Neuritogenic activity of a genipin derivative in retinal ganglion cells is mediated by retinoic acid receptor β expression through nitric oxide/S-nitrosylation signaling

Inflammation and apoptosis, two key events induced by hyperglycemia mediated reactive nitrogen species in RGC-5 cells

Nitrosative stress plays a critical role in retinal injury in high glucose (HG) environment of eye, but the mechanisms remain poorly understood. Here we tested the hypothesis that HG induced reactive nitrogen species (RNS) production acts as a key functional mediator of antioxidant depletion, mitochondrial dysfunction, biomolecule damage, inflammation and apoptosis. Our findings illustrated that exposure of cultured RGC-5 cells to HG significantly disrupts the antioxidant defense mechanism and mitochondrial machineries by increasing the loss of mitochondrial membrane potential (ΔѰM) and elevating mitochondrial mass. Furthermore, we used biochemical tools to analyze the changes in metabolites, sulfur amino acids (SAAs) such as L-glutathione (GSH) and L-cysteine (Cys), in the presence of HG environment. These metabolic changes were followed by an increase in glycolytic flux that is phosphofructokinase-2 (PFK-2) activity. Moreover, HG exposure results in a significant disruption of protein carbonylation (PC) and lipid peroxidation (LPO), downregulation of OGG1 and increase in 8-OHdG accumulations in RGC-5 cells. In addition, our results demonstrated that HG environment coinciding with increased expression of inflammatory mediators, cell cycle deregulation, decreased in cell viability and expression of FoxOs, increased lysosomal content leading to apoptosis. Pre-treatment of selective inhibitors of RNS significantly reduced the HG-induced cell cycle deregulation and apoptosis in RGC-5 cells. Collectively, these results illustrated that accumulated RNS exacerbates the antioxidant depletion, mitochondrial dysfunction, biomolecule damage, inflammation and apoptosis induced by HG exposure in RGC-5 cells. Treatment of pharmacological inhibitors attenuated the HG induced in retinal cells.

Conjugation of tacrine with genipin derivative not only enhances effects on AChE but also leads to autophagy against Alzheimer's disease

Seven tacrine/CHR21 conjugates have been designed and synthesized. Compound 8-7 was confirmed as the most active AChE inhibitor with IC₅₀ value of 5.8 ± 1.4 nM, which was 7.72-fold stronger than tacrine. It was also shown as a strong BuChE inhibitor (IC₅₀ value of 3.7 ± 1.3 nM). 8-7 was clearly highlighted not only as an excellent ChEs inhibitor, but also as a good modulator on protein expression of AChE, p53, Bax, Bcl-2, LC3, p62, and ULK, indicating its functions against programmed cell apoptosis and decrease of autophagy. 8-7 significantly reversed the glutamate-induced dysfunctions including excessive calcium influx and release from internal organelles, overproduction of nitric oxide (NO) and Aβ high molecular weight oligomer. This compound can penetrate blood-brain barrier (BBB). The in vivo hepatotoxicity assay indicated that 8-7 was much less toxic than tacrine. Altogether, these data strongly support that 8-7 is a potential multitarget-directed ligand (MTDL) for treating Alzheimer's disease (AD).

A novel function of neuroglobin for neuroregeneration in mice after optic nerve injury

Neuroglobin (Ngb) is a recently discovered heme protein in the vertebrate brain that can bind to oxygen molecules. Mammalian Ngb plays a crucial role in neuroprotection under conditions of oxidative stress. To investigate other potential functions of Ngb, we investigated the mouse retinal Ngb system following optic nerve injury. In the retina of control mice, Ngb immunoreactivity was limited to the retinal ganglion cell (RGC) layer, and this immunoreactivity rapidly decreased to less than 50% of the control level 5 days after optic nerve injury. On the basis of this decrease, we designed in vivo experiments with enhanced expression of Ngb using adult mouse retina. The enhanced expression of Ngb was achieved by injecting chimeric human Ngb protein, which included the cell membrane-penetrating module of fish Ngb. One-day pretreatment with chimeric Ngb increased immunoreactivity levels of Ngb two-fold in mouse RGCs and increased the number of surviving RGCs three-fold by 14 days after optic nerve injury compared with vehicle controls. Furthermore, in the mouse retinas showing enhanced Ngb expression, several regenerating central optic axons exhibited outgrowth and were found to pass through the nerve crush site 14 days after nerve injury. No such regenerating optic axons were observed in the control mouse optic nerve during the same time frame. The data obtained from in vivo experiments strongly indicate that mammalian Ngb has neuroprotective and neuroregenerative properties.

