Facile in situ formation of hybrid gels for direct-forming tissue engineering.

Development of bioactive hydrogel as extracellular matrix (ECM) is a very important field for cell-based therapy. In this study, we provided a facile method based on sol-gel process for fabricating bioactive composite hydrogels. The composite hydrogels were composed of sol-gel derived silica and biopolymer. Different amounts of silica solution (20-80wt%) were mixed with 2% polymer sol (alginate) followed by aging and gelation to form a network so that the alginate-silica hybrid mixture could form a gel without any additional crosslinking process. The self-gelation time of the hybrid hydrogel measured by rheometer was reduced as the content of silica was increased. Such hydrogels had highly porous and interconnected structures. Their strut showed uniform surface texture. Under physiological conditions (PBS, 37°C), these hybrid hydrogels exhibited long-term stability compared to alginate hydrogels as control. The mechanical properties of these hydrogels such as compressive strength, compressive modulus, and work of fracture were significantly enhanced by hybridization with sol-gel derived silica. In vitro cell tests revealed that these hybrid hydrogels exhibited improved cell adhesion and proliferation behaviors compared to pure alginate hydrogel cross-linked by CaCl2 solution. Furthermore, cell encapsulation within these hydrogels revealed that their alginate-silica composite provided suitable microenvironment for cell survival.

Proteomic and Biochemical Studies of Lysine Malonylation Suggest Its Malonic Aciduria-associated Regulatory Role in Mitochondrial Function and Fatty Acid Oxidation.

The protein substrates of sirtuin 5-regulated lysine malonylation (Kmal) remain unknown, hindering its functional analysis. In this study, we carried out proteomic screening, which identified 4042 Kmal sites on 1426 proteins in mouse liver and 4943 Kmal sites on 1822 proteins in human fibroblasts. Increased malonyl-CoA levels in malonyl-CoA decarboxylase (MCD)-deficient cells induces Kmal levels in substrate proteins. We identified 461 Kmal sites showing more than a 2-fold increase in response to MCD deficiency as well as 1452 Kmal sites detected only in MCD-/- fibroblast but not MCD+/+ cells, suggesting a pathogenic role of Kmal in MCD deficiency. Cells with increased lysine malonylation displayed impaired mitochondrial function and fatty acid oxidation, suggesting that lysine malonylation plays a role in pathophysiology of malonic aciduria. Our study establishes an association between Kmal and a genetic disease and offers a rich resource for elucidating the contribution of the Kmal pathway and malonyl-CoA to cellular physiology and human diseases.

Lysine glutarylation is a protein posttranslational modification regulated by SIRT5.

We report the identification and characterization of a five-carbon protein posttranslational modification (PTM) called lysine glutarylation (Kglu). This protein modification was detected by immunoblot and mass spectrometry (MS), and then comprehensively validated by chemical and biochemical methods. We demonstrated that the previously annotated deacetylase, sirtuin 5 (SIRT5), is a lysine deglutarylase. Proteome-wide analysis identified 683 Kglu sites in 191 proteins and showed that Kglu is highly enriched on metabolic enzymes and mitochondrial proteins. We validated carbamoyl phosphate synthase 1 (CPS1), the rate-limiting enzyme in urea cycle, as a glutarylated protein and demonstrated that CPS1 is targeted by SIRT5 for deglutarylation. We further showed that glutarylation suppresses CPS1 enzymatic activity in cell lines, mice, and a model of glutaric acidemia type I disease, the last of which has elevated glutaric acid and glutaryl-CoA. This study expands the landscape of lysine acyl modifications and increases our understanding of the deacylase SIRT5.

SIRT5-mediated lysine desuccinylation impacts diverse metabolic pathways.

Protein function is regulated by diverse posttranslational modifications. The mitochondrial sirtuin SIRT5 removes malonyl and succinyl moieties from target lysines. The spectrum of protein substrates subject to these modifications is unknown. We report systematic profiling of the mammalian succinylome, identifying 2,565 succinylation sites on 779 proteins. Most of these do not overlap with acetylation sites, suggesting differential regulation of succinylation and acetylation. Our analysis reveals potential impacts of lysine succinylation on enzymes involved in mitochondrial metabolism; e.g., amino acid degradation, the tricarboxylic acid cycle (TCA) cycle, and fatty acid metabolism. Lysine succinylation is also present on cytosolic and nuclear proteins; indeed, we show that a substantial fraction of SIRT5 is extramitochondrial. SIRT5 represses biochemical activity of, and cellular respiration through, two protein complexes identified in our analysis, pyruvate dehydrogenase complex and succinate dehydrogenase. Our data reveal widespread roles for lysine succinylation in regulating metabolism and potentially other cellular functions.

