Ocular neuroprotection by siRNA targeting caspase-2

RNA interference--a silent but an efficient therapeutic tool

RNA interference (RNAi) is an evolutionary conserved gene regulation pathway that has emerged as an important discovery in the field of molecular biology. One of the important advantages of RNAi in therapy is that it brings about efficient downregulation of gene expression by targeting complementary transcripts in comparison with other antisense-based techniques. RNAi can be can be achieved by introducing chemically synthesized small interfering RNAs (siRNAs) into a cell system. A more stable knockdown effect can be brought about by the use of plasmid or viral vectors encoding the siRNA. RNAi has been used in reverse genetics to understand the function of specific genes and also as a therapeutic tool in treating human diseases. This review provides a brief insight into the therapeutic applications of RNAi against debilitating diseases.

What can we learn about stroke from retinal ischemia models?

Retinal ischemia is a very useful model to study the impact of various cell death pathways, such as apoptosis and necrosis, in the ischemic retina. However, it is important to note that the retina is formed as an outpouching of the diencephalon and is part of the central nervous system. As such, the cell death pathways initiated in response to ischemic damage in the retina reflect those found in other areas of the central nervous system undergoing similar trauma. The retina is also more accessible than other areas of the central nervous system, thus making it a simpler model to work with and study. By utilizing the retinal model, we can greatly increase our knowledge of the cell death processes initiated by ischemia which lead to degeneration in the central nervous system. This paper examines work that has been done so far to characterize various aspects of cell death in the retinal ischemia model, such as various pathways which are activated, and the role neurotrophic factors, and discusses how these are relevant to the treatment of ischemic damage in both the retina and the greater central nervous system.

Pharmacological inhibition of caspase-2 protects axotomised retinal ganglion cells from apoptosis in adult rats

Severing the axons of retinal ganglion cells (RGC) by crushing the optic nerve (ONC) causes the majority of RGC to degenerate and die, primarily by apoptosis. We showed recently that after ONC in adult rats, caspase-2 activation occurred specifically in RGC while no localisation of caspase-3 was observed in ganglion cells but in cells of the inner nuclear layer. We further showed that inhibition of caspase-2 using a single injection of stably modified siRNA to caspase-2 protected almost all RGC from death at 7 days, offering significant protection for up to 1 month after ONC. In the present study, we confirmed that cleaved caspase-2 was localised and activated in RGC (and occasional neurons in the inner nuclear layer), while TUNEL⁺ RGC were also observed after ONC. We then investigated if suppression of caspase-2 using serial intravitreal injections of the pharmacological inhibitor z-VDVAD-fmk (z-VDVAD) protected RGC from death for 15 days after ONC. Treatment of eyes with z-VDVAD suppressed cleaved caspase-2 activation by >85% at 3-4 days after ONC. Increasing concentrations of z-VDVAD protected greater numbers of RGC from death at 15 days after ONC, up to a maximum of 60% using 4000 ng/ml of z-VDVAD, compared to PBS treated controls. The 15-day treatment with 4000 ng/ml of z-VDVAD after ONC suppressed levels of cleaved caspase-2 but no significant changes in levels of cleaved caspase-3, -6, -7 or -8 were detected. Although suppression of caspase-2 protected 60% of RGC from death, RGC axon regeneration was not promoted. These results suggest that caspase-2 specifically mediates death of RGC after ONC and that suppression of caspase-2 may be a useful therapeutic strategy to enhance RGC survival not only after axotomy but also in diseases where RGC death occurs such as glaucoma and optic neuritis.

Evaluation of presumptive biomarkers of oxidative stress, immune response and apoptosis in primary open-angle glaucoma

There is growing interest on the correlation among oxidative stress, inflammation, apoptosis and primary open-angle glaucoma initiation and progression. Reactive oxygen species are formed in the eyes following a wide variety of stressors, and are largely implicated in glaucoma pathogenesis. Immune-inflammatory response mediators have recently become a target of ophthalmologic concern, including glaucoma. Much attention has been derived to the role of specific pro and anti-apoptotic molecules in glaucoma. This article reviews the early evidence suggesting that reactive oxygen species, immune inflammatory response mediators, and apoptogenic molecules are engaged in glaucoma disease. Moreover, further research concerning the functions, effectors and signaling pathways of the above molecules and their interactions, may lead to specifically develop targeted screening tools based on presumptive biomarkers and surrogate endpoints against primary open-angle glaucoma progression and blindness.

Stimulating axonal regeneration of mature retinal ganglion cells and overcoming inhibitory signaling

Like other neurons of the central nervous system (CNS), retinal ganglion cells (RGCs) are normally unable to regenerate injured axons and instead undergo apoptotic cell death. This regenerative failure leads to lifelong visual deficits after optic nerve damage and is partially attributable to factors located in the inhibitory environment of the forming glial scar and myelin as well as to an insufficient intrinsic ability for axonal regrowth. In addition to its ophthalmological relevance, the optic nerve has long been used as a favorable paradigm for studying regenerative failure in the CNS as a whole. Findings over the last 15 years have shown that, under certain circumstances, mature RGCs can be transformed into an active regenerative state enabling these neurons to survive axotomy and to regenerate axons in the optic nerve. Moreover, combinatorial treatments overcoming the inhibitory environment of the glial scar and optic nerve myelin, together with approaches activating the intrinsic growth program, can further enhance the amount of regeneration in vivo. These findings are encouraging and open the possibility that clinically meaningful regenerationmay become achievable in the future.

