Alexander S. Raikhel
Professor
|
Molecular Endocrinology Molecular Immunology Transcriptional Control Biomedical Research Molecular Biology of Insect Disease Vectors
M.S. St. Petersburg (Leningrad) University, St. Petersburg, Russia, 1970
Office: 338 Entomology |
NIH MERIT Award
Editor-in-Chief, Insect Biochemistry and Molecular Biology, Pergamon/Elsevier Science
Editorial Board, Annual Review of Entomology, Annual Reviews
Associate Editor, Encyclopedia of Hormones, Academic Press
World Health Organization Advising Committee
Michigan State University Distinguished Faculty Award
Recognition Award in Physiology, Biochemistry and Toxicology, Entomological Society of America
NIH Tropical Medicine and Parasitology Study Section, Panel Member
NIH, Special Review Committee, Tropical Disease Research Units (Centers)
Organizer, Keystone Symposia "The Genetic Manipulation of Insects"
Mosquitoes transmit numerous diseases; some of them are among the most threatening in modern times. Malaria is particularly devastating, taking a heavy toll on the human population in many parts of the world by infecting over 300 million and killing over 2 million people each year. The situation in Sub-Saharan Africa, where 90% of all malaria cases occur, has become a human catastrophe. Diseases caused by mosquito-borne viruses, most importantly Dengue fever, are reaching disastrous levels in South and Central America. In the US, the West Nile encephalitis virus is rapidly spreading westward. Lymphatic filariasis, a nematode-based disease transmitted by mosquitoes, affects millions of people in world's tropical regions. The major reasons for this tragic situation are the unavailability of effective vaccines for malaria and other mosquito-borne diseases and the development of insecticide and drug resistance by the vectors and pathogens, respectively. Therefore, there is an urgent need to explore every possible avenue for developing novel control strategies against these mosquito-borne menacing diseases. My research is focusing on understanding the molecular basis of events in the mosquito reproduction linked to a blood meal and pathogen transmission. Mosquitoes serve as vectors of many devastating human diseases because they need blood feeding for development of eggs. This requirement of a blood meal, called anautogeny, results in a highly regulated cyclicity of egg production as each cycle is tightly coupled with blood intake. Anautogeny is one of most fundamental phenomena underlying the vectorial capacity of mosquitoes. Therefore, understanding the molecular and genetic basis of anautogeny is of great importance for the future development of novel approaches to vector and pathogen control.
Recent Reviews:
- Raikhel, A.S., Kokoza, V.A., Zhu, J, Martin, D., Wang, S-F., Li, C., Sun, G., Ahmed, A., Dittmer., N and Attardo, G. 2002. Molecular biology of mosquito vitellogenesis: from basic studies to genetic engineering of anti-pathogen immunity. Insect Biochem. Molec. Biol. 32: 1275-1286..
- Raikhel, A.S. 2002. Vitellogenesis of Disease Vectors, from Cell Biology to Genes. In: The Biology of Vectors. (B. Beaty and W. Marquardt, eds). Academic Press, San Diego (in Press).
- Shin, S. W., V. Kokoza, and A. S. Raikhel. 2003. Reverse genetics of mosquito innate immunity. J. Exp. Biol. 206: 3835-3843.
- Raikhel, A. S., L. McGurk, and M. Bownes. 2003. Ecdystesteroid action insect reproduction. In: Encyclopedia of Hormones.(H. L. Henry and A. W. Norman, eds/). Academic Press. Vol. 1: pp. 451-459.
Yolk Proteins and their Receptors
My research has focused on biochemical and molecular understanding of mechanism of egg maturation in mosquitoes. Cell biological studies of yolk protein precursor synthesis by the insect metabolic tissue and their accumulation by developing oocytes were followed by biochemical investigation of vitellogenin (Vg) biosynthesis, the major yolk protein precursor. The precursor-product relationship and the major steps in biosynthesis of mosquito Vg as well as the analysis of its protein sequence have been pursued. A protease responsible for specific cleavage of the pro-Vg molecule was identified. This was the first demonstration that subtilisin-like pro-protein convertases, serine proteases implicated in proteolytic processing of neurohormone precursors, are involved in the processing of a major secretory protein.
A novel biological phenomenon, discovered by my research group, is that the fat body, an insect metabolic tissue analogous to the vertebrate liver, produces several pro-enzymes in addition to Vg. These pro-enzymes are secreted into the circulatory system, accumulated by developing oocytes and stored in yolk spheres. At the onset of embryonic development, these proteases are activated to degrade vitellogenin, providing the embryo with nutrients.
In my laboratory, the first insect Vg receptor (VgR), which represents a novel group of membrane receptors belonging to the family of low-density lipoprotein (LDL) receptors, has been isolated and characterized. We are interested in understanding the binding properties of VgR and other yolk protein receptors in the mosquito oocytes. Protein modeling is being used together with binding studies to elucidate unique features of these LDL-related receptors. Transgenic studies are being used to study the regulation of VgR gene expression in germ-line cells of the mosquito ovary.
