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. Author manuscript; available in PMC: 2009 Nov 1.
Published in final edited form as: Biochim Biophys Acta. 2008 Apr 8;1779(11):678–681. doi: 10.1016/j.bbagrm.2008.03.009

Regulation of mammalian microRNA processing and function by cellular signaling and subcellular localization

Neil R Smalheiser 1
PMCID: PMC2628772  NIHMSID: NIHMS80093  PMID: 18433727

Abstract

For many microRNAs, in many normal tissues and in cancer cells, the cellular levels of mature microRNAs are not simply determined by transcription of microRNA genes. This mini-review will discuss how microRNA biogenesis and function can be regulated by various nuclear and cytoplasmic processing events, including emerging evidence that microRNA pathway components can be selectively regulated by control of their subcellular localization and by modifications that occur during dynamic cellular signaling. Finally, I will briefly summarize studies of microRNAs in synaptic fractions of adult mouse forebrain, which may serve as a model for other cell types as well.

Introduction

The purpose of this brief mini-review is to discuss some of the current gaps in research on the cellular regulation of microRNA processing and function -- particularly those questions that do not appear to be currently under study by many investigators.

The general sequence of microRNA biogenesis and function has been well characterized [e.g., see reviews in refs. 1–4]. In mammals, the genes encoding microRNAs are transcribed by pol II as primary transcripts, or pri-miRs, which appear to be induced and regulated by transcription factors in a fashion that is similar, if not identical, to that of conventional mRNAs (though a few microRNA genes may be transcribed by pol III [5]). The primary transcripts are processed in the nucleus by the RNAse III enzyme Drosha working with co-factors (DGCR8 and RNA helicase) to form microRNA precursors, or pre-miRs, ~70–110 nt. in length (though some intronic pri-miRs bypass Drosha processing and are spliced out of the surrounding introns [6]). MicroRNA precursors have characteristic stem-loop secondary structures and are exported via exportin 5 to the cytoplasm.

The microRNA precursors are processed further within the cytoplasm by a different RNAse III enzyme, dicer, and its co-factors (PACT and TRBP) to form small dsRNAs ~22 nt. in length. The two strands become separated and one strand is incorporated into the RNA Induced Silencing Complex (RISC). The core component of RISC is an Argonaute homologue. (In mammals, this is called eIF2c; four eIF2c isoforms are expressed and all four are associated with RISC, though only eIF2c2 has “slicer” activity directed against dsRNAs.) In turn, microRNA-loaded RISC binds to target mRNAs and regulates their translation by one of several different mechanisms [7]. Until recently, it was thought that microRNAs always inhibit translation, but recent evidence suggests that under some conditions, microRNAs may actually stimulate translation of the target mRNAs [8, see also 9, 10]. Also, until recently, it was thought that microRNAs targeted only cytoplasmic messenger RNAs; however, at least one microRNA is transported actively into the nucleus [11] and it appears that microRNAs may target cytoplasmic noncoding RNAs, including repeat-derived RNAs [12–14].

1. Regulation of cellular microRNA levels by nuclear events: RNA editing, Drosha processing and nuclear export of pri-miRs and pre-miRs

A large and growing literature has demonstrated that for many individual microRNAs, the levels of primary transcripts, precursors and mature microRNAs may show little, if any, correlation within a given tissue or cell type. These observations strongly suggest that the levels of mature microRNAs are not simply determined by transcription of microRNA genes; rather, one or more steps in the processing pathway can be critical for regulating the final microRNA levels that are expressed within a cell. (These observations are also consistent with the possibility that pri-miRs or pre-miRs may have direct biological functions, though there is currently no experimental evidence for this.)

At least three different regulated steps in nuclear microRNA processing have been documented experimentally:

  1. In some cases, site-specific RNA editing of individual pri-miRs and/or pre-miRs occurs in a tissue-specific pattern [15, 16]. The edited pri-miRs cannot be cleaved by Drosha [16] and the pre-miRs cannot be cleaved further by dicer [15], which therefore blocks the production of mature microRNAs in certain tissues.

  2. In other cases, certain pri-miRs are transported out of the nucleus into the cytoplasm [17, 18]. Drosha is generally presumed to be a nuclear protein, so if cytoplasmic pri-miRs fail to be processed further, they may be recognized as aberrant and destroyed without creating pre-miRs [18]. (Note, however, that it has not been excluded that pri-miRs may have independent biological functions in the cytoplasm, nor that Drosha can also be expressed in the cytoplasm under certain conditions as well.)

