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Method Article

Isolation of Mouse Retinal Capillaries and Subendothelial Matrix for Stiffness Measurement Using Atomic Force Microscopy

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DOI:

10.3791/66922

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July 12th, 2024

 ,  ,  ,  , 
1Department of Ophthalmology, University of California, Los Angeles, 2Doheny Eye Institute, 3Molecular, Cellular, and Integrative Physiology Interdepartmental PhD Program, University of California, Los Angeles, 4Department of Bioengineering, University of Pennsylvania

In This Article

Summary

We recently identified retinal capillary stiffening as a new paradigm for retinal dysfunction associated with diabetes. This protocol elaborates the steps for isolation of mouse retinal capillaries and the subendothelial matrix from retinal endothelial cultures, followed by a description of the stiffness measurement technique using atomic force microscopy.

Abstract

Retinal capillary degeneration is a clinical hallmark of the early stages of diabetic retinopathy (DR). Our recent studies have revealed that diabetes-induced retinal capillary stiffening plays a crucial and previously unrecognized causal role in inflammation-mediated degeneration of retinal capillaries. The increase in retinal capillary stiffness results from the overexpression of lysyl oxidase, an enzyme that crosslinks and stiffens the subendothelial matrix. Since tackling DR at the early stage is expected to prevent or slow down DR progression and associated vision loss, subendothelial matrix, and capillary stiffness represent relevant and novel therapeutic targets for early DR management. Further, direct measurement of retinal capillary stiffness can serve as a crucial preclinical validation step for the development of new imaging techniques for non-invasive assessment of retinal capillary stiffness in animal and human subjects. With this view in mind, we here provide a detailed protocol for the isolation and stiffness measurement of mouse retinal capillaries and subendothelial matrix using atomic force microscopy.

Introduction

Retinal capillaries are essential for maintaining retinal homeostasis and visual function. Indeed, their degeneration in early diabetes is strongly implicated in the development of vision-threatening complications of diabetic retinopathy (DR), a microvascular condition that affects nearly 40% of all individuals with diabetes1. Vascular inflammation contributes significantly to retinal capillary degeneration in DR. Past studies have demonstrated an important role for aberrant molecular and biochemical cues in diabetes-induced retinal vascular inflammation2,3. However, recent work has introduced a new paradigm for DR pathogenesis that identifies retinal capillary stiffening as a crucial yet previously unrecognized determinant of retinal vascular inflammation and degeneration4,5,6.

Specifically, the diabetes-induced increase in retinal capillary stiffness is caused by the upregulation of collagen crosslinking enzyme lysyl oxidase (LOX) in retinal endothelial cells (ECs), which stiffens the subendothelial matrix (basement membrane)4,5,6. Matrix stiffening, in turn, stiffens the overlying retinal ECs (due to mechanical reciprocity), thus leading to the overall increase in retinal capillary stiffness4. Crucially, this diabetes-induced retinal capillary stiffening alone can promote retinal EC activation and inflammation-mediated EC death. This mechanical regulation of retinal EC defects can be attributed to altered endothelial mechanotransduction, the process by which mechanical cues are converted into biochemical signals to produce a biological response7,8,9. Importantly, altered EC mechanical cues and subendothelial matrix structure have also been implicated in choroidal vascular degeneration associated with early age-related macular degeneration (AMD)10,11,12, which attests to the broader implications of vascular mechanobiology in degenerative retinal diseases.

Notably, retinal capillary stiffening occurs early on in diabetes, which coincides with the onset of retinal inflammation. Thus, the increase in retinal capillary stiffness may serve as both a therapeutic target and an early diagnostic marker for DR. To this end, it is important to obtain reliable and direct stiffness measurements of retinal capillaries and subendothelial matrix. This can be achieved by using an atomic force microscope (AFM), which offers a unique, sensitive, accurate, and reliable technique to directly measure the stiffness of cells, extracellular matrix, and tissues13. An AFM applies minute (nanoNewton-level) indentation force on the sample whose stiffness determines the extent to which the indenting AFM cantilever bends- the stiffer the sample, the more the cantilever bends, and vice versa. We have used AFM extensively to measure the stiffness of cultured endothelial cells, subendothelial matrices, and isolated mouse retinal capillaries4,5,6,11,12. These AFM stiffness measurements have helped identify endothelial mechanobiology as a key determinant of DR and AMD pathogenesis. To help broaden the scope of mechanobiology in vision research, here we provide a step-by-step guide on the use of AFM for stiffness measurements of isolated mouse retinal capillaries and subendothelial matrix.

