No abstract

The number of neurons in the mammalian brain is determined by a balance between cell proliferation and programmed cell death. Recent studies indicated that Bcl-XL prevents, whereas Caspase-3 mediates, cell death in the developing nervous system, but whether Bcl-X L directly blocks the apoptotic function of Caspase-3 in vivo is not known. To examine this question, we generated bcl-x͞caspase-3 double mutants and found that caspase-3 deficiency abrogated the increased apoptosis of postmitotic neurons but not the increased hematopoietic cell death and embryonic lethality caused by the bcl-x mutation. In contrast, caspase-3, but not bcl-x, deficiency changed the normal incidence of neuronal progenitor cell apoptosis, consistent with the lack of expression of Bcl-X L in the proliferative population of the embryonic cortex. Thus, although Caspase-3 is epistatically downstream to Bcl-XL in postmitotic neurons, it independently regulates apoptosis of neuronal founder cells. Taken together, these results establish a role of programmed cell death in regulating the size of progenitor population in the central nervous system, a function that is distinct from the classic role of cell death in matching postmitotic neuronal population with postsynaptic targets. P rogrammed cell death (apoptosis) is an important mechanism in mammalian nervous system development (1, 2). First, programmed cell death adjusts postmitotic neuron number to match the size of their peripheral targets (3). Second, early brain-region-specific apoptosis such as cell death in the lateral edges of the hindbrain neural fold is essential for normal neural tube closure (4). Finally, the incidence of apoptosis within the proliferative ventricular zones (VZ) suggests a potential role of programmed cell death in regulating the size of the progenitor pool (5). Given its importance in normal brain development, the mechanism of apoptosis has been a subject of active investigation.Recent gene-targeting studies identified Bcl-X L as a critical antiapoptotic factor and Bax, Apaf-1, Caspase-9, and Caspase-3 as key proapoptotic molecules during normal brain development (6-12). Mice lacking bcl-x die as embryos and experience massive death of hematopoietic cells and postmitotic neurons (6). In addition, Bcl-X L -deficient neurons are markedly susceptible to trophic factor withdrawal in vitro (13). In contrast, mice lacking Bax show decreased cell death without obvious malformations (14), whereas targeted disruptions of apaf-1, caspase-9, or caspase-3 lead to decreased neuronal apoptosis in the embryonic nervous system and gross structural abnormalities (8-12).In the nematode Caenorhabditis elegans, the homologues of Bax (EGL-1), Bcl-X L (CED-9), Apaf-1 (CED-4), and caspases (CED-3) have analogous effects on programmed cell death and form a linear cell death pathway (15). Thus, it is an intriguing possibility that Bax, Bcl-X L , Apaf-1, Caspase-9, and Caspase-3 constitute an evolutionary conserved cell death pathway during brain development in mammals. Consistent with this ...

Programmed cell death is critical for normal nervous system development and is regulated by Bcl-2 and Caspase family members. Targeted disruption of bcl-x L , an antiapoptotic bcl-2 gene family member, causes massive death of immature neurons in the developing nervous system whereas disruption of caspase-9, a proapoptotic caspase gene family member, leads to decreased neuronal apoptosis and neurodevelopmental abnormalities. To determine whether Bcl-X L and Caspase-9 interact in an obligate pathway of neuronal apoptosis, bcl-x/ caspase-9 double homozygous mutants were generated. The increased apoptosis of immature neurons observed in Bcl-X Ldeficient embryos was completely prevented by concomitant Caspase-9 deficiency. In contrast, bcl-x Ϫ /Ϫ /caspase-9 Ϫ /Ϫ embryonic mice exhibited an expanded ventricular zone and neuronal malformations identical to that observed in mice lacking only Caspase-9. These results indicate both epistatic and independent actions of Bcl-X L and Caspase-9 in neuronal programmed cell death.To examine Bcl-2 and Caspase family-dependent apoptotic pathways in telencephalic neurons, we compared the effects of cytosine arabinoside (AraC), a known neuronal apoptosis inducer, on wild-type, Bcl-X L -, Bax-, Caspase-9-, Caspase-3-, and p53-deficient telencephalic neurons in vitro. AraC caused extensive apoptosis of wild-type and Bcl-X L -deficient neurons. p53-and Bax-deficient neurons showed marked protection from AraC-induced death, whereas Caspase-9-and Caspase-3-deficient neurons showed minimal or no protection, respectively. These findings contrast with our previous investigation of AraC-induced apoptosis of telencephalic neural precursor cells in which death was completely blocked by p53 or Caspase-9 deficiency but not Bax deficiency. In total, these results indicate a transition from Caspase-9-to Bax-and Bcl-X L -mediated neuronal apoptosis.