Unravelling the stromal-nerve interactions in the human diabetic cornea

Effects of hypoxia in the diabetic corneal stroma microenvironment

Corneal dysfunctions associated with Diabetes Mellitus (DM), termed diabetic keratopathy (DK), can cause impaired vision and/or blindness. Hypoxia affects both Type 1 (T1DM) and Type 2 (T2DM) surprisingly, the role of hypoxia in DK is unexplored. The aim of this study was to examine the impact of hypoxia in vitro on primary human corneal stromal cells derived from Healthy (HCFs), and diabetic (T1DMs and T2DMs) subjects, by exposing them to normoxic (21% O2) or hypoxic (2% O2) conditions through 2D and 3D in vitro models. Our data revealed that hypoxia affected T2DMs by slowing their wound healing capacity, leading to significant alterations in oxidative stress-related markers, mitochondrial health, cellular homeostasis, and endoplasmic reticulum health (ER) along with fibrotic development. In T1DMs, hypoxia significantly modulated markers related to membrane permeabilization, oxidative stress via apoptotic marker (BAX), and protein degradation. Hypoxic environment induced oxidative stress (NOQ1 mediated reduction of superoxide in T1DMs and Nrf2 mediated oxidative stress in T2DMs), modulation in mitochondrial health (Heat shock protein 27 (HSP27), and dysregulation of cellular homeostasis (HSP90) in both T1DMs and T2DMs. This data underscores the significant impact of hypoxia on the diabetic cornea. Further studies are warranted to delineate the complex interactions.

Cell sheet-based approach to study the diabetic corneal stroma

Prolonged hyperglycemia during diabetes mellitus (DM) is associated with severe complications that may affect both the anterior and posterior ocular segments, leading to impaired vision or blindness. The cornea is a vital part of the eye that has a dual role as a protective transparent barrier and as a major refractive structure and is likewise negatively affected by hyperglycemia in DM. Understanding the cellular and molecular mechanisms underlying the phenotypic changes associated with DM is critical to developing targeted therapies to promote tissue integrity. In this proof-of-concept study, we applied a cell sheet-based approach to generate stacked constructs of physiological corneal thickness using primary human corneal fibroblasts isolated from cadaveric control (healthy), Type 1 DM and Type 2 DM corneal tissues. Self-assembled corneal stromal sheets were generated after 2 weeks in culture, isolated, and subsequently assembled to create stacked constructs, which were evaluated using transmission electron microscopy. Analysis of gene expression patterns revealed significant downregulation of fibrotic markers, α-smooth muscle actin, and collagen type 3, with stacking in Type 2 DM constructs when compared to controls. IGF1 expression was significantly upregulated in Type 2 DM constructs compared to controls with a significant reduction induced by stacking. This study describes the development of a thicker, self-assembled corneal stromal construct as a platform to evaluate phenotypic differences associated with DM-derived corneal fibroblasts and enable the development of targeted therapeutics to promote corneal integrity.

Monocarboxylate Transporters: Role and Regulation in Corneal Diabetes

Diabetes mellitus (DM) is a group of metabolic diseases that is known to cause structural and functional ocular complications. In the human cornea, DM-related complications affect the epithelium, stroma, and nerves. Monocarboxylate transporters (MCTs) are a family of proton-linked plasma membrane transporters that carry monocarboxylates across plasma membranes. In the context of corneal health and disease, their role, presence, and function are largely undetermined and solely focused on the most common MCT isoforms, 1 through 4. In this study, we investigated the regulation of MCT1, 2, 4, 5, 8, and 10, in corneal DM, using established 3D self-assembled extracellular matrix (ECM) in vitro models. Primary stromal corneal fibroblasts were isolated from healthy (HCFs), type I (T1DMs), and type II (T2DMs) DM donors. Monoculture 3D constructs were created by stimulating stromal cells on transwells with stable vitamin C for two or four weeks. Coculture 3D constructs were created by adding SH-SY5Y neurons at two different densities, 12 k and 500 k, on top of the monocultures. Our data showed significant upregulation of MCT1 at 4 weeks for HCF, T1DM, and T2DM monocultures, as well as the 500 k nerve cocultures. MCT8 was significantly upregulated in HCF and T1DM monocultures and all of the 500 k nerve cocultures. Further, MCT10 was only expressed at 4 weeks for all cocultures and was limited to HCFs and T1DMs in monocultures. Immunofluorescence analysis showed cytoplasmic MCT expression for all cell types and significant downregulation of both MCT2 and MCT4 in HCFs, when compared to T1DMs and T2DMs. Herein, we reveal the existence and modulation of MCTs in the human diabetic cornea in vitro. Changes appeared dependent on neuronal density, suggesting that MCTs are very likely critical to the neuronal defects observed in diabetic keratopathy/neuropathy. Further studies are warranted in order to fully delineate the role of MCTs in corneal diabetes.

