Nerve growth factor metabolic dysfunction in Down's syndrome brains

Basal forebrain cholinergic neurons play a key role in cognition. This neuronal system is highly dependent on nerve growth factor (NGF) for its synaptic integrity and the phenotypic maintenance of its cell bodies. Basal forebrain cholinergic neurons progressively degenerate in Alzheimer's disease and Down's syndrome, and their atrophy contributes to the manifestation of dementia. Paradoxically, in Alzheimer's disease brains, the synthesis of NGF is not affected and there is abundance of the NGF precursor (proNGF). We have shown that this phenomenon is due to a deficit in NGF's extracellular metabolism which compromises proNGF maturation and exacerbates its subsequent degradation. We hypothesized that a similar imbalance should be present in Down's syndrome. Using a combination of quantitative reverse transcription-polymerase chain reaction, enzyme-linked immunosorbent assay, western blotting and zymography, we have investigated signs of NGF metabolic dysfunction in post-mortem brains from the temporal (n = 14), frontal (n = 34) and parietal (n = 20) cortex obtained from Down's syndrome subjects and age-matched controls (age range 31-68 years). We further examined primary cultures of human fetal Down's syndrome cortex (17-21 gestational age weeks). We report a significant increase in proNGF levels in human Down's syndrome brains, with a concomitant reduction in the levels of plasminogen and tissue plasminogen activator mRNA as well as an increment in neuroserpin expression; enzymes that partake in proNGF maturation. Down's syndrome brains also exhibited elevated zymogenic activity of MMP9, the major NGF-degrading protease. Our results indicate a failure in proNGF maturation in Down's syndrome brains and a likely enhanced proteolytic degradation of NGF, changes which can compromise the trophic support of basal forebrain cholinergic neurons. The alterations in proNGF and MMP9 were also present in cultures of Down's syndrome fetal cortex; suggesting that this trophic compromise may be amenable to rescue, before frank dementia onset. Our study thus provides a novel paradigm for cholinergic neuroprotection in Alzheimer's disease and Down's syndrome.