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Juxtaparanodal clustering of Shaker-like K+ channels in myelinated axons depends on Caspr2 and TAG-1.

Poliak S, Salomon D, Elhanany H, Sabanay H, Kiernan B, Pevny L, Stewart CL, Xu X, Chiu SY, Shrager P, Furley AJ, Peles E - J. Cell Biol. (2003)

Bottom Line: In myelinated axons, K+ channels are concealed under the myelin sheath in the juxtaparanodal region, where they are associated with Caspr2, a member of the neurexin superfamily.Deletion of Caspr2 in mice by gene targeting revealed that it is required to maintain K+ channels at this location.These results demonstrate that Caspr2 and TAG-1 form a scaffold that is necessary to maintain K+ channels at the juxtaparanodal region, suggesting that axon-glia interactions mediated by these proteins allow myelinating glial cells to organize ion channels in the underlying axonal membrane.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Cell Biology, The Weizmann Institute of Science, Rehovot 76100, Israel.

ABSTRACT
In myelinated axons, K+ channels are concealed under the myelin sheath in the juxtaparanodal region, where they are associated with Caspr2, a member of the neurexin superfamily. Deletion of Caspr2 in mice by gene targeting revealed that it is required to maintain K+ channels at this location. Furthermore, we show that the localization of Caspr2 and clustering of K+ channels at the juxtaparanodal region depends on the presence of TAG-1, an immunoglobulin-like cell adhesion molecule that binds Caspr2. These results demonstrate that Caspr2 and TAG-1 form a scaffold that is necessary to maintain K+ channels at the juxtaparanodal region, suggesting that axon-glia interactions mediated by these proteins allow myelinating glial cells to organize ion channels in the underlying axonal membrane.

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Distribution of K+ channels in the CNS. Sections of optic nerve from wild-type (A, C, E, G, I, and K) or Caspr2- mice (B, D, F, H, J, and L) were double labeled with antibodies to Caspr (C, D, I, and J) and Kvβ2 (G and H), or Kv1.2 (A and B). Merge images are shown in panels E, F, K, and L. The images were obtained under the same exposure conditions. Note the decrease in K+ channels staining in Caspr2−/− nerves. Inset in L shows measurements of the fluorescence intensity labeling of K+ channels in Caspr2- compared with wild-type nerves in integrated optical density units. Three images at 40× were used for each genotype, and errors are given as ± SEM. Inset in F shows the levels of Kv1.2 protein detected by immunoblots in sciatic nerve lysates from wild-type (+/+) or Caspr2- (−/−) mice. Bar, 20 μm.
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fig3: Distribution of K+ channels in the CNS. Sections of optic nerve from wild-type (A, C, E, G, I, and K) or Caspr2- mice (B, D, F, H, J, and L) were double labeled with antibodies to Caspr (C, D, I, and J) and Kvβ2 (G and H), or Kv1.2 (A and B). Merge images are shown in panels E, F, K, and L. The images were obtained under the same exposure conditions. Note the decrease in K+ channels staining in Caspr2−/− nerves. Inset in L shows measurements of the fluorescence intensity labeling of K+ channels in Caspr2- compared with wild-type nerves in integrated optical density units. Three images at 40× were used for each genotype, and errors are given as ± SEM. Inset in F shows the levels of Kv1.2 protein detected by immunoblots in sciatic nerve lysates from wild-type (+/+) or Caspr2- (−/−) mice. Bar, 20 μm.

Mentions: Next, we examined whether Caspr2 is required for the correct localization of K+ channels along myelinated axons. Double immunolabeling of optic nerve sections from Caspr2−/− mice with antibodies to Caspr and Kv1.2 or Kvβ2 showed that while Caspr was present at the paranodes, these two K+ channel subunits were not concentrated at the juxtaparanodal region as they were in wild-type nerves (Fig. 3). Instead, weak staining of K+ channels was occasionally detected along the internodes, but not in the paranodes, indicating that these channels were redistributed along the internodal region. In agreement, Western blot analysis revealed that optic nerves of wild-type and Caspr2−/− mice express similar levels of Kv1.2 (Fig. 3 F, inset). Similar results were obtained using antibodies to Kv1.1 (unpublished data). Occasionally, some weak juxtaparanodal accumulation of K+ channels was observed in 5–10% of the sites. Overall, there was a striking decrease in the fluorescence intensity of juxtaparanodal K+ channels in the mutant (Fig. 3 L, inset; WT 122 ± 28 vs. KO 1 ± 0.6 integrated optical density units), demonstrating that K+ channels were not clustered at the juxtaparanodal region in the central nervous system (CNS). As expected, there was also a significant decrease in the measured total area of K+ channels-labeled juxtaparanodes in these nerves (WT 583 ± 125 μm2/field of view vs. KO 4.8 ± 2.4 μm2/field of view). In heterozygous nerves, K+ channels were normally located at this site, although a reduction in the intensity of staining was observed (unpublished data).


