Using the same anti-peptide sera we can detect expression of endogenous Asp 2 in SH-SY5Y cells stably expressing the 695 isoform of APP (SH-SY5Y APP-695) and in COS-7 cells expressing the 751 isoform of APP (COS-7 APP-751). The level of Asp 2 is increased upon transient transfection with the protein.
APP clearly localizes to the Golgi/endoplasmic reticulum region as revealed by distinctive juxtanuclear staining and a more generalized reticular staining throughout the cell (Figs. 6b and 6g). Asp 2 shows essentially the same subcellular distribution as revealed by simultaneous detection of myc-tagged Asp 2 and APP in COS-7 APP-751 cells (compare Figs. 6f and 6g), and merging of the confocal images for APP and Asp 2 indicates colocalization (Fig. 6h).
Transient transfection of SH-SY5Y APP-695 cells with Asp 2 (Fig. 2a) results in a significant increase in the secretion of sAPPb (Fig. 2b) consistent with Asp 2 being b-secretase. To demonstrate that this increase in sAPPb is linked to the proteolytic activity of Asp 2, we mutated each of the proposed catalytic aspartic residues at positions 25 and 215 (determined by comparison with the position of the known catalytic aspartyl residues in pepsin) to asparagine. Both mutants and the wild-type Asp 2 are expressed to similar levels (Fig. 2a).
However, expression of either of the Asp 2 mutants does not produce the increase in the secretion of sAPPb (Fig. 2b) seen for wild-type Asp 2. In contrast to this clear effect on sAPPb, Asp 2 has no effect on the secretion of soluble APPa or on full-length APP in the cell (data not shown).
However, while there is a significant increase in the production of CTFb in the presence of wild-type Asp 2, there is no increase in the presence of the mutant enzymes (Fig. 4b).
In contrast to the expression of Asp 2, cathepsin D does not cause an increase in the secretion of sAPPb (Fig. 2d).
We have examined the distribution of Asp 2 in AD hippocampus using a polyclonal antiserum raised to a peptide sequence derived from Asp 2 (see Experimental Methods).We see clear neuronal staining but there is no staining associated with astrocytes, microglia, or oligodendrocytes (Figs. 5a and 5b).
BEL Commons is developed and maintained in an academic capacity by Charles Tapley Hoyt and Daniel Domingo-Fernández at the Fraunhofer SCAI Department of Bioinformatics with support from the IMI project, AETIONOMY. It is built on top of PyBEL, an open source project. Please feel free to contact us here to give us feedback or report any issues. Also, see our Publishing Notes and Data Protection information.
If you find BEL Commons useful in your work, please consider citing: Hoyt, C. T., Domingo-Fernández, D., & Hofmann-Apitius, M. (2018). BEL Commons: an environment for exploration and analysis of networks encoded in Biological Expression Language. Database, 2018(3), 1–11.