Representative Publications:
- Cho, W.L., Tsao, S.M., Hays, A.R., Walter, R, Chen, J.S., Snigirevskaya, E.S. and Raikhel, A.S. 1999. Mosquito cathepsin B-like protease involved in embryonic degradation of vitellin is produced as a latent extraovarian precursor. J. Biol. Chem. 274:13311-13321.
- Sappington, T.W. and Raikhel, A.S. 1998. Ligand-binding domains in vitellogenin receptors and other LDL-receptor family members share a common ancestral ordering of cysteine-rich repeats. J. Molec. Evol. 46: 476-487.
- Sappington, T.W., Kokoza, V.A., Cho, W.L. and Raikhel, A.S. 1996. Molecular characterization of the mosquito vitellogenin receptor reveals unexpected high homology to the Drosophila yolk protein receptor. Proc. Natl. Acad. USA 93: 8934-8939
- Chen, J.-S. and Raikhel, A.S. 1996. Subunit cleavage of insect pro-vitellogenin by a subtilisin-like convertase; characterization of the mosquito convertase. Proc. Natl. Acad. USA 93: 6186-6190.
- Seo, S. J., Cheon, H. M., J. Sun, T. Sappington, and A. S. Raikhel (2003). Tissue- and stage-specific expression of two lipophorin receptor variants with 7 and 8 ligand-binding repeats in the adult female mosquito, Aedes aegypti. J. Biol. Chem (in press).
- Attardo, G., S. Higgs, K. A. Klingler, D. L. Vanlandingham, and A. S. Raikhel. (2003). RNAi-mediated knockdown of a GATA factor reveals a link to anautogeny in the mosquito, Aedes aegypti. Proc. Natl. Acad. USA (in press). .
Molecular Endocrinology
The egg maturation in mosquitoes is cyclic and a blood meal is required for the activation of each cycle. The insect steroid hormone ecdysone plays a central stage in regulation of these developmental events. In my laboratory, we study the molecular basis for hormonal regulation of egg maturation in mosquitoes. In particular, we are interested how nuclear receptors mediate the action of ecdysone in gene activation and repression during vitellogenesis. The functional ecdysteroid receptor is a heterodimer of two nuclear receptors, ecdysone receptor (EcR) and the retinoid X receptor homolog, Ultraspiracle (USP). The mosquito EcR-USP heterodimer is capable of binding to various EcREs to modulate ecdysone regulation of target genes. The binding of the EcR/USP heterodimer to the regulatory region of the Vg gene is required for its activation. The early genes E74 and E75 are induced in response to blood feeding in vitellogenic tissues, the fat body and ovary, and their products are involved in mediating the ecdysteroid response during vitellogenesis. Early genes E74 and E75 are necessary for a high level of the Vg gene expression. Our research has shown that the cyclicity of vitellogenic ecdysteroid-mediated signaling in the mosquito fat body is regulated through USP, which sequentially forms inactive or active heterodimers with either repressors or the activator (EcR), respectively.
Representative Publications:
- Zhu, J., Miura, K., Chen, L., and Raikhel, A.S. 2000. AHR38, a homolog of NGFI-B, inhibits formation of the functional ecdysteroid receptor in the mosquito Aedes aegypti. EMBO J 19: 253-262.
- Wang, S.F., Ayer, S., Segraves, W.A., Williams, D., and Raikhel, A.S. 2000. Molecular determinants of differential ligand sensitivity of insect ecdysteroid receptors. Molec. Cell. Biol., 20: 3870-3879.
- Wang, S.F., Li, C., Zhu, J., Miura, K., Miksicek, R. J., and Raikhel, A.S. 2000. Differential expression and regulation of by 20-hydroxyecdysone of mosquito Ultraspiracle isoforms. Develop. Biol. 218: 99-113.
- Sun, G., Zhu, J.S., Li, C., L., Tu, J. and Raikhel, A.S. 2002. Two isoforms of the early E74 gene, an Ets transcription factor homologue, are utilized in the ecdysteroid hierarchy governing vitellogenesis of the mosquito, Aedes aegypti. Molec. Cell. Endocrinol. 190: 147-157.
- Zhu, J. S., K. Miura, L. Chen, and A. S. Raikhel. 2003. Cyclicity of mosquito vitellogenic ecdysteroid-mediated signaling is modulated by alternative dimerization of the RXR homolog, Ultraspiracle. Proc. Nalt. Acad. USA, 100: 544-549.
- Zhu, J., L. Chen, and A. S. Raikhel. 2003. Post-transcriptional control of the competence factor bFtz-F1 by juvenile hormone in the mosquito Aedes aegypti. Accepted for publication to Proc. Natl. Acad. USA (in press).
