Treatment with both TGF-β1 and PDGF-BB disrupts the stiffness-dependent myofibroblast differentiation of corneal keratocytes

During corneal wound healing, stromal keratocytes transform into a repair phenotype that is driven by the release of cytokines, like transforming growth factor-beta 1 (TGF-β1) and platelet-derived growth factor-BB (PDGF-BB). Previous work has shown that TGF-β1 promotes the myofibroblast differentiation of corneal keratocytes in a manner that depends on PDGF signaling. In addition, changes in mechanical properties are known to regulate the TGF-β1-mediated differentiation of cultured keratocytes. While PDGF signaling acts synergistically with TGF-β1 during myofibroblast differentiation, how treatment with multiple growth factors affects stiffness-dependent differences in keratocyte behavior is unknown. Here, we treated primary corneal keratocytes with PDGF-BB and TGF-β1 and cultured them on polyacrylamide (PA) substrata of different stiffnesses. In the presence of TGF-β1 alone, the cells underwent stiffness-dependent myofibroblast differentiation. On stiff substrata, the cells developed robust stress fibers, exhibited high levels of ⍺-SMA staining, formed large focal adhesions (FAs), and exerted elevated contractile forces, whereas cells in a compliant microenvironment showed low levels of ⍺-SMA immunofluorescence, formed smaller focal adhesions, and exerted decreased contractile forces. When the cultured keratocytes were treated simultaneously with PDGF-BB however, increased levels of ⍺-SMA staining and stress fiber formation were observed on compliant substrata, even though the cells did not exhibit elevated contractility or focal adhesion size. Pharmacological inhibition of PDGF signaling disrupted the myofibroblast differentiation of cells cultured on substrata of all stiffnesses. These results indicate that treatment with PDGF-BB can decouple molecular markers of myofibroblast differentiation from the elevated contractile phenotype otherwise associated with these cells, suggesting that crosstalk in the mechanotransductive signaling pathways downstream of TGF-β1 and PDGF-BB can regulate the stiffness-dependent differentiation of cultured keratocytes.

Statement of Significance

In vitro experiments have shown that changes in ECM stiffness can regulate the differentiation of myofibroblasts. Typically, these assays involve the use of individual growth factors, but it is unclear how stiffness-dependent differences in cell behavior are affected by multiple cytokines. Here, we used primary corneal keratocytes to show that treatment with both TGF-β1 and PDGF-BB disrupts the dependency of myofibroblast differentiation on substratum stiffness. In the presence of both growth factors, keratocytes on soft substrates exhibited elevated ⍺-SMA immunofluorescence without a corresponding increase in contractility or focal adhesion formation. This result suggests that molecular markers of myofibroblast differentiation can be dissociated from the elevated contractile behavior associated with the myofibroblast phenotype, suggesting potential crosstalk in mechanotransductive signaling pathways downstream of TGF-β1 and PDGF-BB.

Investigating transcriptional differences in mechanotransductive and ECM related genes in cultured primary corneal keratocytes, fibroblasts and myofibroblasts

After a stromal injury in the cornea, the release of growth factors and pro-inflammatory cytokines typically results in the activation of quiescent keratocytes toward migratory fibroblast and/or fibrotic myofibroblast phenotypes. The persistence of the myofibroblast phenotype can lead to corneal fibrosis and scarring, which are leading causes of blindness worldwide. The primary goal of this study was to establish comprehensive transcriptional profiles for cultured corneal keratocytes, fibroblasts, and myofibroblasts to gain insights into the mechanisms through which changes in phenotype may occur. Here, we cultured primary rabbit corneal keratocytes on collagen-coated glass coverslips in serum free media (SF), serum containing media (FBS), or in the presence of TGF-β1 to induce keratocyte, fibroblast, and myofibroblast phenotypes, respectively. Total RNA was collected and sent to Novogene for bulk RNA sequencing. Subsequent bioinformatic analysis included gene expression quantification, differential expression, and functional analysis. When comparing FBS and TGF-β1 conditions to SF, genes characteristic of a quiescent keratocyte phenotype were downregulated (e.g. KERA, LUM, ALDH1A1), while genes commonly associated with fibroblasts or myofibroblasts were upregulated (e.g. VIM, TNC, FN1, ITGA5, ACTA2). Functional analysis of genes differentially expressed between fibroblasts and keratocytes highlighted pathways related to proliferation (e.g. DNA replication, PI3K-Akt signaling) and cell migration (e.g. Rap1 signaling, ECM-receptor interactions). Enriched pathways for the comparison of myofibroblasts to keratocytes included focal adhesion, regulation of actin cytoskeleton, hippo signaling, and ECM-receptor interaction pathways. Together, these pathways support changes in cytoskeletal organization, cell contractility, mechanotransduction, and cell-ECM interactions in myofibroblasts compared to keratocytes. Overall, these data demonstrate that there are distinct transcriptional differences between cultured corneal keratocytes, fibroblasts, and myofibroblasts. In our initial analysis, we have identified genes and signaling pathways that may play important roles in keratocyte differentiation, including many related to proliferation, cell mechanical activity, and ECM interactions. Furthermore, our findings reveal novel markers for each cell type as well as possible targets for modulating cell behavior and differentiation to promote physiological corneal wound healing.

Fabrication of Micropatterns of Aligned Collagen Fibrils

Many tissues in vivo contain aligned structures such as filaments, fibrils, and fibers, which expose cells to anisotropic structural and topographical cues that range from the nanometer to micrometer scales. Understanding how cell behavior is regulated by these cues during physiological and pathological processes (e.g., wound healing, cancer invasion) requires substrates that can expose cells to anisotropic cues over several length scales. In this study, we developed a novel method of fabricating micropatterns of aligned collagen fibrils of different geometry onto PDMS-coated glass coverslips that allowed us to investigate the roles of topography and confinement on corneal cell behavior. When corneal cells were cultured on micropatterns of aligned collagen fibrils in the absence of confinement, the degree of cell alignment increased from 40 ± 14 to 82 ± 5% as the size of the micropattern width decreased from 750 to 50 μm. Although the cell area (∼2500 μm2), cell length (∼160 μm), and projected nuclear area (∼175 μm2) were relatively constant on the different micropattern widths, cells displayed an increased aspect ratio as the width of the aligned collagen fibril micropatterns decreased. We also observed that the morphology of cells adhering to the surrounding uncoated PDMS was dependent upon both the size of the aligned collagen fibril micropattern and the distance from the micropatterns. When corneal cells were confined to the micropatterns of aligned collagen fibrils by a Pluronic coating to passivate the surrounding area, a similar trend in increasing cell alignment was observed (35 ± 10 to 89 ± 2%). However, the projected nuclear area decreased significantly (∼210 to 130 μm2) as the micropattern width decreased from 750 to 50 μm. The development of this method allows for the deposition of aligned collagen fibril micropatterns of different geometries on a transparent and elastic substrate and provides an excellent model system to investigate the role of anisotropic cues in cell behavior.

The impact of UV cross-linking on corneal stromal cell migration, differentiation and patterning

Previous studies have demonstrated that UV cross-linking (CXL) increases stromal stiffness and produces alterations in extracellular matrix (ECM) microstructure. In order to investigate how CXL impacts both keratocyte differentiation and patterning within the stroma, and fibroblast migration and myofibroblast differentiation on top of the stroma, we combined CXL with superficial phototherapeutic keratectomy (PTK) in a rabbit model. Twenty-six rabbits underwent a 6 mm diameter, 70 μm deep phototherapeutic keratectomy (PTK) with an excimer laser to remove the epithelium and anterior basement membrane. In 14 rabbits, standard CXL was performed in the same eye immediately after PTK. Contralateral eyes served as controls. In vivo confocal microscopy through focusing (CMTF) was used to analyze corneal epithelial and stromal thickness, as well as stromal keratocyte activation and corneal haze. CMTF scans were collected pre-operatively, and from 7 to 120 days after the procedure. A subset of rabbits was sacrificed at each time point, and corneas were fixed and labeled in situ for multiphoton fluorescence microscopy and second harmonic generation imaging. In vivo and in situ imaging demonstrated that haze after PTK was primarily derived from a layer of myofibroblasts that formed on top of the native stroma. Over time, this fibrotic layer was remodeled into more transparent stromal lamellae, and quiescent cells replaced myofibroblasts. Migrating cells within the native stroma underneath the photoablated area were elongated, co-aligned with collagen, and lacked stress fibers. In contrast, following PTK + CXL, haze was derived primarily from highly reflective necrotic "ghost cells" in the anterior stroma, and fibrosis on top of the photoablated stroma was not observed at any time point evaluated. Cells formed clusters as they migrated into the cross-linked stromal tissue and expressed stress fibers; some cells at the edge of the CXL area also expressed α-SM actin, suggesting myofibroblast transformation. Stromal thickness increased significantly between 21 and 90 days after PTK + CXL (P < 0.001) and was over 35 μm higher than baseline at Day 90 (P < 0.05). Overall, these data suggest that cross-linking inhibits interlamellar cell movement, and that these changes lead to a disruption of normal keratocyte patterning and increased activation during stromal repopulation. Interestingly, CXL also prevents PTK-induced fibrosis on top of the stroma, and results in long term increases in stromal thickness in the rabbit model.