  3. In some cases, pri-miRs are not processed by Drosha and remain in the nucleus [19, 20].

  4. Finally, in some cases, pre-miRs are formed, but they appear to remain within the nucleus without being transported into the cytoplasm [21, 22]. These pre-miRs are not subjected to RNA editing and the basis for the block remains unclear.

These nuclear mechanisms are specific in two senses: they may be restricted to specific cell types, and may affect individual microRNAs but not others within the same cell. They have been described both in normal tissues and in cancer cells. However, no one has yet reported whether nuclear processing steps can be regulated by cellular events occurring in real-time (cf. below)

2. Regulation of cellular microRNA levels by subcellular localization of microRNA pathway components

There is also growing evidence that microRNA levels or localization can be regulated by cytoplasmic processing steps:

  1. Little is known regarding the normal half-life of microRNA precursors once in the cytoplasm. However, several studies have reported that microRNA precursors are associated with proteins, including fragile X mental retardation protein (FMRP) and Argonaute homologues [23, 24], which may provide a basis for their transport within cells and may regulate their interaction with dicer. FMRP interacts with multiple components of the microRNA machinery including dicer, RISC, pre-miRs and mature microRNAs, and at least some of its functions in regulating protein synthesis and dendritic spine development appear to be mediated by microRNAs [25, 26]. Note that mature microRNA levels are grossly normal within brains of neonatal mice lacking functional FMRP [27]. This does not exclude a role for FMRP in processing pre-miRs, however, since FMRP homologues FXR1P and FXR2P also bind pre-miRs [28] and are expressed in brain even in FMRP-null mice.

  2. Obernosterer et al [29] have shown that a particular microRNA, mir-138, expresses pre-miRs that fail to be processed into mature microRNA selectively in brain tissue, due to the presence of a cytoplasmic inhibitor that prevents cleavage by dicer [30]. This inhibitor is specific for mir-138 and did not affect another microRNA tested. It is not known whether cytoplasmic inhibitors play a more widespread role in regulating the interaction of dicer with pre-miRs in other situations.

  3. Both Argonaute homologues and dicer have been reported to be associated with ER and Golgi membranes in various cell types [31–33], which could conceivably facilitate the local formation or action of microRNAs in their vicinity.

  4. There is also experimental evidence that dicer and Argonaute homologues are associated with cytoskeletal elements within mammalian cells, and that cellular activation can lead to the liberation of these elements into the cytoplasm [34] (see below). Genetic evidence in C. elegans has shown that mutations in various cytoskeletal proteins can alter cellular microRNA levels, though no direct interactions of microRNAs or pre-miRs with the cytoskeleton have been reported in that species [35].

  5. Finally, Argonaute homologues are associated with a number of proteins and RNAs that are not obviously related to RISC function (e.g., gemins [36, 37] and mitochondrial transfer RNA [38]). Possibly these may represent entirely different functions of Argonaute homologues, or may represent complexes which sequester Argonaute and thus inhibit RISC formation.

Once dicer has cleaved the pre-miR into a short dsRNA hybrid, and one strand (i.e., the mature microRNA) has been incorporated into RISC, the mature microRNA may potentially regulate its target mRNAs. However, while little is known about the turnover rate of RISC that is bound to its targets, literally nothing is known regarding the fate or turnover of loaded RISC that is NOT bound to a target mRNA. Conceivably lack of suitable target mRNAs may cause an accelerated loss of the corresponding microRNAs [39]. In post-mitotic cells, the time course of RISC repression seems to last at least several weeks [40], suggesting that RISC is extremely stable, although it is not clear whether this refers to target-bound or unbound RISC. The observation that levels of mature microRNAs are typically 20–50 times higher than the corresponding steady-state levels of the pre-miR [41] also implies that mature microRNAs are very stable within cells. It is not currently known whether the turnover of microRNAs or RISC can be regulated by cellular events.

