Phosphatidylinositol 3-Kinase and Akt Nonautonomously Promote Perineurial Glial Growth inDrosophilaPeripheral Nerves

Abstract

Drosophila peripheral nerves, structured similarly to their mammalian counterparts, comprise a layer of motor and sensory axons wrapped by an inner peripheral glia (analogous to the mammalian Schwann cell) and an outer perineurial glia (analogous to the mammalian perineurium). Growth and proliferation within mammalian peripheral nerves are increased by Ras pathway activation: loss-of-function mutations in Nf1, which encodes the Ras inhibitor neurofibromin, cause the human genetic disorder neurofibromatosis, which is characterized by formation of neurofibromas (tumors of peripheral nerves). However, the signaling pathways that control nerve growth downstream of Ras remain incompletely characterized. Here we show that expression specifically within the Drosophila peripheral glia of the constitutively active Ras V12 increases perineurial glial thickness. Using chromosomal loss-of-function mutations and transgenes encoding dominant-negative and constitutively active proteins, we show that this nonautonomous effect of Ras V12 is mediated by the Ras effector phosphatidylinositol 3-kinase (PI3K) and its downstream kinase Akt. We also show that the nonautonomous, growthpromoting effects of activated PI3K are suppressed by coexpression within the peripheral glia of FOXO ϩ (forkhead box O) a transcription factor inhibited by Akt-dependent phosphorylation. We suggest that Ras-PI3K-Akt activity in the peripheral glia promotes growth of the perineurial glia by inhibiting FOXO. In mammalian peripheral nerves, the Schwann cell releases several growth factors that affect the proliferative properties of neighbors. Some of these factors are oversecreted in Nf1 mutants. Our results raise the possibility that neurofibroma formation in individuals with neurofibromatosis might result in part from a Ras-PI3K-Akt-dependent inhibition of FOXO within Schwann cells.

Neurofibromatosis-like phenotype in Drosophila caused by lack of glucosylceramide extension

“…S4). In Drosophila, constitutive activation of Ras in subperineurial glia causes an NF1-like phenotype with glial growth mediated by the Ras effector PI3K and its downstream kinase Akt (22). Because we observed that the egh peripheral nerve phenotype was reminiscent of NF1, we hypothesized that egh is needed for the function of neurofibromin or other factors controlling Ras signaling.…”

“…We hypothesized that loss of Egh activity caused overproliferation of glial cells by dysregulation of NF1 leading to enhanced activation of Ras signaling, which in turn would activate PI3K and Akt and down-regulate FOXO. It has previously been shown that complete activation of Ras through Gli-Gal4-driven expression of activated Ras, Ras V12 , leads to overgrowth of the outer glial layer (22). We therefore tested whether Ras V12 could induce a nerve phenotype similar to egh.…”

“…3 B and G and Table S1). As Gli-Gal4-driven expression of UAS-Ras V12 provides maximum activation of Ras (22), this suggests that the growth signal induced by loss of egh is not relayed solely through Ras.…”

“…Akt is a key transducer of PI3K activity and has previously been implicated in PI3K-induced overgrowth of the outer glial layer (22). To analyze whether Akt is required for transmission of the aberrant signal associated with loss of egh, we reduced the Akt gene dosage by introducing one copy of the partial loss-of-function Akt 04226 allele.…”

The Metabotropic Glutamate Receptor Activates the Lipid Kinase PI3K in Drosophila Motor Neurons Through the Calcium/Calmodulin-Dependent Protein Kinase II and the Nonreceptor Tyrosine Protein Kinase DFak

“…Transmission electron microscopy was performed as described previously (Lavery et al 2007;Howlett et al 2008). Briefly, larvae were grown, selected, and dissected as described above, fixed with glutaraldehyde and paraformaldehyde, stained with 0.5% OsO 4 and 2% uranyl acetate, and embedded in an eponate 12-araldite mixture.…”