Glial and neural response in short-term human retinal detachment

Histologic and Immunohistochemical Characterization of GA-Like Pathology in the Rat Subretinal Sodium Iodate Model

Purpose

Geographic atrophy (GA) is an advanced form of dry age-related macular degeneration with multifactorial etiology and no well-established treatment. A model recapitulating the hallmarks would serve as a key to understanding the underlying pathologic mechanisms better. In this report, we further characterized our previously reported subretinal sodium iodate model of GA.

Methods

Retinal degeneration was induced in rats (6-8 weeks old) by subretinal injections of NaIO3 as described previously. Animals were sacrificed at 3, 8 and 12 weeks after injection and eyes were fixed or cryopreserved. Some choroids were processed as flatmounts while other eyes were cryopreserved, sectioned, and immunolabeled with a panel of antibodies. Finally, some eyes were prepared for transmission electron microscopic (TEM) analysis.

Results

NaIO3 subretinal injection resulted in a well-defined focal area of retinal pigment epithelium (RPE) degeneration surrounded by viable RPE. These atrophic lesions expanded over time. RPE morphologic changes at the border consisted of hypertrophy, multilayering, and the possible development of a migrating phenotype. Immunostaining of retinal sections demonstrated external limiting membrane descent, outer retinal tubulation (ORT), and extension of Müller cells toward RPE forming a glial membrane in the subretinal space of the atrophic area. TEM findings demonstrated RPE autophagy, cellular constituents of ORT, glial membranes, basal laminar deposits, and defects in Bruch's membrane.

Conclusions

In this study, we showed pathologic features of a rodent model resembling human GA in a temporal order through histology, immunofluorescence, and TEM analysis and gained insights into the cellular and subcellular levels of the GA-like phenotypes.

Translational Relevance

Despite its acute nature, the expansion of atrophy and the GA-like border in this rat model makes it ideal for studying disease progression and provides a treatment window to test potential therapeutics for GA.

Subretinal Glial Membranes in Eyes With Geographic Atrophy

Purpose

Müller cells create the external limiting membrane (ELM) by forming junctions with photoreceptor cells. This study evaluated the relationship between focal photoreceptors and RPE loss in geographic atrophy (GA) and Müller cell extension into the subretinal space.

Methods

Human donor eyes with no retinal disease or geographic atrophy (GA) were fixed and the eye cups imaged. The retinal posterior pole was stained for glial fibrillary acidic protein (GFAP; astrocytes and activated Müller cells) and vimentin (Müller cells) while the submacular choroids were labeled with Ulex Europaeus Agglutinin lectin (blood vessels). Choroids and retinas were imaged using a Zeiss 710 confocal microscope. Additional eyes were cryopreserved or processed for transmission electron microscopy (TEM) to better visualize the Müller cells.

Results

Vimentin staining of aged control retinas (n = 4) revealed a panretinal cobblestone-like ELM. While this pattern was also observed in the GA retinas (n = 7), each also had a distinct area in which vimentin⁺ and vimentin⁺/GFAP⁺ processes created a subretinal membrane. Subretinal glial membranes closely matched areas of RPE atrophy in the gross photos. Choroidal vascular loss was also evident in these atrophic areas. Smaller glial projections were noted, which correlated with drusen in gross photos. The presence of glia in the subretinal space was confirmed by TEM and cross cross-section immunohistochemistry.

Conclusions

In eyes with GA, subretinal Müller cell membranes present in areas of RPE atrophy may be a Müller cell attempt to replace the ELM. These membranes could interfere with treatments such as stem cell therapy.

