Turtle interacts with borderless in regulating glial extension and axon ensheathment

Abstract

Proper recognition between axons and glial processes is required for the establishment of axon ensheathment in the developing nervous system. Recent studies have begun to reveal molecular events underlying developmental control of axon-glia recognition. In our previous work, we showed that the transmembrane protein Borderless (Bdl) is specifically expressed in wrapping glia (WG), and is required for the extension of glial processes and the ensheathment of photoreceptor axons in the developing Drosophila visual system. The exact mechanism by which Bdl mediates axon-glia recognition, however, remains unknown. Here, we present evidence showing that Bdl interacts with the Ig transmembrane protein Turtle (Tutl). Tutl is specifically expressed in photoreceptor axons. Loss of tutl in photoreceptors, like loss of bdl in WG, disrupts glial extension and axon ensheatment. Epistasis analysis shows that Tutl interacts genetically with Bdl. Tutl interacts with Bdl in trans in cultured cells. We propose that Tutl interacts with Bdl in mediating axon-glia recognition for WG extension and axon ensheathment.

An RNAi screen for secreted factors and cell-surface players in coordinating neuron and glia development in Drosophila

“…knocking down CadN disrupted the organization of R-cell axonal terminals in the optic lobe, and also caused abnormal axonal terminal morphology. However, the fact that WG projected normally in CadN knockdown animals suggests that unlike that of Tutl and Bdl [9,10], the adhesive property of CadN is not involved in mediating the recognition between Rcell axons and WG for the extension of WG processes.…”

“…And Tutl protein expressed on R-cell axons binds to the WG-specific Bdl in promoting WG extension [9]. Thus, a decrease in the number of R cells may reduce the levels of R-cell-derived factors for WG differentiation and extension.…”

“…Recent studies identify several neuron-derived factors that coordinate the development of R cells and WG [8,9]. It is shown that the neuron-derived FGF8like ligand Thisbe promotes the differentiation of PG into WG, which migrate along the surface of R-cell axons and subsequently insulate R-cell axons [8].…”

“…It is shown that the neuron-derived FGF8like ligand Thisbe promotes the differentiation of PG into WG, which migrate along the surface of R-cell axons and subsequently insulate R-cell axons [8]. Our previous studies reveal that the immunoglobulin (Ig) superfamily transmembrane protein Turtle (Tutl) expressed on R-cell axons binds to the WG-specific cell-surface receptor Borderless (Bdl) to promote WG extension and axonal ensheathment [9,10]. While it is reported that WG also plays an active role in regulating the topographic projection of R-cell axons in the optic lobe [11], the underlying mechanisms remain unclear.…”

Drosophila glia: Few cell types and many conserved functions

“…Furthermore, EGF secreted by photoreceptor neurons activates EGFR signaling in wrapping glia. In response, wrapping glial cells secrete insulin like peptides to induce neuronal differentiation in the lamina and thereby function as a signaling relay (Fernandes, Chen, Rossi, Zipfel, & Desplan, ). Thus, wrapping glial cells are not only necessary for photoreceptor clustering and insulation but also function in the communication between the different cell types.…”