Degeneration of basal forebrain cholinergic neurons (BFCNs) contributes to cognitive dysfunction in Alzheimer's disease (AD) and Down's syndrome (DS). We used Ts65Dn and Ts1Cje mouse models of DS to show that the increased dose of the amyloid precursor protein gene, App, acts to markedly decrease NGF retrograde transport and cause degeneration of BFCNs. NGF transport was also decreased in mice expressing wild-type human APP or a familial AD-linked mutant APP; while significant, the decreases were less marked and there was no evident degeneration of BFCNs. Because of evidence suggesting that the NGF transport defect was intra-axonal, we explored within cholinergic axons the status of early endosomes (EEs). NGF-containing EEs were enlarged in Ts65Dn mice and their App content was increased. Our study thus provides evidence for a pathogenic mechanism for DS in which increased expression of App, in the context of trisomy, causes abnormal transport of NGF and cholinergic neurodegeneration.

The survival, differentiation, and maintenance of responsive neurons are regulated by nerve growth factor (NGF), which is secreted by the target and interacts with receptors on the axon tip. It is uncertain how the NGF signal is communicated retrogradely from distal axons to neuron cell bodies. Retrograde transport of activated receptors in endocytic vesicles could convey the signal. However, little is known about endocytosis of NGF receptors, and there is no evidence that NGF receptors continue to signal after endocytosis. We have examined early events in the membrane traffic of NGF and its receptor, gp140(TrkA) (TrkA), in PC12 cells. NGF induced rapid and extensive endocytosis of TrkA in these cells, and the receptor subsequently moved into small organelles located near the plasma membrane. Some of these organelles contained clathrin and alpha-adaptin, which implies that TrkA is internalized by clathrin-mediated endocytosis. Using mechanical permeabilization and fractionation, intracellular organelles derived from endocytosis were separated from the plasma membrane. After NGF treatment, NGF was bound to TrkA in endocytic organelles, and TrkA was tyrosine-phosphorylated and bound to PLC-gamma1, suggesting that these receptors were competent to initiate signal transduction. These studies raise the possibility that NGF induces formation of signaling endosomes containing activated TrkA. They are an important first step in elucidating the molecular mechanism of NGF retrograde signaling.

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Emerging evidence suggests that the p75 neurotrophin receptor (p75 NTR ) mediates cell death; however, it is not known whether p75NTR negatively regulates other neuronal phenotypes. We found that mice null for p75 NTR displayed highly significant increases in the size of basal forebrain cholinergic neurons, including those that are TrkA-positive. Cholinergic hippocampal target innervation also was increased significantly. Activity of the cholinergic neurotransmitter synthetic enzyme choline acetyltransferase (ChAT) was increased in both the medial septum and hippocampus. Upregulation of these cholinergic features was not associated with increased basal forebrain or hippocampal target NGF levels. In contrast, striatal cholinergic neurons, which do not express p75 NTR , showed no difference in neuronal number, size, or ChAT activity between wild-type and p75NTR null mutant mice. These findings indicate that p75 NTR negatively regulates cholinergic neuronal phenotype of the basal forebrain cholinergic neurons, including cell size, target innervation, and neurotransmitter synthesis.

Down syndrome (trisomy 21) is the most common cause of mental retardation in children and leads to marked deficits in contextual learning and memory. In rodents, these tasks require the hippocampus and are mediated by several inputs, particularly those originating in the locus coeruleus. These afferents mainly use norepinephrine as a transmitter. To explore the basis for contextual learning defects in Down syndrome, we examined the Ts65Dn mouse model. These mice, which have three copies of a fragment of mouse chromosome 16, exhibited significant deficits in contextual learning together with dysfunction and degeneration of locus coeruleus neurons. However, the postsynaptic targets of innervation remained responsive to noradrenergic receptor agonists. Indeed, despite advanced locus coeruleus degeneration, we were able to reverse contextual learning failure by using a prodrug for norepinephrine called l-threo-3,4-dihydroxyphenylserine, or xamoterol, a beta(1)-adrenergic receptor partial agonist. Moreover, an increased gene dosage of App, in the context of Down syndrome, was necessary for locus coeruleus degeneration. Our findings raise the possibility that restoring norepinephrine-mediated neurotransmission could reverse cognitive dysfunction in Down syndrome.