Effects of Topography and PDGF on the Response of Corneal Keratocytes to Fibronectin-Coated Surfaces

During corneal wound healing, corneal keratocytes are exposed to both biophysical and soluble cues that cause them to transform from a quiescent state to a repair phenotype. How keratocytes integrate these multiple cues simultaneously is not well understood. To investigate this process, primary rabbit corneal keratocytes were cultured on substrates patterned with aligned collagen fibrils and coated with adsorbed fibronectin. After 2 or 5 days of culture, keratocytes were fixed and stained to assess changes in cell morphology and markers of myofibroblastic activation by fluorescence microscopy. Initially, adsorbed fibronectin had an activating effect on the keratocytes as evidenced by changes in cell shape, stress fiber formation, and expression of alpha-smooth muscle actin (α-SMA). The magnitude of these effects depended upon substrate topography (i.e., flat substrate vs aligned collagen fibrils) and decreased with culture time. When keratocytes were simultaneously exposed to adsorbed fibronectin and soluble platelet-derived growth factor-BB (PDGF-BB), the cells elongated and had reduced expression of stress fibers and α-SMA. In the presence of PDGF-BB, keratocytes plated on the aligned collagen fibrils elongated in the direction of the fibrils. These results provide new information on how keratocytes respond to multiple simultaneous cues and how the anisotropic topography of aligned collagen fibrils influences keratocyte behavior.

Temporal and Spatial Assessment of the Corneal Response to UV Cross-Linking Using 3-Dimensional In Vivo Confocal Microscopy

ABSTRACT

The goal of this study was to evaluate the temporal and spatial pattern of wound healing following UV corneal cross-linking (CXL) using 3-dimensional (3-D) confocal imaging in vivo. Using a modified Heidelberg Retinal Tomograph with Rostock Corneal Module confocal microscope, we performed 3-D scans on two patients at multiple time points after CXL. Patient 1 showed a normal post-CXL wound healing response, with initial subbasal nerve loss and keratocyte apoptosis in the anterior stroma, followed by partial restoration of both the nerve plexus and stromal keratocytes by 6 months. In patient 2, in addition to anterior corneal damage, pyknotic nuclei were observed in the posterior stroma 7 days after CXL. Acellular areas were present in the posterior stroma at 3 months, with only partial keratocyte repopulation at 6 months. Regeneration of the subbasal nerve plexus was also delayed. Three-dimensional confocal imaging allowed these unusual wound healing responses to be identified in the absence of any corresponding clinical observations.

ECM stiffness modulates the proliferation but not the motility of primary corneal keratocytes in response to PDGF-BB

During corneal wound healing, keratocytes present within the corneal stroma become activated into a repair phenotype upon the release of growth factors, such as transforming growth factor-beta 1 (TGF-β1) and platelet-derived growth factor-BB (PDGF-BB). The process of injury and repair can lead to changes in the mechanical properties of the tissue, and previous work has shown that the TGF-β1-mediated myofibroblast differentiation of corneal keratocytes depends on substratum stiffness. It is still unclear, however, if changes in stiffness can modulate keratocyte behavior in response to other growth factors, such as PDGF-BB. Here, we used a polyacrylamide (PA) gel system to determine whether changes in stiffness influence the proliferation and motility of primary corneal keratocytes treated with PDGF-BB. In the presence of PDGF-BB, cells on stiffer substrata exhibited a more elongated morphology and had higher rates of proliferation than cells in a more compliant microenvironment. Using a freeze-injury to assay cell motility, however, we did not observe any stiffness-dependent differences in the migration of keratocytes treated with PDGF-BB. Taken together, these data highlight the importance of biophysical cues during corneal wound healing and suggest that keratocytes respond differently to changes in ECM stiffness in the presence of different growth factors.

Signaling Downstream of Focal Adhesions Regulates Stiffness-Dependent Differences in the TGF-β1-Mediated Myofibroblast Differentiation of Corneal Keratocytes

Following injury and refractive surgery, corneal wound healing can initiate a protracted fibrotic response that interferes with ocular function. This fibrosis is related, in part, to the myofibroblast differentiation of corneal keratocytes in response to transforming growth factor beta 1 (TGF-β1). Previous studies have shown that changes in the mechanical properties of the extracellular matrix (ECM) can regulate this process, but the mechanotransductive pathways that govern stiffness-dependent changes in keratocyte differentiation remain unclear. Here, we used a polyacrylamide (PA) gel system to investigate how mechanosensing via focal adhesions (FAs) regulates the stiffness-dependent myofibroblast differentiation of primary corneal keratocytes treated with TGF-β1. Soft (1 kPa) and stiff (10 kPa) PA substrata were fabricated on glass coverslips, plated with corneal keratocytes, and cultured in defined serum free media with or without exogenous TGF-β1. In some experiments, an inhibitor of focal adhesion kinase (FAK) activation was also added to the media. Cells were fixed and stained for F-actin, as well as markers for myofibroblast differentiation (α-SMA), actomyosin contractility phosphorylated myosin light chain (pMLC), focal adhesions (vinculin), or Smad activity (pSmad3). We also used traction force microscopy (TFM) to quantify cellular traction stresses. Treatment with TGF-β1 elicited stiffness-dependent differences in the number, size, and subcellular distribution of FAs, but not in the nuclear localization of pSmad3. On stiff substrata, cells exhibited large FAs distributed throughout the entire cell body, while on soft gels, the FAs were smaller, fewer in number, and localized primarily to the distal tips of thin cellular extensions. Larger and increased numbers of FAs correlated with elevated traction stresses, increased levels of α-SMA immunofluorescence, and more prominent and broadly distributed pMLC staining. Inhibition of FAK disrupted stiffness-dependent differences in keratocyte contractility, FA patterning, and myofibroblast differentiation in the presence of TGF-β1. Taken together, these data suggest that signaling downstream of FAs has important implications for the stiffness-dependent myofibroblast differentiation of corneal keratocytes.

Evaluation of the Repeatability of the LacryDiag Ocular Surface Analyzer for Assessment of the Meibomian Glands and Tear Film

Purpose

This study aimed to determine the intra- and interobserver repeatability of the new LacryDiag Ocular Surface Analyzer and compare it to a similar all-in-one device, the OCULUS Keratograph 5M.

Methods

Thirty healthy subjects aged 18 years and above were recruited for this study. All patients were free of any existing ocular pathology. The LacryDiag Ocular Surface Analyzer was used to evaluate tear meniscus height, interferometry, noninvasive tear break-up time (NIBUT), and meibography. The same or analogous exams were performed using the OCULUS Keratograph 5M. Test equivalation was used to compare data from corresponding examinations. Paired t-tests and coefficient of variation were used to determine inter- and intraobserver repeatability. Bland-Altman analysis was used to determine level of agreement between devices.

Results

There were no differences in mean values for tear meniscus height, NIBUT, or tear film interferometry between observers for either device. Significant differences were found between observers for meibography when using the LacryDiag (P = 0.008 for percent loss calculation and P = 0.004 for grading scale). Intra-observer variability for NIBUT was significantly higher for the Keratograph (P = 0.0003 for observer A and P < 0.0001 for observer B).

Conclusions

There was a good correlation but poor agreement between devices for a given observer. This was likely influenced by the use of repeated testing and the non-dry eye cohort.

Translational Relevance

Both the repeatability of the testing device and the use of multiple outcome measures are essential for the diagnosis and monitoring of patients with dry eye disease (DED).

Infectious keratitis after corneal crosslinking: systematic review

Corneal crosslinking is a U.S. Food and Drug Administration--approved therapy to stiffen the cornea and prevent progression of corneal ectasia in patients with keratoconus. The standard procedure involves removal of the corneal epithelium (epithelial-off) prior to treatment. Variations to the standard procedure include accelerated crosslinking and transepithelial procedures. This study reviewed what is known regarding the risk for infection after epithelial-off crosslinking, the spectrum of pathogens, and clinical outcomes. 26 publications were identified. All eyes were fit with a bandage contact lens postoperatively. Available data indicate that the overall frequency of infectious keratitis after epithelium-off crosslinking is low. Bacterial infections are the most common, with a mean time of presentation of 4.8 days postoperatively. The use of steroids and bandage contact lenses in the immediate postoperative period and/or a history of atopic or herpetic disease were associated with infection. These patients require intense postoperative care with prophylactic antiviral therapy when appropriate.

