Donald E. Kuhn

miR-1 Overexpression Enhances Ca2+ Release and Promotes Cardiac Arrhythmogenesis by Targeting PP2A Regulatory Subunit B56α and Causing CaMKII-Dependent Hyperphosphorylation of RyR2

MicroRNAs are small endogenous noncoding RNAs that regulate protein expression by hybridization to imprecise complementary sequences of target mRNAs. Changes in abundance of muscle-specific microRNA, miR-1, have been implicated in cardiac disease, including arrhythmia and heart failure. However, the specific molecular targets and cellular mechanisms involved in the action of miR-1 in the heart are only beginning to emerge. In this study we investigated the effects of increased expression of miR-1 on excitation–contraction coupling and Ca2+ cycling in rat ventricular myocytes using methods of electrophysiology, Ca2+ imaging and quantitative immunoblotting. Adenoviral-mediated overexpression of miR-1 in myocytes resulted in a marked increase in the amplitude of the inward Ca2+ current, flattening of Ca2+ transients voltage dependence, and enhanced frequency of spontaneous Ca2+ sparks while reducing the sarcoplasmic reticulum Ca2+ content as compared with control. In the presence of isoproterenol, rhythmically paced, miR-1–overexpressing myocytes exhibited spontaneous arrhythmogenic oscillations of intracellular Ca2+, events that occurred rarely in control myocytes under the same conditions. The effects of miR-1 were completely reversed by the CaMKII inhibitor KN93. Although phosphorylation of phospholamban was not altered, miR-1 overexpression increased phosphorylation of the ryanodine receptor (RyR2) at S2814 (Ca2+/calmodulin-dependent protein kinase) but not at S2808 (protein kinase A). Overexpression of miR-1 was accompanied by a selective decrease in expression of the protein phosphatase PP2A regulatory subunit B56α involved in PP2A targeting to specialized subcellular domains. We conclude that miR-1 enhances cardiac excitation–contraction coupling by selectively increasing phosphorylation of the L-type and RyR2 channels via disrupting localization of PP2A activity to these channels.

Human chromosome 21-derived miRNAs are overexpressed in down syndrome brains and hearts.

Down syndrome (DS), or Trisomy 21, is the most common genetic cause of cognitive impairment and congenital heart defects in the human population. To date, the contribution of microRNAs (miRNAs) in DS has not been investigated. Bioinformatic analyses demonstrate that human chromosome 21 (Hsa21) harbors five miRNA genes; miR-99a, let-7c, miR-125b-2, miR-155, and miR-802. MiRNA expression profiling, miRNA RT-PCR, and miRNA in situ hybridization experiments demonstrate that these miRNAs are overexpressed in fetal brain and heart specimens from individuals with DS when compared with age- and sex-matched controls. We hypothesize that trisomic 21 gene dosage overexpression of Hsa21-derived miRNAs results in the decreased expression of specific target proteins and contribute, in part, to features of the neuronal and cardiac DS phenotype. Importantly, Hsa21-derived miRNAs may provide novel therapeutic targets in the treatment of individuals with DS.

Differential iron transport into phagosomes isolated from the RAW264.7 macrophage cell lines transfected with Nramp1Gly169 or Nramp1Asp169.

The transport of iron by RAW264.7 macrophage cell lines transfected with either Nramp1Gly169 (resistant) or Nramp1Asp169 (susceptible) alleles was assessed. We found no difference between resistant and susceptible cells in the rate of Fe import or export when Fe transport was measured in intact cells. In contrast, the rate of Fe import by latex‐bead phagosomes isolated from resistant cells was more than double the rate by latex‐bead phagosomes from susceptible cells. Similarly, phagosomes isolated from resistant cells that had been pre‐labeled with 55Fe‐citrate before phagocytosis contained up to four times as much Fe as the corresponding phagosomes from susceptible cells. Phagocytosis of Mycobacterium avium was accompanied by an increase in the production of hydroxyl radicals by Nramp1Gly169 ‐transfected macrophages but not by macrophages transfected with the susceptible allele. These results are consistent with the hypothesis that Nramp1 functions to transport Fe into the bacterium‐containing phagosome where it serves as a catalyst for the Haber‐Weiss reaction, which accounts for the increased capacity of these cells to limit mycobacterial growth. J. Leukoc. Biol. 66: 113–119; 1999.

Chromosome 21-derived microRNAs provide an etiological basis for aberrant protein expression in human down syndrome brains.

