Sorting out signals in fly endosomes

Ligands and receptors that mediate cell–cell interactions during development are removed from the cell surface by endocytosis. Subsequently, many of these internalized proteins are detected in multivesicular bodies (MVBs). Recent work in different organisms has elucidated some aspects of MVB biogenesis and trafficking. This review discusses some intriguing links between these findings, the sorting of proteins in endocytic trafficking, and the regulation of signaling pathways in Drosophila .     

Developmental Signalling: Sorting out the signals

New insights into the developmental roles played by the TGF-β family of signalling molecules come from the identification in Drosophila of two transmembrane receptors encoded by the thick veins and saxophone genes.     

Sorting out sound

Speech analysis is a prototypical categorical mechanism that has been examined behaviorally in work going back to Haskins Laboratory studies in the 1950s: such work examined the perception of continua between phonemes and demonstrated sharp discontinuities consistent with categorical perception. In this issue of Neuron, Raizada and Poldrack examine analysis mechanisms for such processing by measurement of the fMRI BOLD response in the boundary region between different phonemes and argue for a specific amplification mechanism for this type of categorical perception.     

Sorting out small RNAs

Small RNAs carry out their functions by guiding Argonaute (AGO) proteins to their targets. Diverse types of small RNAs and multiple AGO proteins exist in most eukaryotic species, but how small RNAs are sorted into specific AGO complexes remains unclear. Two papers in this issue (Mi et al., 2008; Montgomery et al., 2008) now reveal the importance of the 5' terminal nucleotide of the small RNA in the sorting process in Arabidopsis.     

Sorting of signals from thermosensitive areas.

Of the several models proposed for the neural regulation of temperature in cold-exposed animals, two have been previously restated in dynamic form using CSMP. Subsequently, computer simulations have led to the design and execution of experiments for selection of the more appropriate model for cold-exposed rats. These experiments, as described in the present paper, have been interpreted as being consistent with the model which sorts signals from thermosensitive areas and channels the selected signals over separate neural pathways to independently control each mode of heat production. Since this model requires multiplication of neural signals, possible neuronal mechanisms which may underline such multiplication are discussed. In addition, parameter variation to account for febril responses and rate sensitivity have been evaluated.     

Sorting out bacterial viability with optical tweezers

We have developed a method, using laser, optical tweezers and direct microscopic analysis of reproductive potential and membrane integrity, to assess single-cell viability in a stationary-phase Escherichia coli population. It is demonstrated here that a reduction in cell integrity, determined by using the fluorescent nucleic acid stain propidium iodide, correlated well with a reduction in cell proliferating potential during the stationary-phase period studied. Moreover, the same cells that exhibited reduced integrity were found to be the ones that failed to divide upon nutrient addition. A small but significant number of the intact cells (496 of 7,466 [6.6%]) failed to replicate. In other words, we did not find evidence for the existence of a large population of intact but nonculturable cells during the stationary-phase period studied but it is clear that reproductive ability can be lost prior to the loss of membrane integrity. In addition, about 1% of the stationary-phase cells were able to divide only once upon nutrient addition, and in a few cases, only one of the two cells produced by division was able to divide a second time, indicating that localized cell deterioration, inherited by only one of the daughters, may occur. The usefulness of the optical trapping methodology in elucidating the mechanisms involved in stationary-phase-induced bacterial death and population heterogeneity is discussed.