LM22B-10 promotes corneal nerve regeneration through in vitro 3D co-culture model and in vivo corneal injury model

Corneal nerve wounding often causes abnormalities in the cornea and even blindness in severe cases. In this study, we construct a dorsal root ganglion-corneal stromal cell (DRG-CSC, DS) co-culture 3D model to explore the mechanism of corneal nerve regeneration. Firstly, this model consists of DRG collagen grafts sandwiched by orthogonally stacked and orderly arranged CSC-laden plastic compressed collagen. Nerve bundles extend into the entire corneal stroma within 14 days, and they also have orthogonal patterns. This nerve prevents CSCs from apoptosis in the serum withdrawal medium. The conditioned medium (CM) for CSCs in collagen scaffolds contains NT-3, IL-6, and other factors. Among them, NT-3 notably promotes the activation of ERK-CREB in the DRG, leading to the growth of nerve bundles, and IL-6 induces the upregulation of anti-apoptotic genes. Then, LM22B-10, an activator of the NT-3 receptor TrkB/TrkC, can also activate ERK-CREB to enhance nerve growth. After administering LM22B-10 eye drops to regular and diabetic mice with corneal wounding, LM22B-10 significantly improves the healing speed of the corneal epithelium, corneal sensitivity, and corneal nerve density. Overall, the DS co-culture model provides a promising platform and tools for the exploration of corneal physiological and pathological mechanisms, as well as the verification of drug effects in vitro. Meanwhile, we confirm that LM22B-10, as a non-peptide small molecule, has future potential in nerve wound repair. STATEMENT OF SIGNIFICANCE: The cornea accounts for most of the refractive power of the eye. Corneal nerves play an important role in maintaining corneal homeostasis. Once the corneal nerves are damaged, the corneal epithelium and stroma develop lesions. However, the mechanism of the interaction between corneal nerves and corneal cells is still not fully understood. Here, we construct a corneal stroma-nerve co-culture in vitro model and reveal that NT-3 expressed by stromal cells promotes nerve growth by activating the ERK-CREB pathway in nerves. LM22B-10, an activator of NT-3 receptors, can also induce nerve growth in vitro. Moreover, it is used as eye drops to enhance corneal epithelial wound healing, corneal nerve sensitivity and density of nerve plexus in corneal nerve wounding model in vivo.

The Corneal Changes in Diabetic Patients

Diabetes mellitus (DM) represents a systemic disorder which afects different organs. Ocular complications of the DM are the worldwide leading cause of blindness. The most common complications are diabetic retinopathy, diabetic cataract, neovascular glaucoma. Recently many investigations point out that DM can cause comlications at ocular surface as well. Condition such as decreased corneal sensitivity, dry eye or neurotrophic corneal ulceraction are the main clinical manifestations of the diabetic keratopathy (DK). Untreated, these conditions can lead to serious visual acuity decrease. Pathological processes, based on chronic inflammation, due to chronic hyperglycemia, are the main step in the process of DK development. Adequate treatment of the main disease - DM is an imperative in maintaining the healthy cornea without subjective sensations of diabetic patients.

The effect of high-fat diet-induced metabolic disturbance on corneal neuroimmune features

PURPOSE

The highly innervated cornea is susceptible to nerve loss secondary to systemic diseases such as diabetes and metabolic disturbances caused by high-fat diet. In this study, we characterize the effect of high-fat diet on the mouse corneal neuroimmune phenotype, including changes to corneal nerve density and resident immune cells, alongside the clinical assessment of corneal thickness and endothelial cell density.

METHODS

Male C57Bl6/J mice, aged 10 weeks, were fed a high-fat diet (60 kcal% fat, 5.2 kcal/g) or control diet (10 kcal%, 3.8 kcal/g) for 16 weeks. At the study endpoint, metabolic parameters (HbA1c, weight, fasting glucose, body fat) were measured to confirm metabolic disturbance. Clinical imaging of the anterior segment was performed using optical coherence tomography to measure the corneal epithelial and stromal thickness. Corneal sensory nerves were visualized using flatmount immunostaining and confocal microscopy. The topographical distribution and density of sensory nerves (BIII-tubulin⁺), intraepithelial CD45⁺ and MHC- II⁺ cells, stromal macrophages (IBA1⁺CD206⁺) and endothelial cells (ZO-1⁺) were analysed using FIJI.

