Axonal-SMN (a-SMN), a protein isoform of the survival motor neuron gene, is specifically involved in axonogenesis

Abstract

Spinal muscular atrophy (SMA) is an autosomal recessive disease of childhood due to loss of the telomeric survival motor neuron gene, SMN1. The general functions of the main SMN1 protein product, full-length SMN (FL-SMN), do not explain the selective motoneuronal loss of SMA. We identified axonal-SMN (a-SMN), an alternatively spliced SMN form, preferentially encoded by the SMN1 gene in humans. The a-SMN transcript and protein are down-regulated during early development in different tissues. In the spinal cord, the a-SMN protein is selectively expressed in motor neurons and mainly localized in axons. Forced expression of a-SMN stimulates motor neuron axonogenesis in a time-dependent fashion and induces axonal-like growth in non-neuronal cells. Exons 2b and 3 are essential for the axonogenic effects. This discovery indicates an unexpected complexity of the SMN gene system and may help in understanding the pathogenesis of SMA.alternative splicing ͉ neurodegeneration ͉ spinal muscular atrophy ͉ intron retention ͉ axonal sprouting S pinal muscular atrophy (SMA) is an autosomal recessive disease of childhood causing selective motor neuron death. SMA is the leading genetic cause of infant mortality, with an incidence of 1:10,000 and a carrier frequency of 1:50 (1). The telomeric survival motor neuron gene (SMN1) is the SMA disease gene, and the duplicated centromeric gene, SMN2, governs the severity of the disease (2, 3). Current knowledge suggests that SMN1 codes for a single functional protein [full-length SMN (FL-SMN)], and the major product of SMN2 is ⌬7-SMN (lacking the C terminus), an unstable protein of minor significance (4, 5).The FL-SMN protein is expressed in the cytoplasm and nucleus of all cells (6, 7) and is involved in spliceosomal assembly and pre-mRNAs maturation (8-10). Other proposed roles include general cellular functions (11-15). How reduced FL-SMN levels lead to selective degeneration of motor neurons in SMA remains undetermined. FL-SMN may serve motor neuron-specific functions, perhaps interacting with partner proteins selectively expressed in these cells. However, these functions and the partner proteins are still not known. Additional protein products of the SMN genes have been postulated (16)(17)(18)(19). The identification of SMN proteins acting specifically in motor neurons would be a major step forward in understanding SMA.We identify and characterize an SMN transcript (GenBank accession no. AY876898) originating from the retention of SMN intron 3 and encoding the axonal-SMN (a-SMN) protein. In the spinal cord, a-SMN is selectively expressed in developing motor neurons and mainly localized in axons. This is confirmed by overexpression experiments in cultured cells. These also demonstrate the following: (i) a-SMN stimulates motor neuron axonogenesis in a time-dependent fashion, and (ii) a-SMN induces axonal-like growth in non-neuronal cells such as HeLa. The specific localization and function suggest a role in motor neurons. Because human a-SMN is a specific product of the SMN1 gene, its lo...

Abnormal motoneuron migration, differentiation, and axon outgrowth in spinal muscular atrophy

“…Five SMA-I and control cases were included in our previous study where we analyzed presence of apoptosis in gray matter of the anterior horns of the spinal cord [46]. The diagnosis of SMA was made on the basis of clinical findings (progressive muscle weakness and hypotonia), electromyoneurography, muscle biopsy, and was confirmed by genetic analysis of blood samples.…”

“…SMN is found to be most abundant in the cytoplasm and nucleus of alpha motoneurons [1,2,53]. It is involved in various cellular processes, of which four seem to be best documented: assembly of spliceosomal small nuclear ribonucleoproteins (snRNPs) and pre-mRNA processing (splicing) [16], activation of transcription/translation [6], apoptosis [21,22,28,42,54], and axonal transport [2,8,19,34,37,39,46]. Which of these functions is most important for the pathogenesis of SMA is not clear.…”

Pathogenesis of proximal autosomal recessive spinal muscular atrophy

“…The asymmetric SMN staining demonstrated in the germinative neuroepithelium further supported a role for SMN in neuronal migration and differentiation [23]. Finally, new approaches to study SMN protein function, particularly the use of antisense morpholino and synthetic RNA injections to knockdown SMN levels in the zebrafish model of SMA, have confirmed the hypothesis that, in addition to its role in snRNP biogenesis, SMN protein has an additional and independent function which is critical for motor neuron axon outgrowth [10,11,30,54,74]. Moreover, the severity of motor axon defects was shown to correlate with decreased survival of the zebrafish, suggesting that SMN protein is not only needed for motor neuron outgrowth, but perhaps also in its maintenance of, and interactions with, muscle [11], which is further supported by the finding of increased vulnerability of the neuromuscular juntions in mouse SMA models [60].…”

“…Moreover, the severity of motor axon defects was shown to correlate with decreased survival of the zebrafish, suggesting that SMN protein is not only needed for motor neuron outgrowth, but perhaps also in its maintenance of, and interactions with, muscle [11], which is further supported by the finding of increased vulnerability of the neuromuscular juntions in mouse SMA models [60]. One of the most recent exciting new findings has been the identification of an SMN transcript which encodes the axonal-SMN protein which is found to be selectively expressed in developing spinal cord motoneurons and mainly localized in axons [74]. The presence of diverse types of SMN complexes that have functions in neurons, other than its well characterized role to assemble snRNPs was recently further confirmed using high-resolution fluorescence of an SMN-Gemin complex in neuronal processes and growth cones of embryonic stem cell-derived motor neurons [95].…”

“…This hypothesis stems from observations demonstrating the accumulation of the SMN protein in the axons and growth cones of motor neuron like cells in vitro and anterior horn cells in vivo [6,7,10,11,24,30,54,64,68,74,94], and its interaction with the heterogeneous nuclear riboprotein Q and R (hnRNP-R) and β-actin mRNA, targeting them into the growth cones [68]. According to the pathogenetic model proposed in this review, the selective motoneuron axonopathy appears to be far more important than ubiquitous nuclear splicing deficit.…”