An assembled complex IV maintains the stability and activity of complex I in mammalian mitochondria.

In the mammalian mitochondrial electron transfer system, the majority of electrons enter at complex I, go through complexes III and IV, and are finally delivered to oxygen. Previously we generated several mouse cell lines with suppressed expression of the nuclearly encoded subunit 4 of complex IV. This led to a loss of assembly of complex IV and its defective function. Interestingly, we found that the level of assembled complex I and its activity were also significantly reduced, whereas levels and activity of complex III were normal or up-regulated. The structural and functional dependence of complex I on complex IV was verified using a human cell line carrying a nonsense mutation in the mitochondrially encoded complex IV subunit 1 gene. Our work documents that, although there is no direct electron transfer between them, an assembled complex IV helps to maintain complex I in mammalian cells.

Cytochrome c oxidase subunit IV is essential for assembly and respiratory function of the enzyme complex.

Cytochrome c oxidase or complex IV, catalyzes the final step in mitochondrial electron transfer chain, and is regarded as one of the major regulation sites for oxidative phosphorylation. This enzyme is controlled by both nuclear and mitochondrial genomes. Among its 13 subunits, three are encoded by mitochondrial DNA and ten by nuclear DNA. In this work, an RNA interference approach was taken which led to the generation of mouse A9 cell derivatives with suppressed expression of nuclear-encoded subunit IV (COX IV) of this complex. The amounts of this subunit are decrease by 86% to 94% of normal level. A detail biosynthetic and functional analysis of several cell lines with suppressed COX IV expression revealed a loss of assembly of cytochrome c oxidase complex and, correspondingly, a reduction in cytochrome c oxidase-dependent respiration and total respiration. Furthermore, dysfunctional cytochrome c oxidase in the cells leads to a compromised mitochondrial membrane potential, a decreased ATP level, and failure to grow in galactose medium. Interestingly, suppression of COX IV expression also sensitizes the cells to apoptosis. These observations provide the evidence of the essential role of the COX IV subunit for a functional cytochrome c oxidase complex and also demonstrate a tight control of cytochrome c oxidase over oxidative phosphorylation. Finally, our results further shed some insights into the pathogenic mechanism of the diseases caused by dysfunctional cytochrome c oxidase complex.

Alteration of a macrophages inflammatory protein-related protein-2 (MRP-2) response by high fat and cholesterol diet in mice.

Macrophage inflammatory protein-related protein-2 (MRP-2) is a new member of the CC chemokine family that is recently identified in murine macrophages. MRP-2 is involved in leukocyte trafficking and activation, which can be implicated in inflammatory diseases including atherosclerosis. Little is known about the involvement of this novel chemokine MRP-2 in the pathogenesis of atherosclerosis. To explore the possible association of the MRP-2 with atherosclerosis, we investigated the effects of atherogenic diet on MRP-2 expression in mice. Male C57BL/6 mice were fed a high fat and cholesterol diet (20% fat and 1.5% cholesterol) or a control diet based on AIN-76 for 5, 10, or 14 weeks. The levels of total cholesterol, LDL cholesterol, and F2-isoprostanes in plasma were measured using appropriate enzymatic assays. Tumor necrosis factor alpha (TNF alpha) and MCP-1 release by peritoneal macrophages was determined by ELISA. The mRNA expression level of the MRP-2 was measured by RT-PCR. The levels of total cholesterol, LDL-cholesterol, and 8-iso-prostaglandin F2 alpha in plasma, well-known indexes of atherosclerosis, were significantly increased in the high fat and cholesterol diet group compared to those in the control. A significant increase in the TNF alpha and MCP-1 production by macrophages was also observed in the group fed high fat and cholesterol diet. The mRNA expression of MRP-2 was upregulated by oxLDL treatment in vitro and feeding a high fat and cholesterol diet in vivo at the late stage of atherosclerosis. These results suggest that MRP-2 may be an important contributing factor in the development of atherosclerosis.