The relationship between SMN, the spinal muscular atrophy protein, and nuclear coiled bodies in differentiated tissues and cultured cells

The spinal muscular atrophy protein, SMN, is a cytoplasmic protein that is also found in distinct nuclear structures called "gems." Gems are closely a ssociated with nuclear coiled bodies and both may have a direct role in snR NP maturation and pre-RNA splicing. There has been some controversy over wh ether gems and coiled bodies colocalize or form adjacent/independent struct ures in HeLa and other cultured cells. Using a new panel of antibodies agai nst SMN and antibodies against coilin-p80, a systematic and quantitative st udy of adult differentiated tissues has shown that gems always colocalize w ith coiled bodies. In some tissues, a small. proportion of coiled bodies (< 10%) had no SMN, but independent or adjacent gems were not found. The most striking observation, how ever, was that many cell types appear to have nei ther gems nor coiled bodies (e.g., cardiac and smooth muscle, blood vessels , stomach, and spleen) and this expression pattern is conserved across huma n, rabbit, and pig species. This shows that assembly of distinct nuclear bo dies is not essential for RNA splicing and supports the view that they may be storage sites for reserves of essential proteins and snRNPs. Overexpress ion of SMN in COS-7 cells produced supernumerary nuclear bodies, most of wh ich also contained coilin-p80, confirming the close relationship between ge ms and coiled bodies. However, when SMN is reduced to very low levels in ty pe I SMA fibroblasts, coiled bodies are still formed. Overall, the data sug gest that gem/coiled body formation is not determined by high cytoplasmic S MN concentrations or high metabolic activity alone and that a differentiati on-specific factor may control their formation. (C) 2000 Academic Press.