Collagen XII Is a Regulator of Corneal Stroma Structure and Function

The coordinated activities of collagen VI and XII in maintenance of tissue structure, function and repair: evidence for a physical interaction

Collagen VI and collagen XII are structurally complex collagens of the extracellular matrix (ECM). Like all collagens, type VI and XII both possess triple-helical components that facilitate participation in the ECM network, but collagen VI and XII are distinct from the more abundant fibrillar collagens in that they also possess arrays of structurally globular modules with the capacity to propagate signaling to attached cells. Cell attachment to collagen VI and XII is known to regulate protective, proliferative or developmental processes through a variety of mechanisms, but a growing body of genetic and biochemical evidence suggests that at least some of these phenomena may be potentiated through mechanisms that require coordinated interaction between the two collagens. For example, genetic studies in humans have identified forms of myopathic Ehlers-Danlos syndrome with overlapping phenotypes that result from mutations in either collagen VI or XII, and biochemical and cell-based studies have identified accessory molecules that could form bridging interactions between the two collagens. However, the demonstration of a direct or ternary structural interaction between collagen VI or XII has not yet been reported. This Hypothesis and Theory review article examines the evidence that supports the existence of a functional complex between type VI and XII collagen in the ECM and discusses potential biological implications.

Squishy matters - Corneal mechanobiology in health and disease

The cornea, as a dynamic and responsive tissue, constantly interacts with mechanical forces in order to maintain its structural integrity, barrier function, transparency and refractive power. Cells within the cornea sense and respond to various mechanical forces that fundamentally regulate their morphology and fate in development, homeostasis and pathophysiology. Corneal cells also dynamically regulate their extracellular matrix (ECM) with ensuing cell-ECM crosstalk as the matrix serves as a dynamic signaling reservoir providing biophysical and biochemical cues to corneal cells. Here we provide an overview of mechanotransduction signaling pathways then delve into the recent advances in corneal mechanobiology, focusing on the interplay between mechanical forces and responses of the corneal epithelial, stromal, and endothelial cells. We also identify species-specific differences in corneal biomechanics and mechanotransduction to facilitate identification of optimal animal models to study corneal wound healing, disease, and novel therapeutic interventions. Finally, we identify key knowledge gaps and therapeutic opportunities in corneal mechanobiology that are pressing for the research community to address especially pertinent within the domains of limbal stem cell deficiency, keratoconus and Fuchs' endothelial corneal dystrophy. By furthering our understanding corneal mechanobiology, we can contextualize discoveries regarding corneal diseases as well as innovative treatments for them.

Collagen type XII is undetectable in keratoconus Bowman's layer

PURPOSE

Corneal biomechanical failure is the hallmark of keratoconus (KC); however, the cause of this failure remains elusive. Collagen type XII (COL12A1), which localises to Bowman's layer (BL), is thought to function in stress-bearing areas, such as BL. Given the putative protective role of COL12A1 in biomechanical stability, this study aims to characterise COL12A1 expression in all corneal layers involved in KC.

METHODS

TaqMan quantitative PCR was performed on 31 corneal epithelium samples of progressive KC and myopic control eyes. Tissue microarrays were constructed using full-thickness corneas from 61 KC cases during keratoplasty and 18 non-KC autopsy eyes and stained with an antibody specific to COL12A1. Additionally, COL12A1 was knocked out in vitro in immortalised HEK293 cells.

RESULTS

COL12A1 expression was reduced at transcript levels in KC epithelium compared with controls (ratio: 0.58, p<0.03). Immunohistochemical studies demonstrated that COL12A1 protein expression in BL was undetectable, with reduced expression in KC epithelium, basement membrane and stroma.

CONCLUSIONS

The apparent absence of COL12A1 in KC BL, together with the functional importance that COL12A1 is thought to have in stress bearing areas, suggests that COL12A1 may play a role in the pathogenesis of KC. Further studies are necessary to investigate the mechanisms that lead to COL12A1 dysregulation in KC.

