| Research Abstract |
SMN protein, the product of the SMA gene, is considered to play a crucial role in biogenesis of spliceosomes with the SMN-interacting protein, SIP1. To clarify splicing mechanism in neuron, we searched neural splicing factors and cloned two new SR proteins, Neural-salient SR proteins (NSSR) 1 and 2, which are present at higher levels in brain and testis. During the differentiation, NSSR 1 is detected only in the neuronal stage. Both the purified recombinant NSSR 1 and 2 proteins enhance the in vitro splicing activity of nuclear extract. Moreover, overexpression of NSSR 2 prevents the inclusion of either the Flip or Flop exons in the splicing of the GluR-B gene, resulting in an increase in the abnormal exon-skipping product. In contrast, transient transfection with NSSR 1 promotes the inclusion of the Flip exon so that the abnormal product is spliced to the mature spliced form. This suppression of exon skipping by NSSR 1 is observed even with cotransfection of NSSR 2. Results indicate that NSSR 1 may play a crucial role in the regulation of alternative splicing in neurons.
Next, to clarify relevancy of pathogenesis and SIP1, we identified three novel splicing variants of the SIP1 (SIP-β、γ and δ), in addition to the full-length SIP1-α (280AA). We examined the expression levels of these splicing variants in various normal human tissues and in muscle samples from patients with SMA and ALS. The SIP1-α was a major product and ubiquitiously expressed. In contrast, SIP-β and γ were detected at very low expression level. In patients with SMA and ALS, the SIP-α was dramatically decreased(17%, 19%) compared to the controls, while the SIP-β was significantly increased(34%, 32%) in both diseases. These findings suggest that aberrant alternative splicing event in SIP1 occur in the motor neuron disease and contribute the pathological process of SMA and ALS.
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