Differential requirement for Apaf1 and Bcl-X(L) in the regulation of programmed cell death during development

Apaf1 plays a negative regulatory role in T cell responses by suppressing activation of antigen-stimulated T cells

Apaf1 is a critical component of the apoptosome and initiates apoptosis downstream mitochondrial damages. Although the importance of Apaf1 in embryonic development was shown, the role of Apaf1 in immune responses, especially T cell responses, has yet to be elucidated. We generated T cell-specific Apaf1-deficient mice (Lck-Cre-Apaf1^f/f mice) and examined the antigen-specific delayed-type hypersensitivity (DTH). Lck-Cre-Apaf1^f/f mice exhibited exacerbation of DTH responses as compared with Apaf1-sufficient control mice. In Lck-Cre-Apaf1^f/f mice, antigen-specific T cells proliferated more, and produced more inflammatory cytokines than control T cells. Apaf1-deficient T cells from antigen-immunized mice showed higher percentages of activation phenotypes upon restimulation in vitro. Apaf1-deficient T cells from naive (non-immunized) mice also showed higher proliferation activity and cytokine production over control cells. The impact of Apaf1-deficiency in T cells, however, was not restored by a pan-caspase inhibitor, suggesting that the role of Apaf1 in T cell responses was caspase-independent/non-apoptotic. These data collectively demonstrated that Apaf1 is a negative regulator of T cell responses and implicated Apaf1 as a potential target for immunosuppressive drug discovery.

The transcription factor c-Myc suppresses MiR-23b and MiR-27b transcription during fetal distress and increases the sensitivity of neurons to hypoxia-induced apoptosis

Previous studies reported that the expression of miR-23b-27b cluster was downregulated in embryonic brain cortices during hypoxia-induced neuronal apoptosis. However, the mechanism underlying this downregulation is not completely understood. Here, we report that the transcription factor c-Myc plays an important role in regulating the expression of miR-23b-27b cluster during hypoxia. First, the c-Myc protein level was significantly elevated in embryonic brain cortices in a mouse model of fetal distress. Second, forced overexpression or knockdown of c-Myc could suppress or increase the expression of miR-23b-27b cluster polynucleotides. Third, we identified 2 conserved c-Myc binding sites (E-boxes) in the enhancer and promoter regions of miR-23b-27b cluster in the mouse genome. Finally, we showed that elevated c-Myc expression led to an increase in the Apaf-1 level by suppressing miR-23b-27b cluster expression and that this enhanced neuronal sensitivity to apoptosis. In summary, our study demonstrates that c-Myc may suppress the expression of the miR-23b-27b cluster, resulting in additional neuronal apoptosis during hypoxia.

Triiodothyronine (T3) action on aquatic locomotor behavior during metamorphosis of the bullfrog Rana catesbeiana

Thyroid hormones--particularly triiodothyronine, T3--play a critical role in the morphological transformations comprising metamorphosis in larval bullfrogs (Rana catesbeiana). Traditional staging criteria for anuran larvae incompletely distinguish physiological and behavioral changes during growth. We therefore first developed a new parameter to describe larval growth, the developmental index (DI), which is simply the ratio between the tail length of the larva and its head diameter. Using the DI we were able to identify two distinct populations classifying the larvae during growth along a continuous linear scale with a cutoff value of DI at 2.8. Classification based on the DI, used in this study, proved an effective complement to existing classifications based on developmental staging into pre- or pro-metamorphic stages. Exposure to T3 in the water induced a rapid (beginning within 5 min) and significant decrease (approximately 20-40%) in locomotor activity, measured as total distance traversed and velocity. The largest decrease occurred in more developed larvae (DI<2.8). To determine correlated changes in the neuromuscular junctions during metamorphosis and apoptotic tail loss, miniature endplate currents from tail muscle were recorded during acute exposure to a hypertonic solution, which simulates an apoptotic volume decrease. Our results support a role for T3 in regulating larval locomotor activity during development, and suggest an enhanced response to volume depletion at the neuromuscular junction of older larvae (DI<2.8) compared to younger animals (DI> or =2.8). We discuss the significance of the possible role of an apoptotic volume decrease at the level of the neuromuscular junction.

The involvement of cell death and survival in neural tube defects: a distinct role for apoptosis and autophagy?

Neural tube defects (NTDs), such as spina bifida (SB) or exencephaly, are common congenital malformations leading to infant mortality or severe disability. The etiology of NTDs is multifactorial with a strong genetic component. More than 70 NTD mouse models have been reported, suggesting the involvement of distinct pathogenetic mechanisms, including faulty cell death regulation. In this review, we focus on the contribution of functional genomics in elucidating the role of apoptosis and autophagy genes in neurodevelopment. On the basis of compared phenotypical analysis, here we discuss the relative importance of a tuned control of both apoptosome-mediated cell death and basal autophagy for regulating the correct morphogenesis and cell number in developing central nervous system (CNS). The pharmacological modulation of genes involved in these processes may thus represent a novel strategy for interfering with the occurrence of NTDs.

