Mushroom body defect is required in parallel to Netrin for midline axon guidance in Drosophila

Abstract

The outgrowth of many neurons within the central nervous system is initially directed towards or away from the cells lying at the midline. Recent genetic evidence suggests that a simple model of differential sensitivity to the conserved Netrin attractants and Slit repellents is insufficient to explain the guidance of all axons at the midline. In the Drosophila embryonic ventral nerve cord, many axons still cross the midline in the absence of the Netrin genes (NetA and NetB) or their receptor frazzled. Here we show that mutation of mushroom body defect (mud) dramatically enhances the phenotype of Netrin or frazzled mutants, resulting in many more axons failing to cross the midline, although mutations in mud alone have little effect. This suggests that mud, which encodes a microtubule-binding coiled-coil protein homologous to NuMA and LIN-5, is an essential component of a Netrin-independent pathway that acts in parallel to promote midline crossing. We demonstrate that this novel role of Mud in axon guidance is independent of its previously described role in neural precursor development. These studies identify a parallel pathway controlling midline guidance in Drosophila and highlight a novel role for Mud potentially acting downstream of Frizzled to aid axon guidance.

Midline axon guidance in the Drosophila embryonic central nervous system

“…The microtubule-binding protein Mushroom body defect (Mud) is expressed in post-mitotic neurons of the ventral nerve cord throughout embryogenesis. Mud is able to regulate the orientation of microtubule-based spindle fiber formation, and its function in neurons may be to connect information about polarity with actin motor complexes in order to orient microtubule structures within neurons to encourage directed growth [43]. Deletion of mud alone has little or no effect on midline crossing, but strongly enhances midline crossing defects of NetAB mutants.…”

“…Deletion of mud alone has little or no effect on midline crossing, but strongly enhances midline crossing defects of NetAB mutants. Mud has been associated with signaling downstream of Frizzled (Fz) [46], and genetic interaction experiments suggest that Fz and Mud may function in the same pathway to promote midline crossing, parallel to Net and Fra, and could cooperate with Fmi [42,43].…”

Variability in Neural Circuit Formation

“…In model organisms, such cryptic functions can often be revealed through analysis of epistatic interactions in genetically sensitized backgrounds. These kinds of screens and analyses have provided the means to identify guidance pathways operating in parallel (e.g., Winberg et al 1998;Yu et al 2000;Cate et al 2016) or to elucidate biochemical pathways connecting cues and receptors to cellular responses (for review, see Zang et al 2021).…”

Asymmetric activity of NetrinB controls laterality of the Drosophila brain

“…In this study, we focus on the role of the Netrin (Net) pathway. Indeed, gene network reconstruction from positive hits showed a prevalence for Net pathway genes, including the two receptors (Frazzled 40 and Unc-5 41 ), as well as known transducers (RhoGEF64C 42 , mud 43 , Pak 44 , and Src64B 45 )(Fig. 3b, c).…”