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Neurite fasciculation mediated by complexes of axonin-1 and Ng cell adhesion molecule.

Kunz S, Spirig M, Ginsburg C, Buchstaller A, Berger P, Lanz R, Rader C, Vogt L, Kunz B, Sonderegger P - J. Cell Biol. (1998)

Bottom Line: In contrast, the axonin-1-NgCAM interaction excluded axonin-1/axonin-1 binding.These results and the examination of the coclustering of axonin-1 and NgCAM at cell contacts, suggest that intercellular contact is mediated by a symmetric axonin-12/NgCAM2 tetramer, in which homophilic NgCAM binding across the extracellular space occurs simultaneously with a cis-heterophilic interaction of axonin-1 and NgCAM.The enhanced neurite fasciculation after overexpression of NgCAM by adenoviral vectors indicates that NgCAM is the limiting component for the formation of the axonin-12/NgCAM2 complexes and, thus, neurite fasciculation in DRG neurons.

View Article: PubMed Central - PubMed

Affiliation: Institute of Biochemistry, University of Zurich, CH-8057 Zurich, Switzerland.

ABSTRACT
Neural cell adhesion molecules composed of immunoglobulin and fibronectin type III-like domains have been implicated in cell adhesion, neurite outgrowth, and fasciculation. Axonin-1 and Ng cell adhesion molecule (NgCAM), two molecules with predominantly axonal expression exhibit homophilic interactions across the extracellular space (axonin- 1/axonin-1 and NgCAM/NgCAM) and a heterophilic interaction (axonin-1-NgCAM) that occurs exclusively in the plane of the same membrane (cis-interaction). Using domain deletion mutants we localized the NgCAM homophilic binding in the Ig domains 1-4 whereas heterophilic binding to axonin-1 was localized in the Ig domains 2-4 and the third FnIII domain. The NgCAM-NgCAM interaction could be established simultaneously with the axonin-1-NgCAM interaction. In contrast, the axonin-1-NgCAM interaction excluded axonin-1/axonin-1 binding. These results and the examination of the coclustering of axonin-1 and NgCAM at cell contacts, suggest that intercellular contact is mediated by a symmetric axonin-12/NgCAM2 tetramer, in which homophilic NgCAM binding across the extracellular space occurs simultaneously with a cis-heterophilic interaction of axonin-1 and NgCAM. The enhanced neurite fasciculation after overexpression of NgCAM by adenoviral vectors indicates that NgCAM is the limiting component for the formation of the axonin-12/NgCAM2 complexes and, thus, neurite fasciculation in DRG neurons.

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Molecular models of complexes formed by axonin-1 and NgCAM. (A) Previous studies revealed  that a major proportion of monomeric  axonin-1 has a backfolded, horseshoe-like conformation in which the six Ig  domains fold back towards the membrane. The four NH2-terminal Ig domains (light gray) are shown in close  physical association. This conglomerate  bears the NgCAM binding site (after  Rader et al., 1996). (B) In one model  for a cis-axonin-1–NgCAM complex  the domain conglomerate formed by  the four NH2-terminal Ig domains of  axonin-1 interacts directly with the Ig  domains 2-4 and the third FnIII-like domain of NgCAM, Fn3 (dark gray). In  this model, a partially backfolded conformation of axonin-1 would be required. NgCAM is thought to bend  around the Ig1-4 conglomerate of axonin-1 to allow a simultaneous interaction of Ig2-4 and Fn3 of NgCAM with  Ig1-4 of axonin-1. (C) An alternative  model for the cis-axonin-1–NgCAM  complex in which the axonin-1 domains  Ig1-4 interact only with Ig2-4 of NgCAM. The absence of a direct interaction between Fn3 of NgCAM and axonin-1 Ig1-4 suggests a partially upfolded  conformation for axonin-1 in this model  of the complex. (D) Models for the presumptive axonin-1–NgCAM tetramer  that is formed at cell contacts. Intercellular binding is mediated by NgCAM  homophilic interaction that can occur simultaneously with the cis-binding between axonin-1 and NgCAM. The cis interaction between  axonin-1 and NgCAM may involve a direct interaction of the axonin-1 domains Ig1-4 with Ig2-4 and Fn3 of NgCAM (B, left model) or  only a binding between axonin-1 and Ig2-4 of NgCAM (C, right model).
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Figure 13: Molecular models of complexes formed by axonin-1 and NgCAM. (A) Previous studies revealed that a major proportion of monomeric axonin-1 has a backfolded, horseshoe-like conformation in which the six Ig domains fold back towards the membrane. The four NH2-terminal Ig domains (light gray) are shown in close physical association. This conglomerate bears the NgCAM binding site (after Rader et al., 1996). (B) In one model for a cis-axonin-1–NgCAM complex the domain conglomerate formed by the four NH2-terminal Ig domains of axonin-1 interacts directly with the Ig domains 2-4 and the third FnIII-like domain of NgCAM, Fn3 (dark gray). In this model, a partially backfolded conformation of axonin-1 would be required. NgCAM is thought to bend around the Ig1-4 conglomerate of axonin-1 to allow a simultaneous interaction of Ig2-4 and Fn3 of NgCAM with Ig1-4 of axonin-1. (C) An alternative model for the cis-axonin-1–NgCAM complex in which the axonin-1 domains Ig1-4 interact only with Ig2-4 of NgCAM. The absence of a direct interaction between Fn3 of NgCAM and axonin-1 Ig1-4 suggests a partially upfolded conformation for axonin-1 in this model of the complex. (D) Models for the presumptive axonin-1–NgCAM tetramer that is formed at cell contacts. Intercellular binding is mediated by NgCAM homophilic interaction that can occur simultaneously with the cis-binding between axonin-1 and NgCAM. The cis interaction between axonin-1 and NgCAM may involve a direct interaction of the axonin-1 domains Ig1-4 with Ig2-4 and Fn3 of NgCAM (B, left model) or only a binding between axonin-1 and Ig2-4 of NgCAM (C, right model).

