Expression of Neurotrophic Factors and their Receptors in Keratoconic Cornea

Identification of hub genes and molecular pathways in keratoconus by integrating bioinformatics and literature mining at the RNA level

OBJECTIVE

Keratoconus (KC) is a condition characterized by progressive corneal steepening and thinning. However, its pathophysiological mechanism remains vague. We mainly performed literature mining to extract bioinformatic and related data on KC at the RNA level. The objective of this study was to explore the potential pathological mechanisms of KC by identifying hub genes and key molecular pathways at the RNA level.

METHODS

We performed an exhaustive search of the PubMed database and identified studies that pertained to gene transcripts derived from diverse corneal layers in patients with KC. The identified differentially expressed genes were intersected, and overlapping genes were extracted for further analyses. Significantly enriched genes were screened using "Gene Ontology" (GO) and "Kyoto Encyclopedia of Genes and Genomes" (KEGG) analysis with the "Database for Annotation, Visualization, and Integrated Discovery" (DAVID) database. A protein-protein interaction (PPI) network was constructed for the significantly enriched genes using the STRING database. The PPI network was visualized using the Cytoscape software, and hub genes were screened via betweenness centrality values. Pathways that play a critical role in the pathophysiology of KC were discovered using the GO and KEGG analyses of the hub genes.

RESULTS

68 overlapping genes were obtained. Fifty genes were significantly enriched in 67 biological processes, and 16 genes were identified in 7 KEGG pathways. Moreover, 14 nodes and 32 edges were identified via the PPI network constructed using the STRING database. Multiple analyses identified 4 hub genes, 12 enriched biological processes, and 6 KEGG pathways. GO enrichment analysis showed that the hub genes are mainly involved in the positive regulation of apoptotic process, and KEGG analysis showed that the hub genes are primarily associated with the interleukin-17 (IL-17) and tumor necrosis factor (TNF) pathways. Overall, the matrix metalloproteinase 9, IL-6, estrogen receptor 1, and prostaglandin-endoperoxide synthase 2 were the potential important genes associated with KC.

CONCLUSION

Four genes, matrix metalloproteinase 9, IL-6, estrogen receptor 1, and prostaglandin endoperoxide synthase 2, as well as IL-17 and TNF pathways, are critical in the development of KC. Inflammation and apoptosis may contribute to the pathogenesis of KC.

Neuroimmune crosstalk in the cornea: The role of immune cells in corneal nerve maintenance during homeostasis and inflammation

In the cornea, resident immune cells are in close proximity to sensory nerves, consistent with their important roles in the maintenance of nerves in both homeostasis and inflammation. Using in vivo confocal microscopy in humans, and ex vivo immunostaining and fluorescent reporter mice to visualize corneal sensory nerves and immune cells, remarkable progress has been made to advance our understanding of the physical and functional interactions between corneal nerves and immune cells. In this review, we summarize and discuss recent studies relating to corneal immune cells and sensory nerves, and their interactions in health and disease. In particular, we consider how disrupted corneal nerve axons can induce immune cell activity, including in dendritic cells, macrophages and other infiltrating cells, directly and/or indirectly by releasing neuropeptides such as substance P and calcitonin gene-related peptide. We summarize growing evidence that the role of corneal intraepithelial immune cells is likely different in corneal wound healing versus other inflammatory-dominated conditions. The role of different types of macrophages is also discussed, including how stromal macrophages with anti-inflammatory phenotypes communicate with corneal nerves to provide neuroprotection, while macrophages with pro-inflammatory phenotypes, along with other infiltrating cells including neutrophils and CD4⁺ T cells, can be inhibitory to corneal re-innervation. Finally, this review considers the bidirectional interactions between corneal immune cells and corneal nerves, and how leveraging this interaction could represent a potential therapeutic approach for corneal neuropathy.

Systematically Displaying the Pathogenesis of Keratoconus via Multi-Level Related Gene Enrichment-Based Review

Keratoconus (KC) is an etiologically heterogeneous corneal ectatic disorder. To systematically display the pathogenesis of keratoconus (KC), this study reviewed all the reported genes involved in KC, and performed an enrichment analysis of genes identified at the genome, transcription, and protein levels respectively. Combined analysis of multi-level results revealed their shared genes, gene ontology (GO), and pathway terms, to explore the possible pathogenesis of KC. After an initial search, 80 candidate genes, 2,933 transcriptional differential genes, and 947 differential proteins were collected. The candidate genes were significantly enriched in extracellular matrix (ECM) related terms, Wnt signaling pathway and cytokine activities. The enriched GO/pathway terms of transcription and protein levels highlight the importance of ECM, cell adhesion, and inflammatory once again. Combined analysis of multi-levels identified 13 genes, 43 GOs, and 12 pathways. The pathogenic relationships among these overlapping factors maybe as follows. The gene mutations/variants caused insufficient protein dosage or abnormal function, together with environmental stimulation, leading to the related functions and pathways changes in the corneal cells. These included response to the glucocorticoid and reactive oxygen species; regulation of various signaling (P13K-AKT, MAPK and NF-kappaB), apoptosis and aging; upregulation of cytokines and collagen-related enzymes; and downregulation of collagen and other ECM-related proteins. These undoubtedly lead to a reduction of extracellular components and induction of cell apoptosis, resulting in the loosening and thinning of corneal tissue structure. This study, in addition to providing information about the genes involved, also provides an integrated insight into the gene-based etiology and pathogenesis of KC.

