Gary Ruvkun and Victor Ambros won the 2024 Nobel Prize in Physiology/Medicine “for the discovery of microRNA and its role in post-transcriptional gene regulation”. Image on the left by John Sears is adapted from the source and shared under a CC by 4.0 license. Image on the right by Arthur Petron is adapted from the source and shared under a CC by 4.0 license.
All multicellular animals begin their lives as a single cell, the zygote. The zygote undergoes controlled divisions to give rise to the whole organism. However, every cell in an individual contains the same genetic material encoded by its DNA (deoxyribonucleic acid) sequence but can have widely different functions. How does each cell decide its fate? How do cells in the skin develop and behave differently from cells in the kidneys? The overarching answer to this question is gene regulation.
Genes within the DNA sequence encode proteins that play an important role in cell structure and function. The gene sequence is used to make a complementary mRNA sequence (messenger ribonucleic acid) during transcription, which acts as the template for protein synthesis during translation. Gene regulation is the process by which genes required (not required) for a cell’s function can be turned on (off). For a long time, proteins were known to be key factors in gene regulation. Proteins called transcription factors bind to DNA and regulate its function by promoting or stopping transcription. However, other regulatory mechanisms are active after mRNA is synthesized, which fall under post-transcriptional regulation. The 2024 Nobel Prize in Physiology and Medicine was awarded to Gary Ruvkun from the University of Harvard and Victor Ambros from the University of Massachusetts for “the discovery of microRNA and its role in post-transcriptional gene regulation.”
Ruvkun and Ambros carried out their research on a millimeter-long worm called Caenorhabditis elegans. This worm was developed as a model organism in biology by Sydney Brenner, a Nobel Prize winner himself, who also happens to be the academic grandparent of Ruvkun and Ambros. The duo were working as post-docs in the lab of H Robert Horvitz at the Massachusetts Institute of Technology when they discovered a pair of genes, lin-14 and lin-4, which seemed to be involved in gene regulation. Development in C. elegans larvae is divided into four stages named L1 to L4. Mutations in these genes lead to heterochronic defects where specific cell types appear earlier as compared to the normal course of development. When lin-4 is not functional, worms do not develop a vulva as they repeat the L1 stage and accumulate internal eggs. On the other hand, if lin-14 is not functional, specific cell types start developing precociously, hampering the worm’s growth. A gain of function mutation in lin-14 causes the same effect as a loss of function mutation in lin-4, suggesting that lin-4 regulates the function of the lin-14 gene.
Ruvkun and Ambros used Caenorhabditiselegans as a model system to study mutations in a pair of interacting genes called lin-14 and lin-4 that influence development. Image by Zeynep F. Altun from source shared under a CC by 2.5 license.
At the time Ambros and Ruvkun were carrying out their research, transcription factors were considered the primary regulators of gene expression. However, there also were hypotheses and observations regarding the role of RNA in gene expression. Roy J Britten and Eric H Davidson hypothesized the possible role of RNA in gene regulation in 1969 , and plant molecular biologists demonstrated that two mRNA strands transcribed from either direction of a DNA sequence bind together to affect transcription. The novelty of Ambros’s and Ruvkun’s research was to demonstrate the presence of such a process in animals and identify its exact mechanism.
After their postdoctoral stint at the Horvitz lab, Ambros and Ruvkun established independent labs of their own. Ruvkun settled at Harvard University and chose to study lin-14 as the target of his research. His group made multiple lin-14 mutants and painstakingly found their location on the C. elegans genetic map. Once this was done, they noticed an interesting pattern. All the gain of function mutations mapped to an untranslated region in the tail end of the lin-14 mRNA. Ambros also joined Harvard University and was working on lin-4, the regulator of lin-14, which came with its challenges. In the pre-genomics era, it was a herculean task to map lin-4 since it occupies a tiny region of the genome only about a 100 nucleotides long. Its length was puzzling because such a short sequence could not possibly code for a protein. How was it achieving its function then?
The eureka moment happened during a phone call between Ambros and Ruvkun on 11 June 1992. When discussing their findings, they observed that a 22-nucleotide-long portion of the lin-4 RNA, called microRNA, is partially complementary to the untranslated tail end of lin-14. Et voilà! After transcription, complementary regions of the two mRNA molecules must be binding together preventing protein synthesis in lin-14, thus regulating its expression. They independently published their findings in the same issue of the journal Cell in 1993.
microRNA binds to complementary regions of mRNA preventing its translation into protein. This pathway of post-transcriptional gene regulation was discovered by Gary Ruvkun and Victor Ambros in the worm Caenorhabditiselegans. Image by KajsaMollersen from source shared under a CC by 4.0 license.
Back then, the reception of their now seminal work was lukewarm at best. Most molecular biologists studying animals did not seem to care for a pathway found in a mere worm and plant molecular biologists were trying to understand their own RNA-based observations. Ambros was denied tenure at Harvard, after which he worked at Dartmouth College before settling at the University of Massachusetts.
Their findings garnered attention when Ruvkun’s group subsequently identified another regulatory pair, let-7 microRNA and lin-41, conserved across evolutionary lineages. In the meantime, Ambros’s lab showed that lin-4 microRNA also regulates another gene called lin-28, suggesting a paradigm of microRNA regulation networks that are phylogenetically conserved. Following the theme of post-transcription gene regulation, Andrew Z Fire and Craig C Mello discovered RNA interference, by which double-stranded RNA is involved in gene silencing, and won the 2006 Nobel Prize in Physiology and Medicine.
The puzzle of RNA-based regulation remains to be completely solved and there may be more prizes on the horizon for researchers finding the next missing piece.
References:
Lee, R. C., Feinbaum, R. L., & Ambros, V. (1993). The C. elegans heterochronic gene lin-4 encodes small RNAs with antisense complementarity to lin-14. Cell, 75(5), 843–854. https://doi.org/10.1016/0092-8674(93)90529-y
Wightman, B., Ha, I., & Ruvkun, G. (1993). Posttranscriptional regulation of the heterochronic gene lin-14 by lin-4 mediates temporal pattern formation in C. elegans. Cell, 75(5), 855–862. https://doi.org/10.1016/0092-8674(93)90530-4