Mentions: Based on these results we suggest the following molecular model: to establish a concomitant binding of Ig2-4 and Fn3 of NgCAM with the NH2-terminal Ig1-4 domain conglomerate of axonin-1, with both molecules being anchored to the same membrane by their COOH-terminal end, a backfolded structure of axonin-1 would be required. Interestingly, negative staining electron microscopy had revealed that monomeric axonin-1 has a backfolded, horseshoe-shaped domain-arrangement (Fig. 13 A; Rader et al., 1996). Thus, monomeric axonin-1 can readily fit its NH2-terminal domain conglomerate into a hypothetical binding pocket of NgCAM when both molecules are bound to the same membrane (Fig. 13 B). Although a direct interaction between Fn3 of NgCAM and the Ig1-4 domain conglomerate appears likely, we cannot exclude the possibility that axonin-1 interacts with NgCAM exclusively via a binding site localized in Ig2-4 of NgCAM. In the latter case a partial upfolding of axonin-1 from its horseshoe like conformation in the monomeric state would be required (Fig. 13 C). However, compared with the situation in a cis-axonin-1–NgCAM complex, a considerable conformational change of the axonin-1 molecule would be required to establish a contact with NgCAM residing in the membrane of an opposed cell.


Neurite fasciculation mediated by complexes of axonin-1 and Ng cell adhesion molecule.

Kunz S, Spirig M, Ginsburg C, Buchstaller A, Berger P, Lanz R, Rader C, Vogt L, Kunz B, Sonderegger P - J. Cell Biol. (1998)

Molecular models of complexes formed by axonin-1 and NgCAM. (A) Previous studies revealed  that a major proportion of monomeric  axonin-1 has a backfolded, horseshoe-like conformation in which the six Ig  domains fold back towards the membrane. The four NH2-terminal Ig domains (light gray) are shown in close  physical association. This conglomerate  bears the NgCAM binding site (after  Rader et al., 1996). (B) In one model  for a cis-axonin-1–NgCAM complex  the domain conglomerate formed by  the four NH2-terminal Ig domains of  axonin-1 interacts directly with the Ig  domains 2-4 and the third FnIII-like domain of NgCAM, Fn3 (dark gray). In  this model, a partially backfolded conformation of axonin-1 would be required. NgCAM is thought to bend  around the Ig1-4 conglomerate of axonin-1 to allow a simultaneous interaction of Ig2-4 and Fn3 of NgCAM with  Ig1-4 of axonin-1. (C) An alternative  model for the cis-axonin-1–NgCAM  complex in which the axonin-1 domains  Ig1-4 interact only with Ig2-4 of NgCAM. The absence of a direct interaction between Fn3 of NgCAM and axonin-1 Ig1-4 suggests a partially upfolded  conformation for axonin-1 in this model  of the complex. (D) Models for the presumptive axonin-1–NgCAM tetramer  that is formed at cell contacts. Intercellular binding is mediated by NgCAM  homophilic interaction that can occur simultaneously with the cis-binding between axonin-1 and NgCAM. The cis interaction between  axonin-1 and NgCAM may involve a direct interaction of the axonin-1 domains Ig1-4 with Ig2-4 and Fn3 of NgCAM (B, left model) or  only a binding between axonin-1 and Ig2-4 of NgCAM (C, right model).
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Related In: Results  -  Collection

