463

A selection of interesting papers that were published inthe two months before our press date in major journalsmost likely to report significant results in neurobiology.

• of special interest•• of outstanding interest

Current Opinion in Neurobiology 2002, 12:463–470

Selected by Jack PriceInstitute of Psychiatry, London, UK

e-mail: J.Price@iop.kcl.ac.uk

•• LIM factor Lhx3 contributes to the specification of motorneuron and interneuron identity through cell-type-specificprotein–protein interactions. Thaler JP, Lee S-K, Jurata LW,Gill GN, Pfaff SL: Cell 2002, 110:237-249Significance: An insight into the mechanism by which LIM factors determine neuronal fate in the spinal cord.Findings: Previous work has indicated that the fate of ventralspinal cord neurons is linked primarily to dorsoventral pattering.This results in adjacent groups of precursor cells acquiring distinct specifications and thereby giving rise to different populations of spinal neurons. But how are the different identities of the precursor populations translated into the different neuronal fates of their progeny? We have known for awhile that members of the LIM class of homeodomain factorsare involved in this specification, but their role is not simple, aseach neuronal type expresses not a single specifying factor, buta number of factors. It seemed, therefore, that a combinatorialcode was in operation whereby the expression of a specificcombination of factors constituted a particular fate. In this study,Thaler et al. clarify the mechanism whereby a particular combination specifies one such fate choice. V2 interneuronsand motor neurons are two populations of ventral cord cells thatarise from neighbouring precursor populations. Both popula-tions express the LIM factor, Lhx3 (and a related factor Lhx4),whereas only motor neurons express another LIM factor, Isl1.Both populations also express NLI, a LIM domain interactingprotein. This study shows that V2 fate is driven by a tetramericcomplex of proteins composed of two Lhx3 and two NLI sub-units, whereas motor neuron fate is driven by a hexamericcomplex composed of two Lhx3, two Isl1, and two NLI subunits.

•• Conversion of cerebral cortex into basal ganglia inEmx2–/– Pax6sey/sey double-mutant mice. Muzio L,Di Benedetto B, Stoykova A, Boncinelli E, Gruss P,Mallamaci A: Nat Neurosci 2002, 5:737-745Significance: A preliminary answer to the question of what drives the cerebral cortex to have properties distinct from thoseof neighbouring forebrain regions.Findings: The mammalian telencephalon is made up of a number of distinct regions: the cerebrum and basal gangliaamong them. They develop from neighbouring regions of neuroepithelium. So, what are the genes that direct the twoembryonic tissues to become such different structures? A number of candidates have emerged over recent years, and theauthors of this study take a close look at two of them: Pax6 andEmx2. Their expression patterns plus extensive studies of theirknockout phenotypes have led experimenters to suggest thatthese two genes play fundamental roles in specifying corticalfate. In this study, the authors look at the phenotypes of double-mutants in which both genes are knocked out. They use aseries of marker genes to show that in these double-knockoutmice, the cortex takes on a striatum-like structure and the cortical neurons become respecified as subpallial neurons. Theconclusion they draw is that these two genes, acting along parallel pathways, promote corticogenesis. They leave open thepossibility that other genes, such as Lhx2, might also be significant. Nonetheless, the broad conclusion is that funda-mentally Pax6 and Emx2 make cortex different from striatum.

• Distinct actions of Emx1, Emx2, and Pax6 in regulating thespecification of areas in the developing neocortex.Bishop KM, Rubenstein JLR, O’Leary DDM: J Neurosci 2002,22:7627-7638.Significance: A different perspective on the actions of Emxgenes and Pax6, attributing their influence on cortical develop-ment to their role in area specification.Findings: A fundamental property of mammalian cerebral cortex is that it is organised into functional areas — somatosensory,motor, visual, etc. — and the issue of what drives the process ofarea specification (or arealisation as it is sometimes called) hasa long and chequered history in neuroscience research. Muchof the discussion over many years centred on whether areaspecification was the result of the interaction between cortexand incoming thalamic afferents. Although there is considerableevidence for some role for thalamic axons, most recentresearch suggests that area specification emerges from theinteraction of an intrinsic cortical program with influences fromneighbouring signalling centres such as the cortical hem. Thisconclusion has led to a search for the intrinsic genetic factorsthat drive cortical area specification. Both Emx2 and Pax6 areimplicated in this process for no better reason than they areexpressed in a gradient across the cortical field, although forEmx2 in particular, there is already experimental data that cortical areas shift if levels of its expression are manipulated. Inthis study, a series of marker genes are employed that are differentially expressed across the rostrocaudal dimension ofthe cortical plate. Although not markers of cortical areas per se,their expression is likely to reflect the specification process.Using mouse knockouts, the authors find that in Emx2 mutants,

NeurobiologyPaper alert

Contents (chosen by)

463 Development (Price)464 Cognitive neuroscience (Silva, Kushner and Reber)465 Signalling mechanisms (Häusser and Bezzi)466 Sensory systems (Wood)467 Neuronal and glial cell biology (Bredt)467 Motor systems (Chafee and Merchant)468 Neurobiology of behaviour (Goodwin and Kyriacou)469 Neurobiology of disease (Kempermann and Winkler)

Development

marker expression boundaries shift caudally, whereas in Pax6mutants the boundaries shift rostrally. Emx1, a gene closelyrelated to Emx2 and also expressed in the developing cortex,gives no such shifts, and serves as a useful control. The impli-cation of this study is that Emx2 and Pax6 are part of the areaspecification process, acting to give cortex a more rostral orcaudal identity respectively. Presumably they act antagonisti-cally during the specification process. Considered inconjunction with the work of Muzio et al. (see above), a pictureemerges of these two genes acting in concert to specify cortexand to give it its area specification.

