3. Biogenesis of MiRNAs
The biogenesis of miRNAs is a process for producing mature miRNAs, which takes place in the cell nucleus and cytoplasm. The process initiates in the nucleus, where the non-coding part of the host genome is transcribed by RNA polymerase II to produce a long hairpin-like structure of primary miRNA (pri-miRNA) having more than 200 nucleotides in length. Then, RNase III family enzymes Drosha and DGCR8 complex (DiGeorge Syndrome Critical Region 8) cropped the pri-miRNA into precursor miRNA (pre-miRNA) consisting of approximately 70 nucleotides
[8]. Next, the pre-miRNA hairpin loop is transported into the cytoplasm by exportin-5. In the cytoplasm, the complex of Dicer and Trans-Activation Responsive RNA-Binding Protein (TRBP) cut the hairpin-like structure and processes the pre-miRNA into unstable 14–21 nucleotide long duplex miRNA structure and separated into two strands, mature miRNA guide strand and the miRNA passenger strand. After strand separation, the guide strand (5′end) of the miRNA/miRNA duplexis combined with the RNA induced silencing complex (RISC) attached with Argonaute protein which then interacts with the 3′ un-translated region (3′UTR) target site of the mRNA leads to the translational repression and mRNA degradation. In contrast, the passenger strand is often degraded. As miRNAs can affect multiple RNAs, a single miRNA can potentially impact numerous regulatory functions
[9] (
Figure 1)
[10].
Figure 1. Pathway of biogenesis of microRNAs (miRNAs). In the nucleus, the non-coding part is transcribed by RNA polymerase II to form a hairpin-like structure called pri-miRNA. Then, enzymes Drosha and DGCR8 complex cleaved the pri-miRNA into pre-miRNA. From the nucleus, the pre-miRNA is exported by exportin 5 into the cytoplasm, and the complex of enzyme Dicer and Trans-Activation Responsive RNA-Binding Protein (TRBP) cuts the hairpin-like structure and processes the pre-miRNA into two unstable mature miRNA arms into two strands, mature miRNA guide strand and the miRNA passenger strand. After strand separation, the guide strand (5′end) of the miRNA/miRNA duplex is complexed with RISC attached with Argonaute protein. The miRNA RISC complex facilitates base-pairing interaction between miRNA and mRNA, which leads to gene regulation.
MiRNAs have a primary role in regulating gene expression in the human body by partially binding to mRNA, usually within the 3′ untranslated region and thus preventing their translation to protein
[7]. Interestingly, a single miRNA can bind to many mRNAs and can regulate the expression of a large number of genes, and any mRNA can be regulated by several miRNAs simultaneously
[11]. Moreover, miRNAs recognize the target mRNA accurately and subsequently down-regulate the gene expression either by translational repression or mRNA cleavage
[7]. The backbone of the miRNA-mediated gene silencing primarily depends on the degree of complementarities between miRNA and mRNA. The high degree of complementarity between miRNA and target mRNA enables the AGO complex protein to break the target mRNA, which fixes the process of gene silencing. The Argonaute(AGO) protein complex has four isoforms, out of which only AGO 2 (known as slicer) can break the target Mrna
[12]. The low degree of complementarity between miRNA and mRNA promotes the translational repression mechanism by binding with the target mRNA.
MiRNAs can be tissue-specific or species-specific. Such miRNA-122 and miRNA-124 is highly distributed in the liver and neurological tissues, respectively
[13]. Researchers have revealed that miR-1found in humans; flies and worms are tissue-specific. This miRNA has been evolutionally conserved and has played an important role in muscles. On the other hand, there are many miRNAs, even though they are conserved and have hundreds of non-conserved miRNA genes. These non-conserved miRNAs might be playing a critical species-specific function
[14]. While many studies have hinted the vital role of miRNAs in the pathogenesis of numerous human parasitic or bacterial or viral infections, the specific functions of miRNAs in these infections are still unclear.
