The circadian clock output deubiquitylating enzyme USP2-45 regulates intestinal Ca2+ absorption

In order to identify molecular targets of USP2-45 in the small intestine, we carried out a quantitative proteomics iTraq (Isobaric tag for relative and absolute quantitation) screen to compare proteins from WT and knockout mice. With this analysis we identified, among others, the scaffolding protein Na(+)/H(+) Exchange Regulatory cofactor NHERF4 to be up-regulated (4.81-fold) in the duodenum of Usp2-KO mice. Methods : Usp2-KO and WT littermates males were entrained to 12h L:12h D for at least two weeks in light-light housing boxes. Mice were sacrificed by cervical disolcation at ZT13 under red dim light and duodenal mucosa was dissected by scraping. The tissue samples were quickly frozen in liquid nitrogen before total protein extraction. Proteins were extracted in a lysis buffer containing 8M Urea, 20mM Tris-HCl, pH 7.5, 14 µg/ml Aprotinin, 0.7 µg /ml PepstatinA 0.7 µg /ml Leupeptin, 1 mM NaVO4, 10 mM Na-Pyrophosphate, 50 µM MG132 and 1mM PMSF. Protein concentration was measured by Bradford quantitation. Relative quantitation of proteins in 4 pools of 3 KO and 4 pools of 3 WT tissue samples was carried out by iTRAQ (Isobaric Tags for Relative and Absolute Quantitation). After reduction and alkylation of cysteines, proteins were precipitated, redissolved in 8M Urea buffer and digested with trypsin (1:50 w:w) overnight. 80ug of digested material for each sample was labeled by reaction with one vial of iTRAQ 8-plex reagent (ABSciex). Samples were pooled , desalted and fractionated by off-gel electrofocusing. The 24 fractions obtained were analysed by nanoflow liquid chromatography-tandem mass spectrometry (nanoLC-MS/MS) on a hybrid linear trap LTQ-Orbitrap Velos mass spectrometer (Thermo Fisher Scientific, Bremen, Germany) on a two-hour gradient. Full MS survey scans were performed at 60’000 resolution and the ten most intense multiply charged precursor ions detected in the full MS survey scan were selected for HCD (Higher energy Collision Dissociation) fragmentation and Orbitrap analysis (7500 resolution). Each spectrum was acquired at two relative collision energies (35% and 45%) and the spectra were summed. Data files were analysed with MaxQuant 1.3.0.5 incorporating the Andromeda search engine. Further data analysis was performed using the R statistical programming language version 2.15.2 (R core team 2012). Intensities for the reporter ions 113 to 121 were normalized using the Variance Stabilizing method (R package version 3.26.0). Proteins with non-zero intensities in all channels were evaluated for differential expression using the Local-Pooled-Error method (R package version 1.32.0) followed by correction for multiple testing according to Benjamini and Hochberg.