**3. How Lassa virus replicates**

Lassa virus adopts a replication strategy known as "*Ambisense*," which is very rapid and demonstrates the replication of temporal control [24]. The initial stage of transcription of mRNA copies of the negative sense gene, which makes sufficient deposition of virus proteins for the next stage of the replication process. Subsequently, L and NP proteins are translated from the mRNA. Copies of the viral complementary RNA (vcRNA) are made from the positive sense gene. Negative-sense progeny are produced by templates of RNA copies while mRNA is synthesized from it. The mRNAs produced from vcRNA are later translated to synthesize the Z and GP proteins. The temporal controls enhance the production of proteins spikes lastly, and therefore, delay the recognition by host immune system [25].

protein utilizes viral RNA templates that consist of genomic RNA encapsidated by the viral nucleocapsid protein NP and comprises viral ribonucleoprotein [21, 30]. L polymerase of arenaviruses contains the SDD motif feature of all RNA-dependent RNA polymerases (RdRp) [10]. When infected, the viral agent RNP is inserted into the cytoplasm of the host cell, the L polymerase associated with the viral RNP starts transcription from the genome promoter located at the 3′-end of each genomic RNA segment, L and S. The 5′ and 3′ terminal 19 nt viral promoter regions of both RNA segments required for the recognition and binding by the viral polymerase exhibit a high degree of conservation among the arenaviruses. The genome segments have a high complementary 5′- and 3′-ends (19 nt) have been predicted to form panhandle structures [10, 11]. Transcription stops at the distal side of the stem-loop (SL) structure within the intergenomic region (IGR). The L polymerase adopts a replicase mode and moves across the IGR to create a full-length complementary antigenomic RNA (agRNA) that serves as a template for mRNAs synthesis of viral genes encoded in genomic orientation, GPC and Z, from the S and L segments, respectively, and for the synthesis of full-length genomic RNA (gRNA) [13, 18]. This SL structure has been discovered to stabilize the 3′-termini of the viral mRNAs [10]. The primary transcription leads in the mRNA synthesis of viral genes, which is encoded in antigenomic orientation, NP and L polymerase, from the S and L segments, respectively. The viral agent uses a cap snatching strategy to acquire the cap structures of cellular mRNAs. Cap snatching is facilitated by the endonuclease activity of the L polymerase that is co-factored by the cap binding activity of NP. Therefore, LASV synthesizes capped nonpolyadenylated mRNAs. Both gRNA and agRNA of the viral agent contain a nontemplate G residue at their 5′-ends. The proposed "prime and realign" mechanism includes the production of a pppGPCOH dinucleotide primer from the CG nucleotides at positions +2 and +3 of the 3′-end genome promoter sequence, that is, then realigned such that its 3′-terminal COH is opposite the genome 3′-terminal G residue, and the realigned pppGPCOH then acts as a primer for a complementary RNA strand production. The matrix protein Z is not part of the viral genome transcription and replication but shows a dose-dependent inhibitory effect on viral RNA production. This inhibitory effect of Z has been

A Reemerging Lassa Virus: Aspects of Its Structure, Replication, Pathogenicity and Diagnosis

http://dx.doi.org/10.5772/intechopen.79072

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The Lassa virus is well-known to cause Lassa fever [31]. Its symptoms include flu-like illness characterized by fever, general body weakness, cough, tonsillitis, headache, and gastrointestinal disorders. Hemorrhagic manifestations are other features of Lassa fever, which include

The virus pathogenesis is still unclear, but it has been shown that the virus chiefly target the antigen-presenting cells (mainly dendritic cells) and endothelial cells [32]. Lassa virus infects most tissues in the human body when gained entry. It starts with the mucosa, intestine, lungs, and urinary system, and then moves to the vascular system. There are findings that the viral agent can prevent a host's innate immune system by NP activity [33]. Usually, when a microbe penetrates a host, the innate defense system detects the pathogen-associated molecular patterns (PAMPs) and aggravates the response of the immune system. One of the

proposed for Old and New World arenaviruses [27].

**4. An aspect of Lassa virus pathogenicity**

vascular permeability [10].

The virus enters into the host cell by means of the cell-surface receptor (alpha-dystroglycan alpha-DG), which is a versatile receptor for proteins of the extracellular matrix [26]. Receptor recognition is based on a specific sugar modification of alpha-DG by a group of glycosyltransferases known as the LARGE proteins. Specific variants of the genes encoding these proteins appear to be under positive selection in West Africa, where the virus is endemic [26]. Enveloped viruses makes use of clathrin-coated pits mostly to enter cells and bind to their receptors in a pH dependent way while Lymphocytic choriomeningitis and Lassa virus makes use of the endocytotic pathway independent of caveolin, clathrin, dynamin, and actin. The viruses are quickly delivered once within the cell to endosomes via vesicular trafficking albeit, which is extensively independent of the tiny GTPases Rab5 and Rab7. pH-dependent membrane binding happens on contact with the endosome, which is mediated by the enveloped glycoprotein, and at the acidic pH, the endosome fuses the lysosome protein LAMP1 that yields in membrane bind and endosome escape [19].

