Xenopus Bcl-X _L selectively protects Rohon-Beard neurons from metamorphic degeneration

Abstract

Amphibian metamorphosis involves extensive, but selective, neuronal death and turnover, thus sharing many features with mammalian postnatal development. The antiapoptotic protein Bcl-X L plays an important role in postnatal mammalian neuronal survival. It is therefore of interest that accumulation of the mRNA encoding the Xenopus Bcl-X L homologue, termed xR11, increases abruptly in the nervous system, but not in other tissues, during metamorphosis in Xenopus tadpoles. This observation raises the intriguing possibility that xR11 selectively regulates neuronal survival during postembryonic development. To investigate this hypothesis, we overexpressed xR11 in vivo as a green fluorescent protein (GFP)-xR11 fusion protein by using somatic and germinal transgenesis. Somatic gene transfer showed that the fusion protein was effective in counteracting, in a dose-dependent manner, the proapoptotic effects of coexpressed Bax. When GFP-xR11 was expressed from the neuronal ␤-tubulin promoter by germinal transgenesis we observed neuronal specific expression that was maintained throughout metamorphosis and beyond, into juvenile and adult stages. Confocal microscopy showed GFP-xR11 to be exclusively localized in the mitochondria. Our findings show that GFP-xR11 significantly prolonged Rohon-Beard neuron survival up to the climax of metamorphosis, even in the regressing tadpole tail, whereas in controls these neurons disappeared in early metamorphosis. However, GFP-xR11 expression did not modify the fate of spinal cord motoneurons. The selective protection of Rohon-Beard neurons reveals cell-specific apoptotic pathways and offers approaches to further analyze programmed neuronal turnover during postembryonic development.transgenesis ͉ fusion protein ͉ green fluorescent protein ͉ Mauthner

Thyroid hormone regulation of apoptotic tissue remodeling during anuran metamorphosis

“…The two bcl-2 family members cloned in Xenopus belong to the subfamily of cell death inhibitors [48], [49]. They are capably of inhibiting cell death when expressed in tissue culture cells or in Xenopus tadpoles.…”

Developmental cell death during Xenopus metamorphosis involves BID cleavage and caspase 2 and 8 activation

“…BCL-2 and BCL-XL (xR11) are functionally identical in this assay. xR11 injection alone does not affect luciferase expression in the absence of apoptosis (Coen et al, 2001). These data show that Xenopus BCL-XL is able to antagonize tBID-induced apoptosis in vivo.…”

“…First, Bax expression increases as metamorphosis progresses (Sachs et al, 1997) and second, an antisense approach confirmed the implication of Bax as an actor in the mechanism of muscle loss (Sachs et al, 2004). Furthermore, XR11 can abrogate the proapoptotic effect of Bax when coexpressed in muscle (Coen et al, 2001), and muscle loss can be delayed by overexpression of XR11 during natural metamorphosis (Rowe et al, 2002). .…”

“…Apoptosis was quantified by coinjection of a pcDNA3-LUC, a vector that constitutively expresses luciferase. We have shown previously that the decrease in luciferase activity reflects the degree of apoptosis in muscle cells (Sachs et al, 1997;Coen et al, 2001).…”

“…In the brain, the mRNA encoding the Xenopus Bcl-XL homologue, xR11, increases during mid-metamorphosis and postmetamorphosis (CruzReyes and Tata, 1995). Using transgenic tadpoles, we have shown previously that a subset of neuronal cells can be selectively protected from apoptosis by xR11 overexpression (Coen et al, 2001). Moreover, xR11 can inhibit cell death of tail muscle cells (Rowe et al, 2002;Nakajima and Yaoita, 2003).…”