Keratocyte mechanobiology

In vivo, corneal keratocytes reside within a complex 3D extracellular matrix (ECM) consisting of highly aligned collagen lamellae, growth factors, and other extracellular matrix components, and are subjected to various mechanical stimuli during developmental morphogenesis, fluctuations in intraocular pressure, and wound healing. The process by which keratocytes convert changes in mechanical stimuli (e.g. local topography, applied force, ECM stiffness) into biochemical signaling is known as mechanotransduction. Activation of the various mechanotransductive pathways can produce changes in cell migration, proliferation, and differentiation. Here we review how corneal keratocytes respond to and integrate different biochemical and biophysical factors. We first highlight how growth factors and other cytokines regulate the activity of Rho GTPases, cytoskeletal remodeling, and ultimately the mechanical phenotype of keratocytes. We then discuss how changes in the mechanical properties of the ECM have been shown to regulate keratocyte behavior in sophisticated 2D and 3D experimental models of the corneal microenvironment. Finally, we discuss how ECM topography and protein composition can modulate cell phenotypes, and review the different methods of fabricating in vitro mimics of corneal ECM topography, novel approaches for examining topographical effects in vivo, and the impact of different ECM glycoproteins and proteoglycans on keratocyte behavior.

ECM Stiffness Controls the Activation and Contractility of Corneal Keratocytes in Response to TGF-β1

After surgery or traumatic injury, corneal wound healing can cause a scarring response that stiffens the tissue and impairs ocular function. This fibrosis is caused in part by the activation of corneal keratocytes from a native mechanically quiescent state to an activated myofibroblastic state. This transformation is tied to signaling downstream of transforming growth factor-β1 (TGF-β1). Here, to better understand how biochemical and biophysical cues interact to regulate keratocyte activation and contractility, we cultured primary rabbit corneal keratocytes on flexible substrata of varying stiffness in the presence (or absence) of TGF-β1. Time-lapse fluorescence microscopy was used to assess changes in keratocyte morphology, as well as to quantify the dynamic traction stresses exerted by cells under different experimental conditions. In other experiments, keratocytes were fixed after 5 days of culture and stained for markers of both contractility and myofibroblastic activation. Treatment with TGF-β1 elicited distinct phenotypes on substrata of different stiffnesses. Cells on soft (1 kPa) gels formed fewer stress fibers and retained a more dendritic morphology, indicative of a quiescent keratocyte phenotype. Keratocytes cultured on stiff (10 kPa) gels or collagen-coated glass coverslips, however, had broad morphologies, formed abundant stress fibers, exhibited greater levels of α-smooth muscle actin (α-SMA) expression, and exerted larger traction forces. Confocal images of phospho-myosin light chain (pMLC) immunofluorescence, moreover, revealed stiffness-dependent differences in the subcellular distribution of actomyosin contractility, with pMLC localized at the tips of thin cellular processes in mechanically quiescent cells. Importantly, keratocytes cultured in the absence of TGF-β1 showed no stiffness-dependent differences in α-SMA immunofluorescence, suggesting that a stiff microenvironment alone is insufficient to induce myofibroblastic activation. Taken together, these data suggest that changes in ECM stiffness can modulate the morphology, cytoskeletal organization, and subcellular pattern of force generation in corneal keratocytes treated with TGF-β1.

Outcomes of resident-performed laser-assisted vs traditional phacoemulsification

PURPOSE

To compare the effectiveness of femtosecond laser-assisted cataract surgery (FLACS) and conventional phacoemulsification cataract surgery (CPS) by resident surgeons.

SETTING

Parkland Memorial Health and Hospital System, Dallas, Texas, USA.

DESIGN

Prospective randomized study.

METHODS

All surgeries to be performed by postgraduate year 3 and year 4 residents from October 2015 through June 2017 were eligible for inclusion. Patients were required to complete postoperative day 1, week 1, month 1, and month 3 visits. Specular microscopy was performed preoperatively and postoperatively. Surgeries were filmed, and each step was timed and compared. Surgeon and patient surveys were filled out postoperatively.

RESULTS

Of the 135 eyes of 96 subjects enrolled in the study, 64 eyes received FLACS and 71 eyes received CPS. There was no significant difference in corrected distance visual acuity (CDVA), either preoperatively or at the postoperative day 1, week 1, month 1, or month 3 visits (P = .469, .539, .701, .777, and .777, respectively). Cumulated dissipated energy and irrigation fluid usage were not different between FLACS and CPS (P = .521 and .368, respectively), nor was there a difference in the reduction of endothelial cell counts postoperatively (P = .881). Wound creation (P = .014), cortical cleanup (P = .009), and IOL implantation (P = .031) were faster in the CPS group. Survey results indicated that the overall patient experience was similar for FLACS and CPS.

CONCLUSIONS

This first prospective randomized trial evaluating resident-performed FLACS shows that, in resident hands, FLACS provides similar results to CPS regarding visual acuity, endothelial cell loss, operative time, patient satisfaction, and surgical complication rate.

Positive toxicology and reactive serology in tissue donors: a retrospective study over a 3-year period

Assessment of donor suitability and criteria development for tissue donation evaluation which appropriately addresses the risk factors for disease transmission, especially high risk for Hepatitis B or C, HIV or other transmissible diseases as defined by the Food and Drug Administration, FDA, is a continuing concern for tissue banks. The relationship of drug use, especially IV drugs, has been determined to be associated with an increased possibility of reactive serology (Centers for Disease Control and Prevention (USCDC) in Division of Viral Hepatitis, National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention. Hepatitis C questions and answers for health professionals. https://www.cdc.gov/hepatitis/hcv/hcvfaq.htm; Centers for Disease Control and Prevention (USCDC) in infectious diseases, opioids and injection drug use, 2018. https://www.cdc.gov/pwid/opioid-use.html; HIH National Institute on Drug Abuse in Health Consequences of Drug Misuse, 2017. https://www.drugabuse.gov/related-topics/health-consequences-drug-misuse). Therefore, prior drug use determined by medical social history screening frequently results in deferral of a potential donor even when the route of drug administration has not been determined to be intravenous. Because of the association of drug use in numerous cases, which come under Medical Examiner jurisdiction, a possible rule out of a number of otherwise suitable medical examiner cases could occur. This retrospective review of medical examiner cases, tissue bank referrals and tissue donors in a 3-year period examines the relationship, if any, between reactive serology and positive toxicology results. These results would appear to indicate assessment of donor medical social history screening is effective in reducing recovery of high-risk donors.

A high-throughput microfluidic method for fabricating aligned collagen fibrils to study Keratocyte behavior

In vivo, keratocytes are surrounded by aligned type I collagen fibrils that are organized into lamellae. A growing body of literature suggests that the unique topography of the corneal stroma is an important regulator of keratocyte behavior. In this study we describe a microfluidic method to deposit aligned fibrils of type I collagen onto glass coverslips. This high-throughput method allowed for the simultaneous coating of up to eight substrates with aligned collagen fibrils. When these substrates were integrated into a PDMS microwell culture system they provided a platform for high-resolution imaging of keratocyte behavior. Through the use of wide-field fluorescence and differential interference contrast microscopy, we observed that the density of collagen fibrils deposited was dependent upon both the perfusion shear rate of collagen and the time of perfusion. In contrast, a similar degree of fibril alignment was observed over a range of shear rates. When primary normal rabbit keratocytes (NRK) were seeded on substrates with a high density of aligned collagen fibrils and cultured in the presence of platelet derived growth factor (PDGF) the keratocytes displayed an elongated cell body that was co-aligned with the underlying collagen fibrils. In contrast, when NRK were cultured on substrates with a low density of aligned collagen fibrils, the cells showed no preferential orientation. These results suggest that this simple and inexpensive method can provide a general platform to study how simultaneous exposure to topographical and soluble cues influence cell behavior.

An In Vitro Model for Assessing Corneal Keratocyte Spreading and Migration on Aligned Fibrillar Collagen

Background: Corneal stromal cells (keratocytes) are responsible for developing and maintaining normal corneal structure and transparency, and for repairing the tissue after injury. Corneal keratocytes reside between highly aligned collagen lamellae in vivo. In addition to growth factors and other soluble biochemical factors, feedback from the extracellular matrix (ECM) itself has been shown to modulate corneal keratocyte behavior. Methods: In this study, we fabricate aligned collagen substrates using a microfluidics approach and assess their impact on corneal keratocyte morphology, cytoskeletal organization, and patterning after stimulation with platelet derived growth factor (PDGF) or transforming growth factor beta 1 (TGFβ). We also use time-lapse imaging to visualize the dynamic interactions between cells and fibrillar collagen during wound repopulation following an in vitro freeze injury. Results: Significant co-alignment between keratocytes and aligned collagen fibrils was detected, and the degree of cell/ECM co-alignment further increased in the presence of PDGF or TGFβ. Freeze injury produced an area of cell death without disrupting the collagen. High magnification, time-lapse differential interference contrast (DIC) imaging allowed cell movement and subcellular interactions with the underlying collagen fibrils to be directly visualized. Conclusions: With continued development, this experimental model could be an important tool for accessing how the integration of multiple biophysical and biochemical signals regulate corneal keratocyte differentiation.

