We thank Drs. Daniel Hass and Jim Hurley for the idea of testing O2 solubility in new versus conditioned media as a control. We thank Dr. Magali Saint-Geniez for editorial input on the manuscript. We thank Scott Szalay at Instrument and Electronic Services Core, Kellogg Eye Center, for retrofitting the hypoxia chamber with the Resipher USB cable. No federal funds were used for HFT research. The Electronic Services Core is supported by P30 EY007003 from the National Eye Institute. This work is supported by an unrestricted departmental grant from Research to Prevent Blindness (RPB). J.M.L.M. is supported by the James Grosfeld Initiative for Dry Age-Related Macular Degeneration, the E. Matilda Ziegler Foundation for the Blind, an Eversight eye-bank grant, a K08EY033420 grant from the National Eye Institute, and support from Dee and Dickson Brown as well as the David and Lisa Drews Discovering Hope Foundation. D.Y.S. is supported by the UNSW Scientia Program. L.A.K. is supported by the Iraty Award, Monte J. Wallace, Michel Plantevin, an R01EY027739 grant from the National Eye institute, and the Department of Defense Army Medical Research Acquisition Activity VR220059.

Novel OCR Monitoring System for Differentiated RPE Cultures

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X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Modulation+of+pyruvate+kinase+M2+activity+as+a+therapy+in+a+primary+RPE+cell+culture+model+of+proliferative+vitreoretinopathy.">Modulation of pyruvate kinase M2 activity as a therapy in a primary RPE cell culture model of proliferative vitreoretinopathy.</a> Invest Ophthalmol Vis Sci. 62 (8), 2213 (2021).</li><li>Gupta, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Progress+in+stem+cells-based+replacement+therapy+for+retinal+pigment+epithelium:+in+vitro+differentiation+to+in+vivo+delivery.">Progress in stem cells-based replacement therapy for retinal pigment epithelium: in vitro differentiation to in vivo delivery.</a> Stem Cells Transl Med. 12 (8), 536-552 (2023).</li><li>Wilson, D. F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Oxygen+distribution+and+vascular+injury+in+the+mouse+eye+measured+by+phosphorescence-lifetime+imaging.">Oxygen distribution and vascular injury in the mouse eye measured by phosphorescence-lifetime imaging.</a> Appl Opt. 44 (25), 5239-5248 (2005).</li><li>Hass, D. T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Medium+depth+influences+O2+availability+and+metabolism+in+human+RPE+cultures.">Medium depth influences O2 availability and metabolism in human RPE cultures.</a> Invest Ophthalmol Vis Sci. 64 (14), 4 (2023).</li><li>Shu, D. Y., Butcher, E. R., Saint-Geniez, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Suppression+of+PGC-1&#945;+drives+metabolic+dysfunction+in+TGF&#946;2-Induced+EMT+of+retinal+pigment+epithelial+cells.">Suppression of PGC-1&#945; drives metabolic dysfunction in TGF&#946;2-Induced EMT of retinal pigment epithelial cells.</a> Int J Mol Sci. 22 (9), 4701 (2021).</li><li>Shu, D. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dimethyl+fumarate+blocks+tumor+necrosis+factor-alpha-driven+inflammation+and+metabolic+rewiring+in+the+retinal+pigment+epithelium.">Dimethyl fumarate blocks tumor necrosis factor-alpha-driven inflammation and metabolic rewiring in the retinal pigment epithelium.</a> Front Mol Neurosci. 15, 896786 (2022).</li><li>Ng, P. Q., Saint-Geniez, M., Kim, L. A., Shu, D. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Divergent+metabolomic+signatures+of+TGF&#946;2+and+TNF&#945;+in+the+induction+of+retinal+epithelial-mesenchymal+transition.">Divergent metabolomic signatures of TGF&#946;2 and TNF&#945; in the induction of retinal epithelial-mesenchymal transition.</a> Metabolites. 13 (2), 213 (2023).</li><li>Kar, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Choriocapillaris+impairment+is+associated+with+delayed+rod-mediated+dark+adaptation+in+age-related+macular+degeneration.">Choriocapillaris impairment is associated with delayed rod-mediated dark adaptation in age-related macular degeneration.