Dr. Todd Reynolds

See Also: Curriculum Vitae

Major Research Interests

My lab is interested in the molecular mechanisms that govern biofilm formation and pathogenesis in fungi.  There lab focuses on two main projects.  1) Elucidating the roles that proteins involved in cell wall construction and signal transduction play in biofilm formation. 2) Understanding the role of phospholipid metabolism in fungal virulence. We are exploring these questions in the model yeast Saccharomyces cerevisiae and the fungal pathogens Candida albicans and Candida glabrata. 

Importance to medicine

Yeasts of the genus Candida are the most common causes of fungal infections in humans, and C. albicans and C. glabrata are the two most commonly isolated Candida species.  Unlike many other fungal pathogens, Candida species are normal commensal residents of the oral, gastrointestinal, and genitourinary tracts of humans.  However, under certain conditions, such as immune deficiency, Candida can become a very dangerous pathogen. Candida species are the fourth most common cause of catheter-related bloodstream infection in intensive care units, and these infections are very serious, with mortality rates as a high as 40%. The strong association of Candida infections with catheters is believed to be due to Candida species forming biofilms in the lumens of the catheters. Candida species also cause oral infections (thrush) in immunocompromised patients including those with Acquired ImmunoDeficiency Syndrome (AIDS), and they are the most common cause of fungal vaginitis in women.

Role of S. cerevisiae as a model fungus

Saccharomyces cerevisiae, or bakers’ yeast, has been a very powerful tool for discovering genes and proteins important for a myriad of processes relevant to virulence in fungal pathogens of the Candida and Cryptococcus genera.  For example, in C. albicans, filamentous growth is crucial for virulence, and signal transduction pathways governing filamentous growth such as Rim101p, the filamentous growth MAP kinase (fMAPK), and protein kinase A (PKA), were all discovered first in S. cerevisiae, and then homologs were discovered in C. albicans.  Cell wall associated adhesins, which are crucial virulence factors in Candida species, and are involved in biofilms, filamentous growth, and adhering to the host, were also discovered first in S. cerevisiae, and their homologs were found in Candida. S. cerevisiae has served a similar role in discovering Candida genes involved in drug resistance, cell wall construction, immune evasion, basic lipid metabolism, and other functions. 

Project 1:  Biofilm formation

Biofilms are surface-attached communities of microbial cells that form in association with boundaries such as solid-liquid or liquid-air interfaces.  Biofilms are common among many microbes, and in clinical settings present enormous problems because the cells in the biofilms become inherently more resistant to antimicrobials.  Biofilm formation on intravascular catheters is associated with the large percentage of bloodstream infections caused by Candida albicans.  It is often impossible to eradicate a catheter-associated Candida infection unless the catheter is removed, which can be dangerous in very ill patients. 

In contrast to Candida, fungal biofilms formed by S. cerevisiae can have some useful purposes.  Part of the fermentation process for making Sherri wines involves the formation of biofilms on the surface of wine by specific strains of S. cerevisiae.  These yeasts can withstand high concentrations of alcohol, and it is possible that yeast biofilms might serve a future purpose in creating biofilm-based bioethanol fermentors for alternative fuel production.

Our lab is currently studying fungal biofilms using S. cerevisiae as a model. We have found that bakers’ yeast generates a complex biofilm consisting of aggregates of cells that adhere to one another and the agar surface.  This generates complex flower-like patterns in the center of the biofilm, which is surrounded by a rim of growing cells that spread over the surface as they divide. This growth and spreading is referred to as sliding motility.  The three main aims of this research are to discover 1)  the environmental cues that drive biofilm formation 2) the signaling pathways that regulate this process in response to such cues 3) the proteins and molecules in the fungal cell wall that facilitate adhesion and biofilm formation, and the mechanisms by which they work.

1. Glucose and pH gradients within the biofilm are important environmental cues that drive this process, and this correlates with adhesion and the formation of patterns. Several glucose sensing pathways such as the kinases Snf1p and Yak1p are involved in this process as is the ras homolog Ras2p (Reynolds, et al 2008 Euk Cell 7: 122-130). 
   
