Production Of Mature miRNAs Research Articles

An efficient CRISPR-Cas12a-mediated MicroRNA knockout strategy in plants.

In recent years, the CRISPR-Cas9 nuclease has been used to knock out MicroRNA (miRNA) genes in plants, greatly promoting the study of miRNA function. However, due to its propensity for generating small insertions and deletions, Cas9 is not well-suited for achieving a complete knockout of miRNA genes. By contrast, CRISPR-Cas12a nuclease generates larger deletions, which could significantly disrupt the secondary structure of pre-miRNA and prevent the production of mature miRNAs. Through the case study of OsMIR390 in rice, we confirmed that Cas12a is a more efficient tool than Cas9 in generating knockout mutants of a miRNA gene. To further demonstrate CRISPR-Cas12a-mediated knockout of miRNA genes in rice, we targeted nine OsMIRNA genes that have different spaciotemporal expression and have not been previously investigated via genetic knockout approaches. With CRISPR-Cas12a, up to 100% genome editing efficiency was observed at these miRNA loci. The resulting larger deletions suggest Cas12a robustly generated null alleles of miRNA genes. Transcriptome profiling of the miRNA mutants, as well as phenotypic analysis of the rice grains revealed the function of these miRNAs in controlling gene expression and regulating grain quality and seed development. This study established CRISPR-Cas12a as an efficient tool for genetic knockout of miRNA genes in plants.

The Enrichment of miRNA-Targeted mRNAs in Translationally Less Active over More Active Polysomes.

miRNAs moderately inhibit the translation and enhance the degradation of their target mRNAs via cognate binding sites located predominantly in the 3'-untranslated regions (UTR). Paradoxically, miRNA targets are also polysome-associated. We studied the polysome association by the comparative translationally less-active light- and more-active heavy-polysome profiling of a wild type (WT) human cell line and its isogenic mutant (MT) with a disrupted DICER1 gene and, thus, mature miRNA production. As expected, the open reading frame (ORF) length is a major determinant of light- to heavy-polysome mRNA abundance ratios, but is rendered less powerful in WT than in MT cells by miRNA-regulatory activities. We also observed that miRNAs tend to target mRNAs with longer ORFs, and that adjusting the mRNA abundance ratio with the ORF length improves its correlation with the 3'-UTR miRNA-binding-site count. In WT cells, miRNA-targeted mRNAs exhibit higher abundance in light relative to heavy polysomes, i.e., light-polysome enrichment. In MT cells, the DICER1 disruption not only significantly abrogated the light-polysome enrichment, but also narrowed the mRNA abundance ratio value range. Additionally, the abrogation of the enrichment due to the DICER1 gene disruption, i.e., the decreases of the ORF-length-adjusted mRNA abundance ratio from WT to MT cells, exhibits a nearly perfect linear correlation with the 3'-UTR binding-site count. Transcription factors and protein kinases are the top two most enriched mRNA groups. Taken together, the results provide evidence for the light-polysome enrichment of miRNA-targeted mRNAs to reconcile polysome association and moderate translation inhibition, and that ORF length is an important, though currently under-appreciated, transcriptome regulation parameter.

Discovery of a Novel Small-Molecule Inhibitor Disrupting TRBP-Dicer Interaction against Hepatocellular Carcinoma via the Modulation of microRNA Biogenesis.

MicroRNAs (miRNAs) are key players in human hepatocellular carcinoma (HCC) tumorigenesis. Therefore, small molecules targeting components of miRNA biogenesis may provide new therapeutic means for HCC treatment. By a high-throughput screening and structural simplification, we identified a small molecule, CIB-3b, which suppresses the growth and metastasis of HCC in vitro and in vivo by modulating expression profiles of miRNAome and proteome in HCC cells. Mechanistically, CIB-3b physically binds to transactivation response (TAR) RNA-binding protein 2 (TRBP) and disrupts the TRBP-Dicer interaction, thereby altering the activity of Dicer and mature miRNA production. Structure-activity relationship study via the synthesis of 45 CIB-3b derivatives showed that some compounds exhibited a similar inhibitory effect on miRNA biogenesis to CIB-3b. These results support TRBP as a potential therapeutic target in HCC and warrant further development of CIB-3b along with its analogues as a novel therapeutic strategy for the treatment of HCC.

