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].