Assessment of Corneal Stromal Remodeling and Regeneration after Photorefractive Keratectomy

This study utilizes high resolution multi-dimensional imaging to identify temporal and spatial changes in cell/extracellular matrix (ECM) patterning mediating cell migration, fibrosis, remodeling and regeneration during wound healing. Photorefractive keratectomy (PRK) was performed on rabbits. In some cases, 5([4,6-dichlorotriazin-2yl]-amino)fluorescein (DTAF) was applied immediately after surgery to differentiate native vs. cell-secreted collagen. Corneas were assessed 3–180 days postoperatively using in vivo confocal microscopy, and cell/ECM patterning was evaluated in situ using multiphoton and second harmonic generation (SHG) imaging. 7 days post-PRK, migrating fibroblasts below the ablation site were co-aligned with the stromal lamellae. At day 21, randomly patterned myofibroblasts developed on top of the ablation site; whereas cells underneath were elongated, co-aligned with collagen, and lacked stress fibers. Over time, fibrotic tissue was remodeled into more transparent stromal lamellae. By day 180, stromal thickness was almost completely restored. Stromal regrowth occurred primarily below the ablation interface, and was characterized by co-localization of gaps in DTAF labeling with elongated cells and SHG collagen signaling. Punctate F-actin labeling was detected along cells co-aligned with DTAF and non-DTAF labeled collagen, suggesting cell-ECM interactions. Overall, collagen lamellae appear to provide a template for fibroblast patterning during wound healing that mediates stromal repopulation, regeneration and remodeling.

Fibroblast-fibronectin patterning and network formation in 3D fibrin matrices

PURPOSE

We previously reported that fibroblasts migrating within 3-D collagen matrices move independently, whereas fibroblasts within 3-D fibrin matrices form an interconnected network. Similar networks have been identified previously during in vivo corneal wound healing. In this study, we investigate the role of fibronectin in mediating this mechanism of collective cell spreading, migration and patterning.

METHODS

To assess cell spreading, corneal fibroblasts were plated within fibrillar collagen or fibrin matrices. To assess migration, compacted cell-populated collagen matrices were nested inside cell-free fibrin matrices. Constructs were cultured in serum-free media containing PDGF, with or without RGD peptide, anti-α5 or anti-fibronectin blocking antibodies. In some experiments, LifeAct and fluorescent fibronectin were used to allow dynamic assessment of cell-induced fibronectin reorganization. 3-D and 4-D imaging were used to assess cell mechanical behavior, connectivity, F-actin, α5 integrin and fibronectin organization.

RESULTS

Corneal fibroblasts within 3-D fibrin matrices formed an interconnected network that was lined with cell-secreted fibronectin. Live cell imaging demonstrated that fibronectin tracks were formed at the leading edge of spreading and migrating cells. Furthermore, fibroblasts preferentially migrated through fibronectin tracks laid down by other cells. Interfering with cell-fibronectin binding with RGD, anti α5 integrin or anti fibronectin antibodies inhibited cell spreading and migration through fibrin, but did not affect cell behavior in collagen.

CONCLUSIONS

In this study, a novel mode of cell patterning was identified in which corneal fibroblasts secrete and attach to fibronectin via α5β1 integrin to facilitate spreading and migration within 3-D fibrin matrices, resulting in the formation of localized fibronectin tracks. Other cells use these fibronectin tracks as conduits, resulting in an interconnected cell-fibronectin network.

Second harmonic generation imaging of corneal stroma after infection by Pseudomonas aeruginosa

Pseudomonas aeruginosa is a pathogenic gram-negative organism that has the ability to cause blinding corneal infections following trauma and during contact lens wear. In this study, we investigated the directional movement and orientation of an invasive corneal isolate of P. aeruginosa in the corneal stroma during infection of ex vivo and in vivo rabbit corneas using multiphoton fluorescence and second harmonic generation (SHG) imaging. Ex vivo, rabbit corneas were subject to three partial thickness wounds prior to inoculation. In vivo, New Zealand white rabbits were fit with P. aeruginosa laden contact lenses in the absence of a penetrating wound. At all time points tested, infiltration of the corneal stroma by P. aeruginosa revealed a high degree of alignment between the bacteria and collagen lamellae ex vivo (p < 0.001). In vivo, P. aeruginosa traveled throughout the stroma in discrete regions or bands. Within each region, the bacteria showed good alignment with collagen lamellae (P = 0.002). Interestingly, in both the in vitro and in vivo models, P. aeruginosa did not appear to cross the corneal limbus. Taken together, our findings suggest that P. aeruginosa exploits the precise spacing of collagen lamellae in the central cornea to facilitate spread throughout the stroma.

Temporal and spatial analysis of stromal cell and extracellular matrix patterning following lamellar keratectomy

Extracellular matrix (ECM) supplies both physical and chemical signals to keratocytes which can impact their differentiation to fibroblasts and/or myofibroblasts. It also provides a substrate through which they migrate during wound repair. We have previously shown that following transcorneal freeze injury (FI), migrating corneal fibroblasts align parallel to the stromal lamellae during wound repopulation. In this study, we compare cell and ECM patterning both within and on top of the stroma at different time points following lamellar keratectomy (LK) in the rabbit. Twelve rabbits received LK in one eye. Rabbits were monitored using in vivo confocal microscopy at 3, 7, 21 and 60 days after injury. A subset of animals was sacrificed at each time point to further investigate cell and matrix patterning. Tissue was fixed and labeled in situ with Alexa Fluor 488 phalloidin (for F-actin), and imaged using multiphoton fluorescence and second harmonic generation (SHG) imaging (for collagen). Immediately following LK, cell death occurred in the corneal stroma directly beneath the injury. At 7 and 21 days after LK, analysis of fluorescence (F-actin) and SHG results (collagen) indicated that fibroblasts were co-aligned with the collagen lamellae within this region. In contrast, stromal cells accumulating on top of the stromal wound bed were randomly arranged, contained more prominent stress fibers, and expressed alpha smooth muscle actin (α-SMA) and fibronectin. At 60 days, cells and matrix in this region had become co-aligned into lamellar-like structures; cells were elongated but did not express stress fibers. Corneal haze measured using in vivo confocal microscopy peaked at 21 days after LK, and was significantly reduced by 60 days. Cell morphology and patterning observed in vivo was similar to that observed in situ. Our results suggest that the topography and alignment of the collagen lamellae direct fibroblast patterning during repopulation of the native stroma after LK injury in the rabbit. In contrast, stromal cells accumulating on top of the stromal wound bed initially align randomly and produce a fibrotic ECM. Remarkably, over time, these cells appear to remodel the ECM to produce a lamellar structure that is similar to the native corneal stroma.

Immunomodulatory therapy with anti-interleukin-1 after photorefractive keratectomy in the New Zealand white rabbit

PURPOSE

To examine the safety and efficacy of topical anti-interleukin-1 (anti-IL-1) following photorefractive keratectomy (PRK) in rabbit eyes.

SETTING

Joint Warfighter Refractive Surgery Center, Lackland Air Force Base, Texas, USA.

DESIGN

Experimental study.

METHODS

After standard PRK, 48 eyes of 24 New Zealand white rabbits were divided into 4 treatment arms and 1 control arm. Eyes in the treatment arms were randomized to receive fluorometholone 0.1% or an anti-IL-1 suspension (2.50 mg, 1.25 mg, or 0.25 mg doses) plus standard moxifloxacin, balanced salt solution (BSS), and an ocular lubricant (Systane) 4 times a day. Control eyes received only moxifloxacin, balanced salt solution, and ocular lubricant.

RESULTS

No adverse events were observed with anti-IL-1. The safety of anti-IL-1 was affirmed because there was no statistically significant difference in time to epithelial closure, foam-layer histology and thickness, or final stromal thickness measurements between the anti-IL-1 and the steroid or control groups. No increase in haze was observed with anti-IL-1. There was a trend toward decreased haze with anti-IL-1 at several data points compared with the control and steroid groups. Finally, there was a trend toward less haze in all metrics at almost every timepoint for the 2.50 mg anti-IL-1 group compared with lesser concentrations.

CONCLUSIONS

Anti-IL-1 therapy might be a safe, effective alternative to steroids for haze prevention after PRK. Of the doses studied, 2.50 mg of anti-IL-1 4 times a day appeared to be most effective. Further studies in human eyes are needed.

FINANCIAL DISCLOSURE

Dr. Reilly has been a consultant to Alcon Laboratories, Inc., and Abbott Medical Optics, Inc. None of the authors has a financial or proprietary interest in any material or method mentioned.

