TRENDS in Cell Biology Vol.12 No.9 September 2002

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407Research Update

Meeting Report

Control of gene expression by regulating

mRNA translation is essential for proper

growth and development in species

ranging from yeast to mammals. Recently,

translational regulation has also been

implicated in neuronal function, where

long-lasting changes in synaptic strength

and the formation of memory require new

protein synthesis. Crucial to both

developmental and neurobiological

function, translational control allows for the

asymmetric distribution of gene products

and a rapid and local response to

stimulation – as discussed at a recent

Serono Foundation EMBO Workshop held

in Mallorca, Spain.

Although developmental biologists andneurobiologists both study localizedmRNA translation, their paths rarelycross. To remedy this, Anne Ephrussi(EMBL, Heidelberg, Germany), Cesar deHaro (CBM-UAM, Madrid, Spain) andJoel Richter (UMASS, Worcester, USA)recently organized a joint SeronoFoundation EMBO Workshop held inMallorca, Spain 23–26 May 2002. Thecolocalization of these two groups ofscientists will certainly generateimportant and lasting interactions.

Asymmetric distribution of proteins isachieved, at least in part, by specificallylocalizing translationally dormant mRNA,followed by translational activation. Manylocalized mRNAs contain cis-actingelements (or zipcodes) that drive theirsubcellular localization. But localizing the mRNA by itself is not enough toachieve local distribution of gene products –the mRNA has to be silenced until thedestination is reached and/or theappropriate time arrives, at which point itis de-repressed or translationally activated.These three events are usually carried outthrough multifactorial and multifunctionalprotein complexes that recognize specificmotifs in the mRNA, usually in thenon-coding regions and most often in the3′-untranslated region (3′-UTR). A ‘model’ribonucleoprotein particle (RNP), which inmany cases starts to be assembled in the

nucleus during pre-mRNA processing [1],would include an adaptor that recognizesthe zipcode, a motor that interacts withthe cytoskeleton and drives the RNP to theappropriate location and a repressor thatmaintains the mRNA translationallyinactive. At the appropriate destination,the mRNA would be translated throughdisassembly of the repressor and/or therecruitment of an activator.

Moving targets

Understanding how the mRNAs aretransported requires the visualization ofmoving RNA–protein complexes. This waselegantly accomplished by Robert Singer(Albert Einstein College of Medicine, NY,USA) using high-speed microscopy tovisualize in real-time the behavior ofindividual mRNAs during the zipcode-and microtubule-mediated transport. Therate and speed of transport was dependentupon the number of redundant zipcodes ina particular mRNA. In this case, themRNAs traveled as single molecules andnot as part of large RNP complexes.Translational silencing during localizationwas achieved directly by the transportmachinery, which interacted withlocalization signals within the codingsequence, thus preventing the scanning of ribosomes.

Not a simple on/off switch

In many cases, translation is notcontrolled by one trans-acting factor but,rather, targeting and translation of mRNAis a result of the combined influences ofseveral factors. During oogenesis andembryogenesis, multiple maternalmRNAs are localized in the oocyte, and their coordinated translationalcontrol directs establishment of the bodyaxis and cellular differentiation [2]. Forexample, the localization of bicoid (bcd)transcripts during oogenesis is crucial forembryonic patterning.

mRNA localization, translationalactivation and stability are mediated bycis-acting elements present in the bcdmRNA 3′-UTR that form multiple

stem–loops. A multipeptidic factorrecognizes the stem–loops and localizesthe mRNA in a microtubule-dependentmanner. But this RNP has redundantcomponents both at the RNA and proteinlevels and is stabilized by multiple low-affinity interactions in a combinatorialmanner rather than by a single strongRNA–protein interaction (Paul MacDonald,Univ. of Texas, Austin, USA).

Complex biological processes requirethe coordinated expression of a subset oftranscripts from the entire ‘transcriptome’.A good example of this translationalco-regulation is the broad regulatoryactivity of the PUF family of proteins[Pumilio and Fem-3 mRNA binding factor(FBF)], which supports anterior–posteriorpatterning, sperm–oocyte switching andmitotic proliferation of stem cells [3]. InCaenorhabditis elegans, FBF maintainsmitotically dividing germline cells byrepressing gld-1 mRNA translationthrough the interaction of gld-1 with twoFBF-binding elements present in thegld-1 3′-UTR (Judith Kimble, Univ. ofWisconsin, Madison, USA). In Drosophila,Pumilio inhibits the translation of themRNAs for hunchback and cyclin B1 aswell as the growth of the imaginal discs byinteracting with different co-repressors(Robin Wharton, Duke, Durham, USA).Thus, individual PUF proteins can havemultiple functions, regulating multiplemRNAs by assembling different RNPswith different protein partners.

