RAS activation is both necessary and sufficient to promote perineurial glial growth in Drosophila peripheral nerves

Abstract

Drosophila peripheral nerves contain motor and sensory axons surrounded by two layers of glia: an inner peripheral glia and an outer perineurial glia. Although it is known that intercellular signalling occurs among cells of the peripheral nerve, the effects of this signalling on nerve structure is incompletely understood. It was previously shown that growth of the perineurial glia is negatively regulated by five genes: ine, which encodes a putative neurotransmitter transporter, eag, which encodes a potassium channel, push, which encodes a large, Zn2+ finger‐containing protein, amn, which encodes a putative neuropeptide with similarity to pituitary adenylate cyclase activator peptide (PACAP), and NF1, the Drosophila orthologue of human NF1. Single or various double mutants in any of these genes exhibit perineurial glial hypertrophy. We found that the Ras12A/Rase2f heteroallelic combination, which decreases Ras activity, rescues the thick perineurial glia phenotype exhibited in ine; NF1 double mutants. This result demonstrates that Ras activity is required for the effect of NF1 mutations on perineurial glial growth. We also expressed a constitutively active Ras transgene in an ine– background specifically in peripheral glia using a Gal4‐UAS system. These larvae exhibited perineurial glia thickness comparable to that of ine; NF1 double mutants. Thus, expression of constitutively active Ras in peripheral glia mimics the effect of NF1 mutations on perineurial glial growth. These results demonstrate that Ras activity is both necessary and sufficient to activate perineurial glial growth in peripheral nerves and that Ras activity controls perineurial glial growth in a cell nonautonomous manner.