Relevance of histone H1 kinase activity to the G2/M transition during the cell cycle ofDictyostelium discoideum

The implication of histone H1 kinase activity for the G2/M transition during the cell cycle was investigated usingDictyostelium discoideum Ax-2. Histone H1 kinase with its activity was purified from cell extracts by the use of p13^suc1 affinity gel. In the vegetative cell cycle, the activity of histone H1 kinase including Cdc2 kinase was found using synchronized Ax-2 cells to be highest just before the entry into mitosis. The activity also was markedly enhanced just prior to the M phase from which developing cells (possibly prespore cells) reinitiate their cell cycle at the mound-tipped aggregate stage. These results strongly suggest the importance of Cdc2 kinase activity in the G2 to M phase transition during the cell cycle, as the case for other eukaryotic cells.     

Kinetic properties of alternatively spliced isoforms of laccase-2 from Tribolium castaneum and Anopheles gambiae

Laccase-2 is a highly conserved multicopper oxidase that functions in insect cuticle pigmentation and tanning. In many species, alternative splicing gives rise to two laccase-2 isoforms. A comparison of laccase-2 sequences from three orders of insects revealed eleven positions at which there are conserved differences between the A and B isoforms. Homology modeling suggested that these eleven residues are not part of the substrate binding pocket. To determine whether the isoforms have different kinetic properties, we compared the activity of laccase-2 isoforms from Tribolium castaneum and Anopheles gambiae. We partially purified the four laccases as recombinant enzymes and analyzed their ability to oxidize a range of laccase substrates. The predicted endogenous substrates tested were dopamine, N-acetyldopamine (NADA), N-β-alanyldopamine (NBAD) and dopa, which were detected in T. castaneum previously and in A. gambiae as part of this study. Two additional diphenols (catechol and hydroquinone) and one non-phenolic substrate (2,2′-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid)) were also tested. We observed no major differences in substrate specificity between the A and B isoforms. Dopamine, NADA and NBAD were oxidized with catalytic efficiencies ranging from 51 to 550 min^?1 mM^?1. These results support the hypothesis that dopamine, NADA and NBAD are endogenous substrates for both isoforms of laccase-2. Catalytic efficiencies associated with dopa oxidation were low, ranging from 8 to 30 min^?1 mM^?1; in comparison, insect tyrosinase oxidized dopa with a catalytic efficiency of 201 min^?1 mM^?1. We found that dopa had the highest redox potential of the four endogenous substrates, and this property of dopa may explain its poor oxidation by laccase-2. We conclude that laccase-2 splice isoforms are likely to oxidize the same substrates in vivo, and additional experiments will be required to discover any isoform-specific functions.     

Properties of catalytic,linker and chitin-binding domains of insect chitinase

Manduca sexta (tobacco hornworm) chitinase is a glycoprotein that consists of an N-terminal catalytic domain, a Ser/Thr-rich linker region, and a C-terminal chitin-binding domain. To delineate the properties of these domains, we have generated truncated forms of chitinase, which were expressed in insect cells using baculovirus vectors. Three additional recombinant proteins composed of the catalytic domain fused with one or two insect or plant chitin-binding domains (CBDs) were also generated and characterized. The catalytic and chitin-binding activities are independent of each other because each activity is functional separately. When attached to the catalytic domain, the CBD enhanced activity toward the insoluble polymer but not the soluble chitin oligosaccharide primarily through an effect on the Km for the former substrate. The linker region, which connects the two domains, facilitates secretion from the cell and helps to stabilize the enzyme in the presence of gut proteolytic enzymes. The linker region is extensively modified by O-glycosylation and the catalytic domain is moderately N-glycosylated. Immunological studies indicated that the linker region, along with elements of the CBD, is a major immunogenic epitope. The results support the hypothesis that the domain structure of insect chitinase evolved for efficient degradation of the insoluble polysaccharide to soluble oligosaccharides during the molting process.     

