MicroRNA

Perspectives in Health and Diseases

Jaishree Paul,Rohini Muthuswami

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351679879
• Edition: 1
• Topic: Medicine
• Subtopic: Biotechnology in Medicine

This book explores the role of miRNA as therapeutic agents, the progress made in this direction and the problems that need to be addressed for miRNA based therapies to become successful. It also discusses the basic biology of miRNA sythesis, regulation, and their role in disease biology.

1Controlling the Biogenesis of the Smallest Regulators

Ketki Patne and Rohini Muthuswami

1.1Transcription of miRNA Genes

1.2Factors Regulating the Transcription of miRNA Genes

1.3Processing of the Primary miRNA Transcript

1.4Control of miRNA Biogenesis

1.4.1At the Nuclear Processing Step

1.4.2Nuclear Export of Pre-miRNA

1.5Processing of the Precursor miRNA Transcript

1.5.1Control of miRNA Biogenesis at the Cytoplasmic Processing Step

1.5.2RNA Induced Silencing Complex

1.5.3Repression/Decay of Target mRNA

1.6Conclusion

References

Introduction

miRNAs were first discovered in 1993 by Victor Ambros in C. elegans and now are known to be found in plants, animals and some viruses (Lee et al., 2007, 1993). These small (~22 base) single-stranded regulatory RNAs are involved in most cellular processes, including cell growth, development, differentiation, homeostasis and apoptosis (Bartel, 2004). miRNAs adhere with Argonaute proteins to form the RNA-induced silencing complex (RISC) that binds to the target mRNA and mediates post-transcriptional gene regulation by translational inhibition or mRNA degradation (Yang and Lai, 2011). The importance of regulation of miRNAs can be inferred from the observation that alteration in miRNAome (the entire complement of miRNAs expressed by a cell, tissue or organism) of a cell is linked to many diseases including cardiovascular diseases, neurodevelopmental diseases, autoimmune diseases and cancer (Ardekani and Naeini, 2010).

The biogenesis of most of the miRNAs involves two key processing steps, but some miRNAs, which bypass this canonical pathway, are also known (Daugaard and Hansen, 2017).

The first step of canonical miRNA biogenesis pathway (see Figure 1.1) involves transcription of the miRNA gene by RNA polymerase II/III to form a primary transcript (primary miRNA or pri-miRNA) that can be several kilobases long and contains many stem-loop structures, each of which resembles a different precursor miRNA (pre-miRNA) (Borchert et al., 2006; Lee et al., 2004; Winter et al., 2009). The pri-miRNA undergoes the first processing step in the nucleus where it is cleaved by the microprocessor complex to form the pre-miRNA (Denli et al., 2004; Gregory et al., 2004). The microprocessor complex is composed of two key proteins, Drosha and DGCR8, and several additional proteins (Denli et al., 2004; Gregory et al., 2004; Han et al., 2004; Landthaler et al., 2004; Lee et al., 2003). During the processing of pri-miRNA, DGCR8 recognizes and binds to the stem and loop region and helps Drosha to cleave at the base of this stem-loop structure to form a 70 base long pre-miRNA (Lee et al., 2003; Nguyen et al., 2015).

Figure 1.1miRNA biogenesis pathway. Mature miRNAs are produced in a cell by the action of two sets of processing enzymes – Drosha in the nucleus and Dicer in the cytoplasm. Drosha forms the microprocessor complex with DGCR8 and cleaves the pri-miRNA into pre-miRNA. These pre-miRNAs are acted upon by Dicer in cytoplasm to form the miRNA duplex. The thermodynamically less stable strand is then loaded in the RISC complex where it guides Ago2 to its mRNA target causing translational repression or mRNA degradation.

The pre-miRNA is then transported to the cytoplasm by Exportin5-RanGTP complex, where it goes through the final processing step (Bohnsack et al., 2004; Lund et al., 2004; Yi et al., 2003). The cytoplasmic processing step includes cleavage by Dicer-TRBP-PACT complex to form the miRNA duplex (Bernstein et al., 2001; Hutvágner et al., 2001; Ketting et al., 2001; Wilson et al., 2015). This complex then gets linked with the Argonaute protein where the thermodynamically more stable strand of the miRNA duplex is degraded, and the less stable strand forms the RNA induced silencing (RISC) complex (Chendrimada et al., 2005; Gregory et al., 2005; Khvorova et al., 2003). The miRNA then guides the RISC complex to the target mRNA where the Argonaute protein exerts its function, leading to translational inhibition or mRNA degradation (Gregory et al., 2005).

miRNAs are important regulators of most cellular processes. Therefore, the levels of miRNAs need to be tightly regulated, s...