Making Proteins at the Synapse Activity-Regulated Translation and CPEB
Authors: Paul Worley
Journal: Neuron (1998)

Cytoplasmic polyadenylation element binding protein (CPEB) is a sequence-specific RNA binding protein that was originally identified as playing a role in the translational control of c-Mos kinase in oocyte maturation (4de Moor C.H. Richter J.D. Mol. Cell. Biol. 1997; 17: 6419-6426Crossref PubMed Scopus (125) Google Scholar). 12Wu L. Wells D. Tay J. Mendis D. Abbott M.-A. Barnitt A. Quinlan E. Heynen A. Fallon J.R. Richter J.D. Neuron. 1998; 21 (this issue,): 1129-1139Abstract Full Text Full Text PDF PubMed Scopus (404) Google Scholar, in this issue of Neuron, present compelling observations that indicate a role for CPEB in activity-dependent translation of calcium and calmodulin-dependent kinase II (CaMKII) in brain. CPEB is a ∼62 kDa cytoplasmic protein that possesses two RNA binding motifs and a zinc finger–like motif. Together, these motifs are required for binding to a defined RNA sequence (UUUUUAUU), termed the cytoplasmic polyadenylation element (CPE) (6Hake L.E. Mendez R. Richter J.D. Mol. Cell. Biol. 1998; 18: 685-693Crossref PubMed Scopus (138) Google Scholar). The CPE has been described in several mRNAs and is frequently in the 3′ UTR near the polyadenylation signal sequence (AAUAAA). There is variability in the spacing between the CPE and the polyadenylation sequence, and this spacing may be important in defining the activity of CPEB. CPEB binding to mRNA appears to be stimulus independent (6Hake L.E. Mendez R. Richter J.D. Mol. Cell. Biol. 1998; 18: 685-693Crossref PubMed Scopus (138) Google Scholar), and upon appropriate activation of the cell, CPEB induces lengthening of the poly(A) tail of the mRNA by poly(A) polymerase. Poly(A) tail lengthening in turn is linked to 5′ capping of the mRNA and enhanced translation (7Kuge H. Richter J.D. EMBO J. 1995; 14: 6301-6310Crossref PubMed Scopus (87) Google Scholar). Certain mechanistic details regarding activation of CPEB, its facilitation of translation, and its precise binding requirements remain to be determined. With this background, the authors of the present report demonstrate that CPEB binds the 3′ UTR of CaMKII mRNA, that binding is dependent on two sequence elements in the 3′ UTR, and that translation efficiency of engineered CaMKII mRNAs injected in oocytes is dependent on the natural CPE. The authors examine the in vivo relevance of this translation regulatory mechanism by using a classical dark rearing paradigm to demonstrate correlated, activity-dependent poly(A) lengthening of CaMKII mRNA and de novo synthesis of CaMKII protein. The proposed site of CaMKII synthesis at synapses will require independent anatomic confirmation; however, the study reminds us that translation is highly regulated and provides a testable, mechanistic framework for localized, activity-dependent protein synthesis in neurons. Classical studies demonstrated that rapid, de novo protein synthesis is required for long-term memory and suggested that this may occur, at least in part, at individual synapses (10Steward O. Neuron. 1997; 18: 9-12Abstract Full Text Full Text PDF PubMed Scopus (212) Google Scholar). Recent work has defined the discrete temporal and spatial specificity of the response, as well as the synaptic stimulus parameters that underlie a protein synthesis–dependent late phase of homosynaptic potentiation (5Frey U. Morris R.G.M. Nature. 1997; 385: 533-536Crossref PubMed Scopus (1178) Google Scholar). Indeed, it has become a reasonably strong hypothesis that each synapse may be an individual compartment for regulated translation. We have come to anticipate that basic cell biological mechanisms may be employed in multiple tissues and developmental stages. Oocyte maturation is programmed by maternal mRNAs that are blocked from translation during oogenesis but are activated in a coordinate, hierarchical manner following hormonal stimulation (4de Moor C.H. Richter J.D. Mol. Cell. Biol. 1997; 17: 6419-6426Crossref PubMed Scopus (125) Google Scholar). Might such a complex program pertain to individual synapses? It is anticipated that translation of other mRNAs present in dendrites may be similarly regulated by changes in poly(A) tail length. Recent studies suggest that several tens of mRNA species may be present in the dendrite (2Crino P.B. Eberwine J. Neuron. 