Ordering of neuronal apoptosis signaling: a superoxide burst precedes mitochondrial cytochrome c release in a growth factor deprivation model

Cobalamin-Associated Superoxide Scavenging in Neuronal Cells Is a Potential Mechanism for Vitamin B12-Deprivation Optic Neuropathy

Chronic deficiency of vitamin B₁₂ is the only nutritional deficiency definitively proved to cause optic neuropathy and loss of vision. The mechanism by which this occurs is unknown. Optic neuropathies are associated with death of retinal ganglion cells (RGCs), neurons that project their axons along the optic nerve to the brain. Injury to RGC axons causes a burst of intracellular superoxide, which then signals RGC apoptosis. Vitamin B₁₂ (cobalamin) was recently shown to be a superoxide scavenger, with a rate constant similar to superoxide dismutase. Given that vitamin B₁₂ deficiency causes an optic neuropathy through unknown mechanisms and that it is a potent superoxide scavenger, we tested whether cobalamin, a vitamin B₁₂ vitamer, would be neuroprotective in vitro and in vivo. We found that cobalamin scavenged superoxide in neuronal cells in vitro treated with the reduction-oxidation cycling agent menadione. In vivo confocal scanning laser ophthalmoscopy demonstrated that optic nerve transection in Long-Evans rats increased superoxide levels in RGCs. The RGC superoxide burst was significantly reduced by intravitreal cobalamin and resulted in increased RGC survival. These data demonstrate that cobalamin may function as an endogenous neuroprotectant for RGCs through a superoxide-associated mechanism.

Intracellular disulfide reduction by phosphine-borane complexes: Mechanism of action for neuroprotection

Phosphine-borane complexes are novel cell-permeable drugs that protect neurons from axonal injury in vitro and in vivo. These drugs activate the extracellular signal-regulated kinases 1/2 (ERK1/2) cell survival pathway and are therefore neuroprotective, but do not scavenge superoxide. In order to understand the interaction between superoxide signaling of neuronal death and the action of phosphine-borane complexes, their biochemical activity in cell-free and in vitro assays was studied by electron paramagnetic resonance (EPR) spectrometry and using an intracellular dithiol reporter that becomes fluorescent when its disulfide bond is cleaved. These studies demonstrated that bis(3-propionic acid methyl ester) phenylphosphine-borane complex (PB1) and (3-propionic acid methyl ester) diphenylphosphine-borane complex (PB2) are potent intracellular disulfide reducing agents which are cell permeable. EPR and pharmacological studies demonstrated reducing activity but not scavenging of superoxide. Given that phosphine-borane complexes reduce cell injury from mitochondrial superoxide generation but do not scavenge superoxide, this implies a mechanism where an intracellular superoxide burst induces downstream formation of protein disulfides. The redox-dependent cleavage of the disulfides is therefore a novel mechanism of neuroprotection.

Superoxide generation explains common features of optic neuropathies associated with cecocentral scotomas

I have presented above a hypothesis that ties together several disparate optic neuropathies, all characterized by a similar clinical presentation. The hypothesis is predicated on the formation of intracellular superoxide within RGCs as a common pathological pathway for the type of cell death that occurs. The anatomical predisposition of the papillomacular bundle to have elevated superoxide levels is tied to the size of the fibers involved, a hypothesis that also implicates the crossing fibers of the chiasm. Much of this work is speculative and is an interpretation of several experimental studies that have been performed to date. Hopefully, this hypothesis will be developed further, and its validity tested in both experimental models and, ultimately, in humans.

Changes in parvalbumin immunoreactive retinal ganglion cells and amacrine cells after optic nerve injury

Parvalbumin (PARV) is a Ca(2+)-binding protein that may offer resistance to cell death as it primarily functions to maintain Ca(2+) homeostasis. The purpose of this study was to investigate whether PARV expressing retinal ganglion cells (RGCs) would be more resistant to cell death than RGCs that do not express PARV. RGCs in Sprague-Dawley rats were retrogradely labeled with Fluorogold (FG). After 2-28 days following an optic nerve crush (ONC) injury immunohistochemistry was performed on the sections using antibodies against PARV and markers of RGCs. The proportion of retinal ganglion cell layer cells labeled with PARV colocalized with FG or Brn3a and labeled only with PARV (displaced amacrine cells; dACs) were analyzed. PARV staining intensity was measured in ACs, dACs, and RGCs. Double labeling studies revealed that 49% of RGCs and 22% of dACs expressed PARV. There was an immediate reduction in RGC PARV staining after ONC but the overall rate of cell death after 28 days was similar in PARV and non-PARV expressing RGCs. There was no change in PARV AC or dAC number or staining intensity. Although this study suggests that there is no selective survival of the subpopulation of RGCs that contain PARV, there is down-regulation of PARV expression by these RGCs. This suggests that down-regulation of PARV may contribute to RGC death due to a compromised Ca(2+) buffering capacity. Maintaining PARV expression after injury could be an important neuroprotective strategy to improve RGC survival after injury.

