**12. miRNA role in host-viral interactions**

Regulative control of miRNAs over gene expression in every organism is the core reason why pathogens try to hijack them. This takeover may include disruption of miRNA biogenesis pathway, inhibiting a specific host miRNA that hinders pathogen propagation or differential miRNA expression. In case of human, it has been well documented that miR-146 and miR-155 are involved in upregulating immune function in response to various bacterial infections including *H. pylori* [95] and *Salmonella enterica* [96]. Many viruses encode miRNAs, having the potential to not only the viral genomes, which encode them, but also to target host transcripts, which facilitate their replication [97, 98]. Functions of viral miRNA targeting host genes include cell survival through downregulation of apoptotic factors as well as modulation of host immune response [89]. One example is upregulation of GATA4 by WNV-encoded miRNA, which has been found to be very important in virus replication in mosquito cells [88]. Recently, it has been demonstrated that viruses have evolved to encode miRNAs that mimic host miRNAs. Kaposi's sarcoma-associated herpes virus (KSHV) encoded miR-K12-11 that is an ortholog of human miR-155 sharing same seed region homology that will likely be able to regulate all targets of miR-155 [90].

#### **12.1. miRNA role in** *Aedes aegypti* **with an insect***-***specific flavivirus**

Previous studies demonstrated that mosquito miRNAs could be affected by different viral infections. However, those studies were restricted to pathogenic viruses, mainly restricted to the role of differently expressed miRNAs with limited characterization. Limited knowledge is available about the modulation of host miRNAs with respect to insect-specific flaviviruses (ISFs, e.g., Palm Creek virus (PCV)). ISFs share the similar genome organization with other flaviviruses; however, their rate of vertical transmission between insect hosts is higher than other vertebrate-infecting flaviviruses. Although it is still unclear that why ISFs fail to replicate in mammalian cells, it is assumed that ISFs potentially lack to antagonize the host interferon system. Similarly, ISFs express a high adaptation level in insect hosts, which may further limit their replication in vertebrate hosts. Recently, Lee et al. explicated the potential role of miRNAs in Palm Creek virus (PCV) infection using the *Ae. aegypti* mosquito model [99]. They proposed differently expressed miRNAs after combining small-RNA sequencing and bioinformatics tools although the results hardly predicted the potential involvement of PCV infection to alter host miRNAs. They revealed that only one miRNA (i.e., aae-miR-2940-5p) out of 101 reported miRNAs of *Ae. aegypti* that had significantly altered expression over the course of PCV infection. The level of aae-miR-2940-5p was induced within 2 days p.i. and suppressed at 12 days p.i.; however, a different pattern of miRNA screening was demonstrated in *Ae. aegypti* Aa20 cell line without displaying any significant change upon PCV infection. Further *in vitro* miRNA inhibition experiments while using aae-miR-2940-5p inhibitors demonstrated that this miRNA did not directly impact on PCV replication and has no significant role in PCV-*Ae. aegypti* interaction. This variable response to virus infection in cell lines and host mosquito might be due to tissue tropism of the virus. At mRNA target level, the study reported the inconsistent expression level of MetP with miR-2940-5p expression although previously it was reported a potential target of aae-miR-2940-5p with positive interaction. It means that there was no correlation of MetP expression to PCV infection because silencing the gene did not significantly affect the virus replication. The pair MetP--aae-miR-2940-5p also positively regulate West Nile virus (WNV) virion production where aae-miR-2940-5p was selectively downregulated upon WNV infection in infected cells. The MetP human ortholog like M41 ftsh, YME1L, plays an important role as an antiapoptotic factor. It is assumed that the induction of aae-miR-2940-5p in PCV-infected cells during infection boosts MetP transcript levels and thus protects infected cells undergoing apoptosis under stress responses. However, the MetP physiological role is still uncertain in insects and requires further investigation [99].

viruses may contribute to viral oncogenesis. Recently, it has been reported that a flavivirus named West Nile virus encode an miRNA KUN-miR-1 that specifically induces the expres-

Interestingly, virus-encoded miRNAs can regulate their own replication as well their own benefit. For example, simian virus 40 (SV40)-encoded miRNA miR-S1, expressed during late infection, has been shown to downregulate viral T-antigen, which is crucial to evade cytotoxic T cell response [93]. Another example is HvAV-miR-1 encoded by *Heliothis virescens* ascovirus (HvAV), an insect with dsDNA virus that downregulate viral replication by targeting viral DNA polymerase I [94]. Recently, it has been reported that Dengue virus (DENV)-encoded miRNA like viral small RNA DENV-vsRNA-5 plays an important role in the autoregulation of DENV replication by directly targeting dengue virus NS1 protein during late infection [30].

Regulative control of miRNAs over gene expression in every organism is the core reason why pathogens try to hijack them. This takeover may include disruption of miRNA biogenesis pathway, inhibiting a specific host miRNA that hinders pathogen propagation or differential miRNA expression. In case of human, it has been well documented that miR-146 and miR-155 are involved in upregulating immune function in response to various bacterial infections including *H. pylori* [95] and *Salmonella enterica* [96]. Many viruses encode miRNAs, having the potential to not only the viral genomes, which encode them, but also to target host transcripts, which facilitate their replication [97, 98]. Functions of viral miRNA targeting host genes include cell survival through downregulation of apoptotic factors as well as modulation of host immune response [89]. One example is upregulation of GATA4 by WNV-encoded miRNA, which has been found to be very important in virus replication in mosquito cells [88]. Recently, it has been demonstrated that viruses have evolved to encode miRNAs that mimic host miRNAs. Kaposi's sarcoma-associated herpes virus (KSHV) encoded miR-K12-11 that is an ortholog of human miR-155 sharing same seed region homology that will likely be able to

Previous studies demonstrated that mosquito miRNAs could be affected by different viral infections. However, those studies were restricted to pathogenic viruses, mainly restricted to the role of differently expressed miRNAs with limited characterization. Limited knowledge is available about the modulation of host miRNAs with respect to insect-specific flaviviruses (ISFs, e.g., Palm Creek virus (PCV)). ISFs share the similar genome organization with other flaviviruses; however, their rate of vertical transmission between insect hosts is higher than other vertebrate-infecting flaviviruses. Although it is still unclear that why ISFs fail to replicate in mammalian cells, it is assumed that ISFs potentially lack to antagonize the host interferon system. Similarly, ISFs express a high adaptation level in insect hosts, which may further limit their replication in vertebrate hosts. Recently, Lee et al. explicated the potential role

sion of GATA4 in *Ae. albopictus* that in turn facilitates the WNV replication [88].

**12. miRNA role in host-viral interactions**

102 Current Topics in Tropical Emerging Diseases and Travel Medicine

regulate all targets of miR-155 [90].

**12.1. miRNA role in** *Aedes aegypti* **with an insect***-***specific flavivirus**
