Identification of potential miRNAs and malaria infection
Malaria-infected RBCs can develop in malaria parasites compose with two asexual stages in human (blood steam and liver) and sexual stage in the mosquito. In human, these parasites invade into human red blood cells (RBCs) until the mature parasite sequestration using cytoadherence ligand of
P. falciparum Erythrocyte Membrane Protein-1 (PfEMP-1) [
49,
50]. Splenic macrophages are the main of clearance of malaria from blood circulation. The miRNA possibly helps malaria to invade and grow in RBCs via escape from immune responses and defect of opsonization by circulating macrophage [
49,
51‐
53]. Analysis of plasmodium genome demonstrated more than 500 genes [
54]. Two groups reported that
P. falciparum did not have miRNA-sequences in parasite genome [
50,
55]. They made a clone of all RNAs from a mixed stage of malaria-infected RBCs and then tested with bioinformatics method. It showed no matching between those cloned sequences and miRNA structures. There has been no study showing the presence of RNAi-family [siRNA, miRNAs, repeat-associated small interfering RNAs (rasiRNAs), and PIWI-interacting RNAs (piRNAs)] according to the stage of the parasite. In some infectious diseases, interactions between host miRNA and pathogen gene, or vice versa, were reported. For examples, human miRNAs including miR-223 suppress human immunodeficiency virus-1 (HIV-1) mRNA [
56], Epstein-Bar virus miRNA: miR-BHRF1-3 targets human interferon (IFN)-inducible T cell-attracting chemokine (CXCL11) gene expression [
57], and human miR-122 targets to hepatitis C virus (HCV) RNA [
58]. However, it is still not known whether human miRNA interacts with malaria mRNAs. Further examination may bring the miRNA-based diagnosis or therapeutic approaches. The posttranscriptional gene silencing in
Plasmodium parasites changes alternative pathways other than miRNAs. The possibility is that
Plasmodium is utilizing host miRNAs to regulate their gene expression [
49,
59,
60].
The correlation between reported miRNAs was reviewed according to malaria parasite species (Table
1). For
P. falciparum malaria, Rathjen et al. found that the block of miR-451 synthesis pathway by knocking out Ago2 which produces mature miR-451 resulted in the development of severe anaemia in mice [
55]. Similar to the previous study that miR-451 is an essential molecule for erythroid cells since miR-451 was up-regulated during human erythroid differentiation [
61]. Xue et al. [
50] also showed that 36 clones of miRNA were found in infected erythroid cells, not in malaria parasite and the majority of genome composes with 80–90% of A-T rich sequence in
P. falciparum parasites. Both studies did not found any
Plasmodium specific miRNAs; those might be an effect from cells culture method. In erythroid cells, LaMonte et al. and Chapman et al. found that the levels of miR-233 and miR-451 were high in these parasite-infected cells when compared with normal [
49,
62]. They suggested that impaired growth of parasites might be resulted from a block of mRNA translation by miR-451 and miR-223 in human red blood cells. Thus, Rathjen et al. and Xue et al. demonstrated that parasite could diminish miR-451 level in serum, but be accumulated in
Plasmodium-infected RBCs. Similar to Chamnanchanunt et al. observed the lower levels of miR-451 and -16 in serums from 22
P. vivax patients than non-infected subjects [
63]. This group also found downregulation of miR-451 and -16 in red blood cells of
P. vivax patients [
64]. Reducing miR-451 relates to Ago2 in extracellular vesicles (EVs) to stimulate oxidative damage in infected-RBCs [
