February 21, 2006
Fluorescent Nanosensor Detects Cell Death
A team of investigators at Massachusetts General Hospital has developed a nanoparticle that signals when cells are undergoing apoptosis, the kind of cell death triggered by cancer therapies. The new nanoparticles could finally provide oncologists with a rapid assay that could tell them that a given therapy is working. This groundbreaking work was published in the journal Nano Letters.
Too often, oncologists have to wait weeks, or more often months, to learn if the treatment prescribed for a particular patient is working, a predicament that can have dire results. For the patient, receiving a therapy that is not working means unnecessary suffering, both from the tumor continuing to grow and any side effects that accompany the ineffective treatment. Receiving ineffective therapy for longer than needed also delays the start of second-line therapies that might work. Worse still, the failed treatment can trigger genetic defense mechanisms in tumor cells that can render ineffective these second-line therapies using other drugs. This phenomenon is known as cross-resistance.
To develop their apoptosis detector, Ching-Hsuan Tung, Ph.D., Ralph Weissleder, M.D., and Luisa Quinti, Ph.D., honed in on the molecule phosphatidylserine. Phosphatidylserine, or PS, is found normally on the inside of the cell membrane, but this molecule moves to the outside layer of the cell membrane when a cell begins apoptosis. To detect this early indicator of apoptosis, the researchers constructed an artificial small, fluorescent protein that would bind to four different molecules of PS. Test tube experiments showed this construct did indeed bind to PS but not to other membrane components; however, when the investigators tried to use it to detect apoptotic cells, the experiments were a failure. Evidently, their test construct did not bind strongly enough and for a long enough time to apoptotic cells to be detectable.
Next, the researchers turned to a magnetic iron oxide nanoparticle that Weissleder, who is co-director of the MIT-Harvard Center of Cancer Nanotechnology Excellence, has been developing for imaging tumor metastasis. They reasoned that they could attach an average of 41 of the PS-binding molecules to each nanoparticle, which should increase dramatically the ability of the resulting construct to not only bind to PS, but remain stuck to the apoptotic cell through a process called cooperative binding. Cooperative binding is what gives such enormous sticking power to the hundreds of tiny loops and hooks on matching pieces of Velcro®.
In fact, the nanoparticle worked like a charm. When the investigators added the nanoparticle-based PS-binder to cells triggered to undergo apoptosis, they were able to easily identify the apoptotic cells using a fluorescent microscope or a standard cell sorting apparatus. The researchers note that because the nanoparticle they used has already been optimized for use in humans, their new apoptosis detector holds significant promise for use in cancer imaging applications.
This work, which was supported by the National Cancer Institute, is detailed in a paper titled, “A fluorescent nanosensor for apoptotic cells.” This paper was published online in advance of print publication. An abstract is available at the journal’s website.