Gene expression is the process that converts instructions in our DNA into functional products like proteins. Central to this regulation are molecules called microRNAs (miRNAs), which are small, non-coding RNA molecules. Instead of carrying the blueprints for proteins themselves, they act as regulators by silencing or reducing the output of specific genes. By intercepting the messages that carry protein-building instructions, miRNAs provide a layer of control over nearly every biological process. It is estimated that miRNAs regulate about 60% of all human genes, highlighting their widespread impact on cellular life.
The miRNA Pathway to Action
The journey of a microRNA to a functional regulator is a multi-step process that begins in the cell’s nucleus. A gene coding for an miRNA is transcribed into a long primary miRNA (pri-miRNA) transcript. This initial molecule is then recognized by a protein complex, Drosha-DGCR8, which trims the pri-miRNA into a smaller, hairpin-shaped structure called a precursor-miRNA (pre-miRNA).
This pre-miRNA is then transported out of the nucleus and into the cytoplasm. Here, it encounters the enzyme Dicer, which makes a final cut, removing the loop from the pre-miRNA hairpin. This creates a short, double-stranded RNA molecule approximately 22 nucleotides long, the nearly-mature form of the miRNA.
The final step involves the unwinding of this double-stranded molecule. One of the strands, the mature miRNA, is selected and loaded into a large protein assembly called the RNA-Induced Silencing Complex (RISC), while the other strand is degraded. Once loaded into RISC, the mature miRNA acts as a guide to identify specific genetic targets, and the complex carries out the silencing function.
Targeting the Genetic Message
Once armed within the RISC complex, the miRNA finds its specific targets, which are messenger RNA (mRNA) molecules. These mRNA molecules are temporary copies of a gene’s code that carry instructions to the cell’s protein-making machinery. The miRNA-RISC complex intercepts this message before it can be fully translated into a protein.
The accuracy of this process relies on a portion of the miRNA known as the “seed sequence.” This short stretch of six to eight nucleotides is the primary determinant for binding. The seed sequence scans mRNA molecules, looking for complementary binding sites, which are most often found in the 3′ untranslated region (3′ UTR).
The pairing between the miRNA’s seed sequence and the mRNA target site does not have to be perfect. In animals, this binding is often imperfect, creating a slight mismatch. This flexibility allows a single type of miRNA to recognize and bind to hundreds of different mRNA targets, allowing it to coordinate the regulation of a whole network of genes.
Silencing the Gene
After the miRNA-RISC complex binds to its target mRNA, it can silence the gene in one of two ways, depending on the degree of complementarity. When the miRNA and its target site have a near-perfect match, an enzyme within the RISC complex cleaves the mRNA strand. This action triggers its rapid degradation by the cell and prevents any protein from being synthesized from it.
The more common mechanism in animals occurs when the pairing between the miRNA and mRNA is imperfect. In this scenario, the binding of the miRNA-RISC complex does not lead to the immediate destruction of the mRNA, but instead engages in translational repression. The bound complex physically obstructs the cell’s protein-making machinery, the ribosomes, blocking them from moving along the mRNA strand.
This blockade pauses or completely halts the translation of the mRNA into a protein. The RISC complex can also recruit other proteins that modify the mRNA’s structure, such as by shortening its protective poly(A) tail. This modification further marks the mRNA for eventual degradation.
The Role in Health and Disease
The regulatory network controlled by microRNAs is necessary for maintaining normal biological functions. During embryonic development, miRNAs guide the processes of cell differentiation, ensuring that stem cells develop into specialized types like nerve or muscle cells. They are also involved in managing cellular proliferation, metabolism, and immune responses.
Because miRNAs regulate so many genes, disruptions in their levels can have significant consequences. The abnormal expression of specific miRNAs is a factor in a wide range of human diseases. For instance, in many types of cancer, some miRNAs that normally suppress tumor growth are found at low levels, while others that promote cell proliferation are overexpressed. Dysregulation of miRNAs has also been linked to heart disease, diabetes, and neurological disorders, making them a subject of research for understanding disease and a potential target for new therapies.