I am pleased to provide you complimentary one-time access to my article as a PDF file for your own personal use. Any further/multiple distribution, publication or commercial usage of this copyrighted material would require submission of a permission request to the publisher. Timothy M. Miller, MD, PhD

www.sciencemag.org SCIENCE VOL 307 21 JANUARY 2005 361

It is difficult to overstate the impact ofpenicillin and the family of β-lactamantibiotics since their introduction into

clinical medicine in the early 1940s. Thesedrugs act by inhibiting assembly of the pro-tective outer wall of bacteria. Their impacton the treatment of a wide variety of infec-tions has been nothing short of miraculous.But this family of wonder drugs from thelast century may have yet more untappedtherapeutic potential, as Rothstein and col-leagues report in a recent issue of Nature(1). They demonstrate that certain β-lactamantibiotics have potential as neurotherapeu-tics for treating neurological diseases suchas amyotrophic lateral sclerosis (ALS),adult motor neuron disease,and ischemic injury.

The evidence for thisremarkable f inding hasarisen from a unique pub-lic-private partnershipbetween the NationalInstitute of NeurologicDisorders and Stroke of theNIH and a consortium of dis-ease-oriented philanthropic organizations,including the ALS Association, theHuntington’s Disease Society of America,and the Hereditary Disease Foundation. Thisconsortium sponsored a drug screeningeffort that ignored the hundreds of thousandsof compounds within the traditional chemi-cal libraries mined by pharmaceutical com-panies. Instead, the consortium screened1040 bioactive compounds, 750 of whichwere already approved by the FDA for use inhumans. These compounds were then testedfor their efficacy in multiple assays associ-ated with one or more neurological diseasesby 27 separate academic laboratories.

The f irst insight to emerge from thisapproach (see the figure, panel A) was a sur-prising new function for 15 β-lactam antibi-otics, including penicillin and a more modernvariant, ceftriaxone, that enters the brain bycrossing the blood-brain barrier. These

antibiotics selectively induce transcription ofthe gene encoding the EAAT2 glutamatetransporter; other classes of antibiotics do nothave these effects. Glutamate is crucial fornormal signal transmission between manytypes of neurons, including the motor neu-rons whose job is to trigger muscle contrac-tion and whose premature death produces theprogressive paralysis characteristic of ALS.Upper motor neurons extend processes fromthe brain into the spinal cord, where theyform synaptic attachments directly with thelower motor neurons or indirectly throughintermediate neurons. The axonal processesof the lower motor neurons extend out of thespinal cord and form connections with mus-

cle. These neurons communicate with eachother by release from the presynaptic cell ofthe neurotransmitter glutamate (see the fig-ure, panel B), which then binds to receptorsexpressed by the lower motor neuron.Glutamate receptor activation triggers localmembrane depolarization and generation ofan electrical impulse that propagates downthe full length of the neuron, where it stimu-lates release of another neurotransmitter thatprovokes contraction of the muscle. In thespinal cord, a non-neuronal supporting cell,the astrocyte, provides a key element in thissignaling pathway—that is, a rapid off switchfor the glutamate signal. It does this by juxta-posing a fingerlike projection adjacent to thesynapse between the two motor neurons. Onthe surface of this projection are EAAT2 glu-tamate transporters, which are glutamatepumps that allow eff icient recovery ofreleased glutamate and hence rapid silencingof the glutamate signal.

