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Daniel Wall, Ph.D.
Department of Molecular Biology
University of Wyoming
Laramie, WY 82071
A fundamental problem in biology is how individual cells within a multicellular organism interact to coordinate diverse biological processes. To address this topic our laboratory studies the simple and easy to manipulate multicellular organism called Myxococcus xanthus. This soil bacterium preys on other microorganisms in ‘microbial wolf packs.’ However, when nutrients are limiting thousands of cells will aggregate to build fruiting bodies wherein vegetative cells differentiate into environmentally resistant spores. This developmental program requires gliding motility and cell-cell signaling. Our laboratory is particularly interested in how cells coordinate their movements and how the type IV pili motor powers social (S) gliding motility. The coordination of cellular movement is clearly illustrated when cells build fruits, form ripples or simply glide in coherent S-motile swarms (Fig. 1). Cell-cell interactions play a key role in directing these movements. These signaling pathways in turn control the type IV pilus, which drives motility by the retraction of polarly localized pili that in turn pulls the cell forward (Fig. 2). Our laboratory is using molecular genetic approaches to elucidate how the proteins in the type IV pilus function and interact with signaling pathways. In a general context we are also interested in the role type IV pili play as virulence factors and biofilms in diverse bacterial pathogens.
|Fig. 1. Coordinated cell movements are required to form fruits, ripples and S-motile flares.|
|Fig. 2. Model of the type IV pilus.|
One interesting protein that is a focus of our studies is called Tgl. Similar to mutations in many pil genes, tgl mutants are blocked in pilus biogenesis and consequently lack S-motility. However, unlike other pil mutants, tgl mutants can be transiently ‘stimulated’ to produce pili and become motile when they are placed in physical contact with tgl (wt) cells (Fig. 3). Stimulation is phenotypic only; cells remain tgl mutants and upon subsequent rounds of cell division lose their pili and motility. Tgl is a lipoprotein located in the outer membrane (OM) and is involved in protein-protein interactions, including with the PilQ secretin (Fig. 3). Tgl stimulation is enhanced when donor and recipient cells are physically aligned, a process that enriches pole-to-pole contacts between rod-shaped cells and thus facilitates contacts between the pilus apparatus of donor and recipient cells (Fig. 3). Importantly, during simulation the Tgl protein is efficiently transferred from donor to recipient cells. Although the significance of the physical transfer of Tgl between cells is not fully known, it does suggest a possible role in coordinating the movements between cells in a swarm. This role might be mechanical in nature by triggering pilus polymerization and hence governing the direction of movement. Indeed, the piliated pole indicates cellular polarity and represents the cell front. Long-term goals are aimed to understand how cells regulate their movements and the role Tgl protein transfer might play. We are also interested in a mechanistic understanding of the cis and trans factors involved in Tgl transfer. In a broader context, we are interested whether lipoprotein transfer between cells is common in M. xanthus and possibly other bacteria and what biological role(s) it might play.
|Fig. 3. Tgl stimulation model.|
Antibiotic drug discovery & chemical genetics: In the last decade the rapid development of bacterial resistance to antibiotics has generated a critical medical need for new therapeutics. Although antibiotic drug discovery as a whole is beyond the capabilities of a single academic lab there are creative ways to participate. In particular we are interested in studying orphaned antimicrobial compounds described in the literature and in collaborating with synthetic chemistry groups on novel agents. The focus of our studies is on elucidating the mechanisms of action and resistance of novel antibacterial agents. Such findings add value to novel and/or orphaned chemical entities and can serve as a basis for expanded studies. This work builds on the principal investigator’s industrial experience in antibiotic drug discovery and underlies an interest in working with industry on antibiotic development.
Studying the interactions of small molecules with bacteria serves two roles. One role adds value to a potential antibiotic agent by understanding the in vivo target(s) and resistant mechanism(s). On the other hand small molecules also serve as tool reagents for understanding biological systems, an approach called chemical genetics. We have found both applications to be fruitful. In an example of the former, genetically engineered bacteria strains can be made that under- or over-expressed a target gene product which can correspondingly makes the cell hypersensitive or resistant to an antibiotic that acts on that gene product. Such findings provide conclusive evidence for the in vivo mechanism of action. On the other hand, in chemical genetics small molecules can serve as a biological tool. In one example, a novel antibacterial cationic agent was used to identify gain-of-function mutations in the E. coli PmrA two-component transcriptional regulator. These mutants constitutively activated the PmrA regulon that results in enzymatic modification lipopolysaccharides (LPS, endotoxin). Such changes reduce the negative charge on LPS and renders cells resistant to select cationic agents while simultaneously making the cells hypersensitive to the anionic agent deoxycholate. These studies show that Gram-negative bacteria can alter the properties of their OM barrier to change sensitivity profiles to antibacterials. Thus chemical genetics can be used to probe a myriad of processes because their use is reversible/conditional, dose-dependent and specific. The exploitation of chemical genetics on OM biogenesis and function is especially promising because the purpose of the OM is to act as a toxic molecule barrier. Thus genetic changes that alter the OM can have highly specific and informative consequences.
