Autocrine/Paracrine Slit-Robo Signaling Controls Optic Lobe Development in Drosophila melanogaster.

Cell segregation mechanisms play essential roles during the development of the central nervous system (CNS) to support its organization into distinct compartments. The Slit protein is a secreted signal, classically considered a paracrine repellent for axonal growth through Robo receptors. However, its function in the compartmentalization of CNS is less explored. In this work, we show that Slit and Robo3 are expressed in the same neuronal population of the Drosophila optic lobe, where they are required for the correct compartmentalization of optic lobe neuropils by the action of an autocrine/paracrine mechanism. We characterize the endocytic route followed by the Slit/Robo3 complex and detected genetic interactions with genes involved in endocytosis and actin dynamics. Thus, we report that the Slit-Robo3 pathway regulates the morphogenesis of the optic lobe through an atypical autocrine/paracrine mechanism in addition to its role in axon guidance, and in association with proteins of the endocytic pathway and small GTPases.

Slit/Robo Signaling Regulates Multiple Stages of the Development of the Drosophila Motion Detection System.

Neurogenesis is achieved through a sequence of steps that include specification and differentiation of progenitors into mature neurons. Frequently, precursors migrate to distinct positions before terminal differentiation. The Slit-Robo pathway, formed by the secreted ligand Slit and its membrane bound receptor Robo, was first discovered as a regulator of axonal growth. However, today, it is accepted that this pathway can regulate different cellular processes even outside the nervous system. Since most of the studies performed in the nervous system have been focused on axonal and dendritic growth, it is less clear how versatile is this signaling pathway in the developing nervous system. Here we describe the participation of the Slit-Robo pathway in the development of motion sensitive neurons of the Drosophila visual system. We show that Slit and Robo receptors are expressed in different stages during the neurogenesis of motion sensitive neurons. Furthermore, we find that Slit and Robo regulate multiple aspects of their development including neuronal precursor migration, cell segregation between neural stem cells and daughter cells and formation of their connectivity pattern. Specifically, loss of function of slit or robo receptors in differentiated motion sensitive neurons impairs dendritic targeting, while knocking down robo receptors in migratory progenitors or neural stem cells leads to structural defects in the adult optic lobe neuropil, caused by migration and cell segregation defects during larval development. Thus, our work reveals the co-option of the Slit-Robo signaling pathway in distinct developmental stages of a neural lineage.

Cell segregation and boundary formation during nervous system development.

The development of multicellular organisms involves three main events: differentiation, growth, and morphogenesis. These processes need to be coordinated for a correct developmental program to work. Mechanisms of cell segregation and the formation of boundaries during development play essential roles in this coordination, allowing the generation and maintenance of distinct regions in an organism. These mechanisms are also at work in the nervous system. The process of regionalization involves first the patterning of the developing organism through gradients and the expression of transcription factors in specific regions. Once different tissues have been induced, segregation mechanisms may operate to avoid cell mixing between different compartments. Three mechanisms have been proposed to achieve segregation: (1) differential affinity, which mainly involves the expression of distinct pools of adhesion molecules such as members of the cadherin superfamily; (2) contact inhibition, which is largely mediated by Eph-ephrin signaling; and (3) cortical tension, which involves the actomyosin cytoskeleton. In many instances, these mechanisms collaborate in cell segregation. In the last three decades, there have been several advances in our understanding of how cell segregation and boundaries participate in the development of the nervous system. Interestingly, as in other aspects of development, the molecular players are remarkably similar between vertebrates and invertebrates. Here we summarize the main concepts of cell segregation and boundary formation, focusing on the nervous system and highlighting the similarities between vertebrate and invertebrate model organisms.

Slit neuronal secretion coordinates optic lobe morphogenesis in Drosophila.

The complexity of the nervous system requires the coordination of multiple cellular processes during development. Among them, we find boundary formation, axon guidance, cell migration and cell segregation. Understanding how different cell populations such as glial cells, developing neurons and neural stem cells contribute to the formation of boundaries and morphogenesis in the nervous system is a critical question in neurobiology. Slit is an evolutionary conserved protein essential for the development of the nervous system. For signaling, Slit has to bind to its cognate receptor Robo, a single-pass transmembrane protein. Although the Slit/Robo signaling pathway is well known for its involvement in axon guidance, it has also been associated to boundary formation in the Drosophila visual system. In the optic lobe, Slit is expressed in glial cells, positioned at the boundaries between developing neuropils, and in neurons of the medulla ganglia. Although it has been assumed that glial cells provide Slit to the system, the contribution of the neuronal expression has not been tested. Here, we show that, contrary to what was previously thought, Slit protein provided by medulla neurons is also required for boundary formation and morphogenesis of the optic lobe. Furthermore, tissue specific rescue using modified versions of Slit demonstrates that this protein acts at long range and does not require processing by extracellular proteases. Our data shed new light on our understanding of the cellular mechanisms involved in Slit function in the fly visual system morphogenesis.