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Lipids stored in lipid-bodies (LBs) in host cells are potential sources of fatty acids for pathogens. However, the mechanism of recruitment of LBs from the host cells by pathogens to acquire fatty acids is not known. Here, we have found that Leishmania specifically upregulates the expression of host Rab18 and its GEF, TRAPPC9 by downregulating the expression of miR-1914-3p by reducing the level of Dicer in macrophages via their metalloprotease gp63. Our results also show that miR-1914-3p negatively regulates the expression of Rab18 and its GEF in cells. Subsequently, Leishmania containing parasitophorous vacuoles (Ld-PVs) recruit and retain host Rab18 and TRAPPC9. Leishmania infection also induces LB biogenesis in host cells and recruits LBs on Ld-PVs and acquires FLC12-labeled fatty acids from LBs. Moreover, overexpression of miR-1914-3p in macrophages significantly inhibits the recruitment of LBs and thereby suppresses the multiplication of parasites in macrophages as parasites are unable to acquire fatty acids. These results demonstrate a novel mechanism how Leishmania acquire fatty acids from LBs for their growth in macrophages. Author summary: Leishmania donovani which causes fatal disease known as visceral leishmaniasis, is dependent on its host to scavenge fatty acids for its survival. But, how Leishmania get fatty acids from host macrophages is not known. Lipid bodies are unique organelles that contain neutral lipids and fatty acids, therefore, LBs in macrophages could be the potential source of fatty acids for parasites. We have found that Leishmania upregulate the expression of host Rab18 and its GEF by downregulating the expression of miR-1914-3p in infected macrophages via the Ld-gp63-mediated inhibition of dicer expression. As biogenesis of LBs is shown to be regulated by Rab18, we have found that Leishmania infection induces LB biogenesis in macrophages and subsequently recruits LBs along with Rab18 and its GEF on Leishmania containing parasitophorous vacuoles. Finally, parasites-containing vacuoles acquire FLC12-labeled fatty acids from LBs to meet their nutritional requirement. We have also shown that overexpression of miR-1914-3p in macrophages significantly inhibits the recruitment of LBs and thereby suppresses the growth of parasites in macrophages as parasites are unable to acquire nutrients from LBs. These results demonstrate how Leishmania acquire nutrients like fatty acids from LBs for their proliferation in macrophages. [ABSTRACT FROM AUTHOR]

Background: Ixodes ricinus ticks are infected with a large diversity of vertically and horizontally transmitted symbionts. While horizontally transmitted symbionts rely on a vertebrate host for their transmission, vertically transmitted symbionts rely more on the survival of their invertebrate host for transmission. We therefore hypothesized horizontally transmitted symbionts to be associated with increased tick activity to increase host contact rate and vertically transmitted symbionts to be associated with higher tick weight and lipid fraction to promote tick survival. Methods: We used a behavioural assay to record the questing activity of I. ricinus ticks. In addition, we measured weight and lipid fraction and determined the presence of ten symbiont species in these ticks using qPCR, of which six were vertically transmitted and four horizontally transmitted. Results: Vertically transmitted symbionts (e.g. Midichloria mitochondrii) were associated with an increase in tick weight, whereas horizontally transmitted symbionts (e.g. Borrelia burgdorferi sensu lato) were often associated with lower weight and lipid fraction of ticks. Moreover, horizontally transmitted symbionts (e.g. B. burgdorferi s.l.) were associated with increased tick activity, which may benefit pathogen transmission and increases tick-borne disease hazard. Conclusions: Our study shows that horizontally and vertically transmitted symbionts differentially influence the behaviour and physiology of I. ricinus and warrants future research to study the underlying mechanisms and effects on transmission dynamics of tick-borne pathogens. [ABSTRACT FROM AUTHOR]