• Lobe mediates Notch signaling to control domain-specificgrowth in the Drosophila eye disc. Chern JJ, Choi K-W:Development 2002, 129:4005-4013.Significance: The discovery of a novel modulator of Notch signalling in insect eye development.Findings: The insect eye is composed of multiple clusters ofcells called ommatidia, which are organised in mirror symmetryabout a dorsoventral boundary. The boundary arises as a con-sequence of the expression of the morphogens wingless anddecapentaplegic, which are expressed in the dorsal and ventraldomains respectively. As a consequence, expression of thefringe gene becomes restricted to the ventral domain, leadingto an asymmetry to the Notch signalling cascade in the dorsaland ventral domains. Notch function is required for appropriateeye growth, but how these are linked is unclear. This study pre-sents evidence that the Lobe gene plays a role in this coupling.The authors clone and sequence Lobe and show that it medi-ates the proliferative effect of Notch in the ventral domain only,because loss of Lobe function specifically abolishes ventral eyegrowth. The function of Lobe appears to be to regulate serate,the Notch ligand already implicated in Notch signalling at thedorsoventral boundary. Lobe is a novel gene, and its sequencegives away little about its possible molecular interactions.Nonetheless, the authors present evidence for homologues inboth mouse and human. Thus, we seem to have yet anothernew player in the ever-expanding Notch signalling cascade.

Selected by Alcino J Silva and Steven A KushnerUniversity of California-Los Angeles, California, USA

e-mail: Silvaa@mednet.ucla.edu

•• Immunization reverses memory deficits without reducing brain Aββ burden in Alzheimer’s disease model.Dodart J-C, Bales KR, Gannon KS, Greene SJ, DeMattos RB,Mathis C, DeLong CA, Wu S, Wu X, Holtzman DM, Paul SM:Nat Neurosci 2002, 5:452-457.Significance: Memory deficits associated with an Alzheimer’sdisease mouse model can be rapidly reversed with an anti-Aβantibody without reducing brain Aβ deposits.Findings: Brain Aβ deposits are thought to be a critical component of the cognitive impairments associated with AD.Transgenic mice (PDAPP) overexpressing a mutant β-amyloidprecursor protein show age-dependent memory deficits thatare thought to model the cognitive deficits associated withAlzheimer’s disease (AD). In this study, the authors reversedthese memory deficits with a monoclonal antibody (m266)directed against the Aβ peptide. Although wild-type littermatesperformed significantly above chance in an object recognition

task, neither 8 nor 24 month-old transgenic animals discriminatedbetween novel and familiar objects during testing. Treatment ofthe 24 month-old transgenic mice with intraperitoneal injections of m266 once a week for six weeks rescued performance on the object recognition task to wild-type levels.Despite the significant behavioral improvement, this dose hadno detectable effect on the percentage area occupied by Aβdeposits in the brain (Aβ burden). Additional analysis revealedno significant correlation between amyloid deposits andbehavioral performance. Remarkably, a single treatment withm266, 24 hours prior to testing in the object recognition task,also rescued the cognitive deficit. A dose-response curve withm266 revealed that plasma Aβ levels were correlated withbehavioral performance. Acute treatment with m266 was alsoeffective in improving performance on a holeboard learningtask. The authors conclude that the cognitive deficits inPDAPP mice are caused by the presence of a soluble Aβspecies, not amyloid deposits.

•• Requirement for hippocampal CA3 NMDA receptors inassociative memory recall. Nakazawa K, Quirk MC,Chitwood RA, Watanabe M, Yeckel MF, Sun LD, Kato A,Carr CA, Johnston D, Wilson MA, Tonegawa S: Science 2002,297:211-218.Significance: NMDA receptor plasticity in the CA3 region iscritical for pattern completion, the retrieval of entire memorytraces from partial or degraded inputs. Findings: The auto-associative network formed by recurrentcollaterals within area CA3 of the hippocampus has been suggested to serve as a mechanism for pattern completion, theretrieval of entire memory traces from partial or degradedinputs. The goal of this study was to examine the necessity ofsynaptic plasticity at the CA3 recurrent collateral synapses inperforming a task requiring pattern completion. Taking advan-tage of the regional specificity of the endogenous kainatereceptor 1 promoter, the authors constructed a transgenicmouse line expressing Cre recombinase and capable of deletingthe NMDA receptor type 1 selectively in the CA3 region.Electrophysiological analysis in three different hippocampalsubregions demonstrated a nearly complete absence of NMDAcurrents and a NMDA receptor-dependent synaptic plasticitydeficit specifically in the CA3 region. Although performance inthe standard hidden platform version of the Morris water mazewas normal in mutant mice when all visual cues were present,partial removal of these cues revealed a deficit in these mice.Single unit recordings showed that although representations ofplace information in the CA1 region (place fields) formed normally in mutant mice, partial removal of visual cues, that presumably guide the mice to the escape platform, destabilizedthese representations. Thus, with their unique set of genetictools, the authors conclude that plasticity at excitatory recurrentcommissural/associational synapses, specifically within regionCA3 of the hippocampus, is necessary for intact retrieval ofspatial memories under conditions where only a partial subsetof the original cues are available to the animal.

•• The endogenous cannabinoid system controls extinctionof aversive memories. Marsicano G, Wotjak CT, Azad SC,Bisogno T, Rammes G, Cascio MG, Hermann H, Tang J,Hofmann C, Zeiglgansberger W, DiMarzo V, Lutz B: Nature2002, 418:530-534.Significance: A novel mechanism controlling the extinction ofassociative fear memories through endogenous cannabinoids.

