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Beta A4-amyloid protein precursor mRNA isoforms without exon 15 are ubiquitously expressed in rat tissues including
R Sandbrink1, C L Masters, K Beyreuther
1Center for Molecular Biology Heidelberg (ZMBH), University of Heidelberg, Germany.
Abstract:
The beta A4-amyloid protein precursor (APP), the source of the beta A4-amyloid deposits found in Alzheimer brains, constitutes a family of transmembrane glycoproteins generated by alternative splicing. While exon 7 and exon 8 are well known to be alternatively spliced, APP mRNA isoforms without exon 15 were only recently identified in leukocytes and rat brain microglial cells and therefore denoted as L-APP mRNAs. In order to perform a detailed analysis of individual L-APP mRNA isoforms in perfused rat tissues, we developed a quantitative polymerase chain reaction assay from reverse transcribed RNA allowing us to analyze the alternatively spliced region in between exon 6 and 16. In all peripheral tissues examined, L-APP mRNA isoforms were detected comprising between 25% (skeletal muscle) and about 70% (aorta, pancreas) of total APP transcripts. All four possible APP mRNA isoforms without exon 15 were shown to exist, i.e. L-APP752, L-APP733, L-APP696, and L-APP677. L-APP expression in the central nervous system (approximately 4% of total APP mRNA) was then studied in more detail by analyzing different brain regions and tissues and primary cultured brain cells. The only cell type which was shown not to express L-APP mRNA to a detectable level is the neuronal cell. Ubiquitous expression of APP mRNAs lacking exon 15 except for neurons indicates an important function in non-neuronal cells and is especially remarkable since neurons are the primarily affected cells in Alzheimer's disease.
Insights
Researchers identified novel Alzheimer
Area of Science:
- Neuroscience
- Molecular Biology
- Genetics
Background:
- Alzheimer's disease is characterized by beta A4-amyloid deposits in the brain.
- The beta A4-amyloid precursor protein (APP) is the source of these deposits.
- APP is a transmembrane glycoprotein produced through alternative splicing of its mRNA.
Purpose of the Study:
- To investigate the expression and function of L-APP mRNA isoforms, which lack exon 15.
- To analyze the distribution of L-APP mRNA isoforms in various rat tissues and cell types.
- To understand the significance of L-APP expression in non-neuronal cells, particularly in the context of Alzheimer's disease.
Main Methods:
- Development of a quantitative polymerase chain reaction (qPCR) assay.
- Analysis of the alternatively spliced region between exon 6 and 16 of APP mRNA.
- Examination of perfused rat tissues, brain regions, and primary cultured brain cells.
Main Results:
- L-APP mRNA isoforms were detected in all peripheral tissues examined, ranging from 25% to 70% of total APP transcripts.
- All four possible L-APP mRNA isoforms (L-APP752, L-APP733, L-APP696, L-APP677) were identified.
- L-APP mRNA expression in the central nervous system was approximately 4% of total APP mRNA, with neurons being the only cell type that did not express detectable L-APP mRNA.
Conclusions:
- The ubiquitous expression of L-APP mRNA in non-neuronal cells suggests a crucial, yet uncharacterized, function.
- The absence of L-APP mRNA in neurons is noteworthy, given their primary involvement in Alzheimer's disease.
- Further research into L-APP's role in non-neuronal cells may offer new insights into Alzheimer's disease pathogenesis.