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Association of TAG-1 with Caspr2 is essential for the molecular organization of juxtaparanodal regions of myelinated fibers.

Traka M, Goutebroze L, Denisenko N, Bessa M, Nifli A, Havaki S, Iwakura Y, Fukamauchi F, Watanabe K, Soliven B, Girault JA, Karagogeos D - J. Cell Biol. (2003)

Bottom Line: Myelination results in a highly segregated distribution of axonal membrane proteins at nodes of Ranvier.In the absence of TAG-1, axonal Caspr2 did not accumulate at juxtaparanodes, and the normal enrichment of shaker-type K+ channels in these regions was severely disrupted, in the central and peripheral nervous systems.This complex is analogous to that described previously at paranodes, suggesting that similar molecules are crucial for different types of axo-glial interactions.

View Article: PubMed Central - PubMed

Affiliation: Department of Basic Science, University of Crete Medical School, Heraklion 71110, Crete, Greece.

ABSTRACT
Myelination results in a highly segregated distribution of axonal membrane proteins at nodes of Ranvier. Here, we show the role in this process of TAG-1, a glycosyl-phosphatidyl-inositol-anchored cell adhesion molecule. In the absence of TAG-1, axonal Caspr2 did not accumulate at juxtaparanodes, and the normal enrichment of shaker-type K+ channels in these regions was severely disrupted, in the central and peripheral nervous systems. In contrast, the localization of protein 4.1B, an axoplasmic partner of Caspr2, was only moderately altered. TAG-1, which is expressed in both neurons and glia, was able to associate in cis with Caspr2 and in trans with itself. Thus, a tripartite intercellular protein complex, comprised of these two proteins, appears critical for axo-glial contacts at juxtaparanodes. This complex is analogous to that described previously at paranodes, suggesting that similar molecules are crucial for different types of axo-glial interactions.

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Model of the molecular organization of juxtaparanodal regions. This model is the simplest that can account for the data from previous studies and the present work. Caspr2 is enriched in the axolemma, whereas TAG-1 is expressed in both neurons and myelinating glial cells. Experiments in transfected cells show that TAG-1 interacts with Caspr2 in cis, and that TAG-1 exchanges trans interactions with itself (homophilic) but not with Caspr2. The functional importance of these interactions is demonstrated by the absence of Caspr2 enrichment in juxtaparanodal regions in TAG-1 knockout mice. These complexes are associated with other proteins including Kv1.1 and Kv1.2 potassium channels, presumably through a PDZ domain–containing protein (schematically represented here; Poliak et al., 1999; Rasband et al., 2002) and with protein 4.1B (Denisenko-Nehrbass et al., 2003b). Because protein 4.1B was still detected at juxtaparanodes in the absence of TAG-1 and Caspr2, it is likely that it interacts with other proteins, including components of the cytoskeleton (arrow). Correct localization of juxtaparanodal proteins may depend on both axo–glial interactions and binding to axoplasmic cytoskeletal components.
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fig8: Model of the molecular organization of juxtaparanodal regions. This model is the simplest that can account for the data from previous studies and the present work. Caspr2 is enriched in the axolemma, whereas TAG-1 is expressed in both neurons and myelinating glial cells. Experiments in transfected cells show that TAG-1 interacts with Caspr2 in cis, and that TAG-1 exchanges trans interactions with itself (homophilic) but not with Caspr2. The functional importance of these interactions is demonstrated by the absence of Caspr2 enrichment in juxtaparanodal regions in TAG-1 knockout mice. These complexes are associated with other proteins including Kv1.1 and Kv1.2 potassium channels, presumably through a PDZ domain–containing protein (schematically represented here; Poliak et al., 1999; Rasband et al., 2002) and with protein 4.1B (Denisenko-Nehrbass et al., 2003b). Because protein 4.1B was still detected at juxtaparanodes in the absence of TAG-1 and Caspr2, it is likely that it interacts with other proteins, including components of the cytoskeleton (arrow). Correct localization of juxtaparanodal proteins may depend on both axo–glial interactions and binding to axoplasmic cytoskeletal components.

