Participation of p38 MAP kinase, but not p44/42 MAP kinase, in stimulation of corneal epithelial migration by substance P and IGF-1

The Role of Insulin-like Growth Factor (IGF) System in the Corneal Epithelium Homeostasis-From Limbal Epithelial Stem Cells to Therapeutic Applications

The corneal epithelium, comprising three layers of cells, represents the outermost portion of the eye and functions as a vital protective barrier while concurrently serving as a critical refractive structure. Maintaining its homeostasis involves a complex regenerative process facilitated by the functions of the lacrimal gland, tear film, and corneal nerves. Crucially, limbal epithelial stem cells located in the limbus (transitional zone between the cornea and the conjunctiva) are instrumental for the corneal epithelium integrity by replenishing and renewing cells. Re-epithelialization failure results in persistent defects, often associated with various ocular conditions including diabetic keratopathy. The insulin-like growth factor (IGF) system is a sophisticated network of insulin and other proteins essential for numerous physiological processes. This review examines its role in maintaining the corneal epithelium homeostasis, with a special focus on the interplay with corneal limbal stem cells and the potential therapeutic applications of the system components.

Modulating the tachykinin: Role of substance P and neurokinin receptor expression in ocular surface disorders

Substance P (SP) is a tachykinin expressed by various cells in the nervous and immune systems. SP is predominantly released by neurons and exerts its biological and immunological effects through the neurokinin receptors, primarily the neurokinin-1 receptor (NK1R). SP is essential for maintaining ocular surface homeostasis, and its reduced levels in disorders like diabetic neuropathy disrupt the corneal tissue. It also plays an essential role in promoting corneal wound healing by promoting the migration of keratocytes. In this review, we briefly discuss the structure, expression, and function of SP and its principal receptor NK1R. In addition, SP induces pro-inflammatory effects through autocrine or paracrine action on the immune cells in various ocular surface pathologies, including dry eye disease, herpes simplex virus keratitis, and Pseudomonas keratitis. We provide an in-depth review of the pathogenic role of SP in various ocular surface diseases and several new approaches developed to counter the immune-mediated effects of SP either through modulating its production or blocking its target receptor.

Mechanistic investigations of diabetic ocular surface diseases

With the global prevalence of diabetes mellitus over recent decades, more patients suffered from various diabetic complications, including diabetic ocular surface diseases that may seriously affect the quality of life and even vision sight. The major diabetic ocular surface diseases include diabetic keratopathy and dry eye. Diabetic keratopathy is characterized with the delayed corneal epithelial wound healing, reduced corneal nerve density, decreased corneal sensation and feeling of burning or dryness. Diabetic dry eye is manifested as the reduction of tear secretion accompanied with the ocular discomfort. The early clinical symptoms include dry eye and corneal nerve degeneration, suggesting the early diagnosis should be focused on the examination of confocal microscopy and dry eye symptoms. The pathogenesis of diabetic keratopathy involves the accumulation of advanced glycation end-products, impaired neurotrophic innervations and limbal stem cell function, and dysregulated growth factor signaling, and inflammation alterations. Diabetic dry eye may be associated with the abnormal mitochondrial metabolism of lacrimal gland caused by the overactivation of sympathetic nervous system. Considering the important roles of the dense innervations in the homeostatic maintenance of cornea and lacrimal gland, further studies on the neuroepithelial and neuroimmune interactions will reveal the predominant pathogenic mechanisms and develop the targeting intervention strategies of diabetic ocular surface complications.

The impact of hyperglycemia on the corneal epithelium: Molecular mechanisms and insight

Type 2 Diabetes Mellitus (T2DM) is reaching epidemic levels worldwide and with it, there is a significant increase in complications associated with the disease. T2DM affects virtually all organ systems including the eye. While frequently overlooked, diabetic keratopathy is the most common ocular complication of diabetes and can manifest in mild to severe forms, the latter of which poses a major threat to vision. As the initial barrier between the environment and the eye, the corneal epithelium functions in innate immune defense. Compromise of this barrier may predispose the cornea to infection and can hinder the refractive capabilities of the eye. The clinical burden in patients with diabetic keratopathy lies primarily in the inability of the corneal epithelium to repair damage and maintain its tight barrier function. Current therapies for diabetic keratopathy are supportive, centering on the prevention of infection and promotion of an optimal healing environment. With no clear disease-modifying agent identified as of yet, a thorough understanding of the pathophysiology that underlies the development of diabetic keratopathy at the cellular level is critical to identify and develop potential therapeutic agents capable of promoting corneal re-epithelialization to accelerate the wound healing process. The focus of this review is to examine what is known regarding the cellular and molecular mechanisms needed to maintain epithelial homeostasis and how it goes awry in diabetes.

G2A Attenuates Propionibacterium acnes Induction of Inflammatory Cytokines in Human Monocytes

Background

Acne vulgaris is a disease of the pilosebaceous unit characterized by increased sebum production, hyperkeratinization, and immune responses to Propionibacterium acnes (PA). Here, we explore a possible mechanism by which a lipid receptor, G2A, regulates immune responses to a commensal bacterium.

