Lysine acetyltransferase GCN5b interacts with AP2 factors and is required for Toxoplasma gondii proliferation

Histone acetylation has been linked to developmental changes in gene expression and is a validated drug target of apicomplexan parasites, but little is known about the roles of individual histone modifying enzymes and how they are recruited to target genes. The protozoan parasite Toxoplasma gondii (phylum Apicomplexa) is unusual among invertebrates in possessing two GCN5-family lysine acetyltransferases (KATs). While GCN5a is required for gene expression in response to alkaline stress, this KAT is dispensable for parasite proliferation in normal culture conditions. In contrast, GCN5b cannot be disrupted, suggesting it is essential for Toxoplasma viability. To further explore the function of GCN5b, we generated clonal parasites expressing an inducible HA-tagged dominant-negative form of GCN5b containing a point mutation that ablates enzymatic activity (E703G). Stabilization of this dominant-negative GCN5b was mediated through ligand-binding to a destabilization domain (dd) fused to the protein. Induced accumulation of the _ddHAGCN5b(E703G) protein led to a rapid arrest in parasite replication. Growth arrest was accompanied by a decrease in histone H3 acetylation at specific lysine residues as well as reduced expression of GCN5b target genes in GCN5b(E703G) parasites, which were identified using chromatin immunoprecipitation coupled with microarray hybridization (ChIP-chip). Proteomics studies revealed that GCN5b interacts with AP2-domain proteins, apicomplexan plant-like transcription factors, as well as a “core complex” that includes the co-activator ADA2-A, TFIID subunits, LEO1 polymerase-associated factor (Paf1) subunit, and RRM proteins. The dominant-negative phenotype of _ddHAGCN5b(E703G) parasites, considered with the proteomics and ChIP-chip data, indicate that GCN5b plays a central role in transcriptional and chromatin remodeling complexes. We conclude that GCN5b has a non-redundant and indispensable role in regulating gene expression required during the Toxoplasma lytic cycle.

Toxoplasma gondii is a protozoan parasite that causes significant opportunistic infection in AIDS and other immunocompromised patients. Acute episodes of toxoplasmosis stem from tissue destruction caused by the rapidly growing form of the parasite, the tachyzoite. In this study, we identify a lysine acetyltransferase (KAT) enzyme called GCN5b that is an essential driver of tachyzoite proliferation. Our studies show that GCN5b is present at a wide variety of parasite genes and that expression of defective GCN5b compromises gene expression through its diminished ability to acetylate histone proteins. We also identified the likely mechanism by which GCN5b is recruited to target genes by co-purifying this KAT with plant-like AP2-domain proteins, a subset of which function as DNA-binding transcription factors in Apicomplexa. Our findings demonstrate that KATs play a critical role in parasite replication, which leads to tissue destruction and acute disease in the host. Parasite KAT enzyme complexes may therefore serve as attractive targets for future drug development.

Regions of intrinsic disorder help identify a novel nuclear localization signal in Toxoplasma gondii histone acetyltransferase TgGCN5-B

We have previously shown that protozoan parasites, such as Toxoplasma gondii, contain a high prevalence of intrinsically disordered regions in their predicted proteins. Here, we determine that both TgGCN5-family histone acetyltransferases (HATs) contain unusually high levels of intrinsic disorder. A previously identified basic-rich nuclear localization signal (NLS) in the N-terminus of TgGCN5-A is located within such a region of predicted disorder, but this NLS is not conserved in TgGCN5-B. We therefore analyzed the intrinsically disordered regions of TgGCN5-B for basic-rich sequences that could be indicative of a functional NLS, and this led to the identification of a novel NLS for TgGCN5-B, RPAENKKRGR. The functionality of the GCN5-B NLS was validated experimentally and has predictive value. These studies demonstrate that basic-rich sequences within regions predicted to be intrinsically disordered constitute criteria for a candidate NLS.

