Membrane secretases

Stromelysin 3 (MMP11) Endothelin Convertase Membrane Secretases ... 

APP undergoes proteolytic processing by several secretases. First, the bulk of the ectodomain needs to be removed by membrane-bound a- or (3-secretases leading to secreted forms of APP and membrane-bound C-terminal fragments a-CTF or (3-CTF, respectively. Regulated intramembrane proteolysis (RIP) of the (3-CTF by y-secretase occurs only after ectodomain shedding and releases the A(3 pqrtide from the membrane (Fig. 2). 

Alzheimer s Disease. Figure 2 A(3 is derived from the APP by the sequential action of proteolytic activities exerted by (3- and y-secretases. APP-CTF is (C99) produced after cleavage of the APP by (3-secretase and represents the substrate of the y-secretase. The yellow box marks membrane embedded amino acid residues of A(3 peptide. Scissors represent the main cleavage sites of (3- and y-secretase, e.g. the e- and y-cleavages at positions 49,46, 42, 40 and 38. 

APP is normally cleaved within the A/1 sequence by an unidentified protease, so-called a-secretase, so that most of the extracellular APP is released in a soluble form into the extracellular fluid (see Checler 1995). When jS-amyloid is formed another protease (jS) splits APP so that the complete A/1 sequence persists at the extracellular end of the remaining membrane and intracellular APP chain. This is then cleaved by anaother protease (y-secretase) to release the jS-amyloid (Fig. 18.5). Potentiation of a-or blockage of jS- and y-secretase could reduce the production of A/1 which becomes insoluble and is precipitated (see Hardy 1997). 

Figure 18.5 Schematic representation of possible cleavage sites of APP by a, and y-secretase and the production of j5-amyloid protein. (I) This shows the disposition of APP molecules in 695, 751 and 770 amino-acid chain lengths. Much of it is extracellular. The /1-amyloid (A/I4) sequence is partly extracellular and partly in the membrane. (II) An enlargement of the /1-amyloid sequence. (Ill) Normal cleavage of APP by a-secretase occurs in the centre of A/I4 sequence to release the extracellular APP while the remaining membrane and intracellular chain is broken down by y-secretase to give two short proteins that are quickly broken down. (IV) In Alzheimer s disease ji rather than a-secretase activity splits off the extracellular APP to leave the full AP4 sequence remaining attached to the residual membrane and intracellular chain. 42/43 amino acid )S-amyloid sequence is then split off by y-secretase activity...

These must be worthwhile objectives and the recent identification by a number of research groups (see Skovronsky and Lee 2000 for description and details) of P-secretase as the membrane-bound aspartyl protease (RACE), S-site APP cleaving enzyme, paves the way for developing possible chemical inhibitors of its activity for experimental and clinical evaluation, although that remains for the future. 

Yan, R., Bienkowski, M. J., Shuck, M. E. et al. Membrane-anchored aspartyl protease with Alzheimer s disease P-secretase activity. Nature 402 533-537,1999. 

Numerous y-secretase inhibitors featuring sulfonamide- and sulfone-based scaffolds have been disclosed. Bicyclononane thiophene sulfonamide 40 reduced brain Ap in transgenic mice by 50% after a dose of 100 mg/kg [100]. High potency (A p IC50 = 0.5 nM) and improved oral activity (ID50 = 17 mg/kg) was found in a series of related sulfamides represented by 41 [101]. Tetrahydroquinoline (42) and piperidine (43-44) sulfonamides have been developed [102-104]. Elaboration of the piperidine series with the cyclopropyl substituent present in 44 improved in vitro potency (Aft IC50 = 2.1 nM in membrane assay) and in vivo activity in transgenic mice (plasma Ap = 2% of control after oral dose of 30 mg/kg). Reductions of A p in the cortex were reported to be comparable to those observed in plasma. 

The axonal transport of APP in neurons is mediated by the direct binding of APP to the kinesin light chain subunit of kinesin I. An axonal membrane compartment contains APP, P-secretase, and PSl, and the fast anterograde axonal transport of this compartment is mediated by APP and kinesin I. APP proteolysis in this... 

Kamal, A., Almenar-Queralt, A., LeBlanc, J.F., Roberts, E.A., Goldstein, L.S.B. (2001) Kinesin-mediated axonal transport of a membrane compartment containing beta-secretase and presenilin-1 requires APP. Nature, 414, 643-648. 

Wahrle, S., Das, P., Nyborg, A.C., et al. (2002) Cholesterol-dependent gamma-secretase activity in buoyant cholesterol-rich membrane microdomains. Neurobiol. Dis., 9, 11-23. 

Vetrivel, K.S., Cheng, H., Lin, W., et al. (2004) Association of gamma-secretase with Upid rafts in post-Golgi and endosome membranes. J. Biol. Chem., 279, 44945-44954. 

A protein known as the amyloid precursor protein (APP) spans the plasma membrane of the neurone. It possesses an extracellular domain but its function is unknown. The extracellular domain is partially hydrolised by proteolytic enzymes, known as secretases. One of the products is the amyloid peptide, of which there are two forms. The larger form, contains 42 amino acids and readily polymerises to form plaques in the extracellular space, damaging the neurones. Some sufferers possess a mutated form of the APP protein which more readily produces the larger peptide upon proteolysis, so that more toxic plaques are produced. It is the progressive accumulation of these plaques that is considered to be one cause of Alzheimer s disease. 

Strong evidence suggests that the main constituent of the plaque, P-amyloid peptide (AP), exerts a prominent role in the cause, initiation and progression of AD. Thus AD appears to arise from the abnormal deposition of a protein. Ap is derived from proteolytic cleavage of amyloid precursor protein (APP), an integral membrane protein. APP is cleaved by the sequential actions of three unique proteases, called a-, p-, and y-secretases. Each secretase cleaves at a unique site (see figure 8.7). 

A relatively small minority of APP molecules enter the p-secretase pathway in which p-secretase cleaves APP and releases a soluble fragment, sAPPp. The C-terminal membrane-bound C99 peptide is then cleaved by y-secretase within the transmembrane domain, and two major isoforms of 40 and 42 amino acid lengths with different C-termini, Ap40 and Ap42, are generated. Based on the amino acid sequence, p-secretase is predicted to be a type I transmembrane protein with the active site on the lumenal side of... 

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