Mentions: Our findings in COS-7 cells demonstrate that TAG-1 can exchange cis interactions with Caspr2. This ability supports an association between the two proteins in the axolemma because TAG-1 is expressed in several types of neurons (Dodd et al., 1988; Karagogeos et al., 1991), including adult neurons of the dorsal root ganglia and their projections (unpublished data) and spinal motor neurons (Traka et al., 2002). However, TAG-1 is also expressed in Schwann cells and oligodendrocytes and could be expected to exchange trans interactions with Caspr2. We tested this possibility using TAG-1-Fc chimeras and did not observe any binding, suggesting that in these conditions the two proteins interact directly only if they are present in the same membrane, in the same orientation. Yet, in these assays, TAG-1-Fc was readily capable to bind to membrane-bound TAG-1, in the absence or presence of cotransfected Caspr2. Thus, our results are compatible with a model in which TAG-1 interacts in cis with Caspr2 in the axolemma and in trans, through homophilic interaction, with another molecule of TAG-1 in the glial membrane (Fig. 8). A precedent for this type of interaction has been shown to occur between TAG-1 and L1 (Malhotra et al., 1998). In that case, the trans homophilic interaction between TAG-1 molecules resulted in cis activation of L1, inducing its binding to ankyrin. Although the model depicted in Fig. 8 is the simplest that accounts for all the presently available data, the possibility that additional components are part of this macromolecular complex cannot be excluded, as TAG-1 has been shown to interact with several other extracellular proteins (Milev et al., 1996; Pavlou et al., 2002). In addition, the association of Caspr2 with protein 4.1B (Denisenko-Nehrbass et al., 2003b) suggests that the TAG-1–Caspr2 ternary complexes may be attached to the cytoskeleton through this protein that has the capability to interact with actin and spectrin (Gimm et al., 2002). In TAG-1 knockout mice, we found that protein 4.1B was still present in juxtaparanodal regions although Caspr2-IR was not accumulated in these regions. This observation indicates that additional targeting mechanisms account for the localization of protein 4.1B to juxtaparanodes, and that the presence of protein 4.1B is not sufficient to induce the accumulation of Caspr2 in these regions. Therefore, we suggest that the combination of two complementary mechanisms may be required for the normal localization of the TAG-1–Caspr2 ternary complexes at juxtaparanodes: an axo–glial, TAG-1–mediated, homophilic interaction, and the anchoring of Caspr2 to cytoskeletal elements in the axon that may be found only in the vicinity of nodes of Ranvier. A prediction of this model is that TAG-1 localization should be altered in the absence of Caspr2 or of protein 4.1B.


Association of TAG-1 with Caspr2 is essential for the molecular organization of juxtaparanodal regions of myelinated fibers.

Traka M, Goutebroze L, Denisenko N, Bessa M, Nifli A, Havaki S, Iwakura Y, Fukamauchi F, Watanabe K, Soliven B, Girault JA, Karagogeos D - J. Cell Biol. (2003)

Model of the molecular organization of juxtaparanodal regions. This model is the simplest that can account for the data from previous studies and the present work. Caspr2 is enriched in the axolemma, whereas TAG-1 is expressed in both neurons and myelinating glial cells. Experiments in transfected cells show that TAG-1 interacts with Caspr2 in cis, and that TAG-1 exchanges trans interactions with itself (homophilic) but not with Caspr2. The functional importance of these interactions is demonstrated by the absence of Caspr2 enrichment in juxtaparanodal regions in TAG-1 knockout mice. These complexes are associated with other proteins including Kv1.1 and Kv1.2 potassium channels, presumably through a PDZ domain–containing protein (schematically represented here; Poliak et al., 1999; Rasband et al., 2002) and with protein 4.1B (Denisenko-Nehrbass et al., 2003b). Because protein 4.1B was still detected at juxtaparanodes in the absence of TAG-1 and Caspr2, it is likely that it interacts with other proteins, including components of the cytoskeleton (arrow). Correct localization of juxtaparanodal proteins may depend on both axo–glial interactions and binding to axoplasmic cytoskeletal components.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2172849&req=5