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Protocol

All animal procedures were performed in accordance with the Association for Research in Vision and Ophthalmology (ARVO) Statement for the Use of Animals in Ophthalmic and Vision Research and approved by the Institutional Animal and Care Use Committees (IACUC; protocol number ARC-2020-030) at the University of California, Los Angeles (OLAW institution animal welfare assurance number A3196-01). The following protocol has been performed using retinal capillaries isolated from adult (20-week-old) male C57BL/6J mice weighing ~25 g (diabetic mice) and ~32 g (nondiabetic mice; Jackson Laboratory).

1. Isolation of mouse retinal capillaries for AFM stiffness measurement (Days 1-4)

NOTE: This protocol, reported in a recent study4, details the enucleation and mild fixation of the mouse eye, retinal isolation, and trypsin digestion, and subsequent mounting of the resultant retinal vasculature on microscopy slides for AFM stiffness measurement.

  1. Enucleation and mild fixation (Day 1)
    1. After euthanizing the mouse with carbon dioxide exposure followed by cervical dislocation, insert micro forceps behind the eyeball, hold onto the muscular attachment, and carefully pull out the eye without pinching the micro forceps too tightly, which might sever the optic nerve.
    2. Place the enucleated eye in 5% (v/v) formalin in PBS for 24 h at 4 °C for mild fixation.
      NOTE: Based on experience4, unfixed or more mildly fixed eyes yield fragile capillaries that become fragmented during AFM sample preparation and stiffness measurement.
  2. Retinal isolation (Day 2)
    1. Place the fixed eye on a piece of wax paper under a dissecting microscope. Next, using micro forceps, hold the remnants of the muscle and optic nerve attached to the outside of the posterior eye and orient the eye so that the cornea faces one side (Figure 1).
    2. Using a surgical blade, make an incision 1-2 mm behind and parallel to the limbus (cornea-sclera junction). Next, while holding the eye in place with the micro forceps, apply a downward force with the blade and continue pressing until the anterior segment of the eye is totally separated from the posterior end (Figure 1). Discard the anterior segment and lens. Do not saw back and forth as that may cause retinal damage.
    3. Transfer the posterior segment (sclera, choroid, and retina) into a 6 cm dish filled with PBS (pH 7.4).
    4. Using a micro spatula and simultaneously gently holding the optic nerve with micro forceps, scoop out the retina. Store the retina in a 2 mL microcentrifuge tube containing PBS at 4 °C until capillary isolation.
       
