Identification of Mob2, a Novel Regulator of Larval Neuromuscular Junction Morphology, in Natural Populations of Drosophila melanogaster

Abstract

Although evolutionary changes must take place in neural connectivity and synaptic architecture as nervous systems become more complex, we lack understanding of the general principles and specific mechanisms by which these changes occur. Previously, we found that morphology of the larval neuromuscular junction (NMJ) varies extensively among different species of Drosophila but is relatively conserved within a species. To identify specific genes as candidates that might underlie phenotypic differences in NMJ morphology among Drosophila species, we performed a genetic analysis on one of two phenotypic variants we found among 20 natural isolates of Drosophila melanogaster. We discovered genetic polymorphisms for both positive and negative regulators of NMJ growth segregating within the variant line. Focusing on one subline, that displayed NMJ overgrowth, we mapped the phenotype to Mob2 [Monopolar spindle (Mps) one binding protein 2)], a gene encoding a Nuclear Dbf2 (Dumbbell formation 2)-Related (NDR) kinase activator. We confirmed this identification by transformation rescue experiments and showed that presynaptic expression of Mob2 is necessary and sufficient to regulate NMJ growth. Mob2 interacts in a dominant, dose-dependent manner with tricornered but not with warts, to cause NMJ overgrowth, suggesting that Mob2 specifically functions in combination with the former NDR kinase to regulate NMJ development. These results demonstrate the feasibility and utility of identifying genetic variants affecting NMJ morphology in natural populations of Drosophila. These variants can lead to discovery of new genes and molecular mechanisms that regulate NMJ development while also providing new information that can advance our understanding of mechanisms that underlie nervous system evolution.

Mob Family Proteins: Regulatory Partners in Hippo and Hippo-Like Intracellular Signaling Pathways

“…Additional genetic evidence that fly Mob2 is required for Tricornered activation comes from studies of the larval neuromuscular junctions, where synaptic contacts form between motor neuron axons and muscle. Campbell and Ganetzky (2013) showed that mob2 is required for normal development of neuromuscular junctions. Each junction involves multiple neuronal contacts called synaptic boutons.…”

The Power of Natural Variation for Model Organism Biology

“…This and other mapping resources for model organism communities [55, 54, 28, 56, 48, 49, 57] not only enable mapping of phenotypic differences in parental lines, but can unmask cryptic variation that provides a powerful complement to laboratory mutagenesis (e.g. [58, 59]).…”

Tricornered Kinase Regulates Synapse Development by Regulating the Levels of Wiskott-Aldrich Syndrome Protein

“…Homozygous trc 1 mutants, a previously characterized loss-of-function point mutant allele[ 29 ], do not typically survive beyond the late second or early third-instar stage[ 22 ] and are also much smaller compared to age-matched wild-type (WT) larvae. This is likely because Trc regulates multiple processes that are important for survival[ 19 ].…”

“…Our previous data indicated that Tsc2 acts presynaptically to regulate synapse growth[ 10 ]. In addition, Mob2, an interactor of Trc 1 , can regulate synapse growth presynaptically[ 22 ]. These data suggest that the presynaptic pool of Trc may be required to regulate synapse number.…”

“…In sensory neurons, mTORC2 regulates Trc activity to regulate dendritic tiling [ 21 ]. A recent study hinted at the possibility that Trc kinase may function at the fly NMJs[ 22 ], but its mechanism of action and more importantly, whether it can mediate TSC signaling in neurons remains unknown.…”