Time course modifications in organotypic culture of human neuroretina

The purpose of this study was to characterize organ culture of human neuroretina and to establish survival and early degeneration patterns of neural and glial cells. Sixteen neuroretina explants were prepared from 2 postmortem eyes of 2 individuals. Four explants were used as fresh retina controls, and 12 were evaluated at 3, 6, and 9 days of culture. Neuroretina explants (5 × 5 mm) were cultured in Transwell(®) dishes with the photoreceptor layer facing the supporting membrane. Culture medium (Neurobasal A-based) was maintained in contact with the membrane beneath the explant. Cryostat and ultrathin sections were prepared for immunohistochemistry and electron microscopy. Neuroretinal modifications were evaluated after toluidine blue staining and after immunostaining for neuronal and glial cell markers. Ultrastructural changes were analyzed by electron microscopy. From 0 to 9 days in culture, there was progressive retinal degeneration, including early pyknosis of photoreceptor nuclei, cellular vacuolization in the ganglion cell layer, decrease of both plexiform layer thicknesses, disruption and truncation of photoreceptor outer segments (OS), and marked reduction in the number of nuclei at both nuclear layers where the cells were less densely packed. At 3 days there was swelling of cone OS with impairment of pedicles, loss of axons and dendrites of horizontal and rod bipolar cells that stained for calbindin (CB) and protein kinase C (PKC-α), respectively. After 9 days, horizontal cells were pyknotic and without terminal tips. There were similar degenerative processes in the outer plexiform layer for rod bipolar cells and loss of axon terminal lateral varicosities in the inner plexiform layer. Glial fibrillary acidic protein (GFAP) staining did not reveal a dramatic increase of gliosis in Müller cells. However, some Müller cells were CB immunoreactive at 6 days of culture. Over 9 days of culture, human neuroretina explants underwent morphological changes in photoreceptors, particularly the OS and axon terminals, and in postsynaptic horizontal and bipolar cells. These early changes, not previously described in cultured human samples, reproduce some celullar modifications after retinal damage. Thus, this model may be suitable to evaluate therapeutic agents during retinal degeneration processes.

Apoptosis and other cell death mechanisms after retinal detachment: implications for photoreceptor rescue

Retinal detachment (RD) is one of the most common causes of blindness. This separation of the neurosensory retina from its underlying retinal pigment epithelium results in photoreceptor loss, which is the basis of permanent visual impairment. This review explores the various cell death mechanisms in photoreceptor death associated with RD. One of the major mechanisms is apoptosis, mediated by the intrinsic pathway, the Fas signalling pathway and/or the caspase-independent pathway. Other pathways of mechanisms include endoplasmic reticulum stress-mediated cell death, programmed necrosis and cytokine-related pathways. Understanding the mechanism of RD-associated photoreceptor death is likely to help us improve the current therapies or devise new strategies for this sight-threatening condition.

RhoA inactivation prevents photoreceptor axon retraction in an in vitro model of acute retinal detachment

PURPOSE

An early injury response to retinal detachment is disruption of synaptic connectivity between photoreceptors and second-order neurons. Most dramatic is the retraction of rod cell axons and their terminals away from the outer synaptic layer and toward their cell bodies. This study tested whether axonal retraction in detached retina was due to the activation of the small GTPase RhoA and was preventable using RhoA antagonists.

METHODS

Retinal detachments were created in in vitro preparations of porcine eyecups. RhoA activation was determined with a Rhotekin binding assay. To block axon retraction, drugs were applied to neural retinal explants either before or after detachment from the retinal pigment epithelium. Presynaptic movement was quantified by image analysis of double-labeled retinas examined with confocal microscopy.

RESULTS

Active RhoA increases transiently after detachment followed by morphologic evidence of axonal retraction over the next 24 hours. Pretreating the retina with a RhoA antagonist, CT-04, or a Rho kinase inhibitor, Y27632, at multiple concentrations significantly inhibited axonal retraction. Reducing calcium influx through L-type calcium channels with nicardipine also blocked retraction. To create a more plausible therapeutic scenario, drug treatments were delayed and applied after retinal detachment. The Rho kinase inhibitor, but not nicardipine, significantly blocked rod axonal retraction when applied up to 6 hours after detachment.

CONCLUSIONS

Thus, RhoA and downstream Rho kinase activity constitute part of the mechanism that produces rod axonal retraction in retinal explants. Treatments that manipulate RhoA signaling may promote synaptic stability after retinal detachment.