4. MiRNAs during Malaria Infection
The malaria parasites have a complex life cycle, including sexual and asexual stages in female
Anopheles mosquitoes and humans. At both transcriptional and post-transcriptional levels, this complex life cycle progress requires tight regulation of gene expression
[12]. The asexual erythrocyte stage, including intraerythrocytic development, involves significant interaction between host erythrocyte cells and parasites. Some material is exchanged between host erythrocytes and parasites during these interactions. Curiously, this interaction leads to the transfer of human miRNA from erythrocytes into the parasite. In humans,
plasmodium parasites invade into erythrocytes (RBCs) using cytoadherence ligand of
P. Falciparum Erythrocyte Membrane Protein-1 (PfEMP-1 until the mature parasite sequestration
[15]. Approximately200 human miRNAs are found in RBCs lacking nucleus and transcription/translation machinery
[15]. Some chimeric fusions are formed between
Plasmodium falciparum mRNA and human miRNAs, which are transferred into the
Plasmodium, resulting in translational inhibition. Sequencing and bioinformatics searches revealed that the
P. Falciparum genome lacks the main enzymes, such as Dicer and AGO, required for RNA interference that induce sequence-specific gene silencing by double-stranded RNA. Indeed, parasite
Plasmodium falciparum imports these necessary enzymes along with miRNAs, including let 7a and miR-15a from the human RBC to regulate its gene expression, for example, target the
Plasmodium gene Rad54
[16]. Moreover, the cyclic AMP signaling pathway plays a significant role in parasite development by activating molecules such as the cAMP-dependent protein kinase (PKA). In
P. falciparum, two subunits of PKA, namely, cAMP-dependent catalytic (PKA-C) and regulatory (PKA-R), have been recognized, and the regulation of PKA-R levels is critical for
plasmodium survival. MiR-451 integration into
P. falciparum AMP-dependent protein kinase (PKA-R) transcript reduced the translation of regulatory PKA activity and reduced the parasite growth
[17][18].
Reports revealed that high levels of specific human miR-451, miR-223 and let-7i are found in both HbAS (having sickle cell trait) and HbSS (having sickle cell disease) erythrocytes compared with normal red blood cells and negatively regulate the malaria infection
[19]. Rathjen et al. (2006) have performed small RNA expression profiling from parasite-infected RBCs and healthy RBCs and found a high amount of human miRNA, most abundantly miRNA 451, both in infected RBCs and unparasitized RBCs. Therefore, miR-451 could be functional in the differentiation of erythroid cells
[20]. Despite this, some miRNA like miR-451, miR-106, miR-16, miR-92, miR-7b, miR-144, miR-142, let-7f, let-7a and miR-91 are down-regulated, whereas miR-223 and miR-19b are upregulated in red blood cells of malaria parasite-infected patients. MiR-92 and miR-17 regulate the TGF-β signalling pathway related to malaria
[21][22]. One more study revealed the role of two erythrocytic miRNAs mimic, miRNA-150-3p and miR-107-5p, which retarded malaria parasite growth when highly present in erythrocytes
[23]. Malaria parasite released tissue-specific miRNAs into the bloodstream, and dysregulation of miRNAs such as miR-16, miR-150, miR-155, miR-223 and miR-451 are associated with immune-related genes expression including, AGO-1, AGO-2, CD36, IL4, IFN-CD80, CD86, PfEMP-1, ANG-1 and ANG-2 during malaria infections.
