Light-Activated Molecular Machines Kill Pathogenic Fungi

In the game-turned-TV series, The Last of Us, post-apocalyptic survivors Joel and Ellie make their way across the U.S., outrunning zombie hordes while coping with societal collapse, all caused by a nasty mutation in the parasitic Cordyceps fungus, leading to an even nastier mass infection.

Luckily, in everyday life, a stubborn athlete’s foot infection cannot turn any of us into mildewy zombies. However, COVID, climate change, and other factors have made invasive fungal infections a growing public health concern, and as fungi become increasingly resistant to existing drugs, new antifungals are urgently needed.

James Tour is the T. T. and W. F. Chao Professor of Chemistry and a professor of materials science and nanoengineering at Rice University. Courtesy of Jeff Fitlow/Rice University.

According to research conducted by Rice University professor James Tour and his collaborators, light-activated nanoscale drills could make fungal infections easier to treat. Based on the work of Nobel laureate Bernard Feringa, the inventor of molecular motors, the Tour group’s molecular machines are nanoscale compounds with paddle-like chains of atoms that move in a single direction when exposed to visible light, causing a drilling motion that allows the machines to bore into the surface of cells.

Upon activation with 405-nm visible light, the molecular machines perform unidirectional drilling into the cell at 2 million to 3 million cycles per second. The molecular machines bind to fungal mitochondria in the cell, causing the fungal cell to disintegrate by disrupting its metabolism. The fungi-killing molecular machines can be remotely controlled by adjusting the light dose, with higher light doses enhancing antifungal activity.

Schematic representation of the mechanisms by which light-activated molecular machines kill fungi. Molecular machines bind to fungal mitochondria, decreasing adenosine triphosphate (ATP) production and impairing the function of energy-dependent transporters that control the movement of ions, such as calcium. This leads to the influx of water, which causes the organelles to swell and eventually the cells to burst. Courtesy of Tour Group/Rice University.

“Our molecules differ from conventional antifungals in that they specifically target what we call the powerhouses of the cell, that is, the mitochondria,” Ana Santos, a Rice alumna who is currently a postdoctoral fellow at Fundación Instituto de Investigación Sanitaria Islas Baleares, said.

This targeted attack causes mitochondrial dysfunction, calcium overload, and, ultimately, cell death.

“By targeting the mitochondria, our molecules disrupt the cell’s metabolism, resulting in an overall energy imbalance that leads to an uncontrolled flow of water and ions such as calcium into the cell, eventually causing the cell to explode,” Santos said.

The molecular machines were found to rapidly kill planktonic and biofilm fungi without the fungi developing resistance. Fungal biofilms are highly resistant to antifungal drugs and host immune defenses, making the treatment of biofilm-associated infections particularly challenging.

In addition to providing a direct antifungal effect, the molecular machines were found to strengthen the effects of conventional antifungals, in part by impairing efflux pump function. The molecular machines remained stable over 20 cycles of repeated treatment, suggesting that fungal resistance to molecular machines is not easily achieved.

Ultrastructural changes induced by light-activated molecular machines in the fungus Candida albicans, detected by transmission electron microscopy, compared to a solvent control (1% dimethyl sulfoxide). Courtesy of Matthew Meyer/Electron Microscopy Facilities, Rice University.

In experiments, the molecular machines were found to synergize with conventional antifungals in vivo, reducing mortality and fungal burden associated with systemic fungal infections, and ex vivo, outperforming conventional antifungals in reducing the fungal load in an infected porcine model.

The Tour group originally developed the molecular machines to treat antibiotic-resistant infectious bacteria and cancer cells. They have been shown to be just as good at fighting infectious fungi.

“Dr. Tour posed the question of whether they can also kill fungi, which had never been explored before,” Santos said. “Our study is the first to show that, indeed, these molecules can also be effective against fungi.”

Most conventional antifungal agents come with undesirable side effects. A therapeutic approach combining sublethal molecular machines to sensitize cells to conventional antifungals could mitigate the side effects of existing antifungal therapies. The ability of molecular machines to enhance the effect of conventional antifungal drugs by targeting a distinct process in the cell or preventing efflux supports their use as dual mode-of-action antifungals that could provide a new therapeutic option to combat pan-resistant fungal strains.

The research was published in Advanced Science (www.doi.org/10.1002/advs.202205781).