Krogh cylinders in retinal development, panretinal hypoperfusion and diabetic retinopathy

The volume of cells that a length of capillary supplies with O(2) is called a Krogh cylinder. This geometric 'tissue unit' was named after the Danish zoophysiologist and Nobel laureate August Krogh who made important discoveries in the fields of external and internal respiration in the first half of the last century. Krogh's ideas concerning tissue O(2) distribution can be extrapolated to retinal oxygenation by larger vessels (including arterioles, arteries and even veins) and by vessel groups within higher-order 'microvascular units' (including the choroid). During retinal development, for example, the difference in pO(2) levels within arteries and capillaries determines Krogh cylinders of different radius and establishes the periarterial capillary-free zone of His. The O(2) supply to the venous end of a tissue unit may be compromised during periods of reduced perfusion, increased O(2) consumption or hypoxaemia, resulting in an 'anoxic corner' of the Krogh cylinder. A funnel of hypometabolic (and therefore hypoxia-tolerant) cells will likely intervene between the necrotic cells and unaffected cells located closer to the O(2) source. Macular perivenular whitening heralds anoxic corners and/or hypoxic funnels owing to hypoperfusion within second-order microvascular units. In eyes with extensive retinal capillary closure from diabetes, Krogh cylinders surround the medium-sized arteries and veins that form arteriovenous shunts while traversing the midperipheral retina. These isolated tissue units incorporate an outer sheath of hypoxic cells within which vascular endothelial growth factor is upregulated. This 'angiogenic sheath' expands following retinal detachment; it corresponds to the hypoxia-tolerant funnel within capillary-based tissue units and to the cerebral penumbra after stroke.

Changes in adeno-associated virus-mediated gene delivery in retinal degeneration

Gene therapies for retinal degeneration have relied on subretinal delivery of viral vectors carrying therapeutic DNA. The subretinal injection is clearly not ideal as it limits the viral transduction profile to a focal region at the injection site and negatively affects the neural retina by detaching it from the supportive retinal pigment epithelium (RPE). We assessed changes in adeno-associated virus (AAV) dispersion and transduction in the degenerating rat retina after intravitreal delivery. We observed a significant increase in AAV-mediated gene transfer in the diseased compared with normal retina, the extent of which depends on the AAV serotype injected. We also identified key structural changes that correspond to increased viral infectivity. Particle diffusion and transgene accumulation in normal and diseased retina were monitored via fluorescent labeling of viral capsids and quantitative PCR. Viral particles were observed to accumulate at the vitreoretinal junction in normal retina, whereas particles spread into the outer retina and RPE in degenerated tissue. Immunohistochemistry illustrates remarkable changes in the architecture of the inner limiting membrane, which are likely to underlie the increased viral transduction in diseased retina. These data highlight the importance of characterizing gene delivery vectors in diseased tissue as structural and biochemical changes can alter viral vector transduction patterns. Furthermore, these results indicate that gene delivery to the outer nuclear layer may be achieved by noninvasive intravitreal AAV administration in the diseased state.

CO2-induced ion and fluid transport in human retinal pigment epithelium

In the intact eye, the transition from light to dark alters pH, [Ca²⁺], and [K] in the subretinal space (SRS) separating the photoreceptor outer segments and the apical membrane of the retinal pigment epithelium (RPE). In addition to these changes, oxygen consumption in the retina increases with a concomitant release of CO₂ and H₂O into the SRS. The RPE maintains SRS pH and volume homeostasis by transporting these metabolic byproducts to the choroidal blood supply. In vitro, we mimicked the transition from light to dark by increasing apical bath CO₂ from 5 to 13%; this maneuver decreased cell pH from 7.37 ± 0.05 to 7.14 ± 0.06 (n = 13). Our analysis of native and cultured fetal human RPE shows that the apical membrane is significantly more permeable (≈10-fold; n = 7) to CO₂ than the basolateral membrane, perhaps due to its larger exposed surface area. The limited CO₂ diffusion at the basolateral membrane promotes carbonic anhydrase–mediated HCO₃ transport by a basolateral membrane Na/nHCO₃ cotransporter. The activity of this transporter was increased by elevating apical bath CO₂ and was reduced by dorzolamide. Increasing apical bath CO₂ also increased intracellular Na from 15.7 ± 3.3 to 24.0 ± 5.3 mM (n = 6; P < 0.05) by increasing apical membrane Na uptake. The CO₂-induced acidification also inhibited the basolateral membrane Cl/HCO₃ exchanger and increased net steady-state fluid absorption from 2.8 ± 1.6 to 6.7 ± 2.3 µl × cm⁻² × hr⁻¹ (n = 5; P < 0.05). The present experiments show how the RPE can accommodate the increased retinal production of CO₂ and H₂O in the dark, thus preventing acidosis in the SRS. This homeostatic process would preserve the close anatomical relationship between photoreceptor outer segments and RPE in the dark and light, thus protecting the health of the photoreceptors.