In the context of DNA damage, while XIAP levels remain stable, strong Apaf‐1 induction by p53 permits caspase activation because of efficient apoptosome engagement . Broadly speaking, the survival pathways that repress the expression of BH3‐only proteins in healthy neurons are the PI3K/Akt and the MEK/ERK pathways. JNKs, particularly JNK3 isoforms, activate the intrinsic pathway through direct phosphorylation of pro‐ and anti‐apoptotic Bcl2 family members or through phosphorylation and activation of c‐Jun which induces BH3‐only proteins transcription. The study compares different discourses on models of intensification that differ in the role nature plays in the actual design of the systems. For simplicity, these thresholds can be considered to fall into three distinct developmental stages of increasing apoptotic restriction: (a) NPCs (mitotic); (b) young neurons (post‐mitotic and establishing neuronal connections); and (c) mature neurons (post‐mitotic and fully integrated in the neural network). CHOP, a transcription factor upregulated in response to ER stress, is also capable of directly transactivating Bim and Puma promoters [150, 151, 154]. When texting this station, message & data rates apply. These include the BH3‐only proteins Bim and Bad and the anti‐apoptotic protein Bcl‐2. As the mammalian neuronal apoptosis pathway has been best studied in young neurons (e.g., neonatal sympathetic neurons, embryonic dorsal root ganglion neurons, embryonic cortical neurons), this review will focus primarily on the mechanisms by which the intrinsic apoptotic pathway is activated and regulated in young neurons. However, unlike Mcl‐1, loss of Bcl‐2 does not result in widespread apoptosis in the developing brain. Specifically, for example, JNK activation results in the phosphorylation of c‐Jun on Ser63/73 [11, 12, 14, 15, 17, 20], the subsequent translocation of c‐Jun to the nucleus and its transcription‐mediated activation of the BH3‐only genes. Interestingly, another cyclin‐dependent kinase, Cdk4, which is able to induce E2F1‐dependent transcriptional upregulation of B‐Myb transcription factor, is also required for Bim upregulation and neuronal death [156, 157]. One of these mechanisms involves phosphatases of the DUSP (dual specificity phosphatases)/MKP (MAPK phosphatases) family. Typically, these kinases facilitate FoxO nuclear translocation by relieving either Akt‐ or 14‐3‐3‐mediated inhibition or sequestration of FoxO transcription factors.
Exactly why the restriction of Apaf‐1 is reversible in these situations in unclear, but it could be that the presence of Apaf‐1 may enable death to occur specifically by apoptosis, thus reducing the overall inflammation that may otherwise occur during neuronal injury or disease situations. How, then, are neurons able to survive cytosolic cyt c when other cell types cannot? Phosphorylation of Bim on Ser65 by ERK1/2 leads to interaction with the ubiquitin ligase TRIM2, followed by polyubiquitination and proteasomal degradation of Bim. However, not all neuronal precursors are completely dependent on Mcl‐1 for survival as deletion of Mcl‐1 in CGN precursors does not result in increased apoptosis . Characterization of a Cul9–Parkin double knockout mouse model for Parkinson’s disease. Interestingly, while Bax and Bak appear to have similar capabilities to induce MOMP and release cyt c, there are important differences in their mechanisms of activation. Thus, analyzing the mechanisms allowing the mitochondrial pathway to become activated in a very restricted fashion, as well as identifying the differences in its activation in apoptotic versus non‐apoptotic context, will provide significant insight into neuronal survival and function in physiological and pathological situations. Interestingly, some of the mechanisms evoked in this review have also been observed in other post‐mitotic systems including cardiomyocytes and myotubes [77, 262, 268]. Copyright © 2014 Elsevier B.V. All rights reserved. Although absence of Bcl‐w does not induce noticeable neuronal loss in the mature nervous system, it nonetheless sensitizes adult hippocampal neurons to seizure‐induced apoptosis . These include cIAP1, cIAP2 and XIAP, and even among these, XIAP has the most potent activity to inhibit apoptosis via it ability to directly bind to and inhibit the apoptotic caspases [243-245]. need to be unpacked. Strategies towards agricultural intensification differ on the definitions of sustainability and the variables included in its evaluation.
In addition to inactivating the pro‐apoptotic functions of cyt c by altering its redox state, neurons can also target cytosolic cyt c for degradation . Since neuronal apoptosis is known to be blocked with RNA and protein synthesis inhibitors [133-141], transcriptional induction of the BH3‐only proteins is thought to be a key event that enables neurons to undergo apoptosis. Indeed, genetic deletion of Bax alone confers neuronal protection in response to multiple stresses [83-87]. On the other hand, Bak is present on the surface of mitochondria and its activation only requires its oligomerization [100, 101]. The JNK pathway has also been implicated in Casp3 re‐expression in response to multiple neurotoxic insults [52, 53, 72, 141]. These findings suggest the existence of potentially conserved developmental regulatory pathways responsible for restricting the apoptotic machinery in post‐mitotic cells. It would be interesting to understand what are the signals and molecular mechanisms that initiate and drive this progressive switch toward a more restrictive state.
Interestingly, JNKs can also phosphorylate and activate p53, on Ser15, and induce the transcription of its pro‐apoptotic target genes (e.g., Puma, Bax) to mediate apoptosis in neurons [26-30]. Additionally, XIAP can also be inhibited by posttranslational mechanisms as in animal models of PD and PD patient samples, XIAP was found to be inactivated by nitrosylation of the cysteine residues in the BIR domain . Restricted during embryonic development, Bcl‐w expression greatly increases postnatally in maturing post‐mitotic neurons [111, 112]. While Jnk1 and Jnk2 are ubiquitously expressed, Jnk3 expression is mainly restricted to the brain and testis, suggesting non‐overlapping function with JNK1 and JNK2. Importantly, a subset of the pro‐apoptotic members of the Bcl‐2 family, known as the BH3‐only proteins, are sentinels of cellular stresses and their role is to activate the intracellular apoptotic pathway. Effector Casp3 is without a doubt the caspase member that is the most reported to be upregulated at the transcriptional level in response to neuronal insult. Levels of Bax also decrease in the brain postnatally but are maintained at low levels [77, 81, 88-91].
Interestingly, the antagonistic action of Akt is not limited to JNK activation alone. While apoptosis during nervous system development is critical, the threshold required to induce apoptosis becomes much greater once the mature nervous system is established, resulting in markedly less neuronal cell death. Indeed, caspases are often re‐expressed in the mature nervous system in response to various apoptotic stimuli. Interestingly, these neurons are able to fully recover from apoptosis upon the re‐addition of NGF, despite the complete release of cyt c from their mitochondria [178-180].
Induction of Puma by p53 has also been reported in the mature brain following seizures  or MPTP‐induced neurotoxicity , but also in vitro in response to NGF withdrawal  or amyloid β . Akt also exerts its anti‐apoptotic function in neurons through repression of the FoxO transcription factors and inactivation of the BH3‐only protein Bad [32, 56-59]. Are Marlins Built to Get Hot in Short, 60-Game Slate? Smac/DIABLO and Omi/HtrA2 are both involved in neutralizing XIAP (X‐linked inhibitor of apoptosis), a cytosolic protein that inhibits caspases. Caspases have nonetheless been shown to be activated and involved, at least in part, in response to multiple triggers in mature neurons. For example, neuronal Akt and ERK5 repress FoxO3a transcription factor and suppression of Akt or ERK5 is sufficient to promote Bim expression [56, 65-68, 158].
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