Jonathan Whitfield

A c-Jun dominant negative mutant protects sympathetic neurons against programmed cell death.

Sympathetic neurons depend on nerve growth factor (NGF) for survival and die by apoptosis in its absence. We have investigated the pattern of expression of the Jun and Fos family of transcription factors in dying sympathetic neurons using antibodies specific for each family member. When sympathetic neurons are deprived of NGF, the level of c-Jun protein significantly increases, whereas the levels of the other members of the Jun and Fos family remain relatively constant. c-Jun also becomes more phosphorylated, probably on its amino terminal transactivation domain. When microinjected into sympathetic neurons, an expression vector for a c-Jun dominant negative mutant protects them against NGF withdrawal-induced death, indicating that AP-1 activity is essential for neuronal cell death. Furthermore, overexpression of the full-length c-Jun protein is, in itself, sufficient to induce apoptosis in sympathetic neurons.

Dominant-negative c-Jun promotes neuronal survival by reducing BIM expression and inhibiting mitochondrial cytochrome c release.

Sympathetic neurons require nerve growth factor for survival and die by apoptosis in its absence. Key steps in the death pathway include c-Jun activation, mitochondrial cytochrome c release, and caspase activation. Here, we show that neurons rescued from NGF withdrawal-induced apoptosis by expression of dominant-negative c-Jun do not release cytochrome c from their mitochondria. Furthermore, we find that the mRNA for BIM(EL), a proapoptotic BCL-2 family member, increases in level after NGF withdrawal and that this is reduced by dominant-negative c-Jun. Finally, overexpression of BIM(EL) in neurons induces cytochrome c redistribution and apoptosis in the presence of NGF, and neurons injected with Bim antisense oligonucleotides or isolated from Bim(-/-) knockout mice die more slowly after NGF withdrawal.

Role of the Jun Kinase Pathway in the Regulation of c-Jun Expression and Apoptosis in Sympathetic Neurons

When deprived of nerve growth factor (NGF), developing sympathetic neurons die by apoptosis. This death is associated with an increase in the level of c-Jun protein and is blocked by expression of a c-Jun dominant negative mutant. Here we have investigated whether NGF withdrawal activates Jun kinases, a family of stress-activated protein kinases that can stimulate the transcriptional activity of c-Jun by phosphorylating serines 63 and 73 in the transactivation domain and which can activate c-jun gene expression. We found that sympathetic neurons contained high basal levels of Jun kinase activity that increased further after NGF deprivation. In contrast, p38 kinase, another stress-activated protein kinase that can also stimulate c-jun gene expression, was not activated after NGF withdrawal. Consistent with Jun kinase activation, we found using a phospho-c-Jun-specific antibody that c-Jun was phosphorylated on serine 63 after NGF withdrawal. Furthermore, expression of a constitutively active form of MEK kinase 1 (MEKK1), which strongly activates the Jun kinase pathway, increased c-Jun protein levels and c-Jun phosphorylation and induced apoptosis in the presence of NGF. This death could be prevented by co-expression of SEKAL, a dominant negative mutant of SAPK/ERK kinase 1 (SEK1), an activator of Jun kinase that is a target of MEKK1. In contrast, expression of SEKAL alone did not prevent c-Jun expression, increases in c-Jun phosphorylation, or cell death after NGF withdrawal. Thus, activation of Jun kinase and increases in c-Jun phosphorylation and c-Jun protein levels occur at the same time after NGF withdrawal, but c-Jun levels and phosphorylation are regulated by an SEK1-independent pathway.

c-Jun and the transcriptional control of neuronal apoptosis.