3. Regulation of the microRNA pathway by cellular signaling

Relatively little attention has been focused to date on the dynamic, physiological, real-time regulation of microRNAs within cells in response to cellular signaling, but this is likely to be a major arena for future investigation:

Proteases

At least two studies have reported that neuronal stimulation leading to intracellular calcium fluxes can activate proteases that cleave microRNA pathway components. In the case of adult mouse forebrain neurons, Lugli et al. [34] found that activation of calpain leads to the liberation of dicer bound to the postsynaptic density and to the activation of its RNAse III enzymatic activity. In contrast, in Drosophila mushroom body neurons, Ashraf et al. [42] found that calcium activates proteasomal degradation of armitage, a RNA helicase that is part of RISC. Further research is needed to learn whether the effects of synaptic stimulation in a neuron overall lead to repression of translation, de-repression, or both. In this regard, the case of FMRP may be instructive: Synaptic stimulation causes an initial, transient burst of translation of FMRP 7[43] followed rapidly by loss of FMRP back to basal levels [44, 45] that may reflect, at least in part, degradation by proteosomes [45].

Phosphorylation

In some respects, the interaction of microRNAs with cellular kinases and phosphatases has been well studied. MicroRNAs prominently regulate target mRNAs that encode signaling proteins. Conversely, kinases and phosphatases regulate transcription factors, which in turn are responsible for activating microRNA gene transcription (e.g., p53, NFkappaB, CREB, c-Myc, DAF-16/FOXO, etc.). However, to my knowledge, no studies have examined whether phosphorylation of microRNA pathway components provides a dynamic means of regulating overall microRNA function in living cells, even though numerous individual microRNA pathway components have been shown to be modified by phosphorylation. For example:

  1. p68 RNA helicase is a co-factor of Drosha in pri-miR processing [46] and is required for strand separation following dicer cleavage [47]; the ability of p68 to bind and unwind RNA is regulated by phosphorylation events [48, 49].

  2. Unphosphorylated FMRP is associated with actively translating polyribosomes while a fraction of phosphorylated FMRP is associated with apparently stalled polyribosomes [50].

  3. PACT and TRBP are co-factors of dicer [51] that are activated via phosphorylation by stimuli that elicit an interferon response.

  4. The nuclear phosphatase PPM1G/PP2Cgamma interacts with and dephosphorylates Gemin3, at least within the SMN complex [52].

Phosphorylation is likely to be a global regulator of cellular microRNA levels. For example, it is tempting to speculate that the facultative shift of microRNAs from repression to activation of protein translation may have its basis in phosphorylation and/or arginine methylation of one or more microRNA pathway-associated components such as FXR1P or HuR [8–10].

4. Regulation of microRNAs in synaptic fractions of adult mouse forebrain: a model for other cell types?

An increasing body of evidence indicates that synaptic plasticity and dendritic spine growth depend upon regulation of specific protein synthesis near or within dendritic spines, and emerging evidence suggests that microRNAs play roles in these processes [reviewed in ref. 53]. For example, in mouse, a specific microRNA, mir-134, regulates a specific dendritic mRNA, LIMK1, which is implicated in growth of dendritic spines [54]. Our studies of the microRNA pathway in synaptic fractions of adult mouse forebrain [34, 55] have identified a number of regulatory steps that, to date, have not been investigated in other systems. The key findings are as follows:

  1. Dicer, the RNAse III enzyme that cleaves pre-miRs into mature microRNAs, is expressed within dendritic spines and is highly enriched at postsynaptic densities (PSDs). The RISC core component, the Argonaute homologue eIF2c, is also expressed within spines and PSDs [34, 55].

  2. Dicer associated with isolated PSDs lacks RNAse III activity. However, the activity is cryptic, since upon treatment with the calcium-dependent protease calpain, dicer is liberated from PSDs in both full-length form and as partial proteolytic fragments, and its enzyme activity is activated. The RISC component eIF2c is also liberated from PSDs in full-length form by calpain [34].

  3. Treatment of a synaptic fraction (synaptoneurosomes) with calcium, or NMDA stimulation of acute hippocampal slices, causes the mobilization of full-length dicer and the appearance of discrete dicer fragments in a calcium- and calpain-dependent manner [34].

These experiments and others led us to propose a model in which neuronal activity at excitatory synapses allows in calcium, which activates calpain, which liberates and activates dicer and eIF2c from PSDs. Dicer would then act upon locally present microRNA precursors to form mature microRNAs, which would become incorporated into RISC and bind to mRNA targets on nearby polyribosomes [34].

