Two Odorant-Binding Protein Genes in Mosquitoes: Comparative Genomics, Expression, and Function
Insect Odorant-Binding Proteins (OBPs) are small, water-soluble molecules that solubilize hydrophobic odorant molecules in the sensillum lymph and transport them to their cognate receptors in the olfactory receptor neurons. With the availability of the genome sequence of the African malaria mosquito, Anopheles gambiae, there has been a profound interest in the characterization and functional analyses of Obp genes in order to understand the molecular basis of mosquito host-seeking behavior. However, no direct evidence has been found for specific functions of any mosquito OBPs.
In this study, I describe the comparative genomics and expression analyses on two mosquito Obp genes (Obp1 and Obp7) as well as efforts to determine their functions. Both of these Obp genes were identified in Anopheles stephensi and only Obp7 gene was identified in Anopheles quadriannulatus by screening bacterial artificial chromosome (BAC) libraries of these species. Comparative analyses revealed several interesting features including segments of conserved non-coding sequences (CNSs) that contain potential regulatory elements relevant to olfactory tissue development and blood-feeding.
The expression profiles of these genes were examined in detail in the Asian malaria mosquito An. stephensi. Obp1 and Obp7 transcripts were significantly higher in females than male mosquitoes and they were predominantly found in the antenna, which is the primary olfactory organ of mosquitoes. Twenty-four hours after a blood meal, mRNA levels of these two genes were significantly reduced in the maxillary palp and proboscis, referred to as secondary olfactory organs of mosquitoes. These findings collectively indicate that Obp1 and Obp7 genes in An. stephensi likely function in female olfactory response and may be involved in behaviors related to blood-feeding.
To investigate the function of these Obp genes more directly, a Sindbis virus based expression system is established to knockdown the two Obp gene orthologs in Aedes aegypti. The effective knockdown of Obp1 and Obp7 genes (8 and 100-fold, respectively) is accomplished in female mosquito olfactory tissues. The potential for a systematic analysis of the molecular players involved in mosquito olfaction using this newly developed technique is discussed. Such analysis will provide the foundation for interfering with mosquito host-seeking behavior for the prevention of disease transmission.