Source: Dissertation Abstracts International, Volume: 61-01, Section: B, page: 0183.

Director: Richard Anthony Flavell.

Caspases are a group of intracellular cysteine proteases that mediates the execution of apoptosis, the cell suicidal mechanism by which superfluous and dangerous cells are eliminated to ensure the proper development and homeostasis of all multicellular organisms. While it is well established that the activation of these otherwise latent proteases is the critical event during apoptotic events, the exact involvement of individual caspases during this process is poorly elucidated.

Using gene-targeting technique, we have successfully generated mice deficient in either caspase-3 or caspase-6 that allowed us to examine the precise involvement of these two caspases in mediating mammalian apoptosis. Although deficiency in either caspase-3 or caspase-6 apparently did not affect the normal development and function of the immune system in mice, our study revealed a crucial role of caspase-3 in neuronal cell death during early embryogenesis. Lack of caspase-3 expression, but not that of caspase-6, resulted in a drastically reduced number of apoptotic neurons in the proliferative zone of the ventricular area as early as embryonic day 12, and led to subsequent developmental defects and even premature lethality associated with supernumerary.

In addition to its critical role in brain development, caspase-3 was also found to be indispensable for many, but not all, cellular changes associated with apoptosis. While caspase-3-/- epatocytes and thymocytes readily died upon induction of apoptosis, they did not exhibit several diagnostic features of apoptosis such as cytoplasmic membrane blebbing and nuclear fragmentation. Moreover, internucleosomal DNA fragmentation is also significantly delayed. The occurrence of such aberrant apoptosis in caspase-3 -/- cells is most likely due to the defective cleavage of several key cellular proteins in the absence of caspase-3, as revealed by our biochemical analysis.

We further examined the molecular ordering of the caspase cascade induced by dexamethasone in thymocytes and the compensatory pathways of caspase activation in hepatocytes. While our results strongly supported the hypothesis based on in vitro studies that caspases are activated in a sequential manner in which upstream caspases proteolytically activate downstream caspases, they also revealed, for the first time, that there exist additional pathways that can lead to compensatory activation of downstream caspases.