  3. Retinal rinses
    1. To rinse one retina, fill up six wells of a 12-well plate with 1 mL of double distilled H2O (ddH2O). Next, using an inverted Pasteur pipette, transfer the retina from the 2 mL microcentrifuge tube into the first well.
    2. Rinse the retina on the orbital shaker at 120 RPM for 30 min at room temperature (RT).
    3. Using a P1000 pipette, carefully pipette water up and down adjacent to the retina, blowing water at the retina to cause gentle agitation.
    4. Using the inverted glass pipette, transfer the retina to the second well and repeat steps 1.3.2 and 1.3.3 4 times, with each rinse done in a fresh (ddH2O-containing) well.
    5. Finally, transfer the retina to the sixth well and rinse it overnight (O/N) at RT on the orbital shaker set at 100 RPM to facilitate the separation of retinal neuroglia from blood vessels during trypsin digestion.
  4. Retinal trypsin digestion (Day 3)
    1. Prepare a 10% (w/v) trypsin solution by dissolving trypsin 1:250 powder in Tris buffer (pH 8; 0.1 M). Mix gently by inverting the conical tube several times to minimize bubble formation, which makes it harder to confirm trypsin solubility.
    2. Equilibrate the 10% trypsin solution in a 37 °C water bath for 10-15 min. Once the trypsin powder has completely dissolved, filter the solution using a 0.2 µm syringe filter.
    3. In a 12-well plate, add 2 mL/well/retina of the filtered 10% trypsin solution. Add some extra trypsin solution in the neighboring well (to equilibrate the glass pipette walls for subsequent steps).
    4. Add 3 mL of ddH2O in three wells of a 6-well plate (dedicate these three wells for one retina).
    5. In a 15 mL conical tube, insert a P200 tip before adding 4 mL of the filtered 10% trypsin solution. Transfer this conical tube to the 37 °C bath to allow the tip to be soaked in trypsin for 2-3 min, as that helps prevent the retina from sticking to tip walls in the latter steps.
    6. After O/N rinsing of the retina (from step 1.3.5), use a P1000 pipette to carefully pipette water up and down adjacent to the retina, squirting water on the retina to cause gentle agitation.
    7. Using an inverted glass pipette, transfer the rinsed retina to trypsin-containing-well of the 12-well plate (from step 1.4.3). Ensure that the retina is transferred with a minimal amount of residual ddH2O so as not to dilute the trypsin solution significantly. Incubate for 3 h at 37 °C.
    8. Centering the trypsin-soaked P200 tip (from step 1.4.5) on the optic nerve area, gently pipette the entire vascular network up and down to dissociate the non-vascular tissue14.
    9. Using an inverted glass pipette pre-rinsed in trypsin (by pipetting trypsin up and down 5 times), transfer the retinal vasculature to the first ddH2O-containing well of the 6-well plate (from step 1.4.4).
    10. Swirl the plate and pipette the vasculature up and down using an inverted trypsin-rinsed glass pipette to remove any residual non-vascular tissue.
    11. Using the glass pipette, transfer the retinal vasculature to the second ddH2O-containing well of the 6-well plate and store at 4 °C until mounting on the slide for AFM stiffness measurement.
      NOTE: Hydrated retinal vasculature (in PBS) is structurally intact at 4 °C for up to 2 weeks. However, routine stiffness measurements should be made from freshly isolated capillaries.
  5. Mounting the retinal vasculature for AFM stiffness measurement (Day 4)
    1. Using an inverted trypsin-rinsed glass pipette, transfer the retinal vasculature to the third ddH2O-containing well of the 6-well plate to remove any residual trypsin solution.
    2. Thoroughly clean (using ddH2O and wiper tissue) a charged surface microscopy slide that will be used for mounting the vascular network for AFM stiffness measurement.
      NOTE: The charged surface of the slide provides strong binding for the vascular network during AFM measurement.
    3. Label the slide and draw a 4-5 cm circular region around the center with a hydrophobic pen to avoid water spillage during the subsequent steps.
    4. Using an inverted trypsin-rinsed glass pipette, carefully transfer the retinal vasculature to the center of the marked region.
    5. Using a P200 pipette, carefully remove as much excess water as possible from one edge without accidentally aspirating the vasculature into the tip.
    6. Place the slide in a biosafety cabinet (BSL-2 hood) close to the front side (filtered mesh) and let the residual water evaporate.
    7. Once the vasculature is almost dry, under a phase contrast microscope (4x magnification), carefully rehydrate the vasculature by slowly adding ddH2O with a P200 pipette on to one side of the marked region. Adding ddH2O directly on the vasculature causes it to detach.
    8. Take phase contrast images of the rehydrated vasculature at 4x and 20x magnifications to ensure it has good structural integrity and is properly spread out (not folding on itself) and attached to glass slide, which are essential criteria for reliable AFM stiffness measurement.

