Mutations in tumor suppressor BRCA1 lead to breast and/or ovarian cancer. Here we show that loss of BRCA1 in mice results in transcriptional derepression of the tandemly repeated satellite DNA. BRCA1 deficiency is accompanied by reduction of condensed DNA regions in the genome and loss of ubiquitylation of histone H2A at satellite repeats. BRCA1 binds to satellite DNA regions in vivo and ubiquitylates H2A in vitro. Ectopic expression of an H2A fused to ubiquitin reverses the effects of BRCA1 loss, suggesting that BRCA1 maintains heterochromatin structure via ubiquitylation of histone H2A. Satellite DNA derepression was also observed mouse and human BRCA1 deficient breast cancers. Ectopic expression of satellite DNA can phenocopy BRCA1 loss in centrosome amplification, cell cycle checkpoint defects, DNA damage and genomic instability. We propose that the role of BRCA1 in maintaining global heterochromatin integrity accounts for many of its tumor suppressor functions.

Brain power: An image‐based screen of a chemical library was carried out to identify molecules that regulate the differentiation of adult rat hippocampal neural stem cells. Neuropathiazol (1) induces selective neuronal differentiation and competitively suppresses astrocyte differentiation. This compound should be a useful tool to study the processes that control stem‐cell fate both in vitro and in vivo.

Proteases that degrade the amyloid-␤ peptide (A␤) 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 A␤ accumulation (ϳ30-to 50-fold) and pathological deposition in rodents. The A␤-degrading enzyme neprilysin (NEP) is the best known target of these inhibitors. However, genetic ablation of NEP results in only modest increases (ϳ1.5-to 2-fold) in A␤, 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 A␤. We investigated the role of NEP2 in A␤ degradation in vivo through the use of gene knockout and transgenic mice. Mice deficient for the NEP2 gene showed significant elevations in total A␤ species in the hippocampus and brainstem/diencephalon (ϳ1.5-fold). Increases in A␤ accumulation were more dramatic in NEP2 knockout mice crossbred with APP transgenic mice. In NEP/NEP2 double-knockout mice, A␤ levels were marginally increased (ϳ1.5-to 2-fold), compared with NEP Alzheimer's disease (AD) is a neurodegenerative disorder currently affecting more than 26 million people worldwide and, as advances in modern medicine prolong lifespan, this number is expected to quadruple by 2050. 1 A major factor believed to be involved in the progression of AD pathology is the accumulation of amyloid-␤ peptide (A␤). Studying the mechanisms of A␤ clearance is, therefore, very important to understanding AD.Currently, enzymatic degradation is thought to play an integral role in the removal of A␤. Of the A␤-degrading enzymes, neprilysin (NEP) has been shown to be highly critical for cerebral A␤ control.2 NEP expression has also been inversely correlated with amyloid pathology in humans and mice, and NEP gene transfer has been reported to reduce amyloid pathology in transgenic mice (reviewed by Marr and Spencer 3 ). Despite the importance of NEP-mediated A␤ degradation, NEP knockout (KO) mice show only moderately elevated A␤ levels (1.5-to 2-fold), insufficient to cause plaque deposition. 4 However, when treated with thiorphan, an NEP endopeptidase inhibitor, mice and rats demonstrate pathological accumulations of A␤ after only 1 month. 2,5 This was also found in mice treated with phosphoramidon, another NEP inhibitor. 6 These results indicate that there may exist additional NEPlike endopeptidases in the metalloprotease 13 (M13) family that are central to the A␤ clearance pathway.The NEP homolog neprilysin 2 (NEP2) is one such endopeptidase. NEP2 (also known as MMEL1/2, SEP, NL1, NE-PLP) possesses a 55% sequence identity to NEP and has been shown to degrade vasoactive peptides. 7,8 In addition, the membrane-bound ␣-splice form of murine NEP2 has demonstrated A␤-degrading properties in membrane fractions. 9 In transduced HEK293T cells, our research group previously sho...

Beta-amyloid (Aβ) degrading endopeptidases are thought to protect against Alzheimer 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 Aβ deposition in rodents. Neprilysin is the best known target of T/P; however neprilysin knockout results in only modest Aβ increases insufficient to induce deposition. Therefore, other endopeptidases targeted by T/P must be critical for Aß catabolism. Another candidate is the T/P sensitive membrane metallo-endopeptidase-like protein (MMEL), a close homolog of neprilysin.The endopeptidase properties of β and γ 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 Aβ 42 and Aβ 40 by hMMEL-β but not hMMEL-γ. hMMEL-β activity was found at the extracellular surface with no significant secreted activity. hMMEL-γ was not expressed at the extracellular surface. Finally, it was found that hMMEL cleaves Aβ near the α-secretase site (producing Aβ 1-17 >> Aβ 1-16 ). These data establish hMMEL as a mediator of Aβ catabolism and raises the possibility of its involvement in the etiology of Alzheimer's disease and as a target for intervention.

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