Sequences of cDNAs and expression of genes encoding chitin synthase and chitinase in the midgut of Spodoptera frugiperda

The focus of this study was on the characterization and expression of genes encoding enzymes responsible for the synthesis and degradation of chitin, chitin synthase (SfCHSB) and chitinase (SfCHI), respectively, in the midgut of the fall armyworm, Spodoptera frugiperda. Sequences of cDNAs for SfCHSB and SfCHI were determined by amplification of overlapping PCR fragments and the expression patterns of these two genes were analyzed during insect development by RT-PCR. SfCHSB encodes a protein of 1523 amino acids containing several transmembrane segments, whereas SfCHI encodes a protein of 555 amino acids composed of a catalytic domain, a linker region and a chitin-binding domain. SfCHSB is expressed in the midgut during the feeding stages, whereas SfCHI is expressed during the wandering and pupal stages. Both genes are expressed along the whole midgut. Chitin staining revealed that this polysaccharide is present in the peritrophic membrane (PM) only when SfCHSB is expressed. There is little or no chitin in the midgut when SfCHI is expressed. These results support the hypothesis that SfCHSB is responsible for PM chitin synthesis during the larval feeding stages and SfCHI carries out PM chitin degradation during larval-pupal molting, suggesting mutually exclusive temporal patterns of expression of these genes.     

Mechanical properties of the beetle elytron, a biological composite material

We determined the relationship between composition and mechanical properties of elytra (modified forewings that are composed primarily of highly sclerotized dorsal and less sclerotized ventral cuticles) from the beetles Tribolium castaneum (red flour beetle) and Tenebrio molitor (yellow mealworm). Elytra of both species have similar mechanical properties at comparable stages of maturation (tanning). Shortly after adult eclosion, the elytron of Tenebrio is ductile and soft with a Young's modulus (E) of 44 ± 8 MPa, but it becomes brittle and stiff with an E of 2400 ± 1100 MPa when fully tanned. With increasing tanning, dynamic elastic moduli (E') increase nearly 20-fold, whereas the frequency dependence of E' diminishes. These results support the hypothesis that cuticle tanning involves cross-linking of components, while drying to minimize plasticization has a lesser impact on cuticular stiffening and frequency dependence. Suppression of the tanning enzymes laccase-2 (TcLac2) or aspartate 1-decarboxylase (TcADC) in Tribolium altered mechanical characteristics consistent with hypotheses that (1) ADC suppression favors formation of melanic pigment with a decrease in protein cross-linking and (2) Lac2 suppression reduces both cuticular pigmentation and protein cross-linking.     

MASP-1 Induces a Unique Cytokine Pattern in Endothelial Cells: A Novel Link between Complement System and Neutrophil Granulocytes

Microbial infection urges prompt intervention by the immune system. The complement cascade and neutrophil granulocytes are the predominant contributors to this immediate anti-microbial action. We have previously shown that mannan-binding lectin-associated serine protease-1 (MASP-1), the most abundant enzyme of the complement lectin pathway, can induce p38-MAPK activation, NFkappaB signaling, and Ca²⁺-mobilization in endothelial cells. Since neutrophil chemotaxis and transmigration depends on endothelial cell activation, we aimed to explore whether recombinant MASP-1 (rMASP-1) is able to induce cytokine production and subsequent neutrophil chemotaxis in human umbilical vein endothelial cells (HUVEC). We found that HUVECs activated by rMASP-1 secreted IL-6 and IL-8, but not IL-1alpha, IL-1ra, TNFalpha and MCP-1. rMASP-1 induced dose-dependent IL-6 and IL-8 production with different kinetics. rMASP-1 triggered IL-6 and IL-8 production was regulated predominantly by the p38-MAPK pathway. Moreover, the supernatant of rMASP-1-stimulated HUVECs activated the chemotaxis of neutrophil granulocytes as an integrated effect of cytokine production. Our results implicate that besides initializing the complement lectin pathway, MASP-1 may activate neutrophils indirectly, via the endothelial cells, which link these effective antimicrobial host defense mechanisms.     

