We are interested in studying the assembly and intracellular traffic of ribonucleoproteins, which represents a fundamental process for correct nuclear organization in eukaryotes. We are particularly interested in the biogenesis of the spliceosomal snRNPs required for nuclear pre-mRNAs splicing. A key player in early steps of this process is the SMN protein, the product of the Survival of Motor Neuron gene, whose deletions or mutations are responsible for Spinal Muscular Atrophy (SMA). Our projects are aimed at the characterization of molecular defects associated to the loss of the SMN protein and at the identification of new factors involved in snRNP biogenesis and in the formation of a functional SMN complex. As models, we use mammalian cells (HeLa, cells derived from SMA patients, NSC34 motor-neuron like cells) and the fission yeast genetic system.
The SMN complex is required for the formation of the Sm core complex common to all snRNPs and the association of this complex to the snRNAs. We have shown that, in HeLa cells, depletion of the SMN protein as well as other factors involved in snRNPs biogenesis induces disruption of Cajal bodies, a structure required for snRNPs maturation steps. In the process of investigating the function of other factors in snRNPs biogenesis, we also found that the proteosomal machinery could be involved in the production of a small isoform of the Tgs1 hypermethylase, the enzyme responsible for snRNA and snoRNA m3G cap formation.
Although it is clearly established that a reduced amount of SMN protein is responsible for SMA, the molecular mechanism by which this deficiency induces the specific degeneration of motor neurons remains unknown. It has been proposed that SMN possesses a neuron-specific role in axonal mRNA transport in addition to its function in snRNPs assembly. Accordingly, we found that SMN co-localizes with granules containing numerous proteins involved in diverse aspects of mRNA metabolism. It is also possible that SMA results from splicing defects of pre-mRNA specifically affecting motor neurons function. Using a model organism, we could indeed show that cells carrying a temperature-degron allele of SMN show defects in the synthesis of snRNPs leading to splicing defects of some but not all pre-mRNA.
The goals of our future research are: (i) to characterize new factors required for SMN complex formation, (ii) to study the functional relationships between the SMN complex, snRNP biogenesis and SMA, (iii) to uncover genes whose mRNA show altered splicing and/or localization in Spinal Muscular Atrophy.