The commissural axons project toward and across the floor plate. They then turn into the longitudinal axis, extending along the contralateral side of the floor plate. F-spondin, a protein produced and secreted by the floor plate, promotes adhesion and neurite extension of commissural neurons in vitro. Injection of purified F-spondin protein into the lumen of the spinal cord of chicken embryos in ovo resulted in longitudinal turning of commissural axons before reaching the floor plate, whereas neutralizing antibody (Ab) injections caused lateral turning at the contralateral floor plate boundary. These combined in vitro and in vivo results suggest that F-spondin is required to prevent the lateral drifting of the commissural axons after having crossed the floor plate.

Open channel block is a process in which ions

In Drosophila, a phospholipase C (PLC)-mediated signaling cascade, couples photo-excitation of rhodopsin to the opening of the transient receptor potential (TRP) and TRP-like (TRPL) channels. A lipid product of PLC, diacylglycerol (DAG), and its metabolites, polyunsaturated fatty acids (PUFAs) may function as second messengers of channel activation. However, how can one separate between the increase in putative second messengers, change in pH, and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P 2 ) depletion when exploring the TRPL gating mechanism? To answer this question we co-expressed the TRPL channels together with the muscarinic (M1) receptor, enabling the openings of TRPL channels via G-protein activation of PLC. To dissect PLC activation of TRPL into its molecular components, we used a powerful method that reduced plasma membraneassociated PI(4,5)P 2 in HEK cells within seconds without activating PLC. Upon the addition of a dimerizing drug, PI(4,5)P 2 was selectively hydrolyzed in the cell membrane without producing DAG, inositol trisphosphate, or calcium signals. We show that PI(4,5)P 2 is not an inhibitor of TRPL channel activation. PI(4,5)P 2 hydrolysis combined with either acidification or application of DAG analogs failed to activate the channels, whereas PUFA did activate the channels. Moreover, a reduction in PI(4,5)P 2 levels or inhibition of DAG lipase during PLC activity suppressed the PLC-activated TRPL current. This suggests that PI(4,5)P 2 is a crucial substrate for PLC-mediated activation of the channels, whereas PUFA may function as the channel activator. Together, this study defines a narrow range of possible mechanisms for TRPL gating.The Drosophila transient receptor potential (TRP) 2 -like (TRPL) channel is a 1124-amino acid-long membrane protein.It was discovered independently from the TRP channel in a screen for calmodulin-binding proteins (1). This protein is expressed predominantly in photoreceptor cells and is one of the two founding members of the TRP channel super family (for review, see Ref.2). TRP channels detect changes in the environment in response to a myriad of stimuli, including light (3-6).Drosophila visual transduction is a phosphoinositide-signaling cascade mediated by G q and phospholipase C (PLC) (7). The activation of PLC by the G q protein promotes the opening of the light activated channels, TRP and TRPL, which depolarize the cell (Refs. 8 and 9; see also supplemental Fig. S1).The TRPL channel shows different activation states in different expression systems. Expression of TRPL channels in Sf9 (10, 11), S2 (12, 13), and COS (14) cells results in constitutively active channels. On the other hand, the expression of the channels in HEK cells results in channels that are in their closed state (14, 15), much like the situation in photoreceptor cells, in the dark (8). It is still unclear what leads to this spontaneous activity of the TRPL channel in these expression systems. This is a point that might be important for understanding channel activation.Activation of PL...

Recent studies in Drosophila melanogaster retina indicate that absorption of light causes the translocation of signaling molecules and actin from the photoreceptor's signaling membrane to the cytosol, but the underlying mechanisms are not fully understood. As ezrin-radixin-moesin (ERM) proteins are known to regulate actin–membrane interactions in a signal-dependent manner, we analyzed the role of Dmoesin, the unique D. melanogaster ERM, in response to light. We report that the illumination of dark-raised flies triggers the dissociation of Dmoesin from the light-sensitive transient receptor potential (TRP) and TRP-like channels, followed by the migration of Dmoesin from the membrane to the cytoplasm. Furthermore, we show that light-activated migration of Dmoesin results from the dephosphorylation of a conserved threonine in Dmoesin. The expression of a Dmoesin mutant form that impairs this phosphorylation inhibits Dmoesin movement and leads to light-induced retinal degeneration. Thus, our data strongly suggest that the light- and phosphorylation-dependent dynamic association of Dmoesin to membrane channels is involved in maintenance of the photoreceptor cells.

Light adaptation is a process that enables photoreceptor cells to operate over a wide range of light intensities without saturation. In invertebrate photoreceptors, fast adaptation is mediated by a Ca 2ϩ -dependent negative-feedback mechanism, which mainly affects the terminal steps of the cascade. Therefore, the response to each photon is smaller as light intensity increases, accommodating both high sensitivity and a vast dynamic range. Here, we describe a novel type of adaptation, which is mediated by one of the first steps in the phototransduction cascade affecting the sensitivity to absorbed photons. Long exposure to light resulted in dramatic reduction in the probability of each absorbed photon to elicit a response, whereas the size and shape of each single photon response did not change. To dissect the molecular mechanism underlying this form of adaptation we used a series of Drosophila mutants. Genetic dissection showed a pivotal role for light-induced translocation of G q ␣ between the signaling membrane and the cytosol. Biochemical studies revealed that the sensitivity to light depends on membrane G q ␣ concentration, which was modulated either by light or by mutations that impaired its targeting to the membrane. We conclude that long-term adaptation is mediated by the movement of G q ␣ from the signaling membrane to the cytosol, thereby reducing the probability of each photon to elicit a response. The slow time scale of this adaptation fits well with day/night light intensity changes, because there is no need to maintain single photon sensitivity during daytime.