A role for complexes of survival of motor neurons (SMN) protein with gemins and profilin in neurite-like cytoplasmic extensions of cultured nerve cells.
Spinal muscular atrophy (SMA) is caused by reduced levels of SMN (survival of motor neurons protein) and consequent loss of motor neurons. SMN is involved in snRNP transport and nuclear RNA splicing, but axonal transport of SMN has also been shown to occur in motor neurons. SMN also binds to the small actin-binding protein, profilin. We now show that SMN and profilin II co-localise in the cytoplasm of differentiating rat PC12 cells and in neurite-like extensions, especially at their growth cones. Many components of known SMN complexes were also found in these extensions, including gemin2 (SIP-1), gemin6, gemin7 and unrip (unr-interacting protein). Coilin p80 and Sm core protein immunoreactivity, however, were seen only in the nucleus. SMN is known to associate with beta-actin mRNA and specific hnRNPs in axons and in neurite extensions of cultured nerve cells, and SMN also stimulates neurite outgrowth in cultures. Our results are therefore consistent with SMN complexes, rather than SMN alone, being involved in the transport of actin mRNPs along the axon as in the transport of snRNPs into the nucleus by similar SMN complexes. Antisense knockdown of profilin I and II isoforms inhibited neurite outgrowth of PC12 cells and caused accumulation of SMN and its associated proteins in cytoplasmic aggregates. BIAcore studies demonstrated a high affinity interaction of SMN with profilin IIa, the isoform present in developing neurons. Pathogenic missense mutations in SMN, or deletion of exons 5 and 7, prevented this interaction. The interaction is functional in that SMN can modulate actin polymerisation in vitro by reducing the inhibitory effect of profilin IIa. This suggests that reduced SMN in SMA might cause axonal pathfinding defects by disturbing the normal regulation of microfilament growth by profilins.
Biochemistry Group, N. E. Wales Institute, Mold Road, Wrexham LL11 2AW, UK.
Aarti
A
Sharma
Anja
A
Lambrechts
Le Thi
le T
Hao
Thanh T
TT
Le
Caroline A
CA
Sewry
Christophe
C
Ampe
Arthur H M
AH
Burghes
Glenn E
GE
Morris
D000595:Amino Acid Sequence; D000818:Animals; D017362:Cyclic AMP Response Element-Binding Protein / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008807:Mice, Inbred BALB C; D008969:Molecular Sequence Data; D009046:Motor Neurons / Q000378:metabolism* / Q000473:pathology / Q000648:ultrastructure; D009134:Muscular Atrophy, Spinal / Q000378:metabolism / Q000473:pathology; D009363:Neoplasm Proteins / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D016501:Neurites / Q000378:metabolism* / Q000648:ultrastructure; D009687:Nuclear Proteins / Q000378:metabolism*; D016716:PC12 Cells; D051304:Profilins / Q000378:metabolism*; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D011817:Rabbits; D051381:Rats; D055532:SMN Complex Proteins; D055533:Survival of Motor Neuron 1 Protein
D017362:Cyclic AMP Response Element-Binding Protein; D009363:Neoplasm Proteins; D009419:Nerve Tissue Proteins; D009687:Nuclear Proteins; D051304:Profilins; D016601:RNA-Binding Proteins; D055532:SMN Complex Proteins; C403447:STRAP protein, human; C526473:Smn1 protein, rat; D055533:Survival of Motor Neuron 1 Protein