Keratocan-deficient mice display alterations in corneal structure

Keratan sulfate, an electrosensory neurosentient bioresponsive cell instructive glycosaminoglycan

The roles of keratan sulfate (KS) as a proton detection glycosaminoglycan in neurosensory processes in the central and peripheral nervous systems is reviewed. The functional properties of the KS-proteoglycans aggrecan, phosphacan, podocalyxcin as components of perineuronal nets in neurosensory processes in neuronal plasticity, cognitive learning and memory are also discussed. KS-glycoconjugate neurosensory gels used in electrolocation in elasmobranch fish species and KS substituted mucin like conjugates in some tissue contexts in mammals need to be considered in sensory signalling. Parallels are drawn between KS's roles in elasmobranch fish neurosensory processes and its roles in mammalian electro mechanical transduction of acoustic liquid displacement signals in the cochlea by the tectorial membrane and stereocilia of sensory inner and outer hair cells into neural signals for sound interpretation. The sophisticated structural and functional proteins which maintain the unique high precision physical properties of stereocilia in the detection, transmittance and interpretation of acoustic signals in the hearing process are important. The maintenance of the material properties of stereocilia are essential in sound transmission processes. Specific, emerging roles for low sulfation KS in sensory bioregulation are contrasted with the properties of high charge density KS isoforms. Some speculations are made on how the molecular and electrical properties of KS may be of potential application in futuristic nanoelectronic, memristor technology in advanced ultrafast computing devices with low energy requirements in nanomachines, nanobots or molecular switches which could be potentially useful in artificial synapse development. Application of KS in such innovative areas in bioregulation are eagerly awaited.

Lumican, a Multifunctional Cell Instructive Biomarker Proteoglycan Has Novel Roles as a Marker of the Hypercoagulative State of Long Covid Disease

This study has reviewed the many roles of lumican as a biomarker of tissue pathology in health and disease. Lumican is a structure regulatory proteoglycan of collagen-rich tissues, with cell instructive properties through interactions with a number of cell surface receptors in tissue repair, thereby regulating cell proliferation, differentiation, inflammation and the innate and humoral immune systems to combat infection. The exponential increase in publications in the last decade dealing with lumican testify to its role as a pleiotropic biomarker regulatory protein. Recent findings show lumican has novel roles as a biomarker of the hypercoagulative state that occurs in SARS CoV-2 infections; thus, it may also prove useful in the delineation of the complex tissue changes that characterize COVID-19 disease. Lumican may be useful as a prognostic and diagnostic biomarker of long COVID disease and its sequelae.

Animal Models for the Study of Keratoconus

Keratoconus (KC) is characterized by localized, central thinning and cone-like protrusion of the cornea. Its precise etiology remains undetermined, although both genetic and environmental factors are known to contribute to disease susceptibility. Due to KC's complex nature, there is currently no ideal animal model to represent both the corneal phenotype and underlying pathophysiology. Attempts to establish a KC model have involved mice, rats, and rabbits, with some additional novel animals suggested. Genetic animal models have only been attempted in mice. Similarly, spontaneously occurring animal models for KC have only been discovered in mice. Models generated using chemical or environmental treatments have been attempted in mice, rats, and rabbits. Among several methods used to induce KC in animals, ultraviolet radiation exposure and treatment with collagenase are some of the most prevalent. There is a clear need for an experimental model animal to elucidate the underlying mechanisms behind the development and progression of keratoconus. An appropriate animal model could also aid in the development of treatments to slow or arrest the disorder.

Molecular cues for immune cells from small leucine-rich repeat proteoglycans in their extracellular matrix-associated and free forms

In this review we highlight emerging immune regulatory functions of lumican, keratocan, fibromodulin, biglycan and decorin, which are members of the small leucine-rich proteoglycans (SLRP) of the extracellular matrix (ECM). These SLRPs have been studied extensively as collagen-fibril regulatory structural components of the skin, cornea, bone and cartilage in homeostasis. However, SLRPs released from a remodeling ECM, or synthesized by activated fibroblasts and immune cells contribute to an ECM-free pool in tissues and circulation, that may have a significant, but poorly understood foot print in inflammation and disease. Their molecular interactions and the signaling networks they influence also require investigations. Here we present studies on the leucine-rich repeat (LRR) motifs of SLRP core proteins, their evolutionary and functional relationships with other LRR pathogen recognition receptors, such as the toll-like receptors (TLRs) to bring some molecular clarity in the immune regulatory functions of SLRPs. We discuss molecular interactions of fragments and intact SLRPs, and how some of these interactions are likely modulated by glycosaminoglycan side chains. We integrate findings on molecular interactions of these SLRPs together with what is known about their presence in circulation and lymph nodes (LN), which are important sites of immune cell regulation. Recent bulk and single cell RNA sequencing studies have identified subsets of stromal reticular cells that express these SLRPs within LNs. An understanding of the cellular source, molecular interactions and signaling consequences will lead to a fundamental understanding of how SLRPs modulate immune responses, and to therapeutic tools based on these SLRPs in the future.

Single-Cell Transcriptomics Reveals Cellular Heterogeneity and Complex Cell-Cell Communication Networks in the Mouse Cornea

Purpose

To generate a single-cell RNA-sequencing (scRNA-seq) map and construct cell-cell communication networks of mouse corneas.

Methods

C57BL/6 mouse corneas were dissociated to single cells and subjected to scRNA-seq. Cell populations were clustered and annotated for bioinformatic analysis using the R package "Seurat." Differential expression patterns were validated and spatially mapped with whole-mount immunofluorescence staining. Global intercellular signaling networks were constructed using CellChat.

Results

Unbiased clustering of scRNA-seq transcriptomes of 14,732 cells from 40 corneas revealed 17 cell clusters of six major cell types: nine epithelial cell, three keratocyte, two corneal endothelial cell, and one each of immune cell, vascular endothelial cell, and fibroblast clusters. The nine epithelial cell subtypes included quiescent limbal stem cells, transit-amplifying cells, and differentiated cells from corneas and two minor conjunctival epithelial clusters. CellChat analysis provided an atlas of the complex intercellular signaling communications among all cell types.

Conclusions

We constructed a complete single-cell transcriptomic map and the complex signaling cross-talk among all cell types of the cornea, which can be used as a foundation atlas for further research on the cornea. This study also deepens the understanding of the cellular heterogeneity and heterotypic cell-cell interaction within corneas.

Keratocyte-Derived Myofibroblasts: Functional Differences With Their Fibroblast Precursors

Purpose

In this study, we aim to elucidate functional differences between fibroblasts and myofibroblasts derived from a keratocyte lineage to better understand corneal scarring.

Methods

Corneal fibroblasts, derived from a novel triple transgenic conditional KeraRT/tetO-Cre/mTmG mouse strain that allows isolation and tracking of keratocyte lineage, were expanded, and transformed by exposure to transforming growth factor (TGF)-β1 to myofibroblasts. The composition and organization of a fibroblast-built matrix, deposited by fibroblasts in vitro, was analyzed and compared to the composition of an in vitro matrix built by myofibroblasts. Second harmonic generation microscopy (SHG) was used to study collagen organization in deposited matrix. Different extracellular matrix proteins, expressed by fibroblasts or myofibroblasts, were analyzed and quantified. Functional assays compared latent (TGF-β) activation, in vitro wound healing, chemotaxis, and proliferation between fibroblasts and myofibroblasts.

Results

We found significant differences in cell morphology between fibroblasts and myofibroblasts. Fibroblasts expressed and deposited significantly higher quantities of fibril forming corneal collagens I and V. In contrast, myofibroblasts expressed and deposited higher quantities of fibronectin and other non-collagenous matrix components. A significant difference in the activation of latent TGF-β activation exists between fibroblasts and myofibroblasts when measured with a functional luciferase assay. Fibroblasts and myofibroblasts differ in their morphology, extracellular matrix synthesis, and deposition, activation of latent TGF-β, and chemotaxis.

Conclusions

The differences in the expression and deposition of extracellular matrix components by fibroblasts and myofibroblasts are likely related to critical roles they play during different stages of corneal wound healing.

