Developmental changes in the localization of activated C-JUN N-terminal kinase (JNK/SAPK) in the chick spinal cord

c-Jun N-terminal kinase signaling regulates events associated with both health and degeneration in motoneurons

The c-Jun N-terminal kinases (JNKs) are activated by various stimuli and are critical for neuronal development as well as for death following a stressful stimulus. Here, we have evaluated JNK activity in both healthy and dying motoneurons from developing chick embryos and found no apparent difference in overall JNK activity between the conditions, suggesting that this pathway maybe critical in both circumstances. Pharmacological inhibition of JNK in healthy motoneurons supplied with trophic support resulted in decreased mitochondrial membrane potential, neurite outgrowth, and phosphorylation of microtubule-associated protein 1B. On the other hand, in motoneurons deprived of trophic support, inhibition of JNK attenuated caspase activation, and nuclear condensation. We also examined the role of JNK's downstream substrate c-Jun in mediating these events. While c-Jun expression and phosphorylation were greater in cells supplied with trophic support as compared with those deprived, inhibition of c-Jun had no effect on nuclear condensation in dying cells or neurite outgrowth in healthy cells, suggesting that JNK's role in these events is independent of c-Jun. Together, our data underscore the dualistic nature of JNK signaling that is critical for both survival and degenerative changes in motoneurons.

Involvement of c-Jun-JNK pathways in the regulation of programmed cell death of developing chick embryo spinal cord motoneurons

Key features of developmentally regulated programmed cell death (PCD) have been described for the first time in the chick nervous system. JNK/c-Jun pathway was involved in early events determining normal and pathological neuronal death as shown in experimental models. In the chick embryo, PCD of motoneurons (MNs) in ovo occurs within a well-defined temporal window and can be subjected to experimental manipulation. Taking advantage of this in vivo system, we explored the role of c-Jun and JNK pathway in the regulation of PCD in MNs. By using specific antibodies against phospho-c-Jun (Ser 63, 73) and JNK we demonstrated that before MNs acquire apoptotic phenotype there is an increase in c-Jun. Blockage of neuromuscular activity by the GABA agonist muscimol reduces PCD and diminishes c-Jun immunoreactivity in MNs. Extensive induction of PCD, either due to injection of beta-bungarotoxin or limb bud removal, is also preceded by an increase in c-Jun immunoreactivity that is also associated with upregulation of phospho-c-Jun and JNK. Translocation of JNK from cytoplasm to MN nuclei was also detected. After acute application of beta-bungarotoxin, which is a strong apoptotic stimulus for MNs, c-Jun phosphorylation occurs on serine 73, whereas serine 63 is the main site for c-Jun phosphorylation after limb bud removal. These results demonstrated that the JNK/c-Jun pathway is involved in the decision phase of normal and induced apoptosis in MNs. Pharmacological interventions involving this pathway should be explored as a potential therapeutic target for promoting MN survival.

Activation of c-Jun N-terminal kinase by cadmium in mouse embryo neural cells in vitro

Members of the c-Jun NH(2)-terminal kinase (JNK) signalling pathway have been found to be stimulated by a variety of stresses, including heavy metals, hyperthermia, and UV-irradiation. In the present study, we examined whether exposure of micromass cultures of mouse embryonic midbrain cells to a known teratogen, cadmium, leads to the phosphorylation and activation of JNK. Midbrain cells exposed to 0.5, 1, 2, or 4μM cadmium chloride (CdCl(2)) showed a dose-dependent decline in cell numbers, cell viability and differentiation after 5 days. In cells exposed to 4μM CdCl(2) for up to 1h, the level of phosphorylated JNK increased by 15min and peaked at 30min exposure time, as determined by a phospho-specific anti-JNK antibody, while the total amount of JNK protein did not change. This phosphorylated JNK was active, as shown by a corresponding increase in the level of c-Jun phosphorylated on Ser63 in a kinase assay. These results demonstrate that CdCl(2) induces a rapid and transient activation of the JNK pathway in primary embryonic neuron cell cultures.

Developmental regulation of activated ERK expression in the spinal cord and dorsal root ganglion of the chick embryo

Mitogen-activated protein kinases (MAPKs) are involved in the intracellular pathways that respond to various extracellular signals. Extracellular signal-regulated kinase (ERK) is a member of MAPKs and has various functions in neural development. However, the in vivo distribution of the activated form of ERK (p-ERK) in the developing nervous system is not well understood. Here, we investigated the expression of p-ERK in the spinal cord and dorsal root ganglion (DRG) of chick embryos. In the spinal cord, p-ERK-positive cells appeared in the ventral ventricular zone on embryonic day 4 (E4). From E6 onward, they appeared in the gray matter and in the white matter, suggesting migration from the ventricular zone. A double labeling method revealed that these p-ERK-positive cells included oligodendrocyte precursors. In the dorsal horn, p-ERK-positive small cells appeared on E6. Subsequently, the positive cells in the dorsal horn increased transiently in number and then decreased markedly by E10. Motoneurons also expressed p-ERK transiently on E7. In the DRG, weak p-ERK immunoreaction appeared in the ventrolateral region on E5. From E6, the immunoreactivity became stronger and by E9 intense p-ERK-positive cells were observed throughout the DRG. These data provide a neuroanatomical framework to begin to examine the in vivo role of ERK in neural development.