Antagonistic Functions of MBP and CNP Establish Cytosolic Channels in CNS Myelin

Abstract

The myelin sheath is a multilamellar plasma membrane extension of highly specialized glial cells laid down in regularly spaced segments along axons. Recent studies indicate that myelin is metabolically active and capable of communicating with the underlying axon. To be functionally connected to the neuron, oligodendrocytes maintain non-compacted myelin as cytoplasmic nanochannels. Here, we used high-pressure freezing for electron microscopy to study these cytoplasmic regions within myelin close to their native state. We identified 2,'3'-cyclic nucleotide 3'-phosphodiesterase (CNP), an oligodendrocyte-specific protein previously implicated in the maintenance of axonal integrity, as an essential factor in generating and maintaining cytoplasm within the myelin compartment. We provide evidence that CNP directly associates with and organizes the actin cytoskeleton, thereby providing an intracellular strut that counteracts membrane compaction by myelin basic protein (MBP). Our study provides a molecular and structural framework for understanding how myelin maintains its cytoplasm to function as an active axon-glial unit.

Keywords: CNP; MBP; axons; demyelinating diseases; glia; myelin; neurodegeneration; oligodendrocytes.

Graphical abstract

Figure 1

Ratio of CNP and MBP Determine the Amount of Cytoplasm within Myelin Sheaths (A and B) Electron micrographs of high-pressure-frozen optic nerves of wild-type (A) and CNP-deficient (B) animals showing differences in the number of cytoplasmic channels at P10 (cytoplasmic channels in red). (C) Quantification of the number of axons with myelin sheaths containing cytoplasmic regions in cross-sections of wild-type, CNP-deficient, and shiverer heterozygous (Mbp^+/−) animals over the course of myelin formation in the optic nerve. (D–I) Electron micrographs of high-pressure-frozen cervical spinal cord of wild-type (D and E), CNP-deficient (F and G), and shiverer (Mbp^+/−) heterozygous (H and I) at P60 and P180 (degenerated axons are marked by an “X,” and cytoplasmic channels are in red). (J and K) Quantification of the number of axons with myelin sheaths containing cytoplasmic regions in the spinal cord of wild-type, CNP-deficient, and shiverer (Mbp^+/−) heterozygous animals at P60 (J) and P180 (K). (C, J, and K) Bars show mean ± SEM (n = 3–7; 220–370 axons per animals; ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001; one-way ANOVA with post hoc Tukey). Scale bars represent 500 nm (A and B) and 1 μm (D–I). See also Figure S1.

Figure 2

Antagonistic Function of CNP and MBP in Membrane Compaction (A and B) Biomimetic membrane system to reconstitute the function of MBP in vitro. MBP is sandwiched between supported lipid bilayers (SLBs; mimicking the inner leaflet composition of myelin) and giant unilamellar vesicles (GUVs). (A) SLBs were coated with 7 μM membrane-anchored GFP (R3-GFP), followed by the addition of purified recombinant 14-kDa MBP (0.4 μM), on top of which GUVs composed of PS and PC in 1:2 molar ratios (0.1 mol% of DHPE-Texas red was used to visualize the GUVs) were added. After 30 min, all the GUVs burst on the SLBs (burst GUVs appear as red fluorescent areas). (B) SLBs were coated with 7 μM membrane-anchored CNP (R3-CNP) followed by the addition of purified recombinant 14-kDa MBP (0.4 μM), on top of which GUVs were added. After 30 min, R3-CNP partially prevented the bursting of the GUVs (non-burst GUVs appear as dark spheres with fluorescent rim). Bursted GUVs are marked by a white arrowhead, whereas unbursted GUVs are marked by a black arrowhead. Scale bar, 10 μm. (C) Quantification of GUV bursting over time. (D) Model illustrating the biomimetic membrane system. Bars show mean ± SEM (n = 3 coverslips analyzed per condition; differences between groups: ^∗∗∗p < 0.001; two-way ANOVA). See also Figure S2.

