The occurrence of submicrovillar endotube (modified terminal web) and associated cytoskeletal structures in the intestinal epithelia of nematodes

Abstract

The intestines of 22 genera of nematodes from five different orders were examined for the presence of an endotube, the submicrovillar entity previously described for Haemonchus contortus , a member of the order Strongylida. The endotube can be obtained by blunt dissection as a complex with the microvilli essentially free of the rest of the cytoplasm. Representatives of all three suborders and eight families of the order Strongylida possessed an endotube but of the representatives of the four other parasitic orders and the one free-living group examined only one genus, Strongyloides (Rhabditida) possessed this structure. The thickness of the endotube ranged from about 80 nm in Metastrongylus up to 6 μm in Strongylus . In all samples the filamentous cores of the microvilli, whether formed into an axial bundle (as usual) or dispersed in a net (as in Dictyocaulus ), which extended 0.1-0.5 μm below the base of the microvilli terminated in the luminal surface of the endotube. The basal side of the endotube was usually associated with a layer of microfibrils. The depth and distribution of the microfibrillar layer determined the extent to which the endotube-brush-border complexes could be dissected free from other cytoplasmic components. There was electron microscopic evidence for an endotube-like entity not associated with the microvilli in the intestine of Syphacia (Ascarida). A survey of published electron micrographs of nematode intestines indicated that the true submicrovillar endotube occurred only in members of the order Strongylida and the genus Strongyloides (Rhabditida) in which the structure here described as an endotube has previously been described as terminal web.

Isomin: a novel cytoplasmic intermediate filament protein from an arthropod species

“…The filamentous terminal web found in I. maculatus is ultrastructurally very similar to that described in some nematodes (compare our results with Figure 2 in [16]). Notably, the occurrence of such a peculiar array of IFs in the sub-microvillar region of the intestinal epithelium has not, so far, been detected in any other invertebrate group, although most phyla are likely to express IF proteins in their internal epithelia [9].…”

“…is strongly reminiscent of the terminal web described in the intestine of some nematode species [16]. In Caenorhabditis elegans , the web contains a group of IF proteins [17].…”

“…This organ is subjected to a substantial mechanical stress from all directions, due both to the transport of nutrients and to the peculiar body plan organization and motility of these organisms, which are dominated by the high hydrostatic pressure present within their body cavity. The terminal web possesses peculiar mechanical properties able to provide the apical region of the cell with enhanced stability, as it is also indicated by the possibility to dissect, from various nematode species, the so-called endotube, a single and mechanically resistant unit which consists of both microvilli and the apical layer of the epithelium, including the web [16]. …”

The novel intestinal filament organizer IFO-1 contributes to epithelial integrity in concert with ERM-1 and DLG-1

“…On top rest the microvillar rootlets that are connected to each other via the terminal web, which consists of a mesh of various cytoskeletal filaments (Hirokawa et al, 1982;Kormish et al, 2010;McGhee, 2007). The C. elegans intestine is particularly rich in IFs and is characterized by the nematode-specific electrondense and IF-containing endotube Munn and Greenwood, 1984). To identify and examine molecular regulators that are responsible for the highly specific subcellular enrichment of the intestinal IFs in C. elegans, a mutagenesis screen was performed using an IFB-2::CFP reporter that is expressed exclusively in the intestinal endotube (Hüsken et al, 2008).…”

Strategies for vaccination against gastro-intestinal nematodes

“…Clearly, however, if antibodies were to be induced to antigens at the luminal surface they might well act to the detriment of the parasite. Electron microscopy revealed that the surface of the microvilli of blood-ingesting stages is coated with material with characteristic structure (subsequently shown to be a polymeric protein) which has been named contortin (46). By means of electron microscopy, using the rapid negative staining technique to track the protein, a simple differential centrifugation scheme was developed which yielded a fraction substantially enriched in contortin from homogenates of adult worms.…”