Excessive glutamate levels in the synapseand associated repetitive firing of neuronsresults in excitotoxic injury to neurons, a fea-

M E D I C I N E

Treating NeurodegenerativeDiseases with Antibiotics

Timothy M. Miller and Don W. Cleveland

The authors are at the Ludwig Institute for CancerResearch and the Department of Medicine andNeurosciences, University of California, San Diego, LaJolla, CA 92093, USA. E-mail: dcleveland@ucsd.edu

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Reinventing the β-lactam heroes of the last century. (A) Rapid drug discovery by subjecting com-pounds already approved by the FDA to a battery of new biological screening assays. (B) Communicationbetween motor neurons via the excitatory neurotransmitter glutamate. Glutamate released from theending of one motor neuron binds to glutamate receptors expressed on the surface of a downstreammotor neuron, triggering its activation. Excitotoxic damage results from excessive firing from thesereceptors, leading to the continued influx of calcium ions and resulting in neuronal injury.The glutamatetransporter protein EAAT2 recovers synaptic glutamate such that glutamate neurotransmission is rap-idly silenced and the neurons are protected from excess stimulation.Certain β-lactam antibiotics induceexpression of the glutamate transporter, reducing the risk of excitotoxic damage to neurons.

Published by AAAS

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ture of many neurological disorders includingstroke, spinal cord injury, and ALS (2).Excitotoxicity is one of the best links betweenthe rare familial form of ALS (caused bymutations in the gene encoding superoxidedismutase) and the more common sporadicform of this disease (3). This realization camefrom studies in the early 1990s that showedincreased glutamate in the fluid surroundingthe brain and spinal cord of patients with spo-radic ALS (4, 5). Similarly, in a rat model offamilial ALS, animals develop focal loss ofthe EAAT2 glutamate transporter in regionsof the spinal cord that house motor neurons(6). Indeed, the only approved medication fortreating ALS, the drug riluzole, is thought toact by limiting synaptic glutamate release.The effectiveness of this drug, however, hasbeen disappointing, extending survival ofALS patients by a mere 3 months (7).

The remarkable discovery by Rothsteinand co-workers offers renewed hope for amore effective therapy for ALS and otherneurological diseases. A ceftriaxone-inducedincrease in expression of glutamate trans-porters by astroctyes enhanced the clearanceof glutamate in spinal cord explants and,more important, slowed loss of musclestrength and modestly extended survival ofALS mice. Any benefit from increased gluta-mate clearance also extends to other types ofneuronal damage with an excitotoxic compo-nent—for example, the damage that accom-panies decreased blood flow typical of stroke

(frequently referred to as ischemia). A ceftri-axone-mediated increase in glutamate clear-ance also decreased neuronal death inducedby oxygen deprivation, at least in cell culture.

The encouraging preclinical data from theALS mouse and other models of neurologicaldisease have prompted a clinical trial combin-ing all three phases. This is expected to beginthis spring with a safety and efficacy study ofceftriaxone in the treatment of ALS. The datagenerated by the consortium represent a phe-nomenally quick turnaround from initial drugscreening (started in early 2002) to actual usein patients. This reflects the major advantageof screening FDA-approved drugs whosesafety profiles are already known. Even moreencouraging for the Rothstein et al. findingsare the excellent safety profiles of the β-lactam antibiotics in humans. Drug toxicity iscostly and time-consuming to exclude and, inthe end, is often the Achilles’ heel that sinksthe development of promising new therapeu-tics. In addition, spotting unwanted sideeffects is challenging even after undertakingmultiple preclinical and clinical trials. Thislesson was learned most recently with therealization of the increased risk of stroke andheart attack in people taking commonly pre-scribed anti-inflammatory drugs (8). Againstthis backdrop are the β-lactam antibiotics,first identified with the discovery of peni-cillin in 1928 and now among the mostwidely used modern pharmaceuticals.Although data about the safety of long-term

ceftriaxone use still need to be collected, thebest predictor of safety is a long history ofsafe use in humans. Our vast experience withshort-term β-lactam antibiotic treatment pre-dicts that very few problems should arise overthe long term.

The discovery of new modes of action forthe β-lactam antibiotic family offers two addi-tional lessons for biomedical researchers. Thefirst is unproven but predictable: A systematicscreen of easily accessible chemical com-pounds already approved by the FDA mayreveal common therapeutics with new poten-tial applications. The second is more surpris-ing: Some of these compounds may act bytranscriptional induction of key proteins.Searching for transcriptional up-regulation isnot an approach generally thought attractive indrug screening. With that in mind, last cen-tury’s miracle drug, the β-lactams, may wellrise to one of the big challenges of this cen-tury: slowing the progression of neurologicaldiseases whose treatment has so far evaded theworld’s best efforts.

