Elevated Neuropeptides in Dry Eye Disease and Their Clinical Correlations

Alpha- and beta-melanocyte stimulating hormone positively impact lipogenesis of meibomian gland cells in vitro and ex vivo

PURPOSE

The meibomian glands produce a lipid-rich secretion that forms the superficial layer of the tear film, preventing excessive evaporation. Dysfunction of these glands (MGD) is the primary cause of dry eye disease (DED), a growing public health concern. Currently, there are limited pharmacological treatments for DED. However, α-/β-melanocyte-stimulating hormones (α-/β-MSH), ligands of the melanocortin receptors (MCR), are known to regulate lipogenesis and differentiation in sebaceous glands. This study investigated the influence of α-/β-MSH on exocrine secretion in human meibomian glands.

METHODS

Immunohistochemistry and RT-PCR for MCR expression were performed in human meibomian glands and an immortalized human meibomian gland epithelial cell line (ihMGECs). The effects of α-/β-MSH (agonists) and JNJ-10229570 (antagonist) in ihMGECs on lipid production and MCR response were analyzed using Oil-Red-O staining, transmission electron microscopy, qPCR, and a cAMP assay. Additionally, the effect of α-/β-MSH on an ex vivo organotypic slice culture (OSC) of human eyelids was investigated.

RESULTS

MCR expression was confirmed in human meibomian glands. Stimulation with α-/β-MSH increased cAMP levels and MCR expression. α-/β-MSH dose-dependently induced lipid production in ihMGECs and OSC, resulting in increased lipid droplet formation and upregulation of lipogenesis markers. Co-administration of JNJ-10229570 suppressed this effect.

CONCLUSION

Our data show for the first time that human meibomian glands express MCRs and that stimulation/inhibition of MCRs alters cAMP response, MCR expression, and lipogenesis markers, thereby affecting the genesis of meibum. Therefore, α-/β-MSH positively impacts meibum production and should be considered in the context of changes in glandular secretion in MGD and potential treatments.

Comparison of tear cytokines and neuropeptides, ocular surface parameters, and corneal nerve structure in patients with early-stage diabetes mellitus and control subjects

PURPOSE

This study aimed to assess the impact of diabetes mellitus (DM) on the corneal nerve, ocular surface, and tear cytokine and substance P (SP) levels and to compare the findings with those in control subjects.

METHODS

This cross-sectional study included 23 patients diagnosed with DM within the last 5 years and who had no systemic involvement, including diabetic retinopathy, and 22 control subjects. The ocular surface and tear film were assessed using the Ocular Surface Disease Index (OSDI) questionnaire, ocular surface staining, Schirmer, and corneal sensitivity. In vivo confocal microscopy was used to assess the architecture of the corneal nerves. The tear levels of cytokines were examined by Luminex and SP levels were measured by ELISA.

RESULTS

Both groups had similar OSDI scores, ocular surface staining, Schirmer, and corneal sensitivity measurements. The patient group had higher corneal nerve tortuosity (p = 0.015) but showed no significant difference in short or long nerve fibre density compared with the control group. Tear IL-6 and IL-8 levels were higher in the DM group (p = 0.002 and p = 0.01, respectively), whereas tear SP levels were lower in the DM group (p = 0.05). The tear SP level exhibited a strong positive correlation with total and long nerve fibre parameters (both p = 0.00).

CONCLUSIONS

The study results indicated that DM affected corneal nerve structure, tear SP, and inflammatory cytokine levels. The corneal nerves were affected and the tear SP level decreased even in patients without peripheral neuropathy, which is one of the most common complications of DM. In addition, ocular surface inflammation was observed in patients with DM, despite no ocular surface symptoms.

Novel treatments for dry eye syndrome

Dry eye syndrome (DES) is a prevalent and multifactorial disease that leads to a self-perpetuating cycle of inflammation and damage to the ocular surface. This results in symptoms such as redness, burning, and blurred vision, which can negatively affect a patient's quality of life. While treatments are available to manage DES, they only temporarily relieve symptoms. Furthermore, long-term use of certain medications can cause harm to the ocular surface. Therefore, there is a need for safer and effective treatments for DES. This review highlights the latest advancements in DES therapy, providing valuable insights into ongoing efforts to improve patient outcomes.

