Friday Science Review: March 23, 2012
March 23, 2012
Posted by on
When the human genome was sequenced there was surprise that only 23 thousand protein-encoding genes were found. However, it was followed by an added emphasis on the other mechanisms used by eukaryotic organisms to generate proteome diversity and especially the creation of different protein isoforms through alternative splicing of exons in precursor mRNA transcripts. Indeed, in humans it is estimated that 95% of genes containing more than one exon are subject to alternative splicing and splicing of the same gene varies by tissue type.
Given its importance, characterizing the molecular mechanism by which tissue specific splicing occurs and its effect on downstream function is therefore an important area of research and the study carried out by researchers from McGill University and Laval University is helping to fill some of the current gaps. Alternative splicing is regulated by RNA binding proteins, one of which is Sam68, a member of the signal transduction activator of RNA (STAR) family. Sam68-/- mice are leaner than normal mice and are protected from obesity, insulin resistance and glucose intolerance. In their paper published in Molecular Cell, they explored the alternative splicing events regulated by Sam68 to explain its unexpected physiological roles.
By carrying out genome-wide exon usage profiling in the white adipose tissue, they were able to show that Sam68-/- mouse cells had lower levels of mTOR, which resulted from the introduction of a premature termination codon due to intron 5 from the mTOR gene not being excised. The lower levels of mTOR had downstream consequences, including reduced insulin-stimulated S6 and Akt phosphorylation, which reduced adipogenesis.
mTOR inhibitors are of course in the clinic (and in multiple clinical trials) for use in immune suppression and cancer, but is weight loss about to be added to that list? A recent paper from the University of Geneva shows that the chronic exposure to the mTOR inhibitor rapamycin does result in reduced specific fat mass in rats, but at the cost of rather serious systemic consequences including skeletal muscle insulin resistance.
While probably not initiating the rush for development of the new perfect diet pill, this research on tissue specific gene-splicing continues to add to our understanding of the various levels at which signaling pathways are regulated and their tissue dependence.
This week also saw the announcement of the 2012 Canada Gairdner Awards, which recognized contributions to our understanding of the genetic control of circadian rhythms; of how sensory and motor neurons communicate; and the role of Fc receptors in immune response and autoimmune diseases. The awards also recognized the efforts of two scientists for their role in reducing the toll of infectious diseases. Details can be seen on the Gairdner website here.
- Activation of neuronal P2X7 receptor-pannexin-1 mediates death of enteric neurons during colitis. Nature Medicine. University of Calgary
- Cell-Surface Proteomics Identifies Lineage-Specific Markers of Embryo-Derived Stem Cells. Developmental Cell. University of Toronto
- Direct observation of multiple misfolding pathways in a single prion protein molecule. PNAS. University of Alberta