Extracellular matrix proteins as substrate modulate the pattern of calcium channel expression in cultured rat retinal pigment epithelial cells

Human Adult Retinal Pigment Epithelial Stem Cell-Derived RPE Monolayers Exhibit Key Physiological Characteristics of Native Tissue

PURPOSE

We tested what native features have been preserved with a new culture protocol for adult human RPE.

METHODS

We cultured RPE from adult human eyes. Standard protocols for immunohistochemistry, electron microscopy, electrophysiology, fluid transport, and ELISA were used.

RESULTS

Confluent monolayers of adult human RPE cultures exhibit characteristics of native RPE. Immunohistochemistry demonstrated polarized expression of RPE markers. Electron microscopy illustrated characteristics of native RPE. The mean transepithelial potential (TEP) was 1.19 ± 0.24 mV (mean ± SEM, n = 31), apical positive, and the mean transepithelial resistance (RT) was 178.7 ± 9.9 Ω·cm2 (mean ± SEM, n = 31). Application of 100 μM adenosine triphosphate (ATP) apically increased net fluid absorption (Jv) by 6.11 ± 0.53 μL·cm2·h-1 (mean ± SEM, n = 6) and TEP by 0.33 ± 0.048 mV (mean ± SEM, n = 25). Gene expression of cultured RPE was comparable to native adult RPE (n = 5); however, native RPE RNA was harvested between 24 and 40 hours after death and, therefore, may not accurately reflect healthy native RPE. Vascular endothelial growth factor secreted preferentially basally 2582 ± 146 pg/mL/d, compared to an apical secretion of 1548 ± 162 pg/mL/d (n = 14, P < 0.01), while PEDF preferentially secreted apically 1487 ± 280 ng/mL/d compared to a basolateral secretion of 864 ± 132 ng/mL/d (n = 14, P < 0.01).

CONCLUSIONS

The new culture model preserves native RPE morphology, electrophysiology, and gene and protein expression patterns, and may be a useful model to study RPE physiology, disease, and transplantation.

Cell culture conditions affect RPE phagocytic function

BACKGROUND

Changes in the phenotype of retinal pigment epithelium (RPE) cells in vitro are associated with medium conditions and changes in function. Main goals in RPE tissue engineering are cell propagation in serum-free defined culture conditions, resulting in cells exhibiting differentiated morphology and functioning in vitro.

METHODS

To compare the effects of various media and supplements on cell function, an optimized high-throughput phagocytosis assay was developed. Adult human SV40-RPE cells were cultured. Test media included: MEM(E), DMEM, F99, SFM and hSFM, with or without supplements. SNAFL-2 labelled OS were added to RPE in vitro for 4 h and phagocytic binding and uptake were measured.

RESULTS

RPE phagocytosis was of different magnitude depending on the serum-free basic cell culture media in the following order: hSFM, SFM > DMEM, MEM > F99. Choroid-conditioned medium (ChCM) decreased phagocytosis dose dependently. Whereas 1% retinal extract (RE) supplementation increased, higher concentrations decreased phagocytosis. Addition of 10% FCS increased phagocytosis. 15% ChCM quenched the stimulation induced by 10% FCS, an effect which could be reversed by the addition of 1% RE.

CONCLUSIONS

Cell culture media and RPE environmental factors exert substantial and differential alteration of RPE phagocytic ability. Phagocytosis in a serum-free defined medium is superior to unsupplemented basic media, but still differs from serum-supplemented media (F99RPE) designed for cell propagation. We conclude that media SFM or hSFM promoted phagocytosis most, and application of FCS or 1% RE supports phagocytosis. Unknown factors from neighbouring tissues (retina and choroid) affect phagocytosis differently, suggesting a role in retinal pathogenesis. The results will support identification of specific environmental factors and facilitate design of cell culture media.

Effect of extracellular matrix elements on angiotensin II-induced calcium release in vascular smooth muscle cells from normotensive and hypertensive rats

The interaction of the vascular smooth muscle cells (VSMCs) with the components of the matrix determines several functions of the cell, such as growth and differentiation. In contrast, an alteration in angiotensin (Ang) II-induced Ca(2+) mechanisms in VSMCs was reported in genetic hypertension. In this study, we wished to assess the effect of different components of the extracellular matrix on the increase of [Ca(2+)](i) induced by Ang II in VSMCs from spontaneously hypertensive rats (SHR) compared with those from normotensive Wistar-Kyoto rats (WKY). Results demonstrate for the first time that elements of the extracellular matrix modulate the Ang II-induced Ca(2+) transport mechanisms. This modulation is different in cells from WKY compared with those from SHR. Thus, growing cells from SHR on collagen I, collagen IV, fibronectin, vitronectin, or Matrigel induced a significant decrease in Ang II-induced Ca(2+) release from internal stores, whereas in cells from WKY, no effect could be observed except for those grown on collagen I, which increased Ca(2+) release. Fibronectin and vitronectin, however, induced a decrease in Ang II-induced Ca(2+) influx in WKY, whereas no effect could be observed in SHR. Conversely, collagen I and collagen IV induced an increase in this influx in SHR but not in WKY, whereas Matrigel increased the influx in both strains. These results suggest a modulation of the Ang II-associated signaling events by the matrix elements via the focal adhesion points. The understanding of these synergies should provide insight into issues such as development of hypertrophy of large vessels in hypertension.

