To investigate the role of microRNAs in regulating oligodendrocyte (OL) differentiation and myelination, we utilized transgenic mice in which microRNA processing was disrupted in OL precursor cells (OPCs) and OLs by targeted deletion of Dicer1. We found that inhibition of OPC-OL miRNA processing disrupts normal CNS myelination, and that OPCs lacking mature miRNAs fail to differentiate normally in vitro. We identified three miRNAs, miR-219, miR-138, and miR-338, that are induced 10–100x during OL differentiation; the most strongly induced of these, miR-219, is necessary and sufficient to promote OL differentiation, and partially rescues OL differentiation defects caused by total miRNA loss. miR-219 directly represses the expression of PDGFRα, Sox6, FoxJ3, and ZFP238 proteins, all of which normally help to promote OPC proliferation. Together, these findings show that miR-219 plays a critical role in coupling differentiation to proliferation arrest in the OL lineage, enabling the rapid transition from proliferating OPCs to myelinating OLs.

MicroRNAs (miRNAs) attenuate gene expression by means of translational inhibition and mRNA degradation. They are abundant, highly conserved, and predicted to regulate a large number of transcripts. Several hundred miRNA classes are known, and many are associated with cell proliferation and differentiation. Many exhibit tissue-specific expression, which aids in evaluating their functions, and it has been assumed that their high level of sequence conservation implies a high level of expression conservation. A limited amount of data supports this, although discrepancies do exist. By comparing the expression of Ϸ100 miRNAs in medaka and chicken with existing data for zebrafish and mouse, we conclude that the timing and location of miRNA expression is not strictly conserved. In some instances, differences in expression are associated with changes in miRNA copy number, genomic context, or both between species. Variation in miRNA expression is more pronounced the greater the differences in physiology, and it is enticing to speculate that changes in miRNA expression may play a role in shaping the physiological differences produced during animal development.chick ͉ evolution ͉ medaka ͉ miRNA ͉ zebrafish

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This article is available online at http://www.jlr.org Coronary atherosclerosis is the most prevalent disease in industrialized societies. Although numerous advances have been made in understanding the underlying causes of atherosclerosis and treatment thereof, this condition still remains the leading cause of death in the Western world. The most important risk factor for atherosclerosis is hyperlipidemia ( 1 ). Development of atherosclerosis correlates with high levels of low density lipoprotein cholesterol (LDL). As a result, several therapies have been developed for management of LDL levels. Among these, statins are most widely used ( 2 ). However, there is a range of statin response in humans, and a subset of familial hyperlipidemia patients is unresponsive to statins, prompting the development of additional therapies.

The ␥ complex of the Escherichia coli DNA polymerase III holoenzyme assembles the ␤ sliding clamp onto DNA in an ATP hydrolysis-driven reaction. Interactions between ␥ complex and primer/template DNA are investigated using fluorescence depolarization to measure binding of ␥ complex to different DNA substrates under steady-state and presteady-state conditions. Surprisingly, ␥ complex has a much higher affinity for singlestranded DNA (K d in the nM range) than for a primed template (K d in the M range) under steady-state conditions. However, when examined on a millisecond time scale, we find that ␥ complex initially binds very rapidly and with high affinity to primer/template DNA but is converted subsequently to a much lower affinity DNA binding state. Presteady-state data reveals an effective dissociation constant of 1.5 nM for the initial binding of ␥ complex to DNA and a dissociation constant of 5.7 M for the low affinity DNA binding state. Experiments using nonhydrolyzable ATP␥S show that ATP binding converts ␥ complex from a low affinity "inactive" to high affinity "active" DNA binding state while ATP hydrolysis has the reverse effect, thus allowing cycling between active and inactive DNA binding forms at steady-state. We propose that a DNA-triggered switch between active and inactive states of ␥ complex provides a two-tiered mechanism enabling ␥ complex to recognize primed template sites and load ␤, while preventing ␥ complex from competing with DNA polymerase III core for binding a newly loaded ␤⅐DNA complex.Escherichia coli pol III HE 1 is responsible for replicating the E. coli genome (reviewed in Ref. 1-3). Pol III HE contains pol III core along with accessory proteins. Pol III core is made up of three subunits, ␣, ⑀, and . The ␣ subunit possesses 5Ј 3 3Ј polymerase activity (4) and the ⑀ subunit contains 3Ј 3 5Ј proofreading exonuclease activity (5, 6), while the function of the subunit is not clear (7,8). Synthesis by pol III core is extremely inefficient owing to its low intrinsic processivity ϳ10 to 20 nt (9). The addition of accessory proteins dramatically increases pol III core processivity to several thousand nt, thereby providing a high replication efficiency necessary for genome duplication (4, 10). Accessory proteins enhance pol III core processivity by tethering it to a primed DNA template. Tethering of pol III core to DNA is accomplished by ␤ protein, functioning as a sliding clamp, and by ␥ complex, required for loading ␤ onto p/t DNA. X-ray data show that ␤ sliding clamp is a ring-shaped homodimer with an inner diameter of about 35 Å, large enough to encircle duplex DNA (11). By encircling DNA, ␤ dimer effectively clamps pol III core to the template through direct protein-protein interactions with the ␣ subunit. Biochemical evidence suggests that ␤ exists as a dimer when free in solution (12). The ␤ dimer must be assembled onto DNA by the activity of the ␥ complex clamp loader which is composed of 5 different polypeptides having a stoichiometry of ␥ 2 ␦␦' (13-15). In addition to its clamp l...