Scientists from NTU School of Chemistry, Chemical Engineering & Biotechnology (CCEB) have found a potential new way to treat fungal infections. While studying the biological mechanisms of the fungus Candida albicans, the team discovered a protein sensor on its cell surface that can be targeted to prevent this normally harmless organism from becoming dangerous.
Back row (L-R): Christopher Adamson, Nanyang Assistant Professor Qiao Yuan
Front row (L-R): Evan Wei Long Ng, Li Lanxin
What is Candida Albicans?
The fungus, Candida albicans, is a yeast that lives in and on our bodies — in small amounts on our skin, in our mouths and gut — and is a natural part of our bodies’ microbiome.
C. albicans is described as a pathobiont, or an opportunistic microorganism. In small amounts, it is usually harmless. However, the fungus can take advantage of situations such as when our immune systems have weakened or when we have taken antibiotics and can grow out of control and cause infections, or candidiasis, of which there are several types. These include oral thrush, an infection that affects the mouth and throat; mucocutaneous candidiasis, which affects our skin and mucus membranes; and genital yeast infections.
These infections are common, and symptoms are usually mild, frequently manifesting as skin rashes, itching, or burning sensations. Treatment is straightforward, often just requiring a round of antifungal medication.
However, these yeast infections can become more serious or even life-threatening, especially in patients with very weakened immune systems. An invasive candidiasis infection can affect vital organs like the heart and brain, causing severe inflammation and resulting in diseases like endocarditis and meningitis. Unfortunately, over 200,000 people die each year from invasive candidiasis.
A shape changer
C. albicans is highly responsive to environmental cues and signals, which can trigger the fungus to a undergo morphogenic transition, a biological process in which the microorganism changes its shape.
The fungus is generally harmless in its round form. However, when it senses changes in its environment, C. albicans can transform into a hyphal shape, which is long and branching. In this form, the fungus becomes dangerous to the human body: it can penetrate and break through tissue lining, and its elongated shape makes it difficult for immune cells to ingest.
It is therefore imperative to identify the exact conditions or triggers that cause C. albicans to change its shape so that we can develop ways to stop the fungus before it reaches its more dangerous hyphal form.
What causes C. albicans to change its shape?
Our digestive system has a rich microbiome full of bacteria, viruses, fungi, and other bacterial materials, including peptidoglycan fragments. These fragments, which usually make up the tough cell walls of bacteria, are shed into the gut microbiome when bacteria grow, divide, or are attacked. Far from being just waste products, these fragments are also important signalling molecules that can influence the behaviour of other denizens in our gut microbiome.
In their efforts to study the causes behind C. albicans’ shape changing, the team from NTU CCEB, led by Nanyang Assistant Professor Qiao Yuan, previously carried out in-vitro experiments in which β-lactam antibiotics, like penicillin, were introduced to gut microbiomes. β-lactam antibiotics work by preventing the formation of strong rigid bacterial cell walls, which eventually causes these cells to burst. This releases a massive amount of peptidoglycan fragments from the dying bacteria into the gut microbiome.
The team found that this surge in fragments can trigger C. albicans to start changing its shape. This explains why candidiasis infections are more common in individuals who have taken certain types of antibiotics.
How does C. albicans know when to change its shape?
Scientists have identified an enzyme within the fungus called Cyr1, which is part of the fungus’ shape changing signalling pathway, also known as the cAMP-PKA pathway. Cyr1 initiates this entire pathway when it detects and binds with peptidoglycan fragments. This binding triggers the synthesis of cAMP, a molecule that acts as a “second messenger.’ cAMP then activates protein kinase A (PKA), which causes a series of biochemical reactions in the cell leading to C. albicans’ shape change.
To date, Cyr1 has been the only peptidoglycan sensor identified within the fungus. While this enzyme can sense and bind with peptidoglycan fragments, it is a cytosolic molecule, which means that it exists inside the cell. Therefore, any peptidoglycan fragments it interacts with must first be transported into the cell. Until now, the process behind that transportation has remained a mystery.
The Opt “gates” and the surface sensor, Ssy1
In their study, the team from NTU CCEB identified oligopeptide transporters, or Opts, as the long sought after peptidoglycan fragment transporters. Opts are a family of specialised proteins found in the cell membranes of fungi like C. albicans. They act as “gates,” controlling the movement of molecules across the cell membrane. While there are many types of Opts in C. albicans’ cell membrane, the team found that Opt4 is particularly important for transporting peptidoglycan fragments into the cell.
Peptidoglycan fragments are a good source of nitrogen, which fungi like C. albicans need to grow. During their experiments, the team wanted to see what would happen if the fungus no longer had Opts to take in peptidoglycan fragments. Indeed, samples without these “gates” were unable to take in the fragments.
However, the team discovered that even when peptidoglycan fragments were unable to be taken up by the C. albicans samples, the fungus was still triggered to grow into its hyphal form. This suggests that there is a sensor on the fungus’ cell surface that can detect peptidoglycan fragments and still trigger hyphal growth.
The team identified this sensor as Ssy1, a protein located on the fungus’ cell membrane surface. This was confirmed by removing the Ssy1 gene from another sample of C. albicans and exposing it to a large amount of peptidoglycan fragments. This sample remained in its round yeast form and did not change into its hyphal form. In another sample with one copy of the Ssy1 gene added back in, the fungus regained its ability to grow into its hyphal form when exposed to peptidoglycan fragments.
What does this mean for fungal infections in the future?
While the team is still working on understanding how Ssy1 communicates with other cellular components inside the fungus, the identification of this cell surface sensor remains a major breakthrough.
Scientists can one day start to develop small-molecule inhibitor drugs to target Ssy1 and block hyphal growth. Such drugs could prevent C. albicans from becoming dangerous, even in situations where antibiotics cause large amounts of peptidoglycan fragments to be released into the gut environment. This would be immensely beneficial in protecting patients who have taken strong antibiotics from suffering further dangerous fungal infections.
To read more about their study, click here.