Translational profiling of retinal ganglion cell optic nerve regeneration in Xenopus laevis

Unlike adult mammals, adult frogs regrow their optic nerve following a crush injury, making Xenopus laevis a compelling model for studying the molecular mechanisms that underlie neuronal regeneration. Using Translational Ribosome Affinity Purification (TRAP), a method to isolate ribosome-associated mRNAs from a target cell population, we have generated a transcriptional profile by RNA-Seq for retinal ganglion cells (RGC) during the period of recovery following an optic nerve injury. Based on bioinformatic analysis using the Xenopus laevis 9.1 genome assembly, our results reveal a profound shift in the composition of ribosome-associated mRNAs during the early stages of RGC regeneration. As factors involved in cell signaling are rapidly down-regulated, those involved in protein biosynthesis are up-regulated alongside key initiators of axon development. Using the new genome assembly, we were also able to analyze gene expression profiles of homeologous gene pairs arising from a whole-genome duplication in the Xenopus lineage. Here we see evidence of divergence in regulatory control among a significant proportion of pairs. Our data should provide a valuable resource for identifying genes involved in the regeneration process to target for future functional studies, in both naturally regenerative and non-regenerative vertebrates.

Therapeutic Potential of Genipin in Central Neurodegenerative Diseases

Central neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), are one of the biggest health problems worldwide. Currently, there is no cure for these diseases. The Gardenia jasminoides fruit is a common herbal medicine in traditional Chinese medicine (TCM), and a variety of preparations are used as treatments for central nervous system (CNS) diseases. Pharmacokinetic studies suggest genipin is one of the main effective ingredients of G. jasminoides fruit extract (GFE). Accumulated research data show that genipin possesses a range of key pharmacological properties, such as anti-inflammatory, neuroprotective, neurogenic, antidiabetic, and antidepressant effects. Thus, genipin shows therapeutic potential for central neurodegenerative diseases. We review the pharmacological actions of genipin for the treatment of neurodegenerative diseases of the CNS. We also describe the potential mechanisms underlying these effects.

Gardenamide A Protects RGC-5 Cells from H₂O₂-Induced Oxidative Stress Insults by Activating PI3K/Akt/eNOS Signaling Pathway

Gardenamide A (GA) protects the rat retinal ganglion (RGC-5) cells against cell apoptosis induced by H₂O₂. The protective effect of GA was completely abrogated by the specific phosphoinositide 3-kinase (PI3K) inhibitor LY294002, and the specific protein kinase B (Akt) inhibitor Akt VIII respectively, indicating that the protective mechanism of GA is mediated by the PI3K/Akt signaling pathway. The specific extracellular signal-regulated kinase (ERK1/2) inhibitor PD98059 could not block the neuroprotection of GA. GA attenuated the levels of reactive oxygen species (ROS) and malondialdehyde (MDA) induced by H₂O₂. Western blotting showed that GA promoted the phosphorylation of ERK1/2, Akt and endothelial nitric oxide synthase (eNOS), respectively, and effectively reversed the H₂O₂-inhibited phosphorylation of these three proteins. LY294002 completely inhibited the GA-activated phosphorylation of Akt, while only partially inhibiting eNOS. This evidence implies that eNOS may be activated directly by GA. PD98059 attenuated only partially the GA-induced phosphorylation of ERK1/2 with/without the presence of H₂O₂, indicating that GA may activate ERK1/2 directly. All these results put together confirm that GA protects RGC-5 cells from H₂O₂ insults via the activation of PI3K/Akt/eNOS signaling pathway. Whether the ERK1/2 signaling pathway is involved requires further investigations.

Differential roles of Aβ processing in hypoxia-induced axonal damage

Axonopathy is a common and early phase in neurodegenerative and traumatic CNS diseases. Recent work suggests that amyloid β (Aβ) produced from amyloid precursor protein (APP) may be a critical downstream mediator of CNS axonopathy in CNS diseases, particularly those associated with hypoxia. We critically tested this hypothesis in an adult retinal explant system that preserves the three-dimensional organization of the retina while permitting direct imaging of two cardinal features of early-stage axonopathy: axonal structural integrity and axonal transport capacity. Using this system, we found via pharmacological inhibition and genetic deletion of APP that production of Aβ is a necessary step in structural compromise of retinal ganglion cell (RGC) axons induced by the disease-relevant stressor hypoxia. However, identical blockade of Aβ production was not sufficient to protect axons from associated hypoxia-induced reduction in axonal transport. Thus, Aβ mediates distinct facets of hypoxia-induced axonopathy and may represent a functionally selective pharmacological target for therapies directed against early-stage axonopathy in CNS diseases.

