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The Journal of Neuroscience, October 25, 2006, 26(43):11014-11022; doi:10.1523/JNEUROSCI.1637-06.2006
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Neurobiology of Disease
Survival Motor Neuron Function in Motor Axons Is Independent of Functions Required for Small Nuclear Ribonucleoprotein Biogenesis
Tessa L. Carrel,1
Michelle L. McWhorter,1
Eileen Workman,3
Honglai Zhang,4
Elizabeth C. Wolstencroft,6
Christian Lorson,6
Gary J. Bassell,5
Arthur H. M. Burghes,3 and
Christine E. Beattie1,2
1Center for Molecular Neurobiology and Departments of 2Neuroscience and 3Molecular and Cellular Biochemistry, The Ohio State University, Columbus, Ohio 43210, 4Department of Neuroscience, Albert Einstein College of Medicine, Bronx, New York 10461, 5Departments of Cell Biology and Neurology, Emory University, Atlanta, Georgia 30322, and 6Department of Veterinary Pathobiology, University of Missouri, Columbia, Missouri 65211-5120
Correspondence should be addressed to Christine E. Beattie, 190 Rightmire Hall, 1060 Carmack Road, Columbus, OH 43210. Email: beattie.24{at}osu.edu
Spinal muscular atrophy (SMA) is a motor neuron degenerative disease caused by low levels of the survival motor neuron (SMN) protein and is linked to mutations or loss of SMN1 and retention of SMN2. How low levels of SMN cause SMA is unclear. SMN functions in small nuclear ribonucleoprotein (snRNP) biogenesis, but recent studies indicate that SMN may also function in axons. We showed previously that decreasing Smn levels in zebrafish using morpholinos (MO) results in motor axon defects. To determine how Smn functions in motor axon outgrowth, we coinjected smn MO with various human SMN RNAs and assayed the effect on motor axons. Wild-type SMN rescues motor axon defects caused by Smn reduction in zebrafish. Consistent with these defects playing a role in SMA, SMN lacking exon 7, the predominant form from the SMN2 gene, and human SMA mutations do not rescue defective motor axons. Moreover, the severity of the motor axon defects correlates with decreased longevity. We also show that a conserved region in SMN exon 7, QNQKE, is critical for motor axon outgrowth. To address the function of SMN important for motor axon outgrowth, we determined the ability of different SMN forms to oligomerization and bind Sm protein, functions required for snRNP biogenesis. We identified mutations that failed to rescue motor axon defects but retained snRNP function. Thus, we have dissociated the snRNP function of SMN from its function in motor axons. These data indicate that SMN has a novel function in motor axons that is relevant to SMA and is independent of snRNP biosynthesis.
Received April 17, 2006;
revised Sept. 15, 2006;
accepted Sept. 17, 2006.
Correspondence should be addressed to Christine E. Beattie, 190 Rightmire Hall, 1060 Carmack Road, Columbus, OH 43210. Email: beattie.24{at}osu.edu
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