NP is the most dominant viral protein in virions and infected cells, which consists the main structural component of the viral ribonucleoprotein (RNP), which plays an important role in the RNA synthesis of the virus. Subsequently, the function required to assist replication of the virus, at least two viral proteins (NP and Z), have been proposed to modulate the cell response of the host to infection. Experimental data has showed that NP has a role in virusinduced inhibition of type I IFN signaling [27]. This role has been mapped to the C-terminal domain of NP with a folding that camouflages the DEDDH family of 3′-5′ exoribonucleases. The little RING finger protein Z is the arenavirus counterpart of the matrix (M) protein of other negative sense RNA viruses. The Z protein of lymphocytic choriomeningitis virus (LCMV) interacts with the promyelocytic leukemia (PML) protein as well as the eukaryotic translation initiation factor 4E (eIF4E) in infected cells and has been observed to function in the noncytopathic nature of LCMV infection and repression of cap-dependent translation [22, 28, 29]. Functional assays reported the exonuclease activity of LASV NP that has been proposed as important for its type I IFN counteracting functions [29].

The replication cycle of Lassa virus is similar to the Old World arenaviruses. It was reported that virus internalization is limited upon cholesterol depletion. Dystroglycan, which is later cleaved into alpha-dystroglycan and beta-dystroglycan is initially expressed in most cells to mature tissues, which provides molecular bridge between the actin-based cytoskeleton and extracellular matrix (ECM) [26, 30]. After the viral agent enters the cell by alpha-dystroglycan-mediated endocytosis, low-pH domain enhances pH-dependent membrane bind and releases ribonucleoprotein of the virus (RNP) complex into the cytoplasm. The enzymatic machinery for RNA synthesis in arenaviruses is housed within a single L polymerase protein. This 250–450 kDa protein utilizes viral RNA templates that consist of genomic RNA encapsidated by the viral nucleocapsid protein NP and comprises viral ribonucleoprotein [21, 30]. L polymerase of arenaviruses contains the SDD motif feature of all RNA-dependent RNA polymerases (RdRp) [10]. When infected, the viral agent RNP is inserted into the cytoplasm of the host cell, the L polymerase associated with the viral RNP starts transcription from the genome promoter located at the 3′-end of each genomic RNA segment, L and S. The 5′ and 3′ terminal 19 nt viral promoter regions of both RNA segments required for the recognition and binding by the viral polymerase exhibit a high degree of conservation among the arenaviruses. The genome segments have a high complementary 5′- and 3′-ends (19 nt) have been predicted to form panhandle structures [10, 11]. Transcription stops at the distal side of the stem-loop (SL) structure within the intergenomic region (IGR). The L polymerase adopts a replicase mode and moves across the IGR to create a full-length complementary antigenomic RNA (agRNA) that serves as a template for mRNAs synthesis of viral genes encoded in genomic orientation, GPC and Z, from the S and L segments, respectively, and for the synthesis of full-length genomic RNA (gRNA) [13, 18]. This SL structure has been discovered to stabilize the 3′-termini of the viral mRNAs [10]. The primary transcription leads in the mRNA synthesis of viral genes, which is encoded in antigenomic orientation, NP and L polymerase, from the S and L segments, respectively. The viral agent uses a cap snatching strategy to acquire the cap structures of cellular mRNAs. Cap snatching is facilitated by the endonuclease activity of the L polymerase that is co-factored by the cap binding activity of NP. Therefore, LASV synthesizes capped nonpolyadenylated mRNAs. Both gRNA and agRNA of the viral agent contain a nontemplate G residue at their 5′-ends. The proposed "prime and realign" mechanism includes the production of a pppGPCOH dinucleotide primer from the CG nucleotides at positions +2 and +3 of the 3′-end genome promoter sequence, that is, then realigned such that its 3′-terminal COH is opposite the genome 3′-terminal G residue, and the realigned pppGPCOH then acts as a primer for a complementary RNA strand production. The matrix protein Z is not part of the viral genome transcription and replication but shows a dose-dependent inhibitory effect on viral RNA production. This inhibitory effect of Z has been proposed for Old and New World arenaviruses [27].
