Transient Receptor Potential Vanilloid-1 Channels Facilitate Axonal Degeneration of Corneal Sensory Nerves in Dry Eye

Corneal Sensory Receptors and Pharmacological Therapies to Modulate Ocular Pain

Nociceptors respond to noxious stimuli and transmit pain signals to the central nervous system. In the cornea, the nociceptors located in the most external layer provide a myriad of sensation modalities. Damage to these corneal nerve fibers can induce neuropathic pain. In response, corneal nerves become sensitized to previously non-noxious stimuli. Assessing corneal pain origin is a complex ophthalmic challenge due to variations in its causes and manifestations. Current FDA-approved therapies for corneal nociceptive pain, such as acetaminophen and NSAIDs, provide only broad-acting relief with unwanted side effects, highlighting the need for precision medicine for corneal nociceptive pain. A few targeted treatments, including perfluorohexyloctane (F6H8) eye drops and Optive Plus (TRPV1 antagonist), are FDA-approved, while others are in preclinical development. Treatments that target signaling pathways related to neurotrophic factors, such as nerve growth factors and ion channels, such as the transient receptor potential (TRP) family or tropomyosin receptor kinase A, may provide a potential combinatory therapeutic approach. This review describes the roles of nociceptors in corneal pain. In addition, it evaluates molecules within nociceptor signaling pathways for their potential to serve as targets for efficient therapeutic strategies for corneal nociceptive pain aimed at modulating neurotrophic factors and nociceptive channel sensitivity.

Correlation among ocular surface changes and systemic hematologic indexes and disease activity in primary Sjögren's syndrome: a cross-sectional study

BACKGROUND

To explore the relationship among ocular surface changes, systemic hematologic indexes, and disease activity in primary Sjögren's syndrome patients.

METHODS

Thirty-three primary Sjögren's syndrome patients and 36 healthy controls were recruited in this cross-sectional study. All participants underwent complete ocular surface testing, including dry eye symptoms and signs, tear multi-cytokine analysis, and conjunctival impression cytology (CIC). Multiple systemic hematologic indexes and disease activity were also evaluated, including autoantibodies, immune cells, the EULAR Sjögren's Syndrome Patient Reported Index (ESSPRI), and the EULAR Sjögren's Syndrome Disease Activity Index (ESSDAI).

RESULTS

Primary Sjögren's syndrome patients exhibited significant dry eye, severe conjunctivochalasis, decreased goblet cell density, and severe squamous epithelial on the ocular surface. Interferon-inducible T cell alpha chemoattractant (I-TAC), granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin (IL)-1β, IL-5, IL-8, IL-10, IL-13, IL-21, C-C motif chemokine ligand (CCL)4, interferon-gamma (IFN-γ), CCL20, and tumor necrosis factor-gamma (TNF-α) in the tear fluid of pSS patients changed significantly. Correlation analysis showed that anti-SSA was relevant to ocular surface disease index (OSDI) score, tear break-up time (TBUT), and meibomian gland secretion (MGS). CD8+ T cell percentages were relevant to TBUT and corneal fluorescein staining score (CFS). IL-8, IL-13, CCL4, and TNF-α were correlated with RF-IgA. IL-1β, CCL4, and TNF-α were correlated with CD8+ T cell counts. IL-5 and CCL20 were correlated with the ratio of helper T cells and suppressor T cells. Tear I-TAC, IL-8, CCL20, and TNF-α were significantly correlated with the ESSDAI of different domains.

CONCLUSIONS

Our results revealed that the ocular surface changes in pSS patients were significantly correlated with systemic hematologic indexes and disease activity.

Targeting α7 Nicotinic Acetylcholine Receptor for Modulating the Neuroinflammation of Dry Eye Disease Via Macrophages

Purpose

Patients with dry eye disease (DED) often exhibit neurological abnormalities and may even suffer from neuropathic pain and pain-related anxiety or depression. The α-7 nicotinic acetylcholine receptor (α7nAChR) is a pivotal regulator in the anti-inflammatory pathway connecting the nervous and immune systems, Here, we investigate the potential of α7nAChR agonist as a novel treatment for DED.

Methods

We induced DED model by unilateral excision of extraorbital lachrymal gland in C57/BL6 mice. After seven days of treatment, RNA sequencing was performed to identify differentially expressed genes in the cornea of lacrimal gland excision (LGE) mice. Quantitative polymerase chain reaction, Western blotting, and flow cytometry tests were carried out to elucidate neuroinflammation changes after α7nAChR activation. Corneal nerve abnormalities were assessed by corneal esthesiometry and immunofluorescence staining.

