microRNAs emerge as potent post-transcriptional gene regulators

There is nothing more precise than a Swiss watch — besides the pattern of development of the nematode worm Caenorhabditis elegans, one of the most studied and useful animal models. The postembryonic development of C. elegans entails passage through four accurately coordinated larval stages (L1–L4) interspersed with moults. In the mid‑1980s, many scientists were interested in genetic aberrations that could alter the precise timing of C. elegans development. Genes that, when manipulated, could delay or advance the nematode’s cell cycle and developmental-stage progression were called heterochronic genes. As expected at the time, most of these genes encode proteins.

In 1993, Victor Ambros and colleagues demonstrated that downregulation of the protein LIN‑14 was crucial for the progression from the first larval stage (L1) to the second larval stage (L2). Loss‑of‑function mutations in lin‑14 cause C. elegans to skip a beat, starting development from L2. On the other hand, mutations in another gene, lin‑4, halted developmental progression indefinitely at the L1 stage. Surprisingly, lin‑4 did not encode a protein; instead, it is transcribed into a small non-coding RNA with sequence complementarity to the 3ʹ untranslated region (3ʹ UTR) of lin‑14. Lin‑4 was the first microRNA to be discovered.

At the same time, Gary Ruvkun and colleagues showed that binding of lin-4 to the 3ʹ UTR is essential for LIN‑14 downregulation. Both teams correctly hypothesised that lin‑4 pairs through antisense complementarity to the 3ʹ UTR of lin‑14, and forms an RNA duplex that leads to translational repression of lin‑14. Although lin-4 binding did not affect the overall mRNA levels of lin‑14, it decreased LIN‑14 protein expression, subsequently causing progression from L1 to L2.

This novel mechanism of post-transcriptionally regulating gene expression was shown, in both articles, to be conserved in several worm species, but at the time it was mostly thought to be a nematode oddity. During the 1990s, a second microRNA regulating C. elegans development was identified and named let‑7. In the case of let‑7 mutant nematodes, larvae stopped just short of becoming adult worms. Lin-4 and let-7 were quite different from each other but, in 2000, Ruvkun and colleagues found homologues in the genomes of Drosophila melanogaster and Homo sapiens. Although humans have no heterochronic genes, fruit flies do, and the temporal expression profile of let‑7 was shown to be conserved between worms and fruit flies.

Since the discovery of lin‑4 and let‑7, many microRNAs have been identified. This family of small non-coding RNAs is involved in the regulation of diverse biological processes, and includes many potential therapeutic targets — not bad for what were originally thought to be mere worm time-keepers.