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Membrane domain organization of myelinated axons requires βII spectrin.

Zhang C, Susuki K, Zollinger DR, Dupree JL, Rasband MN - J. Cell Biol. (2013)

Bottom Line: Surprisingly, the K(+) channels and their associated proteins redistributed into paranodes where they colocalized with intact Caspr-labeled axoglial junctions.Furthermore, electron microscopic analysis of the junctions showed intact paranodal septate-like junctions.Thus, the paranodal spectrin-based submembranous cytoskeleton comprises the paranodal barriers required for myelinated axon domain organization.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Neuroscience, Baylor College of Medicine, Houston, TX 77030.

ABSTRACT
The precise and remarkable subdivision of myelinated axons into molecularly and functionally distinct membrane domains depends on axoglial junctions that function as barriers. However, the molecular basis of these barriers remains poorly understood. Here, we report that genetic ablation and loss of axonal βII spectrin eradicated the paranodal barrier that normally separates juxtaparanodal K(+) channel protein complexes located beneath the myelin sheath from Na(+) channels located at nodes of Ranvier. Surprisingly, the K(+) channels and their associated proteins redistributed into paranodes where they colocalized with intact Caspr-labeled axoglial junctions. Furthermore, electron microscopic analysis of the junctions showed intact paranodal septate-like junctions. Thus, the paranodal spectrin-based submembranous cytoskeleton comprises the paranodal barriers required for myelinated axon domain organization.

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βII spectrin–deficient PNS axons have intact paranodal junctions and transverse bands. (A and B) Electron microscopy of dorsal root nodes of Ranvier from control and Avil-Cre;SPNB2f/f mice. Mutant mice have widened nodes (brackets), but intact transverse bands (white arrowheads). Bar, 1 µm. (C) Immunostaining of control and Avil-Cre;SPNB2f/f dorsal and ventral roots using antibodies against Caspr (red) and Na+ channels (PanNav; green). Bar, 3 µm. (D) Quantification of node length in 5-mo-old dorsal and ventral roots. Error bars indicate ± SEM. *, P < 0.01. n = 4 control, 4 cKO. A total of 132 nodes were counted for control dorsal and ventral roots, and cKO ventral roots. A total of 124 nodes were counted for cKO dorsal roots. (E and F) Quantification of paranode and node length in 1-yr-old dorsal and ventral roots. No difference was observed in paranode length, but node length was significantly increased. Error bars indicate ± SEM, *, P < 0.01. n = 4 control, 4 cKO. A total of 100 nodes and 100 paranodes were counted for each genotype.
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fig4: βII spectrin–deficient PNS axons have intact paranodal junctions and transverse bands. (A and B) Electron microscopy of dorsal root nodes of Ranvier from control and Avil-Cre;SPNB2f/f mice. Mutant mice have widened nodes (brackets), but intact transverse bands (white arrowheads). Bar, 1 µm. (C) Immunostaining of control and Avil-Cre;SPNB2f/f dorsal and ventral roots using antibodies against Caspr (red) and Na+ channels (PanNav; green). Bar, 3 µm. (D) Quantification of node length in 5-mo-old dorsal and ventral roots. Error bars indicate ± SEM. *, P < 0.01. n = 4 control, 4 cKO. A total of 132 nodes were counted for control dorsal and ventral roots, and cKO ventral roots. A total of 124 nodes were counted for cKO dorsal roots. (E and F) Quantification of paranode and node length in 1-yr-old dorsal and ventral roots. No difference was observed in paranode length, but node length was significantly increased. Error bars indicate ± SEM, *, P < 0.01. n = 4 control, 4 cKO. A total of 100 nodes and 100 paranodes were counted for each genotype.

Mentions: The formation of membrane domains along axons is thought to depend on the septate-like junctions found at paranodes (Rosenbluth, 2009). To determine if these axoglial junctions are intact in axons lacking βII spectrin, we performed electron microscopic analyses of longitudinal sections of myelinated axons from control and cKO adult dorsal roots. We found that in both genotypes the paranodal myelin loops and the nodal domains were well preserved, despite a widened node in the cKO (Fig. 4 A). Remarkably, close examination of the paranodes in cKO roots also revealed transverse bands (Fig. 4 B, arrowheads), a hallmark of intact septate-like, paranodal junctions (Mierzwa et al., 2010). Thus, in βII spectrin cKO mice, Kv1 channels enter into nodal and paranodal domains despite normal myelination and intact paranodal junctions, including transverse bands.


Membrane domain organization of myelinated axons requires βII spectrin.