In Vivo Confocal Microscopy Evaluation in Patients with Keratoconus

Keratoconus is the most common primary corneal ectasia characterized by progressive focal thinning. Patients experience increased irregular astigmatism, decreased visual acuity and corneal sensitivity. Corneal collagen crosslinking (CXL), a minimally invasive procedure, is effective in halting disease progression. Historically, keratoconus research was confined to ex vivo settings. In vivo confocal microscopy (IVCM) has been used to examine the corneal microstructure clinically. In this review, we discuss keratoconus cellular changes evaluated by IVCM before and after CXL. Cellular changes before CXL include decreased keratocyte and nerve densities, disorganized subbasal nerves with thickening, increased nerve tortuosity and shortened nerve fibre length. Repopulation of keratocytes occurs up to 1 year post procedure. IVCM also correlates corneal nerve status to functional corneal sensitivity. Immediately after CXL, there is reduced nerve density and keratocyte absence due to mechanical removal of the epithelium and CXL effect. Nerve regeneration begins after 1 month, with nerve fibre densities recovering to pre-operative levels between 6 months to 1 year and remains stable up to 5 years. Nerves remain tortuous and nerve densities are reduced. Corneal sensitivity is reduced immediately postoperatively but recovers with nerve regeneration. Our article provides comprehensive review on the use of IVCM imaging in keratoconus patients.

The effect of biodegradable silk fibroin-based scaffolds containing glial cell line-derived neurotrophic factor (GDNF) on the corneal regeneration process

Corneal injury due to ocular trauma or infection is one of the most challenging vision impairing pathologies. The aim of the work was to study the effect of biodegradable silk fibroin-based scaffolds containing GDNF on the corneal regeneration process. During cultivate the highest keratocytes proliferative activity was registered with scaffolds containing 250 ng/ml and 500 ng/ml GDNF. In mice with an experimental model of epithelial-stromal damage to the cornea, silk fibroin-based scaffolds containing GDNF in various concentrations were used (in groups 1, 2 and 3 silk fibroin-based scaffolds containing GDNF in a concentration of 50 ng/ml, 250 ng/ml and 500 ng/ml, respectively; in group 4 - silk fibroin-based scaffolds without GDNF; in group 5 - a solution of GDNF with concentration of 500 ng/ml; group 6- control). The area of the corneal epithelial defect in groups 2, 3, and 5 was less than in the other groups. The most pronounced positive immunohistochemical reaction with antibodies to Bcl2, Bax, phosphoERK1/2 and phospho-JNK1/2, Ki67, Gap43 was observed in groups 2 and 3. Thus, silk fibroin-based scaffolds with GDNF stimulate the epithelialization process, proliferative activity of epithelial cells and keratocytes, accelerate the formation of the stromal nerve plexus and exhibit anti-apoptotic activity.

Xanthine oxidase enzyme activity in keratoconic corneal epithelium

PURPOSE

To assess the activity of xanthine oxidase (XO) enzyme in keratoconic corneal epithelium and to evaluate its relationship with the keratoconus (KC) severity.

METHODS

This prospective and randomized study included 66 eyes of 54 KC patients who received corneal collagen cross-linking treatment and 43 eyes of 32 patients who underwent photorefractive keratectomy due to their refractive error. During surgical procedures, the corneal epithelium was mechanically scraped and gathered to analyze the XO enzyme activity spectrophotometrically. The KC group was subdivided into three groups (stages 1, 2, and 3) according to the Amsler-Krumeich classification. The results were compared between the KC and the control group and in between KC subgroups.

RESULTS

No significant differences in age and gender were found between the KC and control groups (p = 0.064 and p = 0.296, respectively). The mean XO activity levels of the KC and control groups were 173.57 ± 87.61 and 223.70 ± 99.52 mIU/mg, respectively (p < 0.001). In KC group, 33 eyes were at stage 1, 19 were at stage 2, and 14 were at stage 3. No significant difference was observed between KC subgroups regarding XO activity levels (p = 0.681).

CONCLUSION

In this study, our findings revealed that ultraviolet-related pro-oxidant XO enzyme may have a role in the etiopathogenesis of KC. Further studies are needed to support our result.

CLINICAL TRIALS REGISTRATION

When we started this study in 2018, we did not have a "Clinical Trials Registration." However, we have ethics committee approval (date: 21. 02. 2018/No: 22).

Is Keratoconus an Inflammatory Disease? The Implication of Inflammatory Pathways

PURPOSE

Keratoconus is considered a non-inflammatory condition. Recently however, increased proinflammatory cytokines have been detected in the tears of keratoconic patients and clinical and immunohistochemical observations reported infiltration of matured dendritic cells and leukocytes. Our laboratory utilized cytokine antibody arrays to elucidate the inflammatory aspects of keratoconus.

