Potassium regulates axon-oligodendrocyte signaling and metabolic coupling in white matter

Abstract

The integrity of myelinated axons relies on homeostatic support from oligodendrocytes (OLs), which is essential for brain function. However, the mechanisms by which OLs detect axonal spiking and rapidly control axon-OL metabolic coupling are largely unknown. Here, we combine optic nerve electrophysiology and two-photon imaging to study activity-dependent calcium (Ca2+) dynamics in OLs and metabolite fluxes in myelinated axons. Both high-frequency axonal firing and extracellular potassium (K+) elevations trigger a fast Ca2+response in OLs that is facilitated by barium-sensitive, inwardly rectifying K+channels. Using OL-specific Kir4.1 knockout mice (Kir4.1 cKO) we now demonstrate that, in addition to being crucial for K+clearance, oligodendroglial Kir4.1 regulates axonal energy metabolism and long-term axonal integrity. Before the manifestation of axonal damage, we observed reduced glucose transporter GLUT1 and monocarboxylate transporter MCT1 expression in myelin of young Kir4.1 cKO mice, suggesting early deficits in metabolite supply to axons. Strikingly, we found lower resting lactate levels and activity-induced lactate surges in optic nerve axons of young Kir4.1 cKO mice. Moreover, both axonal glucose uptake and consumption were hampered in the absence of oligodendroglial Kir4.1, uncovering a new role of OLs in regulating axonal glucose metabolism. Our findings reveal a novel model of axon-OL signaling and metabolic coupling in which OLs detect high-frequency axonal activity through K+signaling, which is critical in adjusting the axon-OL metabolic unit and in preserving long-term axonal health.

A myelinic channel system for motor-driven organelle transport

“…Myelin-associated peroxisomes are essential for the long-term integrity of axons, particularly faster spiking axons, which are most vulnerable to peroxisomal defects (Kassmann et al, 2007) Axonal spiking activity and glutamatergic signalling also trigger neuroprotective exosome release and oligodendroglial metabolic support (Frühbeis et al, 2020; Saab et al, 2016); the latter as an oligodendroglial calcium response (Looser et al, 2022) that is seen also during developmental myelination (Baraban et al, 2018; Krasnow et al, 2018). As a simple cellular model to examine the response of myelin-associated peroxisomes to changes in neuronal firing, we used myelinating cell cultures, in which neuronal electrical activity and neuronal energy consumption can be enhanced with the GABA receptor blocker picrotoxin (PTX; 100 μM) or diminished with tetrodotoxin (TTX; 1 μM) (Figure 4A and B).…”