   
2. In addition, it has been found that association with the surface of the agar plate during biofilm formation affects the expression of a number of genes that are controlled by the transcription factors of the inositol regulon, Opi1p, Ino2p, and Ino4p.  The inositol regulon has been found to control biofilm formation in yeast (Reynolds 2006 Euk Cell 5: 1266-1275).
   
   
3. Proteins in the inositol regulon and the glucose sensing pathways described above control biofilm formation by regulating the expression of a cell wall adhesion protein (adhesin) called Flo11p, that is essential for biofilm formation (Reynolds, et al 2001 Science 291: 878-881).  We are currently exploring how these various signaling pathways and transcription factors interact with one another to regulate Flo11p. 
   
   
4. In addition, we now have evidence that other cell wall-associated proteins are required for biofilm formation in addition to Flo11p.  Thus, Flo11p is necessary, but not sufficient, for biofilm formation.  We are in the process of characterizing these proteins and the mechanism by which they interact with Flo11p and facilitate biofilm formation. 

We are using genomic, genetic, and biochemical approaches to explore the points listed above.  The genes, proteins, and molecules that we discover in S. cerevisiae that are needed for biofilm formation will serve as a guide for discovering homologs needed for biofilm formation in Candida albicans and/or Candida glabrata.

Project 2: The role of lipid metabolism in fungal pathogenesis

In order to cause disease fungal pathogens must be able to grow and proliferate in the host.  This requires the acquisition of essential nutrients such as nucleotides, amino acids, and lipid precursors.  Phospholipids are a major component of the membranes in virtually all organisms, and can serve as important signaling molecules as well.  Thus, fungi must have mechanisms for acquiring phospholipid precursor molecules from the host in order to proliferate and/or cause disease.  Some of the enzymes utilized by fungi to make phospholipids are not conserved in humans, and therefore may be novel drug targets.  There are only five classes of antifungals in common use, and a combination of fungal resistance and host toxicity has limited several of them. Therefore, there is an urgent need for more antifungal targets and agents. 

We have focused on pathways for making and regulating the synthesis of the phospholipids phosphatidylinositol (PI), phosphatidylserine (PS), phosphatidylethanolamine (PE), and phosphatidylcholine (PC).  These pathways have been very well worked out in Saccharomyces cerevisiae, so this yeast serves as an excellent template from which to explore the roles of similar pathways in Candida albicans and Candida glabrata in virulence.

1. We have discovered that in C. albicans, there are two pathways for acquiring inositol, an essential precursor for phosphatidylinositol (PI) synthesis.   Either pathway is sufficient for C. albicans to cause disease in a host.  (A) In the de novo pathway, C. albicans makes inositol from glucose-6-phosphate by the enzyme inositol-3-phosphate synthase, Ino1p.  (B) In the import pathway, inositol is transported into the cell from the host.  Either pathway is sufficient to cause disease (Chen, et al, 2008 Infect Immun 76: 2793-2801).   This is in stark contrast to other microbial pathogens such as Mycobacterium tuberculosis and Trypansoma brucei, where the de novo pathway is essential for causing disease or growth, respectively (Reynolds, 2009 Microbiology 155:1386-1396).
   
   
2. We are exploring the roles of inositol regulon homologs in regulating phospholipid biosynthesis and/or virulence in both C. albicans and C. glabrata.  These studies will determine how these transcriptional regulatory pathways affect virulence and will allow for a comparison of the pathways between several fungal species (Bethea, E. K., et al, submitted).
   
   
3. We are determining the roles for the PS, PE, and PC phospholipid synthesis pathways in the virulence of C. albicans and C. glabrata (Chen, Y-L., et al, submitted). 

Altogether, these studies will allow us to understand how phospholipid biosynthesis and regulation affect virulence in these extremely important fungal pathogens.

Dr. Todd Reynolds

Assistant Professor
Ph.D., 1999
Vanderbilt University

M409 Walters Life Sciences Knoxville, Tennessee
37996-0845

Phone: 865-974-4025
Fax: 865-974-4007
Email: treynol6@utk.edu