Corneal Fibroblast Migration Patterns During Intrastromal Wound Healing Correlate With ECM Structure and Alignment

PURPOSE

To assess keratocyte backscattering, alignment, morphology, and connectivity in vivo following a full-thickness corneal injury using the Heidelberg Retina Tomograph Rostock Cornea Module (HRT-RCM), and to correlate these findings with en bloc three-dimensional (3-D) confocal fluorescence and second harmonic generation (SHG) imaging.

METHODS

Rabbit corneas were scanned in vivo both before and 3, 7, 14, and 28 days after transcorneal freeze injury (FI), which damages all corneal cell layers. Corneal tissue was also fixed and labeled for f-actin and nuclei en bloc, and imaged using 3-D confocal fluorescence microscopy and SHG imaging.

RESULTS

Using the modified HRT-RCM, full-thickness scans of all cell layers were consistently obtained. Following FI, stromal cells repopulating the damaged tissue assumed an elongated fibroblastic morphology, and a significant increase in cellular light scattering was measured. This stromal haze gradually decreased as wound healing progressed. Parallel, interconnected streams of aligned corneal fibroblasts were observed both in vivo (from HRT-RCM reflection images) and ex vivo (from f-actin and nuclear labeling) during wound healing, particularly in the posterior cornea. Second harmonic generation imaging demonstrated that these cells were aligned parallel to the collagen lamellae.

CONCLUSIONS

The modified HRT-RCM allows in vivo measurements of sublayer thickness, assessment of cell morphology, alignment and connectivity, and estimation of stromal backscatter during wound healing. In this study, these in vivo observations led to the novel finding that the pattern of corneal fibroblast alignment is highly correlated with lamellar organization, suggesting contact guidance of intrastromal migration that may facilitate more rapid wound repopulation.

In Vivo Confocal Microscopy of the Cornea: New Developments in Image Acquisition, Reconstruction, and Analysis Using the HRT-Rostock Corneal Module

The optical sectioning ability of confocal microscopy allows high magnification images to be obtained from different depths within a thick tissue specimen and is thus ideally suited to the study of intact tissue in living subjects. In vivo confocal microscopy has been used in a variety of corneal research and clinical applications since its development over 25 years ago. In this article we review the latest developments in quantitative corneal imaging with the Heidelberg Retinal Tomograph with Rostock Corneal Module (HRT-RCM). We provide an overview of the unique strengths and weaknesses of the HRT-RCM. We discuss techniques for performing 3-D imaging with the HRT-RCM, including hardware and software modifications that allow full-thickness confocal microscopy through-focusing (CMTF) of the cornea, which can provide quantitative measurements of corneal sublayer thicknesses, stromal cell and extracellular matrix backscatter, and depth-dependent changes in corneal keratocyte density. We also review current approaches for quantitative imaging of the subbasal nerve plexus, which require a combination of advanced image acquisition and analysis procedures, including wide-field mapping and 3-D reconstruction of nerve structures. The development of new hardware, software, and acquisition techniques continues to expand the number of applications of the HRT-RCM for quantitative in vivo corneal imaging at the cellular level. Knowledge of these rapidly evolving strategies should benefit corneal clinicians and basic scientists alike.

Effect of HDAC Inhibitors on Corneal Keratocyte Mechanical Phenotypes in 3-D Collagen Matrices

PURPOSE

Histone deacetylase inhibitors (HDAC) have been shown to inhibit the TGFβ-induced myofibroblast transformation of corneal fibroblasts in 2-D culture. However, the effect of HDAC inhibitors on keratocyte spreading, contraction, and matrix remodeling in 3-D culture has not been directly assessed. The goal of this study was to investigate the effects of the HDAC inhibitors Trichostatin A (TSA) and Vorinostat (SAHA) on corneal keratocyte mechanical phenotypes in 3-D culture using defined serum-free culture conditions.

METHODS

Rabbit corneal keratocytes were plated within standard rat tail type I collagen matrices (2.5 mg/ml) or compressed collagen matrices (~100 mg/ml) and cultured for up to 4 days in serum-free media, PDGF BB, TGFβ1, and either 50 nM TSA, 10 μM SAHA, or vehicle (DMSO). F-actin, α-SM-actin, and collagen fibrils were imaged using confocal microscopy. Cell morphology and global matrix contraction were quantified digitally. The expression of α-SM-actin was assessed using western blotting.

RESULTS

Corneal keratocytes in 3-D matrices had a quiescent mechanical phenotype, as indicated by a dendritic morphology, a lack of stress fibers, and minimal cell-induced matrix remodeling. This phenotype was generally maintained following the addition of TSA or SAHA. TGFβ1 induced a contractile phenotype, as indicated by a loss of dendritic cell processes, the development of stress fibers, and significant matrix compaction. In contrast, cells cultured in TGFβ1 plus TSA or SAHA remained dendritic and did not form stress fibers or induce ECM compaction. Western blotting showed that the expression of α-SM actin after treatment with TGFβ1 was inhibited by TSA and SAHA. PDGF BB stimulated the elongation of keratocytes and the extension of dendritic processes within 3-D matrices without inducing stress fiber formation or collagen reorganization. This spreading response was maintained in the presence of TSA or SAHA.

CONCLUSIONS

Overall, HDAC inhibitors appear to mitigate the effects of TGFβ1 on the transformation of corneal keratocytes to a contractile, myofibroblast phenotype in both compliant and rigid 3-D matrices while preserving normal cell spreading and their ability to respond to the pro-migratory growth factor PDGF.

Fibroblastic Transformation of Corneal Keratocytes by Rac Inhibition is Modulated by Extracellular Matrix Structure and Stiffness

The goal of this study was to investigate how alterations in extracellular matrix (ECM) biophysical properties modulate corneal keratocyte phenotypes in response to specific wound healing cytokines and Rho GTPases. Rabbit corneal keratocytes were plated within standard collagen matrices (2.5 mg/mL) or compressed collagen matrices (~100 mg/mL) and cultured in serum-free media, PDGF BB, IGF, FGF2 or TGFβ1, with or without the Rac1 inhibitor NSC23766 and/or the Rho kinase inhibitor Y-27632. After 1 to 4 days, cells were labeled for F-actin and imaged using confocal microscopy. Keratocytes within standard collagen matrices (which are highly compliant) maintained a dendritic phenotype following culture in serum-free media, PDGF, IGF and FGF, but developed stress fibers in TGFβ1. Keratocytes within compressed collagen (which has high stiffness and low porosity) maintained a dendritic phenotype following culture in serum-free media, PDGF and IGF, but developed stress fibers in both FGF and TGFβ1. The Rac inhibitor had no significant impact on growth factor responses in compliant matrices. Within compressed collagen matrices however, the Rac inhibitor induced fibroblastic transformation in serum-free media, PDGF and IGF. Fibroblast and myofibroblast transformation was blocked by Rho kinase inhibition. Overall, keratocyte growth factor responses appear to be regulated by both the interplay between Rho and Rac signaling, and the structural and mechanical properties of the ECM.

Mechanical interactions and crosstalk between corneal keratocytes and the extracellular matrix

The generation of cellular forces and the application of these physical forces to the ECM play a central role in mediating matrix patterning and remodeling during fundamental processes such as developmental morphogenesis and wound healing. In addition to growth factors and other biochemical factors that can modulate the keratocyte mechanical phenotype, another key player in the regulation of cell-induced ECM patterning is the mechanical state of the ECM itself. In this review we provide an overview of the biochemical and biophysical factors regulating the mechanical interactions between corneal keratocytes and the stromal ECM at the cellular level. We first provide an overview of how Rho GTPases regulate the sub-cellular pattern of force generation by corneal keratocytes, and the impact these forces have on the surrounding ECM. We next review how feedback from local matrix structural and mechanical properties can modulate keratocyte phenotype and mechanical activity. Throughout this review, we provide examples of how these biophysical interactions may contribute to clinical outcomes, with a focus on corneal wound healing.

The Role of Thrombin and Cell Contractility in Regulating Clustering and Collective Migration of Corneal Fibroblasts in Different ECM Environments

PURPOSE

We previously reported that extracellular matrix composition (fibrin versus collagen) modulates the pattern of corneal fibroblast spreading and migration in 3-D culture. In this study, we investigate the role of thrombin and cell contractility in mediating these differences in cell behavior.

METHODS

To assess cell spreading, corneal fibroblasts were plated on top of fibrillar collagen and fibrin matrices. To assess 3-dimensional cell migration, compacted collagen matrices seeded with corneal fibroblasts were embedded inside acellular collagen or fibrin matrices. Constructs were cultured in serum-free media containing platelet-derived growth factor (PDGF), with or without thrombin, the Rho kinase inhibitor Y-27632, and/or the myosin II inhibitor blebbistatin. We used 3-dimensional and 4-dimensional imaging to assess cell mechanical behavior, connectivity and cytoskeletal organization.

