Perfusion-cultured bovine anterior segments as an ex vivo model for studying glucocorticoid-induced ocular hypertension and glaucoma

Glucocorticoid-induced glaucoma is a subtype of secondary open angle glaucoma. Either topical or systemic administration of glucocorticoids may induce (OHT) in susceptible persons, some of whom develop optic neuropathy/glaucoma even after glucocorticoid withdrawal. Although glucocorticoid-induced glaucoma is considered a secondary glaucoma, it is closely associated with primary open angle glaucoma (POAG). First, early studies showed that glucocorticoids induce OHT in 90% of POAG patients.1–4 Furthermore, glucocorticoid responsiveness is an important risk factor for POAG.5,6 Second, POAG and glucocorticoid-induced glaucoma share similar clinical presentations, including open anterior chamber angle, IOP elevation, characteristic optic neuropathy, and loss of peripheral vision.7,8 Third, the elevated IOP in both cases is primarily due to damage to the trabecular meshwork (TM).9 The TM plays an important role in IOP regulation. It is the key component of the conventional aqueous humor outflow pathway and contributes to the majority of outflow resistance. Compromised TM function dramatically increases outflow resistance, which leads to IOP elevation. Pathologic changes in the TM, including loss of TM cells, thickening of TM beams, deposition of plaque-like materials, excessive extracellular matrix (ECM) accumulation, and increased cross-linked actin networks (CLANs), are found in glucocorticoid-induced glaucoma as well as in POAG.7,8 Therefore, studying glucocorticoid-induced glaucoma will not only help us to understand this disease but will also provide insightful information about POAG. A number of models have been developed for studying glucocorticoid-induced glaucoma. These can be divided into in vitro, in vivo, and ex vivo models. In vitro models use cultured TM cells. These models are simple and easy to maintain, but they may not reflect in vivo conditions. Instead, in vivo models are most relevant to human glucocorticoid-induced glaucoma. Monkeys,10,11 rabbits,12 mice,13 rats,14 cats,15,16 cows, and sheep17–20 develop glucocorticoid-induced OHT. However, these models cost more and usually need at least 2 to 4 weeks to develop OHT. Ex vivo models combine both the pros and the cons of the two previous models. They provide better simulation of the physiological conditions than in vitro cell cultures and require less time and cost than in vivo models. Perfusion-cultured human anterior segments have been frequently used as an ex vivo model in glucocorticoid-induced glaucoma research.10,21,22 However, human donor eyes are limited by their availability and high cost. More important, because “healthy” human donor eyes are prioritized for corneal transplantation, those available for research are not of the best quality. Therefore, we were looking for eyes from other species as alternatives. In contrast to human donor eyes, bovine eyes are inexpensive and readily available. Because of their large size, surgical manipulation and sample collection are easy. Studies using perfusion-cultured bovine anterior segments showed that the bovine TM (BTM) is directly involved in regulating the outflow pathway.23–25 BTM cells share many properties with human TM cells,26 including dexamethasone (DEX) induction of ECM proteins.27 Recently, Wade et al.28 reported DEX-induced CLAN formation in confluent BTM cultures, which is a unique feature of the TM. Their findings further prove the validity of this model. In addition, an in vivo study showed that glucocorticoids can induce OHT in bovine eyes.18 Based on the advantages described here, we decided to test whether perfusion-cultured bovine anterior segments are suitable for studying glucocorticoid-induced OHT and trabecular outflow research.