2. Obtaining subendothelial matrix from retinal microvascular endothelial cell (REC) cultures (Days 1-17)

NOTE: This protocol, adapted from Beacham et al.15 and reported in recent studies4,5,6, describes REC culture on modified glass coverslips, followed by decellularization to obtain subendothelial matrix for subsequent AFM stiffness measurement.

  1. Preparation of gelatin-coated glass coverslips and cell plating (Day 1)
    NOTE: Perform the gelatin coating and subsequent PBS++ rinsing steps in a tissue culture hood before moving to a chemical fume hood for subsequent glutaraldehyde and ethanolamine treatment steps.
    1. Place one autoclaved 12 mm diameter circular glass coverslip per well of a sterile 24-well plate and add 500 µL of pre-warmed sterile 0.2% (w/v) gelatin (diluted in PBS containing calcium and Magnesium; PBS++) to each well containing coverslip. Incubate for 1 h at 37 °C.
    2. Aspirate the gelatin solution, rinse the coverslips once with sterile PBS++, and crosslink the coated gelatin by adding 500 µL of 1% (v/v) glutaraldehyde for 30 min at RT.
    3. Collect and properly dispose (as per institutional guidelines) the glutaraldehyde solution before rinsing the coverslips with PBS++ for 5 min on an orbital shaker at 100 RPM in RT. Repeat this rinsing step 5 times. During the first three rinses, lift the coverslips using a sterile tweezer to allow thorough rinsing of the glutaraldehyde. Any residual amount can reduce cell viability.
    4. Add 800 µL of 1 M ethanolamine to the gelatin-crosslinked coverslip for 30 min at RT to quench any remaining traces of glutaraldehyde in the well.
    5. Collect and properly dispose (as per institutional guidelines) of the ethanolamine solution and rinse the coverslips 5 times with PBS++ for 5 min on an orbital shaker at 100 rpm in RT. During the first three rinses, lift the coverslips using a sterile tweezer to allow thorough rinsing of the ethanolamine. Any residual amount can reduce cell viability.
    6. The crosslinked coverslips are now ready for cell plating.
      NOTE: Although we routinely plate cells immediately after coverslip preparation, it may be worthwhile to compare cell plating on crosslinked gelatin coverslips stored in PBS++ at 4 °C for 1-2 days.
    7. Under sterile conditions in a tissue culture hood, plate retinal microvascular endothelial cells (RECs) in 500 µL medium/well at a density that achieves 100% confluence within 24 h, as confirmed by phase contrast microscopy.
      NOTE: Reaching confluence within 24 h is important as the resulting REC quiescence will increase the ability of cells to secrete matrix (refer to the following step). In our experience with human RECs, a plating density of 4 x 104 cells/cm2 is sufficient to reach confluence within 24 h. However, as the REC size varies with species (e.g., mouse RECs are significantly smaller), the initial plating density may need to be optimized.
  2. Subendothelial matrix production by REC culture (Day 2-16)
    1. After cells reach confluence (~24 h), replace the culture medium with fresh medium supplemented with sterile-filtered ascorbic acid (final concentration of 200 µg/mL).
      NOTE: Any REC treatment with disease risk factors (e.g., high glucose, advanced glycation end products, etc.) or pharmacological agents can begin now, along with ascorbic acid treatment.
    2. Change the ascorbic acid-supplemented culture medium every other day for 15 days.
      NOTE: Ascorbic acid is unstable in solution. Prepare fresh 100X stock solution every other day for medium supplementation.
  3. Decellularization of REC cultures to obtain the subendothelial matrix (Day 16)
    1. After 15 days of ascorbic acid treatment, remove the medium and rinse the cells with calcium/magnesium-free PBS.
    2. Decellularize the REC cultures by adding 250 µL/per well of warm decellularization buffer (20 mM ammonium hydroxide and 0.5% Triton X-100 in PBS) for ~2-3 min. Confirm the removal of cells under a phase contrast microscope (10x magnification).
    3. Gently remove the decellularization buffer without disturbing the REC-secreted subendothelial matrix and add 0.5 mL of PBS to each well.
      NOTE: To ensure the subendothelial matrix does not detach during this and the subsequent rinsing steps, perform gentle rinsing only with a P1000 pipette. Do not perform vacuum aspiration.
    4. Store the plates at 4 °C overnight to stabilize the REC-secreted matrix.
  4. DNase treatment of subendothelial matrix to remove cellular debris (Day 17)
    1. Rinse the subendothelial matrix from step 2.3.4 with 800 µL of PBS/well.
    2. Gently remove the PBS and incubate the matrix with 200 µL of RNase-free DNase I (30 K units) at 37 °C for 30 min to remove all traces of cellular debris. Check the efficiency of DNase treatment by carefully looking for cell debris under a phase contrast microscope.
    3. Gently remove the DNase I solution and rinse the subendothelial matrix twice with 800 µL of PBS/well.
    4. Check that the macroscale fibrous subendothelial matrix is visible under a phase contrast microscope (10x magnification). Visualization of the finer nanoscale matrix fibers requires AFM topographical scanning or high-resolution confocal imaging of immunolabeled matrix proteins at 100x magnification.
    5. Immediately use the fresh (unfixed) subendothelial matrix for AFM stiffness measurement.
    6. Following AFM measurement, fix matrix samples with 1% (v/v) paraformaldehyde (15 min at room temperature) and stored at 4 °C for immunolabeling with antibodies against subendothelial matrix proteins and/or crosslinking enzymes.

