A novel microRNA boosts hyper-ß-oxidation of fatty acids in liver by impeding CEP350-mediated sequestration of PPARα and thus restricts chronic hepatitis C.

Imbalance in lipid metabolism induces steatosis in liver during Chronic hepatitis C (CHC). Contribution of microRNAs in regulating lipid homoeostasis and liver disease progression is well established using small RNA-transcriptome data. Owing to the complexity in the development of liver diseases, the existence and functional importance of yet undiscovered regulatory miRNAs in disease pathogenesis was explored in this study using the unmapped sequences of the transcriptome data of HCV-HCC liver tissues following miRDeep2.pl pipeline. MicroRNA-c12 derived from the first intron of LGR5 of chromosome 12 was identified as one of the miRNA like sequences retrieved in this analysis that showed human specific origin. Northern blot hybridization has proved its existence in the hepatic cell line. Enrichment of premiR-c12 in dicer-deficient cells and miR-c12 in Ago2-RISC complex clearly suggested that it followed canonical miRNA biogenesis pathway and accomplished its regulatory function. Expression of this miRNA was quite low in CHC tissues than normal liver implying HCV-proteins might be regulating its biogenesis. Promoter scanning and ChIP analysis further revealed that under expression of p53 and hyper-methylation of STAT3 binding site upon HCV infection restricted its expression in CHC tissues. Centrosomal protein 350 (CEP350), which sequestered PPARα, was identified as one of the targets of miR-c12 using Miranda and validated by luciferase assay/western blot analysis. Furthermore, reduced triglyceride accumulation and enhanced PPARα mediated transcription of ß-oxidation genes upon restoration of miR-c12 in liver cells suggested its role in lipid catabolism. Thus this study is reporting miR-c12 for the first time and showed its' protective role during chronic HCV infection.

P-body-induced inactivation of let-7a miRNP prevents the death of growth factor-deprived neuronal cells.

RNA processing bodies (P-bodies) are cytoplasmic RNA granules in eukaryotic cells that regulate gene expression by executing the translation suppression and degradation of mRNAs that are targeted to these bodies. P-bodies can also serve as storage sites for translationally repressed mRNAs both in mammalian cells and yeast cells. In this report, a unique role of mammalian P-bodies is documented. Depletion of P-body components dedifferentiate nerve growth factor-treated PC12 cells, whereas ectopic expression of P-body components induces the neuronal differentiation of precursor cells. Trophic factor withdrawal from differentiated cells induces a decrease in cellular P-body size and numbers that are coupled with dedifferentiation and cell death. Here, we report how the expression of P-body proteins-by ensuring the phosphorylation of argonaute protein 2 and the subsequent inactivation let-7a miRNPs-prevents the apoptotic death of growth factor-depleted neuronal cells.-Patranabis, S., Bhattacharyya, S. N. P-body-induced inactivation of let-7a miRNP prevents the death of growth factor-deprived neuronal cells.

Phosphorylation of Ago2 and Subsequent Inactivation of let-7a RNP-Specific MicroRNAs Control Differentiation of Mammalian Sympathetic Neurons.

MicroRNAs (miRNAs) are small regulatory RNAs that regulate gene expression posttranscriptionally by base pairing to the target mRNAs in animal cells. KRas, an oncogene known to be repressed by let-7a miRNAs, is expressed and needed for the differentiation of mammalian sympathetic neurons and PC12 cells. We documented a loss of let-7a activity during this differentiation process without any significant change in the cellular level of let-7a miRNA. However, the level of Ago2, an essential component that is associated with miRNAs to form RNP-specific miRNA (miRNP) complexes, shows an increase with neuronal differentiation. In this study, differentiation-induced phosphorylation and the subsequent loss of miRNA from Ago2 were noted, and these accounted for the loss of miRNA activity in differentiating neurons. Neuronal differentiation induces the phosphorylation of mitogen-activated protein kinase p38 and the downstream kinase mitogen- and stress-activated protein kinase 1 (MSK1). This in turn upregulates the phosphorylation of Ago2 and ensures the dissociation of miRNA from Ago2 in neuronal cells. MSK1-mediated miRNP inactivation is a prerequisite for the differentiation of neuronal cells, where let-7a miRNA gets unloaded from Ago2 to ensure the upregulation of KRas, a target of let-7a. We noted that the inactivation of let-7a is both necessary and sufficient for the differentiation of sympathetic neurons.

Correction of translational defects in patient-derived mutant mitochondria by complex-mediated import of a cytoplasmic tRNA.

A variety of clinical disorders result from mutations in mitochondrial tRNA genes, leading to translational defects. We show here that a protein complex from the kinetoplastid protozoon Leishmania induces specific, ATP-dependent import of human cytoplasmic tRNA(1)(Lys) into human mitochondria in vitro. The imported tRNA undergoes efficient aminoacylation within the organelle and supports organellar protein synthesis. Moreover, translation in mitochondria from patients with myclonic epilepsy with ragged red fibers (MERRF) and Kearns-Sayre syndrome (KSS), containing mutant tRNA(Lys) genes, is stimulated to near-wild-type levels and the formation of aberrant polypeptides suppressed by complex-mediated import. These results suggest a novel way to introduce RNAs for the modulation of mitochondrial gene expression.

The complexity of mitochondrial tRNA import.

Import of nucleus-encoded, cytoplasmic tRNAs into mitochondria to compensate evolutionary loss of the corresponding mitochondrial genes has been documented in a large number of species. Although the phenomenon has been known for more than 25 years, it was only recently that the mechanism of tRNA import started receiving the sustained attention of workers investigating yeast, protozoal and higher plant systems. The purpose of this review is to summarize recent developments that shed new light on the selectivity of the process, the identity of the import apparatus and the nature of the bioenergetic transactions leading to tRNA translocation, and to build a working model of the import complex suggested by these observations.

Mitochondrial differentiation in kinetoplastid protozoa: a plethora of RNA controls.

Differentiation of kinetoplastid protozoa during their complex life cycles is accompanied by stepwise changes in mitochondrial functions. Recent studies have begun to reveal multilevel post-transcriptional regulatory mechanisms by which the expression of the nuclear and mitochondrially encoded components of respiratory enzymes is coordinated, as well as the identities of some general and gene-specific factors controlling mitochondrial differentiation.

Allosteric regulation of tRNA import: interactions between tRNA domains at the inner membrane of Leishmania mitochondria.

Import of nucleus-encoded tRNAs into the mitochondria of the kinetoplastid protozoon Leishmania involves recognition of specific import signals by the membrane-bound import machinery. Multiple signals on different tRNA domains may be present, and further, importable RNAs interact positively (Type I) or negatively (Type II) with one another at the inner membrane in vitro. By co-transfection assays, it is shown here that tRNA(Tyr) (Type I) transiently stimulates the rate of entry of tRNA(Ile) (Type II) into Leishmania mitochondria in transfected cells, and conversely, is inhibited by tRNA(Ile). Truncation and mutagenesis experiments led to the co-localization of the effector and import activities of tRNA(Tyr) to the D domain, and those of tRNA(Ile) to the variable region-T domain (V-T region), indicating that both activities originate from a single RNA-receptor interaction. A third tRNA, human tRNA(Lys), is imported into Leishmania mitochondria in vitro as well as in vivo. This tRNA has Type I and Type II motifs in the D domain and the V-T region, respectively, and shows both Type I and Type II effector activities. Such dual-type tRNAs may interact simultaneously with the Type I and Type II binding sites of the inner membrane import machinery.