Research models of sulfur mustard- and nitrogen mustard-induced ocular injuries and potential therapeutics

Chloropicrin induced ocular injury: Biomarkers, potential mechanisms, and treatments

Ocular tissue, especially the cornea, is overly sensitive to chemical exposures. The availability and adoption of chemical threat agent chloropicrin (CP) is growing in the United States as a pesticide and fumigant; thereby increasing the risk of its use in warfare, terrorist attacks and non-intentional exposure. Exposure to CP results in immediate ocular, respiratory, and dermal injury; however, we lack knowledge on its mechanism of toxicity as well as of its breakdown products like chlorine and phosgene, and effective therapies are elusive. Herein, we have reviewed the recent findings on exposure route, toxicity and likely mechanisms of CP induced ocular toxicity based on other vesicating chemical warfare agents that cause ocular injury. We have focused on the implication of their toxicity and mechanistic outcomes in the ocular tissue, especially the cornea, which could be useful in the development of broad-spectrum effective therapeutic options. We have discussed on the potential countermeasures, overall hallmarks and challenges involved in studying ocular injuries from chemical threat agent exposures. Finally, we reviewed useful available technologies and methods that can assist in the identification of effective medical countermeasures for chemical threat agents related ocular injuries.

Nitrogen Mustard-Induced Ex Vivo Human Cornea Injury Model and Therapeutic Intervention by Dexamethasone

Sulfur mustard (SM), a vesicating agent first used during World War I, remains a potent threat as a chemical weapon to cause intentional/accidental chemical emergencies. Eyes are extremely susceptible to SM toxicity. Nitrogen mustard (NM), a bifunctional alkylating agent and potent analog of SM, is used in laboratories to study mustard vesicant-induced ocular toxicity. Previously, we showed that SM-/NM-induced injuries (in vivo and ex vivo rabbit corneas) are reversed upon treatment with dexamethasone (DEX), a US Food and Drug Administration-approved, steroidal anti-inflammatory drug. Here, we optimized NM injuries in ex vivo human corneas and assessed DEX efficacy. For injury optimization, one cornea (randomly selected from paired eyes) was exposed to NM: 100 nmoles for 2 hours or 4 hours, and 200 nmoles for 2 hours, and the other cornea served as a control. Injuries were assessed 24 hours post NM-exposure. NM 100 nmoles exposure for 2 hours was found to cause optimal corneal injury (epithelial thinning [∼69%]; epithelial-stromal separation [6-fold increase]). In protein arrays studies, 24 proteins displayed ≥40% change in their expression in NM exposed corneas compared with controls. DEX administration initiated 2 hours post NM exposure and every 8 hours thereafter until 24 hours post-exposure reversed NM-induced corneal epithelial-stromal separation [2-fold decrease]). Of the 24 proteins dysregulated upon NM exposure, six proteins (delta-like canonical Notch ligand 1, FGFbasic, CD54, CCL7, endostatin, receptor tyrosine-protein kinase erbB-4) associated with angiogenesis, immune/inflammatory responses, and cell differentiation/proliferation, showed significant reversal upon DEX treatment (Student's t test; P ≤ 0.05). Complementing our animal model studies, DEX was shown to mitigate vesicant-induced toxicities in ex vivo human corneas. SIGNIFICANCE STATEMENT: Nitrogen mustard (NM) exposure-induced injuries were optimized in an ex vivo human cornea culture model and studies were carried out at 24 h post 100 nmoles NM exposure. Dexamethasone (DEX) administration (started 2 h post NM exposure and every 8 h thereafter) reversed NM-induced corneal injuries. Molecular mediators of DEX action were associated with angiogenesis, immune/inflammatory responses, and cell differentiation/proliferation, indicating DEX aids wound healing via reversing vesicant-induced neovascularization (delta-like canonical Notch ligand 1 and FGF basic) and leukocyte infiltration (CD54 and CCL7).

Cellular senescence implication in mustard keratopathy

Mustard agents are vesicants that were used in warfare multiple times. They are potent alkylating agents that activate cellular pathways of apoptosis, increase oxidative stress, and induce inflammation. Eyes are particularly susceptible to mustard exposure with a wide range of ocular surface damage. Three main categories of mustard-related eye injuries are acute, chronic, and delayed-onset manifestations. Mustard keratopathy (MK) is a known complication characterized by corneal opacification, ulceration, thinning, and neovascularization that can lead to severe vision loss and discomfort. Recently, a few reports demonstrated the role of senescence induction as a new pathological mechanism in mustard-related injuries that could affect wound healing. We ran the first murine model of delayed-onset MK and nitrogen mustard-induced senescence, evaluating the pathological signs of senescence in the cornea using beta-galactosidase staining. Our results suggest that nitrogen mustard exposure causes senescence in the corneal cells, which could be the underlying mechanism for chronic and late-onset ocular surface damage. We also found a significant correlation between the percentage of positive beta-galactosidase staining and the degree of fibrosis in the cornea. This provides valuable insight into the possible role of anti-senescence drugs in the near future for accelerating corneal healing and restricting fibrosis in patients with mustard keratopathy.

A corneo-retinal hypercitrullination axis underlies ocular injury to nitrogen mustard

The vesicant sulfur mustard (SM) is a chemical warfare agent that causes acute and chronic injury to the cornea and proximal anterior segment structures. Despite clinical evidence of SM-exposure causing unexplained retinal deficits, there have been no animal studies conducted to examine the retinal toxicity of this vesciant. The cardinal hallmark of retinal response to stressors or injury is the activation of reactive gliosis, a cellular process largely governed by Müller glia. Previously we showed that corneal exposure to sodium hydroxide elicits rapid induction of reactive gliosis and results in retinal degeneration in a dose-related manner. Based on this evidence, we hypothesized that the vesicant nitrogen mustard (NM), an analog of SM, may also elicit reactive gliosis. To test this idea, we developed a mouse model of NM ocular injury and investigated corneal and retinal effects focusing on citrullination, a posttranslational modification (PTM) of proteins. This PTM was recently linked to alkali injury and has also been shown to occur in retinal degenerative conditions. Here, we demonstrate that corneal exposure to 1% NM causes a synchronous activation of citrullination in both the cornea and retina with hypercitrullination becoming apparent temporally and manifesting with altered cellular expression characteristics. A key finding is that ocular citrullination occurs acutely as early as 1-h post-injury in both the cornea and retina, which underscores a need for expeditious interception of this acute corneal and retinal response. Moreover, exploiting dose response and temporal studies, we uncoupled NM-induced retinal citrullination from its induction of retinal gliosis. Our findings demonstrate that hypercitrullination is a common corneo-retinal mechanism that sensitizes the eye to NM injury and suggests that counteracting hypercitrullination may provide a suitable countermeasure to vesicant injury.