Michael D L Johnson

Michael D L Johnson

Associate Professor, Applied BioSciences - GIDP
Associate Professor, BIO5 Institute
Associate Professor, Immunobiology
Member of the Graduate Faculty
Primary Department
Department Affiliations
(520) 626-3779

Work Summary

Metals such as calcium and iron are essential to living organisms. Some metals in excess, like copper, are detrimental to bacteria. My laboratory studies this phenomenon in Streptococcus pneumoniae to find novels method for killing pathogenic bacteria.

Research Interest

Metals serve as vital nutrients to all biological systems. During infections, bacteria must not only acquire all metals necessary for survival from within the host, such as calcium or manganese, but must also efflux metals that are toxic or in excess such as copper. The overall goal of my laboratory is to investigate how bacteria maintain homeostasis within the metal milieu. This goal involves determining how metals are processed, the orchestrated response during metal sensing, and the role that the host plays in this process during infection. Understanding how bacteria interact with metals during infections will identify novel therapeutic strategies against bacterial infections. Keywords: Infectious Diseases, Antibiotic resistance, Bacterial Pneumonia


Johnson, M. D., Kehl-Fie, T. E., & Rosch, J. W. (2015). Copper intoxication inhibits aerobic nucleotide synthesis in Streptococcus pneumoniae. Metallomics : integrated biometal science, 7(5), 786-94.

Copper is universally toxic in excess, a feature exploited by the human immune system to facilitate bacterial clearance. The mechanism of copper intoxication remains unknown for many bacterial species. Here, we demonstrate that copper toxicity in Streptococcus pneumoniae is independent from oxidative stress but, rather, is the result of copper inhibiting the aerobic dNTP biosynthetic pathway. Furthermore, we show that copper-intoxicated S. pneumoniae is rescued by manganese, which is an essential metal in the aerobic nucleotide synthesis pathway. These data provide insight into new targets to enhance copper-mediated toxicity during bacterial clearance.

Hess, P. R., Barnes, C., Woolard, M. D., Johnson, M. D., Cullen, J. M., Collins, E. J., & Frelinger, J. A. (2007). Selective deletion of antigen-specific CD8+ T cells by MHC class I tetramers coupled to the type I ribosome-inactivating protein saporin. Blood, 109(8), 3300-7.

CD8+ cytotoxic T lymphocytes (CTLs) are important effector cells responsible for tissue destruction in several autoimmune and allograft-related diseases. To discover if pathogenic T cells could be selectively deleted, we investigated the ability of a toxin coupled to major histocompatibility complex (MHC) class I tetramers to kill antigen-specific CD8+ T cells. H2-D(b) tetramers were assembled using streptavidin conjugated to the ribosome-inactivating protein (RIP) saporin (SAP). These tetramers inhibited ribosome activity in vitro, retained the T-cell receptor (TCR)-binding specificity of their nontoxic counterparts, and were internalized by 100% of target cells, leading to cell death in 72 hours. Cytotoxicity was dependent on the tetramer dose and avidity for the T cell. A single injection of the SAP-coupled tetramer eliminated more than 75% of cognate, but not control, T cells. This work demonstrates the therapeutic potential of cytotoxic tetramers to selectively eradicate pathogenic clonotypes while leaving overall T-cell immunity intact.

Echlin, H., Frank, M. W., Iverson, A., Chang, T., Johnson, M. D., Rock, C. O., & Rosch, J. W. (2016). Pyruvate Oxidase as a Critical Link between Metabolism and Capsule Biosynthesis in Streptococcus pneumoniae. PLoS pathogens, 12(10), e1005951.

The pneumococcus is one of the most prodigious producers of hydrogen peroxide amongst bacterial pathogens. Hydrogen peroxide production by the pneumococcus has been implicated in antibiotic synergism, competition between other bacterial colonizers of the nasopharynx, and damage to epithelial cells. However, the role during invasive disease has been less clear with mutants defective in hydrogen peroxide production demonstrating both attenuation and heightened invasive disease capacity depending upon strain and serotype background. This work resolves these conflicting observations by demonstrating that the main hydrogen peroxide producing enzyme of the pneumococcus, SpxB, is required for capsule formation in a strain dependent manner. Capsule production by strains harboring capsules with acetylated sugars was dependent upon the presence of spxB while capsule production in serotypes lacking such linkages were not. The spxB mutant had significantly lower steady-state cellular levels of acetyl-CoA, suggesting that loss of capsule arises from dysregulation of this intermediary metabolite. This conclusion is corroborated by deletion of pdhC, which also resulted in lower steady-state acetyl-CoA levels and phenocopied the capsule expression profile of the spxB mutant. Capsule and acetyl-CoA levels were restored in the spxB and lctO (lactate oxidase) double mutant, supporting the connection between central metabolism and capsule formation. Taken together, these data show that the defect in pathogenesis in the spxB mutant is due to a metabolic imbalance that attenuates capsule formation and not to reduced hydrogen peroxide formation.

