Apaf-1 deficiency and neural tube closure defects are found in fog mice

Abstract

The forebrain overgrowth mutation ( fog) was originally described as a spontaneous autosomal recessive mutation mapping to mouse chromosome 10 that produces forebrain defects, facial defects, and spina bifida. Although the fog mutant has been characterized and available to investigators for several years, the underlying mutation causing the pathology has not been known. Because of its phenotypic resemblance to apoptotic protease activating factor-1 (Apaf-1) knockout mice, we have investigated the possibility that the fog mutation is in the Apaf-1 gene. Allelic complementation, Western blot analysis, and caspase activation assays indicate that fog mutant mice lack Apaf-1 activity. Northern blot and reverse transcription-PCR analysis show that Apaf-1 mRNA is aberrantly processed, resulting in greatly reduced expression levels of normal Apaf-1 mRNA. These findings are strongly suggestive of the fog mutation being a hypomorphic Apaf-1 defect and implicate neural progenitor cell death in the pathogenesis of spina bifida-a common human congenital malformation. Because a complete deficiency in Apaf-1 usually results in perinatal lethality and fog͞fog mice more readily survive into adulthood, these mutants serve as a valuable model with which apoptotic cell death can be studied in vivo.spina bifida ͉ neural development ͉ apoptosis ͉ caspase ͉ cell death N eural tube closure defects account for a significant portion of the congenital malformations that affect humans (1). Such defects include spina bifida, exencephaly, cranioschisis, and hydrocephaly. The cause and inheritance of these defects are complex, reflecting the complex developmental events required for neural tube formation and closure.Neural plate formation, neural fold elevation, neural tube closure, and neural crest cell migration are just a few of the operations that are tightly regulated during neurulation. Many mouse models have been used to study neural tube defects (2). The mutants studied have either arisen spontaneously or have been generated by using targeted gene disruption. One such mouse model is the forebrain overgrowth ( fog) mutant, which has a spontaneous mutation in an uncharacterized gene and is transmitted in an autosomal recessive manner (3).Apoptotic cell death has been shown to play a critical role in neurogenesis. Specifically, proteins required for the cytochromec-mediated apoptotic pathway [such as apoptotic protease activating factor-1 (Apaf-1), caspase-9, and caspase-3] are important for neural tube closure in vivo (4-9). In this biochemical cascade, cytochrome c released from the mitochondria is recruited to Apaf-1. In the presence of dATP, the cytochrome-c͞Apaf-1 complex is able to procure and promote activation of procaspase-9 to caspase-9. The activation of caspase-9 then results in the subsequent activation of caspase-3, a terminal event in apoptotic cell death. In mice, Apaf-1 deficiency results in a dramatic elevation in the number of neural progenitor cells resulting in the thickening of the neuroepithelium (9). Because morph...

Apoptosome inactivation rescues proneural and neural cells from neurodegeneration

“…3,4 Apaf1 involvement in brain-mass control in a dosage-dependent manner was also confirmed by the analysis of the hypomorphic mutant fog (forebrain overgrowth), in which Apaf1 levels are strongly reduced. 27 We wondered whether Apaf1 is involved in the main route of NPCs to death, as suggested by the knockout mouse phenotype, and whether the path to death could be somehow prevented or reverted in Apaf1 À/À cells. Additionally, we wanted to know what was the quality of surviving Apaf1 À/À cells committed to die, and whether they retained their potential to differentiate.…”

Role of the pro-survival molecule Bfl-1 in melanoma

“…The apoptosome, although originally believed to be essential for Bcl-2-regulated apoptosis 171,172 , has more recently been defined as merely an amplifier of caspase activation. This was determined through studies which showed that in the absence of APAF1, or pro-caspase 9, cells could still die through Bcl-2 family driven apoptosis in lymphocytes and post mitotic neurones 173,174 . The apoptosome therefore appears to have more impact on apoptosis occurring in some cell types, such as lymphocytes, than in others, for example neuronal precursors 173 .…”

Apaf1 apoptotic function critically limits Sonic hedgehog signaling during craniofacial development

“…Moreover, our allele phenocopies previously characterized Apaf1 deletion alleles that exhibit defects in neural tube closure, delayed interdigital web removal, embryonic lethality and craniofacial malformations. [18][19][20][21][22] Our data suggest that Shh-producing cells persist and direct a lateral outgrowth of the FNP that results in the craniofacial defects we observe. Because Apaf1 yautja encodes a stable, non-functional protein, this allele provides a valuable tool for examining apoptosome formation and activation.…”

“…The Apaf1 fog allele is functionally deficient in Apaf1 protein due to aberrant transcript processing. [21][22] Embryos heteroallelic for the yautja and fog alleles failed to complement and recapitulated features of both phenotypes: we saw defects of the forebrain, lumbosacral and facial regions, including forebrain encephalocele, exencephaly, open fourth ventricle, compressed forebrain ventricles, spina bifida with or without a kinked tail and wide or split face ( Figure 2d). We saw these features either in isolation or in complex, consistent with the individual mutant phenotypes for yautja and fog.…”

“…[18][19] Loss of Apaf1 causes decreased apoptosis, best documented in the embryonic neuroectoderm, which results in failed neural tube closure and cleft palate, among other morphological defects. [18][19][20][21][22] Although the cleft palate is attributed to decreased apoptosis between the two palatal shelves resulting in failed fusion, 20 details of the craniofacial defects due to Apaf1 deficiency remain to be determined.…”