Stromal matrix directs corneal fibroblasts to re-express keratocan after injury and transplantation

Every tissue has an extracellular matrix (ECM) with certain properties unique to it - the tissue 'niche' - that are necessary for normal function. A distinct specific population of quiescent keratocan-expressing keratocytes populate the corneal stroma during homeostasis to maintain corneal function. However, during wound healing, when there is alteration of the niche conditions, keratocytes undergo apoptosis, and activated corneal fibroblasts and myofibroblasts attempt to restore tissue integrity and function. It is unknown what the fate of activated and temporary fibroblasts and myofibroblasts is after the wound healing process has resolved. In this study, we used several strategies to elucidate the cellular dynamics of corneal wound healing and the fate of corneal fibroblasts. We injured the cornea of a novel mouse model that allows cell-lineage tracing, and we transplanted a cell suspension of in vitro-expanded corneal fibroblasts that could be tracked after being relocated into normal stroma. These transplanted fibroblasts regained expression of keratocan in vivo when relocated to a normal stromal niche. These findings suggest that transformed fibroblasts maintain plasticity and can be induced to a keratocyte phenotype once relocated to an ECM with normal signaling ECM.

Ablation of collagen XII disturbs joint extracellular matrix organization and causes patellar subluxation

Collagen XII, belonging to the fibril-associated collagens, is a homotrimeric secreted extracellular matrix (ECM) protein encoded by the COL12A1 gene. Mutations in the human COL12A1 gene cause an Ehlers-Danlos/myopathy overlap syndrome leading to skeletal abnormalities and muscle weakness. Here, we studied the role of collagen XII in joint pathophysiology by analyzing collagen XII deficient mice and human patients. We found that collagen XII is widely expressed across multiple connective tissue of the developing joint. Lack of collagen XII in mice destabilizes tendons and the femoral trochlear groove to induce patellar subluxation in the patellofemoral joint. These changes are associated with an ECM damage response in tendon and secondary quadriceps muscle degeneration. Moreover, patellar subluxation was also identified as a clinical feature of human patients with collagen XII deficiency. The results provide an explanation for joint hyperlaxity in mice and human patients with collagen XII deficiency.

Tenascins and osteopontin in biological response in cornea

The structural composition, integrity and regular curvature of the cornea contribute to the maintenance of its transparency and vision. Disruption of its integrity caused by injury results in scarring, inflammation and neovascularization followed by losses in transparency. These sight compromising effects is caused by dysfunctional corneal resident cell responses induced by the wound healing process. Upregulation of growth factors/cytokines and neuropeptides affect development of aberrant behavior. These factors trigger keratocytes to first transform into activated fibroblasts and then to myofibroblasts. Myofibroblasts express extracellular matrix components for tissue repair and contract the tissue to facilitate wound closure. Proper remodeling following primary repair is critical for restoration of transparency and visual function. Extracellular matrix components contributing to the healing process are divided into two groups; a group of classical tissue structural components and matrix macromolecules that modulate cell behaviors/activities besides being integrated into the matrix structure. The latter components are designated as matricellular proteins. Their functionality is elicited through mechanisms which modulate the scaffold integrity, cell behaviors, activation/inactivation of either growth factors or cytoplasmic signaling regulation. We discuss here the functional roles of matricellular proteins in mediating injury-induced corneal tissue repair. The roles are described of major matricellular proteins, which include tenascin C, tenascin X and osteopontin. Focus is directed towards dealing with their roles in modulating individual activities of wound healing-related growth factors, e. g., transforming growth factor β (TGF β). Modulation of matricellular protein functions could encompass a potential novel strategy to improve the outcome of injury-induced corneal wound healing.

Proteomic profiling of retina and retinal pigment epithelium combined embryonic tissue to facilitate ocular disease gene discovery

To expedite gene discovery in eye development and its associated defects, we previously developed a bioinformatics resource-tool iSyTE (integrated Systems Tool for Eye gene discovery). However, iSyTE is presently limited to lens tissue and is predominantly based on transcriptomics datasets. Therefore, to extend iSyTE to other eye tissues on the proteome level, we performed high-throughput tandem mass spectrometry (MS/MS) on mouse embryonic day (E)14.5 retina and retinal pigment epithelium combined tissue and identified an average of 3300 proteins per sample (n = 5). High-throughput expression profiling-based gene discovery approaches-involving either transcriptomics or proteomics-pose a key challenge of prioritizing candidates from thousands of RNA/proteins expressed. To address this, we used MS/MS proteome data from mouse whole embryonic body (WB) as a reference dataset and performed comparative analysis-termed "in silico WB-subtraction"-with the retina proteome dataset. In silico WB-subtraction identified 90 high-priority proteins with retina-enriched expression at stringency criteria of ≥ 2.5 average spectral counts, ≥ 2.0 fold-enrichment, false discovery rate < 0.01. These top candidates represent a pool of retina-enriched proteins, several of which are associated with retinal biology and/or defects (e.g., Aldh1a1, Ank2, Ank3, Dcn, Dync2h1, Egfr, Ephb2, Fbln5, Fbn2, Hras, Igf2bp1, Msi1, Rbp1, Rlbp1, Tenm3, Yap1, etc.), indicating the effectiveness of this approach. Importantly, in silico WB-subtraction also identified several new high-priority candidates with potential regulatory function in retina development. Finally, proteins exhibiting expression or enriched-expression in the retina are made accessible in a user-friendly manner at iSyTE ( https://research.bioinformatics.udel.edu/iSyTE/ ), to allow effective visualization of this information and facilitate eye gene discovery.

Dielectric properties of the non-glycated and in vitro methylglyoxal-glycated cornea of the rabbit eye

The dielectric properties of the non-glycated and in vitro methylglyoxal-glycated cornea of the rabbit eye were tested in the frequency range of 200 Hz to 100 kHz of the electric field and at temperatures of 25 to 140 °C. The denaturation temperature (T_d) for the non-glycated cornea and the non-enzymatically glycated cornea are approximately 45 and 55 °C, respectively. The mechanism of proton conduction up to T_d in a glycated cornea requires more energy, i.e. more than twice the activation energy (ΔH) than in non-glycated tissue. The dielectric spectra for both examined tissues showed the same characteristic frequency of about 7 kHz assigned to the orientation relaxation time of the polar side groups inside the corneal stroma. These results may be useful in the surgical treatment of the cornea using conductive keratoplasty and in tissue engineering for clinical applications to regenerate this tissue. The medical use of these physico-biological techniques is important because the human cornea protects all eye tissues from various environmental factors.

Genipin increases extracellular matrix synthesis preventing corneal perforation

PURPOSE

Corneal melting and perforation are feared sight-threatening complications of infections, autoimmune disease, and severe burns. Assess the use of genipin in treating stromal melt.

METHODS

A model for corneal wound healing was created through epithelial debridement and mechanical burring to injure the corneal stromal matrix in adult mice. Murine corneas were then treated with varying concentrations of genipin, a natural occurring crosslinking agent, to investigate the effects that matrix crosslinking using genipin has in wound healing and scar formation. Genipin was used in patients with active corneal melting.

RESULTS

Corneas treated with higher concentrations of genipin were found to develop denser stromal scarring in a mouse model. In human corneas, genipin promoted stromal synthesis and prevention of continuous melt. Genipin mechanisms of action create a favorable environment for upregulation of matrix synthesis and corneal scarring.

CONCLUSION

Our data suggest that genipin increases matrix synthesis and inhibits the activation of latent transforming growth factor-β. These findings are translated to patients with severe corneal melting.

Elevated TGFβ signaling contributes to ocular anterior segment dysgenesis in Col4a1 mutant mice

Ocular anterior segment dysgenesis (ASD) refers to a collection of developmental disorders affecting the anterior structures of the eye. Although a number of genes have been implicated in the etiology of ASD, the underlying pathogenetic mechanisms remain unclear. Mutations in genes encoding collagen type IV alpha 1 (COL4A1) and alpha 2 (COL4A2) cause Gould syndrome, a multi-system disorder that often includes ocular manifestations such as ASD and glaucoma. COL4A1 and COL4A2 are abundant basement membrane proteins that provide structural support to tissues and modulate signaling through interactions with other extracellular matrix proteins, growth factors, and cell surface receptors. In this study, we used a combination of histological, molecular, genetic and pharmacological approaches to demonstrate that altered TGFβ signaling contributes to ASD in mouse models of Gould syndrome. We show that TGFβ signaling was elevated in anterior segments from Col4a1 mutant mice and that genetically reducing TGFβ signaling partially prevented ASD. Notably, we identified distinct roles for TGFβ1 and TGFβ2 in ocular defects observed in Col4a1 mutant mice. Importantly, we show that pharmacologically promoting type IV collagen secretion or reducing TGFβ signaling ameliorated ocular pathology in Col4a1 mutant mice. Overall, our findings demonstrate that altered TGFβ signaling contributes to COL4A1-related ocular dysgenesis and implicate this pathway as a potential therapeutic target for the treatment of Gould syndrome.

