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

Abstract

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

Gata2 is required for the development of inner ear semicircular ducts and the surrounding perilymphatic space

“…In addition, Gata2 has been shown to bind to Bcl-X promoter in leukemic cells (Koga et al, 2007). Bcl-X is strongly expressed in the inner ear epithelium during development at E10.5-E12.5, and its inactivation leads to developmental defects especially in the posterior duct (Cecconi et al, 2004). Here, we verified whether Bcl-X gene products were present also at E14.5 in inner ear epithelium.…”

“…We did not observe any clear decrease in inner mesenchyme cell death in Gata2 cko embryos to explain the maturation defect observed. In addition, there were no evident clearance problems in the vestibule or the cochlea in the absence of programmed cell death in Apaf1-and caspase9-deficient ears (Cecconi et al, 2004 and data not shown) strongly suggesting that Apaf1/caspase9-driven apoptosis is not a major player in the perilymphatic clearance process.…”

“…Programmed cell death has been detected in restricted areas of the otic epithelium during early inner ear morphogenesis in both rodents and chicken (Martin and Swanson, 1993;Fekete et al, 1997;Cecconi et al, 2004;León et al, 2004). Developmental cell death in mouse inner ear occurs mainly through the Apa f1/caspase 9 apoptosome pathway and when components of this pathway are inactivated, severe morphological defects appear in the ''sculpting'' of the epithelial structures and in the growth of the semicircular ducts (Cecconi et al, 2004).…”

“…Developmental cell death in mouse inner ear occurs mainly through the Apa f1/caspase 9 apoptosome pathway and when components of this pathway are inactivated, severe morphological defects appear in the ''sculpting'' of the epithelial structures and in the growth of the semicircular ducts (Cecconi et al, 2004).…”

“…When caspase9 or Apaf1 are inactivated, no programmed cell death is observed by TUNEL in the developing inner ear and severe defects in its epithelial morphogenesis occur (Cecconi et al, 2004). In contrast to otic epithelium, very little or no cell death can be observed in the surrounding inner mesenchyme throughout development (Cecconi et al, 2004 and this work), although apoptosis has been proposed to be the clearance mechanism to generate the vestibular (Chang et al, 2002) and cochlear perilymphatic spaces (Nikolic et al, 2000). We did not observe any clear decrease in inner mesenchyme cell death in Gata2 cko embryos to explain the maturation defect observed.…”