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be an important component of that relation- adiponectin seems to only affect energy lism11. Efficient use of metabolic fuels ship. In general, plasma adiponectin levels are expenditure. requires that the appearance of ingested inversely proportional to the adipose mass3; The demonstration that adiponectin acts nutrients or release of fuels from adipose or obese individuals have low levels, which tend in the CNS extends its function beyond the liver be coordinated with their utilization. to increase with weight loss4.Circulating lev- mediation of insulin sensitivity in liver and Obesity results when excess calorie con- els of adiponectin, particularly multimeric, muscle. Hence adiponectin seems to increase sumption is no longer coordinated with total high molecular weight forms5,correlate reli- glucose clearance through parallel actions in energy expenditure. Diabetes results when the ably with insulin sensitivity, especially in the the peripheral circulation (increasing insulin appearance of glucose in the blood is no liver. Treatment with drugs of the thiazoli- sensitivity) and the brain (increasing the rate longer appropriately coordinated with its denedione class increases the activity of of fuel oxidation). While it is widely accepted uptake and utilization in peripheral tissues. plasma adiponectin and improves insulin that the brain is the primary site for the con- The actions of adipokines such as sensitivity, as does systemic administration of trol of food intake, the notion that the brain adiponectin in the CNS are an important part adiponectin3. strongly influences glucose homeostasis is of this coordination, and their dysregulation Qi et al.2 now show that circulating not widely held9.The study of Qi et al. pro- may influence the development of both obe- adiponectin enters the cerebrospinal fluid vides yet another reason to postulate that the sity and diabetes.

and modulates the activity of neurons in sev- link between obesity and diabetes may 1. Zhang, Y. et al. Nature 372, 425–432 (1994). eral subnuclei of the hypothalamus. One key involve sites other than the pancreatic islet or 2. Qi, Y. et al. Nat. Med. 10, 524–529 observation was that centrally administered the primary target tissues of insulin such as 3. Pajvani, U.B. & Scherer, P.E. Curr. Diab. Rep. 3, 207–213 (2003). adiponectin (and to a lesser extent peptide muscle or liver. 4. Havel, P.J. Diabetes 53, S143–S151 (2004). administered peripherally) reduced body Adiponectin may still have important 5. Pajvani, U.B. et al. J. Biol. Chem. 279, weight and body fat of normal mice and mice direct actions at peripheral tissues; but a 12152–12162 (2004). 6. Ollmann, M. et al. Science 278, 135–138 (1997). with elevated glucose and adipose mass due model for adipokines is emerging that 7. Halaas, J.L. et al. Proc. Natl. Acad. Sci. USA 94, to genetic leptin deficiency. In contrast to the includes coordinated actions in the brain that 8878–8883 (1997).

http://www.nature.com/naturemedicine 8. Seeley, R.J. & Woods, S.C. Nat. Rev. Neurosci. 4, action of central leptin, weight loss following complement peripheral actions (Fig. 1). 901–909 (2003). central adiponectin administration was Leptin, for instance, appears to directly 9. Obici, S. & Rossetti, L. Endocrinology 144, mediated by increased energy expenditure increase fatty acid oxidation in muscle and 5172–5178 (2003). 10 10. Lee, Y. et al. J. Biol. Chem. 276, 5629–5635 rather than reduced food intake. adipose tissue ,while acting in the CNS to (2001). Importantly, central adiponectin also reduce food intake and alter muscle metabo- 11 Minokoshi, Y. et al. Nature 415, 339–343 (2002). elicited a significant decrease in blood glu- cose, independent of peripheral adiponectin actions to improve glucose metabolism. These findings identify the brain as an important target for adiponectin, tying the actions of adiponectin in the brain to the reg- The mosquito’s innate sting ulation of body weight and glucose home- ostasis. One corollary of this observation is Allan Saul