</a> Invest Ophthalmol Vis Sci. 64 (12), 41 (2023).</li><li>Curcio, C. A., Kar, D., Owsley, C., Sloan, K. R., Ach, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Age-related+macular+degeneration,+a+mathematically+tractable+disease.">Age-related macular degeneration, a mathematically tractable disease.</a> Invest Ophthalmol Vis Sci. 65 (3), 4 (2024).</li><li>Maminishkis, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Confluent+monolayers+of+cultured+human+fetal+retinal+pigment+epithelium+exhibit+morphology+and+physiology+of+native+tissue.">Confluent monolayers of cultured human fetal retinal pigment epithelium exhibit morphology and physiology of native tissue.</a> Invest Ophthalmol Vis Sci. 47 (8), 3612-3624 (2006).</li><li>Maminishkis, A., Miller, S. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Experimental+models+for+study+of+retinal+pigment+epithelial+physiology+and+pathophysiology.">Experimental models for study of retinal pigment epithelial physiology and pathophysiology.</a> J Vis Exp: JoVE. (45), e2032 (2010).</li><li>Zhang, Q., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+platform+for+assessing+outer+segment+fate+in+primary+human+fetal+RPE+cultures.">A platform for assessing outer segment fate in primary human fetal RPE cultures.</a> Exp Eye Res. 178, 212-222 (2019).</li><li>Sharma, R., Bose, D., Montford, J., Ortolan, D., Bharti, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Triphasic+developmentally+guided+protocol+to+generate+retinal+pigment+epithelium+from+induced+pluripotent+stem+cells.">Triphasic developmentally guided protocol to generate retinal pigment epithelium from induced pluripotent stem cells.</a> STAR Protoc. 3 (3), 101582 (2022).</li><li>Marchetti, P., Fovez, Q., Germain, N., Khamari, R., Kluza, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitochondrial+spare+respiratory+capacity:+Mechanisms,+regulation,+and+significance+in+non-transformed+and+cancer+cells.">Mitochondrial spare respiratory capacity: Mechanisms, regulation, and significance in non-transformed and cancer cells.</a> FASEB J. 34 (10), 13106-13124 (2020).</li><li>Rosenfeld, P. J., Trivizki, O., Gregori, G., Wang, R. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+update+on+the+hemodynamic+model+of+age-related+macular+degeneration.">An update on the hemodynamic model of age-related macular degeneration.</a> Am J Ophthalmol. 235, 291-299 (2022).</li><li>Fitch, T. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Real-time+analysis+of+bioenergetics+in+primary+human+retinal+pigment+epithelial+cells+using+high-resolution+respirometry.">Real-time analysis of bioenergetics in primary human retinal pigment epithelial cells using high-resolution respirometry.</a> J Vis Exp: JoVE. (192), (2023).</li><li>Calton, M. A., Beaulieu, M. O., Benchorin, G., Vollrath, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Method+for+measuring+extracellular+flux+from+intact+polarized+epithelial+monolayers.">Method for measuring extracellular flux from intact polarized epithelial monolayers.</a> Mol Vis. 24, 425-433 (2018).</li><li>Grumbine, M. K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Maintaining+and+assessing+various+tissue+and+cell+types+of+the+eye+using+a+novel+pumpless+fluidics+system.">Maintaining and assessing various tissue and cell types of the eye using a novel pumpless fluidics system.</a> J Vis Exp: JoVE. 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Richard A. Bryan and Kin Lo are employees of Lucid Scientific, which manufactures the Resipher system.

Mitochondrial metabolism of the RPE plays a critical role in the pathogenesis of common blinding retinal diseases, including AMD and PVR. Modeling RPE mitochondrial metabolism in vitro allows one to isolate its metabolic state from those of surrounding tissues, along with subjecting the tissue to different disease-simulating insults in a controlled manner. Such in vitro modeling of RPE mitochondrial metabolism has been facilitated by the advent of high-fidelity human primary and iPSC-RPE cultures that a...

