Multiprotein Complexes of the Survival of Motor Neuron Protein SMN with Gemins Traffic to Neuronal Processes and Growth Cones of Motor Neurons

ArticleinThe Journal of Neuroscience : The Official Journal of the Society for Neuroscience 26(33):8622-32 · September 2006with5 Reads
DOI: 10.1523/JNEUROSCI.3967-05.2006 · Source: PubMed
Spinal muscular atrophy (SMA), a progressive neurodegenerative disease affecting motor neurons, is caused by mutations or deletions of the SMN1 gene encoding the survival of motor neuron (SMN) protein. In immortalized non-neuronal cell lines, SMN has been shown to form a ribonucleoprotein (RNP) complex with Gemin proteins, which is essential for the assembly of small nuclear RNPs (snRNPs). An additional function of SMN in neurons has been hypothesized to facilitate assembly of localized messenger RNP complexes. We have shown that SMN is localized in granules that are actively transported into neuronal processes and growth cones. In cultured motor neurons, SMN granules colocalized with ribonucleoprotein Gemin proteins but not spliceosomal Sm proteins needed for snRNP assembly. Quantitative analysis of endogenous protein colocalization in growth cones after three-dimensional reconstructions revealed a statistically nonrandom association of SMN with Gemin2 (40%) and Gemin3 (48%). SMN and Gemin containing granules distributed to both axons and dendrites of differentiated motor neurons. A direct interaction between SMN and Gemin2 within single granules was indicated by fluorescence resonance energy transfer analysis of fluorescently tagged and overexpressed proteins. High-speed dual-channel imaging of live neurons depicted the rapid and bidirectional transport of the SMN-Gemin complex. The N terminus of SMN was required for the recruitment of Gemin2 into cytoplasmic granules and enhanced Gemin2 stability. These findings provide new insight into the molecular composition of distinct SMN multiprotein complexes in neurons and motivation to investigate deficiencies of localized RNPs in SMA.
    • "How axonal and synaptic functions are affected and why motor neurons specifically degenerate in SMA remain largely unclear. SMN protein, which has a housekeeping role in mRNA splicing, is concentrated in discrete foci called gems in the nucleus of many cell types (Akten et al., 2011; Zhang et al., 2006). In addition to its nuclear location, SMN protein is also present in the dendrites and axons of neurons and is associated with microtubules (Fan and Simard, 2002; Zhang et al., 2003). "
    [Show abstract] [Hide abstract] ABSTRACT: Spinal muscular atrophy (SMA), characterized by specific degeneration of spinal motor neurons, is caused by mutations in the survival motor neuron 1 (SMN1) gene and subsequent decreased levels of functional SMN. How the deficiency of SMN, a ubiquitously expressed protein, leads to spinal motor neuron-specific degeneration in SMA patients remains unknown. In this study, we examined the role of SMN on mitochondrial axonal transport and morphology in human motor neurons by generating SMA type 1 patient-specific induced pluripotent stem cells (iPSCs) and then differentiating these cells into spinal motor neurons. The initial specification of spinal motor neurons was not affected, but these SMA spinal motor neurons specifically degenerated following long-term culture. Moreover, at an early stage in SMA spinal motor neurons, but not in SMA forebrain neurons, mitochondrial number, area, and transport were significantly reduced in axons. Knocking down of SMN expression led to similar mitochondrial defects in spinal motor neurons derived from human embryonic stem cells (hESCs), confirming that SMN deficiency results in impaired mitochondrial dynamics. Finally, the application of N-acetylcysteine (NAC) mitigated the impaired mitochondrial transport and morphology, and then rescued motor neuron degeneration in SMA long-term cultures. Furthermore, NAC ameliorated the reduced mitochondrial membrane potential in SMA spinal motor neurons, suggesting that NAC may rescue apoptosis and motor neuron degeneration by improving mitochondrial health. Together, our data demonstrate that SMN deficiency results in abnormal mitochondrial transport and morphology and subsequent reduced mitochondrial health, which are implicated in the specific degeneration of spinal motor neurons in SMA.
    Full-text · Article · Nov 2015
    • "Additionally, these genes might represent disease modifiers and thus non-SMN targets for potential new therapeutic strategies. Some reports have demonstrated that SMN also localizes to cytoplasmic or neuritic granules in neurons suggesting that SMN might also play a neuron-specific role in mRNA transport or local mRNA processing30313233. A recent report also indicated that reduced levels of SMN are associated with an increase in microRNA-183 expression , causing downregulation of mammalian target of rapamycin and, eventually, reduced global translation in neurons [34] . "
    [Show abstract] [Hide abstract] ABSTRACT: Spinal muscular atrophy (SMA) is an inherited neuromuscular disorder pathologically characterized by the degeneration of motor neurons in the spinal cord and muscle atrophy. Motor neuron loss often results in severe muscle weakness causing affected infants to die before reaching 2 years of age. Patients with milder forms of SMA exhibit slowly progressive muscle weakness over many years. SMA is caused by the loss of SMN1 and the retention of at least 1 copy of a highly homologous SMN2. An alternative splicing event in the pre-mRNA arising from SMN2 results in the production of low levels of functional SMN protein. To date, there are no effective treatments available to treat patients with SMA. However, over the last 2 decades, the development of SMA mouse models and the identification of therapeutic targets have resulted in a promising drug pipeline for SMA. Here, we highlight some of the therapeutic strategies that have been developed to activate SMN2 expression, modulate splicing of the SMN2 pre-mRNA, or replace SMN1 by gene therapy. After 2 decades of translational research, we now stand within reach of a treatment for SMA. Electronic supplementary material The online version of this article (doi:10.1007/s13311-015-0337-y) contains supplementary material, which is available to authorized users.
    Full-text · Article · Jan 2015
    • "Moreover, axonal defects in Smn-knocked down zebrafish embryos are corrected by overexpression of mutant SMNs which are incapable of snRNP assembly [27]. The core protein components of snRNPs, the Sm proteins, do not colocalize with SMN in neuronal processes [28]. Taken together, these observations suggest that SMN may have a unique function in neurons that is independent of snRNP biogenesis. "
    [Show abstract] [Hide abstract] ABSTRACT: Proximal spinal muscular atrophy (SMA) is an early onset, autosomal recessive motor neuron disease caused by loss of or mutation in SMN1 (survival motor neuron 1). Despite understanding the genetic basis underlying this disease, it is still not known why motor neurons (MNs) are selectively affected by the loss of the ubiquitously expressed SMN protein. Using a mouse embryonic stem cell (mESC) model for severe SMA, the RNA transcript profiles (transcriptomes) between control and severe SMA (SMN2+/+;mSmn-/-) mESC-derived MNs were compared in this study using massively parallel RNA sequencing (RNA-Seq). The MN differentiation efficiencies between control and severe SMA mESCs were similar. RNA-Seq analysis identified 3,094 upregulated and 6,964 downregulated transcripts in SMA mESC-derived MNs when compared against control cells. Pathway and network analysis of the differentially expressed RNA transcripts showed that pluripotency and cell proliferation transcripts were significantly increased in SMA MNs while transcripts related to neuronal development and activity were reduced. The differential expression of selected transcripts such as Crabp1, Crabp2 and Nkx2.2 was validated in a second mESC model for SMA as well as in the spinal cords of low copy SMN2 severe SMA mice. Furthermore, the levels of these selected transcripts were restored in high copy SMN2 rescue mouse spinal cords when compared against low copy SMN2 severe SMA mice. These findings suggest that SMN deficiency affects processes critical for normal development and maintenance of MNs.
    Full-text · Article · Sep 2014
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