Isoforms of terminal selector Mamo control axon guidance during adult Drosophila memory center construction via Semaphorin-1a.

In animals undergoing metamorphosis, the appearance of the nervous system is coincidently transformed by the morphogenesis of neurons. Such morphogenic alterations are exemplified in three types of intrinsic neurons in the Drosophila memory center. In contrast to the well-characterized remodeling of γ neurons, the morphogenesis of α/ß and α'/ß' neurons has not been adequately explored. Here, we show that mamo, a BTB-zinc finger transcription factor that acts as a terminal selector for α'/ß' neurons, controls the formation of the correct axonal pattern of α'/ß' neurons. Intriguingly, specific Mamo isoforms are preferentially expressed in α'/ß' neurons to regulate the expression of axon guidance molecule Semaphorin-1a. This action directs proper axon guidance in α'/ß' neurons, which is also crucial for wiring of α'/ß' neurons with downstream neurons. Taken together, our results provide molecular insights into how neurons establish correct axonal patterns in circuitry assembly during adult memory center construction.

Semaphorin-1a prevents Drosophila olfactory projection neuron dendrites from mis-targeting into select antennal lobe regions.

Elucidating how appropriate neurite patterns are generated in neurons of the olfactory system is crucial for comprehending the construction of the olfactory map. In the Drosophila olfactory system, projection neurons (PNs), primarily derived from four neural stem cells (called neuroblasts), populate their cell bodies surrounding to and distribute their dendrites in distinct but overlapping patterns within the primary olfactory center of the brain, the antennal lobe (AL). However, it remains unclear whether the same molecular mechanisms are employed to generate the appropriate dendritic patterns in discrete AL glomeruli among PNs produced from different neuroblasts. Here, by examining a previously explored transmembrane protein Semaphorin-1a (Sema-1a) which was proposed to globally control initial PN dendritic targeting along the dorsolateral-to-ventromedial axis of the AL, we discover a new role for Sema-1a in preventing dendrites of both uni-glomerular and poly-glomerular PNs from aberrant invasion into select AL regions and, intriguingly, this Sema-1a-deficient dendritic mis-targeting phenotype seems to associate with the origins of PNs from which they are derived. Further, ectopic expression of Sema-1a resulted in PN dendritic mis-projection from a select AL region into adjacent glomeruli, strengthening the idea that Sema-1a plays an essential role in preventing abnormal dendritic accumulation in select AL regions. Taken together, these results demonstrate that Sema-1a repulsion keeps dendrites of different types of PNs away from each other, enabling the same types of PN dendrites to be sorted into destined AL glomeruli and permitting for functional assembly of olfactory circuitry.

Overview of MARCM-Related Technologies in Drosophila Neurobiological Research.

Mosaic analysis with a repressible cell marker (MARCM)-related technologies are positive genetic mosaic labeling systems that have been widely applied in studies of Drosophila brain development and neural circuit formation to identify diverse neuronal types, reconstruct neural lineages, and investigate the function of genes and molecules. Two types of MARCM-related technologies have been developed: single-colored and twin-colored. Single-colored MARCM technologies label one of two twin daughter cells in otherwise unmarked background tissues through site-specific recombination of homologous chromosomes during mitosis of progenitors. On the other hand, twin-colored genetic mosaic technologies label both twin daughter cells with two distinct colors, enabling the retrieval of useful information from both progenitor-derived cells and their subsequent clones. In this overview, we describe the principles and usage guidelines for MARCM-related technologies in order to help researchers employ these powerful genetic mosaic systems in their investigations of intricate neurobiological topics. © 2020 by John Wiley & Sons, Inc.

Cell Lineage Analyses and Gene Function Studies Using Twin-spot MARCM.

Mosaic analysis with a repressible cell marker (MARCM) is a positive mosaic labeling system that has been widely applied in Drosophila neurobiological studies to depict intricate morphologies and to manipulate the function of genes in subsets of neurons within otherwise unmarked and unperturbed organisms. Genetic mosaics generated in the MARCM system are mediated through site-specific recombination between homologous chromosomes within dividing precursor cells to produce both marked (MARCM clones) and unmarked daughter cells during mitosis. An extension of the MARCM method, called twin-spot MARCM (tsMARCM), labels both of the twin cells derived from a common progenitor with two distinct colors. This technique was developed to enable the retrieval of useful information from both hemi-lineages. By comprehensively analyzing different pairs of tsMARCM clones, the tsMARCM system permits high-resolution neural lineage mapping to reveal the exact birth-order of the labeled neurons produced from common progenitor cells. Furthermore, the tsMARCM system also extends gene function studies by permitting the phenotypic analysis of identical neurons of different animals. Here, we describe how to apply the tsMARCM system to facilitate studies of neural development in Drosophila.

Morphogenetic Studies of the Drosophila DA1 Ventral Olfactory Projection Neuron.

In the Drosophila olfactory system, odorant information is sensed by olfactory sensory neurons and relayed from the primary olfactory center, the antennal lobe (AL), to higher olfactory centers via olfactory projection neurons (PNs). A major portion of the AL is constituted with dendrites of four groups of PNs, anterodorsal PNs (adPNs), lateral PNs (lPNs), lateroventral PNs (lvPNs) and ventral PNs (vPNs). Previous studies have been focused on the development and function of adPNs and lPNs, while the investigation on those of lvPNs and vPNs received less attention. Here, we study the molecular and cellular mechanisms underlying the morphogenesis of a putative male-pheromone responding vPN, the DA1 vPN. Using an intersection strategy to remove background neurons labeled within a DA1 vPN-containing GAL4 line, we depicted morphological changes of the DA1 vPN that occurs at the pupal stage. We then conducted a pilot screen using RNA interference knock-down approach to identify cell surface molecules, including Down syndrome cell adhesion molecule 1 and Semaphorin-1a, that might play essential roles for the DA1 vPN morphogenesis. Taken together, by revealing molecular and cellular basis of the DA1 vPN morphogenesis, we should provide insights into future comprehension of how vPNs are assembled into the olfactory neural circuitry.