Abstract
The role of myelination for axonal conduction is well-established in projection neurons but little is known about its significance in GABAergic interneurons. Myelination is discontinuous along interneuron axons and the mechanisms controlling myelin patterning and segregation of ion channels at the nodes of Ranvier have not been elucidated. Protein 4.1B is implicated in the organization of the nodes of Ranvier as a linker between paranodal and juxtaparanodal membrane proteins to the spectrin cytoskeleton. In the present study, 4.1B KO mice are used as a genetic model to analyze the functional role of myelin in Lhx6-positive parvalbumin and somatostatin neurons, two major classes of GABAergic neurons in the hippocampus. We show that deletion of 4.1B induces disruption of juxtaparanodal K+ channel clustering and mislocalization of nodal or heminodal Na+ channels. Strikingly, 4.1B-deficiency causes loss of myelin in GABAergic axons in the hippocampus. In particular, stratum oriens O-LM cells display severe axonal dysmyelination and a reduced excitability. This reduced excitability is associated with a decrease in occurrence probability of small amplitude synaptic inhibitory events on pyramidal cells. In contrast, stratum pyramidale fast-spiking basket cells do not appear affected. The aberrant myelination of hippocampal interneurons is also correlated with impairment of spatial memory in 4.1B KO mice. In conclusion, our results indicate a class-specific effect of dysmyelination on the excitability of hippocampal interneurons associated with a functional alteration of inhibitory drive and impairment of spatial memory.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
We show now MBP immunostaining at higher power in Figure 1, with insets showing the myelin pattern in the CA1 hippocampus and dentate gyrus of 4.1B-deficient mice. A massive loss of myelin was shown in the stratum radiatum, and an apparent slight dysmyelination was also observed in the molecular layer of the dentate gyrus, which receives afferent axons from the entorhinal cortex. We have now examined the density of Olig2-positive cells in both the CA1 region and dentate gyrus in Figure 3. Indeed, it has been reported that the neuronal progenitors of the granule cell layer might contribute to the generation of oligodendrocytes in demyelinating pathological conditions such as in the cuprizone toxic model (Jessberger et al., 2008; Klein et al., 2020). The pattern of MBP-positive axons was apparently slightly disorganized in the molecular layer and not in the hilus of the dentate gyrus in 4.1B-/- mice at P35 (Fig. 3D). We did not observe any difference in the density of Olig2-positive cells in the hilus, granule cell or molecular layers of the dentate gyrus (Fig. 3F). These results suggest that the developmental alteration of myelin induced by the loss of the axonal cytoskeletal linker 4.1B may not be associated with change in oligodendrogenesis. (Results: Lanes 207-214). Accordingly, this comment was added at the end of the Discussion: The severe loss of myelin in interneurons of the hippocampus may be associated with a dysfunction in the inhibitory network. An apparent slight disorganization of myelin was also observed in the molecular layer of the dentate gyrus, likely corresponding to inputs from the entorhinal cortex. Although we do not exclude a dysmyelination effect of 4.1B deficiency of extra-hippocampal afferences connecting the hippocampus, our results highlight the role of myelin in O-LM interneurons in fine tuning the hippocampal inhibitory drive that may be involved in spatial exploration and memory. (Discussion: lanes 590-595). We have also improved Figure 2I and J by adding 3D-reconstructions of O-LM myelinated axons to better illustrate the reduction of myelin coverage induced by 4.1B deficiency.