464 Paper alert

Cognitive neuroscience

Findings: The authors use both genetics and pharmacology toexamine the role of endogenous cannabinoids (CBs) in auditoryfear conditioning where animals learn to fear a neutral stimulus(a tone; conditioned stimulus [CS]) by its association with afootshock. In the first set of experiments, the authors showedthat deletion of the CB1 receptor does not affect acquisition ofauditory fear conditioning but substantially impairs its extinction.The impairment of extinction was significant both short-term(within daily three minute extinction sessions) and long-term(across days). Similarly, a CB1 antagonist (SR141716A) wasshown to impair both short-term and long-term extinction without impairing acquisition. The authors also examined theincrease in endogenous cannabinoid levels following re-expo-sure to the tone 24 hours after associative training. Samplesfrom the basolateral amygdala, but not from the medial prefrontalcortex, demonstrated a specific increase in endogenouscannabinoids following re-exposure to the CS. This effect waslimited to animals receiving paired tone/shock presentations ontraining and was not mediated by the presentation of a CS notpaired with a footshock. In vitro electrophysiological analyses inamygdala slices from CB1 knockout mice demonstratedincreased long-term potentiation, which appeared to be medi-ated by a decrease in GABAA inhibitory currents. The authorspropose that the endogenous cannabinoid system facilitatesextinction of aversive memories by affecting inhibitory networksin the amygdala. (See also Kempermann and Winkler pp 469.)

Selected by Paul J ReberNorthwestern University, Evanston, Illinois, USA

e-mail: preber@nwu.edu

Requirement for hippocampal CA3 NMDA Receptors inassociative memory recall. Nakazawa K, Quirk MC,Chitwood RA, Watanabe M, Yeckel MF, Sun LD, Kato A,Carr CA, Johnston D, Wilson MA, Tonegawa S: Science 2002,297:211-218.Significance: CA3 pyramidal cells play a critical role in patterncompletion in support of associative memory.Findings: Genetically engineered mice with a selective ablationof the NMDA receptor gene in pyramidal CA3 hippocampal neu-rons were tested for learning ability using the Morris water maze.The knockout mice acquired and retained memory of the escaperoute at a normal rate when a set of four spatial cues was avail-able for orientation within the maze. However, when only a singlecue was available, the knockout mice were impaired in escapeperformance relative to wild-type mice and also exhibited areduction in place-related activity in CA1 pyramidal cells. Thisresult provides strong evidence for the hypothesized role of CA3pyramidal cells in pattern completion to support associativememory. (See also Silva and Kushner pp 464.)

Selected by Michael HäusserUniversity College London, London, UK

e-mail: m.hausser@ucl.ac.uk

•• Regulation of AMPA receptor lateral movements.Borgdorff AJ, Choquet D: Nature 2002, 417:649-653.Significance: A direct demonstration that single AMPA-typeglutamate receptors move rapidly on the surface membrane ofneurons and that the delivery of AMPA receptors to synapsesmay be via lateral movement from extrasynaptic locations.

Findings: In a brilliant technical achievement, the authors monitored the movement of single AMPA receptors by coatinglatex beads with GluR2 AMPA receptor antibodies and trackingmovement of the beads as they bound to AMPA receptors onthe surface of hippocampal neurons in culture. AMPA receptorsalternated between rapid random movement and stationaryperiods when the receptors reached a synapse. Importantly,local elevations of calcium immobilised the receptors, suggestingthat a similar process may be activated by the calcium elevations that trigger synaptic plasticity.

•• Directionally selective calcium signals in dendrites ofstarburst amacrine cells. Euler T, Detwiler PB, Denk W:Nature 2002, 418:845-52.Significance: Direct evidence that starburst amacrine cells inthe retina show directionally selective local dendritic calciumsignals, demonstrating that directional selectivity occursupstream from ganglion cells and that dendrites can act asindependent computational compartments.Findings: Retinal ganglion cells show direction selectivity intheir responses to moving stimuli, but the origin of this selectivityhas long been controversial and remains a classic problem incomputational neuroscience. Here, the authors combinedwhole-cell recordings from the soma of starburst amacrine cellsin intact rabbit retina with two-photon imaging of dendritic calcium signals. Dendrites could generate local calcium signalsin response to localised light stimuli. In response to movingstimuli, individual dendritic branches could discriminatebetween different directions of image motion, while the somaticvoltage response was not directionally selective.

•• Transmitter-evoked local calcium release stabilizesdeveloping dendrites. Lohmann C, Myhr KL, Wong RO:Nature 2002, 418:177-181.Significance: Demonstration that local calcium release fromstores triggered by synaptic input stabilises dendritic structureduring the period of synapse formation.Findings: The authors filled embryonic retinal ganglion cells withcalcium indicators using ballistic loading and found local andglobal dendritic calcium increases. Local signals were shown tobe due to calcium-induced calcium release (CICR) from storestriggered by synaptic activation of nicotinic acetylcholine recep-tors. Blocking CICR and local calcium signals caused retractionof dendrites. In contrast, global signals were insensitive to CICRblockers, but were sensitive to tetrodotoxin, which did not affectdendritic structure. Finally, focal uncaging of calcium also triggered CICR and prevented dendritic retraction, confirmingthat local calcium signalling regulates dendritic structure.