© 2004 Nature Publishing Group that peripherally administered adiponectin may owe some of its beneficial effect to its A mosquito similar to complement, a mammalian immune-fighting actions in the brain. Another is that substance, enables the insect to fend off the malaria parasite. adiponectin signaling directly links the regu- lation of glucose and energy homeostasis, Among the areas in the world hit by malaria, Only very few of the malaria parasites such that disruptions of adiponectin func- Africa carries the greatest burden. In addi- ingested by a mosquito can infect, develop tion could be important in the etiology of tion to the difficulties associated with and subsequently be transmitted to a new ver- both obesity and diabetes. healthcare delivery and disease control in tebrate host. These survivors are the sexual An important component of the body economically disadvantaged nations, the stages, the gametocytes. The bulk of the weight regulatory systems are melanocortin continent is home to mosquitoes that are ingested parasites, the asexual blood stages, receptors and the ligands for those recep- particularly efficient in transmitting are destroyed during digestion of the blood tors; reductions in melanocortin receptor malaria: Anopheles gambiae and its close rel- meal. Soon after the meal, gametocytes signaling results in weight gain and agouti atives. In a recent issue of Cell1,Levashina respond to a drop in temperature and to mos- mice are obese because of decreased and colleagues describe one of the defenses quito factors to such as xanthurenic acid by melanocortin signaling6.Analogous to the that a parasite encounters during its devel- undergoing a rapid transformation, ruptur- situation with leptin7,Qi et al. find that opment in the mosquito. They show that the ing their host red cells to become free-living agouti mice were insensitive to the actions TEP1 protein, previously implicated in a gametes. Fertilization of the gametes follows, of adiponectin in the brain. This suggests mosquito defense against bacteria, also and within a few hours these transform into a that leptin and adiponectin share common combats the malaria parasite. motile form, the ookinete. mediators in the central nervous system. About a day after the mosquito feeds, the However, leptin and adiponectin elicit dif- ookinete penetrates the mosquito gut wall to ferent changes in expression for ligands Allan Saul is at the Malaria Vaccine Development establish an oocyst attached to the outside of of melanocortin receptors8.Consistent with Branch, National Insitute of Allergy and Infectious the gut. Over the space of 1–2 weeks, massive this difference, leptin decreases food intake Diseases, Rockville, Maryland 20852, USA. parasite replication takes place within the and increases energy expenditure, whereas e-mail: [email protected] oocysts, which then rupture, releasing

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structural and functional characteristics, parasites traverse the gut wall, but then a including a conserved thioester bond melanization reaction encapsulates the involved in anchoring TEP1 to target malaria parasite7. pathogens6.In an earlier study, Levashina Levashina and colleagues were able to and her colleagues showed that TEP1 show that ookinetes penetrating the gut expression is greatly induced by exposure become labeled with TEP1. Surprisingly, to heat-killed Escherichia coli, that binding even in the sensitive A. gambiae line, only a of TEP1 to E. coli is disrupted by disruption relatively small proportion of the ookinetes of the thioester bond with methylamine, (20% at 32 hours after feeding) expressed and that phagocytosis of E. coli by a mos- GFP,indicating that the majority were being quito cell line was greatly reduced by RNAi killed at this stage. These parasites disappear with TEP16. without leaving the melanized capsules In their current study, the authors exam- present in the resistant line, so presumably ined whether this same protein participates in the dead parasites are lysed. In contrast, the mosquito attack against malaria parasites. silencing of TEP1 using For this study, the authors did not use RNAi resulted in a fivefold increase in the Plasmodium falciparum,which causes most number of viable oocysts, suggesting that malaria deaths in humans, but the related essentially all of the ookinetes transformed Plasmodium berghei,a parasite of an African into oocysts.