Juxtaparanodal clustering of Shaker-like K+ channels in myelinated axons depends on Caspr2 and TAG-1.

Poliak S, Salomon D, Elhanany H, Sabanay H, Kiernan B, Pevny L, Stewart CL, Xu X, Chiu SY, Shrager P, Furley AJ, Peles E - J. Cell Biol. (2003)

Distribution of K+ channels in the CNS. Sections of optic nerve from wild-type (A, C, E, G, I, and K) or Caspr2- mice (B, D, F, H, J, and L) were double labeled with antibodies to Caspr (C, D, I, and J) and Kvβ2 (G and H), or Kv1.2 (A and B). Merge images are shown in panels E, F, K, and L. The images were obtained under the same exposure conditions. Note the decrease in K+ channels staining in Caspr2−/− nerves. Inset in L shows measurements of the fluorescence intensity labeling of K+ channels in Caspr2- compared with wild-type nerves in integrated optical density units. Three images at 40× were used for each genotype, and errors are given as ± SEM. Inset in F shows the levels of Kv1.2 protein detected by immunoblots in sciatic nerve lysates from wild-type (+/+) or Caspr2- (−/−) mice. Bar, 20 μm.
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Related In: Results  -  Collection

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fig3: Distribution of K+ channels in the CNS. Sections of optic nerve from wild-type (A, C, E, G, I, and K) or Caspr2- mice (B, D, F, H, J, and L) were double labeled with antibodies to Caspr (C, D, I, and J) and Kvβ2 (G and H), or Kv1.2 (A and B). Merge images are shown in panels E, F, K, and L. The images were obtained under the same exposure conditions. Note the decrease in K+ channels staining in Caspr2−/− nerves. Inset in L shows measurements of the fluorescence intensity labeling of K+ channels in Caspr2- compared with wild-type nerves in integrated optical density units. Three images at 40× were used for each genotype, and errors are given as ± SEM. Inset in F shows the levels of Kv1.2 protein detected by immunoblots in sciatic nerve lysates from wild-type (+/+) or Caspr2- (−/−) mice. Bar, 20 μm.
Mentions: Next, we examined whether Caspr2 is required for the correct localization of K+ channels along myelinated axons. Double immunolabeling of optic nerve sections from Caspr2−/− mice with antibodies to Caspr and Kv1.2 or Kvβ2 showed that while Caspr was present at the paranodes, these two K+ channel subunits were not concentrated at the juxtaparanodal region as they were in wild-type nerves (Fig. 3). Instead, weak staining of K+ channels was occasionally detected along the internodes, but not in the paranodes, indicating that these channels were redistributed along the internodal region. In agreement, Western blot analysis revealed that optic nerves of wild-type and Caspr2−/− mice express similar levels of Kv1.2 (Fig. 3 F, inset). Similar results were obtained using antibodies to Kv1.1 (unpublished data). Occasionally, some weak juxtaparanodal accumulation of K+ channels was observed in 5–10% of the sites. Overall, there was a striking decrease in the fluorescence intensity of juxtaparanodal K+ channels in the mutant (Fig. 3 L, inset; WT 122 ± 28 vs. KO 1 ± 0.6 integrated optical density units), demonstrating that K+ channels were not clustered at the juxtaparanodal region in the central nervous system (CNS). As expected, there was also a significant decrease in the measured total area of K+ channels-labeled juxtaparanodes in these nerves (WT 583 ± 125 μm2/field of view vs. KO 4.8 ± 2.4 μm2/field of view). In heterozygous nerves, K+ channels were normally located at this site, although a reduction in the intensity of staining was observed (unpublished data).

Bottom Line: In myelinated axons, K+ channels are concealed under the myelin sheath in the juxtaparanodal region, where they are associated with Caspr2, a member of the neurexin superfamily.Deletion of Caspr2 in mice by gene targeting revealed that it is required to maintain K+ channels at this location.These results demonstrate that Caspr2 and TAG-1 form a scaffold that is necessary to maintain K+ channels at the juxtaparanodal region, suggesting that axon-glia interactions mediated by these proteins allow myelinating glial cells to organize ion channels in the underlying axonal membrane.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Cell Biology, The Weizmann Institute of Science, Rehovot 76100, Israel.

ABSTRACT
In myelinated axons, K+ channels are concealed under the myelin sheath in the juxtaparanodal region, where they are associated with Caspr2, a member of the neurexin superfamily. Deletion of Caspr2 in mice by gene targeting revealed that it is required to maintain K+ channels at this location. Furthermore, we show that the localization of Caspr2 and clustering of K+ channels at the juxtaparanodal region depends on the presence of TAG-1, an immunoglobulin-like cell adhesion molecule that binds Caspr2. These results demonstrate that Caspr2 and TAG-1 form a scaffold that is necessary to maintain K+ channels at the juxtaparanodal region, suggesting that axon-glia interactions mediated by these proteins allow myelinating glial cells to organize ion channels in the underlying axonal membrane.

Show MeSH
Related in: MedlinePlus