Role of aquaporins in corneal healing post chemical injury

Abstract

Aquaporins (AQPs) are transmembrane water channel proteins that regulate the movement of water through the plasma membrane in various tissues including cornea. The cornea is avascular and has specialized microcirculatory mechanisms for homeostasis. AQPs regulate corneal hydration and transparency for normal vision. Currently, there are 13 known isoforms of AQPs that can be subclassified as orthodox AQPs, aquaglyceroporins (AQGPs), or supraquaporins (SAQPs)/unorthodox AQPs. AQPs are implicated in keratocyte function, inflammation, edema, angiogenesis, microvessel proliferation, and the wound-healing process in the cornea. AQPs play an important role in wound healing by facilitating the movement of corneal stromal keratocytes by squeezing through tight stromal matrix and narrow extracellular spaces to the wound site. Deficiency of AQPs can cause reduced concentration of hepatocyte growth factor (HGF) leading to reduced epithelial proliferation, reduced/impaired keratocyte migration, reduced number of keratocytes in the injury site, delayed and abnormal wound healing process. Dysregulated AQPs cause dysfunction in osmolar homeostasis as well as wound healing mechanisms. The cornea is a transparent avascular tissue that constitutes the anterior aspect of the outer covering of the eye and aids in two-thirds of visual light refraction. Being the outermost layer of the eye, the cornea is prone to injury. Of the 13 AQP isoforms, AQP1 is expressed in the stromal keratocytes and endothelial cells, and AQP3 and AQP5 are expressed in epithelial cells in the human cornea. AQPs can facilitate wound healing through aid in cellular migration, proliferation, migration, extracellular matrix (ECM) remodeling and autophagy mechanism. Corneal wound healing post-chemical injury requires an integrative and coordinated activity of the epithelium, stromal keratocytes, endothelium, ECM, and a battery of cytokines and growth factors to restore corneal transparency. If the chemical injury is mild, the cornea will heal with normal clarity, but severe injuries can lead to partial and/or permanent loss of corneal functions. Currently, the role of AQPs in corneal wound healing is poorly understood in the context of chemical injury. This review discusses the current literature and the role of AQPs in corneal homeostasis, wound repair, and potential therapeutic target for acute and chronic corneal injuries.

Keywords: Aquaporins; Corneal endothelium; Corneal epithelium; Corneal wound healing; Inflammation; Stroma; Sulfur mustard; Wound healing.

Figure 1:

Schematic diagram depicting the relative location of AQPs expression in the cornea. The corneal epithelium (red) expresses AQP3, AQP5, AQP7, and AQP11. The stroma (blue) expresses AQP1 and AQP3. Corneal endothelium (purple) expresses AQP1, AQP4, and AQP7.

Figure 2:

Corneal thickness (µm) in naïve (NT) and SM-exposed treatment groups in vivo rabbit corneas on day 3, day 7, and day14 post SM-exposure (n=6). All rabbits received a target of 200 mg-min/m³ SM vapor exposure for 8 minutes. All the results were analyzed by one-way analysis of variance (ANOVA) followed by Tukey-Kramer multiple comparisons using GraphPad program. p<0.05 was considered statistically significant. (*** p < 0.01, naïve control Vs SM treated).

Figure 3:

mRNA levels of AQP1, AQP3, and AQP5 normalized to β-actin in naïve (NT) and SM-exposed treatment groups on day 3, day 7, and day14 post SM-exposure in New Zealand White rabbits. Six SM-exposed corneas and six naïve corneas from each group were used for quantitative polymerase chain reaction (q-PCR), and each sample was tested in triplicate. All the results were analyzed by one-way analysis of variance (ANOVA) followed by Tukey-Kramer multiple comparisons using the GraphPad Instat 3 program. p<0.05 was considered statistically significant (** p < 0.01, *** p < 0.01, naïve control Vs SM treated).