The retinal pigment epithelium (RPE) is a monolayer of functionally postmitotic, highly polarized epithelial cells that form a barrier between light-sensitive photoreceptors in the retina and their blood circulation, a capillary bed termed the choriocapillaris. Like the role of glia-supporting neurons, the RPE carries out myriad functions to support photoreceptors, including phagocytosis of photoreceptor outer segments, transport of nutrients and metabolic support for photoreceptors, and secretion of essential growth factors, all critical for maintaining visual function.
Degeneration of the RPE underlies several common degenerative disorders of vision. In age-related macular degeneration (AMD), one of the most common causes of incurable vision loss in the world, the RPE dies, and overlying photoreceptors therefore suffer a secondary degeneration. In proliferative vitreoretinopathy (PVR), the RPE instead exits its normally quiescent postmitotic state, proliferating and dedifferentiating into a mesenchymal state (a so-called epithelial-to-mesenchymal transition [EMT]) with alterations in its metabolism. RPE dedifferentiation causes a loss of RPE support to photoreceptors while also triggering a more fibrotic state. This results in both photoreceptor degeneration and RPE-induced scarring, both of which trigger vision loss1,2.
A major part of the RPE&#39;s support to photoreceptors is metabolic, and metabolic dysregulation is a critical factor in numerous retinal diseases, including AMD and PVR. The RPE serves as a regulatory barrier between photoreceptors and their source of oxygen and nutrients, the choriocapillaris. Thus, what the RPE chooses to metabolize versus what the RPE chooses to pass through from the choriocapillaris to the photoreceptors strongly governs photoreceptor metabolism and survival. Numerous studies have shown that the RPE is heavily dependent on mitochondrial metabolism for its normal health, and that photoreceptors instead rely heavily on glycolysis3. This has introduced the concept of complementary, intertwined metabolic states between photoreceptors and the RPE. Specifically, the RPE reduces its metabolism of preferred photoreceptor metabolic substrates and instead utilizes the byproducts of photoreceptor metabolism combined with the metabolites that photoreceptors do not consume. In diseases such as PVR and AMD, evidence strongly suggests that the RPE becomes more glycolytic and less dependent on mitochondrial metabolism; this shift towards RPE glycolysis may starve photoreceptors of metabolites it needs, triggering degeneration4,5,6. Given how interdependent RPE and photoreceptor metabolism are and how much altered metabolism underlies retinal disease, there is strong interest in modeling and manipulating RPE metabolism for therapeutic purposes.
While studying RPE mitochondrial metabolism in vivo is ideal, many aspects of RPE mitochondrial metabolism can only be practically probed in an in vitro culture system. Significant progress towards high-fidelity RPE cultures has been made in the past several decades, to the point that the most carefully groomed RPE cultures are now being used for cell replacement therapy in human clinical trials7. To maintain such high-fidelity cultures, the RPE needs to be grown on particular substrates in particular media for months prior to experimentation. With these conditions, RPE cultures are maximally differentiated and polarized, approximating in vivo RPE. Unfortunately, there is no equipment currently available that can measure mitochondrial metabolism specifically from the RPE in vivo. While oxygen monitoring of the retinal capillary network has been achieved in vivo using electron paramagnetic resonance (EPR) oximetry8, this is not possible for RPE analysis. Differences between RPE metabolism in vivo and in vitro are not well-described, but RPE cultures have been shown to have high mitochondrial activity, similar to RPE in vivo3,9, suggesting significant insight into RPE mitochondrial metabolism can be gained using high-fidelity RPE cultures.
