Lian-Yong Gao

Assistant Professor

Research Interests: Host-pathogen interactions; Molecular/cellular mechanisms of Mycobacteria pathogenesis

Infections caused by Mycobacteria remain a major human health problem. Mycobacterium tuberculosis alone infects ~1/3 of the world's population and causes 2-3 million deaths each year. Further threats come from the emergence of multi-drug resistant strains and co-infections with AIDS patients, which awaits desperately the development of new generation treatment and prevention of the infections. Given the significance of the disease and decades of research effort, the molecular and cellular mechanisms of Mycobacteria pathogenesis remain largely unknown. This is in large part due to the extremely slow growth rate of M. tuberculosis, its strict requirement for laboratory containment, and lack of opportune tools for the studies. We have developed a M. marinum-zebrafish infection model that has significantly advanced the study of Mycobacteria pathogenesis. M. marinum infects fish and frogs naturally with many pathological features similar to human tuberculosis. We have compelling evidence demonstrating that the M. marinum-zebrafish model is reliable, facile, safe, and rapid for studying molecular and cell biology of Mycobacteria pathogenesis and host responses to the infections.

Being intracellular pathogens primarily infecting host macrophages, Mycobacteria have evolved complex parasitic lifestyle and sophisticated mechanisms to combat host defense machinery. For example, Mycobacteria utilize multiple macrophage receptors for efficient invasion of the cells. Within macrophages, Mycobacteria inhibit acidification of the bacteria-containing phagosome and its maturation to phago-lysosome. For optimal intracellular growth, Mycobacteria inhibit macrophage apoptotic cell death program and undergo profound alterations of gene expression and secretion of bacterial toxins to modulate host defense mechanisms. Furthermore, Mycobacteria are able to spread from a cell to another and to cause dissemination of infection. The molecular mechanisms underlining those processes are just beginning to be understood. We have exploited the M. marinum model and successfully developed transposon mutagenesis to identify Mycobacteria genes necessary for 1) invasion of macrophages; 2) intracellular replication in macrophages; 3) inhibition of macrophage apoptosis; 4) regulation of Mycobacteria gene expression; 5) secretion of Mycobacteria cyto-toxins; and 6) Mycobacteria cell-to-cell spread and dissemination of infection. We have characterized many of these genes in detail. One of our significant findings is our demonstration that for all of the M. marinum mutants tested we could successfully complement their defects by expressing corresponding M. tuberculosis gene homologues, demonstrating the relevance and strength of this model. Our studies have extended significantly current knowledge of the molecular and cellular mechanisms of Mycobacteria pathogenesis.

Among the M. marinum genes that we first characterized is kasB encoding an enzyme catalyzing the elongation of mycolic acids, which are major components of the Mycobacteria cell wall crucial for protection of the bacteria. We have shown that inactivation of kasB not only significantly reduces bacterial intracellular growth in macrophage but also renders the bacteria hypersensitive (over 100-fold) to antibiotics. Further studies to develop novel drugs targeting kasB for therapeutic treatment of multi-drug resistant Mycobacteria are greatly warranted. A second locus studied in detail is mip that contains two genes required for Mycobacteria to both invade and persist in macrophage. Recently, we have characterized in detail a third gene cluster of ~20 genes that are required for Mycobacteria to spread from cell to cell in cell cultures and to cause disseminated infection in zebrafish. Further studies of this gene cluster have revealed that their encoded proteins constitute a novel protein secretion system for exporting Mycobacteria cyto-toxins that resolve the bacteria-containing phagosome membrane to promote Mycobacteria cell-to-cell spread via actin-based motility, similar to Listeria. This gene cluster extends beyond a previously described M. tuberculosis virulence locus RD1 and is thereby named extRD1 (extended RD1). Ongoing research on extRD1 addresses the following important questions: 1) What are the cyto-toxins secreted by extRD1 required for resolution of the bacteria-containing phagosome membrane? 2) How do the cyto-toxins interact with and resolve the host cell membrane? 3) How do the cyto-toxins contribute to disseminated infection in the host? and 4) How are cyto-toxins secreted by the extRD1 secretion system. These studies will be carried out using our established M. marinum-zebrafish model system, and meanwhile the significant findings will be confirmed in the M. tuberculosis system and applied to development of more efficient treatment and prevention of M. tuberculosis infection. Finally, one of our long-term goals is to use the M. marinum-zebrafish model to study Mycobacteria persistence, reactivation, and transmission.