RESULTS

Thrombin stimulated increased contractility of corneal fibroblasts. Thrombin also induced Rho kinase-dependent clustering of cells plated on top of compliant collagen matrices, but not on rigid substrates. In contrast, cells on fibrin matrices coalesced into clusters even when Rho kinase was inhibited. In nested matrices, cells always migrated independently through collagen, even in the presence of thrombin. In contrast, cells migrating into fibrin formed an interconnected network. Both Y-27632 and blebbistatin reduced the migration rate in fibrin, but cells continued to migrate collectively.

CONCLUSIONS

The results suggest that while thrombin-induced actomyosin contraction can induce clustering of fibroblasts plated on top of compliant collagen matrices, it does not induce collective cell migration inside 3-D collagen constructs. Furthermore, increased contractility is not required for clustering or collective migration of corneal fibroblasts interacting with fibin.

TCF4 Triplet Repeat Expansion and Nuclear RNA Foci in Fuchs' Endothelial Corneal Dystrophy

PURPOSE

Expansion of the intronic CTG18.1 triplet repeat locus within TCF4 contributes significant risk to the development of Fuchs' endothelial corneal dystrophy (FECD) in Eurasian populations, but the mechanisms by which the expanded repeats result in degeneration of the endothelium have been hitherto unknown. The purpose of this study was to examine FECD endothelial samples for the presence of RNA nuclear foci, the hallmark of toxic RNA, as well as evidence of haploinsufficiency of TCF4.

METHODS

Using fluorescence in situ hybridization, we examined for the presence of nuclear RNA foci containing expanded CUG transcripts in corneal endothelial samples from FECD subjects with CTG18.1 expansion. We also examined for any changes in expression levels of TCF4 by quantitative real-time PCR.

RESULTS

Numerous discrete nuclear RNA foci were identified in endothelial samples of FECD subjects (n = 8) harboring the CTG18.1 expansion, but not in controls lacking the expansion (n = 5) (P = 7.8 × 10(-4)). Percentage of cells with foci in expansion-positive endothelial samples ranged from 33% to 88%. RNA foci were absent in endothelial samples from an FECD subject without CTG18.1 expansion and a subject with endothelial dysfunction without FECD. Expression of the constitutive TCF4 exon encoding the basic helix-loop-helix domain was unaltered with CTG18.1 expansion.

CONCLUSIONS

Our findings suggest that the RNA nuclear foci are pathognomonic for CTG18.1 expansion-mediated endothelial disease. The RNA nuclear foci have been previously found only in rare neurodegenerative disorders caused by repeat expansions. Our detection of abundant ribonuclear foci in FECD implicates a role for toxic RNA in this common disease.

Pseudomonas aeruginosa infectious keratitis in a high oxygen transmissible rigid contact lens rabbit model

PURPOSE

To establish a rabbit model of infectious Pseudomonas aeruginosa keratitis using ultrahigh oxygen transmissible rigid lenses and characterize the frequency and severity of infection when compared to a non-oxygen transmissible lens material.

METHODS

Rabbits were fit with rigid lenses composed of ultrahigh and non-oxygen transmissible materials. Prior to wear, lenses were inoculated with an invasive corneal isolate of P. aeruginosa stably conjugated to green fluorescent protein (GFP). Corneas were examined before and after lens wear using a modified Heidelberg Rostock Tomograph in vivo confocal microscope. Viable bacteria adherent to unworn and worn lenses were assessed by standard plate counts. The presence of P. aeruginosa-GFP and myeloperoxidase-labeled neutrophils in infected corneal tissue was evaluated using laser scanning confocal microscopy.

RESULTS

The frequency and severity of infectious keratitis was significantly greater with inoculated ultrahigh oxygen transmissible lenses. Infection severity was associated with increasing neutrophil infiltration and in severe cases, corneal melting. In vivo confocal microscopic analysis of control corneas following lens wear confirmed that hypoxic lens wear was associated with mechanical surface damage, whereas no ocular surface damage was evident in the high-oxygen lens group.

CONCLUSIONS

These data indicate that in the absence of adequate tear clearance, the presence of P. aeruginosa trapped under the lens overrides the protective effects of oxygen on surface epithelial cells. These findings also suggest that alternative pathophysiological mechanisms exist whereby changes under the lens in the absence of frank hypoxic damage result in P. aeruginosa infection in the otherwise healthy corneal epithelium.

Experimental models for investigating intra-stromal migration of corneal keratocytes, fibroblasts and myofibroblasts

Following laser vision correction, corneal keratocytes must repopulate areas of cell loss by migrating through the intact corneal stroma, and this can impact corneal shape and transparency. In this study, we evaluate 3D culture models for simulating this process in vitro. Buttons (8 mm diameter) were first punched out of keratocyte populated compressed collagen matrices, exposed to a 3 mm diameter freeze injury, and cultured in serum-free media (basal media) or media supplemented with 10% FBS, TGFβ1 or PDGF BB. Following freeze injury, a region of cell death was observed in the center of the constructs. Although cells readily migrated on top of the matrices to cover the wound area, a limited amount of cell migration was observed within the constructs. We next developed a novel “sandwich” model, which better mimics the native lamellar architecture of the cornea. Using this model, significant migration was observed under all conditions studied. In both models, cells in TGFβ and 10% FBS developed stress fibers; whereas cells in PDGF were more dendritic. PDGF stimulated the most inter-lamellar migration in the sandwich construct. Overall, these models provide insights into the complex interplay between growth factors, cell mechanical phenotypes and the structural properties of the ECM.

MMP regulation of corneal keratocyte motility and mechanics in 3-D collagen matrices

Previous studies have shown that platelet derived growth factor (PDGF) can stimulate corneal keratocyte spreading and migration within 3-D collagen matrices, without inducing transformation to a contractile, fibroblastic phenotype. The goal of this study was to investigate the role of matrix metalloproteinases (MMPs) in regulating PDGF-induced changes in keratocyte motility and mechanical differentiation. Rabbit corneal keratocytes were isolated and cultured in serum-free media (S-) to maintain their quiescent phenotype. A nested collagen matrix construct was used to assess 3-D cell migration, and a standard collagen matrix model was used to assess cell morphology and cell-mediated matrix contraction. In both cases constructs were cultured in S- supplemented with PDGF, with or without the broad spectrum MMP inhibitors GM6001 or BB-94. After 4 days, f-actin, nuclei and collagen fibrils were imaged using confocal microscopy. To assess sub-cellular mechanical activity (extension and retraction of cell processes), time-lapse DIC imaging was also performed. MT1-MMP expression and MMP-mediated collagen degradation were also examined. Results demonstrated that neither GM6001 nor BB-94 affected corneal keratocyte viability or proliferation in 3-D culture. PDGF stimulated elongation and migration of corneal keratocytes within type I collagen matrices, without causing a loss of their dendritic morphology or inducing formation of intracellular stress fibers. Treatment with GM6001 and BB-94 inhibited PDGF-induced keratocyte spreading and migration. Relatively low levels of keratocyte-induced matrix contraction were also maintained in PDGF, and the amount of PDGF-induced collagen degradation was similar to that observed in S- controls. The collagen degradation pattern was consistent with membrane-associated MMP activity, and keratocytes showed positive staining for MT1-MMP, albeit weak. Both matrix contraction and collagen degradation were reduced by MMP inhibition. For most outcome measures, the inhibitory effect of BB-94 was significantly greater than that of GM6001. Overall, the data demonstrate for the first time that even under conditions in which low levels of contractility and extracellular matrix proteolysis are maintained, MMPs still play an important role in mediating cell spreading and migration within 3-D collagen matrices. This appears to be mediated at least in part by membrane-tethered MMPs, such as MT1-MMP.

Techniques for assessing 3-D cell-matrix mechanical interactions in vitro and in vivo

Cellular interactions with extracellular matrices (ECM) through the application of mechanical forces mediate numerous biological processes including developmental morphogenesis, wound healing and cancer metastasis. They also play a key role in the cellular repopulation and/or remodeling of engineered tissues and organs. While 2-D studies can provide important insights into many aspects of cellular mechanobiology, cells reside within 3-D ECMs in vivo, and matrix structure and dimensionality have been shown to impact cell morphology, protein organization and mechanical behavior. Global measurements of cell-induced compaction of 3-D collagen matrices can provide important insights into the regulation of overall cell contractility by various cytokines and signaling pathways. However, to understand how the mechanics of cell spreading, migration, contraction and matrix remodeling are regulated at the molecular level, these processes must also be studied in individual cells. Here we review the evolution and application of techniques for imaging and assessing local cell-matrix mechanical interactions in 3-D culture models, tissue explants and living animals.

Corneal epithelial cell viability following exposure to ophthalmic solutions containing preservatives and/or antihypertensive agents

INTRODUCTION

This in-vitro study compared the toxicity of bimatoprost 0.01% containing benzalkonium chloride (BAK) 0.02% with other commercial BAK-free or BAK-containing prostaglandin analogs.