Collagen Mimetic Peptides Promote Repair of MMP-1-Damaged Collagen in the Rodent Sclera and Optic Nerve Head

The structural and biomechanical properties of collagen-rich ocular tissues, such as the sclera, are integral to ocular function. The degradation of collagen in such tissues is associated with debilitating ophthalmic diseases such as glaucoma and myopia, which often lead to visual impairment. Collagen mimetic peptides (CMPs) have emerged as an effective treatment to repair damaged collagen in tissues of the optic projection, such as the retina and optic nerve. In this study, we used atomic force microscopy (AFM) to assess the potential of CMPs in restoring tissue stiffness in the optic nerve head (ONH), including the peripapillary sclera (PPS) and the glial lamina. Using rat ONH tissue sections, we induced collagen damage with MMP-1, followed by treatment with CMP-3 or vehicle. MMP-1 significantly reduced the Young's modulus of both the PPS and the glial lamina, indicating tissue softening. Subsequent CMP-3 treatment partially restored tissue stiffness in both the PPS and the glial lamina. Immunohistochemical analyses revealed reduced collagen fragmentation after MMP-1 digestion in CMP-3-treated tissues compared to vehicle controls. In summary, these results demonstrate the potential of CMPs to restore collagen stiffness and structure in ONH tissues following enzymatic damage. CMPs may offer a promising therapeutic avenue for preserving vision in ocular disorders involving collagen remodeling and degradation.

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.

Single-Cell RNA Sequencing: Opportunities and Challenges for Studies on Corneal Biology in Health and Disease

The structure and major cell types of the multi-layer human cornea have been extensively studied. However, various cell states in specific cell types and key genes that define the cell states are not fully understood, hindering our comprehension of corneal homeostasis, related diseases, and therapeutic discovery. Single-cell RNA sequencing is a revolutionary and powerful tool for identifying cell states within tissues such as the cornea. This review provides an overview of current single-cell RNA sequencing studies on the human cornea, highlighting similarities and differences between them, and summarizing the key genes that define corneal cell states reported in these studies. In addition, this review discusses the opportunities and challenges of using single-cell RNA sequencing to study corneal biology in health and disease.

Collagen XII regulates stromal wound closure

The role of collagen XII in regulating injury repair and reestablishment of corneal function is unknown. This manuscript aims to investigate the role(s) of collagen XII in the repair of incisional and debridement injuries in an adult mouse model. Two different types of injury in wild type and Col12a1-/- corneas were created to investigate the effects of collagen XII -in wound repair and scar formation-by using clinical photographs, immunohistology, second harmonic generation imaging and electron microscopy. Results showed that collagen XII is a regulator of wound closure after incisional injuries. Absence of collagen XII retarded wound closure and the wound healing process. These findings show that collagen XII regulates fibrillogenesis, CD68 cell lineage infiltration, and myofibroblast survival following injury. In vitro studies suggest that collagen XII regulates deposition of an early and provisional matrix by interacting with two proteins regulating early matrix deposition: fibronectin and LTBP1(latent transforming growth factor β binding protein 1). In conclusion, collagen XII regulates tissue repair in corneal incisional wounds. Understanding the function of collagen XII during wound healing has significant translational value.

Collagen XII mediated cellular and extracellular mechanisms in development, regeneration, and disease

Collagen XII, a fibril-associated collagen with interrupted triple helices (FACIT), influences fibrillogenesis in numerous tissues. In addition to this extracellular function, collagen XII also directly regulates cellular function. Collagen XII is widely expressed in connective tissues, particularly tendons, ligaments, and the periodontium and periosteum, where it is enriched in the pericellular regions. Mutations in the collagen XII gene cause myopathic Ehlers-Danlos syndrome (mEDS), an early-onset disease characterized by overlapping connective tissue abnormalities and muscle weakness. Patients with mEDS exhibit delayed motor development, muscle weakness, joint laxity, hypermobility, joint contractures, and abnormal wound healing. A mEDS mouse model was generated by deletion of the Col12a1 gene, resulting in skeletal and muscle abnormalities with disorganized tissue structures and altered mechanical properties. Extracellularly, collagen XII interacts with collagen I fibrils and regulates collagen fibril spacing and assembly during fibrillogenesis. Evidence for the binding of collagen XII to other EDS-related molecules (e.g., decorin and tenascin X) suggests that disruption of ECM molecular interactions is one of the causes of connective tissue pathology in mEDS. Collagen XII also has been shown to influence cell behavior, such as cell shape and cell-cell communication, by providing physical connection between adjacent cells during tissue development and regeneration. The focus of this review is on the functions of collagen XII in development, regeneration, and disease.

Multiple Optical Elastography Techniques Reveal the Regulation of Corneal Stiffness by Collagen XII

Purpose

Collagen XII plays a role in regulating the structure and mechanical properties of the cornea. In this work, several optical elastography techniques were used to investigate the effect of collagen XII deficiency on the stiffness of the murine cornea.

Methods

A three-prong optical elastography approach was used to investigate the mechanical properties of the cornea. Brillouin microscopy, air-coupled ultrasonic optical coherence elastography (OCE) and heartbeat OCE were used to assess the mechanical properties of wild type (WT) and collagen XII-deficient (Col12a1-/-) murine corneas. The Brillouin frequency shift, elastic wave speed, and compressive strain were all measured as a function of intraocular pressure (IOP).