HnRNPC induces isoform shifts in miR-21-5p leading to cancer development

MicroRNA (miRNA) processing is a critical step in mature miRNA production. Its dysregulation leads to an increase in miRNA isoforms with heterogenous 5′-ends (isomiRs), which can recognize distinct target sites because of their shifted seed sequence. Although some miRNA genes display productive expression of their 5′-isomiRs in cancers, how their production is controlled and how 5′-isomiRs affect tumor progression have yet to be explored. In this study, based on integrative analyses of high-throughput sequencing data produced by our group and publicly available data, we demonstrate that primary miR-21 (pri-miR-21) is processed into the cancer-specific isomiR isomiR-21-5p | ±1, which suppresses growth hormone receptor (GHR) in liver cancer. Treatment with antagomirs against isomiR-21-5p | ±1 inhibited the in vitro tumorigenesis of liver cancer cells and allowed the recovery of GHR, whereas the introduction of isomiR-21-5p | ±1 mimics attenuated these effects. These effects were validated in a mouse model of spontaneous liver cancer. Heterogeneous nuclear ribonucleoprotein C and U2 small nuclear RNA auxiliary factor 2 were predicted to bind upstream of pre-miR-21 via a poly-(U) motif and influence Drosha processing to induce the production of isomiR-21-5p | ±1. Our findings suggest an oncogenic function for the non-canonical isomiR-21-5p | ±1 in liver cancer, and its production was shown to be regulated by hnRNPC.

Abstract C049: A common microRNA-flanking variant (rs13136737) in hsa-miR-302/367 affects risk of prostate cancer progression in young men

Abstract A clinical challenge in prostate cancer (PCa) is identifying the characteristics of primary lesions that give rise to metastatic disease so that patients receive immediate treatment. Genetic variation in microRNA (miRNA) genes can affect mature miRNA biogenesis and/or mRNA targeting, which can consequently influence cancer susceptibility and progression. We identified an under-studied genetic variant (rs13136737, G/T) that is near the 3’ end of the polycistronic primary miRNA, hsa-miR-302/367, which we show to be biofunctional. In this study we investigated the biological effects and clinical consequences of this sequence variant in PCa. We found that production of mature miRNAs from the hsa-miR-302/367 cluster is dependent on rs13136737 genotype, and that the T-allele primary transcript is inefficiently processed with respect to production of mature miR-302d and miR-367. To evaluate the consequences of rs13136737 germ line variation we genotyped the locus in a mixed-race cohort of prostate cancer patients and tested for an association with cancer stage at diagnosis as an indicator of aggressiveness. We observed an age-dependent association between rs13136737 genotype and increased risk of advanced stage PCa which was present only in younger men [<65 years, OR = 3.13, 95% confidence interval 1.46 – 7.52; p=0.003] and replicated this association in an independent data set of African-American men [OR: 1.58; 95% confidence interval 1.10 – 2.26; p=0.013]. When the cancer risk of the polymorphism was stratified with age in an African American population, the 45-54 years of age group shows significance (p=0.0374) whereas there was no significant association in older men. This suggests SNP rs13136737 T allele is associated with risk in early-onset PCa patients. The consistency of this result in discovery and validation datasets, which varied substantially in their racial composition, suggests that rs13136737 genotype may be a robust biomarker for risk of advanced stage PCa among younger men. The rs13136737 risk allele (T) occurs at high frequencies in European, South Asian, Hispanic, and Native American populations (0.51, 0.52, 0.57, and 0.71, respectively) and at relatively low frequency in African/African-American populations (0.12). This indicates that the clinical utility of rs13136737 will be sensitive to patient ancestry due to variation in the frequency of the risk allele. Citation Format: Ronald L Heimark, Kelsey Guest, Virginia Ware, Kari Hernandez, Brenna Rheinheimer, Jessica M Patnode, Raquel Dias, Mitchell Sokoloff, Jason A Wilder. A common microRNA-flanking variant (rs13136737) in hsa-miR-302/367 affects risk of prostate cancer progression in young men [abstract]. In: Proceedings of the Twelfth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2019 Sep 20-23; San Francisco, CA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(6 Suppl_2):Abstract nr C049.