Selected by Paola BezziUniversity of Lausanne, Lausanne, Switzerland

e-mail: Paola.Bezzi@ibcm.unil.ch

• The chemokine receptor CXCR2 controls positioning ofoligodendrocyte precursors in developing spinal cord byarresting their migration. Tsai H, Frost E, To V, Robinson S,Ffrench-Constant C, Geertman R, Ransohoff R, Miller R: Cell2002, 110:373-383.Significance: A chemokine CXCL1, through its receptorCXCR2, inhibits oligodendrocyte precursor migration in thedeveloping spinal cord, thus contributing to the temporal andspatial pattern of myelination in this region.Findings: Successful myelination of the vertebrate CNSrequires long-distance migration, much of which is accomplished

Paper alert 465

Signalling mechanisms

by immature cells. Platelet-derived growth factor (PDGF) is amajor mitogen for oligodendrocyte precursor cells (OPCs),enhances precursor motility and provides chemotactic guid-ance. The proliferative response of spinal cord OPCs to PDGFseems to depend on synergistic stimulation by the chemokineCXCL1. In the CNS, CXCL1 is expressed by white matterastrocytes during the time of migration of OPCs. In this paper,the authors provide evidence that CXCL1 inhibits PDGF-stim-ulated migration of immature spinal cord OPCs both in vivo andin vitro. They show that immature OPCs express the chemokinereceptor CXCR2 and that signaling through this receptor mediates a reversible CXCL1 inhibition of migration. Moreover,in slice cultures of embryonic spinal cord, CXCL1 reduces thenumber of cells that migrate to dorsal regions, and, in the spinalcord white matter of mice lacking CXCR2, the number of differentiated oligodendrocytes is diminished and their distribution altered. Thus, the CXCL1/CXCR2 signal systemcontributes to the temporal and spatial pattern of myelination inthe developing spinal cord. The authors further suggest thatduring migration through the white matter, OPCs encounter anenvironment in which astrocytes transiently express locally highlevels of CXCL1, which, via the CXCR2 receptor, inhibit furthermigration and stimulate proliferation in concert with PDGF.

•• Intracellular calcium stores regulate activity-dependentneuropeptide release form dendrites. Ludwig M, Sabatier N,Bull PM, Landgraf R, Dayanithi G, Leng G: Nature 2002,418:85-89.Significance: The first direct evidence that local intracellularCa2+ increase induces oxytocin release from hypothalamic dendrites without increasing electrical activity of the cell bodyor inducing secretion from the nerve terminals.Findings: Neurons in the supraoptic nucleus (SON) of thehypothalamus project axons to the posterior pituitary, whereoxytocin and vasopressin are secreted from axonal nerve terminals into the systemic circulation. These peptides are alsoreleased in large amounts from dendrites in the SON but secretion at these two sites is not consistently correlated.Oxytocin mobilizes intracellular Ca2+ from thapsigargin-sensitive stores. In this paper, the authors tested whetherincreasing intracellular Ca2+ is a critical step for dendritic oxytocin release. They show that direct application of thapsigargin into the rat SON causes a significant andreversible increase in local oxytocin release; subsequent systemic osmotic stimulation causes a much larger release ofoxytocin in thapsigargin-pretreated animals than in controls. Thepotentiating effect of thapsigargin is also present in the activity-dependent oxytocin release from dendrites and in thedepolarization-dependent secretion of oxytocin from isolatedneural lobes. Thus, mobilization of intracellular Ca2+ is followedby a long-lasting potentiation (priming) of activity- or depolar-ization-dependent dendritic release. To investigate thephysiological significance of priming of activity-dependent den-dritic release, the authors recorded from a single oxytocinneuron, while dialyzing the SON with thapsigargin.Thapsigargin had no effect on the mean electrical dischargerate of cells but statistical analyses revealed an important effecton discharge patterning. Ludwig et al. suggest that under normal conditions activity-dependent dendritic release of oxy-tocin may have a positive feedback effect on electrical activity.To test this hypothesis, they showed that constant collisionstimulation (CCS) applied after thapsigargin treatment causedno increase in the firing rate of oxytocin cells, but did reorganize

spike activity. Finally, the effect of oxytocin on spike patterningduring CCS was similar to that observed with thapsigargin.

Selected by John N WoodUniversity College London, London, UK

e-mail: j.wood@ucl.ac.uk

•• Peripheral axotomy induces only very limited sprouting ofcoarse myelinated afferents into inner lamina II of rat spinalcord. Bao L, Wang HF, Cai HJ, Tong YG, Jin SX, Lu YJ, Grant G,Hokfelt T, Zhang X: Eur J Neurosci 2002, 16:175-185.Significance: How important is rewiring in the dorsal horn forchronic pain?Findings: Axotomy-induced sprouting of myelinated afferents(A-fibres) from laminae III–IV into laminae I–II of the spinal hasbeen considered to be important for the structural basis of neuropathic pain. However, the cholera toxin B subunit (CTB),a neuronal tracer previously used to demonstrate the sproutingof A-fibres, also labels unmyelinated afferents (C-fibres) in lamina II and thin myelinated afferents in lamina I, when appliedafter peripheral nerve transection. In this paper, CTB wasapplied four days before axotomy (pre-injury-labelling), andsprouting was monitored after axotomy. Only a small number ofA-fibres sprouted into inner lamina II making synaptic contactwith dendrites. Neuropeptide Y (NPY) was found in thesesprouts in inner lamina II, an area very rich in Y1 receptor-posi-tive processes. These results suggest that axotomy-inducedsprouting from deeper to superficial layers is only marginal inneuropathic preparations. The limited reorganisation involvespredominantly large NPY immunoreactive dorsal root ganglionneurons sprouting into the Y1 receptor-rich inner lamina II.