More recently, my laboratory has confirmed that a majority of mature microRNAs expressed in adult mouse forebrain are detected within synaptic fractions at levels that are similar to, and in some cases substantially greater than, the levels found in total forebrain homogenate [55]. Similarly, the corresponding pre-miRs are detectably expressed in synaptic fractions at levels that are similar to the levels found in total forebrain homogenate. Within synaptic fractions, mature microRNAs are predominantly found in the cytoplasm whereas the pre-miRs are predominantly associated with PSDs. There was a positive relationship between the extent of synaptic enrichment of a given mature microRNA and its corresponding pre-miR, consistent with the proposal that the mature microRNAs within synaptic fractions derive, at least in part, from local cleavage of pre-miRs [55].

Dendritic spines relate to excitatory synapses made upon pyramidal neurons, and the postsynaptic density (PSD) is admittedly a specialized cytoskeletal structure. However, in a pilot study (Lugli and Smalheiser, unpublished) we treated 293T cells to IFN-gamma (50 ng/ml) for 2 minutes and observed a very large mobilization of full-length dicer into the cytoplasm (i.e., dicer was partitioned into the soluble fraction in greater amounts after IFN-treated cells were lysed with non-ionic detergent Triton X-100). A discrete dicer fragment also appeared in response to IFN treatment (data not shown). This strongly suggests that regulation of the microRNA pathway by cellular signaling and subcellular localization is not specific to neurons but is likely to occur in a wide range of non-neural cells and in response to a wide variety of stimuli.

Acknowledgments

Supported by NIH Grants MH81099 and LM07292. This Human Brain Project/Neuroinformatics research was funded jointly by the National Library of Medicine and the National Institute of Mental Health. The contents of this paper are solely the responsibility of the author and do not necessarily represent the official views of NIH. I apologize for being able to include only a limited number of references to the primary literature on microRNA biogenesis and function.

