Molecular misreading of the ␤-amyloid precursor protein (APP) gene generates mRNA with dinucleotide deletions in GAGAG motifs. The resulting truncated and partly frameshifted APP protein (APP ؉1 ) accumulates in the dystrophic neurites and the neurofibrillary tangles in the cortex and hippocampus of Alzheimer patients. In contrast, we show here that neuronal cells transfected with APP ؉1 proficiently secreted APP ؉1 . Because various secretory APP isoforms are present in cerebrospinal fluid

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... ), this study aimed to determine whether APP ؉1 is also a secretory protein that can be detected in CSF. Post-mortem CSF was obtained at autopsy from 50 non-demented controls and 122 Alzheimer patients; all subjects were staged for neuropathology (Braak score). Unexpectedly, we found that the APP ؉1 level in the CSF of non-demented controls was much higher (1.75 ng/ml) than in the CSF of Alzheimer patients (0.51 ng/ml) (p < 0.001), and the level of APP ؉1 in CSF was inversely correlated with the severity of the neuropathology. Moreover the earliest neuropathological changes are already reflected in a significant decrease of the APP ؉1 level in CSF. These data show that APP ؉1 is normally secreted by neurons, preventing intra-neuronal accumulation of APP ؉1 in brains of nondemented controls without neurofibrillary pathology. Alzheimer's disease (AD) 1 is a progressive neurodegenerative disease and the most common form of dementia in aged populations (1). The major constituent of the extracellular plaques in the brains of AD patients is amyloid ␤ (A␤), which is cleaved from the ␤-amyloid precursor protein (APP) by ␤ and ␥ secretases. Patients suffering from the hereditary forms of AD either carry a mutation in the APP gene or in one of the presenilin genes. These mutations cause an alteration in the proteolytic processing of APP, resulting in the formation of more A␤ 40 and A␤ 42, which are both prone to aggregate and precipitate in the plaques (2, 3). Genomic APP mutations are not the only source of aberrant APP proteins in AD. We (4) and others (5) have reported that small frameshift mutations occur in APP transcripts near short simple repeats. The observed dinucleotide deletions, such as ⌬GA, in the GAGAG motif of exon 9 or 10 of APP, result in the translation of an aberrant APP protein, i.e. APP ϩ1 , which accumulates in the neurofibrillary tangles, neuropil threads, and dystrophic neurites of the neuritic plaques of AD and Down syndrome patients (4, 6). APP ϩ1 is a truncated APP protein of 348 amino acids with a wild-type N terminus and an aberrant C terminus translated in the ϩ1 reading frame. The different isoforms of full-length APP are type I transmembrane proteins, which are proteolytically cleaved by secretases (7, 8) to form secretory APPs and A␤ 40 or A␤ 42 and p3 (9, 10). These secretory APP proteins, sAPP␣ and sAPP␤ (11-13), and A␤ (14) are detectable in cerebrospinal fluid (CSF) as well as in human brain homogenates (15). The present study focuses on APP ϩ1 , which consists of the N-terminal 329 amino acids of the APP 695 , including the signal peptide, and a unique 19amino acid C terminus (Fig. 1) . A major difference between APP ϩ1 and APP, except for their distinct C termini, is the lack of the membrane anchor (amino acids 625-648 of APP 695 ) and the lack of the 〈␤ sequence in APP ϩ1 . The presence of the signal peptide and the absence of the membrane anchor make it very likely that APP ϩ1 is also a secretory protein. Indeed, it has been shown that an APP ϩ1 -enhanced green fluorescent protein fusion protein is readily secreted from rat neuronal cell lines (16). The aim of the present study was to determine whether the endogenous human APP ϩ1 is a secretory protein and can be detected in human CSF. EXPERIMENTAL PROCEDURES Antibodies-The AMY6 peptide (YNVPGHERMGRGRTSSKELA) represents the final 19 amino acids of APP ϩ1 ⌬GA exon 9 (see Fig. 1 ). The peptide was coupled to thyroglobulin by glutaraldehyde, and a rabbit was immunized with a mixture of the coupled peptide and Freund's complete adjuvant (1:1). Several bleedings were collected, and the immunoreactivity of the serum was tested with a spot blot. To the N terminus of the AMY6 peptide an additional tyrosine (Y) was added to allow iodination. 22C11 is a monoclonal antibody directed against