• Identification of gene expression profile of dorsal rootganglion in the rat peripheral axotomy model of neuro-pathic pain. Xiao HS, Huang QH, Zhang FX, Bao L, Lu YJ,Guo C, Yang L, Huang WJ, Fu G, Xu SH, Cheng XP, Yan Qet al.: Proc Natl Acad Sci USA 2002, 99:8360-8365.• Dorsal horn-enriched genes identified by DNA microarray,in situ hybridization and immunohistochemistry. Sun H,Xu J, Della Penna KB, Benz RJ, Kinose F, Holder DJ, Koblan KS,Gerhold DL, Wang H: BMC Neurosci 2002, 3:11. Publishedonline at: http://www.biomedcentral.com/content/pdf/1471-2202-3-11.pdf.Significance: A catalogue of genes that are misregulated inperipheral neurons after axotomy, and genes that are present insecond order sensory neurons of the dorsal horn.Findings: Xiao et al. examined 7523 genes and expressedsequence tags (ESTs) using 33P-labeled probes. Of these, theexpression of 122 genes and 51 ESTs was strongly changedtwo weeks after axotomy in the rat. These genes include neuropeptides, receptors, ion channels, signal transductionmolecules, synaptic vesicle proteins, and others. The upregula-tion of the GABAA receptor α5 subunit, the peripheralbenzodiazepine receptor, the nicotinic acetylcholine receptorα7 subunit, the P2Y1 purinoceptor, the Na+ channel β2 subunit, and the Ca2+ channel α2δ1 subunit all represent inter-esting observations. Sun et al. conducted a large-scalescreening for genes with enriched expression in the dorsalspinal cord, using DNA microarray and quantitative real-timePCR. In addition to genes known to be specifically expressed

466 Paper alert

Sensory systems

in the dorsal spinal cord, other neuropeptides, receptors, ionchannels, and signaling molecules were also found enriched inthe dorsal spinal cord. In situ hybridization and immunohistochemistry revealed the cellular expression of a subset ofthese genes. The regulation of a subset of the genes was alsostudied in the spinal nerve ligation neuropathic pain model. Ingeneral, the genes enriched in the dorsal spinal cord were notamong those found to be upregulated in the spinal nerve ligation model of neuropathic pain.

•• Bradykinin-12-lipoxygenase-VR1 signaling pathway forinflammatory hyperalgesia. Shin J, Cho H, Hwang SW,Jung J, Shin CY, Lee SY, Kim SH, Lee MG, Choi YH, Kim Jet al.: Proc Natl Acad Sci USA 2002, 99:10150-10155.Significance: Gating of the capsaicin receptor TRPV1 bybradykinin via lipid second messengers provides yet anothermechanism for sensory neuron activation via this channel.Findings: The capsaicin-sensitive vanilloid receptor TRPV1 playsan important role in inflammatory pain (hyperalgesia), but theunderlying mechanism is unknown. This study demonstrates thatbradykinin, acting at B2 bradykinin receptors, excites sensorynerve endings by activating capsaicin receptors via production of12-lipoxygenase metabolites of arachidonic acid. This finding iden-tifies a mechanism that might be targeted in the development ofnew therapeutic strategies for the treatment of inflammatory pain.

•• Annexin II light chain regulates sensory neuron-specificsodium channel expression. Okuse K, Malik-Hall M,Baker MD, Poon WY, Kong H, Chao MV, Wood JN: Nature2002, 417:653-656.•• p11, an annexin II subunit, an auxiliary protein associ-ated with the background K(+) channel, TASK-1. Girard C,Tinel N, Terrenoire C, Romey G, Lazdunski M, Borsotto M:EMBO J 2002, 21:4439-4448.Significance: Demonstration that the annexin light chain p11plays a crucial role in the regulation of receptor trafficking forthe voltage-gated sodium channel Nav1.8 and the two-passpotassium channel TASK1.Findings: Annexin II light chain p11 is a regulatory factor thatfacilitates the expression of Nav1.8. p11 binds directly to theamino terminus of Nav1.8 and promotes the translocation ofNav1.8 to the plasma membrane, producing functional chan-nels. The endogenous Nav1.8 current in sensory neurons isinhibited by antisense downregulation of p11 expression. p11also interacts specifically with the TASK1 K+ channel. Thisassociation with p11 requires the integrity of the last three carboxy-terminal amino acids, Ser–Ser–Val, in TASK1.Association with p11 is essential for trafficking of TASK1 to theplasma membrane. p11 association with the TASK1 channelmasks an endoplasmic reticulum retention signal identified asLys–Arg–Arg that precedes the Ser–Ser–Val sequence. Thisnovel role of p11 adds another unsuspected function to themany roles of annexin subunits.

Selected by David S BredtUniversity of California at San Francisco, San Francisco, California

e-mail: bredt@phy.ucsf.edu

•• Regulation of AChR clustering by dishevelled interactingwith MuSK and PAK1. Luo Z, Wang Q, Zhou J, Wang J, Luo Z,

Liu M, He X, Wynshaw-Boris A, Xiong W, Lu B, Mei L: Neuron2002, 35:489.Significance: Description of the cellular signaling cascade thatmediates acetylcholine receptor clustering in response to agrin.Findings: Previous studies have shown that agrin binding to amuscle specific kinase (MuSK) induces clustering of acetylcholinereceptors (AChRs) at neuromuscular junctions; however, the cellular mechanism for this has been uncertain. This study showsthat MuSK binds to dishevelled, which in turn interacts with adownstream kinase PAK1. Disrupting these interactions orinhibiting PAK1 activity attenuates AChR clustering in Xenopusmyocytes or cultured mammalian myotubes. These results iden-tify important roles for dishevelled and PAK1 in agrin-inducedAChR clustering.

