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

摘要

The pH profile, gross structure, ultrastructure and immunolabelling of A. aegypti larval midgut were described as a first step in analysing the role of plasma membrane H+ V-ATPase in the alkalization of the gut, nutrient uptake and ionic regulation.Binding of an antibody to H+ V-ATPase subunit E colocalizes with 'portasomes' (～10 nm in diameter), which are thought to correspond to the V1 part of the H+ V-ATPase. In gastric caeca (pH 8), both antibody-binding sites and portasomes are located apically; in the anterior midgut (pH 10-11), they are located basally; and in the posterior midgut (pH～8) they are again located apically. The hypothesis that the energization of alkalization is mediated by an H+ V-ATPase is supported by the inability of larvae to maintain the high pH after 72 h in 10μM bafilomycin B1. Confirming earlier reports, the two principal epithelial cell types are designated as 'columnar' and 'cuboidal' cells. The apical plasma membranes (microvilli) of epithelial cells in the gastric caeca and basal infoldings of anterior midgut are invaded by mitochondria that lie within～20 nm of the portasome-studded plasma membranes. The colocalization of V-ATPase-immunolabelling sites and portasomes to specific plasma membranes within so-called 'mitochondria-rich' cells of gastric caeca and anterior midgut suggests that midgut alkalization in mosquitoes is achieved by molecular mechanisms similar to those that have been described in Manduca sexta larvae, even though the gross structure of the midgut and the localization of the V-ATPase are dissimilar in the two species. In M. sexta larvae, the high alkalinity is thought to break down dietary tannins, which block nutrient absorption; it may play a similar role in plant-detritus-feeding mosquito larvae. The colocalization of immunolabelling sites and portasomes, together with the presence of long, 'absorptive-type' microvilli in the posterior midgut, suggest that the V-ATPase energizes nutrient uptake there.