Could AFM be used routinely for medical imaging?

David L. Shenkenberg

Lead investigator Simon Scheuring and colleagues from the Institut Curie and from the Centre Hospitalier National d’Ophtalmologie des Quinze-Vingts, both in Paris, recently used atomic force microscopy (AFM) to view the molecular basis of cataracts that occur because of aging, demonstrating that the instrument can be used for subnanometer-resolution medical imaging.

A cataract is an opacity of the eye’s lens. Patients with cataracts view the world as a person with normal vision would through a fogged-up window. Cataracts are very common among the elderly, and the population of developed countries is aging rapidly, so this important disease will become ever more significant with time. Currently, cataracts can be removed only with surgery because the body cannot easily repair lens cells, some of the oldest cells in the body. Scheuring said that cataracts are the leading cause of blindness in countries where most patients cannot afford surgery.

Researchers used AFM to image the membrane topography of a senile cataract from a human patient. One can see arrays of 5-nm cross-shaped aquaporin-0 proteins. Gap junction connexon proteins, however, are completely absent. This absence is the molecular basis of senile cataracts. Image courtesy of Simon Scheuring.

Cataracts result from various causes. Before the AFM study, it was known that senile cataracts, which result from aging, are caused by a degradation of the membrane proteins called connexons, which consist of protein bundles that form bridges called gap junctions between healthy lens cells. On either end of each connexon is an array of cross-shaped membrane proteins called aquaporin-0, which acts as a water channel, whereas connexons transport nutrients to the cells and carry waste out of the cells. When connexons degrade, waste builds up between the cells, and nutrients cannot reach them, resulting in a cataract. Scheuring’s team visualized at a subnanometer resolution the region of the lens normally containing connexons.

The researchers performed AFM on a cataract surgically removed from an 82-year-old man, using a Veeco instrument and Olympus cantilevers. The instrument scanned the cataract membrane surface, resulting in images of the membrane proteins. The results of the analysis were confirmed with an Applied Biosystems mass spectrometer.

They observed only intact aquaporin-0 and did not find any connexons in the patient’s tissue. “We are very proud that we identified the molecular basis of cataracts in this patient,” Scheuring said.

He believes that a cascade of degradation gradually leads to the disappearance of connexons. As proteolytic enzymes gradually break down the connexons, waste builds up, and the chemical balance shifts to favor degradation.

Scheuring said that, although numerous medical imaging techniques exist, such methods generally have poor resolution, whereas AFM can visualize membrane proteins with subnanometer resolution. He added that many pathologies originate from molecular causes and that this technique allows the visualization of individual molecules. “We believe that it can be a medical imaging tool,” he said. He said that the major obstacle right now to the technique’s becoming a widespread medical imaging tool is that it requires operation by a dedicated expert.

Journal of Molecular Biology, Nov. 16, 2007, pp. 162-169.