METHODS

Six test solutions were evaluated: travoprost 0.004% with polyquaternium-1 0.001% (PQ), PQ, bimatoprost 0.01% with BAK 0.02%, latanoprost 0.005% with BAK 0.02%, tafluprost 0.0015% preservative free (PF), and BAK 0.02%. Phosphate-buffered saline (PBS) was the live control and 70% methanol was the dead control. Confluent human corneal epithelial cells were incubated with test solutions (diluted 1:5 or 1:10 with PBS) or control solutions for 10 or 25 min, after which cells were fluorescently labeled to distinguish live and dead cells. Data were expressed as a percentage of PBS live-cell fluorescence for automated readouts. Live and dead cells were manually counted for numeric analyses.

RESULTS

For 1:5 and 1:10 dilutions using automated readout, cells exposed to bimatoprost with BAK, latanoprost with BAK, and BAK alone demonstrated significant reductions in the live cell signal compared with PBS, travoprost with PQ, and PQ alone (all P < 0.001). They also demonstrated significantly greater toxicity than tafluprost PF for 1:5 dilutions (all P < 0.001) and 1:10 dilutions (P ≤ 0.02), except for 1:10-diluted bimatoprost with BAK (P = 0.41). For 1:5 dilutions using manual cell count, cells exposed to bimatoprost with BAK demonstrated significant reductions in the percentage of live cells compared with PBS (P = 0.02). For 1:10 dilutions using manual cell count, cells exposed to bimatoprost with BAK, latanoprost with BAK, and BAK alone demonstrated significantly greater toxicity than PBS, travoprost with PQ, PQ alone, and tafluprost PF (all P ≤ 0.03). No significant differences were observed among PBS, travoprost with PQ, and PQ alone under any test conditions (P ≤ 0.63).

CONCLUSION

This study demonstrated that BAKcontaining solutions, including bimatoprost 0.01% with BAK, were toxic to human corneal epithelial cells, whereas BAK-free solutions showed little to no evidence of toxicity.

Individual versus collective fibroblast spreading and migration: regulation by matrix composition in 3D culture

Extracellular matrix (ECM) supplies both physical and chemical signals to cells and provides a substrate through which fibroblasts migrate during wound repair. To directly assess how ECM composition regulates this process, we used a nested 3D matrix model in which cell-populated collagen buttons were embedded in cell-free collagen or fibrin matrices. Time-lapse microscopy was used to record the dynamic pattern of cell migration into the outer matrices, and 3D confocal imaging was used to assess cell connectivity and cytoskeletal organization. Corneal fibroblasts stimulated with PDGF migrated more rapidly into collagen as compared to fibrin. In addition, the pattern of fibroblast migration into fibrin and collagen ECMs was strikingly different. Corneal fibroblasts migrating into collagen matrices developed dendritic processes and moved independently, whereas cells migrating into fibrin matrices had a more fusiform morphology and formed an interconnected meshwork. A similar pattern was observed when using dermal fibroblasts, suggesting that this response is not unique to corneal cells. We next cultured corneal fibroblasts within and on top of standard collagen and fibrin matrices to assess the impact of ECM composition on the cell spreading response. Similar differences in cell morphology and connectivity were observed – cells remained separated on collagen but coalesced into clusters on fibrin. Cadherin was localized to junctions between interconnected cells, whereas fibronectin was present both between cells and at the tips of extending cell processes. Cells on fibrin matrices also developed more prominent stress fibers than those on collagen matrices. Importantly, these spreading and migration patterns were consistently observed on both rigid and compliant substrates, thus differences in ECM mechanical stiffness were not the underlying cause. Overall, these results demonstrate for the first time that ECM protein composition alone (collagen vs. fibrin) can induce a switch from individual to collective fibroblast spreading and migration in 3D culture. Similar processes may also influence cell behavior during wound healing, development, tumor invasion and repopulation of engineered tissues.

Corneal stromal cells use both high- and low-contractility migration mechanisms in 3-D collagen matrices

Corneal keratocyte migration can impact both corneal clarity and refractive outcome following injury or refractive surgery. In this study, we investigated how culture conditions, ECM properties, and Rho kinase activity regulate the mechanics of keratocyte migration, using a nested collagen matrix model. Time-lapse imaging demonstrated that both serum and PDGF stimulate keratocyte migration into the outer matrix. Although the velocity of cell migration was similar, cells in serum were bipolar and induced significant matrix deformation during migration, whereas PDGF induced extension of branching dendritic processes with smaller, more localized force generation. These differences in cell-induced matrix reorganization were verified with a global matrix contraction assay and confocal reflection imaging, using both bovine and rat tail collagen. When constructs were detached from the substrate to lower the effective stiffness, migration was significantly reduced in serum; but was unchanged in PDGF. These differences in migration mechanics were mediated, in part, by Rho kinase. Overall, corneal keratocytes can effectively migrate through collagen matrices using varying degrees of cellular force generation. Low-contractility migration may facilitate keratocyte repopulation of the stroma following surgery or injury, without altering the structural and mechanical properties that are critical to maintaining corneal transparency.

Growth factor regulation of corneal keratocyte mechanical phenotypes in 3-D collagen matrices

PURPOSE

To assess how wound healing cytokines and the extracellular matrix (ECM) environment regulate the keratocyte mechanical phenotype.

METHODS

Rabbit corneal keratocytes were plated within standard bovine or rat tail type I collagen matrices (2.5 mg/mL), compressed collagen matrices (approximately 100 mg/mL), or on collagen-coated dishes and cultured for up to 7 days in serum-free media, platelet derived growth factor BB (PDGF BB), insulin-like growth factor (IGF), TGFβ1, TGFβ2, or FGF2. F-actin, α-smooth muscle actin (α-SMA) and collagen fibrils were imaged using confocal microscopy. Cell morphology, local matrix reorganization, and global matrix contraction were quantified digitally.

RESULTS

IGF and PDGF BB stimulated elongation of keratocytes and extension of dendritic processes within 3-D matrices, without inducing stress fiber formation or collagen reorganization. In contrast, treatment with TGFβ1 and TGFβ2 increased keratocyte contractility, as indicated by stress fiber formation and matrix compaction and alignment. This transformation was enhanced at higher cell densities within standard 3-D matrices, in which α-SMA was incorporated into stress fibers. In contrast, α-SMA was expressed within compressed 3-D matrices even at low cell density. FGF2 did not produce significant cytoskeletal or matrix reorganization in standard 3-D matrices; however, stress fibers were consistently expressed within compressed collagen matrices and on rigid two-dimensional substrates. Inhibiting Rho kinase blocked both TGFβ- and FGF2-induced stress fiber formation.

CONCLUSIONS

Keratocytes cultured in IGF or PDGF BB maintain a quiescent mechanical phenotype over a range of matrix environments. In contrast, the mechanical phenotypes induced by FGF and TGFβ vary in response to the structural and/or mechanical properties of the ECM.

A reconstituted telomerase-immortalized human corneal epithelium in vivo: a pilot study

PURPOSE

Telomerase-immortalized human corneal epithelial cells have been reported to stratify and differentiate in vitro similar to native tissue. The purpose of this study was to assess the ability of a telomerase-immortalized human corneal epithelial cell line to generate a full thickness epithelium in vivo in athymic mice.

METHODS

Telomerized corneal epithelial cells were transduced with a retroviral vector encoding the herpes simplex thymidine kinase gene. Efficacy of the thymidine kinase suicide gene was confirmed using a live/dead assay. The epithelium was mechanically removed from athymic nude mice and remaining cells were treated with mitomycin C to prevent re-epithelialization. Telomerized corneal epithelial cells were seeded onto the denuded cornea and allowed to adhere for 4 and 24 hours. Cellular attachment was assessed using a fluorescent cell tracker. Stratification and differentiation were assessed after 7 days using phalloidin and a mouse monoclonal antibody to K3.

RESULTS

Telomerized corneal epithelial cells were visualized across the denuded stromal surface at 4 and 24 hours, with multi-layering evident at the latter time point. No epithelium was present in the non-treated eye. After 7 days post-transplantation cells stratified into a multilayered epithelium, with positive K3 expression in basal and suprabasal cells. Treatment with ganciclovir induced significant loss of viability in vitro.

CONCLUSIONS

The findings in this pilot study demonstrate that telomerized corneal epithelial cells possess the capacity to reconstitute a stratified corneal epithelium in vivo. The introduction of thymidine kinase allowed for the successful induction of cell death in proliferating cells in vitro. Collectively, these data suggest that a telomerase-immortalized corneal epithelial cell line transduced with thymidine kinase represents a potential model for studying differentiation and epithelial-niche interactions in vivo with potential applications in tissue engineering.