Reprinted with permission of Cell Press rodent. This mosquito is much easier to The effect of TEP1 silencing in the resist- Figure 1 Parasite invasion. GFP-labelled maintain in the laboratory and allows the ant line was even more striking. No Plasmodium berghei in the salivary glands of a insect stages to be more easily studied. In melanized parasites were observed, and large susceptible Anopheles gambiae mosquito. addition, the authors enhanced parasite iden- numbers of oocysts developed normally. http://www.nature.com/naturemedicine tification and provided a marker of viability Intriguingly, the authors found that TEP1 sporozoites. These make their way into the by using a transgenic P. berghei that expresses itself may be one of the several that salivary glands (Fig. 1), ready for transmis- the fluorescent protein GFP. confers the resistance phenotype. They sion to another vertebrate host. At each The study also made use of A. gambiae found many differences between the stage, there are substantial losses in parasite lines that had been selected for susceptibility sequences of TEP1 from sensitive and refrac- numbers2.For a few hours after the blood or resistance to infection by malaria para- tory mosquitoes, especially in the part of the meal, parasites are still vulnerable to com- sites. In the resistant mosquitoes, malaria protein containing the critical thioester plement and antibody in the blood meal. Later, components of the mosquito’s immune system join the attack1 (Fig. 2). Even in areas with high transmission, most mosquitoes are infected with only a few Oocyst

oocysts. Ookinetes

© 2004 Nature Publishing Group 3 Analysis of the A. gambiae genome, cou- TEP1s LRIM1 pled with comparative genomic analysis of Lysed parasite the Drosophila melanogaster genome, has Macrogamete Nitric oxide Melanized identified multiple gene families encoding Microgametes Mosquito proteases parasite involved in the mosquito host Anti-microbial TEP1r LRIM1 defenses4.These proteins constitute at least peptides Anti-gamete two signaling pathways, the Toll and Imd antibody pathways, and three killing mechanisms, Complement phagocytosis, lysis and humoral melanization TEP1 (in which parasites are entombed in a layer of

melanin pigment). Functional analysis of Ann Thomson these genes has been greatly enhanced by another recent development: direct gene silencing by RNA interference (RNAi). Figure 2 Malaria parasites survive multiple points of attack to infect a mosquito. Parasite present Injection of double-stranded RNA into the in the blood soon after the mosquito feeds (such as the micro- and macrogametes) can be killed by complement and antibodies ingested in the blood. The survivors are fertilized and differentiate hemolymph of adult mosquitoes results in into ookinetes. By the time this occurs, complement is destroyed by mosquito proteases. However, silencing of the target gene, allowing a rela- the ookinetes are still not safe: they may be killed by these proteases or by mosquito antimicrobial tively rapid analysis of loss of function5. peptides. Malaria infection also induces nitric oxide synthetase and the resulting NO kills Among the proteins identified by these parasites as they traverse the mosquito gut cells. Levashina and colleagues have found that the procedures, one, TEP1, is strikingly similar mosquito complement-like protein, TEP1, present in the hemolymph (fluid bathing the outside of to C3, a component of the mammalian the gut) binds to parasites. Parasites with high levels of bound TEP1 (red-coated parasites) are lysed in susceptible (TEP1s) A. gambiae. In a refractory line (carrying the TEP1r allele), all complement system. In vertebrates, this parasites are either lysed or are neutralized by melanization. Other components of the mosquito component of the innate immune system immune systems, such as LMIR1, are required for the killing of TEP1-targeted parasites. In TEP1 binds to foreign organisms, targeting them knockout mosquitoes, apparently all parasites that traverse the gut develop into oocysts in both for destruction. TEP1 and C3 have similar refractory and susceptible A. gambiae.