1996; 17: 1173-1187Abstract Full Text Full Text PDF PubMed Scopus (221) Google Scholar), and there is suggestive evidence that CaMKII (9Ouyang Y. Kantor D. Harris K.M. Schuman E.M. Kennedy M.B. J. Neurosci. 1997; 17: 5416-5427Crossref PubMed Google Scholar), Arc (11Steward O. Wallace C.S. Lyford G.L. Worley P.F. Neuron. 1998; 21: 741-751Abstract Full Text Full Text PDF PubMed Scopus (629) Google Scholar), and CREB (3Crino P. Khodakhah K. Becker K. Ginsberg S. Hemby S. Eberwine J. Proc. Natl. Acad. Sci. USA. 1998; 95: 2313-2318Crossref PubMed Scopus (125) Google Scholar) may be synthesized locally in dendrites. The present study supplies a relatively rapid and simple PCR-based assay for the CPEB-linked mecha-nism. Thus, it will be straightforward to determine which of the many candidate dendritic mRNAs may be regulated by CPEB. It may also be possible to adapt this assay to define the site of CPEB activity using in situ PCR followed by hybridization with conditions that detect long poly(A) sequences. Moreover, it should be possible to directly test whether dendritic translation of identified mRNAs is dependent on CPEB mechanisms, for example, by expressing the modified mRNA as a trans-gene in mice. If the CPEB-linked mechanism is correct, we would anticipate that activity-induced synthesis of CaMKII will be selectively disrupted by single nucleotide substitutions in the 3′ UTR. It should be noted that other translation regulatory mechanisms may also be relevant at synapses. For example, components of the rapamycin-sensitive S6 kinase pathway are also present in spines (1Burnett P.E. Blackshaw S. Lai M.M. Qureshi I.A. Burnett A.F. Sabatini D.M. Snyder S.H. Proc. Natl. Acad. Sci. USA. 1998; 95: 8351-8356Crossref PubMed Scopus (105) Google Scholar). These other translation regulatory mechanisms should be considered in light of the fact that of the aforementioned dendritic mRNAs, only CaMKII has a recognizable CPEB site. Locally synthesized proteins may be hypothesized to exert a regulatory role on the function of preexisting proteins in the spine and synapse. The association between CaMKII activity and synaptic plasticity is well documented, and the 70% increase in CaMKII protein levels reported by 12Wu L. Wells D. Tay J. Mendis D. Abbott M.-A. Barnitt A. Quinlan E. Heynen A. Fallon J.R. Richter J.D. Neuron. 1998; 21 (this issue,): 1129-1139Abstract Full Text Full Text PDF PubMed Scopus (404) Google Scholar could plausibly alter synaptic function. The functional contribution of other putative dendritically synthesized proteins is less clear. Arc (also termed Arg 3.1) is an immediate-early gene (IEG) that possesses features of a cytoskeletal protein (8Lyford G.L. Yamagata K. Kaufmann W.E. Barnes C.A. Sanders L.K. Copeland N.G. Gilbert D.J. Jenkins N.A. Lanahan A.A. Worley P.F. Neuron. 1995; 14: 433-445Abstract Full Text PDF PubMed Scopus (940) Google Scholar). In recent studies, we find that Arc binds to the C-terminal association domain of CaMKII, suggesting that Arc functions as a physical tether for CaMKII. Because Arc mRNA is rapidly and transiently induced by synaptic activity, it has provided a unique tool to study mechanisms that may target newly synthesized mRNA. Arc mRNA is transcriptionally induced by synaptic NMDA receptor activation (8Lyford G.L. Yamagata K. Kaufmann W.E. Barnes C.A. Sanders L.K. Copeland N.G. Gilbert D.J. Jenkins N.A. Lanahan A.A. Worley P.F. Neuron. 1995; 14: 433-445Abstract Full Text PDF PubMed Scopus (940) Google Scholar). The mRNA is distributed to distal dendrites over the time course of ∼1 hr. Arc mRNA is then selectively accumulated in response to localized synaptic input (11Steward O. Wallace C.S. Lyford G.L. Worley P.F. Neuron. 1998; 21: 741-751Abstract Full Text Full Text PDF PubMed Scopus (629) Google Scholar). The synaptic pharmacology mediating localized Arc mRNA accumulation remains to be determined, as does the potential generality of the RNA targeting phenomena. The fact that Arc is regulated as an IEG suggests an additional level of complexity, in that it may define a time window during which the translational response is modified in previously active neurons. In view of the great interest and potential of the local synaptic translation hypothesis, the present description of an activity-dependent CPEB-linked translational mech-anism is a welcome new insight that is likely to usher rapid progress in this field.