Transcriptional Control
Activation of transcription of hormonally controlled, tissue-specific genes involves synergistic interactions of sequence-specific transcription factors with enhancer/promoter elements of these genes. In my laboratory, we are interested to understand what exact combination of transcription factors of hormonal, stage-specific, and tissue-specific regulatory pathways bring specific expression of mosquito fat body genes. Elucidation of molecular mechanisms underlying stage- and tissue-specific expression of genes activated by a blood meal is of great importance for current efforts directed towards utilizing molecular genetics to develop novel strategies of mosquito and pathogen control. Regulatory regions of such genes can be used to express anti-pathogen effector molecules in engineered vectors in a precise temporal and spatial manner, designed to maximally affect a pathogen. The fat body is a particularly important target for engineering anti-pathogen properties because in insects, it is a potent secretory tissue releasing its products to the hemolymph, an environment or a crossroad for most pathogens. In my laboratory, we utilized various molecular and genetic tools to study cis-regulatory sites of the Vg gene responsible for stage- and fat body-specific activation of this gene via a blood-meal-triggered cascade. This research serves as the foundation for future design of mosquito-specific expression cassettes with predicted stage- and tissue specificity at the desired levels of transgene expression.
 |
 |
Representative Publications:
- Attardo, G., S. Higgs, K. A. Klingler, D. L. Vanlandingham, and A. S. Raikhel. 2003. RNAi-mediated knockdown of a GATA factor reveals a link to anautogeny in the mosquito, Aedes aegypti. Proc. Natl. Acad. USA (in press).
- Martin, D. Piulachs, M.D., Raikhel, A.S. 2001. A novel GATA factor transcriptionally represses yolk protein precursor genes in the mosquito Aedes aegypti via interaction with CtBP corepressor. Molec. Cell. Biol. 21: 164-174.
- Kokoza, V.A., Martin, D., Mienaltowski, M., Ahmed, A., Morton, C. and Raikhel, A.S. 2001. Transcriptional regulation of the mosquito Aedes aegypti vitellogenin gene by a blood-meal triggered cascade. 2001. Gene 274: 47-65.
- Kapitskaya, M.Z., Dittmer, N., Deitsch, K.W., Cho, W.L. and Raikhel, A.S. 1998. Three isoforms of the HNF-4 homologue highly expressed in the mosquito during egg maturation. J. Biol. Chem. 273: 29801-29810.
Molecular Immunology
Activation of innate immune factors in both mammals and insects shares a conserved pathway in which Rel/NF-?B transcription factors are chief regulators. Natural immune factors can be used as potential anti-pathogen effector molecules in engineered vectors. However, little is known about the pathways regulating the immune responses in mosquitoes despite the enormous importance of such knowledge for our understanding of the immune system of these disease vectors. The Rel/NF-?B transcription factor Relish performs a central role in the acute-phase response to microbial challenge by activating immune antibacterial peptides. In my laboratory, we cloned and characterized the Aedes Relish gene. This gene gives rise to alternatively spliced transcripts encoding different proteins similar to the mammalian p105 and p100 Rel/NF-kB transcription factors. We have generated Relish-mediated immune deficiency (RIMD) in transgenic Aedes by utilizing the cDNA encoding the Rel-type transcript that contained only the Rel homology domains without the IkB-like domain. The ΔRel transgene that was driven by the Vg promoter and the Vg-ΔRel transgene expression was highly activated by blood feeding. The blood-fed transgenic mosquitoes were extremely susceptible to infection by Gram (-) bacteria. We have engineered stable transformant lines of Aedes aegypti, in which the regulatory region of the Vg gene activates high-level fat body-specific expression of the potent anti-bacterial peptides, defensin and cecropin, in response to a blood meal. These studies have opened the door for future studies of the molecular and genetic basis of immunity in mosquitoes using reverse genetics.
Representative Publications:
- Ahmed, A., Martin, D., Manetti, A., Lee, W.-J., Mathiopoulos, K., Muller, H.M., Kafatos, F.C., Raikhel, A.S. and Brey, P.T. 1999. Genomic structure and ecdysone regulation of prophenoloxidase 1 gene in the malaria vector Anopheles gambiae. Proc. Natl. Acad. USA 96: 14795 - 14800.
- Kokoza, V.A., Ahmed, A., Cho, W.-L., Jasinskiene, N., James, A. and Raikhel, A.S. 2000. Engineering blood meal-activated systemic immunity in the yellow fever mosquito, Aedes aegypti. Proc. Natl. Acad. USA 97: 1624-1629.
- Shin, S. W., Kokoza, V., Ahmed, A., and Raikhel, A.S. 2002. Characterization of three alternatively spliced isoforms of the Rel/NF-kB transcription factor Relish from the mosquito Aedes aegypti. Proc. Natl. Acad. USA 99: 9978-9983.
- Shin, S. W., V. Kokoza, Y. Lobkov, and A. S. Raikhel. 2003. Relish-mediated immunodeficiency in the transgenic mosquito, Aedes aegypti. Proc. Natl. Acad. USA 100: 2616-2632.