fig8: Model of the molecular organization of juxtaparanodal regions. This model is the simplest that can account for the data from previous studies and the present work. Caspr2 is enriched in the axolemma, whereas TAG-1 is expressed in both neurons and myelinating glial cells. Experiments in transfected cells show that TAG-1 interacts with Caspr2 in cis, and that TAG-1 exchanges trans interactions with itself (homophilic) but not with Caspr2. The functional importance of these interactions is demonstrated by the absence of Caspr2 enrichment in juxtaparanodal regions in TAG-1 knockout mice. These complexes are associated with other proteins including Kv1.1 and Kv1.2 potassium channels, presumably through a PDZ domain–containing protein (schematically represented here; Poliak et al., 1999; Rasband et al., 2002) and with protein 4.1B (Denisenko-Nehrbass et al., 2003b). Because protein 4.1B was still detected at juxtaparanodes in the absence of TAG-1 and Caspr2, it is likely that it interacts with other proteins, including components of the cytoskeleton (arrow). Correct localization of juxtaparanodal proteins may depend on both axo–glial interactions and binding to axoplasmic cytoskeletal components.
Mentions: Our findings in COS-7 cells demonstrate that TAG-1 can exchange cis interactions with Caspr2. This ability supports an association between the two proteins in the axolemma because TAG-1 is expressed in several types of neurons (Dodd et al., 1988; Karagogeos et al., 1991), including adult neurons of the dorsal root ganglia and their projections (unpublished data) and spinal motor neurons (Traka et al., 2002). However, TAG-1 is also expressed in Schwann cells and oligodendrocytes and could be expected to exchange trans interactions with Caspr2. We tested this possibility using TAG-1-Fc chimeras and did not observe any binding, suggesting that in these conditions the two proteins interact directly only if they are present in the same membrane, in the same orientation. Yet, in these assays, TAG-1-Fc was readily capable to bind to membrane-bound TAG-1, in the absence or presence of cotransfected Caspr2. Thus, our results are compatible with a model in which TAG-1 interacts in cis with Caspr2 in the axolemma and in trans, through homophilic interaction, with another molecule of TAG-1 in the glial membrane (Fig. 8). A precedent for this type of interaction has been shown to occur between TAG-1 and L1 (Malhotra et al., 1998). In that case, the trans homophilic interaction between TAG-1 molecules resulted in cis activation of L1, inducing its binding to ankyrin. Although the model depicted in Fig. 8 is the simplest that accounts for all the presently available data, the possibility that additional components are part of this macromolecular complex cannot be excluded, as TAG-1 has been shown to interact with several other extracellular proteins (Milev et al., 1996; Pavlou et al., 2002). In addition, the association of Caspr2 with protein 4.1B (Denisenko-Nehrbass et al., 2003b) suggests that the TAG-1–Caspr2 ternary complexes may be attached to the cytoskeleton through this protein that has the capability to interact with actin and spectrin (Gimm et al., 2002). In TAG-1 knockout mice, we found that protein 4.1B was still present in juxtaparanodal regions although Caspr2-IR was not accumulated in these regions. This observation indicates that additional targeting mechanisms account for the localization of protein 4.1B to juxtaparanodes, and that the presence of protein 4.1B is not sufficient to induce the accumulation of Caspr2 in these regions. Therefore, we suggest that the combination of two complementary mechanisms may be required for the normal localization of the TAG-1–Caspr2 ternary complexes at juxtaparanodes: an axo–glial, TAG-1–mediated, homophilic interaction, and the anchoring of Caspr2 to cytoskeletal elements in the axon that may be found only in the vicinity of nodes of Ranvier. A prediction of this model is that TAG-1 localization should be altered in the absence of Caspr2 or of protein 4.1B.

Bottom Line: Myelination results in a highly segregated distribution of axonal membrane proteins at nodes of Ranvier.In the absence of TAG-1, axonal Caspr2 did not accumulate at juxtaparanodes, and the normal enrichment of shaker-type K+ channels in these regions was severely disrupted, in the central and peripheral nervous systems.This complex is analogous to that described previously at paranodes, suggesting that similar molecules are crucial for different types of axo-glial interactions.

View Article: PubMed Central - PubMed

Affiliation: Department of Basic Science, University of Crete Medical School, Heraklion 71110, Crete, Greece.

ABSTRACT
Myelination results in a highly segregated distribution of axonal membrane proteins at nodes of Ranvier. Here, we show the role in this process of TAG-1, a glycosyl-phosphatidyl-inositol-anchored cell adhesion molecule. In the absence of TAG-1, axonal Caspr2 did not accumulate at juxtaparanodes, and the normal enrichment of shaker-type K+ channels in these regions was severely disrupted, in the central and peripheral nervous systems. In contrast, the localization of protein 4.1B, an axoplasmic partner of Caspr2, was only moderately altered. TAG-1, which is expressed in both neurons and glia, was able to associate in cis with Caspr2 and in trans with itself. Thus, a tripartite intercellular protein complex, comprised of these two proteins, appears critical for axo-glial contacts at juxtaparanodes. This complex is analogous to that described previously at paranodes, suggesting that similar molecules are crucial for different types of axo-glial interactions.

Show MeSH
Related in: MedlinePlus