Iron Wars: Hand-to-Hand Combat for Vital Nutrients

            You have likely inhaled several fungal spores while reading this sentence. Fungi are ubiquitous members of our environments, present almost everywhere humans live. In order to reproduce, these organisms create airborne spores that can disperse and spread. Luckily, inhalation of these fungal spores typically has no effect on the person that inhales them. However, in the case of an immunocompromised person, fungi can cause harmful infections in people (1). Aspergillus fumigatus is an example of such a fungus, a common organism found in close association with humans. This fungus can cause a disease, known as aspergillosis, in immunocompromised individuals, when the fungus infects various parts of the body (1). However, in rare cases, a healthy person can be infected with Aspergillus in the cornea of the eye. This disease, called fungal keratitis, causes eye pain, blurred vision, and clouding of the cornea. If left untreated, this infection can lead to severe impairment of vision or even blindness (2).

 

Fungal keratitis in human cornea

            When Aspergillus infects another organism, like a human or another susceptible mammal, it needs to find all the food and nutrients it needs from sources inside that organism’s body. Like most other organisms, Aspergillus requires iron in order to survive and since other organisms need iron as well, the fungus can usually find a source of this iron in the host organism’s blood and fluids. Here, iron can exist in a free-floating soluble form or attached to proteins that transport or store the molecule. During an Apsergillus infection, since both the fungus and the host need iron, the two organisms enter into a heated battle for the iron present in the host’s body. Both sides employ different weapons and tactics in order to win over the iron source and use it for themselves.

            To pick up iron from its environment, Aspergillus uses a molecule called a siderophore. On the outside of the cell, these structures act like iron magnets for the fungus, attracting and binding soluble iron in the fungus’ surroundings. On the inside of the cell, siderophores help store iron for the cells to use later (3). This action makes extracellular siderophores in particular very important when iron is low in the surroundings. Without the ability to bind iron and bring it in to the cell, Aspergillus becomes less virulent (3).
           Humans have their own iron-carrying and iron-storing molecules to do battle with, called transferrin and ferritin, respectively (4). However, the iron-binding strength of the siderophore gives the Aspergillus an advantage in hand-to-hand combat for iron. This is demonstrated in mice infected with Aspergillus, when levels of iron in the mouse blood decreased when the fungus was introduced. Additionally, when iron is added to the mouse’s blood, the severity of the Aspergillus infection increased, suggesting that the fungus thrived on this increase in iron (5). With the fungus stealing iron so efficiently, the human body needs to find a way to level the playing field, and it does so by targeting siderophores.

            Lipocalin-1 is a molecule produced by human cells that can bind fungal siderophores. Blocking the siderophores with this molecule renders them useless in iron binding, impairing Aspergillus’ ability to take up iron. This effect was shown in mice that were treated with human Lipocalin-1 during Aspergillus infection. Mice that received topical treatment of the human produced molecule had a decrease in the amount of fungus that grew in their cornea (5). This suggest that Lipocalin-1 is an effective weapon in combating siderophore use by Aspergillus during infection.

Some interactions between siderophores, iron,and host cell molecules

            As a tactical fighter, Aspergillus needs ways to deploy its siderophores only when it needs them. When iron is scarce in the environment, the fungus wants to produce an abundance of siderophores to give itself every advantage in trapping what little iron there is. However, when iron is common and the fungus doesn’t need to waste its energy trying to find iron, Aspergillus makes fewer siderophores. In addition, too much iron uptake can be toxic to a cell, so it is important to regulate this process.To do this, the fungus utilizes a system to sense the iron in the environment and respond to those levels. For Aspergillus, this system hinges on a transcriptional regulator called SreA. This gene set is activated by high iron levels in the surroundings and represses the uptake and storage of iron by the fungus. When SreA is deleted, iron levels in the cell build up under high-iron conditions, creating harmful circumstances for the fungus (3). SreA presents a potential target for warfare against Aspergillus; inactivating SreA could turn this fungus’ own weapons against it and make iron acquisition toxic for fungal cells.

            Aspergillus also needs specific equipment to build its siderophores. The biosynthetic pathway that makes extracellular siderophores in this fungus incorporates the molecule mevalonate at a crucial step in the process. Without the inclusion of this molecule in its siderophores, the fungus seems to be unable to carry out infection. This was demonstrated by testing in mice. Deleting genes which encode the enzymes that incorporate mevalonate into siderophores resulted in an Aspergillus strain that does not infect the corneas of mice, suggesting that mevalonate plays a crucial role in the virulence of this fungus (5).

            Realizing the importance of siderophores and their biosynthesis as factors in Aspergillus infection can help identify treatments for these infections. Humans naturally produce one compound, Lipocalin-1, which targets siderophores and helps fight infection. Other compounds with similar actions can also be used to target siderophores and treat Aspergillus infections. Perhaps the most powerful weapons at our disposal are statins and iron chelator compounds. Statins work to block the enzymes that incorporate mevalonate into extracellular siderophores. When these compounds are applied topically to mice that are infected with Aspergillus, the severity of the infection is significantly decreased (5). Iron chelators act in a similar fashion to siderophores, binding iron with high affinity and sequestering it. These compounds can compete with fungal siderophores for iron and prevent the siderophores from binding it. Topical treatment with iron chelator compounds in Aspergillus infected mice also decreased the severity of these infections. Furthermore, when these two treatments were combined, they seemed to have additive effects, decreasing the severity of Aspergillus infection more so than either did separately (5).

            Understanding the balance of iron use between the mammalian host and an infecting fungus gives crucial information needed to develop treatments for these infections. Identifying how fungi use and regulate siderophores during infection presents clear targets for the next strike against these invaders. This war tactic is not limited solely to Aspergillus either, but can have effects in fighting related adversaries such as Fusarium oxysporum, a common plant pathogen (plos). With this inside information, we can stage an attack to win the war against these fungal pathogens, starting with the battle for iron.

References 

1.       "Aspergillosis." Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 09 Jan. 2012. Web. 02 Dec. 2013

2.       “Fungal Keratitis.”Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 09 Jan. 2012. Web. 02 Dec. 2013 

3.       Schrettl, M., Kim, H. S., Eisendle, M., Kragl, C., Nierman, W. C., Heinekamp, T., Werner, E. R., Jacobsen, I., Illmer, P., Yi, H., Brakhage, A. A. and Haas, H. (2008), SreA-mediated iron regulation in Aspergillus fumigatus. Molecular Microbiology, 70: 27–43

4.       Hentze MW, Muckenthaler MU, Galy B, Camaschella C (2010) Two to tango: regulation of Mammalian Iron Metabolism. Cell 142: 24-38 

5.       Leal SM Jr, Roy S, Vareechon C, Carrion SdJ, Clark H, et al. (2013) Targeting Iron Acquisition Blocks Infection with the Fungal Pathogens Aspergillus fumigatus and Fusarium oxysporum. PLoS Pathog 9(7): e1003436. doi:10.1371/journal.ppat.1003436