Caspace-3 deficiency rescues peripheral nervous system defect in pRb nullizygous mice.

Abstract

The retinoblastoma tumour suppressor protein, pRb, is a key regulator of cell cycle and has been implicated in the terminal differentiation of neuronal cells. Mice nullizygous for pRb die by E14.5 from haematopoietic and neurological defects attributed to failed differentiation (Jacks et al., 1992; Lee et al., 1992; Clarke et al., 1992). Previous studies by Macleod et al., (1996) have demonstrated that the loss of p53 protects pRb-deficient central nervous system (CNS) neurons but not peripheral nervous system (PNS) neurons from cell death. Thus, the mechanisms by which PNS neurons undergo apoptosis in response to pRb deficiency remain unknown. In view of the pivotal role of caspase-3 in the regulation of neuronal apoptosis during development, we examined its function in the execution of the widespread neuronal cell death induced by pRb deficiency. Our results support a number of conclusions: First, we show that caspase-3 becomes activated in all neuronal populations undergoing apoptosis. Second, caspase-3 deficiency does not extend the life span of pRb null embryos, as double null mutants exhibit high rates of liver apoptosis resulting in erythropoietic failure. Third, pRb/caspase-3 double mutant neurons of the CNS exhibit widespread apoptosis similar to that seen in pRb mutants alone, thus caspase-3 deficiency does not protect this population from apoptosis. Finally, in contrast to the CNS, neurons of the PNS including those comprising the trigeminal ganglia (TG) and the dorsal root ganglia (DRG) are protected from apoptosis in pRb/caspase-3 double mutant embryos. Examination of the mechanistic differences between these two cell types revealed that CNS neurons invoke compensatory caspase activity that is not found in PNS neurons. These findings suggest that PNS neurons are dependent upon caspase-3 for the execution of apoptosis and that caspase-3 may serve as a key therapeutic target for neuroprotection following injury of this cell type.