In this modular context, the sameRNA-binding protein can mediatetranslational repression, translationalactivation and mRNA localization byinteracting with different mRNAsequences and/or different co-regulators.An example is osk mRNA, where thefunctionally interacting repressor elementlocated in the 3′-UTR and the de-repressorelement present in the 5′-UTR recruittrans-acting complexes that sharecommon proteins (Anne Ephrussi, EMBLHeidelberg, Germany). Even morestriking is the case of the ‘cytoplasmicpolyadenylation element’ (CPE) [4],

Location, location, location: translational control in

development and neurobiology

Raúl Méndez and David Wells

a short nucleotide sequence located in the3′-UTR of germinal cell, embryonic andneuronal mRNAs. The CPE-bindingprotein CPEB can mediate mRNAsilencing, translational activation and transport, depending upon theparticular co-regulator employed. Thus, the recruitment of ‘cleavage andpolyadenylation specificity factor’ (CPSF)by CPEB during M phase directscytoplasmic polyadenylation andtranslational activation of CPE-containingmRNAs, whereas, in interphase, CPEBrecruits Maskin to block cap-dependenttranslation (Joel Richter, UMASS,Worcester, USA).

In neurons, CPEB has been implicatedin the translational regulation ofα-calcium/calmodulin-dependent kinaseII (α-CaMKII) mRNA during synapticplasticity [5]. Yi-Shuian Huang (Universityof Massachusetts, Worcester, USA)

presented evidence that CPEB is alsoinvolved in the targeting of CPE-containingmRNAs to dendrites as part of largegranules that include CPEB, Maskin andthe motor proteins kinesin and dynein.Taken together with earlier work, this suggests that there are severalregulatory sequences within the 3′-UTR of α-CaMKII and that targeting andtranslation are likely to arise from theinterplay of several factors.

Translation of the fittest

Trans-acting factors do not regulatetranslation in isolation. Each mRNA hasto compete for the limited translationalmachinery. Thus, changes in the locallevels or activity of general translationinitiation factors such as the cap-bindingprotein eIF4E (Nahum Sonenberg,McGill, Montreal, Canada) [6] or theeukaryotic initiation factor eIF2

(Cesar de Haro) also affect the translationof localized mRNAs. This was demonstratedin an elegant set of experiments byChristoph Schuster (Max-Planck,Tübingen, Germany), who demonstratedthe sub-synaptic localization of eIF4E,poly(A)-binding protein (PABP) and themRNA encoding the glutamate receptorsubunit DGluR-IIA. In addition,overexpression of either eIF4E or PABPpostsynaptically leads to an increase inDGluR-IIA protein levels that correlatedwith an increase in the size of theneuromuscular junctions (NMJs) (Fig. 1).Taken together, these data suggest thatsubsynaptic translation can instructjunctional growth and might act to locallyorganize longterm synaptic changes [7].

The mRNA encoding α-CaMKII islocalized to dendrites, and the protein hasbeen implicated as a key regulator of someforms of synaptic plasticity and memoryformation. Erin Schuman (Caltech,Pasadena, CA, USA) presented evidencethat the 5′ and 3′-UTRs of the α-CaMKIImRNA could confer both dendriticlocalization and translational regulationin hippocampal neurons in culture. Inaddition, new dendritic protein synthesiscould be elicited by the application ofbrain-derived neurotrophic factor (BDNF)even in dendrites that had been physicallytransected from the cell body. Furthermore,Nobuyuki Takei (Niigata Univ, Niigata,Japan) demonstrated that BDNFincreases the overall rate of translation by influencing the phosphorylation state of eIF4E, 4E-binding protein 1(4EBP1) and eEF2. These reports suggestthat BDNF stimulation activatestrans-acting factors bound to the UTRs ofα-CaMKII that together impart a greaterability to compete for the enhancedtranslational machinery.

Finding the targets

Another focus of current research indevelopment and neurobiology is theidentification of the mRNAs that aretargets for the translational controlmechanisms discussed above. Theidentification of the mRNAs regulated indendrites is still unknown. However, Jim Eberwine (Univ. of Pennsylvania, PA,USA) discussed his recent work utilizingmicroarray technology to begin toelucidate the cadre of mRNA in dendrites.He estimates that, of the roughly 12 000genes expressed in any given neuron,approximately 500 are targeted to the

TRENDS in Cell Biology Vol.12 No.9 September 2002

http://tcb.trends.com

408 Research Update

(a)

(b)

(c)

PABPαHRP

αHRPWild type

pabpP970/+

Fig. 1. Subsynaptic protein synthesis is a local organizer of pre- and postsynaptic changes. Confocal images ofneuromuscular junctions (NMJs) of mid-third-instar Drosophila larvae immunolabeled with an antibody recognizingthe translation factor poly(A)-binding protein PABP (red) and an antibody recognizing a pan-neural epitope (anti-HRP;green). (a) Large subsynaptic aggregates of PABP highlight sites of local protein synthesis that lead to specificalterations in the subunit composition of postsynaptic glutamate receptors and to the presynaptic recruitment ofT-bars [8]. (b, c) Local changes in protein synthesis are associated with morphological changes at the junctional level.Morphology of the NMJ in control wild-type(b) and (c) animals with genetically stimulated subsynaptic translation(pabpP970/+). Overexpression of PABP in the postsynaptic muscle cell results in the apparent addition of entirepresynaptic boutons and branches. Thus, subsynaptic protein synthesis organizes local functional andmorphological changes at developing NMJs of Drosophila. Images courtesy of C. Schuster.

dendrites. Amassing such a large number of mRNAs in the dendrite would provide the neuron with a diversepool of gene products whose synthesiscould be tightly regulated temporally and spatially. Furthermore, he showedthat translation of the dendriticallytargeted mRNAs takes place in localizedhotspots where the same RNA istranslated at a rate different to that in the cell soma.

