Day 2 of the Duke Mycology Symposium has wound to a close, [DAY 1 HERE] and I am sitting on my porch contemplating the afternoon’s lectures:
“Pathogen recombination during the amphibian Chytridiomycosis pandemic: Why change what’s working?”
A genetics perspective on Bd, a fungus responsible for widespread amphibian mortality. Apparently one of the factors in its spread is the abundance and transport of bullfrogs (raised for food) and xenopus frogs (used in medical research), which can carry the disease without being killed by it. The recent spread is caused by a single Bd strain which reproduces by cloning itself – it should therefor be genetically uniform. Yet, in practice Bd has a ‘dynamic genome’. This led to discussion led to mechanisms for genetic change without sex, such as mitotic crossover and gene conversion.
“Pathogenicity factors in the chytrid fungus and amphibian pathogen B. dendrobatidis”
Further discussion of Bd, this time from a molecular / genomic perspective. Perhaps the most interesting part was evidence that chytrids contain rhodopsin, a light-sensitive pigment.  I was also alerted to the existence of the 1000 fungal genomes project.
“Pleiotropic roles of the UPR pathway in Cryptococcus”
UPR is the unfolded protein response – when there are bits of proteins floating around inside a cell, it’s a bad sign. Maybe those proteins were torn apart by heat, or a toxin. This talk looked at the responses of Cryptococcus to the presence of the UPRs. In some cases, they release ‘chaperones’, proteins which help other molecules assemble correctly. Or, they might release dedegredation enzymes to clean up the mess. In extreme cases, they may even trigger apoptosis, a sort of cellular suicide.
“The adaptive value of Flo11‐dependent flocculation and adhesion in yeast”
Proteins on the surface of certain yeast cells act to let the cells stick together and form clusters, which then fall out of their liquid medium. The gene for this surface protein is under considerable epigenetic control – there was a really beautiful picture the speaker presented, in which genetically identical yeast cells nonetheless have different levels of gene expression. Additionally, this phenomenon is an example of the green beard effect.
“Fear the Titans: When bad yeast get worse”
Titan cells are variants of cryptococcus. as much as 20 times as large as typical cells. By themselves, Titans are actually non-virulent. However, they may lie dormant in the body – the speaker found them lurking in the lungs of their test mice – and they appear to provide some protection
against FOR normal, virulent cells. They are not only bigger, but are not phagocytosed by immune cells, and they are resistant to chemical attacks by the immune system. This is something interesting I learned: I’d known that the immune system creates reactive oxygen compounds as part of its chemical weaponry, but I was not previously aware that it uses nitric oxide in a similar manner.
“Nickel homeostasis in Cryptococcus neoformans”
A recurring theme at this symposium, and one that I appreciate greatly, is the role of metals in biochemistry. Nickle is apparently important for C. neoformans, which uses it as an active site in its urease enzyme. Urease, in turn, is essential for virulence. This leads to an interesting question: since humans have no known nickel metalloproteins, how does cryptococcus maintain its nickel supply? Another observation the speaker made was that metals have an increasing binding affinity, which means that it’s critical that the right metals get to the right proteins. Indeed, the speaker presented an experiment showing that cobalt competes with nickel for the urease binding site, and in doing so inhibits urease activity.
“The structural challenge of the C. neoformans capsule and melanin”
The speaker began by pointing out a knowledge gap connected with size scales. For smallish molecules, there are easy analytical methods available; for entities large enough for microscopy, there’s microscopy. But in between, mesoscopic phenomena, like the capsule structure of Cryptococcus, things are messier. They described their research into cryptococcal capsules, showing for example that they become denser and less permeable with age, owing to changes in their polysaccharide composition. They then went into one of my all time favorite mycological topics, the relationship between melanin and high energy radiation. Not that long ago, fungi were discovered in Chernobyl which were capable of using melanin as a radiosynthetic pigment (Dadachova et al. 2007), similar to how plants use chlorophyll as a photosynthetic pigment. The speaker covered this topic, the impact of growth substrate on melanin form (the pigment is structurally amorphous), and presented some unpublished research showing that mice could withstand deadly doses of radiation… if they ate melanin-rich black fungi. Whoa!
That wraps it up for now. I’ll be back soon to talk about a couple of my favorite posters from the poster session!
- Further Reading:
Dadachova E, Bryan RA, Huang X, Moadel T, Schweitzer AD, Aisen P, Nosanchuk JD, & Casadevall A (2007). Ionizing radiation changes the electronic properties of melanin and enhances the growth of melanized fungi. PloS one, 2 (5) PMID: 17520016