Extracellular MicroRNAs: A Stable and Diverse Source of Transcriptional Control

MicroRNAs (miRNAs) are a highly conserved class of small (19–25 nucleotides) non-coding RNAs that play critical roles in post-translational gene regulation. Dysregulation of miRNA expression has been widely implicated in the development and progression of numerous diseases, particularly cancer, positioning them as promising candidates for diagnostic and prognostic applications. In parallel, miRNAs are frequently detected in extracellular vesicles (EVs), where they contribute to intercellular communication and have emerged as attractive non-invasive biomarkers. Importantly, EV-associated miRNA profiles do not always directly mirror intracellular miRNA abundance. While altered cellular expression can influence EV-miRNA content, selective and regulated sorting mechanisms also actively shape EV cargo composition. These include sequence- and motif-based recognition elements (such as EXOmotifs), RNA-binding proteins (including hnRNPA2B1, YBX1, and SYNCRIP), and lipid-associated pathways such as ceramide-dependent mechanisms. Together, these processes enable the preferential packaging of specific miRNAs into EVs, independent of their relative cellular expression levels. This review therefore integrates both perspectives: it summarizes current evidence supporting dysregulated miRNAs detected in EVs as disease-associated biomarkers and critically examines the molecular mechanisms governing miRNA sorting into EVs. By clarifying the interplay between cellular miRNA dysregulation and active EV loading processes, we highlight the complexity underlying EV-miRNA signatures and underscore the need for standardized mechanistic frameworks to improve their translational utility in cancer diagnostics and beyond.