Mask, the Drosophila Ankyrin Repeat and KH domain-containing protein, regulates microtubule dynamics

Abstract

Proper regulation of microtubule (MT) dynamics is vital for essential cellular processes and neuronal activities, including axonal transport as well as synaptic growth and remodeling. Here we demonstrate that Mask negatively regulates MT stability and maintains a balanced dynamics of MT length and architecture in both fly larval muscles and motor neurons. In larval muscles, loss of mask increases MT length, and genetically altering mask levels modifies the Tau-induced MT fragmentation. In motor neurons, loss of mask function reduces the number of End-Binding Protein 1 (EB1)-positive MT plus-end structures in the axons and also results in overexpansion of the presynaptic terminal at larval neuromuscular junctions (NMJs). mask shows strong genetic interaction with stathmin (stai), a neuronal modulator of MT dynamics, in regulation of axon transportation and synaptic terminal stability. Our structure/function analysis on Mask revealed that truncated Mask transgenes carrying only its N-terminal portion that contains the two Ankyrin repeats domains is able to rescue the MT-related mask loss-of-function defects in larval muscles and NMJs, suggesting an essential role of the Ankyrin repeats domains in mediating Masks MT stability-regulating function. Furthermore, we discovered that Mask negatively regulates the abundance of the microtubule-associated protein Jupiter in motor neuron axons, and that neuronal knocking down of Jupiter partially suppresses mask loss of function phenotypes at the larval NMJs. Together, our study identified Mask as a novel regulator for microtubule stability and dynamics. Author SummaryMicrotubules (MTs) are an essential part of the cellular cytoskeleton, providing the structural basis for critical cellular processes and functionality. A series of factors are required to orchestrate the assembly and disassembly of MTs. Here, we identified Mask as a novel regulator of MT dynamics in Drosophila. Mask shows prominent interplay with two important modulators of MT, Tau and Stathmin (Stai). These findings not only support the role of Mask as a novel microtubule regulator but also provide the foundation to explore future therapeutic strategies in mitigating deficits related to dysfunction of Tau and/or Stai, both of which are linked to human neurodegenerative disorders. Further analyses on Mask demonstrated that Jupiters localization to the MTs in the axons is negatively affected by Mask, and that reducing Jupiter level is able to partially suppress synaptic defects caused by mask mutant. Together, these data imply that Masks function in regulating MT dynamics requires Jupiter.