The expression of Rbm3, a glycine-rich RNA-binding protein, is enhanced under conditions of mild hypothermia, and Rbm3 has been postulated to facilitate protein synthesis at colder temperatures. To investigate this possibility, Rbm3 was overexpressed as a c-Myc fusion protein in mouse neuroblastoma N2a cells. Cells expressing this fusion protein showed a 3-fold increase in protein synthesis at both 37°C and 32°C compared with control cells. Although polysome profiles of cells expressing the fusion protein and control cells were similar, several differences were noted, suggesting that Rbm3 might enhance the association of 40S and 60S ribosomal subunits at 32°C. Studies to assess a direct interaction of Rbm3 with ribosomes showed that a fraction of Rbm3 was associated with 60S ribosomal subunits in an RNA-independent manner. It appeared unlikely that this association could explain the global enhancement of protein synthesis, however, because cells expressing the Rbm3 fusion protein showed no substantial increase in the size of their monosome and polysome peaks, suggesting that similar numbers of mRNAs were being translated at approximately the same rates. In contrast, a complex that sedimented between the top of the gradient and 40S subunits was less abundant in cells expressing recombinant Rbm3. Further analysis showed that the RNA component of this fraction was microRNA. We discuss the possibility that Rbm3 expression alters global protein synthesis by affecting microRNA levels and suggest that both Rbm3 and microRNAs are part of a homeostatic mechanism that regulates global levels of protein synthesis under normal and cold-stress conditions. ribosome ͉ polysome ͉ hypothermia ͉ RNA-binding protein M ammalian cells exposed to mild hypothermia show a general inhibition of protein synthesis (1, 2) and a concomitant increase in the expression of a small number of cold-shock mRNAs and proteins (3). These proteins appear to protect cells by enabling them to more effectively tolerate cold-stress conditions (reviewed in refs. 3 and 4). In addition, it has been suggested that some cold-shock proteins may have important roles in human testis and skin, where cell temperatures are typically 30-34°C and Ϸ33°C, respectively (3).One of the mRNAs induced by mild hypothermia encodes the RNA binding motif protein 3 (Rbm3) (5-7). This protein is expressed in a wide variety of human fetal tissues (5) and is subjected to both transcriptional and translational regulation in response to mild hypothermia (6, 8). Rbm3 contains a glycinerich carboxyl terminus and can bind to both RNA and DNA via an amino-terminal RNA binding domain (ref. 5 and references therein). This protein is a member of a family of glycine-rich RNA binding proteins that include the cold-inducible RNAbinding protein (CIRP) (6,7,9,10). Although the function of this family of proteins is not yet known, it has been suggested that they affect both transcription and translation (11-13) and that they may facilitate translation at colder temperatures by functioning as RNA c...

MicroRNAs (miRs) are evolutionarily conserved, non-coding RNA molecules of approximately 21 nucleotides that regulate the expression of genes that are involved in various biological processes, such as cell proliferation and differentiation. Previously, we reported the presence of a heterogeneous population of mRNAs present in the axons and nerve terminals of primary sympathetic neurons to include the nuclear-encoded mitochondrial mRNA coding for COXIV. Sequence analysis of the 3′UTR of this mRNA revealed the presence of a putative binding site for miR-338, a brain-specific microRNA. Transfection of precursor miR-338 into the axons of primary sympathetic neurons decreases COXIV mRNA and protein levels and results in a decrease in mitochondrial activity, as measured by the reduction of ATP levels. Conversely, the transfection of synthetic anti-miR oligonucleotides that inhibit miR-338 increases COXIV levels, and results in a significant increase in oxidative phosphorylation and also norepinephrine uptake in the axons. Our results point to a molecular mechanism by which this microRNA participates in the regulation of axonal respiration and function by modulating the levels of COXIV, a protein which plays a key role in the assembly of the mitochondrial cytochrome c oxidase complex IV.

MicroRNAs (miRNAs) constitute a class of small, non-coding RNAs that act as post-transcriptional regulators of gene expression. In neurons, the functions of individual miRNAs are just beginning to emerge, and recent studies have elucidated roles for neural miRNAs at various stages of neuronal development and maturation, including neurite outgrowth, dendritogenesis, and spine formation. Notably, miRNAs regulate mRNA translation locally in the axosomal and synaptodendritic compartments, and thereby contribute to the dynamic spatial organization of axonal and dendritic structures and their function. Given the critical role for miRNAs in regulating early brain development and in mediating synaptic plasticity later in life, it is tempting to speculate that the pathology of neurological disorders is affected by altered expression or functioning of miRNAs. Here we provide an overview of recently identified mechanisms of neuronal development and plasticity involving miRNAs, and the consequences of miRNA dysregulation.