Retinal ischemia

D1 (10,17S-docosatriene) from DHA using tandem liquid chromatography-photodiode array-electrospray ionization-tandem mass spectrometry (LC-PDA-ESI-MS-MS)-based lipidomic analysis have been documented in ischemic brain [4] and retinal pigment epithelium [5], This new lipid is called neuroprotectin D1 (1) because of its neuro-protectiveproperties in brain ischemia-reperfusion [4] and in oxidative stress-challenged retinal pigment epithelial cells [5] (2) because of its potent ability to inactivate proapoptotic signaling (see apoptosis, Ch. 35) [5] and (3) because it is the first identified neuroprotective mediator derived from DHA. 

Rieger JM, Shah AR, Gidday JM (2002) Ischemia-reperfusion injury of retinal endothelium by cyclooxygenase- and xanthine oxidase-derived superoxide. Exp. Eye Res. 74 493-501. 

Retinal ischemia, lipid hydroperoxides, 612 Rhenium catalysts... 

Condeelis J (1993) Life at the leading edge the formation of cell protrusions. Annu Rev Cell Biol 9 411 44 Da T, Verkman AS (2004) Aquaporin-4 gene disruption in mice protects against impaired retinal function and cell death after ischemia. Invest Ophthalmol Vis Sci 45 4477 483 Deen PM, Verdijk MA, Knoers NV, Wieringa B, Monnens LA, van Os CH, van Oost BA (1994) Requirement of human renal water channel aquaporin-2 for vasopressin-dependent concentration of urine. Science 264 92-95... 

III. Blockade of Excitotoxicity Sustains the PI3-K/Akt Prosurvival Pathway in Retinal Ischemia... 

Several evidence underlie the crucial role of excessive glutamate release under ischemic conditions in the brain (Aarts el al., 2003 Camacho and Massieu, 2006 Dirnagl et al., 1999) and, concurrently, experimental data sustain a comparable role for glutamate under retinal ischemia (Adachi et al., 1998 Louzada-Junior et al., 1992). However, it should be stressed that the mechanisms underlying tolerance to ischemia may differ in brain and in the retina, the latter being more tolerant to ischemia than the former (Iijima et al., 2000 Osborne et al., 2004). 

Accumulation of extracellular glutamate during retinal ischemia was first described by Louzada-Junior and collaborators in rabbit (Louzada-Junior et al., 1992) and, subsequently, corroborated by data obtained by Adachi et al. (1998) in cat. 

Fig. 1. Representative fluorescent photomicrographs of retinal whole mounts showing the loss of Fluorogold (FG)-labeled RGCs in ischemic retina of rat. RGCs were retrogradely labeled with the fluorescent dye FG injected, under stereotaxic guidance, bilaterally into the superior colliculus of a rat 4 days after 50 min ischemia and sacrificed after additional 4 days. Obvious reduction of FG-labeled RGCs is evident in the retina undergone ischemia/reperfusion (panel B) as compared to the contralateral, nonischemic, retina (panel A). Photomicrographs were obtained from the peripheral area of the superior quadrant of the retina. Scale bar 50 fim.

Fig. 2. Transient retinal ischemia increases intravitreal glutamate in rat. Neurochemical data from microdialysis experiments carried out in anesthetized rats to demonstrate that ischemia/reperfusion insult increases intravitreal glutamate. The extracellular level of glutamate shows a moderate increase during the first 10 min of ischemia, more evident toward the end of the ischemic period, to reach statistical significance at 10 and 150 min of reperfusion. Baseline glutamate concentrations (basal values) are the mean concentrations obtained by averaging six samples collected consecutively at 10 min intervals immediately before the onset of ischemia (n — 6 rats). Glutamate values (pM) are expressed as mean S.E.M. Statistical significance was assessed by ANOVA followed by Dunnett s test for multiple comparisons. P < 0.05 and P < 0.001 versus basal values.

The central role played by GLAST in the prevention of glutamate neurotoxicity after ischemia is further supported by the observation that GLAST deficient mice show more severe damage after pressure-induced retinal ischemia compared to GLT-1 deficient and wild-type animals (Harada et al., 1998). 

It has been recendy demonstrated that transient retinal ischemia leads to the formation of free radicals and lipid peroxidation, and administration of free radical scavengers or metal chelators prevents the associated RGCs death (Banin et al., 2000 Celebi et al., 2002 Shibuki et al., 2000). 

Activation of calpain is usually associated with the progression of a necrotic type of cell death (Wang, 2000). However, neuronal necrosis and apoptosis occur in parallel after ischemic injury in vitro and in vivo (Charriaut-Marlangue et al., 1996). Retinal ischemia causes precocious necrosis of neurons in the ganglion cell layer (GCL) and INL, whereas apoptosis appears as the delayed component of neuronal death associated with transient retinal ischemia (Joo et al., 1999). 

Morphological features of apoptosis following retinal ischemia, such as DNA fragmentation, nuclear condensation, and chromatin marginalization have been... 

Nakazawa et al. (2005) have reported a causative role for Akt deactivation in RGC death showing the mutual exclusion between TUNEL positive and Akt-activated positive cells. Therefore, the transient deactivation of Akt after retinal ischemia, that is coincident with the activation of the proapoptotic target protein GSK-3/3 (Russo et al., 2008a), may represent one of the early events triggering the death machinery. 

Treatment with the PI3-K inhibitor wortmannin reduces the number of surviving RGCs after retinal ischemia/reperfusion suggesting that, Akt activation is indeed endowed with RGCs neuroprotective properties and represents a prosurvival response of the retina to the ischemic injury (Russo et al., 2008a). 

Fig. 3. Changes in Akt and GSK-3/3 phosphorylation induced by transient retinal ischemia and effect ofintravitreal application of MK801 in rat. Animals were subjected to retinal ischemia for 50 min in the right eye (R) and reperfusion was allowed for 1, 6, or 24 h. The left eye (L) was used as control. (A) Phosphorylation of Akt on Ser473 is significantly diminished after retinal ischemia and is accompanied by a transient dephosphorylation (activation) of GSK-3/3. During the reperfusion phase, Akt activation is increased within 1 h whereas GSK-3(3 phosphorylation status returns to basal level. (B) Intravitreal treatment with MK801 enhances the phosphorylation of Akt reported after I h reperfusion. Histograms show the results of the densitometric analysis of immunoreactive bands. P < 0.05 versus vehicle.

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