Glucose consumption in cultured corneal cells

Insulin facilitates corneal wound healing in the diabetic environment through the RTK-PI3K/Akt/mTOR axis in vitro

Diabetic patients can develop degenerative corneal changes, termed diabetic keratopathy, during the course of their disease. Topical insulin has been shown to reduce corneal wound area and restore sensitivity in diabetic rats, and both the insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF-1R) stimulate cell signaling of the PI3K-Akt pathway. The purpose of this study was to assess a mechanism by which improved wound healing occurs by characterizing expression within the PI3K-Akt pathway in corneal epithelial and stromal cells. In vitro scratch tests were used to evaluate wound healing outcomes under variable glucose conditions in the presence or absence of insulin. Protein expression of intracellular kinases in the PI3K pathway, stromal cell markers, and GLUT-1 was evaluated by immunoblotting.TGF-β1 expression was evaluated by ELISA. Insulin promoted in vitro wound healing in all cell types. In human corneal epithelial cells, insulin did not induce PI3K-Akt signaling; however, in all other cell types evaluated, insulin increased expression of PI3K-Akt signaling proteins compared to vehicle control. Fibroblasts variably expressed α-SMA under all treatment conditions, with significant increases in α-SMA and TGF-β1 occurring in a dose-dependent manner with glucose concentration. These results indicate that insulin can promote corneal cellular migration and proliferation by inducing Akt signaling. Exogenous insulin therapy may serve as a novel target of therapeutic intervention for diabetic keratopathy.

Glutaminolysis is Essential for Energy Production and Ion Transport in Human Corneal Endothelium

Corneal endothelium (CE) is among the most metabolically active tissues in the body. This elevated metabolic rate helps the CE maintain corneal transparency by its ion and fluid transport properties, which when disrupted, leads to visual impairment. Here we demonstrate that glutamine catabolism (glutaminolysis) through TCA cycle generates a large fraction of the ATP needed to maintain CE function, and this glutaminolysis is severely disrupted in cells deficient in NH₃:H⁺ cotransporter Solute Carrier Family 4 Member 11 (SLC4A11). Considering SLC4A11 mutations leads to corneal endothelial dystrophy and sensorineural deafness, our results indicate that SLC4A11-associated developmental and degenerative disorders result from altered glutamine catabolism. Overall, our results describe an important metabolic mechanism that provides CE cells with the energy required to maintain high level transport activity, reveal a direct link between glutamine metabolism and developmental and degenerative neuronal diseases, and suggest an approach for protecting the CE during ophthalmic surgeries.

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Glutamine contributes half of TCA cycle intermediates in human corneal endothelium.

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Glutamine catabolism supplies significant ATP that fuels the endothelial pump function.

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SLC4A11 (NH₃:2H⁺ cotransporter) knockout shows ammonia related oxidative damage.

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Loss of SLC4A11 transporter disrupts expression of glutaminolysis enzymes.

The corneal endothelium (CE) is responsible for maintaining corneal transparency through the action of active transport processes. We report that CE metabolizes the amino acid glutamine producing ATP in support of active transport. In the mouse model of CHED (Congenital Hereditary Endothelial Dystrophy), which manifests corneal edema and loss of transparency, glutamine metabolism is disrupted due to loss of SLC4A11, an NH₃:2H⁺ transporter. This work sheds light on potential clinical therapies to facilitate CE function, the pathogenesis of CHED and Fuchs' Endothelial Corneal Dystrophy, and suggests that the ammonia handling capacity of SLC4A11 is essential for efficient metabolism of glutamine.

Tear Dynamics Model

PURPOSE

Quantitative understanding of tear dynamics may help in developing better ophthalmic drug delivery vehicles and dry eye treatments. This paper attempts to develop a comprehensive model that can predict the effect of physiological parameters on various issues related to tear dynamics.

METHODS

The model is based on mass balances of water and solutes such as glucose, sodium, potassium, and chloride. The mass balances require models for the drainage of fluid through the canaliculi and the transport of tears and solutes through the conjunctiva. The model parameters are obtained by simulating Ussing-chamber experiments. The differential equations for the unsteady balances are solved numerically.

RESULTS

The model predicts that under normal conditions, the tear volume, tear osmolarity and potential are 7.1 microl, 297.6 mM, and -15.1 mV, respectively. The model also predicts that the conjunctiva is secretory and contributes about 25% of the total tear production. We also predict the effect of evaporation on tear physiology and show that an increase in evaporation increases osmolarity, reduces tear volume, and increases conjuctival secretion. Additionally, the new tear dynamics model helps assess the effect of osmolarity of the instilled drops, insertion of punctum plugs and use of moisture chamber as treatments for dry eyes. Furthermore, the model is used to predict the effect of modulation of specific transport pathways, which is proposed as a potential remedy for dry eyes, on conjuctival secretion and total tear volume.

CONCLUSIONS

Most of the predicted results agree with the reported experimental results, at least qualitatively. However, some predictions disagree with experiments suggesting that further improvements in the model are needed. The model developed in this paper can improve our understanding of tear dynamics and also serve as a tool to evaluate the efficacy of various modalities at treating dry eyes.

Modeling glucose distribution in the cornea

The central cornea obtains its glucose by diffusion through the cornea from the aqueous humor to the epithelium. The diffusion of glucose in the cornea is analogous to the flow of current in an electrical resistance network. The cellular consumption of glucose can be compared to shunting a portion of the charge to electrical ground. An electrical analog model of the cornea was developed to predict the availability of glucose to the epithelium and the distribution of glucose in the stroma. The glucose constant concentration lines in the normal stroma are parallel to the corneal surface and have decreasing values from 880 to 580 micrograms/ml. The effects on epithelial glucose concentration by implanting an intracorneal lens (ICL) of varying diameter, depth, permeability and thickness can be modeled. Glucose permeability through the intracorneal lens has the most significant effect on glucose availability. The ICL profile i.e. power, can also be an important fact in determining glucose availability. A minus power design requires a thin central lens zone with a thick peripheral zone. The design results in relatively more glucose flux through the optical zone of the lens and thus improves central epithelial glucose availability.