Apoptosis: The importance of nuclear medicine

Apoptosis is a genetically controlled, energy-dependent process which remov es unwanted cells from the body. Because of its orderly progression, apopto sis is also known as programmed cell death or cell suicide. Once initiated, apoptosis is characterized by a series of biochemical and morphological ch anges involving the cytoplasm, nucleus and cell membrane. Cytoplasmic chang es include cytoskeletal disruption, cytoplasmic shrinkage and condensation; prominent changes in the nucleus include peripheral chromatin clumping and inter-nucleosomal DNA cleavage (DNA ladder formation); and membrane change s include the expression of phosphatidylserine on the outer surface of the cell membrane and blebbing (resulting in the formation of cell membrane-bou nd vesicles or apoptotic bodies). These events allow the cell to digest and package itself into membrane-bound packets containing autodigested cytopla sm and DNA, which can then be easily absorbed by adjacent cells or phagocyt es. An endogenous human protein, annexin V (molecular weight approximately 35,0 00), has an affinity of about 10(-9) M for phosphatidylserine exposed on th e surface of apoptotic cells. Annexin V can be labelled with radionuclides such as iodine or technetium, or positron emitting agents. Experimental stu dies in cells confirm that fluorescence and Tc-99(m)-labelled annexin have comparable affinity for apoptotic cells. In vivo studies with Tc-99(m)-labe lled annexin confirm that radiolabelled annexin V can be used to image apop totic cells/tissues ill vivo. In this article, we review experimental data using annexin V imaging and discuss its possible future use to identify apo ptosis in vivo. ((C) 2000 Lippincott Williams & Wilkins).