Related Experiment Videos
The predominant form of the amyloid beta-protein precursor in human brain is protease nexin 2
W E Van Nostrand1, J S Farrow, S L Wagner
1Department of Microbiology, University of California, Irvine 92717.
Abstract:
The amyloid beta protein and the amyloid beta-protein precursor (APP) are major constituents of senile plaques and cerebrovascular deposits in patients with Alzheimer disease and Down syndrome. Most human tissues contain mRNA that encodes forms of APP that contain the Kunitz protease inhibitor (KPI+) domain. A major 120-kDa protein corresponding to this KPI+ mRNA is also found in these tissues. This protein is identical to the protease inhibitor protease nexin 2. Brain contains an additional mRNA species that encodes a form of APP that lacks the KPI domain (KPI-). This latter mRNA has been suggested to encode a 105-kDa KPI- form of APP protein also found in brain. Using protease inhibitory functional assays, we show that both the 105-kDa and 120-kDa APP proteins in normal and Alzheimer disease brain contain the KPI domain. Moreover, KPI domain-specific precipitation assays reveal that KPI- forms of APP protein represent less than 14% of total brain APP. Lastly, an enriched fraction from total brain homogenate contains proteolytic activity that can process the purified 120-kDa KPI+ form of APP into a 105-kDa form, resulting in a high-molecular-mass doublet identical to that seen in brain. These findings indicate that although KPI- APP mRNA is abundant in brain, little corresponding protein is present. Thus, KPI+ APP protein (equivalent to protease nexin 2) is the predominant form of APP in human brain.
Insights
Alzheimer disease and Down syndrome share amyloid beta-protein precursor (APP) in brain deposits. KPI+ APP protein, also known as protease nexin 2, is the predominant form in human brain, not KPI- APP.
Area of Science:
- Neuroscience
- Molecular Biology
- Biochemistry
Background:
- Amyloid beta-protein precursor (APP) and amyloid beta protein are key in Alzheimer disease and Down syndrome pathology.
- Both Kunitz protease inhibitor-positive (KPI+) and KPI-negative (KPI-) forms of APP are found in human tissues, with distinct mRNA species encoding them.
Purpose of the Study:
- To determine the predominant form of APP protein in human brain.
- To investigate the presence and processing of KPI+ and KPI- forms of APP in normal and Alzheimer disease brains.
Main Methods:
- Protease inhibitory functional assays to identify KPI domains in APP proteins.
- KPI domain-specific precipitation assays to quantify KPI- APP forms.
- Enriched brain homogenate fractions to assess proteolytic processing of APP.
Main Results:
- Both 105-kDa and 120-kDa APP proteins in normal and Alzheimer disease brains contain the KPI domain.
- KPI- forms of APP constitute less than 14% of total brain APP.
- Proteolytic activity in brain homogenates processes 120-kDa KPI+ APP into a 105-kDa form.
Conclusions:
- KPI+ APP protein, equivalent to protease nexin 2, is the predominant form of APP in the human brain.
- Despite abundant KPI- APP mRNA, the corresponding protein is present in minimal amounts.
- APP processing in the brain generates the observed 105-kDa form from the 120-kDa KPI+ precursor.