1.2 MicroRNA
MicroRNAs are small endogenous non-coding RNAs that are 18 to 25 nucleotides in size, with high conservation and specificity[6]. MiRNA are crucial regulators of gene expression as post-transcriptional silencing by binding and inhibiting target genes’ translation, widely involved in growth development and pathological processes[7].
Intergenic miRNAs be transcribed with their own promoter independently, whereas intragenic miRNAs be transcribed together with the host gene. Long chain primary transcripts (pri-miRNAs) be transcribed and then cuts by Drosha protein complex and generates pre-miRNA, after transported to cytoplasm from nucleus, pre-miRNA being cut and modified by Dicer enzymes to form mature miRNAs[8]. By partially complementary binding to the target gene’s 3’ non-coding region (3’UTR), miRISCs (miRNA-induced silencing complexes) leading to transcriptional repression of the target mRNA with no impact on mRNA stability. Completely complementary binding directly cleaves target mRNA. Besides,miRNA also could lead mRNA deadenylation, transcriptional repression or cleavage may trigger the deadenylative processes of mRNA[9]. It has been also reported that miRNAs binding to the 5’-UTR of mRNA may activate translation[10]. MiRNAs suppression and activation of target mMiRNA regulate nearly all biological functions, including cell division, proliferation, differentiation, apoptosis, and cell cycle[11].
MiRNA involved in genes regulation networks, miR-21, miR-29 family, miR-27(a/b), miR-34 in oral fluids were shown to be biomarkers in the tooth movement, modulating the process of osteoblastogenesis, osteoclastogenesis, and extra-cellular matrix conformation post-transcriptionally, and regulating the Physiological processes of orthodontic-related bone and tissue remodeling[12]. MiR-1 and miR-133, act as co-transcriptional and co-regulatory factors[13], that are highly expressed in injured myocardial tissue, encapsulated in exosomes and released into circulation, mediating the mobilization of bone marrow progenitor cells from bone marrow to the peripheral circulation and participating in the repair of myocardial ischemia[14].