Striking morphological transformations characterize the invasion of a red blood cell by the malaria parasite. Shortly after the infection, parasite-induced membranes appear in the cytosol of the affected host erythrocyte. One intensely investigated membrane type, commonly called Maurer's clefts, has a slit-like morphology and can be arranged in the form of extended three-dimensional membrane stacks or networks. Here we report the three-dimensional reconstruction of a second membrane type, giant or extended membrane rings/loops, that have only occasionally been described on single ultrathin sections, however that have never been systematically examined so far. Serial ultrathin sectioning of P. falciparum-infected red blood cells, subsequent three-dimensional reconstructions, and in addition examination of Giemsa-stained blood films revealed that intraerythrocytic membrane rings/loops are not isolated structures but are locally in contact with the parasite. They consist either of the parasitophorous vacuolar membrane alone or contain the parasitophorous vacuolar membrane including the plasma membrane of the parasite and small amounts of parasite cytoplasm. We demonstrate that membrane rings/loops represent surface extensions of the parasite that maybe involved in ring stage parasite formation and Maurer's cleft generation at least in a subset of infected red blood cells., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Takophrya infusionum like all other Suctoria lacks an oral cavity. Its feeding apparatus consists of tentacles, long narrow tubes through which the contents of the living prey are ingested. For normal growth, reproduction, and longevity of clones, Tokophrya needs supplements deriving from the medium in addition to living prey. Since Tokophrya lacks a mouth, these supplements can reach the cytoplasm only through the complex structure of the cortex, which is composed of a three-membraned pellicle and a dense epiplasm. In addition, external to the cortex, an extraneous coat covers the whole organism. Only the outer pellicular plasma membrane is continuous; the other two and the epiplasm are interrupted by the outer plasma membrane which invaginates at intervals forming the so-called pits. The invaginated plasma membrane dips down into the cytoplasm where it extends to form a saccule. Experiments with cationized ferritin and Thorotrast provide evidence that internalization of these macromolecules takes place through the pits by pinocytosis. The membrane of the saccules of the pits forms invaginations which pinch off giving rise to small, flattened vesicles containing the tracers. The tracers were never found free in the cytoplasm but exclusively in the flat vesicles. These vesicles are thus the vehicles transporting macromolecules from the medium to the cytoplasm. The saccules of the pits are the natural loci of pinocytosis and together with the flattened vesicles perform an important function in Suctoria, supplying the organisms with macromolecules from the medium.

Intracellular sporozoan parasites invade the host cell through the invagination of the plasma membrane of the host and a vacuole is formed which accommodates the entering parasite. The vacuole may disappear and the invaginated membrane of the host then becomes closely apposed to that of the parasite's own membrane. As a result the parasite is covered by two membranes. Members of the class Piroplasmea differ from other Sporozoa in that their trophozoites are covered by a single membrane. By screening numerous sections of intraerythrocytic Babesia microti belonging to the class Piroplasmea, it was found that merozoites of Babesia enter the erythrocytes of hamsters in the same way as those of the other Sporozoa. When a merozoite touches the red blood cell with its anterior end it becomes attached to the membrane of the host, which starts to invaginate and a parasitophorous vacuole is formed. The vacuolar space disappears rapidly and the membrane of the vacuole and that of the parasite become closely adjacent. At this stage the parasite is surrounded by two plasma membranes. The outer membrane derived from the invaginated host membrane disintegrates quickly and the parasite is left with a single membrane throughout its life span.

We determined how long Babesia microti survive in the salivary glands of nymphal Ixodes dammini. Of those ticks held at 21 C, the proportion with demonstrable piroplasms decreased from 95%, prior to 20 weeks post-larval-feeding (p-l-f), to less than 80% at 42 weeks p-l-f. Similarly, the number of infected acini decreased significantly. Nymphal I. dammini were kept alive for as long as 1 year, by transferring them to 4 C, at 20 weeks p-l-f. The proportion of infected ticks at 52 weeks p-l-f was less than half of the proportion of infected nymphs examined prior to 20 weeks p-l-f, and only 1/6 as many acini were infected. Ultrastructural observations of salivary glands from ticks at 44 weeks p-l-f revealed that B. microti parasites in older ticks remain in the sporoblast meshwork phase; such parasites rarely differentiated into sporozoites. Degenerating parasites containing autophagic vacuoles were also observed in older ticks.