A decade of epigenetic research in Toxoplasma gondii

In the past 10 years, the field of parasitology has witnessed an explosion of studies investigating gene regulation. In this review, we will describe recent advances largely stemming from the study of Toxoplasma gondii, a significant opportunistic pathogen and useful model for other apicomplexan protozoa. Surprising findings have emerged, including the discovery of a wealth of epigenetic machinery in these primitive eukaryotes, unusual histone variants, and a battery of plant-like transcription factors. We will elaborate on how these unusual features impact parasite physiology and potential therapeutics as we summarize some of the key discoveries from the last decade. We will close by proposing a few questions to address in the next 10 years.

Translation regulation by eukaryotic initiation factor-2 kinases in the development of latent cysts in Toxoplasma gondii

A key problem in the treatment of numerous pathogenic eukaryotes centers on their development into latent forms during stress. For example, the opportunistic protist Toxoplasma gondii converts to latent cysts (bradyzoites) responsible for recrudescence of disease. We report that Toxoplasma eukaryotic initiation factor-2alpha (TgIF2alpha) is phosphorylated during stress and establish that protozoan parasites utilize translation control to modulate gene expression during development. Importantly, TgIF2alpha remains phosphorylated in bradyzoites, explaining how these cells maintain their quiescent state. Furthermore, we have characterized novel eIF2 kinases; one in the endoplasmic reticulum and a likely regulator of the unfolded protein response (TgIF2K-A) and another that is a probable responder to cytoplasmic stresses (TgIF2K-B). Significantly, our data suggest that 1) the regulation of protein translation through eIF2 kinases is associated with development, 2) eIF2alpha phosphorylation is employed by cells to maintain a latent state, and 3) endoplasmic reticulum and cytoplasmic stress responses evolved in eukaryotic cells before the early diverging Apicomplexa. Given its importance to pathogenesis, eIF2 kinase-mediated stress responses may provide opportunities for novel therapeutics.

IMP dehydrogenase from the protozoan parasite Toxoplasma gondii

The opportunistic apicomplexan parasite Toxoplasma gondii damages fetuses in utero and threatens immunocompromised individuals. The toxicity associated with standard antitoxoplasmal therapies, which target the folate pathway, underscores the importance of examining alternative pharmacological strategies. Parasitic protozoa cannot synthesize purines de novo; consequently, targeting purine salvage enzymes is a plausible pharmacological strategy. Several enzymes critical to purine metabolism have been studied in T. gondii, but IMP dehydrogenase (IMPDH), which catalyzes the conversion of IMP to XMP, has yet to be characterized. Thus, we have cloned the gene encoding this enzyme in T. gondii. Northern blot analysis shows that two IMPDH transcripts are present in T. gondii tachyzoites. The larger transcript contains an open reading frame of 1,656 nucleotides whose deduced protein sequence consists of 551 amino acids (TgIMPDH). The shorter transcript is an alternative splice product that generates a 371-amino-acid protein lacking the active-site flap (TgIMPDH-S). When TgIMPDH is expressed as a recombinant protein fused to a FLAG tag, the fusion protein localizes to the parasite cytoplasm. Immunoprecipitation with anti-FLAG was employed to purify recombinant TgIMPDH, which converts IMP to XMP as expected. Mycophenolic acid is an uncompetitive inhibitor relative to NAD+, with a intercept inhibition constant (Kii) of 0.03+/-0.004 microM. Tiazofurin and its seleno analog were not inhibitory to the purified enzyme, but adenine dinucleotide analogs such as TAD and the nonhydrolyzable beta-methylene derivatives of TAD or SAD were inhibitory, with Kii values 13- to 60-fold higher than that of mycophenolic acid.

A pilot clinical evaluation of a new injection denture base system

The purpose of this pilot study was to determine if clinically acceptable complete denture prostheses can be consistently fabricated with a new injection system. In the first part of this investigation, 14 patients requiring complete denture fabrication had one prosthesis made with the new injection system and one made with the compression-pack technique. Polish, porosity, and esthetics were evaluated by one of three investigators at the placement visit. The percentage of the intaglio surface in contact with the mucosa before adjustment was also measured. One week after placement, the patients were questioned regarding the prostheses' stability, the presence of any undesirable taste, and overall satisfaction. The second portion of the study involved materials property testing of flexural strength and Izod toughness of conventionally packed and injection-packed specimens processed by the fast cycle or the long cycle. In every case, acceptable results were obtained for the resin's polishability, its lack of porosity, esthetics, stability, lack of unpleasant taste, and overall patient satisfaction with the prostheses. No difference in clinical adaptation was observed. These results indicated that no clinical difference was detectable between the conventionally packed and injection-packed denture bases by the investigators or patients in this study. Physical property testing indicates that the differences between groups may be more affected by the polymerization cycle used than the way that the resin was placed in the mold space.

