The neuropeptide-degrading enzyme NL1 is expressed in specific neurons of mouse brain

NL1 expression level in Nrx1β and the excitability of PV interneurons in mice with POCD

Effects of NL1 on the expression level of neurexin-1β (Nrx1β) and the excitability of parvalbumin (PV) interneurons in mice with postoperative cognitive dysfunction (POCD) were explored. Eighty 10-month-old C57BL/6 male mice were randomly divided into four groups: control group, control+empty vector group (Control+EV), anesthesia surgery+empty vector group (POCD+EV group) and anesthesia+NL1 overexpression group (POCD+NL1 group), (n=20 per group). The behavioral differences of mice in the four groups were analyzed by means of open field test and fear conditioning test. The expression levels of NL1, PV and Nrx1β were detected by western blot analysis, and the binding between NL1 and Nrx1β was determined by co-immunoprecipitation. The expression level of postsynaptic density protein 95 (PSD95) in hippocampus and changes in the excitability of PV interneurons were further detected. Control and Control+EV groups had no significant difference in each index (P>0.05). Compared with Control+EV group, the percentage of cued freezing time in POCD+EV group decreased significantly, while percentage of cued freezing time was significantly increased in POCD+NL1 group and POCD+EV group (P<0.01). The differences in freezing time were not statistically significant among the 4 groups in the tone-related fear test (P>0.05). Then NL1 was overexpressed in mice with POCD, the protein levels of PV, Nrx1β and PSD95 were subsequently increased, and the interaction between NL1 and Nrx1β protein was enhanced, dramatically increasing the excitability of PV interneurons. The overexpression of NL1 can upregulate the expression levels of PV, Nrx1β and PSD95 in mice with POCD, enhance the interaction between NL1 and Nrx1β and further increase the excitability of PV interneurons, thus restoring the hippocampus-dependent memorial and cognitive impairment in POCD.

Neprilysin-2 is an important β-amyloid degrading enzyme

Proteases that degrade the amyloid-β peptide (Aβ) are important in protecting against Alzheimer's disease (AD), and understanding these proteases is critical to understanding AD pathology. Endopeptidases sensitive to inhibition by thiorphan and phosphoramidon are especially important, because these inhibitors induce dramatic Aβ accumulation (∼30- to 50-fold) and pathological deposition in rodents. The Aβ-degrading enzyme neprilysin (NEP) is the best known target of these inhibitors. However, genetic ablation of NEP results in only modest increases (∼1.5- to 2-fold) in Aβ, indicating that other thiorphan/phosphoramidon-sensitive endopeptidases are at work. Of particular interest is the NEP homolog neprilysin 2 (NEP2), which is thiorphan/phosphoramidon-sensitive and degrades Aβ. We investigated the role of NEP2 in Aβ degradation in vivo through the use of gene knockout and transgenic mice. Mice deficient for the NEP2 gene showed significant elevations in total Aβ species in the hippocampus and brainstem/diencephalon (∼1.5-fold). Increases in Aβ accumulation were more dramatic in NEP2 knockout mice crossbred with APP transgenic mice. In NEP/NEP2 double-knockout mice, Aβ levels were marginally increased (∼1.5- to 2-fold), compared with NEP(-/-)/NEP2(+/+) controls. Treatment of these double-knockout mice with phosphoramidon resulted in elevations of Aβ, suggesting that yet other NEP-like Aβ-degrading endopeptidases are contributing to Aβ catabolism.

Human membrane metallo-endopeptidase-like protein degrades both beta-amyloid 42 and beta-amyloid 40

Beta-amyloid (Abeta) degrading endopeptidases are thought to protect against Alzheimer's disease (AD) and are potentially therapeutic. Of particular interest are endopeptidases that are blocked by thiorphan and phosphoramidon (T/P), as these inhibitors rapidly induce Abeta deposition in rodents. Neprilysin (NEP) is the best known target of T/P; however neprilysin knockout results in only modest Abeta increases insufficient to induce deposition. Therefore, other endopeptidases targeted by T/P must be critical for Abeta catabolism. Another candidate is the T/P sensitive membrane metallo-endopeptidase-like protein (MMEL), a close homolog of neprilysin. The endopeptidase properties of beta and gamma splice forms of human MMEL were determined in HEK293T cells transduced with the human cDNAs for the two splice forms; this showed degradation of both Abeta(42) and Abeta(40) by hMMEL-beta but not hMMEL-gamma. hMMEL-beta activity was found at the extracellular surface with no significant secreted activity. hMMEL-gamma was not expressed at the extracellular surface. Finally, it was found that hMMEL cleaves Abeta near the alpha-secretase site (producing Abeta(1-17)>>Abeta(1-16)). These data establish hMMEL as a mediator of Abeta catabolism and raise the possibility of its involvement in the etiology of AD and as a target for intervention.

Reduced fertility in male mice deficient in the zinc metallopeptidase NL1

Members of the M13 family of zinc metalloendopeptidases have been shown to play critical roles in the metabolism of various neuropeptides and peptide hormones, and they have been identified as important therapeutic targets. Recently, a mouse NL1 protein, a novel member of the family, was identified and shown to be expressed mainly in the testis as a secreted protein. To define its physiological role(s), we used a gene targeting strategy to disrupt the endogenous murine Nl1 gene by homologous recombination and generate Nl1 mutant mice. The Nl1(-/-) mice were viable and developed normally, suggesting that zygotic expression of Nl1 is not required for development. However, Nl1(-/-) males produced smaller litters than their wild-type siblings, indicating specific male fertility problems. Reduced fertility may be explained by two impaired processes, decreased egg fertilization and perturbed early development of fertilized eggs. These two phenotypes did not result from gross anatomical modifications of the testis or from impaired spermatogenesis. Basic sperm parameters were also normal. Thus, our findings suggest that one of the roles of NL1 in mice is related to sperm function and that NL1 modulates the processes of fertilization and early embryonic development in vivo.