Dying a thousand deaths - Radionuclide imaging of apoptosis

Programmed cell death, apoptosis, is an inducible, organized, energy requir ing form of demise that results in the disappearance of a cell without the induction of an inflammatory response. Apoptotic cell death is strikingly d ifferent than necrotic death, which is disorderly, does not require energy and results in local inflammation, usually secondary to sudden release of i ntracellular contents. Apoptosis is induced when cells undergo severe injur y to their nucleus, as occurs following exposure to gamma or X-radiation, o r mitcochondria, as occurs in a variety of viral illnesses. Apoptosis can a lso be induced by external signals, such as interaction of fas ligand with fas receptors. Once the cell is committed to apoptosis, the caspase enzyme cascade is activated. An early effect of caspase activation is the rapid ex pression of phosphatidylserine on the external leaflet of the cell membrane . Membrane bound phosphatidylserine expression serves as a signal to surrou nding cells, identifying the expressing cell as undergoing apoptosis. A def iciency or an excess of programmed cell death is an integral component of a utoimmune disorders, transplant rejection and cancer. A technique to image programmed cell death would be useful to assist in the development of drugs designed to treat these diseases, and to monitor the effectiveness of ther apy. The sudden expression of phosphatidylserine on the cell membrane is a target that could be used for this purpose. A 35 kD physiologic protein, An nexin V Lipocortin, binds with nanomolar affinity to membrane bound phospha tidylserine. Annexin V has been radiolabeled with Technetium-99m by direct coupling to free sulfhydryl groups, and through the hydrazinonicatinamide a nd N2S2 linking agents. The biodistribution of the agents labeled with each of the methods is slightly different. In all cases the radiopharmaceutical binds to cells undergoing apoptosis itt vitro, and permits imaging of the process in experimental animals.