The low-density lipoprotein receptor-related protein 1 (LRP1) interactome in the human cornea

The human cornea is responsible for approximately 70% of the eye's optical power and, together with the lens, constitutes the only transparent tissue in the human body. Low-density lipoprotein receptor-related protein 1 (LRP1), a large, multitalented endocytic receptor, is expressed throughout the human cornea, yet its role in the cornea remains unknown. More than 30 years ago, LRP1 was purified by exploiting its affinity for the activated form of the protease inhibitor alpha-2-macroblulin (A2M), and the original purification protocol is generally referred to in studies involving full-length LRP1. Here, we provide a novel and simplified LRP1 purification protocol based on LRP1's affinity for receptor-related protein (RAP) that produces significantly higher yields of authentic LRP1. Purified LRP1 was used to map its unknown interactome in the human cornea. Corneal proteins extracted under physiologically relevant conditions were subjected to LRP1 affinity pull-down, and LRP1 ligand candidates were identified by LC-MS/MS. A total of 28 LRP1 ligand candidates were found, including 22 novel ligands. The LRP1 corneal interactome suggests a novel role for LRP1 as a regulator of the corneal immune response, structure, and ultimately corneal transparency.

Small Leucine-Rich Proteoglycans in Tendon Wound Healing

Significance: Tendon injury possesses a high morbidity rate and is difficult to achieve a satisfying prognosis with currently available treatment strategies. Current approaches used for tendon healing always lead to the formation of fibrovascular scar tissue, which significantly compromises the biomechanics of the healed tendon. Moreover, the related functional deficiency deteriorates over time with an increased injury recurrence risk. Small leucine-rich proteoglycans (SLRPs) link and interact with collagen fibrils to regulate tendon structure and biomechanics, which can provide a new and promising method in the field of tendon injury management. Recent Advances: The effect of SLRPs on tendon development has been extensively investigated. SLRP deficiency impairs tendon collagen fibril structure and biomechanic properties, while administration of SLRPs generally benefits tendon wound healing and regains better mechanical properties. Critical Issues: Current knowledge on the role of SLRPs in tendon development and regeneration mostly comes from uninjured knockout mice, and mainly focuses on the morphology description of collagen fibril profile and mechanical properties. Little is known about the regulatory mechanism on the molecular level. Future Directions: This article reviews the current knowledge in this highly translational topic and provides an evidence-based conclusion, thereby encouraging in-depth investigations of SLRPs in tendons and the development of SLRP-based treatments for desired tendon healing.

Regenerative therapy for the Cornea

The cornea is the outmost layer of the eye, unique in its transparency and strength. The cornea not only transmits the light essential for vision, also refracts light, giving focus to images. Each of the three layers of the cornea has properties essential for the function of vision. Although the epithelium can often recover from injury quickly by cell division, loss of limbal stem cells can cause severe corneal surface abnormalities leading to corneal blindness. Disruption of the stromal extracellular matrix and loss of cells determining this structure, the keratocytes, leads to corneal opacity. Corneal endothelium is the inner part of the cornea without self-renewal capacity. It is very important to maintain corneal dehydration and transparency. Permanent damage to the corneal stroma or endothelium can be effectively treated by corneal transplantation; however, there are drawbacks to this procedure, including a shortage of donors, the need for continuing treatment to prevent rejection, and limits to the survival of the graft, averaging 10-20 years. There exists a need for new strategies to promote regeneration of the stromal structure and restore vision. This review highlights critical contributions in regenerative medicine with the aim of corneal reconstruction after injury or disease. These approaches include corneal stromal stem cells, corneal limbal stem cells, embryonic stem cells, and other adult stem cells, as well as induced pluripotent stem cells. Stem cell-derived trophic factors in the forms of secretomes or exosomes for corneal regeneration are also discussed. Corneal sensory nerve regeneration promoting corneal transparency is discussed. This article provides description of the up-to-date options for corneal regeneration and presents exciting possible avenues for future studies toward clinical applications for corneal regeneration.

Molecular characteristics and spatial distribution of adult human corneal cell subtypes

Bulk RNA sequencing of a tissue captures the gene expression profile from all cell types combined. Single-cell RNA sequencing identifies discrete cell-signatures based on transcriptomic identities. Six adult human corneas were processed for single-cell RNAseq and 16 cell clusters were bioinformatically identified. Based on their transcriptomic signatures and RNAscope results using representative cluster marker genes on human cornea cross-sections, these clusters were confirmed to be stromal keratocytes, endothelium, several subtypes of corneal epithelium, conjunctival epithelium, and supportive cells in the limbal stem cell niche. The complexity of the epithelial cell layer was captured by eight distinct corneal clusters and three conjunctival clusters. These were further characterized by enriched biological pathways and molecular characteristics which revealed novel groupings related to development, function, and location within the epithelial layer. Moreover, epithelial subtypes were found to reflect their initial generation in the limbal region, differentiation, and migration through to mature epithelial cells. The single-cell map of the human cornea deepens the knowledge of the cellular subsets of the cornea on a whole genome transcriptional level. This information can be applied to better understand normal corneal biology, serve as a reference to understand corneal disease pathology, and provide potential insights into therapeutic approaches.

Characterisation of key proteoglycans in the cranial cruciate ligaments (CCLs) from two dog breeds with different predispositions to CCL disease and rupture

Cranial cruciate ligament disease and rupture (CCLD/R) is one of the most common orthopaedic conditions in dogs, eventually leading to osteoarthritis of the stifle joint. Certain dog breeds such as the Staffordshire bull terrier have an increased risk of developing CCLD/R. Previous studies into CCLD/R have found that glycosaminoglycan levels were elevated in cranial cruciate ligament (CCL) tissue from high-risk breeds when compared to the CCL from a low-risk breed to CCLD/R. Our objective was to determine specific proteoglycans/glycosaminoglycans in the CCL and to see whether their content was altered in dog breeds with differing predispositions to CCLD/R. Disease-free CCLs from Staffordshire bull terriers (moderate/high-risk to CCLD/R) and Greyhounds (low-risk to CCLD/R) were collected and key proteoglycan/glycosaminoglycans were determined by semi-quantitative Western blotting, quantitative biochemistry, quantitative reverse transcription polymerase chain reaction, and immunohistochemistry. Gene expression of fibromodulin (P = 0.03), aggrecan (P = 0.0003), and chondroitin-6-sulphate stubs (P = 0.01) were significantly increased, and for fibromodulin this correlated with an increase in protein content in Staffordshire bull terriers compared to Greyhound CCLs (P = 0.02). Decorin (P = 0.03) and ADAMTS-4 (P = 0.04) gene expression were significantly increased in Greyhounds compared to Staffordshire bull terrier CCLs. The increase of specific proteoglycans and glycosaminoglycans within the Staffordshire bull terrier CCLs may indicate a response to higher compressive loads, potentially altering their risk to traumatic injury. The higher decorin content in the Greyhound CCLs is essential for maintaining collagen fibril strength, while the increase of ADAMTS-4 indicates a higher rate of turnover helping to regulate normal CCL homeostasis in Greyhounds.

Wounding Induces Facultative Opn5-Dependent Circadian Photoreception in the Murine Cornea

Purpose

Autonomous molecular circadian clocks are present in the majority of mammalian tissues. These clocks are synchronized to phases appropriate for their physiologic role by internal systemic cues, external environmental cues, or both. The circadian clocks of the in vivo mouse cornea synchronize to the phase of the brain's master clock primarily through systemic cues, but ex vivo corneal clocks entrain to environmental light cycles. We evaluated the underlying mechanisms of this difference.

