p21ras supports the survival of chick embryonic motor neurones

Neuronal activation of Ras regulates synaptic connectivity

A synRas mouse model was used expressing constitutively activated Ha-Ras (Val12 mutation) in neurons to investigate the role of Ras-MAPkinase signalling for neuronal connectivity in adult brain. Expression of the transgene in the cortex of these mice starts after neuronal differentiation is completed and allows to directly investigate the effects of enhanced Ras activity in differentiated neurons. Activation of Ha-Ras induced an increase in soma size which was sensitive to MEK inhibitor in postnatal organotypic cultures. Adult cortical pyramidal neurons showed complex structural rearrangements associated with an increased size and ramification of dendritic arborization. Dendritic spine density was elevated and correlated with a twofold increase in number of synapses. In acute brain slices of the somatosensory and of the visual cortex, extracellular field potentials were recorded from layer II/III neurons. The input-output relation of synaptically evoked field potentials revealed a significantly higher basal excitability of the transgenic mice cortex compared to wild-type animals. In whole cell patch clamp preparations, the frequency of AMPA receptor-mediated spontaneous excitatory postsynaptic currents was increased while the ratio between NMDA and AMPA-receptor mediated signal amplitude was unchanged. A pronounced depression of paired pulse facilitation indicated that Ras contributes to changes at the presynaptic site. Furthermore, synRas mice showed an increased synaptic long-term potentiation, which was sensitive to blockers of NMDA-receptors and of MEK. We conclude that neuronal Ras is a common switch of plasticity in adult mammalian brain sculpturing neuronal architecture and synaptic connectivity in concert with tuning synaptic efficacy.

Conditional gene ablation of Stat3 reveals differential signaling requirements for survival of motoneurons during development and after nerve injury in the adult

Members of the ciliary neurotrophic factor (CNTF)/leukemia inhibitory factor (LIF)/cardiotrophin gene family are potent survival factors for embryonic and lesioned motoneurons. These factors act via receptor complexes involving gp130 and LIFR-beta and ligand binding leads to activation of various signaling pathways, including phosphorylation of Stat3. The role of Stat3 in neuronal survival was investigated in mice by Cre-mediated gene ablation in motoneurons. Cre is expressed under the neurofilament light chain (NF-L) promoter, starting around E12 when these neurons become dependent on neurotrophic support. Loss of motoneurons during the embryonic period of naturally occurring cell death is not enhanced in NF-L-Cre; Stat3(flox/KO) mice although motoneurons isolated from these mice need higher concentrations of CNTF for maximal survival in culture. In contrast, motoneuron survival is significantly reduced after facial nerve lesion in the adult. These neurons, however, can be rescued by the addition of neurotrophic factors, including CNTF. Stat3 is essential for upregulation of Reg-2 and Bcl-xl expression in lesioned motoneurons. Our data show that Stat3 activation plays an essential role for motoneuron survival after nerve lesion in postnatal life but not during embryonic development, indicating that signaling requirements for motoneuron survival change during maturation.

JNK activation is associated with intracellular beta-amyloid accumulation

c-Jun has been implicated in the pathogenesis of Alzheimer's disease (AD), but the upstream cascade leading to c-Jun activation in AD is not known. Activation of c-Jun N-terminal kinase (JNK) is obviously a candidate for the upstream event. We tested this possibility focusing on PS1-linked AD. First, we observed that JNK is actually activated in cerebral neurons of PS1-linked AD patients, using immunohistochemistry and Western blot analyses with anti-activated JNK antibodies. We analyzed the relationship between beta-amyloid (beta A) and JNK activation by using aged transgenic mice overexpressing mutant (M146L) PS1 and human AD brains. The mice showed no neuronal loss but a very few diffuse beta A deposits, corresponding to the early stage of PS1-linked AD brain. Some neurons were reactive for anti-beta A antibodies in the cerebral cortex. Interestingly, JNK activation was observed in neurons showing intracellular beta A immunoreactivity in transgenic mice. Association between intracellular beta A and JNK activation was confirmed in cortical neurons of sporadic and PS1-linked AD patients. Furthermore, introduction of beta A peptides into the primary culture cortical neurons induced JNK activation and cell death. Collectively, these results suggested that intracellular beta A accumulation might trigger JNK activation leading to neuronal death.

