MicroRNAs (miRNAs) are tiny, non-coding RNA molecules found within cells. These molecules powerfully regulate gene expression by fine-tuning the production of proteins, ensuring cellular processes unfold correctly. Understanding how these molecules are generated within the cell, a process known as miRNA biogenesis, provides insight into their widespread biological importance.
The Journey of miRNA Creation
miRNA creation begins in the cell’s nucleus, where specific genes are transcribed by RNA polymerase II. This initial transcript is a long RNA molecule called a primary microRNA (pri-miRNA), which forms a hairpin-like structure. These pri-miRNAs can be hundreds or thousands of nucleotides long, resembling messenger RNAs with caps and polyadenylated tails.
Following transcription, the pri-miRNA undergoes its first processing step within the nucleus. A protein complex known as the Microprocessor complex, comprising the RNase III enzyme Drosha and its cofactor DGCR8, recognizes and cleaves the pri-miRNA. This cleavage event trims the pri-miRNA into a shorter, ~70-nucleotide hairpin precursor called a precursor microRNA (pre-miRNA). DGCR8 recognizes specific sequence motifs within the pri-miRNA, guiding Drosha to the correct cleavage site.
Once formed, the pre-miRNA is transported out of the nucleus into the cytoplasm. This export is mediated by Exportin-5, a transport protein working with RanGTP. This nuclear export ensures that subsequent processing of the pre-miRNA occurs in the appropriate cellular compartment for its final maturation.
In the cytoplasm, the pre-miRNA encounters another enzyme, Dicer. Dicer acts upon the hairpin-shaped pre-miRNA, cleaving off its terminal loop structure. This results in the formation of a short, double-stranded microRNA duplex, typically 19 to 25 nucleotides in length.
This double-stranded miRNA duplex is unwound, and one of its two strands, the mature miRNA, is selectively loaded into a protein complex called the RNA-induced silencing complex (RISC). The other strand, the passenger strand, is degraded. Argonaute (AGO) family proteins are components of the RISC complex. They bind the mature miRNA and mediate its gene-silencing functions.
How miRNAs Control Genes
Once incorporated into the RISC complex, the mature miRNA guides the complex to specific messenger RNA (mRNA) targets within the cell. The miRNA achieves this specificity by recognizing complementary sequences, found in the 3′-untranslated region (3′-UTR) of the target mRNA. This binding interaction is fundamental to miRNA-mediated gene regulation.
The degree of complementarity between the miRNA and its target mRNA determines the outcome of gene regulation. When there is perfect or near-perfect sequence complementarity, the RISC complex, guided by the miRNA, leads to the degradation of the target mRNA. This breakdown prevents it from being translated into a protein, effectively silencing the gene’s expression.
In cases where the complementarity between the miRNA and its target mRNA is imperfect, the primary mechanism of gene regulation shifts to translational repression. Here, the mRNA is not necessarily degraded, but its translation into protein is inhibited. This means the genetic message is present, but protein synthesis is inhibited, leading to reduced protein production.
This dual mechanism allows miRNAs to exert precise and broad control over gene expression throughout the cell. They can fine-tune the levels of thousands of different proteins, influencing many biological processes, from cellular development and differentiation to proliferation and programmed cell death. Their ability to regulate gene expression at the post-transcriptional level provides an additional layer of control beyond initial gene transcription.
When miRNA Biogenesis Goes Awry
The process of miRNA biogenesis is tightly regulated, ensuring that correct miRNAs are produced at appropriate levels. Disruptions in this pathway, or imbalances in the quantities of mature miRNAs, can have consequences for cellular function and overall health. Maintaining the proper balance of miRNA expression is important for cellular equilibrium.
When components of the miRNA biogenesis machinery, such as Drosha or Dicer, are faulty or their activity is altered, it can lead to overproduction or insufficient supply of specific miRNAs. Such dysregulation can impact the expression of genes normally controlled by these miRNAs, potentially disrupting cellular processes. This imbalance can contribute to the onset and progression of various diseases.
For instance, dysregulated miRNA levels are observed in various types of cancer. Some miRNAs can act as oncogenes, promoting uncontrolled cell growth when overexpressed, while others can function as tumor suppressors, inhibiting growth when their levels are too low. Beyond cancer, errors in miRNA biogenesis or altered miRNA levels have been linked to neurodegenerative disorders, where precise control of gene expression is important for neuronal health.
Cardiovascular diseases and developmental abnormalities have also been associated with disruptions in miRNA pathways. Understanding these pathological deviations from normal miRNA biogenesis provides insights into disease mechanisms. This knowledge may contribute to the development of new approaches for diagnosing diseases or identifying potential therapeutic targets.