Results

All three optical elastography modalities measured a significantly decreased stiffness in the Col12a1-/- compared to the WT (P < 0.01 for all three modalities). The optical coherence elastography techniques showed that mean stiffness increased as a function of IOP; however, Brillouin microscopy showed no discernable trend in Brillouin frequency shift as a function of IOP.

Conclusions

Our approach suggests that the absence of collagen XII significantly softens the cornea. Although both optical coherence elastography techniques showed an expected increase in corneal stiffness as a function of IOP, Brillouin microscopy did not show such a relationship, suggesting that the Brillouin longitudinal modulus may not be affected by changes in IOP. Future work will focus on multimodal biomechanical models, evaluating the effects of other collagen types on corneal stiffness, and in vivo measurements.

Targeted conditional collagen XII deletion alters tendon function

Collagen XII is a fibril-associated collagen with interrupted triple helices (FACIT). This non-fibrillar collagen is a homotrimer composed of three α1(XII) chains assembled into a collagenous molecule with a C terminal collagenous domain and a large N terminal non-collagenous domain. During tendon development and growth, collagen XII is broadly expressed throughout the extracellular matrix and enriched pericellularly around tenocytes. Tendons in a global Col12a1 ^-/- knockout model demonstrated disrupted fibril and fiber structure and disordered tenocyte organization, highlighting the critical regulatory roles of collagen XII in determining tendon structure and function. However, muscle and bone also are affected in the collagen XII knockout model. Therefore, secondary effects on tendon due to involvement of bone and muscle may occur in the global knockout. The global knockout does not allow the definition of intrinsic mechanisms involving collagen XII in tendon versus extrinsic roles involving muscle and bone. To address this limitation, we created and characterized a conditional Col12a1-null mouse model to permit the spatial and temporal manipulation of Col12a1 expression. Collagen XII knockout was targeted to tendons by breeding conditional Col12a1 ^flox/flox mice with Scleraxis-Cre (Scx-Cre) mice to yield a tendon-specific Col12a1-null mouse line, Col12a1 ^Δten/Δten . Both mRNA and protein expression in Col12a1 ^Δten/Δten mice decreased to near baseline levels in flexor digitorum longus tendons (FDL). Collagen XII immuno-localization revealed an absence of reactivity in the tendon proper, but there was reactivity in the cells of the surrounding peritenon. This supports a targeted knockout in tenocytes while peritenon cells from a non-tendon lineage were not targeted and retained collagen XII expression. The tendon-targeted, Col12a1 ^Δten/Δten  mice had significantly reduced forelimb grip strength, altered gait and a significant decrease in biomechanical properties. While the observed decrease in tendon modulus suggests that differences in tendon material properties in the absence of Col12a1 expression underlie the functional deficiencies. Together, these findings suggest an intrinsic role for collagen XII critical for development of a functional tendon.

Type XX Collagen Is Elevated in Circulation of Patients with Solid Tumors

In the tumor microenvironment, the extracellular matrix (ECM) has been recognized as an important part of cancer development. The dominant ECM proteins are the 28 types of collagens, each with a unique function in tissue architecture. Type XX collagen, however, is poorly characterized, and little is known about its involvement in cancer. We developed an ELISA quantifying type XX collagen, named PRO-C20, using a monoclonal antibody raised against the C-terminus. PRO-C20 and PRO-C1, an ELISA targeting the N-terminal pro-peptide of type I collagen, was measured in sera of 219 patients with various solid cancer types and compared to sera levels of 33 healthy controls. PRO-C20 was subsequently measured in a separate cohort comprising 36 patients with pancreatic ductal adenocarcinoma (PDAC) and compared to 20 healthy controls and 11 patients with chronic pancreatitis. PRO-C20 was significantly elevated in all cancers tested: bladder, breast, colorectal, head and neck, kidney, lung, melanoma, ovarian, pancreatic, prostate, and stomach cancer (p < 0.01−p < 0.0001). PRO-C1 was only elevated in patients with ovarian cancer. PRO-C20 could discriminate between patients and healthy controls with AUROC values ranging from 0.76 to 0.92. Elevated levels were confirmed in a separate cohort of patients with PDAC (p < 0.0001). High PRO-C20 levels (above 2.57 nM) were predictive of poor survival after adjusting for the presence of metastasis, age, and sex (HR: 4.25, 95% CI: 1.52−11.9, p-value: 0.006). Circulating type XX collagen is elevated in sera of patients with various types of cancer and has prognostic value in PDAC. If validated, PRO-C20 may be a novel biomarker for patients with solid tumors and can help understand the ECM biology of cancer.