Methods

Molecular circadian clocks of mouse corneas were evaluated in vivo and ex vivo for response to environmental light. The presence of opsins and effect of genetic deletion of opsins were evaluated for influence on circadian photoresponses. Opn5-expressing cells were identified using Opn5^Cre;Ai14 mice and RT-PCR, and they were characterized using immunocytochemistry.

Results

Molecular circadian clocks of the cornea remain in phase with behavioral circadian locomotor rhythms in vivo but are photoentrainable in tissue culture. After full-thickness incision or epithelial debridement, expression of the opsin photopigment Opn5 is induced in the cornea in a subset of preexisting epithelial cells adjacent to the wound site. This induction coincides with conferral of direct, short-wavelength light sensitivity to the circadian clocks throughout the cornea.

Conclusions

Corneal circadian rhythms become photosensitive after wounding. Opn5 gene function (but not Opn3 or Opn4 function) is necessary for induced photosensitivity. These results demonstrate that opsin-dependent direct light sensitivity can be facultatively induced in the murine cornea.

Corneal stromal wound healing: Major regulators and therapeutic targets

Corneal stromal wound healing is a complex event that occurs to restore the transparency of an injured cornea. It involves immediate apoptosis of keratocytes followed by their activation, proliferation, migration, and trans-differentiation to myofibroblasts. Myofibroblasts contract to close the wound and secrete extracellular matrix and proteinases to remodel it. Released proteinases may degenerate the basement membrane allowing an influx of cytokines from overlying epithelium. Immune cells infiltrate the wound to clear cellular debris and prevent infections. Gradually basement membrane regenerates, myofibroblasts and immune cells disappear, abnormal matrix is resorbed, and transparency of the cornea is restored. Often this cascade deregulates and corneal opacity results. Factors that prevent corneal opacity after an injury have always intrigued the researchers. They hold clinical relevance as they can guide the outcomes of corneal surgeries. Studies in the past have shed light on the role of various factors in stromal healing. TGFβ (transforming growth factor-beta) signaling is the central player guiding stromal responses. Other major regulators include myofibroblasts, basement membrane, collagen fibrils, small leucine-rich proteoglycans, biophysical cues, proteins derived from extracellular matrix, and membrane channels. The knowledge about their roles helped to develop novel therapies to prevent corneal opacity. This article reviews the role of major regulators that determine the outcome of stromal healing. It also discusses emerging therapies that modulate the role of these regulators to prevent stromal opacity.

Corneal fibroblasts: Function and markers

Corneal stromal keratocytes contribute to the maintenance of corneal transparency and shape by synthesizing and degrading extracellular matrix. They are quiescent in the healthy cornea, but they become activated in response to insults from the external environment that breach the corneal epithelium, with such activation being associated with phenotypic transformation into fibroblasts. Corneal fibroblasts (activated keratocytes) act as sentinel cells to sense various external stimuli-including damage-associated molecular patterns derived from injured cells, pathogen-associated molecular patterns of infectious microorganisms, and inflammatory mediators such as cytokines-under pathological conditions such as trauma, infection, and allergy. The expression of various chemokines and adhesion molecules by corneal fibroblasts determines the selective recruitment and activation of inflammatory cells in a manner dependent on the type of insult. In infectious keratitis, the interaction of corneal fibroblasts with various components of microbes and with cytokines derived from infiltrated inflammatory cells results in excessive degradation of stromal collagen and consequent corneal ulceration. Corneal fibroblasts distinguish between type 1 and type 2 inflammation through recognition of corresponding cytokines, with their activation by type 2 cytokines contributing to the pathogenesis of corneal lesions in severe ocular allergic diseases. Pharmacological targeting of corneal fibroblast function is thus a potential novel therapeutic approach to prevention of excessive corneal stromal inflammation, damage, and scarring.

Composition, structure and function of the corneal stroma

No other tissue in the body depends more on the composition and organization of the extracellular matrix (ECM) for normal structure and function than the corneal stroma. The precise arrangement and orientation of collagen fibrils, lamellae and keratocytes that occurs during development and is needed in adults to maintain stromal function is dependent on the regulated interaction of multiple ECM components that contribute to attain the unique properties of the cornea: transparency, shape, mechanical strength, and avascularity. This review summarizes the contribution of different ECM components, their structure, regulation and function in modulating the properties of the corneal stroma. Fibril forming collagens (I, III, V), fibril associated collagens with interrupted triple helices (XII and XIV), network forming collagens (IV, VI and VIII) as well as small leucine-rich proteoglycans (SLRP) expressed in the stroma: decorin, biglycan, lumican, keratocan, and fibromodulin are some of the ECM components reviewed in this manuscript. There are spatial and temporal differences in the expression of these ECM components, as well as interactions among them that contribute to stromal function. Unique regions within the stroma like Bowman's layer and Descemet's layer are discussed. To define the complexity of corneal stroma composition and structure as well as the relationship to function is a daunting task. Our knowledge is expanding, and we expect that this review provides a comprehensive overview of current knowledge, definition of gaps and suggests future research directions.

Regulation of Keratocyte Phenotype and Cell Behavior by Substrate Stiffness

Corneal tissue engineering is an alternative way to solve the problem of lack of corneal donor tissue in corneal transplantation. Keratocytes with a normal phenotype and function in tissue-engineered cornea would be critical for corneal regeneration. Although the role of extracellular/substrate material stiffness is well-known for the regulation of the cell phenotype and cell behavior in many different cell types, its effects in keratocyte culture have not yet been thoroughly studied. This project studied the effect of substrate stiffness on the keratocyte phenotype marker expression and typical cell behavior (cell adhesion, proliferation, and migration), and the possible mechanisms involved. Human primary keratocytes were cultured on tissue culture plastic (TCP, ∼10⁶ kPa) or on plates with the stiffness equivalent of physiological human corneal stroma (25 kPa) or vitreous body (1 kPa). The expression of keratocyte phenotype markers, cell adhesion, proliferation, and migration were compared. The results showed that the stiffness of the substrate material regulates the phenotype marker expression and cell behavior of cultured keratocytes. Physiological corneal stiffness (25 kPa) superiorly preserved the cell phenotype when compared to the TCP and 1 kPa group. Keratocytes had a larger cell area when cultured on 25 kPa plates as compared to on TCP. Treatment of cells with NSC 23766 (Rac1 inhibitor) mimicked the response in the cell phenotype and behavior seen in the transition from soft materials to stiff materials, including the cytoskeletal structure, expression of keratocyte phenotype markers, and cell behavior. In conclusion, this study shows that substrate stiffness regulates the cell phenotype marker expression and cell behavior of keratocytes by Rac1-mediated cytoskeletal reorganization. This knowledge contributes to the development of corneal tissue engineering.

A Role for Monosaccharides in Nucleation Inhibition and Transport of Collagen

Background: Collagenous tissues are composed of precisely oriented, tightly packed collagen fibril bundles to confer the maximal strength within the smallest volume. While this compact form benefits mobility, it consequentially restricts vascularity and cell density to a minimally viable level in some regions. These tissues reside in a homeostatic state with an unstable equilibrium, where perturbations to structure or molecular milieu cause descension into a long-term compromised state. Several studies have shown that glycosaminoglycans are key molecules required for healthy tissue maintenance. Our long-term goal is to determine if glycosaminoglycans serve a critical function of stabilizing soluble monomeric collagen in the interstitial fluid that bathes tissue for immediate availability in tissue development and repair in vivo. Materials and Methods: To test glycosaminoglycan and collagen interactions at the most fundamental level, we have explored the effect of the monosaccharides that populate the glycosaminoglycans of the extracellular matrix on collagen assembly kinetics, pre-established matrix stability, and collagen incorporation into a preassembled matrix. Results: Results showed that monosaccharides increased the threshold concentration required for spontaneous polymerization by at least three orders of magnitude. When the monosaccharides were introduced to a pre-existing collagen network, fibrillar dissociation was undetectable. Fluorescent-labeling studies illustrated that in the presence of the saccharide solution, soluble collagen maintains the functional capacity to integrate into a pre-existing network. Conclusion: This work demonstrates a feasible role for glycosaminoglycans in supporting tissue remodeling and highlights the potential importance of age-related deterioration of glycosaminoglycan biosynthesis in reference to the homeostasis of collagen-based tissues.

