Apoptosis has an important role during development to regulate cell number. In differentiated cells, however, activation of autophagy has a critical role by enabling cells to remain functional following stress. In this study, we show that the antiapoptotic BCL-2 homologue MCL-1 has a key role in controlling both processes in a developmentally regulated manner. Specifically, MCL-1 degradation is an early event not only following induction of apoptosis, but also under nutrient deprivation conditions where MCL-1 levels regulate activation of autophagy. Furthermore, deletion of MCL-1 in cortical neurons of transgenic mice activates a robust autophagic response. This autophagic response can, however, be converted to apoptosis by either reducing the levels of the autophagy regulator Beclin-1, or by a concomitant activation of BAX. Our results define a pathway whereby MCL-1 has a key role in determining cell fate, by coordinately regulating apoptosis and autophagy.

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Correct cell cycle regulation and terminal mitosis are critical for nervous system development. The retinoblastoma (Rb) protein is a key regulator of these processes, as Rb-/- embryos die by E15.5, exhibiting gross hematopoietic and neurological defects. The extensive apoptosis in Rb-/- embryos has been attributed to aberrant S phase entry resulting in conflicting growth control signals in differentiating cells. To assess the role of Rb in cortical development in the absence of other embryonic defects, we examined mice with telencephalon-specific Rb deletions. Animals carrying a floxed Rb allele were interbred with mice in which cre was knocked into the Foxg1 locus. Unlike germline knockouts, mice specifically deleted for Rb in the developing telencephalon survived until birth. In these mutants, Rb-/- progenitor cells divided ectopically, but were able to survive and differentiate. Mutant brains exhibited enhanced cellularity due to increased proliferation of neuroblasts. These studies demonstrate that: (i) cell cycle deregulation during differentiation does not necessitate apoptosis; (ii) Rb-deficient mutants exhibit enhanced neuroblast proliferation; and (iii) terminal mitosis may not be required to initiate differentiation.

Mitochondria release proteins that propagate both caspase-dependent and caspase-independent cell death pathways. AIF (apoptosisinducing factor) is an important caspase-independent death regulator in multiple neuronal injury pathways. Presently, there is considerable controversy as to whether AIF is neuroprotective or proapoptotic in neuronal injury, such as oxidative stress or excitotoxicity. To evaluate the role of AIF in BAX-dependent (DNA damage induced) and BAX-independent (excitotoxic) neuronal death, we used Harlequin (Hq) mice, which are hypomorphic for AIF. Neurons carrying double mutations for Hq/Apaf1 Ϫ/Ϫ (apoptosis proteases-activating factor) are impaired in both caspase-dependent and AIF-mediated mitochondrial cell death pathways. These mutant cells exhibit extended neuroprotection against DNA damage, as well as glutamate-induced excitotoxicity. Specifically, AIF is involved in NMDA-and kainic acid-but not AMPA-induced excitotoxicity. In vivo excitotoxic studies using kainic acid-induced seizure showed that Hq mice had significantly less hippocampal damage than wild-type littermates. Our results demonstrate an important role for AIF in both BAXdependent and BAX-independent mechanisms of neuronal injury.

The cell cycle regulatory retinoblastoma (Rb) protein is a key regulator of neural precursor proliferation; however, its role has been expanded to include a novel cell-autonomous role in mediating neuronal migration. We sought to determine the Rb-interacting factors that mediate both the cell cycle and migration defects. E2F1 and E2F3 are likely Rb-interacting candidates that we have shown to be deregulated in the absence of Rb. Using mice with compound null mutations of Rb and E2F1 or E2F3, we asked to what extent either E2F1 or E2F3 interacts with Rb in neurogenesis. Here, we report that E2F1 and E2F3 are both functionally relevant targets in neural precursor proliferation, cell cycle exit, and laminar patterning. Each also partially mediates the Rb requirement for neuronal survival. Neuronal migration, however, is specifically mediated through E2F3, beyond its role in cell cycle regulation. This study not only outlines overlapping and distinct functions for E2Fs in neurogenesis but also is the first to establish a physiologically relevant role for the Rb/E2F pathway beyond cell cycle regulation in vivo.Neurogenesis is a highly regulated process by which neural precursors divide and differentiate, giving rise to the cells that make up the nervous system (reviewed in references 24 and 25). While the role of cell cycle genes in regulating proliferation of neural precursor cells is well appreciated, accumulating data point convincingly to their unique roles in regulating diverse cellular processes, independent of cell cycle regulation (reviewed in reference 63). The retinoblastoma (Rb) tumor suppressor is a key cell cycle regulator that we along with others have shown to play a number of roles in neurodevelopment including proliferation, survival, and, more recently, neuronal migration (7,8,10,18,19,33,40,52). Differentiating Rb-deficient neural precursor cells exhibit delayed cell cycle exit, while the absence of Rb in the telencephalon leads to ectopic proliferation of neural precursor cells and enhanced brain size at midgestation (7,19,52). In a recent study we described a role for Rb in regulating the survival of discrete neuronal subpopulations and a novel cell autonomous role for Rb in regulating neuronal migration (18).The mechanism by which Rb regulates neurogenesis and the extent to which defects in migration and survival are the result of cell cycle deregulation remain unknown. While Rb is known to interact with numerous proteins (reviewed in reference 67), many of which are expressed in quiescent cells or have cell cycle-independent functions, members of the cell cycle regulatory E2F family are likely targets in neurogenesis. The E2F family of transcription factors is comprised of E2Fs 1 to 8; however, E2F1, E2F2, and E2F3, the so-called activating E2Fs, are key Rb-interacting targets best known for their role in promoting cell cycle progression (9, 14, 17, 48, 49, 54; reviewed in reference78). Both E2F1 and E2F3 are likely candidates involved in Rb-mediated regulation of neurogenesis. Deficiency of e...