As all mitochondrial metabolism leads to oxygen consumption, measuring RPE oxygen consumption rates (OCR) is a faithful proxy for mitochondrial metabolism. Measuring OCR in RPE cultures has been difficult, as the conditions that promote maximal RPE polarization and differentiation often preclude long-term accurate OCR measurements with currently available techniques, such as the Seahorse Analyzer. In this methods paper, a novel device, termed the Resipher (hereafter referred to as &#34;the system&#34;), is introduced, which allows continuous measurement of OCR over weeks in RPE grown under conditions that maximally promote polarization and differentiation. The ease with which OCR can be measured by this system in RPE culture conditions that maximally promote RPE differentiation and polarization is unique among existing OCR-measuring devices.
This paper provides tips and tricks for using the system with RPE cultures, followed by a demonstration of two particular applications. First, RPE EMT, mimicking PVR, is triggered by exposure to transforming growth factor-beta (TGF&#946;)1,10,11,12. The system is used to monitor how RPE metabolism evolves during the EMT process. Second, the role of hypoxia in RPE metabolism is explored using this system. Hypoxia is an important contributor to the pathogenesis of AMD, as the choriocapillaris thins with age13,14. Combining this system with hypoxia chambers allows one to model altered RPE mitochondrial metabolism with the subtle hypoxia that accompanies aging. Finally, an online calculator using Resipher data is introduced to allow one to determine whether RPE cultures are in hypoxic, normoxic, or hyperoxic conditions. Such information is critical for drawing any conclusions about RPE metabolism from in vitro RPE culture studies.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>0.25% Trypsin-EDTA</td>
			<td>Gibco</td>
			<td>#25-200-056</td>
			<td></td>
		</tr>
		<tr>
			<td>3,3&#39;,5-triiodo-L-thyronine sodium salt</td>
			<td>Sigma</td>
			<td>T5516</td>
			<td></td>
		</tr>
		<tr>
			<td>32-channel Resipher lid</td>
			<td>Lucid Scientific</td>
			<td>NS32-101A for Falcon</td>
			<td></td>
		</tr>
		<tr>
			<td>Antimycin A from streptomyces sp.</td>
			<td>Sigma</td>
			<td>A8674-25MG</td>
			<td>Inhibitor of Complex III of the electron transport chain</td>
		</tr>
		<tr>
			<td>BAM15</td>
			<td>Sigma</td>
			<td>SML1760-5MG</td>
			<td>Uncoupling agent to increase mitochondrial respiration</td>
		</tr>
		<tr>
			<td>DMSO, cell culture grade</td>
			<td>Sigma-aldrich</td>
			<td>D4540-100ML</td>
			<td>Vehicle for reconstituting mitochondrial drugs</td>
		</tr>
		<tr>
			<td>Extracellular matrix coating substrates: 
			Synthemax II-SC</td>
			<td>Corning</td>
			<td>#3535</td>
			<td>Extracellular matrix for hfRPE</td>
		</tr>
		<tr>
			<td>Extracellular matrix coating substrates: Vitronectin</td>
			<td>Gibco</td>
			<td>A14700</td>
			<td>Extracellular matrix for iPSC-RPE</td>
		</tr>
		<tr>
			<td>FCCP</td>
			<td>Sigma</td>
			<td>C2920-10MG</td>
			<td>Uncoupling agent to increase mitochondrial respiration</td>
		</tr>
		<tr>
			<td>Fetal Bovine Serum (Bio-Techne S11550H)</td>
			<td>Bio-Techne</td>
			<td>S11550H</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrocortisone-Cyclodextrin</td>
			<td>Sigma</td>
			<td>H0396</td>
			<td></td>
		</tr>
		<tr>
			<td>Hypoxia chamber</td>
			<td>Embrient Inc.</td>
			<td>MIC-101</td>
			<td></td>
		</tr>
		<tr>
			<td>N1 Media Supplement</td>
			<td>Sigma</td>
			<td>N6530</td>
			<td></td>
		</tr>
		<tr>
			<td>Non-Essential Amino Acids Solution</td>
			<td>Gibco</td>
			<td>11140050</td>
			<td></td>
		</tr>
		<tr>
			<td>O2 sensor</td>
			<td>Sensit technology or Forensics Detectors</td>
			<td>P100 or FD-90A-O2</td>
			<td></td>
		</tr>
		<tr>
			<td>Penicillin-Streptomycin</td>
			<td>Gibco</td>
			<td>15140-122</td>
			<td></td>
		</tr>
		<tr>
			<td>Recombinant human TGF&#946;2</td>
			<td>Peprotech</td>
			<td>100-35B</td>
			<td>Transforming growth factor beta-2 to induce epithelial-mesenchymal transition</td>
		</tr>
		<tr>
			<td>Rotenone</td>
			<td>Sigma</td>
			<td>R8875-1G</td>
			<td>Inhibitor of Complex I of the electron transport chain</td>
		</tr>
		<tr>
			<td>System-compatible plate</td>
			<td>Corning</td>
			<td>#353072</td>
			<td></td>
		</tr>
		<tr>
			<td>Taurine</td>
			<td>Sigma</td>
			<td>T8691</td>
			<td></td>
		</tr>
		<tr>
			<td>&#945;MEM (Alpha Modification of Eagle&#39;s Media)</td>
			<td>Corning</td>
			<td>15-012-CV</td>
			<td></td>
		</tr>
	</tbody>
</table>