Keratocyte biology

The study of corneal stromal keratocytes is motivated by its strong association with corneal health and visual function. They play a dominant role in the maintenance of corneal homeostasis and transparency through the production of collagens, proteoglycans and corneal crystallins. Trauma-induced apoptosis of keratocytes and replacement by fibroblasts and myofibroblasts disrupt the stromal matrix organization, resulting in corneal haze formation and vision loss. It is, therefore, important to understand the biology and behaviours of keratocytes and the associated stromal cell types (like fibroblasts, myofibroblasts, stromal stem cells) in wound healing, corneal pathologies (including keratoconus, keratitis, endothelial disorders) as well as different ophthalmic situations (such as collagen crosslinking/photodynamic treatment, keratoplasty and refractive surgery, and topical medications). The recent development of ex vivo propagation of keratocytes and stromal stem cells, and their translational applications, either via stromal injection or incorporated in bioscaffold, have been shown to restore the corneal transparency and regenerate native stromal tissue in animal models of corneal haze and other disorders.

Small Leucine-Rich Proteoglycans in Skin Wound Healing

Healing of cutaneous wounds is a complex and well-coordinated process requiring cooperation among multiple cells from different lineages and delicately orchestrated signaling transduction of a diversity of growth factors, cytokines, and extracellular matrix (ECM) at the wound site. Most skin wound healing in adults is imperfect, characterized by scar formation which results in significant functional and psychological sequelae. Thus, the reconstruction of the damaged skin to its original state is of concern to doctors and scientists. Beyond the traditional treatments such as corticosteroid injection and radiation therapy, several growth factors or cytokines-based anti-scarring products are being or have been tested in clinical trials to optimize skin wound healing. Unfortunately, all have been unsatisfactory to date. Currently, accumulating evidence suggests that the ECM not only functions as the structural component of the tissue but also actively modulates signal transduction and regulates cellular behaviors, and thus, ECM should be considered as an alternative target for wound management pharmacotherapy. Of particular interest are small leucine-rich proteoglycans (SLRPs), a group of the ECM, which exist in a wide range of connecting tissues, including the skin. This manuscript summarizes the most current knowledge of SLRPs regarding their spatial-temporal expression in the skin, as well as lessons learned from the genetically modified animal models simulating human skin pathologies. In this review, particular focus is given on the diverse roles of SLRP in skin wound healing, such as anti-inflammation, pro-angiogenesis, pro-migration, pro-contraction, and orchestrate transforming growth factor (TGF)β signal transduction, since cumulative investigations have indicated their therapeutic potential on reducing scar formation in cutaneous wounds. By conducting this review, we intend to gain insight into the potential application of SLRPs in cutaneous wound healing management which may pave the way for the development of a new generation of pharmaceuticals to benefit the patients suffering from skin wounds and their sequelae.

Cell-independent matrix configuration in early corneal development

Mechanisms controlling the spatial configuration of the remarkably ordered collagen-rich extracellular matrix of the transparent cornea remain incompletely understood. We previously described the assembly of the emerging corneal matrix in the mid and late stages of embryogenesis and concluded that collagen fibril organisation was driven by cell-directed mechanisms. Here, the early stages of corneal morphogenesis were examined by serial block face scanning electron microscopy of embryonic chick corneas starting at embryonic day three (E3), followed by a Fourier transform analysis of three-dimensional datasets and theoretical considerations of factors that influence matrix formation. Eyes developing normally and eyes that had the lens surgically removed at E3 were studied. Uniformly thin collagen fibrils are deposited by surface ectoderm-derived corneal epithelium in the primary stroma of the developing chick cornea and form an acellular matrix with a striking micro-lamellar orthogonal arrangement. Fourier transform analysis supported this observation and indicated that adjacent micro-lamellae display a clockwise rotation of fibril orientation, depth-wise below the epithelium. We present a model which attempts to explain how, in the absence of cells in the primary stroma, collagen organisation might be influenced by cell-independent, intrinsic mechanisms, such as fibril axial charge derived from associated proteoglycans. On a supra-lamellar scale, fine cords of non-collagenous filamentous matrix were detected over large tissue volumes. These extend into the developing cornea from the epithelial basal lamina and appear to associate with the neural crest cells that migrate inwardly to form, first the corneal endothelium and then keratocytes which synthesise the mature, secondary corneal stroma. In a small number of experimental specimens, matrix cords were present even when periocular neural crest cell migration and corneal morphogenesis had been perturbed following removal of the lens at E3.

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Highly-ordered connective tissue appears early in development of the avian cornea.

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Cell-independent mechanisms may contribute to the organisation of collagen fibrils into an orthogonal array.

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Matrix cords from epithelium into stroma contact invading neural crest cells.

Dynamic changes of the extracellular matrix during corneal wound healing

The extracellular matrix (ECM) confers transparency to the cornea because of the precise organization of collagen fibrils and a wide variety of proteoglycans. We monitored the corneal wound healing process after alkali burns in rabbits. We analyzed the location and expression of collagens and proteoglycans, the clinical impact, and the recovery of optical transparency. After the animals received both general and ocular topical anesthesia, the central cornea of the left eye was burned by placing an 8-mm diameter filter paper soaked in 0.5 N NaOH for 60 s. The eyes were evaluated under a surgical microscope at 1, 3, and 6 months after burning. At each time point, the clinical conditions of the burned and control corneas were observed. The arrangement of collagen fibers in the corneal stroma was visualized by Picrosirius-red staining, Gomori's silver impregnation and transmission electronic microscopy. Corneal light transmittance was also measured. Myofibroblasts presence was analyzed by immunohistochemistry. mRNA expression levels of collagen types I and III, lumican, decorin, keratocan and alpha-smooth muscle actin were determined by quantitative real-time polymerase chain reaction. One month after alkali burn, the ECM was disorganized and filled with lacunae containing different types of cells and collagen type III fibers in the wound area. Corneal opacities were present with attendant loss of light transmittance. Collagen and proteoglycan mRNA expression levels were up-regulated. After three months, wound healing progress was indicated by reduced corneal opacity, increased light transmittance, reorganization of collagen fibers and only collagen type I expression levels were at control levels. After six months, the wound area ECM morphology was similar to controls, but transmittance values remained low, denoting incomplete restoration of the stromal architecture. This multidisciplinary study of the stromal wound healing process revealed changes in corneal transmittance, collagen organization, myofibroblasts presence and ECM composition at 1, 3, and 6 months after alkali burning. Documenting wound resolution during the six-month period provided reliable information that can be used to test new therapies.

Novel variants in the KERA gene cause autosomal recessive cornea plana in a Chinese family: A case report

Autosomal recessive cornea plana is a very rare hereditary ocular disease, characterized by a flattened corneal curvature, marked hyperopia due to low refractive power and frequently consequent accommodative esotropia. Other features include various cornea anterior segment abnormalities, without systemic problems. The purpose of the present study was to investigate the clinical and molecular alterations in a Chinese family with cornea plana. Full ophthalmic examinations of the patients were performed, including slit-lamp examination, fundus examination and ocular ultrasound. Whole-exome sequencing data were screened for pathological variants in the proband, which were confirmed by Sanger sequencing. One novel missense mutation, c.242A>G (p.N81S) and another novel 7 base-pair deletion mutation, c.772-779del (p.G258Cfs*30), were detected in the keratocan (KERA) gene; two affected siblings inherited these variations in a compound heterozygous state, which were derived from the clinically unaffected heterozygous father (c.772_779del) and mother (c.242A>G), respectively. Neither mutation was observed in unrelated healthy controls (n=200). Multiple computer software predictions supported the pathogenicity of the two variants. Furthermore, protein modeling prediction was performed to better understand the molecular basis of cornea plana, particularly the importance of the leucine-rich repeat domain. This study presents the 14th pathogenic KERA mutations identified worldwide and the first in East Asia so far, to the best of our knowledge. These findings guided prenatal diagnosis for the family in question and expand on the variant spectrum of KERA, therefore facilitating genetic counseling.