Footnotes

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References

  • 1.Bushati N, Cohen SM. microRNA functions. Ann Rev Cell Dev Biol. 2007;23:175–205. doi: 10.1146/annurev.cellbio.23.090506.123406. [DOI] [PubMed] [Google Scholar]
  • 2.Bartel DP. MicroRNAs: genomics, biogenesis, mechanism, and function. Cell. 2004;116:281–297. doi: 10.1016/s0092-8674(04)00045-5. [DOI] [PubMed] [Google Scholar]
  • 3.Kim VN. MicroRNA biogenesis: coordinated cropping and dicing. Nature Rev Mol Cell Biol. 2005;6:376–385. doi: 10.1038/nrm1644. [DOI] [PubMed] [Google Scholar]
  • 4.Lai EC. microRNAs: runts of the genome assert themselves. Current Biol. 2003;13:R925–936. doi: 10.1016/j.cub.2003.11.017. [DOI] [PubMed] [Google Scholar]
  • 5.Borchert GM, Lanier W, Davidson BL. RNA polymerase III transcribes human microRNAs. Nature Struct Mol Biol. 2006;13:1097–1101. doi: 10.1038/nsmb1167. [DOI] [PubMed] [Google Scholar]
  • 6.Ruby JG, Jan CH, Bartel DP. Intronic microRNA precursors that bypass Drosha processing. Nature. 2007;448:83–86. doi: 10.1038/nature05983. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 7.Pillai RS, Bhattacharyya SN, Filipowicz W. Repression of protein synthesis by miRNAs: how many mechanisms? Trends Cell Biol. 2007;17:118–126. doi: 10.1016/j.tcb.2006.12.007. [DOI] [PubMed] [Google Scholar]
  • 8.Vasudevan S, Tong Y, Steitz JA. Switching from Repression to Activation: MicroRNAs Can Up-Regulate Translation. Science. 2007;318:1931–1934. doi: 10.1126/science.1149460. [DOI] [PubMed] [Google Scholar]
  • 9.Vasudevan S, Steitz JA. AU-rich-element-mediated upregulation of translation by FXR1 and Argonaute 2. Cell. 2007;128:1105–1118. doi: 10.1016/j.cell.2007.01.038. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 10.Bhattacharyya SN, Habermacher R, Martine U, Closs EI, Filipowicz W. Relief of microRNA-mediated translational repression in human cells subjected to stress. Cell. 2006;125:1111–1124. doi: 10.1016/j.cell.2006.04.031. [DOI] [PubMed] [Google Scholar]
  • 11.Hwang HW, Wentzel EA, Mendell JT. A hexanucleotide element directs microRNA nuclear import. Science. 2007;315:97–100. doi: 10.1126/science.1136235. [DOI] [PubMed] [Google Scholar]
  • 12.Smalheiser NR, Torvik VI. Alu elements within human mRNAs are probable microRNA targets. Trends Genet. 2006;22:532–536. doi: 10.1016/j.tig.2006.08.007. [DOI] [PubMed] [Google Scholar]
  • 13.Goodier JL, Zhang L, Vetter MR, Kazazian HH., Jr LINE-1 ORF1 protein localizes in stress granules with other RNA-binding proteins, including components of RNA interference RNA-induced silencing complex. Mol Cell Biol. 2007;27:6469–6483. doi: 10.1128/MCB.00332-07. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 14.Calabrese JM, Seila AC, Yeo GW, Sharp PA. RNA sequence analysis defines Dicer's role in mouse embryonic stem cells. Proc Natl Acad Sci U S A. 2007;104:18097–18102. doi: 10.1073/pnas.0709193104. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 15.Kawahara Y, Zinshteyn B, Chendrimada TP, Shiekhattar R, Nishikura K. RNA editing of the microRNA-151 precursor blocks cleavage by the Dicer-TRBP complex. EMBO Rep. 2007;8:763–769. doi: 10.1038/sj.embor.7401011. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 16.Yang W, Chendrimada TP, Wang Q, Higuchi M, Seeburg PH, Shiekhattar R, Nishikura K. Modulation of microRNA processing and expression through RNA editing by ADAR deaminases. Nature Struct Mol Biol. 2006;13:13–21. doi: 10.1038/nsmb1041. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 17.Eis PS, Tam W, Sun L, Chadburn A, Li Z, Gomez MF, Lund E, Dahlberg JE. Accumulation of miR-155 and BIC RNA in human B cell lymphomas. Proc Natl Acad Sci U S A. 2005;102:3627–3632. doi: 10.1073/pnas.0500613102. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 18.Barthelson RA, Lambert GM, Vanier C, Lynch RM, Galbraith DW. Comparison of the contributions of the nuclear and cytoplasmic compartments to global gene expression in human cells. BMC Genomics. 2007;8:340. doi: 10.1186/1471-2164-8-340. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 19.Kluiver J, van den Berg A, de Jong D, Blokzijl T, Harms G, Bouwman E, Jacobs S, Poppema S, Kroesen BJ. Regulation of pri-microRNA BIC transcription and processing in Burkitt lymphoma. Oncogene. 2007;26:3769–3776. doi: 10.1038/sj.onc.1210147. [DOI] [PubMed] [Google Scholar]
