|Fig. 1 from the Elisocovich et al minireview in The Journal of Biological Chemistry, showing the many roads of mRNA.
In a minireview in The Journal of Biological Chemistry, Carolina Eliscovich, Adina Buxbaum, Zachary Katz and Robert Singer at the Albert Einstein College of Medicine of Yeshiva University explain the importance of localizing elements of mRNA sequences and how advanced biochemical and cell-imaging techniques are being used to better understand mRNA movement.
Localizing elements in mRNA sequences are important for cell-fate determination, directed cell movement and tissue functionality. They also play key roles in embryonic patterning and somatic cell differentiation. mRNA localization also can restrict protein production, amplify protein concentration and direct protein integration into macromolecular complexes.
RNA-binding proteins are multifunctional regulators responsible for processing, localizing and controlling translation of mRNA targets. RNA-binding proteins recognize specific cis-acting mRNA localization elements, or as the authors call them, “zipcodes.” These zipcodes, just a few nucleotides in length, can be contained in simple elements or in secondary structures or stem loops. While many localizing elements have been characterized in multiple models, neither a specific sequence pattern nor structural pattern has been closed in on to date.
However, the authors emphasize that technological advances, such as cross-linking and immunoprecipitation, or CLIP, have identified localizing elements and isolated RNA-protein complexes under physiological conditions. Fusion of a localizing element to a reporter RNA helps visualize localization patterns within a cell. These techniques are good but limited, as they cannot elucidate binding specificity and affinity. This is overcome by using live imaging methodologies, including high-resolution and live microscopy.
High-resolution fluorescence in situ hybridization, or FISH, has shown that as many as 70 percent of analyzed mRNAs in the Drosophila embryo, for instance, demonstrate subcellular localization. Using live microscopy, single mRNA molecules have been studied, showing that mRNA localization occurs through directional transport along cytoskeletal elements, random diffusion and local trapping of mRNAs, vertorial export from the nucleus and trapping, or local protection from degradation. Live imaging has been used to study movement within the nucleus, where mRNA move by diffusion; the cytoplasm, where diffusion is faster because the environment is less restrictive; and neurons, where active transport localizes mRNA into dendrites.
By combining different methodologies, researchers are getting a clearer picture of how mRNA localizes and how that localization is related to gene expression, which one day may lead to customized treatments for diseases.