Cell motility and mechanics in three-dimensional collagen matrices

Fibrous connective tissues provide mechanical support and frameworks for other tissues of the body and play an integral role in normal tissue physiology and pathology. Three-dimensional collagen matrices exhibit mechanical and structural features that resemble fibrous connective tissue and have become an important model system to study cell behavior in a tissue-like environment. This review focuses on motile and mechanical interactions between cells&#x2014;especially fibroblasts&#x2014;and collagen matrices. We describe several matrix contraction models, the interactions between fibroblasts and collagen fibrils at global and subcellular levels, unique features of mechanical feedback between cells and the matrix, and the impact of the cell-matrix tension state on cell morphology and mechanical behavior. We develop a conceptual framework to explain the balance between cell migration and collagen translocation including the concept of promigratory and procontractile growth factor environments. Finally, we review the significance of these concepts for the physiology of wound repair.

Inducible macropinocytosis of hyaluronan in B16-F10 melanoma cells

Hyaluronan (HA) is a glycosaminoglycan composed of N-acetylglucosamine and glucuronic acid subunits. Endocytosis is thought to play an essential role in the catabolism of HA due to the intracellular compartmentalization of the HA degrading hyaluronidase enzymes. Previous investigations have shown that keratinocytes, chondrocytes and breast tumor cell lines endocytose HA via the cell surface glycoprotein, CD44. However, other cell types endocytose HA using a CD44-independent mechanism that remains to be defined. The purpose of this study was to investigate HA endocytosis in B16-F10 melanoma cells. We found that B16-F10 melanoma cells expressed CD44 on their surfaces. Unexpectedly, CD44 did not play a role in the endocytosis of HA. Electron microscopy studies revealed that B16-F10 melanoma cells exhibited membrane ruffling, a characteristic feature of macropinocytosis, only after incubating the cells with the HA co-polymer. Moreover, B16-F10 melanoma cells endocytosed HA via macropinocytosis as assessed by drug inhibition studies and the co-localization of fluorescently labeled HA with fluorescent tracers under confocal microscopy. Based on these results, we conclude that induced macropinocytosis may provide a previously unrecognized avenue for HA endocytosis in some cell types.

Rho Kinase Regulation of Fibroblast Migratory Mechanics in Fibrillar Collagen Matrices

Migration of activated corneal fibroblasts plays an important role in matrix patterning during embryonic development and wound repopulation following injury or refractive surgery. In this study, we investigate the role of Rho kinase in regulating fibroblast migration mechanics, by modifying a previously described nested collagen matrix model to facilitate dynamic imaging of cell-matrix interactions.Human corneal fibroblasts were cultured in nested matrices with media containing either 1% fetal bovine serum (FBS), or 1% FBS plus the Rho kinase inhibitor Y-27632. Time-lapse DIC imaging of cell and extracellular matrix (ECM) movements was performed for up to 72 hours. In addition, static confocal imaging was used to assess 3-D cell morphology and local matrix reorganization.In 1% FBS, significant tractional forces were generated during migration, as indicated by inward displacement and reorganization of collagen in front of cells. When Rho kinase was inhibited, cells became more elongated, and extended dendritic processes into the outer matrix. Interestingly, these dendritic cells were still able to generate tractional forces at their leading edge, whereas cell translocation was substantially reduced. Overall, the data suggests that Rho kinase impacts 3-D fibroblast migration by affecting morphology, polarization, and mechanical coordination between the leading and trailing edges of cells.

Growth factor regulation of corneal keratocyte differentiation and migration in compressed collagen matrices

PURPOSE

To evaluate a novel 3D culture model of the corneal stroma and apply it to investigate how key wound-healing growth factors regulate the mechanics of corneal keratocyte migration.

METHODS

Rabbit corneal keratocytes were seeded within collagen matrices that were compacted using external compression. Six-millimeter-diameter buttons were then incubated in media supplemented with 10% FBS, TGFbeta1, TGFbeta2, platelet-derived growth factor (PDGF), or no growth factor (control). After 1, 3, or 7 days, matrices were labeled with phalloidin and a nucleic acid dye, and were imaged using laser confocal microscopy. To study cell migration, buttons were nested within acellular uncompressed outer collagen matrices before growth factor stimulation.

RESULTS

Corneal keratocytes in basal media within compressed matrices had a broad, convoluted cell body and thin dendritic processes. In contrast, cells in 10% FBS developed a bipolar fibroblastic morphology. Treatment with TGFbeta induced the formation of stress fibers expressing alpha-smooth muscle actin, suggesting myofibroblast transformation. PDGF induced keratocyte elongation without inducing stress fiber formation. Both 10% FBS and PDGF stimulated significant keratocyte migration through the uncompressed outer matrix, but 10% FBS produced more cell-induced collagen matrix reorganization. TGFbeta induced the smallest increase in migration and the greatest matrix reorganization.

CONCLUSIONS

Corneal keratocytes are able to differentiate normally and respond to growth factors within compressed collagen matrices, which provide a high-stiffness, 3D environment, similar to native stromal tissue. In addition, nesting these matrices provides a unique platform for investigating the mechanics of keratocyte migration after exposure to specific wound-healing cytokines.

Cell motility and mechanics in three-dimensional collagen matrices

Fibrous connective tissues provide mechanical support and frameworks for other tissues of the body and play an integral role in normal tissue physiology and pathology. Three-dimensional collagen matrices exhibit mechanical and structural features that resemble fibrous connective tissue and have become an important model system to study cell behavior in a tissue-like environment. This review focuses on motile and mechanical interactions between cells&#x2014;especially fibroblasts&#x2014;and collagen matrices. We describe several matrix contraction models, the interactions between fibroblasts and collagen fibrils at global and subcellular levels, unique features of mechanical feedback between cells and the matrix, and the impact of the cell-matrix tension state on cell morphology and mechanical behavior. We develop a conceptual framework to explain the balance between cell migration and collagen translocation including the concept of promigratory and procontractile growth factor environments. Finally, we review the significance of these concepts for the physiology of wound repair.

Characterization of corneal keratocyte morphology and mechanical activity within 3-D collagen matrices

The purpose of this study was to assess quantitatively the differences in morphology, cytoskeletal organization and mechanical behavior between quiescent corneal keratocytes and activated fibroblasts in a 3-D culture model. Primary cultures of rabbit corneal keratocytes and fibroblasts were plated inside type I collagen matrices in serum-free media or 10% FBS, and allowed to spread for 1-5 days. Following F-actin labeling using phalloidin, and immunolabeling of tubulin, alpha-smooth muscle actin or connexin 43, fluorescent and reflected light (for collagen fibrils) 3-D optical section images were acquired using laser confocal microscopy. In other experiments, dynamic imaging was performed using differential interference contrast microscopy, and finite element modeling was used to map ECM deformations. Corneal keratocytes developed a stellate morphology with numerous cell processes that ran a tortuous path between and along collagen fibrils without any apparent impact on their alignment. Fibroblasts on the other hand, had a more bipolar morphology with pseudopodial processes (P </= 0.001). Time-lapse imaging of keratocytes revealed occasional extension and retraction of dendritic processes with only transient displacements of collagen fibrils, whereas fibroblasts exerted stronger myosin II-dependent contractile forces (P < 0.01), causing increased compaction and alignment of collagen at the ends of the pseudopodia (P < 0.001). At high cell density, both keratocytes and fibroblasts appeared to form a 3-D network connected via gap junctions. Overall, this experimental model provides a unique platform for quantitative investigation of the morphological, cytoskeletal and contractile behavior of corneal keratocytes (i.e. their mechanical phenotype) in a 3-D microenvironment.

Remote-controlled scanning and automated confocal microscopy through focusing using a modified HRT rostock corneal module

PURPOSE

To modify the HRT-II Confocal Microscope with Rostock Corneal Module (HRT-RCM) to allow computerized control of the focal plane position (depth) within the cornea.

METHODS

A threaded housing on the HRT-RCM microscope is normally rotated by hand to change the focal plane position within the cornea. This piece was removed to allow the front housing of the microscope to move freely. A linear actuator (Oriel Encoder Mike) was then attached to the side of the microscope and coupled to a drive shaft that was connected to the front housing. The actuator was connected to an Oriel 18011 Encoder Mike controller, which was interfaced to a PC. Software was developed to allow control and display of the focal plane position using this PC, while image acquisition software was run on the HRT-RCM PC. The instrument was tested on one human volunteer.

RESULTS

The modified instrument successfully allowed computer-controlled focusing throughout the entire cornea. Through-focus sequences could be collected online and analyzed and reconstructed three dimensionally off-line using modified confocal microscopy through-focusing software.

CONCLUSIONS

Although this is only a prototype instrument, it significantly improves the examination procedure by allowing completely "hands-free" operation of the HRT-RCM microscope. The data also demonstrate the feasibility of performing quantitative z-axis scans through the full thickness of the cornea with the HRT-RCM. Given the higher contrast images and improved optical sectioning of the HRT-RCM as compared with other instruments, these capabilities could have widespread application.