Transgenic activation of Ras in neurons promotes hypertrophy and protects from lesion-induced degeneration

Ras is a universal eukaryotic intracellular protein integrating extracellular signals from multiple receptor types. To investigate its role in the adult central nervous system, constitutively activated V12-Ha-Ras was expressed selectively in neurons of transgenic mice via a synapsin promoter. Ras-transgene protein expression increased postnatally, reaching a four- to fivefold elevation at day 40 and persisting at this level, thereafter. Neuronal Ras was constitutively active and a corresponding activating phosphorylation of mitogen-activated kinase was observed, but there were no changes in the activity of phosphoinositide 3-kinase, the phosphorylation of its target kinase Akt/PKB, or expression of the anti-apoptotic proteins Bcl-2 or Bcl-X(L). Neuronal Ras activation did not alter the total number of neurons, but induced cell soma hypertrophy, which resulted in a 14.5% increase of total brain volume. Choline acetyltransferase and tyrosine hydroxylase activities were increased, as well as neuropeptide Y expression. Degeneration of motorneurons was completely prevented after facial nerve lesion in Ras-transgenic mice. Furthermore, neurotoxin-induced degeneration of dopaminergic substantia nigra neurons and their striatal projections was greatly attenuated. Thus, the Ras signaling pathway mimics neurotrophic effects and triggers neuroprotective mechanisms in adult mice. Neuronal Ras activation might become a tool to stabilize donor neurons for neural transplantation and to protect neuronal populations in neurodegenerative diseases.

Motoneuron apoptosis is blocked by CEP-1347 (KT 7515), a novel inhibitor of the JNK signaling pathway

Neurons undergoing apoptosis can be rescued by trophic factors that simultaneously increase the activity of extracellular signal-regulated kinase (ERK) and decrease c-Jun N-terminal kinase (JNK) and p38. We identified a molecule, CEP-1347 (KT7515), that rescues motoneurons undergoing apoptosis and investigated its effect on ERK1 and JNK1 activity. Cultured rat embryonic motoneurons, in the absence of trophic factor, began to die 24-48 hr after plating. During the first 24 hr ERK1 activity was unchanged, whereas JNK1 activity increased fourfold. CEP-1347 completely rescued motoneurons for at least 72 hr with an EC50 of 20 +/- 2 nM. CEP-1347 did not alter ERK1 activity but rapidly inhibited JNK1 activation. The IC50 of CEP-1347 for JNK1 activation was the same as the EC50 for motoneuron survival. Inhibition of JNK1 activation by CEP-1347 was not selective to motoneurons. CEP-1347 also inhibited JNK1 activity in Cos7 cells under conditions of ultraviolet irradiation, osmotic shock, and inhibition of glycosylation. Inhibition by CEP-1347 of the JNK1 signaling pathway appeared to be selective, because CEP-1347 did not inhibit p38-regulated mitogen-activated protein kinase-activated protein kinase-2 (MAPKAP2) activity in Cos7 cells subjected to osmotic shock. The direct molecular target of CEP-1347 was not JNK1, because CEP-1347 did not inhibit JNK1 activity in Cos7 cells cotransfected with MEKK1 and JNK1 cDNA constructs. This is the first demonstration of a small organic molecule that promotes motoneuron survival and that simultaneously inhibits the JNK1 signaling cascade.

Activated phosphatidylinositol 3-kinase and Akt kinase promote survival of superior cervical neurons

The signaling pathways that mediate the ability of NGF to support survival of dependent neurons are not yet completely clear. However previous work has shown that the c-Jun pathway is activated after NGF withdrawal, and blocking this pathway blocks neuronal cell death. In this paper we show that over-expression in sympathetic neurons of phosphatidylinositol (PI) 3-kinase or its downstream effector Akt kinase blocks cell death after NGF withdrawal, in spite of the fact that the c-Jun pathway is activated. Yet, neither the PI 3-kinase inhibitor LY294002 nor a dominant negative PI 3-kinase cause sympathetic neurons to die if they are maintained in NGF. Thus, although NGF may regulate multiple pathways involved in neuronal survival, stimulation of the PI 3-kinase pathway is sufficient to allow cells to survive in the absence of this factor.