There has been considerable interest in the molecular mechanisms of apoptosis in mammalian neurons because this form of neuronal cell death is important for the normal development of the nervous system and because inappropriate neuronal apoptosis may contribute to the pathology of human neurodegenerative diseases. The aim of recent research has been to identify the key components of the cell death machinery in neurons and understand how the cell death programme is regulated by intracellular signalling pathways activated by the binding of neurotrophins or death factors to specific cell surface receptors. The aim of this commentary was to review research that has investigated the role of the Jun N-terminal kinase (JNK)/c-Jun signalling pathway in neuronal apoptosis, focusing in particular on work carried out with developing sympathetic neurons. Experiments with sympathetic neurons cultured in vitro, as well as with cerebellar granule neurons and differentiated PC12 cells, have demonstrated that JNK/c-Jun signalling can promote apoptosis following survival factor withdrawal. In addition, experiments with Jnk(-/-) knockout mice have provided evidence that Jnk3 may be required for apoptosis in the hippocampus in vivo following injection of kainic acid, an excitotoxin, and that Jnk1 and Jnk2 are required for apoptosis in the developing embryonic neural tube. However, in the embryonic forebrain, Jnk1 and Jnk2 have the opposite function and are necessary for the survival of developing cortical neurons. These results suggest that JNKs and c-Jun are important regulators of the cell death programme in the mammalian nervous system, but that their biological effects depend on the neuronal type and stage of development.

Direct inhibition of c‐Jun N‐terminal kinase in sympathetic neurones prevents c‐jun promoter activation and NGF withdrawal‐induced death

c‐Jun N‐terminal kinases (JNKs) regulate gene expression by phosphorylating transcription factors, such as c‐Jun. Studies with Jnk knockout mice suggest that JNK activity may be required for excitotoxin‐induced apoptosis in the adult hippocampus and for apoptosis in the developing embryonic neural tube. Here we investigate the role of JNKs in classical neurotrophin‐regulated developmental neuronal death by using nerve growth factor (NGF)‐dependent sympathetic neurones. In this system, NGF withdrawal leads to an increase in JNK activity, an increase in c‐Jun protein levels and c‐Jun N‐terminal phosphorylation before the cell death commitment point, and c‐Jun activity is required for cell death. To inhibit JNK activity in sympathetic neurones we have used two different JNK inhibitors that act by distinct mechanisms: the compound SB 203580 and the JNK binding domain (JBD) of JNK interacting protein 1 (JIP‐1). We demonstrate that JNK activity is required for c‐Jun phosphorylation, c‐jun promoter activation and NGF withdrawal‐induced apoptosis. We also show that ATF‐2, a c‐Jun dimerization partner that can regulate c‐jun gene expression, is activated following NGF deprivation. Finally, by co‐expressing the JBD and a regulatable c‐Jun dominant negative mutant we demonstrate that JNK and AP‐1 function in the same pro‐apoptotic signalling pathway after NGF withdrawal.

Methods for culturing primary sympathetic neurons and for determining neuronal viability.

Developing nerve growth factor (NGF)-dependent sympathetic neurons are one of the best-studied in vitro models of neuronal apoptosis and have been used to identify key components of the neuronal cell death pathway. This chapter describes how to prepare purified cultures of primary sympathetic neurons and how to induce apoptosis by NGF deprivation. In addition, a simple method for measuring neuronal viability based on the live/dead assay is also described. This can be used for assessing the effect of small molecule inhibitors of protein kinases, caspases and other enzymes, on NGF withdrawal-induced death.

Immunocytochemical techniques for studying apoptosis in primary sympathetic neurons.

Developing sympathetic neurons, which depend on nerve growth factor for survival, are one of the best studied in vitro models of neuronal apoptosis and have been extensively used for cellular and molecular studies of the neuronal death pathway. Important apoptotic events after nerve growth factor withdrawal include the release of proapoptotic proteins, such as cytochrome c, from the mitochondria and the activation of caspases, followed by nuclear DNA fragmentation and chromatin condensation. In this chapter, we describe immunocytochemical techniques for studying apoptotic DNA fragmentation, changes in nuclear morphology, and mitochondrial cytochrome c release at the single cell level using sympathetic neurons cultured on glass coverslips.