3. AFM stiffness measurement

NOTE: This protocol, adapted from a standard AFM user manual and reported in recent studies4,5, details the acquisition and analysis of stiffness data from retinal capillaries and subendothelial matrix using an AFM and data analysis software. Although the steps outlined below are based on a specific model of AFM (see Table of Materials), the underlying principles are generally applicable to all AFM models.

  1. Cantilever probe selection
    ​NOTE: Soft biological samples require soft cantilevers that can bend upon sample indentation while the probe dimensions are selected to match the dimensions of the sample.
    1. For stiffness measurement of 5-8 µm diameter mouse retinal vessels, use a 1 µm radius cantilever probe with a spring constant (k) of 0.2-0.3 N/m. For stiffness measurement of a subendothelial matrix composed of nanoscale fibers, use a 70 nm radius cantilever probe with a spring constant (k) of 0.06-0.1 N/m.
  2. Cantilever mounting
    1. On the AFM computer, open the software that controls the AFM unit and the camera attached to a phase contrast microscope that is used to visualize the cantilever and sample.
    2. Fix the cantilever holder onto the cantilever changing stand and mount the selected cantilever on the holder using watchmaker forceps under a stereoscope without physically damaging the cantilever. Tighten the screw on the holder to secure the cantilever.
    3. Place and lock the cantilever holder on the AFM head that is resting on the stand.
    4. Using the step motor function in the AFM software, withdraw the AFM head to the highest point and set that position as point zero in the software. This step prevents the AFM cantilever from accidentally hitting the sample stage while mounting the AFM head (next step).
    5. Carefully lift the AFM head from its stand and mount it on the AFM sample stage by placing the legs in their respective slots.
  3. Laser alignment
    ​NOTE: The initial laser alignment is performed without any sample (i.e., in Air mode) as it ensures a more precise alignment of the laser beam on the cantilever tip (by preventing laser refraction in liquid). However, as biological samples are immersed in a medium that has a different refractive index than air, it is important to realign the laser in the medium before stiffness measurement.
    1. For laser alignment in air, place the AFM head on the sample stage and use the 10x objective and attached camera to visualize the cantilever on the monitor in live mode. The infrared laser is hidden from the user's view but is visible with a CCD camera.
    2. Select the Contact Mode Force Spectroscopy function on the software and open the window entitled Laser alignment.
    3. Using the two screws marked laser lateral on the AFM head, focus the laser beam on the cantilever such that the SUM value in the laser alignment window is the highest. To achieve this, focus the laser at or around the center of the cantilever.
    4. Using the Detector adjustment screws, adjust the photodetector such that the laser spot is at the center of the laser alignment window.
    5. Using the Mirror adjustment screw, adjust the mirror to ensure that the SUM value is at the highest possible value.
      NOTE: A high SUM value ensures a sensitive and accurate assessment of sample stiffness. Thus, both the laser alignment and mirror adjustment steps should be performed to obtain the highest SUM value.
    6. Next, for laser alignment in liquid, add the desired culture medium in a dish and mount it firmly on stage to prevent any vibration or drift, which creates measurement artifacts during sample indentation.
    7. While looking at the live camera feed focused on the cantilever, lower it into the liquid using the step motor function.
    8. Repeat steps 3.3.4 and 3.3.5 to ensure that the SUM value remains the highest and the laser spot is at the center of the laser alignment window.
  4. Calibration of cantilever spring constant
    NOTE: Although cantilever probes come with a manufacturer-calibrated spring constant (k), it is good practice to independently verify its value (in liquid) before the start of a measurement. Use a clean glass surface that minimizes interactions between the cantilever probe and the glass surface.
    1. The setpoint force applied by the cantilever sets the maximum laser deflection from the cantilever that is allowed for a force indentation. Set it initially at 1.5 V. If the cantilever fails to approach at this given setpoint force, increase it incrementally until it indents the sample and generates a force indentation curve.
    2. Z Length signifies the maximum distance by which the z-piezo withdraws after the cantilever has reached the setpoint during sample indentation. Z length should be at least enough to ensure the cantilever probe separates cleanly from the sample. For calibration of spring constant on a glass surface, set Z Length at 1 µm.