A peritrophic membrane (PM) has been demonstrated in the gut of feeding larvae, nymphs, and adults of the tick Ixodes dammini. This is the first report of a PM in ticks. This temporary structure divides the lumen of the gut into two compartments, an endoperitrophic space, the lumen proper, and an ectoperitrophic space located between the PM and the epithelial cells of the gut wall. The PM is a mechanical barrier and even such small particles as ribosomes derived from ingested reticulocytes are retained in the lumen proper; they are never found in the ectoperitrophic compartment. In Ixodes dammini fed on hamsters infected with Babesia microti some of the parasites are found in the ectoperitrophic space. This passage is accomplished by a highly specialized organelle, the arrowhead, which develops in some Babesia during their metamorphosis in the gut of the vector. The arrowhead, while passing through the PM, changes its fine structure and loses its internal organization as if releasing some of its contents. Its disintegration continues and it disappears shortly after the Babesia have entered the epithelial cells. Only Babesia equipped with the arrowhead structure are able to cross the PM. This is the first documented case of a parasite traversing a solidified PM.

Tokophrya infusionum, a suctorian, is deprived of a mouth opening. The uptake of ferritin from the medium is accomplished through pits, invaginations of the plasma membrane which are permanent structures. From the pits containing ferritin, flat vesicles are pinched off transporting the ferritin to the cytoplasm.

During feeding a peritrophic membrane (PM) is formed in the gut of the tick Ixodes dammini, dividing the lumen of the gut into an ecto- and endoperitrophic space. Babesia and all food particles ingested with the blood meal by the tick are retained in the endoperitrophic space, the lumen proper. Only Babesia equipped with a highly specialized organelle, the arrowhead, are able to pass the PM and enter the ectoperitrophic compartment. During the crossing of the PM the arrowhead loses its density, suggesting that enzymes released from it dissolve the polymers in the PM, making passage of the parasite through this barrier possible. In the ectoperitrophic space the arrowhead of Babesia touches the epithelial cell. At the point of contact the membrane of the host cell starts to invaginate, and simultaneously the arrowhead's fine structure loses its highly organized pattern. The growing host membrane encircles the parasite and the arrowhead diminishes progressively in size. When the piroplasm is inside the host cell, the arrowhead can no longer be found. During invasion the host membrane often touches the parasite's plasma membrane at the site of a coiled structure, and the host membrane becomes ruptured and the nearby host cytoplasm appears to be lysed. Babesia inside the host cell is covered solely by its own plasma membrane; the invaginated host membrane is missing. It is postulated that the latter disintegrates during invasion by the parasite through the action of enzymes from the coiled structure. The parasite is surrounded by a halo of homogeneous material deriving most probably from the lysed host cytoplasm.

Tick larvae were permitted to feed on infected hamsters and then allowed to molt. Nymphs were examined just prior to feeding on uninfected hamsters or at timed intervals thereafter. Invasion of the salivary gland by B. microti occurs before feeding of the nymph begins, and development of the parasite is further stimulated by feeding. The sporoblast forms a massive multinucleated meshwork which ramifies throughout the large host cell. No separation of the meshwork into multiple subdivisions, termed "cytomeres" by other workers, has been detected. Instead the specialized organelles characteristic of sporozoites, namely micronemes, rhoptries, and segments of double membrane appear in the meshwork itself and gradually become organized into sporozoite anlagen which protrude from its surface. At the same time the meshwork shortens and thickens giving rise to large compact undifferentiated bodies whose surface is also studded with sporozoite anlagen. Sporozoites thus originate either from the meshwork or from the undifferentiated bodies. In either case large lobate nuclei send projections into the anlagen as they protrude from the surface of the sporoblast. In a final step the mature sporozoites arise by simultaneous nuclear and cytoplasmic divisions. There is no separate stage of schizogony and the process is one of true budding.

Protozoa of the closely related genera Babesia and Theileria are intraerythrocytic parasites of vertebrates. They have a complex life cycle that includes development in an intermediate vector host, a tick. Whether sexual stages occur in the tick has been a subject of great controversy. The small size of the organism and the complexity of developmental stages in the gut of the tick have prevented a definitive solution of this problem. By means of a simple and straightforward although time-consuming method, it became possible to demonstrate gametes and their sexual fusion in Babesia microti developing in the gut of larvae of the tick Ixodes dammini. Tick larvae fed on hamsters infected with a human strain of B. microti were fixed and processed for electron microscopy. It was found that some of the parasites formed a unique structure shaped like an arrowhead. Because it was suspected that these forms might represent gametes, a search was made for pairs of parasites that were fusing and with each member of the pair emerging from a different erythrocyte. Such a fusing pair could not possibly represent a parasite undergoing division. By study of serial sections such pairs were indeed found. In every case one member of the pair of gametes had an arrowhead structure. This proves sexuality of B. microti and makes highly likely its existence in all members of the genera Babesia and Theileria.