NF90 modulates processing of a subset of human pri-miRNAs.

MicroRNAs (miRNAs) are predicted to regulate the expression of >60% of mammalian genes and play fundamental roles in most biological processes. Deregulation of miRNA expression is a hallmark of most cancers and further investigation of mechanisms controlling miRNA biogenesis is needed. The double stranded RNA-binding protein, NF90 has been shown to act as a competitor of Microprocessor for a limited number of primary miRNAs (pri-miRNAs). Here, we show that NF90 has a more widespread effect on pri-miRNA biogenesis than previously thought. Genome-wide approaches revealed that NF90 is associated with the stem region of 38 pri-miRNAs, in a manner that is largely exclusive of Microprocessor. Following loss of NF90, 22 NF90-bound pri-miRNAs showed increased abundance of mature miRNA products. NF90-targeted pri-miRNAs are highly stable, having a lower free energy and fewer mismatches compared to all pri-miRNAs. Mutations leading to less stable structures reduced NF90 binding while increasing pri-miRNA stability led to acquisition of NF90 association, as determined by RNA electrophoretic mobility shift assay (EMSA). NF90-bound and downregulated pri-miRNAs are embedded in introns of host genes and expression of several host genes is concomitantly reduced. These data suggest that NF90 controls the processing of a subset of highly stable, intronic miRNAs.

Chloroplast-to-Nucleus Signaling Regulates MicroRNA Biogenesis in Arabidopsis

As integral regulators in plant development and stress response, microRNAs (miRNAs) themselves need to be tightly regulated. Here, we show that tocopherols (vitamin E), lipid-soluble antioxidants synthesized from tyrosine in chloroplasts, positively regulate the biogenesis of miRNAs. Tocopherols are required for the accumulation of 3'-phosphoadenosine 5'-phosphate (PAP), a retrograde inhibitor of the nuclear exoribonucleases (XRN), which may protect primary miRNAs from being degraded and promote mature miRNA production. Such regulation is involved in heat-induced accumulation of miR398 and plant acquisition of heat tolerance. Our study reveals a chloroplast-to-nucleus signaling mechanism that favors miRNA biogenesis under heatand possibly other environmental perturbations.

Evolutionary Divergence of Brain-specific Precursor miRNAs Drives Efficient Processing and Production of Mature miRNAs in Human

The hallmark of human evolution encompasses the dramatic increase in brain size and complexity. The intricate interplays of micro-RNAs (miRNAs) and their target genes are indispensable in brain development. Sequence divergence in distinct structural regions of Brain-specific precursor miRNAs (pre-miRNAs) and its consequence in the production of corresponding mature miRNAs in human are unknown. To address these questions, first we classified miRNAs into three categories based on tissue expression: Brain-specific (expressed exclusively in brain), Non-brain (expressed in Non-brain tissues) and Common (expressed in all tissues) and compared the sequence divergence of different structural regions (basal segment, lower and upper stem, internal and terminal loop) of categorized pre-miRNAs across human, non-human primates and rodents. Our analysis revealed that unpaired regions of Brain-specific pre-miRNAs in human bear traces of relatively high rate of evolutionary divergence compared to those in other species. Cross-tissue expression analysis unveiled the higher expression of the Brain-specific miRNAs in human compared to other species. Intriguingly, in human brain, expression levels of these miRNAs superseded the levels of the ubiquitously expressed “Common-miRNAs”. Further analysis revealed that presence of certain motif and nucleotide preference in the Brain-specific pre-miRNAs may favor DROSHA and DICER to ameliorate miRNA processing. The higher processing efficiency of human Brain-specific miRNAs was reflected as an elevated production of corresponding mature miRNAs in the human brain. Finally, re-construction of gene-regulatory network uncovers different pathways driven by Brain-specific miRNAs that may contribute to the development of brain in human.