Robert and Jennifer Darnell(Rockefeller, New York, USA) took avariety of approaches, confirmed byanalyzing genetically null animals, to identify the mRNA targets for thefragile X mental retardation protein(FMRP). They showed that the RGG boxRNA-binding domain of FMRP binds to anintramolecular G-quartet structure, andthe investigators used this interaction toidentify candidate FMRP-regulatedmRNAs. Indeed, their data suggest that itis the failure of FMRP to properly regulatemRNA translation of specific geneproducts that leads to fragile X mentalretardation syndrome.

Concluding remarks

It is clear that translational control bymulticomponent mRNPs constitutes aversatile mechanism to regulate geneexpression. Moreover, the modularity ofthe mRNP components allows for a systemto coordinately regulate multiple mRNAs.This EMBO workshop highlighted thepotential of this fast-growing field ofresearch that, no doubt, will expand toother complex biological problems where avery precise temporal and spatial controlof multi-gene expression is required.

Acknowledgements

We deeply regret that, owing to spacelimitations, only a few of the excellentpresentations are highlighted in this report.

References

1 Jansen, R.P. (2001) mRNA localization: messageon the move. Nat. Rev. Mol. Cell. Biol. 2, 247–256

2 Johnstone, O. and Lasko, P. (2001) Translationalregulation and RNA localization in Drosophilaoocytes and embryos. Annu. Rev. Genet. 35,365–406

3 Wickens, M. et al. (2002) A PUF family portrait:3′UTR regulation as a way of life. Trends Genet.18, 150–157

4 Mendez, R. and Richter, J.D. (2001) Translationalcontrol by CPEB: a means to the end. Nat. Rev.Mol. Cell Biol. 2, 521–529

5 Wu, L. et al. (1998) CPEB-mediated cytoplasmicpolyadenylation and the regulation of experience-dependent translation of alpha-CaMKII mRNA atsynapses. Neuron 21, 1129–1139

6 Gingras, A.C. et al. (1999). eIF4 initiation factors:effectors of mRNA recruitment to ribosomes andregulators of translation. Annu. Rev. Biochem. 68,913–963

7 Sigrist, S. et al. (2000) Postsynaptic translationaffects the efficacy and morphology ofneuromuscular junctions. Nature 405, 1062–1064

8 Sigrist, S. et al. The postsynaptic glutamatereceptor subunit DGluR-IIA mediates long-termplasticity in Drosophila. J. Neurosci. (in press)

Raúl Méndez

Center for Genomic Regulation (CRG).Program of Gene Expression. Passeig Maritim 37-49, 08003 Barcelona,Spain.e-mail: raul.mendez@crg.es

David Wells

Dept of Molecular, Cellular andDevelopmental Biology, Yale University, New Haven, CT 06520-8103, USA.

TRENDS in Cell Biology Vol.12 No.9 September 2002

http://tcb.trends.com

409Research Update

‘A Trends Guide to Imaging Technologies’

Published online in the August 2002 issue of Trends in Biotechnology:

visit http://journals.bmn.com/supp

The convergence of imaging techniques with molecular biology, biochemistry and computing is a rapidly growing area.This Trends Guide to Imaging Technologies outlines the latest developments and applications within the life sciences.The collection features nine reviews that cover areas such as molecular imaging and atomic-scale imaging, and the latestdevelopments in the application of imaging to neurosciences and in small-animal models. These integrated approachesprovide great promise for insights into biological processes and in the diagnosis of disease.

Editorial

A Trends Guide to Imaging Technologies

Aaron Fenster

Reviews

Functional CT: physiological models

Ting-Yim Lee

In vivo imaging of gene delivery and expression

Alexei Bogdanov, Jr and Ralph Weissleder

Bioluminescent indicators for in vivo measurements of

gene expression

Caitlin E. O’Connell-Rodwell, Stacy M. Burns, Michael H.Bachmann and Christopher H. Contag

Magnetic resonance microscopy and histology of the CNS

Jeff W.M. Bulte, Peter C.M. van Zijl and Susumu Mori

In vivo ultrasound biomicroscopy in developmental

biology

Daniel H. Turnbull and F. Stuart Foster

Micro-CT in small animal and specimen imaging

David W. Holdsworth and Michael M. Thornton

In vivo veritas: electron cryotomography of cells

Jürgen M. Plitzko, Achilleas S. Frangakis, Stephan Nickell,Friedrich Förster, Ariane Gross and Wolfgang Baumeister

Biomolecular imaging using atomic force microscopy

Daniel J. Müller and Kurt Anderson