Changes in Müller cell plasma membrane specializations during subretinal scar formation in the rabbit

Targeting the Notch and TGF-β signaling pathways to prevent retinal fibrosis in vitro and in vivo

Rationale: The Notch and transforming growth factor-β (TGFβ) signaling pathways are two intracellular mechanisms that control fibrosis in general but whether they play a major role in retinal fibrosis is less clear. Here we study how these two signaling pathways regulate Müller cell-dominated retinal fibrosis in vitro and in vivo.

Methods: Human MIO-M1 Müller cells were treated with Notch ligands and TGFβ1, either alone or in combination. Western blots were performed to study changes in γ-secretase proteases, Notch downstream effectors, endogenous TGFβ1, phosphorylated Smad3 (p-Smad3) and extracellular matrix (ECM) proteins. We also studied the effects of RO4929097, a selective γ-secretase inhibitor, on expression of ECM proteins after ligand stimulation. Müller cell viability was studied by AlamarBlue and cytotoxicity by lactate cytotoxicity assays. Finally, we studied changes in Notch and TGFβ signaling and tested the effect of intravitreal injections of the Notch pathway inhibitor RO4929097 on retinal fibrosis resulted from Sodium iodate (NaIO₃)-induced retinal injury in mice. We also studied the safety of intravitreal injections of RO4929097 in normal mice.

Results: Treatment of Müller cells with Notch ligands upregulated γ-secretase proteases and Notch downstream effectors, with increased expression of endogenous TGFβ1, TGFβ receptors and p-Smad3. TGFβ1 upregulated the expression of proteins associated with both signaling pathways in a similar manner. Notch ligands and TGFβ1 had additive effects on overexpression of ECM proteins in Müller cells which were inhibited by RO4929097. Notch and TGFβ ligands stimulated Müller cell proliferation which was inhibited by RO4929097 without damaging the cells. NaIO₃-induced retinal injury activated both Notch and TGFβ signaling pathways in vivo. Intravitreal injection of RO4929097 prevented Müller cell gliosis and inhibited overexpression of ECM proteins in this murine model. We found no safety concerns for up to 17 days after an intravitreal injection of RO4929097.

Conclusions: Inhibiting Notch signaling might be an effective way to prevent retinal fibrosis. This study is of clinical significance in developing a treatment for preventing fibrosis in proliferative vitreoretinopathy, proliferative diabetic retinopathy and wet age-related macular degeneration.

Peripapillary distribution of Muller cells within the retinal nerve fiber layer in human eyes

The purpose of this study was to describe the distribution of Muller cell within the peripapillary retinal nerve fiber layer (RNFL) in human eyes. Eleven unpaired normal postmortem eyes were recruited into this study. Each eye was sectioned using the "umbrella technique" to obtain a concentric peripapillary ring centered on the optic disc, with a diameter of 3.0 mm. Immunohistochemistry with anti- CRALBP stained Muller cell within each ring. The RNFL thickness measurements around the peripapillary ring were: 262.5, 339.4, 285.4 and 347.5 μm for the temporal, superior, nasal and inferior quadrants, respectively. Muller cell were found to be unevenly distributed in the peripapillary RNFL of normal eyes. The relative Muller cell staining to the thickness of each measured segment (16.6%, 15.2%, 21.3%, and 17.9% for the temporal, superior, nasal and inferior quadrants, respectively) showed a significant increase in the nasal quadrant. The RNFL thickness measurements obtained using imaging techniques reflect the amount of axonal tissue present in this layer. In this study we highlight that around 20% of RNFL thickness is composed of non-axonal contents which do not represent neuronal tissue, nor are they necessarily lost in the glaucomatous process. More so, the ratio of the Muller cell component to the total RNFL thickness varies around the peripapillary RNFL ring, demonstrating the lowest relative content of Muller cell superiorly and the highest content nasally. Further studies should compare the amount and distribution of Muller cell in normal versus glaucomatous eyes.

Loss of inwardly rectifying potassium currents by human retinal glial cells in diseases of the eye

We compared the inward K+ currents of Müller glial cells from healthy and pathologically changed human retinas. To this purpose, the whole-cell voltage-clamp technique was performed on noncultured Müller cells acutely isolated from human retinas. Cells originated from retinas of four healthy organ donors and of 24 patients suffering from different vitreoretinal and chorioretinal diseases. Müller cells from organ donors displayed inward K+ currents in the whole-cell mode similar to those found in other species. In contrast, this pattern was clearly changed in the Müller cells from patient retinas. In whole-cell recordings many Müller cells had strongly decreased inward K+ current amplitudes or lost these currents completely. Thus, the mean input resistance of Müller cells from patients was significantly increased to 1,129 +/- 812 M omega, compared to 279 +/- 174 M omega in Müller cells from healthy organ donor retinas. Accordingly, since the membrane potential is mainly determined by the K+ inward conductance in healthy Müller cells, a large amount of Müller cells from patient retinas had a membrane potential which was significantly lower than that of Müller cells from control eyes. The mean membrane potentials were -37 +/- 24 mV and -63 +/- 25 mV for patient and donor Müller cells, respectively. The newly described membrane characteristic changes of Müller cells from patient eyes are assumed to interfere severely with normal retinal function: (1) the retinal K+ homeostasis, which is partly regulated by the Müller cell-mediated spatial buffering, should be disturbed, and (2) the diminished membrane potential should influence voltage-dependent transporter systems of the Müller cells, e.g., the Na(+)-dependent glutamate uptake.

Reorganization of actin microfilaments and microtubules in regenerating retinal pigment epithelium

Retinal pigment epithelium (RPE) regenerating after experimental damage in rabbits exhibits major changes in cell shape, polarity and junctions--features that depend on the cytoskeleton. This report correlates these changes with the redistribution of actin microfilaments and microtubules, using electron microscopy and confocal laser scanning microscopy. We compare immature cells with the more mature cells that form the new epithelial monolayer. Two populations of immature RPE cells are interspersed at the edge of the regenerating RPE sheet. One population of immature cells makes few junctions with their neighbors or the basement membrane. They form pseudopodia and exhibit a prominent network of actin microfilaments beneath the plasma membrane. These cells are probably motile and advance the epithelial sheet. Another population of immature cells contains numerous stress fibers that insert into large basement membrane attachments. The cells make focal adhesions with their neighbors, rather than the junctional complexes characteristic of mature RPE cells. These cells are probably not motile and mature into the cells forming the new monolayer--cuboidal cells with numerous basal folds and apical villi and a complete belt of intercellular junctions. Stress fibers are lost as the circumferential bundle associated with the zonula adherens re-forms. Microtubules, which form prominent longitudinal bundles running through the processes of immature cells, take on the meshwork organization characteristic of mature RPE as the immature cells differentiate.