Nuclear PTEN enhances the maturation of a microRNA regulon to limit MyD88-dependent susceptibility to sepsis.

Sepsis-induced organ damage is caused by systemic inflammatory response syndrome (SIRS), which results in substantial comorbidities. Therefore, it is of medical importance to identify molecular brakes that can be exploited to dampen inflammation and prevent the development of SIRS. We investigated the role of phosphatase and tensin homolog (PTEN) in suppressing SIRS, increasing microbial clearance, and preventing lung damage. Septic patients and mice with sepsis exhibited increased PTEN expression in leukocytes. Myeloid-specific Pten deletion in an animal model of sepsis increased bacterial loads and cytokine production, which depended on enhanced myeloid differentiation primary response gene 88 (MyD88) abundance and resulted in mortality. PTEN-mediated induction of the microRNAs (miRNAs) miR125b and miR203b reduced the abundance of MyD88. Loss- and gain-of-function assays demonstrated that PTEN induced miRNA production by associating with and facilitating the nuclear localization of Drosha-Dgcr8, part of the miRNA-processing complex. Reconstitution of PTEN-deficient mouse embryonic fibroblasts with a mutant form of PTEN that does not localize to the nucleus resulted in retention of Drosha-Dgcr8 in the cytoplasm and impaired production of mature miRNAs. Thus, we identified a regulatory pathway involving nuclear PTEN-mediated miRNA generation that limits the production of MyD88 and thereby limits sepsis-associated mortality.

Bioinformatics Study of Structural Patterns in Plant MicroRNA Precursors.

According to the RNA world theory, RNAs which stored genetic information and catalyzed chemical reactions had their contribution in the formation of current living organisms. In recent years, researchers studied this molecule diversity, i.a. focusing on small non-coding regulatory RNAs. Among them, of particular interest is evolutionarily ancient, 19–24 nt molecule of microRNA (miRNA). It has been already recognized as a regulator of gene expression in eukaryotes. In plants, miRNA plays a key role in the response to stress conditions and it participates in the process of growth and development. MicroRNAs originate from primary transcripts (pri-miRNA) encoded in the nuclear genome. They are processed from single-stranded stem-loop RNA precursors containing hairpin structures. While the mechanism of mature miRNA production in animals is better understood, its biogenesis in plants remains less clear. Herein, we present the results of bioinformatics analysis aimed at discovering how plant microRNAs are recognized within their precursors (pre-miRNAs). The study has been focused on sequential and structural motif identification in the neighbourhood of microRNA.

Requirement for Dicer in Maintenance of Monosynaptic Sensory-Motor Circuits in the Spinal Cord

In contrast to our knowledge of mechanisms governing circuit formation, our understanding of how neural circuits are maintained is limited. Here, we show that Dicer, an RNaseIII protein required for processing microRNAs (miRNAs), is essential for maintenance of the spinal monosynaptic stretch reflex circuit in which group Ia proprioceptive sensory neurons form direct connections with motor neurons. Inpostnatal mice lacking Dicer in proprioceptor sensory neurons, there are no obvious defects in specificity or formation of monosynaptic sensory-motor connections. However, these circuits degrade through synapse loss and retraction of proprioceptive axonal projections from the ventral spinal cord. Peripheral terminals are also impaired without retracting from muscle targets. Interestingly, despite these central and peripheral axonal defects, proprioceptive neurons survive in the absence of Dicer-processed miRNAs. These findings reveal that Dicer, through its production of mature miRNAs, plays a key role in the maintenance of monosynaptic sensory-motor circuits.