Heat shock protein 70 induction by valproic acid delays photoreceptor cell death by N-methyl-N-nitrosourea in mice

Retinal degenerative diseases (RDs) are a group of inherited diseases characterized by the loss of photoreceptor cells. Selective photoreceptor loss can be induced in mice by an intraperitoneal injection of N-methyl-N-nitrosourea (MNU) and, because of its selectivity, this model is widely used to study the mechanism of RDs. Although it is known that calcium-calpain activation and lipid peroxidation are involved in the initiation of cell death, the precise mechanisms of this process remain unknown. Heat shock protein 70 (HSP70) has been shown to function as a chaperone molecule to protect cells against environmental and physiological stresses. In this study, we investigated the role of HSP70 on photoreceptor cell death in mice. HSP70 induction by valproic acid, a histone deacetylase inhibitor, attenuated the photoreceptor cell death by MNU through inhibition of apoptotic caspase signals. Furthermore, HSP70 itself was rapidly and calpain-dependently cleaved after MNU treatment. Therefore, HSP70 induction by valproic acid was dually effective against MNU-induced photoreceptor cell loss as a result of its anti-apoptotic actions and its ability to prevent HSP70 degradation. These findings might help lead us to a better understanding of the pathogenic mechanism of RDs. Retinal degenerative diseases are characterized by the loss of photoreceptor cells. We proposed the following cascade for N-methyl-N-nitrosourea (MNU)-induced photoreceptor cell death: MNU gives rise to cleavage of heat shock protein 70 (HSP70); HSP70 induction by valproic acid (VPA) is dually effective against MNU-induced photoreceptor cell loss because of its anti-apoptotic actions and its ability to prevent HSP70 degradation. We hope that the present study heralds a new era in developing therapeutic tools against retinal degenerative diseases.

Role of protein S-nitrosylation in central nervous system survival and regeneration

The retina has been regarded as 'an approachable part of the brain' for investigating central nervous system (CNS). The optic nerve injury is a well-accepted model to study the mechanisms of neural degeneration and/or axonal regeneration after trauma in the CNS. Nitric oxide (NO) is a gaseous messenger molecule biosynthesized from L-arginine and molecular oxygen by NO synthase. Many reports suggest that excess production of NO plays a crucial role in neuronal cell death including in death of retinal ganglion cells (RGCs). In contrast, several lines of evidence indicate that NO can prevent neuronal death. In general, NO mediates neuroprotection through two main signaling pathways: the NO/cyclic guanosine monophosphate (cGMP) pathway and the S-nitrosylation pathway. Especially, whether S-nitrosylation of proteins promotes RGCs survival and its axonal regeneration after injury is unclear. Thus, we focused on the S-nitrosylation-dependent mechanism of RGCs survival and axonal regeneration by NO after nerve injury.

Nipradilol promotes axon regeneration through S-nitrosylation of PTEN in retinal ganglion cells

Nipradilol (Nip) is registered as an anti-glaucoma agent. More recently, a protective effect of Nip has been demonstrated in retinal ganglion cells (RGCs) mediated by S-nitrosylation of antioxidative-related Keap1 protein due to its nitric oxide (NO)-donating effect. It also has been reported that Nip promoted axon outgrowth in cat RGCs. However, the detailed mechanism remains unclear. NO physiologically regulates numerous cellular responses through S-nitrosylation of protein at cysteine residues. It has been reported that phosphatase and tensin homologue deleted on chromosome 10 (PTEN) deletion strongly showed axon regeneration after optic nerve injury. PTEN inactivation by S-nitrosylation results in the accumulation of phosphatidylinositol (3, 4, 5) triphosphate (PIP3) and the activation of Akt/mammalian target of rapamycin (mTOR) signaling. The ribosomal S6 kinase 1 (S6K) which can monitor as phospho-S6 (pS6) is one of major target of mTOR. In this study, we investigated the possibility that Nip can promote axon outgrowth in RGCs by Akt/mTOR signaling thorough S-nitrosylation of PTEN.

