Antibody to H+V-ATPase subunit E colocalizes with portasomes in alkaline larval midgut of a freshwater mosquito (Aedes aegypti L.)

The pH profile, gross structure, ultrastructure and immunolabeling of the m osquito (Aedes aegypti) larval midgut are described as a first step in anal yzing the role of plasma membrane H+ V-ATPase in the alkalization of the gu t, nutrient uptake and ionic regulation. Binding of an antibody to Hf V-ATP ase subunit E colocalizes with 'portasomes' (approximately 10 nm in diamete r), which are thought to correspond to the V-1 part of the H+ V-ATPase, In gastric caeca (pH 8), both antibody-binding sites and portasomes are locate d apically; in the anterior midgut (pH 10-11), they are located basally; an d in the posterior midgut (pH approximate to 8) they are again located apic ally. The hypothesis that the energization of alkalization is mediated by a n H+ V-ATPase is supported by the inability of larvae to maintain the high pH after 72 h in 10 mu M bafilomycin B1. Confirming earlier reports, the tw o principal epithelial cell types are designated as 'columnar' and 'cuboida l' cells. The apical plasma membranes (microvilli) of epithelial cells in t he gastric caeca and basal infoldings of anterior midgut are invaded by mit ochondria that lie within approximately 20 nm of the portasome-studded plas ma membranes. The colocalization of V-ATPase-immunolabeling sites and portasomes to speci fic plasma membranes within so-called 'mitochondria-rich' cells of gastric caeca and anterior midgut suggests that midgut alkalization in mosquitoes i s achieved by molecular mechanisms similar to those that have been describe d in caterpillars, even though the gross structure of the midgut and the lo calization of the V-ATPase are dissimilar in the two species. In caterpilla rs, the high alkalinity is thought to break down dietary tannins, which blo ck nutrient absorption; it may play a similar role in plant-detritus-feedin g mosquito larvae. The colocalization of immunolabeling sites and portasome s, together with the presence of long, 'absorptive-type' microvilli in the posterior midgut, suggest that the V-ATPase energizes nutrient uptake there .