Postdoctoral Fellow, Stanford University, 1998
Ph.D., University of Utah, 1994
B.A., Sonoma State University, 1988
Assistant Professor, University of Wyoming
Principal Scientist, Anadys Pharmaceuticals
Senior Scientist, Elitra Pharmaceuticals
Pathak, D.T., Wei, X., Bucuvalas, A., Haft, D., Gerloff, D. L. and Wall, D. 2012. Cell contact-dependent outer membrane exchange in myxobacteria: Genetic determinants and mechanism. PLoS Genetics 8:e1002626.
Xiao, Y., Gerth, K., Müller and Wall, D., 2012. Myxobacterium-produced antibiotic TA (myxovirescin) inhibits type II signal peptidase. Antimicrob. Agents Chemother.56:2014-2021.
Pathak, D.T and Wall, D. 2012. Identification of the cglC, cglD, cglE and cglF genes and their role in cell contact-dependent gliding motility in Myxococcus xanthus. J. Bacteriol. 194:1940-1949.
Xiao, Y., Wei, X., Ebright, R.H. and Wall, D., 2011. Antibiotic production by myxobacteria plays a role in predation. J. Bacteriol. 193:4626-4633.
Wei, X., Pathak, D. T. and Wall, D., 2011. Heterologous protein transfer within structured myxobacteria biofilms. Mol. Microbiol. 81:315-326.
Xu, H., Trawick, J.D., Haselbeck, R.J., Forsyth, R.A., Yamamoto, R.T., Archer, R., Patterson, J., Allen, M., Froelich, J.M., Taylor, I., Nakaji, D., Maile, R., Kedar, G.C., Pilcher, M., Brown-Driver, V., McCarthy, M., Files, A., Robbins, D., King, P., Sillaots, S., Malone, C., Zamudio, C.S., Roemer, T., Wang, L., Youngman, P.J. and Wall, D. 2010. Staphylococcus aureus TargetArray: Comprehensive differential essential gene expression as a mechanistic tool to profile antibacterials. Antimicrob. Agents Chemother. 54:3659-3670.
Wall, D. 2009. Recombinant DNA, Basic Procedures. In Encyclopedia of Microbiology, 3rd Edition, edited by Moselio Schaechter, Oxford: Elsevier pp 271-280.
Zhou, Y., Chow, C., Murphy, Sun, Z., D., Bertolini, T., Froelich, J. M., Webber, S. E., Hermann, T. and Wall, D. 2008. Antibacterial activity in serum of the 3,5-diamino-piperidine translation inhibitors. Bioorg. Med. Chem. Lett. 18:3369-3375.
Zhou, Y., Gregor, V., Ayida, B., Winters, G. C., Sun, Z., Murphy, D., Haley, G., Bailey, D., Froelich, J. M., Fish, S., Webber, S. E., Hermann, T. and Wall, D. 2007. Synthesis and SAR of 3,5-diamino-piperidine derivatives: Novel antibacterial translation inhibitors as aminoglycoside mimetics. Bioorg. Med. Chem. Lett. 17:1206-1210.
Zhou, Y., Sun, Z., Froelich, J. M., Hermann, T. and Wall, D. 2006. Structure-activity relationships of novel antibacterial translation inhibitors: 3,5-diamino-piperidinyl triazines. Bioorg. Med. Chem. Lett. 16:5451-5456.
Nudleman, E., Wall, D. and Kaiser, D. 2006. Polar assembly of the type IV pilus secretin in Myxococcus xanthus. Mol. Microbiol. 60:16-29.
Froelich, J. M., Khoa, T., and Wall, D. 2006. A pmrA constitutive mutant sensitizes Escherichia coli to deoxycholic acid. J. Bacteriol. 188:1180-1183.
Nudleman, E., Wall, D. and Kaiser, D. 2005. Cell-to-cell transfer of bacterial outer membrane proteins. Science 309:125-127.