Intracellular localization and trafficking of serine proteinase AhSub and cysteine proteinase AhCP of Acanthamoeba healyi

Identification and characterization of a surface-associated, subtilisin-like serine protease inTrichomonas vaginalis

Trichomonas vaginalisis a protozoan parasite causing trichomonosis, a sexually transmitted infection in humans. This parasite has numerous proteases, most of which are cysteine proteases that appear to be involved in adherence and cytotoxicity of host cells. In this report we identify and characterize a putative subtilisin-like serine protease (SUB1). Thesub1gene encodes a 101-kDa protein.In silicoanalyses predict signal and pro-peptides at the N-terminus, and a transmembrane helix at the carboxy-terminal region. Thesub1gene was found as single copy by Southern analysis, albeit additional serine protease related genes are annotated in theT. vaginalisgenome. The expression ofsub1could only be detected by RT-PCR and Ribonuclease Protection Assays, suggesting a low abundant mRNA. Thesub1gene transcription start site was correctly assigned by RPA. The transcript abundance was found to be modulated by the availability of iron in the growth medium. Antibodies raised to a specific SUB1 peptide recognized a single protein band (~82 kDa) in Western blots, possibly representing the mature form of the protein. Immunofluorescence showed SUB1 on the trichomonad surface, and in dispersed vesicles throughout the cytoplasm. A bioinformatic analysis of genes annotated as serine proteases in theT. vaginalisgenome is also presented. To our knowledge this is the first putative serine protease experimentally described forT. vaginalis.

Major role for cysteine proteases during the early phase of Acanthamoeba castellanii encystment

Acanthamoeba castellanii is a facultative pathogen that has a two-stage life cycle comprising the vegetatively growing trophozoite stage and the dormant cyst stage. Cysts are formed when the cell encounters unfavorable conditions, such as environmental stress or food deprivation. Due to their rigid double-layered wall, Acanthamoeba cysts are highly resistant to antiamoebic drugs. This is problematic as cysts can survive initially successful chemotherapeutic treatment and cause relapse of the disease. We studied the Acanthamoeba encystment process by using two-dimensional gel electrophoresis (2DE) and found that most changes in the protein content occur early in the process. Truncated actin isoforms were found to abound in the encysting cell, and the levels of translation elongation factor 2 (EF2) were sharply decreased, indicating that the rate of protein synthesis must be low at this stage. In the advanced stage of encystment, however, EF2 levels and the trophozoite proteome were partly restored. The protease inhibitors PMSF (phenylmethylsulfonyl fluoride) and E64d [(2S,3S)-trans-epoxysuccinyl-L-leucylamido-3-methylbutane ethyl ester] inhibited the onset of encystment, whereas the protein synthesis inhibitor cycloheximide was ineffective. Changes in the protein profile, similar to those of encysting cells, could be observed with trophozoite homogenates incubated at room temperature for several hours. Interestingly, these changes could be inhibited significantly by cysteine protease inhibitors but not by inhibitors against other proteases. Taken together, we conclude that the encystment process in A. castellanii is of a bipartite nature consisting of an initial phase of autolysis and protein degradation and an advanced stage of restoration accompanied by the expression of encystment-specific genes.

Elastase secretion in Acanthamoeba polyphaga

Acanthamoeba species are frequently isolated from soil and water collections. In the environment, the organisms multiply as phagotrophic trophozoites and encyst under adverse conditions. Several species are known to infect man, causing keratitis and opportunistic diseases. The mechanisms underlying tissue damage and invasion by the amoebae are being elucidated and the involvement of secreted peptidases, particularly serine peptidases, has been demonstrated. Here, elastase activity was examined in Acanthamoeba-conditioned medium (ACM), making use of elastin-Congo red (ECR) and synthetic peptide p-nitroanilide substrates. ACM hydrolysed ECR over a broad pH range and optimally at a pH of 7.5 and above. Indicating the activity of serine and metallopeptidases, Congo red release was potently inhibited by PMSF, antipain, chymostatin and 1,10-phenanthroline, partially reduced by elastatinal and EDTA, and unaffected by 1,7-phenanthroline and E-64. Screening with synthetic substrates mainly showed the activity of serine peptidases. ACM efficiently hydrolysed Suc-Ala(2)-Pro-Leu-pNA and Suc-Ala(2)-Pro-Phe-pNA over a broad pH range (7.0-9.5) and was weakly active against Suc-Ala(3)-pNA, a substrate found to be optimally hydrolysed at a pH around 7.0. Following ammonium sulfate precipitation of ACM proteins and FPLC analysis, the majority of the ECR-splitting activity, characterised as serine peptidases, bound to CM-sepharose and co-eluted with part of the Suc-Ala(2)-Pro-Phe-pNA-hydrolysing activity in a gradient of 0-0.6M NaCl. In the corresponding FPLC fractions, serine peptidases resolving in the region of 70-130kDa were detected in gelatin gels. Overall, the results demonstrate that trophozoites secrete elastases, and additionally suggest the high molecular weight serine peptidases as possible elastase candidates.