    3. Z speed refers to the speed at which the z-piezo moves the cantilever vertically down towards the sample during force indentation. It should be optimized because a very low speed will primarily capture the viscous behavior of the sample, while a very high speed will primarily capture the elastic behavior. The optimal speed is expected to capture the true viscoelastic nature of biological samples. Set the Z speed to 2 µm/s for the viscoelastic biological samples.
    4. The Target height on the Z range indicates the approximate distance from the sample at which the z-piezo rests after measuring a force curve and before moving to a different location on the sample. The Z Range target height should be greater than the height of the sample features. Set target height on the Z range at 7.5 µm.
    5. Using the Step Motor function, bring the cantilever fairly close to the sample surface (judging by the focus in phase contrast image at 10x magnification) before selecting the Approach function in the contact mode force spectroscopy window to bring down the cantilever in smaller increments of 15 µm.
    6. Click Acquire to capture a force curve.
    7. Select the Force Curve, open it in the calibration manager window, and select Contact-based Mode in the method section.
    8. Using the Select-fit Range function, select the Cantilever Retraction Curve for a linear curve fit. Next, check the Sensitivity Check box to convert the force unit from V to N.
    9. Lift the cantilever by 100-200 µm in the liquid and select the Thermal Noise function. Again, using Select Fit Range, fit the thermal noise bell curve with a Lorentz curve. After curve fitting, select the Spring Constant (k) box. Confirm that spring constant (k) is close to the manufacturer's value and note it down for future reference. After calibration, the unit for setpoint will change from mV to nN.
      ​NOTE: Thermal noise is the natural frequency of the cantilever at a particular temperature. Correction for thermal noise is required for accurate spring constant measurement.
  5. Acquisition of force-distance curves
    1. Place the slide-mounted sample (retinal vessels or subendothelial matrix) on the AFM stage and select Contact Mode Force Spectroscopy in the software's experiment section.
    2. Set the setpoint force at 0.5 nN and click Approach, which is significantly higher than the thermal deflection of both SAA-SPH 1 µm and PFQNM-LC-A 70 nm cantilever probes, ensuring a smooth cantilever approach towards the sample.
    3. After the cantilever has detected the surface and returned to its resting target height, set the setpoint at 0.2 nN for the stiffer SAA-SPH 1 µm cantilever probe or 0.1 nN for the softer PFQNM-LC-A 70 nm cantilever probe. This setpoint adjustment ensures that both stiff and soft cantilevers bend readily during sample indentation (refer to section 3.1).
    4. Set Z Length at ~2.5 µm to ensure clean separation of the cantilever probe from biological samples during retraction (refer to step 3.4.2) and Z speed at 2 µm/s (refer to step 3.4.3).
    5. Using the stage screws on the AFM stage, carefully position the cantilever probe at a desired location on the sample.
      NOTE: Since capillaries do not always spread flat on the slide, it is safe to withdraw the cantilever a further 10-20 µm from its resting target height before moving the stage.
    6. Click Approach to first bring the cantilever closer to the sample and then click Acquire to capture the force curves for the desired locations. Save all force curves for analysis.
  6. Data analysis
    1. Open the force curve in the data processing software.
    2. Select the Retraction Force Curve for data analysis (similar to step 3.4.8).
    3. Using the Data smoothing function, smoothen the force curve with a Gaussian filter to remove the unwanted noise in the acquired data.
    4. Using the Baseline subtraction function, adjust the value of the slope and magnitude of the (non-contact) baseline portion of the force curve to zero.
    5. Click on the Contact Point function to automatically bring the contact point of the force curve to the (0,0) coordinates on X- and Y-axes.
    6. Using the Vertical Tip Position function, calculate the actual vertical position of the cantilever on the Y-axis by correcting for any sample indentation.
    7. On the processed force curve, apply the Elasticity Fit function by first selecting the tip shape and radius (based on the selected cantilever probe), followed by force curve fitting using the Hertz/Sneddon model.
      1. If matrix indentation by the 70 nm radius probe exceeds 70 nm, which is typically the case, select the Paraboloid tip shape. For capillary stiffness measurement, sample indentation by the 1 µm radius probe never exceeds 1 µm, so select the Sphere tip shape. Further, if the contact point of the fitted curve does not coincide with the contact point of the actual retraction curve, select the Shift Curve check box.
    8. Note and save the value of Young's modulus (stiffness).

