The Role of mRNA Alternative Splicing in Macrophages Infected with Mycobacterium tuberculosis: A Field Needing to Be Discovered

Abstract

Mycobacterium tuberculosis (Mtb) is one of the major causes of human death. In its battle with humans, Mtb has fully adapted to its host and developed ways to evade the immune system. At the same time, the human immune system has developed ways to respond to Mtb. The immune system responds to viral and bacterial infections through a variety of mechanisms, one of which is alternative splicing. In this study, we summarized the overall changes in alternative splicing of the transcriptome after macrophages were infected with Mtb. We found that after infection with Mtb, cells undergo changes, including (1) directly reducing the expression of splicing factors, which affects the regulation of gene expression, (2) altering the original function of proteins through splicing, which can involve gene truncation or changes in protein domains, and (3) expressing unique isoforms that may contribute to the identification and development of tuberculosis biomarkers. Moreover, alternative splicing regulation of immune-related genes, such as IL-4, IL-7, IL-7R, and IL-12R, may be an important factor affecting the activation or dormancy state of Mtb. These will help to fully understand the immune response to Mtb infection, which is crucial for the development of tuberculosis biomarkers and new drug targets.

Keywords: Mycobacterium tuberculosis; alternative splicing; macrophages.

Figure 1

Overview of splicing reactions and involved regulatory signals/proteins. (a) The assembly and disassembly of the spliceosome during the entire splicing reaction. The figure shows the progressive interaction of spliceosome small ribonucleoprotein (snRNP) particles (U1, U2, U4, U5, and U6; brown, green, yellow, pink, and purple, respectively) during the removal of introns from pre-mRNA containing two exons (white and black). The name of the spliceosome complex and the catalytic steps of the reaction are marked. (b) The cis-acting elements stabilize or disrupt the assembly of splicing bodies on the pre-mRNA through positive and negative signals. The diagram illustrates a common segment of eukaryotic precursor mRNA, which comprises one exon flanked by two introns. Factors that promote splicing reactions at nearby splice sites, such as serine-arginine repeat (SR) proteins (shown in dark purple), frequently bind to Intronic and Exonic Splicing Enhancers (ISE and ESE, indicated in purple). Intronic and Exonic Splicing Silencers (ISS and ESS; pink) are often constrained by factors that inhibit splicing from nearby splice sites, such as heterogeneous nuclear ribonucleoprotein particles (hnRNP) proteins (red). The * indicates a catalytically active complex.

Figure 2

Graphical description of alternative splicing. (a) Normal splicing. (b) Exon skipping or inclusion. (c) Alternative 5′SS selection. (d) Alternative 3′SS selection. (e) Mutually exclusive splicing. (f) Retained intron. (g) Alternative promoters. (h) Alternative polyA. Grey rectangles represent component exons. Yellow and green rectangles represent variable exons. The black solid line indicates introns. The thin solid lines and dotted lines located on exons represent different splicing patterns.

Figure 3

Alternative splicing of macrophages caused by Mtb infection. The difference in alternative splicing between Healthy people, TB patients, and Healthy TB contacts. The spherical structure of the lungs in TB patients is granuloma. The red arrows indicate increased protein expression.

Figure 4

Different isoforms of IL32 and their interactions. The nine isoforms of IL-32 are listed from top to bottom: IL-32α, IL-32β, IL-32γ, IL-32δ, IL-32ε, IL-32ζ, IL-32η, IL-32θ, and IL-32s. The 1–8 exons of IL32 are arranged from left to right. ATG is the start codon. TGA is a stop codon. The black arcs with arrows represent interactions between different isoforms.

Figure 5

The protective role for IL-32 isoforms upon Mtb infection. Macrophage response after Mtb infection. The yellow rod is Mycobacterium tuberculosis. Cathelicidin and DEFB4 are antimicrobial peptides.