Lassa mammarenavirus

Lassa virus structure and genome.Figure by Fehling et al., 2012[14]

Lassa viruses[15][16] are enveloped, single-stranded, bisegmented, ambisense RNA viruses. Their genome[17] is contained in two RNA segments that code for two proteins each, one in each sense, for a total of four viral proteins.[18] The large segment encodes a small zinc finger protein (Z) that regulates transcription and replication,[19][20] and the RNA polymerase (L). The small segment encodes the nucleoprotein (NP) and the surface glycoprotein precursor (GP, also known as the viral spike), which is proteolytically cleaved into the envelope glycoproteins GP1 and GP2 that bind to the alpha-dystroglycan receptor and mediate host cell entry.[21]

Lassa fever causes hemorrhagic fever frequently shown by immunosuppression. Lassa mammarenavirus replicates very rapidly, and demonstrates temporal control in replication.[22] The first replication step is transcription of mRNA copies of the negative- or minus-sense genome. This ensures an adequate supply of viral proteins for subsequent steps of replication, as the NP and L proteins are translated from the mRNA. The positive- or plus-sense genome, then makes viral complementary RNA (vcRNA) copies of itself. The RNA copies are a template for producing negative-sense progeny, but mRNA is also synthesized from it. The mRNA synthesized from vcRNA are translated to make the GP and Z proteins. This temporal control allows the spike proteins to be produced last, and therefore, delay recognition by the host immune system.

Nucleotide studies of the genome have shown that Lassa has four lineages: three found in Nigeria and the fourth in Guinea, Liberia, and Sierra Leone. The Nigerian strains seem likely to have been ancestral to the others but additional work is required to confirm this.[23]

Entry mechanisms of Old World and New World arenaviruses.

Lassa mammarenavirus gains entry into the host cell by means of the cell-surface receptor the alpha-dystroglycan (alpha-DG),[21] a versatile receptor for proteins of the extracellular matrix. It shares this receptor with the prototypic Old World arenavirus lymphocytic choriomeningitis virus. Receptor recognition depends on a specific sugar modification of alpha-dystroglycan 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 Lassa is endemic.[24] Alpha-dystroglycan is also used as a receptor by viruses of the New World clade C arenaviruses (Oliveros and Latino viruses). In contrast, the New World arenaviruses of clades A and B, which include the important viruses Machupo, Guanarito, Junin, and Sabia in addition to the non pathogenic Amapari virus, use the transferrin receptor 1. A small aliphatic amino acid at the GP1 glycoprotein amino acid position 260 is required for high-affinity binding to alpha-DG. In addition, GP1 amino acid position 259 also appears to be important, since all arenaviruses showing high-affinity alpha-DG binding possess a bulky aromatic amino acid (tyrosine or phenylalanine) at this position.

Unlike most enveloped viruses which use clathrin coated pits for cellular entry and bind to their receptors in a pH dependent fashion, Lassa and lymphocytic choriomeningitis virus instead use an endocytotic pathway independent of clathrin, caveolin, dynamin and actin. Once within the cell the viruses are rapidly delivered to endosomes via vesicular trafficking albeit one that is largely independent of the small GTPases Rab5 and Rab7. On contact with the endosome pH-dependent membrane fusion occurs mediated by the envelope glycoprotein, which at the lower pH of the endosome binds the lysosome protein LAMP1 which results in membrane fusion and escape from the endosome.

Lassa virus life cycle. Figure by Fehling et al., 2012[14]

The life cycle of Lassa mammarenavirus is similar to the Old World arenaviruses. Lassa mammarenavirus enters the cell by the receptor-mediated endocytosis. Which endocytotic pathway is used is not known yet, but at least the cellular entry is sensitive to cholesterol depletion. It was reported that virus internalization is limited upon cholesterol depletion. The receptor used for cell entry is alpha-dystroglycan, a highly conserved and ubiquitously expressed cell surface receptor for extracellular matrix proteins. Dystroglycan, which is later cleaved into alpha-dystroglycan and beta-dystroglycan is originally expressed in most cells to mature tissues, and it provides molecular link between the ECM and the actin-based cytoskeleton.[25] After the virus enters the cell by alpha-dystroglycan mediated endocytosis, the low-pH environment triggers pH-dependent membrane fusion and releases RNP (viral ribonucleoprotein) complex into the cytoplasm. Viral RNA is unpacked, and replication and transcription initiate in the cytoplasm.[25] As replication starts, both S and L RNA genomes synthesize the antigenomic S and L RNAs, and from the antigenomic RNAs, genomic S and L RNA are synthesized. Both genomic and antigenomic RNAs are needed for transcription and translation. The S RNA encodes GP and NP (viral nucleocapsid protein) proteins, while L RNA encodes Z and L proteins. The L protein most likely represents the viral RNA-dependent RNA polymerase.[26] When the cell is infected by the virus, L polymerase is associated with the viral RNP and initiates the transcription of the genomic RNA. The 5’ and 3’ terminal 19 nt viral promoter regions of both RNA segments are necessary for recognition and binding of the viral polymerase. The primary transcription first transcribes mRNAs from the genomic S and L RNAs, which code NP and L proteins, respectively. Transcription terminates at the stem-loop (SL) structure within the intergenomic region. Arenaviruses use a cap snatching strategy to gain the cap structures from the cellular mRNAs, and it is mediated by the endonuclease activity of the L polymerase and the cap binding activity of NP. Antigenomic RNA transcribes viral genes GPC and Z, encoded in genomic orientation, from S and L segments respectively. The antigenomic RNA also serves as the template for the replication.[27] After translation of GPC, it is posttranslationally modified in the endoplasmic reticulum. GPC is cleaved into GP1 and GP2 at the later stage of the secretory pathway. It has been reported that the cellular protease SKI-1/S1P is responsible for this cleavage. The cleaved glycoproteins are incorporated into the virion envelope when the virus buds and release from the cell membrane.[26]