MiRNAs are significantly involved in several biological processes by regulation of gene expression. These are secreted in biological fluids like plasma, serum, cerebrospinal fluid, milk, urine, saliva, seminal fluid, tears and blood
[8]. These are called circulating or extracellular miRNAs. Plasma and serum miRNAs are found in different extracellular vesicles fluids such as exosomes, microvesicles, and apoptotic bodies based on their size and cellular origin. Extracellular vesicles are small membrane-bound particles released from many types of cells. Extracellular vesicles derived miRNA, including plasma miRNA, are highly stable and consist of proteins, lipids and nucleic acids
[24]. Many studies revealed that elevated release of extracellular vesicles from various cell types, including endothelial cells, RBCs and platelets, containing small RNAs, correlates with malaria severity
[25][26]. For example, the elevated level of extracellular vesicles derived miRNA-15-5p and miRNA-150b-5p were found in
Plasmodium vivax-infected patients while let-7a-5p was upregulated in both
Pv and
pf infected patients in Thailand
[27]. A recent study explored thatMiR-451 and miR16 are the most abundant miRNAs in normal plasma and normal RBCs
[24]. In contrast to another study, a significant decrease in the level of miRNA-16 and miRNA-145 in plasma of
Plasmodium vivax infected patients compared to normal individuals but no significant difference for
pf infected patients with normal individuals
[3][28]. Therefore, a diminished level of miRNA-145 leads to lower growth of the
Plasmodium parasite in in-vitro
[29]. EVs released from infected RBCs carry functional miRNAs with Argonaute 2 complex protein in the bloodstream, and these EVs could be absorbed by the host endothelial cells. EVs delivered miRNAs alter the expression level of the target proteins and affect endothelial cell functions (
Figure 2)
[30]. Another study, Wang and colleagues suggested that more amounts of microparticles (extracellular vesicles) were released from a culture medium of malaria-infected RBCs than the normal RBCs. Human AGO 2 protein was found to bind with hundreds of miRNAs in these microparticlesand this miRNA-hAGO2, which is a component of the miRISC complex were transferred into the
Plasmodium falciparum [25]. AGO proteins (Argonaute proteins) are usually associated with small RNAs and are involved in post-transcriptional gene-silencing processes. When bound to miRNAs, AGO proteins also stabilize and protect miRNAs from degradation in animals and plants. Many studies also reported that EVs miRNAs could be transferred from one species to another and play crucial roles in cell-to-cell communication
[31].
Figure 2. (
A) About 200 miRNAs are found in human RBCs lacking nucleus and transcription/translation machinery. (
B)
Plasmodium-infected human red blood cells (RBCs). Transfer of miRNAs from RBC to
Plasmodium parasite and inhibit translation. (
C) Transfer of miRNAs from
Plasmodium-infected RBC to endothelial cells. (Reproduced from Bayer-Santos, E.; Marini, M. M.; da Silveira, J. F., Non-coding RNAs in host–pathogen interactions: subversion of mammalian cell functions by protozoan parasites. Frontiers in microbiology 2017, used under CC BY 4.0 with modification and addition of Normal RBCs, Microvesicles, Exosomes and Host derived EVs
[30]).
Several studies were also performed on an animal model for indicating miRNA expression in the malaria model. Cerebral malaria (CM) is a fatal complication of
Plasmodium infection, mostly affecting children. MiRNAs play an important role in regulating immune cell responses against infection. The infection of
Plasmodium chabaudi and
P.ANKA in the mice experiment model showed changes in the miRNA expression and caused the development of protective immunity against the
Plasmodium parasite. The up-regulation ofmiR-27a, miR-150 and let7i was seen in the brain of
Plasmodium berghei infected CBA mice that cause cerebral malaria when compared to non-cerebral malaria (NCB) and non-infected (NI) samples
[32]. Another study also analyzed the change in the expression profile of some miRNA by using the microarrays technique in microvesicles from
Plasmodium ANKA and
P.yoelii infected CBA mice that cause cerebral malaria and severe malaria and found miR-146a and miR-193b were rich in microvesicles from cerebral malaria-infected mice when compared with non-cerebral malaria and non-infected mice
[33]. MiRNAs may regulate biological pathways related to innate and adaptive immunity. Other studies analyzed dysregulated twelve, miRNAs. (MiR-21-5p,-18a-5p,-19a-3p, -20b-5p, -142-3p, -27a-5p, -152-3p, -193a-5p, -155-5p,-218-1-3p, -543, and -411-5p) between non-infected (NI) and cerebral malaria (CM). Three miRNAs, miR-142-3p, miR-27a-5p andmiR-19a-3p, were considerably upregulated in CM mice compared to NI and NCM. These miRNAs are significantly involved in some cerebral malaria-like adherens junctions, FoxO, TGF-β and endocytosis pathways
[34]. MiRNA-146a plays an important role in innate immunity responses, including IL-receptor-associated kinase (IRAK)-1 and TNF receptor-associated factor (TRAF)-6 (
Table 1)
[35].
Table 1. List of miRNAs studies conducted on humans and animal model experiments to explore various miRNA regulations and their functions.