Zhang C, Susuki K, Zollinger DR, Dupree JL, Rasband MN - J. Cell Biol. (2013)

βII spectrin–deficient PNS axons have intact paranodal junctions and transverse bands. (A and B) Electron microscopy of dorsal root nodes of Ranvier from control and Avil-Cre;SPNB2f/f mice. Mutant mice have widened nodes (brackets), but intact transverse bands (white arrowheads). Bar, 1 µm. (C) Immunostaining of control and Avil-Cre;SPNB2f/f dorsal and ventral roots using antibodies against Caspr (red) and Na+ channels (PanNav; green). Bar, 3 µm. (D) Quantification of node length in 5-mo-old dorsal and ventral roots. Error bars indicate ± SEM. *, P < 0.01. n = 4 control, 4 cKO. A total of 132 nodes were counted for control dorsal and ventral roots, and cKO ventral roots. A total of 124 nodes were counted for cKO dorsal roots. (E and F) Quantification of paranode and node length in 1-yr-old dorsal and ventral roots. No difference was observed in paranode length, but node length was significantly increased. Error bars indicate ± SEM, *, P < 0.01. n = 4 control, 4 cKO. A total of 100 nodes and 100 paranodes were counted for each genotype.
© Copyright Policy - openaccess
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC3824014&req=5

fig4: βII spectrin–deficient PNS axons have intact paranodal junctions and transverse bands. (A and B) Electron microscopy of dorsal root nodes of Ranvier from control and Avil-Cre;SPNB2f/f mice. Mutant mice have widened nodes (brackets), but intact transverse bands (white arrowheads). Bar, 1 µm. (C) Immunostaining of control and Avil-Cre;SPNB2f/f dorsal and ventral roots using antibodies against Caspr (red) and Na+ channels (PanNav; green). Bar, 3 µm. (D) Quantification of node length in 5-mo-old dorsal and ventral roots. Error bars indicate ± SEM. *, P < 0.01. n = 4 control, 4 cKO. A total of 132 nodes were counted for control dorsal and ventral roots, and cKO ventral roots. A total of 124 nodes were counted for cKO dorsal roots. (E and F) Quantification of paranode and node length in 1-yr-old dorsal and ventral roots. No difference was observed in paranode length, but node length was significantly increased. Error bars indicate ± SEM, *, P < 0.01. n = 4 control, 4 cKO. A total of 100 nodes and 100 paranodes were counted for each genotype.
Mentions: The formation of membrane domains along axons is thought to depend on the septate-like junctions found at paranodes (Rosenbluth, 2009). To determine if these axoglial junctions are intact in axons lacking βII spectrin, we performed electron microscopic analyses of longitudinal sections of myelinated axons from control and cKO adult dorsal roots. We found that in both genotypes the paranodal myelin loops and the nodal domains were well preserved, despite a widened node in the cKO (Fig. 4 A). Remarkably, close examination of the paranodes in cKO roots also revealed transverse bands (Fig. 4 B, arrowheads), a hallmark of intact septate-like, paranodal junctions (Mierzwa et al., 2010). Thus, in βII spectrin cKO mice, Kv1 channels enter into nodal and paranodal domains despite normal myelination and intact paranodal junctions, including transverse bands.

Bottom Line: Surprisingly, the K(+) channels and their associated proteins redistributed into paranodes where they colocalized with intact Caspr-labeled axoglial junctions.Furthermore, electron microscopic analysis of the junctions showed intact paranodal septate-like junctions.Thus, the paranodal spectrin-based submembranous cytoskeleton comprises the paranodal barriers required for myelinated axon domain organization.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Neuroscience, Baylor College of Medicine, Houston, TX 77030.

ABSTRACT
The precise and remarkable subdivision of myelinated axons into molecularly and functionally distinct membrane domains depends on axoglial junctions that function as barriers. However, the molecular basis of these barriers remains poorly understood. Here, we report that genetic ablation and loss of axonal βII spectrin eradicated the paranodal barrier that normally separates juxtaparanodal K(+) channel protein complexes located beneath the myelin sheath from Na(+) channels located at nodes of Ranvier. Surprisingly, the K(+) channels and their associated proteins redistributed into paranodes where they colocalized with intact Caspr-labeled axoglial junctions. Furthermore, electron microscopic analysis of the junctions showed intact paranodal septate-like junctions. Thus, the paranodal spectrin-based submembranous cytoskeleton comprises the paranodal barriers required for myelinated axon domain organization.

Show MeSH
Related in: MedlinePlus