From a syncytium to mononucleate cells and back: Yki and JNK in symphony

Alary muscle syncytia in Drosophila larvae undergo a remarkable process of dedifferentiation into single cells that then fuse to become ventral longitudinal muscle in the adult. In this issue, Schaub et al. (2019. J. Cell Biol. https://doi.org/10.1083/jcb.201905048) identify the Hippo and JNK signaling pathways as key regulators of this process of developmental remodeling of cell fate.

Analysis and modeling of the multisegmental coordination of shortening behavior in the medicinal leech. I. Motor output pattern

1. To understand how a multisegmental animal coordinates motor activity over more than one segment, we studied shortening behavior in the medicinal leech, in which several segments contract longitudinally in response to a moderately strong mechanical stimulus. 2. We first demonstrated that the neuronal activity responsible for shortening behavior occurred in semi-intact and isolated nerve cord preparations, and then characterized the responses of motor neurons in isolated preparations. The motor output during shortening was simultaneous excitation of motor neurons innervating dorsal longitudinal muscle and of motor neurons innervating ventral longitudinal muscle. 3. The stronger the stimulus, the more segments produced the shortening motor output, with the segments nearest the stimulus recruited first. 4. Although the shortening response was produced in several segments near the site of stimulation, it was never produced in the stimulated segment, where the local bending motor output pattern was produced. The motor pattern suggests that shortening, initially considered a very simple behavior, requires the involvement of at least few segmentally iterated interneurons.

Calcium Action Potentials in the Skeletal Muscle Fibres of the Stick Insect Carausius Morosus

The ionic requirements for the generation of action potentials in the ventral longitudinal muscle fibres of the stick insect, Carausius morosus, were investigated. Ca-free Ringer rapidly and reversibly abolished the action potential. In the presence of tetraethylammonium (TEA) ions (to suppress outward currents) the overshoot of the action potential changed 26 mV for a 10-fold change in [Ca]o. The maximum rate of rise of the action potential (measured in TEA Ringer) showed saturation at high [Ca]o. Cobaltous ions (20 mM) and the organic Ca antagonist D 600 (5×10−4g/ml) reversibly inhibited the action potential; the inhibitory effect of 1 mM-La3+ was irreversible. Barium and strontium, but not magnesium, were able to substitute for calcium as charge carriers. These results suggest that an inward movement of Ca2+ underlies the action potential of Carausius fibres.