[1] Johns Hopkins Univ, Sch Med, Howard Hughes Med Inst, Baltimore, MD 21205 USA
[2] Johns Hopkins Univ, Sch Med, Dept Neurosci, Baltimore, MD 21205 USA
[3] Forschungszentrum Julich, D-52425 Julich, Germany
[4] Johns Hopkins Univ, Sch Med, Dept Mol Biol & Genet, Baltimore, MD 21205 USA