Collagen XII Regulates Corneal Stromal Structure by Modulating Transforming Growth Factor-β Activity

Collagen XII is a regulator of corneal stroma structure and function. The current study examined the role of collagen XII in regulating corneal stromal transforming growth factor (TGF)-β activation and latency. Specifically, with the use of conventional collagen XII null mouse model, the role of collagen XII in the regulation of TGF-β latency and activity in vivo was investigated. Functional quantification of latent TGF-β in stromal matrix was performed by using transformed mink lung reporter cells that produce luciferase as a function of active TGF-β. Col12a1 knockdown with shRNA was used to test the role of collagen XII in TGF-β activation. Col12a1^-/- hypertrophic stromata were observed with keratocyte hyperplasia. Increased collagen fibril forward signal was found by second harmonic generation microscopy in the absence of collagen XII. Collagen XII regulated mRNA synthesis of Serpine1, Col1a1, and Col5a1 and deposition of collagens in the extracellular matrix. A functional plasminogen activator inhibitor luciferase assay showed that collagen XII is necessary for latent TGF-β storage in the extracellular matrix and that collagen XII down-regulates active TGF-β. Collagen XII dictates stromal structure and function by regulating TGF-β activity. A hypertrophic phenotype in Col12a1^-/- corneal tissue can be explained by abnormal up-regulation of TGF-β activation and decreased latent storage.

Downregulation of collagen XI during late postnatal corneal development is followed by upregulation after injury

Collagen XI plays a role in nucleating collagen fibrils and in controlling fibril diameter. The aim of this research was to elucidate the role that collagen XI plays in corneal fibrillogenesis during development and following injury. The temporal and spatial expression of collagen XI was evaluated in C57BL/6 wild-type mice. For wound-healing studies in adult mice, stromal injuries were created using techniques that avoid caustic chemicals. The temporal expression and spatial localization of collagen XI was studied following injury in a Col11a1 inducible knockout mouse model. We found that collagen XI expression occurs during early maturation and is upregulated after stromal injury in areas of regeneration and remodeling. Abnormal fibrillogenesis with new fibrils of heterogeneous size and shape occurs after injury in a decreased collagen XI matrix. In conclusion, collagen XI is expressed in the stroma during development and following injury in adults, and is a regulator of collagen fibrillogenesis in regenerating corneal tissue.

Collagen XIV Is an Intrinsic Regulator of Corneal Stromal Structure and Function

Collagen XIV is poorly characterized in the body, and the current knowledge of its function in the cornea is limited. The aim of the current study was to elucidate the role(s) of collagen XIV in regulating corneal stromal structure and function. Analysis of collagen XIV expression, temporal and spatial, was performed at different postnatal days (Ps) in wild-type C57BL/6 mouse corneal stromas and after injury. Conventional collagen XIV null mice were used to inquire the roles that collagen XIV plays in fibrillogenesis, fibril packing, and tissue mechanics. Fibril assembly and packing as well as stromal organization were evaluated using transmission electron microscopy and second harmonic generation microscopy. Atomic force microscopy was used to assess stromal stiffness. Col14a1 mRNA expression was present at P4 to P10 and decreased at P30. No immunoreactivity was noted at P150. Abnormal collagen fibril assembly with a shift toward larger-diameter fibrils and increased interfibrillar spacing in the absence of collagen XIV was found. Second harmonic generation microscopy showed impaired fibrillogenesis in the collagen XIV null stroma. Mechanical testing suggested that collagen XIV confers stiffness to stromal tissue. Expression of collagen XIV is up-regulated following injury. This study indicates that collagen XIV plays a regulatory role in corneal development and in the function of the adult cornea. The expression of collagen XIV is recapitulated during wound healing.

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.

Effective elastic modulus of an intact cornea related to indentation behavior: A comparison between the Hertz model and Johnson-Kendall-Roberts model

In this study, a macro-indentation test on the submillimeter scale was performed to analyze the indentation behavior of an intact cornea under physiological pressures. The Hertz and Johnson-Kendall-Roberts (JKR) models were employed to solve the elastic modulus (E) of the intact cornea. The relevant detailed analysis showed that the JKR model, which accounted for the contribution from the adhesion energy, could be used to obtain the E values that were more than two-folds of those obtained from the Hertz model, which only considered the external force. Compared with the uniaxial tension test in vitro, unlike the elastic Hertz-model, the E values under physiological pressures that were obtained with the JKR model were between the lower and upper limits of corneal material. This phenomenon indicated that the JKR model could be used to obtain reasonably effective E values of an intact cornea under physiological pressures.

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.