METHODS

Protein was extracted from 42 corneal buttons (14 keratoconic and 28 non-keratoconic) and incubated with cytokine antibody arrays scanning 120 cytokines. Mann Whitney U test with a p-value of <0.05 was considered significant.

RESULTS

Pathways for wound healing, neuroprotection, angiogenesis, and inflammation were activated in keratoconic samples with 23 cytokines showing significant elevation. Fifteen were expressed only in keratoconus with 8 cytokines elevated 1.7-42-fold.

CONCLUSION

This study identified elevated inflammatory pathways covering immune responses in keratoconus. Our results support the evidence for inflammatory pathway activation in keratoconus and a possible redefinition of keratoconus as a chronic inflammatory corneal disease.

IL-6 Contributes to Corneal Nerve Degeneration after Herpes Simplex Virus Type I Infection

Herpes simplex virus type 1 (HSV-1) is a leading cause of neurotrophic keratitis characterized by decreased corneal sensation because of damage to the corneal sensory fibers. We and others have reported regression of corneal nerves during acute HSV-1 infection. To determine whether denervation is caused directly by the virus or indirectly by the elicited immune response, mice were infected with HSV-1 and topically treated with dexamethasone (DEX) or control eye drops. Corneal sensitivity was measured using a Cochet-Bonnet esthesiometer and nerve network structure via immunohistochemistry. Corneas were assessed for viral content by plaque assay, leukocyte influx by flow cytometry, and content of chemokines and inflammatory cytokines by suspension array. DEX significantly preserved corneal nerve structure and sensitivity on infection. DEX reduced myeloid and T-cell populations in the cornea and did not affect viral contents at 4 and 8 days post infection. The elevated protein contents of chemokines and inflammatory cytokines on infection were greatly suppressed by DEX. Subconjunctival delivery of neutralizing antibody against IL-6 to infected mice resulted in partial preservation of corneal nerve structure and sensitivity. Our study supports a role for the immune response, but not local virus replication in the development of HSV-1-induced neurotrophic keratitis. IL-6 is one of the factors produced by the elicited inflammatory response to HSV-1 infection contributing to nerve regression.

Ciliary Neurotrophic Factor Promotes the Migration of Corneal Epithelial Stem/progenitor Cells by Up-regulation of MMPs through the Phosphorylation of Akt

The migration of limbal epithelial stem cells is important for the homeostasis and regeneration of corneal epithelium. Ciliary neurotrophic factor (CNTF) has been found to promote corneal epithelial wound healing by activating corneal epithelial stem/progenitor cells. However, the possible effect of CNTF on the migration of corneal epithelial stem/progenitor cells is not clear. This study found the expression of CNTF in mouse corneal epithelial stem/progenitor cells (TKE2) to be up-regulated after injury, on both gene and protein level. CNTF promoted migration of TKE2 in a dose-dependent manner and the peak was seen at 10 ng/ml. The phosphorylation level of Akt (p-Akt), and the expression of MMP3 and MMP14, were up-regulated after CNTF treatment both in vitro and in vivo. Akt and MMP3 inhibitor treatment delayed the migration effect by CNTF. Finally, a decreased expression of MMP3 and MMP14 was observed when Akt inhibitor was applied both in vitro and in vivo. This study provides new insights into the role of CNTF on the migration of corneal epithelial stem/progenitor cells and its inherent mechanism of Up-regulation of matrix metalloproteinases through the Akt signalling pathway.

Dendritic cell dysfunction and diabetic sensory neuropathy in the cornea

Diabetic peripheral neuropathy (DPN) often leads to neurotrophic ulcerations in the cornea and skin; however, the underlying cellular mechanisms of this complication are poorly understood. Here, we used post-wound corneal sensory degeneration and regeneration as a model and tested the hypothesis that diabetes adversely affects DC populations and infiltration, resulting in disrupted DC-nerve communication and DPN. In streptozotocin-induced type 1 diabetic mice, there was a substantial reduction in sensory nerve density and the number of intraepithelial DCs in unwounded (UW) corneas. In wounded corneas, diabetes markedly delayed sensory nerve regeneration and reduced the number of infiltrating DCs, which were a major source of ciliary neurotrophic factor (CNTF) in the cornea. While CNTF neutralization retarded reinnervation in normal corneas, exogenous CNTF accelerated nerve regeneration in the wounded corneas of diabetic mice and healthy animals, in which DCs had been locally depleted. Moreover, blockade of the CNTF-specific receptor CNTFRα induced sensory nerve degeneration and retarded regeneration in normal corneas. Soluble CNTFRα also partially restored the branching of diabetes-suppressed sensory nerve endings and regeneration in the diabetic corneas. Collectively, our data show that DCs mediate sensory nerve innervation and regeneration through CNTF and that diabetes reduces DC populations in UW and wounded corneas, resulting in decreased CNTF and impaired sensory nerve innervation and regeneration.