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The presenilin-1 (PS-1) gene on chromosome 14 carries mutations which cosegregate with early-onset familial Alzheimer's disease. We quantified PS-1 mRNA in post-mortem mid-temporal and superior frontal cortices from 14 Alzheimer's disease subjects, 9 non-demented controls and 5 subjects with other neurological diseases using solution hybridisation-RNase protection assay. APP and APLP2 mRNAs had previously been quantified in these samples (Johnston et al. (1996) Mol. Brain Res., in press) and subjects were apolipoprotein E (APOE) genotyped. There were no significant differences between PS-1 mRNA levels per pg total RNA in mid-temporal or superior frontal cortices of the Alzheimer's disease subjects, compared to controls. PS-1 mRNA levels corresponded to 10% of total APP and 30% of APLP2 mRNA levels, and were not significantly affected by age, post-mortem delay, tissue pH, or APOE genotype. PS-1 mRNA showed significant positive correlations with APP and APLP2 mRNA levels in mid-temporal cortex and with APP mRNA in superior frontal cortex. This may reflect a co-regulation of the expression of these genes, or the fact that they are expressed in similar neuronal populations.

The human amyloid precursor-like protein 2 (APLP2) is a member of the Alzheimer's disease amyloid precursor protein (APP) gene family. The human APLP2 ectodomain (sAPLP2) was expressed in the yeast Pichia pastoris and the recombinant sAPLP2 was purified from the culture medium in a single step by metal-chelating Sepharose chromatography. The neuritotrophic activity of APLP2 was compared to the APP isoforms sAPP695 and sAPP751 on chick sympathetic neurones. APLP2 had neurite outgrowth-promoting activity similar to that of the APP isoforms. This suggests that APP and APLP2 have a similar or related role and supports the idea of a redundancy in function between the APP-gene family proteins.

Amyloid beta-peptide (Abeta), which plays a central role in Alzheimer's disease, is generated by presenilin-dependent gamma-secretase cleavage of beta-amyloid precursor protein (betaAPP). We report that the presenilins (PS1 and PS2) also regulate Abeta degradation. Presenilin-deficient cells fail to degrade Abeta and have drastic reductions in the transcription, expression, and activity of neprilysin, a key Abeta-degrading enzyme. Neprilysin activity and expression are also lowered by gamma-secretase inhibitors and by PS1/PS2 deficiency in mouse brain. Neprilysin activity is restored by transient expression of PS1 or PS2 and by expression of the amyloid intracellular domain (AICD), which is cogenerated with Abeta, during gamma-secretase cleavage of betaAPP. Neprilysin gene promoters are transactivated by AICDs from APP-like proteins (APP, APLP1, and APLP2), but not by Abeta or by the gamma-secretase cleavage products of Notch, N- or E- cadherins. The presenilin-dependent regulation of neprilysin, mediated by AICDs, provides a physiological means to modulate Abeta levels with varying levels of gamma-secretase activity.

Synaptic dysfunction and synapse loss are key features of Alzheimer’s pathogenesis. Previously, we showed an essential function of APP and APLP2 for synaptic plasticity, learning and memory. Here, we used organotypic hippocampal cultures to investigate the specific role(s) of APP family members and their fragments for dendritic complexity and spine formation of principal neurons within the hippocampus. Whereas CA1 neurons from APLP1-KO or APLP2-KO mice showed normal neuronal morphology and spine density, APP-KO mice revealed a highly reduced dendritic complexity in mid-apical dendrites. Despite unaltered morphology of APLP2-KO neurons, combined APP/APLP2-DKO mutants showed an additional branching defect in proximal apical dendrites, indicating redundancy and a combined function of APP and APLP2 for dendritic architecture. Remarkably, APP-KO neurons showed a pronounced decrease in spine density and reductions in the number of mushroom spines. No further decrease in spine density, however, was detectable in APP/APLP2-DKO mice. Mechanistically, using APPsα-KI mice lacking transmembrane APP and expressing solely the secreted APPsα fragment we demonstrate that APPsα expression alone is sufficient to prevent the defects in spine density observed in APP-KO mice. Collectively, these studies reveal a combined role of APP and APLP2 for dendritic architecture and a unique function of secreted APPs for spine density.

Recent studies using animal models indicated that the members of the amyloid precursor protein (APP) gene family are important for the formation, maintenance, and plasticity of synapses. Despite this, the specific role of the APP homologues APLP1 and APLP2 within the CNS and PNS is still poorly understood. In contrast to the subtle phenotypes of single mutants, double knockout mice (DKO) lacking APP/APLP2 or APLP1/APLP2 die within the first day after birth. Whereas APP/APLP2-DKO mice show severe deficits of neuromuscular morphology and transmission, the underlying cause of lethality of APLP1/APLP2-DKO mice remains unclear. Since expression of both proteins was confirmed by in situ hybridization, we aimed to test the role of APLP1/APLP2 in the formation and maintenance of synapses in the brainstem, and assessed a potential dysfunction of the most vital central neuronal network in APLP1/APLP2-DKO mice by analyzing the respiratory network of the medulla. We performed in vivo unrestrained whole body plethysmography in newborn APLP1/APLP2-DKO mice at postnatal day zero. Additionally, we directly tested the activity of the respiratory network in an acute slice preparation that includes the pre-Botzinger complex. In both sets of experiments, no significant differences were detected regarding respiratory rate and cycle variability, strongly arguing against central respiratory problems as the primary cause of death of APLP1/APLP2-DKO mice. Thus, we conclude that APLP1 and APLP2 are dispensable for the development of the network and the generation of a normal breathing rhythm.