A PI3-Kinase–Mediated Negative Feedback Regulates Neuronal Excitability

Abstract

Use-dependent downregulation of neuronal activity (negative feedback) can act as a homeostatic mechanism to maintain neuronal activity at a particular specified value. Disruption of this negative feedback might lead to neurological pathologies, such as epilepsy, but the precise mechanisms by which this feedback can occur remain incompletely understood. At one glutamatergic synapse, the Drosophila neuromuscular junction, a mutation in the group II metabotropic glutamate receptor gene (DmGluRA) increased motor neuron excitability by disrupting an autocrine, glutamate-mediated negative feedback. We show that DmGluRA mutations increase neuronal excitability by preventing PI3 kinase (PI3K) activation and consequently hyperactivating the transcription factor Foxo. Furthermore, glutamate application increases levels of phospho-Akt, a product of PI3K signaling, within motor nerve terminals in a DmGluRA-dependent manner. Finally, we show that PI3K increases both axon diameter and synapse number via the Tor/S6 kinase pathway, but not Foxo. In humans, PI3K and group II mGluRs are implicated in epilepsy, neurofibromatosis, autism, schizophrenia, and other neurological disorders; however, neither the link between group II mGluRs and PI3K, nor the role of PI3K-dependent regulation of Foxo in the control of neuronal excitability, had been previously reported. Our work suggests that some of the deficits in these neurological disorders might result from disruption of glutamate-mediated homeostasis of neuronal excitability. Use-dependent downregulation of neuronal excitability (negative feedback) can act to maintain neuronal activity within specified levels. Disruption of this homeostasis can lead to neurological disorders, such as epilepsy. Here, we report a novel mechanism for negative feedback control of excitability in the Drosophila larval motor neuron. In this mechanism, activation by the excitatory neurotransmitter glutamate of metabotropic glutamate receptors (mGluRs) located at motor nerve terminals decreases excitability by activating PI3 kinase (PI3K), consequently causing the phosphorylation and inhibition of the transcription factor Foxo. Foxo inhibition, in turn, decreases neuronal excitability. These observations are of interest for two reasons. First, our observation that PI3K activity regulates neuronal excitability is of interest because altered PI3K activity is implicated in a number of neurological disorders, such as autism and neurofibromatosis. Our results raise the possibility that altered excitability might contribute to the deficits in these disorders. Second, our observation that group II metabotropic glutamate receptors (mGluRs) activate PI3K is of interest because group II mGluRs are implicated in epilepsy, anxiety disorders, and schizophrenia. Yet the downstream signaling pathways affected by these treatments are incompletely understood. Our results raise the possibility that the PI3K pathway might be an essential mediator of signalling by these mGluRs.