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p53 is essential for developmental neuron death as regulated by the TrkA and p75 neurotrophin receptors.

Aloyz RS, Bamji SX, Pozniak CD, Toma JG, Atwal J, Kaplan DR, Miller FD - J. Cell Biol. (1998)

Bottom Line: NGF withdrawal also results in elevation of a known p53 target, the apoptotic protein Bax.Finally, when p53 levels are reduced or absent in p53+/- or p53-/- mice, naturally occurring sympathetic neuron death is inhibited.Thus, p53 is an essential common component of two receptor-mediated signal transduction cascades that converge on the MEKK-JNK pathway to regulate the developmental death of sympathetic neurons.

View Article: PubMed Central - PubMed

Affiliation: Center for Neuronal Survival, Montreal Neurological Institute, McGill University, Montreal, Quebec, Canada H3A 2B4.

ABSTRACT
Naturally occurring sympathetic neuron death is the result of two apoptotic signaling events: one normally suppressed by NGF/TrkA survival signals, and a second activated by the p75 neurotrophin receptor. Here we demonstrate that the p53 tumor suppressor protein, likely as induced by the MEKK-JNK pathway, is an essential component of both of these apoptotic signaling cascades. In cultured neonatal sympathetic neurons, p53 protein levels are elevated in response to both NGF withdrawal and p75NTR activation. NGF withdrawal also results in elevation of a known p53 target, the apoptotic protein Bax. Functional ablation of p53 using the adenovirus E1B55K protein inhibits neuronal apoptosis as induced by either NGF withdrawal or p75 activation. Direct stimulation of the MEKK-JNK pathway using activated MEKK1 has similar effects; p53 and Bax are increased and the subsequent neuronal apoptosis can be rescued by E1B55K. Expression of p53 in sympathetic neurons indicates that p53 functions downstream of JNK and upstream of Bax. Finally, when p53 levels are reduced or absent in p53+/- or p53-/- mice, naturally occurring sympathetic neuron death is inhibited. Thus, p53 is an essential common component of two receptor-mediated signal transduction cascades that converge on the MEKK-JNK pathway to regulate the developmental death of sympathetic neurons.

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Model of sympathetic neuron apoptosis induced by NGF withdrawal or  p75NTR activation. Both of  these apoptotic stimuli increase p53 and Bax protein  levels, and require elevated  p53 levels to efficiently promote cell death. Neuronal  death induced by activated  MEKK1 also increases p53  and Bax protein levels, and  requires elevated levels of  p53 protein. NGF withdrawal-mediated sympathetic neuron death increases  JNK and c-jun phosphorylation and activity, and requires c-jun (Estus et al., 1994; Ham et al.,  1995). p75NTR activation in sympathetic neurons also leads to  hyperphosphorylation of c-jun, presumably via JNK (Bamji et al.,  1998). Bax is also essential for sympathetic neuron apoptosis in  vivo and in vitro (Deckwerth et al., 1996; Easton et al., 1997). The  precise MEKK or JNK family member in these pathways is not  known, and other intermediate proteins, such as SEK1 (Sanchez  et al., 1994) are likely involved as intermediate steps in this hypothetical cascade. Moreover, although this model shows JNK signaling to p53 via c-jun, recent work indicates that JNK can directly interact with and phosphorylate p53 (Hu et al., 1997). p53  may induce death by increasing Bax protein or activity, or via  other p53 target proteins (Polyak et al., 1997).
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Figure 7: Model of sympathetic neuron apoptosis induced by NGF withdrawal or p75NTR activation. Both of these apoptotic stimuli increase p53 and Bax protein levels, and require elevated p53 levels to efficiently promote cell death. Neuronal death induced by activated MEKK1 also increases p53 and Bax protein levels, and requires elevated levels of p53 protein. NGF withdrawal-mediated sympathetic neuron death increases JNK and c-jun phosphorylation and activity, and requires c-jun (Estus et al., 1994; Ham et al., 1995). p75NTR activation in sympathetic neurons also leads to hyperphosphorylation of c-jun, presumably via JNK (Bamji et al., 1998). Bax is also essential for sympathetic neuron apoptosis in vivo and in vitro (Deckwerth et al., 1996; Easton et al., 1997). The precise MEKK or JNK family member in these pathways is not known, and other intermediate proteins, such as SEK1 (Sanchez et al., 1994) are likely involved as intermediate steps in this hypothetical cascade. Moreover, although this model shows JNK signaling to p53 via c-jun, recent work indicates that JNK can directly interact with and phosphorylate p53 (Hu et al., 1997). p53 may induce death by increasing Bax protein or activity, or via other p53 target proteins (Polyak et al., 1997).