Babesia microti is a highly polymorphic organism. To unravel its fine structure and the function of organelles it was necessary to resort often to serial sections. A single plasma membrane covers the organism. In trophozoites approaching reproduction, segments of double membranes can be found below the plasma membrane. In electron micrographs of poor resolution these segments of double membranes look like pieces of thick membranes and they were often thought to be a thick 2nd membrane. Before the segments of double membranes appear 2 other organelles are formed in older trophozoites: micronemes and rhoptries. There are indications that these structures originate from vesicles of the Golgi apparatus. Large dense bodies of the same structure as the host cytoplasm are not food vacuoles but merely invaginations of host cytoplasm, as found in serial sections and in organisms removed from the host cell. Feeding in Babesia seems to take place by a special organelle composed of tightly coiled double membranes located partly inside and partly outside the parasite. It is assumed that extracellular digestion of host cytoplasm take place through this organelle. The nucleus remains undifferentiated throughout the whole intraerythrocytic stage. It becomes irregular, loboid, but does not divide and remains a single body until the late stage of reproduction when only a small portion, a bud, extends into the forming merozoite.

In a previous paper (8) an organized structure was described in the macronuclei of certain old organisms of Tokophrya infusionum. It was found that the same honeycomb structure appears in great abundance in the macronuclei of overfed organisms. This permitted a better three-dimensional reconstruction of the described structure. Since the defined structure may be experimentally induced, it offers an opportunity for further more detailed studies as to its nature and meaning.

OsO(4) has many advantages over Carnoy's fixative mixture for the Feulgen nuclear staining in the protozoan Tokophrya infusionum. While Carnoy's fluid used prior to the Feulgen reaction produces shrinkage of the macronucleus and coarse clumping of its chromatin bodies, OsO(4) preserves faithfully the size and shape of the macronucleus and its chromatin material. This finding seems to be of special importance in view of the fact that electron microscopy relies on OsO(4) fixation. The satisfactory preservation of structured detail in Feulgen-stained preparations is of importance for the correlation of histochemical and morphological information.

Blood collected from rats infected with Plasmodium berghei was centrifuged and the pellet was fixed for 1 hour in 1 per cent buffered OsO(4) with 4.9 per cent sucrose. The material was embedded in n-butyl methacrylate and the resulting blocks sectioned for electron microscopy. The parasites were found to contain, in almost all sections, oval bodies of the same density and structure as the host cytoplasm. Continuity between these bodies and the host cytoplasm was found in a number of electron micrographs, showing that the bodies are formed by invagination of the double plasma membrane of the parasite. In this way the host cell is incorporated by phagotrophy into food vacuoles within the parasite. Hematin, the residue of hemoglobin digestion, was never observed inside the food vacuole but in small vesicles lying around it and sometimes connected with it. The vesicles are pinched off from the food vacuole proper and are the site of hemoglobin digestion. The active double limiting membrane is responsible not only for the formation of food vacuoles but also for the presence of two new structures. One is composed of two to six concentric double wavy membranes originating from the plasma membrane. Since no typical mitochondria were found in P. berghei, it is assumed that the concentric structure performs mitochondrial functions. The other structure appears as a sausage-shaped vacuole surrounded by two membranes of the same thickness, density, and spacing as the limiting membrane of the body. The cytoplasm of the parasite is rich in vesicles of endoplasmic reticulum and Palade's small particles. Its nucleus is of low density and encased in a double membrane. The host cells (reticulocytes) have mitochondria with numerous cristae mitochondriales. In many infected and intact reticulocytes ferritin was found in vacuoles, mitochondria, canaliculi, or scattered in the cytoplasm.