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Results

Mouse retinal capillaries
AFM stiffness measurement of isolated retinal capillaries involves sample handling steps that could potentially damage their mechanostructural integrity. To prevent this and thereby ensure the feasibility, reliability, and reproducibility of AFM measurements, the enucleated eyes are fixed in 5% formalin overnight at 4 °C prior to vessel isolation. This mild fixation protocol with reduced formalin concentration, low fixation temperature, limited fixation time, and lack of cor...

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Discussion

AFM has been widely used to measure disease-associated changes in the stiffness of larger vessels, such as the aorta and arteries16. These findings have helped establish the role of endothelial mechanobiology in cardiovascular complications such as atherosclerosis17. Based on these findings, we have begun to investigate the previously unrecognized role of endothelial mechanobiology in the development of retinal microvascular lesions in early DR. Success in this pursuit, how...

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Disclosures

The authors have nothing to disclose.

Acknowledgements

This work was supported by National Eye Institute/NIH grant R01EY028242 (to K.G.), Research to Prevent Blindness/International Retinal Research Foundation Catalyst Award for Innovative Research Approaches for AMD (to K.G.), The Stephen Ryan Initiative for Macular Research (RIMR) Special Grant from W.M. Keck Foundation (to Doheny Eye Institute), and Ursula Mandel Fellowship and UCLA Graduate Council Diversity Fellowship (to I.S.T.). This work was also supported by an Unrestricted Grant from Research to Prevent Blindness, Inc. to the Department of Ophthalmology at UCLA. The content in this article is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Retinal Capillary Isolation
0.22 µm PVDF syringe filterMerck MilliporeSLGVM33RSLow Protein Binding Durapore
10X Dulbecco's Phosphate Buffered Saline without calcium % magnesiumCorning20-031-CVFinal concentration 1X, pH 7.4
12-well plateFalcon Corning353043
15 mL centrifuge tubeCorning430791Rnase-Dnase-free, Nonpyrogenic
20 mL Luer-Lok TIP syringeBD302830
5 3/4 inch Disposable Borosilicate Glass/Non-sterile Pasteur pipetteFisherBrand13-678-20A
60x15 mm Tissue Culture DishFalcon Corning353002
6-well plateFalcon Corning353046
Aqua-Hold 2 Pap - 13 mL PenScientific Device Laboratory9804-02
Blade holderX-ACTO
Carbon Steel Surgical Blade #10Bard-Parker371110
Dental WaxElectron Microscopy Sciences50-949-027
Dissecting microscopeAm-scope