•• Subunit-specific NMDA receptor trafficking to synapses.Barria A, Malinow R: Neuron 2002, 35:345.Significance: Definition of the subunit-specific rules governingNMDA insertion into hippocampal neurons. Findings: Using optically and electrophysiologically taggedNMDA receptor subunits infected into hippocampal slice neuronal cultures, the authors determine the rules that controlsynaptic incorporation of these receptors. NR1 subunits do notgo to synapses alone, they are driven to synapses by coex-pression with either NR2A or NR2B subunits. During normaldevelopment, NR2A receptors replace NR2B receptors andthis requires synaptic activity. Interestingly, glutamate binding toNR2B rather than calcium influx through the receptor is essential for replacement. Thus, as with AMPA receptors, subunit-specific trafficking determines the synaptic expressionof NMDA receptors.

• Polyribosomes redistribute from dendritic shafts intospines with enlarged synapses during LTP in developing rathippocampal sites. Ostroff L, Fiala J, Allwardt B, Harris K:Neuron 2002, 35:535.Significance: Demonstration that the protein synthesis machineryredistributes to synaptic spines during long-term potentiation.Findings: To assess the presence of polyribosomes in dendriticspines as a potential indicator of increased local protein synthesis in synaptic plasticity, the authors assessed the ultra-structure of synapses in hippocampus before and afterlong-term potentiation (LTP). The authors found a dramaticincrease in the percentage of spines containing polyribosomesfollowing LTP and a commensurate loss of polyribosomes fromdendritic shafts. Furthermore, the spines containing polyribo-somes showed enlarged postsynaptic densities. These studiessuggest that increased protein synthesis during LTP may modulate synaptic morphology and contribute to changes insynaptic strength.

Selected by Matthew Chafee* and Hugo MerchantBrain Sciences Center, VAMC, Minneapolis, Minnesota, USA

*e-mail: chafe001@tc.umn.edu

• Training and synchrony in the motor system. Schieber MH:J Neurosci 2002, 22:5277-5281.Significance: Synchronization among cells with connections tospinal motoneuron pools is larger in monkeys trained to performfinger and wrist movements for long periods of time than in amonkey trained for less than a year.

Paper alert 467

Neuronal and glial cell biology

Motor systems

Findings: The activity of neurons in the motor cortex and theelectromyogram (EMG) of forearm and hand muscles wererecorded in two sets of monkeys. One set was trained for morethan five years to perform 12 finger and wrist movements, theother was trained for less than a year to perform six fingermovements. The results show; first, that the spike-triggeredaverages (STAs) of the EMG were more prevalent in monkeyswith longer training; second, in the monkey with shorter training,most of the STAs had onset latencies and peak widths corre-sponding to relatively pure postspike effects; and third, themajority of the STAs of monkeys with longer training had anonset latency that was too early and a peak width that was toowide to represent a pure postspike effect from the trigger neuron. This last result indicates that other neurons, also connected to the motoneuronal pool, discharge in synchronywith the recorded motor cortical cell.

• A real-time state predictor in motor control: study of saccadic eye movements during reaching movements. Ariff G,Donchin O, Nanayakkara T, Shadmehr R: J Neurosci 2002,22:7721-7729.Significance: Immediately after a brief perturbation of the hand,the brain is able to predict where the limb will be in the near future.Saccades are an unbiased, real-time estimator of this prediction.Findings: Human subjects performed unseen reaching move-ments while tracking with the eyes the hand trajectory withoutvisual feedback. In unperturbed reaching movements, the saccade at time t provided an unbiased estimate of the handposition at t + 196 ms. When a 50 ms random force perturba-tion to the moving hand was applied, the saccades showed thefollowing: a sharp inhibition at 100 ms after perturbation, and alarge increase of occurrence at 170 ms after perturbation. Thesesaccades were an unbiased estimator of the hand position atsaccade time t + 150ms. These results suggest that the eyebehavior during reaching is an indicator that the brain computesan estimate of the future limb position, based on an internalmodel that depends on real-time proprioceptive feedback.

• Adaptation to a visuomotor shift depends on the startingposture. Baradac P, Wolpert DM: J Neurophysiol 2002,88:973-981.Significance: Evidence that motor adaptation corrects movementin a manner that is dependent upon the joint angles that specifythe posture of the entire limb, and not just the limb endpoint.Findings: In learning to compensate for an artificial change invisual feedback, the motor system might adapt to this changeby recalibrating movement trajectories with respect to handposition, or with respect to the joint angles that specify the tra-jectory. In this study, subjects produced pointing movements toa target in a virtual three-dimensional display. Visual feedback offinger position was provided (on some trials). Subjects learnedto hit the target after an offset was introduced between actualand displayed finger position. They were unaware of this adap-tation. After adapting to the change in visual feedback whilemaking the reach from one starting arm posture, the subjectswere tested with different initial arm postures. The start and endpoints of the movements, and thus the path of the hand throughspace, were constant in all cases. The interesting result wasthat the adaptation of the subjects was specific for the startingarm posture. The degree of adaptation that was seen was afunction of the difference between a given starting arm posture,and the starting posture adopted while adaptation was initiallyacquired. This indicates that motor adaptation modified the

visuomotor transformation in a circumscribed region of jointspace, and that when the subject was forced to produce the same hand movement through different arm postures, adaptation broke down.

