The amyloid beta-peptide (Ab) of 39 to 43aa (Ab40, Ab42, Ab43) is constitutively produced in brain upon proteolysis of the b-amyloid precursor protein (APP). In the young and healthy humans, Ab is rapidly catabolized before it can be deposited in the brain. However, upon aging or the onset of familial Alzheimer's Disease, alterations in either synthesis or degradation/clearance of Ab may contribute to amyloid depositions in the brain. Ab is susceptible to a number of in vivo and in vitro proteases like cathepsin-D and M-13 metalloproteases. The M-13 family includes several members of zinc-dependent proteases like damage-induced neuronal endopeptidase (DINE), product of phosphate regulating gene with homologies to endopeptidases on the X chromosome (PHEX), neutral endopeptidase 24.11 or neprilysin (NEP) and neprilysin-like proteases (NEPLs). NEPLs (alpha, -beta, gamma) arise from the alternative splicing of a single NELPS gene and show ~54% sequence homology. M13 members are generally type II transmembrane proteins consisting of a single polypeptide chain with Zinc binding HEXXH motif, a short cytoplasmic tail, a transmembrane segment and an extra-cytoplasmic domain containing the enzyme active site.
NEPL-b (variously called SEP/NL1/NEPII) is type II transmembrane enzyme containing a single polypeptide chain of 765aa (~110kD) with cytoplasmic and transmembrane domains and a large extracellular C-terminal core containing the peptidase active site. The aa sequence is 65.1 % identical to mouse NEP. In comparison to NEPL-a, NEPL-b has an additional and unique sequence of 23-aa (41-63 aa) after the TM domain. There are two splice variants of NEP-b; a secreted isoform of 126kD containing a 23 aa secretion signal sequence and a membrane associated isoform of 110kD. The secreted isoform is much more glycosylated than the membrane isoform. Testis is the only tissue where the soluble/secreted isoform of NEP-b is predominant. Unlike NEP, NEPL-b has no proteolytic activity to Ab however both enzymes can cleave Leu5-enkephalin.
