Figure 3.

Mechanisms by which rotaviruses cause diarrhea. A). Events that occur following rotavirus infection of enterocytes are shown in order from left to right. Not all events are shown in each cell. 1) Infection of the initial cell by luminal virus leads to virus entry, uncoating, transcription, translation of viral proteins, formation of viroplasms (Vi), and apical release of virus and viral protein. Nonstructural protein 4 (NSP4, red triangle) and virus particles are released by a nonclassical secretory pathway. Intracellular NSP4 also induces the release of Ca²⁺, from internal stores, primarily the endoplasmic reticulum, leading to increasing intracellular calcium [Ca²⁺]i. 2) Another outcome can result from a cell being infected with virus. NSP4 produced by the infection disrupts tight junctions, allowing paracellular flow of water and electrolytes (blue arrow). 3) NSP4 released from previously infected cells binds to a specific receptor and triggers a signaling cascade through phospholipase C (PLC) and inositol phosphatase (IP)₃ that results in release of Ca²⁺ and an increase in [Ca²⁺]i. Intracellular expression of NSP4 increases [Ca²⁺]i through a PLC-independent mechanism. The increase in [Ca²⁺]i also disrupts the microvillar cytoskeleton. 4) A crypt cell (brown) can be acted on directly by NSP4 or NSP4 can stimulate the enteric nervous system (ENS), which in turn signals an increase in [Ca²⁺]i that induces Cl⁻ secretion. Panel B shows the normal architecture of the small intestine, without the circulatory system shown. This panel shows the ENS and its ganglia in the different submucosal levels. Panel C shows a reflex arc in the ENS that can receive signals from the villus epithelium and activate the crypt epithelium. Inset 1 shows a whole-mount of an adult mouse small intestinal villus, stained with antibody to the gene product 9.5 neuroendocrine marker to reveal the rich innervation (yellow). Inset 2 shows that infected villus enterocytes can stimulate the ENS by the basolateral release of NSP4 or other effector molecules. The integrin α2β1 can bind NSP4 and elicit diarrhea in neonatal mice. Adapted with permission from Ramig, 2004. ⁹⁷