Role of Cytoplasmic Dyneins in Neuronal Force Generation

Abstract

Cytoplasmic dynein is a motor protein classically involved in transport of cargo towards the minus end of microtubules. This project focuses on the role of dynein in forward translocation of cytoskeletal meshwork in growth cones. This pushing action of dynein, which is antagonistic to the pulling action of contractile myosin II in the growth cone, help balance forces in the growth cone to aid in its forward translocation. Many mechanisms have been proposed to explain how dynein telescopes microtubules towards the growth cone periphery, hence creating a pushing force. But there is no systematic study proving any of these mechanisms. Also, there are no quantitative studies that measure forces exerted by dynein in pushing membranes. This project uses a filopodial bead pulling assay to calculate quantitatively the filopodial contractility upon inhibition of dynein. The study shows that dynein inhibition maintains filopodial contractility in terms of pull step size and velocity but the frequency of pulls per unit time is observed to increases. A traction assay is also employed to qualitatively state that the traction is lower in dynein depleted neurons. This decrease in traction force exerted on the substrate in dynein inhibited neurons is substantiated by a showing that the point contacts are weaker in dynein disrupted conditions. Put together, we propose that when dynein is inhibited in neurons, the force balance will be tipped in favor of myosin, which could transiently increase the traction. Weaker point contacts in these neurons start detaching from the substrate soon after myosin takes over, hence conveying a decrease in the traction exerted on the substrate and a consequent stalling of the growth cone.