SIRT1 promotes lipid metabolism and mitochondrial biogenesis in adipocytes and coordinates adipogenesis by targeting key enzymatic pathways.

The NAD+-dependent deacetylase SIRT1 controls key metabolic functions by deacetylating target proteins and strategies that promote SIRT1 function such as SIRT1 overexpression or NAD+ boosters alleviate metabolic complications. We previously reported that SIRT1-depletion in 3T3-L1 preadipocytes led to C-Myc activation, adipocyte hyperplasia, and dysregulated adipocyte metabolism. Here, we characterized SIRT1-depleted adipocytes by quantitative mass spectrometry-based proteomics, gene-expression and biochemical analyses, and mitochondrial studies. We found that SIRT1 promoted mitochondrial biogenesis and respiration in adipocytes and expression of molecules like leptin, adiponectin, matrix metalloproteinases, lipocalin 2, and thyroid responsive protein was SIRT1-dependent. Independent validation of the proteomics dataset uncovered SIRT1-dependence of SREBF1c and PPARα signaling in adipocytes. SIRT1 promoted nicotinamide mononucleotide acetyltransferase 2 (NMNAT2) expression during 3T3-L1 differentiation and constitutively repressed NMNAT1 and 3 levels. Supplementing preadipocytes with the NAD+ booster nicotinamide mononucleotide (NMN) during differentiation increased expression levels of leptin, SIRT1, and PGC-1α and its transcriptional targets, and reduced levels of pro-fibrotic collagens (Col6A1 and Col6A3) in a SIRT1-dependent manner. Investigating the metabolic impact of the functional interaction of SIRT1 with SREBF1c and PPARα and insights into how NAD+ metabolism modulates adipocyte function could potentially lead to new avenues in developing therapeutics for obesity complications.

Cardiolipin depletion-induced changes in the Trypanosoma brucei proteome.

The mitochondrial signature glycerophospholipid, cardiolipin (CL), binds to transporters of the inner mitochondrial membrane and plays a central role in formation and stability of respiratory supercomplexes. Functional and structural requirement of CL for mitochondrial membrane proteins has been studied in vitro using purified reconstituted proteins or in CL synthesis knockout cells that are viable under specific growth conditions. However, no information is available on mitochondrial function, protein stability, or expression levels in cells during CL depletion. In contrast to yeast and mammalian cells, CL synthesis is essential in Trypanosoma brucei. By stable isotope labeling with amino acids in cell culture and mass spectrometry, we analyzed protein levels in T. brucei procyclic forms at different time points during depletion of CL using tightly controllable conditional CL synthase knockout mutants and identified a set of novel CL-dependent proteins (CLDPs) with unknown functions. Depletion of individual CLDPs using knockout or knockdown technologies showed that although CL synthesis is essential, expression of a given CLDP is not. In addition, ablation of CL synthesis leads to respiratory supercomplex instability and altered mitochondrial ultrastructure and function. Our findings suggest that CL may bind to and affect many more proteins in eukaryotes than previously thought.-Schädeli, D., Serricchio, M., Ben Hamidane, H., Loffreda, A., Hemphill, A., Beneke, T., Gluenz, E., Graumann, J., Bütikofer, P. Cardiolipin depletion-induced changes in the Trypanosoma brucei proteome.

UV radiation induces genome-mediated, site-specific cleavage in viral proteins.

Much research has been dedicated to understanding the molecular basis of UV damage to biomolecules, yet many questions remain regarding the specific pathways involved. Here we describe a genome-mediated mechanism that causes site-specific virus protein cleavage upon UV irradiation. Bacteriophage MS2 was disinfected with 254 nm UV, and protein damage was characterized with ESI- and MALDI-based FT-ICR, Orbitrap, and TOF mass spectroscopy. Top-down mass spectrometry of the products identified the backbone cleavage site as Cys46-Ser47 in the virus capsid protein, a location of viral genome-protein interaction. The presence of viral RNA was essential to inducing backbone cleavage. The similar bacteriophage GA did not exhibit site-specific protein cleavage. Based on the major protein fragments identified by accurate mass analysis, a cleavage mechanism is proposed by radical formation. The mechanism involves initial oxidation of the Cys46 side chain followed by hydrogen atom abstraction from Ser47 C(α). Computational protein QM/MM studies confirmed the initial steps of the radical mechanism. Collectively, this study describes a rare incidence of genome-induced protein cleavage without the addition of sensitizers.