Toxoplasma gondii is a protozoan (single-celled) parasite that has permanently infected tens of millions of people in the world. A normal immune response typically controls infection promptly, but the parasite persists as a latent cyst within host tissues. Tissue cysts can form in brain, heart, and skeletal muscle tissue. If immunity should become compromised due to disease (such as AIDS) or immunosuppressive therapies (e.g., cancer chemotherapy or organ transplantation), the acute stage of infection may recrudesce. Thus, Toxoplasma is a significant opportunisitc pathogen. However, there are instances when Toxoplasma threatens normal, healthy individuals as well. For example, Toxoplasma can cause abortion or congenital birth defects; this may occur if a woman becomes infected for the first time during pregnancy. Additionally, emerging studies have demonstrated intriguing alterations in host organism behavior upon infection, or correlations with neurological disorders such as schizophrenia.
Toxoplasma infection may be picked up from cats (the definitive host) or contaminated food/water. People at significant risk (HIV+ or pregnant individuals) should cook meat thoroughly, wash vegetables, and avoid cat litter boxes and gardening.
If you're studying Toxoplasma, I encourage you to make all or portions of your project open. Invite others to help or add their thoughts to your project. Simply create a "child page" to our current project page to begin. Or if you're still in the brainstorming phase, post a blog and invite others you know working in the field to participate online.
Create a "child page" to add a new project about Toxoplasma.
Histone acetyltransferases (HATs) and HAT inhibitors
The significance of studying HATs is underscored by an abundance of genetic studies that implicate them in having a role in disease (for reviews, see (10), (6), and (16)). Consistent with this, some histone deacetylase (HDAC) inhibitors display anti-tumor activity and are being evaluated in clinical trials (8). In addition to regulating transcription, HATs have crucial functions in modulating other DNA processes (7). Histone acetylation machinery may also be a viable target for novel anti-infectives (5).
The impact of the various HATs on cellular physiology and disease would greatly benefit from the identification of specific pharmacological inhibitors, but very few have been described to date (11). Two natural products, anacardic acid and garcinol (a polyprenylated benzophenone), are reported to inhibit both p300/CBP and PCAF in a 5-10 mM range in vitro (1, 2). In contrast, curcumin displays activity against p300/CBP, but not PCAF (3). Subsequent studies suggest that anacardic acid may be a broad-spectrum HAT inhibitor, as it also interferes with the MYST HAT Tip60 (13). Isothiazolones were identified in a high-throughput screen as inhibitors of PCAF and p300 (12), but like the aforementioned compounds, activity against GCN5 was not determined. Moreover, isothiazolones are strongly reactive with thiol groups and hence are likely to have substantial nonspecific effects. Two small molecule inhibitors of GCN5 that have been documented include a butyrolactone (4) and MC1626 (2-methyl-3-carbethoxyquinoline) (9). However, in our hands, the butyrolactone and MC1626 exhibit no inhibition of recombinant yeast GCN5 in a standard in vitro HAT assay (Sullivan, unpublished). As a positive control, parallel HAT assays showed anacardic acid does inhibit yeast GCN5.
Two reports describe systems that can be used in high-throughput format to identify potential HAT inhibitors (14, 15).
We are interested in acquiring HAT inhibitors, especially those that appear to be selective for distinct types of HATs (i.e. GCN5, MYST). Not only would these serve as valuable probes to study histone acetylation in eukaryotic cells, they may also hold promise as novel drugs to combat parasitic disease.
1. Balasubramanyam, K., M. Altaf, R. A. Varier, V. Swaminathan, A. Ravindran, P. P. Sadhale, and T. K. Kundu. 2004. Polyisoprenylated benzophenone, garcinol, a natural histone acetyltransferase inhibitor, represses chromatin transcription and alters global gene expression. J Biol Chem 279:33716-26.
2. Balasubramanyam, K., V. Swaminathan, A. Ranganathan, and T. K. Kundu. 2003. Small molecule modulators of histone acetyltransferase p300. J Biol Chem 278:19134-40.
3. Balasubramanyam, K., R. A. Varier, M. Altaf, V. Swaminathan, N. B. Siddappa, U. Ranga, and T. K. Kundu. 2004. Curcumin, a novel p300/CREB-binding protein-specific inhibitor of acetyltransferase, represses the acetylation of histone/nonhistone proteins and histone acetyltransferase-dependent chromatin transcription. J Biol Chem 279:51163-71.
4. Biel, M., A. Kretsovali, E. Karatzali, J. Papamatheakis, and A. Giannis. 2004. Design, synthesis, and biological evaluation of a small-molecule inhibitor of the histone acetyltransferase Gcn5. Angew Chem Int Ed Engl 43:3974-6.
5. Darkin-Rattray, S. J., A. M. Gurnett, R. W. Myers, P. M. Dulski, T. M. Crumley, J. J. Allocco, C. Cannova, P. T. Meinke, S. L. Colletti, M. A. Bednarek, S. B. Singh, M. A. Goetz, A. W. Dombrowski, J. D. Polishook, and D. M. Schmatz. 1996. Apicidin: a novel antiprotozoal agent that inhibits parasite histone deacetylase. Proc Natl Acad Sci U S A 93:13143-7.
6. Davis, P. K., and R. K. Brackmann. 2003. Chromatin remodeling and cancer. Cancer Biol Ther 2:22-9.
7. Hasan, S., and M. O. Hottiger. 2002. Histone acetyl transferases: a role in DNA repair and DNA replication. J Mol Med 80:463-74.
8. Kristeleit, R., L. Stimson, P. Workman, and W. Aherne. 2004. Histone modification enzymes: novel targets for cancer drugs. Expert Opin Emerg Drugs 9:135-54.
9. Ornaghi, P., D. Rotili, G. Sbardella, A. Mai, and P. Filetici. 2005. A novel Gcn5p inhibitor represses cell growth, gene transcription and histone acetylation in budding yeast. Biochem Pharmacol 70:911-7.
10. Roth, S. Y., J. M. Denu, and C. D. Allis. 2001. Histone acetyltransferases. Annu Rev Biochem 70:81-120.
11. Schafer, S., and M. Jung. 2005. Chromatin modifications as targets for new anticancer drugs. Arch Pharm (Weinheim) 338:347-57.
12. Stimson, L., M. G. Rowlands, Y. M. Newbatt, N. F. Smith, F. I. Raynaud, P. Rogers, V. Bavetsias, S. Gorsuch, M. Jarman, A. Bannister, T. Kouzarides, E. McDonald, P. Workman, and G. W. Aherne. 2005. Isothiazolones as inhibitors of PCAF and p300 histone acetyltransferase activity. Mol Cancer Ther 4:1521-32.
13. Sun, Y., X. Jiang, S. Chen, and B. D. Price. 2006. Inhibition of histone acetyltransferase activity by anacardic acid sensitizes tumor cells to ionizing radiation. FEBS Lett 580:4353-6.
14. Turlais, F., A. Hardcastle, M. Rowlands, Y. Newbatt, A. Bannister, T. Kouzarides, P. Workman, and G. W. Aherne. 2001. High-throughput screening for identification of small molecule inhibitors of histone acetyltransferases using scintillating microplates (FlashPlate). Anal Biochem 298:62-8.
15. Wynne Aherne, G., M. G. Rowlands, L. Stimson, and P. Workman. 2002. Assays for the identification and evaluation of histone acetyltransferase inhibitors. Methods 26:245-53.
16. Yang, X. J. 2004. The diverse superfamily of lysine acetyltransferases and their roles in leukemia and other diseases. Nucleic Acids Res 32:959-976.