Ph.D. Scripps Institution of Oceanography 1977
B.A. University of California, San Diego 1973
Our research focuses on the isolation and structure determination of compounds from marine invertebrates, the detection of their effects on biological systems, and the characterization of their mode of action and structural determinants contributing to their biological activity. Our emphasis on organisms from tropical reefs is based on the hypothesis that competitive environments select for potent biological activities. Reefs are highly diverse ecosystems that are subject to a variety of selective mechanisms such as predation, fouling by microorganisms, and limited benthic substrates for settlement of sessile organisms. A common strategy for competing in such environments adopted by several organisms that are structurally vulnerable to these pressures is the production or accumulation of chemicals that can be used defensively or offensively.
Although the human body and its pathogens are foreign to a typical marine community, many of the biological pathways implicated in diseases such as cancer, infection, and autoimmune disorders are similar enough to those of competitors in tropical reefs that high potency of biological activity is frequently found in marine natural products. While the ecological roles of most marine natural products are unknown, the high potency and selectivity observed in marine natural products and the observation that structure-activity relationships are often highly optimized in marine natural products suggests that acquiring and maintaining high potency in these systems is a dominant selective pressure acting on their biosynthetic pathways. Add in the fact that marine invertebrates often host diverse and complex microbial communities where invasive disruption on a short time scale is a distinct possibility and that sponges and, to a lesser extent, ascidians have long evolutionary histories among the metazoans and it is no wonder that such potent compounds are to be found in these organisms.
Given our broad interests in marine natural products and their biological activities our group has adopted an approach that is both multidisciplinary and collaborative. The basis for most of our work is purifying compounds using predominantly chromatography and other chemical techniques and characterizing their structures using various analytical techniques including one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS), ultraviolet-visible (UV) spectrophotometry, and chemical modification. Structure determination is a challenging but rewarding process that requires careful experimental technique, the ability to organize and correlate disparate items of data, and extensive consultation of structure databases and the chemical literature.
However, making natural product discoveries of high significance demands a broader skill set. There is a need for innovative biological screens to maximize the potential of finding compounds with biomedical value. In order to reduce the likelihood of rediscovering a known compound with a previously documented biological activity, high-throughput assays and high-throughput analytical processes must be coordinated with an efficient dereplication process. Determining the mechanism of action for an agent is necessary before it will be of use as a therapeutic or biochemical tool. Chemical synthesis or modification is often needed to fully characterize a structure or determine structure-activity relationships (SAR).
The usual path for graduate students in the Ireland research group starts with projects involving the isolation and identification of compounds associated with biological activity as detected through screening. The student will then identify a central topic of research involving a marine natural product family or scaffold for which there are pressing questions regarding their structure or activities. They will formulate a plan for studying this topic that often involves working in the laboratory of collaborators to develop the skills needed for proving or refuting their central hypotheses. In the course of doing this, past group members have generally developed an outlook where the scientific problem itself, rather than the techniques known by the researcher, direct the path of study on new projects.
Phenotypic Acitivity of Pyridoacridines in Zebrafish
In collaboration with Ireland group alumnus Dr. Imelda Sandoval in the David Jones research laboratory, fraction libraries from marine invertebrates were screened for phenotypic activity in embryos of the zebrafish Danio rerio. The fraction library was generated using protocols developed in the Ireland laboratory that allow high-throughput fractionation of crude extracts.1–2 The screens identified extracts from Xestospongia cf. carbonaria (Figure 1) as causing abnormal notochord development (Figure 2).
Figure 1. An example of the marine sponge Xestospongia.
Figure 2. Phenotypic abnormalities elicited by fractions from Xestospongia cf. carbonaria in zebrafish embryos. Top– Control. Bottom– Treated embryos. Note abnormalities in backbone and in the size of the yolk sac.
Based on these observations, extraction and isolation was carried out on Xestospongia cf. carbonaria by Dr. Xiaomei Wei. This work led to the isolation of several pyridoacridines, a family of heterocyclic natural products from numerous marine animals that have been noted for disrupting the process of DNA replication.3 The compounds isolated included amphimedine (1),4 neoamphimedine (2),5 and several minor congeners (Figure 3). Interestingly, the two regioisomers 1 and 2 displayed differential activity in the zebrafish assay: Only 1 was capable of eliciting the originally observed phenotype whereas 2 caused death and no additional phenotypes at sublethal doses. It is interesting to note that 2 exhibits activity mediated by topoisomerase II whereas 1 does not.6 This work was recently published in the journal Marine Drugs.7
Figure 3. The two major pyridoacridines isolated from Xestospongia cf. carbonaria amphimedine (1) and neoamphimedine (2).
David Jones- We have had many ongoing collaborations with Professor Jones on exploring the mechanism of action of marine natural products. Our most recent collaborations involve screening for phenotypic activities in zebrafish larvae.
American Society of Pharmacognosy- This is the largest American organization dedicated to natural product research. In conjunction with the American Chemical Society, the ASP provides editorial support for the Journal of Natural Products, the premiere publication in natural product research.
American Chemical Society- The ACS organizes regional and national meetings for the dissemination of chemical knowledge. They also publish many high profile journals in the field of chemistry.
Ireland's Laboratory Website