Differential effect of mature and precursor forms of NGF on phenotype and cholinergic function of cultured rat basal forebrain neurons

The pro- forms of neurotrophins have been thought for a long time as exclusively necessary for the generation of the biologically active mature proteins. Recently, it has been demonstrated that proNGF, abundantly expressed in the brain, retains a biological activity in vitro, but little and contrasting informations have been produced about its effects on neuronal cells. Growing interest has been shown for this molecule, due to its possible involvement in AD pathogenesis In this work we studied the characteristics and biological functions of the precursor form of NGF comparing its effects to that of the mature form. To this aim we analyzed the effects of recombinant proNGF WT (cleavable), MUT (resistant to cleavage) and mature NGF on phenotype and function of basal forebrain cholinergic neurons (BFCNs), one of the few neuronal populations expressing throughout life the whole complement of NGF and proNGF receptors (p75, TrkA and sortilin). The comparison between NGF and proNGFs effects showed that all treatments had no effect on neuronal death/survival and/or differentiation, but had a similar TrkA-dependent effect on the soma size increase, and a TrkA- and p75-dependent ACh release enhancement. Moreover, both NGF and proNGFs increased p75 expression, but through a different activation of p75 or TrkA. Finally, they differently affected cholinergic markers expression (ChAT and CHT) and TrkA expression. Of interest, proNGF MUT increased TrkA expression, highlighting a possible therapeutic use of the molecule in preventing the decrease of TrkA levels observed in early stages of AD. Our results also demonstrated that p75 had an important role in controlling both neurotrophin-induced and constitutive ACh release, suggesting p75 receptor as a possible therapeutic target for the prevention of cholinergic hypofunction typical of AD. In conclusion, this study describes characteristics and biological functions of proNGF and p75 useful to understand their role in physiological and their possible involvement in AD pathological conditions.