Frameshifted β-Amyloid Precursor Protein (APP+1) Is a Secretory Protein, and the Level of APP+1 in Cerebrospinal Fluid Is Linked to Alzheimer Pathology

Abstract

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 (CSF), 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 t...

Frame‐shifted amyloid precursor protein found in Alzheimer's disease and Down's syndrome increases levels of secreted amyloid β40

“…2d, lanes 3 and 6) could be detected in the culture medium. These results are in agreement with our recent publication in which we show that APP +1 is present in the culture medium of transfected neuronal SH-SY5Y cells and in human cerebral spinal fluid (CSF) (Hol et al 2003). Therefore, it can be concluded that also in non-neuronal HEK293 cells APP +1 is a secretory protein.…”

“…To exclude any cell type specific effects, transfection experiments were repeated on human neuronal cell-lines, i.e. SH-SY5Y (Hol et al 2003), SK-N-SH and CHP-100, which gave similar results (data not shown). From these experiments we concluded that APP 695 and APP +1 are highly expressed from our constructs in HEK293 cells.…”

“…This APP +1 located in beaded fibres is reminiscent of secretion and is probably synthesised, transported and released as shown in cell-lines (Hersberger et al 2001). Moreover, recent studies on human post-mortem CSF show that APP +1 levels are significantly higher in CSF of non-demented control patients when compared to AD patients (Hol et al 2003). In addition, a pilot study on human lumbar puncture CSF also showed that levels of APP +1 are significantly higher in CSF of control patients compared to AD patients (Schoonenboom et al 2002).…”

“…We reported that in short simple repeats in APP transcripts, a dinucleotide deletion could occur, such as DGA in the GAGAG motif in exon 9 or 10 of APP, resulting in the translation of a frame-shifted and truncated APP protein (APP +1 ). This APP +1 , which is, in non-demented controls without any pathology, a secretory protein (Hol et al 2003), accumulates in neurofibrillary tangles and in the neuritic plaques in the hippocampus and temporal cortex of patients suffering from Alzheimer's disease pathology, including those with Down's syndrome (DS) (Hol et al 1998;van Leeuwen et al 1998). The presence of APP +1 in these hallmarks suggests that it could be involved in the process of neurodegeneration contributing to hereditary and non-hereditary forms of Alzheimer's pathology.…”

Misframed Proteins and Neurodegeneration: A Novel View on Alzheimer’s and Parkinson’s Diseases

“…APP +1 accumulates in the neurofibrillary tangles and neuropil threads of AD and Down syndrome patients [4] . In cerebrospinal fluid, APP +1 correlated negatively with increasing Braak stages [20] . This means that APP +1 is an additional biomarker for the early diagnosis of AD.…”