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Figure 13: Molecular models of complexes formed by axonin-1 and NgCAM. (A) Previous studies revealed that a major proportion of monomeric axonin-1 has a backfolded, horseshoe-like conformation in which the six Ig domains fold back towards the membrane. The four NH2-terminal Ig domains (light gray) are shown in close physical association. This conglomerate bears the NgCAM binding site (after Rader et al., 1996). (B) In one model for a cis-axonin-1–NgCAM complex the domain conglomerate formed by the four NH2-terminal Ig domains of axonin-1 interacts directly with the Ig domains 2-4 and the third FnIII-like domain of NgCAM, Fn3 (dark gray). In this model, a partially backfolded conformation of axonin-1 would be required. NgCAM is thought to bend around the Ig1-4 conglomerate of axonin-1 to allow a simultaneous interaction of Ig2-4 and Fn3 of NgCAM with Ig1-4 of axonin-1. (C) An alternative model for the cis-axonin-1–NgCAM complex in which the axonin-1 domains Ig1-4 interact only with Ig2-4 of NgCAM. The absence of a direct interaction between Fn3 of NgCAM and axonin-1 Ig1-4 suggests a partially upfolded conformation for axonin-1 in this model of the complex. (D) Models for the presumptive axonin-1–NgCAM tetramer that is formed at cell contacts. Intercellular binding is mediated by NgCAM homophilic interaction that can occur simultaneously with the cis-binding between axonin-1 and NgCAM. The cis interaction between axonin-1 and NgCAM may involve a direct interaction of the axonin-1 domains Ig1-4 with Ig2-4 and Fn3 of NgCAM (B, left model) or only a binding between axonin-1 and Ig2-4 of NgCAM (C, right model).
Mentions: Based on these results we suggest the following molecular model: to establish a concomitant binding of Ig2-4 and Fn3 of NgCAM with the NH2-terminal Ig1-4 domain conglomerate of axonin-1, with both molecules being anchored to the same membrane by their COOH-terminal end, a backfolded structure of axonin-1 would be required. Interestingly, negative staining electron microscopy had revealed that monomeric axonin-1 has a backfolded, horseshoe-shaped domain-arrangement (Fig. 13 A; Rader et al., 1996). Thus, monomeric axonin-1 can readily fit its NH2-terminal domain conglomerate into a hypothetical binding pocket of NgCAM when both molecules are bound to the same membrane (Fig. 13 B). Although a direct interaction between Fn3 of NgCAM and the Ig1-4 domain conglomerate appears likely, we cannot exclude the possibility that axonin-1 interacts with NgCAM exclusively via a binding site localized in Ig2-4 of NgCAM. In the latter case a partial upfolding of axonin-1 from its horseshoe like conformation in the monomeric state would be required (Fig. 13 C). However, compared with the situation in a cis-axonin-1–NgCAM complex, a considerable conformational change of the axonin-1 molecule would be required to establish a contact with NgCAM residing in the membrane of an opposed cell.

Bottom Line: In contrast, the axonin-1-NgCAM interaction excluded axonin-1/axonin-1 binding.These results and the examination of the coclustering of axonin-1 and NgCAM at cell contacts, suggest that intercellular contact is mediated by a symmetric axonin-12/NgCAM2 tetramer, in which homophilic NgCAM binding across the extracellular space occurs simultaneously with a cis-heterophilic interaction of axonin-1 and NgCAM.The enhanced neurite fasciculation after overexpression of NgCAM by adenoviral vectors indicates that NgCAM is the limiting component for the formation of the axonin-12/NgCAM2 complexes and, thus, neurite fasciculation in DRG neurons.

View Article: PubMed Central - PubMed

Affiliation: Institute of Biochemistry, University of Zurich, CH-8057 Zurich, Switzerland.

ABSTRACT
Neural cell adhesion molecules composed of immunoglobulin and fibronectin type III-like domains have been implicated in cell adhesion, neurite outgrowth, and fasciculation. Axonin-1 and Ng cell adhesion molecule (NgCAM), two molecules with predominantly axonal expression exhibit homophilic interactions across the extracellular space (axonin- 1/axonin-1 and NgCAM/NgCAM) and a heterophilic interaction (axonin-1-NgCAM) that occurs exclusively in the plane of the same membrane (cis-interaction). Using domain deletion mutants we localized the NgCAM homophilic binding in the Ig domains 1-4 whereas heterophilic binding to axonin-1 was localized in the Ig domains 2-4 and the third FnIII domain. The NgCAM-NgCAM interaction could be established simultaneously with the axonin-1-NgCAM interaction. In contrast, the axonin-1-NgCAM interaction excluded axonin-1/axonin-1 binding. These results and the examination of the coclustering of axonin-1 and NgCAM at cell contacts, suggest that intercellular contact is mediated by a symmetric axonin-12/NgCAM2 tetramer, in which homophilic NgCAM binding across the extracellular space occurs simultaneously with a cis-heterophilic interaction of axonin-1 and NgCAM. The enhanced neurite fasciculation after overexpression of NgCAM by adenoviral vectors indicates that NgCAM is the limiting component for the formation of the axonin-12/NgCAM2 complexes and, thus, neurite fasciculation in DRG neurons.

Show MeSH
Related in: MedlinePlus