] sequenced outside the apocritan Hymenoptera (data not shown). The cockroach, brown planthopper and mosquito Vgs are still unique in harboring these domains also at the C-termini [14, 15, 33-35, 57, 59]. However, the role of polyserine regions remains unclear. Their presence might serve as good substrates for kinases. In P. americana and L. maderae, the polyserine domains were found to be highly phosphorylated [14, 34, 59, 60] Phosphoserine tract represents extreme concentrations of negative charge [61], which may promote solubility of the Vg [62] or provide a region for chelating essential metal ions such as Ca 2+ and Fe 3+ [63, 64]. There is evidence that dephosphorylation of Vg reduces its uptake by oocytes [65, 66] which suggests that phosphorylated residues may contribute to the interaction between Vg and its receptor (VgR). The comparison of consensus cleavage site motif (RXXR) indicates that its position is particularly conserved near the N-terminus in nearly all non-apocritan insect species [see review: 15], and is flanked by polyserine domains (data not shown), except L. dispar Vg where the cleavage signal (RXXR) exists at the C-terminus [see reviews: 2, 14, 15]. The cleavage in these sites would give polypeptides corresponding to the large and small subunits as is the case of most insect Vgs [see reviews: 14, 15]. Neverthless, in the Hemimetabola members (like cockroaches and the bean bug, R. clavatus) where the pro-Vg is cleaved into several subunits, additional RXXR cleavage motifs resided near the C-terminus or at the center of the primary product are utilized [15, 34, 45]. The cleavage is accomplished by the subtilisin-like endoproteases, proprotein convertases [67, 68], which recognize a consensus R/KXXR/K sequence motif immediately preceding the cleavage site. However, the reason why Vgs are differentially cleaved in different insect species remains unclear. The other common features emerged in insect Vgs include the existence of a GL/ICG motif and cysteine residues that follow at conserved locations near the C-termini [15, 33, 52, 69, 70]. In addition, a DGXR motif has also been observed 17-19 residues upstream of the GL/ICG in all insect Vgs except for that of L. maderae Vgs [34, 60]. In Nilaparvata lugens, Homalodisca coagulata and Solenopsis invicta arginine was missing [see 15]. The GL/ICG motif and cysteine residues were necessary for oligomerization of vertebrate Vns [71, 72]. It was also demonstrated that the DG residues located near the cysteine-rich repeat contributed to establishing appropriate folding of the LDL receptor protein [73]. One possible function of insect Vns oligomerized in eggs would be binding lipids, in which inactive ecdysteroids are enclosed. The ecdysteroids are then released as Vns and undergo proteolysis during embryogenesis [11, 12]. It was proposed that the DG residues (of DGXR motif) together with the GL/ICG motif and cysteine residues might participate in forming the structure necessary for Vns to function properly during embryogenesis [33]. 2.4. Phylogenetic Relationship The phylogenetic relationship of 60 Vgs from 46 arthropod and non-arthropod species was examined based on the distances of Vg sequences (excluding the signal peptide) conducting a phylogenetic analysis using a neighbbour-joining tree construction method with MEGA version 3.1 [74] (Fig. 1). The Vg sequences used were from the following species (accession number in parentheses): Periplaneta americana (Vg1: AB034804; Vg2: AB047401), Leucophaea maderae (Vg1: AB052640; Vg2: AB194976), Blattella germanica (AJ005115), Lethocerus deyrollei (AB425334), Graptopsaltria nigrofuscata (AB026848), Plautia stali (Vg1: AB033498; Vg2: AB033499; Vg3: AB033500), Riptortus clavatus (U97277), Homalodisca coagulata (DQ118408), Nilaparvata lugens (AB353856), Anthonomus grandis (M72980),
20 Hemolymph Proteins and Functional Peptides: Recent Advances in Insects and Other Arthropods Vol. 1, 2012, 20-31 Muhammad Tufail and Makio Takeda (Eds) All rights reserved-© 2012 <strong>Bentham</strong> <strong>Science</strong> Publishers CHAPTER 2 Adipokinetic Hormones and Their Role in Lipid Mobilization in Insects Dick J. Van der Horst * and Kees W. Rodenburg Division of Endocrinology and Metabolism, Department of Biology and Institute of Biomembranes, Utrecht University, Utrecht, The Netherlands Abstract: Insect flight activity involves the mobilization, transport and utilization of endogenous energy reserves at extremely high rates. In insects engaging in long-distance flights, an elevated supply of fuel, particularly lipids, is required for extended periods of time to support the sustained activity of the