long-term monitoring of oxygen consumption rates in highly differentiated and polarized retinal pigment epithelial cultures

Mitochondrial metabolism is critical for the normal function of the retinal pigment epithelium (RPE), a monolayer of cells in the retina important for photoreceptor survival. RPE mitochondrial dysfunction is a hallmark of age-related macular degeneration (AMD), the leading cause of irreversible blindness in the developed world, and proliferative vitreoretinopathy (PVR), a blinding complication of retinal detachments. RPE degenerative conditions have been well-modeled by RPE culture systems that are highly differentiated and polarized to mimic in vivo RPE. However, monitoring oxygen consumption rates (OCR), a proxy for mitochondrial function, has been difficult in such culture systems because the conditions that promote ideal RPE polarization and differentiation do not allow for easy OCR measurements. 
Here, we introduce a novel system, Resipher, to monitor OCR for weeks at a time in well-differentiated RPE cultures while maintaining the RPE on optimal growth substrates and physiologic culture media in a standard cell culture incubator. This system calculates OCR by measuring the oxygen concentration gradient present in the media above cells. We discuss the advantages of this system over other methods for detecting OCR and how to set up the system for measuring OCR in RPE cultures. We cover key tips and tricks for using the system, caution about interpreting the data, and guidelines for troubleshooting unexpected results. 
We also provide an online calculator for extrapolating the level of hypoxia, normoxia, or hyperoxia RPE cultures experience based on the oxygen gradient in the media above cells detected by the system. Finally, we review two applications of the system, measuring the metabolic state of RPE cells in a PVR model and understanding how the RPE metabolically adapts to hypoxia. We anticipate that the use of this system on highly polarized and differentiated RPE cultures will enhance our understanding of RPE mitochondrial metabolism both under physiologic and disease states.

Author Spotlight: Extended Oxygen Consumption Measurement in Retinal Pigment Epithelium Using Resipher

Long-term Monitoring of Oxygen Consumption Rates in Highly Differentiated and Polarized Retinal Pigment Epithelial Cultures

The &#34;sandwich&#34; setup for the Resipher experiment is demonstrated in Figure 2A. Sensing lids with 32 probes corresponding to columns 3, 4, 9, and 10 on the 96-well plate sit between the cell plate and the Device. After connecting to the Hub, the Device activates motors to move the sensing lid up and down, measuring O2 concentration in the media column at a range of heights above the cell monolayer (typically 1-1.5 mm). The O2 gradient is therefore continuously me...

We introduce a novel device for measuring oxygen consumption rates (OCR) in retinal pigment epithelial (RPE) cultures. The device can measure OCR for weeks at a time on RPE grown on standard cell culture plates with standard media while the plates are in a standard cell culture incubator.

Mitochondrial metabolism is critical for the normal function of the retinal pigment epithelium (RPE), a monolayer of cells in the retina important for ...

long-term-monitoring-oxygen-consumption-rates-highly-differentiated

Research

JoVE Journal

Biology

4.5K Views.  University of Michigan, Ann Arbor. We introduce a novel device for measuring oxygen consumption rates (OCR) in retinal pigment epithelial (RPE) cultures. The device can measure OCR for weeks at a time on RPE grown on standard cell culture plates with standard media while the plates are in a standard cell culture incubator. 