Selected by Stephen Goodwin* and Bambos Kyriacou†

*University of Glasgow, Glasgow, UKe-mail: stephen@molgen.gla.ac.uk

†University of Leicester, Leicester, UKe-mail: cpk@leicester.ac.uk

•• Identification of genes involved in Drosophila melanogastergeotaxis, a complex behavioral trait. Toma DP, White KP,Hirsch J, Greenspan RJ: Nat Genet 2002, 31:349-353.Significance: Identification of the single genes that contributeto a complex phenotypic behavioural character, using micro-array analysis of bidirectionally selected lines.Findings: At the dawn of experimental behaviour genetics in the1950s, Jerry Hirsch and his collaborators, using speciallydesigned mazes, selected for lines of Drosophila melanogasterthat were either positively (moving towards gravity) or negativelygeotactic. The selection was slow, evidently because variation atmany loci was reshuffled and selected at each generation, andchromosome substitution analyses revealed that genes spreadover the entire genome were contributing to differences in thisbehaviour. Forty years later, Greenspan and his colleaguesobtained Jerry Hirsch’s lines, that had been reselected, andapplied cDNA microarrays to two lines showing opposite geotactic behaviour. About 250 genes showing consistent differential expression in the two lines were identified, and asmall number were validated with qPCR. Studying the behaviourof existing mutants in four genes that gave differences betweenthe lines, confirmed the array results at a functional level in threegenes, cryptochrome, pigment-dispersing factor, and pendulin,which encodes a nuclear importin. Interestingly, the first twogenes are implicated as input and output factors respectively inthe circadian mechanism. These results are interesting in themselves, but as a concept, the use of microarrays to dissectpolygenic traits that have been initially revealed by selectionexperiments, marries the classic quantitative approach to behavioural genetics, with the modern pragmatic single geneapproach to dissecting the nervous system.

•• The orphan nuclear receptor REV-ERB alpha controlscircadian transcription within the positive limb of the mammalian circadian oscillator. Preitner N, Damiola F, Molina LL,Zakany J, Duboule D, Albrecht U, Schibler U: Cell 2002,110:251-260.Significance: Rev-erbα is the molecule that connects the tworegulatory pathways, composed of at least one negative feedback loop coupled to a feedforward pathway, in the mammalian circadian clock.Findings: Circadian rhythms in mammals are driven by a mole-cular oscillator located in the suprachiasmatic nuclei (SCN).Major components of the oscillator include the mPer and mCrygenes, whose transcripts cycle in abundance with a 24 hperiod, and mClock and Bmal1, whose transcripts also cycle.mCRY acts as a negative autoregulator and represses bothmPer and mCry transcription, whereas mPER and perhapsmCRY may act as positive regulators for Bmal1 gene expression.

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This interlocking of positive and negative loops stabilises theclockworks, and generates cycles for the mPer/mCry and Bmal1transcripts that are in antiphase. Very little is known about howthe Bmal1 cycle is generated, but its promoter contains putativebinding sites for ROR and REV-ERB orphan nuclear receptors,and mobility shift assays using these sequences reveal cyclingprotein–DNA complexes from mouse liver extracts. Levels ofRev-erbα mRNA showed diurnal cycling in the SCN, and proteincycling in the liver, but in antiphase to Bmal1 cycling, suggestingthat REV-ERB is the negative regulator of Bmal1. In transgenicRev-erbα knockout mice, Bmal1 transcript cycling was dramati-cally reduced in both liver and SCN, and some disruption wasseen in both the mClock and mCry1 cycles, but protein cycles inthe latter two molecules were hardly affected. The mutant miceshowed slightly longer free-running periods, but no majordefects in their wheel-running activity, suggesting that tran-script cycles of Bmal1 are not critical for rhythmic behaviourper se, although more dramatic effects were seen in the mutantcircadian phase-shifting by light pulses. Further genetic analysis revealed that Rev-erbα is repressed by mPER protein,thereby activating Bmal1 transcription. As the Rev-erbαpromoter contains three E-boxes (targets for the BMAL1-mCLOCK bHLH regions), then mCLOCK-BMAL1 would alsobe the activators of Rev-erbα, a suggestion that is supported byseveral other lines of circumstantial evidence. Thus, REV-ERBαcouples the negative to the positive limb of the molecular oscillator in the mammal.

Selected by Gerd Kempermann* and Juergen Winkler†

*Max Delbrück Center for Molecular Medicine (MDC), Berlin, Germanye-mail: gerd.kempermann@mdc-berlin.de

†Department of Neurology, University of Regensburg, Germanye-mail: juergen.winkler@klinik.uni-regensburg.de

• Mice with truncated MECP2 recapitulate many Rett syndrome features and display hyperacetylation of histone H3. Shahbazian MD, Young JI, Yuva-Paylor LA,Spencer CM, Antalffy BA, Noebels JL, Armstrong DL, Paylor R,Zoghbi HY: Neuron 2002, 18:243-254.Significance: The unusual case of a murine model for an extremelycomplex human disorder that even mimics surprising details.Findings: Rett syndrome (RTT) is a complex pediatric syndrome,caused by mutations in the methyl–CpG binding protein 2(MECP2). Surprisingly, dysfunction of MECP2 protein, whichinteracts with the histone deacetylase repressing gene tran-scription, leads to a primarily neurological phenotype. To retainpartial MECP2 function, the authors generated a mouse with atruncated MECP2 gene. These mice showed numerous symp-toms found in RTT, from myoclonic seizures, tremor, hypoactivity,and kyphosis to stereotyped forelimb movements as the murineequivalent to the characteristic handwringing movements seenin humans with RTT. Cognitive parameters, such as watermazelearning and fear conditioning were normal. Mutant mice hadelevated tissue-specific acetylation of histone H3, suggestingthat the mutants have an impaired chromatin structure.Importantly, the mouse model was generated in males, where itwould not be confounded by X-chromosome inactivation.