Decoupling internalization, acidification and phagosomal-endosomal/lysosomal fusion during phagocytosis of InlA coated beads in epithelial cells

Background

Phagocytosis has been extensively examined in ‘professional’ phagocytic cells using pH sensitive dyes. However, in many of the previous studies, a separation between the end of internalization, beginning of acidification and completion of phagosomal-endosomal/lysosomal fusion was not clearly established. In addition, very little work has been done to systematically examine phagosomal maturation in ‘non-professional’ phagocytic cells. Therefore, in this study, we developed a simple method to measure and decouple particle internalization, phagosomal acidification and phagosomal-endosomal/lysosomal fusion in Madin-Darby Canine Kidney (MDCK) and Caco-2 epithelial cells.

Methodology/Principal Findings

Our method was developed using a pathogen mimetic system consisting of polystyrene beads coated with Internalin A (InlA), a membrane surface protein from Listeria monocytogenes known to trigger receptor-mediated phagocytosis. We were able to independently measure the rates of internalization, phagosomal acidification and phagosomal-endosomal/lysosomal fusion in epithelial cells by combining the InlA-coated beads (InlA-beads) with antibody quenching, a pH sensitive dye and an endosomal/lysosomal dye. By performing these independent measurements under identical experimental conditions, we were able to decouple the three processes and establish time scales for each. In a separate set of experiments, we exploited the phagosomal acidification process to demonstrate an additional, real-time method for tracking bead binding, internalization and phagosomal acidification.

Conclusions/Significance

Using this method, we found that the time scales for internalization, phagosomal acidification and phagosomal-endosomal/lysosomal fusion ranged from 23–32 min, 3–4 min and 74–120 min, respectively, for MDCK and Caco-2 epithelial cells. Both the static and real-time methods developed here are expected to be readily and broadly applicable, as they simply require fluorophore conjugation to a particle of interest, such as a pathogen or mimetic, in combination with common cell labeling dyes. As such, these methods hold promise for future measurements of receptor-mediated internalization in other cell systems, e.g. pathogen-host systems.

Characterization of a serine proteinase mediating encystation of Acanthamoeba

Members of the genus Acanthamoeba, amphizoic protozoan parasites, are causative agents of granulomatous amoebic encephalitis and amoebic keratitis. Proteinases play a role in various biologic actions in Acanthamoeba, including host tissue destruction, pathogenesis, and digestion of phagocytosed food. Interestingly, we found that encystation of Acanthamoeba was inhibited by the serine proteinase inhibitor phenylmethanesulfonyl fluoride. In this study, we characterize a serine proteinase that is involved in mediating the encystation of Acanthamoeba. This encystation-mediating serine proteinase (EMSP) is shown to be highly expressed during encystation by real-time PCR and Western blot analysis. Chemically synthesized small interfering RNA against EMSP inhibited the expression of EMSP mRNA and significantly reduced the encystation efficiency of Acanthamoeba. An EMSP-enhanced green fluorescent protein fusion protein localized to vesicle-like structures within the amoeba. Using LysoTracker analysis, these vesicular structures were confirmed to be lysosomes. After incubation of the transfected amoeba in encystment media, small fluorescent vesicle-like structures gathered and formed ball-like structures, which were identified as colocalizing with the autophagosome. Taken together, these results indicate that EMSP plays an important role in the differentiation of Acanthamoeba by promoting autolysis.