In vitro fusion of Acanthamoeba phagolysosomes. III. Evidence that cyclic nucleotides and vacuole subpopulations respectively control the rate and the extent of vacuole fusion in Acanthamoeba homogenates

The many lives of nontuberculous mycobacteria

Nontuberculous mycobacteria (NTM) include species that colonize human epithelia, as well as species that are ubiquitous in soil and aquatic environments. NTM that primarily inhabit soil and aquatic environments include the Mycobacterium avium complex (MAC, M. avium and Mycobacterium intracellulare) and the Mycobacterium abscessus complex (MABSC, M. abscessus subspecies abscessus, massiliense, and bolletii), and can be free-living, biofilm-associated, or amoeba-associated. Although NTM are rarely pathogenic in immunocompetent individuals, those who are immunocompromised - due to either an inherited or acquired immunodeficiency - are highly susceptible to NTM infection (NTMI). Several characteristics such as biofilm formation and the ability of select NTM species to form distinct colony morphotypes all may play a role in pathogenesis not observed in the related, well-characterized pathogen Mycobacterium tuberculosis The recognition of different morphotypes of NTM has been established and characterized since the 1950s, but the mechanisms that underlie colony phenotype change and subsequent differences in pathogenicity are just beginning to be explored. Advances in genomic analysis have led to progress in identifying genes important to the pathogenesis and persistence of MAC disease as well as illuminating genetic aspects of different colony morphotypes. Here we review recent literature regarding NTM ecology and transmission, as well as the factors which regulate colony morphotype and pathogenicity.

Fusion between Leishmania amazonensis and Leishmania major parasitophorous vacuoles: live imaging of coinfected macrophages

Protozoan parasites of the genus Leishmania alternate between flagellated, elongated extracellular promastigotes found in insect vectors, and round-shaped amastigotes enclosed in phagolysosome-like Parasitophorous Vacuoles (PVs) of infected mammalian host cells. Leishmania amazonensis amastigotes occupy large PVs which may contain many parasites; in contrast, single amastigotes of Leishmania major lodge in small, tight PVs, which undergo fission as parasites divide. To determine if PVs of these Leishmania species can fuse with each other, mouse macrophages in culture were infected with non-fluorescent L. amazonensis amastigotes and, 48 h later, superinfected with fluorescent L. major amastigotes or promastigotes. Fusion was investigated by time-lapse image acquisition of living cells and inferred from the colocalization of parasites of the two species in the same PVs. Survival, multiplication and differentiation of parasites that did or did not share the same vacuoles were also investigated. Fusion of PVs containing L. amazonensis and L. major amastigotes was not found. However, PVs containing L. major promastigotes did fuse with pre-established L. amazonensis PVs. In these chimeric vacuoles, L. major promastigotes remained motile and multiplied, but did not differentiate into amastigotes. In contrast, in doubly infected cells, within their own, unfused PVs metacyclic-enriched L. major promastigotes, but not log phase promastigotes - which were destroyed - differentiated into proliferating amastigotes. The results indicate that PVs, presumably customized by L. major amastigotes or promastigotes, differ in their ability to fuse with L. amazonensis PVs. Additionally, a species-specific PV was required for L. major destruction or differentiation – a requirement for which mechanisms remain unknown. The observations reported in this paper should be useful in further studies of the interactions between PVs to different species of Leishmania parasites, and of the mechanisms involved in the recognition and fusion of PVs.

Many non-viral intracellular pathogens lodge within cell vesicles known as “parasitophorous vacuoles” (PVs), which exhibit a variety of pathogen-dependent functional and compositional phenotypes. PVs of the protozoan Leishmania are similar to the digestive organelles known as phagolysosomes. We asked if, in phagocytes infected with two different Leishmania species, would the two parasites be found in the same or in separate vacuoles? Of the species chosen, Leishmania amazonensis develops within large vacuoles which shelter many parasites; in contrast, Leishmania major lodges in small PVs containing one or two parasites. In the present experiments, the species and their life-cycle stages (extracellular promastigotes, and intracellular amastigotes) were distinguished by means of fluorescent markers, and the intracellular localization of the parasites was examined in living cells. We report here that, whereas L. major amastigotes remained within their individual vacuoles, L. major promastigotes were delivered to L. amazonensis vacuoles, in which they survived and multiplied but were unable to differentiate into amastigotes. A species-specific vacuole was thus required for L. major differentiation. The model should be useful in cellular and molecular studies of the biology of these parasites and of their parasitophorous vacuoles.