Fibrosis: Shared Lessons From the Lens and Cornea

Regenerative repair in response to wounding involves cell proliferation and migration. This is followed by the reestablishment of cell structure and organization and a dynamic process of remodeling and restoration of the injured cells' extracellular matrix microenvironment and the integration of the newly synthesized matrix into the surrounding tissue. Fibrosis in the lungs, liver, and heart can lead to loss of life and in the eye to loss of vision. Learning to control fibrosis and restore normal tissue function after injury repair remains a goal of research in this area. Here we use knowledge gained using the lens and the cornea to provide insight into how fibrosis develops and clues to how it can be controlled. The lens and cornea are less complex than other tissues that develop life‐threatening fibrosis, but they are well characterized and research using them as model systems to study fibrosis is leading toward an improved understanding of fibrosis. Here we summarize the current state of the literature and how it is leading to promising new treatments. Anat Rec, 2019. © 2019 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Functional Consequences of Keratan Sulfate Sulfation in Electrosensory Tissues and in Neuronal Regulation

Keratan sulfate (KS) is a functional electrosensory and neuro-instructive molecule. Recent studies have identified novel low sulfation KS in auditory and sensory tissues such as the tectorial membrane of the organ of Corti and the Ampullae of Lorenzini in elasmobranch fish. These are extremely sensitive proton gradient detection systems that send signals to neural interfaces to facilitate audition and electrolocation. High and low sulfation KS have differential functional roles in song learning in the immature male zebra song-finch with high charge density KS in song nuclei promoting brain development and cognitive learning. The conductive properties of KS are relevant to the excitable neural phenotype. High sulfation KS interacts with a large number of guidance and neuroregulatory proteins. The KS proteoglycan microtubule associated protein-1B (MAP1B) stabilizes actin and tubulin cytoskeletal development during neuritogenesis. A second 12 span transmembrane synaptic vesicle associated KS proteoglycan (SV2) provides a smart gel storage matrix for the storage of neurotransmitters. MAP1B and SV2 have prominent roles to play in neuroregulation. Aggrecan and phosphacan have roles in perineuronal net formation and in neuroregulation. A greater understanding of the biology of KS may be insightful as to how neural repair might be improved.

Integrated Stress Response and Decreased ECM in Cultured Stromal Cells From Keratoconus Corneas

Purpose

Keratoconus (KC) is a multifactorial disease where progressive thinning and weakening of the cornea leads to loss of visual acuity. Although the underlying etiology is poorly understood, a major endpoint is a dysfunctional stromal connective tissue matrix. Using multiple individual KC corneas, we determined that matrix production by keratocytes is severely impeded due to an altered stress response program.

Methods

KC and donor (DN) stromal keratocytes were cultured in low glucose serum-free medium containing insulin, selenium and transferrin. Fibronectin, collagens and proteins related to their chaperone, processing and export, matrix metalloproteinase, and stress response related proteins were investigated by immunoblotting, immunocytochemistry, hydroxyproline quantification, and gelatin zymography. Multiplexed mass spectrometry was used for global proteomic profiling of 5 individual DN and KC cell culture. Transcription of selected proteins was assayed by qPCR.

Results

DN and KC cells showed comparable survival and growth. However, immunoblotting of selected ECM proteins and global proteomics showed decreased fibronectin, collagens, PCOLCE, ADAMTS2, BMP1, HSP47, other structural and cytoskeletal proteins in KC. Phosphorylated (p) eIF2α, a translation regulator and its target, ATF4 were increased in KC cultured cells and corneal sections.

Conclusions

The profound decrease in structural proteins in cultured KC cells and increase in the p-eIF2α, and ATF4, suggest a stress related blockade in structural proteins not immediately needed for cell survival. Therefore, this cell culture system reveals an intrinsic aggravated stress response with consequent decrease in ECM proteins as potential pathogenic underpinnings in KC.

Aberrant expression of a stabilized β-catenin mutant in keratocytes inhibits mouse corneal epithelial stratification

We previously reported that genetic deletion of β-catenin in mouse corneal keratocytes resulted in precocious corneal epithelial stratification. In this study, to strengthen the notion that corneal keratocyte-derived Wnt/β-catenin signaling regulates corneal epithelial stratification during mouse development, we examined the consequence of conditional overexpression of a stabilized β-catenin mutant (Ctnnb1ΔE3) in corneal keratocytes via a doxycycline (Dox)-inducible compound transgenic mouse strain. Histological analysis showed that conditional overexpression of Ctnnb1ΔE3 in keratocytes inhibited corneal epithelial stratification during postnatal development. Unlike the corneal epithelium of the littermate controls, which consisted of 5-6 cell layers at postnatal day 21 (P21), the mutant corneal epithelium contained 1-2 or 2-3 cell layers after Dox induction from embryonic day 0 (E0) to P21 and from E9 to P21, respectively. X-gal staining revealed that Wnt/β-catenin signaling activity was significantly elevated in the corneal keratocytes of the Dox-induced mutant mice, compared to the littermate controls. Furthermore, RT-qPCR and immunostaining data indicated that the expression of Bmp4 and ΔNp63 was downregulated in the mutant corneas, which was associated with reduced corneal epithelial proliferation in mutant epithelium, as revealed by immunofluorescent staining. However, the expression of Krt12, Krt14 and Pax6 in the mutant corneas was not altered after overexpression of Ctnnb1ΔE3 mutant protein in corneal keratocytes. Overall, mutant β-catenin accumulation in the corneal keratocytes inhibited corneal epithelial stratification probably through downregulation of Bmp4 and ΔNp63 in the corneal epithelium.

Cross-ancestry genome-wide association analysis of corneal thickness strengthens link between complex and Mendelian eye diseases

Central corneal thickness (CCT) is a highly heritable trait associated with complex eye diseases such as keratoconus and glaucoma. We perform a genome-wide association meta-analysis of CCT and identify 19 novel regions. In addition to adding support for known connective tissue-related pathways, pathway analyses uncover previously unreported gene sets. Remarkably, >20% of the CCT-loci are near or within Mendelian disorder genes. These included FBN1, ADAMTS2 and TGFB2 which associate with connective tissue disorders (Marfan, Ehlers-Danlos and Loeys-Dietz syndromes), and the LUM-DCN-KERA gene complex involved in myopia, corneal dystrophies and cornea plana. Using index CCT-increasing variants, we find a significant inverse correlation in effect sizes between CCT and keratoconus (r = -0.62, P = 5.30 × 10-5) but not between CCT and primary open-angle glaucoma (r = -0.17, P = 0.2). Our findings provide evidence for shared genetic influences between CCT and keratoconus, and implicate candidate genes acting in collagen and extracellular matrix regulation.

Confirmation and refinement of the heterozygous deletion of the small leucine-rich proteoglycans associated with posterior amorphous corneal dystrophy

PURPOSE

To present the clinical and cytogenetic features of a previously unreported family with posterior amorphous corneal dystrophy (PACD) associated with a heterozygous deletion of the small leucine-rich proteoglycan (SRLP) genes on chromosome 12.

METHODS

Clinical characterization was performed using slit lamp biomicroscopic and optical coherence tomography (OCT) imaging. Genomic DNA was collected from affected and unaffected family members, and a cytogenomic array was used to identify copy number variations (CNV) present in the PACD locus.

RESULTS

Three members of a Guatemalan family presented with clinical characteristics consistent with PACD: bilateral posterior stromal lamellar opacification, decreased corneal curvature, and iridocorneal adhesions. OCT imaging demonstrated decreased corneal thickness and hyperreflectivity of the posterior third of the corneal stroma. CNV analysis confirmed the presumed clinical diagnosis of PACD by revealing a 0.304 Mb heterozygous deletion in the PACD locus on chromosome 12 that included the four SLRP genes (KERA, LUM, DCN, and EPYC) deleted in each of the PACD families in which CNV analysis has been reported.

CONCLUSIONS

This is the first report of the OCT appearance of PACD and the second confirmation of a heterozygous deletion of chromosome 12q21.33 as the cause of PACD, highlighting the utility of array-based cytogenomics to confirm the suspected clinical diagnosis of PACD. As the smallest previously reported pathogenic deletion was 0.701 Mb, the 0.304-Mb deletion we report is the smallest identified to date and reduces the size of the PACD locus to 0.275 Mb.