Substrate specificity of chitinases from two species of fish, greenling, Hexagrammos otakii, and common mackerel, Scomber japonicus, and the insect, tobacco hornworm, Manduca sexta

Three chitinase isozymes, HoChiA, HoChiB, and HoChiC, were purified from the stomach of the greenling, Hexagrammos otakii, by ammonium sulfate fractionation, followed by column chromatography on Chitopearl Basic BL-03 and CM-Toyopearl 650S. The molecular masses and pIs of HoChiA, HoChiB, and HoChiC are 62 kDa and pH 5.7, 51 kDa and pH 7.6, and 47 kDa and pH 8.8, respectively. Substrate specificities of these chitinases were compared with those of another fish stomach chitinase from the common mackerel, Scomber japonicus (SjChi), as well as two from the tobacco hornworm, Manduca sexta (MsChi535 and MsChi386). The efficiency parameters, kcat/Km, toward glycolchitin for HoChiA and SjChi were larger than those for HoChiB and HoChiC. The relative activities of HoChiA and SjChi toward various forms of chitin were as follows: shrimp shell or crab shell alpha-chitin > beta-chitin > silkworm cuticle alpha-chitin. On the other hand, the relative activities of HoChiB and HoChiC were beta-chitin > silkworm alpha-chitin > shrimp and crab alpha-chitin. MsChi535 preferred silkworm alpha-chitin to shrimp and crab alpha-chitins, and no activity was observed toward beta-chitin. MsChi386, which lacked the C-terminal linker region and the chitin-binding domain, did not hydrolyze silkworm alpha-chitin. These results demonstrate that fish and insect chitinases possess unique substrate specificities that are correlated with their physiological roles in the digestion of food or cuticle.     

Chitin synthases are required for survival, fecundity and egg hatch in the red flour beetle, Tribolium castaneum

The synthesis of chitin, the beta-1,4-linked polymer of N-acetylglucosamine, is catalyzed by chitin synthase (CHS). Chitin is essential for the structural integrity of the exoskeletal cuticle and midgut peritrophic membrane (PM) of insects. To study the functions of the two chitin synthase genes, TcCHS-A and TcCHS-B, during embryonic and adult development in the red flour beetle, Tribolium castaneum, RNA interference (RNAi) experiments were carried out. When dsRNA for TcCHS-A was injected into male or female pharate adults, all insects died 5-7 d after the adult molt, and the females failed to oviposit prior to death. When dsTcCHS-A was injected into young adults 1-2 d post-eclosion, a similar lethal phenotype was obtained after 5 d and no oviposition occurred. When dsTcCHS-A injections were delayed until after adult maturation (7-10 d post-eclosion), the treated females did oviposit and the resulting embryos appeared to develop normally. However, the chitin content of the eggs was dramatically reduced, the embryos became twisted and enlarged, and the eggs did not hatch. Adults treated with dsRNA for TcCHS-B exhibited little or no chitin in their PM and died about 2 wk after injection. None of the TcCHS-B-treated females oviposited, which was probably a secondary effect caused by starvation. These results extend our previous findings that CHS genes are required for all types of molt. The present study also demonstrates that these genes have additional roles in embryonic and adult development.     

Comparison of chitinase isozymes from yam tuber--enzymatic factor controlling the lytic activity of chitinases

To evaluate the anti-pathogen activity of chitinases, we developed a new method for measuring the lytic activity, and investigated the correlation of the lytic activity with the enzymatic properties by using four chitinase isozymes, Chitinases E, F, H1 and G, which had been purified from yam tubers by column chromatography. Chitinases E, F and H1 had high lytic activity against the plant pathogen, Fusarium oxysporum, but Chitinase G did not. Chitinase E, which is the family 19 chitinase, was similar to Chitinases F and G in its antigenecity, but not to Chitinase H1 or H2. Chitinases H1 and H2 were recognized by the anti-Bombyx mori chitinase antibody, suggesting that Chitinases H1 and H2 are family 18 chitinases like B. mori chitinases. Chitinases E, F and H1 had two optimum pH ranges of 3-4 and 7.5-9 toward glycolchitin, but Chitinase G had only one optimum pH value of 5. Chitinases E, F and H1 had higher affinity to the polymer substrate, glycolchitin, than Chitinase G. These results suggest that the lytic activity of plant chitinases may be related to the chitin affinity and probably to the characteristic optimum pH value, or two values, but not related to its classification. The correlation of the lytic activity of a chitinase isozyme with its elicitor specificity is also discussed.