•• The endogenous cannabinoid system controls extinction ofaversive memories. Marsicano G, Wotjak CT, Azad SC,

Bisogno T, Rammes G, Cascio MG, Hermann H, Tang J,Hofmann C, Zieglgansberger W et al.: Nature 2002,418:530-534.Significance: An exploration of how we can forget unwantedmemories.Findings: The molecular mechanisms underlying the extinctionof (aversive) memories are largely known. Mice deficient of the(endo-) cannabinoid receptor 1 (CB1) showed severe impair-ments in memory extinction in auditory fear-conditioning tasks,whereas memory acquisition was normal. Treating wild-typemice with a CB1 antagonist mimicked the mutant phenotype. Aside experiment showed that indeed during the extinction trialof the test, endocannabinoid levels were elevated in brain tissue. In the amygdala of CB1–/– mice, electrophysiologicalcorrelates of learning (long-term potentiation) were more pro-nounced, whereas correlates of inhibitory function (long-termdepression of inhibitory postsynaptic currents) were abolished.The authors conclude that endocannabinoids selectively inhibitinhibitory connections in the amygdala. This is an importantpaper with many implications for psychiatry and with a greatgeneral appeal due to its exploration of how to forget unwantedmemories. (See also Silva and Kushner pp 464.)

• Cognitive slowing in Parkinson’s disease: a behavioralevaluation independent of motor slowing. Sawamoto N,Honda M, Hanakawa T, Fukuyama H, Shibasaki H: J Neurosci2002,22:5198-203.Significance: A clever experimental design allowing assess-ment of cognitive slowing in Parkinson’s disease withoutconfounding the test with the dominating motor impairment.Findings: In this study, Parkinson’s disease (PD) patientswithout bradykinesia (slowness of movements, one of thepossible motor symptoms of PD) were examined and werepresented with a series of visual stimuli, on which they had toreact by performing a mental operation. Both patients andcontrol subjects became worse with increasing speed of thetest, but PD patients performed with significantly lower accu-racy than controls. The study indicates that the decrease inthe speed of performing a given task, as seen in PD patients,is not restricted to motor function, but also affects cognitivesystems. This finding is important, because the issue ofwhether PD patients are genuinely slower in cognitive processing and thus whether or not such symptoms wouldpotentially benefit from particular therapeutic attention hasbeen a subject of long debate.

• Evidence for a genetic association between monoamineoxidase A and restless legs syndrome. Desautels A,Turecki G, Montplaisir J, Brisebois K, Sequeira A, Adam B,Rouleau GA: Neurology 2002, 59:215-219.Significance: Another step towards a better understanding of theinvolvement of the dopamine system in restless legs syndrome.Findings: Restless legs syndrome (RLS) is associated with apredominant nocturnal, unpleasant aching and drawing sensation of the calves and thighs that is shortly suppressedby moving the legs. This compulsion periodically interruptssleep and is ultimately irresistible. In particular, the therapeuticefficacy of dopaminergic-mimetic drugs suggests the involve-ment of the dopaminergic system in this syndrome. Thispopulation-based study links genetic variations of themonoamine oxidase A (MAOA) gene to RLS. In particular, thefunctional variable tandem repeat polymorphism that wasrecently identified in the MAOA gene promoter region

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resulting in an elevated MAOA activity was correlated with theseverity of RLS symptoms. No association of RLS with theMOAB gene was detected.

• An in vitro model of Parkinson’s disease: linking mito-chondrial impairment to altered αα-synuclein metabolismand oxidative damage. Sherer TB, Betarbet R, Stout AK,Lund S, Baptista M, Panov AV, Cookson MR, Greenamyre JT:J Neurosci 2002, 22:7006-7015.Significance: An in vitro model links several etiologic factorsimplicated in PD pathogenesis and may provide a good tool totest future therapeutic agents for the treatment of PD.Findings: Sherer et al. developed a chronic in vitro PD modelbased on exposing human neuroblastoma cells to a low concentration of rotenone, a naturally occurring compound thatis used as an insecticide and specifically inhibits the activity of complex I of the mitochondrial electron transfer chain.Treatment with rotenone lasting four weeks increased levels ofinsoluble α-synuclein and ubiquitin and was paralleled by anincreased occurrence of apoptosis and higher vulnerability tofurther oxidative challenge. These results provide evidence thatchronic low-grade complex I inhibition may be sufficient tomimic several important mechanisms leading to PD.

• Neuroprotective effects of glial cell line-derived neurotrophic factor mediated by an adeno-associated virusvector in a transgenic animal model of amyotrophic lateralsclerosis. Wang LJ, Lu YY, Muramatsu S, Ikeguchi K,Fujimoto K, Okada T, Mizukami H, Matsushita T, Hanazono Y,Kume A et al.: J Neurosci 2002, 22:6920-6928.Significance: Another promising attempt to deliver the mostpotent trophic factor for motoneurons, glial cell line-derivedneurotrophic factor, to an in vivo model of amyotrophic lateralsclerosis-related awaiting to be confirmed in the clinic.Findings: At present, there is no effective therapy available tocure amyotrophic lateral sclerosis (ALS), a fatal neurological dis-order caused by progressive spinal motoneuron degeneration.To test the efficacy of the adeno-associated virus (AAV) genedelivery approach for glial cell line-derived neurotrophic factor(GDNF), the authors used transgenic mice overexpressing themutant Cu/Zn superoxide dismutase gene that is linked to asubtype of familial ALS. Intramuscular AAV–GDNF injectionsresulted in an increased synthesis of GDNF in the musclesassociated with retrograde transport to the correspondingspinal motoneurons. More importantly, transgene expressionprevented the loss of motoneurons leading to a delayed onsetand attenuated manifestation of motor symptoms.

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