[
Cell,
1998]
Muscle contraction is controlled by motor neurons, which are activated by glutamate release in spinal cord. This stimulus triggers action potentials that are propagated down long motor neuron axons to mediate acetylcholine release at distant neuromuscular junctions. This vectorial signaling in a motor neuron requires polarized sorting of proteins to appropriate neuronal domains. Neurotransmitter receptors for glutamate must be targeted to dendrites, while proteins that mediate synthesis and release of acetylcholine are shuttled down axons. Many nonneuronal cells are also polarized, but the two polarized faces are typically much closer than in neurons. For example, intestinal epithelial cells have an apical surface that faces the lumen of the gut and a basolateral surface that contacts the underlying basement membrane. Uptake of nutrients by epithelial cells requires that transporters for glucose and amino acids are expressed selectively at the apical membrane and that Na+/K+ ATPases, which set up ionic gradients essential for resorption, are expressed at the basolateral membrane. Understanding the basis for cellular polarity is important as defects in protein sorting underlie several common human disorders including cystic fibrosis and polycystic kidney disease.
[
Genes Dev,
1999]
Double-strand RNA (dsRNA) is a signal for gene-specific silencing of expression in a number of organisms. This phenomenon was demonstrated recently in Caenorhabditis elegans when dsRNA was injected into the worm and the corresponding gene products disappeared from both the somatic cells of the organism as well as in its F1 progeny. This RNA interference, RNAi, has been generalized to many genes in C. elegans. ds-RNA can also suppress expression of specific genes in plants, a component of the phenomenon called cosuppression. Two recent reports document dsRNA-mediated interference with expression of specific genes in other organisms. Double-strand RNA produced gene-specific phenotypes in Trypanosoma brucei and, very recently, dsRNA-mediated interference was demonstrated in Drosophila. Thus, the RNAi phenomenon is likely to be a general mechanism for gene regulation and may be critical for many developmental and antiviral processes.