Investigating the synergistic effect of combined neurotrophic factor concentration gradients to guide axonal growth

Neurotrophic factors direct axonal growth toward the target tissue by a concentration gradient, which is mediated through different tyrosine kinase cell surface receptors. In this study, well-defined concentration gradients of neurotrophic factors (NFs) allowed us to study the synergistic effect of different NFs (e.g. nerve growth factor [NGF], neurotrophin-3 [NT-3] and brain-derived neurotrophic factor [BDNF]) for axonal guidance of embryonic lumbar dorsal root ganglion cells (DRGs). Effective guidance of DRG axons was achieved with a minimum NGF concentration gradient of 133 ng/ml/mm alone, or combined NGF and NT-3 concentration gradients of 80 ng/ml/mm each. Interestingly, the combined concentration gradients of NGF and BDNF did not show any significant synergism at the concentration gradients studied. The synergism observed between NGF and NT-3 indicates that axons may be guided over a 12.5 mm distance, which is significantly greater than that of 7.5 mm calculated by us for NGF alone or that of 2 mm observed by others.