Presence of Histatin-1 in Human Tears and Association with Aqueous Deficient Dry Eye Diagnosis: A Preliminary Study

Immunopeptides: immunomodulatory strategies and prospects for ocular immunity applications

Immunopeptides have low toxicity, low immunogenicity and targeting, and broad application prospects in drug delivery and assembly, which are diverse in application strategies and drug combinations. Immunopeptides are particularly important for regulating ocular immune homeostasis, as the eye is an immune-privileged organ. Immunopeptides have advantages in adaptive immunity and innate immunity, treating eye immune-related diseases by regulating T cells, B cells, immune checkpoints, and cytokines. This article summarizes the application strategies of immunopeptides in innate immunity and adaptive immunity, including autoimmunity, infection, vaccine strategies, and tumors. Furthermore, it focuses on the mechanisms of immunopeptides in mediating ocular immunity (autoimmune diseases, inflammatory storms, and tumors). Moreover, it reviews immunopeptides' application strategies and the therapeutic potential of immunopeptides in the eye. We expect the immune peptide to get attention in treating eye diseases and to provide a direction for eye disease immune peptide research.

Western blotting: evolution of an old analytical method to a new quantitative tool for biomarker measurements

Western blotting (WB) is a widely used laboratory technique for detecting specific proteins in biological matrices. Recent advances in antibody production, automation, gel and membrane manufacturing and highly sensitive detection platforms have transformed WB from a labor-intensive and qualitative method into a highly reproducible and quantitative assay suitable for biomarker detection. Despite these significant improvements in the capabilities and efficiency of WB, there remain challenges that hinder its widespread application as a research, diagnostic (in two-tiered assays like Lyme disease testing) and drug development tool. This article describes recent innovations introduced to WB methodology and the remaining challenges that prevent its wider adoption for biomarker measurements throughout the drug development process.

Sigma-2 Receptors—From Basic Biology to Therapeutic Target: A Focus on Age-Related Degenerative Diseases

There is a large unmet medical need to develop disease-modifying treatment options for individuals with age-related degenerative diseases of the central nervous system. The sigma-2 receptor (S2R), encoded by TMEM97, is expressed in brain and retinal cells, and regulates cell functions via its co-receptor progesterone receptor membrane component 1 (PGRMC1), and through other protein–protein interactions. Studies describing functions of S2R involve the manipulation of expression or pharmacological modulation using exogenous small-molecule ligands. These studies demonstrate that S2R modulates key pathways involved in age-related diseases including autophagy, trafficking, oxidative stress, and amyloid-β and α-synuclein toxicity. Furthermore, S2R modulation can ameliorate functional deficits in cell-based and animal models of disease. This review summarizes the current evidence-based understanding of S2R biology and function, and its potential as a therapeutic target for age-related degenerative diseases of the central nervous system, including Alzheimer’s disease, α-synucleinopathies, and dry age-related macular degeneration.

Salivary Antimicrobial Peptide Histatin-5 Does Not Display Zn(II)-Dependent or -Independent Activity against Streptococci

Histatin-5 (Hst5) is a member of the histatin superfamily of cationic, His-rich, Zn(II)-binding peptides in human saliva. Hst5 displays antimicrobial activity against fungal and bacterial pathogens, often in a Zn(II)-dependent manner. In contrast, here we showed that under in vitro conditions that are characteristic of human saliva, Hst5 does not kill seven streptococcal species that normally colonize the human oral cavity and oropharynx. We further showed that Zn(II) does not influence this outcome. We then hypothesized that Hst5 exerts more subtle effects on streptococci by modulating Zn(II) availability. We initially proposed that Hst5 contributes to nutritional immunity by limiting nutrient Zn(II) availability and promoting bacterial Zn(II) starvation. By examining the interactions between Hst5 and Streptococcus pyogenes as a model Streptococcus species, we showed that Hst5 does not influence the expression of Zn(II) uptake genes. In addition, Hst5 did not suppress growth of a ΔadcAI mutant strain that is impaired in Zn(II) uptake. These observations establish that Hst5 does not promote Zn(II) starvation. Biochemical examination of purified peptides further confirmed that Hst5 binds Zn(II) with high micromolar affinities and does not compete with the AdcAI high-affinity Zn(II) uptake protein for binding nutrient Zn(II). Instead, we showed that Hst5 weakly limits the availability of excess Zn(II) and suppresses Zn(II) toxicity to a ΔczcD mutant strain that is impaired in Zn(II) efflux. Altogether, our findings led us to reconsider the function of Hst5 as a salivary antimicrobial agent and the role of Zn(II) in Hst5 function.

