MicroRNAs (miRNAs) are small molecules that control the types and amounts of proteins cells produce. They are important regulators in biological processes, influencing gene expression.
Defining MicroRNAs
MicroRNAs are a class of small, non-coding RNA molecules, 20 to 25 nucleotides in length. They are “non-coding” because, unlike messenger RNA (mRNA), they do not carry instructions for making proteins. Instead, miRNAs originate from longer RNA precursors that undergo several processing steps.
The journey begins with its gene in the DNA transcribed into a primary microRNA (pri-miRNA), often forming a hairpin-like structure. This pri-miRNA is processed in the nucleus by a protein complex with Drosha, forming a shorter precursor microRNA (pre-miRNA). The pre-miRNA is then exported to the cytoplasm, where Dicer processes it into the mature microRNA.
How MicroRNAs Function
MicroRNAs exert their effects by binding to messenger RNA (mRNA) molecules in the cell’s cytoplasm. This binding is guided by a “seed region” (nucleotides 2-8 at the miRNA’s 5′ end), which partially complements sequences in the target mRNA’s 3′ untranslated region (UTR). Once bound, the miRNA, as part of the RNA-induced silencing complex (RISC), regulates gene expression.
The interaction leads to two main outcomes: blocking mRNA translation into protein or promoting mRNA degradation. Translational repression prevents protein synthesis, silencing gene expression. Alternatively, the mRNA can be broken down, preventing any protein production. Both mechanisms reduce the protein levels encoded by the target gene.
MicroRNAs: Orchestrators of Cellular Processes
MicroRNAs regulate numerous biological processes within cells. Through gene silencing, miRNAs contribute to cell growth and cellular differentiation, guiding cells to specialize. They also regulate programmed cell death (apoptosis), essential for removing damaged or unneeded cells.
Their influence extends to cellular metabolism, affecting processes like glucose uptake, glycolysis, and lipid metabolism. By fine-tuning gene expression, miRNAs help maintain cellular balance.
MicroRNAs and Human Health
Dysregulation of microRNAs, meaning their expression levels are either too high or too low, has been linked to the development and progression of various human diseases. For instance, altered miRNA levels are observed in many cancers, where they can either promote tumor growth or suppress it. They also contribute to cardiovascular diseases, impacting processes like angiogenesis and fibrosis.
Neurodegenerative disorders, such as Alzheimer’s and Parkinson’s diseases, also show associations with dysregulated miRNAs. Due to their presence in bodily fluids and stability, miRNAs are being investigated as potential biomarkers for disease diagnosis and prognosis. Manipulating miRNA levels to correct imbalances positions them as promising therapeutic targets.