  • 20.Thomson JM, Newman M, Parker JS, Morin-Kensicki EM, Wright T, Hammond SM. Extensive post-transcriptional regulation of microRNAs and its implications for cancer. Genes Dev. 2006;20:2202–2207. doi: 10.1101/gad.1444406. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 21.Tang GQ, Maxwell ES. Xenopus microRNA genes are predominantly located within introns and are differentially expressed in adult frog tissues via post-transcriptional regulation. Genome Res. 2008;18:104–112. doi: 10.1101/gr.6539108. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 22.Lee EJ, Baek M, Gusev Y, Brackett DJ, Nuovo GJ, Schmittgen TD. Systematic evaluation of microRNA processing patterns in tissues, cell lines, and tumors. RNA. 2007;14:35–42. doi: 10.1261/rna.804508. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 23.Jin P, Zarnescu DC, Ceman S, Nakamoto M, Mowrey J, Jongens TA, Nelson DL, Moses K, Warren ST. Biochemical and genetic interaction between the fragile X mental retardation protein and the microRNA pathway. Nature Neurosci. 2004;7:113–117. doi: 10.1038/nn1174. [DOI] [PubMed] [Google Scholar]
  • 24.Wulczyn FG, Smirnova L, Rybak A, Brandt C, Kwidzinski E, Ninnemann O, Strehle M, Seiler A, Schumacher S, Nitsch R. Post-transcriptional regulation of the let-7 microRNA during neural cell specification. FASEB J. 2007;21:415–426. doi: 10.1096/fj.06-6130com. [DOI] [PubMed] [Google Scholar]
  • 25.Qurashi A, Chang S, Jin P. Role of microRNA pathway in mental retardation. ScientificWorldJournal. 2007;7:146–154. doi: 10.1100/tsw.2007.208. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 26.Plante I, Davidovic L, Ouellet DL, Gobeil LA, Tremblay S, Khandjian EW, Provost P. Dicer-Derived MicroRNAs Are Utilized by the Fragile X Mental Retardation Protein for Assembly on Target RNAs. J Biomed Biotechnol. 2006:64347. doi: 10.1155/JBB/2006/64347. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 27.Landgraf P, et al. A mammalian microRNA expression atlas based on small RNA library sequencing. Cell. 2007;129:1401–1414. doi: 10.1016/j.cell.2007.04.040. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 28.Duan R, Jin P. Identification of messenger RNAs and microRNAs associated with fragile X mental retardation protein. Methods Mol Biol. 2006;342:267–276. doi: 10.1385/1-59745-123-1:267. [DOI] [PubMed] [Google Scholar]
  • 29.Obernosterer G, Leuschner PJ, Alenius M, Martinez J. Post-transcriptional regulation of microRNA expression. RNA. 2006;12:1161–1167. doi: 10.1261/rna.2322506. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 30.Leuschner PJ, Martinez J. In vitro analysis of microRNA processing using recombinant Dicer and cytoplasmic extracts of HeLa cells. Methods. 2007;43:105–109. doi: 10.1016/j.ymeth.2007.04.005. [DOI] [PubMed] [Google Scholar]
  • 31.Cikaluk DE, Tahbaz N, Hendricks LC, DiMattia GE, Hansen D, Pilgrim D, Hobman TC. GERp95, a membrane-associated protein that belongs to a family of proteins involved in stem cell differentiation. Mol Biol Cell. 1999;10:3357–3372. doi: 10.1091/mbc.10.10.3357. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 32.Tahbaz N, Kolb FA, Zhang H, Jaronczyk K, Filipowicz W, Hobman TC. Characterization of the interactions between mammalian PAZ PIWI domain proteins and Dicer. EMBO Rep. 2004;5:189–194. doi: 10.1038/sj.embor.7400070. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 33.Barbato C, Ciotti MT, Serafino A, Calissano P, Cogoni C. Dicer expression and localization in post-mitotic neurons. Brain Res. 2007;1175:17–27. doi: 10.1016/j.brainres.2007.07.088. [DOI] [PubMed] [Google Scholar]
  • 34.Lugli G, Larson J, Martone ME, Jones Y, Smalheiser NR. Dicer and eIF2c are enriched at postsynaptic densities in adult mouse brain and are modified by neuronal activity in a calpain-dependent manner. J Neurochem. 2005;94:896–905. doi: 10.1111/j.1471-4159.2005.03224.x. [DOI] [PubMed] [Google Scholar]
  • 35.Parry DH, Xu J, Ruvkun G. A Whole-Genome RNAi Screen for C. elegans miRNA Pathway Genes. Current Biol. 2007;17:2013–2022. doi: 10.1016/j.cub.2007.10.058. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 36.Mourelatos Z, Dostie J, Paushkin S, Sharma A, Charroux B, Abel L, Rappsilber J, Mann M, Dreyfuss G. miRNPs: a novel class of ribonucleoproteins containing numerous microRNAs. Genes Dev. 2002;16:720–728. doi: 10.1101/gad.974702. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 37.Nelson PT, Hatzigeorgiou AG, Mourelatos Z. miRNP:mRNA association in polyribosomes in a human neuronal cell line. RNA. 2004;10:387–394. doi: 10.1261/rna.5181104. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 38.Maniataki E, Mourelatos Z. Human mitochondrial tRNAMet is exported to the cytoplasm and associates with the Argonaute 2 protein. RNA. 2005;11:849–852. doi: 10.1261/rna.2210805. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 39.Michael MZ, O'Connor SM, van Holst Pellekaan NG, Young GP, James RJ. Reduced accumulation of specific microRNAs in colorectal neoplasia. Mol Cancer Res. 2003 Oct 1;12:882–91. [PubMed] [Google Scholar]