Mentions: These experiments demonstrated that JNK, p53 and Bax are all downstream of activated MEKK, and that elevated p53 is essential for MEKK-induced apoptosis. To determine whether p53 is downstream of JNK and upstream of Bax, as we had hypothesized, we took advantage of a recombinant adenovirus expressing human p53 (previously described in Slack et al., 1996). Specifically, sympathetic neurons were cultured for 4 d in 20 ng/ml NGF, were infected with 20 moi of p53-expressing adenovirus and 48 h later, cell lysates were analyzed by Western blots with anti-p53, anti-phosphoJNK or anti-Bax. This analysis revealed that the p53 adenovirus increased expression of p53 to levels similar to those observed after NGF withdrawal (Fig. 5 G), but that this elevated expression of p53 had no effect on JNK phosphorylation in sympathetic neurons maintained in 20 ng/ml NGF (Fig. 5 E). In contrast, infection with the p53 virus caused an increase in the levels of Bax protein (Fig. 5 F) that was similar in magnitude to that observed after NGF withdrawal (Fig. 1 F) or after infection with the activated MEKK1 adenovirus (Fig. 5 D). Together with the previous experiments with activated MEKK1, these experiments indicate that MEKK and JNK are upstream of p53, while Bax is downstream (Fig. 7).


p53 is essential for developmental neuron death as regulated by the TrkA and p75 neurotrophin receptors.

Aloyz RS, Bamji SX, Pozniak CD, Toma JG, Atwal J, Kaplan DR, Miller FD - J. Cell Biol. (1998)

Model of sympathetic neuron apoptosis induced by NGF withdrawal or  p75NTR activation. Both of  these apoptotic stimuli increase p53 and Bax protein  levels, and require elevated  p53 levels to efficiently promote cell death. Neuronal  death induced by activated  MEKK1 also increases p53  and Bax protein levels, and  requires elevated levels of  p53 protein. NGF withdrawal-mediated sympathetic neuron death increases  JNK and c-jun phosphorylation and activity, and requires c-jun (Estus et al., 1994; Ham et al.,  1995). p75NTR activation in sympathetic neurons also leads to  hyperphosphorylation of c-jun, presumably via JNK (Bamji et al.,  1998). Bax is also essential for sympathetic neuron apoptosis in  vivo and in vitro (Deckwerth et al., 1996; Easton et al., 1997). The  precise MEKK or JNK family member in these pathways is not  known, and other intermediate proteins, such as SEK1 (Sanchez  et al., 1994) are likely involved as intermediate steps in this hypothetical cascade. Moreover, although this model shows JNK signaling to p53 via c-jun, recent work indicates that JNK can directly interact with and phosphorylate p53 (Hu et al., 1997). p53  may induce death by increasing Bax protein or activity, or via  other p53 target proteins (Polyak et al., 1997).
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Related In: Results  -  Collection

