Shared and Differential Retinal Responses against Optic Nerve Injury and Ocular Hypertension

Abstract

Glaucoma, one of the leading causes of blindness worldwide, affects primarily retinal ganglion cells (RGCs) and their axons. The pathophysiology of glaucoma is not fully understood, but it is currently believed that damage to RGC axons at the optic nerve head plays a major role. Rodent models to study glaucoma include those that mimic either ocular hypertension or optic nerve injury. Here we review the anatomical loss of the general population of RGCs (that express Brn3a; Brn3a⁺RGCs) and of the intrinsically photosensitive RGCs (that express melanopsin; m⁺RGCs) after chronic (LP-OHT) or acute (A-OHT) ocular hypertension and after complete intraorbital optic nerve transection (ONT) or crush (ONC). Our studies show that all of these insults trigger RGC death. Compared to Brn3a⁺RGCs, m⁺RGCs are more resilient to ONT, ONC, and A-OHT but not to LP-OHT. There are differences in the course of RGC loss both between these RGC types and among injuries. An important difference between the damage caused by ocular hypertension or optic nerve injury appears in the outer retina. Both axotomy and LP-OHT induce selective loss of RGCs but LP-OHT also induces a protracted loss of cone photoreceptors. This review outlines our current understanding of the anatomical changes occurring in rodent models of glaucoma and discusses the advantages of each one and their translational value.

Keywords: Brn3a retinal ganglion cells; acute ocular hypertension; axotomy; chronic ocular hypertension; cone photoreceptors; glaucoma; melanopsin retinal ganglion cells; retinal nerve fiber layer.

Figure 1

Temporal course of RGC loss after axotomy or ocular hypertension. (A) Graph showing the loss of Brn3a+RGCs (left) and melanopsin+RGCs (right) as percent of naive retinas vs. time post-lesion (days) after ONC, ONT, LP-OHT, or AOH in rats and mice. The open circles mark the time points of retinal analysis. Legend as in (B). (B) Graph showing the regression analysis of the loss of Brn3a+RGCs (percent of naive retinas, data in (A) vs. time post-lesion (days) after ONC, ONT, LP-OHT, or AOH in rats and mice. All courses adjust to either a segmental linear or to a linear regression. For ONT, ONC, and LP-OHT in rat the analysis was done with data points >45 days (see A) but the regression graph was cropped at 45 days for clarity. ONC, optic nerve crush at 0.5 mm from the optic head for mice, and 2 mm for rat. ONT: optic nerve transection at 0.5 mm from the optic nerve in both species. LP-OHT: ocular hypertension induced by laser-photocoagulation, in this model the increase of intra-ocular pressure lasts a week and rises on average up to 40 mmHg. AOH: acute ocular hypertension, induced by cannulating the anterior chamber, in this model the intra-ocular pressure was elevated to 76 mmHg for 75 min. X₀ indicates the day when the first linear phase of RGC loss changes to a second, slower, one. m1 and m2 are the slopes of the first and second linear phases, respectively and show the daily loss of RGCs (in percent). Fifty percent survival is the time (days) when, according to the mathematical analysis, half of the RGCs are lost. R² is the fitness of the regression. Albino mice: Swiss. Pigmented mice: C57/BL6. Albino rat: Sprague Dawley. These graphs were constructed using data from Salinas-Navarro et al. (2009c, 2010), Nadal-Nicolás et al. (2009, 2015a), Galindo-Romero et al. (2011, 2013b), Ortín-Martínez et al. (2015), Rovere et al. (2015, 2016a), Valiente-Soriano et al. (2015a,b), and Sánchez-Migallón et al. (2016).

Figure 2

Topography of RGC loss after axotomy or increased intraocular pressure. (A–J) isodensity maps showing the distribution of Brn3a+RGCs intact (A,B) or injured (C–J) retinas from rat (A–I) or mouse (B–J). (A'–J'): neighbor maps showing the distribution of melanopsin+RGCs in the same retinas as (A–J). (C–F'): axotomized retinas analyzed during the first phase of RGC death (C,D', 5 or 7 days) or long after the injury (E,F', 45 days or 6 months). In both species axotomy causes a diffuse loss of both functional types of RGCs. Importantly, the survival percent of m+RGCs is higher than that of Brn3a+RGCs. In rat, shortly after the lesion (C') there are fewer m+RGCs than at 6 months (E') due to the transient downregulation of melanopsin (Nadal-Nicolás et al., 2015b). (G–J'): retinas analyzed after ocular hypertension (OHT) induced by either an acute increase of the intraocular pressure (G,I, 7 days and 45 days) or by laser photocogulation of the epiescleral and perilimbar veins (H,J, 14 or 30 days). OHT causes a sectorial or patchy loss of Brn3a+RGCs and a diffuse loss of m+RGCs. As observed for axotomy, m+RGCs are more resilient than Brn3a+RGCs. Again, in rat melanopsin expression recovers partially with time post-AOH (G' vs. I'). Isodensity maps depict the density of RGCs with a color scale that goes from 0 RGCs/mm² (purple) to ≥3,500 (rat) or ≥4,800 (mouse) RGCs/mm² (red). In the neighbors maps each dot represents a m+RGC and its color the number of neighbors around it from purple (0–1 in mouse or 0–2 in rats) neighbors to red (≥11 rat, or ≥21 mouse) neighbors in a radius of 0.22 mm (rat) or 0.165 mm (mouse). Below each map is shown the total number of RGCs quantified in their corresponding retina. Bar scale for rat in (A) and for mouse in (B). N, nasal; T, temporal; S, superior; I, inferior; d, days; m, months; A–OHT, acute ocular hypertension; LP–OHT, laser photocoagulation induced ocular hypertension. These original isodensity maps were constructed using data from Salinas-Navarro et al. (2009c, 2010), Nadal-Nicolás et al. (2009, 2015a), Galindo-Romero et al. (2011, 2013b), Ortín-Martínez et al. (2015), Rovere et al. (2015, 2016a), Valiente-Soriano et al. (2015a,b), and Sánchez-Migallón et al. (2016).

Figure 3

Ocular hypertension but no optic nerve axotomy, causes the protracted loss of cone photoreceptors (A–C”) : isodensity maps showing the topography of Brn3a+RGCs (A–C), S-cones (A'–C') and L-cones (A”–C”) in the same retina within each group. (A–A”): intact, (B–B”): 6 months after axotomy, (C–C”): 6 months ocular hypertension induced by laser photocoagulation. Isodensity color scale is found in (C–C”) and goes from 0 cells/mm2 (purple) to ≥3,000 RGCs/mm², ≥900 S-cones/mm² or 7,000 L-cones/mm² (red). N, nasal; T, temporal; S, superior; I, inferior; d, days; These original isodensity maps were constructed using data from Ortín-Martínez et al. (2015).