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bond. Clearly TEP1 targets parasites for specifically upregulate the mosquitoes’ through the presence of genetic markers destruction by subsequent lysis or melaniza- innate immunity4.Even without generating such as sickle-cell anemia. This has allowed tion in both the sensitive and refractory lines completely refractory mosquitoes, a reduc- the study of the coevolution of malaria para- (Fig. 1). It is possible that the additional tion in the average number of oocysts devel- sites and their vertebrate hosts. Now, the killing in the refractory mosquitoes may oping in infected mosquitoes may make study of malaria in mosquitoes is proving to relate to the sequence differences between other methods of malaria control, such as be a similarly productive area for under- the different versions of TEP1. transmission-blocking vaccines, work much standing the innate immune system and the In our urgency to find effective control more effectively. coevolution of parasites and their inverte- methods for malaria, there is a strong temp- However, possible applications of this brate hosts. tation to view the importance of this work, work should not overshadow the interesting and related studies of other components of and important basic science that these stud- 1. Blandin, S. et al. Cell 116, 661–670 (2004). 8 2. Sinden, R.E. Cell Microbiol. 4, 713–724 (2002). the mosquito’s immune system , largely in ies are unraveling. The development of 3. Holt, R.A. et al. Science 298, 129–149 (2002). terms of likely practical benefit. These stud- malaria vaccines has been a significant driv- 4. Christophides, C.K., Vlachou, D. & Kafatos, F.C. Immunol. Rev. 198, 127–148 (2004). ies certainly could lead to better control ing force for a better understanding of adap- 5. Blandin, S. et al. EMBO Rep. 3, 852–856 (2002). methods involving the selection of refrac- tive immunity in the vertebrate host. Malaria 6. Levashina, E.A. et al. Cell 104, 709–718 (2001). tory mosquitoes through conventional is one of the few diseases that has exerted a 7. Collins, F.H. et al. Science 234, 607–610 (1986). genetics or creation the of transgenic mos- strong enough evolutionarily pressure on 8. Osta, M.A., Christophides, G.K. & Kafatos, F.C. quitoes, or even the use of compounds to humans that we can readily see its impact Science 303, 2030–2032 (2004).

A COG in the sugar machine http://www.nature.com/naturemedicine Thorsten Marquardt

Defects in a Golgi protein, COG7, underlie a disorder that kills children in the first year of life. The disorder is the first to be defined in a class that will likely expand with future studies (pages 518–523).

Metabolic specialists and pediatric neurol- by a defect of a glycosyltransferase, glycosi- linked glycans. Since the enzymatic defects ogists encounter a patient nearly every day dase or sugar-nucleotide transporter result in glycoproteins and lipids with miss- with a severe and life-threatening multisys- involved in the biosynthesis of N- or O- ing or structurally abnormal glycans, it is tem disease that, despite all efforts, they cannot diagnose. In this issue, Wu and Street et al.1 provide hope by tracing the underpinnings of a previously undefined

© 2004 Nature Publishing Group disorder. The authors describe a congenital disorder of glycosylation (CDG), caused by a defect that, among other effects, uniquely disrupts the machinery that adds sugar CDG-IIa CDG-IId groups to proteins. The work shows that a

simple blood test, careful examination of GlcNAc Fucosyltransferase Galactosyl- Sialyltransferase cells and well-designed experiments can Asn transferase II VIII transferase lead to the discovery of new diseases, and GDP-Fuc UDP-gal CMP-Sial Golgi suggests that similar approaches may crack many of these unsolved cases in the future. Inherited metabolic disorders that affect CDG-IIc glycan biosynthesis have been a topic of intense research since the discovery of the Cytoplasm Cog2 Cog7Cog2 Cog7 Cog1 Cog4 Cog5 Cog8 Cog1 Cog4 Cog5 Cog8 first enzymatic defect in 1995 (ref. 2). Since Cog3 Cog6 Cog3 Cog6 then, 15 different disorders (termed CDG) have been discovered in rapid succession3. In most cases, CDG patients are severely ill Figure 1 Glycosylation reactions in the Golgi. Membrane-anchored glycosyltransferases from birth, and many die within the first modify the N-glycan bound to asparagine (Asn) residues of the protein. Congenital two years of life. The disorders are caused disorders of glycosylation are designated by a red cross. The conserved oligomeric Golgi (COG) complex is attached to the cytoplasmic surface of the Golgi and keeps glycosyltransferases and sugar nucleotide transporters in place. Disruption of the complex caused by COG7 deficiency leads to mislocalization of interacting enzymes and transporters resulting in Thorsten Marquardt is at the University of Münster, hypoglycosylation of glycoproteins. Wu and Street et al.1 have found that COG7 deficiency Department of Pediatrics, Albert-Schweitzer-Str. 33, underlies a severe multisystem disease that kills children shortly after birth. 48149 Münster, Germany. Blue square = N-acetylglucosamine, red circle = mannose, green rhombus = galactose, e-mail: [email protected] red rhombus = sialic acid, gray triangle = fucose.

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