Chronic Nerve Growth Factor Exposure Increases Apoptosis in a Model of In Vitro Induced Conjunctival Myofibroblasts

In the conjunctiva, repeated or prolonged exposure to injury leads to tissue remodeling and fibrosis associated with dryness, lost of corneal transparency and defect of ocular function. At the site of injury, fibroblasts (FB) migrate and differentiate into myofibroblasts (myoFB), contributing to the healing process together with other cell types, cytokines and growth factors. While the physiological deletion of MyoFB is necessary to successfully end the healing process, myoFB prolonged survival characterizes the pathological process of fibrosis. The reason for myoFB persistence is poorly understood. Nerve Growth Factor (NGF), often increased in inflamed stromal conjunctiva, may represent an important molecule both in many inflammatory processes characterized by tissue remodeling and in promoting wound-healing and well-balanced repair in humans. NGF effects are mediated by the specific expression of the NGF neurotrophic tyrosine kinase receptor type 1 (trkA^NGFR) and/or the pan-neurotrophin glycoprotein receptor (p75^NTR). Therefore, a conjunctival myoFB model (TGFβ1-induced myoFB) was developed and characterized for cell viability/proliferation as well as αSMA, p75^NTR and trkA^NGFR expression. MyoFB were exposed to acute and chronic NGF treatment and examined for their p75^NTR/trkA^NGFR, αSMA/TGFβ1 expression, and apoptosis. Both NGF treatments significantly increased the expression of p75^NTR, associated with a deregulation of both αSMA/TGFβ1 genes. Acute and chronic NGF exposures induced apoptosis in p75^NTR expressing myoFB, an effect counteracted by the specific trkA^NGFR and/or p75^NTR inhibitors. Focused single p75^NTR and double trkA^NGFR/p75^NTR knocking-down experiments highlighted the role of p75^NTR in NGF-induced apoptosis. Our current data indicate that NGF is able to trigger in vitro myoFB apoptosis, mainly via p75^NTR. The trkA^NGFR/p75^NTR ratio in favor of p75^NTR characterizes this process. Due to the lack of effective pharmacological agents for balanced tissue repairs, these new findings suggest that NGF might be a suitable therapeutic tool in conditions with impaired tissue healing.