flight muscles. These lipids are mobilized from triacylglycerol (TAG) stores accumulated in cytosolic lipid droplets of fat body cells. As a result of TAG mobilization, the concentration of diacylglycerol (DAG) in the insect blood (hemolymph) is increased progressively and gradually constitutes the principal fuel for flight. Peptide adipokinetic hormones (AKHs), synthesized and stored in neuroendocrine adipokinetic cells in the glandular lobes of the corpus cardiacum, play a crucial role in this process by controlling the process of lipolysis in the fat body and integrating flight energy metabolism. The onset of flight activity triggers the release of AKHs; the binding of these hormones to their G protein-coupled receptors at the fat body target cell membranes induces a number of coordinated signal transduction processes that ultimately result in the activation of fat body TAG lipolysis and the release of DAG on which long-distance flight is dependent. Recent data reveal that the mechanisms guiding mobilization of stored lipids, including the action of lipid droplet-associated TAG lipases, are conserved between insects and mammals. The transport of lipids in insect hemolymph requires specific lipoprotein carriers (lipophorins) that act as a lipid shuttle; the AKHinduced increase in DAG loading during flight activity results in reversible changes in the lipophorin particles, requiring the association of the exchangeable apolipoprotein, apolipophorin III (apoLp-III), to the particle surface to allow increased lipid uptake and efficient transport to the flight muscles. Keywords: Adipokinetic hormones (AKH), structure, biosynthesis, release, AKH-signal transduction, lipid mobilization, lipid metabolism, AKH-induced lipophorin conversion. INTRODUCTION- PERSPECTIVE AND OVERVIEW Insects that engage in long-distance flights provide a fascinating model system for metabolic key processes and their regulation during sustained physical exercise. Active insect flight muscles are among the most energydemanding tissues known, which is reflected in the extremely high metabolic rates sustained during longdistance flight. In addition, the relative simplicity of the insect system offers ample opportunities to investigate the molecular mechanisms of energy metabolism-related processes such as mobilization of endogenous energy reserves and substrate transport, as well as the regulation of these processes by metabolic neurohormones, while the insight obtained may be pertinent also to more complex vertebrate systems. In the last few decades, considerable progress has been made in the areas of biosynthesis, storage and release of the neuropeptide adipokinetic hormones (AKHs), their signal transduction in fat body target cells and metabolic responses of acute storage fat mobilization involving lipid droplet-associated triacylglycerol (TAG) lipases, and the AKHinduced lipid transport processes in the hemolymph, requiring conversions of the diacylglycerol (DAG)enriched specific lipoprotein carriers (lipophorins) that act as a lipid shuttle. These aspects have been detailed in a large number of reviews and will be reviewed briefly in the next sections. The major focus of this review, however, is on the recent advances in the key events in neurohormonal regulation of insect energy metabolism, aiming at providing an integrated, up-to-date view of the role of AKHs in insect lipid mobilization. STRUCTURE, BIOSYNTHESIS AND RELEASE OF AKHs The AKHs comprise a family of structurally related N- and C-blocked small (8-10 amino acid residues *Addresss correspondence to Dick J. Van der Horst: Division of Endocrinology and Metabolism, Department of Biology and Institute of Biomembranes, Utrecht University, Utrecht, The Netherlands; Tel: +31 30 253 3084; Fax: +31 30 253 2837; Email: d.j.vanderhorst@uu.nl
Page 2 and 3: Hemolymph Proteins and Functional P
Page 4 and 5: CONTENTS Foreword i Preface ii List
Page 6 and 7: ii PREFACE The aim of this eBook
Page 8 and 9: iv Giancarlo Lopez-Martinez Departm
Page 14 and 15: Adipokinetic Hormones and Their Rol
Page 16 and 17: 32 Hemolymph Proteins and Functiona
Page 18 and 19: 34 HPFP: Recent Advances in Insects
Page 20 and 21: Immune Response of Insects and Crus
Page 22 and 23: 78 Hemolymph Proteins and Functiona
Page 24 and 25: 80 HPFP: Recent Advances in Insects
Page 26 and 27: Structures and Functions of Insect
Page 30 and 31: Neurohormones and Second Messengers
Page 32 and 33: Role of Conventional and Unconventi
Page 36 and 37: Insulin-Related Peptides in Insects
Page 38 and 39: ENF Peptides HPFP: Recent Advances

chapter 1 - Bentham Science

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?