The Neuroprotective Effect of Tetramethylpyrazine Against Contusive Spinal Cord Injury by Activating PGC-1α in Rats

Tetramethylpyrazine (TMP) has been suggested to have neuroprotective effects against spinal cord injury (SCI); however, few studies have examined these effects and the corresponding mechanism. Therefore, the present study aimed to investigate the neuroprotective effect and underlying mechanism of TMP against contusive SCI. Adult male Sprague-Dawley rats were randomly divided into Sham, normal saline (NS) and TMP groups. Each group was divided into subgroups according to the time of sacrifice: 1, 3, 7, 14, 21 and 28 days post-injury. Laminectomy was performed in all groups, followed by contusive SCI establishment in the TMP and NS groups. TMP (80 mg/kg) was injected thereafter daily from 3 to 7 days post-injury in the TMP group, which was replaced by equal volume of normal saline in the NS group. The Basso-Beattie-Bresnahan (BBB) Locomotor Rating Scale was measured at different time points post-injury to appraise locomotor functional recovery. Quantitative real-time PCR and immunofluorescence were used to assess the spatio-temporal expression of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), while western blot was adopted to detect the effect of TMP on PGC-1α. Neural apoptotic changes and neuronal survival were evaluated using the TUNEL method and Nissl staining, respectively. TMP treatment markedly increased PGC-1α expression, neuronal survival and BBB locomotor scores, while also reducing neural apoptosis. These results demonstrate that TMP is neuroprotective against contusive SCI, with the inhibition of neural apoptosis and increase of neuronal survival. The sustained expression of PGC-1α may partially contribute to the TMP-mediated neuroprotective effect.

Retinal ganglion cell neuroprotection by an angiotensin II blocker in an ex vivo retinal explant model

PURPOSE

An ex vivo organotypic retinal explant model was developed to examine retinal survival mechanisms relevant to glaucoma mediated by the renin angiotensin system in the rodent eye.

METHODS

Eyes from adult Sprague Dawley rats were enucleated immediately post-mortem and used to make four retinal explants per eye. Explants were treated either with irbesartan (10 µM), vehicle or angiotensin II (2 μM) for four days. Retinal ganglion cell density was estimated by βIII tubulin immunohistochemistry. Live imaging of superoxide formation with dihydroethidium (DHE) was performed. Protein expression was determined by Western blotting, and mRNA expression was determined by RT-PCR.

RESULTS

Irbesartan (10 µM) almost doubled ganglion cell survival after four days. Angiotensin II (2 µM) reduced cell survival by 40%. Sholl analysis suggested that irbesartan improved ganglion cell dendritic arborisation compared to control and angiotensin II reduced it. Angiotensin-treated explants showed an intense DHE fluorescence not seen in irbesartan-treated explants. Analysis of protein and mRNA expression determined that the angiotensin II receptor At1R was implicated in modulation of the NADPH-dependent pathway of superoxide generation.

CONCLUSION

Angiotensin II blockers protect retinal ganglion cells in this model and may be worth further investigation as a neuroprotective treatment in models of eye disease.

Expression of classical components of the renin-angiotensin system in the human eye

PURPOSE

The purpose of this study was to determine the relative expression of clinically-relevant components of the renin-angiotensin system (RAS) in the adult human eye.

METHODS

We obtained 14 post-mortem enucleated human eyes from patients whom had no history of inflammatory ocular disease nor pre-mortem ocular infection. We determined the gene expression for prorenin, renin, prorenin receptor, angiotensin-converting enzyme, angiotensinogen and angiotensin II Type 1 receptor, on tissue sections and in cultured human primary retinal pigment epithelial and iris pigment epithelial (RPE/IPE) cell lines, using both qualitative and quantitative reverse transcription polymerase chain reaction (RT-PCR). Protein expression was studied using indirect immunofluorescence (IF).

RESULTS

Almost all components of the classical RAS were found at high levels, at both the transcript and protein level, in the eyes' uvea and retina; and at lower levels in the cornea, conjunctiva and sclera. There was a much lower level of expression in the reference cultured RPE/IPE cells lines.

CONCLUSION

This study describes the distribution of RAS in the normal adult human eye and demonstrates the existence of an independent ocular RAS, with uveal and retinal tissues showing the highest expression of RAS components. These preliminary findings provide scope for examination of additional components of this system in the human eye, as well as possible differential expression under pathological conditions.

Neurite Outgrowth and Neuroprotective Effects of Quercetin from Caesalpinia mimosoides Lamk. on Cultured P19-Derived Neurons

Quercetin has been isolated for the first time from ethyl acetate extract of Caesalpinia mimosoides Lamk. C. mimosoides Lamk. (Fabaceae) or Cha rueat (Thai name) is an indigenous plant found in mixed deciduous forest in northern and north-eastern parts of Thailand. Thai rural people consume its young shoots and leaves as a fresh vegetable, as well as it is used for medicinal purposes.The antioxidant capacity in terms of radical scavenging activity of quercetin was determined as IC₅₀ of 3.18 ± 0.07 µg/mL, which was higher than that of Trolox and ascorbic acid (12.54 ± 0.89 and 10.52 ± 0.48 µg/mL, resp.). The suppressive effect of quercetin on both purified and cellular acetylcholinesterase (AChE) enzymes was investigated as IC₅₀ 56.84 ± 2.64 and 36.60 ± 2.78 µg/mL, respectively. In order to further investigate the protective ability of quercetin on neuronal cells, P19-derived neurons were used as a neuronal model in this study. As a result, quercetin at a very low dose of 1 nM enhanced survival and induced neurite outgrowth of P19-derived neurons. Furthermore, this flavonoid also possessed significant protection against oxidative stress induced by serum deprivation. Altogether, these findings suggest that quercetin is a multifunctional compound and promising valuable drugs candidate for the treatment of neurodegenerative disease.