The Temporal Relation between a Corneal Neuronotrophic Factor and Corneal Nerve Regeneration

Abstract

Epithelial neuronotrophic factor (ENF) is a protein secreted specifically by cultured corneal and conjunctival epithelial cells of rabbit (1,2). It is absent in corneal fibroblasts and endothelial cells (1), or epithelial cells of the lens, retina, skin, and kidney (2). ENF acts selectively on cultured sensory neurons (trigeminal and dorsal root ganglia) by both prolonging their survival and inducing neurite outgrowth (1, 2). It has no effect on sympathetic neurons. ENF also stimulates protein synthesis in cultured trigeminal neurons, such as glycoproteins (independent of sprouting) and proline-labeled proteins (neurite-dependent), including collagens that are deposited extracellularly (3). The newly-synthesized proteins are increasingly distributed to both the particulate (somata) and neurite fractions with time. Indirect evidence suggests that ENF is not NGF, collagen, fibronectin, laminin or certain glycosaminoglycans (4). Presumably, ENF belongs to a class of endogenous trophic factors that are secreted by tissues to initiate and maintain proper innervation by neurons.