Distinct anterograde trafficking pathways of BACE1 and amyloid precursor protein from the TGN and the regulation of amyloid-beta production

Processing of amyloid precursor protein (APP) by the beta-secretase BACE1 is the initial step of the amyloidogenic pathway to generate amyloid-beta (A beta). Although newly synthesized BACE1 and APP are transported along the secretory pathway, it is not known whether BACE1 and APP share the same post-Golgi trafficking pathways or are partitioned into different transport routes. Here we demonstrate that BACE1 exits the Golgi in HeLa cells and primary neurons by a pathway distinct from the trafficking pathway for APP. By using the Retention Using Selective Hooks system, we show that BACE1 is transported from the trans-Golgi network to the plasma membrane in an AP-1- and Arf1/4-dependent manner. Subsequently, BACE1 is endocytosed to early and recycling endosomes. Perturbation of BACE1 post-Golgi trafficking results in an increase in BACE1 cleavage of APP and increased production of both A beta 40 and A beta 42. These findings reveal that Golgi exit of BACE1 and APP in primary neurons is tightly regulated, resulting in their segregation along different transport routes, which limits APP processing

DNA methylation associated with polycomb repression in retinoic acid receptor β silencing

International audienceRetinoic acid receptor β 2 (RARβ2) is a tumor suppressor gene whose loss of expression is recurrent in prostate cancers. Here we studied the epigenetic mechanisms leading to its stable silencing. First, we characterized all RARβ isoforms in 6 human tumor cell lines (prostate DU145, LNCaP, PC3, lung A549, breast Hs578T, and colon HCT116) by RT-PCR and Western blot. We excluded loss of heterozygosity (2D-FISH) and loss of RARa expression, an upstream regulator, as origin of RARβ2 silencing. All data concluded to an epigenetic silencing. In agreement, a DNA methylation inhibitor restored its expression. Second RARβ2 loss of expression was found associated with different epigenetic profiles in LNCaP and DU145 cells. According to bisulfite sequencing and ChIP analysis, we observed heavy methylation (97%) of the RARβ2 promoter with repressive histone mark H3K9me3 in LNCaP. While DNA methylation and polycomb repression are described to be mutually exclusive at CpG-rich promoters, we observed that in DU145, moderate DNA methylation (36%) and H3K9me3 mark were present concomitantly with H3K27me3, a signature of polycomb repression. In summary, we provide new insights on how the RARβ2 promoter is silenced, reveal the existence of two distinct repressive chromatin profiles at the same locus, and support a polycomb-mediated epigenetic repression process in prostate cancer