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Spinal muscular atrophy (SMA) is a neurodegenerative
disease caused by deletion and/or mutation of the survival motor neuron
protein Gene (SMN1) that results in the expression of a truncated protein
lacking the C terminal exon-7. Whereas SMN has been shown to be an important
component of diverse ribonucleoprotein (RNP) complexes, its function
in neurons is unknown. We hypothesize that the active transport of SMN
may be important for neurite outgrowth and that disruption of exon-7
could impair its normal intracellular trafficking. SMN was localized
in granules that were associated with cytoskeletal filament systems
and distributed throughout neurites and growth cones. Live cell imaging
of enhanced green fluorescent protein (EGFP)-SMN granules revealed rapid,
bidirectional and cytoskeletal-dependent movements. Exon-7 was necessary
for localization of SMN into the cytoplasm but was not sufficient for
granule formation and transport. A cytoplasmic targeting signal within
exon-7 was identified that could completely redistribute the nuclear
protein D-box binding factor 1 into the cytoplasm. Neurons transfected
with SMN lacking exon-7 had significantly shorter neurites, a defect
that could be rescued by redirecting the exon-7 deletion mutant into
neurites by a targeting sequence from growth-associated protein-43.
These findings provide the first demonstration of cytoskeletal-based
active transport of SMN in neuronal processes and the function of exon-7
in cytoplasmic localization. Such observations provide motivation to
investigate possible transport defects or inefficiency of SMN associated
RNPs in motor neuron axons in SMA.
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