Curative Effect and Possible Mechanism of Taurine on Early Corneal Alkali Burns

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Curative Effect and Possible Mechanism of Taurine on Early Corneal Alkali Burns

Corneal alkali burns (CABs) represent a significant clinical challenge, often leading to severe complications such as corneal opacity, neovascularization, and blindness. The aggressive aseptic inflammatory response triggered by alkali injury is a critical contributor to corneal damage, emphasizing the need for therapeutic strategies targeting inflammatory pathways. This study investigates the therapeutic potential of taurine (Tau), a naturally occurring amino acid with antioxidant and anti-inflammatory properties, in mitigating corneal injury following alkali burns. The findings highlight Tau’s ability to suppress the NLRP3 inflammasome pathway, thereby reducing the expression of pro-inflammatory cytokines interleukin (IL)-1β and IL-18, and promoting corneal repair.

Study Design and Methodology

A murine model of CABs was established using C57BL/6 mice. Alkali injury was induced by applying filter paper soaked in 1N NaOH to the cornea for 10 seconds. Thirty-five mice were randomly divided into three groups: the normal group (healthy corneas, n=5), the phosphate-buffered saline (PBS) group (PBS treatment post-CAB, n=5 per time point), and the Tau group (5% Tau eye drops post-CAB, n=5 per time point). Corneal injury and repair were assessed at 1, 2, and 5 days post-burn. Evaluations included clinical scoring of corneal opacity, histopathological analysis via hematoxylin-eosin (H&E) staining, and molecular assessments using immunohistochemistry (IHC), immunofluorescence (IF), and real-time quantitative polymerase chain reaction (RT-qPCR).

Clinical and Histopathological Outcomes

Corneal Opacity Scores
Clinical evaluation revealed significant differences in corneal opacity between the PBS and Tau groups. At day 5 post-injury, the PBS group exhibited severe corneal clouding (score: 4), while the Tau group showed moderate haze (score: 2–3). Corneal opacity scores in the Tau group were consistently lower than those in the PBS group from days 1 to 5 (P<0.05) (Figure 1B).

H&E Staining
Histopathological analysis demonstrated structural differences between groups. At day 2, the PBS group displayed complete corneal epithelial defects with disorganized stromal architecture. By day 5, the epithelium remained sparse and irregular. In contrast, the Tau group exhibited partial epithelial damage at day 2, with a relatively orderly, albeit thinned, epithelial layer by day 5 (Figure 1C).

Molecular Mechanisms: NLRP3 Inflammasome Suppression

Protein Expression of Inflammatory Mediators

Immunohistochemistry (IHC)
IHC analysis at day 5 post-injury showed elevated optical density of NLRP3, IL-1β, and IL-18 in the PBS group compared to the Tau group. These proteins localized predominantly in the corneal epithelium and stroma (Figure 1D).

Immunofluorescence (IF)
IF staining at day 2 revealed intense fluorescence signals for NLRP3, IL-1β, and IL-18 in the PBS group, indicating robust inflammasome activation. The fluorescence intensity ratios relative to the normal group were:

  • NLRP3: 12.20 ± 0.90 (P=0.0006)
  • IL-18: 11.39 ± 0.77 (P=0.0004)
  • IL-1β: 10.13 ± 0.67 (P=0.0003)

In the Tau group, fluorescence intensity was significantly reduced:

  • NLRP3: 6.33 ± 0.44 (P=0.0017 vs. PBS)
  • IL-18: 8.64 ± 0.66 (P=0.0012 vs. PBS)
  • IL-1β: 8.00 ± 1.04 (P=0.0037 vs. PBS)

mRNA Expression Analysis

RT-qPCR quantified messenger RNA (mRNA) levels of NLRP3, IL-1β, and IL-18 normalized to glyceraldehyde-3-phosphate dehydrogenase (GAPDH).

At Day 2 Post-Injury

  • PBS Group:
    • NLRP3: 4.81 ± 0.18-fold increase vs. normal (P=0.0003)
    • IL-1β: 3.62 ± 0.09-fold increase (P=0.0003)
    • IL-18: 4.30 ± 0.15-fold increase (P=0.0003)
  • Tau Group:
    • NLRP3: 3.31 ± 0.18-fold (P=0.0007 vs. normal; P=0.0024 vs. PBS)
    • IL-1β: 3.04 ± 0.22-fold (P=0.0005 vs. normal; P=0.0241 vs. PBS)
    • IL-18: 3.00 ± 0.13-fold (P=0.0003 vs. normal; P=0.0032 vs. PBS)

At Day 5 Post-Injury

  • PBS Group:
    • NLRP3: 2.45 ± 0.25-fold (P=0.0004 vs. normal)
    • IL-1β: 1.67 ± 0.22-fold (P=0.0028)
    • IL-18: 2.73 ± 0.29-fold (P=0.0009)
  • Tau Group:
    • NLRP3: 1.39 ± 0.13-fold (P=0.0027 vs. normal; P=0.0029 vs. PBS)
    • IL-1β: 1.08 ± 0.25-fold (P=0.0038 vs. normal; P=0.0052 vs. PBS)
    • IL-18: 1.47 ± 0.06-fold (P=0.0016 vs. normal; P=0.0014 vs. PBS)

These results confirm that Tau suppresses NLRP3 inflammasome activation and downstream cytokine production in a time-dependent manner (Figure 1F).

Discussion

NLRP3 Inflammasome in Corneal Alkali Injury

The NLRP3 inflammasome is a key mediator of aseptic inflammation, driving caspase-1 activation and subsequent maturation of IL-1β and IL-18. In CABs, alkali-induced tissue damage releases danger-associated molecular patterns (DAMPs), activating NLRP3 and perpetuating inflammation. This study demonstrates that NLRP3, IL-1β, and IL-18 are upregulated in corneal tissue post-alkali injury, with peak expression at day 2, followed by gradual decline by day 5.

Taurine’s Multimodal Protective Effects

Tau’s therapeutic efficacy in CABs is attributed to its dual antioxidant and anti-inflammatory properties:

  1. Epithelial Stability: Tau enhances corneal epithelial cell survival by maintaining membrane integrity and counteracting oxidative stress.
  2. NLRP3 Suppression: Tau inhibits NLRP3 inflammasome assembly, reducing caspase-1 activation and IL-1β/IL-18 secretion.
  3. Time-Dependent Modulation: Tau’s effect is most pronounced during early inflammation (day 2), aligning with the peak of NLRP3 activity.

Clinical Implications and Future Directions

The study underscores Tau’s potential as a topical therapeutic for CABs. However, optimal dosing, treatment duration, and long-term safety require further investigation. Comparative studies with other NLRP3 inhibitors, such as butyrate, could elucidate synergistic mechanisms. Additionally, exploring Tau’s role in other corneal pathologies, including infectious keratitis, may broaden its clinical applicability.

Conclusion

This study provides mechanistic evidence that Tau alleviates corneal alkali injury by suppressing the NLRP3 inflammasome pathway, thereby reducing IL-1β and IL-18 levels. These findings position Tau as a promising candidate for early intervention in CABs, offering a foundation for developing targeted therapies to prevent blindness in severe cases.

doi.org/10.1097/CM9.0000000000001570

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