  • 40.Omi K, Tokunaga K, Hohjoh H. Long-lasting RNAi activity in mammalian neurons. FEBS Lett. 2004;558:89–95. doi: 10.1016/S0014-5793(04)00017-1. [DOI] [PubMed] [Google Scholar]
  • 41.Jiang J, Lee EJ, Gusev Y, Schmittgen TD. Real-time expression profiling of microRNA precursors in human cancer cell lines. Nucleic Acids Res. 2005;33:5394–5403. doi: 10.1093/nar/gki863. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 42.Ashraf SI, McLoon AL, Sclarsic SM, Kunes S. Synaptic protein synthesis associated with memory is regulated by the RISC pathway in Drosophila. Cell. 2006;124:191–205. doi: 10.1016/j.cell.2005.12.017. [DOI] [PubMed] [Google Scholar]
  • 43.Vanderklish PW, Edelman GM. Differential translation and fragile X syndrome. Genes Brain Behav. 2005;4:360–384. doi: 10.1111/j.1601-183X.2005.00134.x. [DOI] [PubMed] [Google Scholar]
  • 44.Gabel LA, Won S, Kawai H, McKinney M, Tartakoff AM, Fallon JR. Visual experience regulates transient expression and dendritic localization of fragile X mental retardation protein. J Neurosci. 2004;24:10579–10583. doi: 10.1523/JNEUROSCI.2185-04.2004. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 45.Hou L, Antion MD, Hu D, Spencer CM, Paylor R, Klann E. Dynamic translational and proteasomal regulation of fragile X mental retardation protein controls mGluR-dependent long-term depression. Neuron. 2006;51:441–454. doi: 10.1016/j.neuron.2006.07.005. [DOI] [PubMed] [Google Scholar]
  • 46.Fukuda T, et al. DEAD-box RNA helicase subunits of the Drosha complex are required for processing of rRNA and a subset of microRNAs. Nature Cell Biol. 2007;9:604–611. doi: 10.1038/ncb1577. [DOI] [PubMed] [Google Scholar]
  • 47.Salzman DW, Shubert-Coleman J, Furneaux H. P68 RNA helicase unwinds the human let-7 microRNA precursor duplex and is required for let-7-directed silencing of gene expression. J Biol Chem. 2007;282:32773–32779. doi: 10.1074/jbc.M705054200. [DOI] [PubMed] [Google Scholar]
  • 48.Yang L, Yang J, Huang Y, Liu ZR. Phosphorylation of p68 RNA helicase regulates RNA binding by the C-terminal domain of the protein. Biochem Biophys Res Commun. 2004;314:622–630. doi: 10.1016/j.bbrc.2003.12.129. [DOI] [PubMed] [Google Scholar]
  • 49.Yang L, Lin C, Liu ZR. Signaling to the DEAD box--regulation of DEAD-box p68 RNA helicase by protein phosphorylations. Cell Signal. 2005;17:1495–1504. doi: 10.1016/j.cellsig.2005.03.008. [DOI] [PubMed] [Google Scholar]
  • 50.Ceman S, O'Donnell WT, Reed M, Patton S, Pohl J, Warren ST. Phosphorylation influences the translation state of FMRP-associated polyribosomes. Human Mol Genet. 2003;12:3295–3305. doi: 10.1093/hmg/ddg350. [DOI] [PubMed] [Google Scholar]
  • 51.Lee Y, Hur I, Park SY, Kim YK, Suh MR, Kim VN. The role of PACT in the RNA silencing pathway. EMBO J. 2006;25:522–532. doi: 10.1038/sj.emboj.7600942. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 52.Petri S, Grimmler M, Over S, Fischer U, Gruss OJ. Dephosphorylation of survival motor neurons (SMN) by PPM1G/PP2Cgamma governs Cajal body localization and stability of the SMN complex. J Cell Biol. 2007;179:451–465. doi: 10.1083/jcb.200704163. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 53.Kosik KS. The neuronal microRNA system. Nat Rev Neurosci. 2006;7:911–920. doi: 10.1038/nrn2037. [DOI] [PubMed] [Google Scholar]
  • 54.Schratt GM, Tuebing F, Nigh EA, Kane CG, Sabatini ME, Kiebler M, Greenberg ME. A brain-specific microRNA regulates dendritic spine development. Nature. 2006;439:283–289. doi: 10.1038/nature04367. [DOI] [PubMed] [Google Scholar]
  • 55.Lugli G, Torvik VI, Larson J, Smalheiser NR. Expression of microRNAs and their precursors in synaptic fractions of adult mouse forebrain. J Neurochem. 2008 doi: 10.1111/j.1471-4159.2008.05413.x. in press. [DOI] [PMC free article] [PubMed] [Google Scholar]

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