Junctional complexes in the inner cyst tissue of the cysticercoid of Hymenolepis diminuta (Cestoda)

Role of gap junctions and hemichannels in parasitic infections

In vertebrates, connexins (Cxs) and pannexins (Panxs) are proteins that form gap junction channels and/or hemichannels located at cell-cell interfaces and cell surface, respectively. Similar channel types are formed by innexins in invertebrate cells. These channels serve as pathways for cellular communication that coordinate diverse physiologic processes. However, it is known that many acquired and inherited diseases deregulate Cx and/or Panx channels, condition that frequently worsens the pathological state of vertebrates. Recent evidences suggest that Cx and/or Panx hemichannels play a relevant role in bacterial and viral infections. Nonetheless, little is known about the role of Cx- and Panx-based channels in parasitic infections of vertebrates. In this review, available data on changes in Cx and gap junction channel changes induced by parasitic infections are summarized. Additionally, we describe recent findings that suggest possible roles of hemichannels in parasitic infections. Finally, the possibility of new therapeutic designs based on hemichannel blokers is presented.

Phagocytosis of microvilli of the metacestode ofHymenolepis diminutabyTenebrio molitorhaemocytes

A limited number of metacestodes ofHymenolepis diminutafromTenebrio molitorhad host plasmatocytes and cell debris of coagulocyte origin associated with the microvillar surface. There was evidence of phagocytosis of microvilli by plasmatocytes, involving clathrin-like endocytotic vesicles. In contrast to these otherwise ‘normal’ cysticercoids, occasional batches were ‘sticky’ and, on flushing into saline, rapidly developed large bladders possibly associated with osmotic stress. In addition, numerous small surface blebs were present. These ‘surface-stressed’ cysticercoids also had host plasmatocytes and cell debris associated with the bladder bases and the blebs, and phagocytosis of microvilli was observed. In no case had precipitation of haemolymph, melanization or encapsulation (features typical of the insect immune response) occurred. The possibility that the limited host response indicates a spatially restricted impairment of the surface compatibility of the parasite, and thus insufficient to elicit the full immune response, is discussed.

Maturation of the scolex syncytium in the metacestode ofHymenolepis diminuta, with special reference to microthrix formation

The entire surface of the early developmental stages of the cysticercoid ofHymenolepis diminutabears microvilli. Posteriad differentiation produces a microthrix-bearing surface on the presumptive scolex region, commencing before scolex retraction in cysticercoids inTenebrio molitorkept at 26°C. During sucker development, the scolex syncytial cytoplasm possesses electron-dense discoidal bodies, aligned parallel to the surface. They become associated with the apical membrane, domes are formed and electron-dense spine-like profiles become apparent. Elevation on short shafts follows, and multilaminate baseplates are then discernible. Fewer microvilli are now present and vesicles with asymmetric membranes (denser inner leaflet) occur within the syncytial cytoplasm. Before scolex retraction, this differentiation extends to the level of the suckers; thus the neck, which becomes reflected on retraction, still possesses only microvilli. After retraction, differentiation proceeds rapidly with increased growth of existing microtriches, discoidal bodies possibly contributing to the shaft support material; the reflected neck develops microtriches and the microvilli are shed into the scolex retraction cavity. The ‘asymmetric’ vesicles increase numerically, some fusing with the apical membrane and releasing their contents. Concurrently, apical membrane asymmetry (denser outer leaflet) becomes apparent. The adult-type surface is formed by 3 days post-scolex retraction, coinciding with the earliest time that the metacestode is infective to the final host.

Immunolocalization ofTaenia soliumgap junction innexins

In previous studies, ultrastructural observations revealed a large number of gap junctions (GJs) in the neck and immature proglottid tissues ofTaenia soliumtapeworms. In these helminths, cytoplasmic glycogen sacs are connected by numerous discrete GJs to other cells throughout the maturing strobilar tissue. Discontinuous sucrose gradients were used to purify membrane fractions containing GJs, which were identified by ultrastructural analysis. A trans-membrane peptide sequence from a highly conserved innexin region was used to construct a 20-amino acid synthetic peptide and used to raise polyclonal antibodies in rabbits that recognized both a 55 and a 67 kDa protein in a Western blot of the GJ-enriched pellet. Immunohistochemistry of larval and adult worm sections incubated with antiserum to the synthetic peptide and a secondary anti-rabbit IgG bound to fluorescein, revealed strong binding to the tegumentary surface of the worm, as well as patchy fluorescent areas in the parenchyma. The results indicate that both the tegument of cysticerci and adultT. soliumcontain innexin-rich membranes, which may function as a tegumentary transport system for small molecules.

Hymenolepis diminuta (Cestoda): uptake of cycloleucine by metacestodes

Cycloleucine uptake by metacestodes of H. diminuta of various ages was investigated. Absorption occurs by active mediated transport, mean Kt = 0.28 mM. Vmax values are age-related, and can be correlated to developmental changes. Cycloleucine uptake in the metacestode is very similar to that in the adult worm and the implications of this are discussed.

An ultrastructural analysis of cyst wall development in the metacestode of Hymenolepis diminuta (Cestoda)

A series of development stages (I-XI) have been devised to describe the development of the cyst wall of the metacestode of Hymenolepis diminuta. The cyst wall possesses tegumentary, muscular, fibrous and inner cyst tissues, the developmental rates and differentiation patterns of which are not identical. The tegumentary tissue differentiates posteriorly. Its microvillus-bearing distal cytoplasm remains simple until scolex retraction, after which rapid increase in depth followed by vacuolation occurs and basal membrane infoldings surround Phase 3 fibrogenesis fibrils. Senescence, which also affects the tegumentary cytons, then ensues. The muscle system development is posteriad and maturation, completed before scolex retraction, is followed by myocyton senescence. Posteriorly differentiated fibroblasts commence Phase 1 fibrogenesis after scolex retraction and the primary fibrous zone is fully established within approximately 6 days. Phase 2 and 3 fibrogenesis develop centrifugally, the fibrils of Phase 2 surrounding the tegumentary cytons and myocytons prior to their senescence, and those of Phase 3 lying more peripherally. The inner cyst tissue, established posteriorly, differentiates anteriorly, centripetally and early, the penultimate stage commencing just before scolex retraction, about 6 days after which the final maturation junctional complexes start development. Neither in vitro excystment nor infectivity of the definitive host can be satisfactorily achieved before the initial development of the primary fibrous zone. This may play a skeletal role during excystment, and is shown to be unaffected by the digestive enzymes which cause loss of cytoplasmic integrity in the outer regions of the cyst.