Retinal ischemia
The retina has a high metabolic demand and responds very sensitively to an impaired blood flow (ischemia) and thus an under-supply of nutrients [1, 2]. Therefore, ischemic processes play an important role in the pathophysiology of various eye diseases. These include glaucoma, ocular vascular occlusion and diabetic retinopathy [3]. The blood flow disorder in the retina leads to lower oxygen supply of the tissue, arising cell death, as well as functional and morphological changes in several retinal layers. The subsequent reperfusion of the tissue initiates an increased present of oxygen and associated aggressive oxygen metabolites. These cause toxic effects on neuronal cells [4, 5]. Our research group is working with the ischemia/reperfusion model (I/R-model) to investigate the effects of ischemia on neural retinal cells as well as on the optic nerve (ON) and to understand the molecular mechanism behind this process.
While other studies have shown cell loss at an early point of time [6, 7], previous studies by our group could reveal that retinal ischemia leads also at a late point of time to a loss and damage of different retinal cell types [8]. A loss of retinal ganglion cells by apoptosis (Fig. 1), and a reduced number of amacrine cells could be detected (Fig. 2).
Loss of retinal ganglion cells
Loss of retinal ganglion cells (RGCs). A) Immunohistochemical staining of retinal cross-sections with Brn-3a (RGCs, green) und DAPI (cell nuclei, blue) 21 days after I/R. Fewer Brn-3a+ RGCs were observed in ischemic retinas. B) RGC numbers were significantly reduced in the ischemic eyes compared to controls (p=0.03). *: p<0,05; GCL: retinal ganglion cell layer, INL: inner nuclear layer, ONL: outer nuclear layer; scale bar: 20 µm (Schmid et al, 2104).
Analysis of ChAT-positive amacrine cells
A) ChAT staining of amacrine cells (red) in control and ischemic eyes 21 days after ischemia (DAPI: cell nuclei, blue). Cell bodies and stratification were present in control, but absent in ischemic eyes. B) The analysis showed a significant loss of ChAT+ cells in ischemic retinas. ***: p<0.001; GCL: retinal ganglion cell layer, INL: inner nuclear layer, ONL: outer nuclear layer; scale bar: 20 µm (Schmid et al, 2104).
In addition, an increased number of inflammatory microglia was noted due to the strong tissue damage. Furthermore, a functional loss of the inner retinal cell layers could be detected by ERG measurements (Fig. 3).
Due to the massive retinal damage caused by the ischemic processes, we suppose that the optic nerve is also affected. However, so far, little is known about the severity of ischemia/reperfusion on the optic nerve. Some research groups could observe functional and structural changes in the optic nerve following transient ischemia [9-12]. Our group was able to demonstrate severe tissue damage in the optic nerve after retinal ischemia/reperfusion. In previous studies we noted cell infiltration, demyelination and immigration as well as activation of immune effector cells. Further investigations have to be done to clarify the ischemic effects on the optic nerve and to understand the mechanisms of ischemic processes.
These findings should help to determine new therapies for retinal diseases.
ERG measurements
A) Representative ERG recording at 3 cd*s/m² of a control eye (grey) and a contralateral ischemic eye (black). The arrow represents the start of the light stimulus. A reduction of the a- and b-wave can be noted in the ischemic eye. B) Changes in the amplitude of the a-wave of ischemic eyes at light intensities ranging from 0.1-25 cd*s/m². For 0.1-10 cd*s/m² the amplitude of the a-wave was significantly reduced in the ischemic eyes compared to control. C) Changes of the b-wave amplitude of control and ischemic eyes at 0.1-25 cd*s/m² light intensities. The amplitude of the b-wave was significantly reduced in the ischemic eyes at all intensities. D) Changes in the a-wave latencies of control and ischemic eyes. No differences between the two groups were observed. E) Response latency in the b-wave in control and ischemic eyes. At 0.1 cd*s/m² a significant reduction of the b-wave latency was noted. *: p<0.05; **: p<0.01; ***: p<0.001 (Schmid et al, 2104).
