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The Journal of Neuroscience, February 1, 2006, 26(5):1516-1530; doi:10.1523/JNEUROSCI.4543-05.2006

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Development/Plasticity/Repair
Long-Term Adeno-Associated Viral Vector-Mediated Expression of Truncated TrkB in the Adult Rat Facial Nucleus Results in Motor Neuron Degeneration

Joris De Wit,¹ Ruben Eggers,¹ Robert Evers,¹ Eero Castrén,² and Joost Verhaagen¹

¹Graduate School of Neuroscience, Netherlands Institute for Brain Research, 1105 AZ Amsterdam, The Netherlands, and ²Neuroscience Center, 00014 University of Helsinki, Helsinki, Finland

Correspondence should be addressed to Joost Verhaagen, Netherlands Institute for Brain Research, Meibergdreef 33, 1105 AZ Amsterdam, The Netherlands. Email: j.verhaagen{at}nih.knaw.nl

Adult facial motor neurons continue to express full-length TrkB tyrosine kinase receptor (TrkB.FL), the high-affinity receptor for the neurotrophins BDNF and neurotrophic factor-4/5 (NT-4/5), suggesting that they remain dependent on target-derived and locally produced neurotrophins in adulthood. Studies on the role of TrkB signaling in the adult CNS have been hampered by the early lethality of bdnf, nt-4/5, and trkB knock-out mice. We disrupted TrkB.FL signaling in adult facial motor neurons using adeno-associated viral vector-mediated overexpression of a naturally occurring dominant-negative TrkB receptor, TrkB.T1. Expression of TrkB.T1 resulted in neuronal atrophy and downregulation of NeuN (neuronal-specific nuclear protein) and ChAT expression in facial motor neurons. A subset of transduced neurons displayed signs of motor neuron degeneration that included dendritic beading and rounding of the soma at 2 months of TrkB.T1 expression. Cell counts revealed a significant reduction in motor neuron number in the facial nucleus at 4 months after onset of expression of TrkB.T1, suggesting that a proportion of TrkB.T1-expressing motor neurons became undetectable as a result of severe atrophy or was lost because of cell death. In contrast, overexpression of TrkB.FL did not result in a decrease in facial motor neuron number. Our results indicate that a subset of facial motor neurons remains dependent on TrkB ligands for the maintenance of structural and molecular characteristics in adulthood.

Key words: degeneration; facial nucleus; AAV; TrkB; plasticity; motor neuron

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Received March 9, 2005; revised Dec. 14, 2005; accepted Dec. 18, 2005.

Correspondence should be addressed to Joost Verhaagen, Netherlands Institute for Brain Research, Meibergdreef 33, 1105 AZ Amsterdam, The Netherlands. Email: j.verhaagen{at}nih.knaw.nl

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