Parvovirus B19 binds to cells expressing the glycosphin-golipid blood group antigen P and requires the presence of a5pj integrin as a coreceptor for cell entry. Replication is confined to cells that cycle through the S-phase. This restricts the spectrum of fully permissive host cells mainly to erythroid precursors in the bone marrow and liver of the fetus, and to erythroid progenitors postnatally. The viral transactivating NS1 protein regulates both transcription and replication. It also mediates a multistep cell cycle arrest, which is mainly confined to the Gi phase.
NS1 induces apoptosis, possibly because of its nucleoside-triphosphate binding motif, which triggers pathways also used by tumor necrosis factor-a (TNF-a). Apoptosis contributes to loss of erythroid progenitors. Heterologous NS1 transactivation also involves IL-6 expression, which may have triggering effects on various inflammatory and autoimmune disorders associated with B19 infection.
The minor (VP1) and major (VP2) virion polypeptides are generated from alternatively spliced, partially overlapping mRNA, and autoassemble into virions of 60 capsomeres into which either a negative-stranded or a positive-stranded viral genome in approximately equal ratios is packaged. Replication is unusually efficient, producing virus loads of 108-1013 particles per milliliter of blood during an acute primary infection. The function of the phospholipase A2-like activity in the N-terminal region of VP1 (VP1u) remains unclear, although it may play a role in host cell entry, nuclear transport, and release of mature virions through phospholipase A2-specific secretory pathways. Interestingly, release of cellular phospholipase A2 is stimulated by IL-6, which in turn is upregulated by NS1 in infected cells.
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The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.