SUMOylation of TARBP2 regulates miRNA/siRNA efficiency

Small RNA-induced gene silencing is essential for post-transcriptional regulation of gene expression; however, it remains unclear how miRNA/siRNA efficiency is regulated. Here we show that TARBP2 is SUMOylated at K52, which can be enhanced by its phosphorylation. This modification can stabilize TARBP2 via repressing its K48-linked ubiquitination. We find that TARBP2 SUMOylation does not influence the overall production of mature miRNAs, but it regulates miRNA/siRNA efficiency. SUMOylated TARBP2 recruits Ago2 to constitute the RNA-induced silencing complex (RISC)-loading complex (RLC), and simultaneously promotes more pre-miRNAs to load into the RLC. Consequently, Ago2 is stabilized and miRNAs/siRNAs bound by TARBP2/Dicer is effectively transferred to Ago2. Thus, these processes lead to the formation of the effective RISC for RNA interference (RNAi). Collectively, our data suggest that SUMOylation of TARBP2 is required for regulating miRNA/siRNA efficiency, which is a general mechanism of miRNA/siRNA regulation.

A key gene of the small RNA pathway in the flounder, Paralichthys olivaceus: identification and functional characterization of dicer.

Dicer is critical for producing mature microRNAs (miRNAs) from precursor molecules and small interfering RNAs and plays an important role in controlling development and metabolism. In the present study, we cloned the flounder dicer gene, which is 6585 nucleotides (nt), including a 5'-untranslated region (UTR) of 231 nt, a 3'-UTR of 663 nt and an open reading frame of 5691 nt encoding a polypeptide of 1897 amino acids, and analyzed the conservation and expression pattern of dicer. The tissue distribution analysis indicated that dicer is abundantly expressed in the brain, heart, liver, spleen, stomach, kidney, gill, muscle, intestine and gonad of adult fish. Temporal expression analysis indicated that dicer mRNA is highly expressed during the embryonic and early larval stages, and exhibits low expression during the metamorphic stages. Treatment with thyroid hormone (TH) or thiourea indirectly or directly up-regulated dicer mRNA levels at 17 and 23dph, whereas treatment with TH down-regulated dicer mRNA levels at 36dph. The dicer-specific siRNA significantly down-regulated dicer mRNA and pol-let-7d levels, while pol-let-7d precursor levels were not differentially changed compared with the control (NC). These results demonstrated that dicer plays a key role in development and metabolism through the production of mature miRNAs, providing basic information for further studies concerning the role of dicer in Paralichthys olivaceus development.

MiRNAs in the Pathogenesis of Systemic Lupus Erythematosus.

MicroRNAs (miRNAs) were first discovered as regulatory RNAs that controlled the timing of the larval development of Caenorhabditis elegans. Since then, nearly 30,000 mature miRNA products have been found in many species, including plants, warms, flies and mammals. Currently, miRNAs are well established as endogenous small (~22 nt) noncoding RNAs, which have functions in regulating mRNA stability and translation. Owing to intensive investigations during the last decade, miRNAs were found to play essential roles in regulating many physiological and pathological processes. Systemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by elevated autoantibodies against nuclear antigens and excessive inflammatory responses affecting multiple organs. Although efforts were taken and theories were produced to elucidate the pathogenesis of SLE, we still lack sufficient knowledge about the disease for developing effective therapies for lupus patients. Recent advances indicate that miRNAs are involved in the development of SLE, which gives us new insights into the pathogenesis of SLE and might lead to the finding of new therapeutic targets. Here, we will review recent discoveries about how miRNAs are involved in the pathogenesis of SLE and how it can promote the development of new therapy.

