Elevation of Intracellular Ca2+ Concentration Induced by Hypoxia in Retinal Ganglion Cells

Inhibition of high-voltage-activated calcium currents by acute hypoxia in cultured retinal ganglion cells

Hypoxia is a common factor of numerous ocular diseases that lead to dysfunctions and loss of retinal ganglion cells (RGCs) with subsequent vision loss. High-voltage-activated calcium channels are the main source of calcium entry into neurons. Their activity plays a central role in different signaling processes in health and diseases, such as enzyme activation, gene transcription, synaptic transmission, or the onset of cell death. This study aims to establish and evaluate the initial effect of the early stage of acute hypoxia on somatic HVA calcium currents in cultured RGCs. HVA calcium currents were recorded in RGCs using the whole-cell patch-clamp technique in the voltage-clamp mode. The fast local superfusion was used for a brief (up to 270 s) application of the hypoxic solution (pO₂ < 5 mmHg). The switch from normoxic to hypoxic solutions and vice versa was less than 1 s. The HVA calcium channel activity was inhibited by acute hypoxia in 79% of RGCs (30 of 38 RGCs) in a strong voltage-dependent manner. The level of inhibition was independent of the duration of hypoxia or repeated applications. The hypoxia-induced inhibition of calcium currents had a strong correlation with the duration of hypoxia and showed the transition from reversible to irreversible at 75 s of hypoxia and longer. The results obtained are the first demonstration of the phenomena of HVA calcium current inhibition by acute hypoxia in RGCs and provide a conceptual framework for further research.

Hypoxia Activates Calpains in the Nerve Fiber Layer of Monkey Retinal Explants

PURPOSE

The vascular ischemic hypothesis attributes nerve damage in the retina to decreased blood flow in the ophthalmic artery, reduced oxygenation, and impaired axonal transport. Activation of calpain enzymes contributes to retinal cell death during hypoxia. However, we still do not know in which specific retinal layers calpains are activated. Thus, the purpose of the present study was to investigate where and when calpains are activated in an improved culture model of hypoxic monkey retina.

METHODS

Monkey retinal explants were cultured on microporous membranes with the retinal ganglion cell (RGC) side facing up. Explants were incubated under hypoxic conditions, with or without additional reoxygenation. When it was used, the calpain inhibitor SNJ-1945 was maintained throughout the culture period. Immunohistochemistry and immunoblotting assays for α-spectrin, calpains 1 and 2, calpastatin, β-III tubulin, and γ-synuclein were performed with specific antibodies. Cell death was assessed by TUNEL staining.

RESULTS

Under normoxic conditions, TUNEL-positive cells were minimal in our improved culture conditions. As early as 8 hours after hypoxia, the 150-kDa calpain-specific α-spectrin breakdown product appeared in the nerve fiber layer (NFL), where calpains 1 and 2 were localized. TUNEL-positive RGCs then increased at later time periods. The calpain inhibitor SNJ-1945 ameliorated changes induced by hypoxia or hypoxia/reoxygenation.

CONCLUSIONS

During hypoxia/reoxygenation in an improved, relevant monkey model, calpains were first activated in the NFL, followed by death of the parent RGCs. This observation suggest that calpain-induced degeneration of retinal nerve fibers may be an underlying mechanism for RGC death in hypoxic retinal neuropathies.

Taurine protects transformed rat retinal ganglion cells from hypoxia-induced apoptosis by preventing mitochondrial dysfunction

Hypoxia-induced apoptosis of retinal ganglion cells (RGCs) is the major cause of progressive vision loss in numerous retinal diseases, including glaucoma and diabetic retinopathy. Taurine is a naturally occurring free amino acid that has been shown to have neurotrophic and neuroprotective properties in the retina. We investigated the specific potential for taurine to be protective for immortalized rat retinal ganglion cells (RGC-5) exposed to hypoxia (5% O(2)). Pretreatment of RGC-5 cells with 0.1 mM taurine significantly reduced the extent of apoptosis detected by DAPI staining, MTT, and Annexin V-FITC/PI assays. To further study the mechanism underlying the beneficial effect of taurine, interactions between taurine and the process of mitochondria-mediated apoptosis were examined. Taurine treatment of RGC-5 cells suppressed the induction of the mitochondrial permeability transition (mPT) by reducing intracellular calcium levels and inhibiting the opening of mitochondrial permeability transition pores (mPTPs). Moreover, the loss of mitochondrial membrane potential, a decline in cellular ATP levels, a reduction in the amount of cytochrome c translocated to the cytoplasm and caspase-3 activation were observed in taurine-treated cultures. These results demonstrate the potential for taurine to protect RGCs against hypoxic damage in vivo by preventing mitochondrial dysfunction.