RNA Splicing

RNA splicing, a post-transcriptional process necessary to form a mature mRNA, was discovered in the late 1970s. Two different modes of splicing have been defined, that is, constitutive splicing and alternative splicing. 

Constitutive splicing is the process of removing introns from the pre mRNA, and joining the exons together to form a mature mRNA. 

Alternative splicing, on the other hand, is the process where exons can be included or excluded in different combinations to create a diverse array of mRNA transcripts from a single pre-mRNA and therefore serves as a process to increase the diversity of the transcriptome. 

The estimated number of alternative splicing events in the human transcriptome has risen sharply over the past decades. In the 1980s, it was thought that about 5% of human genes were subjected to alternative splicing. In 2002, this number had risen to 60%, and now, after implementation of next-generation-sequencing technologies, we know that the vast majority, >95% of mRNAs, are subjected to alternative splicing. Nevertheless, the function of a large fraction of these splice isoforms remains to be elucidated. Furthermore, it is anticipated that in different tissues, or in tissues with different disease states, new isoforms still remain to be identified.

1.0Introduction

The process of splicing is highly conserved during evolution. Splicing is more prevalent in multicellular than in unicellular eukaryotes because of the lower number of introncontaining genes in the latter. Later in evolution, alternative splicing becomes more prevalent in vertebrates than in invertebrates. Interestingly, just a single exon-skipping event in the RNA-binding protein (RBP), polypyrimidine tract binding protein has been shown to direct numerous alternative splicing changes between species, indicating that a single splicing event can amplify transcriptome diversity between species. The recent observation that the total number of protein-coding genes does not differ much between species, fueled the hypothesis that alternative splicing largely contributes to organism diversity. And indeed, as we move up the phylogenetic tree, alternative splicing complexity increases, with the highest complexity in primates.

2.0RNA Splicing Definition

RNA splicing is a critical step in molecular biology that converts precursor messenger RNA (pre-mRNA) into mature messenger RNA (mRNA). This process involves the precise removal of introns, which are non-coding segments of RNA, and the subsequent joining of exons, the coding sequences, to form a continuous sequence that can be translated into a protein. RNA splicing takes place in the nucleus of the cell. 

3.0What is RNA Splicing? 

• RNA splicing is a fundamental process in eukaryotic cells that involves the editing of a precursor messenger RNA (pre-mRNA) into a mature messenger RNA (mRNA). This editing process is essential for the removal of introns, which are non-coding regions, from the pre-mRNA. After introns are removed, the remaining exons, which are the coding sequences that will be translated into protein, are joined together. 
• This RNA splicing process allows a single gene to potentially produce multiple variations of proteins through alternative splicing, where the same pre-mRNA can be spliced in different ways to produce different mRNAs. RNA splicing occurs in the nucleus and is a crucial step for the correct expression of genes and the production of functional proteins.
• During the RNA splicing process, the segments known as introns, which do not code for proteins, are excised, while the coding sequences, referred to as exons, are stitched together. This crucial step is facilitated by a complex molecular machine called the spliceosome. Additionally, certain RNA molecules, known as ribozymes, possess the unique ability to catalyse their own splicing.
• The mature mRNA is further processed to enhance its stability and functionality: it undergoes 5' capping, where a modified guanine nucleotide is added to the 5' end, and polyadenylation, where a chain of adenine nucleotides is appended to the 3' end.

4.0Mechanism of RNA Splicing

RNA splicing process in eukaryotic pre-mRNA undergoes three critical modifications which are as follows: 

• 5' Capping: A 7-methylguanosine cap is added to the 5' end of the pre-mRNA. This involves attaching a GTP to the initial nucleotide and adding methyl groups to the guanine residue and the ribose of the first nucleotide. This cap structure consists of several methyl groups.
• Polyadenylation: The 3' end of the mRNA is processed to add a poly-A tail. A signal sequence, typically AAUAAA, located upstream of the cleavage site, guides this process. Proteins recognize this signal, leading to the cutting of the RNA chain and the addition of about 200 adenine nucleotides by poly-A polymerase. Polyadenylation marks the end of transcription and plays roles in regulating translation and mRNA stability.
• Splicing: Introns are removed, and exons are joined. Initially, the pre-mRNA is cleaved at the 5' splice site, and the 5' end of the intron is connected to an adenine nucleotide within the intron to form a loop structure. Then, a cut at the 3' splice site allows the exons to be joined together, excising the intron in a lariat form which is then degraded. Splicing occurs within large complexes called spliceosomes, comprised of proteins and small nuclear RNAs (snRNAs) - U1, U2, U4, U5, and U6 - forming snRNPs that are crucial for the splicing process.

These steps ensure that the pre-mRNA is properly edited and modified to produce mature mRNA capable of directing the synthesis of proteins, highlighting the intricate control mechanisms underlying gene expression in eukaryotic cells.

5.0Importance of RNA Splicing

• Increases protein diversity by allowing a single gene to produce multiple proteins.
• Regulates gene expression, influencing how and when genes are turned on or off.
• Contributes to cell and tissue specificity by generating protein variants needed in different cellular environments.
• Is involved in disease development when splicing errors occur, leading to dysfunctional proteins.
• Plays a significant role in evolution by enabling the complexity and adaptability of organisms through the generation of diverse proteins from a limited number of genes.