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Huntingtons disease and neurogenesis:FGF-2 to the rescue?Albert R. La Spada*Departments of Laboratory Medicine, Medicine, and Neurology, and Center for Neurogenetics and Neurotherapeutics,University of Washington Medical Center, Box 357110, Room NW120, Seattle, WA 98195-7110

Huntingtons disease (HD) isan autosomal dominant in-herited neurodegenerativedisorder characterized by in-

voluntary choreiform movements, cogni-tive impairment, metabolic abnormalities,and a relentlessly progressive course cul-minating in death 10 25 years after on-set. Neuropathology studies of HD haveestablished a stereotypical pattern of neurodegeneration and neuron loss, re- vealing that medium spiny neurons of the striatum are primarily affected fol-lowed by regions of the cerebral cortex

(1). The genetic basis of HD is expan-sion of a CAG trinucleotide repeat within the huntingtin (htt) gene, result-ing in the production of htt protein con-taining an expanded glutamine tract (2).Such polyglutamine tract expansionsare now known to be the cause of nineinherited neurological diseases. As poly-glutamine expansions misfold to pro-duce altered peptide conformers thatare resistant to the normal cellular pro-cesses of protein turnover, aggregatesor inclusions of the aberrant proteinaccumulate within neurons in HD brainregions (3). The occurrence of mis-folded proteins that cannot be properlyturned over is an emerging theme inneurodegenerative disease, as Alzhei-mers disease, Parkinsons disease,amyotrophic lateral sclerosis, prion dis-eases, and polyglutamine repeat diseasessuch as HD all share this feature (4). Although much progress has been madein understanding the molecular geneticbasis of HD, efforts to develop a cure ordefinitive treatment for HD have beencomplicated by an inadequate under-standing of why certain populations of neurons degenerate within the striatumand cortex in HD despite widespreadexpression of the htt mutant proteinthroughout the central nervous system(CNS). Furthermore, polyglutamine-expanded htt can produce a myriad of nuclear and cellular abnormalities, ac-counting for the promulgation of a pan-orama of potential therapies directed atdifferent aspects of the molecular pa-thology (5). One intriguing therapeuticoption yet to be explored in HD isgrowth factor stimulation of endogenousneurogenesis. In this issue of PNAS, Jin et al. (6) present the results of a com-pelling preclinical trial of fibroblast

growth factor 2 (FGF-2) in whichthey demonstrate that growth factor-mediated stimulation of neural stemcell proliferation should be added tothe expanding armamentarium of can-didate HD therapies.

The existence of populations of multi-potent neural stem cells within the adultCNS was first recognized in 1962 (7). As it turns out, dense pools of suchmultipotent neural stem cells occur incircumscribed regions of the brain: thesubventricular zone (SVZ) [or sub-ependymal zone (SEZ)] and the hip-

pocampal subgranular zone (SGZ).Numerous studies performed on multi-potent neural precursors indicate thatthese cells retain the capacity to differ-entiate into a wide range of neuronsand glia (reviewed in ref. 8). Neuronsderived from such neural stem cells arecapable of migrating to various regionsof the CNS, receiving afferent innerva-tion, forming axonal projections, andexpressing neurotransmitters. A decadeof work with such neural stem cellsboth in vivo and ex vivo has revealedthat exposure to various combinationsof growth factors [e.g., FGFs, epider-mal growth factor (EGF), transforminggrowth factors (TGFs)] and/or neuro-trophic factors [e.g., brain-derived neu-rotrophic growth factor (BDNF) andciliary neurotrophic factor (CNTF)] per-mits directed differentiation along eitherneuronal or astrocytic lineages (9). Thesuccess of manipulating stem cells inculture has fueled interest in the ascer-tainment of undifferentiated stem cellsand created a new field known as re-generative medicine in which stem cellsare coaxed into forming cells of any lin-eage (including neural cells) and thendelivered to regions of injury or degen-eration to treat disease. Administrationof growth factors to direct endogenousneural stem cells to differentiate intoneurons or glia [as was done by Jin et al.(6)] thus differs from cell-replacementtherapies based on delivery of ex vivo-derived neural cells to areas of injury ordegeneration.

Because FGF-2 had been shown tostimulate proliferation of neural stemcells differentiated into striatal-like neu-rons and protect striatal neurons in toxin-induced models of HD (10, 11), Jin et al.(6) selected FGF-2 for their study. To

test the ability of FGF-2 to promoteneurogenesis in the SVZ in HD, theyinjected FGF-2 s.c. into HD R6/2 trans-genic mice. The R6/2 mouse model is widely used for therapeutic trials anddisplays an early-onset, rapidly progres-sive HD-like neurological phenotypeculminating in death at 1215 weeks of age (12). The severity of the R6/2 phe-notype stems from significant expressionof a severely truncated version of the httgene, containing the amino-terminal 3%of the htt coding region with an 120-glutamine repeat. Using BrdUrd label-

ing, they observed a 150% increase inneuron stem cell proliferation in FGF-2-treated HD mice compared with un-treated HD mice. This increase wasmore than five times greater than theproliferation increase noted for FGF-2-treated nontransgenic mice, suggestingthat SVZ neural stem cells in HD areprimed to proliferate in accordance withan earlier study that reported increasedneurogenesis in human HD brains (13).Newly generated neurons were double-cortin- and DARPP-32-positive, verifyingtheir migratory nature and striatal-likeproperties. Injections of a fluorescenttracer dye into the globus pallidus con-firmed that the newly generated neurons were sending axonal projections to theproper neuroanatomical region, furthersuggesting that FGF-2 could induce en-dogenous neurogenesis in the SVZ of HD transgenic mice to yield new neu-rons that migrated into the basal gangliaand projected axons to the expectedneuroanatomical location.