RESULTS

High-fat diet mice had significantly higher blood HbA1c, higher body weight, a higher percentage of body fat and elevated fasting glucose compared to the control diet mice. Corneal epithelial and stromal thickness was similar in both groups. The sum length of the basal nerve plexus was lower in the central and peripheral cornea of mice fed a high-fat diet. In contrast, the sum length of superficial nerve terminals was similar between groups. Epithelial immune cell density was two-fold higher in the central corneas of high-fat diet mice compared to control diet mice. IBA1⁺CD206⁺ macrophage density was similar in the anterior stroma of both groups but was significantly higher in the posterior stroma of the peripheral cornea in the high-fat diet mice compared to controls. The percentage of nerve-associated MHC-II⁺ cells in the epithelium and stroma was higher in HFD mice compared to controls. Endothelial cell density was similar in the corneas of high-fat diet mice compared to controls.

CONCLUSION

Together with corneal neuropathy, corneal immune cells in mice fed a high-fat diet were differentially affected depending on their topographical distribution and location within cornea, and appeared in closer proximity to epithelial and stromal nerves, suggesting a local neuroimmune disruption induced by systemic metabolic disturbance.

3D in vitro corneal models: A review of current technologies

Three-dimensional (3D) in vitro models are excellent tools for studying complex biological systems because of their physiological similarity to in vivo studies, cost-effectiveness and decreased reliance on animals. The influence of tissue microenvironment on the cells, cell-cell interaction and the cell-matrix interactions can be elucidated in 3D models, which are difficult to mimic in 2D cultures. In order to develop a 3D model, the required cell types are derived from the tissues or stem cells. A 3D tissue/organ model typically includes all the relevant cell types and the microenvironment corresponding to that tissue/organ. For instance, a full corneal 3D model is expected to have epithelial, stromal, endothelial and nerve cells, along with the extracellular matrix and membrane components associated with the cells. Although it is challenging to develop a corneal 3D model, several attempts have been made and various technologies established which closely mimic the in vivo environment. In this review, three major technologies are highlighted: organotypic cultures, organoids and 3D bioprinting. Also, several combinations of organotypic cultures, such as the epithelium and stroma or endothelium and neural cultures are discussed, along with the disease relevance and potential applications of these models. In the future, new biomaterials will likely promote better cell-cell and cell-matrix interactions in organotypic corneal cultures.

Corneal alteration and pathogenesis in diabetes mellitus

The incidence of diabetes mellitus (DM) and its complications have increased considerably worldwide. Diabetic keratopathy is the major complication of the cornea characterized by delayed corneal wound healing, decreasing corneal epithelial sensitivity, and recurrent corneal ulcers. There is accumulating evidence that diabetic keratopathy is correlated with the hyperglycemic state. Different corneal components may produce different alterations under hyperglycemia. In addition, diabetic nerve alteration may become a novel biomarker of early-stage DM. Abnormalities of the corneal nerve plexus have been associated with diabetic inflammatory states. There is rapidly growing evidence based on investigations of diabetic corneal nerves through in vivo confocal microscopy. Understanding the molecular pathogenesis caused by hyperglycemia may assist in the identification of novel biomarkers, as well as therapeutic targets for early treatment. This review mainly summarizes recent findings on corneal alteration and pathogenesis in DM.

Corneal Epithelial-Stromal Fibroblast Constructs to Study Cell-Cell Communication in Vitro

Cell–cell communication plays a fundamental role in mediating corneal wound healing following injury or infection. Depending on the severity of the wound, regeneration of the cornea and the propensity for scar development are influenced by the acute resolution of the pro-fibrotic response mediated by closure of the wound via cellular and tissue contraction. Damage of the corneal epithelium, basement membrane, and anterior stroma following a superficial keratectomy is known to lead to significant provisional matrix deposition, including secretion of fibronectin and thrombospondin-1, as well as development of a corneal scar. In addition, corneal wounding has previously been shown to promote release of extracellular vesicles from the corneal epithelium, which, in addition to soluble factors, may play a role in promoting tissue regeneration. In this study, we report the development and characterization of a co-culture system of human corneal epithelial cells and corneal stromal fibroblasts cultured for 4 weeks to allow extracellular matrix deposition and tissue maturation. The secretion of provisional matrix components, as well as small and large extracellular vesicles, was apparent within the constructs, suggesting cell–cell communication between epithelial and stromal cell populations. Laminin-1β was highly expressed by the corneal epithelial layer with the presence of notable patches of basement membrane identified by transmission electron microscopy. Interestingly, we identified expression of collagen type III, fibronectin, and thrombospondin-1 along the epithelial–stromal interface similar to observations seen in vivo following a keratectomy, as well as expression of the myofibroblast marker, α-smooth muscle actin, within the stroma. Our results suggest that this corneal epithelial–stromal model may be useful in the study of the biochemical phenomena that occur during corneal wound healing.