Results

The activation of α7nAChR stimulates genes involved in immune-mediated inflammatory progression and neuroregulation, inhibits the expression of transient receptor potential vanilloid-1, reinstates corneal nerve density, and alleviates anxiety-like behaviors associated with severe DED. Furthermore, we demonstrated that α7nAChR agonist restored corneal nerve abnormality and alleviated inflammation response by down-regulating the proportion of CD86+ M1 macrophages (proinflammatory phenotypes).

Conclusions

Our findings underscore the activation of α7nAChR as a pioneering therapeutic approach for preserving corneal nerves balance and controlling inflammation in DED.

Antagonizing NK-1R modulates pain perception following corneal injury

Substance P (SP) expressed by corneal nerves, is an 11-amino acid long neuropeptide from the tachykinin family, encoded by the Tac1 gene, and binds to neurokinin receptors. SP overexpression is associated with various pathological responses in the cornea including vasodilation, pain, inflammation, and angiogenesis in the normally avascular tissue. This study investigates the role of neurokinin-1 receptor (NK-1R) mediated signaling in nociception, nerve regeneration, and neuronal activation following mechanical corneal injury in mice. Corneal injuries were induced in age- and sex-matched C57BL/6 mice by removing corneal epithelium and partial anterior stroma. Following injury, mice were treated with either L-733,060, an NK-1R antagonist, or vehicle, administered topically twice daily for 21 days. Corneal SP levels were measured using ELISA, and nerve regeneration was assessed by quantifying corneal nerve fiber density (CNFD) via β-Tubulin III staining. Gene expression of neuronal markers (ATF3, GFAP, cFos, TRPV1, and TRPM8) in the trigeminal ganglia was measured using qPCR. Pain responses were evaluated using the eye-wiping test (EWT) and palpebral ratio (PR). Results indicated a persistent increase in corneal SP post-injury, significantly reduced by NK-1R antagonism. At 21 days, NK-1R antagonist-treated mice showed higher CNFD, reduced expression of neuronal activation markers, and lower pain perception compared to controls. These findings suggest that SP/NK-1R signaling is critical in corneal nociception post-injury, and its inhibition reduces pain, prevents neuronal hyperactivation, and supports nerve regeneration.

Ion Channels as Potential Drug Targets in Dry Eye Disease and Their Clinical Relevance: A Review

Dry eye disease (DED) is a common multifactorial disorder characterized by a deficiency in the quality and/or quantity of tear fluid. Tear hyperosmolarity, the dysfunction of ion channel proteins, and eye inflammation are primarily responsible for the development and progression of DED. Alterations in the structure and/or function of ion channel receptors (transient receptor potential ankyrin 1 (TRPA1), transient receptor potential melastatin 8 (TRPM8), transient receptor potential vanilloid 1 and 4 (TRPV1 and TRPV4)), and consequent hyperosmolarity of the tears represent the initial step in the development and progression of DED. Hyperosmolarity triggers the activation of ion channel-dependent signaling pathways in corneal epithelial cells and eye-infiltrated immune cells, leading to the activation of transcriptional factors that enhance the expression of genes regulating inflammatory cytokine production, resulting in a potent inflammatory response in the eyes of DED patients. A persistent and untreated detrimental immune response further modifies the structure and function of ion channel proteins, perpetuating tear hyperosmolarity and exacerbating DED symptoms. Accordingly, suppressing immune cell-driven eye inflammation and alleviating tear hyperosmolarity through the modulation of ion channels in DED patients holds promise for developing new therapeutic strategies. Here, we summarize current knowledge about the molecular mechanisms responsible for the inflammation-induced modification of ion channels leading to tear hyperosmolarity and immune cell dysfunction in DED patients. We also emphasize the therapeutic potential of the newly designed immunomodulatory and hypo-osmotic solution d-MAPPS™ Hypo-Osmotic Ophthalmic Solution, which can activate TRPV4 in corneal epithelial cells, stabilize the tear film, enhance natural cytokine communication, and suppress detrimental immune responses, an important novel approach for DED treatment.

