Medical Nanoimaging Pinpoints Cause Of Cataracts

This is the first time that high-resolution AFM imaging of a diseased tissue has yielded information on the single molecule level of the disease. AFM has emerged from the state-of-the-art laboratory to bring us medical nanoimaging. These results are now online in the Journal of Molecular Biology.

The eye’s lens focuses light and forms a sharp image on the retina thanks to the organization and specific properties of its constituent cells (see box overleaf). As in all tissues, cellular exchanges are essential for nutrition and removal of waste products, but in the eye they must nonetheless be adapted to the particular properties of the lens. The membranes of lens cells contain protein assemblies, the aquaporins and connexons₂: the former act as water channels and the latter as channels for metabolites and ions. Together these membrane proteins ensure cell adhesion.

Using atomic force microscopy (AFM), which images the surface of a sample at a precision of one nanometer (one billionth of a meter), Simon Scheuring’s team at the Institut Curie is studying how these protein assemblies function. An atomically sharp tip is scanned over the sample surface and its movements are tracked by a laser. The resulting data can be used to draw a topographical map of the sample. By comparing assemblies of aquaporins and connexons in membranes of healthy and diseased lens cells, Scheuring and colleagues have identified the biological changes that cause cataracts.

In this senile cataract, lack of connexons prevents formation of the channels ensuring cell to cell communication. These molecular modifications explain the lack of adherence between cells, waste accumulation in cells, and the defective transport of water, ions, and metabolites in a lens with a cataract.

This is the first time that high-resolution AFM imaging of diseased tissue has shed light on the molecular cause of a disease at the single membrane protein level. A step towards medical nanoimaging has been taken with atomic force microscopy.

The lens

The specific properties of the eye’s lens cells enable the lens to function correctly. These cells have no nucleus or organelles, such as mitochondria, and are unable to perform certain biochemical functions essential for their nutrition, and therefore depend on transmembrane channels₃ for transport of water, ions, and metabolites, and for waste removal. These cells are full of so-called lens proteins (crystallins), which ensure lens transparency. To avoid any loss of light, the lens is avascular and its network of cells is extremely compact: the gap between neighboring cells must be less than the wavelength of visible light.

The cataract

The cataract results from opacification linked to the hardening of the lens. Age-related (senile) cataracts are by far the commonest, and affect more than one in five of the over-65s, over one in three of the over-75s, and two thirds of people over 85 years of age. Cataracts cause reduced image sharpness, blurred vision, and sensitivity to light and glare. The only effective treatment for cataracts at present is surgery, in which the opaque lens is removed and replaced by an artificial lens. Cataracts are the main cause of blindness in the third world and explain the sight loss of 40% of the world’s 37 million blind.

Notes

1. “Atomic force microscopy of proteins in native membranes” team in the Curie Physical Chemistry research unit UMR 168 CNRS/Institut Curie directed by Jean-François Joanny.
2. A connexon is an assembly of 6 connexin molecules and forms a gap junction between the cytoplasm of two adjacent cells.
3. “The supramolecular architecture of junctional microdomains in native lens membranes” N. Buzhynskyy, R. Hite, T. Walz, S Scheuring. EMBO R. January 2007, vol. 8, p. 51-55.