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Distinct domains of MuSK mediate its abilities to induce and to associate with postsynaptic specializations.

Zhou H, Glass DJ, Yancopoulos GD, Sanes JR - J. Cell Biol. (1999)

Bottom Line: Using this system, we found that sequences in or near the first of four extracellular immunoglobulin-like domains in MuSK are required for agrin responsiveness, whereas sequences in or near the fourth immunoglobulin-like domain are required for interaction with rapsyn.Together, our results indicate that the ectodomain of MuSK mediates both agrin- dependent activation of a complex signal transduction pathway and agrin-independent association of the kinase with other postsynaptic components.These interactions allow MuSK not only to induce a multimolecular AChR-containing complex, but also to localize that complex to a primary scaffold in the postsynaptic membrane.

View Article: PubMed Central - PubMed

Affiliation: Washington University School of Medicine, St. Louis, Missouri 63110, USA.

ABSTRACT
Agrin released from motor nerve terminals activates a muscle-specific receptor tyrosine kinase (MuSK) in muscle cells to trigger formation of the skeletal neuromuscular junction. A key step in synaptogenesis is the aggregation of acetylcholine receptors (AChRs) in the postsynaptic membrane, a process that requires the AChR-associated protein, rapsyn. Here, we mapped domains on MuSK necessary for its interactions with agrin and rapsyn. Myotubes from MuSK(-/)- mutant mice form no AChR clusters in response to agrin, but agrin-responsiveness is restored by the introduction of rat MuSK or a Torpedo orthologue. Thus, MuSK(-/)- myotubes provide an assay system for the structure-function analysis of MuSK. Using this system, we found that sequences in or near the first of four extracellular immunoglobulin-like domains in MuSK are required for agrin responsiveness, whereas sequences in or near the fourth immunoglobulin-like domain are required for interaction with rapsyn. Analysis of the cytoplasmic domain revealed that a recognition site for the phosphotyrosine binding domain-containing proteins is essential for MuSK activity, whereas consensus binding sites for the PSD-95/Dlg/ZO-1-like domain-containing proteins and phosphatidylinositol-3-kinase are dispensable. Together, our results indicate that the ectodomain of MuSK mediates both agrin- dependent activation of a complex signal transduction pathway and agrin-independent association of the kinase with other postsynaptic components. These interactions allow MuSK not only to induce a multimolecular AChR-containing complex, but also to localize that complex to a primary scaffold in the postsynaptic membrane.

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Model of MuSK-mediated postsynaptic differentiation (see Discussion). In a first step, the nerve induces subterminal clustering of MuSK, forming a primary synaptic scaffold. In a second step, nerve-derived Z+ agrin activates MuSK, inducing coclustering of rapsyn and AChRs via intracellular pathways that involve PTB domain–containing proteins but not PDZ domain proteins. In a third step, MuSK uses RATL to recruit rapsyn–AChR clusters to the primary scaffold. Sequences in or near immunoglobulin-like domains 1 are required to mediate associations with agrin, presumably via MASC, whereas juxtamembranous sequences in or near immunoglobulin-like domain 4 are required to mediate associations with rapsyn, presumably via RATL.
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Figure 9: Model of MuSK-mediated postsynaptic differentiation (see Discussion). In a first step, the nerve induces subterminal clustering of MuSK, forming a primary synaptic scaffold. In a second step, nerve-derived Z+ agrin activates MuSK, inducing coclustering of rapsyn and AChRs via intracellular pathways that involve PTB domain–containing proteins but not PDZ domain proteins. In a third step, MuSK uses RATL to recruit rapsyn–AChR clusters to the primary scaffold. Sequences in or near immunoglobulin-like domains 1 are required to mediate associations with agrin, presumably via MASC, whereas juxtamembranous sequences in or near immunoglobulin-like domain 4 are required to mediate associations with rapsyn, presumably via RATL.

Mentions: Our new results, along with those presented previously (Glass et al. 1996, Glass et al. 1997; Apel et al. 1997; Jones et al. 1999), suggest a model in which MuSK plays critical roles in three distinct steps that together lead to the formation of the postsynaptic membrane (Fig. 9). First, a signal from the nerve leads to formation of a primary synaptic scaffold of which MuSK is a component, and for which MuSK is required (Gautam et al. 1995; Moscoso et al. 1995; deChiara et al. 1996; Apel et al. 1997). The observation that MuSK clusters beneath nerve terminals in rapsyn-deficient mutant mice, whereas other components of the synaptic membrane do not, demonstrates that the primary scaffold is assembled by mechanisms distinct from those responsible for AChR clustering. The neural signal that induces formation of the primary scaffold is unknown; it may be agrin, but no published data bear directly on this point. Second, agrin released from the nerve terminal interacts with MuSK, presumably via MASC, to activate MuSK kinase. This interaction requires sequences in or near the first immunoglobulin-like domain of MuSK, and may also depend on the second immunoglobulin-like domain. Once phosphorylated, MuSK recruits PTB domain–containing proteins to initiate a signaling process that leads to formation of aggregates that contain AChRs, rapsyn, and other components of the postsynaptic membrane and cytoskeleton. Third, AChR–rapsyn clusters are recruited to the primary scaffold. This step is mediated by RATL, and requires juxtamembranous sequences of the MuSK ectodomain. It is agrin-independent in that it is mediated by constructs unable to interact with agrin. Although shown as a late step in the model, it probably occurs in parallel with formation of the primary scaffold in vivo (Noakes et al. 1993; Apel et al. 1997; Bowen et al. 1998).


