When cancer invades the brain, normal salt levels are quickly thrown off-kilter, but detecting this change in patients has been difficult. A team of Yale researchers has developed a non-invasive way to observe these changes in close detail at the early stages of cancer growth.
This method, developed in the lab of Fahmeed Hyder, professor of biomedical engineering & radiology and biomedical imaging, builds evidence for how the tumor microenvironment is uniquely shaped to be unfavorable to host cells. Published in Nature Scientific Reports with lead author Muhammad Khan, a biomedical engineering graduate student, it’s a breakthrough that could lead to several new directions in treating cancer, from earlier diagnosis to new drugs for targeting cancer cells.
“To better treat the cancer, we have to understand what the cancer cells like their environment to be, because that’s the environment they thrive in,” Hyder said. “To understand that environment, you have to measure it.”
While most cancer research has involved screening for tumor-specific molecules, often invasively, there have been much fewer attempts to explore altered salinity within the tumor habitat and use it for cancer diagnosis.
Normal sodium levels in the body help maintain numerous functions, including blood pressure, neuronal signaling, and even muscle movement. Salt, composed of sodium and chloride ions, is typically low inside cells, but is high in the blood and the area surrounding the cells. In what’s known as the “transmembrane gradient,” salt levels decrease steadily from outside the cell to the cell’s core. Alternately, in the brain’s blood vessels, in what’s known as the “transendothelial gradient,” salt levels are comparable between blood and the area outside cells. This reinforces the blood-brain barrier, in which the vessels are tightly packed together, making it difficult for anything outside to enter the brain tissue. These salt levels temporarily change during normal bodily function, but critical mechanisms exist to reestablish the normal sodium gradients. Several of these restorative mechanisms rely on adenosine triphosphate (ATP), a high-energy molecule in the body, for fuel.
This distribution of sodium, however, changes more permanently in the habitat of cancer cells. That’s partly because the cancer slows down the production of ATP, making it difficult to maintain the appropriate sodium levels across the cell membrane.
“Because cancer cells have an altered metabolism, ATP is produced less efficiently,” said Khan. “With less ATP in the tumor, you don’t have the restorative processes for maintaining appropriate salt levels.”
This change creates a kind of feedback loop: Less ATP leads to an altered salt balance, which fuels the tumor’s growth, which leads to even less ATP, and that in turn causes further salt imbalance. Finding a non-invasive way to detect this change in the salt balance, however, has been elusive. State-of-the-art magnetic resonance imaging (MRI) techniques can detect sodium in the brain non-invasively, but can’t specify where it is from the inside to the outside of the cell.
“It’s impossible with traditional sodium MRI to say ‘This much sodium is inside the cell,’ or ‘You have this much sodium outside the cell,’” Khan said. “You're measuring the total sodium, but you can’t compartmentalize the sodium MRI signal. And it’s the different sodium compartments that are more important than knowing just the total sodium.”
The Hyder lab tried a new approach that involves injecting a small amount of the chemical compound (TmDOTP5-) into the bloodstream to separate the sodium MRI signals and observe results in the environments of brain tumors (upper left image). Key to the method’s success is that these paramagnetic sensors, like TmDOTP5-, can go from the blood vessels to the extracellular space, but can’t penetrate the cells’ membranes.
“It weakly attracts some sodium ions, and whatever is attracted to it will give a different sodium MRI signal than the sodium that is not attracted to it,” Khan said. “That means you have magnetic distinctions between the sodium in the cells, outside the cells, and in the blood. Because you have now separated the signals, you can quantify them by the cells and characterize the sodium distribution in the tumors.” For the first time, this work shows in the tumor both the transmembrane and transendothelial gradients are altered, compared to normal tissue (image to the right).
Even though the study focused on cancer, the team’s new sodium MRI method can be applied to any situation where there is altered salt balance. Given the ubiquity of salt throughout the body, salt imbalances are thought to be at the root of several pathophysiological states. For instance, sodium levels ultimately affect the immune system, which means that we now have a non-invasive way to probe the body’s reaction to various pathogens. Sodium imbalances are also found in a number of neurodegenerative disorders like Huntington’s Disease and multiple sclerosis. Pathologies like these serve as models for future research which can further validate the necessity of a method like this one developed by the Hyder lab.