Objective

To elucidate the inflammatory properties of G2A in monocytes in response to PA stimulation. Furthermore, our study sought to investigate pathways by which lipids modulate immune responses in response to PA.

Methods

Our studies focused on monocytes collected from human peripheral blood mononuclear cells, the monocytic cell line THP-1, and a lab strain of PA. Our studies involved the use of enzyme-linked immunosorbent, Western blot, reverse transcription polymerase chain reaction, small interfering RNA (siRNA), and microarray analysis of human acne lesions in the measurements of inflammatory markers.

Results

G2A gene expression is higher in acne lesions compared to normal skin and is inducible by the acne therapeutic, 13-cis-retinoic acid. In vitro, PA induces both the Toll-like receptor 2-dependent expression of G2A as well as the production of the G2A ligand, 9-hydroxyoctadecadienoic acid, from human monocytes. G2A gene knockdown through siRNA enhances PA stimulation of interleukin (IL)-6, IL-8, and IL-1β possibly through increased activation of the ERK1/2 MAP kinase and nuclear factor kappa B p65 pathways.

Conclusion

G2A may play a role in quelling inflammatory cytokine response to PA, revealing G2A as a potential attenuator of inflammatory response in a disease associated with a commensal bacterium.

Role of Substance P Neuropeptide in Inflammation, Wound Healing, and Tissue Homeostasis

Substance P (SP) is an undecapeptide present in the CNS and the peripheral nervous system. SP released from the peripheral nerves exerts its biological and immunological activity via high-affinity neurokinin 1 receptor (NK1R). SP is also produced by immune cells and acts as an autocrine or paracrine fashion to regulate the function of immune cells. In addition to its proinflammatory role, SP and its metabolites in combination with insulin-like growth factor-1 are shown to promote the corneal epithelial wound healing. Recently, we showed an altered ocular surface homeostasis in unmanipulated NK1R^-/- mice, suggesting the role of SP-NK1R signaling in ocular surface homeostasis under steady-state. This review summarizes the immunobiology of SP and its effect on immune cells and immunity to microbial infection. In addition, the effect of SP in inflammation, wound healing, and corneal epithelial homeostasis in the eye is discussed.

TFOS DEWS II Sex, Gender, and Hormones Report

One of the most compelling features of dry eye disease (DED) is that it occurs more frequently in women than men. In fact, the female sex is a significant risk factor for the development of DED. This sex-related difference in DED prevalence is attributed in large part to the effects of sex steroids (e.g. androgens, estrogens), hypothalamic-pituitary hormones, glucocorticoids, insulin, insulin-like growth factor 1 and thyroid hormones, as well as to the sex chromosome complement, sex-specific autosomal factors and epigenetics (e.g. microRNAs). In addition to sex, gender also appears to be a risk factor for DED. "Gender" and "sex" are words that are often used interchangeably, but they have distinct meanings. "Gender" refers to a person's self-representation as a man or woman, whereas "sex" distinguishes males and females based on their biological characteristics. Both gender and sex affect DED risk, presentation of the disease, immune responses, pain, care-seeking behaviors, service utilization, and myriad other facets of eye health. Overall, sex, gender and hormones play a major role in the regulation of ocular surface and adnexal tissues, and in the difference in DED prevalence between women and men. The purpose of this Subcommittee report is to review and critique the nature of this role, as well as to recommend areas for future research to advance our understanding of the interrelationships between sex, gender, hormones and DED.

Peptide therapies for ocular surface disturbances based on fibronectin-integrin interactions

The condition of the corneal epithelium is a critical determinant of corneal transparency and clear vision. The corneal epithelium serves as a barrier to protect the eye from external insults, with its smooth surface being essential for its optical properties. Disorders of the corneal epithelium include superficial punctate keratopathy, corneal erosion, and persistent epithelial defects (PEDs). The prompt resolution of these disorders is important for minimization of further damage to the cornea. Currently available treatment modalities for corneal epithelial disorders are based on protection of the ocular surface in order to allow natural healing to proceed. PEDs remain among the most difficult corneal conditions to treat, however. On the basis of characterization of the pathobiology of PEDs at the cell and molecular biological levels, we have strived to develop new modes of treatment for these defects. These treatments rely on two key concepts: provision of a substrate, such as the adhesive glycoprotein fibronectin, for the attachment and migration of corneal epithelial cells, and activation of these cells by biological agents such as the combination of substance P and insulin-like growth factor-1 (IGF-1). Central to both approaches is the role of the fibronectin-integrin system in corneal epithelial wound healing. Determination of the minimum amino acid sequences required for the promotion of corneal epithelial wound closure by fibronectin (PHSRN) and by substance P (FGLM-amide) plus IGF-1 (SSSR) has led to the development of peptide eyedrops for the treatment of PEDs that are free of adverse effects of the parent molecules.

