Programmed Cell Death in Avian Thymocytes: Role of the Apoptotic Endonuclease

Abstract

Apoptosis, or programmed cell death, is a physiological process whereby a target cell dies in response to a specific signal. A prominent model system used to study this process is the glucocorticoid-mediated killing of immature thymocytes. Following glucocorticoid treatment, apoptotic thymocytes undergo a series of distinct morphological alteration including cellular shrinkage, blebbing of the cytoplasmic membrane, and chromatin condensation. The chromatin condensation that occurs during apoptosis is associated with a characteristic endonuclease activity that degrades the genome at inter-nucleosomal sites. To study this characteristic endonuclease activity further, nuclear extracts were prepared from thymocytes of glucocorticoid-treated chicks and nuclease activity present in the protein extracts was analyzed using chicken red blood cell nuclei as a substrate. Using this in vitro assay system, it was demonstrated that the avian endonuclease activity degrades chromatin at internucleosomal sites and can be inhibited by EDTA and zinc ions. Current efforts are focused on purifying the avian apoptotic endonuclease and further characterizing this nuclease activity.