CD4+ T cells drive corneal nerve damage but not epitheliopathy in an acute aqueous-deficient dry eye model

Dry eye disease (DED) is characterized by a dysfunctional tear film in which the corneal epithelium and its abundant nerves are affected by ocular desiccation and inflammation. Although adaptive immunity and specifically CD4+ T cells play a role in DED pathogenesis, the exact contribution of these cells to corneal epithelial and neural damage remains undetermined. To address this, we explored the progression of a surgical DED model in wild-type (WT) and T cell-deficient mice. We observed that adaptive immune-deficient mice developed all aspects of DED comparably to WT mice except for the absence of functional and morphological corneal nerve changes, nerve damage-associated transcriptomic signature in the trigeminal ganglia, and sustained tear cytokine levels. Adoptive transfer of CD4+ T cells from WT DED mice to T cell-deficient mice reproduced corneal nerve damage but not epitheliopathy. Conversely, T cell-deficient mice reconstituted solely with naïve CD4+ T cells developed corneal nerve impairment and epitheliopathy upon DED induction, thus replicating the WT DED phenotype. Collectively, our data show that while corneal neuropathy is driven by CD4+ T cells in DED, corneal epithelial damage develops independently of the adaptive immune response. These findings have implications for T cell-targeting therapies currently in use for DED.

Preventing and treating neurotrophic keratopathy by a single intrastromal injection of AAV-mediated gene therapy

PURPOSE

Neurotrophic keratopathy (NK) is a degenerative corneal condition resulting from corneal nerve injury. Current therapies, including the recombinant human nerve growth factor (rhNGF) therapy, requires continuous administration. This study aims to develop a novel and highly effective gene therapy strategy for the prevention and treatment of NK.

METHODS

Adeno-associated virus (AAV) was transduced into corneal stromal cells by intrastromal injection. Three dimensional corneal wholemount imaging with co-immunostaining of ZO-1 and tubulin was utilized to assess the transduction of AAV.rh10. The efficacy of prevention and treatment of NK by a single intrastromal injection of AAV-Ngf was tested using capsaicin mouse model, herpes simplex keratitis (HSK) model, type Ⅱ diabetes model and alkali burn model. rhNGF eye drops served as the positive control.

RESULTS

Intrastromal injection of AAV.rh10 efficiently transduced the subepithelial nerve plexus and retrogradely transported to the trigeminal ganglion (TG). A single injection of AAV.rh10-Ngf can significantly promote corneal nerve repair, accelerate corneal epithelial repair, reduce corneal stromal edema, and improve corneal sensitivity across the four NK models. The therapeutic effects were consistent with those achieved by continuous administration of rhNGF drops by 6 times daily.

CONCLUSIONS

This proof-of-concept study demonstrates that AAV.rh10-Ngf gene therapy is a promising method for preventing and treating of NK. Our results underline the potential for developing clinical trials to further explore the safety and efficacy of such gene therapy.

Hyaluronate Protects From Benzalkonium Chloride-Induced Ocular Surface Toxicity

Purpose

The purpose of this study was to investigate the effect of sodium hyaluronate (SH) on benzalkonium chloride (BAK)-induced toxicity in the ocular surface epithelium and corneal nerves.

Methods

Ocular surface epithelial cells from Balb/c mice were cultured with 0.1% to 0.4% SH and 0.001% to 0.01% BAK and their metabolic activity, viability, and wound repair capacity were assessed in vitro. Following a controlled corneal wound, re-epithelialization and recovery of epithelial barrier function and mechanosensitivity were measured in Balb/c mice treated with 0.4% SH 3 times/day and 0.01% BAK twice daily for 3 weeks. Nerve morphology was assessed by confocal microscopy of corneal whole mounts.

Results

Whereas BAK exposure reduced metabolic activity, viability, and wound repair ability of ocular epithelial cells in vitro, pretreatment with SH ameliorated BAK toxicity in a concentration-dependent manner. The highest SH concentration partially reversed the effects of 0.01% BAK in vitro and increased the corneal healing rate of BAK-exposed mice. Although all corneal wounds closed after 4 days, continuous SH treatment improved corneal barrier dysfunction 18 days after wounding and accelerated the recovery of corneal mechanical sensitivity to baseline levels in BAK-exposed mice. SH treatment also increased corneal nerve density in the wounded area after 3 weeks.

Conclusions

SH mitigates BAK-associated ocular epithelial and neurotoxicity in a concentration-dependent manner.

Translational Relevance

Commercially available, high-concentration SH formulations may have added benefits in treating BAK-associated ocular surface toxicity.