Kim Baumann

Cell death: Multitasking p53 promotes necrosis

p53 triggers necrosis by inducing the opening of mitochondrial permeability transition pore (PTP).

Stem cells: Multiple routes to pluripotency

Five papers report extensive transcriptomic, epigenomic and proteomic analyses of reprogramming, revealing the existence of several reprogramming routes and multiple unique pluripotent cell states.

Post-translational modifications: Crotonylation versus acetylation

p300 catalyses histone crotonylation, which activates transcription more efficiently than histone acetylation.

Stem cells: A metabolic switch.

Quiescent haematopoietic stem cells use glycolysis and switch to aerobic respiration for differentiation.

Epigenetics: Enhancers under TET control

The ten-eleven-tanslocation (TET) family of proteins regulates enhancer methylation and activity in embryonic stem cells.

Small RNAs: Protecting a healthy circulation

factor 2 (KLF2), which is expressed in endothelial cells under shear stress, has a protective function against atherosclerosis. Dimmeler and colleagues now show that KLF2 induces the expression of the microRNAs (miRNAs) miR-143 and miR-145 in endothelial cells, and that these are transported to adjacent smooth muscle cells to carry out a vasculoprotective function. First, the authors observed that KLF2 regulated several miRNAs in human vascular endothelial cells under conditions of shear stress. Of these miRNAs, the cluster containing miR-143 and miR-145, which are known to be expressed by smooth muscle cells, was the most profoundly upregulated by KLF2 and shear stress. Further experiments confirmed that these miRNAs are expressed by endothelial cells in vivo and are directly activated by KLF2. So how does KLF2-dependent miR-143 and miR-145 expression in endothelial cells affect the vasculature? It had been observed previously that mice lacking KLF2 have a normal endothelium but dysfunctional and disorganized smooth muscle cells. Furthermore, miR-143 and miR-145 are known to be crucial for proper smooth muscle cell function, which suggests a physiological link, mediated by the miRNAs, between endothelial KLF2 expression and the underlying smooth muscle cells. To test this hypothesis, Dimmeler and colleagues analysed the RNA content of extracellular vesicles isolated from the supernatant of endothelial cells overexpressing KLF2 or exposed to shear stress. They found that these vesicles were enriched in miR-143 and miR-145. Furthermore, electron microscopy observations and treatment with phospholipid membrane disruptors showed that the vesicles carrying miR-143 and miR-145 are mostly exosomes, which are small vesicles surrounded by a lipid bilayer. Next, the authors co-cultured endothelial cells and smooth muscle cells, separated by a 0.4 μm pore-size membrane that prevents direct contact or transfer of large vesicles, to show that miRNAs can be actively transferred from endothelial to smooth muscle cells through small vesicles, such as exosomes. Furthermore, when smooth muscle cells were co-cultured with endothelial cells overexpressing KLF2, their levels of miR-143 and miR-145 increased and expression of miR-143 and miR-145 target genes was reduced. This indicates that transfer of miRNAs from endothelial to smooth muscle cells is induced by KLF2 and that this leads to miRNA target gene repression. Blood vessels exposed to laminar blood flow have high shear stress, express KLF2 and are protected from atherosclerosis. To examine whether miRNA-mediated gene repression induces this atheroprotective effect, the authors isolated vesicles from KLF2-expressing mouse endothelial cells and subsequently injected them into Apoe mice kept on a high fat diet. This significantly reduced the number of fatty lesions areas in the aorta and, importantly, the atheroprotective effect was abolished if miRNA expression was inhibited. This work shows that KLF2-expressing endothelial cells protect against atherosclerotic lesions in a miR-143and miR-145-dependen t manner and provides an example of vesicle-mediated miRNA transfer as a means for cell-to-cell signalling. Kim Baumann

Organelle dynamics: Fusing for stability

the function of mitochondria in all organisms studied. Fusion controls mitochondrion morphology, and impaired fusion is associated with neurodegenerative disorders and severe defects in cell respiration. Fusion has also been proposed to increase the tolerance of human cells to high levels of mutations in mitochondrial DNA (mtDNA), but how this may occur has not been examined. Now in Cell, Chen et al. show that impaired mitochondrial fusion causes muscle atrophy and that fusion is required for mtDNA stability and tolerance of mtDNA mutations in mice. Mitochondrial fusion requires the coordinated fusion of the outer and inner membranes. Outer membrane fusion depends on the mitofusins MFN1 and MFN2, and inner membrane fusion requires the dynamin-like protein optic atrophy protein 1 (OPA1). Chen et al. found that mice in which Mfn1 and Mfn2 were specifically disrupted in skeletal muscle are severely undersized and die prematurely. Furthermore, Mfn-deficient muscles are small and have abnormal mitochondria, which are often fragmented or form aggregates that disrupt myofibril arrays. When analysing mtDNA copy number, the authors found that Mfn-deficient muscles contain ~250 copies of mtDNA per nuclear genome instead of ~3,500 copies. This shows that mitochondrial fusion is required for mtDNA stability. This was also seen in OPA1-null cells, indicating that inner membrane fusion, and hence matrix content mixing, is necessary to preserve mtDNA. mtDNA depletion becomes apparent earlier than histological defects, suggesting that it might be at least in part responsible for muscle atrophy. In addition, Mfn-deficient muscles accumulate a considerably high rate of mtDNA mutations and deletions. Interestingly, removal of MFN1 in mice carrying an error-prone mtDNA polymerase (which have a tendency to accumulate higher rates of mtDNA mutations) showed that fusion does indeed increase the tolerance to mtDNA mutations, as lethality was greatly accelerated in the absence of fusion. By analysing the mitochondrial proteome, the authors found that a lack of Mfn proteins causes imbalance and protein content variability across the mitochondrial population and argue that this might be the underlying cause of mtDNA instability. However, the exact molecular mechanisms remain to be determined. Kim Baumann

Autophagy: Mitophagy receptors unravelled.

Optineurin and NDP52 are primary mitophagy receptors that are recruited to damaged mitochondria by PINK1-dependent phospho-ubiquitin.

Gene expression: RNAi as a global transcriptional activator

The CSR-1 RNAi pathway activates transcription at a genome-wide level by controlling RNA polymerase II directionality.

Stem cells: moving out of the niche.

Decaying tracheal branches produce fibroblast growth factor to guide progenitor cells to form new tissue.