Investigation of the utility of an in vitro release test for optimizing semisolid dosage forms

Current literature indicates that an in vitro release test (IVRT) can serve as a research tool during the course of developing topical formulations. The purpose of this study was therefore to investigate the ability of an IVRT to select the topical semisolid formulations with the most rapid release rate of the model drug ketoprofen from two closely related hydrogels in a simulated product development process. Two glycols with distinct differences in their physical-chemical properties, Transcutol P (ethoxydiglycol) and propylene glycol, were incorporated into Carbopol 980 and Poloxamer 407 formulations. The release rate of ketoprofen was determined utilizing different receptor media and conditions, i.e., phosphate buffer pH 7.4, isopropyl myristate (IPM), and a combination of an IPM soaked membrane and phosphate buffer (pH 7.4) as receptor fluid. The results indicated that the conditions chosen could affect greatly the conclusions concerning the formulations. The only observable trend was that Transcutol P-containing formulations tended to permit a faster ketoprofen release than propylene glycol-containing formulations when utilizing IPM as a receptor component. This was attributed to the mutual miscibility of Transcutol P in IPM. It can be concluded that, for the purpose of formulation screening in the early phases of product development, an IVRT will only be useful for predicting the amount of drug available for absorption if the receptor medium has properties that closely mimic human skin. These results illustrate the importance of selecting suitable receptor components and indicate that it may be necessary to consider alternatives to the commonly used synthetic membranes.

Amalgams still viable, safe treatment, controversies notwithstanding

This article addresses the dental amalgam debate from two aspects. The first is a review of the current status regarding the appropriate treatment planning of direct restorations. The second is a discussion of the safety of amalgam and the mercury toxicity concerns. Dental research continues to support the use of amalgam while the search continues for the "ideal" restorative material.

Campylobacter enteritis at a university: transmission from eating chicken and from cats

Campylobacter jejuni is the most common enteric pathogen isolated from university and college students in the United States. During the fall and winter quarters of the 1983-1984 academic year, the authors conducted a case-control study at the University of Georgia, Athens, Georgia, to identify risk factors for C. jejuni enteritis. Students with diarrhea whose cultures yielded C. jejuni were compared with controls matched by age, sex, and residence. A total of 45 case-control pairs were interviewed about exposures during the week before the case's onset of illness. The infections occurred sporadically and were caused by a wide variety of C. jejuni serotypes. Three risk factors were identified: eating fully cooked chicken, eating chicken reported to be raw or undercooked, and contact with a cat or kitten. No case reported drinking raw milk. No significant association was found between illness and the places where chicken meals were prepared or the specific manner in which chicken was cooked. Chicken may be the principal vehicle of transmission for sporadic Campylobacter enteritis among college students.

Hormonal control of spermatogenesis in the cockroach, Periplaneta americana (L.)

A surgical approach was made to the question of humoral regulation of testicular development. Neither the neurosecretory cells nor the prothoracic glands retarded or accelerated development of the testes. The corpora allata maintained the testes in a juvenile state by retarding their development.

Haemolymph esterases in the female larval honeybee, Apis mellifera L., during caste development

The haemolymph esterase patterns were obtained for eight ages during larval development of the two female castes of the honeybee, Apis mellifera L., by starch gel electrophoresis. Qualitatively, the patterns differed markedly with age within and between the castes. Quantitatively, esterase activity is higher in workers up to 72 hours of age and thereafter shows a general decline, whereas in queens the activity is lower up to 72 hours of age and is followed by a general increase. In both castes the activity is slightly increased towards prepupal stage. The significance of these results is discussed in relation to female dimorphism.