Mechanisms of axon-Schwann cell signaling in the squid nerve fiber

Glutamate Activates AMPA Receptor Conductance in the Developing Schwann Cells of the Mammalian Peripheral Nerves

Schwann cells (SCs) are myelinating cells of the PNS. Although SCs are known to express different channels and receptors on their surface, little is known about the activation and function of these proteins. Ionotropic glutamate receptors are thought to play an essential role during development of SC lineage and during peripheral nerve injury, so we sought to study their functional properties. We established a novel preparation of living peripheral nerve slices with preserved cellular architecture and used a patch-clamp technique to study AMPA-receptor (AMPAR)-mediated currents in SCs for the first time. We found that the majority of SCs in the nerves dissected from embryonic and neonatal mice of both sexes respond to the application of glutamate with inward current mediated by Ca²⁺-permeable AMPARs. Using stationary fluctuation analysis (SFA), we demonstrate that single-channel conductance of AMPARs in SCs is 8-11 pS, which is comparable to that in neurons. We further show that, when SCs become myelinating, they downregulate functional AMPARs. This study is the first to demonstrate AMPAR-mediated conductance in SCs of vertebrates, to investigate elementary properties of AMPARs in these cells, and to provide detailed electrophysiological and morphological characterization of SCs at different stages of development.SIGNIFICANCE STATEMENT We provide several important conceptual and technical advances in research on the PNS. We pioneer the first description of AMPA receptor (AMPAR)-mediated currents in the PNS glia of vertebrates and provide new insights into the properties of AMPAR channels in peripheral glia; for example, their Ca²⁺ permeability and single-channel conductance. We describe for the first time the electrophysiological and morphological properties of Schwann cells (SCs) at different stages of development and show that functional AMPARs are expressed only in developing, not mature, SCs. Finally, we introduce a preparation of peripheral nerve slices for patch-clamp recordings. This preparation opens new possibilities for studying the physiology of SCs in animal models and in surgical human samples.

Neuronal activity in the hub of extrasynaptic Schwann cell-axon interactions

The integrity and function of neurons depend on their continuous interactions with glial cells. In the peripheral nervous system glial functions are exerted by Schwann cells (SCs). SCs sense synaptic and extrasynaptic manifestations of action potential propagation and adapt their physiology to support neuronal activity. We review here existing literature data on extrasynaptic bidirectional axon-SC communication, focusing particularly on neuronal activity implications. To shed light on underlying mechanisms, we conduct a thorough analysis of microarray data from SC-rich mouse sciatic nerve at different developmental stages and in neuropathic models. We identify molecules that are potentially involved in SC detection of neuronal activity signals inducing subsequent glial responses. We further suggest that alterations in the activity-dependent axon-SC crosstalk impact on peripheral neuropathies. Together with previously reported data, these observations open new perspectives for deciphering glial mechanisms of neuronal function support.

Neurotransmitter-mediated signaling between axons and glial cells

Neurotransmitter-mediated signaling is not restricted to the synaptic regions of the nervous system but also takes places along fiber tracts lacking vesicular means of releasing neuroactive substances. The first demonstration for dynamic signaling of this type came in the early 1970s from studies by Villegas and co-workers in squid axons and their satellite Schwann cells. In this invertebrate system, glutamate has been identified as the mediator of this signaling in being first released from the active axons thus setting off a series of cascades, leading to a cholinergic activation of the Schwann cell membrane. Recent evidence suggests that receptor-mediated signaling also exists between glial cells and axons in vertebrates. In the frog optic nerve, axonal activity facilitated the activity of glial ion channels. In the neonatal rat optic nerve, electrical activity of axons triggered oscillations in intracellular calcium in a subset of glial cells. These observations have been postulated to reflect receptor-mediated signaling, including a mechanism in which glutamate is released from axons via the reversal of a transporter and induces intracellular calcium spiking in glial cells via metabotropic glutamate receptors. The efficacy of "axon-to-glia" transmission may, like that in "neuron-to-neuron" transmission, be modulated by co-release of multiple neuroactive substances. One possibility is that adenosine, which is known to be released from fiber tracts, can modulate glutamate signaling in white matter by modulating the periaxonal glutamate concentration through an effect on the glial glutamate uptake system.