Ap Degrading Enzymes

Neprilysin (NEP), a plasma membrane glycoprotein of the neutral zinc metalloendopeptidase family, is a major Ap-degrading enzyme in the brain. Viral vectors have been used to express NEP for reduction of Ap deposition. Park et al. [582] produced recombinant soluble NEP from insect cells using an NEP expression vector, which was administered by intracerebral injection into AD mice, resulting in significantly reduced accumulation of Ap and improved behavioral performance. 

Figure 6.9. Combined inhibitors of enkephalin degrading enzymes. ENK (enkephalinase). AP (aminopeptidases), APM, (aminopeptidase M), DAP (dipeptidylaminopeptidase). D,R,S (stereochemical configuration), ° (membrane-bound aminopeptidase). (soluble aminopeptidase).

It was shown that resveratrol lowered the levels of secreted and intracellular amyloid-P (AP) peptides produced from different cell lines [66]. Resveratrol did not inhibit AP production because it has no effect on the AP-producing enzymes P- and y-secretases but promotes intracellular degradation of AP via a mechanism that involves the proteasome. The resveratrol-induced decrease of AP was shown to be prevented by several selective proteasome inhibitors and by siRNA-directed silencing... 

The discovery of this family of E3 enzymes started from the studies on the targeted degradation of the p53 tumor suppressor protein. Ubiquitinylation and degradation of p53 can be mediated by the papillomavirus E6 oncoprotein (see below) in collaboration with a further protein, E6-AP (E6 associated protein). E6-AP was the first member of a large family of E3 enzymes, the Hect (homologous to E6-AP C-terminus ) domain family. These proteins contain an essential active site Cys residue near the C-terminus and one or several WW domains ( see Chapter 8.2.6). 

Fig. 2.17. Degradation of the tumor suppressor protein p53 by the ubiquitin-proteosome system. The oncoprotein E6 of the human papilloma virus (HPV) forms a specific complex with the p53 protein and can thus induce the degradation of p53. The E6-p53 complex is recognized by E6-AP, a E3 enzyme of the ubiquitin pathway, as a target protein, whereby a ubiquitin residue is transferred to a lysine residue of p53. In this process, the E6 protein serves as the recognition element for ubiquitin hgation of p53.

In the kiss-and-run mode exocytosis and endocytosis are directly coupled to each other, while in the case of classical complete vesicle fusion, exocytosis and slow clathrin-mediated endocytosis are timely and spatially separated. However, it appears that also in the latter case exocytosis and endocytosis occur coordinated, as both are stimulated by an increase of the cytoplasmic calcium concentration. It has been shown that after calcium entry the enzyme phospho-inositol-5 kinase Iy, which is enriched in the synapse, catalyzes the synthesis of phosphatidylinos-itol (4,5)-bisphosphate and that this mechanism is important for synaptic vesicle trafficking (Di Paolo et al. 2004). As many proteins involved in clathrin-mediated endocytosis are recruited to the plasma membrane by binding to phosphatidylinosi-tol (4,5)-bisphosphate (e.g., amphiphysin, dynamin, epsin, AP-180, and AP-2) it is attractive to speculate that elevated levels of calcium mediate the recruitment of en-docytic proteins to the plasma membrane by this mechanism. The increased level of phosphatidylinositol (4,5)-bisphosphate could be in part degraded by synaptojanin that thereby initiates the disassembly of the clathrin coat. Hence, calcium-induced transient increases in the level of phosphatidylinositol (4,5)-bisphosphate appear to play a central role for coupling exocytosis to clathrin-mediated endocytosis. In addition, it has been demonstrated that calcium also leads to the dephosphorylation of endocytic proteins as amphiphysin, dynamin, and synaptojanin, which in vitro is important for efficient coat assembly (Cousin and Robinson 2001). 

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