Sequenciamento parcial dos genes TOX e NCOA2 em bubalinos

Os bubalinos são animais com uma importância grande para a produção de alimentos, além da ação sócio-econômica na região tropical. A caracterização de genes possibilita o estudo de características próprias da espécie e o desenvolvimento de tecnologia para sua produção. Esse trabalho estudou parcialmente os genes thymus high mobility group box protein (TOX) e o Nuclear Receptor Coactivator 2 (NCOA2) em femêas bubalinas da raça Murrah pela técnica de PCR-sequenciamento. Seis SNPs foram identificados em cada gene. Dois SNPs adjacentes no gene TOX criaram/destruíram um sitio de produção de um miRNA e são bons candidatos para serem estudos no futuro. A homologia das regiões desses genes com as correspondentes em bovinos é muito alta (99 %).

SNP in pre-miR-1666 decreases mature miRNA expression and is associated with chicken performance.

Polymorphisms in miRNA genes could potentially alter various biological processes by influencing the processing and (or) target selection of miRNAs. The rs14120863 (C > G) mutation, which we characterized in a Gushi-Anka F2 resource population, resides in the precursor region of miR-1666. Association analysis with chicken carcass and growth traits showed that the SNP was significantly associated with carcass weight, evisceration weight, breast muscle weight, leg muscle weight, and body weight at 8 weeks of age, as well as some body size indexes including shank girth, chest breadth, breast bone length, and body slanting length, in the Gushi-Anka F2 resource population. Quantitative RT-PCR results showed that miR-1666 expression levels in muscle tissues differed within various genotypes. Experiment in DF1 cells further confirmed that the SNP in miR-1666 could significantly alter mature miRNA production. Subsequently, using dual-luciferase report assay, we verified that miR-1666 could perform its function through targeting of the CBFB gene. In conclusion, the SNP in the precursor of miR-1666 could significantly reduce mature miR-1666 production. It may further affect the function of miR-1666 through the target gene CBFB, hence it is associated with chicken growth traits.

MicroRNAs in cardiac arrhythmia: DNA sequence variation of MiR-1 and MiR-133A in long QT syndrome

Long QT syndrome (LQTS) is a genetic cardiac condition associated with prolonged ventricular repolarization, primarily a result of perturbations in cardiac ion channels, which predisposes individuals to life-threatening arrhythmias. Using DNA screening and sequencing methods, over 700 different LQTS-causing mutations have been identified in 13 genes worldwide. Despite this, the genetic cause of 30–50% of LQTS is presently unknown. MicroRNAs (miRNAs) are small (∼ 22 nucleotides) noncoding RNAs which post-transcriptionally regulate gene expression by binding complementary sequences within messenger RNAs (mRNAs). The human genome encodes over 1800 miRNAs, which target about 60% of human genes. Consequently, miRNAs are likely to regulate many complex processes in the body, indeed aberrant expression of various miRNA species has been implicated in numerous disease states, including cardiovascular diseases. MiR-1 and MiR-133A are the most abundant miRNAs in the heart and have both been reported to regulate cardiac ion channels. We hypothesized that, as a consequence of their role in regulating cardiac ion channels, genetic variation in the genes which encode MiR-1 and MiR-133A might explain some cases of LQTS. Four miRNA genes (miR-1-1, miR-1-2, miR-133a-1 and miR-133a-2), which encode MiR-1 and MiR-133A, were sequenced in 125 LQTS probands. No genetic variants were identified in miR-1-1 or miR-133a-1; but in miR-1-2 we identified a single substitution (n.100A> G) and in miR-133a-2 we identified two substitutions (n.−19G> A and n.98C> T). None of the variants affect the mature miRNA products. Our findings indicate that sequence variants of miR-1-1, miR-1-2, miR-133a-1 and miR-133a-2 are not a cause of LQTS in this cohort.

Suppression of Dicer increases sensitivity to gefitinib in human lung cancer cells.