Influence of extracellular matrix proteins on membrane potentials and excitability in NG108-15 cells

Previous studies have demonstrated that components of the extracellular matrix can induce neurite extension and cell adhesion in the neuroblastoma x glioma hybrid cell line, NG108-15. Using standard intracellular recording techniques, we examined the resting membrane potential (RMP) and membrane excitability of NG108-15 cells differentiated under serum-free media with representative extracellular matrix (ECM) protein components as the substrate. Surfaces coated with collagen IV and a laminin-1 synthetic peptide induced a significantly (P < 0.05) more hyperpolarized RMP than control polystyrene surfaces. For example, after > or =8 days in culture NG108-15 cells plated on polystyrene exhibited a RMP of -33.2+/-0.8 mV (mean+/-SEM, n=158 cells) whereas cells cultured on the laminin-1 peptide C16 and collagen IV showed a RMP of -37.6+/-0.7 mV (n=157) and -37.5+/-1.5 mV (n=68), respectively. Furthermore, the proportions of cells on ECM substrates showing membrane excitability, i.e. evoked action potentials (APs) and the capability for regular firing, were significantly greater compared to those cells cultured on polystyrene. Among excitable cells cultured on the different substrates, characteristics of the action potentials, such as AP duration, amplitude, and the maximum rate of rise, dV/dtMAX, were examined in detail. While little or no differences were observed between polystyrene and the laminin-1 peptide groups, significant differences in the AP parameters were apparent for collagen IV. For example, dV/dtMAX for polystyrene and the laminin-1 peptide C16 were only 71.7+/-24.5 V/s (n=11) and 59.0+/-8.9 V/s (n=9), respectively, whereas cells cultured on collagen IV surfaces exhibited a dV/dtMAX reaching 156.1+/-22.0 V/s (n=7). These data support a role for ECM components in the maintenance of the RMP and membrane excitability in NG108-15 cells.

Activation of a Cl--conductance by protein kinase-dependent phosphorylation in cultured rat retinal pigment epithelial cells

While chloride conductances are involved in signals of the electroretinogram generated by the retinal pigment epithelium (RPE), patch-clamp experiments of freshly isolated or cultured RPE cells have shown that potassium conductances predominate. The purpose of this study was to investigate mechanisms which activate Cl--conductances in RPE cells. Membrane currents of cultured rat RPE cells were measured using the whole-cell configuration of the patch-clamp technique under extra- and intracellular K+-free conditions. The bath solution was hyperosmolal to the pipette solution to prevent hypoosmotic swelling. Exchange of the physiological intracellular fluid by a pipette solution with physiological levels of ATP (2 mm) induced a continuous increase of membrane conductance. Conductance was blocked by DIDS (1 mm), and showed a reversal potential close to the Nernst potential for Cl-. When the experiments were carried out under conditions in which all cations, and not only potassium, were replaced by NMDG, the same responses could be observed. Current activation was independent of extracellular calcium. Chloride currents were also induced when ATPgammaS or AMP-PNP were used instead of ATP. In the presence of AMP-PNP currents were 10 times smaller than in the presence of ATP or ATPgammaS. In cells preincubated with staurosporine or chelerythrine no currents were induced. Establishing the whole-cell configuration with ATP and with myristoylated PKC substrate in addition, no voltage-dependent currents were activated. We conclude that ATP hydrolysis leads to activation of chloride currents via PKC in the whole-cell configuration. The perforated patch configuration, with the intracellular compartment intact, no currents were induced under otherwise identical experimental conditions. Inhibition of phosphatase by calyculin (10 nm) in the perforated-patch configuration did not change membrane conductance. In the intact cell, chloride conductance is possibly inhibited by a cytosolic factor which is washed out when the whole-cell configuration is established.

Reattachment to a substrate prevents apoptosis of human retinal pigment epithelium

BACKGROUND

Epithelial cells generally fail to survive in suspension. Harvesting human retinal pigment epithelium (RPE) for transplantation may separate the cells from their extracellular matrix and induce apoptosis. We investigated whether reattachment of RPE to a substrate will prevent apoptosis.

METHODS

Second-passage human RPE cells were plated onto tissue culture plastic precoated with extracellular matrix, fibronectin or laminin, uncoated tissue culture plastic, untreated plastic and untreated plastic coated with 4% agarose. Reattachment rates were determined for each substrate 24 h after plating. The TUNEL technique was used to determine apoptosis rates in attached cells, unattached cells and the entire cell population.

RESULTS

Attachment rates were as follows: ECM-coated tissue culture plastic-->fibronectin-coated tissue culture plastic-->laminin-coated tissue culture plastic-->uncoated tissue culture plastic-->untreated plastic-->agarose-coated untreated plastic. Apoptosis rates for the entire cell population increased as the RPE cell attachment rate decreased. The proportion of apoptotic cells in the entire population was inversely related to the percent attached cells (r = -0.95).

CONCLUSION

Reattachment of harvested RPE to a substrate decreased the rate of RPE apoptosis in vitro. RPE cells which are removed from their substrate prior to transplantation must reattach rapidly to a substrate to prevent apoptosis.