SMN regulation in SMA and in response to stress: new paradigms and therapeutic possibilities

Abstract

Low levels of the survival of motor neuron (SMN) protein cause the neurodegenerative disease spinal muscular atrophy (SMA). SMA is a pediatric disease characterized by spinal motor neuron degeneration. SMA exhibits several levels of severity ranging from early antenatal fatality to only mild muscular weakness, and disease prognosis is related directly to the amount of functional SMN protein that a patient is able to express. Current therapies are being developed to increase the production of functional SMN protein; however, understanding the effect that natural stresses have on the production and function of SMN is of critical importance to ensuring that these therapies will have the greatest possible effect for patients. Research has shown that SMN, both on the mRNA and protein level, is highly affected by cellular stress. In this review we will summarize the research that highlights the roles of SMN in the disease process and the response of SMN to various environmental stresses.

Figure 1. SMN1 and SMN2 splicing

SMN1 and SMN2 genes contain a C to T nucleotide alteration in exon7. This alteration changes the splicing of the pre-mRNA, leading to exon inclusion in SMN1 and predominant exon exclusion in SMN2. This alters the isoforms of protein produced, with SMN1 producing mainly full-length protein, which oligomerizes and is stable, and SMN2 producing mainly truncated proteins (depicted by missing wedge), which do not oligomerizes and are rapidly degraded. The small amount of full-length protein that can be produced from SMN2, however, is identical to SMN1 and the two populations can interact (figure adapted from Burghes and Beattie 2009).

Figure 2. SMN1 and SMN2 exon 7 splicing regulators

SMN1 is characterized by predominantly positive splicing regulation. SRSF1 and TRA2B bind directly to exon 7 splicing enhancer regions SE1 and SE2, respectively. TRA2B allows the cooperative binding of hnRNP G and SRSF9, which add further positive influence for exon recognition. These factors help to recruit U1 and U2 snRNPs to the exon boundaries. This snRNP recognition is critical for the inclusion of this exon. On the other hand, SMN2 transcripts contain a nucleotide alteration from SMN1; C>U in SE1 region (marked in red). This nucleotide alteration changes the preferred protein binding partner from the positive SRSF1 to the negative hnRNP A1. Similarly, SAM68 is also capable of binding this region and exerting negative influence. Two more hnRNP A1 sites are known to induce exon 7 exclusion, located in SE2 region and the intronic silencing element, ISS-N1. A GC-rich region overlapping the 5′end of ISS-N1 also associates with long distance interaction (LD-1), The negatively acting proteins decrease the recognition of the exon by U1 and U2 snRNPs, leading to exon 7 exclusion. hnRNP M binds to sequence in SE2 that overlaps the TRA2B binding site and promotes exon 7 inclusion in both SMN1 and SMN2 transcripts.

Figure 3. SMN protein and RNA processes are affected by environmental and microenvironmental stresses

The range of possible SMN-mediated effects produced by stress is likely dependent on the type and magnitude of stress. While starvation and hypoxia reduce SMN protein levels by increasing aberrant splicing of SMN mRNA, oxidative stress may effectively reduce SMN function through inactivation resulting from covalent binding of oligomers. The net result of reduced functional SMN would be predicted to impact the full range processes in which SMN participates. In contrast, metabolic stress resulting from bacterial infection may have a more limited effect due to the specific aggregation of SMN and snRNA.