Wang, H., Li, H., Moore, L. B., Johnson, M. D., Maglich, J. M., Goodwin, B., Ittoop, O. R., Wisely, B., Creech, K., Parks, D. J., Collins, J. L., Willson, T. M., Kalpana, G. V., Venkatesh, M., Xie, W., Cho, S. Y., Roboz, J., Redinbo, M., Moore, J. T., & Mani, S. (2008). The phytoestrogen coumestrol is a naturally occurring antagonist of the human pregnane X receptor. Molecular endocrinology (Baltimore, Md.), 22(4), 838-57.

Antagonizing the action of the human nuclear xenobiotic receptor pregnane X receptor (PXR) may have important clinical implications in preventing drug-drug interactions and improving therapeutic efficacy. We provide evidence that a naturally occurring phytoestrogen, coumestrol, is an antagonist of the nuclear receptor PXR (NR1I2). In transient transfection assays, coumestrol was able to suppress the agonist effects of SR12813 on human PXR activity. PXR activity was assessed and correlated with effects on the metabolism of the anesthetic tribromoethanol and on gene expression in primary human hepatocytes. We found that coumestrol was able to suppress the effects of PXR agonists on the expression of the known PXR target genes, CYP3A4 and CYP2B6, in primary human hepatocytes as well as inhibit metabolism of tribromoethanol in humanized PXR mice. Coumestrol at concentrations above 1.0 microm competed in scintillation proximity assays with a labeled PXR agonist for binding to the ligand-binding cavity. However, mammalian two-hybrid assays and transient transcription data using ligand-binding-cavity mutant forms of PXR show that coumestrol also antagonizes coregulator recruitment. This effect is likely by binding to a surface outside the ligand-binding pocket. Taken together, these data imply that there are antagonist binding site(s) for coumestrol on the surface of PXR. These studies provide the basis for development of novel small molecule inhibitors of PXR with the ultimate goal of clinical applications toward preventing drug-drug interactions.

Meliopoulos, V. A., Van de Velde, L., Van de Velde, N. C., Karlsson, E. A., Neale, G., Vogel, P., Guy, C., Sharma, S., Duan, S., Surman, S. L., Jones, B. G., Johnson, M. D., Bosio, C., Jolly, L., Jenkins, R. G., Hurwitz, J. L., Rosch, J. W., Sheppard, D., Thomas, P. G., , Murray, P. J., et al. (2016). An Epithelial Integrin Regulates the Amplitude of Protective Lung Interferon Responses against Multiple Respiratory Pathogens. PLoS pathogens, 12(8), e1005804.

The healthy lung maintains a steady state of immune readiness to rapidly respond to injury from invaders. Integrins are important for setting the parameters of this resting state, particularly the epithelial-restricted αVβ6 integrin, which is upregulated during injury. Once expressed, αVβ6 moderates acute lung injury (ALI) through as yet undefined molecular mechanisms. We show that the upregulation of β6 during influenza infection is involved in disease pathogenesis. β6-deficient mice (β6 KO) have increased survival during influenza infection likely due to the limited viral spread into the alveolar spaces leading to reduced ALI. Although the β6 KO have morphologically normal lungs, they harbor constitutively activated lung CD11b+ alveolar macrophages (AM) and elevated type I IFN signaling activity, which we traced to the loss of β6-activated transforming growth factor-β (TGF-β). Administration of exogenous TGF-β to β6 KO mice leads to reduced numbers of CD11b+ AMs, decreased type I IFN signaling activity and loss of the protective phenotype during influenza infection. Protection extended to other respiratory pathogens such as Sendai virus and bacterial pneumonia. Our studies demonstrate that the loss of one epithelial protein, αVβ6 integrin, can alter the lung microenvironment during both homeostasis and respiratory infection leading to reduced lung injury and improved survival.