After documenting the effect of FGF-2 on SVZ neural stem cells, Jin et al. (6) evaluated FGF-2 as a treat-ment for HD in a cohort of R6/2 mice,initiating FGF-2 injections at 8 weeksof age. FGF-2 therapy significantly ex-tended average survival and maximumsurvival, reduced tremor, and improvedmotor function (based on rotarod per-formance) in treated HD R6/2 trans-genic mice. As FGF-2 was administeredby s.c. injection and HD R6/2 mice suf-fer from a wide range of metabolic ab-

Conict of interest statement: No conicts declared.

See companion article on page 18189.

*E-mail: laspada@u.washington.edu.

2005 by The National Academy of Sciences of the USA

www.pnas.org cgi doi 10.1073 pnas.0509222102 PNAS December 13, 2005 vol. 102 no. 50 1788917890

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normalities that likely contribute to, if not cause, their early demise, FGF-2sbeneficial effects on survival may repre-sent peripheral effects on tissues andorgans outside the CNS. This seemsespecially plausible because FGF-2 hasbeen implicated in a variety of nonneu-ronal pathways. However, the therapeuticeffects of FGF-2 on the motor pheno-

type likely result from actions within theCNS (Fig. 1). Although Jin et al. (6)provide evidence for the migration of SVZ neural stem cells into the striatumand for proper axonal projection to theglobus pallidus, it seems unlikely thatsuch newly generated neurons could be-come fully functional neurons integratedinto the existing basal ganglia circuitry.More likely, newly generated neuronsforming alongside dysfunctional neuronsretard the degenerative process by aug-menting key prosurvival pathways. Forexample, newly generated neurons mayprovide trophic factor support, enhanceexcitatory amino acid neurotransmitteruptake, and/or restore ionic transmem-brane homeostasis in synaptic microen- vironments. However, because Jin et al.(6) show that FGF-2 supplementationsignificantly diminished cell death inimmortalized striatal-like neurons fromhomozygous HD knock-in mice withoutenhancing cellular proliferation, FGF-2also has neuroprotective properties inde-pendent of neurogenesis. Sorting out thetherapeutic actions of FGF-2 in HD,and determining whether and how newlygenerated neurons from SVZ stem cellssustain degenerating neurons, should bea focus of future studies.

Improvements in the neurologicalphenotype in HD mice were accompaniedby a decrease in nuclear and perinuclearprotein aggregates and by restorationof cannabinoid 1 (CB1) receptor expres-sion. The mechanistic basis of this in-triguing set of findings also deservesfurther consideration. In an induciblemodel of HD, Yamamoto et al. (14)showed that HD mice with pronounced

neurological phenotypes and dysfunc-tional neurons containing considerablequantities of aggregates could recover

normal motor function and eliminateaggregates upon termination of mutanthtt protein expression. Perhaps the FGF-2treatment regimen, either through itsneurogenesis or neuroprotective effects(or both), can sufficiently restore dys-functional neurons to allow normalcellular processes of protein turnover,metabolism, etc. to be reinstated. Be-

cause endogenous and exogenous canna-binoids appear to be neuroprotectiveand can induce neurogenesis (15, 16),restoration of CB1 receptor expressionin FGF-2-treated HD mice raises thequestion of how the cannabinoid path- way might be linked to the recoveryprocess. Future studies could address whether the CB1 receptor effect is aprerequisite for full recovery or is a sim-ply an unrelated epiphenomenon notcasually linked to FGF-2 action.

The study by Jin et al. (6) is notewor-thy for many reasons. Recent studiesdone in neurodegenerative disease

mouse models suggest that simple ma-nipulations such as environmental en-richment or exercise are sufficient toprofoundly impact disease course (1719). As such interventions likely stimu-late endogenous neurogenesis, there isgood reason to believe that neural stemcell reserves already residing in the CNScould be called upon in certain diseasestates to retard progression. That FGF-2displayed therapeutic efficacy throughperipheral administration is encouragingbecause the difficulty of in situ CNSdelivery could conceivably be avoided.However, this is a double-edged sword,

because the need for repeated adminis-tration and concern over untoward pe-ripheral effects may pose a problem formoving such a therapy from the benchto the bedside. Despite the remainingquestions of mechanistic action and po-tential future translation, the work of Jin et al. (6) suggests that FGF-2 andthe role of endogenous neurogenesisin the treatment of neurodegenerativedisease are enticing enough to warrantfurther study and future attention.

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Fig. 1. FGF-2 promotes restoration of neuronfunction in Huntingtons disease. ( A) Schematicillustration of the brain of an HD mouse displays adegenerating neuron to indicate the ongoing andprogressive neuronal dysfunction that character-izes the natural history of this disease. Shown alsois the lateral ventricle (LV) and the resident popu-lation of neural stem cells that are found in thesubventricular zone (SVZ). ( B) After administeringFGF-2, Jin et al. (6) document proliferation of neu-ral stem cells and generation of new neurons thatmigrate to the striatum and establish anatomicallycorrect axon projections. ( C ) Jin et al. (6) thenreport that HD transgenic mice treated with FGF-2

derive signicant therapeutic benet from FGF-2as evidencedby improvedmotor functionand pro-longed survival in comparison with untreatedmice. Although FGF-2 is shown to induce endoge-nous neurogenesis, the role of neurogenesis in re-tardingdiseaseprogressionand themechanism(s)bywhich newly generated neurons and/or FGF-2 pro-duces this outcome remain to be determined.

17890 www.pnas.org cgi doi 10.1073 pnas.0509222102 La Spada