Tear Film MicroRNAs as Potential Biomarkers: A Review

MicroRNAs are non-coding RNAs that serve as regulatory molecules in a variety of pathways such as inflammation, metabolism, homeostasis, cell machinery, and development. With the progression of sequencing methods and modern bioinformatics tools, novel roles of microRNAs in regulatory mechanisms and pathophysiological states continue to expand. Advances in detection methods have further enabled larger adoption of studies utilizing minimal sample volumes, allowing the analysis of microRNAs in low-volume biofluids, such as the aqueous humor and tear fluid. The reported abundance of extracellular microRNAs in these biofluids has prompted studies to explore their biomarker potential. This review compiles the current literature reporting microRNAs in human tear fluid and their association with ocular diseases including dry eye disease, Sjögren's syndrome, keratitis, vernal keratoconjunctivitis, glaucoma, diabetic macular edema, and diabetic retinopathy, as well as non-ocular diseases, including Alzheimer's and breast cancer. We also summarize the known roles of these microRNAs and shed light on the future progression of this field.

Modulation of ocular surface desiccation in a murine model by histatin-5 application

PURPOSE

To determine the efficacy of Histatin-5 (Hst5) peptide treatment in ameliorating dry eye disease (DED) phenotype in an in-vivo mouse model of scopolamine and desiccating stress (SDS) dry eye.

METHODS

SDS was induced in female C57BL/6 mice by subcutaneous injections of scopolamine hydrobromide and exposure to low relative humidity and forced air draft for five days. Mouse eyes were topically treated with synthetic Hst5 peptide or balanced salt solution (BSS) twice a day for four days. Control mice were not exposed to SDS induction and did not receive any treatments. Oregon green dextran (OGD) staining was used to evaluate corneal permeability. Histologically, staining with periodic acid schiff (PAS), immunohistochemistry (IHC) and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), were used to quantify the number of goblet cells (GC), CD45+ immune cells and apoptotic cells respectively in formalin fixed paraffin embedded (FFPE) mouse whole eye sections.

RESULTS

Compared to treatment with BSS, Hst5 treatment significantly lowered corneal epithelial permeability, prevented conjunctival epithelial GC loss, decreased conjunctival CD45+ immune cell infiltration and reduced conjunctival epithelial cell apoptosis.

CONCLUSIONS

Hst5 peptide topical treatment significantly improves the clinical parameters observed in SDS experimental model of DED. This is the first report of the efficacy of Hst5 treatment of dry eye phenotype, and potential novel treatment for DED in the clinic. Hst5 represents a new class of efficacious therapeutic agents, demonstrating pro-epithelial and anti-inflammatory activities at the ocular surface.

Role of ocular surface neurobiology in neuronal-mediated inflammation in dry eye disease

Inflammation is the consequence of dry eye disease regardless of its etiology. Several injurious or harmless processes to the ocular surface neurons promote ocular surface neurogenic inflammation, leading to the vicious cycle of dry eye disease. These processes include the regular release of neuromediators during the conduction of ocular surface sensations, hyperosmolarity-induced ocular surface neuronal damage, neuro-regenerative activities, and neuronal-mediated dendritic cell activities. Neurogenic inflammation appears to be the main culprit, instigating the self-perpetuating inflammation observed in patients with dry eye disease.

Host Defense Peptides at the Ocular Surface: Roles in Health and Major Diseases, and Therapeutic Potentials

Sight is arguably the most important sense in human. Being constantly exposed to the environmental stress, irritants and pathogens, the ocular surface – a specialized functional and anatomical unit composed of tear film, conjunctival and corneal epithelium, lacrimal glands, meibomian glands, and nasolacrimal drainage apparatus – serves as a crucial front-line defense of the eye. Host defense peptides (HDPs), also known as antimicrobial peptides, are evolutionarily conserved molecular components of innate immunity that are found in all classes of life. Since the first discovery of lysozyme in 1922, a wide range of HDPs have been identified at the ocular surface. In addition to their antimicrobial activity, HDPs are increasingly recognized for their wide array of biological functions, including anti-biofilm, immunomodulation, wound healing, and anti-cancer properties. In this review, we provide an updated review on: (1) spectrum and expression of HDPs at the ocular surface; (2) participation of HDPs in ocular surface diseases/conditions such as infectious keratitis, conjunctivitis, dry eye disease, keratoconus, allergic eye disease, rosacea keratitis, and post-ocular surgery; (3) HDPs that are currently in the development pipeline for treatment of ocular diseases and infections; and (4) future potential of HDP-based clinical pharmacotherapy for ocular diseases.