Down syndrome (DS), or Trisomy 21, is the most common genetic cause of cognitive impairment and congenital heart defects in the human population. Bioinformatic annotation has established that human chromosome 21 (Hsa21) harbors five microRNA (miRNAs) genes: miR-99a, let-7c, miR-125b-2, miR-155, and miR-802. Our laboratory recently demonstrated that Hsa21-derived miRNAs are overexpressed in DS brain and heart specimens. The aim of this study was to identify important Hsa21-derived miRNA/mRNA target pairs that may play a role, in part, in mediating the DS phenotype. We demonstrate by luciferase/target mRNA 3′-untranslated region reporter assays, and gain- and loss-of-function experiments that miR-155 and -802 can regulate the expression of the predicted mRNA target, the methyl-CpG-binding protein (MeCP2). We also demonstrate that MeCP2 is underexpressed in DS brain specimens isolated from either humans or mice. We further demonstrate that, as a consequence of attenuated MeCP2 expression, transcriptionally activated and silenced MeCP2 target genes, CREB1/Creb1 and MEF2C/Mef2c, are also aberrantly expressed in these DS brain specimens. Finally, in vivo silencing of endogenous miR-155 or -802, by antagomir intra-ventricular injection, resulted in the normalization of MeCP2 and MeCP2 target gene expression. Taken together, these results suggest that improper repression of MeCP2, secondary to trisomic overexpression of Hsa21-derived miRNAs, may contribute, in part, to the abnormalities in the neurochemistry observed in the brains of DS individuals. Finally these results suggest that selective inactivation of Hsa21-derived miRNAs may provide a novel therapeutic tool in the treatment of DS.

Iron transport into Mycobacterium avium‐containing phagosomes from an Nramp1Gly169‐transfected RAW264.7 macrophage cell line

Nramp1 is an important determinant of innate resistance of macrophages to the growth of intracellular microorganisms. We previously showed that Nramp1 functions to transport iron from the cytoplasm into phagosomes of Mycobacterium avium‐infected macrophages. The purpose of this investigation was to further characterize the factors that regulate Nramp1‐mediated iron transport into phagosomes. Treatment of Nramp1Gly169 macrophages with the lysomotrophic agents chloroquine or ammonium chloride reduced the import of iron significantly. We found that macrophage‐activating cytokines, including TNF‐α, IFN‐γ, IL‐1α, and GM‐CSF, when added prior to M. avium, increased the transport of iron into the phagosome. This increase in iron transport was not a result of an increased amount of Nramp1 protein in the phagosome nor to new protein synthesis. Treatment of Nramp1Gly169‐transfected macrophages with inhibitors of protein kinase C (PKC) diminished the import of iron into the phagosomes. Iron import was inhibited by an anti‐Nramp1 antibody against the putative fourth outer‐loop region of Nramp1 but not by an anti‐Nramp1 antibody against the carboxy terminus. The significance of these results on the orientation of Nramp1 in the phagosome membrane and on the transport of iron is discussed.

Analysis of gene expression differences between utrophin/dystrophin-deficient vs mdx skeletal muscles reveals a specific upregulation of slow muscle genes in limb muscles

Dystrophin deficiency leads to the progressive muscle wasting disease Duchenne muscular dystrophy (DMD). Dystrophin-deficient mdx mice are characterized by skeletal muscle weakness and degeneration but they appear outwardly normal in contrast to DMD patients. Mice lacking both dystrophin and the dystrophin homolog utrophin [double knockout (dko)] have muscle degeneration similar to mdx mice, but they display clinical features similar to DMD patients. Dko limb muscles also lack postsynaptic membrane folding and display fiber-type abnormalities including an abundance of phenotypically oxidative muscle fibers. Extraocular muscles, which are spared in mdx mice, show a significant pathology in dko mice. In this study, microarray analysis was used to characterize gene expression differences between mdx and dko tibialis anterior and extraocular skeletal muscles in an effort to understand the phenotypic differences between these two dystrophic mouse models. Analysis of gene expression differences showed that upregulation of slow muscle genes specifically characterizes dko limb muscle and suggests that upregulation of these genes may directly account for the more severe phenotype of dko mice. To investigate whether any upregulation of slow genes is retained in vitro, independent of postsynaptic membrane abnormalities, we derived mdx and dko primary myogenic cultures and analyzed the expression of Myh7 and Myl2. Real-time reverse transcriptase-polymerase chain reaction analysis demonstrates that transcription of these slow genes is also upregulated in dko vs mdx myotubes. This data suggests that at least part of the fiber-type abnormality is due directly to the combined absence of utrophin and dystrophin and is not an indirect effect of the postsynaptic membrane abnormalities.