JNK2 and JNK3 are major regulators of axonal injury-induced retinal ganglion cell death

Flaum Eye Institute, University of Rochester Medical Center, Rochester, NY 14642, USA.
Neurobiology of Disease (Impact Factor: 5.08). 02/2012; 46(2):393-401. DOI: 10.1016/j.nbd.2012.02.003
Source: PubMed


Glaucoma is a neurodegenerative disease characterized by the apoptotic death of retinal ganglion cells (RGCs). The primary insult to RGCs in glaucoma is thought to occur to their axons as they exit the eye in the optic nerve head. However, pathological signaling pathways that exert central roles in triggering RGC death following axonal injury remain unidentified. It is likely that the first changes to occur following axonal injury are signal relay events that transduce the injury signal from the axon to the cell body. Here we focus on the c-Jun N-terminal kinase (JNK1-3) family, a signaling pathway implicated in axonal injury signaling and neurodegenerative apoptosis, and likely to function as a central node in axonal injury-induced RGC death. We show that JNK signaling is activated immediately after axonal injury in RGC axons at the site of injury. Following its early activation, sustained JNK signaling is observed in axonally-injured RGCs in the form of JUN phosphorylation and upregulation. Using mice lacking specific Jnk isoforms, we show that Jnk2 and Jnk3 are the isoforms activated in injured axons. Combined deficiency of Jnk2 and Jnk3 provides robust long-term protection against axonal injury-induced RGC death and prevents downregulation of the RGC marker, BRN3B, and phosphorylation of JUN. Finally, using Jun deficient mice, we show that JUN-dependent pathways are important for axonal injury-induced RGC death. Together these data demonstrate that JNK signaling is the major early pathway triggering RGC death after axonal injury and may directly link axon injury to transcriptional activity that controls RGC death.

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Available from: Iok-Hou Pang, Mar 05, 2014
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    • "mpared with controls . Indeed , we found that the stress - activated protein kinase JNK , and its targets are likely to play important roles in the answer following injury . Our data corroborate previous observa - tions showing that JNK and c - Jun are highly activated in injured RGCs , and that deletion of Jnk2 and Jnk3 pro - motes RGC survival ( Fernandes et al . , 2012 ) . Here , we confirmed that JNK and its target c - Jun are activated in axotomized RGCs in both the HDAC1 / 2 mutant and control retinae . In addition , the mRNA level of the proa - poptotic target Hrk was also upregulated confirming pre - vious observations in axotomized RGCs ( Wakabayashi et al . , 2002 ) . However , these targets co"
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    ABSTRACT: Histones deacetylases (HDACs), besides their function as epigenetic regulators, deacetylate and critically regulate the activity of nonhistone targets. In particular, HDACs control partially the proapoptotic activity of p53 by balancing its acetylation state. HDAC inhibitors have revealed neuroprotective properties in different models, but the exact mechanisms of action remain poorly understood. We have generated a conditional knockout mouse model targeting retinal ganglion cells (RGCs) to investigate specifically the functional role of HDAC1 and HDAC2 in an acute model of optic nerve injury. Our results demonstrate that combined HDAC1 and HDAC2 ablation promotes survival of axotomized RGCs. Based on global gene expression analyses, we identified the p53-PUMA apoptosis-inducing axis to be strongly activated in axotomized mouse RGCs. Specific HDAC1/2 ablation inhibited this apoptotic pathway by impairing the crucial acetylation status of p53 and reducing PUMA expression, thereby contributing to the ensuing enhanced neuroprotection due to HDAC1/2 depletion. HDAC1/2 inhibition and the affected downstream signaling components emerge as specific targets for developing therapeutic strategies in neuroprotection. © The Author(s) 2015.
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    • "Although the promoter assays in the current study showed that c-Jun and C/EBPβ are upstream regulators to bind the promoter of ETB receptor and activate the transcription of ETB receptor, the direct functional roles of these factors in vivo in glaucoma experimental eyes are still unclear. Recently, lack of JNK2/3 signaling due to deficiency of JNK2/3 or Jun in mice has been shown the protective effects from optic nerve crush-induced RGC death [41]. However, the mechanisms that Jun-mediated pathways activate apoptosis of RGC have not been elucidated. "
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    ABSTRACT: Previous studies showed that the endothelin B receptor (ETB) expression was upregulated and played a key role in neurodegeneration in rodent models of glaucoma. However, the mechanisms underlying upregulation of ETB receptor expression remain largely unknown. Using promoter-reporter assays, the 1258 bp upstream the human ETB promoter region was found to be essential for constitutive expression of ETB receptor gene in human non-pigmented ciliary epithelial cells (HNPE). The -300 to -1 bp and -1258 to -600 bp upstream promoter regions of the ETB receptor appeared to be the key binding regions for transcription factors. In addition, the crucial AP-1 binding site located at -615 to -624 bp upstream promoter was confirmed by luciferase assays and CHIP assays which were performed following overexpression of c-Jun in HNPE cells. Overexpression of either c-Jun or C/EBPβ enhanced the ETB receptor promoter activity, which was reflected in increased mRNA and protein levels of ETB receptor. Furthermore, knock-down of either c-Jun or C/EBPβ in HNPE cells was significantly correlated to decreased mRNA levels of both ETB and ETA receptor. These observations suggest that c-Jun and C/EBPβ are important for regulated expression of the ETB receptor in HNPE cells. In separate experiments, intraocular pressure (IOP) was elevated in one eye of Brown Norway rats while the corresponding contralateral eye served as control. Two weeks of IOP elevation produced increased expression of c-Jun and C/EBPβ in the retinal ganglion cell (RGC) layer from IOP-elevated eyes. The mRNA levels of c-Jun, ETA and ETB receptor were upregulated by 2.2-, 3.1- and 4.4-fold in RGC layers obtained by laser capture microdissection from retinas of eyes with elevated IOP, compared to those from contralateral eyes. Taken together, these data suggest that transcription factor AP-1 plays a key role in elevation of ETB receptor in a rodent model of ocular hypertension.
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    • "However, it is unclear what fraction of p-JNK originating at the injury site is able to reach the cell body. Previous studies have demonstrated that p-JNK can be observed in the optic nerve head 1 h after injury, but p-c-Jun cannot be detected in RGC nuclei until 6 h after crush (Fig. 4 A; Fernandes et al., 2012). In DLK heterozygous animals, the number of p-c-Jun–positive neurons at this time point is significantly reduced (Fig. 4, A and B), demonstrating that DLK protein quantity governs the amount of active JNK reaching the nucleus in the immediate response to injury. "
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    ABSTRACT: Neurons are highly polarized cells that often project axons a considerable distance. To respond to axonal damage, neurons must transmit a retrograde signal to the nucleus to enable a transcriptional stress response. Here we describe a mechanism by which this signal is propagated through injury-induced stabilization of dual leucine zipper-bearing kinase (DLK/MAP3K12). After neuronal insult, specific sites throughout the length of DLK underwent phosphorylation by c-Jun N-terminal kinases (JNKs), which have been shown to be downstream targets of DLK pathway activity. These phosphorylation events resulted in increased DLK abundance via reduction of DLK ubiquitination, which was mediated by the E3 ubiquitin ligase PHR1 and the de-ubiquitinating enzyme USP9X. Abundance of DLK in turn controlled the levels of downstream JNK signaling and apoptosis. Through this feedback mechanism, the ubiquitin-proteasome system is able to provide an additional layer of regulation of retrograde stress signaling to generate a global cellular response to localized external insults.
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