Figure 3

CNP Associates with and Organizes the Actin Cytoskeleton to Reinforce its Antagonistic Function in Membrane Compaction by MBP (A) F-actin in vitro co-sedimentation assay with full-length CNP (fl-CNP) and two constructs of the catalytic domain (CNPcat; CNPcat+C, an extended version of CNPcat by 22 residues). The position of actin is indicated by the black arrowhead in (A), (B), and (D). For comparison, equal fractions of the supernatant (S) and pellet (P) were loaded onto the gel in panels (A), (B), and (D). Please note that fl-CNP and actin are not separated in the gel system used here, good separation can be seen in Figure S2. (B) Co-sedimentation of the CNP N-terminal domain (CNP-N) and the C-terminal catalytic domain (CNPcat). Both domains appear to co-sediment independently with F-actin. (C) Electron micrograph showing negatively stained actin bundles that are decorated with full-length CNP stained with immunogold/monoclonal anti-His. Scale bar, 200 nm. (D) F-actin bundling assay. The catalytic domain induces bundling already at 1 μM (left), as evidenced by the moving of actin from the supernatant to the pellet. Both the inactive catalytic domain mutant (CNPcat-2H, middle) and full-length CNP (fl-CNP, right) also bundle filaments at 1 μM. Note that fl-CNP runs slightly above actin on the gel. All samples contain F-actin. (E) Chemical crosslinking of F-actin and CNP. F-actin and both the catalytic domain and full-length CNP were crosslinked to probe for protein-protein interactions. Actin was activated in samples 1–5, and CNP was activated in samples 1, 2, 9, and 10. When crosslinking reactions contained both actin and either of the CNP variants, a set of higher-molecular-weight species emerged. Arrowheads indicate bands that were picked for mass spectrometric verification, and they were found to contain both actin and CNP (red), actin alone (green), full-length CNP alone (blue), or CNP catalytic domain alone (magenta). F-actin by itself undergoes only minor self-crosslinking (green asterisk), whereas CNP does not (lanes 9–10). (F) Mapping of peptides missing in the crosslinked samples on the surface of F-actin (left) and CNP (right). In F-actin, one of the peptides corresponds to the D-loop (blue), and the other one is close in 3D space (green). In CNP, the two peptides are within the N-terminal PNK domain; one of them (orange) corresponds to the proposed calmodulin (CaM) binding site, and the second one lies nearby (red). The shown model is that of full-length CNP based on small-angle X-ray scattering (Myllykoski et al., 2013). (G and H) SLBs were coated with 7 μM membrane-anchored CNP (R3-CNP) (H) or not (G), followed by the addition of 5 μM F-actin, purified recombinant 14-kDa MBP (0.7 μM), and GUVs. After 30 min, R3-CNP together with F-actin blocked bursting of the GUVs induced by MBP (H), while F-actin alone did not (G). Scale bar, 10 μm. (I) Quantification of GUV bursting over time. Bars show mean ± SD; difference between the groups: ^∗∗∗p < 0.001; two-way ANOVA. (J and K) Electron micrographs of high-pressure-frozen optic nerves of ADF^−/−; CNP-Cre/cofilin^flox/flox mice (K) and control animals (CNP-Cre/cofilin^wt/wt) (J) (cytoplasmic channels in red). Scale bars, 500 nm. (L) Quantification of the number of axons with myelin sheaths containing cytoplasmic regions in cross sections. Bars show mean ± SD (n = 3; 350–470 axons per animal; ^∗∗p < 0.01; t test). See also Figure S3.

Figure 4

Reducing MBP Levels Rescues the Phenotype of CNP-Deficient Mice (A and B) Quantification of the number of myelin sheaths with cytoplasmic regions in the spinal cord of CNP-deficient, CNP null/shiverer heterozygous, and wild-type mice at P60 (A) and P180 (B) (the number of wild-type and CNP-deficient mice shown as a comparison are from Figure 1). Bars show mean ± SD (n = 3; 270–350 axons per animal; ^∗p < 0.05, one-way ANOVA with post hoc Tukey). (C) Electron micrographs of high-pressure-frozen cervical spinal cord of CNP-deficient/shiverer heterozygote animals at P180 (cytoplasmic channels in red). (D–G) Quantification (D) of the number of APP+ spheroids in P75 fimbria in wild-type (E), CNP-deficient (F), and CNP-deficient/shiverer heterozygote mice (G). (H) Electron micrograph of high-pressure-frozen cervical spinal cord of CNP-deficient mice at P180 showing axonal degeneration patterns (indicated by an X). (I and J) Quantification of axonal degeneration and myelin outfolding in wild-type, CNP-deficient, shiverer heterozygote, and CNP-deficient/shiverer heterozygote mice at P60 and P180 in cervical spinal cord of high-pressure-frozen samples (I) and at P180 in the optic nerve (J). (K) Model showing the molecular composition of myelin subdomains and depicting how a complex of CNP and F-actin antagonize membrane compaction driven by MBP and its implication for cytoplasmic channel integrity. Bars show mean ± SEM (n = 3–5; 270–350 axons per animal; ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001; one-way ANOVA with post hoc Tukey test). Scale bars represent 1 μm (C and D) and 10 μm (H–J). See also Figure S4.