Tissue- and Stage-specific Expression of Two Lipophorin Receptor Variants with Seven and Eight Ligand-binding Repeats in the Adult Mosquito

Abstract

We identified two splice variants of lipophorin receptor (LpR) gene products specific to the mosquito fat body (AaLpRfb) and ovary (AaLpRov) with respective molecular masses of 99.3 and 128.9 kDa. Each LpR variant encodes a member of the low density lipoprotein receptor family with five characteristic domains: 1) ligand recognition, 2) epidermal growth factor precursor, 3) putative O-linked sugar, 4) single membrane-spanning domains, and 5) the cytoplasmic tail with a highly conserved internalization signal FDNPVY. Proposed phylogenetic relationships among low density lipoprotein receptor superfamily members suggest that the LpRs of insects are more closely related to vertebrate low density lipoprotein receptors and very low density lipoprotein receptor/vitellogenin receptor than to insect vitellogenin receptor/yolk protein receptors. Two mosquito LpR isoforms differ in their amino termini, the ligand-binding domains, and O-linked sugar domains, which are generated by differential splicing. Polymerase chain reaction and Southern blot hybridization analyses show that these two transcripts originated from a single gene. Significantly, the putative ligand-binding domain consists of seven and eight complement-type, cysteine-rich repeats in AaLpRfb and AaLRov, respectively. Seven cysteine-rich repeats in AaLpRfb are identical to the second through eighth repeats of AaLpRov. Previous analyses (1) have indicated that the AaLpRov transcript is present exclusively in ovarian germ-line cells, nurse cells, and oocytes throughout the previtellogenic and vitellogenic stages, with the peak at 24-30 h after blood meal, coincident with the peak of yolk protein uptake. In contrast, the fat body-specific AaLpRfb transcript expression is restricted to the postvitellogenic period, during which yolk protein production is terminated and the fat body is transformed to a storage depot of lipid, carbohydrate, and protein.