Primary enamel knot cell death in Apaf-1 and caspase-9 deficient mice

During molar development, apoptosis occurs in a well-characterised pattern suggesting several roles for cell death in odontogenesis. However, molecular mechanisms of dental apoptosis are only poorly understood. In this study, Apaf-1 and caspase-9 knockouts were used to uncover the engagement of these members of the apoptotic machinery during early tooth development, concentrating primarily on their function in the apoptotic elimination of primary enamel knot cells. Molar tooth germ morphology, proliferation and apoptosis were investigated on frontal histological sections of murine heads at embryonic days (ED) 15.5, the stage when the primary enamel knot is eliminated apoptotically. In molar tooth germs of both knockouts, no apoptosis was observed according to morphological (haematoxylin-eosin) as well as biochemical criteria (TUNEL). Morphology of the mutant tooth germs, however, was not changed. Additionally, knockout mice showed no changes in proliferation compared to wild type mice. According to our findings on knockout embryos, Apaf-1 and caspase-9 are involved in apoptosis during tooth development; however, they seem dispensable and not necessary for proper tooth shaping. Compensatory or other mechanisms of cell death may act to eliminate the primary enamel knot cells in the absence of Apaf-1 and caspase-9.

Expanding roles of programmed cell death in mammalian neurodevelopment

Programmed cell death is an orchestrated form of cell death in which cells are actively involved in their own demise. During neural development in mammals, many progenitor cells, immature cells or differentiated cells undergo the most clearly characterized type of cell death, apoptosis. Several pathways of apoptosis have been linked to neural development, but according to the numerous and striking phenotypes observed when apoptotic genes are inactivated, the mitochondrial death-route is the most important pathway in this context. Here, we discuss the relative importance of pro-growth/pro-death factors in the control of neural tissue development. We also discuss the impact of studying programmed cell death in development in order to better understand the basis of several human diseases and embryonic defects of the nervous system.

Apaf1-dependent programmed cell death is required for inner ear morphogenesis and growth

During inner ear development programmed cell death occurs in specific areas of the otic epithelium but the significance of it and the molecules involved have remained unclear. We undertook an analysis of mouse mutants in which genes encoding apoptosis-associated molecules have been inactivated. Disruption of the Apaf1 gene led to a dramatic decrease in apoptosis in the inner ear epithelium, severe morphogenetic defects and a significant size reduction of the membranous labyrinth, demonstrating that an Apaf1-dependent apoptotic pathway is necessary for normal inner ear development. This pathway most probably operates through the apoptosome complex because caspase 9 mutant mice suffered similar defects. Inactivation of the Bcl2-like (Bcl2l) gene led to an overall increase in the number of cells undergoing apoptosis but did not cause any major morphogenetic defects. In contrast, decreased apoptosis was observed in specific locations that suffered from developmental deficits, indicating that proapoptotic isoform(s) produced from Bcl2l might have roles in inner ear development. In Apaf1-/-/Bcl2l-/-double mutant embryos, no cell death could be detected in the otic epithelium,demonstrating that the cell death regulated by the anti-apoptotic Bcl2l isoform, Bcl-XL in the otic epithelium is Apaf1-dependent. Furthermore, the otic vesicle failed to close completely in all double mutant embryos analyzed. These results indicate important roles for both Apaf1 and Bcl2l in inner ear development.

Coupling of caspase-9 to Apaf1 in response to loss of pRb or cytotoxic drugs is cell-type-specific

Inactivation of the tumor suppressor Rb in the mouse induces cell death, which depends entirely (in lens, CNS) and only partly (PNS, skeletal muscles) on Apaf1/Ced4, an apoptosomal factor thought to be required for processing procaspase-9 following mitochondrial permeabilization. Here, we report that in response to cytotoxic drugs, Apaf1(-/-) primary myoblasts but not fibroblasts undergo bona fide apoptosis. Cell demise was associated with disruption of mitochondria but not endoplasmic reticulum. Processing of procaspase-9 occurred in Apaf1(-/-) myoblasts but not fibroblasts, and ablation of Casp9 prevented drug-induced apoptosis in both cell types. Deregulation of the Rb pathway by overexpression of E2F1 also induced caspase-9-dependent, Apaf1-independent apoptosis in myoblasts. Despite its requirement for apoptosis in vitro, mutation in Casp9 abrogated cell death in the nervous system and lens but only partly in skeletal muscles of Rb-deficient embryos. In addition, developmental cell death in fetal liver and PNS was not inhibited in Casp9(-/-) embryos. Therefore, loss of pRb elicits apoptosome-dependent and apoptosome-independent cell death, and the requirement and coupling of caspase-9 to Apaf1 are both context-dependent.

The role of Apaf-1 in programmed cell death: from worm to tumor

Apoptosis or programmed cell death is an important process to eliminate unnecessary or hazardous cells. Apaf-1, a mammalian homologue of CED-4 of C. elegans, is the essential adaptor molecule in the mitochondrial pathway of apoptosis. Mice lacking Apaf-1 show accumulation of neurons in the developing central nervous system due to reduced apoptosis. Apaf-1-deficient cells are remarkably resistant to various apoptotic stimuli. Apaf-1-mediated apoptosis plays a role in the prevention of tumorigenesis. However, Apaf-1-independent cell death pathways are also indicated. In this review, we will summarize what has been learned about the role of Apaf-1 by biochemical and genetical approaches.