Phenotypic characterization of the SIRC (Statens Seruminstitut Rabbit Cornea) cell line reveals a mixed epithelial and fibroblastic nature

The aim of the present study was to investigate, in the Statens Seruminstitut Rabbit Cornea (SIRC) cell line, the presence of epithelial and fibroblastic markers, comparing their levels with those of the human Retinal Pigmented Epithelial (ARPE-19) cell line, and the Human Keratocyte (HK) cell line, respectively. SIRC cells, often described as of epithelial origin, are used as a corneal epithelial barrier model to study the permeability of ophthalmic drugs. However, they show a morphology that is more consistent with a fibroblastic cell phenotype, similar to corneal keratocytes. Our comparative analyses of cell type specific markers demonstrated that SIRC do not express cytokeratins 19 and 16 (typical of ARPE-19) and cytokeratin 9 (typical of HK); they do express cytokeratins 3 and 18 common to all three cell lines, and cytokeratin 12 typical of ARPE-19. Tight junction proteins were absent in HK, and lower in SIRC than in ARPE-19. All cell lines expressed the markers lumican and vimentin, with SIRC expressing intermediate levels between HK and ARPE-19; alpha-SMA was highly expressed in all lines. These markers, considered typical of fibroblasts, can be, however, expressed by epithelial cells during wound healing. These results might suggest that long-term in vitro cultivation of cell lines leads to a derangement of their specific phenotype, most likely due to genetic and epigenetic factors. This could be the reason why SIRC cells came to exhibit a hybrid nature between epithelial and fibroblastic cells.

Analysis of KERA in four families with cornea plana identifies two novel mutations

PURPOSE

To identify the molecular genetic cause in four families of various ethnic backgrounds with cornea plana.

METHODS

Detailed ophthalmological examination and direct sequencing of the KERA coding region in five patients of Czech and Turkish origin and their available family members.

RESULTS

Compound heterozygosity for a novel missense mutation c.209C>T; p.(Pro70Leu) and a novel splice site mutation c.887-1G>A in KERA were detected in two affected siblings of Czech origin. In silico analysis supported the pathogenicity of both variants. The second proband of Czech origin harboured c.835C>T; p.(Arg279*) in a homozygous state. Homozygous mutations c.740A>G; p.(Asn247Ser) and c.674C>T; p.(Ile225Thr) were identified in the Turkish probands, both born out of consanguineous marriages. Observed ocular phenotypes were typical of cornea plana with the exception of one Czech patient who also had marked thinning and protrusion in the superior part of the left cornea (mean keratometry 47.2 D). No corneal endothelial cell pathology was found by specular microscopy in seven eyes, in three eyes visualization of the posterior corneal surface was unsuccessful.

CONCLUSION

KERA mutation c.740A>G has been identified to date in three different populations, which makes it the most frequently occurring mutation in patients with cornea plana. Marked corneal thinning and ectasia are a very rare finding in this disorder and longitudinal follow-up needs to be performed to determine its potential progressive nature.

Proteomic profiling of TGFBI-null mouse corneas reveals only minor changes in matrix composition supportive of TGFBI knockdown as therapy against TGFBI-linked corneal dystrophies

TGFBIp is a constituent of the extracellular matrix in many human tissues including the cornea, where it is one of the most abundant proteins expressed. TGFBIp interacts with Type I, II, IV, VI, and XII collagens as well as several members of the integrin family, suggesting it plays an important role in maintaining structural integrity and possibly corneal transparency as well. Significantly, more than 60 point mutations within the TGFBI gene have been reported to result in aberrant TGFBIp folding and aggregation in the cornea, resulting in severe visual impairment and blindness. Several studies have focused on targeting TGFBIp in the cornea as a therapeutic approach to treat TGFBI-linked corneal dystrophies, but the effect of this approach on corneal homeostasis and matrix integrity remained unknown. In the current study, we evaluated the histological and proteomic profiles of corneas from TGFBI-deficient mice as well as potential redundant functions of the paralogous protein POSTN. The absence of TGFBIp in mouse corneas did not grossly affect the collagen scaffold, and POSTN is unable to compensate for loss of TGFBIp. Proteomic comparison of wild-type and TGFBI^-/- mice revealed 11 proteins were differentially regulated, including Type VI and XII collagens. However, as these alterations did not manifest at the macroscopic and behavioral levels, these data support partial or complete TGFBI knockdown as a potential therapy against TGFBI-linked corneal dystrophies. Lastly, in situ hybridization verified TGFBI mRNA in the epithelial cells but not in other cell types, supportive of a therapy directed specifically at this lineage.

Stromal striae: a new insight into corneal physiology and mechanics

We uncover the significance of a previously unappreciated structural feature in corneal stroma, important to its biomechanics. Vogt striae are a known clinical indicator of keratoconus, and consist of dark, vertical lines crossing the corneal depth. However we detected stromal striae in most corneas, not only keratoconus. We observed striae with multiple imaging modalities in 82% of 118 human corneas, with pathology-specific differences. Striae generally depart from anchor points at Descemet’s membrane in the posterior stroma obliquely in a V-shape, whereas in keratoconus, striae depart vertically from posterior toward anterior stroma. Optical coherence tomography shear wave elastography showed discontinuity of rigidity, and second harmonic generation and scanning electron microscopies showed undulation of lamellae at striae locations. Striae visibility decreased beyond physiological pressure and increased beyond physiological hydration. Immunohistology revealed striae to predominantly contain collagen VI, lumican and keratocan. The role of these regions of collagen VI linking sets of lamellae may be to absorb increases in intraocular pressure and external shocks.

Generation and Characterization of a Novel Mouse Line, Keratocan-rtTA (KeraRT), for Corneal Stroma and Tendon Research

Purpose

We created a novel inducible mouse line Keratocan-rtTA (KeraRT) that allows specific genetic modification in corneal keratocytes and tenocytes during development and in adults.

Methods

A gene-targeting vector (Kera- IRES2-rtTA3) was constructed and inserted right after the termination codon of the mouse Kera allele via gene targeting techniques. The resulting KeraRT mouse was crossed to tet-O-Hist1H2B-EGFP (TH2B-EGFP) to obtain KeraRT/TH2B-EGFP compound transgenic mice, in which cells expressing Kera are labeled with green fluorescence protein (GFP) by doxycycline (Dox) induction. The expression patterns of GFP and endogenous Kera were examined in KeraRT/TH2B-EGFP. Moreover, KeraRT was bred with tet-O-TGF-α to generate a double transgenic mouse, KeraRT/tet-O-TGF-α, to overexpress TGF-α in corneal keratocytes upon Dox induction.

Results

Strong GFP-labeled cells were detected in corneal stroma, limbs, and tail when KeraRT/TH2B-EGFP mice were fed Dox chow. There was no GFP in any single transgenic KeraRT or TH2B-EGFP mouse. Histological analysis showed that GFP in the cornea was limited to stromal keratocytes of KeraRT/TH2B-EGFP, which is consistent with Kera expression. Induction of GFP occurred in 24 hours and reached a plateau by 7 days after Dox induction. GFP could be detected 3-months after induction of KeraRT/TH2B-EGFP. Ectopic expression of TGF-α in corneal keratocytes caused hyperplasia in the corneal epithelium and stroma.

Conclusions

The novel Dox inducible KeraRT driver mouse line is a useful genetic tool for gene manipulation and elucidating gene functions in corneal stroma and tendons of limbs and tail during embryonic development, homeostasis and pathogenesis.

Asporin-deficient mice have tougher skin and altered skin glycosaminoglycan content and structure

The main structural component of connective tissues is fibrillar, cross-linked collagen whose fibrillogenesis can be modulated by Small Leucine-Rich Proteins/Proteoglycans (SLRPs). Not all SLRPs’ effects on collagen and extracellular matrix in vivo have been elucidated; one of the less investigated SLRPs is asporin. Here we describe the successful generation of an Aspn^-/- mouse model and the investigation of the Aspn^-/- skin phenotype. Functionally, Aspn^-/- mice had an increased skin mechanical toughness, although there were no structural changes present on histology or immunohistochemistry. Electron microscopy analyses showed 7% thinner collagen fibrils in Aspn^-/- mice (not statistically significant). Several matrix genes were upregulated, including collagens (Col1a1, Col1a2, Col3a1), matrix metalloproteinases (Mmp2, Mmp3) and lysyl oxidases (Lox, Loxl2), while lysyl hydroxylase (Plod2) was downregulated. Intriguingly no differences were observed in collagen protein content or in collagen cross-linking-related lysine oxidation or hydroxylation. The glycosaminoglycan content and structure in Aspn^-/- skin was profoundly altered: chondroitin/dermatan sulfate was more than doubled and had an altered composition, while heparan sulfate was halved and had a decreased sulfation. Also, decorin and biglycan were doubled in Aspn^-/- skin. Overall, asporin deficiency changes skin glycosaminoglycan composition, and decorin and biglycan content, which may explain the changes in skin mechanical properties.

