Superior cervical ganglion-10 protein as a molecular effector of c-Jun N-terminal kinase 1: implications for the therapeutic targeting of Jun N-terminal kinase in nerve regeneration

Stathmins and Motor Neuron Diseases: Pathophysiology and Therapeutic Targets

Motor neuron diseases (MNDs) are a group of fatal, neurodegenerative disorders with different etiology, clinical course and presentation, caused by the loss of upper and lower motor neurons (MNs). MNs are highly specialized cells equipped with long, axonal processes; axonal defects are some of the main players underlying the pathogenesis of these disorders. Microtubules are key components of the neuronal cytoskeleton characterized by dynamic instability, switching between rapid polymerization and shrinkage. Proteins of the stathmin family affect microtubule dynamics regulating the assembly and the dismantling of tubulin. Stathmin-2 (STMN2) is one of the most abundantly expressed genes in MNs. Following axonal injury, STMN2 expression is upregulated, and the protein is transported toward the growth cones of regenerating axons. STMN2 has a critical role in axonal maintenance, and its dysregulation plays an important role in neurodegenerative processes. Stathmin-1 (STMN1) is a ubiquitous protein that is highly expressed during the development of the nervous system, and its phosphorylation controls microtubule dynamics. In the present review, we summarize what is currently known about the involvement of stathmin alterations in MNDs and the potential therapeutic effect of their modulation, with a specific focus on the most common forms of MND, amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA).

The nNOS-p38MAPK pathway is mediated by NOS1AP during neuronal death

Neuronal nitric oxide synthase (nNOS) and p38MAPK are strongly implicated in excitotoxicity, a mechanism common to many neurodegenerative conditions, but the intermediary mechanism is unclear. NOS1AP is encoded by a gene recently associated with sudden cardiac death, diabetes-associated complications, and schizophrenia (Arking et al., 2006; Becker et al., 2008; Brzustowicz, 2008; Lehtinen et al., 2008). Here we find it interacts with p38MAPK-activating kinase MKK3. Excitotoxic stimulus induces recruitment of NOS1AP to nNOS in rat cortical neuron culture. Excitotoxic activation of p38MAPK and subsequent neuronal death are reduced by competing with the nNOS:NOS1AP interaction and by knockdown with NOS1AP-targeting siRNAs. We designed a cell-permeable peptide that competes for the unique PDZ domain of nNOS that interacts with NOS1AP. This peptide inhibits NMDA-induced recruitment of NOS1AP to nNOS and in vivo in rat, doubles surviving tissue in a severe model of neonatal hypoxia-ischemia, a major cause of neonatal death and pediatric disability. The highly unusual sequence specificity of the nNOS:NOS1AP interaction and involvement in excitotoxic signaling may provide future opportunities for generation of neuroprotectants with high specificity.

The JNK inhibitor D-JNKI-1 blocks apoptotic JNK signaling in brain mitochondria

Kainic acid (KA) induced seizures provokes an extensive neuronal degeneration initiated by c-Jun N-terminal kinases (JNK) as central mediators of excitotoxicity. However, the actions of their individual isoforms in cellular organelles including mitochondria remain to be elucidated. Here, we have studied the activation of JNK1, JNK2 and JNK3 and their activators, mitogen-activated protein kinase kinase (MKK) 4/7, in brain mitochondria, cytosolic and nuclear fractions after KA seizures. In the mitochondrial fraction, KA significantly increased the presence of JNK1, JNK3 and MKK4 and stimulated their phosphorylation i.e. activation. The pro-apoptotic proteins, Bim and Bax were induced and, consequently, the ratio Bcl-2-Bax decreased. These changes were paralleled by the release of cytochrome c and cleavage of poly(ADP-ribose)-polymerase (PARP). The JNK peptide inhibitor, D-JNKI-1 (XG-102) reversed these pathological events in the mitochondria and almost completely abolished cytochrome c release and PARP cleavage. Importantly, JNK3, but not JNK1 or JNK2, was associated with Bim in mitochondria and D-JNKI-1 prevented the formation of this apoptotic complex. Apart from of the attenuation of c-Jun phosphorylation in the nucleus, D-JNKI-1 did not affect the level of JNK3 isoform in the nuclear and cytosolic fractions. These findings provide novel insights into the mode of action of individual JNK isoforms in cell organelles and points to the JNK3 pool in mitochondria as a target of the JNK inhibitor D-JNKI-1 to confer neuroprotection.

All JNKs can kill, but nuclear localization is critical for neuronal death

JNKs are implicated in a range of brain pathologies and receive considerable attention as potential therapeutic targets. However, JNKs also regulate physiological and homeostatic processes. An attractive hypothesis from the drug development perspective is that distinct JNK isoforms mediate "physiological" and "pathological" responses. However, this lacks experimental evaluation. Here we investigate the isoforms, subcellular pools, and c-Jun/ATF2 targets of JNK in death of central nervous system neurons following withdrawal of trophic support. We use gene knockouts, gene silencing, subcellularly targeted dominant negative constructs, and pharmacological inhibitors. Combined small interfering RNA knockdown of all JNKs 1, 2, and 3, provides substantial neuroprotection. In contrast, knockdown or knock-out of individual JNKs or two JNKs together does not protect. This explains why the evidence for JNK in neuronal death has to date been largely pharmacological. Complete knockdown of c-Jun and ATF2 using small interfering RNA also fails to protect, casting doubt on c-Jun as a critical effector of JNK in neuronal death. Nonetheless, the death requires nuclear but not cytosolic JNK activity as nuclear dominant negative inhibitors of JNK protect, whereas cytosolic inhibitors only block physiological JNK function. Thus any one of the three JNKs is capable of mediating apoptosis and inhibition of nuclear JNK is protective.