Accumulating evidence is revealing an important role of microRNA (miRNA) in tumor progression and chemotherapeutic resistance. Dicer is a cytoplasmic endoribonuclease type III crucial for production of mature miRNAs. The aberrant expression of Dicer has also been reportedly associated with clinical aggressiveness, prognosis, and patient survival in various cancer types. However, the molecular mechanisms of Dicer in acquired gefitinib resistance are still not clear. In this study, we analyzed the protein level of Dicer between gefitinib-sensitive (PC9) and gefitinib-resistant (PC9/GR) non-small-cell lung cancer (NSCLC) cell lines by Western blot analysis. Silence and overexpression of the Dicer were performed to investigate the effects on gefitinib sensitivity, as assessed by (3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide) assay and sub-G1 assay of flow cytometry. To further explore the mechanism of chemoresistance, we examined whether Dicer knockdown led to modulating specific miRNAs and its miRNA target genes. Dicer expression was significantly increased in PC9/GR compared with PC9 cells. Knockdown of Dicer restores gefitinib sensitivity in resistant cells, and overexpression of Dicer enhances resistance to gefitinib in sensitive cells. Silencing of Dicer induces sensitivity to gefitinib in NSCLC cells through the downregulation of miR-30b/c and miR-221/222 to increase the protein level of caspase-3, resulting in an increase in gefitinib-induced apoptosis. Dicer contributes to the resistance to gefitinib in lung cancer. These results indicate that Dicer may be a target for diagnosis and therapy of patients with resistance to gefitinib.

Multiple products from microRNA transcripts

A single transcript sometimes codes for more than one product. In bacteria, and in a few exceptional animal lineages, many genes are organized into operons: clusters of open reading frames that are transcribed together in a single polycistronic transcript. However, polycistronic transcripts are rare in eukaryotes. One notable exception is that of miRNAs (microRNAs), small RNAs that regulate gene expression at the post-transcriptional level. The primary transcripts of miRNAs commonly produce more than one functional product, by at least three different mechanisms. miRNAs are often produced from polycistronic transcripts together with other miRNA precursors. Also, miRNAs frequently derive from protein-coding gene introns. Finally, each miRNA precursor can produce two mature miRNA products. We argue, in the present review, that miRNAs are frequently hosted in transcripts coding for multiple products because new miRNA precursor sequences that arise by chance in transcribed regions are more likely to become functional miRNAs during evolution.

MicroRNA Regulation and Role in Stem Cell Maintenance, Cardiac Differentiation and Hypertrophy

There are currently 1527 known microRNAs (miRNAs) in human, each of which may regulate hundreds or thousands of target genes. miRNA expression levels vary between cell types; for example, miR- 302 and miR-290 families are highly enriched in embryonic stem cells, while miR-1 is a muscle specific miRNA. miRNA biosynthesis and function are highly regulated and this regulation may be cell type specific. The processing enzymes and factors that recognize features in sequence and secondary structure of the miRNA play key roles in regulating the production of mature miRNA. Mature miRNA enriched in stem cells control stem cell self-renewal as well as their differentiation. Though specific miRNAs have been shown to control differentiation towards various lineages such as neural or skin cells, some of the most well characterized miRNAs have been found in promoting the formation of cardiac cells. In addition, miRNAs also play a critical role in cardiomyocyte hypertrophy, especially in a pathological context. Such miRNAs are predicted to be therapeutic targets for treating cardiovascular diseases. In this review we will discuss how miRNAs act to maintain the stem cell state and also explore the current knowledge of the mechanisms that regulate miRNAs. Furthermore, we will discuss the emerging roles of miRNAs using cardiomyocyte differentiation and maturation as a paradigm. Emphasis will also be given on some of the less ventured areas such as the role of miRNAs in the physiological maturation of cardiomyocytes. These potentially beneficial miRNAs are likely to improve cardiac function in both in vivo and in vitro settings and thus provide additional strategy to treat heart diseases and more importantly serve as a good model for understanding cardiomyocyte maturation in vitro.