65]. Interestingly, Baro et al. [
66] demonstrated that miR-221/222, -24 and -191 were decreased in bone marrow in
P. vivax malaria patients. The numbers of
P. falciparum patients need to more large scale study.
Table 1
Summary of discovery miRNAs among patients and animal experimental studies
Human specimen |
| 2006 |
P.f. parasite in cell culture | miR-451: significantly accumulated in infected RBCs |
| 2008 |
P.f. infected in human erythroid cells | miR-451, let-7b, miR-16, miR-91, miR-142, miR-144, let-7a, let-7f, miR-92, miR106: identified form infected RBCs |
| 2012 | HbAS and HbCC RBCs with p. f. | – | miR-451 and miR-223 |
Chamnanchanunt et al. [ 63] | 2015 | Patients with p.f. and p.v. infection | miR-451 and miR-16 (plasma of p.v. patients than p.f. patients) | – |
Chamnanchanunt et al. [ 64] | 2015 | Patients with malaria infection | miR-451 and miR-16 (RBCs of p.v. patients) | – |
Animal specimen |
| 2011 |
P. chabaudi infected in mice model | miR-10b, let-7a, let-7 g, miR-193a-3p, miR-192, miR-14205p, miR-465d, miR-677, miR-98, miR-694, miR-142-5p, miR-465d, miR-677, miR-98, miR-694, miR-374, miR-450b-5p, miR-464, miR-377, miR-20a, miR-466d-3p: (in liver) | miR-26b, miR-M23-1-5p, miR-1274a: (in liver organ) |
| 2011 |
P. berghei infected in mice model | – | let-7i, miR-27a, miR-150 (in brain organ) |
| 2012 |
P. chabaudi infected in mice model | miR-194, miR-192, miR-193A-3P, miR-145, miR-16, miR-99A, miR-99B, miR-15A, miR-152, let-7G, let-7B, miR-455-3P: (in spleen and liver) | – |
For an animal model of demonstration parasite induces organ failure, mice infected with
P. chabaudi malaria showed that 12 common miRNAs were downregulated in spleen and liver tissues [
67]. A study by Delic et al. found three miRNA species upregulated and 16 miRNA species downregulated [
68]. These findings suggested that miRNAs might be reprogrammed to minimize disease severity after infection. Knowledge of the interaction between falciparum parasite and the human genome could be valuable in malaria control. Furthermore, a study by El-Assaad et al. found that mouse with cerebral malaria had overexpression of miR-27a, miR-150, and let7i levels in brain tissue compared to a mouse with no cerebral malaria [
69]. Thus, miRNAs would have significant roles as biomarkers to predict early host responses and prognosis of malaria infection.
The knowledge of miRNAs as possible disease biomarkers in blood is a promising breakthrough especially the patients with malaria infection. For practical use, the disease criteria for severe falciparum malaria were applied to identify severe falciparum malaria patients from non-severe form [
70] (Table
2). The relationship between candidate miRNAs and severe falciparum malaria is not yet clearly understood, and this might help to predict early critical patients.
Table 2
Criteria for severe or complicated falciparum malaria infection [
4‐
7] and candidate miRNAs
Acidosis/acidemia | Artrial pH <7.3 or presence of acidosis | miR-210 | HIF-dependent trasncriptional regulation |
ARDS or pulmonary edema | The acute lung injury from noncardiogenic causes | miR-181b | NF-kB mediated vascular inflammation |
miR-125b | LPS-induced lung injury |
Cerebral malaria | Imparied consciousness or seizures | miR-210 | Regulation of the revascularization |
miR-27a, miR-23a | Brain activation by EFNA3, NP1 |
miR-150 | Stimulate angiogenic factors |
Renal failure | Urine output <0.4 ml/kg/hour or serum creatinine >3.0 mg/dl | miR 17–92 | Renal progenitors and renaly dysfuntion |
miR-24 | Apoptosis regulation |
Ongoing investigation |
Anemia | Haemoglobin ≤8 g/dl | n.a. | – |
Shock | Blood pressure <90/60 mmHg with the sign of cold, clammy extremities | n.a. | – |
DIC | The presence of DIC phenomenon or spontaneous mucosal bleeding | n.a. | – |
Hyperparasitemia | Presence of parasitized erythrocytes >10% | n.a. | – |
Hypoglycemia | Presence of blood sugar <40 mg/dl | n.a. | – |
Macroscopic hemoglobinuria | The presence of hemolysis in the patients without G6PD deficiency | n.a. | – |