High-throughput screens allow us to understand how transcription factors trigger developmental processes, including cell specification. A major challenge is identification of their binding sites because feedback loops and homeostatic interactions may mask the direct impact of those factors in transcriptome analyses. Moreover, this approach dissects the downstream signaling cascades and facilitates identification of conserved transcriptional programs. Here we show the results and the validation of a DNA adenine methyltransferase identification (DamID) genome-wide screen that identifies the direct targets of Glide/Gcm, a potent transcription factor that controls glia, hemocyte, and tendon cell differentiation in Drosophila. The screen identifies many genes that had not been previously associated with Glide/Gcm and highlights three major signaling pathways interacting with Glide/Gcm: Notch, Hedgehog, and JAK/STAT, which all involve feedback loops. Furthermore, the screen identifies effector molecules that are necessary for cell-cell interactions during late developmental processes and/or in ontogeny. Typically, immunoglobulin (Ig) domain–containing proteins control cell adhesion and axonal navigation. This shows that early and transiently expressed fate determinants not only control other transcription factors that, in turn, implement a specific developmental program but also directly affect late developmental events and cell function. Finally, while the mammalian genome contains two orthologous Gcm genes, their function has been demonstrated in vertebrate-specific tissues, placenta, and parathyroid glands, begging questions on the evolutionary conservation of the Gcm cascade in higher organisms. Here we provide the first evidence for the conservation of Gcm direct targets in humans. In sum, this work uncovers novel aspects of cell specification and sets the basis for further understanding of the role of conserved Gcm gene regulatory cascades.

Cell migration is an essential and highly regulated process. During development, glia and neurons migrate over long distances, in most cases collectively, to reach their final destination and build the sophisticated architecture of the nervous system, the most complex tissue of the body. Collective migration is highly stereotyped and efficient, defects in the process leading to severe human diseases that include mental retardation. This dynamic process entails extensive cell communication and coordination, hence the real challenge is to analyze it in the whole organism and at cellular resolution. We here investigate the impact of the N-cadherin adhesion molecule on collective glial migration using the Drosophila developing wing and cell-type specific manipulation of gene expression. We show that N-cadherin timely accumulates in glial cells and that its levels affect migration efficiency. N-cadherin works as a molecular brake in a dosage dependent manner by negatively controlling actin nucleation and cytoskeleton remodeling through α/β catenins. This is the first in vivo evidence for N-cadherin negatively and cell autonomously controlling collective migration.

Glial cell deficient/Glial cells missing (Glide/Gcm) transcription factor is expressed in all glial precursors of the Drosophila embryo. Gcm is necessary and sufficient to induce glial differentiation but also plays a role in other cell types, by interacting with specific factors. To find potential partners of Gcm which trigger these other pathways, we performed a yeast two-hybrid screen and identified dpias, a gene involved in post-embryonic hematopoiesis. dpias larvae show melanotic tumors due to excess of lamellocytes, a hemocyte lineage that is involved in non-self recognition. We here show that blocking Gcm activity also triggers melanotic tumors and that gcm interacts genetically with dpias. Moreover, the members of the Janus Kinase (JAK)/ Signal Transducer and Activator of Transcription (STAT) pathway, which are known for their role in the vertebrate and invertebrate immune system and are required for dpias-dependent tumor formation, act downstream of Gcm. Altogether, this study identifies an unpredicted role of Gcm, dictated by its cofactor dpias, allowing Gcm to act in a specific pathway. Together with the recent finding that glia act as scavengers during development and in pathological conditions, our data open new perspectives onto the cellular and molecular pathways involved in non-self recognition within and outside the nervous system.

glial cells missing genes (gcm and gcm2) act as the glial fate determinants in the Drosophila embryo. However, their requirement in the adult central nervous system (CNS) is at present not known, except for their role in lamina glia. This is particularly important with respect to two recent sets of data. Adult glial subpopulations differentiate through embryonic glia proliferation. Also, gcm‐gcm2 are required for the differentiation of specific adult neurons. We here show that gcm is expressed in precursors and postmitotic, migrating, cells of the medulla neuropile glia (mng) lineage. It is also expressed in a thoracic glial lineage and in neurons of the ventral nerve cord (VNC). Finally, while gcm is required for gliogenesis in medulla and VNC, it does not seem to be required for the generation of VNC neurons. Developmental Dynamics 236:2101–2108, 2007.

The fly glial cell deficient/glial cell missing ( glide/gcm ) gene codes for a transcription factor that induces gliogenesis. Lack of its product eliminates lateral glial cells in the embryonic nervous system. Here we identify a second gene, glide2 , that is homologous to glide/gcm in the binding domain and that is also necessary and sufficient to promote glial differentiation. glide2 codes for a transcription factor that displays a weaker and delayed expression compared with glide/gcm . The two genes, which are located 27 kb apart and share cis ‐regulatory elements, are able to auto‐ and cross‐regulate, indicating that they form a gene complex. Finally, we show that lack of both products eliminates all lateral glial cells, which means that the two genes contain all the fly lateral glial promoting activity.