Contractile vacuoles are organelles that collect fluid from the cytoplasm and expel it to the outside. After each discharge (systole), they appear again and expand (diastole). They are widely distributed among Protozoa, and have been found also in some fresh water algae, sponges, and recently in some blood cells of the frog, guinea pig, and man. In spite of the extensive work on the contractile vacuole, very little is known concerning its mode of operation. An electron microscope study of a suctorian Tokophrya infusionum provided an opportunity to study thin sections of contractile vacuoles, and in these some structures were found which could be part of a mechanism for the systolic and diastolic motions the organelle displays. In Tokophrya, as in Suctoria and Ciliata in general, the contractile vacuole has a permanent canal connecting it with the outside. The canal appears to have a very elaborate structure and is composed of three parts: (1) a pore; (2) a channel; and (3) a narrow tubule located in a papilla protruding into the cavity of the contractile vacuole. Whereas the pore and channel have fixed dimensions and are permanently widely open, the tubule has a changeable diameter. At diastole it is so narrow (about 25 to 30 mmicro in diameter) that it could be regarded as closed, while at systole it is widely open. It is assumed that the change in diameter is due to the contraction of numerous fine fibrils (about 180 A thick) which are radially disposed around the canal in form of a truncated cone, with its tip at the channel, and its base at the vacuolar membrane. It seems most probable that the broadening of the tubule results in discharge of the content of the contractile vacuole. In the vicinity of the very thin limiting vacuolar membrane, small vesicles and canaliculi of the endoplasmic reticulum, very small dense particles, and mitochondria may be found. In addition, rows of closely packed vesicles are present in this region, and in other parts of the cytoplasm. It is suggested that they might represent dictyosome-like bodies, responsible for withdrawing fluids from the cytoplasm and then conveying them to the contractile vacuole, contributing to its expansion at diastole.

A study of thin sections of Paramecium multimicronucleatum, Tetrahymena pyriformis, Tokophrya infusionum, and Amoeba proteus shows that the mitochondria in all these protozoa are similar in certain aspects of their fine structure to that described in metazoan cells. As in higher organisms the mitochondrion is surrounded by a double limiting membrane and contains protrusions directed inward from the innermost of the double membranes. There are, however, some differences. In a majority of higher organisms the internal structure of mitochondria consists of ridges or cristae mitochondriales and in a few instances only of finger-like projections, or microvilli. In all protozoa described here and elsewhere microvilli represent the dominant structure. They are characteristic therefore of protozoan mitochondria.

The feeding apparatus of Suctoria consists of long, thin, stiff tubes called tentacles. When a swimming prey attaches to the tip of the tentacle a number of events follow in rapid succession. The tentacle broadens, a stream of tiny granules starts to move upward at its periphery to the tip, the prey becomes immobilized and shortly thereafter the cytoplasm of the still living prey begins to flow through the center of the tentacle to the body of the predator. An electron microscope study of the tentacle in Tokophrya infusionum, a protozoan of the subclass Suctoria, has disclosed a number of structural details which help to clarify some of the mechanisms involved in this unusual way of feeding. Each tentacle is composed of two concentric tubes. The lumen of the inner tube is surrounded by 49 tubular fibrils most probably of contractile nature. In the inner tube the cytoplasm of the prey is present during feeding, and in the outer tube are small dense bodies. It was found that the dense bodies originate in the cytoplasm of Tokophrya. They have an elongate, missile-like appearance, pointed at one end, rounded at the other, and are composed of several distinct segments. At the tip of the tentacle they penetrate the plasma membrane, with their pointed ends sticking out. It is assumed that the missile-like bodies play a major role in the feeding process. Their composite structure suggests that they might contain a number of enzymes which most probably are responsible for the various events preceding the actual food intake.

Basal body replication and ciliogenesis in Tokophrya infusionum were studied in synchronized cultures. Basal body replication occurs during the 1st hr of reproduction, which in Tokophrya is by internal budding. The number of basal bodies increases from about 20 to over 300 within this period. New basal bodies develop in association with mature basal bodies; they are formed at right angles to the mature basal body as short "probasal" bodies, which elongate, slant upward, become parallel to the mature basal body, and elongate to the mature size. Ciliogenesis occurs only during reproduction; the nonreproducing adult is not ciliated, and has only 18-25 barren basal bodies. Cilia first appear as short bulges above the basal body. The axonemal structure is incomplete at first, with one or both central microtubules absent, and occasionally the B fibers of the outer doublets are missing. Several accessory fibers are associated with the basal bodies, both in the adult and during reproduction. One of the fibers appears only after the cilia have sprouted. The scheme of basal body replication and ciliogenesis in Tokophrya is compared to that reported in other organisms, and the role of the accessory fibers is discussed.