Selected Publications:

Gao L.Y., Guo S., Morisaki H., Horwitz M.A., Engel J.N., and Brown E.J. (2004). A mycobacterial virulence gene cluster extending RD1 is required for cytolysis, bacterial spreading, and ESAT-6 secretion. Mol. Microbiol. 53(6):1677-1693.

Gao L.-Y., Laval F., Lawson E.H., Groger R.K, Woodruff A., Morisaki J.H., Cox J.S., Daffe M., and Brown E.J. (2003). Requirement for kasB in Mycobacterium Mycolic Acid Biosynthesis, Cell Wall Impermeability, and Intracellular Survival: Implications for therapy. Mol. Microbiol. 49(6):1547-1563.

Stamm L.M., Morisaki J.H., Gao L.-Y., Jeng R.L., McDonald K.L., Roth R., Takeshita S., Heuser J., Welch M.D., and Brown E.J. (2003). Mycobacterium marinum Escapes from Phagosomes and Is Propelled by Actin-based Motility. J. Exp. Med. 198(9):1361-1368.

Gao L.-Y., Groger R., Cox J.S., Beverley S.M., Lawson E.H., and Brown E.J. (2003). Transposon Mutagenesis of Mycobacterium marinum Identifies a Locus Linking Pigmentation and Intracellular Survival. Infect. Immun. 71(2):1-8.

Gao, L.-Y. and Abu Kwaik, Y. (2000). Modulation of host cell apoptosis by intracellular bacterial pathogens. Trends Microbiol. 8(7):306-313.

Venkataraman, C., Gao, L.-Y., Bondada, S., and Abu Kwaik, Y. (1998). Identification of putative cytoskeletal protein homologues in the protozoan host Hartmannella vermiformis as substrates for induced tyrosine phosphatase activity upon attachment to the Legionnaires' disease bacterium, Legionella pneumophila. J. Exp. Med. 188:505-514.

Gao, L.-Y., Harb, O.S., and Abu Kwaik, Y. (1998). Identification of macrophage-specific infectivity loci (mil) of Legionella pneumophila that are not required for infectivity of protozoa. Infect. Immun. 66:883-892.

Positions open:

1. Postdoctoral position: A Postdoctoral position is immediately available for highly motivated and qualified individuals to study the molecular and cellular mechanisms of Mycobacteria pathogenesis. Projects will involve studying how Mycobacteria escape from host cell phagosome and spread from cell to cell and how these processes contribute to pathogenesis of the infection. The successful applicants will need to demonstrate expertise in, via publications in internationally recognized peer-reviewed journals: 1) microbial genetics, 2) standard molecular biology skills including DNA and protein works, and 3) cell biology skills including cell culture and microscopy. Certain experience with human or animal subjects and the cells is a plus. Qualified candidates should have a Ph.D. degree with excellent oral and written communication skills. Salary will be commensurate with experience. The University of Maryland at College Park is the flagship campus of the University of Maryland System. It occupies a beautiful 1,500 acre campus in the heart of the Washington-Baltimore high-tech corridor. Please send curriculum vitae and a brief letter with three names of reference to:

Lian-Yong Gao, Ph.D.
Department of Cell Biology and Molecular Genetics
3109 Microbiology Building
University of Maryland
College Park, MD 20742
Telephone: 301-405-7562
Fax: 301-314-9489
E-mail: lygao@mail.umd.edu

2. Graduate Research Assistantships: My lab has 2-3 new openings for graduate student research assistants and will provide outstanding individual research projects for them. Please contact me for possible lab rotations.