You have grown neurons in a culture dish containing laminin and have induced axon outgrowth. You now incubate the cells in the presence of various concentrations of two secreted proteins (1 and 2). You then fix the cells, stain them with rhodamine-phalloidin (a fluorescent dye that selectively labels actin filaments), and finally use a fluorescence microscope to examine the axons. Some of the axon tips have lost their spread morphology (with lamellipodia shrunk and filopodia retracted) and lack significant fluorescence. You call these "collapsed cones." In the following graph, the percentage of cones that have collapsed is plotted as a function of concentration of proteins 1 and 2. Do you expect these proteins to be chemoattractants or chemorepellents in guiding axonal growth cones? Which one is more potent?
A) Chemoattractants; protein 1 is more potent.
B) Chemoattractants; protein 2 is more potent.
C) Chemorepellents; protein 1 is more potent.
D) Chemorepellents; protein 2 is more potent.
E) Protein 1 is a chemoattractant, and protein 2 is a chemorepellent; protein 2 is more potent.

Question

You have grown neurons in a culture dish containing laminin and have induced axon outgrowth. You now incubate the cells in the presence of various concentrations of two secreted proteins (1 and 2). You then fix the cells, stain them with rhodamine-phalloidin (a fluorescent dye that selectively labels actin filaments), and finally use a fluorescence microscope to examine the axons. Some of the axon tips have lost their spread morphology (with lamellipodia shrunk and filopodia retracted) and lack significant fluorescence. You call these "collapsed cones." In the following graph, the percentage of cones that have collapsed is plotted as a function of concentration of proteins 1 and 2. Do you expect these proteins to be chemoattractants or chemorepellents in guiding axonal growth cones? Which one is more potent? 
A) Chemoattractants; protein 1 is more potent. 
B) Chemoattractants; protein 2 is more potent. 
C) Chemorepellents; protein 1 is more potent. 
D) Chemorepellents; protein 2 is more potent. 
E) Protein 1 is a chemoattractant, and protein 2 is a chemorepellent; protein 2 is more potent.

Quizplus · Accepted Answer

Both of these proteins would be expected to be chemorepellents in guiding the migration of growth cones, as they both increase the rate of collapse. Protein 1 is a more potent repellent.

You Have Grown Neurons in a Culture Dish Containing Laminin