Using light energy as a weapon is a popular trope in science fiction. The planet-killing Death Star from the Star Wars movies is probably the biggest example. In real life, the US military is also investing heavily in high-energy weapons that use lasers to intercept drones and fast missiles.
Besides melting large physical objects and vehicles, light energy can also be harnessed at the molecular level to destroy biological cells. In a radically innovative new form of targeted therapy, researchers at Ohio State University are using light-sensitive proteins to kill cancer cells.
Their target is the mitochondria inside cancer cells. All cells, including mutated cancer cells, rely on mitochondria for energy to survive and reproduce, which naturally makes them an appealing target for researchers looking at ways to kill cancer cells efficiently.
Unfortunately, the mitochondria are a bit of a tough nut to crack with the traditional approach of therapeutic drugs. The novel approach adopted by the research team at the Ohio State University Comprehensive Cancer Center aims to circumvent those defenses.
What Makes the Mitochondria So Resistant to Therapeutic Drugs?
Mitochondria is an oval or cylindrical organelle that generates usable energy for the cells (adenosine triphosphate/ATP) from sugar and fat molecules. Depending on the cell’s energy needs, mitochondria can change their shape into long thread-like structures, or even become highly fragmented.
The sensitive mitochondrial DNA and other enzymes necessary for energy generation are protected by two membranes. Of the two layers, the outer membrane is smooth and relatively permeable, allowing small molecules and metabolites to go through.
Meanwhile, the inner membrane has a folded structure that increases its overall surface area. It is also more selective when it comes to allowing molecules to reach the sensitive mitochondrial matrix that lies at the core.
Primarily due to the selective nature of the inner membrane, only smaller molecules like potassium and fatty acids are allowed inside the mitochondria. Therapeutic drug molecules typically have larger, more complex structures that are easily blocked by the dual membranes.
Instead of attacking the mitochondria directly, existing drugs aim to disrupt its mechanism indirectly, often by targeting specific metabolic pathways like fatty cell oxidation, or the release of cytochrome c for programmed cell death.
Using Electric Currents to Disrupt the Mitochondrial Inner Membrane
One of the lead authors of the study, Professor Lufang Zhou, had discovered in earlier research that the inner membrane was vulnerable to electrical currents. There are specific proteins in living organisms that can facilitate such reactions through a process called ion transfer.
Dr. Zhou’s team used Cationic Channelrhodopsin (CoChR), a type of ion channel protein usually found in certain algae, to disrupt the electrical charge balance of the mitochondrial inner membrane. Since CoChR is a light-sensitive protein, Zhou originally activated it using an external laser.
To make the process work more efficiently from a therapeutic perspective, the Ohio State team had to find ways to deliver a light source along with the protein code into the cells. After experimentation, they landed on the following combination of molecules:
- CoChR genes from a green algae species called Chloromonas oogama
- A luciferase enzyme from a deep-sea shrimp species Oplophorus gracilirostris
Luciferase enzymes are used by fireflies and other organisms to produce light, through a process called bioluminescence. The team packaged blue-light sensitive CoChR genes with a matching light-emitting luciferase, giving the protein its very own activation switch inside the target cells.
The researchers call this new approach mitochondrial luminoptogencis, or mLumiOpto for short. The next step involved figuring out a way to deliver the payload to the cancer cells without causing too much collateral damage to healthy cells.
A Viral Delivery System Designed to Seek Out Cancer Cells
Researchers have used harmless viruses to deliver therapeutic genes since the late 1980s. To send mLumiOpto to cancer cells, Dr. Zhou’s team chose a well-known strain of adeno-associated virus (AAV). These small viruses are not harmful to humans.
To further improve the effectiveness of the virus vector, the AAVs were created using human cells instead of traditional methods that use bacterial or insect cells. This approach reduces the presence of non-human proteins that would trigger an immune response when the AAV is injected into the body.
The AAVs created in Dr. Zhou’s lab had an outer shell that resembled particles that are usually found on human cells. These extracellular vesicles helped the AAVs further blend in with other cells in the blood, thus evading detection by the immune cells during testing.
Last but not least, the viruses also needed a guidance system to ensure that the mLumiOpto payload was only delivered to cancer cells. Targeted anti-cancer therapies often rely on specific receptors found on cancer cells.
The viral delivery system was enhanced with a monoclonal antibody, a lab-produced molecule designed to mimic the immune system’s defenses. In this case, the antibody was designed to seek out a receptor only found in the targeted cancer cells.
Results and Major Implications of the Study for Future Cancer Treatments
The Ohio State research team, which also included Dr. Xiaoguang “Margaret” Liu as lead co-author, tested the mLumoOpto system on two highly aggressive and treatment-resistant cancers – glioblastoma of the brain, and triple-negative breast cancer.
After injecting the AAVs, the mice were given another injection of a chemical designed to activate the luciferase enzyme. The light created by the enzyme, triggered the CoChR protein to weaken and destroy the mitochondria inside the cancer cells.
In lab mice tests, both cancers responded well to the treatment and displayed a significant reduction in tumor size. Further, the monoclonal antibody used for targeting the cancer cells also triggered a secondary immune response against the cancer cells.
The two cancers chosen for this study are among the most aggressive and deadly malignancies, characterized by low survival rates and strong resistance towards traditional treatment approaches. If the mLumiOpto method can replicate its results in future human trials, it could be a game-changer for the treatment of various aggressive cancers.
