Nidogen-2: Location and expression during corneal wound healing

Nidogen-2 is a basement membrane (BM) glycoprotein that could be a key to understanding why defects in BM regeneration occur after severe trauma to the cornea. We monitored the location and expression of nidogen-2 during corneal repair after alkali burn in rabbits. In rabbits that received both general and ocular topical anaesthesia, the central cornea of the left eye was burned by placing an 8-mm diameter filter paper soaked in 0.5 N NaOH for 60 s. Right corneas were used as controls. The eyes were evaluated at 2, 7, 15, and 30 days after burning and analysed by immunohistochemistry for nidogen-2 and alpha-smooth muscle actin, a myofibroblast marker. Nidogen-2 mRNA expression levels were determined by quantitative real-time polymerase chain reaction. In control corneas, nidogen-2-positive cells were in all epithelial layers, the endothelium, and the anterior and posterior stromal regions. At Day 2 after the alkali burn, the wound area epithelium and the peripheral epithelium were made up of only 1 to 2 cell layers, all of them nidogen-2 positive. At Day 7 in the wound area, the epithelium consisted of two cell layers, and the basally located cells were mostly nidogen-2 positive. The greatest change was observed at Day 30. At this time, the ulcer prevalence in the alkali-burned corneas was approximately 50% and the central epithelial defects remained. In unepithelialized corneas, frequent epithelial detachments were present, in which almost of the epithelial cells were nidogen-2 negative. The injured stroma was repopulated by activated stromal cells that synthesized nidogen-2. The nidogen-2 was retained in the newly secreted, but disordered, matrix produced mainly by the myofibroblasts localized in the stroma at 7, 15, and 30 days after burning. Thus, even though nidogen-2 was present, it was unable to contribute to the effective regeneration of the BM.