Phosphorylation of the growth factors bFGF, NGF and BDNF: a prerequisite for their biological activity

The aim of this work was to test whether growth factors such as basic fibroblast growth factor (bFGF), nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) undergo autophosphorylation and whether this affects their biological activity. Incubation of those growth factors with gamma-(32)P]ATP resulted in phosphorylation in vitro. The phosphate bond was resistant to alkaline pH, yet acid-labile. Addition of alkaline phosphatase resulted in time and protein dependent dephosphorylation. Concomitantly, alkaline phosphatase abolished the neuroprotective effect of those growth factors upon oxygen and glucose deprivation and upon staurosporine-induced cell death. For those studies, we were using primary cultures of cortical and hippocampal neurons from embryonic and neonatal rats. Incubation of bFGF with non-hydrolyzable ATP-gammaS resulted in phosphorylation and in neuroprotection resistant to alkaline phosphatase. We conclude that bFGF, NGF and BDNF undergo autophosphorylation on site(s) other than serine, threonine, tyrosine and/or ATP-binding, and that this binding of phosphate is essential for neuroprotection in vivo.