MMP9 and Alzheimer disease: Altered NGF maturation, characterised by pro-NGF accumulation and reduced presence of mature NGF, together with increased NGF degradation, caused by enhanced activity of the matrix metalloprotease MMP9 in AD, have been suggested to impair the trophic support of basal forebrain cholinergic neurons, as demonstrated by the reduction of cortical and hippocampal cholinergic synapses in transgenic AD rats.