Week 5 - P2 - L2 -Molecular Mechanisms and Evolutionary Significance of MicroRNAs in Gene Expression

Introduction to Small RNAs and RNA-RNA Interactions

• RNA molecules possess the ability to interact with other RNA molecules through base pairing, a property that is essential for controlling gene expression and coordinating development within multicellular organisms.

• Small RNAs represent a diverse class of molecules with several critical biological roles:

  • Splicing Regulation: Facilitating the removal of introns from pre-mRNA.

  • Transposon Suppression: Silencing "movable elements" or jumping genes within the genome to maintain genomic stability.

  • MicroRNAs (miRNAs): A specific class of small RNAs that coordinate gene expression across all metazoans (multicellular animals).

• MicroRNAs are encoded in the genome and are initially treated similarly to protein-coding genes during production:

  • They are transcribed by RNA polymerase II.

  • They undergo modifications such as the addition of a polyadenylation (poly-A) tail and a guanosine cap.

Comparative Genomics and Gene Expression Coordination

• A comparison of model organisms like $C. elegans$ (a small nematode approximately $1\,mm$ long) and humans reveals surprising similarities and a key difference in their molecular toolkits:

  • Transcription Factors: Both $C. elegans$ and humans possess roughly the same number of identified transcription factors, approximately $1,500$, which bind to promoters to activate or repress gene expression.

  • Protein Kinases and Phosphatases: There is very little difference in the number of these enzymes, which regulate protein activity by adding or removing phosphate groups.

  • RNA Binding Proteins: Estimates suggest there are around $600$ in both species, though this number is expected to increase as new RNA binding domains are discovered.

  • MicroRNAs: This is the primary class of regulators where significant expansion is observed. While worms have only a few hundred miRNAs, humans have several thousand ($2,000+$).

• The expansion of the miRNA repertoire is suggested to provide the fidelity and complexity required for the development of diverse tissue types and complex body architectures, even with a relatively limited number of transcription factors.

Genomic Organization and Structural Characteristics of MicroRNAs

• MicroRNAs are found in various locations within the genome and can be organized in different ways:

  • Monocistronic Genes: A single gene encodes only one microRNA.

  • Polycistronic Genes: Multiple different microRNAs are encoded within a single transcript. This is analogous to bacterial operons, though they produce functional RNA rather than proteins.

  • Intronic Region MicroRNAs (Mirtrons): Found within the introns of other genes and released during the splicing process.

• Regardless of their genomic origin, these RNAs fold into predictable hairpin structures (stem-loops) due to internal base pairing (e.g., Adenine with Uracil, Guanine with Cytosine). These shapes are recognized by specific cellular processing machinery.

The MicroRNA Biogenesis Pathway

• The production of functional microRNAs involves a multi-step enzymatic process across the nucleus and cytoplasm:

1. Primary MicroRNA (pri-miRNA) Formation: The initial transcript produced by RNA polymerase II, which contains the hairpin structure.

2. Nuclear Processing (The Microprocessor Complex):

   • The complex consists of two primary proteins: DGCR8 (an RNA binding protein that recognizes the hairpin loop and the single-stranded RNA bulge at the top) and Drosha (the catalytic enzyme).

   • Drosha cleaves the pri-miRNA to release the hairpin loop, which is now referred to as a pre-microRNA (pre-miRNA).

3. Nuclear Export:

   • The pre-miRNA interacts with Exportin-5.

   • Exportin-5 facilitates the movement of the pre-miRNA through the nuclear pore and the associated molecular mesh into the cytoplasm.

4. Cytoplasmic Processing:

   • The enzyme Dicer cleaves the loop portion of the hairpin, leaving a double-stranded RNA molecule approximately $22$ nucleotides long.

5. RISC Loading:

   • One strand of the RNA, known as the mature microRNA (or guide strand), is retained and loaded onto a large protein complex called the RNA-induced silencing complex (RISC).

   • The other strand, often called the passenger strand, is discarded and degraded.

Mechanisms of Target mRNA Regulation

• The mature miRNA within the RISC scans the transcriptome to find messenger RNAs (mRNAs) with complementary sequences.

• Binding Location: RISC typically binds to complementary sites located in the $3'$ untranslated region ($3'\,UTR$) of target mRNAs.

• Complementarity and Seed Regions:

  • Binding is characterized by partial complementarity rather than a perfect match.

  • There is a critical region at the $5'$ end of the miRNA with strong complementarity to the mRNA, known as the "seed" sequence.

  • Other regions often contain "bulges" where the sequences do not match perfectly, making the prediction of miRNA targets challenging without experimental verification.

• Regulatory Flexibility:

  • One miRNA can target and regulate multiple different mRNAs.

  • A single mRNA can be targeted by multiple different miRNAs simultaneously.

• This system allows for a highly diverse and refined pattern of gene regulation, controlling how much protein is produced from specific transcripts to coordinate developmental steps.

• Therapeutic Applications: Synthetic microRNAs and microRNA mimics have been developed as therapeutics to exploit these natural regulatory pathways.

Evolutionary Conservation and Significance

• MicroRNAs are highly conserved across vast evolutionary timescales, underscoring their importance in life:

  • The Let-7 Family: This miRNA family is present in worms, flies, mice, and humans.

  • The last common ancestor between worms ($C.\,elegans$) and humans lived approximately $500 \times 10^6$ to $600 \times 10^6$ years ago.

  • The sequence of Let-7, which is approximately $22$ nucleotides long, has remained unchanged throughout this entire period.

• This extreme conservation indicates that the sequence is strictly protected because it regulates critical genes involved in development. In many cases, disrupting Let-7-mediated regulation results in non-viable organisms.

• Expansion and Complexity: While some species lose specific miRNA families (e.g., $C.\,elegans$ is missing the miR-100 family), the overall trend in evolution is an expansion of the miRNA repertoire as body architectures become more complex. This expansion provides the necessary fidelity for regulating the development of intricate biological structures.