Formalin solution, neutral buffered, 10%Millipore SigmaHT501128-4LFinal concentration 5% (v/v)
Kimwipes - wiper tissueKimtech Science34133
Micro spatulaFine Science Tools10089-11
Orbital ShakerLab GeniusSK-O180
PELCO Economy #7 Stainless Steel 115mm  TweezerTed Pella, Inc.5667
Phase contrast microscopeNikon TS2
Purifier Logic+ Class II, Type A2 Biosafety CabinetLabconco302380001
Safe-Lock microcentrifuge tubes 2 mLEppendorf22363352
StereoscopeAmScopeSM-3 Series Zoom Trinocular Stereomicroscope 3.5X-90X
Superfrost Plus microscopy slide - White tab - Pre-cleaned - 25x75x1.0 mmFisherBrand1255015
Tris Buffer, 0.1M solution, pH 7.4 - Biotechnology GradeVWRE553-500MLpH 8 for trypsin solution
Trypsin 1:250 powder Tissue Culture GradeVWRVWRV0458-25G10 % (w/v) trypsin solution
Water Molecular Biology GradeCorning46-000-CM
Subendothelial Matrix
10X PBSCorning20-031-CV
1X PBS with calcium and magnesiumThermo Fisher Scientific14040-117pH 7.4
Ammonium hydroxideSigma-Aldrich338818
Ascorbic AcidSigma-AldrichA4034
Collagen IV antibodyNovus BiologicalsNBP1-26549
DNase IQiagen79254
EthanolamineSigma-Aldrich398136
Fibronectin antibodySigma-AldrichF6140
FluoromountInvitrogen-Thermo Fisher Scientific00-4958-02
GelatinSigma-AldrichG1890
Glass coverslips (12mm)Fisher12-541-000
GlutaraldehydeElectron microscopy Sciences16220
Human retinal endothelial cells (HREC)Cell Systems CorpACBRI 181
MCDB131 mediumCorning15-100-CV
Mouse retinal endothelial cells (mREC)Cell BiologicsC57-6065
Triton X-100Thermo Fisher Scientific BP151-100
TrypsinGibco-Thermo Fisher Scientific25200-056
AFM Measurement
1 µm ProbeBrukerSAA-SPH-1UMA 19 micron tall hemispherical probe with 1
micron end radius, Spring constant 0.25N/m
70 nm LC probeBrukerPFQNM-LC-V2A 19 micron tall hemispherical probe with 70nm end radius,
 Spring constant 0.1N/m
 cameraXCAM familyToupcam1080P HDMI
Desktop to run the cameraAsusAsus desktopIntel i5-6600 CPU , 8GB RAM
Dish holder for coverslipCellvisD29-14-1.5-N29mm glass bottom dish with
 14 mm micro-well
Nanowizard 4BrukerNanowizard 4Bioscience atomic force microscope mounted on an optical microscope for sensitive measurement of the mechanostructural properties (stiffness and topography) of soft biological samples
Phase contrast micrscopeZeissAxiovert 200Inverted microscope with 10X objective

References

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Capillary StiffnessDiabetic RetinopathyVascular StiffnessRetinal Vessel IsolationPhase Contrast MicroscopyDecellularization BufferCollagen Four Matrix
Isolation of Mouse Retinal Capillaries and Subendothelial Matrix for Stiffness Measurement Using Atomic Force Microscopy
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