Neurotransmitter signaling between axons and Schwann cells in mammalian peripheral nerves.
Final Report Abstract
In this project we set up a novel preparation of live longitudinal sciatic nerve slices of the mouse (using know-how of brain slice preparation) and used this preparation to study: (a) Functional expression of ionotropic neurotransmitter receptors in Schwann cells; (b) Fast calcium transients along the axons in mouse sciatic nerve; (c) Possible neurotransmitter-mediated signaling between axons and Schwann cells in the nerves. We demonstrated for the first time that large proportion of Schwann cells in the developing mouse sciatic nerve respond to the application of glutamate with inward current mediated by AMPA/kainate receptors. The single-channel conductance of AMPA/kainate receptors in Schwann cells appeared comparable to the values for the small and medium level conductance of non-NMDA receptors in the glial cells of the central nervous system. Schwann cells responding to glutamate did not label with myelin basic protein, indicating that they comprise the group of immature and/or non-myelinating cells of the lineage. Further, we found that no glutamateinduced inward current can be recorded in more mature myelinating Schwann cells. The most parsimonious explanation of this finding is during maturation Schwann lineage cells downregulate the expression of AMPA/kainate receptors, similar as it also occurs during the development of oligodendrocyte lineage cells in the CNS. Although functional significance of AMPA/kainate receptors in Schwann cells is yet un-known, it is possible that glutamate-mediated signalling through these receptors plays an important role for regulation of proliferation, migration, and maturation of developing Schwann cells, in analogy to as it occurs in the central nervous system. In parallel to studying glutamate receptors on Schwann cells, we were interested to test whether fast Ca2+ transients, mediated by voltage-gated Ca2+ channels (VGCCs) occur along axonal shafts in the peripheral nerves. To address this question we used mouse sciatic nerve slices, Ca2+ indicator Oregon Green BAPTA-1, and 2-photon Ca2+ imaging in fast line scan mode (500 Hz). We found that transient increases in intra-axonal Ca2+ concentration take place along peripheral nerve axons in situ when axons are stimulated electrically with single pulses. Furthermore, we demonstrated for the first time that Ca2+ transients in peripheral nerves are fast, i.e., occur in a millisecond timedomain. Combining Ca2+ imaging and pharmacology with specific blockers of different VGCCs subtypes we showed that Ca2+ transients in peripheral nerves are mediated mainly by N-type and L-type VGCCs. Our immuno-histochemical evidence indicates that L-type VGCCs are located on nonmyelinated sensory fibers, while N-type VGCCs are located on myelinated motor and sensory fibers. We think that VGGCs may be involved in regulation of action potential conduction or frequency in the peripheral nerves, or mediate neurotransmitter release from axonal shafts as it occurs in white matter of the central nervous system (and optic nerve). In line with this hypothesis we have already obtained preliminary evidence that electrical stimulation of sciatic nerve axons triggers small inward current in Schwann cells. Finally, both AMPA/kainate receptors in Schwann cells and VGCCs in peripheral axons may be of significance during pathological conditions, e.g. nerve injury, pain, or peripheral neuropathy.
Publications
- (2016). N- and L-Type Voltage-Gated Calcium Channels Mediate Fast Calcium Transients in Axonal Shafts of Mouse Peripheral Nerve. Front Cell Neurosci., 2, 10, 135
Barzan R, Pfeiffer F, Kukley M.
(See online at https://doi.org/10.3389/fncel.2016.00135)