Antimicrobial Peptide Expression at the Ocular Surface and Their Therapeutic Use in the Treatment of Microbial Keratitis

Microbial keratitis is a common cause of ocular pain and visual impairment worldwide. The ocular surface has a relatively paucicellular microbial community, mostly found in the conjunctiva, while the cornea would be considered relatively sterile. However, in patients with microbial keratitis, the cornea can be infected with multiple pathogens including Staphylococcus aureus, Pseudomonas aeruginosa, and Fusarium sp. Treatment with topical antimicrobials serves as the standard of care for microbial keratitis, however, due to high rates of pathogen resistance to current antimicrobial medications, alternative therapeutic strategies must be developed. Multiple studies have characterized the expression and activity of antimicrobial peptides (AMPs), endogenous peptides with key antimicrobial and wound healing properties, on the ocular surface. Recent studies and clinical trials provide promise for the use of AMPs as therapeutic agents. This article reviews the repertoire of AMPs expressed at the ocular surface, how expression of these AMPs can be modulated, and the potential for harnessing the AMPs as potential therapeutics for patients with microbial keratitis.

Antimicrobial peptides and proteins in human biological fluids

Antimicrobial peptides and proteins (AMPs) are endogenous compounds that have a direct antimicrobial effect on bacteria (e. g., by disrupting bacterial membranes), as well as on fungi and viruses. AMPs are the main component of the innate immunity of living organisms and are produced by both epithelial cells (skin cells, cells of respiratory tract, intestine, urinary and genital tracts) and cells of the immune system and are secreted into secretory fluids. AMPs can also act as chemoattractants for immunocompetent cells (neutrophils, monocytes, T lymphocytes, dendritic cells) in the inflammation site and affect the antigen presenting cells by modulating adaptive T cell immune responses. The representatives of the main 15 AMP classes, that we describe in this review, are the most studied group of the large pool of these compounds. We discuss their localization, expression, and concentration in various biofluids of humans under normal and pathological conditions.

Foundational concepts in the biology of bacterial keratitis

Bacterial infections of the cornea, or bacterial keratitis (BK), are notorious for causing rapidly fulminant disease and permanent vision loss, even among treated patients. In the last sixty years, dramatic upward trajectories in the frequency of BK have been observed internationally, driven in large part by the commercialization of hydrogel contact lenses in the late 1960s. Despite this worsening burden of disease, current evidence-based therapies for BK - including broad-spectrum topical antibiotics and, if indicated, topical corticosteroids - fail to salvage vision in a substantial proportion of affected patients. Amid growing concerns of rapidly diminishing antibiotic utility, there has been renewed interest in urgently needed novel treatments that may improve clinical outcomes on an individual and public health level. Bridging the translational gap in the care of BK requires the identification of new therapeutic targets and rational treatment design, but neither of these aims can be achieved without understanding the complex biological processes that determine how bacterial corneal infections arise, progress, and resolve. In this chapter, we synthesize the current wealth of human and animal experimental data that now inform our understanding of basic BK pathophysiology, in context with modern concepts in ocular immunology and microbiology. By identifying the key molecular determinants of clinical disease, we explore how novel treatments can be developed and translated into routine patient care.

Histatin-1 Attenuates LPS-Induced Inflammatory Signaling in RAW264.7 Macrophages

Macrophages play a critical role in the inflammatory response to environmental triggers, such as lipopolysaccharide (LPS). Inflammatory signaling through macrophages and the innate immune system are increasingly recognized as important contributors to multiple acute and chronic disease processes. Nitric oxide (NO) is a free radical that plays an important role in immune and inflammatory responses as an important intercellular messenger. In addition, NO has an important role in inflammatory responses in mucosal environments such as the ocular surface. Histatin peptides are well-established antimicrobial and wound healing agents. These peptides are important in multiple biological systems, playing roles in responses to the environment and immunomodulation. Given the importance of macrophages in responses to environmental triggers and pathogens, we investigated the effect of histatin-1 (Hst1) on LPS-induced inflammatory responses and the underlying molecular mechanisms in RAW264.7 (RAW) macrophages. LPS-induced inflammatory signaling, NO production and cytokine production in macrophages were tested in response to treatment with Hst1. Hst1 application significantly reduced LPS-induced NO production, inflammatory cytokine production, and inflammatory signaling through the JNK and NF-kB pathways in RAW cells. These results demonstrate that Hst1 can inhibit LPS-induced inflammatory mediator production and MAPK signaling pathways in macrophages.

Histatin-1 is an endogenous ligand of the sigma-2 receptor

The Sigma-2 receptor (S2R) (a.k.a TMEM97) is an important endoplasmic reticular protein involved in cancer, cholesterol processing, cell migration, and neurodegenerative diseases, including Niemann-Pick Type C. While several S2R pharmacologic agents have been discovered, its recent (2017) cloning has limited biological investigation, and no endogenous ligands of the S2R are known. Histatins are a family of endogenous antimicrobial peptides that have numerous important effects in multiple biological systems, including antifungal, antibacterial, cancer pathogenesis, immunomodulation, and wound healing. Histatin-1 (Hst1) has important roles in epithelial wound healing and cell migration, and is the primary wound healing agent in saliva. Little is understood about the downstream machinery that underpins the effects of histatins, and no mammalian receptor is known to date. In this study, we show, using biophysical methods and functional assays, that Hst1 is an endogenous ligand for S2R and that S2R is a mammalian receptor for Hst1.