Promoting optic nerve regeneration

Vision is the most important sense for humans and it is irreversibly impaired by axonal damage of retinal ganglion cells (RGCs) in the optic nerve due to the lack of axonal regeneration. The failure of regeneration is partially attributable to factors located in the inhibitory environment of the forming glial scar and myelin as well as an insufficient intrinsic ability for axonal regrowth. Moreover, RGCs undergo apoptotic cell death after optic nerve injury, eliminating any chance for regeneration. In this review, we discuss the different aspects that cause regenerative failure in the optic nerve. Moreover, we describe discoveries of the last two decades demonstrating that under certain circumstances mature RGCs can be transformed into an active regenerative state allowing these neurons to survive axotomy and to regenerate axons in the injured optic nerve. In this context we focus on the role of the cytokines ciliary neutrophic factor (CNTF) and leukemia inhibitory factor (LIF), their receptors and the downstream signaling pathways. Furthermore, we discuss strategies to overcome inhibitory signaling induced by molecules associated with optic nerve myelin and the glial scar as well as the regenerative outcome after combinatorial treatments. These findings are encouraging and may open the possibility that clinically meaningful regeneration may become achievable one day in the future.

Divergent modulation of neuronal differentiation by caspase-2 and -9

Human Ntera2/cl.D1 (NT2) cells treated with retinoic acid (RA) differentiate towards a well characterized neuronal phenotype sharing many features with human fetal neurons. In view of the emerging role of caspases in murine stem cell/neural precursor differentiation, caspases activity was evaluated during RA differentiation. Caspase-2, -3 and -9 activity was transiently and selectively increased in differentiating and non-apoptotic NT2-cells. SiRNA-mediated selective silencing of either caspase-2 (si-Casp2) or -9 (si-Casp9) was implemented in order to dissect the role of distinct caspases. The RA-induced expression of neuronal markers, i.e. neural cell adhesion molecule (NCAM), microtubule associated protein-2 (MAP2) and tyrosine hydroxylase (TH) mRNAs and proteins, was decreased in si-Casp9, but markedly increased in si-Casp2 cells. During RA-induced NT2 differentiation, the class III histone deacetylase Sirt1, a putative caspase substrate implicated in the regulation of the proneural bHLH MASH1 gene expression, was cleaved to a ∼100 kDa fragment. Sirt1 cleavage was markedly reduced in si-Casp9 cells, even though caspase-3 was normally activated, but was not affected (still cleaved) in si-Casp2 cells, despite a marked reduction of caspase-3 activity. The expression of MASH1 mRNA was higher and occurred earlier in si-Casp2 cells, while was reduced at early time points during differentiation in si-Casp9 cells. Thus, caspase-2 and -9 may perform opposite functions during RA-induced NT2 neuronal differentiation. While caspase-9 activation is relevant for proper neuronal differentiation, likely through the fine tuning of Sirt1 function, caspase-2 activation appears to hinder the RA-induced neuronal differentiation of NT2 cells.

Combined effect of hypothermia and caspase-2 gene deficiency on neonatal hypoxic-ischemic brain injury

INTRODUCTION

[corrected] Hypoxia-ischemia (HI) injury in term infants develops with a delay during the recovery phase, opening up a therapeutic window after the insult. Hypothermia is currently an established neuroprotective treatment in newborns with neonatal encephalopathy (NE), saving one in nine infants from developing neurological deficits. Caspase-2 is an initiator caspase, a key enzyme in the route to destruction and, therefore, theoretically a potential target for a pharmaceutical strategy to prevent HI brain damage.

METHODS

The aim of this study was to explore the neuroprotective efficacy of hypothermia in combination with caspase-2 gene deficiency using the neonatal Rice-Vannucci model of HI injury in mice.

RESULTS

HI brain injury was moderately reduced in caspase-2(-/-) mice as compared with wild-type (WT) mice. Five hours of hypothermia (33 °C ) vs. normothermia (36 °C) directly after HI provided additive protection overall (temperature P = 0.0004, caspase-2 genotype P = 0.0029), in the hippocampus and thalamus, but not in other gray matter regions or white matter. Delayed hypothermia initiated 2 h after HI in combination with caspase-2 gene deficiency reduced injury in the hippocampus, but not in other brain areas.

DISCUSSION

In conclusion, caspase-2 gene deficiency combined with hypothermia provided enhanced neuroprotection as compared with hypothermia alone.

Gene therapy for retinal ganglion cell neuroprotection in glaucoma

Glaucoma is the leading cause of irreversible blindness worldwide. The primary cause of glaucoma is not known, but several risk factors have been identified, including elevated intraocular pressure and age. Loss of vision in glaucoma is caused by the death of retinal ganglion cells (RGCs), the neurons that convey visual information from the retina to the brain. Therapeutic strategies aimed at delaying or halting RGC loss, known as neuroprotection, would be valuable to save vision in glaucoma. In this review, we discuss the significant progress that has been made in the use of gene therapy to understand mechanisms underlying RGC degeneration and to promote the survival of these neurons in experimental models of optic nerve injury.