Membrane trafficking along the phagocytic pathway

Phagosome maturation involves extensive remodelling of the phagosomal membrane as a result of intracellular transport events. Newly formed phagosomes exchange membrane-associated and soluble proteins with early endosomes by fusion. Budding of vesicles from the phagosome and fusion with Golgi-derived vesicles may also contribute to the remodelling of the phagosomal compartment. As a consequence of changes in membrane composition, phagosomes acquire the ability to fuse with late endocytic compartments. In vitro reconstitution and other studies suggest that the trafficking events underlying phagosome maturation require several GTP-binding proteins, including Rab5 and Galphas', NSF-SNAP-SNARE complexes and coatomers.

Bacterial endosymbiont-derived lipopolysaccharides and a protein on symbiosome membranes in newly infected amoebae and their roles in lysosome-symbiosome fusion

Experimental results are presented to support the view that symbiont-derived lipopolysaccharides are involved in the prevention of lysosome-symbiosome fusion in xD amoebae harboring bacterial endosymbionts. Monoclonal antibodies against lipopolysaccharides and a 96-kDa protein present on symbiosome membranes of amoebae were used to monitor the appearance of the membrane-specific components in newly infected amoebae with endosymbionts from xD amoebae. The lipopolysaccharides and protein appeared on the newly forming symbiosome membranes within 3 to 7 days, as detected by indirect immunofluorescence staining with monoclonal antibodies. The lysosome-symbiosome fusion was followed by double staining of two antigens with different monoclonal antibodies applied to the same amoeba. Antilipopolysaccharide monoclonal antibodies were detected by staining with a fluorescein isothiocyanate-conjugated secondary antibody, and a biotinylated anti-lysosomal protein monoclonal antibody was detected by staining with Texas Red-conjugated streptavidin. In xD amoebae injected with an antilipopolysaccharide antibody, lysosomes fused with some of the symbiosomes that did not fuse with lysosomes in noninjected cells.

Activation of adenylate cyclase in Acanthamoeba palestinensis

Preincubation of Acanthamoeba palestinensis homogenates in 0.25M sucrose-TM (2mM MgSO4 and 5mM Tris-HCl, pH 7.4) at 0 degree C for increasing periods of time up to 3 h, leads to a progressive increase in the activity of adenylated cyclase. In contrast, preincubation of isolated membrane fractions enriched in enzyme activity in the same medium results in no activation. However, preincubation of membrane fractions in medium containing a high density of sugars (sucrose, glucose or fructose) mimics the activation obtained with homogenates. The high density sugar activation is time and temperature dependent, and reversible upon return to a low density medium. The high osmotic pressure of the sugars utilized may be a factor, since high concentrations of the sucrose polymer, Ficoll, which has low osmotic activity, causes not activation. Soluble activators, protein synthesis and changes in cyclic nucleotide phosphodiesterase activity were all eliminated as possible effectors of the apparent activation of adenylate cyclase. In contrast to mammalian adenylate cyclase, the endoplasmic reticulum localized enzyme of Acanthamoeba is inhibited by NaF and is unaffected by GTP, adenosine, epinephrine, prostaglandin E1, propranolol, and meclofenamic acid. These data indicate that the adenylate cyclase of Acanthamoeba is structurally different from that of most mammalian cells.

Reconstitution of an endocytic fusion event in a cell-free system

Using a cell-free system we have obtained fusion of vesicles from the endocytic pathway. The fusion is rapid, efficient, and requires ATP. Only vesicles derived from certain positions along the endocytic pathway are capable of fusing. Lysosomes and vesicles derived from the plasma membrane do not fuse.

Cell death: questions for histochemists concerning the causes of the various cytological changes

The question of cell death is accessible to study by histochemists and many questions remain to be resolved. From a physiological point of view, the most important are the causal relationships. (1) At what phase in cell death is the synthesis of RNA disrupted and at what phase is the rate of degradation of RNA increased? (2) Does the disruption of synthesis result from a direct genetic command, or does it result indirectly from gradual deterioration of energy resources or optimal ionic conditions? (3) What properties, presumably of the substrate organelles, marks them for specific absorption into autophagic vacuoles? (4) What proteases and other hydrolases operate currently undetected in the cytoplasm? How are they controlled and regulated? (5) Why does the physiologically dying cell shrink and appear more dense? To what extent is a cell in this state able to regulate any metabolic parameter? The advent of newer, more sensitive and quantitative techniques, and greater attention to the controls and causes as opposed to the phenomena, should help to resolve these questions.

Compartmentalization of cyclic AMP during phagocytosis by human neutrophilic granulocytes

Immunocytochemistry shows that early during phagocytosis of zymosan, adenosine 3',5'-monophosphate (cyclic AMP) appears on the cell surface before the phagosome is internalized. The appearance of cyclic AMP on the cell surface is coincident with that of granule products and regulatory subunit of type I cyclic AMP-dependent protein kinase. Guanosine 3',5'-monophosphate is not associated with the initiation site of phagocytosis, but is observed throughout the granular cytoplasmic region. This sharply localized accumulation of cyclic AMP may serve as a signal for the initiation of phagocytosis.