Diagnostic biomarkers in tear fluid: from sampling to preanalytical processing

Tear fluid is receiving growing attention as a source for novel diagnostic biomarkers. Multiple techniques are available for its collection and impact the composition of acquired samples. We sought to provide a direct comparison of two collection methods with regard to implementation, acceptance, and impact on sample composition. Tear fluid was collected from fifteen healthy volunteers with capillary tubes and Schirmer strips and analyzed for total protein and IgG concentrations. Sampling parameters and perception by test persons were compared. The use of capillary tubes was more convenient for the participants while causing more effort for the collector. Tear flow rates as well as the relative and absolute amount of IgG were higher when Schirmer strips were used. Consecutive collections with Schirmer strips significantly influenced tear flow rates, IgG, and protein concentrations. A moderate correlation was observed between tear flow rates and IgG concentrations for both methods. Samples collected with both methods can be analyzed by isoelectric focusing, a potential diagnostic application in the field of neurology. The specific advantages and limitations of tear fluid sampling with either capillary tubes or Schirmer strips demonstrate the need for a thorough investigation of collection methods with regard to the application of interest.

Exploring the human lacrimal gland using organoids and single-cell sequencing

The lacrimal gland is essential for lubrication and protection of the eye. Disruption of lacrimal fluid production, composition, or release results in dry eye, causing discomfort and damage to the ocular surface. Here, we describe the establishment of long-term 3D organoid culture conditions for mouse and human lacrimal gland. Organoids can be expanded over multiple months and recapitulate morphological and transcriptional features of lacrimal ducts. CRISPR-Cas9-mediated genome editing reveals the master regulator for eye development Pax6 to be required for differentiation of adult lacrimal gland cells. We address cellular heterogeneity of the lacrimal gland by providing a single-cell atlas of human lacrimal gland tissue and organoids. Finally, human lacrimal gland organoids phenocopy the process of tear secretion in response to neurotransmitters and can engraft and produce mature tear products upon orthotopic transplantation in mouse. Together, this study provides an experimental platform to study the (patho-)physiology of the lacrimal gland.

The Role of Endogenous Antimicrobial Peptides in Modulating Innate Immunity of the Ocular Surface in Dry Eye Diseases

The ocular surface has the challenging responsibility of maintaining a clear moist refractive surface while protecting the eye from exogenous pathogens and the environment. Homeostasis of the ocular surface, including its innate immune components, is altered in ocular surface disease states. In this review, we focus on antimicrobial peptides and the role they play in the immune response of the ocular surface during healthy states and dry eye diseases. Antimicrobial peptides are of special interest to the study of the ocular surface because of their various roles that include microbial threat neutralization, wound healing, and immune modulation. This review explores current literature on antimicrobial peptides in ocular surface diseases and discusses their therapeutic potential in ocular surface diseases and dry eye.

Wound Healing Properties of Histatin-5 and Identification of a Functional Domain Required for Histatin-5-Induced Cell Migration

Histatin peptides are endogenous anti-microbial peptides that were originally discovered in the saliva. Aside from their broad anti-microbial properties, these peptides play an important role in multiple biological systems. Different members of this family are thought to have relative specializations, with histatin-5 originally being thought to have mostly anti-fungal properties, and histatin-1 having strong wound healing properties. In this report, we describe the robust wound healing properties of histatin-5 and elucidate a functional domain, which is necessary and sufficient for promoting wound healing. We demonstrate these findings in multiple different cell types in vitro and with a standardized murine corneal wound healing model. Discovery of this wound healing domain and description of this functional role of histatin-5 will support developing therapies.

Expression and Function of Host Defense Peptides at Inflammation Sites

There is a growing interest in the complex role of host defense peptides (HDPs) in the pathophysiology of several immune-mediated inflammatory diseases. The physicochemical properties and selective interaction of HDPs with various receptors define their immunomodulatory effects. However, it is quite challenging to understand their function because some HDPs play opposing pro-inflammatory and anti-inflammatory roles, depending on their expression level within the site of inflammation. While it is known that HDPs maintain constitutive host protection against invading microorganisms, the inducible nature of HDPs in various cells and tissues is an important aspect of the molecular events of inflammation. This review outlines the biological functions and emerging roles of HDPs in different inflammatory conditions. We further discuss the current data on the clinical relevance of impaired HDPs expression in inflammation and selected diseases.