A molecular mechanism of optic nerve regeneration in fish: the retinoid signaling pathway

The fish optic nerve regeneration process takes more than 100 days after axotomy and comprises four stages: neurite sprouting (1-4 days), axonal elongation (5-30 days), synaptic refinement (35-80 days) and functional recovery (100-120 days). We screened genes specifically upregulated in each stage from axotomized fish retina. The mRNAs for heat shock protein 70 and insulin-like growth factor-1 rapidly increased in the retinal ganglion cells soon after axotomy and function as cell-survival factors. Purpurin mRNA rapidly and transiently increased in the photoreceptors and purpurin protein diffusely increased in all nuclear layers at 1-4 days after injury. The purpurin gene has an active retinol-binding site and a signal peptide. Purpurin with retinol functions as a sprouting factor for thin neurites. This neurite-sprouting effect was closely mimicked by retinoic acid and blocked by its inhibitor. We propose that purpurin works as a retinol transporter to supply retinoic acid to damaged RGCs which in turn activates target genes. We also searched for genes involved in the second stage of regeneration. The mRNA of retinoid-signaling molecules increased in retinal ganglion cells at 7-14 days after injury and tissue transglutaminase and neuronal nitric oxide synthase mRNAs, RA-target genes, increased in retinal ganglion cells at 10-30 days after injury. They function as factors for the outgrowth of thick, long neurites. Here we present a retinoid-signaling hypothesis to explain molecular events during the early stages of optic nerve regeneration in fish.

Requirement of retinoic acid receptor β for genipin derivative-induced optic nerve regeneration in adult rat retina

Like other CNS neurons, mature retinal ganglion cells (RGCs) are unable to regenerate their axons after nerve injury due to a diminished intrinsic regenerative capacity. One of the reasons why they lose the capacity for axon regeneration seems to be associated with a dramatic shift in RGCs’ program of gene expression by epigenetic modulation. We recently reported that (1R)-isoPropyloxygenipin (IPRG001), a genipin derivative, has both neuroprotective and neurite outgrowth activities in murine RGC-5 retinal precursor cells. These effects were both mediated by nitric oxide (NO)/S-nitrosylation signaling. Neuritogenic activity was mediated by S-nitrosylation of histone deacetylase-2 (HDAC2), which subsequently induced retinoic acid receptor β (RARβ) expression via chromatin remodeling in vitro. RARβ plays important roles of neural growth and differentiation in development. However, the role of RARβ expression during adult rat optic nerve regeneration is not clear. In the present study, we extended this hypothesis to examine optic nerve regeneration by IPRG001 in adult rat RGCs in vivo. We found a correlation between RARβ expression and neurite outgrowth with age in the developing rat retina. Moreover, we found that IPRG001 significantly induced RARβ expression in adult rat RGCs through the S-nitrosylation of HDAC2 processing mechanism. Concomitant with RARβ expression, adult rat RGCs displayed a regenerative capacity for optic axons in vivo by IPRG001 treatment. These neuritogenic effects of IPRG001 were specifically suppressed by siRNA for RARβ. Thus, the dual neuroprotective and neuritogenic actions of genipin via S-nitrosylation might offer a powerful therapeutic tool for the treatment of RGC degenerative disorders.

Heparin crosslinked chitosan microspheres for the delivery of neural stem cells and growth factors for central nervous system repair

An effective paradigm for transplanting large numbers of neural stem cells after central nervous system (CNS) injury has yet to be established. Biomaterial scaffolds have shown promise in cell transplantation and in regenerative medicine, but improved scaffolds are needed. In this study we designed and optimized multifunctional and biocompatible chitosan-based films and microspheres for the delivery of neural stem cells and growth factors for CNS injuries. The chitosan microspheres were fabricated by coaxial airflow techniques, with the sphere size controlled by varying the syringe needle gauge and the airflow rate. When applying a coaxial airflow at 30 standard cubic feet per hour, ∼300μm diameter spheres were reproducibly generated that were physically stable yet susceptible to enzymatic degradation. Heparin was covalently crosslinked to the chitosan scaffolds using genipin, which bound fibroblast growth factor-2 (FGF-2) with high affinity while retaining its biological activity. At 1μgml(-1) approximately 80% of the FGF-2 bound to the scaffold. A neural stem cell line, GFP+RG3.6 derived from embryonic rat cortex, was used to evaluate cytocompatibility, attachment and survival on the crosslinked chitosan-heparin complex surfaces. The MTT assay and microscopic analysis revealed that the scaffold containing tethered FGF-2 was superior in sustaining survival and growth of neural stem cells compared to standard culture conditions. Altogether, our results demonstrate that this multifunctional scaffold possesses good cytocompatibility and can be used as a growth factor delivery vehicle while supporting neural stem cell attachment and survival.