Diabetes inhibits corneal epithelial cell migration and tight junction formation in mice and human via increasing ROS and impairing Akt signaling

Corneal wounds usually heal quickly; but diabetic patients have more fragile corneas and experience delayed and painful healing. In the present study, we compared the healing capacity of corneal epithelial cells (CECs) between normal and diabetic conditions and the potential mechanisms. Primary murine CEC derived from wild-type and diabetic (db/db) mice, as well as primary human CEC were prepared. Human CEC were exposed to high glucose (30 mM) to mimic diabetic conditions. Cell migration and proliferation were assessed using Scratch test and MTT assays, respectively. Reactive oxygen species (ROS) production in the cells was measured using dichlorofluorescein reagent. Western blot was used to evaluate the expression levels of Akt. Transepithelial electrical resistance (TEER) and zonula occludens-1 (ZO-1) expression were used to determine tight junction integrity. We found that the diabetic CEC displayed significantly slower cell proliferation and migration compared with the normal CEC from both mice and humans. Furthermore, ROS production was markedly increased in CEC grown under diabetic conditions. Treatment with an antioxidant N-acetyl cysteine (NAC, 100 μM) significantly decreased ROS production and increased wound healing in diabetic CEC. Barrier function was significantly reduced in both diabetic mouse and human CEC, while NAC treatment mitigated these effects. We further showed that Akt signaling was impaired in diabetic CEC, which was partially improved by NAC treatment. These results show that diabetic conditions lead to delayed wound-healing capacity of CEC and impaired tight junction formation in both mice and human. Increased ROS production and inhibited Akt signaling may contribute to this outcome, implicating these as potential targets for treating corneal wounds in diabetic patients.

Corneal pain and experimental model development

The cornea is a valuable tissue for studying peripheral sensory nerve structure and regeneration due to its avascularity, transparency, and dense innervation. Somatosensory innervation of the cornea serves to identify changes in environmental stimuli at the ocular surface, thereby promoting barrier function to protect the eye against injury or infection. Due to regulatory demands to screen ocular safety of potential chemical exposure, a need remains to develop functional human tissue models to predict ocular damage and pain using in vitro-based systems to increase throughput and minimize animal use. In this review, we summarize the anatomical and functional roles of corneal innervation in propagation of sensory input, corneal neuropathies associated with pain, and the status of current in vivo and in vitro models. Emphasis is placed on tissue engineering approaches to study the human corneal pain response in vitro with integration of proper cell types, controlled microenvironment, and high-throughput readouts to predict pain induction. Further developments in this field will aid in defining molecular signatures to distinguish acute and chronic pain triggers based on the immune response and epithelial, stromal, and neuronal interactions that occur at the ocular surface that lead to functional outcomes in the brain depending on severity and persistence of the stimulus.

Influence of diabetes mellitus on anterior segment of the eye

Diabetes mellitus (DM) has been emerging as one of the most serious health problems worldwide. Ocular complications of DM are currently one of the major causes of blindness in developed countries, among which diabetic retinopathy is relatively well studied and understood. However, although ocular surface complications of DM are common, diabetic complications of anterior segment of the eye, such as, cornea, conjunctiva, and lacrimal glands, are often overlooked. DM is associated with progressive damage to corneal nerves and epithelial cells, which increases the risk of anterior segment disorders including dry eye disease, corneal erosion, persistent epithelial defects, and even sight-threatening corneal ulcer. In this review, the authors will discuss the association of DM with disorders of anterior segment of the eye. Studies indicating the value of corneal nerve assessment as a sensitive, noninvasive, and repeatable biomarker for diabetic neuropathy will also be introduced. In addition, treatment modalities of anterior segment disorders associated with DM is discussed. The studies introduced in this review suggest that early and periodic screening of the anterior segment of the eye, as well as the retina, is important for the optimal treatment of DM.