Decreased apoptosis in proliferative and postmitotic regions of the caspase 3-deficient embryonic central nervous system

Caspase 3 (CPP32/Yama/apopain), a mammalian homolog of the Caenorhabditis e legans pro-cell death gene ced-3, is required for normal programmed cell de ath (PCD) in the nematode. Its prior deletion by homologous recombination i n mice resulted in embryonic/early postnatal lethality associated with dram atic central nervous system (CNS) hyperplasia, yet a reported subtle decrea se in cell death (Kuida et al. [1996] Nature 384:368-372). By comparison, t he magnitude and distribution of dying cells identified using a DNA end-lab eling technique, in situ end-labeling plus (ISEL+) (Blaschke et al. [1996] Development 122:1165-1174; Blaschke et al. [1998] J. Comp. Neurol. 396:39-5 0), supported an alternative explanation where the loss of caspase 3 functi on produces a more pervasive block in cell death, particularly among neurob lasts. To determine the relationship between loss of caspase 3 and dying ce lls identified by ISEL+, we analyzed caspase 3 +/+, +/-, and -/- embryos fo r normal caspase 3 expression and ISEL+ labeling. Both caspase 3 mRNA and a ctive caspase 3 protein are present throughout the +/+ embryonic CNS, and b oth are absent from -/- embryonic cortices. Quantitation of dying cells ide ntified by ISEL+ reveals a 30% reduction of labeled cells throughout the ca spase 3 -/- embryonic cortices relative to +/+ littermates, Associated with this decrease is marked expansion of the total population of actively prol iferating neuroblasts identified by 5-bromo-2'-deoxyuridine incorporation t hat nevertheless appears to maintain histological features of normal neurog enesis rather than dysregulated, neoplastic growth. These data indicate tha t caspase 3 deficiency results in a pervasive, albeit partial, decrease in embryonic neuroblast apoptosis that can account for the observed phenotypic hyperplasia in -/- embryos, and support the additional operation of caspas e 3-independent PCD mechanisms during embryonic CNS development. (C) 2000 W iley-Liss, Inc.