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Figure 7: Model of sympathetic neuron apoptosis induced by NGF withdrawal or p75NTR activation. Both of these apoptotic stimuli increase p53 and Bax protein levels, and require elevated p53 levels to efficiently promote cell death. Neuronal death induced by activated MEKK1 also increases p53 and Bax protein levels, and requires elevated levels of p53 protein. NGF withdrawal-mediated sympathetic neuron death increases JNK and c-jun phosphorylation and activity, and requires c-jun (Estus et al., 1994; Ham et al., 1995). p75NTR activation in sympathetic neurons also leads to hyperphosphorylation of c-jun, presumably via JNK (Bamji et al., 1998). Bax is also essential for sympathetic neuron apoptosis in vivo and in vitro (Deckwerth et al., 1996; Easton et al., 1997). The precise MEKK or JNK family member in these pathways is not known, and other intermediate proteins, such as SEK1 (Sanchez et al., 1994) are likely involved as intermediate steps in this hypothetical cascade. Moreover, although this model shows JNK signaling to p53 via c-jun, recent work indicates that JNK can directly interact with and phosphorylate p53 (Hu et al., 1997). p53 may induce death by increasing Bax protein or activity, or via other p53 target proteins (Polyak et al., 1997).
Mentions: These experiments demonstrated that JNK, p53 and Bax are all downstream of activated MEKK, and that elevated p53 is essential for MEKK-induced apoptosis. To determine whether p53 is downstream of JNK and upstream of Bax, as we had hypothesized, we took advantage of a recombinant adenovirus expressing human p53 (previously described in Slack et al., 1996). Specifically, sympathetic neurons were cultured for 4 d in 20 ng/ml NGF, were infected with 20 moi of p53-expressing adenovirus and 48 h later, cell lysates were analyzed by Western blots with anti-p53, anti-phosphoJNK or anti-Bax. This analysis revealed that the p53 adenovirus increased expression of p53 to levels similar to those observed after NGF withdrawal (Fig. 5 G), but that this elevated expression of p53 had no effect on JNK phosphorylation in sympathetic neurons maintained in 20 ng/ml NGF (Fig. 5 E). In contrast, infection with the p53 virus caused an increase in the levels of Bax protein (Fig. 5 F) that was similar in magnitude to that observed after NGF withdrawal (Fig. 1 F) or after infection with the activated MEKK1 adenovirus (Fig. 5 D). Together with the previous experiments with activated MEKK1, these experiments indicate that MEKK and JNK are upstream of p53, while Bax is downstream (Fig. 7).

Bottom Line: NGF withdrawal also results in elevation of a known p53 target, the apoptotic protein Bax.Finally, when p53 levels are reduced or absent in p53+/- or p53-/- mice, naturally occurring sympathetic neuron death is inhibited.Thus, p53 is an essential common component of two receptor-mediated signal transduction cascades that converge on the MEKK-JNK pathway to regulate the developmental death of sympathetic neurons.

View Article: PubMed Central - PubMed

Affiliation: Center for Neuronal Survival, Montreal Neurological Institute, McGill University, Montreal, Quebec, Canada H3A 2B4.

ABSTRACT
Naturally occurring sympathetic neuron death is the result of two apoptotic signaling events: one normally suppressed by NGF/TrkA survival signals, and a second activated by the p75 neurotrophin receptor. Here we demonstrate that the p53 tumor suppressor protein, likely as induced by the MEKK-JNK pathway, is an essential component of both of these apoptotic signaling cascades. In cultured neonatal sympathetic neurons, p53 protein levels are elevated in response to both NGF withdrawal and p75NTR activation. NGF withdrawal also results in elevation of a known p53 target, the apoptotic protein Bax. Functional ablation of p53 using the adenovirus E1B55K protein inhibits neuronal apoptosis as induced by either NGF withdrawal or p75 activation. Direct stimulation of the MEKK-JNK pathway using activated MEKK1 has similar effects; p53 and Bax are increased and the subsequent neuronal apoptosis can be rescued by E1B55K. Expression of p53 in sympathetic neurons indicates that p53 functions downstream of JNK and upstream of Bax. Finally, when p53 levels are reduced or absent in p53+/- or p53-/- mice, naturally occurring sympathetic neuron death is inhibited. Thus, p53 is an essential common component of two receptor-mediated signal transduction cascades that converge on the MEKK-JNK pathway to regulate the developmental death of sympathetic neurons.

Show MeSH
Related in: MedlinePlus