Purpose: To investigate neurotrophic growth factors and corresponding receptors in human and rabbit corneal epithelium and stroma.
Methods: Transcription of nerve growth factor (NGF), neurotrophin 3 (NT-3), NT-4, brain-derived neurotrophic factor (BDNF), glial cell line- derived neurotrophic factor (GDNF), and receptors Trk A-E, was investigated by reverse transcription-polymerase chain reaction. DNA dot blot analysis allowed to estimate transcription levels. Single cell proliferation assays were performed using recombinant NGF, BDNF, and GDNF. Mitogen-activated protein kinase signal transduction was investigated with Western blot analysis using antibodies against activated and total extracellular signal-regulated kinase (ERK) 1/2 and the jun N-terminal protein kinase (JNK) 1/2.
Results: Transcription of NGF, NT-3, BDNF, and Trk A, Trk B, Trk C, and Trk E receptors was detected in both ex vivo and cultured epithelium and stroma. Transcription of NT-4 was only detected in epithelium and transcription of GDNF only in stroma. Levels of transcription were higher for NT-3, NT-4, and the Trk receptors and lower for NGF, BDNF, and GDNF. NGF and GDNF stimulated both epithelial colony formation and proliferation, whereas BDNF only enhanced colony formation. Stromal proliferation was enhanced in serum-free medium. In epithelium, predominantly ERK 1 was activated by NGF, GDNF, and BDNF. In stromal cells NGF and GDNF stimulated phosphorylation of ERK 1 and JNK 1.
Conclusions: Neurotrophic factors and tyrosine kinase receptors are transcribed in the human cornea. GDNF and NGF stimulate corneal epithelial proliferation, and the effect of the latter might be mediated by activation of ERK 1. Neurotrophic factors have very specific effects on phosphorylation of ERK and JNK in epithelial and stromal cells. The differential expression of NT-4 and GDNF suggests a regulatory function within the cytokine network of the cornea.