Enhanced wound healing, kinase and stem cell marker expression in diabetic organ-cultured human corneas upon MMP-10 and cathepsin F gene silencing

PURPOSE

Diabetic corneas overexpress proteinases including matrix metalloproteinase-10 (M10) and cathepsin F (CF). Our purpose was to assess if silencing M10 and CF in organ-cultured diabetic corneas using recombinant adenovirus (rAV)-driven small hairpin RNA (rAV-sh) would normalize slow wound healing, and diabetic and stem cell marker expression.

METHODS

Sixteen pairs of organ-cultured autopsy human diabetic corneas (four per group) were treated with rAV-sh. Proteinase genes were silenced either separately, together, or both, in combination (Combo) with rAV-driven c-met gene overexpression. Fellow control corneas received rAV-EGFP. Quantitative RT-PCR confirmed small hairpin RNA (shRNA) silencing effect. Ten days after transfection, 5-mm epithelial wounds were made with n-heptanol and healing time recorded. Diabetic, signaling, and putative stem cell markers were studied by immunofluorescence of corneal cryostat sections.

RESULTS

Proteinase silencing reduced epithelial wound healing time versus rAV-enhanced green fluorescent protein (EGFP) control (23% for rAV-shM10, 31% for rAV-shCF, and 36% for rAV-shM10 + rAV-shCF). Combo treatment was even more efficient (55% reduction). Staining patterns of diabetic markers (α₃β₁ integrin and nidogen-1), and of activated epidermal growth factor receptor and its signaling target activated Akt were normalized upon rAV-sh treatment. Combo treatment also restored normal staining for activated p38. All treatments, especially the combined ones, increased diabetes-altered staining for putative limbal stem cell markers, ΔNp63α, ABCG2, keratins 15 and 17, and laminin γ3 chain.

CONCLUSIONS

Small hairpin RNA silencing of proteinases overexpressed in diabetic corneas enhanced corneal epithelial and stem cell marker staining and accelerated wound healing. Combined therapy with c-met overexpression was even more efficient. Specific corneal gene therapy has a potential for treating diabetic keratopathy.

Normalization of wound healing and diabetic markers in organ cultured human diabetic corneas by adenoviral delivery of c-Met gene

Purpose. Diabetic corneas display altered basement membrane and integrin markers, increased expression of proteinases, decreased hepatocyte growth factor (HGF) receptor, c-met proto-oncogene, and impaired wound healing. Recombinant adenovirus (rAV)-driven c-met overexpression in human organ-cultured corneas was tested for correction of diabetic abnormalities. Methods. Forty-six human corneas obtained postmortem from 23 donors with long-term diabetes (5 with diabetic retinopathy) were organ cultured and transduced with rAV-expressing c-met gene (rAV-cmet) under the cytomegalovirus promoter at approximately 10(8) plaque-forming units per cornea for 48 hours. Each control fellow cornea received control rAV (rAV expressing the beta-galactosidase gene or vector alone). After an additional 4 to 5 days of incubation, 5-mm epithelial wounds were created with n-heptanol, and healing was monitored. The corneas were analyzed afterward by immunohistochemistry and Western blot analysis. Signaling molecule expression and role was examined by immunostaining, phosphokinase antibody arrays, Western blot analysis, and inhibitor analysis. Results. rAV-cmet transduction led to increased epithelial staining for c-met (total, extracellular, and phosphorylated) and normalization of the patterns of select diabetic markers compared with rAV-vector-transduced control fellow corneas. Epithelial wound healing time in c-met-transduced diabetic corneas decreased twofold compared with rAV-vector-transduced corneas and became similar to normal. c-Met action apparently involved increased activation of p38 mitogen-activated protein kinase. c-Met transduction did not change tight junction protein patterns, suggesting unaltered epithelial barrier function. Conclusions. rAV-driven c-met transduction into diabetic corneas appears to restore HGF signaling, normalize diabetic marker patterns, and accelerate wound healing. c-Met gene therapy could be useful for correcting human diabetic corneal abnormalities.

Advances in treatment for neurotrophic keratopathy

PURPOSE OF REVIEW

To review the clinical characteristics and possible new mode of treatments for the corneal epithelial disorders associated with neurotrophic keratopathy.

RECENT FINDINGS

The successful clinical applications of eyedrops containing substance P and insulin-like growth factor-1, or peptides derived therefrom, were reported for the treatment of humans with neurotrophic keratopathy.

SUMMARY

Neurotrophic keratopathy, a degenerative disease of the cornea, is caused by an impairment of function of the trigeminal nerve and an insufficient supply of neural factors. The combination of substance P, a sensory neurotransmitter, and insulin-like growth factor-1 induced a synergistic stimulatory effect on corneal epithelial migration as well as on corneal wound closure in vivo. We discuss the roles of growth factors and substance P, a sensory neurotransmitter released from the trigeminal nerve, in corneal epithelial wound healing as well as the clinical application of these molecules to the treatment of neurotrophic keratopathy.