Distinct domains of MuSK mediate its abilities to induce and to associate with postsynaptic specializations.

Zhou H, Glass DJ, Yancopoulos GD, Sanes JR - J. Cell Biol. (1999)

Model of MuSK-mediated postsynaptic differentiation (see Discussion). In a first step, the nerve induces subterminal clustering of MuSK, forming a primary synaptic scaffold. In a second step, nerve-derived Z+ agrin activates MuSK, inducing coclustering of rapsyn and AChRs via intracellular pathways that involve PTB domain–containing proteins but not PDZ domain proteins. In a third step, MuSK uses RATL to recruit rapsyn–AChR clusters to the primary scaffold. Sequences in or near immunoglobulin-like domains 1 are required to mediate associations with agrin, presumably via MASC, whereas juxtamembranous sequences in or near immunoglobulin-like domain 4 are required to mediate associations with rapsyn, presumably via RATL.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2169478&req=5

Figure 9: Model of MuSK-mediated postsynaptic differentiation (see Discussion). In a first step, the nerve induces subterminal clustering of MuSK, forming a primary synaptic scaffold. In a second step, nerve-derived Z+ agrin activates MuSK, inducing coclustering of rapsyn and AChRs via intracellular pathways that involve PTB domain–containing proteins but not PDZ domain proteins. In a third step, MuSK uses RATL to recruit rapsyn–AChR clusters to the primary scaffold. Sequences in or near immunoglobulin-like domains 1 are required to mediate associations with agrin, presumably via MASC, whereas juxtamembranous sequences in or near immunoglobulin-like domain 4 are required to mediate associations with rapsyn, presumably via RATL.
Mentions: Our new results, along with those presented previously (Glass et al. 1996, Glass et al. 1997; Apel et al. 1997; Jones et al. 1999), suggest a model in which MuSK plays critical roles in three distinct steps that together lead to the formation of the postsynaptic membrane (Fig. 9). First, a signal from the nerve leads to formation of a primary synaptic scaffold of which MuSK is a component, and for which MuSK is required (Gautam et al. 1995; Moscoso et al. 1995; deChiara et al. 1996; Apel et al. 1997). The observation that MuSK clusters beneath nerve terminals in rapsyn-deficient mutant mice, whereas other components of the synaptic membrane do not, demonstrates that the primary scaffold is assembled by mechanisms distinct from those responsible for AChR clustering. The neural signal that induces formation of the primary scaffold is unknown; it may be agrin, but no published data bear directly on this point. Second, agrin released from the nerve terminal interacts with MuSK, presumably via MASC, to activate MuSK kinase. This interaction requires sequences in or near the first immunoglobulin-like domain of MuSK, and may also depend on the second immunoglobulin-like domain. Once phosphorylated, MuSK recruits PTB domain–containing proteins to initiate a signaling process that leads to formation of aggregates that contain AChRs, rapsyn, and other components of the postsynaptic membrane and cytoskeleton. Third, AChR–rapsyn clusters are recruited to the primary scaffold. This step is mediated by RATL, and requires juxtamembranous sequences of the MuSK ectodomain. It is agrin-independent in that it is mediated by constructs unable to interact with agrin. Although shown as a late step in the model, it probably occurs in parallel with formation of the primary scaffold in vivo (Noakes et al. 1993; Apel et al. 1997; Bowen et al. 1998).

Bottom Line: Using this system, we found that sequences in or near the first of four extracellular immunoglobulin-like domains in MuSK are required for agrin responsiveness, whereas sequences in or near the fourth immunoglobulin-like domain are required for interaction with rapsyn.Together, our results indicate that the ectodomain of MuSK mediates both agrin- dependent activation of a complex signal transduction pathway and agrin-independent association of the kinase with other postsynaptic components.These interactions allow MuSK not only to induce a multimolecular AChR-containing complex, but also to localize that complex to a primary scaffold in the postsynaptic membrane.

View Article: PubMed Central - PubMed

Affiliation: Washington University School of Medicine, St. Louis, Missouri 63110, USA.

ABSTRACT
Agrin released from motor nerve terminals activates a muscle-specific receptor tyrosine kinase (MuSK) in muscle cells to trigger formation of the skeletal neuromuscular junction. A key step in synaptogenesis is the aggregation of acetylcholine receptors (AChRs) in the postsynaptic membrane, a process that requires the AChR-associated protein, rapsyn. Here, we mapped domains on MuSK necessary for its interactions with agrin and rapsyn. Myotubes from MuSK(-/)- mutant mice form no AChR clusters in response to agrin, but agrin-responsiveness is restored by the introduction of rat MuSK or a Torpedo orthologue. Thus, MuSK(-/)- myotubes provide an assay system for the structure-function analysis of MuSK. Using this system, we found that sequences in or near the first of four extracellular immunoglobulin-like domains in MuSK are required for agrin responsiveness, whereas sequences in or near the fourth immunoglobulin-like domain are required for interaction with rapsyn. Analysis of the cytoplasmic domain revealed that a recognition site for the phosphotyrosine binding domain-containing proteins is essential for MuSK activity, whereas consensus binding sites for the PSD-95/Dlg/ZO-1-like domain-containing proteins and phosphatidylinositol-3-kinase are dispensable. Together, our results indicate that the ectodomain of MuSK mediates both agrin- dependent activation of a complex signal transduction pathway and agrin-independent association of the kinase with other postsynaptic components. These interactions allow MuSK not only to induce a multimolecular AChR-containing complex, but also to localize that complex to a primary scaffold in the postsynaptic membrane.

Show MeSH