References1. J. D. Rothstein et al., Nature 433, 73 (2005).2. M. P. Mattson, Neuromol. Med. 3, 65 (2003).3. P. R. Heath, P. J. Shaw, Muscle Nerve 26, 438 (2002).4. J. D. Rothstein et al., Ann. Neurol. 28, 18 (1990).5. J. D. Rothstein et al., Ann. Neurol. 30, 224 (1991).6. D. S. Howland et al., Proc. Natl. Acad. Sci. U.S.A. 99,

1604 (2002).7. L. I. Bruijn et al., Annu. Rev. Neurosci. 27, 723 (2004).8. E. J. Topol, N. Engl. J. Med. 351, 1707 (2004).

10.1126/science.1109027

Radiocarbon (14C) dating (1, 2) iswidely used to determine the ages ofsamples that are less than about

50,000 years old. Natural radiocarbon ismainly formed in Earth’s stratospherethrough the interaction of neutrons pro-duced by cosmic rays with 14-nitrogen.However, the rate of radiocarbon produc-tion is not constant (3), nor is its partition-ing among the atmosphere, terrestrial bio-sphere, and oceans. After local corrections[see, for example, (4–6)], radiocarbon agesmust therefore be calibrated to obtain ageson an absolute time scale (7). For decades,

the radiocarbon community has adoptedinternational calibration standards, mostrecently IntCal98 (8). Here, we discuss theinherent limitations faced when usingradiocarbon dates to derive calendar ages.

From modern day to 11,800 years ago,IntCal98 is based on sets of tree-ringchronologies that each cover several thou-sand years and together provide an annuallyresolved, nearly absolute time frame. Thesedata set a quality standard against whichother proposed calibration datasets can bejudged. Prior to 11,800 years ago, IntCal98is based on marine data and contains addi-tional assumptions and uncertainties asso-ciated with the translation of marine datainto atmospheric radiocarbon values.

Here we examine how precisely calen-dar ages can be determined from individualradiocarbon dates. We focus on the tree-

ring section of the IntCal98 calibrationcurve. Between 0 and 8000 years before thepresent (B.P.), the error in this curve is oftenless than 20 years, and—except for a fewbrief intervals—it is less than 30 years overthe past 11,800 years. But as we will show,the range of statistically possible calendarages, or calibrated age ranges, correspon-ding to any particular radiocarbon date canbe larger or smaller, depending on where itfalls on the curve.

We have linearly interpolated the Int-Cal98 curve at intervals of 20 calendaryears and determined the radiocarbon datesthat correspond to the calendar ages. Wethen calibrated these resampled radiocar-bon ages using CALIB v4.4 (4) assumingan uncertainty of ±40 radiocarbon years,which is currently typical of routine dating(calibration 1). We performed a second cal-ibration with a constant uncertainty of ±15radiocarbon years, which is typical of theIntCal98 tree-ring data (calibration 2).

The calibrated age range waxes andwanes (see the first figure) as a result ofvariations in the atmospheric 14C/12C ratio.On average, the 1σ calibrated age range is180 years (minimum 30 years, maximum529 years) for calibration 1 and 140 years

G E O S C I E N C E

The Boon and Baneof Radiocarbon Dating

Tom P. Guilderson, Paula J. Reimer, Tom A. Brown

The authors are at the LLNL Center for AcceleratorMass Spectrometry, University of California,Livermore, CA 94550, USA. T. P. Guilderson is also atthe Institute of Marine Science and Department ofOcean Sciences, University of California, Santa Cruz,CA 95064, USA. E-mail: tguilderson@llnl.gov

21 JANUARY 2005 VOL 307 SCIENCE www.sciencemag.org

P E R S P E C T I V E S

Published by AAAS