Analysis of Clinical Characteristics and Neuropeptides in Patients with Dry Eye with and without Chronic Ocular Pain after FS-LASIK

INTRODUCTION

Chronic ocular pain, particularly prevalent in patients with dry eye disease and post-femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK) surgery, presents with unclear clinical characteristics and an undefined pathogenesis. In this study, we aimed to compare clinical characteristics and tear neuropeptide concentrations in patients with dry eye disease (DED) with and without chronic ocular pain following FS-LASIK, and investigate correlations between ocular pain, clinical characteristics, and tear neuropeptide levels.

METHODS

Thirty-eight post-FS-LASIK patients with DED were assigned to two groups: those with chronic ocular pain and those without chronic ocular pain. Dry eye, ocular pain, and mental health-related parameters were evaluated using specific questionnaires and tests. The morphology of corneal nerves and dendritic cells (DCs) was evaluated by in vivo confocal microscopy. Function of corneal innervation was evaluated by corneal sensitivity. Concentrations of tear cytokines (interleukin [IL]-6, IL-23, IL-17A, and interferon-γ) and neuropeptides (α-melanocyte-stimulating hormone, neurotensin, β-endorphin, oxytocin, and substance P [SP]) were measured using the Luminex assay.

RESULTS

Most patients with chronic ocular pain experienced mild to moderate pain; the most common types included stimulated pain (provoked by wind and light), burning pain, and pressure sensation. More severe dry eye (P < 0.001), anxiety symptoms (P = 0.026), lower Schirmer I test values (P = 0.035), lower corneal nerve density (P = 0.043), and more activated DCs (P = 0.041) were observed in patients with ocular pain. Tear concentrations of SP and oxytocin were significantly higher in patients with ocular pain (P = 0.001, P = 0.021, respectively). Furthermore, significant correlations were observed among ocular pain severity, SP, and anxiety levels.

CONCLUSIONS

Patients with DED after FS-LASIK who have chronic ocular pain show more severe ocular and psychological discomfort and higher tear levels of neuropeptides. Furthermore, ocular pain severity is correlated with tear SP levels.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT05600985.

Chronic Kidney Disease Has No Impact on Tear Film Substance P Concentration in Type 2 Diabetes

PURPOSE

The study aimed to ascertain the potential effects of chronic kidney disease (CKD) on substance P concentration in the tear film of people with type 2 diabetes.

METHODS

Participants were classified into two groups: type 2 diabetes with concurrent chronic kidney disease (T2DM-CKD (n = 25)) and type 2 diabetes without chronic kidney disease (T2DM-no CKD (n = 25)). Ocular surface discomfort assessment, flush tear collection, in-vivo corneal confocal microscopy, and peripheral neuropathy assessment were conducted. Enzyme-linked immunosorbent assays were utilized to ascertain the levels of tear film substance P in collected flush tears. Correlation analysis, hierarchical multiple linear regression analysis, and t-tests or Mann-Whitney U tests were used in the analysis of data for two-group comparisons.

RESULTS

There was no substantial difference between the T2DM-CKD and T2DM-no CKD groups for tear film substance P concentration (4.4 (0.2-50.4) and 5.9 (0.2-47.2) ng/mL, respectively; p = 0.54). No difference was observed in tear film substance P concentration between the low-severity peripheral neuropathy and high-severity peripheral neuropathy groups (4.4 (0.2-50.4) and 3.3 (0.3-40.7) ng/mL, respectively; p = 0.80). Corneal nerve fiber length (9.8 ± 4.6 and 12.4 ± 3.8 mm/mm2, respectively; p = 0.04) and corneal nerve fiber density (14.7 ± 8.5 and 21.1 ± 7.0 no/mm2, respectively; p < 0.01) were reduced significantly in the T2DM-CKD group compared to the T2DM-no CKD group. There were significant differences in corneal nerve fiber density (21.0 ± 8.1 and 15.8 ± 7.7 no/mm2, respectively; p = 0.04) and corneal nerve fiber length (12.9 ± 4.2 and 9.7 ± 3.8 mm/mm2, respectively; p = 0.03) between the low- and high-severity peripheral neuropathy groups.

CONCLUSION

In conclusion, no significant difference in tear film substance P concentration was observed between type 2 diabetes with and without CKD. Corneal nerve loss, however, was more significant in type 2 diabetes with chronic kidney disease compared to type 2 diabetes alone, indicating that corneal nerve morphological measures could serve greater utility as a tool to detect neuropathy and nephropathy-related corneal nerve changes.