Publications
Joachim SC, Wax MB, Boehm N, Dirk DR, Pfeiffer N,. Grus FH (2011). "Upregulation of antibody response to heat shock proteins and tissue antigens in an ocular ischemia model." Invest Ophthalmol Vis Sci 52(6): 3468-3474.
Joachim SC, Jehle T, Boehm N, Gramlich OW, Lagreze WA, Pfeiffer N, Grus FH (2012). "Effect of Ischemia Duration on Autoantibody Response in Rats Undergoing Retinal Ischemia-Reperfusion." Ophthalmic Res 48(2): 67-74.
Schmid H, Renner M, Dick HB, Joachim SC (2014). "Loss of inner retinal neurons after retinal ischemia in rats." Investigative ophthalmology & visual science 55(4): 2777-2787.
Current conference contributions
Schmid H. Renner M, Dick HB, Joachim SC (2014). "Loss of cholinergic amacrine cells in an ischemia-reperfusion animal model." Invest. Ophthalmol. Vis. Sci. 55(5): 1907
Renner M, Stute G, Schmid H, Horstmann H, Dick HB, Joachim SC (2015). "Optic nerve degeneration after retinal ischemia-reperfusion in a rodent model." Investigative Ophthalmology & Visual Science 56(7): 22.
Joachim SC, Renner M, Reinehr S, Stute G, Theiss C, Dick HB (2015). "Ranibizumab treatment protects retinal cells in an ischemia model." Investigative Ophthalmology & Visual Science 56(7): 2478-2478.
Literature
1. Minhas, G., R. Morishita, and A. Anand, Preclinical models to investigate retinal ischemia: advances and drawbacks. Front Neurol, 2012. 3: p. 75.
2. Kaur, C., W.S. Foulds, and E.A. Ling, Hypoxia-ischemia and retinal ganglion cell damage. Clin Ophthalmol, 2008. 2(4): p. 879-89.
3. Osborne, N.N., et al., Retinal ischemia: mechanisms of damage and potential therapeutic strategies. Prog Retin Eye Res, 2004. 23(1): p. 91-147.
4. Belforte, N., et al., Ischemic tolerance protects the rat retina from glaucomatous damage. PLoS One, 2011. 6(8): p. e23763.
5. Joachim, S.C., et al., Effect of ischemia duration on autoantibody response in rats undergoing retinal ischemia-reperfusion. Ophthalmic Res, 2012. 48(2): p. 67-74.
6. Lam, T.T., A.S. Abler, and M.O. Tso, Apoptosis and caspases after ischemia-reperfusion injury in rat retina. Invest Ophthalmol Vis Sci, 1999. 40(5): p. 967-75.
7. Zheng, G.Y., C. Zhang, and Z.G. Li, Early activation of caspase-1 after retinal ischemia and reperfusion injury in mice. Chin Med J (Engl), 2004. 117(5): p. 717-21.
8. Schmid, H., et al., Loss of inner retinal neurons after retinal ischemia in rats. Invest Ophthalmol Vis Sci, 2014. 55(4): p. 2777-87.
9. Adachi, M., et al., High intraocular pressure-induced ischemia and reperfusion injury in the optic nerve and retina in rats. Graefes Arch Clin Exp Ophthalmol, 1996. 234(7): p. 445-51.
10. Grozdanic, S.D., et al., Functional characterization of retina and optic nerve after acute ocular ischemia in rats. Invest Ophthalmol Vis Sci, 2003. 44(6): p. 2597-605.
11. Joachim, S.C., et al., Upregulation of antibody response to heat shock proteins and tissue antigens in an ocular ischemia model. Invest Ophthalmol Vis Sci, 2011. 52(6): p. 3468-74.
12. Wang, Q., et al., Diffusion tensor imaging detected optic nerve injury correlates with decreased compound action potentials after murine retinal ischemia. Invest Ophthalmol Vis Sci, 2012. 53(1): p. 136-42.