A novel model for fluid secretion by the trypanosomatid contractile vacuole apparatus.

Abstract

We have studied fluid secretion by the contractile vacuole apparatus of the trypanosomatid flagellate Leptomonas collosoma with thin sections and freezefracture replicas of cells stabilized by ultrarapid freezing without prior fixation or cryoprotection . The ultrarapid freezing has revealed membrane specializations related to fluid segregation and transport as well as membrane rearrangements which may accompany water expulsion at systole . This osmoregulatory apparatus consists of the spongiome, the contractile vacuole, and the fluid discharge site. The coated tubules of the spongiome converge on the contractile vacuole from all directions . These 60-to 70-nm tubules contain characteristic double rows of I Inm intramembrane particles in a helical configuration which fracture predominantly with the E face . Short double rows of similar particles are also frequently found on both faces of the contractile vacuole itself, in addition to many smaller particles on the P face . The spongiome tubules fuse with the vacuole during the filling stage of each cycle and then detach before secretion . The contractile vacuole membrane is permanently attached to the plasma membrane of the flagellar pocket by a dense adhesion plaque. In some ultrarapidly frozen cells, 20-to 40-nm perforations can be visualized within the plaque and the adjacent membranes during the presumptive time of discharge . The formation of the plaque perforations and the membrane channels occurs without fusion of the vacuole and the plasma membrane and does not require extracellular calcium . On the basis of our results, we have developed a model for water secretion which suggests that the adhesion plaque may induce pore formation in the adjoining lipid bilayers, thereby allowing bulk expulsion of the fluid .KEY WORDS secretion " contractile vacuole freeze-fracture -membrane fusion trypanosomatidThe study of the membrane events during secretion is significant for our understanding of broad physiological problems as well as for molecular models of membrane behavior. Fluid secretion in

Computer Aided Three-dimensional Reconstruction of the Free living Protozoan Bodo sp. (Kinetoplastida: Bodonidae).

“…It is believed that the contractile vacuole of bodonids empties its contents in the flagellar pocket through the area in which periodic densities are observed between the flagellar pocket and the bladder membrane (5,7,19), as observed in the trypanosomatid Leptomonascollosoma (14). It has been assumed that once the bladder is completely filled, "pores" are formed between the two membranesand a systolic process occurs.…”

A Functional Aquaporin Co-Localizes with the Vacuolar Proton Pyrophosphatase to Acidocalcisomes and the Contractile Vacuole Complex of Trypanosoma cruzi

“…Labeling with Antibodies Against Aquaporin-Acidocalcisomes have been linked to the function of a contractile vacuole complex in C. reinhardtii (3) and D. discoideum (4) and the presence of a contractile vacuole complex has been reported in different trypanosomatids (10,11), including T. cruzi (9). However, there has been only occasional mention of this structure (37) even though extensive electron microscopy studies have been done in T. cruzi.…”

“…The pulsation period was found to be between 1 min and 1 min and 15 s (8). Electron micrographs of the contractile vacuole and surrounding spongiome of other trypanosomatids have also been published (9,10).…”

Immunocytochemistry of Proteases in the Study of Leishmania Physiology and Host-Parasite Interaction

“…L. (L.) chagasi serine proteases were also located in contractile vacuoles and in vesicles located at the posterior region of the parasite body, next to the nucleus. The contractile vacuoles are intracellular vesicles immediately adjacent to the plasma membrane of the flagellar pocket and are involved in fluid secretion via this pocket (Linder and Staehelin, 1979). Other serine peptidases, such as the extracellular serine peptidases from L. (L.) amazonensis (Silva-López et al, 2004) (Figure 2) and T. cruzi (Silva-López et al, 2008) employed the same route of secretion, since they were also evidenced in the flagellar pocket and contractile vacuoles.…”