The Changing Face of Aging: Highly Sulfated Glycosaminoglycans Induce Amyloid Formation in a Lattice Corneal Dystrophy Model Protein

Glycosaminoglycans (GAGs) are related to multiple biological functions and diseases. There is growing evidence that GAG concentration and sulfate content increase with age. The destabilizing mutation A546T in the corneal protein TGFBIp leads to lattice-type corneal dystrophy, but symptoms only appear in the fourth decade of life. We hypothesize that this delayed phenotype can be explained by increased GAG sulfation over time. Using in vitro assays with the C-terminal TGFIBIp domain Fas1-4, previously shown to recapitulate many properties of full-length TGFBIp, we find that only long GAGs with multiple sulfate groups on each repeating unit increase the amount of worm-like aggregates and induce long, straight fibrils in A546T. In contrast, GAGs did not induce aggregation of wildtype Fas1-4, suggesting that the finding might be specific for lattice corneal dystrophy mutants. Our results highlight a possible role of changing GAG sulfation in the accumulation of amyloid, which also may have implications for the development of neurodegenerative diseases.

Fell Muir Lecture: Collagen fibril formation in vitro and in vivo

It is a great honour to be awarded the Fell Muir Prize for 2016 by the British Society of Matrix Biology. As recipient of the prize, I am taking the opportunity to write a minireview on collagen fibrillogenesis, which has been the focus of my research for 33 years. This is the process by which triple helical collagen molecules assemble into centimetre-long fibrils in the extracellular matrix of animals. The fibrils appeared a billion years ago at the dawn of multicellular animal life as the primary scaffold for tissue morphogenesis. The fibrils occur in exquisite three-dimensional architectures that match the physical demands of tissues, for example orthogonal lattices in cornea, basket weaves in skin and blood vessels, and parallel bundles in tendon, ligament and nerves. The question of how collagen fibrils are formed was posed at the end of the nineteenth century. Since then, we have learned about the structure of DNA and the peptide bond, understood how plants capture the sun's energy, cloned animals, discovered antibiotics and found ways of editing our genome in the pursuit of new cures for diseases. However, how cells generate tissues from collagen fibrils remains one of the big unsolved mysteries in biology. In this review, I will give a personal account of the topic and highlight some of the approaches that my research group are taking to find new insights.

Acute hypoxia influences collagen and matrix metalloproteinase expression by human keratoconus cells in vitro

Keratoconus (KC) is a progressive corneal ectasia linked to thinning of the central cornea. Hard contact lenses, rigid gas permeable lenses, and scleral lenses are the primary treatment modalities for early to mid- stages of KC to correct refractive error and astigmatism that develops as a result of an irregular corneal structure. These treatments are associated with significant drawbacks, including reduced availability of the tear film and oxygen to the corneal epithelium and stroma. However, it remains unknown whether hypoxia affects corneal integrity in the KC pathobiology. A number of studies have associated elevated oxidative stress with KC both in vitro and ex vivo. We hypothesized that KC-derived corneal fibroblasts are more susceptible to hypoxia-induced oxidative stress compared to healthy controls leading to exacerbation of corneal thinning in KC. This study investigated the effects of hypoxia on ECM secretion, assembly, and matrix metalloproteinase (MMP) expression in human corneal fibroblasts from healthy controls (HCFs) and KC patients (HKCs) in vitro. HCFs and HKCs were cultured in 3D constructs for 3 weeks and maintained or transferred to normoxic (21% O₂) or hypoxic (2% O₂) conditions, respectively, for 1 additional week. At the 4 week time-point, constructs were isolated and probed for Collagen I, III, and V, keratocan and MMP-1, -2, -3, -9, and -13, as well as hypoxia markers, hypoxia inducible factor-1α and lactoferrin. Conditioned media was also collected and probed for Collagen I, III, and V by Western blot. Thickness of the ECM assembled by HCFs and HKCs was measured using immunofluorescence microscopy. Results showed that hypoxia significantly reduced Collagen I secretion in HKCs, as well as upregulated the expression of MMP-1 and -2 with no significant effects on MMP-3, -9, or -13. ECM thickness was reduced in both cell types following 1 week in a low oxygen environment. Our study shows that hypoxia influences collagen and MMP expression by HKCs, which may have consequential effects on ECM structure in the context of KC.

The use of in vivo, ex vivo, in vitro, computational models and volunteer studies in vision research and therapy, and their contribution to the Three Rs

Much is known about mammalian vision, and considerable progress has been achieved in treating many vision disorders, especially those due to changes in the eye, by using various therapeutic methods, including stem cell and gene therapy. While cells and tissues from the main parts of the eye and the visual cortex (VC) can be maintained in culture, and many computer models exist, the current non-animal approaches are severely limiting in the study of visual perception and retinotopic imaging. Some of the early studies with cats and non-human primates (NHPs) are controversial for animal welfare reasons and are of questionable clinical relevance, particularly with respect to the treatment of amblyopia. More recently, the UK Home Office records have shown that attention is now more focused on rodents, especially the mouse. This is likely to be due to the perceived need for genetically-altered animals, rather than to knowledge of the similarities and differences of vision in cats, NHPs and rodents, and the fact that the same techniques can be used for all of the species. We discuss the advantages and limitations of animal and non-animal methods for vision research, and assess their relative contributions to basic knowledge and clinical practice, as well as outlining the opportunities they offer for implementing the principles of the Three Rs (Replacement, Reduction and Refinement).

The Tyrosine Sulfate Domain of Fibromodulin Binds Collagen and Enhances Fibril Formation

Small leucine-rich proteoglycans interact with other extracellular matrix proteins and are important regulators of matrix assembly. Fibromodulin has a key role in connective tissues, binding collagen through two identified binding sites in its leucine-rich repeat domain and regulating collagen fibril formation in vitro and in vivo Some nine tyrosine residues in the fibromodulin N-terminal domain are O-sulfated, a posttranslational modification often involved in protein interactions. The N-terminal domain mimics heparin, binding proteins with clustered basic amino acid residues. Because heparin affects collagen fibril formation, we investigated whether tyrosine sulfate is involved in fibromodulin interactions with collagen. Using full-length fibromodulin and its N-terminal tyrosine-sulfated domain purified from tissue, as well as recombinant fibromodulin fragments, we found that the N-terminal domain binds collagen. The tyrosine-sulfated domain and the leucine-rich repeat domain both bound to three specific sites along the collagen type I molecule, at the N terminus and at 100 and 220 nm from the N terminus. The N-terminal domain shortened the collagen fibril formation lag phase and tyrosine sulfation was required for this effect. The isolated leucine-rich repeat domain inhibited the fibril formation rate, and full-length fibromodulin showed a combination of these effects. The fibrils formed in the presence of fibromodulin or its fragments showed more organized structure. Fibromodulin and its tyrosine sulfate domain remained bound on the formed fiber. Taken together, this suggests a novel, regulatory function for tyrosine sulfation in collagen interaction and control of fibril formation.

Small leucine-rich repeat proteoglycans in corneal inflammation and wound healing

The small leucine rich repeat proteoglycans are major components of the cornea. Lumican, keratocan, decorin, biglycan and osteoglycin are present throughout the adult corneal stroma, and fibromodulin in the peripheral limbal area. In the cornea literature these proteoglycan have been reviewed as structural, collagen fibril-regulating proteins of the cornea. However, these proteoglycans are members of the leucine-rich-repeat superfamily, and share structural similarities with pathogen recognition toll-like receptors. Emerging studies are showing that these have a range of interactions with cell surface receptors, chemokines, growth factors and pathogen associated molecular patterns and are able to regulate host immune response, inflammation and wound healing. This review discusses what is known about their innate immune-related role directly in the cornea, and studies outside the field that find interesting links with innate immune and wound healing responses that are likely to be relevant to the ocular surface. In addition, the review discusses phenotypes of mice with targeted deletion of proteoglycan genes and genetic variants associated with human pathologies.